棒旋星系中心共振點之緻密分子雲與恆星形成

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(4) Contents Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . .. v. Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. vi. List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . .. x. List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiv 1 INTRODUCTION. 1. 1.1 Density Wave . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1. 1.1.1. Material Arms . . . . . . . . . . . . . . . . . . . . . . .. 1. 1.1.2. Lin-Shu Wave . . . . . . . . . . . . . . . . . . . . . . .. 2. 1.2 Bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2. 1.2.1. Orbits in barred potential . . . . . . . . . . . . . . . .. 2. 1.2.2. Gas Flow in Barred Galaxy . . . . . . . . . . . . . . .. 3. 1.2.3. Formation of Rings . . . . . . . . . . . . . . . . . . . .. 3. 1.3 Gas Observational Constraints of Resonances. . . . . . . . . .. 6. 1.3.1. Statistical Results . . . . . . . . . . . . . . . . . . . . .. 6. 1.3.2. The Results of Individual Barred Galaxies . . . . . . .. 7. 1.3.3. x1 and x2 Orbits in the Circumnuclear Region . . . . . 10. 1.4 Properties of NGC 7552 . . . . . . . . . . . . . . . . . . . . . 12. i.

(5) 1.4.1. Systemic Velocity . . . . . . . . . . . . . . . . . . . . . 17. 1.4.2. Nuclear Classification . . . . . . . . . . . . . . . . . . . 18. 1.4.3. Multi-wavelength View of the Central Region of NGC 7552 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20. 2. 1.4.4. The Spectral Energy Distribution of NGC 7552 . . . . 28. 1.4.5. The Resonances of NGC 7552 . . . . . . . . . . . . . . 28. OBSERVATIONS AND DATA REDUCTION. 32. 2.1 SMA Observation and Data Reduction . . . . . . . . . . . . . 32 2.2 ATCA Observation and Data Reduction . . . . . . . . . . . . 36 2.2.1. HCN and HCO+ . . . . . . . . . . . . . . . . . . . . . 36. 2.2.2. Radio Continuum . . . . . . . . . . . . . . . . . . . . . 38. 2.2.3. HI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39. 3 RESULTS. 40. 3.1 HI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.2 Radio Continuum . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.3 HCN and HCO+ . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.4 CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4 PHYSICAL PROPERTIES OF THE CIRCUMNUCLEAR MOLECULAR GAS. 60. 4.1 Emission Recovered in our Interferometric Observations . . . . 60 4.2 Physical Condition . . . . . . . . . . . . . . . . . . . . . . . . 62 4.3 Gravitational Instability . . . . . . . . . . . . . . . . . . . . . 65. ii.

(6) 5 GLOBAL KINEMATICS. 67. 5.1 Introduction of Rotation Curve . . . . . . . . . . . . . . . . . 67 5.2 Dynamical Center and Systemic Velocity of NGC 7552 . . . . 70 5.3 Rotation Curve of NGC 7552 . . . . . . . . . . . . . . . . . . 70 5.4 Dynamical Resonances . . . . . . . . . . . . . . . . . . . . . . 75 6 STAR FORMATION RATE AND TIMESCALE. 78. 6.1 Star Formation Rate . . . . . . . . . . . . . . . . . . . . . . . 78 6.1.1. Recombination Lines . . . . . . . . . . . . . . . . . . . 78. 6.1.2. Infrared . . . . . . . . . . . . . . . . . . . . . . . . . . 81. 6.1.3. Radio . . . . . . . . . . . . . . . . . . . . . . . . . . . 83. 6.2 Star Formation in the Circumnuclear Ring of NGC 7552 . . . 85 6.2.1. Star Formation Rate . . . . . . . . . . . . . . . . . . . 85. 6.2.2. Formation of Dense Molecular Gas and Stars at the Circumnuclear Ring . . . . . . . . . . . . . . . . . . . . 87. 7 SUMMARY. 91. 8 FUTURE WORK. 95. iii.

(7) Acknowledgment This Master thesis is supervised by Prof. Jeremy Lim from the University of Hong Kong and Dr. Satoki Matsushita from the ASIAA. I would like to thank Jeremy, who has taught me for nearly three years. You give me a chance to complete a scientific research. You are a good teacher, teaches me the attitude that scientists should hold and how to think scientific questions logically. Thanks for your patience that you always answer my questions again and again with different ways. I appreciate that you gave me a chance to visit HKU for 3.5 months. The experience let me realize the research and atmosphere in classrooms in one of the best university in the Asia. Thanks to Satoki for all red-penned comments on my paper draft and the thesis. You teach me that we should be scrupulous about every detail in a research and we should be confidence in our own results of the research. And I would like to thank you that you gave a chance to visit the SMA in 2009. The trip indeed enhances my dream to be an astronomer. I also want to thank Prof. Lin-wen Chen from NTNU, who leaded me into the astronomy research world. I learn a lot from your lectures. If I am able to be a teacher in a university in the future, I will remind myself the iv.

(8) attitude of teaching you insisted on. Thanks for your tolerance and support in all my decisions. Thanks to all faculties in ASIAA for fruitful discussions. Thanks to all students and assistances in NTNU and ASIAA, especially the students in Prof. Chen’s group. You always accompany me without any complaint and doubt in my NTNU life for six years. A special thank is for my family. You always allow me go my way. My twin sister, you give me the strongest shoulder to rely on and support me unconditionally. I am not proud of any achievement if I have, but I am proud because I have you.. v.

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(10) Abstract Around 50% of spiral galaxies have bars. Many barred galaxies show circumnuclear starburst rings. Theory predicts that a bar can channel molecular gas towards the central region of the galaxy. However, the collection of molecular gas does not mean stars can form in the molecular clouds. It is because the clouds are not dense enough. Diffuse molecular gas channeled in the circumnuclear starburst ring has to become dense to form stars. Kohno et al. (1999) suggested a picture of formation of dense molecular gas in the circumnuclear starburst ring based on their CO (J = 1 - 0) and HCN (J = 1 - 0) observations of a barred galaxy NGC 6951. In the picture, diffuse gas is channeled into the central region by the bar along the dust lanes. Gas collects at the intersections of the dust lanes and the circumnuclear ring. However, at the intersections, the gas are too turbulent to form dense molecular gas so stars cannot form at the intersections. Some diffuse gas will be driven into the circumnuclear ring where the velocity dispersion is smaller than that at the intersections. Dense molecular gas then can form through gravitational instability. In this case, the dense molecular gas is at the downstream of the intersections. Stars therefore can form in the dense molecular gas. We tested this picture with NGC 7552, which has almost identical morphology vi.

(11) and orientation with NGC 6951. NGC 7552 is a barred galaxy at a distance of ∼22.1 Mpc in the southern hemisphere. The circumnuclear starburst ring is already seen in previous radio continuum and infrared observations. NGC 7552 has the same major morphological features to NGC 6951. NGC 7552 has a strong bar, which extends along the east-west direction. Two spiral arms are seen in NGC 7552. NGC 7552 has two dust lanes along the bar and there is circumnuclear starburst ring at the inner ends of the dust lanes. To test the picture of Kohno et al. (1999), we need to find out where dense molecular gas forms in the circumnuclear ring. Therefore we observed HCN (J = 1 - 0) and HCO+ (J = 1 - 0) with Australia Telescope Compact Array (ATCA). To determine the physical conditions (density and temperature) of the gas, we also observed in. 12. CO (J = 2 - 1) and. 13. CO (J = 2 - 1) with. Submillimeter Array (SMA). Then physical conditions can be estimated from line ratio of HCN to CO. In the first place, we constructed a rotation curve of NGC 7552 with our 12. CO (J = 2 - 1) and HI data to understand the kinematics of this galaxy.. With the derived rotation curve, we can theoretically predict locations of dynamical resonances in NGC 7552. The inner Lindblad resonances (ILRs) and the outer Lindblad resonance (OLR) are in rough agreement with the observations. The circumnuclear ring lies between the outer inner Lindblad resonance (oILR) and inner inner Lindblad resonance (iILR). The OLR is at or beyond the outermost of spiral arms, which is located at around 10 kpc. Then we calculated physical conditions of the molecular gas in the circumnuclear ring with the Large Velocity Gradient (LVG) approximation and. vii.

(12) our. 13. CO (J = 2 - 1) and HCN (J = 1 - 0) observations. The results show. that the molecular gas in the circumnuclear ring is dense (nH2 ∼ 103 to 106 cm−3 ) and warm (> 100 K). The dense molecular gas implies that the stars can form in the circumnuclear ring since the dense molecular gas is the material of star formation. The warm molecular gas indicates that massive stars indeed have formed in this region and heat the surrounding molecular gas. For these results, we estimated star formation rate (SFR) and star formation efficiency (SFE) in the ring. The SFR is around 10 to 20 M" in the ring. Moreover, the SFR in NGC 7552 is dominated by the circumnuclear ring and the SFE of 5 × 10−9 year−1 is high comparing to normal galaxies and extreme starburst galaxies. In the last part, we discuss how the dense molecular gas forms in the circumnuclear ring. The result of our HCN (J = 1 - 0) observation shows that HCN forms at the intersections of the dust lanes and the ring. This result conflicts to the conclusion of Kohno et al. (1999) in NGC 6951. At the same time, new higher angular resolution and sensitivity HCN (J = 1 0) observation shows that the HCN also forms at the intersections in NGC 6951. We therefore suggested two ways to form molecular gas and stars in the ring. First, the molecular gas accumulates at the intersections where the dust lanes and the ring meet. Giant molecular clouds (GMCs) have more chance to collide with each other to form dense molecular gas. Secondly, the dense gas and stars form through the gravitational instability. We calculated the Toomre Q parameter in the ring. The Toomre Q parameter of 0.14 suggests that the gas in the ring is gravitational unstable. Therefore the gas can collapse to form stars.. viii.

(13) In both NGC 7552 and NGC 6951, there is a displacement between radio emission and HCN knots. The deprojected displacement is ∼ 0.## 5 and 3.## 0 in NGC 7552 and NGC 6951, respectively. We estimated the time it would take for the rotating circumnuclear ring to have carried the radio emission knots (the locations of present supernova emission) away from the present location of the HCN knots (the location of present dense molecular gas). The expectation of the timescale is approximately the lifetime of massive stars. The timescale is 3 × 106 year in NGC 6951. However, the timescale of 3 × 105 year in NGC 7552 is one order shorter than the lifetime of massive stars. The result suggests that the initial mass function (IMF) is top heavy in this starburst region in NGC 7552. The systemic error of derived rotational velocity of the ring may also lead to the shorter timescale in our galaxy.. ix.

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(16) List of Figures 1.1 The gas response in a barred galaxy. The are two families of orbits in the system, x1 and x2 , which are parallel and perpendicular to the bar, respectively. The spiral arm forms along the maximum of the gas compression (Binney & Tremaine 1987).. 4. 1.2 The gas elliptical orbits in a barred galaxy. The major axis of the ellipses rotate gradually (Kalnajs 1973). . . . . . . . . . .. 4. 1.3 The signs of gravity torques exerted by the bar in a galaxy. (a) Between the CR and OLR the gas gains angular momentum so the gas flows to exterior. (b) Between the CR and ILR the gas lose angular momentum so it falls into the center. (c) Inside the ILR, the gas gains angular momentum again and drifts outwards. (Binney & Tremaine 1987). . . . . . . . . . .. x. 5.

(17) 1.4 The examples of outer, inner, and nuclear rings. All images are carried out by UK Schmidt Telescope at 4680 ˚ A. Left panel: NGC 1543 shows a faint outer ring. 1.## 0 is equal to 67 pc. Middle panel: The example of inner ring. The NGC 1433 shows a bright inner ring at the radius of the bar end. The linear scale is 62 pc per arcsec. Right panel: The nuclear ring is shown in the NGC 1300. The linear scale is 100 pc per arcsec. (The images and the linear scales are obtained from The NASA/IPAC Extragalactic Database (NED).) . . . . . .. 7. 1.5 The location of the derived OLR in NGC 1253 compared to the HI distribution. The HI image is deprojected for i of 65◦ . The dotted lines shows the uncertainties for the derived positions of the OLR, 16.0+3.8 2.7 kpc (Clemens & Alexander 2001). . . . . 10 1.6 The schematic view of the CO and HCN distributions in circumnuclear ring (Kohno et al. 1999). The molecular gas flows into the circumnuclear ring through the dust lanes. The molecular gas concentrate at the intersection of x1 and x2 orbits. However, this region is too turbulent for dense molecular gas. Some gas will be driven into the x2 orbit. The velocity dispersion in the x2 is smaller. HCN therefore can form in this orbits. Massive star formation then will occur in these dense molecular clouds and emit radio non-therml emission when they are in supernova phase . . . . . . . . . . . . . . . . . . . 13. xi.

(18) 1.7 Comparison of the CO (1-0) and HCN (1-0) distribution (thin contours) and 6-cm radio continuum images (thick contour). The beamsize of 6-cm radio map is 2.## 57 × 1.## 44 with a P.A. = 80◦ , and contour levels are 3, 6, 9, 12, 15, and 18 σ, where 1σ = 60 muJy beam. −1. . The dust lanes are denoted as dashed. lines (Kohno et al. 1999). . . . . . . . . . . . . . . . . . . . . 14 1.8 The left panel is integrated HCN (1-0) emission (black contour) overlaid on CO (2-1) (color scale). The contours are from 3 σ to 23σ in steps of 1 σ = 0.06 Jy beam km s−1 . The black line is the major axis of the bar at P.A. = 100◦ . The grey line is the the major axis of the galaxy (P.A. = 130◦ ) (Krips et al. 2007). The right panel shows the radio image at 8.6 GHz with angular resolution of 2.## 57 ×1.## 44. (Saikia et al. 2002). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.9 The DSS-1 image of Southern Grus Quartet. There are four member galaxies, namely NGC 7552, NGC 7582, NGC 7590, and NGC 7599, in this galaxy group. . . . . . . . . . . . . . . 16 1.10 The HI (21 cm) line profile of NGC 7552. The red solid line indicates the systemic velocity of 1608.0 km s−1 derived by HIPASS project. The dashed red lines are the uncertainties of systemic velocity of ± 5.2 km s−1 . (The data of the HI spectral line was downloaded from on-line HIPASS Public Data Release (http://www.atnf.csiro.au/research/multibeam/release/) and the spectral line was plotted with IDL.). xii. . . . . . . . . . . . . 18.

(19) 1.11 The previous HI observations of Southern Grus Quartet. Top panel: The HI map overlaid on the optical emission (greyscale) from DSS. The NGC 7552 is not included in this image. This observation was made by ATCA with angular resolution of 1.# 7 × 1.# 3 (Koribalski 1996). Middle panel: The velocity field of the Grus quartet (grey scale ranging from 1280 km s−1 (white) to 1850 km s−1 (dark grey)). The observation was also made by ATCA. The beamsize is 34.## 1 × 30.## 6 (Dahlem 2005). Bottom panel: Freeland et al. (2009) used a 23 pointing mosaic to cover 1.◦ 5 × 1.◦ 5 on the sky. The observation was also carried out by ATCA. The angular resolution is 62## × 49## (Freeland et al. 2009).. . . . . . . . . . . . . . . . . . . . 19. 1.12 The diagnostic diagram. The x- and y-axis are log([OIII]/Hβ) versus log( [NII]/Hα). The circles are the HII galaxies. The squares and the triangles are the Syefert galaxies and the LINERs, respectively. The star indicates the line properties of NGC 7552.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21. 1.13 Map of the Einstein Observatory observation. The images is 1 deg2 field. The center of the image is NGC 7552. There are four detected sources. The data have been smoothed by a 32## gaussian (Charles & Phillips 1982). . . . . . . . . . . . . . . . 23 1.14 IRAC2 H-band map shows the presence of a bar within a bar. The inner bar is perpendicular to the primary bar, which is in horizontal direction of the image ( Schinnerer et al. 1997). . . 25. xiii.

(20) 1.15 The left panel is the isophotal optical image of NGC 7552 (Feistein et al. 1990). The inner and outer white contours are 20.5 and 23.0 mag arcsec. −1. , respectively. North is at. left. The arrow points the reference star for polarimetry. The field is 154## × 251## . The right panel is the Hα of NGC 7552 (Feistein et al. 1990). The orientation is the same as the left panel. The E is indicated with an arrow (See details about E in text). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 1.16 Red continuum and continuum-subtracted Hα image. The white bar in the bottom indicates 1 kpc (Hameed & Devereus 1999). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 1.17 Contour map of the 3-cm radio emission. A to D indicate the location of the knots in the circumnuclear ring. The N indicates the center of the galaxy. The peak flux is 2.5 mJy beam−1 . The contour levels are 0.1, 0.3, 0.6, 0.9, 1.2, 1.5, and 2.25 mJy beam−1 . The beamsize is 1.## 36 × 0.## 93 and PA = -5◦ (Forbes et al. 1994a).. . . . . . . . . . . . . . . . . . . . . . . 29. 1.18 Flux distribution in Janskys for NGC 7552 and M82. The open symbols indicates the SED of NGC 7552. The solid symbols indicates the SED of M82 (Feistein et al. 1990). . . . . . 30 3.1 Optical B-band image of NGC 7552 taken at the CTIO, and retrieved from the NASA Extragalactic Database.. xiv. . . . . . . 42.

(21) 3.2 Grayscale channel maps of NGC 7552 in HI. The data was taken by Dahlem (2005), but the maps shown here processed to yield a higher angular resolution than that published by Dahlem (2005) (see the main text). A cross is plotted at s α2000 =23h 16m 10.s .7 and δ2000 = −42◦ 35# 05.## 41, which coincides with the center of the circumnuclear starburst ring (Forbes et al. 1994a) and which we assume to be the dynamical center of the galaxy (see text). The barycentric velocity of each channel is shown in the upper left corner of each panel. The rms noise of each channel is 1.1 mJy, and the synthesized beam 20.## 0 × 20.## 0.. . . . . . . . . . . . . . . . . . . . . . . . . 43. 3.3 Integrated HI intensity (upper panel) and intensity-weighted mean HI velocity field (lower panel), both in color, derived from the channel maps shown in Figure 3.2. A cross is plotted at the center of the galaxy. In the upper panel, contours correspond to the optical B-band image of the galaxy shown in Figure 3.1. In the lower panel, contours (the black and white contours indicate positive and negative respect to the systemic velocity, respectively.) correspond to the intensityweighted mean HI velocity just like the color map and are plotted in steps of 20 km s−1 starting from −110 km s−1 and ending at 90 km s−1 with respect to the systemic velocity of 1580 km s−1 . The synthesized beam is shown at the lower left corner of each panel. . . . . . . . . . . . . . . . . . . . . . . . 44. xv.

(22) 3.4 Radio continuum images of the circumnuclear starburst ring in NGC 7552 at 6 cm (left panel) and 3 cm (right panel), recreated from the data taken by and published in Forbes et al. (1994a). The cross indicates the center of the ring as defined by Forbes et al. (1994a). The synthesized beam is 2.## 0 × 1.## 3 at 6 cm 1.## 1 × 1.## 1 at 3 cm, and shown at the lower left corner of the respective panels. Contour levels are plotted at (3, 7, 15, 20, 30, 40, 50, 60, and 70) × 72 µJy beam−1 (the rms noise level) at 6 cm, and (3, 7, 15, 20, 30, and 40) × 66 µJy beam−1 (rms noise level) at 3 cm. . . . . . . . . . . . . . . . . . . . . . 46 3.5 Channel maps of the central region of NGC 7552 in HCN (J = 1 - 0). Contour levels are plotted at (2, 3, 5, 7, and 10) × 4.5 mJy beam−1 (rms noise level). The barycentric velocity of each channel is shown in the upper left corner of each panel. The synthesized beam 2.## 6 × 2.## 0 at a position angle of 80◦ , and is shown at the lower left corner of the top left panel. A cross is plotted at the center of the galaxy.. xvi. . . . . . . . . . . . . . 48.

(23) 3.6 Integrated HCN (J = 1 - 0) intensity (left panel) and intensityweighted mean HCN (J = 1 - 0) velocity field (right panel) maps, both in color, derived from the channel maps shown in Figure 3.5. In the left panel, contours correspond to the 3-cm continuum image shown in Figure 3.4 but convolved to the same angular resolution as in HCN (J = 1 - 0). In the right panel, contours correspond to the intensity-weighted mean HCN (J = 1 - 0) velocity just like the color map and are plotted in steps of 20 km s−1 starting from −70 km s−1 and ending at 70 km s−1 with respect to the systemic velocity. A cross is plotted at the center of the galaxy. The synthesized beam is shown at the lower left corner of the right panel.. . . 49. 3.7 Channel maps of the central region of NGC 7552 in HCO+ (J = 1 - 0). Contour levels are plotted at (2, 3, 5, 7, and 10) × 4.5 mJy beam−1 (rms noise level). The barycentric velocity of each channel is shown in the upper left corner of each panel. The synthesized beam 2.## 6 × 2.## 0 at a position angle of 80◦ , and is shown at the lower left corner of the top left panel. A cross is plotted at the center of the galaxy.. xvii. . . . . . . . . . . . . . 50.

(24) 3.8 Integrated HCO+ (J = 1 - 0) intensity (left panel) and intensityweighted mean HCO+ (J = 1 - 0) velocity field (right panel) maps, derived from the channel maps shown in Figure 3.7. In the right panel, contours are plotted in steps of 20 km s−1 starting from −70 km s−1 and ending at 70 km s−1 with respect to the systemic velocity. A cross is plotted at the center of the galaxy. The synthesized beam is shown at the lower left corner of each panel. . . . . . . . . . . . . . . . . . . . . . . . 51 3.9 Channel maps of the central region of NGC 7552 in. 13. CO (J. = 2 - 1). Contour levels are plotted at (2, 3, 5, 7, 10, and 12)× 39 mJy beam−1 (rms noise level). The barycentric velocity of each channel is shown in the upper left corner of each panel. The synthesized beam 6.## 9 × 2.## 8 at a position angle of -8.◦ 1, and is shown at the lower left corner of the top left panel. A cross is plotted at the center of the galaxy. . . . . . . . . . . . 55. xviii.

(25) 3.10 Integrated 13 CO (J = 2 - 1) intensity (left panel) and intensityweighted mean. 13. CO (J = 2 - 1) velocity field (right panel),. both in color, derived from the channel maps shown in Figure 3.9. In the left panel, contours correspond to the integrated HCN (J = 1 - 0) intensity image shown in Figure 3.6 but convolved to the same angular resolution as in 13 CO (J = 2 - 1). In the right panel, contours correspond to the intensityweighted mean. 13. CO (J = 2 - 1) velocity field just like the. color map and are plotted in steps of 20 km s−1 starting from −70 km s−1 and ending at 70 km s−1 with respect to the systemic velocity. A cross is plotted at the center of the galaxy. The synthesized beam is shown at the lower left corner of the right panel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3.11 PV-diagrams of HCN (J = 1 - 0) (upper left panel), HCO+ (J = 1 - 0) (upper right panel), 13 CO (J = 2 - 1) (lower left panel) and 12 CO (J = 2 - 1) (lower right panel) along a position angle of 110◦ , which corresponds to the orientation of the kinematic major axis. Negative positions are located eastwards and positive positions located westwards of the galaxy center. The red circles in the. 12. rotation curve of. 12. CO (J = 2 - 1) P-V diagram indicate the. CO (J = 2 - 1). . . . . . . . . . . . . . . . 57. xix.

(26) 3.12 Channel maps of the central region of NGC 7552 in. 12. CO (J. = 2 - 1). Contour levels are plotted at (2, 3, 5, 7, 15, 20, 30, and 40) × 78 mJy beam−1 (rms noise level). The synthesized beam 7.## 0 × 2.## 8 at a position angle of -11.◦9, and is shown at the lower left corner of the top left panel. The barycentric velocity of each channel is shown in the upper left corner of each panel. A cross is plotted at the center of the galaxy. . . . 58 3.13 Integrated 12 CO (J = 2 - 1) intensity plotted in white contours (left panel) and intensity-weighted mean 12 CO (J = 2 - 1) velocity plotted in both color and contours (right panel). In the left panel, white contour levels are plotted at (3, 5, 10, 15, 35, 55, 75, and 80) × 4.5 Jy beam−1 km s−1 , and overlaid on the optical B-band image shown in Figure 3.1. The red contour is the outline of the 8 µm emission from the Spitzer SINGS project published in Kennicutt et al. (2003); this emission traces the dust lane visible as silhouette in the optical image. In the right panel, contours levels are plotted in steps of 20 km s−1 starting from −70 km s−1 and ending at 70 km s−1 with respect to the systemic velocity. A cross is plotted at the center of the galaxy. The synthesized beam is shown at the lower left corner of the right panel. . . . . . . . . . . . . . . . 59. xx.

(27) 4.1 Inferred physical conditions of the molecular hydrogen gas in the circumnuclear starburst ring based on our LVG analysis. The density of the molecular hydrogen gas is plotted as a function of the product Z(13 CO)/(dv/dr), which for the present circumstances has an adopted value of ∼4 × 10−6 (see text). The black contours in each panel indicate the. 13. CO (J = 2. - 1) opacity. The red contour is the ratio in brightness temperature of HCN (J = 1 - 0) to. 13. CO (J = 2 - 1), which. averaged over the ring has a value of 2.0 ± 0.2. The upper left panel are solutions for kinetic temperatures of 10 K, upper right panel 50 K, lower left panel 100 K, and lower right panel 1000 K. For the measured line ratio and the adopted product Z(13 CO)/(dv/dr), solutions can only be found for kinetic temperatures well above 100 K. . . . . . . . . . . . . . . . . . 64 5.1 Rotation Curve of the Milky Way. The dots represent the measured points. The curves indicate the contribution of different components in the Galaxy as well as the sum of all components. The spheroid represents the bulge and stellar halo. The corona represent the dark matter halo. (This figure is adapted from Bradley W. Carroll, and Dale A. Ostlie, An Introduction to Modern Astrophysics (2nd Edition), Benjamin Cummings, 2006) . . . . . . . . . . . . . . . . . . . . . . . . . 69. xxi.

(28) 5.2 Rotation curve derived for NGC 7552 from our. 12. CO (J = 2. - 1) maps, plotted as black circles, and HI maps, plotted as black triangles. The rotational velocity of the circumnuclear starburst ring as inferred from HCN (J = 1 - 0) is plotted as the red circle, and from HCO+ (J = 1 - 0) as the blue circle. We assumed a constant inclination of 25◦ and position angle for the kinematic major axis of 110◦ . The formal uncertainties in the deprojected rotational velocities based on the fitting function used in our maps (see text) are smaller than the size of the individual symbols. . . . . . . . . . . . . . . . . . . . . 73 5.3 Rotation curve of Figure 5.2 projected for an inclination of 25◦ shown in red circles overlaid on a PV-diagram derived from our HI maps at a position angle of 110◦ shown in contours. Contour levels are plotted at 1.5, 4, 7, 10, and 15 × 1.1 mJy beam−1 . . . . . . . . . . . . . . . . . . . . . . . . . . 74. xxii.

(29) 5.4 Diagram used to infer the locations of dynamical resonances. The angular velocity of the galaxy, Ω, based on an interpolation of the rotation curve shown in Figure 5.2 is plotted as the black solid line. The individual measurements that define the Ω − κ/2 curve are plotted as circles (from. 12. CO (J = 2 - 1)). and triangles (from HI), where κ is the epicyclic frequency of the orbit. The Ω−κ/4 curve is plotted as the dash-dotted line, and the Ω + κ/2 curve as the dashed line. Resonances occur where the values of these curves are equal to the pattern speed of the bar, assumed to be equal to the rotational velocity at the ends of the bar as indicated by the red horizontal line. . . 77 6.1 The integrated HCN (J = 1 - 0) intensity of Figure 3.6 (upper panel), and the 3-cm continuum intensity of Figure 3.4 (lower panel), plotted in polar coordinates. The ordinate is polar angle measured from east toward north, and the abscissa the radius from center. The units of the y-axis is 0.## 25.. xxiii. . . . . . . 90.

(30) List of Tables 1.1 Coordinates of the galaxies in the Southern Grus Quartet. . . 12. 1.2 Basic Properties of the galaxies in the Southern Grus Quartet. 14. 2.1 The properties of making moment maps . . . . . . . . . . . . 36. xxiv.

(31) Chapter 1 INTRODUCTION 1.1. Density Wave. 1.1.1. Material Arms. To discuss the density waves, we start from a differentially rotating axisymmetric disk with a flat rotation curve. Assume that a disk rotates at a speed Ω0 , this Ω0 will match up with a rotation speed at a radius Ω(R). If there is a spiral arm, which is made with materials, as the inner particles revolve faster than that at outer part, it will wind up into tight spiral arms. The theory predicts that the pitch angle of the materiel arms is only 0.◦ 25. However, observations show that the pitch angles are from 5◦ to 20◦ (Binney & Tremaine 1987). It implies that spiral arms are not material arms.. 1.

(32) 1.1.2. Lin-Shu Wave. The first theory was developed by Lin & Shu (1964). They suggested that spiral arms are manifestations of spiral density waves in gas and stars of a galaxy. Density wave theory provides an explanation of spiral formation. If we perturb the axisymmetric potential of a rotating disk with a small non-axisymmetric component, the rotation speed of the perturbation is Ωp = Ω0 . The Ωp is called pattern speed. We can derive that the particles move in epicyclic orbits. We can find a corotation radius where Ωp = Ω(R). The radius is known as the corotation resonance (CR). Spiral arm pattern is amplified by resonances between the epicyclic frequencies of the stars and the angular frequency of the spiral. There are two resonances discovered by Lindblad lying inside and outside to the CR (Ωp = Ω - κ/2 and Ωp = Ω + κ/2, where κ is epicyclic frequency of stars).. 1.2 1.2.1. Bar Orbits in barred potential. A bar grows through swing amplification. CR separates the galaxy into two regions. Inside to the CR, the energy and angular momentum of the waves are negative, and they are positive outside to the CR. Contopoulos & Grosbol (1989) reviewed five important orbits for a barred galaxy: 1. At the region where is very close to the center of a galaxy, the orbit is 2.

(33) parallel to the bar. The orbit is called x1 orbit. 2. At the region where is between outer inner Lindblad resonance (iILR) and outer inner Lindblad resonance (iILR), the orbits are perpendicular to the bar. They are known as x2 orbits. 3. At the region where is between oILR and CR, the orbits is parallel to the x1 orbits. 4. At the CR, there are orbits around stable Lagrangian points L4 and L5 . 5. Beyond the CR, the orbits are almost circular.. 1.2.2. Gas Flow in Barred Galaxy. In barred galaxies, the orientations of stellar orbits rotate by 90◦ at each resonance (Figure 1.1). Gas tends to follow these orbits. However, the gas is different from the stars. Gas can collide when they cross because gas is fluid. Therefore the major axis of the ellipses will rotate gradually by 90◦ at each resonance (Kalnajs 1973; See Figure 1.2).. 1.2.3. Formation of Rings. The galactic rings are formed by gas accumulating at resonances, A torque exerted by the bar change sign at each resonance, between the oILR and CR the torque is negative; between the CR and OLR it is positive (See Figure 1.3).. 3.

(34) Figure 1.1 The gas response in a barred galaxy. The are two families of orbits in the system, x1 and x2 , which are parallel and perpendicular to the bar, respectively. The spiral arm forms along the maximum of the gas compression (Binney & Tremaine 1987).. Figure 1.2 The gas elliptical orbits in a barred galaxy. The major axis of the ellipses rotate gradually (Kalnajs 1973). 4.

(35) Figure 1.3 The signs of gravity torques exerted by the bar in a galaxy. (a) Between the CR and OLR the gas gains angular momentum so the gas flows to exterior. (b) Between the CR and ILR the gas lose angular momentum so it falls into the center. (c) Inside the ILR, the gas gains angular momentum again and drifts outwards. (Binney & Tremaine 1987).. 5.

(36) 1.3. Gas Observational Constraints of Resonances. The relation between the resonances and galactic features are suggested in previous studies of other galaxies. According to the bar theory, There is massive pile-up of gas at the ILR and OLR radius.. 1.3.1. Statistical Results. We summarize the results of Buta (1986). Buta (1986) presented the statistical results of around 1200 ring galaxies in the southern sky. There are three types of rings in the galaxies. First of all, the outer rings are the largest rings. Outer rings have diameter ∼2.2 ± 0.4 times larger than the length of bars (Kormendy 1979). The fraction of galaxies with the outer rings is difficult to estimate since the observations were not always deep enough to show the outer rings and the outer rings occur in only a few percent of galaxies. Secondly, the inner rings have diameter similar to the bars. The inner rings are generally more elliptical than the outer rings. The axis ratio of inner rings ranges 0.6 - 0.95 (Buta 1986). Buta (1986) suggested that both inner and outer rings are bluer than the galactic disk and there are many HII regions at the inner and outer rings. The third type of ring suggested by Buta (1986) is nuclear ring. The nuclear ring are usually very small with a few hundred parsec. They are nearly circular. They usually show active star formation. The examples of rings structure are shown in Figure 1.4. NGC 1543 has 6.

(37) Figure 1.4 The examples of outer, inner, and nuclear rings. All images are carried out by UK Schmidt Telescope at 4680 ˚ A. Left panel: NGC 1543 shows a faint outer ring. 1.## 0 is equal to 67 pc. Middle panel: The example of inner ring. The NGC 1433 shows a bright inner ring at the radius of the bar end. The linear scale is 62 pc per arcsec. Right panel: The nuclear ring is shown in the NGC 1300. The linear scale is 100 pc per arcsec. (The images and the linear scales are obtained from The NASA/IPAC Extragalactic Database (NED).) a faint outer ring beyond the bar. NGC 1433 has a inner ring. The inner ring is bright and elliptical in the optical image. NGC 1300 has a nuclear ring in the central of the galaxy. Buta (1986) suggested that the rings are related to the resonances. The outer ring links to the OLR. The inner ring suggests the radius of CR and the nuclear ring is at ILR.. 1.3.2. The Results of Individual Barred Galaxies. The ILRs (or the circumnuclear rings) are often close to the turnover radii of rotation curves (Forbes et al. 1994a, b; Telesco & Decher 1988; Diaz et al. 1999), and the turnover points are closer to nuclei than those in other. 7.

(38) normal galaxies without central starbursts (Storchi-Bergmann, Wilson, & Baldwin 1996), indicating that the galaxies with circumnuclear ring have higher central mass concentration. NGC 1068 is the brightest and closest Seyfert 2 galaxy. The nuclear ringlike structure is detected in many molecular lines (e.g., Planesas et al. 1991; Jackson et al. 1993; Helfer & Blitz 1995; Tacconi et al. 1997) and in radio continuum (e.g. Gallimore et al. 1996). Schinnerer et al. (2000) observed NGC 1068 in. 12. CO ( J = 1 - 0) and. 12. CO (J = 2 - 1) in the central region. with the IRAM interferometer. The angular resolutions are 1.## 4 and 0.## 7 in 12. CO (J = 1 - 0) and. 12. CO (J = 2 - 1), respectively. The nuclear ring-like. structure gas is resolved into two tightly-wound spiral arms with a radius of 20.## 0 (∼1.4 kpc). They used a common assumption that the CR lies at the end of the bar. The pattern speed of 20 ± 10 km s−1 can therefore be derived from the previous HI observation, which provided the velocity field of the entire galaxy. Then the radii of resonances are defined. The radius of ILR is 18.##0 (1.3 kpc), which is near the molecular tightly-wound spiral arms. The result suggests that the molecular gas indeed accumulates at the resonance radius. Diaz et al. (1999) presented a velocity field of the central 2 kpc of a LINER galaxy NGC 1672. NGC 1672 is a barred galaxy in the southern hemisphere. It is a face-on galaxy with i = 40◦ . The star formation activity in the circumnuclear ring is already seen. The rotation curve was constructed with optical measurements. The maximum velocity is around 190 km s−1 at 0.4 kpc in the rotation curve. They concluded that this galaxy has a circumnuclear ring which located near the turnover of the rotation curve and. 8.

(39) the circumnuclear ring also located on iILR (suppose that the CR is at the end of the bar). However, the result of NGC 5728 conflicts with the results above. Schommer et al. (1988) derived the rotation curve of barred Seyfert 2 galaxy NGC 5728, which is a barred galaxy with a starburst ring shown in its colour maps. Two dust lanes are also seen emerging from the nuclear region and the dust lanes are parallel to the bar. By assuming the bar ends at CR, they computed that the nuclear ring of the galaxy is located just inside the ILR. Laine et al. (2002) concluded that 28 ± 5% barred galaxies have inner bars. Diaz et al. (2003) presented an inner bar-like structure, which is approximately perpendicular to the large-scale bar of NGC 1241 in Paα emission image. They computed the Ωp of both primary and inner bars. The derived resonances showed that the circumnuclear ring is located just inside the large-scale ILR. They suggested that the perturbation of a companion of NGC 1241 is modified the dynamics and morphology of the galactic systems from global scale down to the nuclear region. Clemens & Alexander (2001) observed NGC 1253 with the VLA in 21 cm. NGC 1253 is at the distance of ∼23 Mpc. NGC 1253 is a gas-rich interacting system composed of NGC 1253 and NGC 1253A. NGC 1253 and NGC 1253A are classified as SAB(rs) and SB(s), respectively. There is a prominent HI ring with deprojected radius between 12.1 to 15.5 kpc depending on azimuth angle. The HI ring is at the edge of the distribution of HI emission. The Ωp is 19 ± 3 km s−1 kpc−1 in this galaxy. The radii of resonances can be dened once the Ωp is known. The OLR lies within the HI ring. The result is consistent the expectation that the material accumulates in the OLR (Figure. 9.

(40) Figure 1.5 The location of the derived OLR in NGC 1253 compared to the HI distribution. The HI image is deprojected for i of 65◦ . The dotted lines shows the uncertainties for the derived positions of the OLR, 16.0+3.8 2.7 kpc (Clemens & Alexander 2001). 1.5).. 1.3.3. x1 and x2 Orbits in the Circumnuclear Region. Kohno et al. (1999) observed NGC 6951 in CO (J = 1 - 0) and HCN (J = 1 - 0) emission. They suggested that the formation of dense molecular gas in the circumnuclear region is through the gravitational instabilities of molecular gas. They therefore provided a picture to explain the formation of dense molecular gas in the ring in terms of x1 and x2 orbits. The molecular 10.

(41) gas is channeled by the bar into the central region along the dust lanes, which is nearly parallel to the x1 orbit (see Figure 1.6). However, at the contact points where the dust lanes and circumnuclear ring meet (the contact points are also the intersections of x1 and x2 orbits), the velocity dispersion is too turbulent to form dense molecular clouds. For this reason, we can only see the CO (traces diffuse molecular gas) concentrations at the intersections. Some of diffuse molecular gas will be driven into x2 orbit, which is perpendicular to the bar. The gas in x2 orbit become unstable becuase it has smaller velocity dispersion. The dense molecular gas are formed via gravitational instabilities at the x2 orbit. Therefore the HCN concentrations in the x2 appear in the downstream of CO concentrations. This picture is sketched in Figure 1.6. Since the dense molecular gas traced by HCN (J = 1 - 0) is the material of star formation. Stars then can form in the HCN (J = 1 - 0) concentrations. Some of stars are massive stars. They will evolve into supernova phase at the end of their lifetime and emit non-thermal radio emission (synchrotron radiation). Therefore the concentrations of radio emission (6-cm) are also seen at the locations of HCN concentrations. However, the recent high angular resolution and sensitivity CO, HCN (beamsize 2.## 78 × 2.## 27) (Krips et al. 2007) and 6-cm continuum (beamsize 2.## 57 × 1.## 44) (Saikia et al. 2002) observations of NGC 6951 did not support the picture above. In the left panel of the Figure 1.8 the CO and HCN locate at the same position where the dust lane the ring meet which is different from what Kohno et al. (1999) suggested. Furthermore, the new 6-cm observation with comparable resolution to CO and HCN observations shows that the 6-. 11.

(42) Table 1.1. Coordinates of the galaxies in the Southern Grus Quartet. Properties. RA. DEC. NGC 7552 23h 16m 10.s 7. -42◦ 35# 4.## 41. NGC 7582 23h 18m 23.s 5. -42◦ 22# 14.## 0. NGC 7590 23h 18m 54.s 8. -42◦ 14# 21.## 0. NGC 7599 23h 19m 21.s 1. -42◦ 15# 25.## 0. cm continuum emission emitted by supernovae is in the downstream with respect to CO and HCN peaks. We suggest that the different results of the Kohno et al. (1999) and other observations is due to the low angular resolution (3.## 9 × 3.## 1 for CO and 4.## 7 × 4.## 5 for HCN) of the observations in Kohno et al. (1999).. 1.4. Properties of NGC 7552. NGC 7552 is one of the Southern Grus Quartet (NGC 7552, NGC 7582, NGC 7590, and NGC 7599) in the southern hemisphere with distance (D) of 22.1 Mpc (H0 = 73 km s−1 Mpc−1 ; see Figure 1.9). NGC 7552 is a barred galaxy classified as SBbc. The linear scale is 107 pc per arcsecond. Assuming a circular size, It is a low inclination (i = 25◦ ) with the inclination obtained the ratio of optical major to minor axis (a/b closes to 1.14; Feinstein et al. 1990; Forbes et al. 1994a).. 12.

(43) Figure 1.6 The schematic view of the CO and HCN distributions in circumnuclear ring (Kohno et al. 1999). The molecular gas flows into the circumnuclear ring through the dust lanes. The molecular gas concentrate at the intersection of x1 and x2 orbits. However, this region is too turbulent for dense molecular gas. Some gas will be driven into the x2 orbit. The velocity dispersion in the x2 is smaller. HCN therefore can form in this orbits. Massive star formation then will occur in these dense molecular clouds and emit radio non-therml emission when they are in supernova phase .. 13.

(44) Figure 1.7 Comparison of the CO (1-0) and HCN (1-0) distribution (thin contours) and 6-cm radio continuum images (thick contour). The beamsize of 6-cm radio map is 2.## 57 × 1.## 44 with a P.A. = 80◦ , and contour levels are 3, 6, 9, 12, 15, and 18 σ, where 1σ = 60 muJy beam. −1. . The dust lanes are. denoted as dashed lines (Kohno et al. 1999).. Table 1.2. Basic Properties of the galaxies in the Southern Grus Quartet. Properties. Vsys (km s−1 ) Diameter(arcmin). Morphology Spectral Type. NGC 7552 1580 ± 80. 3.4(3.0). SBbc. LINER/HII. NGC 7582 1623 ± 80. 5.0(2.1). SBab. Seyfert 1. NGC 7590 1515 ± 80. 2.7(1.0). SAbc. Seyfert 2. NGC 7599 1753 ± 80. 4.4(1.3). SB(s)c. LINER. 14.

(45) Figure 1.8 The left panel is integrated HCN (1-0) emission (black contour) overlaid on CO (2-1) (color scale). The contours are from 3 σ to 23σ in steps of 1 σ = 0.06 Jy beam km s−1 . The black line is the major axis of the bar at P.A. = 100◦ . The grey line is the the major axis of the galaxy (P.A. = 130◦ ) (Krips et al. 2007). The right panel shows the radio image at 8.6 GHz with angular resolution of 2.## 57 ×1.## 44. (Saikia et al. 2002).. 15.

(46) Figure 1.9 The DSS-1 image of Southern Grus Quartet. There are four member galaxies, namely NGC 7552, NGC 7582, NGC 7590, and NGC 7599, in this galaxy group.. 16.

(47) 1.4.1. Systemic Velocity. The Neutral Hydrogen (HI; λ = 21 cm) was observed in NGC 7552 through HI Parkes All-Sky Survey (HIPASS) (Staveley-Smith et al. 1996). The HIPASS covered the whole southern sky as well as the northern DEC up to +25◦ . HIPASS was carried out with the Australia Telescope National Facility (ANTF) Parkes 64 m telescope from 1997 February to 2001 December. Figure 1.10 shows the HI line profile of NGC 7552. As we can see from Figure 1.10, the HI spectra of NGC 7552 is not symmetric. The flux of the line increases towards high velocity. The solid red line indicates the Vsys derived by the best-gaussion-fit of HI spectra and the dashed red lines represent the error bars of the systemic velocity (Vsys ) coming from the uncertainty of the peak of the gaussian. The Vsys derived from HI line is 1608.0 ± 5.2 km s−1 . The HI observations (Koribalski 1996; Dahlem 2005; Freeland et al. 2009; see Figure 1.11) indicate a tidal interaction between NGC 7552 and the other galaxy members in this group. The interaction may leads to the asymmetry in the HI spectra of NGC 7552. We therefore use other measurement to define the systemic velocity of NGC 7552. Optical stellar photospheric absorption lines are emitted from ionized gas and absorbed by stellar atmosphere. Thus optical lines trace stars in NGC 7552 directly. The optical spectroscopic Vsys is obtained with the du Pont 2.5 m telescope at the Las Campanas Observatory by measuring 109 optical absorption lines absorbed by stellar atmosphere in a range of 3500 - 6500 ˚ Ain Southern Grus Quartet. The optical systemic velocity is 1580 ± 80 km s−1 (Zabludoff & Mulchaey 1998), which is consistent with the HIPASS result. The large uncertainty in optical Vsys is caused by velocity dispersion of stars. 17.

(48) Figure 1.10 The HI (21 cm) line profile of NGC 7552. The red solid line indicates the systemic velocity of 1608.0 km s−1 derived by HIPASS project. The dashed red lines are the uncertainties of systemic velocity of ± 5.2 km s−1 . (The data of the HI spectral line was downloaded from on-line HIPASS Public Data Release (http://www.atnf.csiro.au/research/multibeam/release/) and the spectral line was plotted with IDL.) The 1580 km s−1 is applied as Vsys of NGC 7552.. 1.4.2. Nuclear Classification. The nuclear spectral classification of NGC 7552 is not clear. Ward et al. (1980) showed that this galaxy has low-excitation lines in the nucleus so it could be a HII region galaxy. Durret & Bergeron (1988) observed the nucleus NGC 7552 with the European Southern Observatory (ESO) 3.6 m 18.

(49) Figure 1.11 The previous HI observations of Southern Grus Quartet. Top panel: The HI map overlaid on the optical emission (greyscale) from DSS. The NGC 7552 is not included in this image. This observation was made by ATCA with angular resolution of 1.# 7 × 1.# 3 (Koribalski 1996). Middle panel: The velocity field of the Grus quartet 19 (grey scale ranging from 1280 km s−1 (white) to 1850 km s−1 (dark grey)). The observation was also made by ATCA. The beamsize is 34.## 1 × 30.## 6 (Dahlem 2005). Bottom panel: Freeland et al. (2009) used a 23 pointing mosaic to cover 1.◦ 5 × 1.◦ 5 on the sky. The observation was also carried out by ATCA. The angular resolution is 62## × 49## (Freeland et al. 2009)..

(50) telescope at La Silla, Chile. They classified the galaxy therefore as a weak Low-ionization nuclear emission-line region (LINER; Heckman 1980). The forbidden lines in LINERs tend to arise in less highly ionized atoms compared with AGNs. The LINERs are found at the center of ∼80% of Sa and Sb galaxies. Bonatto, Bica & Alloin (1989) provides the emission line properties in nucleus of NGC 7552. The observation was carried out by the 1.52 m telescope at La Silla. The Hα, Hβ, Hγ, Hδ, [OIII], [NII], [SII]6717˚ A, and [SII]6731˚ A lines were detected. However, [OI] emission is not detected in this observation. We used the diagnostic diagram of Veilleux & Osterbrock (1987) with log([OIII]/Hβ) versus log([NII]/Hα) to constrain the spectral classification of the nucleus of NGC 7552. The diagnostic diagram is widely used to distinguish Seyfert, HII, and LINERs galaxies in terms of optical line ratios. The intensities of optical lines we used are from Bonatto, Bica & Alloin (1989). The ratio of log([OIII]/Hβ) and log([NII]/Hα) are -0.7 and -0.2, respectively. The data of known Seyfert galaxies, LINER, and HII galaxies are from Ho (1997). The number of galaxies are Seyfert (50), LINER (81), and HII galaxy (200). The result of diagnostic diagram is shown in the Figure 1.12. NGC 7552 indeed locates between the population of HII galaxies and LINERs.. 1.4.3. Multi-wavelength View of the Central Region of NGC 7552. Starburst ring is a common galactic feature in barred galaxies. The ring is an important tracer of the dynamics in the nuclei of host galaxies (e.g., Buta 20.

(51) Figure 1.12 The diagnostic diagram. The x- and y-axis are log([OIII]/Hβ) versus log( [NII]/Hα). The circles are the HII galaxies. The squares and the triangles are the Syefert galaxies and the LINERs, respectively. The star indicates the line properties of NGC 7552.. 21.

(52) 1986; Kohno et al. 1999; Schinnerer et al. 2000; Sarzi et al. 2007; Mazzuca et al. 2008; Espada et al. 2009), since the ring associates with dynamical resonances in a galaxy. NGC 7552 is well known for its circumnuclear starburst ring (e.g., Forbes et al. 1994a, b; Schinnerer et al. 1997). The AGN activity is weak in this galaxy (Forbes et al. 1994b). Therefore it is possible to study the intrinsic properties of the circumnuclear region without any contamination of emission emitted from the AGN. In this section we introduce the main observational results in literatures of NGC 7552. X-ray The X-ray is detected in the nucleus of NGC 7552 (Charles & Phillips 1982) with the Einstein Observatory. NGC 7552 shows a point source in the X-ray observation (see Figure 1.13). The X-ray luminosity (LX = 8 × 1040 erg s−1 ) of NGC 7552 is ∼30 times larger than that of normal spiral galaxies such as M31. Weedman et al. (1980) suggested a starburst nuclear model for the nucleus of NGC 7714 can be accounted for the X-ray and radio properties of the nucleus of NGC 7552 rather than AGN (Charles & Phillips 1982). They compared NGC 7552 with the nuclear region of M82 (Rieke et al. 1980) and concluded that the ratio of 1415 MHz flux to the X-ray luminosity is identical to that of M82. This ratio can be explained by a few thousand supernova remnants (SNRs) scaling from typical galactic value. Infrared Infrared continuum are better at revealing bulk of stellar population than optical, which highlight luminous massive stars. However, massive stars make 22.

(53) Figure 1.13 Map of the Einstein Observatory observation. The images is 1 deg2 field. The center of the image is NGC 7552. There are four detected sources. The data have been smoothed by a 32## gaussian (Charles & Phillips 1982).. 23.

(54) up only a small fraction of the total stellar mass. Infrared lines have less subject to extinction, and the H2 traces hot molecular hydrogen. Schinnerer et al. (1997) presented JHK continuums, most of the prominent near-infrared K-band lines (Brγ, He, and H2 in emission and. 12. CO,. 13. CO in absorption),. and mid-infrared (N-band continuum) high angular resolution spectroscopic and image data of the nuclear region of NGC 7552. The observations were made by the ESO 3.5 m NTT on La Silla, Chile. The circumnuclear ring is resolved in the observations. Their spectral synthesis of individuals knots in the ring suggested the existence of young stellar clusters. The analysis also shows that the starbursts in the prominent knots have age of ∼1.5 × 107 year and the starburst activity is decaying in the ring. Furthermore, the bar within bar system is seen in the central region of the galaxy in the IRAC2 H-band image (see Figure 1.14). Forbes et al. (1994b) also suggested a inner bar by their H2 observation. The inner bar-like structure is perpendicular to the primary bar. Optical Feistein et al. (1990) presented an optical morphology study of NGC 7552. It is the first work linking the observational galactic features with the dynamical resonances. First of all, there is a prominent bar and two dominant outer arms (see left panel in Figure 1.15). Secondly, there is a giant HII region located in the eastern bar is shown in Hα image (labeled as ”E” in the right panel of Figure 1.15). The HII region also contributes to the radio continuum (3-cm and 6-cm) emission in later observations, indicating the vast amount of supernova explosions. Feistein et al. (1990) suggested that 24.

(55) Figure 1.14 IRAC2 H-band map shows the presence of a bar within a bar. The inner bar is perpendicular to the primary bar, which is in horizontal direction of the image ( Schinnerer et al. 1997).. 25.

(56) the HII regions probably define the CR radius. Thirdly, the center of the galaxy is surrounded by a ring. The ring emits most of the far infrared flux. The ring may suggests the location of ILR of NGC 7552. The Hα and optical continuum observations made by Cerro Tololo InterAmerican Observatory (CTIO) in Chile are showed in Hameed & Devereus (1999) (see Figure 1.16). The observations were made in 24 October 1997 with exposure time of 900 seconds. Hameed & Devereus (1999) suggested that the continuum images of NGC 7552 reveal a uncataloged dwarf galaxy at the end of the northern spiral arm appearing to be interacting with the larger galaxy. However, there is no detectable Hα emission from the dwarf companion. CO Observation Claussen & Sahai (1992) observed the. 12. CO (1-0) emission with Swedish-. ESO15m Submillimeter Telescope (SEST) from NGC 7552. The intensity of 12. CO (1-0) in the nucleus is five times larger than the rest of the galaxy, and. ICO is stronger along the major axis than that of the minor axis, indicating a gaseous bar along the stellar bar. Centimeter Radio Continuum Observation Forbes et al. (1994a) discovered a circumnuclear ring with a size of only 1 kpc in 3-cm and 6-cm radio continuum emissions. The observation were carry out by the ATCA. There are five knots in the ring in 3-cm image, and two northerly knots have the strongest intensity (Figure 1.17). The ring is presented in both NIR and optical color maps (Forbes et al. 1994a). The 26.

(57) Figure 1.15 The left panel is the isophotal optical image of NGC 7552 (Feistein et al. 1990). The inner and outer white contours are 20.5 and 23.0 mag arcsec. −1. , respectively. North is at left. The arrow points the reference star. for polarimetry. The field is 154## × 251## . The right panel is the Hα of NGC 7552 (Feistein et al. 1990). The orientation is the same as the left panel. The E is indicated with an arrow (See details about E in text).. 27.

(58) Figure 1.16 Red continuum and continuum-subtracted Hα image. The white bar in the bottom indicates 1 kpc (Hameed & Devereus 1999). ring can also be seen in its Hα and Brγ line emission (Forbes et al. 1994b).. 1.4.4. The Spectral Energy Distribution of NGC 7552. The overall spectral distribution from X-ray to the radio wavelengths compared with starburst galaxy M82 is provided in Feistein et al. (1990). The SED of NGC 7552 is similar to that observed in M82 (see Figure 1.18).. 1.4.5. The Resonances of NGC 7552. NGC 7552 shows three types of rings, including the nuclear ring, the inner ring and the outer ring (Forbes et al. 1994a). Forbes et al. (1994a) defined the rings with broadband BVRI and Hα optical images carried out by CTIO and radio continuum image from ATCA. Forbes et al. (1994a) suggested the location of circumnuclear ring is also the radius of ILR. There is an inner ring with diameter 15.0 × 9.9 kpc in Hα image, emanating from the end of 28.

(59) Figure 1.17 Contour map of the 3-cm radio emission. A to D indicate the location of the knots in the circumnuclear ring. The N indicates the center of the galaxy. The peak flux is 2.5 mJy beam−1 . The contour levels are 0.1, 0.3, 0.6, 0.9, 1.2, 1.5, and 2.25 mJy beam−1 . The beamsize is 1.## 36 × 0.## 93 and PA = -5◦ (Forbes et al. 1994a).. 29.

(60) Figure 1.18 Flux distribution in Janskys for NGC 7552 and M82. The open symbols indicates the SED of NGC 7552. The solid symbols indicates the SED of M82 (Feistein et al. 1990).. 30.

(61) the bar. Forbes et al. (1994a) suggested that it is CR. In faint levels, there is another weak Hα circular ring in the end of the bar with the diameter 18.0 × 16.0 kpc. Forbes et al. (1994a) suggested that it is outer ring at OLR. In Hα image, there is a prominent HII region in 35.## 0 east of the nucleus, it may implies the location of UHR (Forbes et al. 1994a).. 31.

(62) Chapter 2 OBSERVATIONS AND DATA REDUCTION 2.1. SMA Observation and Data Reduction. We observed NGC 7552 in 12 CO (J = 2-1) and 13 CO (J = 2-1) in the compact configuration of the Submillimeter Array (SMA) on 4 August 2006. Our observation was centered at α2000 = 23h 16m 10.s 7 and δ2000 = −42◦ 35# 05.## 41, coinciding with the center of the circumnuclear starburst ring as defined by Forbes et al. (1994a), and covered a field of diameter 55## (FWHM). Seven of the eight antennas of the rSMA were available during our observation. With baseline lengths ranging from 16 m to 69 m, the largest structure (of uniform brightness) that we expect to be sensitive to is about 16## . The receiver was tuned and the correlator configured to cover the frequency range 227.6– 229.6 GHz in the upper sideband (USB) thus including the 12 CO (J = 2-1) line, and 217.6–219.6 GHz in the lower sideband (LSB) thus including the 32.

(63) 13. CO (J = 2-1) line. The correlator provided a total of 24 spectral windows. (chunks) in each sideband, with a total of 128 channels and a bandwidth of 104 MHz in each chunk. To make the channel maps in 12 CO (J = 2-1), we averaged 10 channels together resulting in a velocity resolution of 10 km s−1 . Because the signal is weaker in 13 CO (J = 2-1), to make the channel maps in this line we averaged 18 channels together resulting in a velocity resolution of 18 km s−1 . We calibrated the data separately for bandpass, amplitude, and phase using SMA-specific MIR tasks adopted from the MMA software package (written in IDL), which was originally developed for the Owens Valley Radio Observatory (OVRO; Scoville et al. 1993). It is important to calibrate the instrument bandpass response to derive accurate spectral line data. A long observation of a unresolved calibrator is required to determine the channel-to-channel gains of the spectral channels. We observed Uranus for bandpass calibration. The calibration is not relevant to the flux of Uranus. It is because the amplitude of spectrum of the calibrator is normalzed to unity before applying the bandpass calibration. Before deriving bandpass solutions from our observation of Uranus, we checked for baseline-based errors by deriving the complex gain (i.e., amplitude and phase) of the telescope during our observation of Uranus using both antenna-based and baseline-based approaches. After discarding 3 chunks from the data that showed abnormally low amplitudes on 10 baselines, we found that the dynamic range of the map for Uranus is superior (by a factor of ∼5) using baseline-based rather than antenna-based solutions (indicating significant baseline-based errors). As a result, we adopted a baseline-based. 33.

(64) approach for deriving calibration solutions for the telescope. For complex gain calibration, we observed the quasars 2235-485 and 2258279 for a duration of 6 min every 30 min. We used 2235-485, which is located at an angular distance of 9.◦ 3 from our target source, for phase calibration. Unfortunately, 2235-485 is too weak for amplitude calibration. We therefore used 2258-279, which is located at a larger angular distance of 15◦ from our target source, for amplitude calibration. Because the amplitude solutions derived from the individual scans did not differ significantly between adjacent scans, we average three scans of 2258-279 at a time to derive more precise amplitude solutions. We derived the absolute flux calibration scale from Uranus, which we assumed had a flux density of 36.836 Jy at the time of our observation. We imaged the data using MIRIAD. MIRIAD was designed and developed by BIMA groups in 1988. To eliminate the sidelobes, we first CLEANed the dirty maps without any preselection to judge which features are likely to be real and which are likely to be sidelobes. First of all, the program finds the peak intensity of the dirty image, then subtract a fraction of f with the shape of the dirty beam from the image. Then repeat this n times until the intensities of the remaining peaks are below a limit. The output of the CLEAN is the best guess of what the emission really looks like. We then placed boxes around those features judged to be real on the basis that they do not appear to resemble the sidelobes of the DIRTY beam, and that their structure changed continuously between adjacent channels. As shown in §3.4, the features in each channel have a sufficiently simple structure that we are confident of having picked out only those that are real. The. 34. 12. CO (J =.

(65) 2 - 1) channel maps have an angular resolution of 7.## 0 × 2.## 8 and a rootmean-square (rms) noise level of 78 mJy beam−1 , and the. 13. CO (J = 2 - 1). channel maps an angular resolution of 6.## 9 × 2.## 8 and a rms noise level of 39 mJy beam−1 . Because the features detected span an angular extent from the pointing center that is significantly smaller than the primary beam, we did not apply a primary beam correction to the maps. The task MOMNT in the AIPS is used to generate moment images from spectral line data cubes. The zeroth, first, and the second moment maps are defined as M0 =. !. I(v)dv,. " I(v)vdv M1 = " , I(v)dv # I(v)(v − M1 )2 dv " M2 = , I(v)dv. (2.1) (2.2) (2.3). respectively. The zeroth, first, and second moment maps are integrated intensity, intensity-weighted mean velocity field, and velocity dispersion maps, respectively. For making moment maps, hanning smoothing was applied and a flux cutoff is defined. By playing different width of smoothing functions and cutoff levels, we adopted final maps which have the largest spatial sizes and the least noise in the velocity fields. The width of smoothing functions (Cellsize) and cutoff level of all molecular lines are displayed the following Table.. 35.

(66) Table 2.1. The properties of making moment maps. Noise (Jy beam−1 km s−1 ). Cellsize. Cutoff (σ). 12. CO (J = 2 - 1). 4.5. 7. 1.5. 13. CO (J = 2 - 1). 3.8. 7. 1.8. HCN (J = 1 - 0). 0.3. 5. 1.5. HCO+ (J = 1 - 0) 0.3. 5. 1.5. HI. 11. 1.0. 2.2 2.2.1. 0.1. ATCA Observation and Data Reduction HCN and HCO+. We observed NGC 7552 in HCN (J = 1 - 0) and HCO+ (J = 1 - 0) with the ATCA on three separate occasions between May and September 2005 in the H75, H168, and H214 configurations. This combination of array configurations provides baseline lengths ranging from 30 m to 200 m, providing sensitivity to structures (with uniform brightnesses) as large as about 23## . Five of the six antennas of the ATCA are equipped with dual polarization receivers in the 3 mm band, covering a frequency range from 85 GHz to 105 GHz. At the wavelength of the HCN (J = 1 - 0) (rest frequency of 88.63 GHz) and HCO+ (J = 1 - 0) lines (rest frequency of 89.18 GHz), the primary beam of the antennas is ∼36## (FWHM). We configured the correlator to observe the HCN (J = 1 - 0) and HCO+ (J = 1 - 0) lines simultaneously in 36.