Figure 3.9 shows our channel maps of the13CO (J = 2 - 1) line. Only a sin-gle compact component is discernible that shifts from west to east across the center of the galaxy with increasing velocity, just like that seen in HCN (J = 1 - 0) and HCO+ (J = 1 - 0) as described above in §3.3. This compact cen-tral component is detectable over the velocity range from about −95 km s−1 to 100 km s−1 with respect to the systemic velocity, comparable to the de-tectable velocity range of the HCN (J = 1 - 0) and HCO+ (J = 1 - 0) lines that trace molecular gas at higher densities.
Figure 3.10 (left panel) shows a map of the integrated13CO (J = 2 - 1) intensity in color, and overlaid in contours the integrated HCN (J = 1 - 0) intensity map of Figure 3.6 but now convolved to the same angular resolution as the 13CO (J = 2 - 1) map. Figure 3.10 (right panel) shows a map of the intensity-weighted mean 13CO (J = 2 - 1) velocity. As can be seen, the compact central component detected in13CO (J = 2 - 1) has a similar size and kinematics to the circumnuclear HCN (J = 1 - 0) (and HCO+ (J = 1 - 0)) emission. In Figure 3.11, we show position-velocity (PV-) diagrams of the HCN (J = 1 - 0), HCO+ (J = 1 - 0), and 13CO (J = 2 - 1) emission along a position angle of 110◦, which corresponds to the orientation of the kinematic major axis. As can be seen, the PV-diagram in13CO (J = 2 - 1) closely resembles that in the higher density tracers of molecular hydrogen gas.
We therefore believe that the observed13CO (J = 2 - 1) emission originates from essentially the same regions of the circumnuclear starburst ring as the observed in the HCN (J = 1 - 0) or HCO+ (J = 1 - 0) emission. Of course, any central depression or hole with the same size as that seen in HCN (J = 1
- 0) and HCO+ (J = 1 - 0) would not be discernible at the angular resolution of our 13CO (J = 2 - 1) observation.
Figure 3.12 shows our channel maps of the12CO (J = 2 - 1) line. Three components are now discernible: a compact central component that is the counterpart of that seen in 13CO (J = 2 - 1), and two arm-like components on the opposite sides of the galactic center with one arm extending east from north and the other arm west from south. The compact central component is detectable over the velocity range from about −125 km s−1 to 135 km s−1 with respect to the systemic velocity, extending somewhat beyond the de-tectable velocity range of its counterparts observed in 13CO (J = 2 - 1), HCN (J = 1 - 0), and HCO+ (J = 1 - 0). By contrast, both the eastern and western arm-like components are detectable over a narrower velocity range than the compact central component in any of the observed molecu-lar lines. The western arm is detectable over the velocity range from about
−85 km s−1 to 55 km s−1 and is therefore preferentially blueshifted, whereas the eastern arm is detectable over the velocity range from about −55 km s−1 to 75 km s−1 and is therefore preferentially redshifted.
Figure 3.13 (left panel) shows a map of the integrated12CO (J = 2 - 1) intensity in white contours overlaid on the optical B-band image of the inner part of the galaxy. Dust lanes are visible as silhouettes in the optical image running east and west of center, as well as in emission at 8 µm as shown by the red contour based on observations by Kennicutt et al. (2003) in their Spitzer SINGS project. The eastern and western 12CO (J = 2 - 1) arms coincide with the innermost regions of these dust lanes. These arms appear to connect with the circumnuclear starburst ring at or near the locations
where we detected two knots of dense molecular gas as traced in HCN (J = 1 - 0) and HCO+ (J = 1 - 0) (see §3.3).
Figure 3.13 (right panel) shows a map of the intensity-weighted mean
12CO (J = 2 - 1) velocity field. At our angular resolution, the kinematics of the compact central component cannot be completely separated from the individual kinematics of the eastern and western molecular arms, and vice versa. The western arm is preferentially blueshifted whereas the eastern arm preferentially redshifted, and therefore both arms share the same overall kinematics as the large-scale galactic disk seen in HI. In Figure 3.11, we show a position-velocity (PV) diagram made from the 12CO (J = 2 - 1) channel maps at a position angle of 110◦. As can be seen, the kinematics of the compact central component is similar to its counterparts in the other observed molecular lines.
We derive the mass of molecular hydrogen gas in all three features de-tected in the standard manner using the Galactic conversion between CO in-tensity and mass of molecular hydrogen gas. This conversion is based on the
12CO (J = 1 - 0) line, and in the present situation requires knowledge of the ratio in 12CO (J = 2 - 1) to 12CO (J = 1 - 0) intensities. Aalto et al. (1995) find a ratio in brightness temperatures between these two lines of 1.2 ± 0.1 based on single-dish observations with the Swedish-ESO 15-m Submillimeter Telescope (SEST) towards the center of NGC 7552. For simplicity, we shall therefore assume a ratio in brightness temperature of unity for 12CO (J = 2 - 1) to 12CO (J = 1 - 0) throughout this manuscript. Accordingly, the com-pact central component (corresponding to the circumnuclear starburst ring, and perhaps regions within), which has an integrated intensity in 12CO (J
CONT: N7552 1339.6 KM/S IPOL 13CO.CM.IMAP.1 PLot file version 5 created 10-JUN-2010 11:10:15
Cont peak flux = 5.0675E-01 JY/BEAM Levs = 3.900E-02 * (2, 3, 5, 7, 10, 12)
1339.6 KM/S1357.2 KM/S1374.9 KM/S1392.5 KM/S1410.2 KM/S1427.8 KM/S1445.5 KM/S1463.1 KM/S
DECLINATION (J2000)
1480.8 KM/S1498.4 KM/S1516.1 KM/S1533.7 KM/S1551.4 KM/S1569.0 KM/S1586.7 KM/S1604.3 KM/S
23 16 11.5 10.5 09.5
Figure 3.9 Channel maps of the central region of NGC 7552 in13CO (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.
= 2 - 1) of 404.7 ± 7.0 Jy km s−1, has a mass in molecular hydrogen gas of (3.2 ± 0.1)×109 M". The integrated12CO (J = 2 - 1) intensity detected over the entire region is 601.1 ± 10.6 Jy km s−1, and hence the mass of molecular gas in the eastern and western arms combined is (1.6 ± 0.2)×109 M".
Figure 3.10 Integrated13CO (J = 2 - 1) intensity (left panel) and intensity-weighted mean 13CO (J = 2 - 1) velocity field (right panel), both in color, derived from the channel maps shown in Figure 3.9. In the left panel, con-tours correspond to the integrated HCN (J = 1 - 0) intensity image shown in Figure 3.6 but convolved to the same angular resolution as in 13CO (J = 2 - 1). In the right panel, contours correspond to the intensity-weighted mean
13CO (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.
Figure 3.11 PV-diagrams of HCN (J = 1 - 0) (upper left panel), HCO+ (J = 1 - 0) (upper right panel),13CO (J = 2 - 1) (lower left panel) and12CO (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 12CO (J = 2 - 1) P-V diagram indicate the rotation curve of 12CO (J = 2 - 1).
CONT: N7552 1420.2 KM/S IPOL 12CO.CM.IMAP.1 PLot file version 1 created 11-JUN-2010 10:49:05
Cont peak flux = 4.0109E+00 JY/BEAM Levs = 7.800E-02 * (2, 3, 7, 15, 20, 30, 40) -42 34 45
35 00 15 30
1420.2 KM/S1430.2 KM/S1440.2 KM/S1450.2 KM/S1460.2 KM/S1470.2 KM/S1480.2 KM/S1490.2 KM/S
-42 34 45 35 00
15 30
1500.2 KM/S1510.2 KM/S1520.2 KM/S1530.2 KM/S1540.2 KM/S1550.2 KM/S1560.2 KM/S1570.2 KM/S
DECLINATION (J2000)
-42 34 45 35 00
15 30
1580.2 KM/S1590.2 KM/S1600.2 KM/S1610.2 KM/S1620.2 KM/S1630.2 KM/S1640.2 KM/S1650.2 KM/S
23 16 13 12 11 10 09
Figure 3.12 Channel maps of the central region of NGC 7552 in 12CO (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.
Figure 3.13 Integrated 12CO (J = 2 - 1) intensity plotted in white contours (left panel) and intensity-weighted mean12CO (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.
Chapter 4
PHYSICAL PROPERTIES OF THE CIRCUMNUCLEAR
MOLECULAR GAS
In this chapter, we infer the physical conditions of the molecular gas in the circumnuclear starburst ring based on the ratio in brightness temperatures of the molecular lines observed. We then investigate the dynamical stability of this gas.
4.1 Emission Recovered in our Interferomet-ric Observations
Line ratios inferred with an interferometer can be compromised if there ex-ists uniformly-bright features with sizes beyond that detectable by even the
shortest baseline. Aalto et al. (1995) have observed the central region of NGC 7552 with the SEST in 12CO (J = 1 - 0), 12CO (J = 2 - 1), 13CO (J
= 1 - 0), 13CO (J = 2 - 1), and HCN (J = 1 - 0). They do not show line profiles for any of the transitions observed, but only quote the line intensities or ratio in brightness temperatures of the different lines. After convolving our channel maps to the same angular resolution as in the observation of the corresponding line with the SEST, we found that our SMA observation recovered 78.5 ± 1.5% of the 12CO (J = 2 - 1) emission detected by the SEST. Given typical systematic uncertainties of about 20% in the absolute flux calibration at millimeter wavelengths, we may well have recovered not just the bulk but essentially all of the circumnuclear emission in 12CO (J = 2 - 1). Our ATCA observation recovered 78.5 ± 10.5% of the HCN (J = 1 - 0) emission detected by the SEST, suggesting once again that the bulk if not all of the circumnuclear emission in this line has been recovered. Sur-prisingly, our SMA observation recovered only 26 ± 10% of the 13CO (J = 2 - 1) emission detected by the SEST, despite the fact that13CO (J = 2 -1) traces higher column densities and therefore more compact regions than
12CO (J = 2 - 1) as evidenced by the maps presented in Figures 3.10. The
13CO (J = 2 - 1) line, however, was detected at a confidence level of only 3σ in the SEST observation, and we therefore do not place a high degree of confidence on this particular comparison.