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

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).

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).

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).

Chapter 2

OBSERVATIONS AND DATA REDUCTION

2.1 SMA Observation and Data Reduction

We observed NGC 7552 in¹²CO (J = 2-1) and¹³CO (J = 2-1) in the compact configuration of the Submillimeter Array (SMA) on 4 August 2006. Our observation was centered at α2000 = 23^h 16^m10.^s7 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 the¹²CO (J = 2-1) line, and 217.6–219.6 GHz in the lower sideband (LSB) thus including the

13CO (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 in¹²CO (J = 2-1), we averaged 10 channels together resulting in a velocity resolution of 10 km s⁻¹. Because the signal is weaker in¹³CO (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 us-ing SMA-specific MIR tasks adopted from the MMA software package (writ-ten 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 chan-nels. 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 calibra-tion. Before deriving bandpass solutions from our observation of Uranus, we checked for baseline-based errors by deriving the complex gain (i.e., ampli-tude 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

approach for deriving calibration solutions for the telescope.

For complex gain calibration, we observed the quasars 2235-485 and 2258-279 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 ¹²CO (J =

2 - 1) channel maps have an angular resolution of 7.^##0 × 2.^##8 and a root-mean-square (rms) noise level of 78 mJy beam⁻¹, and the¹³CO (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

respectively. The zeroth, first, and second moment maps are integrated in-tensity, 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.

Table 2.1. The properties of making moment maps

Noise (Jy beam⁻¹ km s⁻¹) Cellsize Cutoff (σ)

12CO (J = 2 - 1) 4.5 7 1.5

13CO (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 0.1 11 1.0