Tony Wong (UNSW & ATNF), Stuart Ryder (AAO), Michael Dahlem (ATNF), Kotaro Kohno (U. of Tokyo), & Ron Buta (U. of Alabama).

Starbursts are regions of enhanced star formation efficiency, where gas is being converted into stars at much higher rates than can be sustained over billions of years. Aside from merging or dwarf galaxies, one of the most common sites for starbursts is the central kiloparsec of barred spiral galaxies. There they appear as bright circumnuclear rings in emission tracers of ionised gas such as hydrogen recombination lines or radio continuum. A good example is the nearby galaxy NGC 7552 in the southern Grus Quartet, which has been imaged with ATCA at 3cm by Forbes et al. (1994) [see Figure 1(b)]. NGC 7552 has been extensively studied, not only as a starburst but as a classic example of a multiple ringed galaxy: in additional to the circumnuclear ring of 3 arcsec radius, Forbes et al. identify an inner ring of dimensions 150 arcsec x 95 arcsec and an outer ring of dimensions 180 arcsec x 160 arcsec.

In 2005 we observed this galaxy with ATCA in the 3mm lines of HCN and HCO⁺. These lines trace the densest molecular gas and thus provide information on where star formation is likely to occur. The two lines were observed simultaneously, allowing a direct comparison of their line strengths. Both lines were detected at comparable strength with a peak signal-to-noise ratio of ~15. The images, which show both the rotation velocity of the ring and the distribution of dense gas along it, are among the best produced thus far by the ATCA at 3mm. The quality of the data can be directly attributed to three factors. First, we combined data from three configurations of the ATCA to provide nearly complete coverage of the visibility plane from baselines of 30m to 200m. Second, we were able to conduct the observations in good morning or nighttime conditions. Finally, the use of a nearby phase calibrator (within 5 degrees) minimised errors in calibration.

Although analysis is still in progress, a few results are already apparent. First, HCO⁺ shows a somewhat more extended distribution than HCN, even though the two molecules require similar densities for excitation. This may be attributable to the strong ultraviolet radiation near a starburst, which may enhance the HCO⁺ abundance. Second, both molecular tracers display a double or "twin peaks" morphology, which is commonly seen in barred galaxies and has been attributed to crowding of gas orbits as they shift from being oriented parallel to perpendicular to the bar. The twin peaks occur approximately where the bar dust lanes (oriented east-west) intersect the ring (Figure 2). Third, there doesn't seem to be much gas interior to the ring, consistent with the relative lack of star formation there. This indicates that the inflow of gas along the bar which feeds the ring does not continue on to the nucleus, presumably due to orbital "resonances" associated with the bar. Any central black hole is therefore "starved" of fuel by the inability of gas to flow inwards.

As shown in Figure 1(b), the dense gas is distributed somewhat differently from the 3cm radio continuum emission, which is believed to be mostly non-thermal in origin (Forbes et al. 1994). Since extinction does not affect the radio image, the offset may be related to a time delay between the peaks in the dense gas, which are tied to the pattern speed of the bar, and the locations of recent star formation, which may revolve at a different angular velocity. The symmetry of the 3cm emission suggests that there may be a regular timescale on which bursts of star formation occur. Further comparison with recently released Spitzer Space Telescope images, which at a wavelength of 8µm traces the interstellar emission from heated molecular aggregates known as PAH's, is currently underway (see Figure 2).

The distribution of dense molecular gas can differ significantly from that of the more diffuse molecular gas traced by the CO line, as pointed out for instance by Kohno et al. (1999), who compared CO and HCN images of NGC 6951. They found that CO tends to occur in the bar dust lanes as well as the ring, but HCN is concentrated in the circumnuclear ring. This points to relatively low densities in the shocked gas associated with the dust lanes, with higher densities being achieved only once gas falls into more circular orbits in the ring. Already our HCO⁺ data reveal evidence for non-circular gas motions towards the outer edge of emission, in the form of "S"-shaped isovelocity contours [Figure 1(c)]. Upcoming CO(2-1) observations with the Submillimeter Array should provide a clearer picture of how gas is transported in towards the ring.

We thank the ATNF engineers and Narrabri staff for efforts which have led to significant improvements in the 3mm receiver temperatures and antenna gains.

References

Forbes, D. A., Norris, R. P., Williger, G. M., & Smith, R. C. 1994, AJ, 107, 984

Kennicutt, R. C., et al. 2003, PASP, 115, 928

Kohno, K., Kawabe, R., & Vila-Vilaró, B. 1999, ApJ, 511, 157

Figure 1: (a) ATCA HCN intensity image, shown as greyscale (units of Jy/bm km/s) and as contours (spaced by 0.6 Jy/bm km/s). The beam size of 2.6 arcsec x 2 arcsec is shown at lower left. (b) ATCA HCO⁺ contours overlaid on ATCA 3cm image of Forbes et al. (1994), recently reprocessed by us. Units of the 3cm image are Jy per 1.1 arcsec beam. Contours are spaced by 0.6 Jy/bm km/s. (c) Velocity image derived from Gaussian fitting to the HCO⁺ data cube. Units are in km/s, with contours spaced by 20 km/s from 1500 to 1700 km/s.

Figure 2: Spitzer 8µm image from the SINGS Legacy Project (Kennicutt et al. 2003). The dark contours at the centre show the HCN emission. The box represents the region shown in Fig. 1, whereas the circle represents the ATCA field of view.