ISM Studies of Galaxies
Rotation curve of NGC 660. The black symbols represent VLA data, the grey ones MERLIN data.
The large-scale structure of the neutral gas can be investigated by studying the 21-cm neutral hydrogen line in emission. Radio single dish or interferometric observations show the overall distribution of the HI and allow to analyse its dynamics. In order to observe the neutral gas on scales of the order of 10 parsec, the 21-cm line can be studied in absorption using MERLIN at 100 milliarcsecond angular resolution.
The data can, e.g., be used to derive a rotation curve of a galaxy. The figure to the right shows the rotation curve of the peculiar barred spiral galaxy NGC 660. The rotation velocity is plotted against the distance from the dynamic centre of the galaxy. The curve is steeply rising at the beginning, indicating solid body rotation, and coming to a plateau at about 0.2 kpc. According to the Keplerian law, this should be followed by a decrease in velocity as the gas density drops significantly in the outer parts. The fact that the curve stays flat points to the existence of huge amounts of dark matter which cannot be seen, but influence the dynamics of all galaxies. The grey symbols in the inner part represent velocity data points measured in absorption with MERLIN, the black symbols are from VLA observations.
NGC 2366. HI intensity contours plotted over a deep H&alpha image.
Deep optical images allow to study the distribution and intensity of the ionised gas. Usually one finds that the distribution of the neutral gas is much more extended than the one of the ionised gas. A convincing example is the figure to the right which shows a deep H&alpha image of the nearby irregular dwarf galaxy NGC 2366 obtained with the 3.5m telescope at the Calar Alto observatory. Overlaid in black are the HI intensity contours from VLA data.
Another possibility to trace the ionised gas are radio continuum observations. The continuum flux at two different frequencies, e.g., at 1.4 and 5 GHz, can be compared by calculating the spectral index. Depending on its value, the emission can be of two different origins. We differentiate between the synchrotron emission caused by relativistic particles released in a supernova explosion and free-free emission which is a tracer for the ionised gas. The advantage of radio continuum observations against H&alpha studies is that the radio data are not contaminated by dust.