Thomas Williams
This poster presents two of the projects I undertook as part of my PhD, looking at star formation and the interstellar medium in one of the most nearby spiral galaxies, M33. The first is a study of the star formation law, which relates the gas content with star formation, and on the scales of integrated galaxy scales follows a tight power-law relationship, with a power exponent of ~1.5. By making use of resolved measurements, I can study this statistically at high resolution in one galaxy, down to scales where we may expect to see this relationship break down. Unlike other studies, I find that (at least with the molecular gas), the correlation between gas and star formation surface density remains strong, but with much increased scatter. This highlights the various evolutionary stages of regions in a galaxy, which we average over to form the tight relationship seen at integrated scales.
Secondly, I make use of the cosmic dust as a tracer of molecular hydrogen in M33, to produce a giant molecular cloud catalogue. Traditionally, CO is typically used, but in low metallicity galaxies (like M33), it is shielded less from the harsh interstellar radiation field and may be destroyed, becoming a poor tracer of the bulk molecular gas. I find that there is agreement between my work and previous works using CO, but at large galactocentric radii I find clouds that are not present in the CO. I suggest this is indicative of 'CO-dark' gas, and therefore the dust continuum may offer a more representative view of the molecular gas
The second slide covers the first of these studies: the star formation law in M33, a nearby (840kpc away) spiral galaxy, which is the background for these slides. This law relates the gas and star formation surface density, and when looked at on the scales of integrated galaxies follows a tight power-law relationship, with a power-law exponent of ~1.4. In this work, I have combined observations from UV to sub-millimetre to calculate the star formation rate on 100pc scales (i.e. individual star-forming regions), as well as tracers of the atomic and molecular gas to see how this law looks at high resolution. Fig. 1 shows the main result of this paper: from left to right we take molecular gas only, total gas (molecular plus atomic) and the total gas as traced by dust, and from top to bottom the relationship is plotted at a variety of scales, from 100pc to 1kpc. There are two important results here: firstly, that there is a strong scale dependence on the power law slope measured, and secondly that we find a much tighter relationship with molecular gas than total gas. The first of these highlights that the star formation law we typically see on integrated galaxy scales is built up from an average of an ensemble of regions in different evolutionary states, and the second that it is the molecular gas that is the true building block of star formation in galaxies.
The third slide covers the second study: I use the cold dust continuum to create a giant molecular cloud catalogue of M33. Traditionally, CO is used as a proxy for the molecular gas, but in low metallicity environments like M33 this may be unsuitable. In this case, the dust may be a more representative tracer. We find a number of clouds that range in size from 46-280pc, and from 10^4 to 10^7 solar masses. We also find these clouds are forming more inefficiently than other nearby spiral galaxies. Fig. 2 shows one of the main results of this study: a comparison between our clouds and those catalogues from earlier CO studies. Whilst we find good correspondence in the centre of the galaxy, at large galactocentric radii we find clouds where typically the CO does not. We interpret this as a signature of ‘CO-dark’ gas, which corresponds to a large fraction of the total molecular gas in these clouds at further distances. In this situation, it seems as though the dust continuum offers a more representative view of the molecular gas.
The final slide has the take home messages from these studies, as well as the references. Firstly, we see that the correlations between star formation rate and molecular gas surface density remain strong even to 100pc regions, but the scale dependence shows a strong evolutionary diversity in star forming regions in this galaxy. Secondly, M33 appears to be forming GMCs significantly more inefficiently than other local spirals, and has a large amount of CO-dark gas. Thus, we suggest the dust is a useful cross-check with CO in low-metallicity environments.
I hope you have enjoyed my poster! If you have any further questions, you can email me at williams@mpia.de.