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 Thomas Williams

Thomas Williams

Research student, School of Physics and Astronomy

1.06, 53 The Parade, 52 The Parade, Cardiff, CF24 3AB

Although stars make up around 99% of the mass of a galaxy, the effect of the other 1% (the interstellar medium, or ISM) on the stars and their starlight is immense. Stars are believed to form out of dense molecular hydrogen, and we see strong correlations between the amount of gas in a galaxy and its associated star formation rate. Interstellar dust also plays a role in shaping what we see in a galaxy -- this dust aborbs, processes and reemits light from stars at much longer wavelengths. The dust emission contributes some 50% of the total luminosity of a galaxy, despite only making up 1% of the ISM, by mass.

With the advent of new, higher resolution instruments, we can now probe properties down to ~100pc scales in galaxies (which correspond roughly to the size of a Giant Molecular Cloud, the birthplace of massive stars). My work focusses on seeing if the relationships we see averaged over galactic scales still apply at these regions, where we begin to resolve individual star forming regions and clouds. How these relationships evolve with scale can teach us about the nature of galaxy evolution, and help to localise it to particular regimes in a galaxy.

Research interests

My PhD work has primarily focussed on the nearby galaxy M33, the third massive spiral of our Local Group of galaxies, behind Andromeda (M31) and the Milky Way (MW). The projects I have been carrying out (or have recently finished) are:

The Star Formation Law at GMC Scales

Read the paper!

It has long been established that there is a relationship between the amount of gas in a galaxy, and its amount of star formation. In this work, I have brought together data from multiple different observatories, across 4 orders of magnitude in wavelength, to fit the spectral energy distribution (SED) for M33 at scales of 100pc. Doing this allows us to calculate some intrinsic properties of the galaxy, in particular its star formation rate (SFR). Combining this with resolution-matched dust maps, as well as CO (to trace molecular hydrogen) and HI (atomic hydrogen) maps, we can measure the relationship between gas and star formation rate at an unprecedented scale across an entire galaxy. We find that we recover the traditional star formation law at large scales, but find a strong scale dependence on our fitted relationship. This indicates that various regions are at a variety of evolutionary states, and the star formation law we see at galactic scales is built from an average of these states. Unlike previous works, however, we find that these quantities are still well-correlated, which we attribute to a more thorough calculation of the SFR, and tracing more dense molecular gas.

A Dust-Selected GMC Catalogue of M33

Paper coming soon, keep your eyes peeled!

Giant Molecular Clouds, or GMCs, are thought to be the places where dense, molecular hydrogen is turned into stars. In understanding how stars form, it is necessary to understand these star-forming regions. Generally, these molecular clouds are quite compact, with average sizes of around 50pc, and so given telescope resolutions we are limited to surves of local galaxies. Because these clouds are dominated by molecular hydrogen, it's been traditional to use CO (a proxy for the molecular gas) to trace these regions, but in this work I take an alternative approach, by using the dust continuum. With new SCUBA-2 PI data, we can probe down to regions of ~100pc in M33, and resolve individual, massive GMCs. We find that M33 is significantly poorer at forming massive clouds than other galaxies in our local group (M31, the Milky Way), but that these clouds are distributed fairly evenly across the face of M33's disk.

A Radiative Transfer Model of M33

Paper coming soon (I promise!)

Radiative transfer (essentially, the modelling of photons from stars being absorbed, processed and re-emitted by dust) is an important tool for studying the geometry of a galaxy, and how light interacts in that 3D geometry. However, the calculations are computationally very difficult and so large-scale radiative transfer simulations have only become possible recently. I have built up a simple model for M33, consisting of old and young stars, and the dust disk they reside in (with the geometries constrained by input images), and attempted to recreate the observed spectral energy distribution (SED) of the galaxy. We find that even with this simple model, we can well constrain our various parameters and recreate very well the SED of the galaxy. However, on a resolved basis this model fares more poorly, perhaps indicating some change in the properties of stars and dust in the brighter, more compact regions and in the more diffiuse interstellar media. We find a dust mix that is significantly different to the Milky Way, with less small carbon grains -- with M33's low metallicity, these have less shielding from the harsh interstellar radiation field and are likely destroyed. Finally, we can also calculate the average distance a photon travels from a star before being absorbed by a dust grain -- we find this to be in the region of 1kpc.

Some Other Things

Outside of my main PhD work, I’m also a collaborator on two large JCMT programs - JINGLE and HASHTAG. The former is a study of ~200 nearby galaxies in both sub-millimetre continuum and CO (to trace gas), and the latter is currently mapping the whole of Andromeda at sub-mm wavelengths.


  • PX1122 (Mathematical Methods for Physicists) - 2016-17
  • PX1125 (Mathematical Practice for Physical Sciences) - 2017-18
  • PX2139 (Observational Techniques in Astronomy) - 2017-18
  • PX2134 (Structured programming) - 2018-19
Walter Gear

Professor Walter Gear

Honorary Professor

Image of Matthew Smith

Dr Matthew Smith

Astronomy Group