Research
Many galaxies in the universe don't live alone, but stay together in groups or clusters. Such dense environments have different galaxy populations than the field: they harbour an increased fraction of early-type galaxies. This suggests that galaxies in clusters evolve differently and have their star formation quenched more quickly than their counterparts in the field. The goal of my research is to figure out which environmental processes are responsible for this early quenching, and what their effects are on the interstellar medium of galaxies. Below you can find some of my published and ongoing projects (with many more in the pipeline!).
VERTICO: The Virgo Environment Traced In CO Survey
VERTICO is an ALMA large program, led by Dr. Toby Brown, that uses the Compact Array to study the CO in 51 spiral galaxies in the Virgo cluster in detail. Observations for this program have now been completed, and the survey paper has been published. I am leading one of VERTICO's early science projects. It combines data from VERTICO and the VLA Imaging survey of Virgo galaxies in Atomic gas (VIVA) to study how environmental effects visible in the atomic hydrogen affect molecular hydrogen, and how the effects on both phases of the gas are related.
As a key member of the small "Data Products" team I am maintaining the data products pipeline that I wrote for VERTICO. It turns the reduced datacubes into science-ready products, such as moment maps and position-velocity diagrams.
This image I made for the VERTICO website shows a preview of the integrated intensity (moment 0) maps of the galaxies that have thus far been detected. The wide variety of morphologies and inclinations promises some interesting science!
Artist impression of the Herschel spacecraft, which will an unprecedented view of the cold universe, bridging the gap between what can be observed in the infrared from the ground and earlier space missions of this kind.
(Image: © ESA – D. Ducros, 2009)
Extreme gas-to-dust ratios in the Fornax cluster
Having been a PhD candidate at Cardiff University, of course I had to get involved in the dusty universe! In my final PhD project I study the gas-to-dust ratios in the Fornax galaxies we observed with AlFoCS (see below). Again, we collaborate with the Fornax3D team, this time to relate these gas-to-dust ratios to metallicity, estimated from their MUSE observations. This particular relation can tell us a lot about the evolutionary state of the cluster galaxies we are interested in. It turns out that (molecular) gas-to-dust ratios in the Fornax cluster are extremely low (<50)! Interestingly, we see much more common gas-to-dust ratios in the Virgo cluster. Check out the paper for our hypotheses as to why this might be!
AlFoCS meets Fornax3D
We now know that molecular gas can be disturbed and removed from galaxies by environmental processes. The next question is, how does this affect the star formation relation in dense environments? Do galaxies in the Fornax cluster follow "traditional" star formation relations, such as the one found by Kennicutt and Schmidt? To answer this question, we collaborate with the Fornax3D (F3D) team, who have performed amazing Multi Unit Spectroscopic Explorer (MUSE) observations of a subset of the AlFoCS galaxies. From the combination of these two data sets, we have found that many Fornax galaxies, especially less massive galaxies far away from the cluster centre, are forming stars more efficiently than predicted by the Kennicutt-Schmidt relation!
This is the dwarf galaxy FCC090, my favourite galaxy in the AlFoCS sample. It has a spectacular molecular gas tail, as probed by its CO emission (the extent of which is shown in blue). The Hα, whose extent is shown in yellow, was observed by F3D. The optical image in the background is from the FDS. Most galaxies would run out of gas in ~2 Gyr with their current star formation rate. FCC090 is forming stars very efficiently in its stellar body (short depletion times, <0.5 Gyr), but not really forming stars in its molecular gas tail (long depletion times, >10 Gyr).
This is NGC1365, a very large spiral galaxy in the Fornax cluster. I put together a three-colour image from images from the Fornax Deep Survey (FDS), a deep optical survey of the Fornax cluster.
The ALMA Fornax Cluster Survey (AlFoCS)
It is well known that atomic hydrogen (HI) can be disturbed and removed from galaxies due to environmental effects. It is less clear if molecular gas, which is more tightly bound to galaxies (and therefore more difficult to remove), can also be directly affected by these processes. This is an important question, as molecular gas is the direct fuel for star formation, meaning the answer will have strong implications for our understanding of galaxy evolution.
With AlFoCS, a complete Atacama Large Millimetre/submillimetre Array (ALMA)survey of all 30 HI/FIR selected galaxies in the Fornax cluster, we aimed to find out whether it is possible to directly disturb and/or remove molecular gas from galaxies with environmental quenching. We found that the cluster environment is indeed capable of directly disturbing and removing the molecular gas in galaxies, especially in dwarf galaxies, which have shallower potential wells, and are therefore more susceptible to environmental effects.