My work principally revolves around observations of the dust and molecular gas in galaxies, as a way to probe their evolution over cosmic times.
The most extreme molecular outflows in the local universe
Supermassive black holes (SMBHs) are found ubiquitously at the nuclei of all nearby massive elliptical galaxies, and appear to have a close correlation with the properties of the stellar bulge – the so called M-sigma relation. This, together with the discovery that the high-mass end of the galaxy luminosity function shows a steep decline, and with the presence of `red and dead’ galaxies – all of these together suggest that star formation in the massive galaxies is quenched. Feedback from the central SMBH during its actively accreting phase – AGN feedback – is one of the most likely culprits for this quenching.
In my research, I study how this feedback impacts the molecular gas – the fuel for star formation – is impacted by AGN feedback in local Ultra Luminous Infra-red galaxies (ULIRGs). AGN-feedback manifests itself in the form of massive molecular outflows, with outflow velocities > 1000 km/s, which to first order drives out the molecular gas and thus quenches the star formation. ULIRGs are ideal targets in which to study molecular outflows, as they are the closest analog to high-redshift dusty star-forming galaxies in the local universe, and we can get high resolution observations of the outflowing molecular gas in these systems, allowing us to constrain its geometry, its energetics, and the chemistry of the outflow.
Dense molecular gas in high-redshift main-sequence galaxies
Over the past decade, observations have revealed a population of `normal’ star-forming galaxies (SFGs), which fall on a tight relation between stellar mass (M*) and star formation rate (SFR), the so-called star-forming ‘main sequence’. The existence of the galaxy MS implies that the bulk of cosmic star formation at high redshift proceeds in a quasi-steady state, over long timescales (∼1 Gyr) and large spatial scales, while episodes of intense merger-induced starburst activity play a smaller role. A question that then arises is whether galaxies on the main-sequence have a more-efficient mode of star formation, or whether their star formation efficiency is much higher than the star-forming galaxies observed in the local universe.
We use observations of the dense molecular gas, traced by HCN rotational lines, to study the fraction of dense, actively star forming gas in a gas-rich main-sequence galaxy at z = 1.2, results of which were published here. We find, though with modest significance, that while the dense gas fraction is enhanced, the relative efficiency of star formation is lower than expected. These results are consistent with increased turbulence in the ISM of our target galaxy, possibly fueled by infall of molecular gas.