RESEARCH INTERESTS
Our group is interested in the role that environment plays in galaxy evolution. Much of this effort is focused on the scale of galaxy groups. We use multi-wavelength imaging and spectroscopy to study the observed properties of these systems as well as weak gravitational lensing to study the underlying dark matter distribution. Galaxy groups typically only contain a handful of bright galaxies so they’re much more difficult to locate than rich galaxy clusters.
Galaxy Evolution in Groups & Clusters
Galaxy groups are the place where most galaxies live in the local universe. Groups also seem to be the environment in which lots of galaxy evolution takes place. In our research group we look at groups in different surveys to probe galaxy evolution in groups and clusters over the last ~10 billion years.
We’ve been studying the importance of dynamics on galaxy evolution in groups (Hou et al., 2009, 2012, 2013, Roberts et al. 2017). We’re also interested in understanding radial trends in the galaxy properties in groups (Roberts et al. 2015, Joshi et al. 2016), both at large radii where galaxies are falling into groups and being influenced by the group potential for the first time, and in the group cores. We’re finding a diversity of properties for the central galaxies in groups: some groups looks a lot like more massive clusters with a giant red galaxy sitting at the centre of the potential, while other groups are filled with blue star-forming galaxies in their cores and have many galaxies of comparable brightness.The properties of both the central and satellite galaxies are also tied to the dynamical properties of the host group (in prep). We’ve also recently shown that the properties of group galaxies depend on the X-ray brightness of their host system (Roberts et al. 2016) and we’re exploring how X-ray properties of groups correlate with their dynamical state (Roberts et al. 2018).
A sample of most massive group galaxies from the GEEC survey
We’re using radial trends in observed galaxy properties (masses, morphologies, SFRs, AGN fractions) to try and understand the physical processes driving galaxy evolution in rich environments.
Radial trends are complicated to interpret because it’s hard to know whether a galaxy at a given radius is infalling onto a group for the first time or whether it’s already passes close to the group centre and is ‘backsplashing’
We know that many galaxies infall into group and clusters environments in substructures and we can see whether these galaxies have been ‘pre-processed’ (had their properties transformed) in these small substructures before encountering a rich environment (e.g. Hou et al. 2014, Joshi et al. 2017).
We’re also interested in further understanding the role of environment on galaxy evolution by simulating a diversity of groups with the hydrodynamical simulations including the latest prescriptions for feedback. We’ve starting with building a suite of dark matter only simulations and we’ve carefully tracking substructure in group and cluster halos.
We’ve found that galaxies accreted into clusters in smaller groups have lost much more of their mass at the time of accretion than galaxies which are accreted individually (Joshi et al. 2017).
The images of faint background galaxies are distorted by objects along the line of site. The more massive the foreground objects are the more dramatic is the gravitational lensing effect. Using this distortion we can measure the masses of objects in the Universe and can therefore estimate the amount and distribution of dark matter.
Any mass along the line of sight will cause lensing effects, but with the exception of massive clusters this weak distortion is not detectable for single objects. We measure the lensing effect on 10’s of thousands of background galaxies around thousands of foreground galaxies. We do this (Parker et al. 2007) in order to understand the total mass of galaxies is, as well as the shapes of their dark matter halos. Eventually we would like to know the detailed properties of halos for galaxies that live in isolation, groups and rich galaxy clusters. Because lensing can be used to map dark matter that would otherwise be undetectable, lensing measurements allow us to connect observations directly to dark matter theories.
Another issue we can address with weak lensing and dynamics is the location of galaxy group centres. In order to probe radial trends of member properties or study the total masses of groups it is important to identify the centre of their potentials:
Galaxy group centres may be best defined as the location of the most massive galaxy (left), the average position of the member galaxies, weighted by mass (middle) or the location of the peak in X-ray emission (right)
Most galaxies in the universe are either blue and star-forming or red and passively evolving. We recently presented a comprehensive study of galaxies which defy this bimodal trend because they are red in optical colour but actively forming stars (Evans et al. 2018). These galaxies represent ~10% of the local galaxy population and are present in an equal proportion in nearly all environments. Their lack of dependence on environment and the high fraction of these galaxies hosting AGN implies that their properties are likely dominated by internal processes. They share some overlap with the green valley population, but lack the environmental trends seen in green valley galaxies. We have also published a catalogue of red misfit galaxies in the SDSS. We are now studying these galaxies in the sub-mm to probe their cold gas and dust properties (Chown et al. in prep).
sSFR vs. inclination-corrected and k-corrected colour for galaxies in the full sample. (Evans et al. 2018).
BPT diagrams for emission line SDSS galaxies (Evans et al. 2018).
Credit Marie-Pier Neault
Red Misfit Galaxies
Weak Lensing and Dark Matter Halos
