Gas stripping in cluster galaxies:
HI in and around galaxies is escencial in understanding galaxy formation and evolution, as it is the key ingredient for forming stars and its a sensitive tracer of different environmental processes. To address the question where, how and why star-forming spiral galaxies get transformed into passively evolving early-type galaxies, we have embarked on a study of two galaxy clusters, at z=0.2 and the large scale structure in which they are embedded: A Blind Ultra Deep HI Environmental Survey (BUDHIES) with the Westerbork Synthesis Radio telescope (WSRT). The aim is to identify if there are physical mechanisms, like tidal interactions or gas stripping, that govern these transformations. The unique aspect of our study is that, for the first time, we have accurate measurements of the cool gas content which fuels the star formation in galaxies in clusters at intermediate redshift.
I started working in BUDHIES when I was a post-doc at the Osservatorio Astronomico di Padova (INAF), and I am still heavily involved in it (see Jaffe et al. 2012, 2013, 2015, and 2016)
The BUDHIES volume contains 2 clusters: a very massive one, Abell 963, and a low mass cluster full of substructure: Abell 2192. From Abell 2192, we learned that galaxies can start loosing their HI gas in groups, and that they can be already quenched by the time they are accreted into a massive cluster. The cube below shows a 3D map of galaxies in Abell 2192. Different sub-structures are plotted with solid circles of different colours. Blue transparent cubes represent galaxies detected in HI and orange transparent cubes correspond to emission-line galaxies (see Jaffe et al. 2012):
From Abel 963, the most massive structure in our survey, we could instead study the effect of ram-pressure stripping. We did this utilizing phase-space diagrams, as explained below.
Phase-space diagrams:
observations vs. simulations
Position vs. velocity phase-space diagrams of cluster galaxies are great tools to study the orbital histories of the galaxies within the cluster. In such a diagram, galaxies that haven been in the cluster for over a few cluster crossings (i.e. the virialized galaxies) will gather at low clustercentric radius and low velocities, while the recently accreted cluster members will be distributed at higher velocities overall.
In the figure below from Rhee et al. (2017) we show the time since infall of simulated cluster galaxies in different regions of phase-space
In addition to time since infall, phase-space diagrams can be used to characterize the timescale of different physical cluster processes, such as ram-pressure stripping. In Jaffe et al. (2015) we combined cosmological simulations with an analytic prescription of ram-pressure to predict the regions in phase-space where galaxies would be sufficiently stripped of their gas to be undetected by our survey. Our observations show a remarkable agreement with the simulations (as shown in the figure below) and suggest that a significant amount of gas is stripped during the first passage of the galaxy into the cluster.
In Jaffe et al. (2016) we have also studied in fine details the effect of the evolution of environment in BUDHIES galaxies, and found that a significant amounf of galaxies are quenched in groups prior entering the cluster.
In Yoon et al. (2017) we further studied the stripping sequence in phase-space using high resolution HI morphologies of galaxies in the Virgo cluster from the VIVA survey.
