WALLABY SWG3

The influence of environment on the HI content of galaxies

Virginia KilbornBunker Bay, Nov, 2009

SWG3: Galaxy Environments (groups, interactions/mergers)

Ben Bekhti, Blake, Bouchard, Buyle, Colless, English, Henning, Jerjen, Jozsa, Juette, Karachentsev, Kerp, Kilborn, Koribalski, Kraan-Korteweg, Lopez-Sanchez, Meurer, Oosterloo, Popping, Radchaudhury, Rhee, Saikia, Spekkens, van der Hulst, Westmeier, Wilcots, Winkel, Wong

Effect of Environment

• We know that environment affects the evolution of galaxies:– Galaxies in clusters were bluer at high redshift compared to z~0 -

evolution of galaxies in clusters? Galaxies in clusters are redder and older than the field (e.g. Butcher & Oemler)

– Galaxies in clusters are more likely to be early-type (e.g. Dressler 1980).

– Similar effects recently seen in the less-dense group environment

•How will ASKAP be able to investigate these effects?– Observations of the removal of HI from galaxies (gas stripping) – Frequency and environment of galaxy mergers– Extra-galactic HI (in dense, and low-density environments)– Evolution of HI content of galaxies from z~0.5 to z~0– HI mass function in different environments (Zwaan et al.)

Stripping the HI from galaxies

• What are the main methods to strip gas from galaxies?

– Harassment - frequent, high-speed encounters in clusters– Strangulation - gradual gas-loss from a galaxy into the potential well

of a cluster or group– Ram pressure stripping– Tidal interactions

• In particular, these last 2 effects can be seen in high-resolution HI observations

Chung 2007

Vollmer et al 2004

Investigating gas stripping without mapping

• The closest few to ten thousand galaxies will be mapped well enough to see tidal and ram pressure stripping effects in the imaging

• The other 100s of thousand galaxies will be unresolved - how to test if they are losing gas?

• Use the optical properties to determine the expected HI content of a galaxy of a particular optical classification (or some other classification scheme)

• This requires accurate matching of HI detection with optical catalogues, and homogeneous optical data

• A good project for SkyMapper

Previous “HI deficiency” measures

• In the mid-80s, Haynes and Giovanelli looked at defHI in clusters, finding HI deficient spirals near the centres of a number of clustersdefHI = MHI(expected) - MHI (obs)

• Their “expected HI mass” based on the relationship of a few tens of galaxies of each optical classification

• In 2005, new “expected HI mass” relations determined using HIPASS BGC - over 100 galaxies in each relationship

• This data applied to galaxy group work, and found that there are HI deficient galaxies in groups too - so its not just clusters that are stripping HI from galaxies

HI deficiency in groups: comparison with X-ray and v

• HI def in groups - can’t be due to ram pressure stripping (entirely) as:– HI def galaxies found in groups without X-rays– No relationship seen between HI def fraction and X-ray

emission/velocity dispersion of groups

clustersclusters

Global galaxy evolution• Until now, deep HI studies have concentrated on

particular structures- clusters, and more recently groups and filaments

• ASKAP will make it possible to study galaxy evolution across the continuum of all environments

Ryder et al. 2002

Giovanelli et al. 2006, in prep

Kilborn et al 2000

English et al. 2009

Oosterloo & van Gorkum 08