Galaxy Groups in HICAT Jamie Stevens

Outline Introduction Group-finding in HICAT HIPASS group properties Star formation properties Summary

Introduction The HIPASS Catalogue (HICAT) contains 4315 galaxies, 4065 of which are certainly real Primarily H I -rich late-type galaxies –Can galaxy groups be identified using only these galaxies?

HIPASS Catalogue

Galaxy Groups Generally dominated by late-type galaxies Galaxies can be affected by various processes –galaxy-galaxy interactions –strangulation –mergers How do these processes affect the H I in the group galaxies? Compare with –compact groups: same processes, higher density, H I may be changed/removed, and moved around –clusters: add ram-pressure stripping, H I is rarely found within cluster cores

Group-finding in HICAT Hierarchical group-finder based on the method of Gourgoulhon (1992) –relied on estimates of the galaxies’ mass –not possible with the information available in HICAT Two mass-independent methods –crossing-time: (4/  ) (R/  v ) –number density: (3/4  ) (N/R 3 )

HIPASS Groups Crossing-time groups have crossing-times < 0.30 H 0 -1 Number density groups have densities > 3.16 Mpc -3

Velocity Distribution

HYPERLEDA

Group Properties

Group H I Properties [(2) -1/2  v n g  R g 2 ] -1

Galaxy Properties

median field morph = Sd median group morph = Scd

Interpretation The HIPASS groups are serene –collisions of large galaxies may be very rare Higher luminosity galaxies cluster more strongly –observed before (Norberg et al. 2001) Early-type fraction increases in denser environments –morphology-density relation (Dressler 1980) H I dependence on luminosity isn’t radically affected

Star Formation Can estimate star formation rate (SFR) of the HICAT galaxies with radio continuum luminosity, or far-infrared (FIR) luminosity

Star Formation – H I Mass

surface density of SFR correlates with surface density of gas mass – the Schmidt Law (Kennicutt 1998) global SFR – H I mass relation –H I is gas reservoir for SF –young stars disassociate H 2 into H I (Allen 2002) –observed relation consistent with disassociation models (Taylor & Webster 2005)

SFR Depression Star formation rate is depressed in dense environments (Lewis 2002) Unlikely to be due to tidal interactions –gas strangulation?

Summary Loose groups found in HICAT using hierarchical group-finder –widely separated galaxies, which rarely interact More luminous, earlier-type galaxies cluster more strongly –no great difference between group and field galaxies Star formation depressed in group galaxies –may be gas strangulation, but dependence exists on HI mass