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 HI-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 HI in the group galaxies?

• Compare with– compact groups: same processes, higher density, HI

may be changed/removed, and moved around– clusters: add ram-pressure stripping, HI 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/R3)

HIPASS Groups

Crossing-time

groups have crossing-times < 0.30 H0-1

Number density

groups have densities > 3.16 Mpc-3

Velocity Distribution

Group Properties

Group HI Properties

[(2)-1/2vngRg2]-1

Galaxy Properties

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)

• HI 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 – HI Mass

Star Formation – HI Mass

• surface density of SFR correlates with surface density of gas mass – the Schmidt Law (Kennicutt 1998)

• global SFR – HI mass relation– HI is gas reservoir for SF– young stars disassociate H2 into HI (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