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