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Galaxy Groups in HICAT

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

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Galaxy Groups in HICAT

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  1. Galaxy Groups in HICAT Jamie Stevens

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

  3. 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?

  4. HIPASS Catalogue

  5. 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

  6. 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)

  7. HIPASS Groups Crossing-time groups have crossing-times < 0.30 H0-1 Number density groups have densities > 3.16 Mpc-3

  8. Velocity Distribution

  9. Group Properties

  10. Group HI Properties [(2)-1/2vngRg2]-1

  11. Galaxy Properties

  12. Galaxy Properties median field morph = Sd median group morph = Scd

  13. 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

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

  15. Star Formation – HI Mass

  16. 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)

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

  18. 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

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