Galaxies in LowDensity Environments
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Transcript of Galaxies in LowDensity Environments
Galaxies in LowDensity Environments
Duncan Forbes
Swinburne University
Low Density Environments
• Poorly studied relative to clusters• Star formation suppressed in clusters• Suppression occurs at group-like
densities (2dF, SDSS)• What is the timescale ?• Are galaxies in groups pre-
processed ?• What is the physical mechanism ?
Physical Processes in Groups
• Ram pressure stripping • Interactions & Harassment• Mergers• Group tidal field• Strangulation/Suffocation • Overlapping dark matter halos
People…in Crete• Trevor Ponman, Trevor Miles, Habib
Khosroshahi, Louise Nolan (Birmingham)
• Frazer Pearce (Nottingham)• Warrick Couch (UNSW)• Ewan O’Sullivan (CfA)• Sarah Brough, Robert Proctor, Virginia
Kilborn (Swinburne)
...a loose group of people studying loose groups of galaxies
GEMS
Group Evolution Multi-wavelength Study
Aim: to understand how the group environment affects galaxy evolution and how groups themselves evolve.
Method: multi-wavelength data and mock catalogues.
GEMS Sample Selection
• Catalogues of Galaxy Groups • 14 < D < 43 Mpc• ROSAT PSPC 10,000sec=> 60 nearby groups with a range of X-
ray properties, ie dynamically young groups with no IGM to virialised old groups with a hot, dense IGM. Includes both compact and loose groups.
GEMS Dataset
• ROSAT imaging (1.5 degrees)• INT/ESO2.2m Optical imaging (0.5
degrees)• Parkes HI mapping (5.5 degrees)• ATCA HI follow-up • UK Schmidt 6dF/AAT 2dF spectra• XMM/Chandra imaging
X-ray Imaging
60 Groups
TX, LX, Z to R500
Osmond & Ponman 2004
Halo Stripping ?
Helsdon etal. 2001
Optical Imaging
30 Groups
B,R,I filters
Structural Properties
Khosroshahi etal. 2004R/RvirialPoster 3.15
Luminosity Functions
Miles etal. 2004
Low LX
groups
Low sigma
Lots of spirals
Current merging
Pre-virialised
Poster 3.19
HI Imaging
• 17 Groups mapped with the Parkes multibeam instrument over 5.5 degrees
• Mass limit of ~ 108 Msun
• 2x deeper than HIPASS survey• 10x better velocity resolution than
HIPASS• 15 arcmin beam (hence ATCA followup)• Further details next talk…
NGC 3783 Group
New Galaxies
• In 16/17 groups searched to date:• 30 new group galaxies (~ 10% new)• 20 new redshifts• 10 not optically catalogued
• a few potential HI clouds M ~ 108 Msun
• IGM HI is rare, but some tidal bridges=> small effect on LF faint end => `missing satellites’ are not HI clouds (CenA
group: a few M ~ 106 Msun HI clouds)
New Galaxy in NGC 5044 group
MHI = 109 Msun
V = 2750 km/s
MHI/LB = 1.7
McKay etal 2004
Virial Radius
• Calculate Rvirial for groups using σ or TX ?
• Statistical problems calculating σ for low N systems
• Systematic bias in σ• Physical reasons for a low σ (dynamical
friction, orientation effects)• Xrays only probe small fraction of virial
radius => do both ! (including new HI galaxies)
NGC 1566 group
no IGM
Rvirial (σ)
NGC 5044 group
NGC 3783 group
Isolated Galaxies• Early-type galaxy, V < 9,000 km/s, B < 14No neighbours within:• 700 km/s• 0.67 Mpc in plane of the sky• 2 B mags (factor of 6 in mass)Formation?• old, undisturbed• recent merger• collapsed group/fossil
Scaling RelationsColour-magnitude Relation
Fundamental Plane
Reda, Forbes etal. 2004
Fossil Groups ?
• Typical isolated galaxy MB = -20.5
• One potential collapsed group:NGC 1132MB = -22.0, log LX = 43.0 erg/s
ΔM12 = 2.2, featureless morphology,
old stellar population
Conclusions
• Xray halos of ellipticals not stripped
• Optical LF dip in young, low LX groups
• HI mapping finds new group galaxies
• Many group galaxies beyond Rvirial
(backsplash galaxies?)• Most isolated ellipticals obey scaling
relations, and are not collapsed groups
Web sites
• www.sr.bham.ac.uk/gems/
• astronomy.swin.edu.au/staff/dforbes