Transcript Document 7679488

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
Poster 3.15
R/Rvirial
Khosroshahi etal. 2004
Luminosity Functions
Low LX groups
Low sigma
Lots of
spirals
Current
merging
Pre-virialised
Poster 3.19
Miles etal. 2004
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 < 14
No 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 Relations
Colour-magnitude Relation
Fundamental Plane
Reda, Forbes etal. 2004
Fossil Groups ?
• Typical isolated galaxy MB = -20.5
• One potential collapsed group:
NGC 1132
MB = -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