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Sept 17th, Galaxies in Real Life & Simulations
Galaxy Growth: The role of
environment
Simone Weinmann (MPA Garching)
Collaborators: Guinevere Kauffmann, Frank van den Bosch,
Anna Pasquali, Dan McIntosh, Xiaohu Yang, Houjun Mo
Environment affects galaxy evolution
Distinguish centrals and satellites:
centrals are the most massive
galaxy in their group.
Many environmental processes have been proposed.
Perhaps the most natural one is starvation (or strangulation):
Infalling gas is mainly accreted by the central galaxy.
Satellites galaxies (slowly) starve.
This is the only environmental process currently included in
semi-analytical models.
Is this good enough? What about the morphology-density
relation? What about stripping of ISM, harassement... ?
The SDSS DR4 group catalogue
(Yang et al. 07)
Iterative group finder (Yang et al. 2005)
Method has been carefully tested using Mock Galaxy Redshift
Surveys constructed with the Conditional Luminosity Function.
Our sample: ~ 280,000 galaxies in ~ 50,000 groups.
Group masses estimated using the summed stellar mass for
each group.
Relation between morphology and environment revisited
Environment quantified to first order by distinguishing
satellite and central galaxies in the group catalogue.
Morphological indicator: Concentration (=R90/R50)
Satellites more concentrated than centrals @ fixed stellar
mass.
Relation between morphology and environment revisited
Environment quantified to first order by distinguishing
satellite and central galaxies in the group catalogue.
Morphological indicator: Concentration (=R90/R50)
Satellites more concentrated than centrals @ fixed stellar
mass. However: Fraction of galaxies with C>3
~ is the same!
Ellipticals are not produced by environmental
processes acting on satellites
Average radial profiles of SDSS galaxies
bright
The average surface
brightness profiles of
C<3 galaxies
(with log(M)~10.9)
show:
Satellites are fainter, and
more so towards outskirts.
Explains increased C.
faint
What is the origin
of this difference?
Average radial profiles of SDSS galaxies
red
The
T average colour
profile of the same
galaxies show:
Colours of satellite
galaxies redder
across the disk.
Central bulge-dominated
region is unchanged.
blue
Average radial profiles of SDSS galaxies
Average stellar
T
mass profiles
show:
Mass distribution
in satellites and centrals
is very similar.
No major rearrangement
of mass distribution
by environmental process ->
evidence against
harassement, tidal stripping,
disk instabilities.
Average radial profiles of SDSS galaxies
Average stellar
T
mass profiles
show:
Mass distribution
in satellites and centrals
is very similar.
No major rearrangement
of mass distribution
by environmental process ->
evidence against
harassement, tidal stripping,
disk instabilities.
Process affecting star formation can explain environmental
dependencies of galaxy concentration!
Ram-pressure stripping of ISM
vs. starvation
Ram-pressure stripping of ISM
vs. starvation
Starvation: Affects fuel for future star formation.
Star formation decreases slowly (~ 2 Gyr)
Ram-pressure stripping of ISM
vs. starvation
Ram pressure stripping of ISM: Affects fuel for current
star formation.
Leads to fast decline of SF in affected regions.
How fast does the star formation decline?
We model colour profiles using Bruzual & Charlot 03 SPS and
different SF histories.
T
red
Slow (e-folding time
data:
2.5 Gyr) decrease of
SFR reproduces
model: infall 9 Gyr ago
satellite profile well!
infall 1 Gyr ago
Colour difference
increases with radius
due to stronger
reaction in young
parts of galaxy.
blue
How fast does the star formation decline?
Alternative option: Fast truncation of star formation
red
Truncation only
1 Gyr ago still leads
to colours which
are much too red.
T
model results for
different
infall times
average sat
blue
How fast does the star formation decline?
Alternative option: Fast truncation of star formation
red
Truncation only
1 Gyr ago still leads
to colours which
are much too red.
Environmental
processes do not
lead to fast
star formation
truncation.
T
model results for
different
infall times
average sat
blue
Summary
• Ellipticals not produced by environmental effects –
T
violent environmental processes not needed.
• Remaining concentration differences can be explained
by slowly decreasing SF
• Starvation is dominant environmental process
and is able to explain what remains of "morphology
- density relation"
Semi-analytical models have included the key
environmental process.
But: Starvation is clearly over-efficient in most SAMs.
(Adressed by Font et al. 2008, Guo et al. in prep)