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 Sept 17th, Galaxies in Real Life & Simulations

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) Group masses estimated using the summed stellar mass for each group. 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.

Relation between morphology and environment revisited Satellites more concentrated than fixed stellar mass. Environment quantified to first order by distinguishing satellite and central galaxies in the group catalogue. Morphological indicator: Concentration (=R90/R50)

Ellipticals are not produced by environmental processes acting on satellites Relation between morphology and environment revisited Satellites more concentrated than fixed stellar mass. However: Fraction of galaxies with C>3 is the same! Environment quantified to first order by distinguishing satellite and central galaxies in the group catalogue. Morphological indicator: Concentration (=R90/R50) ~

The average surface brightness profiles of C<3 galaxies (with log(M)~10.9) show: Satellites are fainter, and more so towards out- skirts. Explains increased C. What is the origin of this difference? Average radial profiles of SDSS galaxies bright faint

Average radial profiles of SDSS galaxies T The average colour profile of the same galaxies show: Colours of satellite galaxies redder across the disk. Central bulge-dominated region is unchanged. blue red

Average radial profiles of SDSS galaxies T Average stellar 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 T Average stellar 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

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. Slow (e-folding time 2.5 Gyr) decrease of SFR reproduces satellite profile well! Colour difference increases with radius due to stronger reaction in young parts of galaxy. T model: infall 9 Gyr ago infall 1 Gyr ago data: blue red

How fast does the star formation decline? Truncation only 1 Gyr ago still leads to colours which are much too red. Alternative option: Fast truncation of star formation T average sat average cen blue red model results for different infall times

How fast does the star formation decline? Truncation only 1 Gyr ago still leads to colours which are much too red. Alternative option: Fast truncation of star formation Environmental processes do not lead to fast star formation truncation. T average sat average cen blue red model results for different infall times

Summary T Ellipticals not produced by environmental effects – 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" But: Starvation is clearly over-efficient in most SAMs. (Adressed by Font et al. 2008, Guo et al. in prep) Semi-analytical models have included the key environmental process.