Redshift Evolution Of The Morphology Density Relation Peter

Redshift Evolution Of The Morphology Density Relation Peter Capak B. Mobasher, R. Abraham, R. Ellis, K. Sheth, N. Scoville Postdoctoral Fellow California Institute of Technology

What Does Morphology Tell Us? • The light distribution of a galaxy • Linked to the orbits of the stars – Spherical galaxies are dynamically relaxed – Disk galaxies are not • Disk galaxies can become spheroids through interactions • Evolution of the morphological fraction traces the interaction history

Morphology Density Relation • Higher Spheroid (E + S 0) galaxies in high density regions (Dressler et. Al. 1980) – Galaxies in dense regions are more relaxed • Seen up to z~1 (Dressler et. Al. 1996, Smith et. Al. 2004, Postman et. Al. 2005) • Possible differential evolution with redshift (Smith et. al. 2004) – Systematic Effects • Morphological classification • Density measurement

Morphological Classification • Eyeball classification is the traditional method – Not practical for ~500, 000 galaxies – Need automated classifier – Not free from systematic effects • COSMOS is only one band (F 814 W) – Classifier must be independent of band shifting • Surface brightness dimming a problem – Classifier must be independent of surface brightness

Morphological Classification • Chose to use Gini and Asymetry – Gini is similar to concentration but considers total light distribution • Similar to Abraham et. Al. 1996 and CAS system used by GEMS

Petrosian Parameters • Petrosian raidus free of surface brightness dimming effects – Eye and isophotal parameters are sensitive to these – Defined at first minima in enclosed flux divided by radius • Gini takes overall light distribution into account – Cleanly divides E+S 0 population from spirals and Irregular galaxies

Eyeball Bias

Density Estimator • • Used a version of Dressler’s projected density – Area defined by 10 th nearest neighbor a with Mv<-21. 2 at z=1 Count out from center until there are statistically 10 objects at the same redshift as the object of interest – Density error is constant with density – Works well at high density, fails at “critical” low density – Lowest density determined by redshift accuracy

Morphology Density Relation • Reproduce the morphology density relation at middle densities at all redshifts • Smith et. Al. working below the “Critical” density

Evolution!

Differential Evolution • E+S 0 fraction grows more rapidly in dense regions • Evolution is slower than expected if proportional to the number of interactions • No indication of “Critical” density above which cluster physics becomes important

Conclusions • Elliptical and spiral galaxies can be separated with single band morphologies • Density can be measured with photometric redshift accuracy • Morphology Density relation is differentially evolving – Slower than expected from a simple interaction model – However No “Critical” density down to 3 galaxies per Mpc 2