Redshift Evolution Of The Morphology Density Relation
Peter CapakB. 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 + S0) 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 (F814W)– 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+S0
population from spirals and Irregular galaxies
Eyeball Bias
Density Estimator
• Used a version of Dressler’s projected density – Area defined by 10th 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+S0 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 Mpc2