Galaxies and their Environments Nick Cowan UW Astronomy January 26, 2007 Nick Cowan UW Astronomy January 26, 2007

Reasonable Questions Why care about environments? What is an environment? How do we determine environments? How exactly do environments effect galaxies?

Galaxy Environments Galaxies come in all sorts of shapes and sizes. Whether these differences are due to nature or nurture, the environment of the galaxy is bound to be involved. Understanding the observed properties of galaxies therefore requires an understanding of the environment in which they formed and evolved. To first order the environment is simply the local density of matter (read: other galaxies).

Environmental Relations Morphology-Density Relation Color-Density Relation Magnitude-Density Relation

The Environment-Galaxy Relation Nature –Halo mass? –Halo angular momentum? Nurture –Relative velocity of hierarchically merging clumps? –Ram pressure stripping? –Close encounters? –Harassment? –Strangulation? –Star formation?

Cosmic Evolution Survey (COSMOS) HST’s largest contiguous survey. Near-IR ACS imaging of 1.7 sqr deg. 0.1 arcsec absolute astrometry arcsec pixels for morphology arcsec pixels for weak lensing. Field also imaged with Subaru and CFHT, KPNO and CTIO. Luminosity, size, color, morphology, nuclear activity for galaxies out to z= ,000 objects to I = 26

Characterizing the COSMOS Environment at z=0.73 SUBARU imaging. XMM-Newton imaging. High-res HST images (weak lensing). Get  10 in [h 2 Mpc -2 ]

Lensing vs. X-Ray Emission

Morphology-Density Relation

Calibrating Morphology

Color-Magnitude Plots

Bimodal Colors

Red Late-Types

Ellipticity of Late-Types

Blue Early-Types

Early-Types and Environment

Kormendy Relation Coma Cluster Constant M B =-20

Color-Mass Relationship

Conclusions There is a clear Red Sequence in the COSMOS z = 0.7 slice. There is a Blue Sequence (as opposed to a Blue Cloud). The bimodality is more obvious when one considers morphology rather than color. The individual sequences are insensitive to environment, but their relative population does depend on local density. Red Late-Types are usually edge-on spirals with lots of dust (these are 33% of “red” galaxies!). Blue Early-Types are not precursors of giant ellipticals, but might turn into S0, bulges and dwarf ellipticals. Slope of Blue Sequence is governed by star-formation. Slope of Red Sequence is governed by metallicity.

References Cassata et al (astro-ph/ ) Guzzo et al (astro-ph/ )