Evolution of Luminous Galaxy Pairs out to z=1.2 in the HST/ACS COSMOS Field Jeyhan Kartaltepe, IfA, Hawaii Dave Sanders, IfA, Hawaii Nick Scoville, Caltech Venice, Italy 2006 March 28 Galaxies and Structures Through Cosmic Times

Background Role of mergers in galaxy formation and evolution Methods: –Morphological approach Look for irregular or peculiar galaxies Select galaxies in the process of merging Morphology difficult to classify at high z Morphological K-correction –Pair statistics Look for number of close pairs Probes galaxies that will eventually merge Linked to merger rate by timescale of process

Background Merger rates & pair evolution Parameterized as: (1+z) m & (1+z) n, respectively Previous work finds wide range of results: m, n = 0-6 Nearby Examples: The Mice & NGC 520

The COSMOS Field Previous studies –Very small samples –Small range in z The COSMOS field has –High resolution ACS images and catalog –Contiguous 2-deg 2 field – over 1 million galaxies! –Multiwavelength ground and space-based follow-up Ground based photometry catalog Photometric redshifts Ideal survey for this study!

Method: Selection of Sample Galaxies brighter than M v = (~L * ) –Allows for a complete sample out to z limit –Probes most massive galaxies Reliable photometric redshifts –Detected in 4 bands (including Ks) –I < 26 –Small errors in z (~ 0.05 out to z=1.2)  97, 066 galaxies in sample

Method: Searching for Pairs Ground based catalog –Use ground based catalog for photometric redshifts –Find pairs with a projected separation < 50 kpc in 11 redshift bins (0.1 < z < 1.2) –Misses some pairs < 3” apart due to blending ACS Catalog –Find pairs missed in ground based catalog (0.1” < sep < 0.3”) –Match to ground based catalog to get one photoz for pair –Visual inspection to remove artifacts

Method: Searching for Pairs Local Sample –Catalog of pairs from SDSS (Allam et al. 2004) –Applied same search criteria –In 0 < z < 0.1  22 pairs over 462 square degrees Random line of sight galaxy pairs –Calculate number of random pairs expected at various separations and redshifts

Results At higher separations, numbers match random Low number statistics at low redshift

12 Galaxy Pairs Selected from all z-bins, various separations Selected based on presence of obvious signs of interaction

12 Galaxy Pairs Selected from all z-bins, various separations Selected based on lack of obvious signs of interaction

Results Power law is not a good fit Evidence of LSS? What is happening at z > 0.8 Value of n can range from 4-8! Evidence for strong evolution! Total of 3,990 pairs found! Evolution ~ (1+z) n  Slope, n = 4.2 ± 0.69

Conclusion Slope, n = 4.2 ± 0.69 Power law not best fit – range n ~ 4-8 In contrast with results that show zero/weak evolution but consistent with results that show strong evolution –Differences in selection criteria? Also consistent with strong evolution of –ULIGs (n=7.2 ± 3.6: Kim & Sanders 1998) –QSOs (n~6-8: Schmidt & Green 1983)  Evidence suggests that these are formed by the merger of equal mass, ~L * galaxy pairs (e.g. Ishida 2004 & Guyon, Sanders & Stockton 2005)

Future Work Remove L * criterion for comparison Push out to higher redshifts Morphological criterion Spectroscopic redshifts  kinematic pairs Explore effects of LSS IR properties (morphologies, luminosities) –Spitzer observations –Ongoing ground based nIR observations

Acknowledgments For all of their help and support with this project, thanks go to: Peter Capak Lisa Kewley The COSMOS team