Evolution of Luminous Galaxy Pairs out to z=1.2 in the HST/ACS COSMOS Field

1. Venice, Italy 2006 March 28 Galaxies and Structures Through Cosmic Times 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

2. 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

3. 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

4. The COSMOS Field • Previous studies • Very small samples • Small range in z • The COSMOS field has • High resolution ACS images and catalog • Contiguous 2-deg2 field – over 1 million galaxies! • Multiwavelength ground and space-based follow-up • Ground based photometry catalog • Photometric redshifts • Ideal survey for this study!

5. Method: Selection of Sample • Galaxies brighter than Mv = -20.4 (~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

6. 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

7. 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

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

9. 12 Galaxy Pairs Selected from all z-bins, various separations Selected based on presence of obvious signs of interaction

10. 12 Galaxy Pairs Selected from all z-bins, various separations Selected based on lack of obvious signs of interaction

11. 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 Results

12. Conclusion • Slope, n = 4.2 ± 0.69 • Power law not best fit – range n ~ 4-8 • Inconsistent 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)

13. 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

14. Distribution of Pairs All pairs in field 0.7 < z < 0.8 bin

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

16. Summary of Previous Results

17. Table of Results