Searching for Interacting Early Type Galaxies at Late Cosmic Times

Abstract

Major gas-poor (dry) merging between two comparably massive spheroidal galaxies are post u-lated to be the central mechanism responsible for the assembly of the most massive (M[subscript star,tot] ≥ 10¹¹ M[subscript ☉]) elliptical galaxies. Numerical simulations predict that these mergers may occur at late cosmic times and typically in dense environments. Previous work based on a complete sample of high-mass (M[subscript star] ≥ 5 x 10¹⁰ M[subscript ☉]) galaxies with z ≤ 0.12 selected from the Yang et al. (2007) SDSS Galaxy Group Catalog and analyzed for residual asymmetric features in SDSS r-band images provided a lower limit to the frequency of massive pairs with interaction signatures residing in groups and clusters with halo masses M[subscript halo] ≥ 2.5 x 10¹³ M[subscript ☉]. The tidal sgnatures of such interactions may often be too faint to be clearly detected at the sensitivity of SDSS imaging data. To improve constraints and test the identification of dry merging, we obtained V-band images 1.5 mag deeper than SDSS for a random selection of 27 pairs with no SDSS tidal signatures, plus a subset of six previously identified interactions. Using GALFIT image residuals, we visually identify interaction signatures with surface brightnesses down to ~26.5 mag arcsec⁻² at S/N ≥ 5. We confirmed 80% of previous interaction identifications based on shallower SDSS imaging, and identified three new systems with mutual tidal signatures that were previously classified as non-interacting. Applying these deeper statistics indicates that the merger frequency for massive pairs with projected separation ≤ 30kpc increases from 16 ± 3% to 23 ± 6% combined with the shallower imaging data. The merger frequency further increases to 35 ± 16%, when the massive pairs share the same dark matter halo and have similar redshifts. We found that using SDSS alone would underestimate the merger frequency by 30%.