A Close-Pair Analysis of Damp Mergers at Intermediate Redshifts

Abstract

We have studied the kinematics of ∼2800 candidate close-pair galaxies at 0.1 < z < 1.2 identified from the Canada-France-Hawaii Telescope Legacy Survey fields. Spectra of these systems were obtained using spectrometers on the 6.5 m Magellan and 5 m Hale telescopes. These data allow us to constrain the rate of dry mergers at intermediate redshifts and to test the "hot halo" model for quenching of star formation. Using virial radii estimated from the correlation between dynamical and stellar masses published by Leauthaud et al., we find that around 1/5 of our candidate pairs are likely to share a common dark matter halo (our metric for close physical association). These pairs are divided into red-red, blue-red, and blue-blue systems using the rest-frame colors classification method introduced in Chou et al.. Galaxies classified as red in our sample have very low star formation rates, but they need not be totally quiescent, and hence we refer to them as "damp," rather than "dry," systems. After correcting for known selection effects, the fraction of blue-blue pairs is significantly greater than that of red-red and blue-red pairs. Red-red pairs are almost entirely absent from our sample, suggesting that damp mergers are rare at z ∼ 0.5. Our data support models with a short merging timescale (<0.5 Gyr) in which star formation is enhanced in the early phase of mergers, but quenched in the late phase. Hot halo models may explain this behavior, but only if virial shocks that heat gas are inefficient until major mergers are nearly complete.

Galaxy and Mass Assembly (GAMA): merging galaxies and their properties

“…Similar claims of a greater frequency of red galaxies in pairs were earlier reported by Darg et al (2010a) and Ellison et al (2010). Chou et al (2012) also find that close pairs tend to quench, and not enhance, star formation in their sample. Bergvall et al (2003) argued that there is only a modest enhancement in star formation rates for a sample of highly interacting objects.…”

“…upper limit of < 0.8% per Gyr, which is consistent with flat evolution of the dry merger rate out to at least z = 0.6. Chou et al (2012) also confirm the slow growth of the dry merger rate as a function of redshift out to z = 0.7. It is therefore unlikely that the red sequence is formed by dry mergers at least in the past 1/3 of the Hubble time.…”

“…We confirm this from our analysis of BPT classes where there is a lower fraction of star-forming galaxies among close pairs compared to galaxies in the same volume. Bergvall et al (2003) found only a modest degree of enhancement of star formation for interacting pairs, contributing little to the luminosity; Lin et al (2007) also find only a modest enhancement of star formation in galaxy pairs in a wide range of merger stages; Li et al (2008) instead find that the enhancement in star formation, in a sample of star-forming galaxies, increases with decreasing separation but does not depend on the luminosity ratio, although it is stronger for lower-mass galaxies; for massive galaxies, Robaina et al (2009) find that only 10% of star formation is due to major interactions (albeit at higher redshifts than studied here); Patton et al (2011) also detect an excess of red galaxies in pairs and attribute this to denser environments and hence more significant spheroids (because of the morphology-density relation); Chou et al (2012) confirm that most close pairs are red and not blue; Deng & Zhang (2013) also finds evidence that interactions are not generally responsible for star formation bursts in two volume-limited samples drawn from the SDSS; Cluver et al (2014) study star formation for galaxies in groups and claim that galaxies with a close neighbour have lower star formation rates. Similarly, we find that, by comparing the distributions of Sérsic indices for galaxies in pairs and their parent samples, there is a relative lack of 'disks' and excess of 'spheroids' among paired galaxies as in Patton et al (2011).…”

“…This leads to a gradual suppression of star formation as gas is exhausted or driven out, while dynamical relaxation leads to secular evolution of spheroids without mergers. Chou et al (2012) also suggest that in their sample star formation is triggered at large radii and then suppressed to form red close pairs. Robotham et al (2013) point out the role of mergers and interactions in suppressing star formation in most galaxy pairs, while Trihn et al (2013) present an analysis of the properties of single and widely spaced paired galaxies in the SDSS and find evidence of delayed quenching, over timescales of a few Gyrs, consistent with a 'harassment' scenario as proposed here.…”

The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time

“…opposed to red on red 'dry mergers'). One approach to measuring merger rates more precisely is to take a close pair catalogue and confirm the kinematic association spectroscopically using slit spectroscopy (Chou et al 2012). An interesting hybrid approach was used by López-Sanjuan et al (2013), where they took advantage of the fact that their IFS maps were wider field than typical to count close kinematic pairs (within ß20 kpc and 500 km s −1 ) as Kinematics of High-z Star-Forming Galaxies 37 well as advanced ongoing mergers for star-forming galaxies with 10 10−10.5 M ࣻ .…”