MASSIV: Mass Assembly Survey with SINFONI in VVDS

Abstract

Context. The contribution of the merging process to the early phase of galaxy assembly at z > 1 and, in particular, to the build-up of the red sequence, still needs to be accurately assessed. Aims. We aim to measure the major merger rate of star-forming galaxies at 0.9 < z < 1.8, using close pairs identified from integral field spectroscopy (IFS). Methods. We use the velocity field maps obtained with SINFONI/VLT on the MASSIV sample, selected from the star-forming population in the VVDS. We identify physical pairs of galaxies from the measurement of the relative velocity and the projected separation (r p ) of the galaxies in the pair. Using the well constrained selection function of the MASSIV sample, we derive at a mean redshift up to z = 1.54 the gas-rich major merger fraction (luminosity ratio μ = L 2 /L 1 ≥ 1/4), and the gas-rich major merger rate using merger time scales from cosmological simulations. Results. We find a high gas-rich major merger fraction of 20.8 +15.2 −6.8 %, 20.1 +8.0 −5.1 %, and 22.0 +13.7 −7.3 % for close pairs with r p ≤ 20 h −1 kpc in redshift ranges z = [0.94, 1.06], [1.2, 1.5), and [1.5, 1.8), respectively. This translates into a gas-rich major merger rate of 0.116 +0.084 −0.038 Gyr −1 , 0.147 +0.058 −0.037 Gyr −1 , and 0.127 +0.079 −0.042 Gyr −1 at z = 1.03, 1.32, and 1.54, respectively. Combining our results with previous studies at z < 1, the gas-rich major merger rate evolves as (1 + z) n , with n = 3.95 ± 0.12, up to z = 1.5. From these results we infer that ∼35% of the star-forming galaxies with stellar masses M = 10 10 −10 10.5 M have undergone a major merger since z ∼ 1.5. We develop a simple model that shows that, assuming that all gas-rich major mergers lead to early-type galaxies, the combined effect of gas-rich and dry mergers is able to explain most of the evolution in the number density of massive early-type galaxies since z ∼ 1.5, with our measured gas-rich merger rate accounting for about two-thirds of this evolution. Conclusions. Merging of star-forming galaxies is frequent at around the peak in star formation activity. Our results show that gasrich mergers make an important contribution to the growth of massive galaxies since z ∼ 1.5, particularly on the build-up of the red sequence.

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

“…The close pair studies mentioned above suggest the merger fraction is closer to 4% at z ß0.6 (e.g. see Figure 24 of López-Sanjuan et al 2013) and if the timescales are similar then these should be comparable and clearly they are not. Puech et al argued that a fraction of their objects were in what they called a 'postfusion' phase, these should be compared to close pairs at an earlier epoch (z ß1.1) which lessens the tension due to the fast evolution in pairs with redshift.…”

“…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 ࣻ . Good agreement was found by López-Sanjuan et al between their kinematic and other's photometric surveys at 1 < z < 1.5.…”

PHYSICAL PROPERTIES OF A PILOT SAMPLE OF SPECTROSCOPIC CLOSE PAIR GALAXIES ATz∼ 2

“…When the velocity separation between the two components in such a pair is 500  km s −1 (see the discussion by Lin et al 2004;Lotz et al 2008), numerical simulations suggest that such systems should predominantly trace major galaxy-galaxy mergers during their first pericentric passage and before final coalescence (Lotz et al 2008(Lotz et al , 2010. Indeed, when merger rates derived from such close pairs (e.g., Bundy et al 2009;de Ravel et al 2009;Williams et al 2011;López-Sanjuan et al 2013;Tasca et al 2014) are combined with physically motivated timescales for interaction, the overall agreement on merger rates between different studies is relatively good (Lotz et al 2011).…”

Evidence for major mergers of galaxies at 2 ≲z< 4 in the VVDS and VUDS surveys

“…At redshifts beyond z ∼ 1, only a few direct identifications of pairs and measurements of the merger fraction and merger rate exist. López-Sanjuan et al (2013) report a high pair fraction of ∼20−22% at 1 < z < 1.8 from 3D spectroscopy measurements in MASSIV (Mass Assembly Survey with Sinfoni in VVDS, Contini et al 2012). At higher, redshifts Conselice et al (2003) has used the CAS (concentration, asymmetry clumpiness) methodology, which relies on image shapes and the expected signature of on-going or past mergers, to perform a measurement of the merger fraction up to z ∼ 3.…”