A Direct Measurement of Major Galaxy Mergers atz3

Abstract

This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z ∼ 3. We identify major mergers using the model-independent CAS (concentration, asymmetry, clumpiness) physical morphological system on galaxies detected, and photometrically selected, in the WFPC2 and NICMOS Hubble Deep Field North. We specifically use the asymmetric distributions of rest-frame optical light measured through the asymmetry parameter (A) to determine the fraction of galaxies undergoing major mergers as a function of redshift (z), stellar mass (M ⋆ ), and absolute magnitude (M B ). We find that the fraction of galaxies consistent with undergoing a major merger increases with redshift for all galaxies, but most significantly, at 5 -10 σ confidence, for the most luminous and massive systems. The highest merger fractions we find are 40% -50% for galaxies with M B < −21, or M ⋆ > 10 10 M ⊙ at z > 2.5, i.e., objects identified as Lyman-break galaxies. Using these results, we model the merger fraction evolution in the form: f m (A,M ⋆ ,M B , z) = f 0 × (1 + z) mA . We find m A values ∼ 4 − 6 for the most luminous and massive galaxies, while lower mass and less luminous galaxies have smaller m A values. We use these merger fractions, combined with merger time scales calculated from N-body simulations, to derive galaxy merger rates to z ∼ 3. We also use stellar masses of HDF-N galaxies to determine the mass accretion rate of field galaxies involved in major mergers. We find an average stellar mass accretion rate ofṀ G ∼ 4 × 10 8 M ⊙ Gyr −1 galaxy −1 at z ∼ 1 for galaxies with stellar masses M ⋆ > 10 9 M ⊙ . This accretion rate changes with redshift as:Ṁ G = 1.6 × 10 8 (1 + z) 0.99±0.32 M ⊙ Gyr −1 galaxy −1 . We also find that the fraction of stellar mass density in galaxies involved in major mergers increases with redshift, with a peak mass fraction ∼ 0.5 for the brightest, M B < −21, and most massive, M ⋆ > 10 10 M ⊙ , systems near z ∼ 2.5. By comparing merger fractions predicted in Cold Dark Matter semi-analytic models with our results we find a reasonably good agreement for the largest and brightest systems, although we find more low-mass galaxy mergers at lower redshifts than what these models predict.

Massive close pairs measure rapid galaxy assembly in mergers at high redshift

“…Assuming a roughly constant observability timescale, i.e., the duration a merging pair is identified in a catalog, these observations of galaxy pairs matched well with expectations (e.g., Hopkins et al 2010;Lotz et al 2011;Robotham et al 2014). Moreover, different windows on the merger process, such as asymmetric morphologies (Conselice et al 2003) and post-starburst signatures (Snyder et al 2011) all generally agree with this picture, so long as we adopt the proper observability timescales.…”

Tides in Colliding Galaxies

“…Conselice et al, 2003). This requires a reliable algorithm to automatically measure the degree of perturbation.…”

Galaxy And Mass Assembly (GAMA): the effect of close interactions on star formation in galaxies

“…As galaxies interact and merge hierarchically over cosmic time-scales, these interaction events leave strong imprints on the galaxies involved, modifying their morphology (e.g. Conselice et al 2003;Conselice, Blackburne & Papovich 2005;Lotz et al 2008a,b;Mortlock et al 2013, or see review in Conselice 2014), triggering active galactic nuclei (AGN; e.g. Kazantzidis et al 2005;Hopkins et al 2008;Medling et al 2013, however cf.…”