Role of galaxy mergers in cosmic star formation history

We present a morphology study of intermediate-redshift (0.2 < z < 1.2) luminous infrared galaxies (LIRGs) and general field galaxies in the GOODS fields using a revised asymmetry measurement method optimized for deep fields. By taking careful account of the importance of the underlying sky-background structures, our new method does not suffer from systematic bias and offers small uncertainties. By redshifting local LIRGs and low-redshift GOODS galaxies to different higher redshifts, we have found that the redshift dependence of the galaxy asymmetry due to surface-brightness dimming is a function of the asymmetry itself, with larger corrections for more asymmetric objects. By applying redshift-, infrared (IR)-luminosity- and optical-brightness-dependent asymmetry corrections, we have found that intermediate-redshift LIRGs generally show highly asymmetric morphologies, with implied merger fractions ~50% up to z = 1.2, although they are slightly more symmetric than local LIRGs. For general field galaxies, we find an almost constant relatively high merger fraction (20%-30%). The B-band luminosity functions (LFs) of galaxy mergers are derived at different redshifts up to z = 1.2 and confirm the weak evolution of the merger fraction after breaking the luminosity-density degeneracy. The IR LFs of galaxy mergers are also derived, indicating a larger merger fraction at higher IR luminosity. The integral of the merger IR LFs indicates a dramatic evolution of the merger-induced IR energy density [(1 + z)^~(5-6)], and that galaxy mergers start to dominate the cosmic IR energy density at z greater than or ~ 1.

CANDELS: The progenitors of compact quiescent galaxies at z ~ 2

We combine high-resolution Hubble Space Telescope/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (M_*> 10^10 M_☉) galaxies at redshifts z = 1.4-3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates (SFRs) qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5-3. At z≲2, cSFGs present SFR = 100-200 M_☉ yr^–1, yet their specific star formation rates (sSFR ~ 10^–9 yr^–1) are typically half that of other massive SFGs at the same epoch, and host X-ray luminous active galactic nuclei (AGNs) 30 times (~30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 10^8 yr). The cSFGs are continuously being formed at z = 2-3 and fade to cQGs down to z ~ 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary tracks of QG formation: an early (z≲2), formation path of rapidly quenched cSFGs fading into cQGs that later enlarge within the quiescent phase, and a late-arrival (z≳2) path in which larger SFGs form extended QGs without passing through a compact state.

Satellites around massive galaxies since z∼ 2

The accretion of minor satellites has been postulated as the most likely mechanism to explain the significant size evolution of massive galaxies over cosmic time. Using a sample of 629 massive (M_star~ 10^11 M_⊙) galaxies from the near-infrared Palomar/DEEP-2 survey, we explore what fraction of these objects have satellites with 0.01 < M_sat/M_central < 1 (1:100) up to z= 1 and what fraction have satellites with 0.1 < M_sat/M_central < 1 (1:10) up to z= 2 within a projected radial distance of 100 kpc. We find that the fraction of massive galaxies with satellites, after background correction, remains basically constant and close to 30 per cent for satellites with a mass ratio down to 1:100 up to z= 1, and close to 15 per cent for satellites with a 1:10 mass ratio up to z= 2. The family of spheroid-like massive galaxies presents a 2–3 times larger fraction of objects with satellites than the group of disc-like massive galaxies. A crude estimation of the number of 1:3 mergers a massive spheroid-like galaxy has experienced since z~2 is around 2. For a disc-like galaxy this number decreases to ~1.

Are luminous radio-loud active galactic nuclei triggered by galaxy interactions?

We present the results of a comparison between the optical morphologies of a complete sample of 46 southern 2 Jy radio galaxies at intermediate redshifts (0.05 < z < 0.7) and those of two control samples of quiescent early-type galaxies: 55 ellipticals at redshifts z ≤ 0.01 from the Observations of Bright Ellipticals at Yale (OBEY) survey, and 107 early-type galaxies at redshifts 0.2 < z < 0.7 in the Extended Groth Strip (EGS). Based on these comparisons, we discuss the role of galaxy interactions in the triggering of powerful radio galaxies (PRGs). We find that a significant fraction of quiescent ellipticals at low and intermediate redshifts show evidence for disturbed morphologies at relatively high surface brightness levels, which are likely the result of past or on-going galaxy interactions. However, the morphological features detected in the galaxy hosts of the PRGs (e.g. tidal tails, shells, bridges, etc.) are up to 2 mag brighter than those present in their quiescent counterparts. Indeed, if we consider the same surface brightness limits, the fraction of disturbed morphologies is considerably smaller in the quiescent population (53 per cent at z < 0.2 and 48 per cent at 0.2 ≤ z < 0.7) than in the PRGs (93 per cent at z < 0.2 and 95 per cent at 0.2 ≤ z < 0.7 considering strong-line radio galaxies only). This supports a scenario in which PRGs represent a fleeting active phase of a subset of the elliptical galaxies that have recently undergone mergers/interactions. However, we demonstrate that only a small proportion (≲20 per cent) of disturbed early-type galaxies are capable of hosting powerful radio sources.