Dry Mergers in GEMS: The Dynamical Evolution of Massive Early‐Type Galaxies (original) (raw)
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Astrophysical Journal, 2008
We study the redshift evolution of galaxy pair fractions and merger rates for different types of galaxies using kinematic pairs selected from the DEEP2 Redshift Survey. By parameterizing the evolution of the pair fraction as (1+z)^{m}, we find that the companion rate increases mildly with redshift with m = 0.41+-0.20 for all galaxies with -21 < M_B^{e} < -19. Blue galaxies show slightly faster evolution in the blue companion rate with m = 1.27+-0.35 while red galaxies have had fewer red companions in the past as evidenced by the negative slope m = -0.92+-0.59. We find that at low redshift the pair fraction within the red sequence exceeds that of the blue cloud, indicating a higher merger probability among red galaxies compared to that among the blue galaxies. With further assumptions on the merger timescale and the fraction of pairs that will merge, the galaxy major merger rates for 0.1 < z <1.2 are estimated to be ~10^{-3}h^{3}Mpc^{-3}Gyr^{-1} with a factor of 2 uncertainty. At z ~ 1.1, 68% of mergers are wet, 8% of mergers are dry, and 24% of mergers are mixed, compared to 31% wet mergers, 25% dry mergers, and 44% mixed mergers at z ~ 0.1. The growth of dry merger rates with decreasing redshift is mainly due to the increase in the co-moving number density of red galaxies over time. About 22% to 54% of present-day L^{*} galaxies have experienced major mergers since z ~ 1.2, depending on the definition of major mergers. Moreover, 24% of the red galaxies at the present epoch have had dry mergers with luminosity ratios between 1:4 and 4:1 since z ~ 1. Our results also suggest that the wet mergers and/or mixed mergers may be partially responsible for producing red galaxies with intermediate masses while a significant portion of massive red galaxies is assembled through dry mergers at later times.
The Astrophysical Journal, 2010
We study the environment of wet, dry, and mixed galaxy mergers at 0.75 < z < 1.2 using close pairs in the DEEP2 Galaxy Redshift Survey, aiming to establish a clear picture of how the cosmic evolution of various merger types relate to the observed large-scale extra-galactic environment and its role in the growth of redsequence galaxies. We find that the typical environment of mixed and dry mergers is denser than that of wet mergers, mostly due to the color-density relation. While the galaxy companion rate (N c ) is observed to increase with overdensity, using N-body simulations we find that the fraction of pairs that will eventually merge decreases with the local density, predominantly because interlopers are more common in dense environments. After taking into account the merger probability of pairs as a function of local density, we find only marginal environment dependence of the fractional merger rate for wet mergers over the redshift range we have probed. On the other hand, the fractional dry merger rate increases rapidly with local density due to the increased population of red galaxies in dense environments. In other words, while wet mergers transform galaxies from the blue cloud into the red sequence at a similar fractional rate across different environments (assuming that the success rate of wet mergers to yield red galaxies does not depend on environment), the dry and mixed mergers are most effective in overdense regions. We also find that the environment distribution of K+A galaxies is similar to that of wet mergers alone and of wet+mixed mergers, suggesting a possible connection between K+A galaxies and wet and/or wet+mixed mergers. Based on our results, we therefore expect that the properties, including structures and masses, of red-sequence galaxies should be different between those in underdense regions and in overdense regions since the dry mergers are significantly more important in dense environments. We conclude that, as early as z ∼ 1, high-density regions are the preferred environment in which dry mergers occur, and that present-day red-sequence galaxies in overdense environments have, on average, undergone 1.2±0.3 dry mergers since this time, accounting for (38±10)% of their mass accretion in the last 8 billion years. Our findings suggest that dry mergers are crucial in the mass-assembly of massive red galaxies in dense environments, such as Brightest Cluster Galaxies (BCGs) in galaxy groups and clusters.
Dry Mergers and the Formation of Early-Type Galaxies: Constraints from Lensing and Dynamics
Astrophysical Journal - ASTROPHYS J, 2009
Dissipationless (gas-free or "dry") mergers have been suggested to play a major role in the formation and evolution of early-type galaxies, particularly in growing their mass and size without altering their stellar populations. We perform a new test of the dry-merger hypothesis by comparing N-body simulations of realistic systems to empirical constraints provided by recent studies of lens early-type galaxies. We find that major and minor dry mergers (1) preserve the nearly isothermal structure (rhotot vprop r -2) of early-type galaxies within the observed scatter, (2) do not change more than the observed scatter the ratio between total mass M and "virial" mass R esigma2 e2/2G (where R e is the half-light radius and sigmae2 is the projected velocity dispersion), (3) strongly increase galaxy sizes (R e vprop M 0.85±0.17) and weakly increase velocity dispersions (sigmae2 vprop M 0.06±0.08) with mass, thus moving galaxies away from the local observed M-R e and M-sigm...
Hierarchical models predict that massive early-type galaxies (mETGs) derive from the most massive and violent merging sequences occurred in the Universe. However, the role of wet, mixed, and dry major mergers in the assembly of mETGs is questioned by some recent observations. We have developed a semi-analytical model to test the feasibility of the major-merger origin hypothesis for mETGs, just accounting for the effects on galaxy evolution of the major mergers strictly reported by observations. The model proves that it is feasible to reproduce the observed number density evolution of mETGs since z ∼ 1, just accounting for the coordinated effects of wet/mixed/dry major mergers. It can also reconcile the different assembly redshifts derived by hierarchical models and by mass downsizing data for mETGs, just considering that a mETG observed at a certain redshift is not necessarily in place since then. The model predicts that wet major mergers have controlled the mETGs buildup since z∼1, although dry and mixed mergers have also played an essential role in it. The bulk of this assembly took place at 0.7< z <1, being nearly frozen at z 0.7 due to the negligible number of major mergers occurred per existing mETG since then. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.
Hierarchical models predict that massive early-type galaxies (mETGs) derive from the most massive and violent merging sequences occurred in the Universe. However, the role of wet, mixed, and dry major mergers in the assembly of mETGs is questioned by some recent observations. We have developed a semi-analytical model to test the feasibility of the major-merger origin hypothesis for mETGs, just accounting for the effects on galaxy evolution of the major mergers strictly reported by observations. The model proves that it is feasible to reproduce the observed number density evolution of mETGs since z ∼ 1, just accounting for the coordinated effects of wet/mixed/dry major mergers. It can also reconcile the different assembly redshifts derived by hierarchical models and by mass downsizing data for mETGs, just considering that a mETG observed at a certain redshift is not necessarily in place since then. The model predicts that wet major mergers have controlled the mETGs buildup since z∼1, although dry and mixed mergers have also played an essential role in it. The bulk of this assembly took place at 0.7< z <1, being nearly frozen at z 0.7 due to the negligible number of major mergers occurred per existing mETG since then. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.
Context. Several studies have tried to ascertain whether or not the increase in abundance of the early-type galaxies (E-S0a's) with time is mainly due to major mergers, reaching opposite conclusions. Aims. We have tested it directly through semi-analytical modelling, by studying how the massive early-type galaxies with log(M * /M ⊙ ) > 11 at z ∼ 0 (mETGs) would have evolved backwards-in-time, under the hypothesis that each major merger gives place to an early-type galaxy. Methods. The study was carried out just considering the major mergers strictly reported by observations at each redshift, and assuming that gas-rich major mergers experience transitory phases as dust-reddened, star-forming galaxies (DSFs). Results. The model is able to reproduce the observed evolution of the galaxy LFs at z 1, simultaneously for different rest-frame bands (B, I, and K) and for different selection criteria on color and morphology. It also provides a framework in which apparentlycontradictory results on the recent evolution of the luminosity function (LF) of massive, red galaxies can be reconciled, just considering that observational samples of red galaxies can be significantly contaminated by DSFs. The model proves that it is feasible to build up ∼ 50-60% of the present-day number density of mETGs at z 1 through the coordinated action of wet, mixed, and dry major mergers, fulfilling global trends that are in general agreement with mass-downsizing. The bulk of this assembly takes place during ∼ 1 Gyr elapsed at 0.8 < z < 1, providing a straightforward explanation to the observational fact that redshift z ∼ 0.8 is a transition epoch in the formation of mETGs. The gas-rich progenitors of these recently-assembled mETGs reproduce naturally the observational excess by a factor of ∼ 4-5 of late-type galaxies at 0.8 < z < 1, as compared to pure luminosity evolution (PLE) models. Conclusions. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive-end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.
The Role of Mergers in Early-Type Galaxy Evolution and Black Hole Growth
The Astrophysical Journal, 2010
Models of galaxy formation invoke the major merger of gas-rich progenitor galaxies as the trigger for significant phases of black hole growth and the associated feedback that suppresses star formation to create red spheroidal remnants. However, the observational evidence for the connection between mergers and active galactic nucleus (AGN) phases is not clear. We analyze a sample of low-mass early-type galaxies known to be in the process of migrating from the blue cloud to the red sequence via an AGN phase in the green valley. Using deeper imaging from SDSS Stripe 82, we show that the fraction of objects with major morphological disturbances is high during the early starburst phase, but declines rapidly to the background level seen in quiescent early-type galaxies by the time of substantial AGN radiation several hundred Myr after the starburst. This observation empirically links the AGN activity in low-redshift early-type galaxies to a significant merger event in the recent past. The large time delay between the merger-driven starburst and the peak of AGN activity allows for the merger features to decay to the background and hence may explain the weak link between merger features and AGN activity in the literature.
The Role of Mergers in Galaxy Evolution
The Evolution of Galaxies, 2001
In the last decade the importance of mergers in the evolution of galaxies has become evident. In this paper we illustrate this importance by showing examples of merging galaxies, both local and at increasing redshift. However before getting carried away by the charms of the hierarchical model in which large galaxies have been built up by successive mergers of smaller objects, it is worth looking at what stellar population synthesis can tell us. Here I show that, using indices which allow us to separate the effects of metallicity and age on the spectra of the stellar populations of galaxies, we can show that the most massive galaxies have the oldest stellar populations, an effect which is enhanced within galaxy clusters and is maximized within the most massive clusters. These measurements imply that a model where mergers (even "dry" mergers) are the main driver for galaxy evolution cannot be giving us anything like a valid picture. The role of mergers must be considerably more subtle than one would infer from the standard semi-analytic models of galaxy evolution within a cosmological framework.
Galaxy and Mass Assembly (GAMA): merging galaxies and their properties
We derive the close pair fractions and volume merger rates for galaxies in the Galaxy and Mass Assembly (GAMA) survey with -23 < Mr < -17 (ΩM = 0.27, ΩΛ = 0.73, H0 = 100 km s-1 Mpc-1) at 0.01 < z < 0.22 (look-back time of <2 Gyr). The merger fraction is approximately 1.5 per cent Gyr-1 at all luminosities (assuming 50 per cent of pairs merge) and the volume merger rate is ≈3.5 × 10-4 Mpc-3 Gyr-1. We examine how the merger rate varies by luminosity and morphology. Dry mergers (between red/spheroidal galaxies) are found to be uncommon and to decrease with decreasing luminosity. Fainter mergers are wet, between blue/discy galaxies. Damp mergers (one of each type) follow the average of dry and wet mergers. In the brighter luminosity bin (-23 < Mr < -20), the merger rate evolution is flat, irrespective of colour or morphology, out to z ˜ 0.2. The makeup of the merging population does not appear to change over this redshift range. Galaxy growth by major mergers appears comparatively unimportant and dry mergers are unlikely to be significant in the buildup of the red sequence over the past 2 Gyr. We compare the colour, morphology, environmental density and degree of activity (BPT class, Baldwin, Phillips & Terlevich) of galaxies in pairs to those of more isolated objects in the same volume. Galaxies in close pairs tend to be both redder and slightly more spheroid dominated than the comparison sample. We suggest that this may be due to `harassment' in multiple previous passes prior to the current close interaction. Galaxy pairs do not appear to prefer significantly denser environments. There is no evidence of an enhancement in the AGN fraction in pairs, compared to other galaxies in the same volume.
The Merger Rate of Massive Galaxies
The Astrophysical Journal, 2006
We calculate the projected two point correlation function for samples of luminous and massive galaxies in the COMBO-17 photometric redshift survey, focusing particularly on the amplitude of the correlation function at small projected radii and exploring the constraints such measurements can place on the galaxy merger rate. For nearly volume-limited samples with 0.4 < z < 0.8, we find that 4±1% of luminous M B < −20 galaxies are in close physical pairs (with real space separation of < 30 proper kpc). The corresponding fraction for massive galaxies with M * > 2.5 × 10 10 M ⊙ is 5±1%. Incorporating close pair fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid evolution of the merger fraction of massive galaxies between z = 0.8 and the present day. Assuming that the major merger timescale is of order the dynamical timescale for close massive galaxy pairs, we tentatively infer that ∼ 50% (70%) of all galaxies with present-day masses M * > 5 × 10 10 M ⊙ (remnants of mergers between galaxies with M * > 2.5 × 10 10 M ⊙ ) have undergone a major merger since z = 0.8 (1): major mergers between massive galaxies are a significant driver of galaxy evolution over the last eight billion years.