Galaxy and Mass Assembly (GAMA): merging galaxies and their properties (original) (raw)
Related papers
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.
Monthly Notices of the Royal Astronomical Society, 2010
Following the study of hereafter D09a) we explore the environments, optical colours, stellar masses, star formation and AGN activity in a sample of 3003 pairs of merging galaxies drawn from the SDSS using visual classifications from the Galaxy Zoo project. While D09a found that the spiral-to-elliptical ratio in (major) mergers appeared higher than that of the global galaxy population, no significant differences are found between the environmental distributions of mergers and a randomly selected control sample. This makes the high occurrence of spirals in mergers unlikely to be an environmental effect and must, therefore, arise from differing time-scales of detectability for spirals and ellipticals. We find that merging galaxies have a wider spread in colour than the global galaxy population, with a significant blue tail resulting from intense star formation in spiral mergers. Galaxies classed as star-forming using their emission-line properties have average star-formation rates approximately doubled by the merger process though star formation is negligibly enhanced in merging elliptical galaxies. We conclude that the internal properties of galaxies significantly affect the time-scales over which merging systems can be detected (as suggested by recent theoretical studies) which leads to spirals being 'over-observed' in mergers. We also suggest that the transition mass 3 × 10 10 M ⊙ , noted by , below which ellipticals are rare could be linked to disc survival/destruction in mergers.
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.
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.
Dry Mergers in GEMS: The Dynamical Evolution of Massive Early‐Type Galaxies
The Astrophysical Journal, 2006
We have used the 28 ′ × 28 ′ Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric redshift survey to constrain the incidence of major mergers between spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A set of N-body merger simulations was used to explore the morphological signatures of such interactions: they are recognizable either as < 5 kpc separation close pairs or because of broad, low surface brightness tidal features and asymmetries. Data with the depth and resolution of GEMS are sensitive to dry mergers between galaxies with M V −20.5 for z 0.7; dry mergers at higher redshifts are not easily recovered in single-orbit HST imaging. Six dry mergers (12 galaxies) with luminosity ratios between 1:1 and 4:1 were found from a sample of 379 red early-type galaxies with M V < −20.5 and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of dry merging are visible for ∼ 150 Myr and we use this timescale to convert the observed merger incidence into a rate. On this basis we find that present day spheroidal galaxies with M V < −20.5 on average have undergone between 0.5 and 2 major dry mergers since z ∼ 0.7. We have compared this result with the predictions of a Cold Dark Matter based semi-analytic galaxy formation model. The model reproduces the observed declining major merger fraction of bright galaxies and the space density of luminous early-type galaxies reasonably well. The predicted dry merger fraction is consistent with our observational result. Hence, hierarchical models predict and observations now show that major dry mergers are an important driver of the evolution of massive early-type galaxies in recent epochs.
A scaling relation between merger rate of galaxies and their close pair count
2013
We study how to measure the galaxy merger rate from the observed close pair count. Using a highresolution N-body/SPH cosmological simulation, we find an accurate scaling relation between galaxy pair counts and merger rates down to a stellar mass ratio of about 1:30. The relation explicitly accounts for the dependence on redshift (or time), on pair separation, and on mass of the two galaxies in a pair. With this relation, one can easily obtain the mean merger timescale for a close pair of galaxies. The use of virial masses, instead of stellar masses, is motivated by the fact that the dynamical friction time scale is mainly determined by the dark matter surrounding central and satellite galaxies. This fact can also minimize the error induced by uncertainties in modeling star formation in the simulation. Since the virial mass can be read from the well-established relation between the virial masses and the stellar masses in observation, our scaling relation can be easily applied to observations to obtain the merger rate and merger time scale. For major merger pairs (1:1-1:4) of galaxies above a stellar mass of 4 × 10 10 h −1 M ⊙ at z = 0.1, it takes about 0.31 Gyr to merge for pairs within a projected distance of 20 h −1 kpc with stellar mass ratio of 1:1, while the time taken goes up to 1.6 Gyr for mergers with stellar mass ratio of 1:4. Our results indicate that a single timescale usually used in literature is not accurate to describe mergers with the stellar mass ratio spanning even a narrow range from 1:1 to 1:4.
Galaxy And Mass Assembly (GAMA): galaxy close pairs, mergers and the future fate of stellar mass
We use a highly complete subset of the Galaxy And Mass Assembly II (GAMA-II) redshift sample to fully describe the stellar mass dependence of close pairs and mergers between 108 and 1012 M⊙. Using the analytic form of this fit we investigate the total stellar mass accreting on to more massive galaxies across all mass ratios. Depending on how conservatively we select our robust merging systems, the fraction of mass merging on to more massive companions is 2.0-5.6 per cent. Using the GAMA-II data we see no significant evidence for a change in the close pair fraction between redshift z = 0.05 and 0.2. However, we find a systematically higher fraction of galaxies in similar mass close pairs compared to published results over a similar redshift baseline. Using a compendium of data and the function γM = A(1 + z)m to predict the major close pair fraction, we find fitting parameters of A = 0.021 ± 0.001 and m = 1.53 ± 0.08, which represents a higher low-redshift normalization and shallower power-law slope than recent literature values. We find that the relative importance of in situ star formation versus galaxy merging is inversely correlated, with star formation dominating the addition of stellar material below M^* and merger accretion events dominating beyond M^*. We find mergers have a measurable impact on the whole extent of the galaxy stellar mass function (GSMF), manifest as a deepening of the `dip' in the GSMF over the next ˜Gyr and an increase in M^* by as much as 0.01-0.05 dex.
Very Small Scale Clustering and Merger Rate of Luminous Red Galaxies
The Astrophysical Journal, 2006
We present the small-scale (0.01 < r < 8 h −1 Mpc) projected correlation function w p (r p ) and real space correlation function ξ(r) of 24520 luminous early-type galaxies from the Sloan Digital Sky Survey Luminous Red Galaxy (LRG) sample ( 0.16 < z < 0.36). "Fiber collision" incompleteness of the SDSS spectroscopic sample at scales smaller than 55 arcsec prevents measurements of the correlation function for LRGs on scales smaller than ∼ 0.3 Mpc by the usual methods. In this work, we cross-correlate the spectroscopic sample with the imaging sample, with a weighting scheme to account for the collisions, extensively tested against mock catalogs. We correct for photometric biases in the SDSS imaging of close galaxy pairs. We find that the correlation function ξ(r) is surprisingly close to a r −2 power law over more than 4 orders of magnitude in separation r. This result is too steep at small scales to be explained in current versions of the halo model for galaxy clustering. We infer an LRG-LRG merger rate of 0.6 Gyr −1 Gpc −3 for this sample. This result suggests that the LRG-LRG mergers are not the main mode of mass growth for LRGs at z < 0.36.
The Astrophysical Journal, 2007
We compare the use of galaxy asymmetry and pair proximity for measuring galaxy merger fractions and rates for a volume limited sample of 3184 galaxies with −21 < M B − 5 log h < −18 mag. and 0.010 < z < 0.123 drawn from the Millennium Galaxy Catalogue. Our findings are that: (i) Galaxies in close pairs are generally more asymmetric than isolated galaxies and the degree of asymmetry increases for closer pairs. At least 35% of close pairs (with projected separation of less than 20 h −1 kpc and velocity difference of less than 500 km s −1 ) show significant asymmetry and are therefore likely to be physically bound.