C. López-Sanjuan

The galaxy major merger fraction to z 1

Aims. The importance of disc-disc major mergers in galaxy evolution remains uncertain. We study the major merger fraction in a SPITZER/IRAC-selected catalogue in the GOODS-S field up to z ~ 1 for luminosity- and mass-limited samples. Methods. We select disc-disc merger remnants on the basis of morphological asymmetries/distortions, and address three main sources of systematic errors: (i) we explicitly apply morphological K-corrections; (ii) we measure asymmetries in galaxies artificially redshifted to z_d = 1.0 to deal with loss of morphological information with redshift; and (iii) we take into account the observational errors in z and A, which tend to overestimate the merger fraction, though use of maximum likelihood techniques. Results. We obtain morphological merger fractions (f_m^mph) below 0.06 up to z ~ 1. Parameterizing the merger fraction evolution with redshift as f_m^mph (z) = f_m^mph (0)(1 + z)^m, we find that m = 1.8 ± 0.5 for M(B)≤ -20 galaxies, while m = 5.4 ± 0.4 for M_* ≥ 10^10 M_⨀ galaxies. When we translate our merger fractions to merger rates (R_m^mph), their evolution, parameterized as R_m^mph (z) = R_m^mph (0)(1+ z)^n, is quite similar in both cases: n = 3.3 ± 0.8 for M(B) ≤ -20 galaxies, and n = 3.5 ± 0.4 for M_* ≥10^10 M_⨀ galaxies. Conclusions. Our results imply that only similar to 8% of todays M(star) ≥ 10^10 M_⨀ galaxies have undergone a disc-disc major merger since z ~ 1. In addition, ~ 21% of M_* ≥ 10(10) M_⨀ galaxies at z ~ 1 have undergone one of these mergers since z similar to 1.5. This suggests that disc-disc major mergers are not the dominant process in the evolution of M_* ≥ 10(10) M_⨀ galaxies since z 1, with only 0.2 disc-disc major mergers per galaxy, but may be an important process at z > 1, with ~ 1 merger per galaxy at 1 < z < 3.

The ALHAMBRA Survey: Bayesian photometric redshifts with 23 bands for 3 deg2

The ALHAMBRA (Advance Large Homogeneous Area Medium Band Re dshift Astronomical) survey has observed 8 different regions of the sky, incl uding sections of the COSMOS, DEEP2, ELAIS, GOODS-N, SDSS and Groth fields using a new photometric system with 20 contiguous �300 ˚ A filters covering the optical range, combining them with deep JHKs imaging. The observations, carried out with the Calar Alto 3.5m telescope using the wide field (0.25 deg 2 FOV) optical camera LAICA and the NIR instrument Omega-2000, correspond to �700hrs of on-target science images. The photometric system was specifically designed to maximize the effective depth of the survey in terms of accurate spectral-type and photometric redshift estimation along with the capability of identi fication of relatively faint emission lines. Here we present multicolor photometry and photometric redshifts for �438,000 galaxies, detected in synthetic F814W images, complete down to a magnitude I�24.5AB, carefully taking into account realistic noise estimates, and correct ing by PSF and aperture effects with the ColorPro software. The photometric zeropoints have been calibrated using stellar transformation equations and refined internally, using a new tech nique based on the highly robust photometric redshifts measured for emission line galaxies. We calculate photometric redshifts with the BPZ2.0 code, which includes new empirically calibrated galaxy templates and priors. —

THE MINOR ROLE OF GAS-RICH MAJOR MERGERS IN THE RISE OF INTERMEDIATE-MASS EARLY TYPES AT z ⩽ 1

We study the evolution of galaxy structure since z � 1 to the present. From a Great Observatories Origins Deel Survey South (GOODS-S) multi-band catalog we define (blue) luminosity- and massweighted samples, limited by MB � − 20 and M? � 10 10 M� , comprising 1122 and 987 galaxies, respectively. We extract early-type (ET; E/S0/Sa) and late-type (LT; Sb-Irr) subsamples by their position in the concentration–asymmetry plane, in which galaxies exhibit a clear bimodality. We find that the ET fraction, fET, rises with cosmic time, with a corresponding decrease in the LT fraction, fLT, in both luminosity- and mass-selected samples. However, the evolution of the comoving number density is very different: the decrease in the total number density of MB � − 20 galaxies since z = 1 is due to the decrease in the LT population, which accounts for � 75% of the total star formation rate

ROBUST DETERMINATION OF THE MAJOR MERGER FRACTION AT Z = 0.6 IN THE GROTH STRIP

We measure the fraction of galaxies undergoing disk-disk major mergers (∫^mph_m) at intermediate redshifts ((0.35 ≤ z < 0.85) by studying the asymmetry index A of galaxy images. Results are provided for B- and K_s-band absolute magnitude selected samples from the Groth strip in the galaxy origins and young assembly photometric survey. Three sources of systematic error are carefully addressed and quantified. The effects of the large errors in the photometric redshifts and asymmetry indices are corrected with maximum-likelihood techniques. Biases linked to the redshift degradation of the morphological information in the images are treated by measuring asymmetries on images artificially redshifted to a reference redshift of z_d = 0.75. Morphological K-corrections are further constrained by remaining within redshifts where the images sample redward of 4000 angstrom. We find that: (1) our data allow for a robust merger fraction to be provided for a single redshift bin, which we center at z = 0.6. (2) Merger fractions at that z have lower values than previous determinations: ∫_m^mph = 0.045^-0.011_+0.014 for M_B ≤ -20 galaxies, and f_m^mph = 0.031(-0.009)(+0.013) for M(Ks) <= - 23.5 galaxies. And, (3) failure to address the effects of the large observational errors leads to overestimating f_m^mph by factors of 10%-60%. Combining our results with those on other B-band selected samples, and parameterizing the merger fraction evolution as ∫_m^mph (z) = ∫_m^mph (0)(1 + z)^m, we obtain that m = 2.9 ± 0.8, and ∫_m^mph (0) = 0.012 ± 0.004. For an assumed merger timescale between 0.35 0.6 Gyr, these values imply that only 20%-35% of present-day M_B ≤ -20 galaxies have undergone a disk-disk major merger since z ∽ 1. Assuming a K_s -band mass-to- light ratio not varying with luminosity, we infer that the merger rate of galaxies with stellarmass M(star) greater than or similar to 3.5 x 10^10 M_☉ is R_m = 1.6_-0.6^+0.9 x 10^-4 Mpc^-3 Gyr^-1 at z = 0.6. When we compare with previous studies at similar redshifts, we find that the merger rate decreases when mass increases.

On the buildup of massive early-type galaxies at z<~1. I- Reconciling their hierarchical assembly with mass-downsizing

Context. Several studies have tried to ascertain whether the increase in abundance of the early-type galaxies (E-S0as) with time is mainly due to major mergers, but have reached opposite conclusions. Aims. We have tested it directly through semi-analytical modelling, quantifying the possible contribution of the observed major mergers to the evolution of the high-mass end of the galaxy luminosity function (LF). Methods. The model analyses the backwards-in-time evolution of the massive early-type galaxies with log(M_*/M_⨀) > 11 at z∼0 (mETGs) under the hypothesis that each major merger leads to an early-type galaxy. The model considers only the major mergers strictly reported by observations at each redshift and assumes 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 colour and morphology. It also provides a framework in which apparently contradictory results on the recent evolution of the LF of massive red galaxies can be reconciled, just considering that observed 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 for the observed 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 the observed excess by a factor of ∼4-5 of late-type galaxies at 0.8 < z < 1 naturally, as compared to pure luminosity evolution (PLE) models. Conclusions. The model suggests that major mergers have been the main driver for the observed mass migration from the massive end of the blue galaxy cloud to that of the red sequence in the past ∼8 Gyr.

Evolutionary paths among different red galaxy types at 0.3 < z < 1.5 and the late buildup of massive E-S0s through major mergers

Some recent observations seem to disagree with hierarchical theories of galaxy formation about the role played by major mergers in the late buildup of massive E-S0s. We re-address this question by analysing the morphology, structural distortion level and star formation enhancement of a sample of massive galaxies (M_* > 5x10^10M_⨀) lying on the Red Sequence and its surroundings at 0.3 10^11 M_⨀ at z = 0 through gas-rich major mergers has frozen since z similar to 0.6. All these facts support that major mergers have played a dominant role in the definitive buildup of present-day E-S0s with M-* > 10(11) M-circle dot at 0.6 < z < 1.2, in good agreement with hierarchical scenarios of galaxy formation.

The ALHAMBRA survey : evolution of galaxy clustering since z ~ 1.

PA-M was supported by an ERC StG Grant (DEGAS-259586). PN acknowledges the support of the Royal Society through the award of a University Research Fellowship and the European Research Council, through receipt of a Starting Grant (DEGAS-259586). This work was supported by the Science and Technology Facilities Council (grant number ST/F001166/1), by the Generalitat Valenciana (project of excellence Prometeo 2009/064), by the Junta de Andalucia (Excellence Project P08-TIC-3531) and by the SpanishMinistry for Science and Innovation (grantsAYA2010-22111-C03-01 and CSD2007-00060).

Spectro-photometric close pairs in GOODS-S: major and minor companions of intermediate-mass galaxies

Aims. Recent work has shown that major mergers of disc galaxies can only account for ~20% of the growth of the galaxy red sequence between z = 1 and z = 0. Our goal here is to provide merger frequencies that encompass both major and minor mergers, derived from close pair statistics. We aim to show that reliable close pair statistics can be derived from galaxy catalogues with mixed spectroscopic and photometric redshifts. Methods. We use B-band luminosity- and mass-limited samples from a Spitzer/IRAC-selected catalogue of GOODS-S. We present a new methodology for computing the number of close companions, , when spectroscopic redshift information is partial. The methodology extends the one used in spectroscopic surveys to make use of photometric redshift information. We select as close companions those galaxies separated by 6 h-1 kpc < rp < 21 h-1 kpc in the sky plane and with a difference Δ v ≤ 500 km s-1 in redshift space. Results. We provide for four different B-band-selected samples. It increases with luminosity, in good agreement with previous estimations from spectroscopic surveys. The evolution of with redshift is faster in more luminous samples. We provide of ≥ 1010 galaxies, finding that the number including minor companions (, mass ratio μ ≥ 1/10) is roughly two times the number of major companions alone (, mass ratio μ ≥ 1/3) in the range 0.2 ≤ z < 1.1. We compare the major merger rate derived by close pairs with the one computed by morphological criteria, finding that both approaches provide similar merger rates for field galaxies when the progenitor bias is taken into account. Finally, we estimate that the total (major+minor) merger rate is ~1.7 times the major merger rate. Conclusions. Only 30% to 50% of the ≥ 1010 early-type (E/S0/Sa) galaxies that appear between z = 1 and z = 0 may have undergone a major or a minor merger. Half of the red sequence growth since z = 1 is therefore unrelated to mergers.

The ALHAMBRA survey: accurate merger fractions derived by PDF analysis of photometrically close pairs

Aims. Our goal is to develop and test a novel methodology to compute accurate close pair fractions with photometric redshifts. Methods. We improve the current methodologies to estimate the merger fraction fm from photometric redshifts by (i) using the full probability distribution functions (PDFs) of the sources in redshift space, (ii) including the variation in the luminosity of the sources with z in both the selection of the samples and in the luminosity ratio constrain, and (iii) splitting individual PDFs into red and blue spectral templates to deal robustly with colour selections. We test the performance of our new methodology with the PDFs provided by the ALHAMBRA photometric survey. Results. The merger fractions and rates from the ALHAMBRA survey are in excellent agreement with those from spectroscopic work, both for the general population and for red and blue galaxies. With the merger rate of bright (MB 20 1:1z) galaxies evolving as (1 + z) n , the power-law index n is larger for blue galaxies (n = 2:7 0:5) than for red galaxies (n = 1:3 0:4), confirming previous results. Integrating the merger rate over cosmic time, we find that the average number of mergers per galaxy since z = 1 is N red m = 0:57 0:05 for red galaxies and N blue m = 0:26 0:02 for blue galaxies. Conclusions. Our new methodology exploits statistically all the available information provided by photometric redshift codes and provides accurate measurements of the merger fraction by close pairs only using photometric redshifts. Current and future photometric surveys will benefit of this new methodology.

Galaxy clusters and groups in the ALHAMBRA survey

Ministerio de Economia y Competitividad AYA2010-22111-C03-02, AYA2010-15169, AYA2012-30789, AYA2013-48623-C2-2, AYA2013-42227-P, AYA2013-40611-P, AYA2011-29517-C03-01, AYA2014-58861-C3-1, AYA2010-15081