Cesar E. Garcia-Dabo

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 [O [CSC]ii[/CSC]] λ3727 Luminosity Function of the Local Universe

The measurement of the Star Formation Rate density of the Universe is of prime importance in understanding the formation and evolution of galaxies. The [OII]3727 emission line flux, easy to measure up to z~1.4 within deep redshift surveys in the optical and up to z~5.4 in the near infrared, offers a reliable means of characterizing the star formation properties of high-z objects. In order to provide the high-z studies with a local reference, we have measured total [OII]3727 fluxes for the well analyzed local sample of star-forming galaxies from the Universidad Complutense de Madrid Survey. This data is used to derive the [OII]3727 luminosity function for local star-forming galaxies. When compared with similar luminosity densities published for redshift up to z~1, the overall evolution already observed in the star formation activity of the Universe is confirmed.The measurement of the star formation rate density of the universe is of prime importance in understanding the formation and evolution of galaxies. The [O II] ƛ3727 emission-line flux, easy to measure up to z ≈ 1.4 within deep redshift surveys in the optical and up to z ≈ 5/4 in the near-infrared, offers a reliable means of characterizing the star formation properties of high-z objects. In order to provide the high-z studies with a local reference, we have measured total [O II] ƛ3727 fluxes for the well-analyzed local sample of star-forming galaxies from the Universidad Complutense de Madrid Survey. These data are used to derive the [O II] ƛ3727 luminosity function for local star-forming galaxies. When compared with similar luminosity densities published for redshift up to z ≈ 1, the overall evolution already observed in the star formation activity of the universe is confirmed.

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.

Goya Survey: U and B Number Counts in the Groth-Westphal Strip

We present U and B galaxy differential number counts from a field of ~900 arcmin2, based on GOYA Survey imaging of the HST Groth-Westphal strip. Source detection efficiency corrections as a function of the object size have been applied. A variation of the half-exposure image method has been devised to identify and remove spurious detections. Achieved 50% detection efficiencies are 24.8 mag in U and 25.5 mag in B in the Vega system. Number count slopes are d log N/dm = 0.50 ± 0.02 for B = 21.0-24.5 and d log N/dm = 0.48 ± 0.03 for U = 21.0-24.0. Simple number count models are presented that simultaneously reproduce the counts over 15 mag in U and B and over 10 mag in Ks, using a Λ-dominated cosmology and SDSS local luminosity functions. Only by setting a recent zf ~ 1.5 formation redshift for early-type, red galaxies do the models reproduce the change of slope observed at Ks = 17.5 in NIR counts. A moderate optical depth (τB = 0.6) for all galaxy types ensures that the recent formation for elliptical galaxies does not leave a signature in the U or B number counts, which are featureless at intermediate magnitudes. No ad hoc disappearing populations are needed to explain the counts if number evolution is introduced using an observationally based z evolution of the merger fraction.

The Merger Fraction Evolution up to z ~ 1

We present results on the disk–disk major merger fraction evolution up to z∼1 in SPITZER/IRAC selected samples in the GOODS–S field. We pick as merger remnants sources with high asymmetry (A). We take into account the experimental errors in photometric redshift and index A, that tend to overestimate the merger fraction, by maximum likelihood techniques, and avoid the loss of information with redshift (degradation of spatial resolution and cosmological dimming) by artificially redshifting all sources to a representative redshift, z d=1. We define absolute B-band and mass selected samples, for which we obtain a very different merger fraction evolution: f m mph(z,M B ≤−20)=0.013(1+z)1.8, while f m mph(z,M ⋆>1010M⊙)=0.001(1+z)5.4. These results implies that only∼20% (8%) of today’s M B ≤−20 (M ⋆>1010M⊙) galaxies have undergone a disk–disk major merger since z=1. Combined with high redshift data in the literature, we expect 1.2−0.3 +0.4 disk–disk major mergers since z∼3 for M ⋆>1010M⊙ galaxies, with almost all the merger activity before z = 1.