The Ha Luminosity Function and Star Formation Rate at z\sim 0.2

We have measured the Ha+[N II] fluxes of the I-selected Canada-France Redshift Survey (CFRS) galaxies lying at a redshift z below 0.3, and hence derived the Ha luminosity function. The magnitude limits of the CFRS mean that only the galaxies with M(B) > -21 mag were observed at these redshifts. We obtained a total Ha luminosity density of at least 10^{39.44\pm 0.04} erg/s/Mpc^{3} at a mean z=0.2 for galaxies with rest-fame EW(Ha+[N II]) > 10 Angs. This is twice the value found in the local universe by Gallego et al. 1995. Our Ha star formation rate, derived from Madau (1997) is higher than the UV observations at same z, implying a UV dust extinction of about 1 mag. We found a strong correlation between the Ha luminosity and the absolute magnitude in the B-band: M(B(AB)) = 46.7 - 1.6 log L(Ha). This work will serve as a basis of future studies of Ha luminosity distributions measured from optically-selected spectroscopic surveys of the distant universe, and it will provide a better understanding of the physical processes responsible for the observed galaxy evolution.Comment: Accepted for publication in ApJ, 14 pages, LaTeX (macro aas2pp4.sty), 6 figure

Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys - IV. Influence of mergers in the evolution of faint field galaxies from z~1

Hubble Space Telescope images of a sample of 285 galaxies with measured redshifts from the Canada-France Redshift Survey (CFRS) and Autofib-Low Dispersion Spectrograph Survey (LDSS) redshift surveys are analysed to derive the evolution of the merger fraction out to redshifts z~1. We have performed visual and machine-based merger identifications, as well as counts of bright pairs of galaxies with magnitude differences δm<=1.5mag. We find that the pair fraction increases with redshift, with up to ~20per cent of the galaxies being in physical pairs at z~0.75-1. We derive a merger fraction varying with redshift as ~(1+z)3.2+/-0.6, after correction for line-of-sight contamination, in excellent agreement with the merger fraction derived from the visual classification of mergers for which m=3.4+/-0.6. After correcting for seeing effects on the ground-based selection of survey galaxies, we conclude that the pair fraction evolves as ~(1+z)2.7+/-0.6. This implies that an average L* galaxy will have undergone 0.8-1.8 merger events from z=1 to z=0, with 0.5 to 1.2 merger events occuring in a 2-Gyr time-span at around z~0.9. This result is consistent with predictions from semi-analytical models of galaxy formation. From the simple coaddition of the observed luminosities of the galaxies in pairs, physical mergers are computed to lead to a brightening of 0.5mag for each pair on average, and a boost in star formation rate of a factor of 2, as derived from the average [Oii] equivalent widths. Mergers of galaxies are therefore contributing significantly to the evolution of both the luminosity function and luminosity density of the Universe out to z~1

Recent stellar mass assembly of low-mass star-forming galaxies at redshifts 0.3 < z < 0.9

The epoch when low-mass star forming galaxies (LMSFGs) form the bulk of their stellar mass is uncertain. While some models predict an early formation, others favor a delayed scenario until later ages of the universe. We present constraints on the star formation histories (SFHs) of a sample of LMSFGs obtained through the analysis of their spectral energy distributions using a novel approach that (1) consistently combines photometric (broadband) and spectroscopic (equivalent widths of emission lines) data, and (2) uses physically motivated SFHs with non-uniform variations of the star formation rate (SFR) as a function of time. The sample includes 31 spectroscopically confirmed LMSFGs (7.3 < log M*/Msun < 8.0) at 0.3 < z_spec < 0.9 in the Extended-Chandra Deep Field-South field (E-CDF-S). Among them, 24 were selected with photometric stellar mass log M*/Msun < 8.0, 0.3 < z_phot < 1.0, and NB816 < 26 AB mag; the remaining 7 were selected as blue compact dwarfs (BCDs) within the same photometric redshift and magnitude ranges. We also study a secondary sample of 43 more massive spectroscopically confirmed galaxies (8.0 < log M*/Msun < 9.1), selected with the same criteria. The SFRs and stellar masses derived for both samples place our targets on the standard main sequence of star forming galaxies. The median SFH of LMSFGs at intermediate redshifts appears to form 90% of the median stellar mass inferred for the sample in the 0.5-1.8 Gyr immediately preceding the observation. These results suggest a recent stellar mass assembly for LMSFGs, consistent with the cosmological downsizing trends. We find similar median SFH timescales for the more massive secondary sample.Comment: 7 pages, 3 figures, 2 tables; ApJ, in pres

Hubble Space Telescope Imaging of the CFRS and LDSS Redshift Surveys. II. Structural Parameters and the Evolution of Disk Galaxies to Z approximately 1

Several aspects of the evolution of star-forming galaxies are studied using measures of the two-dimensional surface brightness profiles extracted from Hubble Space Telescope images of a sample of 341 faint objects selected from the CFRS and LDSS redshift surveys. The galaxies have 0 3.2 h^{-1}_{50} kpc, where the sample is most complete and where the disk and bulge decompositions are most reliable. This result, which is strengthened by inclusion of the local de Jong et al. size function, suggests that the scale lengths of typical disks cannot have grown substantially with cosmic epoch since z ~ 1, unless a corresponding number of large disks have been destroyed through merging. In addition to a roughly constant number density, the galaxies with large disks, alpha -1 >= 4 h^{-1}_{50} kpc, have, as a set, properties consistent with the idea that they are similar galaxies observed at different cosmic epochs. However, on average, they show higher B-band disk surface brightnesses, bluer overall (U-V) colors, higher [O II] lambda 3727 equivalent widths, and less regular morphologies at high redshift than at low redshift, suggesting an increase in the star formation rate by a factor of about 3 to z ~ 0.7. This is consistent with the expectations of recent models for the evolution of the disk of the Milky Way Galaxy. The evolution of the large disk galaxies with scale lengths alpha -1 >= 4 h^{-1}_{50} kpc, is probably not sufficient to account for the evolution of the overall luminosity function of galaxies over the interval 0 < z < 1, especially if Omega ~ 1. Analysis of the half-light radii of all the galaxies in the sample and construction of the bivariate size-luminosity function suggests that larger changes in the galaxy population are due to smaller galaxies, those with half-light radii around 5 h^{-1}_{50} kpc (i.e., disk scale lengths of 3 h^{-1}_{50} kpc or less)

Discovery of Massive, Mostly Star-formation Quenched Galaxies with Extremely Large Lyman-alpha Equivalent Widths at z ~ 3

We report a discovery of 6 massive galaxies with both extremely large Lya equivalent width and evolved stellar population at z ~ 3. These MAssive Extremely STrong Lya emitting Objects (MAESTLOs) have been discovered in our large-volume systematic survey for strong Lya emitters (LAEs) with twelve optical intermediate-band data taken with Subaru/Suprime-Cam in the COSMOS field. Based on the SED fitting analysis for these LAEs, it is found that these MAESTLOs have (1) large rest-frame equivalent width of EW_0(Lya) ~ 100--300 A, (2) M_star ~ 10^10.5--10^11.1 M_sun, and (3) relatively low specific star formation rates of SFR/M_star ~ 0.03--1 Gyr^-1. Three of the 6 MAESTLOs have extended Ly$\alpha$ emission with a radius of several kpc although they show very compact morphology in the HST/ACS images, which correspond to the rest-frame UV continuum. Since the MAESTLOs do not show any evidence for AGNs, the observed extended Lya emission is likely to be caused by star formation process including the superwind activity. We suggest that this new class of LAEs, MAESTLOs, provides a missing link from star-forming to passively evolving galaxies at the peak era of the cosmic star-formation history.Comment: Accepted for publication in ApJ Letters on 15th July, 2015. 6 pages including 3 figures and 2 table

SHARDS: A global view of the star formation activity at z~0.84 and z~1.23

In this paper, we present a comprehensive analysis of star-forming galaxies (SFGs) at intermediate redshifts (z~1). We combine the ultra-deep optical spectro-photometric data from the Survey for High-z Absorption Red and Dead Sources (SHARDS) with deep UV-to-FIR observations in the GOODS-N field. Exploiting two of the 25 SHARDS medium-band filters, F687W17 and F823W17, we select [OII] emission line galaxies at z~0.84 and z~1.23 and characterize their physical properties. Their rest-frame equivalent widths (EW$_{\mathrm{rf}}$([OII])), line fluxes, luminosities, star formation rates (SFRs) and dust attenuation properties are investigated. The evolution of the EW$_{\mathrm{rf}}$([OII]) closely follows the SFR density evolution of the universe, with a trend of EW$_{\mathrm{rf}}$([OII])$\propto$(1+z)$^3$ up to redshift z~1, followed by a possible flattening. The SF properties of the galaxies selected on the basis of their [OII] emission are compared with complementary samples of SFGs selected by their MIR and FIR emission, and also with a general mass-selected sample of galaxies at the same redshifts. We demonstrate observationally that the UVJ diagram (or, similarly, a cut in the specific SFR) is only partially able to distinguish the quiescent galaxies from the SFGs. The SFR-M$_*$ relation is investigated for the different samples, yelding a logarithmic slope ~1, in good agreement with previous results. The dust attenuations derived from different SFR indicators (UV(1600), UV(2800), [OII], IR) are compared and show clear trends with respect to both the stellar mass and total SFR, with more massive and highly star-forming galaxies being affected by stronger dust attenuation.Comment: Replaced to match the accepted version (24 pages, 1 table, 17 figures). Published in ApJ, 812, 155 (2015): http://stacks.iop.org/0004-637X/812/15

Post-starburst galaxies: more than just an interesting curiosity

From the VIMOS VLT DEEP Survey (VVDS) we select a sample of 16 galaxies with spectra which identify them as having recently undergone a strong starburst and subsequent fast quenching of star formation. These post-starburst galaxies lie in the redshift range 0.510^9.75Msun. They have a number density of 1x10^-4 per Mpc^3, almost two orders of magnitude sparser than the full galaxy population with the same mass limit. We compare with simulations to show that the galaxies are consistent with being the descendants of gas rich major mergers. Starburst mass fractions must be larger than ~5-10% and decay times shorter than ~10^8 years for post-starburst spectral signatures to be observed in the simulations. We find that the presence of black hole feedback does not greatly affect the evolution of the simulated merger remnants through the post-starburst phase. The multiwavelength spectral energy distributions of the post-starburst galaxies show that 5/16 have completely ceased the formation of new stars. These 5 galaxies correspond to a mass flux entering the red-sequence of rhodot(A->Q, PSB) = 0.0038Msun/Mpc^3/yr, assuming the defining spectroscopic features are detectable for 0.35Gyr. If the galaxies subsequently remain on the red sequence, this accounts for 38(+4/-11)% of the growth rate of the red sequence. Finally, we compare our high redshift results with a sample of galaxies with 0.05<z<0.1 observed in the SDSS and UKIDSS surveys. We find a very strong redshift evolution: the mass density of strong post-starburst galaxies is 230 times lower at z~0.07 than at z~0.7.Comment: 18 pages, 12 figures, to match version accepted to MNRAS. Minor reordering of text in places and Sec 2.2 on SPH simulation comparisons expande