ABSTRACT We present a near-infrared spectroscopic study of a stellar mass selected sample of galaxies at z 1 utilizing the Long-slit Intermediate Resolution Infrared Spectrograph multi-object ...spectrograph on the William Herschel Telescope. We detect continuum, and the Hα line for our sample, which is one of the better direct tracers of star formation in external galaxies. We spectroscopically measure the Hα emission from 41 massive (M * > 1010.5M) galaxies taken from the POWIR Survey with spectroscopic redshifts 0.4 < z spec < 1.4. We correct our Hα fluxes for dust extinction by using multiwavelength data, and investigate star formation rate (SFR) trends with mass and colour. We find a drop in the fraction of massive galaxies with M * > 1011M which are detected in Hα emission at z < 0.9. We furthermore find that the fraction of galaxies with Hα emission drops steadily and significantly with redder (U -B ) colours at z 1, and that the specific SFR (SSFR) drops with increasing (U -B ) colour for galaxies at all masses. By investigating the SFR-mass relation, we find that the SFR is roughly constant with mass, in possible contrast to previous work, and that the SSFR is lower in the most massive galaxies. The scatter in the SFR versus mass relationship is very small for those systems with ongoing star formation, which suggests that star formation in the most massive galaxies at z 1 shuts off rather abruptly over <1Gyr, without an obvious gradual decline. We furthermore investigate the SFR as a function of (U -B ) colour divided into different mass bins, revealing a tracer of the epoch of transition from star forming to passive, as a form of star formation 'downsizing'. This suggests that the shut off of star formation occurs before the change in a galaxy's colour. We find that galaxy stellar mass is the primary driving mechanism behind the star formation history for these galaxies and discuss several possible mechanisms for regulating this process. PUBLICATION ABSTRACT
We study the relationship between galaxy colour, stellar mass and local galaxy density in a deep near-infrared imaging survey up to a redshift of z similar to 3 using the GOODS (Great Observatories ...Origins Deep Survey) NICMOS (Near Infrared Camera and Multi-Object Spectrometer) Survey (GNS). The GNS is a deep near-infrared Hubble Space Telescope survey imaging a total of 45 arcmin2 of the GOODS fields, reaching a stellar mass completeness limit of M*= 109.5Modot at z= 3. Using these data, we measure galaxy local densities based on galaxy counts within a fixed aperture, as well as the distance to the third, fifth and seventh nearest neighbour. We compare the average rest-frame (U-B) colour and fraction of blue galaxies in different local densities and at different stellar masses. We find a strong correlation between colour and stellar mass at all redshifts up to z similar to 3. Massive red galaxies are already in place at z similar to 3 at the expected location of the red sequence in the colour-magnitude diagram, although they are star forming. We do not find a strong correlation between colour and local density; however, there may be evidence that the highest overdensities are populated by a higher fraction of blue galaxies than average or underdense areas. This could be indicating that the colour-density relation at high redshift is reversed with respect to lower redshifts (z < 1), where higher densities are found to have lower blue fractions. Our data suggest that the possible higher blue fraction at extreme overdensities might be due to a lack of massive red galaxies at the highest local densities.
We present the results of an extended narrow-band H{\alpha} study of the massive galaxy cluster XMMU J2235.3-2557 at z = 1.39. This paper represents a follow up study to our previous investigation of ...star-formation in the cluster centre, extending our analysis out to a projected cluster radius of 1.5 Mpc. Using the Near InfraRed Imager and Spectrograph (NIRI) on Gemini North we obtained deep H narrow-band imaging corresponding to the rest-frame wavelength of H{\alpha} at the cluster's redshift. We identify a total of 163 potential cluster members in both pointings, excluding stars based on their near-IR colours derived from VLT/HAWK-I imaging. Of these 163 objects 14 are spectroscopically confirmed cluster members, and 20% are excess line-emitters. We find no evidence of star formation activity within a radius of 200 kpc of the brightest cluster galaxy in the cluster core. Dust-corrected star formation rates (SFR) of excess emitters outside this cluster quenching radius, RQ \sim 200 kpc, are on average = 2.7 \pm 1.0 M\odot yr-1, but do not show evidence of increasing star-formation rates toward the extreme 1.5 Mpc radius of the cluster. No individual cluster galaxy exceeds an SFR of 6 M\odot yr-1 . Massive galaxies (log M\ast /M\odot > 10.75) all have low specific SFRs (SSFRs, i.e. SFR per unit stellar mass). At fixed stellar mass, galaxies in the cluster centre have lower SSFRs than the rest of the cluster galaxies, which in turn have lower SSFRs than field galaxies at the same redshift by a factor of a few to 10. For the first time we can demonstrate through measurements of individual SFRs that already at very early epochs (at an age of the Universe of \sim4.5 Gyr) the suppression of star-formation is an effect of the cluster environment which persists at fixed galaxy stellar mass. Erratum added after the original paper
Maximizing the public impact of astronomy projects in the next decade requires NSF-funded centers to support the development of online, mobile-friendly outreach and education activities. EPO teams ...with astronomy, education, and web development expertise should be in place to build accessible programs at scale and support astronomers doing outreach.
In Galaxy And Mass Assembly Data Release 4 (GAMA DR4), we make available our full spectroscopic redshift sample. This includes 248682 galaxy spectra, and, in combination with earlier surveys, results ...in 330542 redshifts across five sky regions covering ~250deg^2. The redshift density, is the highest available over such a sustained area, has exceptionally high completeness (95 per cent to r_KIDS=19.65mag), and is well suited for the study of galaxy mergers, galaxy groups, and the low redshift (z<0.25) galaxy population. DR4 includes 32 value-added tables or Data Management Units (DMUs) that provide a number of measured and derived data products including GALEX, ESO KiDS, ESO VIKING, WISE and Herschel Space Observatory imaging. Within this release, we provide visual morphologies for 15330 galaxies to z<0.08, photometric redshift estimates for all 18million objects to r_KIDS~25mag, and stellar velocity dispersions for 111830 galaxies. We conclude by deriving the total galaxy stellar mass function (GSMF) and its sub-division by morphological class (elliptical, compact-bulge and disc, diffuse-bulge and disc, and disc only). This extends our previous measurement of the total GSMF down to 10^6.75 M_sol h^-2_70 and we find a total stellar mass density of rho_*=(2.97+/-0.04)x10^8 M_sol h_70 Mpc^-3 or Omega_*=(2.17+/-0.03)x10^-3 h^-1_70. We conclude that at z<0.1, the Universe has converted 4.9+/-0.1 per cent of the baryonic mass implied by Big Bang Nucleosynthesis into stars that are gravitationally bound within the galaxy population.
PDF
