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68 - Mauro Stefanon 2014
We build a Spitzer IRAC complete catalog of objects, obtained by complementing the $K_mathrm{s}$-band selected UltraVISTA catalog with objects detected in IRAC only. With the aim of identifying massive (i.e., $log(M_*/M_odot)>11$) galaxies at $4<z<7$ , we consider the systematic effects on the measured photometric redshifts from the introduction of an old and dusty SED template and from the introduction of a bayesian prior taking into account the brightness of the objects, as well as the systematic effects from different star formation histories (SFHs) and from nebular emission lines in the recovery of stellar population parameters. We show that our results are most affected by the bayesian luminosity prior, while nebular emission lines and SFHs only introduce a small dispersion in the measurements. Specifically, the number of $4<z<7$ galaxies ranges from 52 to 382 depending on the adopted configuration. Using these results we investigate, for the first time, the evolution of the massive end of the stellar mass functions (SMFs) at $4<z<7$. Given the rarity of very massive galaxies in the early universe, major contributions to the total error budget come from cosmic variance and poisson noise. The SMF obtained without the introduction of the bayesian luminosity prior does not show any evolution from $zsim6.5$ to $zsim 3.5$, implying that massive galaxies could already be present when the Universe was $sim0.9$~Gyr old. However, the introduction of the bayesian luminosity prior reduces the number of $z>4$ galaxies with best fit masses $log(M_*/M_odot)>11$ by 83%, implying a rapid growth of very massive galaxies in the first 1.5 Gyr of cosmic history. From the stellar-mass complete sample, we identify one candidate of a very massive ($log(M_*/M_odot)sim11.5$), quiescent galaxy at $zsim5.4$, with MIPS $24mu$m detection suggesting the presence of a powerful obscured AGN.
We present the first spectroscopic confirmation of an ultra-massive galaxy at redshift z>3 using data from Keck-NIRSPEC, VLT-Xshooter, and GTC-Osiris. We detect strong [OIII] and Ly$alpha$ emission, and weak [OII], CIV, and HeII, placing C1-23152 at a spectroscopic redshift of $z_{spec}$=3.351. The modeling of the emission-line corrected spectral energy distribution results in a best-fit stellar mass of $M_{*}=3.1^{+0.6}_{-0.7}times10^{11} M_{odot}$, a star-formation rate of <7 $M_{odot} yr^{-1}$, and negligible dust extinction. The stars appear to have formed in a short intense burst ~300-500 Myr prior to the observation epoch, setting the formation redshift of this galaxy at z~4.1. From the analysis of the line ratios and widths, and the observed flux at 24$mu$m, we confirm the presence of a luminous hidden active galactic nucleus (AGN), with bolometric luminosity of ~$10^{46}erg s^{-1}$. Potential contamination to the observed SED from the AGN continuum is constrained, placing a lower limit on the stellar mass of $2times10^{11} M_{odot}$. HST/WFC3 $H_{160}$ and ACS $I_{814}$ images are modeled, resulting in an effective radius of $r_{e}$~1 kpc in the $H_{160}$ band and a Sersic index n~4.4. This object may be a prototype of the progenitors of local most massive elliptical galaxies in the first 2 Gyr of cosmic history, having formed most of its stars at z>4 in a highly dissipative, intense, and short burst of star formation. C1-23152 is completing its transition to a post-starburst phase while hosting a powerful AGN, potentially responsible for the quenching of the star formation activity.
[Abridged] Using public data from the NMBS and CANDELS surveys, we study the population of massive galaxies at z>3 to identify the potential progenitors of z~2 compact, massive, quiescent (CMQ) galaxies, furthering our understanding of the evolution of massive galaxies. Our work is enabled by high-resolution CANDELS images and accurate photometric redshifts, stellar masses and star formation rates (SFRs) from 37-band NMBS photometry. The total number of z>3 massive galaxies is consistent with the number of massive quiescent (MQ) galaxies at z~2, implying that the SFRs for all of these galaxies must be much lower by z~2. We discover 4 CMQ galaxies at z>3, pushing back the time for which such galaxies have been observed. However, the volume density for these galaxies is significantly less than that of galaxies at z<2 with similar masses, SFRs, and sizes, implying that additional CMQ galaxies must be created in the ~1 Gyr between z=3 and z=2. We find 5 star-forming galaxies at z~3 that are compact (Re<1.4 kpc) and have stellar mass M*>10^(10.6)Msun, likely to become members of the CMQ galaxy population at z~2. We evolve the stellar masses and SFRs of each individual z>3 galaxy adopting 5 different star formation histories (SFHs) and studying the resulting population of massive galaxies at z=2.3. We find that declining or truncated SFHs are necessary to match the observed number density of MQ galaxies at z~2, whereas a constant SFH results in a number density significantly smaller than observed. All of our assumed SFHs imply number densities of CMQ galaxies at z~2 that are consistent with the observed number density. Better agreement with the observed number density of CMQ galaxies at z~2 is obtained if merging is included in the analysis and better still if star formation quenching is assumed to shortly follow the merging event, as implied by recent models of formation of MQ galaxies.
136 - Danilo Marchesini 2012
We present the rest-frame V-band luminosity function (LF) of galaxies at 0.4<z<4.0, measured from a near-infrared selected sample constructed from the NMBS, the FIRES, the FIREWORKS, and the ultra-deep NICMOS and WFC3 observations in the HDFN, HUDF, and GOODS-CDFS, all having high-quality optical to mid-infrared data. This unique sample combines data from surveys with a large range of depths and areas in a self-consistent way, allowing us to (1) minimize the uncertainties due to cosmic variance; and (2) simultaneously constrain the bright and faint ends with unprecedented accuracy over the targeted redshift range, probing the LF down to 0.1 L* at z~3.9. We find that (1) the faint end is fairly flat and with a constant slope from z=4, with alpha= -1.27 +/- 0.05; (2) the characteristic magnitude has dimmed by 1.3 mag from z~3.7 to z=0.1; (3) the characteristic density has increased by a factor of ~8 from z~3.7 to z=0.1, with 50% of this increase from z~4 to z~1.8; and (4) the luminosity density peaks at z~1-1.5, increasing by a factor of ~4 from z=4.0 to z~1-1.5, and subsequently decreasing by a factor of ~1.5 by z=0.1. We find no evidence for a steepening of the faint-end slope with redshift out to z=4, in contrast with previous observational claims and theoretical predictions. The constant faint-end slope suggests that the efficiency of stellar feedback may evolve with redshift. Alternative interpretations are discussed, such as different masses of the halos hosting faint galaxies at low and high redshifts and/or environmental effects.
We present the rest-frame J- and H-band luminosity function (LF) of field galaxies, based on a deep multi-wavelength composite sample from the MUSYC, FIRES and FIREWORKS survey public catalogues, covering a total area of 450 arcmin^2. The availabilit y of flux measurements in the Spitzer IRAC 3.6, 4.5, 5.8, and 8 um channels allows us to compute absolute magnitudes in the rest-frame J and H bands up to z=3.5 minimizing the dependence on the stellar evolution models. We compute the LF in the four redshift bins 1.5<z<2.0, 2.0<z<2.5, 2.5<z<3.0 and 3.0<z<3.5. Combining our results with those already available at lower redshifts, we find that (1) the faint end slope is consistent with being constant up to z=3.5, with alpha=-1.05+/-0.03 for the rest-frame J band and alpha=-1.15+/-0.02 for the rest-frame H band; (2) the normalization phi* decreases by a factor of 6 between z=0 and z~1.75 and by a factor 3 between z~1.75 and z=3.25; (3) the characteristic magnitude M* shows a brightening from z=0 to z~2 followed by a slow dimming to z=3.25. We finally compute the luminosity density (LD) in both rest-frame J and H bands. The analysis of our results together with those available in the literature shows that the LD is approximately constant up to z~1, and it then decreases by a factor of 6 up to z=3.5.
[Abridged] We present the evolution of the stellar mass function (SMF) of galaxies from z=4.0 to z=1.3 measured from a sample constructed from the deep NIR MUSYC, the FIRES, and the GOODS-CDFS surveys, all having very high-quality optical to mid-infr ared data. This sample, unique in that it combines data from surveys with a large range of depths and areas in a self-consistent way, allowed us to 1) minimize the uncertainty due to cosmic variance and empirically quantify its contribution to the total error budget; 2) simultaneously probe the high-mass end and the low-mass end (down to ~0.05 times the characteristic stellar mass) of the SMF with good statistics; and 3) empirically derive the redshift-dependent completeness limits in stellar mass. We provide, for the first time, a comprehensive analysis of random and systematic uncertainties affecting the derived SMFs. We find that the mass density evolves by a factor of ~17(+7,-10) since z=4.0, mostly driven by a change in the normalization Phi*. If only random errors are taken into account, we find evidence for mass-dependent evolution, with the low-mass end evolving more rapidly than the high-mass end. However, we show that this result is no longer robust when systematic uncertainties due to the SED-modeling assumptions are taken into account. Taking our results at face value, we find that they are in conflict with semi-analytic models of galaxy formation. The models predict SMFs that are in general too steep, with too many low-mass galaxies and too few high-mass galaxies. The discrepancy at the high-mass end is susceptible to uncertainties in the models and the data, but the discrepancy at the low-mass end may be more difficult to explain.
Deep near-infrared imaging surveys allow us to select and study distant galaxies in the rest-frame optical, and have transformed our understanding of the early Universe. As the vast majority of K- or IRAC-selected galaxies is too faint for spectrosco py, the interpretation of these surveys relies almost exclusively on photometric redshifts determined from fitting templates to the broad-band photometry. The best-achieved accuracy of these redshifts Delta(z)/(1+z) ~ 0.06 at z>1.5, which is sufficient for determining the broad characteristics of the galaxy population but not for measuring accurate rest-frame colors, stellar population parameters, or the local galaxy density. We have started a near-infrared imaging survey with the NEWFIRM camera on the Kitt Peak 4m telescope to greatly improve the accuracy of photometric redshifts in the range 1.5<z<3.5. The survey uses five medium-bandwidth filters, which provide crude spectra over the wavelength range 1-1.8 micron for all objects in the 27.6 x 27.6 arcmin NEWFIRM field. In this first paper, we illustrate the technique by showing medium band NEWFIRM photometry of several galaxies at 1.7<z<2.7 from the near-infrared spectroscopic sample of Kriek et al. (2008). The filters unambiguously pinpoint the location of the redshifted Balmer break in these galaxies, enabling very accurate redshift measurements. The full survey will provide similar data for ~8000 faint K-selected galaxies at z>1.5 in the COSMOS and AEGIS fields. The filter set also enables efficient selection of exotic objects such as high redshift quasars, galaxies dominated by emission lines, and very cool brown dwarfs; we show that late T and candidate Y dwarfs could be identified using only two of the filters.
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