Aims. An unbiased and detailed characterization of the galaxy luminosity function (LF) is a basic requirement in many astrophysical issues: it is of particular interest in assessing the role of the ...environment in the evolution of the LF of different galaxy types. Methods. We studied the evolution in the B band LF to redshift $z\sim 1$ in the zCOSMOS 10k sample, for which both accurate galaxy classifications (spectrophotometric and morphological) and a detailed description of the local density field are available. Results. The global B band LF exhibits a brightening of ~0.7 mag in M* from $z\sim 0.2 $ to $z\sim 0.9$. At low redshifts ($z<0.35$), spectrophotometric late types dominate at faint magnitudes ($M_{B_{AB}} > -20$), while the bright end is populated mainly by spectrophotometric early types. At higher redshift, spectrophotometric late-type galaxies evolve significantly and, at redshift $z\sim 1$,the contribution from the various types to the bright end of the LF is comparable. The evolution for spectrophotometric early-type galaxies is in both luminosity and normalization: M* brightens by ~0.6 mag but $\phi^*$ decreases by a factor ~1.7 between the first and the last redshift bin. A similar behaviour is exhibited by spectrophotometric late-type galaxies, but with an opposite trend for the normalization: a brightening of ~0.5 mag is present in M*, while $\phi^*$ increases by a factor ~1.8. Studying the role of the environment, we find that the global LF of galaxies in overdense regions has always a brighter M* and a flatter slope. In low density environments, the main contribution to the LF is from blue galaxies, while for high density environments there is an important contribution from red galaxies to the bright end. The differences between the global LF in the two environments are not due to only a difference in the relative numbers of red and blue galaxies, but also to their relative luminosity distributions: the value of M* for both types in underdense regions is always fainter than in overdense environments. These results indicate that galaxies of the same type in different environments have different properties. We also detect a differential evolution in blue galaxies in different environments: the evolution in their LF is similar in underdense and overdense regions between $z\sim 0.25$ and $z\sim 0.55$, and is mainly in luminosity. In contrast, between $z\sim 0.55$ and $z\sim 0.85$ there is little luminosity evolution but there is significant evolution in $\phi^*$, that is, however, different between the two environments: in overdense regions $\phi^*$ increases by a factor ~1.6, while in underdense regions this increase reaches a factor ~2.8. Analyzing the blue galaxy population in more detail, we find that this evolution is driven mainly by the bluest types. Conclusions. The “specular” evolution of late- and early-type galaxies is consistent with a scenario where a part of blue galaxies is transformed in red galaxies with increasing cosmic time, without significant changes in the fraction of intermediate-type galaxies. The bulk of this tranformation in overdense regions probably happened before $z\sim 1$, while it is still ongoing at lower redshifts in underdense environments.
We present strong empirical evidence for a physical connection between the occurrence of a starburst (SB) and a luminous active galactic nucleus (AGN) phase. Drawing infrared (IR), X-ray and ...optically selected samples from COSMOS, we find that the locus of type II AGN hosts in the optical colour-magnitude (U − V/V) and colour-colour (U − V/V − J) space significantly overlaps with that of IR-luminous (L
IR > 1010 L) galaxies. Based on our observations, we propose that, when simultaneously building their black hole and stellar masses, type II AGN hosts are located in the same part of colour-colour space as dusty star-forming galaxies. In fact, our results show that IR-luminous galaxies at z < 1.5 are on average three times more likely to host a type II AGN (L
X > 1042 erg s−1) than would be expected serendipitously, if AGN and star-formation events were unrelated. In addition, the optical and IR properties of the AGN/SB hybrid systems tentatively suggest that the AGN phase might be coeval with a particularly active phase in a galaxy's star-formation history. Interestingly, we also find a significant fraction of type II AGN hosts offset from the dusty galaxy sequence in colour-colour space, possibly representing a transitional or post-SB phase in galaxy evolution. Our findings are consistent with a scenario whereby AGN play a role in the termination of star formation in massive galaxies.
Dust-obscured galaxies (DOGs) are an ultraviolet-faint, infrared-bright galaxy population that reside at z ~ 2 and are believed to be in a phase of dusty star-forming and active galactic nucleus ...(AGN) activity. We present far-infrared (far-IR) observations of a complete sample of DOGs in the 2 deg super(2) of the Cosmic Evolution Survey. The 3077 DOGs have left angle bracketzright angle bracket= 1.9 + or - 0.3 and are selected from 24 mu m and r super(+) observations using a color cut of r super(+) - 24 > or =, slanted 7.5 (AB mag) and S sub(24) > or =, slanted 100 mu Jy. Based on the near-IR spectral energy distributions, 47% are bump DOGs (star formation dominated) and 10% are power-law DOGs (AGN-dominated). We use SPIRE far-IR photometry from the Herschel Multi-tiered Extragalactic Survey to calculate the IR luminosity and characteristic dust temperature for the 1572 (51%) DOGs that are detected at 250 mu m (> or =, slanted3sigma). For the remaining 1505 (49%) that are undetected, we perform a median stacking analysis to probe fainter luminosities. Herschel-detected and undetected DOGs have average luminosities of (2.8 + or - 0.4) x 10 super(12) L sub(middot in circle) and (0.77 + or - 0.08) x 10 super(12) L sub(middot in circle), and dust temperatures of (33 + or - 7) K and (37 + or - 5) K, respectively. The IR luminosity function for DOGs with S sub(24) > or =, slanted 100 mu Jy is calculated, using far-IR observations and stacking. DOGs contribute 10%-30% to the total star formation rate (SFR) density of the universe at z = 1.5-2.5, dominated by 250 mu m detected and bump DOGs. For comparison, DOGs contribute 30% to the SFR density for all z = 1.5-2.5 galaxies with S sub(24) > or =, slanted 100 mu Jy. DOGs have a large scatter about the star formation main sequence and their specific SFRs show that the observed phase of star formation could be responsible for their total observed stellar mass at z ~ 2.
We present three bright z +-dropout candidates selected from deep near-infrared (NIR) imaging of the COSMOS 2 deg2 field. All three objects match the 0.8-8 Delta *mm colors of other published z > 7 ...candidates but are 3 mag brighter, facilitating further study. Deep spectroscopy of two of the candidates covering 0.64-1.02 Delta *mm with Keck-DEIMOS and all three covering 0.94-1.10 Delta *mm and 1.52-1.80 Delta *mm with Keck-NIRSPEC detects weak spectral features tentatively identified as Ly Delta *a at z = 6.95 and z = 7.69 in two of the objects. The third object is placed at z ~ 1.6 based on a 24 Delta *mm and weak optical detection. A comparison with the spectral energy distributions of known z < 7 galaxies, including objects with strong spectral lines, large extinction, and large systematic uncertainties in the photometry, yields no objects with similar colors. However, the Delta *l > 1 Delta *mm properties of all three objects can be matched to optically detected sources with photometric redshifts at z ~ 1.8, so the non-detection in the i + and z + bands is the primary factor which favors a z > 7 solution. If any of these objects are at z ~ 7, the bright end of the luminosity function is significantly higher at z > 7 than suggested by previous studies, but consistent within the statistical uncertainty and the dark matter halo distribution. If these objects are at low redshift, the Lyman break selection must be contaminated by a previously unknown population of low-redshift objects with very strong breaks in their broadband spectral energy distributions and blue NIR colors. The implications of this result on luminosity function evolution at high redshift are discussed. We show that the primary limitation of z > 7 galaxy searches with broad filters is the depth of the available optical data.
We investigate the effects of the environment on star formation in a sample of massive luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) with S(24 {mu}m) >80 {mu}Jy and i {sup +} < 24 ...in the COSMOS field. We exploit the accurate photometric redshifts in COSMOS to characterize the galaxy environment and study the evolution of the fraction of LIRGs and ULIRGs in different environments in the redshift range z = 0.3-1.2 and in bins of stellar mass. We find that the environment plays a role in the star formation processes and evolution of LIRGs and ULIRGs. We find an overall increase of the ULIRG+LIRG fraction in an optically selected sample with increasing redshift, as expected from the evolution of the star formation rate (SFR) density. We find that the ULIRG+LIRG fraction decreases with increasing density up to z {approx} 1, and that the dependence on density flattens with increasing redshift. We do not observe the reversal of the SFR density relation up to z = 1 in massive LIRGs and ULIRGs, suggesting that such reversal might occur at higher redshift in this infrared luminosity range.
Context. A substantial amount of the stellar mass growth across cosmic time occurred within dust-enshrouded environments. So far, identification of complete samples of distant star-forming galaxies ...from the short wavelength range has been strongly biased by the effect of dust extinction. Nevertheless, the exact amount of star-forming activity that took place in high-redshift dusty galaxies but that has currently been missed by optical surveys has barely been explored. Aims. Our goal is to determine the number of luminous star-forming galaxies at 1.5 ≲ z ≲ 3 that are potentially missed by the traditional color selection techniques because of dust extinction. We also aim at quantifying the contribution of these sources to the IR luminosity and cosmic star formation density at high redshift. Methods. We based our work on a sample of 24 μm sources brighter than 80 μJy and taken from the Spitzer survey of the COSMOS field. Almost all of these sources have accurate photometric redshifts. We applied to this mid-IR selected sample the BzK and BM/BX criteria, as well as the selections of the IRAC peakers and the Optically-Faint IR-bright (OFIR) galaxies. We analyzed the fraction of sources identified with these techniques. We also computed 8 μm rest-frame luminosity from the 24 μm fluxes of our sources, and considering the relationships between L8 μm and LPaα and between L8 μm and LIR, we derived ρIR and then ρSFR for our MIPS sources. Results. The BzK criterion offers an almost complete (~90%) identification of the 24 μm sources at 1.4 < z < 2.5. In contrast, the BM/BX criterion misses 50% of the MIPS sources. We attribute this bias to the effect of extinction, which reddens the typical colors of galaxies. The contribution of these two selections to the IR luminosity density produced by all the sources brighter than 80 μJy are on the same order. Moreover the criterion based on the presence of a stellar bump in their spectra (IRAC peakers) misses up to 40% of the IR luminosity density, while only 25% of the IR luminosity density at z ~ 2 is produced by OFIR galaxies characterized by extreme mid-IR to optical flux ratios. Conclusions. Color selections of distant star-forming galaxies must be used with care given the substantial bias they can suffer. In particular, the effect of dust extinction strongly affects the completeness of identifications at the bright end of the bolometric luminosity function, which implies large and uncertain extrapolations to account for the contribution of dusty galaxies missed by these selections. In the context of forthcoming facilities that will operate at long wavelengths (e.g., JWST, ALMA, SAFARI, EVLA, SKA), this emphasizes the importance of minimizing the extinction biases when probing the activity of star formation in the early Universe.
Based on broadband/narrowband photometry and Keck DEIMOS spectroscopy, we report a redshift of z = 4.64+0.06 --0.08 for AzTEC/COSMOS 1, the brightest submillimeter galaxy (SMG) in the AzTEC/COSMOS ...field. In addition to the COSMOS-survey X-ray to radio data, we report observations of the source with Herschel/PACS (100, 160 Delta *mm), CSO/SHARC II (350 Delta *mm), and CARMA and PdBI (3 mm). We do not detect CO(5 -> 4) line emission in the covered redshift ranges, 4.56-4.76 (PdBI/CARMA) and 4.94-5.02 (CARMA). If the line is within this bandwidth, this sets 3 Delta *s upper limits on the gas mass to 8 X 109 M and 5 X 1010 M , respectively (assuming similar conditions as observed in z ~ 2 SMGs). This could be explained by a low CO-excitation in the source. Our analysis of the UV-IR spectral energy distribution of AzTEC 1 shows that it is an extremely young (50 Myr), massive (M * ~ 1011 M ), but compact (2 kpc) galaxy, forming stars at a rate of ~1300 M yr--1. Our results imply that AzTEC 1 is forming stars in a 'gravitationally bound' regime in which gravity prohibits the formation of a superwind, leading to matter accumulation within the galaxy and further generations of star formation.
ABSTRACT We present the COSMOS201524 catalog, which contains precise photometric redshifts and stellar masses for more than half a million objects over the 2deg2 COSMOS field. Including new images ...from the UltraVISTA-DR2 survey, Y-band images from Subaru/Hyper-Suprime-Cam, and infrared data from the Spitzer Large Area Survey with the Hyper-Suprime-Cam Spitzer legacy program, this near-infrared-selected catalog is highly optimized for the study of galaxy evolution and environments in the early universe. To maximize catalog completeness for bluer objects and at higher redshifts, objects have been detected on a χ2 sum of the and z++ images. The catalog contains objects in the 1.5 deg2 UltraVISTA-DR2 region and objects are detected in the "ultra-deep stripes" (0.62 deg2) at (3 , 3″, AB magnitude). Through a comparison with the zCOSMOS-bright spectroscopic redshifts, we measure a photometric redshift precision of = 0.007 and a catastrophic failure fraction of %. At , using the unique database of spectroscopic redshifts in COSMOS, we find = 0.021 and . The deepest regions reach a 90% completeness limit of to z = 4. Detailed comparisons of the color distributions, number counts, and clustering show excellent agreement with the literature in the same mass ranges. COSMOS2015 represents a unique, publicly available, valuable resource with which to investigate the evolution of galaxies within their environment back to the earliest stages of the history of the universe. The COSMOS2015 catalog is distributed via anonymous ftp and through the usual astronomical archive systems (CDS, ESO Phase 3, IRSA).
We have identified a complex galaxy cluster system in the COSMOS field via a wide-angle tail (WAT) radio galaxy, consistent with the idea that WAT galaxies can be used as tracers of clusters. The WAT ...galaxy, CWAT-01, is coincident with an elliptical galaxy resolved in the HST ACS image. Using the COSMOS multiwavelength data set, we derive the radio properties of CWAT-01 and use the optical and X-ray data to Investigate its host environment. The cluster hosting CWAT-01 is part of a larger assembly consisting of a minimum of four X-ray luminous clusters within similar to 2 Mpc distance. We apply hydrodynamic models that combine ram pressure and buoyancy forces on CWAT-01. These models explain the shape of the radio jets only if the galaxy's velocity relative to the intracluster medium (ICM) is in the range of about 300-550 km s super(-1), which is higher than expected for brightest cluster galaxies (BCGs) in relaxed systems. This indicates that the CWAT-01 host cluster is not relaxed but is possibly dynamically young. We argue that such a velocity could have been induced through subcluster merger within the CWAT-01 parent cluster and/or cluster-cluster interactions. Our results strongly indicate that we are witnessing the formation of a large cluster from an assembly of multiple clusters, consistent with the hierarchical scenario of structure formation. We estimate the total mass of the final cluster to be approximately 20% of the mass of the Coma Cluster.
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