Mid-IR Enhanced Galaxies in the Coma & Virgo Cluster: lenticulars with a high star formation rate

We explore the properties of early-type galaxies (ETGs), including ellipticals (E) and lenticulars (S0), in rich environments such as clusters of galaxies (Virgo and Coma). The L_24/L_K distribution of ETGs in both Virgo and Coma clusters shows that some S0s have a much larger L_24/L_K ratio (0.5 to ~2 dex) than the bulk of the ETG population. This could be interpreted as an enhanced star formation rate in these lenticulars. We compare the optical colors of galaxies in these two clusters and investigate the nature of these sources with a large L24/L_K ratio by looking at their spatial distribution within the cluster, by analyzing their optical spectra and by looking at their optical colors compared to late-types. We obtain 10 Coma and 3 Virgo early-type sources with larger L24/L_K ratios than the bulk of their population. We call these sources Mid-Infrared Enhanced Galaxies (MIEGs). In Coma, they are mostly located in the South-West part of the cluster where a substructure is falling onto the main cluster. MIEGs present lower g-r color than the rest of the ETG sample, because of a blue continuum. We interpret the excess L24/L_K ratio as evidence for an enhanced star-formation induced as a consequence of their infall into the main cluster.Comment: Accepted for publication in Ap

Deep Spitzer 24 μm COSMOS Imaging. I. The Evolution of Luminous Dusty Galaxies—Confronting the Models

We present the first results obtained from the identification of ~30,000 sources in the Spitzer/24 μm observations of the COSMOS field at S_(24 μm) ≳ 80 μJy. Using accurate photometric redshifts (σ_ z ~ 0.12 at z ~ 2 for 24 μm sources with i ^+ ≳ 25 mag AB) and simple extrapolations of the number counts at faint fluxes, we resolve with unprecedented detail the buildup of the mid-infrared background across cosmic ages. We find that ~50% and ~80% of the 24 μm background intensity originate from galaxies at z ≳ 1 and z ≳ 2, respectively, supporting the scenario where highly obscured sources at very high redshifts (z ≳ 2) contribute only marginally to the cosmic infrared background. Assuming flux-limited selections at optical wavelengths, we also find that the fraction of i ^+-band sources with 24 μm detection strongly increases up to z ~ 2 as a consequence of the rapid evolution that star-forming galaxies have undergone with look-back time. Nonetheless, this rising trend shows a clear break at z ~ 1.3, probably due to k-correction effects implied by the complexity of spectral energy distributions in the mid-infrared. Finally, we compare our results with the predictions from different models of galaxy formation. We note that semianalytical formalisms currently fail to reproduce the redshift distributions observed at 24 μm. Furthermore, the simulated galaxies at S _(24 μm) > 80 μJy exhibit R–K colors much bluer than observed and the predicted K-band fluxes are systematically underestimated at z ≳ 0.5. Unless these discrepancies mainly result from an incorrect treatment of extinction in the models they may reflect an underestimate of the predicted density of high-redshift massive sources with strong ongoing star formation, which would point to more fundamental processes and/or parameters (e.g., initial mass function, critical density to form stars, feedback,...) that are still not fully controlled in the simulations. The most recent backward evolution scenarios reproduce reasonably well the flux/redshift distribution of 24 μm sources up to z ~ 3, although none of them is able to exactly match our results at all redshifts

The abundance of compact quiescent galaxies since z∼0.6z\sim0.6

International audienceWe set out to quantify the number density of quiescent massive compact galaxies at intermediate redshifts. We determine structural parameters based on i-band imaging using the Canada–France–Hawaii Telescope (CFHT) equatorial Sloan Digital Sky Survey (SDSS) Stripe 82 (CS82) survey (∼170 deg^2) taking advantage of an exquisite median seeing of ∼0.6 arcsec. We select compact massive (M_⋆ > 5 × 10^10 M_⊙) galaxies within the redshift range of 0.2 10. We systematically measure a factor of ∼5 more compacts at the same redshift than what was previously reported on smaller fields with Hubble Space Telescope (HST) imaging, which are more affected by cosmic variance. This means that the decrease in number density from z ∼ 1.5 to z ∼ 0.2 might be only of a factor of ∼2–5, significantly smaller than what was previously reported. This supports progenitor bias as the main contributor to the size evolution. This milder decrease is roughly compatible with the predictions from recent numerical simulations. Only the most extreme compact galaxies, with R_eff  10^10.7 M_⊙, appear to drop in number by a factor of ∼20 and hence likely experience a noticeable size evolution

Type 2 AGN host galaxies in Chandra-COSMOS

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title \u27Type 2 AGN host galaxies in the Chandra-COSMOS Legacy Survey: no evidence of AGN-driven quenching.\u27 (bibcode: 2017ApJ...841..102S

Type 2 AGN Host Galaxies in the Chandra-COSMOS Legacy Survey: No Evidence of AGN-driven Quenching

We investigate the star formation properties of a large sample of ~2300 X-ray-selected Type 2 Active Galactic Nuclei (AGNs) host galaxies out to $z\sim 3$ in the Chandra COSMOS Legacy Survey in order to understand the connection between the star formation and nuclear activity. Making use of the existing multi-wavelength photometric data available in the COSMOS field, we perform a multi-component modeling from far-infrared to near-ultraviolet using a nuclear dust torus model, a stellar population model and a starburst model of the spectral energy distributions (SEDs). Through detailed analyses of SEDs, we derive the stellar masses and the star formation rates (SFRs) of Type 2 AGN host galaxies. The stellar mass of our sample is in the range of $9\lt \mathrm{log}\,{M}_{\mathrm{stellar}}/{M}_{\odot }\lt 12$ with uncertainties of ~0.19 dex. We find that Type 2 AGN host galaxies have, on average, similar SFRs compared to the normal star-forming galaxies with similar M stellar and redshift ranges, suggesting no significant evidence for enhancement or quenching of star formation. This could be interpreted in a scenario, where the relative massive galaxies have already experienced substantial growth at higher redshift ($z\gt 3$), and grow slowly through secular fueling processes hosting moderate-luminosity AGNs

The Composite Nature of Dust-obscured Galaxies (DOGs) at z ∼ 2–3 in the COSMOS Field. II. The AGN Fraction

We present the X-ray properties of 108 Dust-Obscured Galaxies (DOGs; F 24 μm/F R > 1000) in the COSMOS field, all of which are detected in at least three far-infrared bands with the Herschel Observatory. Out of the entire sample, 22 are individually detected in the hard 2–8 keV X-ray band by the Chandra COSMOS Legacy survey, allowing us to classify them as AGN. Six (27%) of them are Compton-thick AGN candidates with column densities N H > 1024 cm−2, while 15 are moderately obscured AGNs with 1022 < N H < 1024 cm−2. Additionally, we estimate AGN contributions to the IR luminosity (8–1000 μm rest-frame) greater than 20% for 19 DOGs based on SED decomposition using Spitzer/MIPS 24 μm and the five Herschel bands (100–500 μm). Only 7 of these are detected in X-rays individually. We performed an X-ray stacking analysis for the 86 undetected DOGs. We find that the AGN fraction in DOGs increases with 24 μm flux and that it is higher than that of the general 24 μm population. However, no significant difference is found when considering only X-ray detections. This strongly motivates the combined use of X-ray and far-IR surveys to successfully probe a wider population of AGNs, particularly for the most obscured ones

DEEP SPITZER 24 \u3bcm COSMOS IMAGING. I. THE EVOLUTION OF LUMINOUS DUSTY GALAXIES\u2014CONFRONTING THE MODELS

We present the first results obtained from the identification of ~30,000 sources in the Spitzer/24 \u3bcm observations of the COSMOS field at S 24 \u3bcm gsim 80 \u3bcJy. Using accurate photometric redshifts (\u3c3 z ~ 0.12 at z ~ 2 for 24 \u3bcm sources with i + lsim 25 mag AB) and simple extrapolations of the number counts at faint fluxes, we resolve with unprecedented detail the buildup of the mid-infrared background across cosmic ages. We find that ~50% and ~80% of the 24 \u3bcm background intensity originate from galaxies at z lsim 1 and z lsim 2, respectively, supporting the scenario where highly obscured sources at very high redshifts (z gsim 2) contribute only marginally to the cosmic infrared background. Assuming flux-limited selections at optical wavelengths, we also find that the fraction of i +-band sources with 24 \u3bcm detection strongly increases up to z ~ 2 as a consequence of the rapid evolution that star-forming galaxies have undergone with look-back time. Nonetheless, this rising trend shows a clear break at z ~ 1.3, probably due to k-correction effects implied by the complexity of spectral energy distributions in the mid-infrared. Finally, we compare our results with the predictions from different models of galaxy formation. We note that semianalytical formalisms currently fail to reproduce the redshift distributions observed at 24 \u3bcm. Furthermore, the simulated galaxies at S 24 \u3bcm > 80 \u3bcJy exhibit R-K colors much bluer than observed and the predicted K-band fluxes are systematically underestimated at z gsim 0.5. Unless these discrepancies mainly result from an incorrect treatment of extinction in the models they may reflect an underestimate of the predicted density of high-redshift massive sources with strong ongoing star formation, which would point to more fundamental processes and/or parameters (e.g., initial mass function, critical density to form stars, feedback,...) that are still not fully controlled in the simulations. The most recent backward evolution scenarios reproduce reasonably well the flux/redshift distribution of 24 \u3bcm sources up to z ~ 3, although none of them is able to exactly match our results at all redshifts