Searcharxiv⌕ Search

arXiv subjects

R. R. Chary

Publications and source records attributed to R. R. Chary.

13 recordsLinked to original sources
arXiv2014

The galaxy stellar mass function at 3.5<z<7.5 in the CANDELS/UDS, GOODS-South, and HUDF fields

The galaxy stellar mass function (GSMF) at high-z provides key information on star-formation history and mass assembly in the young Universe. We aimed to use the unique combination of deep optical/NIR/MIR imaging provided by HST, Spitzer and the VLT in the CANDELS-UDS, GOODS-South, and HUDF fields to determine the GSMF over the redshift range 3.5 4. These results confirm the unique synergy of the CANDELS+HUDF, HUGS, and SEDS surveys for the discovery and study of moderate/low-mass galaxies at high redshifts.

astro-ph.GA↗
arXiv2014

The Hawk-I UDS and GOODS Survey (HUGS): Survey design and deep K-band number counts

We present the results of a new, ultra-deep, near-infrared imaging survey executed with the Hawk-I imager at the ESO VLT, of which we make all the data public. This survey, named HUGS (Hawk-I UDS and GOODS Survey), provides deep, high-quality imaging in the K and Y bands over the CANDELS UDS and GOODS-South fields. We describe here the survey strategy, the data reduction process, and the data quality. HUGS delivers the deepest and highest quality K-band images ever collected over areas of cosmological interest, and ideally complements the CANDELS data set in terms of image quality and depth. The seeing is exceptional and homogeneous, confined to the range 0.38"-0.43". In the deepest region of the GOODS-S field, (which includes most of the HUDF) the K-band exposure time exceeds 80 hours of integration, yielding a 1-sigma magnitude limit of ~28.0 mag/sqarcsec. In the UDS field the survey matches the shallower depth of the CANDELS images reaching a 1-sigma limit per sq.arcsec of ~27.3mag in the K band and ~28.3mag in the Y-band, We show that the HUGS observations are well matched to the depth of the CANDELS WFC3/IR data, since the majority of even the faintest galaxies detected in the CANDELS H-band images are also detected in HUGS. We present the K-band galaxy number counts produced by combining the HUGS data from the two fields. We show that the slope of the number counts depends sensitively on the assumed distribution of galaxy sizes, with potential impact on the estimated extra-galactic background light (abridged).

astro-ph.GA↗
arXiv2013

The deepest Herschel-PACS far-infrared survey: number counts and infrared luminosity functions from combined PEP/GOODS-H observations

We present results from the deepest Herschel-PACS (Photodetector Array Camera and Spectrometer) far-infrared blank field extragalactic survey, obtained by combining observations of the GOODS (Great Observatories Origins Deep Survey) fields from the PACS Evolutionary Probe (PEP) and GOODS-Herschel key programmes. We describe data reduction and the construction of images and catalogues. In the deepest parts of the GOODS-S field, the catalogues reach 3-sigma depths of 0.9, 0.6 and 1.3 mJy at 70, 100 and 160 um, respectively, and resolve ~75% of the cosmic infrared background at 100um and 160um into individually detected sources. We use these data to estimate the PACS confusion noise, to derive the PACS number counts down to unprecedented depths and to determine the infrared luminosity function of galaxies down to LIR=10^11 Lsun at z~1 and LIR=10^12 Lsun at z~2, respectively. For the infrared luminosity function of galaxies, our deep Herschel far-infrared observations are fundamental because they provide more accurate infrared luminosity estimates than those previously obtained from mid-infrared observations. Maps and source catalogues (>3-sigma) are now publicly released. Combined with the large wealth of multi-wavelength data available for the GOODS fields, these data provide a powerful new tool for studying galaxy evolution over a broad range of redshifts.

astro-ph.CO↗
arXiv2012

GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies

We present here a new spectral energy distribution (SED) fitting approach that we adopt to select radio-excess sources amongst distant star-forming galaxies in the GOODS-Herschel (North) field and to reveal the presence of hidden, highly obscured AGN. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 um detections using some of the deepest Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to date, we have robustly identified a sample of 51 radio-excess AGN (~1300 deg^-2) out to redshift z~3. These radio-excess AGN have a significantly lower far-IR/radio ratio (q<1.68) than the typical relation observed for star-forming galaxies (q~2.2). We find that ~45% of these radio-excess sources have a dominant AGN component in the mid-IR band, while for the remainders the excess radio emission is the only indicator of AGN activity. The fraction of radio-excess AGN increases with X-ray luminosity reaching ~60% at Lx~10^44-10^45 erg/s, making these sources an important part of the total AGN population. However, almost half (24/51) of these radio-excess AGN are not detected in the deep Chandra X-ray data, suggesting that some of these sources might be heavily obscured. We also find that the specific star formation rates (sSFRs) of the radio-excess AGN are on average lower that those observed for X-ray selected AGN hosts, indicating that our sources are forming stars more slowly than typical AGN hosts, and possibly their star formation is progressively quenching.

astro-ph.CO↗
arXiv2011

Planck early results XIV: ERCSC validation and extreme radio sources

Planck's all sky surveys at 30-857 GHz provide an unprecedented opportunity to follow the radio spectra of a large sample of extragalactic sources to frequencies 2-20 times higher than allowed by past, large area, ground-based surveys. We combine the results of the Planck Early Release Compact Source Catalog (ERCSC) with quasi-simultaneous ground-based observations, as well as archival data, at frequencies below or overlapping Planck frequency bands, to validate the astrometry and photometry of the ERCSC radio sources and study the spectral features shown in this new frequency window opened by Planck. The ERCSC source positions and flux density scales are found to be consistent with the ground-based observations. We present and discuss the spectral energy distributions (SEDs) of a sample of "extreme" radio sources to illustrate the richness of the ERCSC for the study of extragalactic radio sources. Variability is found to play a role in the unusual spectral features of some of these sources.

astro-ph.CO↗
arXiv2011

Planck early results. IX. XMM-Newton follow-up for validation of Planck cluster candidates

We present the XMM-Newton follow-up for confirmation of Planck cluster candidates. Twenty-five candidates have been observed to date using snapshot (~10 ksec) exposures, ten as part of a pilot programme to sample a low range of signal-to-noise ratios (4 5 candidates. The sensitivity and spatial resolution of XMM-Newton allows unambiguous discrimination between clusters and false candidates. The 4 false candidates have S/N <= 4.1. A total of 21 candidates are confirmed as extended X-ray sources. Seventeen are single clusters, the majority of which are found to have highly irregular and disturbed morphologies (about ~70%). The remaining four sources are multiple systems, including the unexpected discovery of a supercluster at z=0.45. For 20 sources we are able to derive a redshift estimate from the X-ray Fe K line (albeit of variable quality). The new clusters span the redshift range 0.09 <= z <= 0.54, with a median redshift of z~0.37. A first determination is made of their X-ray properties including the characteristic size, which is used to improve the estimate of the SZ Compton parameter, Y_SZ. The follow-up validation programme has helped to optimise the Planck candidate selection process. It has also provided a preview of the X-ray properties of these newly-discovered clusters, allowing comparison with their SZ properties, and to the X-ray and SZ properties of known clusters observed in the Planck survey. Our results suggest that Planck may have started to reveal a non-negligible population of massive dynamically perturbed objects that is under-represented in X-ray surveys. However, despite their particular properties, these new clusters appear to follow the Y_SZ-Y_X relation established for X-ray selected objects, where Y_X is the product of the gas mass and temperature.

astro-ph.CO↗
arXiv2011

Insights on the Formation, Evolution, and Activity of Massive Galaxies From Ultra-Compact and Disky Galaxies at z=2-3

We present our results on the structure and activity of massive galaxies at z=1-3 using one of the largest (166 with M_star>=5e10 M_sun) and most diverse samples of massive galaxies derived from the GOODS-NICMOS survey: (1) Sersic fits to deep NIC3/F160W images indicate that the rest-frame optical structures of massive galaxies are very different at z=2-3 compared to z~0. Approximately 40% of massive galaxies are ultra-compact (r_e<=2 kpc), compared to less than 1% at z~0. Furthermore, most (~65%) systems at z=2-3 have a low Sersic index n<=2, compared to ~13% at z~0. We present evidence that the n<=2 systems at z=2-3 likely contain prominent disks, unlike most massive z~0 systems. (2) There is a correlation between structure and star formation rates (SFR). The majority (~85%) of non-AGN massive galaxies at z=2-3, with SFR high enough to yield a 5 sigma (30 micro Jy) 24 micron Spitzer detection have low n<=2. Such n<=2 systems host the highest SFR. (3) The frequency of AGN is ~40% at z=2-3. Most (~65%) AGN hosts have disky (n<=2) morphologies. Ultra-compact galaxies appear quiescent in terms of both AGN activity and star formation. (4) Large stellar surface densities imply massive galaxies at z=2-3 formed via rapid, highly dissipative events at z>2. The large fraction of n<=2 disky systems suggests cold mode accretion complements gas-rich major mergers at z>2. In order for massive galaxies at z=2-3 to evolve into present-day massive E/S0s, they need to significantly increase (n, r_e). Dry minor and major mergers may play an important role in this process.

astro-ph.CO↗
arXiv2011

Evolution of the dusty infrared luminosity function from z=0 to z=2.3 using observations from Spitzer

We derive the evolution of the infrared (IR) luminosity function (LF) over the last 4/5ths of cosmic time, using deep 24um and 70um imaging of the GOODS North and South fields. We use an extraction technique based on prior source positions at shorter wavelengths to build the 24 and 70um source catalogs. The majority (93%) of the sources have a spectroscopic (39%) or a photometric redshift (54%) and, in our redshift range of interest (i.e., 1.3<z<2.3) ~20% of the sources have a spectroscopic redshifts. To extend our study to lower 70um luminosities we perform a stacking analysis and we characterize the observed L_24/(1+z) vs L_70/(1+z) correlation. Using spectral energy distribution templates which best fit this correlation, we derive the IR luminosity of sources from their 24 and 70 um fluxes. We then compute the IR LF at z=1.55+/-0.25 and z=2.05+/-0.25. The redshift evolution of the IR LF from z=1.3 to z=2.3 is consistent with a luminosity evolution proportional to (1+z)^1.0+/-0.9 combined with a density evolution proportional to (1+z)^-1.1+/-1.5. At z~2, luminous IR galaxies (LIRGs: 10^11Lsun< LIR <10^12Lsun) are still the main contributors to the total comoving IR luminosity density (IR LD) of the Universe. At z~2, LIRGs and ultra-luminous IR galaxies (ULIRGs: 10^12Lsun< LIR) account for ~49% and ~17% respectively of the total IR LD of the Universe. Combined with previous results for galaxies at z<1.3 and assuming a constant conversion between the IR luminosity and star-formation rate (SFR) of a galaxy, we study the evolution of the SFR density of the Universe from z=0 to z=2.3. We find that the SFR density of the Universe strongly increased with redshift from z=0 to z=1.3, but is nearly constant at higher redshift out to z=2.3. As part of the online material accompanying this article, we present source catalogs at 24um and 70um for both the GOODS-North and -South fields.

astro-ph.CO↗
arXiv2010

Balancing the Energy Budget: Star-Formation versus AGN in High Redshift Infrared Luminous Galaxies

We present deep {\it Spitzer} mid-infrared spectroscopy, along with 16, 24, 70, and 850\,$\micron$\ photometry, for 22 galaxies located in the Great Observatories Origins Deep Survey-North (GOODS-N) field. The sample spans a redshift range of $0.6\la z \la 2.6$, 24~$μ$m flux densities between $\sim$0.2$-$1.2 mJy, and consists of submillimeter galaxies (SMGs), X-ray or optically selected active galactic nuclei (AGN), and optically faint ($z_{AB}>25$\,mag) sources. We find that infrared (IR; $8-1000~\micron$) luminosities derived by fitting local spectral energy distributions (SEDs) with 24~$\micron$ photometry alone are well matched to those when additional mid-infrared spectroscopic and longer wavelength photometric data is used for galaxies having $z\la1.4$ and 24~$\micron$-derived IR luminosities typically $\la 3\times 10^{12}~L_{\sun}$. However, for galaxies in the redshift range between $1.4\la z \la 2.6$, typically having 24~$\micron$-derived IR luminosities $\ga 3\times 10^{12}~L_{\sun}$, IR luminosities are overestimated by an average factor of $\sim$5 when SED fitting with 24~$\micron$ photometry alone. This result arises partly due to the fact that high redshift galaxies exhibit aromatic feature equivalent widths that are large compared to local galaxies of similar luminosities. Through a spectral decomposition of mid-infrared spectroscopic data, we are able to isolate the fraction of IR luminosity arising from an AGN as opposed to star formation activity. This fraction is only able to account for $\sim$30\% of the total IR luminosity among the entire sample.

astro-ph.CO↗
arXiv2008

Reliable Identification of Compton-thick Quasars at z~2: Spitzer Mid-IR spectroscopy of HDF-oMD49

Many models that seek to explain the origin of the unresolved X-ray background predict that Compton-thick Active Galactic Nuclei (AGNs) are ubiquitious at high redshift. However, few distant Compton-thick AGNs have been reliably identified to date. Here we present Spitzer-IRS spectroscopy and 3.6-70um photometry of a z=2.2 optically identified AGN (HDF-oMD49) that is formally undetected in the 2Ms Chandra Deep Field-North (CDF-N) survey. The Spitzer-IRS spectrum and spectral energy distribution of this object is AGN dominated, and a comparison of the energetics at X-ray wavelengths to those derived from mid-infrared (mid-IR) and optical spectroscopy shows that the AGN is intrinsically luminous (L_X~3x10^44 erg/s) but heavily absorbed by Compton-thick material (N_H>>10^24 cm^{-2}); i.e., this object is a Compton-thick quasar. Adopting the same approach that we applied to HDF-oMD49, we found a further six objects at z~2-2.5 in the literature that are also X-ray weak/undetected but have evidence for AGN activity from optical and/or mid-IR spectroscopy, and show that all of these sources are also Compton-thick quasars with L_X>10^44 erg/s. On the basis of the definition of Daddi etal. (2007), these Compton-thick quasars would be classified as mid-IR excess galaxies, and our study provides the first spectroscopic confirmation of Compton-thick AGN activity in a subsample of these z~2 mid-IR bright galaxies. Using the four objects that lie in the CDF-N field, we estimate the space-density of Compton-thick quasars [Phi~(0.7-2.5)x10^-5 Mpc^-3 for L_X>10^44 erg/s objects at z~2-2.5] and show that Compton-thick accretion is as ubiquitious as unobscured accretion in the distant Universe.

astro-ph↗
arXiv2006

Mid infrared properties of distant infrared luminous galaxies

We present evidence that the mid infrared (MIR) is a good tracer of the total infrared luminosity, L(IR), and star formation rate (SFR), of galaxies up to z 1.3. We use deep MIR images from the Infrared Space Observatory (ISO) and the Spitzer Space Telescope in the Northern field of the Great Observatories Origins Deep Survey (GOODS-N) together with VLA radio data to compute three independant estimates of L(IR). The L(IR,MIR) derived from the observed 15 and/or 24 um flux densities using a library of template SEDs, and L(IR,radio), derived from the radio (1.4 and/or 8.5 GHz) using the radio-far infrared correlation, agree with a 1-sigma dispersion of 40 %. We use the k-correction as a tool to probe different parts of the MIR spectral energy distribution (SED) of galaxies as a function of their redshift and find that on average distant galaxies present MIR SEDs very similar to local ones. However, in the redshift range z= 0.4-1.2, L(IR,24um) is in better agreement with L(IR,radio) than L(IR,15 um) by 20 %, suggesting that the warm dust continuum is a better tracer of the SFR than the broad emission features due to polycyclic aromatic hydrocarbons (PAHs). We find marginal evidence for an evolution with redshift of the MIR SEDs: two thirds of the distant galaxies exhibit rest-frame MIR colors (L(12 um)/L(7 um) and L(10 um)/L(15 um) luminosity ratios) below the median value measured for local galaxies. Possible explanations are examined but these results are not sufficient to constrain the physics of the emitting regions. We compare three commonly used SED libraries which reproduce the color-luminosity correlations of local galaxies with our data and discuss possible refinements to the relative intensities of PAHs, warm dust continuum and silicate absorption.

astro-ph↗
arXiv2000

Tentative Detection of the Cosmic Infrared Background at 2.2 and 3.5 microns Using Ground Based and Space Based Observations

The Cosmic InfraRed Background (CIRB) is the sum total of the redshifted and reprocessed short wavelength radiation from the era of galaxy formation, and hence contains vital information about the history of galactic evolution. One of the main problems associated with estimating an isotropic CIRB in the near infrared (1-5 microns) is the unknown contribution from stars within our own galaxy. The optimal observational window to search for a background in the near-IR is at 3.5 microns since that is the wavelength region where the other main foreground, the zodiacal dust emission, is the least. It is not possible to map out the entire 3.5 micron sky at a resolution which will accurately estimate the flux from stars. However, since the CIRB is presumably isotropic, it can potentially be detected by selecting a smaller field and imaging it at good resolution to estimate the stellar intensity. We selected a 2x2 degree "dark spot" near the North Galactic Pole which had the least intensity at 3.5 microns after a zodiacal light model was subtracted from the all-sky maps generated by the Diffuse InfraRed Background Experiment (DIRBE). The measured total intensity of the few bright stars in this field was combined with a model for the contribution from dimmer stars and subtracted from the zodi-subtracted DIRBE map. The contribution from the interstellar medium was also subtracted leaving a residual intensity at 2.2 microns of: 16.4+/-4.4 kJy/sr or 22.4+/-6 nW/m^2/sr, and at 3.5 microns: 12.8+/-3.8 kJy/sr or 11+/-3.3 nW/m^2/sr. [Abridged]

astro-ph↗