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F. Shankar

Publications and source records attributed to F. Shankar.

At least 91 records · Page 5Linked to original sources
arXiv2015

A catalog of visual-like morphologies in the 5 CANDELS fields using deep-learning

We present a catalog of visual like H-band morphologies of $\sim50.000$ galaxies ($H_{f160w}<24.5$) in the 5 CANDELS fields (GOODS-N, GOODS-S, UDS, EGS and COSMOS). Morphologies are estimated with Convolutional Neural Networks (ConvNets). The median redshift of the sample is $ \sim1.25$. The algorithm is trained on GOODS-S for which visual classifications are publicly available and then applied to the other 4 fields. Following the CANDELS main morphology classification scheme, our model retrieves the probabilities for each galaxy of having a spheroid, a disk, presenting an irregularity, being compact or point source and being unclassifiable. ConvNets are able to predict the fractions of votes given a galaxy image with zero bias and $\sim10\%$ scatter. The fraction of miss-classifications is less than $1\%$. Our classification scheme represents a major improvement with respect to CAS (Concentration-Asymmetry-Smoothness)-based methods, which hit a $20-30\%$ contamination limit at high z. The catalog is released with the present paper via the $\href{http://rainbowx.fis.ucm.es/Rainbow_navigator_public}{Rainbow\,database}$

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arXiv2015

Black Hole and Galaxy Coevolution from Continuity Equation and Abundance Matching

[abridged] We investigate the coevolution of galaxies and hosted supermassive black holes throughout the history of the Universe by a statistical approach based on the continuity equation and the abundance matching technique. Specifically, we present analytical solutions of the continuity equation without source term to reconstruct the supermassive black hole (BH) mass function from the AGN luminosity functions. Such an approach includes physically-motivated AGN lightcurves tested on independent datasets, which describe the evolution of the Eddington ratio and radiative efficiency from slim- to thin-disc conditions. We nicely reproduce the local estimates of the BH mass function, the AGN duty cycle as a function of mass and redshift, along with the Eddington ratio function and the fraction of galaxies with given stellar mass hosting an AGN with given Eddington ratio. We exploit the same approach to reconstruct the observed stellar mass function at different redshift from the UV and far-IR luminosity functions associated to star formation in galaxies. These results imply that the buildup of stars and BHs in galaxies occurs via in-situ processes, with dry mergers playing a marginal role at least for stellar masses < 3 10^11 M_sun and BH masses < 10^9 M_sun, where the statistical data are more secure and less biased by systematic errors. In addition, we develop an improved abundance matching technique to link the stellar and BH content of galaxies to the gravitationally dominant dark matter component. The resulting relationships constitute a testbed for galaxy evolution models, highlighting the complementary role of stellar and AGN feedback in the star formation process. Finally, the clustering properties of BHs and galaxies are found to be in full agreement with current observations, so further validating our results from the continuity equation.

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arXiv2015

The cosmic growth of the active black hole population at 1<z<2 in zCOSMOS, VVDS and SDSS

We present a census of the active black hole population at 1<z<2, by constructing the bivariate distribution function of black hole mass and Eddington ratio, employing a maximum likelihood fitting technique. The study of the active black hole mass function (BHMF) and the Eddington ratio distribution function (ERDF) allows us to clearly disentangle the active galactic nuclei (AGN) downsizing phenomenon, present in the AGN luminosity function, into its physical processes of black hole mass downsizing and accretion rate evolution. We are utilizing type-1 AGN samples from three optical surveys (VVDS, zCOSMOS and SDSS), that cover a wide range of 3 dex in luminosity over our redshift interval of interest. We investigate the cosmic evolution of the AGN population as a function of AGN luminosity, black hole mass and accretion rate. Compared to z = 0, we find a distinct change in the shape of the BHMF and the ERDF, consistent with downsizing in black hole mass. The active fraction or duty cycle of type-1 AGN at z~1.5 is almost flat as a function of black hole mass, while it shows a strong decrease with increasing mass at z=0. We are witnessing a phase of intense black hole growth, which is largely driven by the onset of AGN activity in massive black holes towards z=2. We finally compare our results to numerical simulations and semi-empirical models and while we find reasonable agreement over certain parameter ranges, we highlight the need to refine these models in order to match our observations.

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arXiv2014

Clustering properties of moderate luminosity X-ray selected Type 1 and Type 2 AGN at z~3

We investigate, for the first time at z~3, the clustering properties of 189 Type 1 and 157 Type 2 X-ray active galactic nuclei (AGN) of moderate luminosity (log = 45.3 erg/s), with photometric or spectroscopic redshifts in the range 2.2<z<6.8. These samples are based on Chandra and XMM-Newton data in COSMOS. We find that Type 1 and Type 2 COSMOS AGN at z=3 inhabit DMHs with typical mass of logMh = 12.84+0.10/-0.11 and 11.73+0.39/-0.45 Msun/h, respectively. This result requires a drop in the halo masses of Type 1 and 2 COSMOS AGN at z~3 compared to z<2 XMM COSMOS AGN with similar luminosities. Additionally, we infer that unobscured COSMOS AGN at z~3 reside in 10 times more massive halos compared to obscured COSMOS AGN, at 2.6sigma level. This result extends to z~3 that found in COSMOS at z<2, and rules out the picture in which obscuration is purely an orientation effect. A model which assumes that the AGN activity is triggered by major mergers is quite successful in predicting both the low halo mass of COSMOS AGN and the typical mass of luminous SDSS quasars at z~3, with the latter inhabiting more massive halos respect to moderate luminosity AGN. Alternatively we can argue, at least for Type 1 COSMOS AGN, that they are possibly representative of an early phase of fast (i.e. Eddington limited) BH growth induced by cosmic cold flows or disk instabilities. Given the moderate luminosity, these new fast growing BHs have masses of e7-8 Msun at z~3 which might evolve into e8.5-9 Msun mass BHs at z=0. Following our clustering measurements, we argue that this fast BH growth at z~3 in AGN with moderate luminosity occurs in DMHs with typical mass of 6 times e12 Msun/h.

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arXiv2014

Systematic effects on the size-luminosity relation: dependence on model fitting and morphology

We quantify the systematics in the size-luminosity relation of galaxies in the SDSS main sample which arise from fitting different 1- and 2-component model profiles to the images. In objects brighter than L*, fitting a single Sersic profile to what is really a two-component SerExp system leads to biases: the half-light radius is increasingly overestimated as n of the fitted single component increases; it is also overestimated at B/T ~ 0.6. However, the net effect on the R-L relation is small, except for the most luminous tail, where it curves upwards towards larger sizes. We also study how this relation depends on morphological type. Our analysis is one of the first to use Bayesian-classifier derived weights, rather than hard cuts, to define morphology. Crudely, there appear to be only two relations: one for early-types (Es, S0s and Sa's) and another for late-types (Sbs and Scds). However, closer inspection shows that within the early-type sample S0s tend to be 15% smaller than Es of the same luminosity, and, among faint late types, Sbs are more than 25% smaller than Scds. Neither the early- nor the late-type relations are pure power-laws: both show significant curvature, which we quantify. However, the R-L relations of the bulges of early-types are almost pure power laws. Our analysis confirms that two mass scales are special for early-type galaxies: M* = 3e10 and 2e11 Msun. These same mass scales are also special for late types: there is almost no correlation between R and M* below the former, and almost no late-types above the latter. We also show that the intrinsic scatter around the relation decreases at large luminosity and/or stellar mass; this should provide additional constraints on models of how the most massive galaxies formed.

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arXiv2014

Galaxy size trends as a consequence of cosmology

We show that recently documented trends in galaxy sizes with mass and redshift can be understood in terms of the influence of underlying cosmic evolution; a holistic view which is complimentary to interpretations involving the accumulation of discreet evolutionary processes acting on individual objects. Using standard cosmology theory, supported with results from the Millennium simulations, we derive expected size trends for collapsed cosmic structures, emphasising the important distinction between these trends and the assembly paths of individual regions. We then argue that the observed variation in the stellar mass content of these structures can be understood to first order in terms of natural limitations of cooling and feedback. But whilst these relative masses vary by orders of magnitude, galaxy and host radii have been found to correlate linearly. We explain how these two aspects will lead to galaxy sizes that closely follow observed trends and their evolution, comparing directly with the COSMOS and SDSS surveys. Thus we conclude that the observed minimum radius for galaxies, the evolving trend in size as a function of mass for intermediate systems, and the observed increase in the sizes of massive galaxies, may all be considered an emergent consequence of the cosmic expansion.

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arXiv2013

The massive end of the luminosity and stellar mass functions: Dependence on the fit to the light profile

In addition to the large systematic differences arising from assumptions about the stellar mass-to-light ratio, the massive end of the stellar mass function is rather sensitive to how one fits the light profiles of the most luminous galaxies. We quantify this by comparing the luminosity and stellar mass functions based on SDSS cmodel magnitudes, and PyMorph single-Sersic and Sersic-Exponential fits to the surface brightness profiles of galaxies in the SDSS. The PyMorph fits return more light, so that the predicted masses are larger than when cmodel magnitudes are used. As a result, the total stellar mass density at z~0.1 is about 1.2x larger than in our previous analysis of the SDSS. The differences are most pronounced at the massive end, where the measured number density of objects having M* > 6 x 10^{11} Msun is ~5x larger. Alternatively, at number densities of 10^{-6} Mpc^{-3}, the limiting stellar mass is 2x larger. The differences with respect to fits by other authors, typically based on Petrosian-like magnitudes, are even more dramatic, although some of these differences are due to sky-subtraction problems, and are sometimes masked by large differences in the assumed $M_*/L$ (even after scaling to the same IMF). Our results impact studies of the growth and assembly of stellar mass in galaxies, and of the relation between stellar and halo mass, so we provide simple analytic fits to these new luminosity and stellar mass functions and quantify how they depend on morphology, as well as the binned counts in electronic format.

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arXiv2013

Larger sizes of massive quiescent early-type galaxies in clusters than in the field at 0.8 < z < 1.5

[abridged] The mass-size relation of early-type galaxies (ETGs) has been largely studied in the last years to probe the mass assembly of the most massive objects in the Universe. In this paper, we focus on the mass-size relation of quiescent massive ETGs (Mstar/Msol > 3*10^10) living in massive clusters (M200 ~ 10^14 Mstar) at 0.8< z <1.5, as compared to those living in the field at the same epoch. Our sample contains ~ 400 ETGs in clusters and the same number in the field. Therefore, our sample is approximately an order of magnitude larger than previous studies in the same redshift range for galaxy clusters. We find that ETGs living in clusters are between ~30-50% larger than galaxies with the same stellar mass residing in the field. We parametrize the size using the mass-normalized size, gamma=Re/Mstar^0.57. The gamma distributions in both environments peak at the same position but the distributions in clusters are more skewed towards larger sizes. Since this size difference is not observed in the local Universe, the size evolution at fixed stellar mass from z~1.5 of cluster galaxies is less steep ((1+z)-0.53pm0.04) than the evolution of field galaxies ((1+z)-0.92pm0.04). The size difference seems to be essentially driven by the galaxies residing in the clusters cores (R<0.5*R200). If part of the size evolution is due to mergers, the difference we see between cluster and field galaxies could be due to higher merger rates in clusters at higher redshift, probably during the formation phase of the clusters when velocity dispersions are lower. We cannot exclude however that the difference is driven by newly quenched galaxies which are quenched more efficiently in clusters. The implications of these results for the hierarchical growth of ETGs will be discussed in a companion paper.

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arXiv2012

Occupation of X-ray selected galaxy groups by X-ray AGN

We present the first direct measurement of the mean Halo Occupation Distribution (HOD) of X-ray selected AGN in the COSMOS field at z < 1, based on the association of 41 XMM and 17 C-COSMOS AGN with member galaxies of 189 X-ray detected galaxy groups from XMM and Chandra data. We model the mean AGN occupation in the halo mass range logM_200[Msun] = 13-14.5 with a rolling-off power-law with the best fit index alpha = 0.06(-0.22;0.36) and normalization parameter f_a = 0.05(0.04;0.06). We find the mean HOD of AGN among central galaxies to be modelled by a softened step function at logMh > logMmin = 12.75 (12.10,12.95) Msun while for the satellite AGN HOD we find a preference for an increasing AGN fraction with Mh suggesting that the average number of AGN in satellite galaxies grows slower (alpha_s < 0.6) than the linear proportion (alpha_s = 1) observed for the satellite HOD of samples of galaxies. We present an estimate of the projected auto correlation function (ACF) of galaxy groups over the range of r_p = 0.1-40 Mpc/h at = 0.5. We use the large-scale clustering signal to verify the agreement between the group bias estimated by using the observed galaxy groups ACF and the value derived from the group mass estimates. We perform a measurement of the projected AGN-galaxy group cross-correlation function, excluding from the analysis AGN that are within galaxy groups and we model the 2-halo term of the clustering signal with the mean AGN HOD based on our results.

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arXiv2012

Accreting SMBHs in the COSMOS field and the connection to their host galaxies

Using the wide multi-band photometry available in the COSMOS field we explore the host galaxy properties of a large sample of Active Galactic Nuclei (AGN) obtained by combining X-ray and optical spectroscopic selections. Based on a careful study of their Spectral Energy Distribution (SED), which has been parametrized using a 2-component (AGN+galaxy) model fit, we derived dust-corrected rest-frame magnitudes, colors, stellar masses and star formation rates (SFRs). We find that AGN hosts span a large range of stellar masses and SFRs. No color-bimodality is seen at any redshift in the AGN hosts, which are found to be mainly massive, red galaxies. Once accounting for the color-mass degeneracy in well defined mass-matched samples, we find a residual marginal enhancement of AGN incidence in redder galaxies with lower specific star formation rates, and we argue that this result might emerge because of our ability to properly account for AGN light contamination and dust extinction. Interestingly, we find that the probability for a galaxy to host a black hole growing at any given "specific accretion rate" (i.e. the ratio of X-ray luminosity to the host stellar mass) is almost independent of the host galaxy mass, while it decreases as a power-law with Lx/M. By analyzing the normalization of such probability distribution, we show how the incidence of AGN increases with redshift as rapidly as (1+z)^4, in close resemblance with the overall evolution of the specific star formation rate of the entire galaxy population. Although AGN activity and star formation in galaxies do appear to have a common triggering mechanism, at least in a statistical sense, within the COSMOS sample we do not find strong evidence of any 'smoking gun' signaling powerful AGN influence on the star-forming properties of their hosts galaxies.

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arXiv2012

Bolometric luminosities and Eddington ratios of X-ray selected Active Galactic Nuclei in the XMM-COSMOS survey

Bolometric luminosities and Eddington ratios of both X-ray selected broad-line (Type-1) and narrow-line (Type-2) AGN from the XMM-Newton survey in the COSMOS field are presented. The sample is composed by 929 AGN (382 Type-1 AGN and 547 Type-2 AGN) and it covers a wide range of redshifts, X-ray luminosities and absorbing column densities. About 65% of the sources are spectroscopically identified as either Type-1 or Type-2 AGN (83% and 52% respectively), while accurate photometric redshifts are available for the rest of the sample. The study of such a large sample of X-ray selected AGN with a high quality multi-wavelength coverage from the far-infrared (now with the inclusion of Herschel data at 100 micron and 160 micron) to the optical-UV allows us to obtain accurate estimates of bolometric luminosities, bolometric corrections and Eddington ratios. The kbol-Lbol relations derived in the present work are calibrated for the first time against a sizable AGN sample, and rely on observed redshifts, X-ray luminosities and column density distributions. We find that kbol is significantly lower at high Lbol with respect to previous estimates by Marconi et al. (2004) and Hopkins et al. (2007). Black hole masses and Eddington ratios are available for 170 Type-1 AGN, while black hole masses for Type-2 AGN are computed for 481 objects using the black hole mass-stellar mass relation and the morphological information. We confirm a trend between kbol and lambda_Edd, with lower hard X-ray bolometric corrections at lower Eddington ratios for both Type-1 and Type-2 AGN. We find that, on average, Eddington ratio increases with redshift for all Types of AGN at any given Mbh, while no clear evolution with redshift is seen at any given Lbol.

astro-ph.CO↗
arXiv2012

Black Holes in Pseudobulges: demography and models

There is mounting evidence that a significant fraction of Black Holes (BHs) today live in late-type galaxies, including bulge-less galaxies and those hosting pseudobulges, and are significantly undermassive with respect to the scaling relations followed by their counterpart BHs in classical bulges of similar stellar (or even bulge) mass. Here we discuss the predictions of two state-of-the-art hierarchical galaxy formation models in which BHs grow via mergers and, in one, also via disk instability. Our aim is to understand if the wealth of new data on local BH demography is consistent with standard models. We follow the merger trees of representative subsamples of BHs and compute the fractional contributions of different processes to the final BH mass. We show that the model in which BHs always closely follow the growth of their host bulges, also during late disk instabilities (i.e., bars), produces too narrow a distribution of BHs at fixed stellar mass to account for the numerous low-mass BHs now detected in later-type galaxies. Models with a looser connection between BH growth and bar instability instead predict the existence of a larger number of undermassive BHs, in better agreement with the observations. The scatter in the updated local BH-bulge mass relation (with no restriction on galaxy type) appears to be quite large when including later-type systems, but it can still be managed to be reproduced within current hierarchical models. However, the fuelling of BHs during the late bar-instability mode needs to be better quantified/improved to properly fit the data. We conclude discussing how the possibly large number of BHs in later type galaxies demands for an in-depth revision of the local BH mass function and its modelling.

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arXiv2011

Early-type galaxies at z~1.3. IV. Scaling relations in different environments

We present the Kormendy and mass-size relations for early-type galaxies (ETGs) as a function of environment at z~1.3. Our sample includes 76 visually classified ETGs with masses 10^10 < M/Msun < 10^11.5, selected in the Lynx supercluster and in the GOODS/CDF-S field, 31 ETGs in clusters, 18 in groups and 27 in the field, all with multi-wavelength photometry and HST/ACS observations. The Kormendy relation, in place at z~1.3, does not depend on the environment. The mass-size relation reveals that ETGs overall appear to be more compact in denser environments: cluster ETGs have sizes on average around 30-50% smaller than those of the local universe, and a distribution with a smaller scatter, whereas field ETGs show a mass-size relation with a similar distribution than the local one. Our results imply that (1) the mass-size relation in the field did not evolve overall from z ~ 1.3 to present; this is interesting and in contrast to the trend found at higher masses from previous works; (2) in denser environments, either ETGs have increased their size by 30-50%, on average, and spread their distributions, or more ETGs have been formed within the dense environment from not ETG progenitors or larger galaxies have been accreted to a pristine compact population to reproduce the mass-size relation observed in the local Universe. Our results are driven by galaxies with masses M<2*10^11Msun and those with masses M~10^11Msun follow the same trends that the entire sample. Following Valentinuzzi et al. definition of superdense ETGs, around 35-45% of our cluster sample is made of superdense ETGs.

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arXiv2011

Faint high-redshift AGN in the Chandra Deep Field South: the evolution of the AGN luminosity function and black hole demography

We present detection and analysis of faint X-ray sources in the Chandra deep field south (CDFS) using the 4 Msec Chandra observation and adopting a new detection algorithm, based on a targeted search at the position of known high-z galaxies. This optimized technique results in the identification of 54 z>3 AGNs, 29 of which are new detections. Applying stringent completeness criteria, we derive AGN luminosity functions in the redshift bins 3-4, 4-5 and >5.8 and for 42.75 3 (18+17-10%). Their optical counterparts are not strongly reddened and we thus conclude that the size of the X-ray absorber is likely smaller than the dust sublimation radius. We finally report the discovery of a highly star-forming galaxy at z=3.47. If confirmed, this would be one of the farthest objects in which stellar sources are detected in X-rays.

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arXiv2011

The Population of High Redshift AGNs in the Chandra COSMOS survey

We present the high redshift (3 3. 81 sources are selected in the 0.5-2 keV band, 14 are selected in the 2-10 keV and 6 in the 0.5-10 keV bands. We sample the high luminosity (log L_(2-10 keV)>44.15 erg/s) space density up to z~5 and a fainter luminosity range (43.5 3. We find that the space density of high-luminosity AGNs declines exponentially at all the redshifts, confirming the trend observed for optically selected quasars. At lower luminosity, the measured space density is not conclusive, and a larger sample of faint sources is needed. Comparisons with optical luminosity functions and BH formation models are presented together with prospects for future surveys.

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arXiv2011

The XMM-Newton Wide field survey in the COSMOS field: redshift evolution of AGN bias and subdominant role of mergers in triggering moderate luminosity AGN at redshift up to 2.2

We present a study of the redshift evolution of the projected correlation function of 593 X-ray selected AGN with I_AB<23 and spectroscopic redshifts z<4, extracted from the 0.5-2 keV X-ray mosaic of the 2.13 deg^2 XMM-COSMOS survey. We introduce a method to estimate the average bias of the AGN sample and the mass of AGN hosting halos, solving the sample variance using the halo model and taking into account the growth of the structure over time. We find evidence of a redshift evolution of the bias factor for the total population of XMM-COSMOS AGN from b(z=0.92)=2.30 +/- 0.11 to b(z=1.94)=4.37 +/- 0.27 with an average mass of the hosting DM halos logM [h^-1 M_sun] ~ 13.12 +/- 0.12 that remains constant at all z < 2. Splitting our sample into broad optical lines AGN (BL), AGN without broad optical lines (NL) and X-ray unobscured and obscured AGN, we observe an increase of the bias with redshift in the range z=0.7-2.25 and z=0.6-1.5 which corresponds to a constant halo mass logM [h^-1 M_sun] ~ 13.28 +/- 0.07 and logM [h^-1 M_sun] ~ 13.00 +/- 0.06 for BL /X-ray unobscured AGN and NL/X-ray obscured AGN, respectively. The theoretical models which assume a quasar phase triggered by major mergers can not reproduce the high bias factors and DM halo masses found for X-ray selected BL AGN with L_BOL ~ 2e45 erg s^-1. Our work extends up to z ~ 2.2 the z <= 1 statement that, for moderate luminosity X-ray selected BL AGN, the contribution from major mergers is outnumbered by other processes, possibly secular such as tidal disruptions or disk instabilities.

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arXiv2011

Evidence of major dry mergers at M* > 2 x 10^11 Msun from curvature in early-type galaxy scaling relations?

For early-type galaxies, the correlations between stellar mass and size, velocity dispersion, surface brightness, color, axis ratio and color-gradient all indicate that two mass scales, M* = 3 x 10^10 Msun and M* = 2 x 10^11 Msun, are special. The smaller scale could mark the transition between wet and dry mergers, or it could be related to the interplay between SN and AGN feedback, although quantitative measures of this transition may be affected by morphological contamination. At the more massive scale, mean axis ratios and color gradients are maximal, and above it, the colors are redder, the sizes larger and the velocity dispersions smaller than expected based on the scaling at lower M*. In contrast, the color-sigma relation, and indeed, most scaling relations with sigma, are not curved: they are well-described by a single power law, or in some cases, are almost completely flat. When major dry mergers change masses, sizes, axis ratios and color gradients, they are expected to change the colors or velocity dispersions much less. Therefore, the fact that scaling relations at sigma > 150 km/s show no features, whereas the size-M*, b/a-M*, color-M* and color gradient-M* relations do, suggests that M* = 2 x 10^11 Msun is the scale above which major dry mergers dominate the assembly histories of early-type galaxies.

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