arXiv subjects

Francesco Shankar

Publications and source records attributed to Francesco Shankar.

At least 109 records · Page 6Linked to original sources

arXiv2011

Constraints on star-formation driven galaxy winds from the mass-metallicity relation at z=0

We extend a chemical evolution model relating galaxy stellar mass and gas-phase oxygen abundance (the mass-metallicity relation) to explicitly consider the mass-dependence of galaxy gas fractions and outflows. Using empirically derived scalings of galaxy mass with halo virial velocity in conjunction with the most recent observations of z~0 total galaxy cold gas fractions and the mass-metallicity relation, we place stringent global constraints on the magnitude and scaling of the efficiency with which star forming galaxies expel metals. We demonstrate that under the assumptions that metal accretion is negligible and the stellar initial mass function does not vary, efficient outflows are required to reproduce the mass-metallicity relation; without winds, gas-to-stellar mass ratios >~ 0.3 dex higher than observed are needed. Moreover, z=0 gas fractions are low enough that while they have some effect on the magnitude of outflows required, the slope of the gas fraction--stellar mass relation does not strongly affect our conclusions on how the wind efficiencies must scale with galaxy mass. Despite systematic uncertainties in the normalization and slope of the mass-metallicity relation, we show that the metal expulsion efficiency zetaw=(Zw/Zg)etaw (where Zw is the wind metallicitiy and Zg is the interstellar medium metallicity) must be both high and scale steeply with mass. Specifically, we show that zetaw >> 1 and zetaw proportional to vvir^-3 or steeper. In contrast, momentum- or energy-driven outflow models suggest that etaw should scale as vvir^-1 or vvir^-2, respectively, implying that the Zw-Mstar relation should be shallower than the Zg-Mstar relation. [abridged]

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arXiv2011

A tale of two populations: the stellar mass of central and satellite galaxies

We develop a new empirical methodology to study the relation between the stellar mass of galaxies and the mass of their host subhaloes. Our approach is similar to abundance matching, and is based on assigning a stellar mass to each subhalo within a large cosmological N-body simulation. However, we significantly extend the method to include a different treatment for central and satellite galaxies, allowing the stellar mass of satellite galaxies to depend on both the host subhalo mass, and on the halo mass. Unlike in previous studies, our models are constrained by two observations: the stellar mass function of galaxies, and their auto-correlation functions (CFs). We present results for ~10^6 different successful models, showing that the uncertainty in the host subhalo mass reaches a factor of ~10 for a given stellar mass. Our results thus indicate that the host subhalo mass of central and satellite galaxies is poorly constrained by using information coming solely from the abundance and CFs of galaxies. In addition, we demonstrate that the fraction of stellar mass relative to the universal baryon fraction can vary between different models, and can reach ~0.6 for a specific population of low mass galaxies. We conclude that additional observational constraints are needed, in order to better constrain the mass relation between haloes and galaxies. These might include weak lensing, satellite kinematics, or measures of clustering other than the CFs. Moreover, CFs based on larger surveys with a better estimate of the systematic uncertainties are needed.

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arXiv2011

Suzaku Observations of Three FeLoBAL QSOs, SDSS J0943+5417, J1352+4239, and J1723+5553

We present Suzaku observations of three iron low-ionization broad absorption line quasars (FeLoBALs). We detect J1723+5553 (3σ) in the observed 2--10 keV band, and constrain its intrinsic nh column density to nh > 6\times10^{23} \cmsq by modeling its X-ray hardness ratio. We study the broadband spectral index, aox, between the X-ray and UV bands by combining the X-ray measurements and the UV flux extrapolated from 2MASS magnitudes, assuming a range of intrinsic column densities, and then comparing the aox values for the three FeLoBALs with those from a large sample of normal quasars. We find that the FeLoBALs are consistent with the spectral energy distribution (SED) of normal quasars if the intrinsic nh column densities are nh > 7\times10^{23} \cmsq for J0943+5417, nh > 2\times10^{24} \cmsq for J1352+4293, and 6\times10^{23} < \nh < 3\times10^{24} \cmsq for J1723+5553. At these large intrinsic column densities, the optical depth from Thomson scattering can reach \sim6, which will significantly modulate the UV flux. Our results suggest that the X-ray absorbing material is located at a different place from the UV absorbing wind, likely between the X-ray and UV emitting regions. We find a significant kinetic feedback efficiency for FeLoBALs, indicating that the outflows are an important feedback mechanism in quasars.

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arXiv2011

The Intrinsic Fractions and Radio Properties of Low Ionization Broad Absorption Line Quasars

Low-ionization (MgII, FeII, FeIII) broad absorption line quasars (LoBALs) probe a relatively obscured quasar population, and could be at an early evolutionary stage for quasars. We study the intrinsic fractions of LoBALs using the SDSS, 2MASS, and FIRST surveys. We find that the LoBAL fractions of the near infra-red (NIR) and radio samples are approximately 5--7 times higher than those measured in the optical sample. This suggests that the fractions measured in the NIR and radio bands are closer to the intrinsic fractions of the populations, and that the optical fractions are significantly biased due to obscuration effects, similar to high-ionization broad absorption line quasars (HiBALs). We also find that the LoBAL fractions decrease with increasing radio luminosities, again, similar to HiBALs. In addition, we find tentative evidence for high fractions of LoBALs at high NIR luminosities, especially for FeLoBALs with a fraction of ~18 per cent at M_K_s < -31 mag. This population of NIR luminous LoBALs may be at an early evolutionary stage of quasar evolution. We use a two-component model of LoBALs including a pure geometric component and a luminosity dependent component at high NIR luminosities, and obtain better fits than those from a pure geometric model. Therefore, the LoBAL population can be modelled as a hybrid of both the geometric and evolutionary models, where the geometric component constitutes 3.4\pm0.3, 5.8\pm0.4, and 1.5\pm0.3 per cent of the quasar population for BI-LoBALs, AI-LoBALs, and FeLoBALs, respectively. Considering a population of obscured quasars that do not enter the SDSS survey, which could have a much higher LoBAL fraction, we expect that intrinsic fraction of LoBALs could be even higher.

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arXiv2010

On the radiative efficiencies, Eddington ratios, and duty cycles of luminous high-redshift quasars

We investigate the characteristic radiative efficiency ε, Eddington ratio λ, and duty cycle P_0 of high-redshift active galactic nuclei (AGN), drawing on measurements of the AGN luminosity function at z=3-6 and, especially, on recent measurements of quasar clustering at z=3-4.5 from the Sloan Digital Sky Survey. The free parameters of our models are ε, λ, and the normalization, scatter, and redshift evolution of the relation between black hole mass \mbh and halo virial velocity V_vir. We compute the luminosity function from the implied growth of the black hole mass function and the quasar correlation length from the bias of the host halos. We test our adopted formulae for the halo mass function and halo bias against measurements from the large N-body simulation developed by the MICE collaboration. The strong clustering of AGNs observed at z=3 and, especially, at z=4 implies that massive black holes reside in rare, massive dark matter halos. Reproducing the observed luminosity function then requires high efficiency εand/or low Eddington ratio λ, with a lower limit (based on 2σagreement with the measured z=4 correlation length) ε> 0.7λ/(1+0.7λ), implying ε> 0.17 for λ> 0.25. Successful models predict high duty cycles, P_0~0.2, 0.5, and 0.9 at z=3.1, 4.5 and 6, respectively, and they require that the fraction of halo baryons locked in the central black hole is much larger than the locally observed value. The rapid drop in the abundance of the massive and rare host halos at z>7 implies a proportionally rapid decline in the number density of luminous quasars, much stronger than simple extrapolations of the z=3-6 luminosity function would predict. (abridged)

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arXiv2010

Constraints on black hole duty cycles and the black hole-halo relation from SDSS quasar clustering

We use Shen et al.'s (2009) measurements of luminosity-dependent clustering in the SDSS Data Release 5 Quasar Catalog, at redshifts 0.4 < z < 2.5, to constrain the relation between quasar luminosity and host halo mass and to infer the duty cycle f_opt, the fraction of black holes that shine as optically luminous quasars at a given time. We assume a monotonic mean relation between quasar luminosity and host halo mass, with log-normal scatter Σ. For specified f_opt and Σ, matching the observed quasar space density determines the normalization of the luminosity-halo mass relation, from which we predict the clustering bias. The data show no change of bias between the faint and bright halves of the quasar sample but a modest increase in bias for the brightest 10%. At the mean redshift z=1.45 of the sample, the data can be well described either by models with small intrinsic scatter (Σ=0.1 dex) and a duty cycle f_opt=6*10^(-4) or by models with much larger duty cycles and larger values of the scatter. "Continuity equation" models of the black hole mass population imply f_opt > 2*10^(-3) in this range of masses and redshifts, and the combination of this constraint with the clustering measurements implies scatter Σ> 0.4 dex. These findings contrast with those inferred from the much stronger clustering of high-luminosity quasars at z~4, which require minimal scatter between luminosity and halo mass and duty cycles close to one.

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arXiv2010

Further constraining galaxy evolution models through the Size Function of SDSS Early-type galaxies

We discuss how the effective radius Phi(Re) function (ERF) recently worked out by Bernardi et al. (2009) represents a new testbed to improve the current understanding of Semi-analytic Models of Galaxy formation. In particular, we here show that a detailed hierarchical model of structure formation can broadly reproduce the correct peak in the size distribution of local early-type galaxies, although it significantly overpredicts the number of very compact and very large galaxies. This in turn is reflected in the predicted size-mass relation, much flatter than the observed one, due to too large (~3 kpc) low-mass galaxies (<10^11 \msun), and to a non-negligible fraction of compact (< 0.5-1 kpc) and massive galaxies (> 10^11 \msun). We also find that the latter discrepancy is smaller than previously claimed, and limited to only ultracompact (Re < 0.5 kpc) galaxies when considering elliptical-dominated samples. We explore several causes behind these effects. We conclude that the former problem might be linked to the initial conditions, given that large and low-mass galaxies are present at all epochs in the model. The survival of compact and massive galaxies might instead be linked to their very old ages and peculiar merger histories. Overall, knowledge of the galactic stellar mass {\em and} size distributions allows a better understanding of where and how to improve models.

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arXiv2009

Sizes and ages of SDSS ellipticals: Comparison with hierarchical galaxy formation models

In a sample of about 45,700 early-type galaxies extracted from SDSS, we find that the shape, normalization, and dispersion around the mean size-stellar mass relation is the same for young and old systems, provided the stellar mass is greater than 3*10^10 Msun. This is difficult to reproduce in pure passive evolution models, which generically predict older galaxies to be much more compact than younger ones of the same stellar mass. However, this aspect of our measurements is well reproduced by hierarchical models of galaxy formation. Whereas the models predict more compact galaxies at high redshifts, subsequent minor, dry mergers increase the sizes of the more massive objects, resulting in a flat size-age relation at the present time. At lower masses, the models predict that mergers are less frequent, so that the expected anti-correlation between age and size is not completely erased. This is in good agreement with our data: below 3*10^10 Msun, the effective radius R_e is a factor of ~2 lower for older galaxies. These successes of the models are offset by the fact that the predicted sizes have other serious problems, which we discuss.

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arXiv2009

The relative growth of Optical and Radio Quasars in SDSS

We cross-correlate the SDSS DR3 quasar sample with FIRST and the Vestergaard et al. black hole (BH) mass sample to compare the mean accretion histories of optical and radio quasars. We find significant statistical evidence that radio quasars have a higher mean Eddington ratio Lambda at z > 2 with respect to optical quasars, while the situation is clearly reverse at z < 1. At z > 2 radio quasars happen to be less massive than optical quasars; however, as redshift decreases radio quasars appear in increasingly more massive BHs with respect to optical quasars. These two trends imply that radio sources are not a mere random subsample of optical quasars. No clear correlation between radio activity and BH mass and/or accretion rate is evident from our data, pointing to other BH properties, possibly the spin, as the driver of radio activity. We have checked that our main results do not depend on any evident bias. We perform detailed modelling of reasonable accretion histories for optical and radio quasars, finding that radio quasars grow by a factor of a few, at the most, since z ~ 4. The comparison between the predicted mass function of active radio quasars and the observed optical luminosity function of radio quasars, implies a significantly lower probability for lower mass BHs to be radio loud at all epochs, in agreement with what is observed in the local universe.

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arXiv2009

On merger bias and the clustering of quasars

We use the large catalogues of haloes available for the Millennium Simulation to test whether recently merged haloes exhibit stronger large-scale clustering than other haloes of the same mass. This effect could help to understand the very strong clustering of quasars at high redshift. However, we find no statistically significant excess bias for recently merged haloes over the redshift range 2 < z < 5, with the most massive haloes showing an excess of at most ~5%. We also consider galaxies extracted from a semianalytic model built on the Millennium Simulation. At fixed stellar mass, we find an excess bias of ~ 20-30% for recently merged objects, decreasing with increasing stellar mass. The fact that recently-merged galaxies are found in systematically more massive haloes than other galaxies of the same stellar mass accounts for about half of this signal, and perhaps more for high-mass galaxies. The weak merger bias of massive systems suggests that objects of merger-driven nature, such as quasars, do not cluster significantly differently than other objects of the same characteristic mass. We discuss the implications of these results for the interpretation of clustering data with respect to quasar duty cycles, visibility times, and evolution in the black hole-host mass relation.

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arXiv2009

Binary Quasars at High Redshift I: 24 New Quasar Pairs at z ~ 3-4

The clustering of quasars on small scales yields fundamental constraints on models of quasar evolution and the buildup of supermassive black holes. This paper describes the first systematic survey to discover high redshift binary quasars. Using color-selection and photometric redshift techniques, we searched 8142 deg^2 of SDSS imaging data for binary quasar candidates, and confirmed them with follow-up spectroscopy. Our sample of 27 high redshift binaries (24 of them new discoveries) at redshifts 2.9 < z < 4.3 with proper transverse separations 10 kpc < R_{\perp} < 650 kpc increases the number of such objects known by an order of magnitude. Eight members of this sample are very close pairs with R_{\perp} < 100 kpc, and of these close systems four are at z > 3.5. The completeness and efficiency of our well-defined selection algorithm are quantified using simulated photometry and we find that our sample is ~ 50% complete. Our companion paper uses this knowledge to make the first measurement of the small scale clustering (R < 1 Mpc/h comoving) of high-redshift quasars. High redshift binaries constitute exponentially rare coincidences of two extreme (M >~ 10^9 Msun) supermassive black holes. At z ~ 4 there is about one close binary per 10 Gpc^3, thus these could be the highest sigma peaks, the analogs of superclusters, in the early Universe.

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arXiv2009

Binary Quasars at High Redshift II: Sub-Mpc Clustering at z ~ 3-4

We present measurements of the small-scale (0.1<~ r <~ 1 Mpc/h) quasar two-point correlation function at z>2.9, for a flux-limited (i<21) sample of 15 binary quasars compiled by Hennawi et al. (2009). The amplitude of the small-scale clustering increases from z ~ 3 to z ~ 4. The small-scale clustering amplitude is comparable to or lower than power-law extrapolations (with slope gamma=2) from the large-scale correlation function of the i<20.2 quasar sample from the Sloan Digital Sky Survey. Using simple prescriptions relating quasars to dark matter halos, we model the observed small-scale clustering with halo occupation models. Reproducing the large-scale clustering amplitude requires that the active fraction of the black holes in the central galaxies of halos is near unity, but the level of small-scale clustering favors an active fraction of black holes in satellite galaxies 0.1 <~ f_s <~ 0.5 at z >~ 3.

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arXiv2009

The Demography of Super-Massive Black Holes: Growing Monsters at the Heart of Galaxies

Supermassive black holes (BHs) appear to be ubiquitous at the center of all galaxies which have been observed at high enough sensitivities and resolution with the Hubble Space Telescope. Their masses are found to be tightly linked with the masses and velocity dispersions of their host galaxies. On the other hand, BHs are widely held to constitute the central engines of quasars and active galactic nuclei (AGN) in general. It is however still unclear how BHs have grown, and whether they have co-evolved with their hosts. In this Review I discuss how, in ways independent of specific models, constraints on the growth history of BHs and their host galaxies have been set by matching the statistics of local BHs to the emissivity, number density, and clustering properties of AGNs at different cosmological epochs. I also present some new results obtained through a novel numerical code which evolves the BH mass function and clustering adopting broad distributions of Eddington ratios. I finally review BH evolution in a wider cosmological context, connecting BH growth to galaxy evolution.

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arXiv2009

The age dependence of the size-stellar mass relation and some implications

We use a sample of about 48,000 SDSS early-type galaxies to show that older galaxies have smaller half-light radii re and larger velocity dispersions sigma than younger ones of the same stellar mass Mstar. We use the age-corrected luminosity Lcorr as a proxy for Mstar to minimize biases: below Lcorr~1e11 Lsun, galaxies with age ~11 Gyrs have re smaller by 40% and sigma larger by 25%, compared to galaxies that are ~4 Gyrs younger. The sizes and velocity dispersions of more luminous galaxies vary by less than 15%, whatever their age, a challenge for current galaxy formation models. A closer check reveals that the lowering in the dispersion is caused by older galaxies that show a significant departure from the re--Lcorr and sigma--Lcorr relations at high Lcorr. Such features might find an explanation in models where more massive galaxies undergo more minor mergers than less massive galaxies at late times, thus causing a break in the homology. In terms of the Fundamental Plane of early-type galaxies, the data indicate that all galaxies show a significant and similar increase in the dynamical-to-stellar mass ratio with increasing mass, independent of their age. However, older galaxies have smaller Mdyn/Mstar ratios than objects which formed more recently. These findings may suggest that lower mass galaxies and, at fixed stellar mass, higher redshift galaxies, formed from gas-richer progenitors, thus underwent more dissipation and contraction.

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arXiv2008

Quasar Clustering from SDSS DR5: Dependences on Physical Properties

Using a homogenous sample of 38,208 quasars with a sky coverage of $4000 {\rm deg^2}$ drawn from the SDSS Data Release Five quasar catalog, we study the dependence of quasar clustering on luminosity, virial black hole mass, quasar color, and radio loudness. At $z<2.5$, quasar clustering depends weakly on luminosity and virial black hole mass, with typical uncertainty levels $\sim 10%$ for the measured correlation lengths. These weak dependences are consistent with models in which substantial scatter between quasar luminosity, virial black hole mass and the host dark matter halo mass has diluted any clustering difference, where halo mass is assumed to be the relevant quantity that best correlates with clustering strength. However, the most luminous and most massive quasars are more strongly clustered (at the $\sim 2σ$ level) than the remainder of the sample, which we attribute to the rapid increase of the bias factor at the high-mass end of host halos. We do not observe a strong dependence of clustering strength on quasar colors within our sample. On the other hand, radio-loud quasars are more strongly clustered than are radio-quiet quasars matched in redshift and optical luminosity (or virial black hole mass), consistent with local observations of radio galaxies and radio-loud type 2 AGN. Thus radio-loud quasars reside in more massive and denser environments in the biased halo clustering picture. Using the Sheth et al.(2001) formula for the linear halo bias, the estimated host halo mass for radio-loud quasars is $\sim 10^{13} h^{-1}M_\odot$, compared to $\sim 2\times 10^{12} h^{-1}M_\odot$ for radio-quiet quasar hosts at $z\sim 1.5$.

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arXiv2008

Self-Consistent Models of the AGN and Black Hole Populations: Duty Cycles, Accretion Rates, and the Mean Radiative Efficiency

We construct evolutionary models of the populations of AGN and supermassive black holes, in which the black hole mass function grows at the rate implied by the observed luminosity function, given assumptions about the radiative efficiency and the Eddington ratio. We draw on a variety of recent X-ray and optical measurements to estimate the bolometric AGN luminosity function and compare to X-ray background data and the independent estimate of Hopkins et al. (2007) to assess remaining systematic uncertainties. The integrated AGN emissivity closely tracks the cosmic star formation history, suggesting that star formation and black hole growth are closely linked at all redshifts. Observational uncertainties in the local black hole mass function remain substantial, with estimates of the integrated black hole mass density ρ_BH spanning the range 3-5.5x10^5 Msun/Mpc^3. We find good agreement with estimates of the local mass function for a reference model where all active black holes have efficiency \eps=0.065 and L_bol/L_Edd~0.4. In this model, the duty cycle of 10^9 Msun black holes declines from 0.07 at z=3 to 0.004 at z=1 and 0.0001 at z=0. The decline is shallower for less massive black holes, a signature of "downsizing" evolution in which more massive black holes build their mass earlier. The predicted duty cycles and AGN clustering bias in this model are in reasonable accord with observational estimates. If the typical Eddington ratio declines at z<2, then the "downsizing" of black hole growth is less pronounced. Matching the integrated AGN emissivity to the local black hole mass density implies \eps=0.075 (ρ_BH/4.5x10^5 Msun/Mpc^3)^{-1} for our standard luminosity function estimate (25% higher for Hopkins et al.'s), lower than the values \eps=0.16-0.20 predicted by MHD simulations of disk accretion.

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arXiv2008

Dependence of the BALQSO fraction on Radio Luminosity

We find that the fraction of classical Broad Absorption Line quasars (BALQSOs) among the FIRST radio sources in the Sloan Data Release 3, is 20.5^{+7.3}_{-5.9}% at the faintest radio powers detected (L_{\rm 1.4 GHz}~10^{32} erg/s), and rapidly drops to <8% at L_{\rm 1.4 GHz}~3*10^{33} erg/s. Similarly, adopting the broader Absorption Index (AI) definition of Trump et al. (2006) we find the fraction of radio BALQSOs to be 44^{+8.1}_{-7.8}% reducing to 23.1^{+7.3}_{-6.1}% at high luminosities. While the high fraction at low radio power is consistent with the recent near-IR estimates by Dai et al. (2008), the lower fraction at high radio powers is intriguing and confirms previous claims based on smaller samples. The trend is independent of the redshift range, the optical and radio flux selection limits, or the exact definition of a radio match. We also find that at fixed optical magnitude, the highest bins of radio luminosity are preferentially populated by non-BALQSOs, consistent with the overall trend. We do find, however, that those quasars identified as AI-BALQSOs but \emph{not} under the classical definition, do not show a significant drop in their fraction as a function of radio power, further supporting independent claims for which these sources, characterized by lower equivalent width, may represent an independent class with respect to the classical BALQSOs. We find the balnicity index, a measure of the absorption trough in BALQSOs, and the mean maximum wind velocity to be roughly constant at all radio powers. We discuss several plausible physical models which may explain the observed fast drop in the fraction of the classical BALQSOs with increasing radio power, \emph{although no one is entirely satisfactory}. (abridged).

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arXiv2008

The Evolution of the M_BH-sigma relation Inferred from the Age Distribution of Local Early-Type Galaxies and AGN Evolution

We utilize the local velocity dispersion function (VDF) of spheroids, together with their inferred age--distributions, to predict the VDF at higher redshifts (0 1.3 excluded at the 99% confidence level. A direct match between the characteristic BH mass in the VDF--based and quasar LF--based BH mass functions also yields a mean Eddington ratio of lambda ~ 0.5-1 that is roughly constant within 0 0.23 for alpha>1.5. abridged

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