arXiv subjects

Francesco Shankar

Publications and source records attributed to Francesco Shankar.

At least 55 records · Page 3Linked to original sources

arXiv2022

A panchromatic view of infrared quasars: excess star formation and radio emission in the most heavily obscured systems

To understand the Active Galactic Nuclei (AGN) phenomenon and their impact on the evolution of galaxies, a complete AGN census is required; however, finding heavily obscured AGNs is observationally challenging. Here we use the deep and extensive multi-wavelength data in the COSMOS field to select a complete sample of 578 infrared (IR) quasars ($L_{\rm AGN,IR}>10^{45}\rm \: erg\: s^{-1}$) at $z<3$, with minimal obscuration bias, using detailed UV-to-far IR spectral energy distribution (SED) fitting. We complement our SED constraints with X-ray and radio observations to further investigate the properties of the sample. Overall, 322 of the IR quasars are detected by Chandra and have individual X-ray spectral constraints. From a combination of X-ray stacking and $L_{\rm 2-10\rm keV}$ - $L_{\rm 6\: μm}$ analyses, we show that the majority of the X-ray faint and undetected quasars are heavily obscured (many are likely Compton thick), highlighting the effectiveness of the mid-IR band to find obscured AGNs. We find that 355 ($\approx$61%) IR quasars are obscured ($N_{\rm H}>10^{22}\rm \: cm^{-2}$) and identify differences in the average properties between the obscured and unobscured quasars: (1) obscured quasars have star-formation rates $\approx 3$ times higher than unobscured systems for no significant difference in stellar mass and (2) obscured quasars have stronger radio emission than unobscured systems, with a radio-loudness parameter $\approx 0.2 \rm \: dex$ higher. These results are inconsistent with a simple orientation model but in general agreement with either extreme host-galaxy obscuration towards the obscured quasars or a scenario where obscured quasars are an early phase in the evolution of quasars.

astro-ph.GA↗

arXiv2022

Testing the key role of the stellar mass-halo mass relation in galaxy merger rates and morphologies via DECODE, a novel Discrete statistical sEmi-empiriCal mODEl

The relative roles of mergers and star formation in regulating galaxy growth are still a matter of intense debate. We here present our DECODE, a new Discrete statistical sEmi-empiriCal mODEl specifically designed to predict rapidly and efficiently, in a full cosmological context, galaxy assembly and merger histories for any given input stellar mass-halo mass (SMHM) relation. DECODE generates object-by-object dark matter merger trees (hence discrete) from accurate subhalo mass and infall redshift probability functions (hence statistical) for all subhaloes, including those residing within other subhaloes, with virtually no resolution limits on mass or volume. Merger trees are then converted into galaxy assembly histories via an input, redshift dependent SMHM relation, which is highly sensitive to the significant systematics in the galaxy stellar mass function and on its evolution with cosmic time. DECODE can accurately reproduce the predicted mean galaxy merger rates and assembly histories of hydrodynamic simulations and semi-analytic models, when adopting in input their SMHM relations. In the present work we use DECODE to prove that only SMHM relations implied by stellar mass functions characterized by large abundances of massive galaxies and significant redshift evolution, at least at $M_\star \gtrsim 10^{11} \, M_\odot$, can simultaneously reproduce the local abundances of satellite galaxies, the galaxy (major merger) pairs since $z \sim 3$, and the growth of Brightest Cluster Galaxies. The same models can also reproduce the local fraction of elliptical galaxies, on the assumption that these are strictly formed by major mergers, but not the full bulge-to-disc ratio distributions, which require additional processes.

astro-ph.CO↗

arXiv2022

Coincidence between morphology and star-formation activity through cosmic time: the impact of the bulge growth

The origin of the quenching in galaxies is still highly debated. Different scenarios and processes are proposed. We use multi-band (400-1600 nm) bulge-disc decompositions of massive galaxies in the redshift range 0 0.2) are equally distributed in number between star forming and passive regions. The vast majority of bulges in the Main Sequence galaxies are quiescent, while star formation is localized in the disc component. Our current findings underline a strong correlation between the presence of the bulge and the star formation state of the galaxy. A bulge, if present, is often quiescent, independently of the morphology or the star formation activity of the host galaxy. Additionally, if a galaxy is quiescent, with a large probability, is hosting a bulge. Conversely, if the galaxy has a disky shape is highly probable to be star forming.

astro-ph.GA↗

arXiv2022

An Eddington ratio-driven origin for the ${\rm L}_{\rm X}-{\rm M}_{*}$ relation in quiescent and star forming active galaxies

A mild correlation exists in active galaxies between the mean black hole accretion, as traced by the mean X-ray luminosity $\left<{\rm L}_{\rm X}\right>$, and the host galaxy stellar mass M$_*$, characterised by a normalisation steadily decreasing with cosmic time and lower in more quiescent galaxies. We create comprehensive semi-empirical mock catalogues of active black holes to pin down which parameters control the shape and evolution of the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation of X-ray detected active galaxies. We find that the normalisation of the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation is largely independent of the fraction of active galaxies (the duty cycle), but strongly dependent on the mean Eddington ratio, when adopting a constant underlying M$_{\rm BH}-{\rm M}_*$ relation as suggested by observational studies. The data point to a decreasing mean Eddington ratio with cosmic time and with galaxy stellar mass at fixed redshift. Our data can be reproduced by black holes and galaxies evolving on similar M$_{\rm BH}-{\rm M}_*$ relations but progressively decreasing their average Eddington ratios, mean X-ray luminosities, and specific star formation rates, when moving from the starburst to the quiescent phase. Models consistent with the observed $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation and independent measurements of the mean Eddington ratios, are characterised by M$_{\rm BH}-{\rm M}_*$ relations lower than those derived from dynamically measured local black holes. Our results point to the $\left<{\rm L}_{\rm X}\right>-{\rm M}_*$ relation as a powerful diagnostic to: 1) probe black hole-galaxy scaling relations and the level of accretion onto black holes; 2) efficiently break the degeneracies between duty cycles and accretion rates in cosmological models of black holes.

astro-ph.GA↗

arXiv2022

The low-end of the black hole mass function at cosmic dawn

Understanding the formation and growth of supermassive black holes (SMBHs) at high redshift represents a major challenge for theoretical models. In this work we investigate the early evolution of the first SMBHs by constraining their distribution in mass and luminosity at $z > 4$. In particular, we focus on the poorly explored low-mass end of the nuclear black hole (BH) distribution down to $z \simeq 4$, and explore its connection with the nature of the first BH seeds and the processes governing their mass growth. To this aim, we have developed CAT (Cosmic Archaeology Tool), a new semi-analytic model that describes the formation of the first stars and black holes in a self-consistent way and follows the co-evolution of nuclear BHs and their host galaxies for a representative population at $z > 4$. We find that current observational constraints favour models where the growth of BH seeds is Eddington limited and occurs at the Bondi-Hoyle-Lyttleton rate or where super-Eddington accretion occurs via a slim disk during gas rich galaxy mergers. The main difference between these two model variants lies at the low-end of the predicted mass and luminosity functions at $4 \le z \le 6$, where a clear gap appears in the first model, reflecting the stunted growth of light BH seeds formed as remnants of the first stars. Detecting this signature will be extremely challenging even for the future generation of space observatories, such as JWST, Athena and Lynx.

astro-ph.GA↗

arXiv2021

The weak dependence of velocity dispersion on disk fractions, mass-to-light ratio and redshift: Implications for galaxy and black hole evolution

Velocity dispersion ($σ$) is a key driver for galaxy structure and evolution. We here present a comprehensive semi-empirical approach to compute $σ$ via detailed Jeans modelling assuming both a constant and scale-dependent mass-to-light ratio $M^*/L$. We compare with a large sample of local galaxies from MaNGA and find that both models can reproduce the Faber-Jackson (FJ) relation and the weak dependence of $σ$ on bulge-to-total ratio $B/T$ (for $B/T\gtrsim 0.25$). The dynamical-to-stellar mass ratio within $R\lesssim R_e$ can be fully accounted for by a gradient in $M^*/L$. We then build velocity dispersion evolutionary tracks $σ_{ap}[M^*,z]$ (within an aperture) along the main progenitor dark matter haloes assigning stellar masses, effective radii and Sersic indices via a variety of abundance matching and empirically motivated relations. We find: 1) clear evidence for downsizing in $σ_{ap}[M^*,z]$ along the progenitor tracks; 2) at fixed stellar mass $σ\propto(1+z)^{0.2-0.3}$ depending on the presence or not of a gradient in $M^*/L$. We extract $σ_{ap}[M^*,z]$ from the TNG50 hydrodynamic simulation and find very similar results to our models with constant $M^*/L$. The increasing dark matter fraction within $R_e$ tends to flatten the $σ_{ap}[M^*,z]$ along the progenitors at $z \gtrsim 1$ in constant $M^*/L$ models, while $σ_{ap}[M^*,z]$ have a steeper evolution in the presence of a stellar gradient. We then show that a combination of mergers and gas accretion are likely responsible for the constant or increasing $σ_{ap}[M^*,z]$ with time. Finally, our $σ_{ap}[M^*,z]$ are consistent with a nearly constant and steep $M_{bh}-σ$ relation at $z\lesssim 2$, with black hole masses derived from the $L_X-M^*$ relation.

astro-ph.GA↗

arXiv2021

The Black Hole Mass Function Across Cosmic Times I. Stellar Black Holes and Light Seed Distribution

This is the first paper in a series aimed at modeling the black hole (BH) mass function, from the stellar to the intermediate to the (super)massive regime. In the present work we focus on stellar BHs and provide an ab-initio computation of their mass function across cosmic times. Specifically, we exploit the state-of-the-art stellar and binary evolutionary code \texttt{SEVN}, and couple its outputs with redshift-dependent galaxy statistics and empirical scaling relations involving galaxy metallicity, star-formation rate and stellar mass. The resulting relic mass function ${\rm d}N/{\rm d}V{\rm d}\log m_\bullet$ as a function of the BH mass $m_\bullet$ features a rather flat shape up to $m_\bullet\approx 50\, M_\odot$ and then a log-normal decline for larger masses, while its overall normalization at a given mass increases with decreasing redshift. We highlight the contribution to the local mass function from isolated stars evolving into BHs and from binary stellar systems ending up in single or binary BHs. We also include the distortion on the mass function induced by binary BH mergers, finding that it has a minor effect at the high-mass end. We estimate a local stellar BH relic mass density of $ρ_\bullet\approx 5\times 10^7\, M_\odot$ Mpc$^{-3}$, which exceeds by more than two orders of magnitude that in supermassive BHs; this translates into an energy density parameter $Ω_\bullet\approx 4\times 10^{-4}$, implying that the total mass in stellar BHs amounts to $\lesssim 1\%$ of the local baryonic matter. We show how our mass function for merging BH binaries compares with the recent estimates from gravitational wave observations by LIGO/Virgo, and discuss the possible implications for dynamical formation of BH binaries in dense environments like star clusters. [abridged]

astro-ph.GA↗

arXiv2021

Rejuvenation triggers nuclear activity in nearby galaxies

Feedback, in particular from active galactic nuclei (AGN), is believed to play a crucial role in the evolution of galaxies. In the local Universe, many galaxies with an AGN are indeed observed to reside in the so-called green valley, usually interpreted as a transition phase from a blue star-forming to a red quenched state. We use data from the Sloan Digital Sky Survey to show that such an interpretation requires substantial revision. Optically-selected nearby AGN galaxies follow exponentially declining star formation histories, as normal galaxies of similar stellar and dark matter halo mass, reaching in the recent past ($\sim$0.1 Gyr ago) star formation rate levels consistent with a quiescent population. However, we find that local AGN galaxies have experienced a sudden increase in their star formation rate, unfolding on timescales similar to those typical of AGN activity, suggesting that both star formation and AGN activity were triggered simultaneously. We find that this quenching followed by an enhancement in the star formation rate is common to AGN galaxies and more pronounced in early type galaxies. Our results demonstrate that local AGN galaxies are not just a simple transition type between star-forming and quiescent galaxies as previously postulated.

astro-ph.GA↗

arXiv2021

Probing modified gravity with magnetically levitated resonators

We present an experimental procedure, based on Meissner effect levitation of neodymium ferromagnets, as a method of measuring the gravitational interactions between mg masses. The scheme consists of two superconducting lead traps, with a magnet levitating in each trap. The levitating magnets behave as harmonic oscillators, and by carefully driving the motion of one magnet on resonance with the other, we find that it should be easily possible to measure the gravitational field produced by a 4~mg sphere, with the gravitational attraction from masses as small as 30~$μ$g predicted to be measurable within realistic a realistic measurement time frame. We apply this acceleration sensitivity to one concrete example and show the ability of testing models of modified Newtonian dynamics.

gr-qc↗

arXiv2021

The role of scatter and satellites in shaping the large-scale clustering of X-ray AGN as a function of host galaxy stellar mass

The co-evolution between central supermassive black holes (BH), their host galaxies, and dark matter halos is still a matter of intense debate. Present theoretical models suffer from large uncertainties and degeneracies, for example, between the fraction of accreting sources and their characteristic accretion rate. In recent work we showed that Active Galactic Nuclei (AGN) clustering represents a powerful tool to break degeneracies when analysed in terms of mean BH mass, and that AGN bias at fixed stellar mass is largely independent of most of the input parameters, such as the AGN duty cycle and the mean scaling between BH mass and host galaxy stellar mass. In this paper we take advantage of our improved semi-empirical methodology and recent clustering data derived from large AGN samples at $z \sim 1.2$, demonstrate that the AGN bias as a function of host galaxy stellar mass is a crucial diagnostic of the BH--galaxy connection, and is highly dependent on the scatter around the BH mass--galaxy mass scaling relation and on the relative fraction of satellite and central active BHs. Current data at $z \sim 1.2$ favour relatively high values of AGN in satellites, pointing to a major role of disc instabilities in triggering AGN, unless a high minimum host halo mass is assumed. The data are not decisive on the magnitude/covariance of the BH-galaxy scatter at $z \sim 1.2$ and intermediate host masses $M_\mathrm{star} \lesssim 10^{11} \,\mathrm{M}_\odot$. However, future surveys like Euclid/LSST will be pivotal in shedding light on the BH--galaxy co-evolution.

astro-ph.GA↗

arXiv2021

The size function of massive satellites from the $R_e-R_h$ and $M_{star}-M_h$ relations: constraining the role of environment

In previous work we showed that a semi-empirical model in which galaxies in host dark matter haloes are assigned stellar masses via a stellar mass-halo mass (SMHM) relation and sizes ($R_e$) via a linear and tight $R_e-R_h$ relation, can faithfully reproduce the size function of local SDSS central galaxies and the strong size evolution of massive galaxies (MGs, $M_{\rm star}>10^{11.2}M_\odot$). In this third paper of the series, we focus on the population of satellite MGs. We find that without any additional calibration and irrespective of the exact SMHM relation, fraction of quenched galaxies or level of stellar stripping, the same model is able to reproduce the local size function of quiescent satellite MGs in SDSS. In addition, the same model can reproduce the puzzling weak dependence of mean size on host halo mass for both central and satellite galaxies. The model also matches the size function of starforming satellite MGs, after assuming that some of them transform into massive lenticulars in a few Gyr after infalling in the group/cluster environment. However, the vast majority of satellite lenticulars is predicted to form before infall. The $R_e-R_h$ appears to be fundamental to connect galaxies and their host haloes.

astro-ph.GA↗

arXiv2021

Building robust AGN mock catalogs to unveil black hole evolution and for survey planning

The statistical distributions of active galactic nuclei (AGN), i.e. accreting supermassive black holes (BHs), in mass, space and time, are controlled by a series of key properties, namely the BH-galaxy scaling relations, Eddington ratio distributions and fraction of active BHs (duty cycle). Shedding light on these properties yields strong constraints on the AGN triggering mechanisms whilst providing a clear baseline to create useful mock catalogues for the planning of large galaxy surveys. We here delineate a robust methodology to create mock AGN catalogs built on top of large N-body dark matter simulations via state-of-the-art semi-empirical models. We show that by using as independent tests the AGN clustering at fixed X-ray luminosity, galaxy stellar mass and BH mass, along with the fraction of AGN in groups and clusters, it is possible to significantly narrow down the choice in the relation between black hole mass and host galaxy stellar mass, the duty cycle, and the average Eddington ratio distribution, delivering well-suited constraints to guide cosmological models for the co-evolution of BHs and galaxies. Avoiding such a step-by-step methodology inevitably leads to strong degeneracies in the final mock catalogs, severely limiting their usefulness in understanding AGN evolution and in survey planning and testing.

astro-ph.CO↗

arXiv2020

The relationship between fine galaxy stellar morphology and star formation activity in cosmological simulations: a deep learning view

Hydrodynamical simulations of galaxy formation and evolution attempt to fully model the physics that shapes galaxies. The agreement between the morphology of simulated and real galaxies, and the way the morphological types are distributed across galaxy scaling relations are important probes of our knowledge of galaxy formation physics. Here we propose an unsupervised deep learning approach to perform a stringent test of the fine morphological structure of galaxies coming from the Illustris and IllustrisTNG (TNG100 and TNG50) simulations against observations from a subsample of the Sloan Digital Sky Survey. Our framework is based on PixelCNN, an autoregressive model for image generation with an explicit likelihood. We adopt a strategy that combines the output of two PixelCNN networks in a metric that isolates the fine morphological details of galaxies from the sky background. We are able to \emph{quantitatively} identify the improvements of IllustrisTNG, particularly in the high-resolution TNG50 run, over the original Illustris. However, we find that the fine details of galaxy structure are still different between observed and simulated galaxies. This difference is driven by small, more spheroidal, and quenched galaxies which are globally less accurate regardless of resolution and which have experienced little improvement between the three simulations explored. We speculate that this disagreement, that is less severe for quenched disky galaxies, may stem from a still too coarse numerical resolution, which struggles to properly capture the inner, dense regions of quenched spheroidal galaxies.

astro-ph.GA↗

arXiv2020

Using Unreal Engine to Visualize a Cosmological Volume

In this work we present `Astera', a cosmological visualization tool that renders a mock universe in real time using Unreal Engine 4. The large scale structure of the cosmic web is hard to visualize in two dimensions, and a 3D real time projection of this distribution allows for an unprecedented view of the large scale universe, with visually accurate galaxies placed in a dynamic 3D world. The underlying data are based on empirical relations assigned using results from N-Body dark matter simulations, and are matched to galaxies with similar morphologies and sizes, images of which are extracted from the Sloan Digital Sky Survey. Within Unreal Engine 4, galaxy images are transformed into textures and dynamic materials (with appropriate transparency) that are applied to static mesh objects with appropriate sizes and locations. To ensure excellent performance, these static meshes are `instanced' to utilize the full capabilities of a graphics processing unit. Additional components include a dynamic system for representing accelerated-time active galactic nuclei. The end result is a visually realistic large scale universe that can be explored by a user in real time, with accurate large scale structure. Astera is not yet ready for public release, but we are exploring options to make different versions of the code available for both research and outreach applications.

astro-ph.GA↗

arXiv2020

Accretion History of AGN III: Radiative Efficiency and AGN Contribution to Reionization

The cosmic history of supermassive black hole (SMBH) growth is important for understanding galaxy evolution, reionization and the physics of accretion. Recent NuSTAR, Swift-BAT and \textit{Chandra} hard X-ray surveys have provided new constraints on the space density of heavily obscured Active Galactic Nuclei (AGN). Using the new X-ray luminosity function derived from these data, we here estimate the accretion efficiency of SMBHs and their contribution to reionization. We calculate the total ionizing radiation from active galactic nuclei (AGN) as a function of redshift, based on the X radiation and distribution of obscuring column density, converted to UV wavelengths. Limiting the luminosity function to unobscured AGN only, our results agree with current UV luminosity functions of unobscured AGN. For realistic assumptions about the escape fraction, the contribution of all AGN to cosmic reionization is $\sim4$ times lower than the galaxy contribution (23% at $z\sim6$). Our results also offer an observationally constrained prescription that can be used in simulations or models of galaxy evolution. To estimate the average efficiency with which supermassive black holes convert mass to light, we compare the total radiated energy, converted from X-ray light using a bolometric correction, to the most recent local black hole mass density. The most likely value, $η\sim 0.3-0.34$, approaches the theoretical limit for a maximally rotating Kerr black hole, $η=0.42$, implying that on average growing supermassive black holes are spinning rapidly.

astro-ph.HE↗

arXiv2020

Correlation of structure and stellar properties of galaxies in Stripe 82

Establishing a correlation (or lack thereof) between the bimodal colour distribution of galaxies and their structural parameters is crucial to understand the origin of bimodality. To achieve that, we have performed 2D mass-based structural decomposition (bulge+disc) of all disc galaxies (total$=$1263) in the Herschel imaging area of the Stripe 82 region using $K_s$ band images from the VICS82 survey. The scaling relations thus derived are found to reflect the internal kinematics and are employed in combination to select an indubitable set of classical and pseudo bulge hosting disc galaxies. The rest of the galaxies ($<20\%$) are marked as discs with "ambiguous" bulges. Pseudo and classical bulge disc galaxies exhibit clear bimodality in terms of all stellar parameters ($M_*$, sSFR, $r-K_s$). All pseudo bulge disc galaxies are blue and star-forming and all classical bulge disc galaxies are red and quiescent with less than $5\%$ digressions. Ambiguous bulge disc galaxies are intermittent to pseudo and classical bulge disc galaxies in the distribution of all structural and stellar parameters. $Δ$$\langleμ_{eb}\rangle$ - based on the placement of bulges on the Kormendy relation - is found to be the most efficient single structural indicator of both bulge type and stellar activity. The placement of ambiguous bulge disc galaxies on scaling relations and fundamental plane, in addition to their peculiar stellar properties suggest that they are dominantly a part of the green valley.

astro-ph.GA↗

arXiv2020

The Rise of Active Galactic Nuclei in the GAlaxy Evolution and Assembly semi-analytic model

We present a new implementation of the GAlaxy Evolution and Assembly (GAEA) semi-analytic model, that features an improved modelling of the process of cold gas accretion onto supermassive black hole (SMBHs), derived from both analytic arguments and high-resolution simulations. We consider different scenarios for the loss of angular momentum required for the available cold gas to be accreted onto the central SMBHs, and we compare different combinations of triggering mechanisms, including galaxy mergers and disc instabilities in star forming discs. We compare our predictions with the luminosity function (LF) observed for Active Galactic Nuclei (AGN) and we confirm that a non-instantaneous accretion timescale (either in the form of a low-angular momentum reservoir or as an assumed light curve evolution) is needed in order to reproduce the measured evolution of the AGN-LF and the so-called AGN-downsizing trend. Moreover, we also study the impact of AGN feedback, in the form of AGN-driven outflows, on the SF properties of model galaxies, using prescriptions derived both from empirical studies or from numerical experiments. We show that AGN-driven outflows are effective in suppressing the residual star formation rate in massive galaxies ($> 10^{11} M_\odot$) without changing their overall assembly history. These winds also affect the SFR of lower mass galaxies, resulting in a too large fraction of passive galaxies at $< 10^{10} M_\odot$. Finally, we study the Eddington ratio distribution as a function of SMBH mass, showing that only objects more massive than $10^8 M_\odot$ are already in a self-regulated state as inferred from observations.

astro-ph.CO↗

arXiv2020

The case for the fundamental $M_{\rm BH}$-$σ$ relation

Strong scaling relations between host galaxy properties (such as stellar mass, bulge mass, luminosity, effective radius etc) and their nuclear supermassive black hole's mass point towards a close co-evolution. In this work, we first review previous efforts supporting the fundamental importance of the relation between supermassive black hole mass and stellar velocity dispersion ($M_{\rm BH}$-$σ_{\rm e}$). We then present further original work supporting this claim via analysis of residuals and principal component analysis applied to some among the latest compilations of local galaxy samples with dynamically measured supermassive black hole masses. We conclude with a review of the main physical scenarios in favour of the existence of a $M_{\rm BH}$-$σ_{\rm e}$ relation, with a focus on momentum-driven outflows.

astro-ph.GA↗