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David M. Alexander

Publications and source records attributed to David M. Alexander.

At least 37 records · Page 2Linked to original sources
arXiv2024

The NuSTAR Serendipitous Survey: the 80-month catalog and source properties of the high-energy emitting AGN and quasar population

We present a catalog of hard X-ray serendipitous sources detected in the first 80 months of observations by the Nuclear Spectroscopic Telescope Array (NuSTAR). The NuSTAR serendipitous survey 80-month (NSS80) catalog has an unprecedented $\sim$ 62 Ms of effective exposure time over 894 unique fields (a factor of three increase over the 40-month catalog), with an areal coverage of $\sim $36 deg$^2$, larger than all NuSTAR extragalactic surveys. NSS80 provides 1274 hard X-ray sources in the $3-24$ keV band (822 new detections compared to the previous 40-month catalog). Approximately 76% of the NuSTAR sources have lower-energy ($<10$ keV) X-ray counterparts from Chandra, XMM-Newton, and Swift-XRT. We have undertaken an extensive campaign of ground-based spectroscopic follow-up to obtain new source redshifts and classifications for 427 sources. Combining these with existing archival spectroscopy provides redshifts for 550 NSS80 sources, of which 547 are classified. The sample is primarily composed of active galactic nuclei (AGN), detected over a large range in redshift ($z$ = 0.012-3.43), but also includes 58 spectroscopically confirmed Galactic sources. In addition, five AGN/galaxy pairs, one dual AGN system, one BL Lac candidate, and a hotspot of 4C 74.26 (radio quasar) have been identified. The median rest-frame $10-40$ keV luminosity and redshift of the NSS80 are $\langle{L_\mathrm{10-40 keV}}\rangle$ = 1.2 $\times$ 10$^{44}$ erg s$^{-1}$ and $\langle z \rangle = 0.56$. We investigate the optical properties and construct composite optical spectra to search for subtle signatures not present in the individual spectra, finding an excess of redder BL AGN compared to optical quasar surveys predominantly due to the presence of the host-galaxy and, at least in part, due to dust obscuration.

astro-ph.HE↗
arXiv2024

Cosmic evolution of radio-excess AGNs in quiescent and star-forming galaxies across $0 < z < 4$

Recent deep and wide radio surveys extend the studies for radio-excess active galactic nuclei (radio-AGNs) to lower luminosities and higher redshifts, providing new insights into the abundance and physical origin of radio-AGNs. Here we focus on the cosmic evolution, physical properties and AGN-host galaxy connections of radio-AGNs selected from a sample of ~ 500,000 galaxies at 0 < z < 4 in GOODS-N, GOODS-S, and COSMOS fields. Combining deep radio data with multi-band, de-blended far-infrared (FIR) and sub-millimeter data, we identify 1162 radio-AGNs through radio excess relative to the FIR-radio relation. We study the cosmic evolution of 1.4 GHz radio luminosity functions (RLFs) for star-forming galaxies (SFGs) and radio-AGNs, which are well described by a pure luminosity evolution of $L_*\propto (1+z)^{-0.31z+3.41}$ and a pure density evolution of $Φ_*\propto (1+z)^{-0.80z+2.88}$, respectively. We derive the turnover luminosity above which the number density of radio-AGNs surpasses that of SFGs. This crossover luminosity increases as increasing redshift, from $10^{22.9}$ W Hz$^{-1}$ at z ~ 0 to $10^{25.2}$ W Hz$^{-1}$ at z ~ 4. At full redshift range (0 < z < 4), we further derive the probability ($p_{radio}$) of SFGs and quiescent galaxies (QGs) hosting a radio-AGN as a function of stellar mass ($M_*$), radio luminosity ($L_R$), and redshift (z), which yields $p_{radio}\propto (1+z)^{3.54}M_*^{1.02}L_R^{-0.90}$ for SFGs, and $p_{radio}\propto (1+z)^{2.38}M_*^{1.39}L_R^{-0.60}$ for QGs, respectively. It indicates that radio-AGNs in QGs prefer to reside in more massive galaxies with larger $L_R$ than those in SFGs, and radio-AGN fraction increases towards higher redshift in both SFGs and QGs with a more rapid increase in SFGs. Further, we find that the radio-AGN fraction depends on accretion states of BHs and redshift in SFGs, while in QGs it also depends on BH (or galaxy) mass.

astro-ph.GA↗
arXiv2023

The High Energy X-ray Probe (HEX-P): Bringing the Cosmic X-ray Background into Focus

Since the discovery of the cosmic X-ray background (CXB), astronomers have strived to understand the accreting supermassive black holes (SMBHs) contributing to its peak in the 10-40 keV band. Existing soft X-ray telescopes could study this population up to only 10 keV, and, while NuSTAR (focusing on 3--24 keV) made great progress, it also left significant uncertainties in characterizing the hard X-ray population, crucial for calibrating current population synthesis models. This paper presents an in-depth analysis of simulations of two extragalactic surveys (deep and wide) with the High-Energy X-ray Probe (HEX-P), each observed for 2 Ms. Applying established source detection techniques, we show that HEX-P surveys will reach a flux of $\sim$10$^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the 10-40 keV band, an order of magnitude fainter than current NuSTAR surveys. With the large sample of new hard X-ray detected sources ($\sim2000$), we showcase HEX-P's ability to resolve more than 80% of the CXB up to 40 keV into individual sources. The expected precision of HEX-P's resolved background measurement will allow us to distinguish between population synthesis models of SMBH growth. HEX-P leverages accurate broadband (0.5-40 keV) spectral analysis and the combination of soft and hard X-ray colors to provide obscuration constraints even for the fainter sources, with the overall objective of measuring the Compton-thick fraction. With unprecedented sensitivity in the 10--40 keV band, HEX-P will explore the hard X-ray emission from AGN to flux limits never reached before, thus expanding the parameter space for serendipitous discoveries. Consequently, it is plausible that new models will be needed to capture the population HEX-P will unveil.

astro-ph.HE↗
arXiv2023

Probing the roles of orientation and multi-scale gas distributions in shaping the obscuration of Active Galactic Nuclei through cosmic time

The origin of obscuration in Active Galactic Nuclei (AGN) is still an open debate. In particular, it is unclear what drives the relative contributions to the line-of-sight column densities from galaxy-scale and torus-linked obscuration. The latter source is expected to play a significant role in Unification Models, while the former is thought to be relevant in both Unification and Evolutionary Models. In this work, we make use of a combination of cosmological semi-analytic models and semi-empirical prescriptions for the properties of galaxies and AGN, to study AGN obscuration. We consider a detailed object-by-object modelling of AGN evolution, including different AGN light curves (LCs), gas density profiles, and also AGN feedback-induced gas cavities. Irrespective of our assumptions on specific AGN LC or galaxy gas fractions, we find that, on the strict assumption of an exponential profile for the gas component, galaxy-scale obscuration alone can hardly reproduce the fraction of $\log (N_{\rm H}/$cm$^{-2}) \geq 24$ sources at least at $z\lesssim3$. This requires an additional torus component with a thickness that decreases with luminosity to match the data. The torus should be present in all evolutionary stages of a visible AGN to be effective, although galaxy-scale gas obscuration may be sufficient to reproduce the obscured fraction with $22<\log (N_{\rm H}/$cm$^{-2})<24$ (Compton-thin, CTN) if we assume extremely compact gas disc components. The claimed drop of CTN fractions with increasing luminosity does not appear to be a consequence of AGN feedback, but rather of gas reservoirs becoming more compact with decreasing stellar mass.

astro-ph.GA↗
arXiv2023

Obscuration beyond the nucleus: infrared quasars can be buried in extreme compact starbursts

In the standard quasar model, the accretion disk obscuration is due to the canonical dusty torus. Here, we argue that a substantial part of the quasar obscuration can come from the interstellar medium (ISM) when the quasars are embedded in compact starbursts. We use an obscuration-unbiased sample of 578 infrared (IR) quasars at $z\approx 1-3$ and archival ALMA submillimeter host galaxy sizes to investigate the ISM contribution to the quasar obscuration. We calculate SFR and ISM column densities for the IR quasars and a control sample of submillimeter galaxies (SMGs) not hosting quasar activity and show that: (1) the quasar obscured fraction is constant up to $\rm SFR\approx 300 \: M_{\odot} \: yr^{-1}$, and then increases towards higher SFR, suggesting that the ISM obscuration plays a significant role in starburst host galaxies, and (2) at $\rm SFR\gtrsim 300 \: M_{\odot} \: yr^{-1}$, the SMGs and IR quasars have similarly compact submillimeter sizes ($R_{\rm e}\approx 0.5-3\rm \: kpc$) and, consequently, the ISM can heavily obscure the quasar, even reaching Compton-thick ($N_{\rm H}>10^{24} \rm \: cm^{-2}$) levels in extreme cases. Based on our results, we infer that $\approx 10-30\%$ of the IR quasars with $\rm SFR\gtrsim 300 \: M_{\odot} \: yr^{-1}$ are obscured solely by the ISM.

astro-ph.GA↗
arXiv2023

A new discovery space opened by eROSITA: Ionised AGN outflows from X-ray selected samples

In the context of an evolutionary model, the outflow phase of an Active Galactic Nuclei (AGN) occurs at the peak of its activity, once the central SMBH is massive enough to generate sufficient power to counterbalance the potential well of the host galaxy. This phase plays a vital role in galaxy evolution. We aim to apply various selection methods to isolate powerful AGNs in the feedback phase, trace and characterise their outflows, and explore the link between AGN luminosity and outflow properties. We applied a combination of methods to the eROSITA Final Equatorial Depth survey (eFEDS) catalogue and isolated ~1400 candidates at z>0.5 out of ~11750 AGNs (~12\%). We tested the robustness of our selection on the small subsample of 50 sources with available good quality SDSS spectra at 0.5 1\%. We find that the majority of the outflows have momentum flux ratios lower than 20 which rules out an energy-conserving nature. Our present work points to the unequivocal existence of a rather short AGN outflow phase, paving the way towards a new avenue to dissect AGN outflows in large samples within eROSITA and beyond.

astro-ph.GA↗
arXiv2023

Detecting and Characterizing Mg II absorption in DESI Survey Validation Quasar Spectra

We present findings of the detection of Magnesium II (Mg II, λ = 2796, 2803 Å) absorbers from the early data release of the Dark Energy Spectroscopic Instrument (DESI). DESI is projected to obtain spectroscopy of approximately 3 million quasars (QSOs), of which over 99% are anticipated to be at redshifts greater than z > 0.3, such that DESI would be able to observe an associated or intervening Mg II absorber illuminated by the background QSO. We have developed an autonomous supplementary spectral pipeline that detects these systems through an initial line-fitting process and then confirms the line properties using a Markov Chain Monte Carlo sampler. Based upon a visual inspection of the resulting systems, we estimate that this sample has a purity greater than 99%. We have also investigated the completeness of our sample in regard to both the signal-to-noise properties of the input spectra and the rest-frame equivalent width (W0) of the absorber systems. From a parent catalog containing 83,207 quasars, we detect a total of 23,921 Mg II absorption systems following a series of quality cuts. Extrapolating from this occurrence rate of 28.8% implies a catalog at the completion of the five-year DESI survey that will contain over eight hundred thousand Mg II absorbers. The cataloging of these systems will enable significant further research because they carry information regarding circumgalactic medium environments, the distribution of intervening galaxies, and the growth of metallicity across the redshift range 0.3 < z < 2.5.

astro-ph.GA↗
arXiv2023

Revisiting the Properties of X-ray AGN in the SSA22 Protocluster: Normal SMBH and Host-Galaxy Growth for AGN in a $z=3.09$ Overdensity

We analyze the physical properties of 8 X-ray selected active galactic nuclei (AGN) and one candidate protoquasar system (ADF22A1) in the $z = 3.09$ SSA22 protocluster by fitting their X-ray-to-IR spectral energy distributions (SEDs) using our SED fitting code, Lightning. We recover star formation histories (SFH) for 7 of these systems which are well-fit by composite stellar population plus AGN models. We find indications that 4/9 of the SSA22 AGN systems we study have host galaxies below the main sequence, with $\rm SFR/SFR_{MS} \leq -0.4$. The remaining SSA22 systems, including ADF22A1, are consistent with obscured supermassive black hole (SMBH) growth in star forming galaxies. We estimate the SMBH accretion rates and masses, and compare the properties and SFH of the 9 protocluster AGN systems with X-ray detected AGN candidates in the Chandra Deep Fields (CDF), finding that the distributions of SMBH growth rates, star formation rates, SMBH masses, and stellar masses for the protocluster AGN are consistent with field AGN. We constrain the ratio between the sample-averaged SSA22 SMBH mass and CDF SMBH mass to $<1.41$. While the AGN are located near the density peaks of the protocluster, we find no statistically significant trends between the AGN or host galaxy properties and their location in the protocluster. We interpret the similarity of the protocluster and field AGN populations together with existing results as suggesting that the protocluster and field AGN co-evolve with their hosts in the same ways, while AGN-triggering events are more likely in the protocluster.

astro-ph.GA↗
arXiv2023

Performance of the Quasar Spectral Templates for the Dark Energy Spectroscopic Instrument

Millions of quasar spectra will be collected by the Dark Energy Spectroscopic Instrument (DESI), leading to a four-fold increase in the number of known quasars. High accuracy quasar classification is essential to tighten constraints on cosmological parameters measured at the highest redshifts DESI observes ($z>2.0$). We present the spectral templates for identification and redshift estimation of quasars in the DESI Year 1 data release. The quasar templates are comprised of two quasar eigenspectra sets, trained on spectra from the Sloan Digital Sky Survey. The sets are specialized to reconstruct quasar spectral variation observed over separate yet overlapping redshift ranges and, together, are capable of identifying DESI quasars from $0.05 < z <7.0$. The new quasar templates show significant improvement over the previous DESI quasar templates regarding catastrophic failure rates, redshift precision and accuracy, quasar completeness, and the contamination fraction in the final quasar sample.

astro-ph.IM↗
arXiv2023

Identification and Characterization of a Large Sample of Distant Active Dwarf Galaxies in XMM-SERVS

Active dwarf galaxies are important because they contribute to the evolution of dwarf galaxies and can reveal their hosted massive black holes. However, the sample size of such sources beyond the local universe is still highly limited. In this work, we search for active dwarf galaxies in the recently completed XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS). XMM-SERVS is currently the largest medium-depth X-ray survey covering 13 $\mathrm{deg}^2$ in three extragalactic fields, which all have well-characterized multi-wavelength information. After considering several factors that may lead to misidentifications, we identify 73 active dwarf galaxies at $z<1$, which constitutes the currently largest X-ray-selected sample beyond the local universe. Our sources are generally less obscured than predictions based on the massive-AGN (active galactic nucleus) X-ray luminosity function and have a low radio-excess fraction. We find that our sources reside in similar environments to inactive dwarf galaxies. We further quantify the accretion distribution of the dwarf-galaxy population after considering various selection effects and find that it decreases with X-ray luminosity, but redshift evolution cannot be statistically confirmed. Depending upon how we define an AGN, the active fraction may or may not show a strong dependence on stellar mass. Their Eddington ratios and X-ray bolometric corrections significantly deviate from the expected relation, which is likely caused by several large underlying systematic biases when estimating the relevant parameters for dwarf galaxies. Throughout this work, we also highlight problems in reliably measuring photometric redshifts and overcoming strong selection effects for distant active dwarf galaxies.

astro-ph.GA↗
arXiv2023

The Most Obscured AGNs in the XMM-SERVS Fields

We perform X-ray spectral analyses to derive characteristics (e.g., column density, X-ray luminosity) of $\approx$10,200 active galactic nuclei (AGNs) in the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS), which was designed to investigate the growth of supermassive black holes across a wide dynamic range of cosmic environments. Using physical torus models (e.g., Borus02) and a Bayesian approach, we uncover 22 representative Compton-thick (CT; $N_{\rm H} \;>\; 1.5\times10^{24}\; \rm cm^{-2}$) AGN candidates with good signal-to-noise ratios as well as a large sample of 136 heavily obscured AGNs. We also find an increasing CT fraction (\fct ) from low ($z<0.75$) to high ($z>0.75$) redshift. Our CT candidates tend to show hard X-ray spectral shapes and dust extinction in their SED fits, which may shed light on the connection between AGN obscuration and host-galaxy evolution.

astro-ph.GA↗
arXiv2023

The hidden side of cosmic star formation at z > 3: Bridging optically-dark and Lyman break galaxies with GOODS-ALMA

Our current understanding of the cosmic star formation history at z>3 is primarily based on UV-selected galaxies (i.e., LBGs). Recent studies of H-dropouts have revealed that we may be missing a large proportion of star formation that is taking place in massive galaxies at z>3. In this work, we extend the H-dropout criterion to lower masses to select optically dark/faint galaxies (OFGs), in order to complete the census between LBGs and H-dropouts. Our criterion (H> 26.5 mag & [4.5] < 25 mag) combined with a de-blending technique is designed to select not only extremely dust-obscured massive galaxies but also normal star-forming galaxies. In total, we identified 27 OFGs at z_phot > 3 (z_med=4.1) in the GOODS-ALMA field, covering a wide distribution of stellar masses with log($M_{\star}$/$M_{\odot}$) = 9.4-11.1. We find that up to 75% of the OFGs with log($M_{\star}$/$M_{\odot}$) = 9.5-10.5 were neglected by previous LBGs and H-dropout selection techniques. After performing stacking analyses, the OFGs exhibit shorter gas depletion timescales, slightly lower gas fractions, and lower dust temperatures than typical star-forming galaxies. Their SFR_tot (SFR_ IR+SFR_UV) is much larger than SFR_UVcorr (corrected for dust extinction), with SFR_tot/SFR_UVcorr = $8\pm1$, suggesting the presence of hidden dust regions in the OFGs that absorb all UV photons. The average dust size measured by a circular Gaussian model fit is R_e(1.13 mm)=1.01$\pm$0.05 kpc. We find that the cosmic SFRD at z>3 contributed by massive OFGs is at least two orders of magnitude higher than the one contributed by equivalently massive LBGs. Finally, we calculate the combined contribution of OFGs and LBGs to the cosmic SFRD at z=4-5 to be 4 $\times$ 10$^{-2}$ $M_{\odot}$ yr$^{-1}$Mpc$^{-3}$, which is about 0.15 dex (43%) higher than the SFRD derived from UV-selected samples alone at the same redshift.

astro-ph.GA↗
arXiv2023

Host Dark Matter Halos of WISE-selected Obscured & Unobscured Quasars: Evidence for Evolution

Obscuration in quasars may arise from steep viewing angles along the dusty torus, or instead may represent a distinct phase of supermassive black hole growth. We test these scenarios by probing the host dark matter halo environments of $\sim 1.4$ million WISE-selected obscured and unobscured quasars at $\langle z \rangle = 1.4$ using angular clustering measurements as well as cross-correlation measurements of quasar positions with the gravitational lensing of the cosmic microwave background (CMB). We interpret these signals within a halo occupation distribution (HOD) framework to conclude that obscured systems reside in more massive effective halos ($ \sim 10^{12.9} h^{-1} M_{\odot}$) than their unobscured counterparts ($ \sim 10^{12.6} h^{-1} M_{\odot}$), though we do not detect a difference in the satellite fraction. We find excellent agreement between the clustering and lensing analyses and show that this implies the observed difference is robust to uncertainties in the obscured quasar redshift distribution, highlighting the power of combining angular clustering and weak lensing measurements. This finding appears in tension with models that ascribe obscuration exclusively to orientation of the dusty torus along the line-of-sight, and instead may be consistent with the notion that some obscured quasars are attenuated by galaxy-scale or circumnuclear material during an evolutionary phase.

astro-ph.GA↗
arXiv2023

The Target-selection Pipeline for the Dark Energy Spectroscopic Instrument

In 2021 May, the Dark Energy Spectroscopic Instrument (DESI) began a 5 yr survey of approximately 50 million total extragalactic and Galactic targets. The primary DESI dark-time targets are emission line galaxies (ELGs), luminous red galaxies (LRGs) and quasars (QSOs). In bright time, DESI will focus on two surveys known as the Bright Galaxy Survey (BGS) and the Milky Way Survey (MWS). DESI also observes a selection of "secondary" targets for bespoke science goals. This paper gives an overview of the publicly available pipeline (desitarget) used to process targets for DESI observations. Highlights include details of the different DESI survey targeting phases, the targeting ID (TARGETID) used to define unique targets, the bitmasks used to indicate a particular type of target, the data model and structure of DESI targeting files, and examples of how to access and use the desitarget code base. This paper will also describe "supporting" DESI target classes, such as standard stars, sky locations, and random catalogs that mimic the angular selection function of DESI targets. The DESI target selection pipeline is complex and sizable; this paper attempts to summarize the most salient information required to understand and work with DESI targeting data.

astro-ph.IM↗
arXiv2023

The DESI Survey Validation: Results from Visual Inspection of Bright Galaxies, Luminous Red Galaxies, and Emission Line Galaxies

The Dark Energy Spectroscopic Instrument (DESI) Survey has obtained a set of spectroscopic measurements of galaxies to validate the final survey design and target selections. To assist in these tasks, we visually inspect (VI) DESI spectra of approximately 2,500 bright galaxies, 3,500 luminous red galaxies (LRGs), and 10,000 emission line galaxies (ELGs), to obtain robust redshift identifications. We then utilize the VI redshift information to characterize the performance of the DESI operation. Based on the VI catalogs, our results show that the final survey design yields samples of bright galaxies, LRGs, and ELGs with purity greater than $99\%$. Moreover, we demonstrate that the precision of the redshift measurements is approximately 10 km/s for bright galaxies and ELGs and approximately 40 km/s for LRGs. The average redshift accuracy is within 10 km/s for the three types of galaxies. The VI process also helps improve the quality of the DESI data by identifying spurious spectral features introduced by the pipeline. Finally, we show examples of unexpected real astronomical objects, such as Ly$α$ emitters and strong lensing candidates, identified by VI. These results demonstrate the importance and utility of visually inspecting data from incoming and upcoming surveys, especially during their early operation phases.

astro-ph.CO↗
arXiv2022

The DESI Survey Validation: Results from Visual Inspection of the Quasar Survey Spectra

A key component of the Dark Energy Spectroscopic Instrument (DESI) survey validation (SV) is a detailed visual inspection (VI) of the optical spectroscopic data to quantify key survey metrics. In this paper we present results from VI of the quasar survey using deep coadded SV spectra. We show that the majority (~70%) of the main-survey targets are spectroscopically confirmed as quasars, with ~16% galaxies, ~6% stars, and ~8% low-quality spectra lacking reliable features. A non-negligible fraction of the quasars are misidentified by the standard spectroscopic pipeline but we show that the majority can be recovered using post-pipeline "afterburner" quasar-identification approaches. We combine these "afterburners" with our standard pipeline to create a modified pipeline to improve the overall quasar yield. At the depth of the main DESI survey both pipelines achieve a good-redshift purity (reliable redshifts measured within 3000 km/s) of ~99%; however, the modified pipeline recovers ~94% of the visually inspected quasars, as compared to ~86% from the standard pipeline. We demonstrate that both pipelines achieve an median redshift precision and accuracy of ~100 km/s and ~70 km/s, respectively. We constructed composite spectra to investigate why some quasars are missed by the standard spectroscopic pipeline and find that they are more host-galaxy dominated (i.e., distant analogs of "Seyfert galaxies") and/or dust reddened than the standard-pipeline quasars. We also show example spectra to demonstrate the overall diversity of the DESI quasar sample and provide strong-lensing candidates where two targets contribute to a single spectrum.

astro-ph.GA↗
arXiv2022

The demographics of obscured AGN from X-ray spectroscopy guided by multiwavelength information

A complete census of Active Galactic Nuclei (AGN) is a prerequisite for understanding the growth of supermassive black holes across cosmic time. A significant challenge toward this goal is the whereabouts of heavily obscured AGN that remain uncertain. This paper sets new constraints on the demographics of this population by developing a methodology that combines X-ray spectral information with priors derived from multiwavelength observations. We select X-ray AGN in the Chandra COSMOS Legacy survey and fit their $2.2-500μm$ spectral energy distributions with galaxy and AGN templates to determine the mid-infrared ($6μm$) luminosity of the AGN component. Empirical correlations between X-ray and $6μm$ luminosities are then adopted to infer the intrinsic accretion luminosity at X-rays for individual AGN. This is used as prior information in our Bayesian X-ray spectral analysis to estimate physical properties, such as line-of-sight obscuration. Our approach breaks the degeneracies between accretion luminosity and obscuration that affect X-ray spectral analysis, particularly for the most heavily obscured (Compton-Thick) AGN with low photon counts X-ray spectra. The X-ray spectral results are then combined with the selection function of the Chandra COSMOS Legacy survey to derive the AGN space density and a Compton-Thick fraction of $21.0^{+16.1}_{-9.9}\%$ at redshifts $z<0.5$. At higher redshift, our analysis suggests upper limits to the Compton-Thick AGN fraction of $\le 40\%$. These estimates are at the low end of the range of values determined in the literature and underline the importance of multiwavelength approaches for tackling the challenge of heavily obscured AGN demographics.

astro-ph.GA↗
arXiv2022

Does the lockstep growth between black holes and bulges create their mass relation?

Recent studies have revealed a strong relation between sample-averaged black-hole (BH) accretion rate (BHAR) and star formation rate (SFR) among bulge-dominated galaxies, i.e., "lockstep" BH-bulge growth, in the distant universe. This relation might be closely related to the BH-bulge mass correlation observed in the local universe. To understand further BH-bulge coevolution, we present ALMA CO(2-1) or CO(3-2) observations of 7 star-forming bulge-dominated galaxies at z=0.5-2.5. Using the ALMA data, we detect significant ($>3σ$) CO emission from 4 objects. For our sample of 7 galaxies, we measure (or constrain with upper limits) their CO line fluxes and estimate molecular gas masses ($M_{gas}$). We also estimate their stellar masses ($M_{star}$) and SFRs by modelling their spectral energy distributions (SEDs). Using these physical properties, we derive the gas-depletion timescales ($t_{dep} = M_{gas}/SFR$) and compare them with the bulge/BH growth timescales ($t_{grow} = M_{star}/SFR \sim M_{BH}/BHAR$). Our sample generally has $t_{dep}$ shorter than $t_{grow}$ by a median factor of $\gtrsim 4$, indicating that the cold gas will be depleted before significant bulge/BH growth takes place. This result suggests that the BH-bulge lockstep growth is mainly responsible for maintaining their mass relation, not creating it. We note that our sample is small and limited to $z<2.5$; JWST and ALMA will be able to probe to higher redshifts in the near future.

astro-ph.GA↗