arXiv subjects

F. E. Bauer

Publications and source records attributed to F. E. Bauer.

At least 55 records · Page 3Linked to original sources

arXiv2024

The cold interstellar medium of a normal sub-$L^\star$ galaxy at the end of reionization

We present the results of a ~60-hr observational campaign with ALMA targeting a spectroscopically confirmed and lensed sub-$L^\star$ galaxy at z=6.07, identified during the ALMA Lensing Cluster Survey (ALCS). We sample the dust continuum emission from rest frame 90 to 370 $μ$m at six different frequencies and set constraining upper limits on the molecular gas line emission and content via CO(7-6) and [CI](2-1) for two lensed images with $μ\gtrsim20$. Complementing these sub-mm observations with deep optical and near-IR photometry and spectroscopy with JWST, we find this galaxy to form stars at a rate of SFR~7 Msun/yr, ~50-70% of which is obscured by dust. This is consistent with what is expected for a $M_\star$~7.5$\times10^{8}$ Msun object by extrapolating the $M_\star$-obscured SFR fraction relation at z<2.5 and with observations at 5<z<7. The dust temperature of ~50K is similar to that of more massive galaxies at similar redshifts, although with large uncertainties and with possible negative gradients. We measure a dust mass of $M_{\rm dust}$~1.5$\times10^6$ Msun and, by combining [CI], [CII], and a dynamical estimate, a gas mass of ~2$\times10^9$ Msun. Their ratio is in good agreement with the predictions from models in the literature. The $M_{\rm dust}$/$M_\star$ fraction of ~0.002 and the young stellar age are consistent with dust production via supernovae. Also, models predict a number density of galaxies with $M_{\rm dust}\sim10^{6}$ Msun at z=6 in agreement with our estimate from the parent ALCS survey. The combination of lensing and multiwavelength observations allow us to probe luminosity regimes up to two orders of magnitude lower than what has been explored so far for field galaxies at similar redshifts. Our results serve as a benchmark for future observations of faint sub-$L^\star$ galaxy population that might have driven the reionization of the Universe. [Abridged]

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arXiv2024

The eROSITA Final Equatorial Depth Survey (eFEDS): the hard X-ray selected sample

During its calibration and performance verification phase, the eROSITA instrument aboard the SRG satellite performed a uniform wide--area X-ray survey of approximately 140 deg$^{2}$ in a region of the sky known as the eROSITA Final Equatorial Depth Survey (eFEDS). The primary aim of eFEDS is to demonstrate the scientific performance to be expected at the end of the 8-pass eROSITA all sky survey. This will provide the first focussed image of the whole sky in the hard X-ray ($>2$~keV) bandpass. The expected source population in this energy range is thus of great interest, particularly for AGN studies. We use the 2.3--5 keV selection presented by Brunner et al. (2022) to construct a sample of 246 point-like hard X-ray sources for further study and characterization. These are classified as either extragalactic ($\sim 90$~\%) or Galactic ($\sim 10$~\%), with the former consisting overwhelmingly of AGN and the latter active stars. We concentrate our further analysis on the extragalactic/AGN sample, describing their X-ray and multiwavelength properties and comparing them to the eFEDS main AGN sample selected in the softer 0.2-2.3 keV band. The eROSITA hard band selects a subsample of sources that is a factor $>10$ brighter than the eFEDS main sample. The AGN within the hard population reach up to $z=3.2$ but on the whole are relatively nearby, with median $z$=0.34 compared to $z$=0.94 for the main sample. The hard survey probes typical luminosities in the range $\log L_{\rm X} = 43-46$. X-ray spectral analysis shows significant intrinsic absorption (with $\log N_{\rm H}>21$) in $\sim 20$~\% of the sources, with a hard X-ray power law continuum with mean $<Γ>=1.83\pm0.04$, typical of AGN, but slightly harder than the soft-selected eROSITA sample. (abridged)

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arXiv2024

Probing a Magnetar Origin for the population of Extragalactic Fast X-ray Transients detected by Chandra

Twenty-two extragalactic fast X-ray transients (FXTs) have now been discovered from two decades of Chandra data (analyzing ~259 Ms of data), with 17 associated with distant galaxies (>100 Mpc). Different mechanisms and progenitors have been proposed to explain their properties; nevertheless, after analyzing their timing, spectral parameters, host-galaxy properties, luminosity function, and volumetric rates, their nature remains uncertain. We interpret a sub-sample of nine FXTs that show a plateau or a fast-rise light curve within the framework of a binary neutron star (BNS) merger magnetar model. We fit their light curves and derive magnetar (magnetic field and initial rotational period) and ejecta (ejecta mass and opacity) parameters. This model predicts two zones: an orientation-dependent free zone (where the magnetar spin-down X-ray photons escape freely to the observer) and a trapped zone (where the X-ray photons are initially obscured and only escape freely once the ejecta material becomes optically thin). We argue that six FXTs show properties consistent with the free zone and three FXTs with the trapped zone. This sub-sample of FXTs has a similar distribution of magnetic fields and initial rotation periods to those inferred for short gamma-ray bursts (SGRBs), suggesting a possible association. We compare the predicted ejecta emission fed by the magnetar emission (called merger-nova) to the optical and near-infrared upper limits of two FXTs, XRT 141001 and XRT 210423 where contemporaneous optical observations are available. The non-detections place lower limits on the redshifts of XRT 141001 and XRT 210423 of z>1.5 and >0.1, respectively. If the magnetar remnants lose energy via gravitational waves, it should be possible to detect similar objects with the current advanced LIGO detectors out to a redshift z<0.03, while future GW detectors will be able to detect them out to z=0.5.

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arXiv2023

XMM-Newton-discovered Fast X-ray Transients: Host galaxies and limits on contemporaneous detections of optical counterparts

Extragalactic fast X-ray transients (FXTs) are a class of soft (0.3-10 keV) X-ray transients lasting a few hundred seconds to several hours. Several progenitor mechanisms have been suggested to produce FXTs, including supernova shock breakouts, binary neutron star mergers, or tidal disruptions involving an intermediate-mass black hole and a white dwarf. We present detailed host studies, including spectroscopic observations of the host galaxies of 7 XMM-Newton-discovered FXTs. The candidate hosts lie at redshifts 0.0928 $< z <$ 0.645 implying peak X-ray luminosities of 10$^{43}$ erg s$^{-1}$ $< L_X <$ 10$^{45}$ erg s$^{-1}$,and physical offsets of 1 kpc < $r_\mathrm{proj}$ < 22 kpc. These observations increase the number of FXTs with a spectroscopic redshift measurement by a factor of 2, although we note that one event is re-identified as a Galactic flare star. We infer host star formation rates and stellar masses by fitting the combined spectroscopic and archival photometric data. We also report on a contemporaneous optical counterpart search to the FXTs in Pan-STARRS and ATLAS by performing forced photometry at the position of the FXTs. We do not find any counterpart in our search. Given our constraints, including peak X-ray luminosities, optical limits, and host properties, we find that XRT 110621 is consistent with a SN SBO event. Spectroscopic redshifts of likely host galaxies for four events imply peak X-ray luminosities that are too high to be consistent with SN SBOs, but we are unable to discard either the BNS or WD-IMBH TDE scenarios for these FXTs.

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arXiv2023

BASS XLII: The relation between the covering factor of dusty gas and the Eddington ratio in nearby active galactic nuclei

Accreting supermassive black holes (SMBHs) located at the center of galaxies are typically surrounded by large quantities of gas and dust. The structure and evolution of this circumnuclear material can be studied at different wavelengths, from the submillimeter to the X-rays. Recent X-ray studies have shown that the covering factor of the obscuring material tends to decrease with increasing Eddington ratio, likely due to radiative feedback on dusty gas. Here we study a sample of 549 nearby (z<0.1) hard X-ray (14-195 keV) selected non-blazar active galactic nuclei (AGN), and use the ratio between the AGN infrared and bolometric luminosity as a proxy of the covering factor. We find that, in agreement with what has been found by X-ray studies of the same sample, the covering factor decreases with increasing Eddington ratio. We also confirm previous findings which showed that obscured AGN typically have larger covering factors than unobscured sources. Finally, we find that the median covering factors of AGN located in different regions of the column density-Eddington ratio diagram are in good agreement with what would be expected from a radiation-regulated growth of SMBHs.

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arXiv2023

AT2022aedm and a new class of luminous, fast-cooling transients in elliptical galaxies

We present the discovery and extensive follow-up of a remarkable fast-evolving optical transient, AT2022aedm, detected by the Asteroid Terrestrial impact Last Alert Survey (ATLAS). AT2022aedm exhibited a rise time of $9\pm1$ days in the ATLAS $o$-band, reaching a luminous peak with $M_g\approx-22$ mag. It faded by 2 magnitudes in $g$-band during the next 15 days. These timescales are consistent with other rapidly evolving transients, though the luminosity is extreme. Most surprisingly, the host galaxy is a massive elliptical with negligible current star formation. X-ray and radio observations rule out a relativistic AT2018cow-like explosion. A spectrum in the first few days after explosion showed short-lived He II emission resembling young core-collapse supernovae, but obvious broad supernova features never developed; later spectra showed only a fast-cooling continuum and narrow, blue-shifted absorption lines, possibly arising in a wind with $v\approx2700$ km s$^{-1}$. We identify two further transients in the literature (Dougie in particular, as well as AT2020bot) that share similarities in their luminosities, timescales, colour evolution and largely featureless spectra, and propose that these may constitute a new class of transients: luminous fast-coolers (LFCs). All three events occurred in passive galaxies at offsets of $\sim4-10$ kpc from the nucleus, posing a challenge for progenitor models involving massive stars or massive black holes. The light curves and spectra appear to be consistent with shock breakout emission, though usually this mechanism is associated with core-collapse supernovae. The encounter of a star with a stellar mass black hole may provide a promising alternative explanation.

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arXiv2023

Panning for gold, but finding helium: discovery of the ultra-stripped supernova SN2019wxt from gravitational-wave follow-up observations

We present the results from multi-wavelength observations of a transient discovered during the follow-up of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN2019wxt, a young transient in a galaxy whose sky position (in the 80\% GW contour) and distance ($\sim$150\,Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient's tightly constrained age, its relatively faint peak magnitude ($M_i \sim -16.7$\,mag) and the $r-$band decline rate of $\sim 1$\,mag per 5\,days appeared suggestive of a compact binary merger. However, SN2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of $\sim 0.1\,M_\odot$, with $^{56}$Ni comprising $\sim 20\%$ of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitors that could give rise to the observed properties of SN2019wxt, and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling electromagnetic counterparts to GW events from transients such as SN2019wxt is challenging: in a bid to characterise the level of contamination, we estimated the rate of events with properties comparable to those of SN2019wxt and found that $\sim 1$ such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500\,Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.

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arXiv2023

2mm Observations and the Search for High-Redshift Dusty Star-forming Galaxies

Finding high-redshift (z>>4) dusty star-forming galaxies is extremely challenging. It has recently been suggested that millimeter selections may be the best approach, since the negative K-correction makes galaxies at a given far-infrared (FIR) luminosity brighter at z>4 than those at z=2-3. Here we analyze this issue using a deep ALMA 2mm sample obtained by targeting ALMA 870um priors (these priors were the result of targeting SCUBA-2 850um sources) in the GOODS-S. We construct the prior-based 2mm galaxy number counts and compare them with published blank field-based 2mm counts, finding good agreement down to 0.2mJy. Only a fraction of the current 2mm extragalactic background light is resolved, and we estimate what observational depths may be needed to resolve it fully. By complementing the 2mm ALMA data with a deep SCUBA-2 450um sample in the GOODS-S, we exploit the steep gradient with redshift of the 2mm to 450um flux density ratio to estimate redshifts for these galaxies without spectroscopic or robust optical/near-infrared photometric redshifts. Our observations measure galaxies with star formation rates in excess of 250 solar masses per year. For these galaxies, the star formation rate densities fall by a factor of 9 from z=2-3 to z=5-6.

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arXiv2023

Unbiased surveys of dust-enshrouded galaxies using ALMA

The ALMA lensing cluster survey (ALCS) is a 96-hr large program dedicated to uncovering and characterizing intrinsically faint continuum sources and line emitters with the assistance of gravitational lensing. All 33 cluster fields were selected from HST/Spitzer treasury programs including CLASH, Hubble Frontier Fields, and RELICS, which also have Herschel and Chandra coverages. The total sky area surveyed reaches $\sim$133 arcmin$^2$ down to a depth of $\sim$60 $μ$Jy beam$^{-1}$ (1$σ$) at 1.2 mm, yielding 141 secure blind detections of continuum sources and additional 39 sources aided by priors. We present scientific motivation, survey design, the status of spectroscopy follow-up observations, and number counts down to $\sim$7 $μ$Jy. Synergies with JWST are also discussed.

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arXiv2023

Dust Properties of 870 Micron Selected Galaxies in the GOODS-S

We analyze the dust properties of 57 870 $μ$m selected dusty star-forming galaxies in the GOODS-S using new deep ALMA 1.2 mm, 2 mm, and 3 mm continuum imaging together with other far-infrared through millimeter data. We fit the spectral energy distributions (SEDs) with optically thin modified blackbodies to constrain the emissivity indices and effective dust temperatures, finding a median emissivity index of $β= 1.78^{+0.43}_{-0.25}$ and a median temperature of $T_d = 33.6^{+12.1}_{-5.4}$ K. We observe a negative correlation between $β$ and $T_d$. By testing several SED models, we determine that the derived emissivity indices can be influenced by opacity assumptions. Our temperature measurements are consistent with no evolution in dust temperature with redshift.

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arXiv2023

The ALMA Frontier Fields Survey. VI. Lensing-corrected 1.1mm number counts in Abell 2744, MACSJ0416.1-2403, MACSJ1149.5+2223, Abell 370 and Abell S1063

[abridged] Probing the faint end of the number counts at mm wavelengths is important to identify the origin of the extragalactic background light in this regime. Aided by strong gravitational lensing, ALMA observations towards massive galaxy clusters have opened a window to disentangle this origin, allowing to resolve sub-mJy dusty star-forming galaxies. We aim to derive number counts at 1.1 mm down to flux densities fainter than 0.1 mJy, based on ALMA observations towards five Hubble Frontier Fields (FF) galaxy clusters, following a statistical approach to correct for lensing effects. We created a source catalog that includes 29 ALMA 1.1 mm continuum detections down to a 4.5sigma significance. We derived source intrinsic flux densities using public lensing models. We folded the uncertainties in both magnifications and source redshifts into the number counts through Monte Carlo simulations. We derive cumulative number counts over two orders of magnitude down to 0.01 mJy after correction for lensing effects. Cosmic variance estimates are all exceeded by uncertainties in our median combined cumulative counts that come from both our Monte Carlo simulations and Poisson statistics. Our number counts are consistent to 1sigma with most of recent ALMA estimates and galaxy evolution models. However, below 0.1 mJy, they are lower by 0.4 dex compared to two deep ALMA studies but consistent with ASPECS-LP to 1sigma. Importantly, the flattening found for our cumulative counts extends further to 0.01 mJy. Our results bring further support in line of the flattening of the number counts reported previously by us and ASPECS-LP, which has been interpreted by a recent galaxy evolution model as a measurement of the "knee" of the infrared luminosity function at high redshift. Our estimates of the contribution to the EBL in the FFs suggest that we may be resolving most of the EBL at 1.1mm down to 0.01 mJy.

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arXiv2023

Extragalactic FXT Candidates Discovered by Chandra (2014-2022)

Extragalactic fast X-ray transients (FXTs) are short flashes of X-ray photons of unknown origin that last a few minutes to hours. We extend the search for extragalactic FXTs from Quirola et al. 2022 (Paper I; based on sources in the Chandra Source Catalog 2.0, CSC2) to further Chandra archival data between 2014-2022. We extract X-ray data using a method similar to that employed by CSC2 and apply identical search criteria as in Paper I. We report the detection of eight FXT candidates, with peak 0.3-10 keV fluxes between 1$\times$10$^{-13}$ to 1$\times$10$^{-11}$ erg cm$^{-2}$ s$^{-1}$ and $T_{90}$ values from 0.3 to 12.1 ks. This sample of FXTs has likely redshifts between 0.7 to 1.8. Three FXT candidates exhibit light curves with a plateau (${\approx}$1-3 ks duration) followed by a power-law decay and X-ray spectral softening, similar to what was observed for a few previously reported FXTs in Paper I. In light of the new, expanded source lists (eight FXTs with known redshifts from Paper I and this work), we update the event sky rates derived in Paper I, finding 36.9$_{-8.3}^{+9.7}$ deg$^{-2}$ yr$^{-1}$ for the extragalactic samples for a limiting flux of ${\gtrsim}$1${\times}$10$^{-13}$ erg cm$^{-2}$ s$^{-1}$, calculate the first FXT X-ray luminosity function, and compare the volumetric density rate between FXTs and other transient classes. Our latest Chandra-detected extragalactic FXT candidates boost the total Chandra sample by $\sim$50 %, and appear to have a similar diversity of possible progenitors.

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arXiv2023

Persistent and occasional: searching for the variable population of the ZTF/4MOST sky using ZTF data release 11

We present a variability, color and morphology based classifier, designed to identify transients, persistently variable, and non-variable sources, from the Zwicky Transient Facility (ZTF) Data Release 11 (DR11) light curves of extended and point sources. The main motivation to develop this model was to identify active galactic nuclei (AGN) at different redshift ranges to be observed by the 4MOST ChANGES project. Still, it serves as a more general time-domain astronomy study. The model uses nine colors computed from CatWISE and PS1, a morphology score from PS1, and 61 single-band variability features computed from the ZTF DR11 g and r light curves. We trained two versions of the model, one for each ZTF band. We used a hierarchical local classifier per parent node approach, where each node was composed of a balanced random forest model. We adopted a 17-class taxonomy, including non-variable stars and galaxies, three transient classes, five classes of stochastic variables, and seven classes of periodic variables. The macro averaged precision, recall and F1-score are 0.61, 0.75, and 0.62 for the g-band model, and 0.60, 0.74, and 0.61, for the r-band model. When grouping the four AGN classes into one single class, its precision, recall, and F1-score are 1.00, 0.95, and 0.97, respectively, for both the g and r bands. We applied the model to all the sources in the ZTF/4MOST overlapping sky, avoiding ZTF fields covering the Galactic bulge, including 86,576,577 light curves in the g-band and 140,409,824 in the r-band. Only 0.73\% of the g-band light curves and 2.62\% of the r-band light curves were classified as stochastic, periodic, or transient with high probability ($P_{init}\geq0.9$). We found that, in general, more reliable results are obtained when using the g-band model. Using the latter, we identified 384,242 AGN candidates, 287,156 of which have $P_{init}\geq0.9$.

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arXiv2023

Multiwavelength monitoring of the nucleus in PBC J2333.9-2343: the giant radio galaxy with a blazar-like core

PBC J2333.9-2343 is a giant radio galaxy at z = 0.047 with a bright central core associated to a blazar nucleus. If the nuclear blazar jet is a new phase of the jet activity, then the small orientation angle suggest a dramatic change of the jet direction. We present observations obtained between September 2018 and January 2019 (cadence larger than three days) with Effeslberg, SMARTS-1.3m, ZTF, ATLAS, Swift, and Fermi-LAT, and between April-July 2019 (daily cadence) with SMARTS-1.3m and ATLAS. Large (>2x) flux increases are observed on timescales shorter than a month, which are interpreted as flaring events. The cross correlation between the SMARTS-1.3m monitoring in the NIR and optical shows that these data do not show significant time lag within the measured errors. A comparison of the optical variability properties between non-blazars and blazars AGN shows that PBC J2333.9-2343 has properties more comparable to the latter. The SED of the nucleus shows two peaks, that were fitted with a one zone leptonic model. Our data and modelling shows that the high energy peak is dominated by External Compton from the dusty torus with mild contribution from Inverse Compton from the jet. The derived jet angle of 3 degrees is also typical of a blazar. Therefore, we confirm the presence of a blazar-like core in the center of this giant radio galaxy, likely a Flat Spectrum Radio Quasar with peculiar properties.

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arXiv2023

The Fast X-ray Transient XRT 210423 and its Host Galaxy

Fast X-ray Transients (FXTs) are X-ray flares with a duration ranging from a few hundred seconds to a few hours. Possible origins include the tidal disruption of a white dwarf by an intermediate-mass black hole, a supernova shock breakout, and a binary neutron star merger. We present the X-ray light curve and spectrum, and deep optical imaging of the FXT XRT 210423, which has been suggested to be powered by a magnetar produced in a binary neutron star merger. Our Very Large Telescope and Gran Telescopio Canarias (GTC) observations began on May 6, 2021, thirteen days after the onset of the flare. No transient optical counterpart is found in the 1" (3$σ$) X-ray uncertainty region of the source to a depth $g_{s}$=27.0 AB mag. A candidate host lies within the 1" X-ray uncertainty region with a magnitude of 25.9 $\pm$ 0.1 in the GTC/HiPERCAM $g_s$-filter. Due to its faintness, it was not detected in other bands, precluding a photometric redshift determination. We detect two additional candidate host galaxies; one with $z_{\rm spec}=1.5082 \pm 0.0001$ and an offset of 4.2$\pm$1" (37$\pm$9 kpc) from the FXT and another one with $z_{\rm phot}=1.04^{+0.22}_{-0.14}$, at an offset of 3.6$\pm$1" (30$\pm$8 kpc). Based on the properties of all the prospective hosts we favour a binary neutron star merger, as previously suggested in the literature, as explanation for XRT 210423.

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arXiv2023

The universal shape of the X-ray variability power spectrum of AGN up to $z\sim 3$

We study the ensemble X-ray variability properties of Active Galactic Nuclei (AGN) over a large range of timescales (20 ks $\leq T\leq$ 14 yrs), redshift ($0\leq z \lesssim 3$), luminosities ($10^{40}$ erg s$^{-1}\leq L_X\leq 10^{46}$ erg s$^{-1}$) and black hole (BH) masses ($10^6 \leq $M$_\odot \leq 10^9$). We propose the use of the variance-frequency diagram, as a viable alternative to the study of the power spectral density (PSD), which is not yet accessible for distant, faint and/or sparsely sampled AGN. We show that the data collected from archival observations and previous literature studies are fully consistent with a universal PSD form which does not show any evidence for systematic evolution of shape or amplitude with redshift or luminosity, even if there may be differences between individual AGN at a given redshift or luminosity. We find new evidence that the PSD bend frequency depends on BH mass and, possibly, on accretion rate. We finally discuss the implications for current and future AGN population and cosmological studies.

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arXiv2023

The gas and stellar content of a metal-poor galaxy at $z=8.496$ as revealed by JWST and ALMA

We present a joint analysis of the galaxy S04590 at $z=8.496$ based on NIRSpec, NIRCam, and NIRISS observations obtained through as part of Early Release Observations programme of the James Webb Space Telescope (JWST) and the far-infrared [CII]-$158μ$m emission line detected by dedicated Atacama Large Millimeter/submillimeter Array (ALMA) observations. We determine the physical properties of S04590 from modelling of the spectral energy distribution (SED) and through the redshifted optical nebular emission lines detected with JWST/NIRSpec. The best-fit SED model reveals a low-mass ($M_\star = 10^{7.2}-10^{8}\,M_{\odot}$) galaxy with a low oxygen abundance of $12+\log{\rm (O/H)} = 7.16^{+0.10}_{-0.12}$ derived from the strong nebular and auroral emission lines. Assuming that [CII] effectively traces the interstellar medium (ISM), we estimate the total gas mass of the galaxy to be $M_{\rm gas} = (8.0\pm 4.0)\times 10^{8}\,M_\odot$ based on the luminosity and spatial extent of [CII]. This yields an exceptionally high gas fraction, $f_{\rm gas} = M_{\rm gas}/(M_{\rm gas} + M_\star) \gtrsim 90\%$, though still consistent within the range expected for its low metallicity. We further derive the metal mass of the galaxy based on the gas mass and gas-phase metallicity, which we find to be consistent with the expected metal production from Type II supernovae. Finally, we make the first constraints on the dust-to-gas (DTG) and dust-to-metals (DTM) ratios of galaxies in the epoch of reionization at $z\gtrsim 6$, showing overall low mass ratios of logDGT $<-3.8$ and logDTM $<-0.5$, though consistent with local scaling relations and in particular the local metal-poor galaxy I Zwicky 18. Our analysis highlights the synergy between ALMA and JWST in characterizing the gas, metal, and stellar content of the first generation of galaxies.

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arXiv2022

Probing the Structure and Evolution of BASS AGN through Eddington Ratios

We constrain the intrinsic Eddington ratio (\lamEdd ) distribution function for local AGN in bins of low and high obscuration (log NH <= 22 and 22 < log NH < 25), using the Swift-BAT 70-month/BASS DR2 survey. We interpret the fraction of obscured AGN in terms of circum-nuclear geometry and temporal evolution. Specifically, at low Eddington ratios (log lamEdd < -2), obscured AGN outnumber unobscured ones by a factor of ~4, reflecting the covering factor of the circum-nuclear material (0.8, or a torus opening angle of ~ 34 degrees). At high Eddington ratios (\log lamEdd > -1), the trend is reversed, with < 30% of AGN having log NH > 22, which we suggest is mainly due to the small fraction of time spent in a highly obscured state. Considering the Eddington ratio distribution function of narrow-line and broad-line AGN from our prior work, we see a qualitatively similar picture. To disentangle temporal and geometric effects at high lamEdd, we explore plausible clearing scenarios such that the time-weighted covering factors agree with the observed population ratio. We find that the low fraction of obscured AGN at high lamEdd is primarily due to the fact that the covering factor drops very rapidly, with more than half the time is spent with < 10% covering factor. We also find that nearly all obscured AGN at high-lamEdd exhibit some broad-lines. We suggest that this is because the height of the depleted torus falls below the height of the broad-line region, making the latter visible from all lines of sight.

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