arXiv subjects

Julia M. Comerford

Publications and source records attributed to Julia M. Comerford.

At least 19 recordsLinked to original sources

arXiv2026

Overmassive supermassive black holes in SDSS close galaxy pairs

Supermassive black holes (SMBHs) and their host-galaxies coevolve through channels such as hierarchical merging and AGN feedback, establishing tight scaling relations between SMBH masses and properties of the host-galaxy. These scaling relations, particularly those with host-galaxy bulge mass (M${_\text{b}}$) and stellar velocity dispersion ($σ$), are widely used to estimate SMBH masses. Galaxy mergers drive gas into the centers of galaxies, enhancing SMBH growth and stellar bulge growth but not necessarily in tandem, so it is necessary to determine if galaxies undergoing a merger still follow SMBH-host-galaxy scaling relations. In this study, we explore scaling relations in galaxy mergers by taking advantage of the well-established value-added catalogs in the Sloan Digital Sky Survey. These catalogs provide close galaxy pairs, AGN broad lines for SMBH mass measurements, and bulge-disk decomposed stellar masses. We compare SMBH mass estimates from M$_\bullet-$M${_\text{b}}$ to SMBH mass estimates from AGN broad lines and find that SMBH mass growth outpaces the bulge for AGN residing in the secondary (less massive) bulge and for smaller physical separations between galaxies in a pair. Using our full sample of close galaxy pairs, we find that M$_\bullet-$M${_\text{b}}$ and M$_\bullet- σ$ predict significantly different fractions of major and minor black hole mergers with M$_\bullet-$M${_\text{b}}$ predicting black hole mass ratios closer to 1:1 and $6-20\%$ more major mergers overall. These results have major implications, including for predictions of astrophysical gravitational-waves and high-redshift overmassive SMBHs.

astro-ph.GA↗

arXiv2026

Beyond the Brightest: A Deep Learning Approach to Identifying Major and Minor Galaxy Mergers in CANDELS at $z \sim 1$

Galaxy mergers play an important role in galaxy evolution. Therefore, accurate merger identifications are paramount for achieving a complete understanding of how galaxies evolve. As we enter the era of large, deep, high-resolution imaging surveys, we can observe mergers extending to even lower masses and higher redshifts. Despite low-mass galaxies being more common, many previous merger identification methods were calibrated for high-mass galaxies, which are easier to identify. To prepare for upcoming surveys, we train a convolutional neural network (CNN) using mock $\textit{HST}$ CANDELS images at $z\sim1$ created from the IllustrisTNG50 cosmological simulation. We successfully identify galaxy mergers between a wide range of galaxies ($10^8M_\odot < M_\star < 10^{12.5}M_\odot$, and $q\geq1:10$), achieving overall accuracy, purity, and completeness of $\sim65\%$. We show, for the first time, that a CNN trained on this diverse set of galaxies is capable of identifying major mergers, especially at early stages (74% accuracy), similar to that of networks trained at lower redshifts and/or higher masses (with accuracies between $66-80$%). We discuss the inherent limits of galaxy merger identification due to orientation angle, finding 98% of mergers are correctly identified from at least one angle, and 61% from the majority of angles. We additionally explore the confounding variables, such as star formation, to consider when applying to real data.This network enables the exploration of the impact of previously overlooked mergers of high mass ratio and low stellar masses on galaxy evolution in CANDELS, and can be expanded to surveys from $\textit{JWST}$, Rubin, $\textit{Roman}$, and $\textit{Euclid}$.

astro-ph.GA↗

arXiv2025

Recoiling Black Hole Candidates from Spatially Offset Broad Emission Lines in MaNGA

From the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, we identify 14 off-nuclear broad (FWHM>1000 km/s) Halpha and/or Hbeta emission line sources that indicate spatially offset active galactic nuclei (AGN) candidates. In addition to massive black holes (MBHs) in on-going galaxy mergers, this selection can also find MBHs that have been ejected from the host galaxy nucleus due to MBH binary coalescence and asymmetric gravitational wave emission or the dynamical `slingshot' mechanism. Recoiling/slingshot MBHs are predicted to affect co-evolution between MBHs and their host galaxies, and they are observational tracers of past binary MBH mergers and gravitational wave emission. This is the first systematic search through an integral field spectroscopy survey for ejected MBHs to enable uniform constraints on their surface densities. We find that 42% (6/14) have optical image counterparts consistent with galaxy stellar cores from infalling MBHs before the close binary MBH stage. The remaining 58% (8/14) have large broad line luminosities relative to their stellar core mass upper limits (~2 times larger than for central AGN), suggesting merger-driven MBH accretion enhancements or potentially ejected MBHs. The signatures of AGN-ionized narrow emission lines for recoil/slingshot candidates are weaker by 68%, which is consistent with the ejected MBH scenario. The broad line projected velocity offsets range from ~10-600 km/s and suggest motion within the host galaxy potentials. Finally, the implied recoiling MBH surface density upper limit is consistent with predictions that assume random spin orientations in MBH binaries.

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arXiv2025

Preferential Accretion onto the Secondary Black Hole Strengthens Gravitational Wave Signals

Pulsar timing arrays have recently found evidence for nanohertz gravitational waves that are consistent with being produced by a cosmological population of binary supermassive black holes (SMBHs). However, the amplitude of this gravitational wave background is larger than predicted from theoretical and empirical models of SMBH binary populations. We investigate preferential accretion onto the secondary, less massive SMBH of the binary as a potential solution to this discrepancy. We carry out the first observationally-based analysis of the effect of preferential accretion on the SMBH binary population, and we find that preferential accretion onto the secondary SMBH increases the binary SMBH mass ratio, causing many minor galaxy mergers to lead to major SMBH mergers. The fraction of SMBH mergers that are major mergers increases by a factor of 2-3 when preferential accretion is included. Further, we find that only a small amount of preferential accretion (10% total SMBH mass growth) is needed to bring the predicted gravitational wave background amplitude into agreement with observations. Preferential accretion has an even larger effect on gravitational wave signals detected by LISA, which will probe SMBH binaries at higher redshifts where the environment is more gas-rich, and can also help explain the rapid build up of overmassive black holes at high redshifts observed by the James Webb Space Telescope. It also shortens the time to the first detection of an individual SMBH binary emitting continuous waves. Preferential accretion strengthens the gravitational wave signals produced by any binary embedded in a circumbinary disk, including LIGO sources.

astro-ph.GA↗

arXiv2025

Spatially Offset Active Galactic Nuclei in the Very Large Array Sky Survey: Tracers of Galaxy Mergers and Wandering Massive Black Holes

The remnants of galaxy mergers may host multiple off-nuclear massive black holes (MBHs), some of which may wander indefinitely within the host galaxy halos. Tracing the population of offset MBHs is essential for understanding how the distribution of MBHs in the Universe evolves through galaxy mergers, the efficiency of binary MBH formation, and the rates at which MBHs are seeded in low-mass satellite galaxies. Offset MBHs can be observationally traced if they are accreting and detectable as spatially offset active galactic nuclei (AGN). In this work, we build the largest uniform sample of spatially offset AGN candidates (328) by matching sources from the Very Large Array Sky Survey (VLASS) to galaxies in the Sloan Digital Sky Survey (SDSS). Based on the radio source surface density, 29+/-3% are unrelated chance projections. The offset AGN occupation fraction is positively correlated with host galaxy stellar mass, consistent with predictions that most offset MBHs will reside in massive halos. However, this trend vanishes, and may reverse, at the lowest stellar masses, potentially reflecting the weaker host galaxy gravitational potentials. The offset AGN occupation fraction shows no significant evolution with orbital radius, and the agreement with predictions suggests a binary MBH formation rate of <0.5 per merger. Finally, for offset MBHs down to masses of 10^5 Solar masses, the occupation fraction is ~30-70 times lower than the expected value assuming all accreted satellites host a MBH. This result may suggest a relatively low MBH seeding efficiency.

astro-ph.GA↗

arXiv2025

Enhanced Star Formation and Black Hole Accretion Rates in Galaxy Mergers in IllustrisTNG50

Many theoretical and observational studies have suggested that galaxy mergers may trigger enhanced star formation or active galactic nuclei (AGN) activity. We present an analysis of merging and nonmerging galaxies from $0.2 \leq z \leq 3$ in the IllustrisTNG50 simulation. These galaxies encompass a range of masses ($M_\star > 10^{8}M_\odot$), multiple merger stages, and mass ratios ($\geq1:10$). We examine the effect that galaxy mergers have on star formation and black hole accretion rates in the TNG50 universe. We additionally investigate how galaxy and black hole mass, merger stage, merger mass ratio, and redshift affect these quantities. Mergers in our sample show excess specific star formation rates (sSFR) at $z \leq 3$ and enhanced specific black hole accretion rates (sBHAR) at $z \lesssim 2$. The difference between sSFRs and sBHARs in the merging sample compared to the non-merging sample increases as redshift decreases. Additionally, we show that these enhancements persist for at least $\sim1$ Gyr after the merger event. Investigating how mergers behave in the TNG50 simulation throughout cosmic time enables both a better appreciation of the importance of spatial resolution in cosmological simulations and a better basis to understand our high-$z$ universe with observations from $\textit{JWST}$.

astro-ph.GA↗

arXiv2025

The impact of applying black hole-host galaxy scaling relations to large galaxy populations

Supermassive black holes (SMBHs) with dynamically measured masses have shown empirical correlations with host galaxy properties. These correlations are often the only method available to estimate SMBH masses and gather statistics for large galaxy populations across a range of redshifts, even though the scaling relations themselves are derived from a small subset of nearby galaxies. Depending on the scaling relation used, estimated SMBH masses can vary significantly. The most widely used scaling relations are the M$_{BH}-$M$_{\mathrm{bulge}}$ and M$_{BH}- σ$ relations, where M$_{\mathrm{bulge}}$ is galaxy bulge mass and $σ$ is the bulge velocity dispersion. In this paper, we determine how severely the choice of scaling relation impacts SMBH mass estimates for different subsets of a large galaxy population. For this analysis we use a sample of $\sim$ 400,000 galaxies, including 1,240 Type 1 AGN from the Sloan Digital Sky Survey. We calculate SMBH masses from M$_{BH}-$M$_{\mathrm{bulge}}$ and M$_{BH}- σ$ and compare to single-epoch virial SMBH masses from broad-line H$β$, which are derived independently of black hole-host galaxy scaling relations. We find that SMBH masses derived from the single-epoch virial relation for H$β$ are better reproduced by M$_{BH}- σ$ than M$_{BH}-$M$_{\mathrm{bulge}}$. Finally, in cases where $σ$ and M$_{\mathrm{bulge}}$ cannot be measured directly, we show that it is possible to infer $σ$ from photometry with more accuracy than we can infer M$_{\mathrm{bulge}}$.

astro-ph.GA↗

arXiv2024

Evolution of the Dual AGN in Mrk 266: A Young AGN and a Rotation Dominated Disk in the SW Nucleus

Dual active galactic nuclei (AGN) offer a unique opportunity to probe the relationship between super massive black holes (SMBH) and their host galaxies as well as the role of major mergers in triggering AGN activity. The confirmed dual AGN Mrk 266 has been studied extensively with multi-wavelength imaging. Now, high spatial resolution IFU spectroscopy of Mrk 266 provides an opportunity to probe the kinematics of both the merger event and AGN feedback. We present for the first time high spatial resolution kinematic maps for both nuclei of Mrk 266 obtained with the Keck OSIRIS IFU spectrograph, utilizing adaptive optics to achieve a resolution of 0.31" and 0.20" for the NE and SW nuclei, respectively. Using the M-sigma relation for mergers, we infer a SMBH mass of approximately 7e7 solar masses for the southwestern nucleus. Additionally, we report that the molecular gas kinematics of the southwestern nucleus are dominated by rotation rather than large-scale chaotic motions. The southwest nucleus also contains both a circumnuclear ring of star formation from which an inflow of molecular gas is likely fueling the AGN and a compact, AGN-dominated outflow of highly ionized gas with a timescale of approximately 2 Myr, significantly shorter than the timescale of the merger. The northeastern nucleus, on the other hand, exhibits complex kinematics related to the merger, including molecular gas that appears to have decoupled from the rotation of the stars. Our results suggest that while the AGN activity in Mrk 266 was likely triggered during the merger, AGN feeding is currently the result of processes internal to each host galaxy, thus resulting in a strong asymmetry between the two nuclei.

astro-ph.GA↗

arXiv2024

Chandra Discovery of a Candidate Hyper-Luminous X-ray Source in MCG+11-11-032

We present a multi-wavelength analysis of MCG+11-11-032, a nearby AGN with the unique classification of both a binary and a dual AGN candidate. With new Chandra observations we aim to resolve any dual AGN system via imaging data, and search for signs of a binary AGN via analysis of the X-ray spectrum. Analyzing the Chandra spectrum, we find no evidence of previously suggested double-peaked Fe K$α$ lines; the spectrum is instead best fit by an absorbed powerlaw with a single Fe K$α$ line, as well as an additional line centered at $\approx$7.5 keV. The Chandra observation reveals faint, soft, and extended X-ray emission, possibly linked to low-level nuclear outflows. Further analysis shows evidence for a compact, hard source -- MCG+11-11-032 X2 -- located 3.27'' from the primary AGN. Modeling MCG+11-11-032 X2 as a compact source, we find that it is relatively luminous ($L_{\text{2$-$10 keV}} = 1.52_{-0.48}^{+0.96}\times 10^{41}$ erg s$^{-1}$), and the location is coincident with an compact and off-nuclear source resolved in Hubble Space Telescope infrared (F105W) and ultraviolet (F621M, F547M) bands. Pairing our X-ray results with a 144 MHz radio detection at the host galaxy location, we observe X-ray and radio properties similar to those of ESO 243-49 HLX-1, suggesting that MCG+11-11-032 X2 may be a hyper-luminous X-ray source. This detection with Chandra highlights the importance of a high-resolution X-ray imager, and how previous binary AGN candidates detected with large-aperture instruments benefit from high-resolution follow-up. Future spatially resolved optical spectra, and deeper X-ray observations, can better constrain the origin of MCG+11-11-032 X2.

astro-ph.HE↗

arXiv2024

Mapping AGN winds: a connection between radio-mode AGN and the AGN feedback cycle

We present a kinematic analysis based on the large Integral Field Spectroscopy (IFS) dataset of SDSS-IV MaNGA (10.000 galaxies). We have compiled a diverse sample of 594 unique Active Galactic Nuclei (AGN), identified through a variety of independent selection techniques, encompassing radio (1.4 GHz) observations, optical emission line diagnostics (BPT), broad Balmer emission lines, mid-infrared colors, and hard X-ray emission. We investigate how ionized gas kinematics behave in these different AGN populations through stacked radial profiles of the [OIII]~5007 emission-line width across each AGN population. We contrast AGN populations against each other (and non-AGN galaxies) by matching samples by stellar mass, [OIII]~5007 luminosity, morphology, and redshift. We find similar kinematics between AGN selected by BPT diagnostics compared to broad-line selected AGN. We also identify a population of non-AGN with similar radial profiles as AGN, indicative of the presence of remnant outflows (or fossil outflows) of a past AGN activity. We find that purely radio-selected AGN display enhanced ionized gas line widths across all radii. This suggests that our radio-selection technique is sensitive to a population where AGN-driven kinematic perturbations have been active for longer durations (potentially due to recurrent activity) than in purely optically selected AGN. This connection between radio activity and extended ionized gas outflow signatures is consistent with recent evidence that suggests radio emission (expected to be diffuse) originated due to shocks from outflows. We conclude that different selection techniques can trace different AGN populations not only in terms of energetics but also in terms of AGN evolutionary stages. Our results are important in the context of AGN duty cycle and highlight IFU data's potential to deepen our knowledge of AGN and galaxy evolution.

astro-ph.GA↗

arXiv2024

A Catalog of Broad Hα and Hβ Active Galactic Nuclei in MaNGA

Broad H$α$ and H$β$ emission lines (FWHM > 1,000 km s$^{-1}$) are incredibly efficient tracers of the high-velocity clouds encircling Active Galactic Nuclei (AGN). As a result, we search for these broad line AGN in the Sloan Digital Sky Survey's Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) catalog. We identify 301 broad-line H$α$ galaxies and 801 broad-line H$β$ galaxies in the catalog. In total, we detect 1,042 unique broad-line galaxies with luminosities between 10$^{37}$ - 10$^{43}$ erg s$^{-1}$; 60 feature both broad H$α$ and broad H$β$ emission. We also determine that the broad line region radius ranges between 0.01 - 46 light days, with a median radius of 0.1 light days (0.02 pc) for our broad H$β$ sample. In addition, we find that both samples feature a higher fraction of galaxy mergers (44% for the broad H$α$ sample and 43% for the broad H$β$ sample), compared to the full MaNGA galaxy sample (26%), which suggests that merger-driven fueling is strongly active in our sample.

astro-ph.GA↗

arXiv2024

An Excess of AGNs Triggered by Galaxy Mergers in MaNGA Galaxies of Stellar Mass $\sim10^{11}$ $M_{\odot}$

To facilitate new studies of galaxy merger driven fueling of active galactic nuclei (AGNs), we present a catalog of 387 AGNs that we have identified in the final population of over $10,000$ $z<0.15$ galaxies observed by the SDSS-IV integral field spectroscopy survey Mapping Nearby Galaxies at Apache Point Observatory (MaNGA). We selected the AGNs via mid-infrared WISE colors, Swift/BAT ultra hard X-ray detections, NVSS and FIRST radio observations, and broad emission lines in SDSS spectra. By combining the MaNGA AGN catalog with a new SDSS catalog of galaxy mergers that were identified based on a suite of hydrodynamical simulations of merging galaxies, we study the link between galaxy mergers and nuclear activity for AGNs above a limiting bolometric luminosity of $10^{44.4}$ erg s$^{-1}$. We find an excess of AGNs in mergers, relative to non-mergers, for galaxies with stellar mass $\sim10^{11}$ $M_{\odot}$, where the AGN excess is somewhat stronger in major mergers than in minor mergers. Further, when we combine minor and major mergers and sort by merger stage, we find that the highest AGN excess occurs in post-coalescence mergers in the highest mass galaxies. However, we find no evidence of a correlation between galaxy mergers and AGN luminosity or accretion rate. In summary, while galaxy mergers overall do appear to trigger or enhance AGN activity more than non-mergers, they do not seem to induce higher levels of accretion or higher luminosities. We provide the MaNGA AGN Catalog and the MaNGA Galaxy Merger Catalog for the community here.

astro-ph.GA↗

arXiv2024

Merger-Driven Growth of Intermediate-mass Black Holes: Constraints from Hubble Space Telescope Imaging of Hyper-luminous X-Ray Sources

Hyper-luminous X-ray sources (HLXs) are extragalactic off-nuclear X-ray sources with luminosities exceeding the theoretical limit for accretion onto stellar-mass compact objects. Many HLXs may represent intermediate-mass black holes (IMBHs) deposited in galaxy halos through mergers, and properties of the stellar cores surrounding HLXs provide powerful constraints on this scenario. Therefore, we have systematically built the largest sample of HLX candidates with archival Hubble Space Telescope (HST) imaging (24) for the first uniform population study of HLX stellar cores down to low masses. Based on their host galaxy redshifts, at least 21 (88%) have stellar core masses >=10^7 Msun and hence are consistent with accretion onto massive black holes from external galaxies. In 50% of the sample, the HST imaging reveals features connecting the HLXs with their host galaxies, strongly suggesting against the background/foreground contaminant possibility in these cases. Assuming a mass scaling relation for active galactic nuclei and accounting for an estimated contamination fraction of 29%, up to ~60% of our sample may be associated with IMBHs. Similar to previously known HLXs, the X-ray luminosities are systematically elevated relative to their stellar core masses, possibly from merger-driven accretion rate enhancements. The least massive stellar cores are preferentially found at larger nuclear offsets and are more likely to remain wandering in their host galaxy halos. The HLX galaxy occupation fraction is ~10^-2 and has a strong inverse mass dependence. Up to three of the HLX candidates (12%) are potentially consistent with formation within globular clusters or with exceptionally luminous X-ray binaries.

astro-ph.GA↗

arXiv2023

A Candidate Dual QSO at Cosmic Noon

We report the discovery of a candidate dual QSO at z=1.889, a redshift that is in the era known as "cosmic noon" where most of the Universe's black hole and stellar mass growth occurred. The source was identified in Hubble Space Telescope WFC3/IR images of a dust-reddened QSO that showed two closely-separated point sources at a projected distance of 0.26", or 2.2 kpc. This red QSO was targeted for imaging to explore whether red QSOs are hosted by merging galaxies. We subsequently obtained a spatially-resolved STIS spectrum of the system, covering the visible spectral range, and verifying the presence of two distinct QSO components. We also obtained high-resolution radio continuum observations with the VLBA at 1.4 GHz (21-cm L band) and found two sources coincident with the optical positions. The sources have similar black hole masses, bolometric luminosities, and radio loudness parameters. However, their colors and reddenings differ significantly. The redder QSO has a higher Eddington ratio, consistent with previous findings. We consider the possibility of gravitational lensing and and find that it would require extreme and unlikely conditions. If confirmed as a bona-fide dual QSO, this system would link dust-reddening to galaxy and supermassive black hole mergers, opening up a new population in which to search for samples of dual AGN.

astro-ph.GA↗

arXiv2023

A Census of WISE-selected Dual and Offset AGN Across the Sky: New Constraints on Merger-Driven Triggering of Obscured AGN

Pairs of galaxies hosting active galactic nuclei (AGN) are powerful probes of merger-driven supermassive black hole (SMBH) growth as they can resolve individual AGN and trace mergers over a large range of physical separations. To exploit this on a large scale for the first time for both obscured and unobscured AGN, we use photometric redshifts of AGN selected by the Wide-field Infrared Survey Explorer (WISE) to find probabilistic pairs (<100 kpc separations) across the sky, along with a comparison sample of inactive galaxy pairs. Our final sample of integrated pair probabilities yields 198 AGN-AGN pairs (dual AGN) and 2767 AGN-galaxy pairs (offset AGN) with uniformly measured AGN and host galaxy physical properties. We find the fraction of galaxy pairs hosting WISE AGN is dominated by offset AGN and significantly elevated above that of inactive galaxies for large host stellar masses. We show how the AGN merger fraction directly increases with AGN extinction for both offset and dual AGN, with up to ~40% of heavily obscured AGN found in galaxy pairs. Elevated AGN merger fractions coincide with increased host specific star formation rates that suggest merger-driven co-evolution of galaxies and SMBHs. Among dual AGN, the most rapid SMBH growth may occur within the less massive galaxy. Relative to stochastic mechanisms, mergers produce an excess of AGN at increasingly smaller separations, especially for obscured AGN (up to a factor of ~5), and augmented by correlated triggering. Finally, this excess is stronger than for lower luminosity optically-selected AGN, regardless of AGN obscuration level.

astro-ph.GA↗

arXiv2023

A Catalog of 71 Coronal Line Galaxies in MaNGA: [NeV] is an Effective AGN Tracer

Despite the importance of AGN in galaxy evolution, accurate AGN identification is often challenging, as common AGN diagnostics can be confused by contributions from star formation and other effects (e.g., Baldwin-Phillips-Terlevich diagrams). However, one promising avenue for identifying AGNs are ``coronal emission lines" (``CLs"), which are highly ionized species of gas with ionization potentials $\ge$ 100 eV. These CLs may serve as excellent signatures for the strong ionizing continuum of AGN. To determine if CLs are in fact strong AGN tracers, we assemble and analyze the largest catalog of optical CL galaxies using the Sloan Digital Sky Survey's Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) catalog. We detect CL emission in 71 MaNGA galaxies, out of the 10,010 unique galaxies from the final MaNGA catalog, with $\ge$ 5$σ$ confidence. In our sample, we measure [NeV]$λ$3347, $λ$3427, [FeVII]$λ$3586, $λ$3760, $λ$6086, and [FeX]$λ$6374 emission and crossmatch the CL galaxies with a catalog of AGNs that were confirmed with broad line, X-ray, IR, and radio observations. We find that [NeV] emission, compared to [FeVII] and [FeX] emission, is best at identifying high luminosity AGN. Moreover, we find that the CL galaxies with the least dust extinction yield the most iron CL detections. We posit that the bulk of the iron CLs are destroyed by dust grains in the galaxies with the highest [OIII] luminosities in our sample, and that AGN in the galaxies with low [OIII] luminosities are possibly too weak to be detected using traditional techniques.

astro-ph.GA↗

arXiv2022

The Redshift Evolution of Ultraluminous X-Ray Sources out to z~0.5: Comparison with X-Ray Binary Populations and Contribution to the Cosmic X-Ray Background

Ultraluminous X-ray sources (ULXs) are thought to be powerful X-ray binaries (XRBs) and may contribute significantly to the redshift-dependent X-ray emission from star forming galaxies. We have assembled a uniform sample of 259 ULXs over the redshift range z=0.002-0.51 to constrain their physical nature and their contribution to the Cosmic X-Ray Background (CXB). The sample is constructed by crossmatching galaxies from the Sloan Digital Sky Survey with the Chandra Source Catalog and selecting off-nuclear X-ray sources after applying astrometric corrections. The fraction of contaminants is ~30% and shows no evolution with redshift. The host galaxy star formation rates (SFRs) are systematically elevated relative to the parent sample when matched in host stellar mass. The specific SFRs suggest a slight preference for high-mass XRBs, and the X-ray luminosity scaling relations with host galaxy stellar mass and SFR indicate that the highest redshift sources represent relatively luminous XRB populations that dominate their host galaxy X-ray emission. The fraction of galaxies hosting at least one ULX of a given luminosity increases with redshift over the full range of our sample, as expected if ULXs are preferentially found in galaxies with high SFRs and low metallicities. At z~0.5, the ULX X-ray flux is consistent with the X-ray emission from star-forming galaxies. Moreover, ULXs may account for up to ~40% of the integrated flux from XRBs in the normal galaxy population out to z~0.5, suggesting they may contribute significantly to the overall ionizing radiation from galaxies.

astro-ph.GA↗

arXiv2022

Towards a More Complete Optical Census of Active Galactic Nuclei, Via Spatially-Resolved Spectroscopy

While emission-line flux ratio diagnostics are the most common technique for identifying active galactic nuclei (AGNs) in optical spectra, applying this approach to single fiber spectra of galaxies can omit entire subpopulations of AGNs. Here, we use spatially resolved spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to construct a sample of 10 galaxies where Baldwin-Philips-Terlevich line flux ratio diagnostics classify each galaxy's central $3^{\prime\prime}$ spectrum as LINER or star forming, while $>10\%$ of the spaxels in the galaxy's MaNGA footprint are classified as Seyfert. We obtain Chandra observations of these 10 galaxies with off-nuclear Seyfert regions to determine whether AGNs are actually present in them. Our main result is that 7-10 (depending on strictness of criteria) of the galaxies host one or more X-ray AGNs, even though none of them were classified as AGNs based on their single-fiber optical spectra. We find that these AGNs were not identified in the single-fiber spectra because they are AGNs in the nuclei of companion galaxies, low luminosity AGNs, dust obscured AGNs, and/or flickering AGNs. In summary, we find that off-nuclear AGN signatures may increase the number of known AGNs by a factor of two over what conventional single nuclear fiber spectra identify. Our results show that spatially resolved spectroscopy can be leveraged to reveal a more complete census of AGNs that are traditionally missed by single fiber spectra.

astro-ph.GA↗