arXiv subjects

Megan R. Sturm

Publications and source records attributed to Megan R. Sturm.

6 recordsLinked to original sources

arXiv2025

Chandra and HST Observations of Radio-Selected (Wandering) Massive Black Hole Candidates in Dwarf Galaxies

We present Chandra X-ray Observatory and Hubble Space Telescope (HST) follow-up observations of 12 dwarf galaxies from Reines et al. (2020) that are potential hosts of radio-selected active galactic nuclei (AGNs), eight of which are non-nuclear and possible ``wandering" black holes (BHs). Our multi-wavelength analysis indicates a heterogeneous sample with five radio sources detected at both X-ray and optical wavelengths within positional uncertainties and non-detections for the remaining objects. Of the radio objects detected in the X-ray/optical, three have multi-wavelength evidence for hosting nuclear massive BHs and one object is consistent with an extreme compact starburst. Only one of the off-nuclear radio sources has a significant optical counterpart and we present Palomar spectroscopy that identifies this object as a background AGN. We cannot definitively determine if the seven remaining off-nuclear radio sources are wandering massive BHs in the target dwarf galaxies or background AGNs, although the three sources with the largest offsets have compact radio cores detected with the Very Large Baseline Array and are consistent with expectations for background AGNs (Sargent et al 2022). Our HST sensitivity limits also allow for wandering massive BHs in the target dwarf galaxies that are hosted by stellar clusters with masses $\lesssim 10^6 M_\odot$.

astro-ph.GA↗

arXiv2025

Magellan Spectroscopy of AGNs in Low-mass Galaxies: Scaling Relations and a Triple-Peaked AGN

In this work, we aim to further populate the low-mass regime of black hole (BH) scaling relations to better understand the formation and growth mechanisms of central supermassive BHs. We target six galaxies that have been previously identified as hosting active galactic nuclei (AGN) based on optical spectroscopy from the Galaxy and Mass Assembly (GAMA) survey or the Sloan Digital Sky Survey (SDSS) with stellar masses reported to be M$_\star < 5 \times 10^9$ M$_\odot$. Using follow-up optical spectroscopy from the Magellan Echellette Spectrograph (MagE), we extract galaxy velocity dispersions ($σ_\star$) and estimate virial BH masses from broad H$α$ emission. We find that the galaxies in our sample do not deviate significantly from either the M$_{BH}-σ_\star$ or M$_{BH}-$M$_{\star}$ scaling relations defined by higher mass galaxies. Additionally, we identify one galaxy with triple-peaked SII, NII, H$α$ and H$β$ emission lines. This spectral shape is not shared by OIII and, in fact, the OIII line appears to have distinct kinematics from the other emission lines. Incorporating the spatial distribution of the various emission lines, we find that the galaxy spectrum is consistent with a prominent central AGN driving an outflow, surrounded by an extended ring/disk of gas predominantly ionized by shocks and/or star formation. This work has implications for the demographics of BHs in low-mass galaxies and the role of AGN feedback.

astro-ph.GA↗

arXiv2025

Star-Forming Nuclear Clusters in Dwarf Galaxies Mimicking AGN Signatures in the Mid-Infrared

Effectively finding and identifying active galactic nuclei (AGNs) in dwarf galaxies is an important step in studying black hole formation and evolution. In this work, we examine four mid-IR-selected AGN candidates in dwarf galaxies with stellar masses between $M_\star \sim 10^8 - 10^9 M_\odot$ , and find that the galaxies are host to nuclear star clusters (NSCs) that are notably rare in how young and massive they are. We perform photometric measurements on the central star clusters in our target galaxies galaxies using Hubble Space Telescope optical and near-IR imaging and compare their observed properties to models of stellar population evolution. We find that these galaxies are host to very massive ($\sim10^7 M_\odot$), extremely young ($\lesssim 8$ Myr), dusty ($0.6 \lesssim \mathrm{A_v} \lesssim 1.8$) nuclear star clusters. Our results indicate that these galactic nuclei have ongoing star-formation, are still at least partially obscured by clouds of gas and dust, and are most likely producing the extremely red AGN-like mid-IR colors. Moreover, prior work has shown that these galaxies do not exhibit X-ray or optical AGN signatures. Therefore, we recommend caution when using mid-IR color-color diagnostics for AGN selection in dwarf galaxies, since, as directly exemplified in this sample, they can be contaminated by massive star clusters with ongoing star formation.

astro-ph.GA↗

arXiv2024

A Breakdown of the Black Hole - Bulge Mass Relation in Local Active Galaxies

We investigate the relation between black hole (BH) mass and bulge stellar mass for a sample of 117 local ($z \sim 0$) galaxies hosting low-luminosity, broad-line active galactic nuclei (AGN). Our sample comes from Reines & Volonteri (2015), who found that, for a given $total$ stellar mass, these AGNs have BH masses more than an order of magnitude smaller than those in early-type galaxies with quiescent BHs. Here we aim to determine whether or not this AGN sample falls on the canonical BH-to-$bulge$ mass relation by utilizing bulge-disk decompositions and determining bulge stellar masses using color-dependent mass-to-light ratios. We find that our AGN sample remains offset by more than an order of magnitude from the $M_\mathrm{BH}-M_\mathrm{bulge}$ relation defined by early-type galaxies with dynamically detected BHs. We caution that using canonical BH-to-bulge mass relations for galaxies other than ellipticals and bulge-dominated systems may lead to highly biased interpretations. This work bears directly to predictions for gravitational wave detections and cosmological simulations that are tied to the local BH-to-bulge mass relations.

astro-ph.GA↗

arXiv2022

CLEAR: Paschen-$β$ Star Formation Rates and Dust Attenuation of Low Redshift Galaxies

We use \Pab\ (1282~nm) observations from the Hubble Space Telescope ($\HST$) G141 grism to study the star-formation and dust attenuation properties of a sample of 29 low-redshift ($z < 0.287$) galaxies in the CANDELS Ly$α$ Emission at Reionization (CLEAR) survey. We first compare the nebular attenuation from $\Pab/\Ha$ with the stellar attenuation inferred from the spectral energy distribution, finding that the galaxies in our sample are consistent with an average ratio of the continuum attenuation to the nebular gas of 0.44, but with a large amount of excess scatter beyond the observational uncertainties. Much of this scatter is linked to a large variation between the nebular dust attenuation as measured by (space-based) $\Pab$ to (ground-based) $\Ha$ to that from (ground-based) $\Ha/\Hb$. This implies there are important differences between attenuation measured from grism-based / wide-aperture $\Pab$ fluxes and the ground-based / slit-measured Balmer decrement. We next compare star-formation rates (SFRs) from $\Pab$ to those from dust-corrected UV. We perform a survival analysis to infer a census of \Pab\ emission implied by both detections and non-detections. We find evidence that galaxies with lower stellar mass have more scatter in their ratio of \Pab\ to attenuation-corrected UV SFRs. When considering our \Pab\ detection limits, this observation supports the idea that lower mass galaxies experience "burstier" star-formation histories. Together, these results show that \Pab\ is a valuable tracer of a galaxy's SFR, probing different timescales of star-formation and potentially revealing star-formation that is otherwise missed by UV and optical tracers.

astro-ph.GA↗

arXiv2022

The Sloan Digital Sky Survey Reverberation Mapping Project: UV-Optical Accretion Disk Measurements with Hubble Space Telescope

We present accretion-disk structure measurements from UV-optical reverberation mapping observations of a sample of eight quasars at 0.24<z<0.85. Ultraviolet photometry comes from two cycles of Hubble Space Telescope monitoring, accompanied by multi-band optical monitoring by the Las Cumbres Observatory network and Liverpool Telescopes. The targets were selected from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project sample with reliable black-hole mass measurements from Hbeta reverberation mapping results. We measure significant lags between the UV and various optical griz bands using JAVELIN and CREAM methods. We use the significant lag results from both methods to fit the accretion-disk structure using a Markov chain Monte Carlo approach. We study the accretion disk as a function of disk normalization, temperature scaling, and efficiency. We find direct evidence for diffuse nebular emission from Balmer and FeII lines over discrete wavelength ranges. We also find that our best-fit disk color profile is broadly consistent with the Shakura \& Sunyaev disk model. We compare our UV-optical lags to the disk sizes inferred from optical-optical lags of the same quasars and find that our results are consistent with these quasars being drawn from a limited high-lag subset of the broader population. Our results are therefore broadly consistent with models that suggest longer disk lags in a subset of quasars, for example, due to a nonzero size of the ionizing corona and/or magnetic heating contributing to the disk response.

astro-ph.HE↗