Search

We test the merger-induced dual active galactic nuclei (dAGN) paradigm using a sample of 35 radio galaxy pairs from the SDSS Stripe 82 field. Using Keck optical spectroscopy, we confirm 21 pairs have consistent redshifts, constituting kinematic pairs; the remaining 14 pairs are line-of-sight projections. We classify the optical spectral signatures via emission line ratios, equivalent widths, and excess of radio power above star-formation predicted outputs. We find 6 galaxies are classified as LINERs and 7 are AGN/starburst composites. Most of the LINERs are retired galaxies, while the composites likely have AGN contribution. All of the kinematic pairs exhibit radio power more than 10$\times$ above the level expected from just star-formation, suggestive of a radio AGN contribution. We also analyze high-resolution (0.3") imaging at 6 GHz from the NSF's Karl G. Jansky Very Large Array for 17 of the kinematic pairs. We find 6 pairs (2 new, 4 previously known) host two separate radio cores, confirming their status as dAGNs. The remaining 11 pairs contain single AGNs, with most exhibiting prominent jets/lobes overlapping their companion. Our final census indicates a dAGN duty cycle slightly higher than predictions of purely stochastic fueling, although a larger sample (potentially culled from VLASS) is needed to fully address the dAGN fraction. We conclude that while dAGNs in the Stripe 82 field are rare, the merger process plays some role in their triggering and it facilitates low to moderate levels of accretion., Comment: 26 pages, 10 figures, resubmitted to ApJ after addressing referee report, supplemental figures/data to be included in online journal

Kpc-scale triple active galactic nuclei (AGNs), potential precursors of gravitationally-bound triple massive black holes (MBHs), are rarely seen objects and believed to play an important role in the evolution of MBHs and their host galaxies. In this work we present a multi-band (3.0, 6.0 10.0, and 15.0 GHz), high-resolution radio imaging of the triple AGN candidate, SDSS J0849+1114, using the Very Large Array. Two of the three nuclei (A and C) are detected at 3.0, 6.0, and 15 GHz for the first time, both exhibiting a steep spectrum over 3--15 GHz (with a spectral index $-0.90 \pm 0.05$ and $-1.03 \pm 0.04$) consistent with a synchrotron origin. Nucleus A, the strongest nucleus among the three, shows a double-sided jet, with the jet orientation changing by $\sim20^{\circ}$ between its inner 1" and the outer 5.5" (8.1 kpc) components, which may be explained as the MBH's angular momentum having been altered by merger-enhanced accretion. Nucleus C also shows a two-sided jet, with the western jet inflating into a radio lobe with an extent of 1.5" (2.2 kpc). The internal energy of the radio lobe is estimated to be $\rm 5.0 \times 10^{55}$ erg, for an equipartition magnetic field strength of $\rm \sim 160\ \mu G$. No significant radio emission is detected at all four frequencies for nucleus B, yielding an upper limit of 15, 15, 15, and 18 $\rm \mu Jy\ beam^{-1}$ at 3.0, 6.0, 10.0, and 15.0 GHz, based on which we constrain the star formation rate in nucleus B to be $\lesssim 0.4~\rm M_{\odot}~yr^{-1}$., Comment: 11 pages, 3 figures, accepted for publication in ApJ. Comments welcome

NGC 4395 is a dwarf galaxy at a distance of about 4.3 Mpc (scale: ~0.021 pc mas$^{-1}$). It hosts an intermediate-mass black hole (IMBH) with a mass between ~10$^4$ and ~10$^5$ solar masses. The early radio observations of NGC 4395 with the very long baseline interferometry (VLBI) network, High Sensitivity Array (HSA), at 1.4 GHz in 2005 showed that its nucleus has a sub-mJy outflow-like feature (E) extending over 15 mas. To probe the possibility of the feature E as a continuous jet with a base physically coupled with the accretion disc, we performed deep VLBI observations with the European VLBI Network (EVN) at 5 GHz, and analysed the archival data obtained with the HSA at 1.4 GHz in 2008, NSF's Karl G. Jansky Very Large Array (VLA) at 12-18 GHz and the Atacama Large Millimetre/submillimetre Array (ALMA) at 237 GHz. The feature E displays more diffuse structure in the HSA image of 2008 and has no compact substructure detected in the EVN image. Together with the optically thin steep spectrum and the extremely large angular offset (about 220 mas) from the accurate optical Gaia position, we explain the feature E as nuclear shocks likely formed by the IMBH's episodic ejection or wide-angle outflow. The VLA and ALMA observations find a sub-mJy pc-scale diffuse feature, possibly tracing a thermal free-free emission region near the IMBH. There is no detection of a jet base at the IMBH position in the VLBI maps. The non-detections give an extremely low luminosity of <=4.7 x 10$^{33}$ erg s$^{-1}$ at 5 GHz and indicate no evidence of a disc-jet coupling on sub-pc scales., Comment: 10 pages, 4 figures, accepted for publication in MNRAS

The merger of two galaxies, each hosting a supermassive black hole (SMBH) of mass $10^6$\,M$_{\odot}$ or more, could yield a bound SMBH binary. For the early-type galaxy NGC\,4472, we study how astrometry with a next-generation Very Large Array (ngVLA) could be used to monitor the reflex motion of the primary SMBH of mass $M_{\rm pri}$, as it is tugged on by the secondary SMBH of mass $M_{\rm sec}$. Casting the orbit of the putative SMBH binary in terms of its period $P$, semimajor axis $a_{\rm bin}$, and mass ratio $q = M_{\rm sec} / M_{\rm pri} \le 1$, we find the following: (1) Orbits with fiducial periods of $P = 4$\,yr and 40\,yr could be spatially resolved and monitored. (2) For a 95\% accuracy of $2\,\mu$as per monitoring epoch, sub-parsec values of $a_{\rm bin}$ could be accessed over a range of mass ratios notionally encompassing major ($q > \frac{1}{4}$) and minor ($q < \frac{1}{4}$) galaxy mergers. (3) If no reflex motion is detected for $M_{\rm pri}$ after 1(10)\,yr of monitoring, a SMBH binary with period $P = 4(40)$\,yr and mass ratio $q > 0.01(0.003)$ could be excluded. This would suggest no present-day evidence for a past major merger like that recently simulated, where scouring by a $q \sim 1$ SMBH binary formed a stellar core with kinematic traits like those of NGC\,4472. (4) Astrometric monitoring could independently check the upper limits on $q$ from searches for continuous gravitational waves from NGC\,4472., Comment: 7 pages, 2 figures, accepted by ApJ; builds upon our ngVLA Memo. #90 (arXiv:2108.07341)

In an EHT study of a Jy-level target, Safarzadeh et al. (2019) show how astrometric monitoring could constrain massive black hole binaries with the wide separations that make them long-lived against gravitational wave losses, and with the small mass ratios expected from merged satellite galaxies. With this ngVLA study, we show how such frontier topics could be explored for the more numerous mJy-level targets, such as NGC\,4472. We also discuss how ngVLA astrometric monitoring could test the upper limits from pulsar timing arrays on gravitational waves from NGC\,4472., Comment: 4 pages, 1 figure, Next Generation Very Large Array (ngVLA) Memo # 90

Intermediate-mass black holes (IMBHs) by definition have masses of $M_{\rm IMBH} \sim 10^{2-5}~M_\odot$, a range with few observational constraints. Finding IMBHs in globular star clusters (GCs) would validate a formation channel for massive black-hole seeds in the early universe. Here, we simulate a 60-hour observation with the next-generation Very Large Array (ngVLA) of 728 GC candidates in the Virgo Cluster galaxy NGC\,4472. Interpreting the radio detection thresholds as signatures of accretion onto IMBHs, we benchmark IMBH mass thresholds in three scenarios and find the following: (1) Radio analogs of ESO\,243-49 HLX-1, a strong IMBH candidate with $M_{\rm IMBH}^{\rm HLX} \sim 10^{4-5}~M_\odot$ in a star cluster, are easy to access in all 728 GC candidates. (2) For the 30 GC candidates with extant X-ray detections, the empirical fundamental-plane relation involving black hole mass plus X-ray and radio luminosities suggests access to $M_{\rm IMBH}^{\rm FP} \sim 10^{1.7-3.6}~M_\odot$, with an uncertainty of 0.44 dex. (3) A fiducial Bondi accretion model was applied to all 728 GC candidates and to radio stacks of GC candidates. This model suggests access to IMBH masses, with a statistical uncertainty of 0.39 dex, of $M_{\rm IMBH}^{\rm B} \sim 10^{4.9-5.1}~M_\odot$ for individual GC candidates and $M_{\rm IMBH}^{\rm B,stack} \sim 10^{4.5}~M_\odot$ for radio stacks of about 100-200 GC candidates. The fiducial Bondi model offers initial guidance, but is subject to additional systematic uncertainties and should be superseded by hydrodynamical simulations of gas flows in GCs., Comment: 8 pages, 6 figures, accepted 2021 June 25 by ApJ

The NSF's Karl G.\ Jansky Very Large Array (VLA) was used at 3~cm to search for accretion signatures from intermediate-mass black holes (IMBHs) in 19 globular star clusters (GCs) in NGC\,3115, an early-type galaxy at a distance of 9.4 Mpc. The 19 have stellar masses $M_{\star} \sim (1.1 - 2.7) \times 10^6~M_\odot$, with a mean $\overline{M_{\star}} \sim 1.8 \times 10^6~M_\odot$. None were detected. An IMBH accretion model was applied to the individual GCs and their radio stack. The radio-stacked GCs have an IMBH mass $\overline{M_{\rm IMBH}} < 1.7 \times 10^5~M_\odot$ and mass fraction $\overline{M_{\rm IMBH}} / \overline{M_{\star}} < 9.5\%$, with each limit being uncertain by a factor of about 2.5. The latter limit contrasts with the extremes of some stripped nuclei, suggesting that the set of stacked GCs in NGC\,3115 is not a set of such nuclei. The radio luminosities of the individual GCs correspond to X-ray luminosities $L_{\rm X} < (3.3 - 10) \times 10^{38}$ erg~s$^{-1}$, with a factor of about 2.5 uncertainty. These limits predicted for putative IMBHs in the GCs are consistent with extant {\em Chandra} observations. Finally, a simulated observation with a next-generation VLA (ngVLA) demonstrates that accretion signatures from IMBHs in GCs can be detected in a radio-only search, yet elude detection in an X-ray-only search due to confusion from X-ray binaries in the GCs., Comment: 8 pages, 4 figures, accepted 2020 July 22 by ApJ

We report the discovery of SDSS J0849+1114 as the first known triple Type 2 Seyfert nucleus. It represents three active black holes that are identified from new spatially resolved optical slit spectroscopy using the Dual Imaging Spectrograph on the 3.5 m telescope at the Apache Point Observatory. We also present new complementary observations including the Hubble Space Telescope Wide Field Camera 3 U- and Y-band imaging, Chandra Advanced CCD Imaging Spectrometer S-array X-ray 0.5--8 keV imaging spectroscopy, and NSF Karl G. Jansky Very Large Array radio 9.0 GHz imaging in its most extended A configuration. These comprehensive multiwavelength observations, when combined together, strongly suggest that all three nuclei are active galactic nuclei. While they are now still at kiloparsec-scale separations, where the host-galaxy gravitational potential dominates, the black holes may evolve into a bound triple system in $\lesssim$2 Gyr. These triple merger systems may explain the overly massive stellar cores that have been observed in some elliptical galaxies such as M87, which are expected to be unique gravitational wave sources. Similar systems may be more common in the early universe, when galaxy mergers are thought to have been more frequent., Comment: 19 pages, 8 figures, 6 tables; ApJ published version

The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution ($\approx$2.5"), sensitivity (a 1$\sigma$ goal of 70 $\mu$Jy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2-4 GHz). The first observations began in September 2017, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hours of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (Declination $>-40^{\circ}$), a total of 33,885 deg$^2$. The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an "on the fly" interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations., Comment: 34 pages, accepted by PASP (modified from prior version to address referee's and coauthor comments). (v2) Minor fixes to author list

Merger simulations predict that tidally induced gas inflows can trigger kpc-scale dual active galactic nuclei (dAGN) in heavily obscured environments. Previously with the Very Large Array, we have confirmed four dAGN with redshifts between $0.04 < z < 0.22$ and projected separations between 4.3 and 9.2 kpc in the SDSS Stripe 82 field. Here, we present $Chandra$ X-ray observations that spatially resolve these dAGN and compare their multi-wavelength properties to those of single AGN from the literature. We detect X-ray emission from six of the individual merger components and obtain upper limits for the remaining two. Combined with previous radio and optical observations, we find that our dAGN have properties similar to nearby low-luminosity AGN, and they agree well with the black hole fundamental plane relation. There are three AGN-dominated X-ray sources, whose X-ray hardness-ratio derived column densities show that two are unobscured and one is obscured. The low obscured fraction suggests these dAGN are no more obscured than single AGN, in contrast to the predictions from simulations. These three sources show an apparent X-ray deficit compared to their mid-infrared continuum and optical [OIII] line luminosities, suggesting higher levels of obscuration, in tension with the hardness-ratio derived column densities. Enhanced mid-infrared and [OIII] luminosities from star formation may explain this deficit. There is ambiguity in the level of obscuration for the remaining five components since their hardness ratios may be affected by non-nuclear X-ray emissions, or are undetected altogether. They require further observations to be fully characterized., Comment: 11 pages, 5 figures, Accepted for publication in the Astrophysical Journal

We have compelling evidence for stellar-mass black holes (BHs) of ~5-80 M_sun that form through the death of massive stars. We also have compelling evidence for so-called supermassive BHs (10^5-10^10 M_sun) that are predominantly found in the centers of galaxies. We have very good reason to believe there must be BHs with masses in the gap between these ranges: the first ~10^9 M_sun BHs are observed only hundreds of millions of years after the Big Bang, and all theoretically viable paths to making supermassive BHs require a stage of "intermediate" mass. However, no BHs have yet been reliably detected in the 100-10}^5 M_sun mass range. Uncovering these intermediate-mass BHs of 10^3-10^5 M_sun is within reach in the coming decade. In this white paper we highlight the crucial role that 30-m class telescopes will play in dynamically detecting intermediate-mass black holes, should they exist., Comment: Science White Paper Submitted for the Astro2020 Decadal Survey on Astronomy and Astrophysics

Massive 10^6-10^10 Msun black holes (BHs) are ubiquitous in local galactic nuclei. They were common by the time the Universe is several Gyr old, and many of them were in place within the first 1~Gyr after the Big Bang. Their quick assembly has been attributed to mechanisms such as the rapid collapse of gas into the nuclei of early protogalaxies, accretion and mergers of stellar-mass BHs accompanying structure formation at early times, and the runaway collapse of early, ultra-dense stellar clusters. The origin of the early massive BHs remains an intriguing and long-standing unsolved puzzle in astrophysics. Here we discuss strategies for discerning between BH seeding models using electromagnetic observations. We argue that the most direct answers will be obtained through detection of BHs with masses M<10^5 Msun at redshifts z>10, where we expect them to first form. Reaching out to these redshifts and down to these masses is crucial, because BHs are expected to lose the memory of their initial assembly by the time they grow well above 10^5 Msun and are incorporated into higher-mass galaxies. The best way to detect 10^4-10^5 Msun BHs at high redshifts is by a sensitive X-ray survey. Critical constraining power is augmented by establishing the properties and the environments of their host galaxies in deep optical/IR imaging surveys. Required OIR data can be obtained with the JWST and WFIRST missions. The required X-ray flux limits (down to 10^{-19} erg/s/cm^2) are accessible only with a next-generation X-ray observatory which has both high (sub-1") angular resolution and high throughput. A combination of deep X-ray and OIR surveys will be capable of probing several generic markers of the BH seed scenarios, and resolving the long-stanging puzzle of their origin. These electromagnetic observations are also highly synergistic with the information from LISA on high-z BH mergers., Comment: Science white paper submitted to the Astro2020 Decadal Survey

Intermediate-mass black holes (IMBHs) have masses of about 100 to 100,000 solar masses. They remain elusive. Observing IMBHs in present-day globular clusters (GCs) would validate a formation channel for seed black holes in the early universe and inform event predictions for gravitational wave facilities. Reaching a large number of GCs per galaxy is key, as models predict that only a few percent will have retained their gravitational-wave fostering IMBHs. Related, many galaxies will need to be examined to establish a robust sample of IMBHs in GCs. These needs can be meet by using a next-generation Very Large Array (ngVLA) to search for IMBHs in the GCs of hundreds of galaxies out to a distance of 25 Mpc. These galaxies hold tens of thousands of GCs in total. We describe how to convert an ngVLA signal from a GC to an IMBH mass according to a semi-empirical accretion model. Simulations of gas flows in GCs would help to improve the robustness of the conversion. Also, self-consistent dynamical models of GCs, with stellar and binary evolution in the presence of IMBHs, would help to improve IMBH retention predictions for present-day GCs., Comment: 7 pages; 2 figures; an Astro2020 Science White Paper. arXiv admin note: substantial text overlap with arXiv:1806.06052

The nearby galaxy NGC 3115 contains a known radio-emitting, low-luminosity active galactic nucleus (AGN), and was recently claimed to host a candidate AGN displaced 14.3 pc from the galaxy's optical photocenter. Our goal is to understand whether this represents a single offset AGN, an AGN in orbit around a central black hole, or something else. We present a new, sensitive (RMS = 4.4 $\mu$Jy beam$^{-1}$) 10 GHz image, which finds evidence for only one AGN. We place a stringent limit on the radio luminosity of any secondary supermassive black hole of $L_{10~\rm{GHz}}<5.8\times10^{33}$ ergs/s. An analysis of the relative positioning of the radio core, X-ray nucleus, and stellar bulge in this galaxy indicate that the radio source is centrally located, and not offset from the galactic bulge. This provides an argument against a single offset AGN in NGC 3115, however does not provide conclusive evidence against the purported offset AGN as an in-spiralling secondary black hole., Comment: Accepted to ApJ

Using the Next Generation Very Large Array (ngVLA), we will make a comprehensive inventory of intermediate-mass black holes (IMBHs) in hundreds of globular cluster systems out to a distance of 25 Mpc. IMBHs have masses of about 100 to 100,000 solar masses. Finding them in globular clusters would validate a formation channel for seed black holes in the early universe and inform event predictions for gravitational wave facilities. Reaching a large number of globular clusters is key, as Fragione et al. (2018) predict that only a few percent will have retained their gravitational-wave fostering IMBHs., Comment: An ngVLA Science Book chapter; 8 pages; 3 figures

We use NSF's Karl G. Jansky Very Large Array to constrain the mass of ionized gas in 206 globular star clusters (GCs) in M81, a nearby spiral galaxy. We detect none of the GCs and impose a typical gas-mass upper limit of 550 solar masses (3-sigma). These findings bear on GC evolution in M81., Comment: 3 pages; 1 figure, accepted by RNAAS

The nearby dwarf galaxy NGC404 harbors a low-luminosity active galactic nucleus (AGN) powered by the lowest-mass (< 150,000 solar-masses) central massive black hole (MBH) with a dynamical mass constraint currently known, thus providing a rare low-redshift analog to the MBH "seeds" that formed in the early Universe. Here, we present new imaging of the nucleus of NGC404 at 12-18 GHz with the Karl G. Jansky Very Large Array (VLA) and observations of the CO(2-1) line with the Atacama Large Millimeter/Submillimeter Array (ALMA). For the first time, we have successfully resolved the nuclear radio emission, revealing a centrally peaked, extended source spanning 17 pc. Combined with previous VLA observations, our new data place a tight constraint on the radio spectral index and indicate an optically-thin synchrotron origin for the emission. The peak of the resolved radio source coincides with the dynamical center of NGC404, the center of a rotating disk of molecular gas, and the position of a compact, hard X-ray source. We also present evidence for shocks in the NGC404 nucleus from archival narrowband HST imaging, Chandra X-ray data, and Spitzer mid-infrared spectroscopy, and discuss possible origins for the shock excitation. Given the morphology, location, and steep spectral index of the resolved radio source, as well as constraints on nuclear star formation from the ALMA CO(2-1) data, we find the most likely scenario for the origin of the radio source in the center of NGC404 to be a radio outflow associated with a confined jet driven by the active nucleus., Comment: Accepted to ApJ - 19 pages, 9 figures

Supermassive binary black holes (BBHs) on sub-parsec scales are prime targets for gravitational wave experiments. They also provide insights on close binary evolution and hierarchical structure formation. Sub-parsec BBHs cannot be spatially resolved but indirect methods can identify candidates. In 2015 Liu et al. reported an optical-continuum periodicity in the quasar PSO J334.2028+01.4075, with the estimated mass and rest-frame period suggesting an orbital separation of about 0.006 pc (0.7 micro-arcsec). The persistence of the quasar's optical periodicity has recently been disfavored over an extended baseline. However, if a radio jet is launched from a sub-parsec BBH, the binary's properties can influence the radio structure on larger scales. Here, we use the Very Long Baseline Array (VLBA) and Karl G. Jansky Very Large Array (VLA) to study the parsec- and kiloparsec-scale emission energized by the quasar's putative BBH. We find two VLBA components separated by 3.6 mas (30 pc), tentatively identifying one as the VLBA "core" from which the other was ejected. The VLBA components contribute to a point-like, time-variable VLA source that is straddled by lobes spanning 8 arcsec (66 kpc). We classify PSO J334.2028+01.4075 as a lobe-dominated quasar, albeit with an atypically large twist of 39 deg between its elongation position angles on parsec- and kiloparsec-scales. By analogy with 3C 207, a well-studied lobe-dominated quasar with a similarly-rare twist, we speculate that PSO J334.2028+01.4075 could be ejecting jet components over an inner cone that traces a precessing jet in a BBH system., Comment: 6 pages, 3 figures, 3 tables. Accepted for publication in MNRAS

We present a 1.4 GHz Karl G. Jansky Very Large Array (VLA) study of a sample of early-type galaxies (ETGs) from the volume- and magnitude-limited ATLAS-3D survey. The radio morphologies of these ETGs at a resolution of 5" are diverse and include sources that are compact on sub-kpc scales, resolved structures similar to those seen in star-forming spiral galaxies, and kpc-scale radio jets/lobes associated with active nuclei. We compare the 1.4 GHz, molecular gas, and infrared (IR) properties of these ETGs. The most CO-rich ATLAS-3D ETGs have radio luminosities consistent with extrapolations from H_2-mass-derived star formation rates from studies of late-type galaxies. These ETGs also follow the radio-IR correlation. However, ETGs with lower molecular gas masses tend to have less radio emission relative to their CO and IR emission compared to spirals. The fraction of galaxies in our sample with high IR-radio ratios is much higher than in previous studies, and cannot be explained by a systematic underestimation of the radio luminosity due to the presence extended, low-surface-brightness emission that was resolved-out in our VLA observations. In addition, we find that the high IR-radio ratios tend to occur at low IR luminosities, but are not associated with low dynamical mass or metallicity. Thus, we have identified a population of ETGs that have a genuine shortfall of radio emission relative to both their IR and molecular gas emission. A number of mechanisms may conspire to cause this radio deficiency, including a bottom-heavy stellar initial mass function, weak magnetic fields, a higher prevalence of environmental effects compared to spirals and enhanced cosmic ray losses., Comment: accepted for publication in MNRAS

Nantais et al. used the Hubble Space Telescope to localize probable globular clusters (GCs) in M81, a spiral galaxy at a distance of 3.63 Mpc. Theory predicts that GCs can host intermediate-mass black holes (IMBHs) with masses M_BH \sim 100 - 100,000 M_sun. Finding IMBHs in GCs could validate a formation channel for seed BHs in the early universe, bolster gravitational-wave predictions for space missions, and test scaling relations between stellar systems and the central BHs they host. We used the NRAO Karl G. Jansky Very Large Array (VLA) to search for the radiative signatures of IMBH accretion from 206 probable GCs in a mosaic of M81. The observing wavelength was 5.5 cm and the spatial resolution was 1.5 arcsec (26.4 pc). None of the individual GCs are detected, nor are weighted-mean image stacks of the 206 GCs and the 49 massive GCs with stellar masses M_star \gtrsim 200,000 M_sun. We apply a semi-empirical model to predict the mass of an IMBH that, if undergoing accretion in the long-lived hard X-ray state, is consistent with a given radio luminosity. The 3$\sigma$ radio-luminosity upper limits correspond to mean IMBH masses of M_BH(all) < 42,000 M_sun for the all-cluster stack and M_BH(massive) < 51,000 M_sun for the massive-cluster stack. We also apply the empirical fundamental-plane relation to two X-ray-detected clusters, finding that their individual IMBH masses at 95% confidence are M_BH < 99,000 M_sun and M_BH < 15,000 M_sun. Finally, no analog of HLX-1, a strong IMBH candidate in an extragalactic star cluster, occurs in any individual GC in M81. This underscores the uniqueness or rarity of the HLX-1 phenomenon., Comment: 23 pages; 7 figures; accepted by AJ

We present the results of a high-resolution, 5 GHz, Karl G. Jansky Very Large Array study of the nuclear radio emission in a representative subset of the Atlas3D survey of early-type galaxies (ETGs). We find that 51 +/- 4% of the ETGs in our sample contain nuclear radio emission with luminosities as low as 10^18 W/Hz. Most of the nuclear radio sources have compact (< 25-110 pc) morphologies, although < 10% display multi-component core+jet or extended jet/lobe structures. Based on the radio continuum properties, as well as optical emission line diagnostics and the nuclear X-ray properties, we conclude that the majority of the central 5 GHz sources detected in the Atlas3D galaxies are associated with the presence of an active galactic nucleus (AGN). However, even at sub-arcsecond spatial resolution, the nuclear radio emission in some cases appears to arise from low-level nuclear star formation rather than an AGN, particularly when molecular gas and a young central stellar population is present. This is in contrast to popular assumptions in the literature that the presence of a compact, unresolved, nuclear radio continuum source universally signifies the presence of an AGN. Additionally, we examine the relationships between the 5 GHz luminosity and various galaxy properties including the molecular gas mass and - for the first time - the global kinematic state. We discuss implications for the growth, triggering, and fueling of radio AGNs, as well as AGN-driven feedback in the continued evolution of nearby ETGs., Comment: Accepted for publication in MNRAS. 53 pages, 16 figures

Representing simultaneous black hole accretion during a merger, binary active galactic nuclei (AGNs) could provide valuable observational constraints to models of galaxy mergers and AGN triggering. High-resolution radio interferometer imaging offers a promising method to identify a large and uniform sample of binary AGNs, because it probes a generic feature of nuclear activity and is free from dust obscuration. Our previous search yielded 52 strong candidates of kpc-scale binaries over the 92 deg$^2$ of the Sloan Digital Sky Survey (SDSS) Stripe 82 area with 2"-resolution Very Large Array (VLA) images. Here we present 0.3"-resolution VLA 6 GHz observations for six candidates that have complete optical spectroscopy. The new data confirm the binary nature of four candidates and identify the other two as line-of-sight projections of radio structures from single AGNs. The four binary AGNs at $z \sim 0.1$ reside in major mergers with projected separations of 4.2-12 kpc. Optical spectral modeling shows that their hosts have stellar masses between $10.3 < log(M_\star/M_\odot) < 11.5$ and velocity dispersions between $120 < \sigma_\star < 320$ km/s. The radio emission is compact ($<$0.4") and show steep spectrum ($-1.8 < \alpha < -0.5$) at 6 GHz. The host galaxy properties and the Eddington-scaled accretion rates broadly correlate with the excitation state, similar to the general radio-AGN population at low redshifts. Our estimated binary AGN fraction indicates that simultaneous accretion occurs $>23^{+15}_{-8}$% of the time when a kpc-scale galaxy pair is detectable as a radio-AGN. The high duty cycle of the binary phase strongly suggests that major mergers can trigger and synchronize black hole accretion., Comment: accepted by ApJ Letters

Forbes et al. recently used the Hubble Space Telescope to localize hundreds of candidate star clusters in NGC 1023, an early-type galaxy at a distance of 11.1 Mpc. Old stars dominate the light of 92% of the clusters and intermediate-age stars dominate the light of the remaining 8%. Theory predicts that clusters with such ages can host intermediate-mass black holes (IMBHs) with masses M_BH \lesssim 10^5 M_sun. To investigate this prediction, we used 264 s of 5.5 GHz data from the Karl G. Jansky Very Large Array (VLA) to search for the radiative signatures of IMBH accretion from 337 candidate clusters in an image spanning 492 arcsec (26 kpc) with a resolution of 0.40 arcsec (22 pc). None of the individual clusters are detected, nor are weighted-mean image stacks of the 311 old clusters, the 26 intermediate-age clusters, and the 20 clusters with stellar masses M_star \gtrsim 7.5 x 10^5 M_sun. The clusters thus lack radio analogs of HLX-1, a strong IMBH candidate in a cluster in the early-type galaxy ESO 243-49. This suggests that HLX-1 is accreting gas related to its cluster's light-dominating young stars. Alternatively, the HLX-1 phenomenon could be so rare that no radio analog is expected in NGC 1023. Also, using a formalism heretofore applied to star clusters in the Milky Way, the radio-luminosity upper limit for the massive-cluster stack corresponds to a mean 3$\sigma$ IMBH mass of M_BH(massive) < 2.3 x 10^5 M_sun, suggesting mean black-hole mass fractions of M_BH(massive)/M_star < 0.05-0.29., Comment: 19 pages; 6 figures; accepted by AJ

Galaxy mergers play an important role in the growth of galaxies and their supermassive black holes. Simulations suggest that tidal interactions could enhance black hole accretion, which can be tested by the fraction of binary active galactic nuclei (AGNs) among galaxy mergers. But determining the fraction requires a statistical sample of binaries. We have identified kpc-scale binary AGNs directly from high-resolution radio imaging. Inside the 92 square deg covered by the high-resolution Very Large Array survey of the Sloan Digital Sky Survey (SDSS) Stripe 82 field, we identified 22 grade A and 30 grade B candidates of binary radio AGNs with angular separations less than 5" (10 kpc at z = 0.1). Eight of the candidates have optical spectra for both components from the SDSS spectroscopic surveys and our Keck program. Two grade B candidates are projected pairs, but the remaining six candidates are all compelling cases of binary AGNs based on either emission line ratios or the excess in radio power compared to the H-alpha-traced star formation rate. Only two of the six binaries were previously discovered by an optical spectroscopic search. Based on these results, we estimate that ~60% of our binary candidates would be confirmed once we obtain complete spectroscopic information. We conclude that wide-area high-resolution radio surveys offer an efficient method to identify large samples of binary AGNs. These radio-selected binary AGNs complement binaries identified at other wavelengths and are useful for understanding the triggering mechanisms of black hole accretion., Comment: ApJ accepted

SDSS J150243.09+111557.3 is a merging system at z = 0.39 that hosts two confirmed AGN, one unobscured and one dust-obscured, offset by several kiloparsecs. Deane et al. recently reported evidence from the European VLBI Network (EVN) that the dust-obscured AGN exhibits two flat-spectrum radio sources, J1502SE/SW, offset by 26 mas (140 pc), with each source being energized by its own supermassive black hole (BH). This intriguing interpretation of a close binary BH was reached after ruling out a double-hotspot scenario, wherein both hotspots are energized by a single, central BH, a configuration occuring in the well-studied Compact Symmetric Objects. When observed with sufficient sensitivity and resolution, an object with double hotspots should have an edge-brightened structure. We report evidence from the Very Long Baseline Array (VLBA) for just such a structure in an image of the obscured AGN with higher sensitivity and resolution than the EVN images. We thus conclude that a double-hotspot scenario should be reconsidered as a viable interpretation for J1502SE/SW, and suggest further VLBA tests of that scenario. A double-hotspot scenario could have broad implications for feedback in obscured AGNs. We also report a VLBA detection of high-brightness-temperature emssion from the unobscured AGN that is offset several kiloparsecs from J1502SE/SW., Comment: 12 pages, 1 figure, accepted by ApJL on 2014 July 27

COSMOS J100043.15+020637.2 is a merger remnant at z = 0.36 with two optical nuclei, NW and SE, offset by 500 mas (2.5 kpc). Prior studies suggest two competing scenarios for these nuclei: (1) SE is an active galactic nucleus (AGN) lost from NW due to a gravitational-wave recoil. (2) NW and SE each contain an AGN, signaling a gravitational-slingshot recoil or inspiralling AGNs. We present new images from the Very Large Array (VLA) at a frequency nu = 9.0 GHz and a FWHM resolution theta = 320 mas (1.6 kpc), and the Very Long Baseline Array (VLBA) at nu = 1.52 GHz and theta = 15 mas (75 pc). The VLA imaging is sensitive to emission driven by AGNs and/or star formation, while the VLBA imaging is sensitive only to AGN-driven emission. No radio emission is detected at these frequencies. Folding in prior results, we find: (a) The properties of SE and its adjacent X-ray feature resemble those of the unobscured AGN in NGC 4151, albeit with a much higher narrow emission-line luminosity. (b) The properties of NW are consistent with it hosting a Compton-thick AGN that warms ambient dust, photoionizes narrow emission-line gas and is free-free absorbed by that gas. Finding (a) is consistent with scenarios (1) and (2). Finding (b) weakens the case for scenario (1) and strengthens the case for scenario (2). Follow-up observations are suggested., Comment: 6 pages; 2 figures; emulateapj.cls; accepted by ApJ

We present new high spatial resolution Karl G. Jansky Very Large Array (VLA) HI absorption and Very Long Baseline Array (VLBA) continuum observations of the Active Galactic Nucleus (AGN)-driven molecular outflow candidate NGC 1266. Although other well-known systems with molecular outflows may be driven by star formation in a central molecular disk, the molecular mass outflow rate reported in Alatalo et al. (2011) in NGC 1266 of 13 M$_{\odot}$ year$^{-1}$ exceeds star formation rate estimates from a variety of tracers. This suggests that an additional energy source, such as an AGN, may play a significant role in powering the outflow. Our high spatial resolution HI absorption data reveal compact absorption against the radio continuum core co-located with the putative AGN, and the presence of a blueshifted spectral component re-affirms that gas is indeed flowing out of the system. Our VLBA observations at 1.65 GHz reveal one continuum source within the densest portion of the molecular gas, with a diameter d < 8 mas (1.2 pc), a radio power $P_{\mathrm{rad}}$ = 1.48 $\times$ 10$^{20}$ W Hz$^{-1}$, and a brightness temperature $T_{\mathrm{b}}$ > 1.5 x 10$^7$ K that is most consistent with an AGN origin. The radio continuum energetics implied by the compact VLBA source, as well as archival VLA continuum observations at lower spatial resolution, further support the possibility that the AGN in NGC 1266 could be driving the molecular outflow. These findings suggest that even low-level AGNs may be able to launch massive outflows in their host galaxies., Comment: 14 pages, 8 figures, Accepted to ApJ

The early-type galaxy NGC 3115, at a distance of 10.2 Mpc, hosts the nearest billion-solar-mass black hole. Wong et al. recently inferred a substantial Bondi accretion rate near the black hole. Bondi-like accretion is thought to fuel outflows, which can be traced through their radio emission. This paper reports the discovery of a radio nucleus in NGC 3115, with a diameter less that 0.17 arcsec (8.4 pc), a luminosity at 8.5 GHz of 3.1 x 10^35 ergs/s and a flat spectrum (alpha = -0.23+/-0.20, flux density ~ frequency^alpha). The radio source coincides with the galaxy's photocenter and candidate X-ray nucleus. The emission is radio-loud, suggesting the presence of an outflow on scales less than 10 pc. On such scales, the Bondi accretion could be impeded by heating due to disruption of the outflow., Comment: 4 pages; 1 figure; emulateapj.cls; to appear in AJ

We present the results of deep, high-resolution, 5 GHz Expanded Very Large Array (EVLA) observations of the nearby, dwarf lenticular galaxy and intermediate mass black hole candidate (M ~4.5 x 10^5 M_sun), NGC 404. For the first time, radio emission at frequencies above 1.4 GHz has been detected in this galaxy. We found a modestly resolved source in the NGC 404 nucleus with a total radio luminosity of 7.6 +/- 0.7 x 10^17 W/Hz at 5 GHz and a spectral index from 5 to 7.45 GHz of alpha = -0.88 +/- 0.30. NGC 404 is only the third central intermediate mass black hole candidate detected in the radio regime with subarcsecond resolution. The position of the radio source is consistent with the optical center of the galaxy and the location of a known, hard X-ray point source (Lx ~1.2 x 10^37 erg/s). The faint radio and X-ray emission could conceivably be produced by an X-ray binary, star formation, a supernova remnant or a low-luminosity AGN powered by an intermediate mass black hole. In light of our new EVLA observations, we find that the most likely scenario is an accreting intermediate mass black hole, with other explanations incompatible with the observed X-ray and/or radio luminosities or statistically unlikely., Comment: Accepted for publication to ApJ

Ibata et al. reported evidence for density and kinematic cusps in the Galactic globular cluster M54, possibly due to the presence of a 9400 solar-mass black hole. Radiative signatures of accretion onto M54's candidate intermediate-mass black hole (IMBH) could bolster the case for its existence. Analysis of new Chandra and recent Hubble Space Telescope astrometry rules out the X-ray counterpart to the candidate IMBH suggested by Ibata et al. If an IMBH exists in M54, then it has an Eddington ratio of L(0.3-8 keV) / L(Edd) < 1.4 x 10^(-10), more similar to that of the candidate IMBH in M15 than that in G1. From new imaging with the NRAO Very Large Array, the luminosity of the candidate IMBH is L(8.5 GHz) < 3.6 x 10^29 ergs/s (3 sigma). Two background active galaxies discovered toward M54 could serve as probes of its intracluster medium., Comment: 4 pages; 2 figures; emulateapj.cls; to appear in AJ

The radio-quiet quasar SDSS J1536+0441A shows two broad-line emission systems, recently interpreted as a binary black hole (BBH) system with a subparsec separation; as a double-peaked emitter; or as both types of systems. The NRAO VLBA was used to search for 8.4 GHz emission from SDSS J1536+0441A, focusing on the optical localization region for the broad-line emission, of area 5400 mas^2 (0.15 kpc^2). One source was detected, with a diameter of less than 1.63 mas (8.5 pc) and a brightness temperature T_b > 1.2 x 10^7 K. New NRAO VLA photometry at 22.5 GHz, and earlier photometry at 8.5 GHz, gives a rising spectral slope of alpha = 0.35+/-0.08. The slope implies an optically thick synchrotron source, with a radius of about 0.04 pc, and thus T_b ~ 5 x 10^10 K. The implied radio-sphere at rest frame 31.2 GHz has a radius of 800 gravitational radii, just below the size of the broad line region in this object. Observations at higher frequencies can probe whether or not the radio-sphere is as compact as expected from the coronal framework for the radio emission of radio-quiet quasars., Comment: 6 pages; 2 figures; emulateapj.cls; to appear in ApJL

We present the first luminous, spatially resolved binary quasar that clearly inhabits an ongoing galaxy merger. SDSS J125455.09+084653.9 and SDSS J125454.87+084652.1 (SDSS J1254+0846 hereafter) are two luminous z=0.44 radio quiet quasars, with a radial velocity difference of just 215 km/s, separated on the sky by 21 kpc in a disturbed host galaxy merger showing obvious tidal tails. The pair was targeted as part of a complete sample of binary quasar candidates with small transverse separations drawn from SDSS DR6 photometry. We present follow-up optical imaging which shows broad, symmetrical tidal arm features spanning some 75 kpc at the quasars' redshift. Numerical modeling suggests that the system consists of two massive disk galaxies prograde to their mutual orbit, caught during the first passage of an active merger. This demonstrates rapid black hole growth during the early stages of a merger between galaxies with pre-existing bulges. Neither of the two luminous nuclei show significant instrinsic absorption by gas or dust in our optical or X-ray observations, illustrating that not all merging quasars will be in an obscured, ultraluminous phase. We find that the Eddington ratio for the fainter component B is rather normal, while for the A component L/LEdd is quite (>3sigma) high compared to quasars of similar luminosity and redshift, possibly evidence for strong merger-triggered accretion. More such mergers should be identifiable at higher redshifts using binary quasars as tracers., Comment: 15 pages, 5 figures, accepted to the Astrophysical Journal for the February 2010 - 20 v710 issue. Latest version corrects author list

Knowledge about the age of presolar grains provides important insights into Galactic chemical evolution and the dynamics of grain formation and destruction processes in the Galaxy. Determination from the abundance of cosmic ray interaction products is straightforward, but in the past has suffered from uncertainties in correcting for recoil losses of spallation products. The problem is less serious in a class of large (tens of micrometer) grains. We describe the correction procedure and summarise results for He and Ne ages of presolar SiC "Jumbo" grains that range from close to zero to ~850 Myr, with the majority being less than 200 Myr. We also discuss the possibility of extending our approach to the majority of smaller SiC grains and explore possible contributions from trapping of cosmic rays., Comment: Publications of the Astronomical Society of Australia, Contribution to PASA special volume "The Origin of Elements Heavier than Iron in honor of the 70th birthday of Roberto Gallino"