arXiv subjects

J. M. Wrobel

Publications and source records attributed to J. M. Wrobel.

At least 19 recordsLinked to original sources

arXiv2025

Inside the Stagnation Radius of the Nearest Billion-Solar-Mass Black Hole

We used the NSF Jansky Very Large Array at a frequency $ν=$ 22\,GHz to study the nearest billion-solar-mass black hole, in the early-type galaxy NGC\,3115 at a distance of 9.7\,Mpc. We localize a faint continuum nucleus, with flux density $S_{\rm 22\,GHz} = 48.2\pm6.4\,μ$Jy, to a FWHM diameter $d_{\rm 22\,GHz} <$ 59\,mas (2.8\,pc). We find no evidence for adjacent emission within a stagnation region of radius $R_{\rm sta} \sim$ 360\,mas (17\,pc) identified in a recent hydrodynamic simulation tailored to NGC\,3115. Within that region, the simulated gas flow developed into an advection-dominated accretion flow (ADAF). The nucleus' luminosity density $L_{\rm 22\,GHz} = 5.4 \times 10^{17}\,\rm W\,Hz^{-1}$ is about 60 times that of Sagittarius\,A$^\star$. The nucleus' spectral index $α_{\rm 10\,GHz}^{\rm 22\,GHz} = -1.85\pm0.18$ ($S_ν\propto ν^α$) indicates optically-thin synchrotron emission. The spectral energy distribution of the nucleus peaks near $ν_{\rm peak} =$ 9\,GHz. Modeling this radio peak as an ADAF implies a black hole mass $M_{\rm ADAF} = (1.2\pm0.2) \times 10^9\,M_\odot$, consistent with previous estimates of $(1-2) \times 10^9\,M_\odot$ from stellar or hot-gas dynamics. Also, the Eddington-scaled accretion rate for NGC\,3115, $\dot{M}_{\rm ADAF}/\dot{M}_{\rm Edd} = 1.2^{+1.0}_{-0.6} \times 10^{-8}$, is about 4-8 times lower than recent estimates for Sagittarius\,A$^\star$.

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arXiv2023

A Suggested Final Configuration for the Very Large Array

If the construction of the ngVLA begins in 2026, its sensitivity is expected to match that of the VLA by late 2029. At that juncture it is anticipated that open-skies observing will cease on the VLA and commence on the ngVLA. We suggest that during 2026-2029 the VLA be held in a customized final configuration encompassing portions of its standard A, B, C and D configurations. Such a final VLA configuration would (1) help minimize the cost of VLA operations and maximize the pace of ngVLA construction and commissioning; (2) help VLA users pivot to the high-resolution, high-frequency research topics that are projected to headline the ngVLA science program; and (3) help mitigate the effects of source confusion during responses to transients in the era of the Rubin Observatory and LIGO A+.

astro-ph.IM↗

arXiv2022

Toward Astrometric Constraints on a Supermassive Black Hole Binary in the Early-Type Galaxy NGC\,4472

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 $2\,μ$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.

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arXiv2021

Astrometric Constraints on a Massive Black Hole Binary in NGC\,4472

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.

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arXiv2021

Accessing Intermediate-Mass Black Holes in 728 Globular Star Clusters in NGC\,4472

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.

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arXiv2020

VLA Limits on Intermediate-Mass Black Holes in 19 Massive Globular Clusters

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.

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arXiv2019

X-ray Properties of Radio-Selected Dual Active Galactic Nuclei

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.

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arXiv2018

Intermediate-Mass Black Holes in Globular Cluster Systems

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.

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arXiv2018

Limits on Ionized Gas in M81's Globular Clusters

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.

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arXiv2017

The Twisted Radio Structure of PSO J334.2028+01.4075, Still a Supermassive Binary Black Hole Candidate

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.

astro-ph.HE↗

arXiv2016

A Very Large Array Search for Intermediate-Mass Black Holes in Globular Clusters in M81

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$σ$ 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.

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arXiv2015

Binary Active Galactic Nuclei in Stripe 82: Constraints on Synchronized Black Hole Accretion in Major Mergers

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 $120 < σ_\star < 320$ km/s. The radio emission is compact ($<$0.4") and show steep spectrum ($-1.8 < α< -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.

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arXiv2015

A Stacked Search for Intermediate-Mass Black Holes in 337 Extragalactic Star Clusters

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$σ$ 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.

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arXiv2014

Radio-Selected Binary Active Galactic Nuclei from the Very Large Array Stripe 82 Survey

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.

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arXiv2014

On the radio properties of the intermediate-mass black hole candidate ESO 243-49 HLX-1

We present follow-up radio observations of ESO 243-49 HLX-1 from 2012 using the Australia Telescope Compact Array (ATCA) and the Karl G. Jansky Very Large Array (VLA). We report the detection of radio emission at the location of HLX-1 during its hard X-ray state using the ATCA. Assuming that the `Fundamental Plane' of accreting black holes is applicable, we provide an independent estimate of the black hole mass of $M_{\rm{BH}}\leq2.8^{+7.5}_{-2.1} \times 10^{6}$ M$_{\odot}$ at 90% confidence. However, we argue that the detected radio emission is likely to be Doppler-boosted and our mass estimate is an upper limit. We discuss other possible origins of the radio emission such as being due to a radio nebula, star formation, or later interaction of the flares with the large-scale environment. None of these were found adequate. The VLA observations were carried out during the X-ray outburst. However, no new radio flare was detected, possibly due to a sparse time sampling. The deepest, combined VLA data suggests a variable radio source and we briefly discuss the properties of the previously detected flares and compare them with microquasars and active galactic nuclei.

astro-ph.HE↗

arXiv2014

Evidence from the Very Long Baseline Array that J1502SE/SW are Double Hotspots, not a Supermassive Binary Black Hole

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.

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arXiv2014

Constraints on Two Active Galactic Nuclei in the Merger Remnant COSMOS J100043.15+020637.2

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.

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arXiv2013

Detection of a High Brightness Temperature Radio Core in the AGN-Driven Molecular Outflow Candidate NGC 1266

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.

astro-ph.CO↗