arXiv subjects

Richard Barvainis

Publications and source records attributed to Richard Barvainis.

18 recordsLinked to original sources

arXiv2013

Evidence for a receding dust sublimation region around a supermassive black hole

The near-IR emission in Type 1 AGNs is thought to be dominated by the thermal radiation from dust grains that are heated by the central engine in the UV/optical and are almost at the sublimation temperature. A brightening of the central source can thus further sublimate the innermost dust, leading to an increase in the radius of the near-IR emitting region. Such changes in radius have been indirectly probed by the measurements of the changes in the time lag between the near-IR and UV/optical light variation. Here we report direct evidence for such a receding sublimation region through the near-IR interferometry of the brightest Type 1 AGN in NGC4151. The increase in radius follows a significant brightening of the central engine with a delay of at least a few years, which is thus the implied destruction timescale of the innermost dust distribution. Compiling historic flux variations and radius measurements, we also infer the reformation timescale for the inner dust distribution to be several years in this galactic nucleus. More specifically and quantitatively, we find that the radius at a given time seems to be correlated with a long-term average of the flux over the previous several (~6) years, instead of the instantaneous flux. Finally, we also report measurements of three more Type 1 AGNs newly observed with the Keck interferometer, as well as the second epoch measurements for three other AGNs.

astro-ph.CO↗

arXiv2012

Probing the innermost dusty structure in AGN with mid-IR and near-IR interferometers

With mid-IR and near-IR long-baseline interferometers, we are now mapping the radial distribution of the dusty accreting material in AGNs at sub-pc scales. We currently focus on Type 1 AGNs, where the innermost region is unobscured and its intrinsic structure can be studied directly. As a first systematic study of Type 1s, we obtained mid-/near-IR data for small samples over ~3-4 orders of magnitudes in UV luminosity L of the central engine. Here we effectively trace the structure by observing dust grains that are radiatively heated by the central engine. Consistent with a naive expectation for such dust grains, the dust sublimation radius R_in is in fact empirically known to be scaling with L^1/2 from the near-IR reverberation measurements, and this is also supported by our near-IR interferometry. Utilizing this empirical relationship, we normalize the radial extent by R_in and eliminate the simple L^1/2 scaling for a direct comparison over the samples. We then find that, in the mid-IR, the overall size in units of R_in seems to become more compact in higher luminosity sources. More specifically, the mid-IR brightness distribution is rather well described by a power-law, and this power-law becomes steeper in higher luminosity objects. The near-IR flux does not seem to be a simple inward extrapolation of the mid-IR power-law component toward shorter wavelengths, but it rather comes from a little distinct brightness concentration at the inner rim region of the dust distribution. Its structure is not well constrained yet, but there is tentative evidence that this inner near-IR-emitting structure has a steeper radial distribution in jet-launching objects. All these should be scrutinized with further observations.

astro-ph.CO↗

arXiv2011

The innermost dusty structure in active galactic nuclei as probed by the Keck interferometer

We are now exploring the inner region of Type 1 active galactic nuclei (AGNs) with the Keck interferometer in the near-infrared. Adding to the four targets previously studied, we report measurements of the K-band (2.2 um) visibilities for four more targets, namely AKN120, IC4329A, Mrk6, and the radio-loud QSO 3C273 at z=0.158. The observed visibilities are quite high for all the targets, which we interpret as an indication of the partial resolution of the dust sublimation region. The effective ring radii derived from the observed visibilities scale approximately with L^1/2, where L is the AGN luminosity. Comparing the radii with those from independent optical-infrared reverberation measurements, these data support our previous claim that the interferometric ring radius is either roughly equal to or slightly larger than the reverberation radius. We interpret the ratio of these two radii for a given L as an approximate probe of the radial distribution of the inner accreting material. We show tentative evidence that this inner radial structure might be closely related to the radio-loudness of the central engine. Finally, we re-observed the brightest Seyfert 1 galaxy NGC4151. Its marginally higher visibility at a shorter projected baseline, compared to our previous measurements obtained one year before, further supports the partial resolution of the inner structure. We did not detect any significant change in the implied emission size when the K-band flux was brightened by a factor of 1.5 over a time interval of one year.

astro-ph.CO↗

arXiv2009

Exploring the inner region of Type 1 AGNs with the Keck interferometer

The exploration of extragalactic objects with long-baseline interferometers in the near-infrared has been very limited. Here we report successful observations with the Keck interferometer at K-band (2.2 um) for four Type 1 AGNs, namely NGC4151, Mrk231, NGC4051, and the QSO IRAS13349+2438 at z=0.108. For the latter three objects, these are the first long-baseline interferometric measurements in the infrared. We detect high visibilities (V^2 ~ 0.8-0.9) for all the four objects, including NGC4151 for which we confirm the high V^2 level measured by Swain et al.(2003). We marginally detect a decrease of V^2 with increasing baseline lengths for NGC4151, although over a very limited range, where the decrease and absolute V^2 are well fitted with a ring model of radius 0.45+/-0.04 mas (0.039+/-0.003 pc). Strikingly, this matches independent radius measurements from optical--infrared reverberations that are thought to be probing the dust sublimation radius. We also show that the effective radius of the other objects, obtained from the same ring model, is either roughly equal to or slightly larger than the reverberation radius as a function of AGN luminosity. This suggests that we are indeed partially resolving the dust sublimation region. The ratio of the effective ring radius to the reverberation radius might also give us an approximate probe for the radial structure of the inner accreting material in each object. This should be scrutinized with further observations.

astro-ph.CO↗

arXiv2009

A Search for H2O Megamasers in High-z Type-2 AGNs

We report a search for H2O megamasers in 274 SDSS type-2 AGNs (0.3 < z < 0.83), half of which can be classified as type-2 QSOs from their [OIII] 5007 luminosity, using the Robert C. Byrd Green Bank Telescope (GBT) and the Effelsberg 100-m radio telescope. Apart from the detection of the extremely luminous water vapor megamaser SDSS J080430.99+360718.1, already reported by Barvainis & Antonucci (2005), we do not find any additional line emission. This high rate of non-detections is compared to the water maser luminosity function created from the 78 water maser galaxies known to date and its extrapolation towards the higher luminosities of "gigamasers" that we would have been able to detect given the sensitivity of our survey. The properties of the known water masers are summarized and discussed with respect to the nature of high-z type-2 AGNs and megamasers in general. In the appendix, we list 173 additional objects (mainly radio galaxies, but also QSOs and galaxies) that were observed with the GBT, the Effelsberg 100-m radio telescope, or Arecibo Observatory without leading to the detection of water maser emission.

astro-ph.GA↗

arXiv2005

Extremely Luminous Water Vapor Emission from a Type 2 Quasar at Redshift z = 0.66

A search for water masers in 47 Sloan Digital Sky Survey Type 2 quasars using the Green Bank Telescope has yielded a detection at a redshift of z = 0.660. This maser is more than an order of magnitude higher in redshift than any previously known and, with a total isotropic luminosity of 23,000 L_sun, also the most powerful. The presence and detectability of water masers in quasars at z ~ 0.3-0.8 may provide a better understanding of quasar molecular tori and disks, as well as fundamental quasar and galaxy properties such as black hole masses. Water masers at cosmologically interesting distances may also eventually provide, via direct distance determinations, a new cosmological observable for testing the reality and properties of dark energy, currently inferred primarily through Type 1a supernova measurements.

astro-ph↗

arXiv2004

VLBA Imaging of Central Engines in Radio Quiet Quasars

We have used the Very Long Baseline Array (VLBA) to image five radio-quiet quasars (RQQs) at milliarcsecond resolution, at frequencies between 1.4 and 5 GHz. These quasars have typical total flux densities of a few millijansky at gigahertz frequencies, and are compact on arcsecond scales. The VLBA images reveal that four of the quasars are dominated by unresolved radio cores, while the fifth has an apparent two-sided jet. Typical core brightness temperatures range from 10^8 K to at least 10^9 K. The compact radio morphologies and X-ray luminosities of many objects in the RQQ sample seem to indicate classical accretion onto black holes as massive as 10^9 solar masses, with emission physics in many ways similar to their radio-loud counterparts. Therefore, the relatively small amount of radiative energy emerging at radio wavelengths in the RQQs may simply be due to the presence of less powerful radio jets.

astro-ph↗

arXiv2004

Radio Variability of Radio Quiet and Radio Loud Quasars

The majority of quasars are weak in their radio emission, with flux densities comparable to those in the optical, and energies far lower. A small fraction, about 10%, are hundreds to thousands of times stronger in the radio. Conventional wisdom holds that there are two classes of quasars, the radio quiets and radio louds, with a deficit of sources having intermediate power. Are there really two separate populations, and if so, is the physics of the radio emission fundamentally different between them? This paper addresses the second question, through a study of radio variability across the full range of radio power, from quiet to loud. The basic findings are that the root mean square amplitude of variability is independent of radio luminosity or radio-to-optical flux density ratio, and that fractionally large variations can occur on timescales of months or less in both radio quiet and radio loud quasars. Combining this with similarities in other indicators, such as radio spectral index and the presence of VLBI-scale components, leads to the suggestion that the physics of radio emission in the inner regions of all quasars is essentially the same, involving a compact, partially opaque core together with a beamed jet.

astro-ph↗

arXiv2002

A CO Survey of Gravitationally Lensed Quasars with the IRAM Interferometer

We present the results of a CO survey of gravitationally lensed quasars, conducted with the Plateau de Bure Interferometer over the last three years. Among the 18 objects surveyed, one was detected in CO line emission, while six were detected in the continuum at 3mm and three in the continuum at 1mm. The low CO detection rate may at least in part be due to uncertainties in the redshifts derived from quasar broad emission lines. The detected CO source, the z=3.2 radio quiet quasar MG0751+2716, is quite strong in the CO(4-3) line and in the millimeter/submillimeter continuum, the latter being emission from cool dust. The integrated CO line flux is 5.96 +- 0.45 Jy.km/s, and the total molecular gas mass is estimated to be in the range M(H_2) = 1.6-3.1 X 10^9 solar masses.

astro-ph↗

arXiv2002

A Submillimeter Survey of Gravitationally Lensed Quasars

Submillimeter (and in some cases millimeter) wavelength continuum measurements are presented for a sample of 40 active galactic nuclei (probably all quasars) lensed by foreground galaxies. The object of this study is to use the lensing boost, anywhere from ~3- 20 times, to detect dust emission from more typical AGNs than the extremely luminous ones currently accessible without lensing. The sources are a mix of radio loud and radio quiet quasars, and, after correction for synchrotron radation (in the few cases where necessary), 23 of the 40 (58%) are detected in dust emission at 850um; 11 are also detected at 450um. Dust luminosities and masses are derived after correction for lensing magnification, and luminosities are plotted against redshift from z = 1 to z = 4.4, the redshift range of the sample. The main conclusions are (1) Monochromatic submillimeter luminosities of quasars are, on average, only a few times greater than those of local IRAS galaxies; (2) Radio quiet and radio loud quasars do not differ significantly in their dust lumimosity; (3) Mean dust luminosities of quasars and radio galaxies over the same redshift range are comparable; (4) Quasars and radio galaxies alike show evidence for more luminous and massive dust sources toward higher redshift, consistent with an early epoch of formation and possibly indicating that the percentage of obscured AGNs increases with redshift.

astro-ph↗

arXiv2000

Radio-Variability in Radio-Quiet Quasars and Low-Luminosity AGN

We report on two surveys of radio-weak AGN to look for radio variability. We find significant variability with an RMS of 10-20% on a timescale of months in radio-quiet and radio-intermediate quasars. This exceeds the variability of radio cores in radio-loud quasars (excluding blazars), which vary only on a few percent level. The variability in radio-quiet quasars confirms that the radio emission in these sources is indeed related to the AGN. The most extremely variable source is the radio-intermediate quasar III Zw 2 which was recently found to contain a relativistic jet. In addition we find large amplitude variabilities (up to 300% peak-to-peak) in a sample of nearby low-luminosity AGN, Liners and dwarf-Seyferts, on a timescale of 1.5 years. The variability could be related to the activity of nuclear jets responding to changing accretion rates. Simultaneous radio/optical/X-ray monitoring also for radio-weak AGN, and not just for blazars, is therefore a potentially powerful tool to study the link between jets and accretion flows.

astro-ph↗

arXiv1999

New CO and Millimeter Continuum Observations of the z=2.394 Radio Galaxy 53W002

The z=2.39 radio galaxy 53W002 lies in a cluster of Ly-alpha emission line objects and may itself be undergoing a major burst of star formation. CO(3--2) emission, at 102 GHz, was detected from 53W002 by Scoville et al. (1997a), who also reported a possible 30 kpc extension and velocity gradient suggesting a rotating gaseous disk. In this paper we present new interferometric CO(3--2) observations which confirm the previous line detection with improved signal-to-noise ratio, but show no evidence for source extension or velocity gradient. The compact nature of the CO source and the molecular mass found in this object are similar to luminous infrared galaxies and other AGNs previously studied.

astro-ph↗

arXiv1999

ISOCAM 15um Search for Distant Infrared Galaxies Lensed by Clusters

In a search for lensed infrared galaxies, ISOCAM images have been obtained toward the rich clusters Abell 2218 and Abell 2219 at 15um. Nine galaxies (four in Abell 2218 and five in Abell 2219) were detected with flux levels in the range 530-1100 microJy. Three of the galaxies detected in Abell 2218 have previously known redshifts; of these one is a foreground galaxy and the other two are lensed background galaxies at z=0.474 and z=1.032. One of the objects detected in the field of Abell 2219 is a faint, optically red, extreme infrared-dominated galaxy with a probable redshift of 1.048.

astro-ph↗

arXiv1997

Molecular Emission Lines from High Redshift AGNs

A brief overview is given of the current status of molecular emission line observations at high redshift. New observations are presented of CO in the gravitationally lensed quasar MG 0414+0534 (detected) and the unlensed quasar PG 1634+706 (not detected). Also noted are the results of a partially completed search for water maser emission in several redshift bands above z=0.4.

astro-ph↗

arXiv1997

Search for Free-Free Absorption Cutoffs from Tori in Three Type 2 Active Galactic Nuclei

We have observed three Type 2 radio loud active galactic nuclei using the VLBA at five frequencies between 1.6 GHz and 15 GHz, in a search for the free-free absorption cutoff predicted by some models for optically-obscuring dusty molecular tori. We find no evidence for sharp cutoffs toward low frequencies in any of the sources. However, one source, OW637, has VLBI components which are steeply falling toward high frequencies (S_nu \propto nu^-2).

astro-ph↗

arXiv1997

Detection of CO(3-2) Emission at z=2.64 from the Gravitationally Lensed Quasar MG 0414+0534

We have detected CO(3-2) line emission from the gravitationally lensed quasar MG 0414+0534 at redshift 2.64, using the IRAM Plateau de Bure Interferometer. The line is broad, with Delta v_FWHM = 580 km/s. The velocity-integrated CO flux is comparable to, but somewhat smaller than, that of IRAS F10214+4724 and the Cloverleaf quasar (H1413+117), both of which are at similar redshifts. The lensed components A1+A2 and B were resolved, and separate spectra are presented for each. We also observed the unlensed radio quiet quasar PG 1634+706 at z=1.33, finding no significant CO emission.

astro-ph↗

arXiv1997

Multiple CO Transitions, CI, and HCN from the Cloverleaf Quasar

New millimeter-wavelength emission lines are reported for the Cloverleaf (H1413+117), a gravitationally lensed quasar at redshift 2.56. High signal-to-noise ratio spectra of four lines in CO (J=3-2, J=4-3, J=5-4, and J=7-6) have been obtained and are modeled using an escape probability formalism. The line brightness temperatures are flat or rising between CO(3-2) and CO(4-3) and then drop to the CO(5-4) and CO(7-6) lines; this falloff suggests that the optical depths in the CO lines are modest (tau_{4-3} < 3), and that the gas is relatively warm (T ~ 100 K) and dense (n_H2 > 3 X 10^3 cm^{-3}). The neutral carbon fine structure line CI(3P1-3P0) was detected with moderate SNR on two separate occasions, and appears secure. The HCN(4-3) line is somewhat more tentative, as it was observed only once and detected with moderate SNR. However, its brightness ratio of 0.25 relative to the CO(4-3) line is very similar to the ratio seen in luminous infrared galaxies. The detectability of the Cloverleaf in molecular and neutral atomic transitions is largely due to high emissivity of the gas and magnification from gravitational lensing, rather than an extremely large mass of gas.

astro-ph↗

arXiv1997

Molecular Gas in Quasar Hosts

The study of molecular gas in quasar host galaxies addresses a number of interesting questions pertaining to the hosts' ISM, to unified schemes relating quasars and IR galaxies, and to the processes fueling nuclear activity. In this contribution I review observations of molecular gas in quasar hosts from z=0.06 to z=4.7. The Cloverleaf quasar at z=2.5 is featured as a case where there are now enough detected transitions (four in CO, and one each in CI and HCN) to allow detailed modeling of physical conditions in the molecular ISM. We find that the CO-emitting gas is warmer, denser, and less optically thick than that found in typical Galactic molecular clouds. These differences are probably due to the presence of the luminous quasar in the nucleus of the Cloverleaf's host galaxy.

astro-ph↗