Michael Eracleous

BLACK HOLES AT THE CENTERS OF NEARBY DWARF GALAXIES

Using a distance-limited portion of the Sloan Digital Sky Survey (SDSS) Data Release 7, we have identified 28 active galactic nuclei (AGNs) in nearby (d < 80 Mpc) low-mass, low-luminosity dwarf galaxies. The accreting objects at the galaxy centers are expected to be intermediate-mass black holes (IMBHs) with M_BH < 1e6 M_sun. The AGNs were selected using several optical emission-line diagnostics after careful modeling of the continuum present in the spectra. We have limited our survey to objects with spectral characteristics similar to those of Seyfert nuclei, excluding emission-line galaxies with ambiguous spectra that could be powered by stellar processes. The host galaxies in our sample are thus the least massive objects in the very local universe certain to contain central black holes. Given our focus on the nearest objects included in the SDSS, our survey is more sensitive to low-luminosity emission than previous optical searches for AGNs in low-mass galaxies. The [O III] lambda5007 luminosities of the Seyfert nuclei in our sample have a median value of L_5007 = 2e5 L_sun and extend down to 1e4 L_sun. Using published data for broad-line IMBH candidates, we have derived an [O III] bolometric correction of log (L_bol/L_5007) = 3.0 +/- 0.3, which is significantly lower than values obtained for high-luminosity AGNs. Applying this correction to our sample, we obtain minimum black-hole mass estimates that fall mainly in the 10^3 M_sun -- 10^4 M_sun range, which is roughly where the predicted mass functions for different black-hole seed formation scenarios overlap the most. In the stellar mass range that includes the bulk of the AGN host galaxies in our sample, we derive a lower limit on the AGN fraction of a few percent, indicating that active nuclei in dwarf galaxies are not as rare as previously thought.

An X-Ray Spectral Survey of Radio-loud Active Galactic Nuclei with ASCA

We present a uniform and systematic analysis of the 0.6-10 keV X-ray spectra of radio-loud active galactic nuclei (AGNs) observed by ASCA. The sample, which is not statistically complete, includes 10 broad-line radio galaxies (BLRGs), five radio-loud quasars (QSRs), nine narrow-line radio galaxies (NLRGs), and 10 radio galaxies (RGs) of mixed FR I and FR II types. For several sources the ASCA data are presented here for the first time. The exposure times of the observations and the fluxes of the objects vary over a wide range; as a result, so does the signal-to-noise ratio of the individual X-ray spectra. At soft X-rays, about 50% of NLRGs and 100% of RGs exhibit thermal plasma emission components, with bimodal distributions of temperatures and luminosities. This indicates that the emission in such an object arises in hot gas either in a surrounding cluster or loose group or in a hot corona, consistent with previous ROSAT and optical results. At energies above 2 keV, a hard power-law component (photon index Γ ~ 1.7-1.8) is detected in 90% of cases. The power-law photon indices and luminosities in BLRGs, QSRs, and NLRGs are similar. This is consistent with simple orientation-based unification schemes for lobe-dominated radio-loud sources in which BLRGs, QSRs, and NLRGs harbor the same type of central engine. Moreover, excess cold absorption in the range 1021-1024 cm-2 is detected in most (but not all) NLRGs, consistent with absorption by obscuring tori, as postulated by unification scenarios. The ASCA data provide initial evidence that the immediate gaseous environment of the X-ray source of BLRGs may be different than in Seyfert 1 galaxies: absorption edges of ionized oxygen, common in the latter, are detected in only one BLRG. Instead we detect large columns of cold gas in a fraction (~44%-60%) of BLRGs and QSRs, comparable to the columns detected in NLRGs, which is puzzling. This difference hints at different physical and/or geometrical properties of the medium around the X-ray source in radio-loud AGNs compared to their radio-quiet counterparts, properties that can be explored further with future X-ray observations. For the full sample, the nuclear X-ray luminosity is correlated with the luminosity of the [O III] emission line, the FIR emission at 12 μm, and the lobe radio power at 5 GHz. The Fe Kα line is detected in 50% of BLRGs and in one QSR, with a large range of intrinsic widths and equivalent widths. In the handful of NLRGs where it is detected, the line is generally unresolved. Comparing the average power-law photon indices of the various classes of radio-loud AGNs to their radio-quiet counterparts from the literature, we find only a weak indication that the ASCA 2-10 keV spectra of BLRGs are flatter than those of Seyfert 1 galaxies of comparable X-ray luminosity. This result is at odds with evidence from samples studied by other authors suggesting that radio-loud AGNs have flatter spectra than radio-quiet ones. Rather, it supports the idea that a beamed synchrotron self-Compton component related to the radio source (jet) is responsible for the flatter slopes in those radio-loud AGNs. We argue that, because of the way those samples were constructed, beamed X-ray emission from the radio jets probably contributed to the observed X-ray spectra. The sample studied here includes six weak-line radio galaxies (WLRGs), powerful radio galaxies characterized by [O III] 4569 and 5007 A of unusually low luminosity and by unusually high [O II]/[O III] line ratios. The ASCA spectra of WLRGs can be generally decomposed into a soft thermal component with kT ~ 1 keV, plus a hard component, described either by a flat (Γ = 1.5) absorbed power law or by a very hot (kT ~ 100 keV) thermal bremsstrahlung model. Their intrinsic luminosities are in the range L2-10 keV ~ 1040-1042 ergs s-1, 2 orders of magnitude lower than in other sources in our sample. If the hard X-ray emission is attributed to a low-luminosity AGN, an interesting possibility is that WLRGs represent an extreme population of radio galaxies in which the central black hole is accreting at a rate well below the Eddington rate.

Luminous Thermal Flares from Quiescent Supermassive Black Holes

A dormant supermassive black hole lurking in the center of a galaxy will be revealed when a star passes close enough to be torn apart by tidal forces, and a flare of electromagnetic radiation is emitted when the bound fraction of the stellar debris falls back onto the black hole and is accreted. Although the tidal disruption of a star is a rare event in a galaxy,≈10^(–4) yr^(–1), observational candidates have emerged in all-sky X-ray and deep ultraviolet (UV) surveys in the form of luminous UV/X-ray flares from otherwise quiescent galaxies. Here we present the third candidate tidal disruption event discovered in the Galaxy Evolution Explorer (GALEX) Deep Imaging Survey: a 1.6 × 10^(43) erg s^(–1) UV/optical flare from a star-forming galaxy at z = 0.1855. The UV/optical spectral energy distribution (SED) during the peak of the flare measured by GALEX and Palomar Large Field Camera imaging can be modeled as a single temperature blackbody with T_(bb) = 1.7 × 10^5 K and a bolometric luminosity of 3 × 10^(45) erg s^(–1), assuming an internal extinction with E(B – V)_(gas) = 0.3. The Chandra upper limit on the X-ray luminosity during the peak of the flare, L_X (2 – 10 keV) M_g > – 18.9) to predict the detection capabilities of upcoming optical synoptic surveys.

Large-Amplitude X-Ray Outbursts from Galactic Nuclei: A Systematic Survey using ROSAT Archival Data

In recent years, luminous X-ray outbursts with variability amplitudes as high as ≈400 have been serendipitously detected from a small number of active and inactive galaxies. These outbursts may result from the tidal disruptions of stars by supermassive black holes, as well as accretion disk instabilities. In order to place the first reliable constraints on the rate of such outbursts in the universe and to test the stellar tidal disruption hypothesis, we have performed a systematic and complete survey for them by cross-correlating ROSAT All-Sky Survey (RASS) and pointed Position Sensitive Proportional Counter data. We have detected five galaxies that were in outburst during the RASS, three of which show no signs of nuclear activity; these objects had been reported on individually in previous studies. After making reasonable corrections for the complicated selection effects, we conclude that the rate of large-amplitude X-ray outbursts from inactive galaxies in the local universe is ≈9.1 × 10-6 galaxy-1 yr-1. This rate is consistent with the predicted rate of stellar tidal disruption events in such galaxies. When only the two active galaxies are considered, we find a rate for active galaxies of ≈8.5 × 10-4 galaxy-1 yr-1. In order to place tighter constraints on these rates, additional outbursts must be detected.

Elliptical accretion disks in active galactic nuclei

We present a calculation of the profiles of emission lines originating in a relativistic, eccentric disk, and show examples of the resulting model profiles. Our calculations are motivated by the fact that in about one-quarter of the double-peaked emission lines observed in radio-loud active galactic nuclei (and in the mildly active nucleus of NGC 1097), the red peak is stronger than the blue peak, which is contrary to the prediction of relativistic, circular disk models. Using the eccentric disk model we fit some of the observed profiles that cannot be fitted with a circular disk model. We propose two possible scenarios for the formation of an eccentric disk in an active galactic nucleus: (a) tidal perturbation of the disk around a supermassive black hole by a smaller binary companion, and (b) formation of an elliptical disk from the debris resulting from the tidal disruption of a star by the central black hole. In the former case we show that the eccentricity can be long-lived because of the presence of the binary companion. In the latter case, although the inner parts of the disk may circularize quickly, we estimate that the outer parts will maintain their eccentricity for times much longer than the local viscous time. We suggest that it may be possible to detect profile variability on much shorter timescales than those ranging from a decade to several centuries by comparing the evolution of the line profile with detailed model predictions. We argue that line-profile variability may also be the most promising discriminant among competing models for the origin of asymmetric, double-peaked emission lines.

THE DISCOVERY OF THE FIRST “CHANGING LOOK” QUASAR: NEW INSIGHTS INTO THE PHYSICS AND PHENOMENOLOGY OF ACTIVE GALACTIC NUCLEI

SDSS J015957.64+003310.5 is an X-ray selected,

Three LINERs under the Chandra X-Ray Microscope

z=0.31

Rejection of the Binary Broad-Line Region Interpretation of Double-peaked Emission Lines in Three Active Galactic Nuclei

AGN from the Stripe 82X survey that transitioned from a Type 1 quasar to a Type 1.9 AGN between 2000 and 2010. This is the most distant AGN, and first quasar, yet observed to have undergone such a dramatic change. We re-observed the source with the double spectrograph on the Palomar 5m telescope in July 2014 and found that the spectrum is unchanged since 2010. From fitting the optical spectra, we find that the AGN flux dropped by a factor of 6 between 2000 and 2010 while the broad H

Evolution of the nuclear accretion disk emission in NGC 1097 : getting closer to the black hole

\alpha

Radiatively inefficient accretion flow in the nucleus of NGC 1097

emission faded and broadened. Serendipitous X-ray observations caught the source in both the bright and dim state, showing a similar 2-10 keV flux diminution as the optical while lacking signatures of obscuration. The optical and X-ray changes coincide with