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An Internet Database of Ultraviolet Continuum Light Curves for Seyfert Galaxies
Using the Multimission Archive at STScI (MAST), we have extractedspectra and determined continuum light curves for 175 Seyfert galaxiesthat have been observed with the International Ultraviolet Explorer andthe Faint Object Spectrograph on the Hubble Space Telescope. To obtainthe light curves as a function of Julian Date, we used fixed bins in theobject's rest frame and measured small regions (between 30 and 60Å) of each spectrum's continuum flux in the range 1150 to 3200Å. We provide access to the UV light curves and other basicinformation about the observations in tabular and graphical form via theInternet at http://www.chara.gsu.edu/PEGA/IUE.
| On the origin of the iron Kα line cores in active galactic nuclei
X-ray observations made with Chandra and XMM-Newton have shown thatthere are relatively narrow cores to the iron Kα emission lines inactive galactic nuclei (AGN). Plausible origins for this core emissioninclude the outer regions of an accretion disc, a parsec-scale moleculartorus, and the optical broad-line region (BLR). Using data from theliterature it is shown that no correlation exists between the FeKα core width and the BLR (specifically Hβ) line width. Thisshows that in general the iron Kα core emission does not arisefrom the BLR. There is a similar lack of correlation between the widthof the Fe Kα core and black hole mass. The average Kα widthis about a factor of 2 lower than the Hβ width. It therefore seemslikely that, in many cases, the narrow core arises in the torus. Thereis a very wide range of observed Fe Kα core widths, however, andthis argues for multiple origins. The simplest explanation for theobserved line profiles in AGN is that they are due to a mixing of verynarrow emission from the inner edge of the torus, and broadened emissionfrom the accretion disc, in varying proportions from object to object.
| The near-infrared spectrum of Mrk 1239: direct evidence of the dusty torus?
We report 0.8-4.5 μm SpeX spectroscopy of the narrow-line Seyfert 1galaxy Mrk 1239. The spectrum is outstanding because the nuclearcontinuum emission in the near-infrared (NIR) is dominated by a strongbump of emission peaking at 2.2 μm, with a strength not reportedbefore in an active galactic nucleus. A comparison of the Mrk 1239spectrum to that of Ark 564 allowed us to conclude that the continuum isstrongly reddened by E(B-V) = 0.54. The excess of emission, confirmed byaperture photometry and additional NIR spectroscopy, follows a simpleblackbody curve at T~ 1200 K. This suggests that we may be observingdirect evidence of dust heated to near to the sublimation temperature,likely produced by the putative torus of the unification model. Althoughother alternatives are also plausible, the lack of star formation, thestrong polarization and low extinction derived for the emission linessupport the scenario where the hot dust is located between the narrowline region and the broad line region.
| Investigating a fluctuating-accretion model for the spectral-timing properties of accreting black hole systems
The fluctuating-accretion model of Lyubarskii and its extension byKotov, Churazov & Gilfanov seek to explain the spectral-timingproperties of the X-ray variability of accreting black holes in terms ofinward-propagating mass accretion fluctuations produced at a broad rangeof radii. The fluctuations modulate the X-ray emitting region as theymove inwards and can produce temporal-frequency-dependent lags betweenenergy bands, and energy-dependent power spectral densities (PSDs) as aresult of the different emissivity profiles, which may be expected atdifferent X-ray energies. Here, we use a simple numerical implementationto investigate in detail the X-ray spectral-timing properties of themodel and their relation to several physically interesting parameters,namely the emissivity profile in different energy bands, the geometricalthickness and viscosity parameter of the accretion flow, the strength ofdamping on the fluctuations and the temporal coherence (measured by the`quality factor', Q) of the fluctuations introduced at each radius. Wefind that a geometrically thick flow with large viscosity parameter isfavoured, and we confirm that the predicted lags are quite robust tochanges in the emissivity profile and physical parameters of theaccretion flow, which may help to explain the similarity of the lagspectra in the low/hard and high/soft states of Cyg X-1. We alsodemonstrate the model regime where the light curves in different energybands are highly spectrally coherent. We compare model predictionsdirectly to X-ray data from the narrow line Seyfert 1 galaxy NGC 4051and the black hole X-ray binary (BHXRB) Cyg X-1 in its high/soft state,and we show that this general scheme can reproduce simultaneously thetime lags and energy-dependence of the PSD.
| Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| XMM-Newton observations of the Seyfert 1 AGN H0557-385
We present XMM-Newton observations of the Seyfert 1 active galacticnucleus (AGN) H0557-385. We have conducted a study into the warmabsorber present in this source, and using high-resolution ReflectionGrating Spectrometer (RGS) data we find that the absorption can becharacterized by two phases: a phase with log ionization parameter ξof 0.50 (where ξ is in units of ergcms-1) and a column of0.2 × 1021cm-2, and a phase with log ξ of1.62 and a column of 1.3 × 1022cm-2. An ironKα line is detected. Neutral absorption is also present in thesource, and we discuss possible origins for this. On the assumption thatthe ionized absorbers originate as an outflow from the inner edge of thetorus, we use a new method for finding the volume filling factor. Bothphases of H0557-385 have small volume filling factors (<=1 per cent).We also derive the volume filling factors for a sample of 23 AGN usingthis assumption and for the absorbers with logξ > 0.7, we findreasonable agreement with the filling factors obtained through thealternative method of equating the momentum flow of the absorbers to themomentum loss of the radiation field. By comparing the filling factorsobtained by the two methods, we infer that some absorbers with logξ< 0.7 occur at significantly larger distances from the nucleus thanthe inner edge of the torus.
| Comptonization and Reprocessing Processes in Accretion Disks: Applications to the Seyfert 1 Galaxies NGC 5548 and NGC 4051
Simultaneous multi-wavelength observations have revealed complexvariability in AGNs. To explain the variability we considered atheoretical model consisting of an inner hot comptonizing corona and anouter thin accretion disk, with interactions between the two componentsin the form of comptonization and reprocessing. We found that thevariability of AGNs is strongly affected by the parameters of the model,namely, the truncated disk radius rmin, the corona radiusrs, the temperature KTe and the optical depthτ0 of the corona. We applied this model to the two bestobserved Seyfert 1 galaxies, NGC 5548 and NGC 4051. Our model canreproduce satisfactory the observed SEDs. Our fits indicate that NGC5548 may have experienced dramatic changes in physical parametersbetween 19891990 and 1998, and that NGC 4051 has a much largertruncated disk radius (700 Schwarzschild radii) than NGC 5548 (severaltens of Schwarzschild radii). Since we adopted a more refined treatmentof the comptonization process rather than simply assuming a cut-offpower law, our results should be more reasonable than the previous ones.
| A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
| A FUSE Survey of High-Latitude Galactic Molecular Hydrogen
Measurements of molecular hydrogen (H2) column densities arepresented for the first six rotational levels (J=0-5) for 73extragalactic targets observed with the Far Ultraviolet SpectroscopicExplorer (FUSE). All of these have a final signal-to-noise ratio largerthan 10 and are located at Galactic latitude |b|>20deg.The individual observations were calibrated with the FUSE calibrationpipeline CalFUSE version 2.1 or higher and then carefully aligned invelocity. The final velocity shifts for all the FUSE segments arelisted. H2 column densities or limits are determined for thesix lowest rotational (J) levels for each H I component in the line ofsight, using a curve-of-growth approach at low column densities(<16.5) and Voigt-profile fitting at higher column densities.Detections include 65 measurements of low-velocity H2 in theGalactic disk and lower halo. Eight sight lines yield nondetections forGalactic H2. The measured column densities range fromlogN(H2)=14 to 20. Strong correlations are found betweenlogN(H2) and T01, the excitation temperature ofthe H2, as well as between logN(H2) and the levelpopulation ratios (log[N(J')/N(J)]). The average fraction ofnuclei in molecular hydrogen [f(H2)] in each sight line iscalculated; however, because there are many H I clouds in each sightline, the physics of the transition from H I to H2 cannot bestudied. Detections also include H2 in 16intermediate-velocity clouds in the Galactic halo (out of 35 IVCs).Molecular hydrogen is seen in one high-velocity cloud (the Leading Armof the Magellanic Stream), although 19 high-velocity clouds areintersected; this strongly suggests that dust is rare or absent in theseobjects. Finally, there are five detections of H2 in externalgalaxies.
| VLBI Images of 49 Radio Supernovae in Arp 220
We have used a very long baseline interferometry (VLBI) array at 18 cmwavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1pc linear resolution, with very high sensitivity. The resulting map hasan rms of 5.5 μJy beam-1, and careful image analysisresults in 49 confirmed point sources ranging in flux density from 1.2mJy down to ~60 μJy. Comparison with high-sensitivity data from 12months earlier reveals at least four new sources. The favoredinterpretation of these sources is that they are radio supernovae, andif all new supernovae are detectable at this sensitivity, a resultingestimate of the supernova rate in the Arp 220 system is 4+/-2 per year.The implied star formation rate is sufficient to power the entireobserved far-infrared luminosity of the galaxy. The two nuclei of Arp220 exhibit striking similarities in their radio properties, althoughthe western nucleus is more compact, and appears to be ~3 times moreluminous than the eastern nucleus. There are also some puzzlingdifferences, and differential free-free absorption, synchrotron aging,and expansion losses may all be playing a role. Comparison with thenearby starburst galaxy M82 supports the hypothesis that the activity inArp 220 is essentially a scaled-up version of that in M82.
| The Star-forming Torus and Stellar Dynamical Black Hole Mass in the Seyfert 1 Nucleus of NGC 3227
We report R~4300 VLT SINFONI adaptive optics integral field K-bandspectroscopy of the nucleus of the Seyfert 1 galaxy NGC 3227 at aspatial resolution of 0.085" (7 pc). We present the morphologies andkinematics of emission lines and absorption features and give the firstderivation of a black hole mass in a Seyfert 1 nucleus from stellardynamics (marginally resolving the black hole's sphere of influence). Weshow that the gas in the nucleus has a mean column density of order1024 cm-2 and that it is geometrically thick, inagreement with the standard ``molecular torus'' scenario. We discusspossible heating processes responsible for maintaining the verticalheight of the torus. We also resolve the nuclear stellar distributionand find that within a few parsecs of the AGN there has been an intensestarburst, the most recent episode of which began ~40 Myr ago but hasnow ceased. The current luminosity of stars within 30 pc of the AGN,~3×109 Lsolar, is comparable to that of theAGN. We argue that the star formation has been occurring in theobscuring material. Finally, we apply Schwarzschild orbit superpositionmodels to our full two-dimensional data and derive the mass of the blackhole, paying careful attention to the input parameters, which are oftenuncertain. Our models yield a 1 σ range for the black hole mass ofMBH=7×106-2×107Msolar.Based on observations at the European Southern Observatory VLT(074.B-9012).
| Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. II. X-Ray and Ultraviolet Continuum Variability
We report on two Chandra observations, and a simultaneous Hubble SpaceTelescope ultraviolet observation, of the dwarf Seyfert 1 galaxy NGC4395. Each Chandra observation had a duration of ~30 ks, with aseparation of ~50 ks. The spectrum was observed to harden between theseobservations via a scaling down of the soft-band flux. Theinterobservation variability is in a different sense from the observedvariability within each observation and is most likely the result ofincreased absorption. Spectral variations were seen during the firstobservation suggesting that the X-ray emission is produced in more thanone disconnected region. We have also reanalyzed a ~17 ks Chandraobservation conducted in 2000. During the three Chandra observations the2-10 keV flux is about a factor of 2 lower than seen during anXMM-Newton observation conducted in 2003. Moreover, the fractionalvariability amplitude exhibited during the XMM-Newton observation issignificantly softer than seen during the Chandra observations. A powerspectral analysis of the first of the two new Chandra observationsrevealed a peak at 341 s with a formal detection significance of 99%. Asimilar peak was seen previously in the 2000 Chandra data. However, thedetection of this feature is tentative given that it was found inneither the second of our two new Chandra observations nor theXMM-Newton data, and it is much narrower than expected. The Hubble SpaceTelescope observation was conducted during part of the second Chandravisit. A zero-lag correlation between the ultraviolet and X-ray fluxeswas detected with a significance of ~99.5%, consistent with thepredictions of the two-phase model for the X-ray emission from activegalactic nuclei.
| On the X-Ray Baldwin Effect for Narrow Fe Kα Emission Lines
Most active galactic nuclei (AGNs) exhibit a narrow Fe Kα line at~6.4 keV in the X-ray spectra, due to the fluorescent emission from coldmaterial far from the inner accretion disk. Using XMM-Newtonobservations, Page et al. found that the equivalent width (EW) of thenarrow Fe Kα line decreases with increasing luminosity(EW~L-0.17+/-0.08), suggesting a decrease in the coveringfactor of the material emitting the line (presumably the torus). Bycombining the archival Chandra HETG observations of 34 type 1 AGNs withXMM observations in the literature, we build a much larger sample with101 AGNs. We find a similar X-ray Baldwin effect in the sample(EW~L-0.2015+/-0.0426) however, we note that theanticorrelation is dominated by the radio-loud AGNs in the sample, whoseX-ray spectra might be contaminated by the relativistic jet. Excludingthe radio-loud AGNs, we find a much weaker anticorrelation(EW~L-0.1019+/-0.0524). We present Monte Carlo simulationsshowing that such a weak anticorrelation can be attributed to therelative short timescale variations of the X-ray continuum.
| A Galactic Origin for the Local Ionized X-Ray Absorbers
Recent Chandra and XMM-Newton observations of distant quasars have shownstrong local (z~0) X-ray absorption lines from highly ionized gas,primarily He-like oxygen. The nature of these X-ray absorbers, i.e.,whether they are part of the hot gas associated with the Milky Way orpart of the intragroup medium in the Local Group, remains a puzzle dueto the uncertainties in the distance. We present in this paper a surveyof 20 AGNs with Chandra and XMM-Newton archival data. About 40% of thetargets show local O VII He α absorption with column densitiesaround 1016 cm-2; in particular, O VII absorptionis present in all the high-quality spectra. We estimate that the skycovering fraction of this O VII-absorbing gas is at least 63%, at 90%confidence, and likely to be unity given enough high-quality spectra. Onthe basis of (1) the expected number of absorbers along sight linestoward distant AGNs, (2) joint analysis with X-ray emissionmeasurements, and (3) mass estimation, we argue that the observed X-rayabsorbers are part of the hot gas associated with our Galaxy. Futureobservations will significantly improve our understanding of thecovering fraction and provide robust tests of this result.
| The Radius-Luminosity Relationship for Active Galactic Nuclei: The Effect of Host-Galaxy Starlight on Luminosity Measurements
We have obtained high-resolution images of the central regions of 14reverberation-mapped active galactic nuclei (AGNs) using the HubbleSpace Telescope Advanced Camera for Surveys High Resolution Camera toaccount for host-galaxy starlight contamination of measured AGNluminosities. We measure the host-galaxy starlight contribution to thecontinuum luminosity at 5100 Å through the typical ground-basedslit position and geometry used in the reverberation-mapping campaigns.We find that removing the starlight contribution results in asignificant correction to the luminosity of each AGN both for lowerluminosity sources, as expected, but also for the higher luminositysources such as the PG quasars. After accounting for the host galaxystarlight, we revisit the well-known broad-line region radius-luminosityrelationship for nearby AGNs. We find the power-law slope of therelationship for the Hβ line to be 0.518+/-0.039, shallower thanwhat was previously reported and consistent with the slope of 0.5expected from the naive theoretical assumption that all AGNs have, onaverage, the same ionizing spectrum and the same ionization parameterand gas density in the Hβ line-emitting region.
| BeppoSAX View of Radio-loud Active Galactic Nuclei
A systematic analysis of a large sample of radio-loud active galacticnuclei (AGNs) available in the BeppoSAX public archive has beenperformed. The sample includes 3 narrow-line radio galaxies (NLRGs), 10broad-line radio galaxies (BLRGs), 6 steep-spectrum radio quasars(SSRQs), and 16 flat-spectrum radio quasars (FSRQs). According to theunified models, these classes correspond to objects with increasingviewing angles. As expected, the presence of a nonthermal beamedcomponent emerges clearly in FSRQs. This class shows in fact afeatureless continuum (with the exception of 3C 273) and a significantlyflatter average spectral slope. However, traces of a nonthermalDoppler-enhanced radiation are elusive in the other classes. We findthat the iron line equivalent widths (EWs) are generally weaker inradio-loud AGNs than in Seyfert 1 galaxies, and we confirm the presenceof an X-ray Baldwin effect; that is, a decrease of EW with the 2-10 keVluminosity from Seyfert galaxies to BLRGs and quasars. Since theEW-L2-10 keV anticorrelation is present also in radio-quietAGNs alone, this effect cannot be ascribed entirely to a strongly beamedjet component. Possible alternative interpretations are explored.
| The MBH-σ* Relation in Local Active Galaxies
We examine whether active galaxies obey the same relation between blackhole mass and stellar velocity dispersion as inactive systems, using thelargest published sample of velocity dispersions for active nuclei todate. The combination of 56 original measurements with objects from theliterature not only increases the sample from the 15 consideredpreviously to 88 objects but allows us to cover an unprecedented rangein both stellar velocity dispersion (30-268 km s-1) and blackhole mass (105-108.6 Msolar). In theMBH-σ* relation of active galaxies, we finda lower zero point than the best-fit relation of Tremaine et al. forinactive galaxies, and an upper limit on the intrinsic scatter of 0.4dex. There is also evidence of a flatter slope at low black hole masses.We discuss potential contributors to the observed offsets, includingvariations in the geometry of the broad-line region, evolution in theMBH-σ* relation, and differential growthbetween black holes and galaxy bulges.
| Determining Central Black Hole Masses in Distant Active Galaxies and Quasars. II. Improved Optical and UV Scaling Relationships
We present four improved empirical relationships useful for estimatingthe central black hole mass in nearby AGNs and distant luminous quasarsalike using either optical or UV single-epoch spectroscopy. These massscaling relationships between line widths and luminosity are based onrecently improved empirical relationships between the broad-line regionsize and luminosities in various energy bands and are calibrated to theimproved mass measurements of nearby AGNs based on emission-linereverberation mapping. The mass scaling relationship based on theHβ line luminosity allows mass estimates for low-redshift sourceswith strong contamination of the optical continuum luminosity by stellaror nonthermal emission, while that based on the C IV λ1549 linedispersion allows mass estimates in cases where only the line dispersion(as opposed to the FWHM) can be reliably determined. We estimate thatthe absolute uncertainties in masses given by these mass scalingrelationships are typically around a factor of 4. We include in anappendix mass estimates for all of the Bright Quasar Survey (PG) quasarsfor which direct reverberation-based mass measurements are notavailable.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the Data Archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Is HCN a True Tracer of Dense Molecular Gas in Luminous and Ultraluminous Infrared Galaxies?
We present the results of the first HCO+ survey probing thedense molecular gas content of a sample of 16 luminous and ultraluminousinfrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN (1-0)observations, had shown that LIRGs and ULIRGs possess a significantlyhigher fraction of dense molecular gas compared to normal galaxies.While the picture issued from HCO+ partly confirms thisresult, we have discovered an intriguing correlation between the HCN(1-0)/HCO+ (1-0) luminosity ratio and the IR luminosity ofthe galaxy (LIR). This trend casts doubts on the use of HCNas an unbiased quantitative tracer of the dense molecular gas content inLIRGs and ULIRGs. A plausible scenario explaining the observed trendimplies that X-rays coming from an embedded active galactic nucleus mayplay a dominant role in the chemistry of molecular gas atLIR>=1012 Lsolar. We discuss theimplications of this result for the understanding of LIRGs, ULIRGs, andhigh-redshift gas-rich galaxies.
| Reverberation Measurements of the Inner Radius of the Dust Torus in Nearby Seyfert 1 Galaxies
The most intense monitoring observations yet made in the optical andnear-infrared wave bands were carried out for Seyfert 1 galaxies NGC5548, NGC 4051, NGC 3227, and NGC 7469 by the MAGNUM telescope, andclear time-delayed responses of the K-band flux variations to the V-bandflux variations were found for all of these galaxies. Their H-K colortemperatures of 1500-1800 K, estimated from their observed fluxvariation gradients, support a view that the bulk of the K flux shouldoriginate in the thermal radiation of hot dust surrounding the centralengine and that the lag time should correspond to light-travel distancebetween them. Cross-correlation analysis measures their lag times to be47-53 (NGC 5548), 11-18 (NGC 4051), about 20 (NGC 3227), and 65-87 (NGC7469) days. The lag times are tightly correlated with the opticalluminosities, as expected from dust reverberation(Δt~L0.5), while weakly with the central virial masses,which suggests that the inner radii of the dust tori around activenuclei have one-to-one correspondences with their central luminosities.In the lag time versus central luminosity diagram, the K-band lag timesplace an upper boundary on the similar lag times of broad emission linesin the literature, which not only supports the unified scheme of AGNsbut also implies a physical transition from the BLR out to the dusttorus that encircles the BLR. Correlated short-term V-band and X-rayflux variations in NGC 5548 are also found with a delay of 1 or 2 days,indicating the thermal reprocessing of X-ray emission by the centralaccretion flow.
| Local and Large-Scale Environment of Seyfert Galaxies
We present a three-dimensional study of the local (<=100h-1 kpc) and the large-scale (<=1 h-1 Mpc)environment of the two main types of Seyfert AGN galaxies. For thispurpose we use 48 Seyfert 1 galaxies (with redshifts in the range0.007<=z<=0.036) and 56 Seyfert 2 galaxies (with0.004<=z<=0.020), located at high galactic latitudes, as well astwo control samples of nonactive galaxies having the same morphological,redshift, and diameter size distributions as the corresponding Seyfertsamples. Using the Center for Astrophysics (CfA2) and Southern SkyRedshift Survey (SSRS) galaxy catalogs (mB~15.5) and our ownspectroscopic observations (mB~18.5), we find that within aprojected distance of 100 h-1 kpc and a radial velocityseparation of δv<~600 km s-1 around each of ourAGNs, the fraction of Seyfert 2 galaxies with a close neighbor issignificantly higher than that of their control (especially within 75h-1 kpc) and Seyfert 1 galaxy samples, confirming a previoustwo-dimensional analysis of Dultzin-Hacyan et al. We also find that thelarge-scale environment around the two types of Seyfert galaxies doesnot vary with respect to their control sample galaxies. However, theSeyfert 2 and control galaxy samples do differ significantly whencompared to the corresponding Seyfert 1 samples. Since the maindifference between these samples is their morphological typedistribution, we argue that the large-scale environmental differencecannot be attributed to differences in nuclear activity but rather totheir different type of host galaxies.
| Evolution of the Circumnuclear Radio Supernova SN 2000ft in NGC 7469
SN 2000ft is the first radio supernova detected in the circumnuclearstarburst of a luminous infrared Seyfert 1 galaxy. It is located at adistance of 600 pc from the QSO-like nucleus of NGC 7469. We report thetemporal evolution of SN 2000ft during the 3 years after its discovery.Although SN 2000ft has exploded in the dusty and very dense environmentthat exists in the nuclear regions of luminous infrared galaxies, itshows the radio evolution properties characteristic of radio supernovaeidentified as Type II supernovae, aside from some foreground free-freeabsorption. The peak luminosity and circumstellar matter opacity of SN2000ft are similar to other compact radio sources detected in luminousinfrared galaxies such as NGC 6240, Arp 299, and Arp 220 and identifiedas Type II supernovae.
| The First INTEGRAL AGN Catalog
We present the first INTEGRAL AGN catalog, based on observationsperformed from launch of the mission in 2002 October until 2004 January.The catalog includes 42 AGNs, of which 10 are Seyfert 1, 17 are Seyfert2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rathersmall, with five detected objects, and only one galaxy cluster and nostarburst galaxies have been detected so far. A complete subset consistsof 32 AGNs with a significance limit of 7 σ in the INTEGRAL ISGRI20-40 keV data. Although the sample is not flux limited, thedistribution of sources shows a ratio of obscured to unobscured AGNs of1.5-2.0, consistent with luminosity-dependent unified models for AGNs.Only four Compton-thick AGNs are found in the sample. Based on theINTEGRAL data presented here, the Seyfert 2 spectra are slightly harder(Γ=1.95+/-0.01) than Seyfert 1.5 (Γ=2.10+/-0.02) and Seyfert1 (Γ=2.11+/-0.05).
| A Sample of IRAS Infrared-selected Seyfert 1.5 Galaxies: Infrared Color α(60, 25)-dominated Eigenvector 1
The well-documented E1 relationships are first extended to infraredcolor α(60, 25) and flux ratio [O III]/Hβn bycomparing emission-line properties to continuum properties in infraredwavelengths. Both direct correlations and a principal component analysisare used in a sample of 50 IRAS IR-selected Seyfert 1.5 galaxies. Inaddition, to confirm the correlations of E1 in Boroson & Green, oureigenvector 1 turns out to be dominated by the mid-infrared colorα(60, 25) and most strongly affected by RFe, [OIII]/Hβn, and EW(Hβb). Our analysisindicates that the objects with large E1 tend to coexist with relativelyyoung nuclear stellar populations, which implies that E1 is related tothe nuclear star formation history. The IR-dominated eigenvector 1 cantherefore be inferred to be interpreted as the ``age'' of an AGN. Inconfirmation of the work of Xu and coworkers, it is clear that theextreme Seyfert galaxies with both large RFe and large [OIII]/Hβn are rare in our universe.
| A FUSE Survey of Interstellar Molecular Hydrogen toward High-Latitude AGNs
We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE)survey of interstellar molecular hydrogen (H2) along 45 sightlines to AGNs at high Galactic latitudes (b>20deg). Most(39 out of 45) of the sight lines show detectable Galactic H2absorption from Lyman and Werner bands between 1000 and 1126 Å,with column densities ranging fromNH2=1014.17 to 1019.82cm-2. In the northern Galactic hemisphere, we identify manyregions of low NH2 (<=1015cm-2) between l=60deg and 180° and atb>54deg. These ``H2 holes'' provide valuable,uncontaminated sight lines for extragalactic UV spectroscopy, and a fewmay be related to the ``Northern Chimney'' (low Na I absorption) and the``Lockman Hole'' (low NHI). A comparison of high-latitudeH2 with 139 OB star sight lines surveyed in the Galactic disksuggests that high-latitude and disk H2 clouds may havedifferent rates of heating, cooling, and UV excitation. For rotationalstates J=0 and 1, the mean excitation temperature at high latitude,=124+/-8 K, is somewhat higher thanthat in the Galactic disk, =86+/-20K. For J>=2, the mean =498+/-28 K, and thecolumn-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate acomparable degree of UV excitation in the disk and low halo for sightlines with NH2>=1018cm-2. The distribution of molecular fractions at highlatitude shows a transition at lower total hydrogen column density(logNhlH~20.38+/-0.13) than in the Galactic disk(logNdiskH~20.7). If the UV radiation fields aresimilar in disk and low halo, this suggests an enhanced H2(dust-catalyzed) formation rate in higher density, compressed clouds,which could be detectable as high-latitude, sheetlike infrared cirrus.
| A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
Seyfert galaxies commonly host compact jets spanning 10-100 pc scales,but larger structures are resolved out in long-baseline aperturesynthesis surveys. Previous, targeted studies showed thatkiloparsec-scale radio structures (KSRs) may be a common feature ofSeyfert and LINER galaxies, and the origin of KSRs may be starbursts oractive galactic nuclei (AGNs). We report a new Very Large Array surveyof a complete sample of Seyfert and LINER galaxies. Out of all of thesurveyed radio-quiet sources, we find that 44% (19 out of 43) showextended radio structures at least 1 kpc in total extent that do notmatch the morphology of the disk or its associated star-forming regions.The detection rate is a lower limit owing to the combined effects ofprojection and resolution. The infrared colors of the KSR host galaxiesare unremarkable compared to other Seyfert galaxies, and the large-scaleoutflows orient randomly with respect to the host galaxy axes. The KSRSeyfert galaxies instead stand out by deviating significantly from thefar-infrared-radio correlation for star-forming galaxies, with tendencytoward radio excess, and they are more likely to have a relativelyluminous, compact radio source in the nucleus; these results argue thatKSRs are powered by the AGNs rather than starbursts. The high detectionrate indicates that Seyfert galaxies generate radio outflows over asignificant fraction of their lifetime, which is much longer than thedynamical timescale of an AGN-powered jet but is comparable instead tothe buoyancy timescale. The likely explanation is that the KSRsoriginate from jet plasma that has been decelerated by interaction withthe nuclear interstellar medium (ISM). Based on a simple ram pressureargument, the kinetic power of the jet on kiloparsec scales is about 3orders of magnitude weaker than the power of the jet on 10-100 pcscales. This result is consistent with the interaction model, in whichcase virtually all of the jet power must be lost to the ISM within theinner kiloparsec.
| The Host Galaxies of Narrow-Line Seyfert 1 Galaxies: Nuclear Dust Morphology and Starburst Rings
We present a study of the nuclear morphology of a sample of narrow- andbroad-line Seyfert 1 galaxies (NLS1s and BLS1s, respectively) based onbroadband images in the Hubble Space Telescope archives. In our previousstudy we found that large-scale stellar bars at >1 kpc from thenucleus are more common in NLS1s than BLS1s. In this paper we find thatNLS1s preferentially have grand-design dust spirals within ~1 kpc oftheir centers. We also find that NLS1s have a higher fraction of nuclearstar-forming rings than BLS1s. We find that many of the morphologicaldifferences are due to the presence or absence of a large-scale stellarbar within the spiral host galaxy. In general, barred Seyfert 1 galaxiestend to have grand-design dust spirals at their centers, confirming theresults of other researchers. The high fraction of grand-design nucleardust spirals and stellar nuclear rings observed in NLS1s' host galaxiessuggests a means for efficient fueling of their nuclei to support theirhigh Eddington ratios.
| Examining the Seyfert-Starburst Connection with Arcsecond-Resolution Radio Continuum Observations
We compare the arcsecond-scale circumnuclear radio continuum propertiesof five Seyfert and five starburst galaxies, concentrating on the searchfor any structures that could imply a spatial or causal connectionbetween the nuclear activity and a circumnuclear starburst ring. Noevidence is found in the radio emission for a link between thetriggering or feeding of nuclear activity and the properties ofcircumnuclear star formation. Conversely, there is no clear evidence ofnuclear outflows or jets triggering activity in the circumnuclear ringsof star formation. Interestingly, the difference in the angle betweenthe apparent orientation of the most elongated radio emission and theorientation of the major axis of the galaxy is on average larger inSeyfert galaxies than in starburst galaxies, and Seyfert galaxies appearto have a larger physical size scale of the circumnuclear radiocontinuum emission. The concentration, asymmetry, and clumpinessparameters of radio continuum emission in Seyfert galaxies andstarbursts are comparable, as are the radial profiles of radio continuumand near-infrared line emission. The circumnuclear star formation andsupernova rates do not depend on the level of nuclear activity. Theradio emission usually traces the near-infrared Brγ andH2 1-0 S(1) line emission on large spatial scales, butlocally their distributions are different, most likely because of theeffects of varying local magnetic fields and dust absorption andscattering.
| The structure and X-ray radiation spectra of illuminated accretion disks in AGN. III. Modeling fractional variability
Context: .Random magnetic flares above the accretion disks of ActiveGalactic Nuclei can account for the production of the primary radiationand for the rapid X-ray variability that have been frequently observedin these objects. The primary component is partly reprocessed in thedisk atmosphere, forming a hot spot underneath the flare source andgiving rise to distinct spectral features. Aims: .Extending thework of Czerny et al. (2004, A&A, 420, 1), we model the fractionalvariability amplitude due to distributions of hot spots co-orbiting onthe accretion disk around a supermassive black hole. We compare ourresults to the observed fractional variability spectrum of the Seyfertgalaxy MCG-6-30-15. Methods: .According to defined radialdistributions, our code samples random positions for the hot spotsacross the disk. The local spot emission is computed as reprocessedradiation coming from a compact primary source above the disk. Thestructure of the hot spot and the anisotropy of the re-emission aretaken into account. We compute the fractional variability spectraexpected from such spot ensembles and investigate dependencies on theparameters describing the radial spot distribution. We consider thefractional variability F{ var} with respect to the spectralmean and the so-called point-to-point definition F{ pp}. Ourmethod includes relativistic corrections due to the curved space-time inthe vicinity of a rotating supermassive black hole at the disk center;the black hole's angular momentum is a free parameter and is subject tothe fitting procedure. Results: .We confirm that therms-variability spectra involve intrinsic randomness at a significantlevel when the number of flares appearing during the total observationtime is too small. Furthermore, the fractional variability expressed byF{ var} is not always compatible with F{ pp}. Inthe special case of MCG-6-30-15, we can reproduce the short-timescalevariability and model the suppressed variability in the energy range ofthe Kα line without any need to postulate reprocessing fartheraway from the center. The presence of the dip in the variabilityspectrum requires an increasing rate of energy production by the flarestoward the center of the disk. The depth of the feature is wellrepresented only if we assume a fast rotation of the central black holeand allow for considerable suppression of the primary flare emission.The modeled line remains consistent with the measured equivalent widthof the iron Kα line complex. The model can reproduce thefrequently observed suppression of the variability in the spectral rangearound 6.5 keV, thereby setting constraints on the black hole spin andon the disk inclination.
| SINFONI adaptive optics integral field spectroscopy of the Circinus Galaxy
Aims.We investigate the star formation activity and the gas and stellardynamics on scales of a few parsecs in the nucleus of the CircinusGalaxy. Methods: .Using the adaptive optics near infrared integralfield spectrometer SINFONI on the VLT, we have obtained observations ofthe Circinus galaxy on scales of a few parsecs and at a spectralresolution of 70 km s-1 FWHM. The physical properties of thenucleus are analyzed by means of line and velocity maps extracted fromthe SINFONI datacube. Starburst models are constrained using theBrγ flux, stellar continuum (as traced via the CO absorptionbandheads longward of 2.3 μm), and radio continuum. Results:.The similarity of the morphologies of the H2 1-0 S(1) 2.12 μm andBrγ 2.17 μm lines to the stellar continuum and also theirkinematics, suggest a common origin in star formation. Within 8 pc ofthe AGN we find there has been a recent starburst in the last 100 Myr,which currently accounts for 1.4% of the galaxy's bolometric luminosity.The similarity of the spatial scales over which the stars and gas existindicates that this star formation is occuring within the torus; andcomparison of the gas column density through the torus to the maximumpossible optical depth to the stars implies the torus is a clumpymedium. The coronal lines show asymmetric profiles with a spatiallycompact narrow component and a spatially extended blue wing. Thesecharacteristics are consistent with some of the emission arising inclouds gravitationally bound to the AGN, and some outflowing incloudlets which have been eroded away from the bound clouds.
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