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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.
| 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.
| On the Lengths, Colors, and Ages of 18 Face-on Bars
Along with a brief analysis we present data obtained from BVRI andKs images of a sample of 19 galaxies (18 barred and 1unbarred), which will be further explored in a future paper. We measuredthe lengths and colors of the bars, created color maps, and estimatedglobal color gradients. Applying a method developed in a companionpaper, we could distinguish for seven galaxies in our sample those whosebars have been recently formed from the ones with already evolved bars.We estimated an average difference in the optical colors between youngand evolved bars that may be translated to an age difference of theorder of 10 Gyr, meaning that bars may be, at least in some cases,long-standing structures. Moreover, our results show that, on average,evolved bars are longer than young bars. This seems to indicate that,during its evolution, a bar grows longer by capturing stars from thedisk, in agreement with recent numerical and analytical results.Although the statistical significance of these results is low, andfurther studies are needed to confirm them, we discuss the implicationsfrom our results on the possibility of bars being a recurrentphenomenon. We also present isophotal contours for all our images aswell as radial profiles of relevant photometric and geometricparameters.
| On the Fraction of X-Ray-obscured Quasars in the Local Universe
Recent wide-area hard X-ray and soft gamma-ray surveys have shown thatthe fraction of X-ray-obscured active galactic nuclei (AGNs) in thelocal universe significantly decreases with intrinsic luminosity. Inthis Letter we point out that two corrections have to be made to thesamples: (1) radio-loud AGNs have to be excluded, since their X-rayemission might be dominated by the jet component, and (2) Compton-thicksources have to be excluded too, since their hard X-ray and softgamma-ray emission are also strongly attenuated by Compton scattering.The soft gamma-ray-selected AGN samples obtained by Swift and INTEGRALprovide the best opportunity to study the fraction of obscured AGNs inthe local universe in the least biased way. We choose these samples tocheck if the corrections could alter the above result on the fraction ofobscured AGNs. We find that before the corrections both samples showsignificant anticorrelation between LX and NH,indicating an obvious decrease in the fraction of obscured AGNs withluminosity. However, after the corrections, we find only marginalevidence of anticorrelation (at the 98% confidence level) in the Swiftsample and no evidence at all in the INTEGRAL sample, which consists ofa comparable number of objects. We conclude that current samples onlyshow a marginal decrease in the fraction of obscured AGNs in the localuniverse and that much larger samples are required in order to reach amore robust conclusion.
| 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.
| Pseudobulges in the Disk Galaxies NGC 7690 and NGC 4593
We present Ks-band surface photometry of NGC 7690 (Hubbletype Sab) and NGC 4593 (SBb). We find that, in both galaxies, a majorpart of the ``bulge'' is as flat as the disk and has approximately thesame color as the inner disk. In other words, the ``bulges'' of thesegalaxies have disklike properties. We conclude that these are examplesof ``pseudobulges,'' that is, products of secular dynamical evolution.Nonaxisymmetries such as bars and oval disks transport disk gas towardthe center. There star formation builds dense stellar components thatlook like-and often are mistaken for-merger-built bulges, but that wereconstructed slowly out of disk material. These pseudobulges can mosteasily be recognized when, as in the present galaxies, they retaindisklike properties. NGC 7690 and NGC 4593 therefore contribute to thegrowing evidence that secular processes help to shape galaxies. NGC 4593contains a nuclear ring of dust that is morphologically similar tonuclear rings of star formation that are seen in many barred and ovalgalaxies. The nuclear dust ring is connected to nearly radial dust lanesin the galaxy's bar. Such dust lanes are a signature of gas inflow. Wesuggest that gas is currently accumulating in the dust ring andhypothesize that the gas ring will starburst in the future. Theobservations of NGC 4593 therefore suggest that major starburst eventsthat contribute to pseudobulge growth can be episodic.Based on observations made with the Anglo-Australian Telescope.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the Data Archive at the Space Telescope ScienceInstitute (STScI). STScI is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS5-26555. Theobservations of NGC 7690 are associated with program IDs 7331 (NICMOS:M. Stiavelli) and 6359 (WFPC2: M. Stiavelli). The observations of NGC4593 are associated with program IDs 7330 (NICMOS: J. Mulchaey), and5479 (WFPC2: M. Malkan).
| 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.
| 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.
| 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.
| 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).
| INTEGRAL IBIS Extragalactic Survey: Active Galactic Nuclei Selected at 20-100 keV
Analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL)Core Programme and public open-time observations performed up to 2005April provides a sample of 62 active galactic nuclei in the 20-100 keVband above a flux limit of ~1.5×10-11 ergscm-2 s-1. Most (42) of the sources in the sampleare Seyfert galaxies, almost equally divided between type 1 and type 2objects; six are blazars, and 14 are still unclassified. Excluding theblazars, the average redshift of our sample is 0.021, while the meanluminosity is logL=43.45. We find that absorption is present in 65% ofthe objects, with 14% of the total sample due to Compton-thick activegalaxies. In agreement with both Swift BAT team results and 2-10 keVstudies, the fraction of absorbed objects decreases with the 20-100 keVluminosity. All Seyfert 2's in our sample are absorbed, as are 33% ofSeyfert 1's. The present data highlight the capability of INTEGRAL toprobe the extragalactic gamma-ray sky and to find new and/or absorbedactive galaxies.Based on observations obtained with INTEGRAL, an ESA project withinstruments and science data center funded by ESA member states(especially the PI countries: Denmark, France, Germany, Italy,Switzerland, Spain), the Czech Republic, and Poland and with theparticipation of Russia and the US.
| 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.
| The small dispersion of the mid IR - hard X-ray correlation in active galactic nuclei
Context: .We investigate mid-infrared and X-ray properties of the dustytorus in unification scenarios for active galactic nuclei. Aims:.We use the relation between mid IR and hard X-ray luminosities toconstrain AGN unification scenarios. Methods: .With VISIR at theVLT, we have obtained the currently highest angular resolution (0.35arcsec FWHM) narrow-band mid infrared images of the nuclei of 8 nearbySeyfert galaxies. Combining these observations with X-ray data from theliterature we study the correlation between their mid IR and hard X-rayluminosities. Results: .We find that the rest frame 12.3 μm(LMIR) and 2-10 keV (LX) luminosities arecorrelated at a highly significant level. The best fit power-law to ourdata is log{LMIR} ∝ (1.60 ± 0.22 )log{LX}, showing a much smaller dispersion than earlierstudies. Conclusions: .The similarity in thelog{LMIR}/log{LX} ratio between Sy1s and Sy2s evenusing high angular resolution MIR data implies that the similarity isintrinsic to AGN and not caused by contamination from extra-nuclearemission. This supports clumpy torus models. The exponent of thecorrelation constrains the inner geometry of the torus.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| Interstellar polarization and the position-angle orientations of type 1 Seyfert galaxies
We comment on recent spectropolarimetric studies that compare theobserved polarization position angles (PAs) of type 1 Seyfert galaxiesnear Hα with the observed orientations of their radio-source axeson the sky. For a Seyfert galaxy in which scattering occurs mainly in anequatorial scattering region, the polarization PA is expected to beparallel to the radio axis, while for a case in which light scatterspredominantly in the polar regions, the Hα polarization PA shouldbe perpendicular to the radio axis. In practice, these correlations aredifficult to establish because a Galactic interstellar polarizationcontribution can introduce a significant uncertainty into thepolarization PA determination, even when the magnitude of interstellarpolarization is small. We show how such uncertainties may affect theanalysis of PA alignments, and present spectropolarimetric observationsof a probe star along the line of sight to the type 1 Seyfert galaxy Mrk871 that allow us to assess the intrinsic Hα polarization and PAof Mrk 871. These results suggest that spectropolarimetric observationsof such probe stars should form an integral part of future polarizationstudies of Seyfert galaxies.
| Constraints on hot metals in the vicinity of the Galaxy
We have searched for evidence of soft X-ray absorption by hot metals inthe vicinity of the Galaxy in the spectra of a small sample of 15 type Iactive galactic nuclei (AGN) observed with the high-resolution X-raygratings on board Chandra. This is an extension of our previous surveyof hot OVII and OVIII absorbing gas in the vicinity of the Galaxy. Thestrongest absorption signatures within a few hundred km s-1of their rest-frame energies are most likely to be due to warm absorbingoutflows from the nearest AGN, which are back-lighting the local hotgas. We emphasize that absorption signatures in the spectra of somedistant AGN that are kinematically consistent with the recessionalvelocity of the AGN are most likely to be due to hot local gas. Alongthe sightline towards PG 1211+143, PDS 456 and MCG-6-30-15 there is avery large absorbing Fe column density which is kinematically consistentwith absorption by hot, local Fe. The sightlines to these three AGN passthrough the limb of the Northern Polar Spur (NPS), a local bubble formedfrom several supernovae which, if rich in Fe, may account for a largelocal Fe column.We obtain limits on the column density of local, highly ionized N, Ne,Mg, Si along all of the sightlines in our sample. We correlate thecolumn density limits with those of highly ionized O along the samesightlines. Assuming the hot local gas is in collisionally ionizedequilibrium, we obtain limits on the temperature and relative abundancesof the metals in the hot local gas. Our limits on the ionic columndensities in the local hot gas seem to be consistent with those observedin the hot halo gas of edge-on normal spiral galaxies.
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
| The relationship between X-ray variability amplitude and black hole mass in active galactic nuclei
We have investigated the relationship between the X-ray variabilityamplitude and black hole mass for a sample of 46 radio-quiet activegalactic nuclei observed by ASCA. 33 of the objects in our sampleexhibited significant variability over a time-scale of ~40 ks. Wedetermined the normalized excess variance in the 2-10 keV light curvesof these objects and found a significant anticorrelation between excessvariance and black hole mass. Unlike most previous studies, we havequantified the variability using nearly the same time-scale for allobjects. Moreover, we provide a prescription for estimating theuncertainties in variance which accounts both for measurementuncertainties and for the stochastic nature of the variability. We alsopresent an analytical method to predict the excess variance from a modelpower spectrum accounting for binning, sampling and windowing effects.Using this, we modelled the variance-mass relation assuming all objectshave a universal twice-broken power spectrum, with the position of thebreaks being dependent on mass. This accounts for the general form ofthe variance-mass relationship but is formally a poor fit and there isconsiderable scatter. We investigated this scatter as a function of theX-ray photon index, luminosity and Eddington ratio. After accounting forthe primary dependence of excess variance on mass, we find nosignificant correlation with either luminosity or X-ray spectral slope.We do find an anticorrelation between excess variance and the Eddingtonratio, although this relation might be an artefact owing to theuncertainties in the mass measurements. It remains to be establishedthat enhanced X-ray variability is a property of objects with steepX-ray slopes or large Eddington ratios. Narrow-line Seyfert 1 galaxies,in particular, are consistent with being more variable than theirbroad-line counterparts solely because they tend to have smaller masses.
| Principal components in active galactic nuclei variability data and the estimation of the flux contributions from different components
It has been found that the near-infrared flux variations of Seyfertgalaxies satisfy relations of the formFi~αij+βijFj,where Fi, Fj are the fluxes in filters i and j;and αi,j, βi,j are constants. Theserelations have been used to estimate the constant contributions of thenon-variable underlying galaxies. The paper attempts a formal treatmentof the estimation procedure, allowing for the possible presence of athird component, namely non-variable hot dust. In an analysis of asample of 38 Seyfert galaxies, inclusion of the hot dust componentimproves the model fit in approximately half the cases. All derived dusttemperatures are below 300 K, in the range 540-860 K or above 1300 K. Anoteworthy feature is the estimation of confidence intervals for thecomponent contributions: this is achieved by bootstrapping. It is alsopointed out that the model implies that such data could be fruitfullyanalysed in terms of principal components.
| FeXXV and FeXXVI lines from low-velocity, photoionized gas in the X-ray spectra of active galactic nuclei
We have calculated the equivalent widths of the absorption linesproduced by FeXXV and FeXXVI in a Compton-thin, low-velocityphotoionized material illuminated by the nuclear continuum in activegalactic nuclei. The results, plotted against the ionization parameterand the column density of the gas, are a complement to those presentedby Bianchi & Matt for the emission lines from the same ionicspecies. As an extension to the work by Bianchi & Matt, we alsopresent a qualitative discussion on the different contributions to theHe-like iron emission line complex in the regimes where recombination orresonant scattering dominates, providing a useful diagnostic tool tomeasure the column density of the gas. Future high-resolution missions(e.g. Astro-E2) will allow us to fully take advantage of these plasmadiagnostics. In the meantime, we compare our results with an up-to-datelist of Compton-thick and unobscured (at least at the iron line energy)Seyfert galaxies with emission and/or absorption lines from H- andHe-like iron observed with Chandra and XMM-Newton.
| The continuum and narrow-line region of the narrow-line Seyfert 1 galaxy Mrk 766
We present the first spectroscopic observations in the interval 0.8-4.0μm, complemented with existing Hubble Space Telescope ultraviolet(UV) and optical spectroscopy, of the narrow-line Seyfert 1 galaxy Mrk766. The near-infrared spectrum is characterized by numerous permittedlines of HI, HeI, HeII and FeII, and forbidden lines of [SII], [SIII]and [FeII] among others. Highly ionized species such as [SiIX], [SiX],[SIX] and [MgVII] are also observed. The continuum emission has acomplex shape, with contributions from the central engine, circumnuclearstellar population and dust. This last component is demonstrated by thepresence of an excess of emission peaking at 2.25 μm, well fitted bya blackbody function with Tbb= 1200 K. That temperature isclose to the evaporation temperature of graphite grains. As such, itprovides strong evidence of hot dust, probably very close to thenucleus. Consistent modelling of the line spectrum and the broad-bandcontinuum by composite models, which account for the photoionizing fluxof the central engine and shocks, shows that shock velocities rangebetween 100 and 500 km s-1, the pre-shock densities between100 and 1000 cm-3 and the radiation fluxes from the activecentre between 109 and 5 × 1012 photoncm-2 s-1 eV-1 at 1 Ryd with spectralindices αUV=-1.5 and αX=-0.4. Adoptingsilicate grains, dust-to-gas ratios are between 10-6 and 4× 10-4 by mass. The emitting clouds are at an averagedistance of 160 pc from the centre, with high-velocity clouds closer andlow-velocity clouds further from the centre. The N/H relative abundancededuced from the fit of the [NII] 6548+/[OIII] 5007+ line ratio could betwice the solar value. On the other hand, Fe is depleted from thegaseous phase by a factor >2, most probably trapped into grains.Ratios of calculated to observed line ratios to Hβ indicate anaverage contribution of the broad-line region to the observed Hβ ofapproximately 40 per cent.
| Radio Identifications of Markarian Galaxies and the Correlation between Radio and Far-Infrared Properties
By checking DSS optical images and NVSS radio images, 782 Markariangalaxies were identified to be NVSS radio sources. A comparison of theradio luminosity at 1.4 GHz and the far-infrared (FIR) luminosity for468 ``normal" galaxies shows a tight correlation. Most of the Seyfertgalaxies and quasars follow the radio-FIR relation deduced from the``normal" galaxy sample, but with a somewhat larger scatter. A total 167Markarian galaxies, comprising 100 ``normal" galaxies, 66 Seyfertgalaxies and one quasar, have either excess radio emission or much lowerFIR spectral index α (25μm, 60μm). These galaxies may beclassified as ``AGN-powered". For ``normal" galaxies, the average qvalue (defined as the log ratio between FIR and radio luminosities) is2.3. There seems a trend for q to slightly decrease with increasingradio luminosity. This may imply that the ongoing active star formationin galaxies with higher radio luminosities is more efficient in heatingthe cosmic-ray electrons.
| The Relation between the Inclinations of Broad Line Regions and the Accretion Disk
According to the standard model, an active galactic nucleus (AGN)consists of an inner accretion disk with a jet around a central massiveblack hole, and a number of outer broad line regions (BLRs) and narrowline regions (NLRs). The geometrical relationship between the BLRs andthe accretion disk is not well understood. Assuming the motion of theBLRs is virialized and its configuration is disk-like, we derived itsinclination to the line of sight for a sample of AGNs from their bulgestellar velocity dispersion, their size of the BLRs and their Hβlinewidth. Compared with the inclination of the accretion disk obtainedfrom the X-ray Fe Kα emission lines, we found that there is nopositive correlation between the two. Our results showed that BLRs arenot coplanar with the accretion disk and that we should be cautious ofusing the BLRs inclination as the disk inclination. The non-coplanargeometry of the outer BLRs and the inner accretion disk provides cluesto the origin of BLRs and the properties of the accretion disk. Ourpreferable interpretation is that BLRs arise out of the outer part of awarped accretion disk.
| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| The Relationship of Hard X-Ray and Optical Line Emission in Low-Redshift Active Galactic Nuclei
In this paper we assess the relationship of the population of activegalactic nuclei (AGNs) selected by hard X-rays to the traditionalpopulation of AGNs with strong optical emission lines. First, we studythe emission-line properties of a new hard-X-ray-selected sample of 47local AGNs (classified optically as Type 1 and 2 AGNs). We find that thehard X-ray (3-20 keV) and [O III] λ5007 optical emission-lineluminosities are well-correlated over a range of about 4 orders ofmagnitude in luminosity (mean luminosity ratio 2.15 dex with a standarddeviation of σ=0.51 dex). Second, we study the hard X-rayproperties of a sample of 55 local AGNs selected from the literature onthe basis of the flux in the [O III] line. The correlation between thehard X-ray (2-10 keV) and [O III] luminosity for the Type 1 AGNs isconsistent with what is seen in the hard-X-ray-selected sample. However,the Type 2 AGNs have a much larger range in the luminosity ratio, andmany are very weak in hard X-rays (as expected for heavily absorbedAGNs). We then compare the hard X-ray (3-20 keV) and [O III] luminosityfunctions of AGNs in the local universe. These have similar faint-endslopes, with a luminosity ratio of 1.60 dex (0.55 dex smaller than themean value for individual hard-X-ray-selected AGNs). We conclude that atlow redshift, selection by narrow optical emission lines will recovermost AGNs selected by hard X-rays (with the exception of BL Lacobjects). However, selection by hard X-rays misses a significantfraction of the local AGN population with strong emission lines.
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