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Публикации по объекту
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.
| Discovery of a 500 Parsec Shell in the Nucleus of Centaurus A
Spitzer Space Telescope mid-infrared images of the radio galaxyCentaurus A reveal a shell-like, bipolar, structure 500 pc to the northand south of the nucleus. This shell is seen in 5.8, 8.0, and 24 μmbroadband images. Such a remarkable shell has not been previouslydetected in a radio galaxy and, if confirmed, would be the firstextragalactic nuclear shell detected at mid-infrared wavelengths.Assuming that it is a coherent expanding structure, we estimate that theshell is a few million years old and has a mass on the order of amillion solar masses. A conservative estimate for the mechanical energyin the wind-driven bubble is 1053 ergs. The shell could havebeen created by a small, few-thousand solar mass, nuclear burst of starformation. Alternatively, the bolometric luminosity of the activenucleus is sufficiently large that it could power the shell. Constraintson the shell's velocity are lacking. However, if the shell is moving at1000 km s-1, then the required mechanical energy would be 100times larger.
| First Detection of PAHs and Warm Molecular Hydrogen in Tidal Dwarf Galaxies
We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope.We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2,7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of~100 carbon atoms with an equal mixture of neutral and ionized PAHs. TheTDGs have a dominant warm (~140 K) dust component in marked contrast tothe cooler (40-60 K) dust found in starburst galaxies. For the firsttime we detect the low-J rotational lines from molecular hydrogen.Adopting LTE, there is ~105 Msolar of ~400 K gas,which is <0.1% of the cold gas mass inferred from 12CO(1-0) measurements. The combination of one-third solar metallicity witha recent (<5 million year) episode of star formation is reflected inthe S and Ne ratios. The excitation is higher than typical values forstarburst galaxies and similar to that found in BCDs. Using the InfraredArray Camera, we identify an additional 13 PAH-rich candidate TDGs.These sources occupy a distinct region of IRAC color space with[3.6]-[4.5]<0.4 and [4.5]-[8.0]>3.2. Their disturbed morphologiessuggest past merger events between companions; for example, NGC 5291 Shas a projected 11 kpc tail. NGC 5291 N and S have stellar masses of(1.5 and 3.0)×108 Msolar, which iscomparable to BCDs, although still roughly 10% of the LMC's stellarmass. The candidate TDGs are an order of magnitude less massive. Thissystem appears to be a remarkable TDG nursery.
| 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 radio structure of radio-quiet quasars
Aims.We investigate the radio emitting structures of radio-quiet activegalactic nuclei with an emphasis on radio-quiet quasars to study theirconnection to Seyfert galaxies. Methods: .We present and analysehigh-sensitivity VLA radio continuum images of 14 radio-quiet quasarsand six Seyfert galaxies. Results: .Many of the low redshiftradio-quiet quasars show radio structures that can be interpreted asjet-like outflows. However, the detection rate of extended radiostructures on arcsecond scales among our sample decreases withincreasing redshift and luminosity, most likely due to a lack ofresolution. The morphologies of the detected radio emission indicatestrong interactions of the jets with the surrounding medium. We alsocompare the radio data of seven quasars with corresponding HST images ofthe [O III] emitting narrow-line region (NLR). We find that the scenarioof interaction between the radio jet and the NLR gas is confirmed in twosources by structures in the NLR gas distribution as previously knownfor Seyfert galaxies. The extended radio structures of radio-quietquasars at sub-arcsecond resolution are by no means different from thatof Seyferts. Among the luminosities studied here, the morphologicalfeatures found are similar in both types of objects while the overallsize of the radio structures increases with luminosity. This supportsthe picture where radio-quiet quasars are the scaled-up versions ofSeyfert galaxies. In addition to known luminosity relations we find acorrelation of the NLR size and the radio size shared by quasars andSeyferts.
| The K-band properties of Seyfert 2 galaxies
Aims. It is well known that the [O iii]λ5007 emission line andhard X-ray (2-10 keV) luminosities are good indicators of AGN activitiesand that the near and mid-infrared emission of AGN originates fromre-radiation of dusty clouds heated by the UV/optical radiation from theaccretion disk. In this paper we present a study of the near-infraredK-band (2.2 μm) properties for a sample of 65 Seyfert 2 galaxies. Methods: .By using the AGN/Bulge/Disk decomposition technique, weanalyzed the 2MASS K_S-band images for Seyfert 2 galaxies in order toderive the K_S-band magnitudes for the central engine, bulge, and diskcomponents. Results: .We find that the K_S-band magnitudes of thecentral AGN component in Seyfert 2 galaxies are tightly correlated withthe [O iii]λ5007 and the hard X-ray luminosities, which suggeststhat the AGN K-band emission is also an excellent indicator of thenuclear activities at least for Seyfert 2 galaxies. We also confirm thegood relation between the central black hole masses and bulge's K-bandmagnitudes for Seyfert 2s.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| Variability of broad and blueshifted component of [OIII]λ5007 in I ZWI
Although the existence of asymmetrical profile of [OIII]λ5007 hasbeen discovered for ages, its filiation and physics are poorlyunderstood. Two new spectra of I ZWI taken on November 16, 2001 and onDecember 3, 2002 were compared with the spectra taken by BG92. Followingresults are obtained. (1) The certain variations of broad [OIII] duringabout 10 years separating the observations are identified. The inferredlength scale of broad [OIII] emitting region ranges from 0.3 to 3 pc. Byassuming a Keplerian motion in line emitting region, the materialemitting broad [OIII] is likely to be located at the transient emissionline region, between BLR and NLR. (2) We find a positive relationbetween the FeII emission and flux of Hβ (or continuum). On theother hand, the parameter RFe decreases with ionizing continuummarginally. (3) We detect a low ionized NLR in I ZWI, because of the lowflux ratios [OIII]n/Hβn (1.7).
| 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.
| On the transmission-dominated to reprocessing-dominated spectral state transitions in Seyfert 2 galaxies
We present Chandra and XMM-Newton observations of a small sample (11objects) of optically selected Seyfert 2 galaxies, for which ASCA andBeppoSAX had suggested Compton-thick obscuration of the active galacticnucleus (AGN). The main goal of this study is to estimate the rate oftransitions between `transmission-dominated' and`reprocessing-dominated' states. We discover one new transition in NGC4939, with a possible additional candidate in NGC 5643. This indicates atypical occurrence rate of at least ~0.02yr-1. Thesetransitions could be due to large changes of the obscuring gas columndensity, or to a transient dimming of the AGN activity, the latterscenario being supported by detailed analysis of the best-studiedevents. Independently of the ultimate mechanism, comparison of theobserved spectral dynamics with Monte Carlo simulations demonstratesthat the obscuring gas is largely inhomogeneous, with multiple absorbingcomponents possibly spread through the whole range of distances from thenucleus between a fraction of parsecs up to several hundred parsecs. Asa by-product of this study, we report the first measurement ever of thecolumn density covering the AGN in NGC 3393 (NH~= 4.4 ×1024cm-2), and the discovery of soft X-rayextended emission, apparently aligned along the host galaxy main axis inNGC 5005. The latter object most likely hosts an historicallymisclassified low-luminosity Compton-thin AGN.
| Galactic Winds
Galactic winds are the primary mechanism by which energy and metals arerecycled in galaxies and are deposited into the intergalactic medium.New observations are revealing the ubiquity of this process,particularly at high redshift. We describe the physics behind thesewinds, discuss the observational evidence for them in nearbystar-forming and active galaxies and in the high-redshift universe, andconsider the implications of energetic winds for the formation andevolution of galaxies and the intergalactic medium. To inspire futureresearch, we conclude with a set of observational and theoreticalchallenges.
| 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.
| The Swift/BAT High-Latitude Survey: First Results
We present preliminary results from the first 3 months of the SwiftBurst Alert Telescope (BAT) high Galactic latitude survey in the 14-195keV band. The survey reaches a flux of ~10-11 ergscm-2 s-1 and has ~2.7 arcmin (90% confidence)positional uncertainties for the faintest sources. This represents themost sensitive survey to date in this energy band. These data confirmthe conjectures that a high-energy-selected active galactic nucleus(AGN) sample would have very different properties from those selected inother bands and that it represents a ``true'' sample of the AGNpopulation. We have identified 86% of the 66 high-latitude sources.Twelve are Galactic-type sources, and 44 can be identified withpreviously known AGNs. All but five of the AGNs have archival X-rayspectra, enabling us to estimate the line-of-sight column densities andother spectral properties. Both of the z>0.11 objects are blazars.The median redshift of the others (excluding radio-loud objects) is0.012. We find that the column density distribution of these AGNs isbimodal, with 64% of the nonblazar sources having column densitiesNH>=1022 cm-2. None of the sourceswith logLX>43.5 (cgs units) show high column densities,and very few of the lower LX sources have low columndensities. Based on these data, we expect the final BAT catalog to have>200 AGNs and reach fluxes of less than ~10-11 ergscm-2 s-1 over the entire sky.
| Molecular Hydrogen Excitation around Active Galactic Nuclei
We report R~3000 Very Large Telescope ISAAC K-band spectroscopy of thenuclei (i.e., central 100-300 pc) of nine galaxies hosting an activegalactic nucleus. For five of these we also present spectra of thecircumnuclear region out to 1 kpc. We have measured a number ofmolecular hydrogen lines in the ν=1-0, 2-1, and 3-2 vibrationaltransitions, as well as the Brγ and He I recombination lines andthe Na I stellar absorption feature. Although only three of the galaxiesare classified as type 1 Seyfert galaxies in the literature, broadBrγ (FWHM>~1000 km s-1) is seen in seven of theobjects. The ν=1-0 emission appears thermalized at temperaturesT~1000 K. However, the ν=2-1 and ν=3-2 emission show evidence ofbeing radiatively excited by far-ultraviolet photons. Thephotodissociation region models that fit the data best are, as for theultraluminous infrared galaxies in Davies et al., those for which theH2 emission arises in dense clouds illuminated by intensefar-ultraviolet radiation. The Na I stellar absorption line is clearlyseen in six of the nuclear spectra of these AGNs, indicating thepresence of a significant population of late-type stars. It is possiblethat these stars are a result of the same episode of star formation thatgave rise to the stars heating the photodissociation regions. It seemsunlikely that the AGN is the dominant source of excitation for thenear-infrared H2 emission: in two of the nuclear spectraH2 was not detected at all, and in general we find noevidence of suppression of the 2-1 S(3) line, which may occur inX-ray-irradiated gas. Our data do not reveal any significant differencebetween the nuclear and circumnuclear line ratios, suggesting that thephysical conditions of the dominant excitation mechanism are similarboth near the AGN and in the larger scale environment around it, andthat star formation is an important process even in the central 100 pcaround AGNs.Based on observations at the European Southern Observatory VLT(69.B-0075).
| Outflows in Active Galactic Nucleus/Starburst-Composite Ultraluminous Infrared Galaxies1,
Galactic superwinds occur in almost all infrared-luminous galaxies withstar formation rates (SFRs) above 10 Msolar yr-1,as shown by studies of the Na I D interstellar absorption line. Wedemonstrate that this result also applies to ultraluminous infraredgalaxies (ULIRGs) that host an active galactic nucleus (AGN) embedded ina strong starburst (SFR>~100 Msolar yr-1) bystudying a sample of 26 Seyfert ULIRGs in Na I D. The infraredluminosity of these galaxies is powered jointly by the AGN andstarburst. We find that there are hints of the influence of the AGN onoutflows in Seyfert 2/starburst composites, but the evidence is not yetstatistically conclusive. The evidence we find is lower wind detectionrates (i.e., wind opening angles) in Seyfert 2 ULIRGs than in galaxiesof comparable LIR, higher velocities than in galaxies ofcomparable SFR, and correlations between the neutral gas and the ionizedgas in the extended narrow-line region. Although the AGN probablycontributes to the outflows in Seyfert 2 ULIRGs, its momentum and energyinjection is equal to or less than that of the starburst. Similarly, theoutflow mechanical luminosity (energy outflow rate) per unit radiativeluminosity is the same for starburst and Seyfert 2 ULIRGs. In the nucleiof Seyfert 1 ULIRGs, we observe small-scale outflows that are poweredsolely by the AGN. However, in Mrk 231, we observe both a high-velocity,small-scale and low-velocity, extended outflow. The latter may bepowered by a starburst or radio jet. These large-scale, lower velocityoutflows certainly exist in other Seyfert 1 ULIRGs, but they are washedout by the light of the nucleus.Some of the observations reported here were obtained at the MMTObservatory, which is a joint facility of the Smithsonian Institutionand the University of Arizona.Some of the observations reported here were obtained at the Kitt PeakNational Observatory, National Optical Astronomy Observatory, which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under cooperative agreement with the National ScienceFoundation.
| The X-Ray Spectral Properties of SCUBA Galaxies
Deep SCUBA surveys have uncovered a large population of massivesubmillimeter-emitting galaxies (SMGs; f850μm>~4 mJy)at z>~1. Although it is generally believed that these galaxies hostintense star formation activity, there is growing evidence that asubstantial fraction also harbor an active galactic nucleus (AGN; i.e.,an accreting super-massive black hole [SMBH]). We present here possiblythe strongest evidence for this viewpoint to date: the combination ofultradeep X-ray observations (the 2 Ms Chandra Deep Field-North) anddeep Keck spectroscopic data of SMGs with radio counterparts. We findthat the majority (~75%) of these radio-selected spectroscopicallyidentified SMGs host AGN activity; the other ~25% have X-ray propertiesconsistent with star formation (X-ray-derived star formation rates of~1300-2700 Msolar yr-1). The AGNs have propertiesgenerally consistent with those of nearby luminous AGNs(Γ~1.8+/-0.5, NH~1020-1024cm-2, and LX~1043-1044.5ergs s-1), and the majority (~80%) are heavily obscured(NH>~1023 cm-2). We constructcomposite rest-frame 2-20 keV spectra for three different obscurationclasses [NH<1023 cm-2,NH=(1-5)×1023 cm-2, andNH>5×1023 cm-2], which revealfeatures not seen in the individual X-ray spectra. An ~1 keV equivalentwidth Fe Kα emission line is seen in the composite X-ray spectrumof the most heavily obscured AGNs, suggesting Compton-thick or nearCompton-thick absorption. Even taking into account the effects ofabsorption, we find that the average X-ray to far-IR luminosity ratio ofthe AGN-classified SMGs (LX/LFIR=0.004) isapproximately 1 order of magnitude below that found for typical quasars.This result suggests that intense star formation activity (of order~1000 Msolar yr-1) dominates the bolometric outputof these SMGs. However, we also explore the possibility that the X-rayto far-IR luminosity ratio of the AGN components is intrinsically lessthan that found for typical quasars and postulate that some SMGs may beAGN dominated. We investigate the implications of our results for thegrowth of massive black holes, discuss the prospects for deeper X-rayobservations, and explore the scientific potential offered by the nextgeneration of X-ray observatories.
| Extranuclear X-Ray Emission in the Edge-on Seyfert Galaxy NGC 2992
We observed the edge-on Seyfert 1.9 galaxy NGC 2992 with the ACIS CCDarray on the Chandra X-Ray Observatory and found several extranuclear(r>~3'') X-ray nebulae within 40" (6.3 kpc for our assumeddistance of 32.5 Mpc) of the nucleus. The net X-ray luminosity from theextranuclear sources is ~2-3×1039 ergs s-1in the 0.3-8.0 keV band. The X-ray core itself (r<~1'') ispositioned at R.A. 9h45m41.06s, decl.-14°19'34.8" (J2000.0) and has a remarkably simple power-lawspectrum with photon index Γ=1.86 and intrinsicNH=7×1021 cm-2. The near-nuclear(3''<~r<~18'') Chandra spectrum is bestmodeled by three components: (1) a direct active galactic nucleus (AGN)component from the wings of the point-spread function or anelectron-scattered AGN component, with Γ fixed at 1.86, (2) coldCompton reflection of the AGN component with intrinsic absorptionNH~1022 cm-2, with approximately thesame 0.3-8.0 keV flux as the direct component, and (3) a 0.5 keVlow-abundance (Z<0.03 Zsolar) thermal plasma, with ~10% ofthe flux of either of the first two components. The X-ray luminosity ofthe third component (the ``soft excess'') is ~1.4×1040ergs s-1, or ~5 times that of all of the detectedextranuclear X-ray sources. We suggest that most (~75%-80%) of the softexcess emission originates from a region between radii of 1" and 3",which is not imaged in our observation due to severe CCD pileup. We alsorequire the cold reflector to be positioned at least 1" (158 pc) fromthe nucleus, since there is no reflection component in the X-ray corespectrum. Much of the extranuclear X-ray emission is coincident withradio structures (nuclear radio bubbles and large-scale radio features),and its soft X-ray luminosity is generally consistent with luminositiesexpected from a starburst-driven wind (with the starburst scaled fromLFIR). However, the AGN in NGC 2992 seems equally likely topower the galactic wind in that object. Furthermore, AGN photoionizationand photoexcitation processes could dominate the soft excess, especiallythe ~75%-80% that is not imaged by our observations.
| Rapid Compton-thick/Compton-thin Transitions in the Seyfert 2 Galaxy NGC 1365
We present multiple Chandra and XMM-Newton observations of the type 1.8Seyfert galaxy NGC 1365, which shows the most dramatic X-ray spectralchanges observed so far in an active galactic nucleus: the sourceswitched from reflection-dominated to transmission-dominated and back injust 6 weeks. During this time the soft thermal component, arising froman ~1 kpc region around the center, remained constant. The reflectioncomponent is constant at all timescales, and its high flux relative tothe primary component implies the presence of thick gas covering a largefraction of the solid angle. The presence of this gas, and the fastvariability timescale, suggest that the Compton-thick to Compton-thinchange is due to variation in the line-of-sight absorber rather than toextreme intrinsic emission variability. We discuss a structure of thecircumnuclear absorber/reflector that can explain the observed X-rayspectral and temporal properties.
| Probing the Dust Obscuration in Seyfert Galaxies using Infrared Spectroscopy. II. Implication for the Unification of Seyfert Galaxies
We report near-IR spectroscopic observations of 11 Seyfert galaxies (sixSeyfert 1s, one Seyfert 1.9, and four Seyfert 2s) and additionalgalaxies (four LINERs, two H II, and one type 2 transition) forcomparison, obtained using the Gemini twin-channel near-IR camera on theShane 3 m telescope at Lick Observatory. With the unique design of theGemini camera, full J and K spectra were taken simultaneously throughthe same slit. This produced accurate line ratios of hydrogenrecombination lines over a large wavelength baseline. For the Seyfert 1s(<=1.5), the line ratios of Paβ/Brγ are not onlycomparable in both broad- and narrow-line regions but also consistentwith case B recombination, indicating little or no reddening in bothnarrow- and broad-line regions. Seyfert 2 (>1.5) galaxies, however,show substantial reddening toward the narrow-line regions. We compareoptical reddening data from the literature and find significant supportfor the dichotomy between Seyfert 1s and Seyfert 2s, at least inlow-luminosity objects. Two different scenarios are explored to explainthe observed difference in reddening: a difference in reddening due toan extended dusty torus under active galactic nucleus unification, and adifference due to a different grain size distribution between the twoSeyfert types. We also discuss a similar potential difference found inthe strength of the 9.7 μm silicate line, along with a possiblecorrelation between the narrow-line reddening and the strength of thesilicate absorption line. We also analyzed CO band head absorptionfeatures longward of 2.3 μm to look for nonstellar contamination andevidence of recent star formation activity. The CO band head in Seyfert1s shows heavy contamination from nonstellar radiation, which iscorrelated with an H-K nuclear color excess. We confirm that the COspectroscopic indices in both Seyfert types do not show evidence ofrecent star formation. Taking the nonstellar contamination into account,there is little evidence from the CO index for a difference in starformation rates in the nuclei of Seyfert 1s and Seyfert 2s in ourlow-luminosity sample.
| A search for changing-look AGN in the Grossan catalog
We observed with XMM-Newton 4 objects selected from the Grossan (1992,Ph.D. Thesis) catalog, with the aim to search for new "changing-look"AGN. The sample includes all the sources which showed in subsequentobservations a flux much lower than the one measured with HEAO A-1: NGC7674, NGC 4968, IRAS 13218+0552 and NGC 1667. None of the sources wascaught in a high flux state during the XMM-Newton observations, whoseanalysis reveal they are all likely Compton-thick objects. We suggestthat, for all the sources, potential problems with the HEAO A-1 sourceidentification and flux measurement prevent us from being certain thatthe HEAO A-1 data represent a putative "high" state for these objects.Nonetheless, based on the high flux state and Compton-thin spectrum ofits GINGA observation, NGC 7674 represents probably the sixth known caseof a "changing-look" Seyfert 2 galaxy. From the X-ray variabilitypattern, we can estimate a likely lower limit of a few parsec to thedistance of the inner walls of the torus in this object. Remarkably,IRAS 13218+0552 was not detected by XMM-Newton, despite being currentlyclassified as a Seyfert 1 with a large [OIII] flux. However, theoriginal classification was likely to be affected by an extreme velocityoutflow component in the emission lines. The object likely harbors anhighly obscured AGN and should be re-classified as a type 2 source.
| Mid-infrared imaging of active galaxies. Active nuclei and embedded star clusters
High resolution, mid-infrared (MIR) images of nine nearby activegalaxies are presented. The data were obtained with the TIMMI 2instrument mounted at the ESO 3.6 m telescope using a set of N-bandnarrow filters. The resulting images have an angular resolution of0.6´´-1´´. The MIR emission has been resolved infour galaxies: NGC 253, NGC 1365, NGC 1808 and NGC 7469. The images showa circumnuclear population of unknown MIR sources in NGC 1365 and NGC1808, coincident with radio sources. These MIR/radio sources areinterpreted in terms of embedded young star clusters. A high-resolutionMIR map of NGC 253 is also presented, and enables the identification ofa previously unknown MIR counterpart to the radio nucleus. Extended MIRemission is detected in NGC 7469, and concurs with previous observationsin the NIR and radio. For this source, an interesting morphologicaldifference between the 10.4 μ m and the 11.9 μ m emission isobserved, suggesting the presence of a dust-rich micro-bar. Our MIRimages of Circinus do not show resolved emission from the nucleus downto an angular scale of 0.5´´. In the case of NGC 2992, anupper limit to the extended MIR emission can be set. We provide new MIRflux measurements for the unresolved AGN in NGC 5995, IZw1 and IIZw136.
| High Spatial Resolution Mid-Infrared Observations of Five Seyfert Galaxies
High spatial resolution images at 12.5 μm of the nuclei of fivenearby Seyfert galaxies-I Zwicky 1, NGC 1320, NGC 2992, M81, and NGC7479-have been obtained with the 10 m Keck Telescope. The angular sizelimits indicate that under typical conditions the Keck Telescope showsan unresolved nucleus for these active galactic nuclei. In all cases,the lower limit to the infrared surface brightness is above3×1012 Lsolar kpc-2 this arguesthat nuclear starbursts do not contribute significantly to the infraredluminosities in these nuclei.
| Reflection Component in the Hard X-Ray Emission from the Seyfert 2 Galaxy Mrk 1210
The Seyfert 2 galaxy Mrk 1210 was found to exhibit a flat hard X-raycomponent by ASCA, although ASCA could not distinguish whether it is anabsorbed direct component or a reflected one. We then observed Mrk 1210with BeppoSAX, and found that the X-ray spectral properties are quitedifferent from those of ASCA, as have been confirmed with XMM-Newton;the flux is significantly higher than that in the ASCA observation, anda clear absorption cut-off appears below 5keV. A bright hard X-rayemission is detected up to 100keV. The reflection component is necessaryto describe the BeppoSAX PDS spectrum, and represents the ASCA hardcomponent very well. Therefore, the hard component in the ASCA spectrumis a reflected one, whose intensity is almost constant over 6 yr. Thisindicates that a dramatic spectral variability is attributed to a largechange of the absorption column density by a factor of >5. ratherthan the variability of the nuclear emission. The change in theabsorption column density means that the torus is not homogeneous, buthas a blobby structure with a typical blob size of < 0.001 pc.
| Unveiling the nature of the highly obscured active galactic nucleus in NGC 5643 with XMM-Newton
We present results from an XMM-Newton observation of the nearby Seyfert2 galaxy NGC 5643. The nucleus exhibits a very flat X-ray continuumabove 2 keV, together with a prominent Kα fluorescent iron line.This indicates heavy obscuration. We measure an absorbing column densityNH in the range 6-10 × 1023 cm-2,either directly covering the nuclear emission, or covering its Comptonreflection. In the latter case, we might be observing a rather unusualgeometry for the absorber, whereby reflection from the inner far side ofa torus is in turn obscured by its near side outer atmosphere. Thenuclear emission might be then either covered by a Compton-thickabsorber, or undergoing a transient state of low activity. A secondsource (christened `X-1' in this paper) at the outskirts of the NGC 5643optical surface outshines the nucleus in X-rays. If belonging to NGC5643, it is the third brightest (LX~ 4 ×1040 erg s-1) known ultraluminous X-ray source.Comparison with past large aperture spectra of NGC 5643 unveils dramaticX-ray spectral changes above 1 keV. We interpret these as due tovariability of the active nucleus and of source X-1 intrinsic X-raypowers by factors of >=10 and 5, respectively.
| The star formation history of Seyfert 2 nuclei
We present a study of the stellar populations in the central ~200 pc ofa large and homogeneous sample comprising 79 nearby galaxies, most ofwhich are Seyfert 2s. The star formation history of these nuclei isreconstructed by means of state-of-the-art population synthesismodelling of their spectra in the 3500-5200 Åinterval. Aquasar-like featureless continuum (FC) is added to the models to accountfor possible scattered light from a hidden active galactic nucleus(AGN).We find the following. (1) The star formation history of Seyfert 2nuclei is remarkably heterogeneous: young starbursts, intermediate-ageand old stellar populations all appear in significant and widely varyingproportions. (2) A significant fraction of the nuclei show a strong FCcomponent, but this FC is not always an indication of a hidden AGN: itcan also betray the presence of a young, dusty starburst. (3) We detectweak broad Hβ emission in several Seyfert 2s after cleaning theobserved spectrum by subtracting the synthesis model. These are mostlikely the weak scattered lines from the hidden broad-line regionenvisaged in the unified model, given that in most of these casesindependent spectropolarimetry data find a hidden Seyfert 1. (4) The FCstrengths obtained by the spectral decomposition are substantiallylarger for the Seyfert 2s which present evidence of broad lines,implying that the scattered non-stellar continuum is also detected. (5)There is no correlation between the star formation in the nucleus andeither the central or overall morphology of the parent galaxies.
| Long-term infrared photometry of Seyferts
Long-term (up to 10 000 d) monitoring has been undertaken for 41Seyferts in the near-infrared (1.25-3.45 μm). All but two showedvariability, with amplitudes at K in the range <0.1 to >1.1 mag.The time-scale for detectable change is from about one week to a fewyears.Where contemporary observations of variability in X-rays, ultraviolet(UV) or visible light exist, it is found that the near-infrared variesin a similar way, though in some cases the shorter-wavelength infrared(IR) bands are diluted by underlying galaxy radiation.A simple cross-correlation study indicates that there is evidence fordelays of up to several hundred d between the variations seen at theshortest wavelengths (U or J) and the longest (L) in many galaxies. Inparticular, the data for Fairall 9 now extend to twice the intervalcovered in earlier publications and the delay between its UV and IRoutputs is seen to persist.An analysis of the fluxes shows that, for any given galaxy, the coloursof the variable component of its nucleus are usually independent of thelevel of activity. The state of activity of the galaxy can beparameterized.Taken over the whole sample, the colours of the variable components fallwithin moderately narrow ranges. In particular, the H-K colour isappropriate to a blackbody of temperature 1600 K. The H-K excess for aheavily reddened nucleus can be determined and used to findEB-V, which can be compared to the values found from thevisible region broad line ratios.Using flux-flux diagrams, the flux within the aperture from theunderlying galaxies can often be determined without the need for modelsurface brightness profiles. In many galaxies it is apparent that theremust be an additional constant contribution from warm dust.
| Spectropolarimetry of Compton-thin Seyfert 2 galaxies
We present new spectropolarimetry of a sample of nearby Compton-thinSeyfert 2 galaxies (i.e. those with NH < 1023cm-2). We show that the detection rate of scattered broadHα in this sample is considerably higher than in Seyfert 2galaxies as a whole. Our results also show that in this low obscurationset it is possible to find scattered broad Hα even when the globalproperties of the galaxy are largely dominated by the host galaxy andnot the active galactic nucleus. These results argue against theexistence of a population of `pure' Seyfert 2 galaxies.
| VLA HI and OVRO CO Interferometry of a Tidal Dwarf Galaxy
We present high resolution interferometric observations of the coolatomic and cold molecular ISM of the TDG candidate Arp 245N, an objectresembling a dwarf galaxy in the northern tidal tail of the interactingsystem NGC 2992/3. We observed the HI line with the NRAO VLA and theCO(1→0) transition with the OVRO millimeter interferometer at5''-6'' angular resolution (750 pc linear resolution). These datacubesoffer the required spatial and velocity resolution to determine whetherthe mass concentration near the tip of the tail is a genuine feature,and hence a good TDG candidate, or an artefact caused by a fortuitousalignment of our line of sight with the direction of the tail. Apreliminary analysis seems to confirm that Arp 245N is aself-gravitating entity.
| Dusty, Radiation Pressure-Dominated Photoionization. I. Model Description, Structure, and Grids
We present the implementation of dusty, radiation pressure-dominatedphotoionization models applicable to the narrow-line regions (NLRs) ofactive galactic nuclei, using the MAPPINGS III code. We give a grid ofthe predicted intensities of the most commonly used diagnostic spectrallines in the UV, optical, and IR, covering a wide range of density,metallicity, the power-law index characterizing the photoionizingsource, and photoionization parameter, for use in the diagnosis of NLRs.We examine the temperature, density, and ionization structure of thesemodels, investigating the effect of variation of these parameters inorder to gain a better understanding of NLR clouds themselves.
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