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Penetrating the Deep Cover of Compton-thick Active Galactic Nuclei
We analyze observations obtained with the Chandra X-Ray Observatory ofbright Compton-thick active galactic nuclei (AGNs), those with columndensities in excess of 1.5×1024 cm-2 alongthe lines of sight. We therefore view the powerful central engines onlyindirectly, even at X-ray energies. Using high spatial resolution andconsidering only galaxies that do not contain circumnuclear starbursts,we reveal the variety of emission AGNs alone may produce. Approximately1% of the continuum's intrinsic flux is detected in reflection in eachcase. The only hard X-ray feature is the prominent Fe Kαfluorescence line, with equivalent width greater than 1 keV in allsources. The Fe line luminosity provides the best X-ray indicator of theunseen intrinsic AGN luminosity. In detail, the morphologies of theextended soft X-ray emission and optical line emission are similar, andline emission dominates the soft X-ray spectra. Thus, we attribute thesoft X-ray emission to material that the central engines photoionize.Because the resulting spectra are complex and do not reveal the AGNsdirectly, crude analysis techniques, such as hardness ratios, wouldmisclassify these galaxies as hosts of intrinsically weak, unabsorbedAGNs and would fail to identify the luminous, absorbed nuclei that arepresent. We demonstrate that a three-band X-ray diagnostic can correctlyclassify Compton-thick AGNs, even when significant soft X-ray lineemission is present. The active nuclei produce most of the galaxies'total observed emission over a broad spectral range, and much of theirlight emerges at far-infrared wavelengths. Stellar contamination of theinfrared emission can be severe, however, making long-wavelength dataalone unreliable indicators of the buried AGN luminosity.
| Size and properties of the narrow-line region in Seyfert-2 galaxies from spatially-resolved optical spectroscopy
Context: .While [O III] narrow-band imaging is commonly used to measurethe size of the narrow-line regions (NLRs) in active galactic nuclei(AGNs), it can be contaminated by emission from surrounding starbursts.Recently, we have shown that long-slit spectroscopy provides a valuablealternative approach to probe the size in terms of AGN photoionisation.Moreover, several parameters of the NLR can be directly accessed.Aims: .We here apply the same methods developed and described for theSeyfert-2 galaxy NGC 1386 to study the NLR of five other Seyfert-2galaxies by using high-sensitivity spatially-resolved opticalspectroscopy obtained at the VLT and the NTT. Methods: .We probethe AGN-photoionisation of the NLR and thus, its "real" size usingdiagnostic line-ratio diagrams. We derive physical properties of the NLRsuch as reddening, ionisation parameter, electron density, and velocityas a function of distance from the nucleus. Results: .For NGC5643, the diagnostic diagrams unveil a similar transition between lineratios falling in the AGN regime and those typical for H II regions asfound for NGC 1386, thus determining the size of the NLR. For the otherfour objects, all measured line ratios fall in the AGN regime. In almostall cases, both electron density and ionisation parameter decrease withradius. Deviations from this general behaviour (such as a secondarypeak) seen in both the ionisation parameter and electron density can beinterpreted as signs of shocks from the interaction of a radio jet andthe NLR gas. In several objects, the gaseous velocity distribution ischaracteristic for rotational motion in an (inclined) emission-line diskin the centre. We compare our results to those of NGC 1386 and show thatthe latter can be considered as prototypical also for this largersample. We discuss our findings in detail for each object.
| 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.
| The soft X-ray/NLR connection: a single photoionized medium?
We present a sample of 8 nearby Seyfert 2 galaxies observed by HST andChandra. All of the sources present soft X-ray emission which iscoincident in extension and overall morphology with the [O iii]emission. The spectral analysis reveals that the soft X-ray emission ofall the objects is likely to be dominated by a photoionized gas. This isstrongly supported by the 190 ks combined XMM-Newton/RGS spectrum of Mrk3, which different diagnostic tools confirm as being produced in a gasin photoionization equilibrium with an important contribution fromresonant scattering. We tested with the code cloudy a simple scenariowhere the same gas photoionized by the nuclear continuum produces boththe soft X-ray and the [O iii] emission. Solutions satisfying theobserved ratio between the two components exist, and require the densityto decrease with radius roughly like r-2, similarly to whatoften found for the Narrow Line Region.
| 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 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 Connection between the Narrow-Line Region and the UV Absorbers in Seyfert Galaxies
We present evidence that the outflowing UV absorbers in Seyfert 1galaxies arise primarily in their inner narrow- (emission-) line regions(NLRs), based on similarities in their locations, kinematics, andphysical conditions. Hubble Space Telescope observations show thatnearly all Seyfert galaxies have bright, central knots of [O III]emission in their NLRs with radii of tens of parsecs. These sizes areconsistent with most previous estimates of the distances of UV (andX-ray) absorbers from their central continuum sources and a recentlyobtained reliable distance of ~25 pc for a UV absorber in the Seyfert 1galaxy NGC 3783. The nuclear emission-line knots in a sample of 10Seyfert galaxies have velocity widths of 300-1100 km s-1(half-width at zero intensity), similar to the radial velocities of mostUV absorbers. The highest radial velocity for a Seyfert UV absorber todate is only -2100 km s-1, which is much lower than typicalbroad-line region (BLR) velocities. There is also mounting evidence thatthe NLR clouds are outflowing from the nucleus, like the UV absorbers.If our hypothesis is correct, then the NLR should have a component witha high global covering factor (Cg) of the continuum sourceand a BLR to match that found from previous surveys of UV absorbers(Cg=0.5-1.0). Using Space Telescope Imaging Spectrographspectra of NGC 4151, obtained when the continuum and BLR fluxes werelow, we find evidence for optically thin gas in its nuclearemission-line knot. We are able to match the line ratios from this gaswith photoionization models that include a component withCg~1 and an ionization parameter and hydrogen column densitythat are typical of UV absorbers.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| Probing the Kinematics of the Narrow-Line Region in Seyfert Galaxies with Slitless Spectroscopy: Observational Results
We present slitless spectra of 10 Seyfert galaxies observed with theSpace Telescope Imaging Spectrograph on the Hubble Space Telescope(HST). The spectra cover the [O III] λλ4959, 5007 emissionlines at a spectral resolving power of λ/Δλ~9000 anda spatial resolution of ~0.1". We compare the slitless spectra withprevious HST narrowband images to determine the velocity shifts anddispersions of the bright emission-line knots in the narrow-line regions(NLRs) of these Seyfert galaxies, which extend out to at least severalhundred parsecs from their nuclei. Many knots are spatially resolvedwith sizes of tenths of arcseconds, corresponding to tens of parsecs,and yet they appear to move coherently with radial velocities betweenzero and +/-1200 km s-1 with respect to the systemicvelocities of their host galaxies. The knots also show a broad range invelocity dispersion, ranging from ~30 km s-1 (the velocityresolution) to ~1000 km s-1 FWHM. Most of the Seyfertgalaxies in this sample show an organized flow pattern, with radialvelocities near zero at the nucleus (defined by the optical continuumpeak) and increasing to maximum blueshifts and redshifts within ~1" ofthe nucleus, followed by a decline to the systemic velocity. However,there are large local variations around this pattern, and in one case(NGC 7212), the radial velocities are nearly chaotic. The emission-lineknots also follow a general trend of decreasing velocity dispersion withincreasing distance from the nucleus. In the Seyfert 2 galaxies, thepresence of blueshifts and redshifts on either side of the nucleusindicates that rotation alone cannot explain the observed radialvelocities. The most straightforward interpretation is that radialoutflow plays an important role in the NLR kinematics. Each of theSeyfert galaxies in this sample (with the exception of Mrk 3) shows abright, compact (FWHM<=0.5") [O III] knot at the position of itsoptical nucleus. These nuclear emission-line knots have radial velocitycentroids near zero, but they typically have the highest velocitydispersions. Their similar properties suggest that they may be a common,distinct component of the NLR.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555. These observations areassociated with proposal GO-8340.
| X-ray obscuration and obscured AGN in the local universe
We discuss the X-ray properties of 49 local (z<0.035) Seyfert 2galaxies with HST/WFC2 high-resolution optical coverage. It includes theresults of 26 still unpublished Chandra and XMM-Newton observations,which yield 25 (22) new X-ray detections in the 0.5-2 keV (2-10 keV)energy band. Our sample covers a range in the 2-10 keV observed flux,F2-10, from 3 × 10-11 to 6 ×10-15 erg cm-2 s-1. The percentage ofobjects that are likely obscured by Compton-thick matter (columndensity, NH ≥ σt-1 ≃1.6 × 1024 cm-2) is ≃50%, and reaches≃80% for log (F2-10) < 12.3. Hence, Kαfluorescent iron lines with large Equivalent Width ({EW} > 0.6 keV)are common in our sample (6 new detections at a confidence level≥2σ). They are explained as due to reflection off theilluminated side of optically thick material. We confirm a correlationbetween the presence of a 100-pc scale nuclear dust in the WFC2 imagesand Compton-thin obscuration. We interpret this correlation as due tothe large covering fraction of gas associated with the dust lanes. TheX-ray spectra of highly obscured AGN invariably present a prominent softexcess emission above the extrapolation of the hard X-ray component.This soft component can account for a very large fraction of the overallX-ray energy budget. As this component is generally unobscured - andtherefore likely produced in extended gas structures - it may lead to asevere underestimation of the nuclear obscuration in z 1 absorbedAGN, if standard X-ray colors are used to classify them. As a by-productof our study, we report the discovery of a soft X-ray, luminous(≃7 × 1040 erg s-1) halo embedding theinteracting galaxy pair Mkn 266.
| 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.
| A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.
| Radio-Excess IRAS Galaxies. II. Host Galaxies
This is the second of a series of papers studying a sample ofradio-excess IRAS galaxies. These galaxies have radio emission in excessof that expected due to star formation, but largely fall between thetraditional categories of radio-loud and radio-quiet active galaxies.R-band images of the hosts of far-infrared (FIR)-luminous radio-excessgalaxies are presented and analyzed. The hosts of the FIR-luminousradio-excess galaxies are luminous galaxies, on average 0.8 mag brighterthan M*R. Their optical luminosities andmorphologies are similar to comparison samples of radio-loud compactsteep-spectrum and gigahertz peaked-spectrum sources and extended radiogalaxies. We find a similar fraction of galaxies in our sample (~70%)with companions or distorted morphologies as in radio-loud comparisonsamples. This is consistent with radio activity being associated withtidal interaction. The majority (65%) of the FIR-luminous radio-excessgalaxies have radio source sizes that are smaller than the optical hostby more than an order of magnitude. These compact radio sources may beyoung precursors to classical radio galaxies or a different populationof radio sources, possibly confined by the host interstellar medium. Thehost galaxy types were determined by analysis of the surface brightnessdistributions. The elliptical hosts have effective surface brightnessesand radii consistent with known ellipticals but inconsistent with apopulation of brightest cluster galaxies. Thus, it is unlikely theseobjects are the precursors of FR I radio galaxies. The disk hosts havesmaller sizes and low radio excesses. However, they have a range ofradio source sizes, which is not expected if they are radio-``loud''Seyfert galaxies.
| Milliarcsec-scale radio structure of a matched sample of Seyfert 1 and Seyfert 2 galaxies
We have obtained mas-scale resolution very long baseline interferometry({VLBI}) images of a sample of Seyfert 1 and Seyfert 2 galaxies at 5 GHz(wavelength, λ = 6 cm). The Seyferts of the two types were chosento be matched in several orientation-independent properties, primarilyin order to rigorously test predictions of the unified scheme. Wedetected all the 15 objects that we observed. In this paper we describethe observations and data reduction procedures, and present the {VLBI}radio images as well as simultaneous Very Large Array images that weobtained for these 15 Seyferts.Full Fig. \ref{maps1} and Tables \ref{vla_param}, \ref{vlbi_param} areonly available in electronic form at http://www.edpsciences.orgTables \ref{vlbi_res} and \ref{vla_res} are also available in electronicform the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/425/99
| Motion and properties of nuclear radio components in Seyfert galaxies seen with VLBI
We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm ofthe Seyfert galaxies NGC 7674, NGC5506, NGC 2110 and Mrk1210 to study their structure and proper motions on pc scalesand to add some constraints on the many possible causes of theradio-quietness of Seyferts. The component configurations inNGC 7674 and NGC 2110 are simple,linear structures, whereas the configurations in NGC5506 and Mrk 1210 have multiple componentswith no clear axis of symmetry. We suggest that NGC7674 is a low-luminosity compact symmetric object. Comparingthe images at different epochs, we find a proper motion in NGC7674 of (0.92±0.07) c between the two centralcomponents separated by 282 pc and, in NGC 5506, wefind a 3 σ upper limit of 0.50 c for the components separated by3.8 pc. Our results confirm and extend earlier work showing that theoutward motion of radio components in Seyfert galaxies isnon-relativistic on pc scales. We briefly discuss whether thisnon-relativistic motion is intrinsic to the jet-formation process orresults from deceleration of an initially relativistic jet byinteraction with the pc or sub-pc scale interstellar medium. We combinedour sample with a list compiled from the literature of VLBI observationsmade of Seyfert galaxies, and found that most Seyfert nuclei have atleast one flat-spectrum component on the VLBI scale, which was not seenin the spectral indices measured at arcsec resolution. We found alsothat the bimodal alignment of pc and kpc radio structures displayed byradio galaxies and quasars is not displayed by this sample of Seyferts,which shows a uniform distribution of misalignment between0° and 90°. The frequent misalignmentcould result from jet precession or from deflection of the jet byinteraction with gas in the interstellar medium.
| Stellar population gradients in Seyfert 2 galaxies: northern sample
We use high signal-to-noise ratio long-slit spectra in theλλ3600-4700 range of the 20 brightest northern Seyfert 2galaxies to study the variation of the stellar population properties asa function of distance from the nucleus. In order to characterize thestellar population and other continuum sources (e.g. featurelesscontinuum, FC) we have measured the equivalent width, W, of sixabsorption features, four continuum colours and their radial variations,and performed spectral population synthesis as a function of distancefrom the nucleus. About half of the sample has CaIIK and G band W valuessmaller at the nucleus than at 1 kpc from it, owing to a youngerpopulation and/or FC. The stellar population synthesis shows that, whileat the nucleus, 75 per cent of the galaxies present contribution >20per cent of ages <=100 Myr and/or of an FC, this proportion decreasesto 45 per cent at 3 kpc. In particular, 55 per cent of the galaxies havea contribution >10 per cent of the 3-Myr/FC component (a degeneratecomponent in which one cannot separate what is caused by an FC or by a3-Myr stellar population) at the nucleus, but only 25 per cent of themhave this contribution at 3 kpc. As a reference, the stellar populationof 10 non-Seyfert galaxies, spanning the Hubble types of the Seyfert(from S0 to Sc) was also studied. A comparison between the stellarpopulation of the Seyferts and that of the non-Seyferts shows systematicdifferences: the contribution of ages younger than 1 Gyr is in mostcases larger in the Seyfert galaxies than in non-Seyferts, not only atthe nucleus but up to 1 kpc from it.
| A Hubble Space Telescope Survey of Extended [O III] λ5007 Emission in a Far-Infrared Selected Sample of Seyfert Galaxies: Observations
We present a Hubble Space Telescope (HST) survey of extended [O III]emission for a sample of 60 Seyfert galaxies (22 Seyfert 1 galaxies and38 Seyfert 2 galaxies), selected based on their far-infrared properties.The observations for 42 of these galaxies were done in a snapshot surveywith WFPC2. The remaining 18 were obtained from the HST archive, most ofwhich were observed with the same configuration. These observationscover 68% of the objects in the sample defined by Kinney et al. andcreate a valuable data set for the study of the narrow-line region (NLR)properties of Seyfert galaxies. In this paper, we present the details ofthe observations, reductions, and measurements. We also discuss theextended structure of individual sources, and the relation of thisemission to the radio and host galaxy morphology. We also address howrepresentative the subsample of [O III]-imaged galaxies is of the entiresample, and possible selection effects that may affect this comparisonof the properties of Seyfert 1 and Seyfert 2 galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| High-energy sources before INTEGRAL. INTEGRAL reference catalog
We describe the INTEGRAL reference catalog which classifies previouslyknown bright X-ray and gamma-ray sources before the launch of INTEGRAL.These sources are, or have been at least once, brighter than ~ 1 mCrababove 3 keV, and are expected to be detected by INTEGRAL. This catalogis being used in the INTEGRAL Quick Look Analysis to discover newsources or significantly variable sources. We compiled several publishedX-ray and gamma-ray catalogs, and surveyed recent publications for newsources. Consequently, there are 1122 sources in our INTEGRAL referencecatalog. In addition to the source positions, we show an approximatespectral model and expected flux for each source, based on which wederive expected INTEGRAL counting rates. Assuming the default instrumentperformances and at least ~ 105 s exposure time for anypart of the sky, we expect that INTEGRAL will detect at least ~ 700sources below 10 keV and ~ 400 sources above 20 keV over the missionlife.The Catalog is available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/411/L59
| Seyfert 2 Galaxies with Spectropolarimetric Observations
We present a compilation of radio, infrared, optical, and hard X-ray(2-10 keV) data for a sample of 90 Seyfert 2 galaxies (Sy2s) withspectropolarimetric observations (41 Sy2s with detection of polarizedbroad lines [PBLs] and 49 without PBLs). Compared to Sy2s without PBLs,Sy2s with PBLs tend to be earlier type spirals and show warmermidinfrared color and significant excess of emissions (including thehard X-ray [2-10 keV], [O III] λ5007, infrared [25 μm], andradio). Our analyses indicate that the majority of Sy2s without PBLs arethose sources having less powerful active galactic nucleus (AGN)activities, most likely caused by a low accretion rate. It implies thatthe detectability of the polarized broad emission lines in Sy2s maydepend on their central AGN activities in most cases. Based on theavailable data, we find no compelling evidence for the presence of twotypes of Sy2s; one of which has been proposed to be intrinsicallydifferent from Sy2s claimed in the unification model.
| Circumnuclear Stellar Population, Morphology, and Environment of Seyfert 2 Galaxies: An Evolutionary Scenario
We investigate the relation between the characteristics of thecircumnuclear stellar population and both the galaxy morphology and thepresence of close companions for a sample of 35 Seyfert 2 nuclei.Fifteen galaxies present unambiguous signatures of recent episodes ofstar formation within ~300 pc of the nucleus. When we relate thisproperty to the Hubble type of the host galaxy, we find that theincidence of recent circumnuclear star formation increases along theHubble sequence; it seems to be greater than that in non-Seyfertgalaxies for the early Hubble types S0 and Sa but similar to that innon-Seyfert galaxies for later Hubble types. In both early-type andlate-type Seyfert galaxies, the presence of recent circumnuclear starformation is related to the galaxy morphology in the inner fewkiloparsecs, as observed in Hubble Space Telescope images through thefilter F606W by Malkan et al., who have assigned a late ``inner Hubbletype'' to most Seyfert 2 galaxies with recent circumnuclear starformation. This new classification is due to the presence of dust lanesand spiral structures in the inner region. The presence of recent starformation around Seyfert 2 nuclei is also related to interactions: amongthe 13 galaxies of the sample with close companions or in mergers, ninehave recent star formation in the nuclear region. These correlationsbetween the presence of companions, the inner morphology, and theincidence of recent star formation suggest an evolutionary scenario inwhich the interaction is responsible for sending gas inward, which bothfeeds the active galactic nucleus and triggers star formation. Thestarburst then fades with time and the composite Seyfert 2+starburstnucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellarpopulation. This scenario can reconcile the hypothesis that the activenucleus in Seyfert galaxies is triggered by interactions with theresults of previous studies, which find only a small excess ofinteracting galaxies in Seyfert samples when compared with non-Seyfertsamples. The large excess can only be found early after the interaction,in the phase in which a composite (Seyfert+starburst) nucleus isobserved.
| Empirical Diagnostics of the Starburst-AGN Connection
We examine a representative sample of 35 Seyfert 2 nuclei. Previous workhas shown that nearly half (15) of these nuclei show the direct (butdifficult to detect) spectroscopic signature at optical/near-UVwavelengths of the hot massive stars that power circumnuclearstarbursts. In the present paper we examine a variety of more easilymeasured quantities for this sample, such as the equivalent widths ofstrong absorption features, continuum colors, emission line equivalentwidths, emission line ratios and profiles, far-IR luminosities, andnear-UV surface brightness. We compare the composite starburst+Seyfert 2nuclei to ``pure'' Seyfert 2 nuclei, Starburst galaxies, and normalgalactic nuclei. Our goals are to verify whether the easily measuredproperties of the composite nuclei are consistent with the expectedimpact of a starburst and to investigate alternative less demandingmethods to infer the presence of starbursts in Seyfert 2 nuclei,applicable to larger or more distant samples. We show that starbursts doindeed leave clear and easily quantifiable imprints on the near-UV tooptical continuum and emission line properties of Seyfert 2's. Compositestarburst+Seyfert 2 systems can be recognized by: (1) a strong``featureless continuum'' (FC), which dilutes the Ca II K line from oldstars in the host's bulge to an equivalent width WK<10Å (2) emission lines whose equivalent widths are intermediatebetween starburst galaxies and ``pure'' Seyfert 2's (3) relatively lowexcitation line ratios, which indicate that part of the gas ionizationin these Seyfert 2's (typically ~50% of Hβ) is due tophotoionization by OB stars; (4) large far-IR luminosities(>~1010 Lsolar) (5) high near-UV surfacebrightness (~103 Lsolar pc-2). Thesecharacteristics are all consistent with the expected impact ofcircumnuclear starbursts on the observed properties of Seyfert 2's.Furthermore, they offer alternative empirical diagnostics of thepresence of circumnuclear starbursts from a few easily measuredquantities.
| Testing the Unified Model with an Infrared-selected Sample of Seyfert Galaxies
We present a series of statistical tests done to a sample of 29 Seyfert1 and 59 Seyfert 2 galaxies selected from mostly isotropic properties,their far-infrared fluxes and warm infrared colors. Such selectioncriteria provide a profound advantage over the criteria used by mostinvestigators in the past, such as ultraviolet excess. These tests weredone using ground-based high-resolution Very Large Array A-configuration3.6 cm radio and optical B and I imaging data. From the relative numberof Seyfert 1's and Seyfert 2's, we calculate that the torus half-openingangle is 48°. We show that, as seen in previous papers, there is alack of edge-on Seyfert 1 galaxies, suggesting that dust and gas alongthe host galaxy disk probably play an important role in hiding somenuclei from direct view. We find that there is no statisticallysignificant difference in the distribution of host galaxy morphologicaltypes and radio luminosities of Seyfert 1's and Seyfert 2's, suggestingthat previous results showing the opposite may have been due toselection effects. The average extension of the radio emission ofSeyfert 1's is smaller than that of Seyfert 2's by a factor of ~2-3, aspredicted by the unified model. A search for galaxies around ourSeyferts allows us to put a lower and an upper limit on the possiblenumber of companions around these galaxies of 19% and 28%, respectively,with no significant difference in the number of companion galaxiesbetween Seyfert 1's and Seyfert 2's. We also show that there is nopreference for the radio jets to be aligned closer to the host galaxydisk axis in late-type Seyferts, unlike results claimed by previouspapers. These results, taken together, provide strong support for aunified model in which type 2 Seyferts contain a torus seen more edge-onthan the torus in type 1 Seyferts.
| The Nuclear and Circumnuclear Stellar Population in Seyfert 2 Galaxies: Implications for the Starburst-Active Galactic Nucleus Connection
We report the results of a spectroscopic investigation of a sample of 20of the brightest type 2 Seyfert nuclei. Our goal is to search for thedirect spectroscopic signature of massive stars and thereby probe therole of circumnuclear starbursts in the Seyfert phenomenon. The methodused is based on the detection of the higher order Balmer lines and He Ilines in absorption and the Wolf-Rayet feature at ~4680 Å inemission. These lines are strong indicators of the presence of young (afew Myr) and intermediate-age (a few 100 Myr) stellar populations. Inover half the sample, we have detected He I and/or strong stellarabsorption features in the high-order (near-UV) Balmer series togetherwith relatively weak lines from an old stellar population. In threeothers we detect a broad emission feature near 4680 Å that is mostplausibly ascribed to a population of Wolf-Rayet stars (the evolveddescendants of the most massive stars). We therefore conclude that theblue and near-UV light of over half of the sample is dominated by youngand/or intermediate age stars. The ``young'' Seyfert 2 galaxies havelarger far-IR luminosities, cooler mid/far-IR colors, and smaller [OIII]/Hβ flux ratios than the ``old'' ones. These differences areconsistent with a starburst playing a significant energetic role in theformer class. We consider the possibility that there may be two distinctsubclasses of Seyfert 2 nuclei (``starbursts'' and ``hidden broadlineregions'' [BLRs]). However, the fact that hidden BLRs have been found inthree of the ``young'' nuclei argues against this and suggests thatnuclear starbursts may be a more general part of the Seyfert phenomenon.
| Stellar populations in Seyfert 2 galaxies. I. Atlas of near-UV spectra
We have carried out a uniform spectroscopic survey of Seyfert 2 galaxiesto study the stellar populations of the host galaxies. New spectra havebeen obtained for 79 Southern galaxies classified as Seyfert 2 galaxies,7 normal galaxies, and 73 stars at a resolution of 2.2 Å over thewavelength region 3500-5300 Å. Cross-correlation between thestellar spectra is performed to group the individual observations into44 synthesis standard spectra. The standard groups include a solarabundance sequence of spectral types from O5 to M3 for dwarfs, giants,and supergiants. Metal-rich and metal-weak F-K giants and dwarfs arealso included. A comparison of the stellar data with previouslypublished spectra is performed both with the individual spectra and thestandard groups. For each galaxy, two distinct spatial regions areconsidered: the nucleus and the external bulge. Spectroscopic variationsfrom one galaxy to another and from the central to the external regionare briefly discussed. It is found that the central region of a Seyfert2 galaxy, after subtracting the bulge stellar population, always shows anear-UV spectrum similar to one of three representative categories: a)many strong emission lines and only two visible absorption lines (Ca IiK and G band) (Sey2e); b) few emission lines, many absorption lines, anda redder continuum than the previous category (Sey2a); c) an almost flatcontinuum and high-order Balmer lines seen in absorption (Sey2b). Theproportion of Seyfert 2 galaxies belonging to each class is found to be22%, 28%, and 50% respectively. We find no significative differencesbetween morphology distributions of Seyfert 2 galaxies with Balmer linesdetected in absorption and the rest of the sample. This quick lookthrough the atlas indicates that half of Seyfert 2 galaxies harbour ayoung stellar population (about or less than 100 Myr) in their centralregion, clearly unveiled by the high order Balmer series seen inabsorption. Based on observations collected at the European SouthernObservatory, Chile (ESO 65.P-0014(A)). Tables 1-3 and 8 and Fig. A.1(Appendix A) are only available in electronic form athttp://www.edpsciences.org
| Jet Directions in Seyfert Galaxies: B and I Imaging Data
We present the results of broadband B and I imaging observations for asample of 88 Seyfert galaxies (29 Seyfert 1s and 59 Seyfert 2s),selected from a mostly isotropic property, the flux at 60 μm. We alsopresent the B and I imaging results for an additional sample of 20Seyfert galaxies (7 Seyfert 1s and 13 Seyfert 2s), selected from theliterature and known to have extended radio emission. The I-band imagesare fitted with ellipses to determine the position angle and ellipticityof the host galaxy major axis. This information will be used in a futurepaper, combined with information from radio observations, to study theorientation of radio jets relative to the planes of their host galaxies.Here we present surface brightness profiles and magnitudes in the B andI bands, as well as mean ellipticities and major axis position angles.
| X-Ray Emission from Seyfert 2 Galaxies with Optical Polarized Broad Lines
We analyze the 0.5-10 keV spectra of six Seyfert 2 galaxies observedwith the X-ray satellite ASCA: Mrk 3, Mrk 348, Mrk 1210, Mrk 477, NGC7212, and Was 49b. These galaxies were selected based on theirpossession of optical polarized broad lines. In the 2-10 keV band, theirspectra are heavily absorbed, with 2-10 keV absorption-corrected X-rayluminosities ranging from 1042 to 1043 ergss-1. The observed X-ray emission is generally only about 1/10that predicted based on their known infrared and Hβ luminosities.This apparent X-ray weakness can be understood if a considerablefraction of the nuclear activity is completely blocked from view bythick matter along our line of sight to the nucleus. All of thesegalaxies possess significant soft X-ray emission whose origin appears tobe scattered light from their nuclear emission. Based on thishypothesis, we estimate a typical scattering efficiency for X-rays to beabout 10%. This efficiency is larger than the few percent found forordinary Seyfert 2 galaxies with no report of optical polarized broadlines. A large scattering efficiency is best explained by an apparentweakness of the hard X-ray luminosity rather than unusually strongscattered light in the soft band. When we estimate the scatteringefficiency using the intrinsic luminosity derived assuming that Comptonscattering dominates the hard X-ray spectrum, as opposed to a purelyabsorbed nuclear continuum, the efficiency can be less and is similar tothat of ordinary Seyfert 2 galaxies. Since the difference between oursample and ordinary Seyfert 2 galaxies can be explained by thedifference of viewing angle, the similar scattering efficiency suggeststhe existence of a scattering region that is larger than the putativedusty torus.
| Jet Directions in Seyfert Galaxies
Here we present the study of the relative angle between the accretiondisk (or radio jet) and the galaxy disk for a sample of Seyfert galaxiesselected from a mostly isotropic property, the 60 μm flux, and warminfrared colors. We used VLA A-array 3.6 cm continuum data andground-based optical imaging, homogeneously observed and reduced tominimize selection effects. For parts of the analysis we enlarged thesample by including galaxies serendipitously selected from theliterature. For each galaxy we have a pair of points (i, δ), whichare the inclination of the galaxy relative to the line of sight and theangle between the jet projected into the plane of the sky and the hostgalaxy major axis, respectively. For some galaxies we also hadinformation about which side of the minor axis is closer to Earth. Thisdata is combined with a statistical technique, developed by us, todetermine the distribution of β angles in three dimensions, theangle between the jet and the host galaxy plane axis. We found from aninitial analysis of the data of the 60 μm sample, where Seyfert 1 and2 galaxies were not differentiated, that the observed distribution of iand δ values can be well represented either by a homogeneoussinβ distribution in the range0deg<=β<=90deg or in0deg<=β<=65deg, but not by anequatorial ring. A more general model, which tested β-distributionsin the range β1<=β<=β2, fordifferent ranges of β1 and β2 values,required β2 to be larger than 65° and gavepreference for β1 smaller than 40°-50°. Animportant result from our analysis was obtained when we determinedwhether the jet was projected against the near or the far side of thegalaxy and differentiated between Seyfert 1 and Seyfert 2 galaxies,which showed that the model could not represent Seyfert 1 galaxiesadequately. We found that the inclusion of viewing angle restrictionsfor Seyfert 1 galaxies, namely, that a galaxy can be recognized as aSeyfert 1 only if the angle between the jet and the line of sight(|φ|) is smaller than a given angle φc and that thegalaxy inclination i is smaller than an angle ic, gave riseto statistically acceptable models. This indication that there is adifference in viewing angle to the central engine between Seyfert 1galaxies and Seyfert 2 galaxies is a direct and independent confirmationof the underlying concepts of the unified model. We discuss possibleexplanations for the misalignment between the accretion disk and thehost galaxy disk: warping of the accretion disk by self-irradiationinstability, by the Bardeen-Petterson effect, or by a misalignedgravitational potential of a nuclear star cluster surrounding the blackhole, as well as feeding of the accretion disk by a misaligned inflow ofgas from minor mergers, capture of individual stars or gas from thenuclear star cluster, and the capture of individual molecular cloudsfrom the host galaxy.
| The hard X-ray emission of luminous infrared galaxies
We present a study of the hard X-ray properties of a sample thatincludes all the Luminous Infrared Galaxies (LIGs, LIR >1011 L_sun) observed in the 2-10 keV energy band (new andarchival data). We find that a significant fraction of the sourcesoptically classified as AGNs do not show any indication of nuclearactivity in the X rays, thus suggesting heavy absorption along our lineof sight. The absence of strong emission in the 20-200 keV band in asubsample of LIGs observed with BeppoSAX suggests that in many casesthese sources are completely Compton thick (N_H > 1025cm-2). From a comparison between the infrared and the X-rayemission we deduce that the mid-IR emission is absorbed by a lowercolumn density than the X-ray emission or, alternatively, that thedust-to-gas ratio is lower than Galactic. We describe a simple modelthat reproduces the IR-X correlation by means of mixed AGN and starburstcontributions and we compare the predictions of this model with theobservational data at X-ray and optical wavelengths. Finally, we discussthe biases that affect the currently available samples of LIGs andbriefly analyze a small unbiased sample, finding that at least 50% ofthe sources host a (weak) AGN.
| Molecular torus in Seyfert galaxies.
Not Available
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| Thomson scattering origin of the polarization of Seyfert spectra.
Not Available
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