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The messy environment of Mrk 6
In recent years it has become clear that understanding the absorptionpresent in active galactic nuclei (AGN) is essential given its bearingon unification models. We present the most recent XMM-Newton observationof Mrk 6, with the goal of understanding the nature and origin of thecomplex absorption intrinsic to this source. X-ray spectral fittingshows that a simple warm absorption model provides an equally goodstatistical representation of the CCD data as a partial covering model.Furthermore, once the Reflection Grating Spectrometer (RGS) data areincluded in the spectral fitting, the simple warm absorber modelprovides a very good fit to the data, without increasing the complexityof the model, in contrast with the partial covering model which requiresthe addition of either a low-metalicity (<0.03 solar) thermal plasmaor low-temperature blackbody emission in order to provide a similarquality fit. The warm absorber is also a considerably more natural wayto explain the variability observed in the X-ray absorbing columndensity between the previous XMM-Newton observation and this one,requiring only a second, higher column density, higher ionization,absorber to be present during the previous XMM-Newton observation. Incomparison, the partial covering models which requires moving, clumpy,material relatively close to the source that result in two distinctlines of sight, with separate absorbing columns that each varyconsiderably without any associated change in their covering fractions,in order to explain the observed variability. We associate the warmabsorber either with an accretion disc wind with densities of~109cm-3, or with an ionized `skin' or atmosphereof the molecular torus with densities of~103-5cm-3.
| Evolutionary unification in composite active galactic nuclei
In this paper, we present an evolutionary unification scenario,involving supermassive black holes (SMBHs) and starbursts (SBs) withoutflow (OF), that seems capable of explaining most of the observationalproperties (of at least part) of active galactic nuclei (AGN).The scenario includes a nuclear/circumnuclear SB closely associated withthe AGN where the narrow-line region (NLR), broad-line region (BLR) andbroad absorption line (BAL) region are produced in part by the OFprocess with shells and in compact supernova remnants (cSNRs).The OF process in BAL quasi-stellar objects (QSOs) with extreme infrared(IR) and FeII emission is studied. In addition, the FeII problemregarding the BLR of AGN is analysed. The correlations between the BAL,IR emission, FeII intensity and the intrinsic properties of the AGN arenot clearly understood. We suggest here that the behaviour of the BAL,IR and FeII emission in AGN can be understood within an evolutionary andcomposite model for AGN.In our model, strong BAL systems and FeII emission are present (andintense) in young IR objects. Parameters like the BALs, IR emission,FeII/Hβ intensity ratio, FeII equivalent width (EW), broad-linewidth, [OIII]λ5007-Å intensity and width, NLR size, X-rayspectral slope in radio quiet (RQ) AGN plus lobe separation, and lobe tocore intensity ratio in radio loud (RL) AGN are proposed to befundamentally time-dependent variables inside time-scales of the orderof 108 yr. Orientation/obscuration effects take the role of asecond parameter providing the segregation between Seyfert 1/Seyfert 2galaxies (Sy1/Sy2) and broad-/narrow-line radio galaxies (BLRG/NLRG).
| A Radio Study of the Seyfert Galaxy Markarian 6: Implications for Seyfert Life Cycles
We have carried out an extensive radio study with the Very Large Arrayon the Seyfert 1.5 galaxy Mrk 6 and imaged a spectacular radio structurein the source. The radio emission occurs on three different spatialscales: ~7.5 kpc bubbles, ~1.5 kpc bubbles lying nearly orthogonal tothem, and a ~1 kpc radio jet lying orthogonal to the kiloparsec-scalebubble. To explain the complex morphology, we first consider a scenarioin which the radio structures are the result of superwinds ejected by anuclear starburst. However, recent Spitzer observations of Mrk 6 providean upper limit to the star formation rate (SFR) of ~5.5Msolar yr-1, an estimate much lower than the SFRof ~33 Msolar yr-1 derived assuming that thebubbles are a result of starburst winds energized by supernovaexplosions. Thus, a starburst alone cannot meet the energy requirementsfor the creation of the bubbles in Mrk 6. We then present anenergetically plausible model wherein the bubbles are a result of energydeposited by the kiloparsec-scale jet as it plows into the interstellarmedium. Finally, we consider a model in which the complex radiostructure is a result of an episodically powered precessing jet thatchanges its orientation. This model is the most attractive as it cannaturally explain the complex radio morphology and is consistent withthe energetics, the spectral index, and the polarization structure.Radio emission in this scenario is a short-lived phenomenon in thelifetime of a Seyfert galaxy, which results from an accretion event.
| The Hard X-Ray 20-40 keV AGN Luminosity Function
We have compiled a complete extragalactic sample based on ~25,000deg2 to a limiting flux of 3×10-11 ergscm-2 s-1 (~7000 deg2 to a flux limit of10-11 ergs cm-2 s-1) in the 20-40 keVband with INTEGRAL. We have constructed a detailed exposure map tocompensate for effects of nonuniform exposure. The flux-number relationis best described by a power law with a slope of α=1.66+/-0.11.The integration of the cumulative flux per unit area leads tof20-40 keV=2.6×10-10 ergs cm-2s-1 sr-1, which is about 1% of the known 20-40 keVX-ray background. We present the first luminosity function of AGNs inthe 20-40 keV energy range, based on 38 extragalactic objects detectedby the imager IBIS-ISGRI on board INTEGRAL. The luminosity functionshows a smoothly connected double-power-law form with an index ofγ1=0.8 below and γ2=2.1 above theturnover luminosity of L*=2.4×1043 ergss-1. The emissivity of all INTEGRAL AGNs per unit volume isW20-40keV(>1041 ergss-1)=2.8×1038 ergs s-1h370 Mpc-3. These results areconsistent with those derived in the 2-20 keV energy band and do notshow a significant contribution by Compton-thick objects. Because thesample used in this study is truly local (z¯=0.022), only limitedconclusions can be drawn for the evolution of AGNs in this energy band.
| 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.
| The First INTEGRAL AGN Catalog
We present the first INTEGRAL AGN catalog, based on observationsperformed from launch of the mission in 2002 October until 2004 January.The catalog includes 42 AGNs, of which 10 are Seyfert 1, 17 are Seyfert2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rathersmall, with five detected objects, and only one galaxy cluster and nostarburst galaxies have been detected so far. A complete subset consistsof 32 AGNs with a significance limit of 7 σ in the INTEGRAL ISGRI20-40 keV data. Although the sample is not flux limited, thedistribution of sources shows a ratio of obscured to unobscured AGNs of1.5-2.0, consistent with luminosity-dependent unified models for AGNs.Only four Compton-thick AGNs are found in the sample. Based on theINTEGRAL data presented here, the Seyfert 2 spectra are slightly harder(Γ=1.95+/-0.01) than Seyfert 1.5 (Γ=2.10+/-0.02) and Seyfert1 (Γ=2.11+/-0.05).
| A Sample of IRAS Infrared-selected Seyfert 1.5 Galaxies: Infrared Color α(60, 25)-dominated Eigenvector 1
The well-documented E1 relationships are first extended to infraredcolor α(60, 25) and flux ratio [O III]/Hβn bycomparing emission-line properties to continuum properties in infraredwavelengths. Both direct correlations and a principal component analysisare used in a sample of 50 IRAS IR-selected Seyfert 1.5 galaxies. Inaddition, to confirm the correlations of E1 in Boroson & Green, oureigenvector 1 turns out to be dominated by the mid-infrared colorα(60, 25) and most strongly affected by RFe, [OIII]/Hβn, and EW(Hβb). Our analysisindicates that the objects with large E1 tend to coexist with relativelyyoung nuclear stellar populations, which implies that E1 is related tothe nuclear star formation history. The IR-dominated eigenvector 1 cantherefore be inferred to be interpreted as the ``age'' of an AGN. Inconfirmation of the work of Xu and coworkers, it is clear that theextreme Seyfert galaxies with both large RFe and large [OIII]/Hβn are rare in our universe.
| 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.
| Spitzer IRS Spectra of a Large Sample of Seyfert Galaxies: A Variety of Infrared Spectral Energy Distributions in the Local Active Galactic Nucleus Population
We are conducting a large observing program with the Spitzer SpaceTelescope to determine the mid- to far-IR spectral energy distributionsof a well-defined sample of 87 nearby, 12 μm-selected Seyfertgalaxies. In this paper we present the results of Spitzer IRSlow-resolution spectroscopy of a statistically representative subsampleof 51 of the galaxies (59%), with an analysis of the continuum shapesand a comparison of the Seyfert types. We find that the spectra clearlydivide into groups based on their continuum shapes and spectralfeatures. The largest group (47% of the sample of 51) shows a very redcontinuum suggestive of cool dust and strong emission featuresattributed to polycyclic aromatic hydrocarbons. Sixteen objects (31%)have a power-law continuum with spectral indices of α5-20μm=-2.3 to -0.9 that flatten to α20-35μm=-1.1 to 0.0 at ~20 μm. Clear silicate emission featuresat 10 and 18 μm are found in two of these objects (Mrk 6 and Mrk335). A further 16% of the sample show power-law continua withunchanging slopes of α5-35 μm=-1.7 to -1.1. Twoobjects are dominated by a broad silicate absorption feature. One objectin the sample shows an unusual spectrum dominated by emission features,which is unlike any of the other spectra. Some spectral features areclearly related to a starburst contribution to the IR spectrum, whilethe mechanisms producing observed power-law continuum shapes, attributedto an active galactic nucleus (AGN) component, may be dust or nonthermalemission. The IR spectral types appear to be related to the Seyferttypes. Principal component analysis results suggest that the relativecontribution of starburst emission may be the dominant cause of variancein the observed spectra. The derived starburst component of eachspectrum, however, contributes <40% of the total flux density. Wecompare the IR emission with the optically thin radio emissionassociated with the AGN and find that Seyfert 1 galaxies have higherratios of IR to radio emission than Seyfert 2 galaxies, as predicted bythe unified model if the torus is optically thick in the mid-IR.However, smooth-density torus models predict a much larger differencebetween Seyfert types 1 and 2 than the factor of 2 difference observedin our sample; the observed factor of ~2 difference between the type 1and type 2 galaxies in their IR-to-radio ratios above 15 μm requiresthe standard smooth-density torus models to be optically thin at thesewavelengths. However, the resulting low torus opacity requires that thehigh observed columns detected in X-ray absorption be produced in gaswith a very low dust-to-gas ratio (perhaps within the dust sublimationregion). On the other hand, our observations may be consistent withclumpy torus models containing a steep radial distribution of opticallythick dense clumps. The selection of our sample at 12 μm, where thetorus may be optically thick, implies that there may beorientation-dependent biases in the sample; however, we do not find thatthe sample is biased toward Seyfert 2 galaxies with more luminouscentral engines, as would be expected. We find that the Seyfert 2galaxies typically show stronger starburst contributions than theSeyfert 1 galaxies in the sample, contrary to what is expected based onthe unified scheme for AGNs. This may be due to the selection effectthat only those Seyfert 2 galaxies with strong starburst contributionshad high enough integrated 12 μm flux densities to fall above theflux limit of the sample.
| 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.
| 4U 1344-60: a bright intermediate Seyfert galaxy at z = 0.012 with a relativistic Fe Kα emission line
We present analysis of the optical and X-ray spectra of the low Galacticlatitude bright (F_2-10 = 3.6 × 10-11 ergcm-2 s-1) source 4U 1344-60. On the basis of theoptical data we propose to classify 4U 1344-60 as an intermediate typeSeyfert galaxy and we measure a value of z = 0.012±0.001 for itsredshift. From the XMM-Newton observation we find that the overall X-rayspectral shape of 4U 1344-60 is complex and can be described by apower-law continuum (Γ≈ 1.55) obscured by two neutralabsorption components (N_Hf ~ 1022 cm-2and N_Hp ~ 4 × 1022 cm-2), thelatter covering only the ~50% of the primary X-ray source. The X-raydata therefore lend support to our classification of 4U 1344-60. Itexhibits a broad and skewed Fe Kα line at ~6.4 keV, which suggeststhe existence of an accretion disk that is able to reprocess the primarycontinuum down to a few gravitational radii. Such a line represents oneof the clearest examples of a relativistic line observed by XMM-Newtonso far. Our analysis has also revealed the marginal presence of twonarrow line-like emission features at ~4.9 and ~5.2 keV.
| 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.
| Equatorial scattering and the structure of the broad-line region in Seyfert nuclei: evidence for a rotating disc
We present detailed scattering models confirming that distinctivevariations in polarization across the broad Hα line, which areobserved in a significant fraction of type 1 Seyfert galaxies, can beunderstood in terms of a rotating line-emitting disc surrounded by acoplanar scattering region (the equatorial scattering region). Thepredicted polarization properties are: (i) averaged over wavelength, theposition angle (PA) of polarization is aligned with the projected discrotation axis and hence also with the radio source axis; (ii) thepolarization PA rotates across the line profile, reaching equal butopposite (relative to the continuum PA) rotations in the blue and redwings; and (iii) the degree of polarization peaks in the line wings andpasses through a minimum in the line core. We identify 11 objects thatexhibit these features to different degrees. In order to reproduce thelarge-amplitude PA rotations observed in some cases, the scatteringregion must closely surround the emission disc and the latter mustitself be a relatively narrow annulus - presumably the Hα-emittingzone of a larger accretion disc. Asymmetries in the polarization spectramay be attributable to several possible causes, including bulk radialinfall in the equatorial scattering region, or contamination by polarscattered light. The broad Hα lines do not, in general, exhibitdouble-peaked profiles, suggesting that a second Hα-emittingcomponent of the broad-line region is present, in addition to the disc.
| Correlations among multiwavelength luminosities of star-forming galaxies
It has been known for two decades that a tight correlation existsbetween global far-infrared (FIR) and radio continuum (1.4 and 4.8 GHz)fluxes/luminosities from star-forming galaxies, which may be explainedby formation activities of massive stars in these galaxies. For thisvery reason, a correlation might also exist between X-ray and FIR/radioglobal luminosities of galaxies. We analyse data from the ROSAT All-SkySurvey and from IRAS to show that such correlation does indeed existbetween FIR (42.5-122.5μm) and soft X-ray (0.1-2.4 keV) luminositiesLX and LFIR in 17 normal star-forming galaxies(NSFGs), including 16 late-type galaxies and one host-dominant Seyfertgalaxy, as well as in 14 active star-forming galaxies (ASFGs) consistingof starburst-dominant Seyfert galaxies. The quantitative difference insuch correlations in NSFGs and in ASFGs may be interpreted in terms ofevolutionary variations from classic starburst galaxies tostarburst-dominant Seyfert galaxies. Meanwhile, some low-luminosityactive galactic nuclei (LLAGNs) tend to exhibit such a correlation thatwe infer for star-forming galaxies, implying that star-formingactivities might be more dominant in LLAGNs. In contrast, AGN-dominantSeyfert galaxies do not show such a LX versus LFIRcorrelation; this is most likely related to accretions towardssupermassive black holes (SMBHs) in galactic nuclei. In order toestablish a physical connection between theLX-LFIR correlation and global star formation rate(SFR) in galaxies, we empirically derive bothLX-LB and LFIR-LB relationswith the blue-band luminosity LB roughly representing themass of a galaxy. It appears that the more massive galaxies are, themore star formation regions exist in these galaxies. The global SFR isnot only associated with the mass of a galaxy but also closely relatedto the level of star-forming activities therein. We propose a relationbetween soft X-ray luminosity and SFR in star-forming galaxies. In orderto probe the LX-LFIR relation, we construct anempirical model in which both FIR and X-ray emissions consist of twocomponents with one being closely associated with star formation and theother one not. Based on this model, we infer a linear relation betweenFIR/soft X-ray in star formation regions and radio luminosities, and geta linear relation between LX and LFIR forstar-forming regions.
| 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.
| Lag-Luminosity Relationship for Interband Lags between Variations in B, V, R, and I Bands in Active Galactic Nuclei
We determine interband lags between variations in the B band andvariations in the V, R, and I bands for 14 active galactic nucleiobserved at the Crimean Astrophysical Observatory. The computed lagsrange from tenths of a day to several days, and it is positive (that is,V, R, and I bands lag behind the B band) in most cases, except for a fewcases for the V filter. In some cases, the lag is greater than zero,with more than 3 σ confidence. The lag is systematically less forthe V filter than for the red filters, and the lag determined from thecross-correlation function (CCF) centroid is systematically greater thanthe lag determined from the CCF peak. We find that the lag scales withluminosity as Lb, where b~0.4-0.5. We attribute this lag tothe light time travel effect, so it reflects the geometrical size of theregion that emits optical continuum. We consider a model in whichoptical emission is mainly reprocessed emission that arises in theaccretion disk heated by an X-ray source above the disk.
| Narrow Iron Kα Lines in Active Galactic Nuclei: Evolving Populations?
We assemble a sample consisting of 66 active galactic nuclei (AGNs) fromthe literature and from the XMM-Newton archive in order to investigatethe origin of the 6.4 keV narrow iron Kα line (NIKAL). The X-rayBaldwin effect of the NIKAL is confirmed in this sample. We find thatthe equivalent width (EW) of the NIKAL is more strongly inverselycorrelated with the Eddington ratio (E) than with the 2-10 keVX-ray luminosity. Our sample favors the dusty torus origin, with theX-ray Baldwin effect being caused by the changing opening angle of thedusty torus. The EW-E relation can be derived from a toy model ofthe dusty torus. If the unification scheme is valid in all AGNs, we canderive the Baldwin effect from the ratio of type II AGNs to the totalpopulation given by Chandra and Hubble Space Telescope deep surveys.Thus, the evolution of populations could be reflected in the NIKAL'sBaldwin effect.
| INTEGRAL observations of the field of the BL Lacertae object S5 0716+714
We have performed observations of the blazar S5 0716+714 with INTEGRALon 2-6 April 2004. In the first months of 2004, the source had increasedsteadily in optical brightness and had undergone two outbursts. Duringthe latter outburst, that occurred in March, it reached the extremelevel of R = 12.1 mag, which triggered our INTEGRAL program. The targethas been detected with IBIS/ISGRI up to 60 keV, with a flux of 3× 10-11 erg s-1 cm-2 in the 30-60keV interval, a factor of 2 higher than observed by the BeppoSAXPDS in October 2000. In the field of S5 0716+714 we have also detectedthe Flat Spectrum Radio Quasar S5 0836+710 and the two Seyfert galaxiesMkn 3 and Mkn 6. Their IBIS/ISGRI spectra are rather flat, albeitconsistent with those measured by BeppoSAX. In the spectrum of Mkn 3 wefind some evidence of a break between 60 and 100 keV,reminiscent of the high energy cut-offs observed in other Seyfertgalaxies. This is the first report of INTEGRAL spectra of weak ActiveGalactic Nuclei.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, Czech Republic and Poland), and with theparticipation of Russia and the USA.
| Seyferts on the edge: polar scattering and orientation-dependent polarization in Seyfert 1 nuclei
We have identified 12 Seyfert 1 galaxies that exhibit opticalpolarization spectra similar to those of Seyfert 2 galaxies in whichpolarized broad lines are detected. We present new spectropolarimetricobservations of three of them: Was 45, Mrk 231 and NGC 3227. Theseobjects appear to be polarized as a result of far-field scattering inthe polar illumination cones of the circumnuclear torus. We estimatethat they represent between 10 and 30 per cent of the Seyfert 1population; they are found amongst all the main spectroscopic subtypes,including narrow-line Seyfert 1 galaxies. We have shown elsewhere thatSeyfert 1 nuclei more commonly have polarization characteristics thatcan be attributed to scattering by a compact `equatorial' scatteringregion located inside the torus. We propose that both equatorial andpolar scattering regions are present in all Seyfert galaxies and arguethat the observed range of polarization properties can be broadlyunderstood as an orientation effect. In this scheme, polar-scatteredSeyfert 1 galaxies represent the transition between unobscured (themajority of type 1) and obscured (type 2) Seyferts. They are viewedthrough the upper layers of the torus and are thus subject to moderateextinction (AV~ 1-4 mag) sufficient to suppress polarizedlight from the equatorial scattering region, but not the broad wings ofthe Balmer lines. The orientation of the polarization position anglerelative to the radio source is broadly consistent with thetwo-component scattering model. More generally, we find that amongstSeyfert 1 galaxies, parallel, perpendicular and intermediateorientations of the polarization position angle relative to the radioaxis occur roughly in the proportions 2:1:1.
| Search for Active Galaxies Using the First Byurakan Survey
The First Byurakan Survey (FBS) low-dispersion spectra allow selectionof different types of objects by their SEDs and/or emission orabsorption lines. This method is especially good for the search of AGNs.Markarian survey, the 2nd part of the FBS, optical identifications ofIRAS sources, as well as cross-correlations of radio, IR, X-ray catalogswith optical data (APS and USNO) revealed a vast amount of new AGNs andtheir candidates. Spectroscopic observations for a number of them haveconfirmed their nature. The digitization of the FBS will make theseworks more efficient. It will be a unique observational database forsearch for new interesting objects, including AGNs.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Circumnuclear Structure and Black Hole Fueling: Hubble Space Telescope NICMOS Imaging of 250 Active and Normal Galaxies
Why are the nuclei of some galaxies more active than others? If mostgalaxies harbor a central massive black hole, the main difference isprobably in how well it is fueled by its surroundings. We investigatethe hypothesis that such a difference can be seen in the detailedcircumnuclear morphologies of galaxies using several quantitativelydefined features, including bars, isophotal twists, boxy and diskyisophotes, and strong nonaxisymmetric features in unsharp-masked images.These diagnostics are applied to 250 high-resolution images of galaxycenters obtained in the near-infrared with NICMOS on the Hubble SpaceTelescope. To guard against the influence of possible biases andselection effects, we have carefully matched samples of Seyfert 1,Seyfert 2, LINER, starburst, and normal galaxies in their basicproperties, taking particular care to ensure that each was observed witha similar average scale (10-15 pc pixel-1). Severalmorphological differences among our five different spectroscopicclassifications emerge from the analysis. The H II/starburst galaxiesshow the strongest deviations from smooth elliptical isophotes, whilethe normal galaxies and LINERs have the least disturbed morphology. TheSeyfert 2s have significantly more twisted isophotes than any othercategory, and the early-type Seyfert 2s are significantly more disturbedthan the early-type Seyfert 1s. The morphological differences betweenSeyfert 1s and Seyfert 2s suggest that more is at work than simply theviewing angle of the central engine. They may correspond to differentevolutionary stages.
| Markarian 1239: A Highly Polarized Narrow-Line Seyfert 1 Galaxy with a Steep X-Ray Spectrum and Strong Ne IX Emission
We report the results of an XMM-Newton observation of the narrow-lineSeyfert 1 galaxy Mrk 1239. This optically highly polarized AGN has oneof the steepest X-ray spectra found in AGNs, withαX=+3.0 based on ROSAT PSPC data. The XMM-Newton EPICpn and MOS data confirm this steep X-ray spectrum. The pn data are bestfitted by a power law with a partial-covering absorption modelsuggesting two light paths between the continuum source and theobserver, one indirect scattered one, which is less absorbed, and ahighly absorbed direct light path. This result agrees with thewavelength-dependent degree of polarization in the optical/UV band.Residuals in the X-ray spectra of all three XMM-Newton EPIC detectorsaround 0.9 keV suggest the presence of an emission-line feature, mostlikely the Ne IX triplet. The detection of Ne IX and the nondetection ofO VII/O VIII suggest a supersolar Ne/O ratio.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contribution directly funded by ESA member statesand NASA.
| Gas near active galactic nuclei: A search for the 4.7 μm CO band
In order to constrain the properties of dense and warm gas around activegalactic nuclei, we have searched Infrared Space Observatory spectra oflocal active galactic nuclei (AGN) for the signature of the 4.7 μmfundamental ro-vibrational band of carbon monoxide. Low resolutionspectra of 31 AGN put upper limits on the presence of wide absorptionbands corresponding to absorption by large columns of warm and dense gasagainst the nuclear dust continuum. High resolution (R2500) spectraof NGC 1068 detect no significant absorption oremission in individual lines, to a 3σ limit of 7% of thecontinuum. The limits set on CO absorption in local AGN are much lowerthan the recent Spitzer Space Telescope detection of strong COabsorption by dense and warm gas in the obscured ultraluminous infraredgalaxy IRAS F00183-7111, despite evidence for dense material on parsecscales near an AGN in both types of objects. This suggests that suchdeep absorptions are not intimately related to the obscuring ``torus''material invoked in local AGN, but rather are a signature of thepeculiar conditions in the circumnuclear region of highly obscuredinfrared galaxies like IRAS F00183-7111. They may reflect fully coveredrather than torus geometries.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, TheNetherlands, and the UK) with the participation of ISAS and NASA.
| The relation between AGN hard X-ray emission and mid-infrared continuum from ISO spectra: Scatter and unification aspects
We use mid-infrared spectral decomposition to separate the 6 μmmid-infrared AGN continuum from the host emission in the ISO lowresolution spectra of 71 active galaxies and compare the results toobserved and intrinsic 2-10 keV hard X-ray fluxes from the literature.We find a correlation between mid-infrared luminosity and absorptioncorrected hard X-ray luminosity, but the scatter is about an order ofmagnitude, significantly larger than previously found with smallerstatistics. Main contributors to this scatter are likely variations inthe geometry of absorbing dust, and AGN variability in combination withnon-simultaneous observations. There is no significant differencebetween type 1 and type 2 objects in the average ratio of mid-infraredand hard X-ray emission, a result which is not consistent with the mostsimple version of a unified scheme in which an optically andgeometrically thick torus dominates the mid-infrared AGN continuum. Mostprobably, significant non-torus contributions to the AGN mid-IRcontinuum are masking the expected difference between the two types ofAGN.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/465Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, TheNetherlands, and the UK) with the participation of ISAS and NASA.
| A joint mid-infrared spectroscopic and X-ray imaging investigation of LINER galaxies
We present a comprehensive comparative high resolution mid-IRspectroscopic and X-ray imaging investigation of LINERs using archivalobservations from the ISO-SWS and the Chandra Advanced CCD ImagingSpectrometer. Although the sample is heterogenous and incomplete, thisis the first comprehensive study of the mid-infrared fine structure lineemission of LINERs. These results have been compared with similarobservations of starburst galaxies and AGN. We find that LINERs veryclearly fall between starbursts and AGN in their mid-IR fine structureline spectra, showing L[OIV]26 μm/LFIR andL[OIV]26 μm/L[NeII]12.8 μm ratios, bothmeasures of the dominant nuclear energy source in dust-enshroudedgalaxies, intermediate between those of AGN and starbursts. Chandraimaging observations of the LINERs reveal hard nuclear point sourcesmorphologically consistent with AGN in most (67%) of the sample, with aclear trend with IR-brightness. Most LINERs that show a single dominanthard compact X-ray core are IR-faint (LFIR/LB <1), whereas most LINERs that show scattered X-ray sources are IR-bright.A comparative X-ray/mid-IR spectroscopic investigation of LINERs revealssome puzzling results. Objects that display strong hard nuclear X-raycores should also display high excitation lines in the IR. However, wefind two LINERs disagree with this expectation. The galaxy NGC 404 showsweak soft X-ray emission consistent with a starburst but has the mostprominent highest excitation mid-IR spectrum of our entire sample. UsingIR emission line diagnostics alone, this galaxy would be classified ashosting a dominant AGN. Conversely, the IR luminous LINER NGC 6240 hasan extremely luminous binary AGN as revealed by the X-rays but showsweak IR emission lines. With the advent of SIRTF, and future IR missionssuch as Herschel and JWST, it is increasingly critical to determine theorigin of these multiwavelength anomalies.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/825Table 3 is only available in electronic form athttp://www.edpsciences.org
| 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.
| The Unified Model and Evolution of Active Galaxies: Implications from a Spectropolarimetric Study
We extend the analysis presented in Paper I of a spectropolarimetricsurvey of the CfA and 12 μm samples of Seyfert 2 galaxies (S2s). Weconfirm that polarized (hidden) broad-line region (HBLR) S2s tend tohave hotter circumnuclear dust temperatures, show mid-IR spectra morecharacteristic of Seyfert 1 galaxies (S1s), and are intrinsically moreluminous than non-HBLR S2s. The levels of obscuration and circumnuclearstar formation, however, appear to be similar between HBLR and non-HBLRS2 galaxies, based on an examination of various observationalindicators. HBLR S2s, on average, share many similar large-scale,presumably isotropic, characteristics with S1s, as would be expected ifthe unified model is correct, while non-HBLR S2s generally do not. Theactive nuclear engines of non-HBLR S2s, then, appear to be truly weakerthan HBLR S2s, which in turn are fully consistent with being S1s viewedfrom another direction. There is also evidence that the fraction ofdetected HBLRs increases with the radio power of the active galacticnucleus. Thus, all S2 galaxies may not be intrinsically similar innature, and we speculate that evolutionary processes may be at work.
| The Host Galaxies of Narrow-Line Seyfert 1 Galaxies: Evidence for Bar-Driven Fueling
We present a study of the host galaxy morphologies of narrow- andbroad-line Seyfert 1 galaxies (NLS1's and BLS1's) based on broadbandoptical images from the Hubble Space Telescope archives. We find thatlarge-scale stellar bars, starting at ~1 kpc from the nucleus, are muchmore common in NLS1's than BLS1's. Furthermore, the fraction of NLS1spirals that have bars increases with decreasing full width athalf-maximum of the broad component of Hβ. These results suggest alink between the large-scale bars, which can support high fueling ratesto the inner kiloparsecs, and the high mass accretion rates associatedwith the supermassive black holes in NLS1's.
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