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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.
| Discovery of Water Maser Emission in Eight AGNs with 70 m Antennas of NASA's Deep Space Network
We report the discovery of water maser emission in eight active galacticnuclei (AGNs) with the 70 m NASA Deep Space Network (DSN) antennas atTidbinbilla, Australia, and Robledo, Spain. The positions of the newlydiscovered masers, measured with the VLA, are consistent with theoptical positions of the host nuclei to within 1 σ (0.3" radio and1.3" optical) and most likely mark the locations of the embedded centralengines. The spectra of two sources, NGC 3393 and NGC 5495, display thecharacteristic spectral signature of emission from an edge-on accretiondisk, with orbital velocities of ~600 and ~400 km s-1,respectively. In a survey with DSN facilities of 630 AGNs selected fromthe NASA Extragalactic Database, we have discovered a total of 15 watermaser sources. The resulting incidence rate of maser emission amongnearby (vsys<7000 km s-1) Seyfert 1.8-2.0 andLINER systems is ~10% for a typical rms noise level of ~14 mJy over 1.3km s-1 spectral channels. As a result of this work, thenumber of nearby AGNs (vsys<7000 km s-1)observed with <20 mJy rms noise has increased from 130 to 449.
| X-Ray Emission from Megamaser Galaxy IC 2560
An observation of the H2O megamaser galaxy IC 2560 with theChandra X-Ray Observatory reveals a complex spectrum composed of softX-ray emission due to multitemperature thermal plasma and a hardcontinuum with strong emission lines. The continuum is most likely aCompton reflection (reprocessing) of primary emission that is completelyabsorbed at least up to 7 keV. The lines can be identified withfluorescence from Si, S, and Fe in the lowest ionization stages. Theequivalent widths of the Si and S lines are broadly compatible withthose anticipated for reprocessing by optically thick cold plasma ofsolar abundances, while the large equivalent width of the Fe linerequires some overabundance of iron. A contribution to the line from atransmitted component cannot be ruled out, but the limits on thestrength of the Compton shoulder make it less likely. From thebolometric luminosity of the nuclear region, we infer that the sourceradiates at 1%-10% of its Eddington luminosity for an adopted centralmass of 3×106 Msolar. The overall spectrumis consistent with the hypotheses that the central engines powering thedetected megamasers in accretion disks are obscured from direct view bythe associated accretion disk material itself and that there is acorrelation between the occurrence of megamaser emission andCompton-thick absorption columns. For the 11 known galaxies with bothcolumn density measurements and maser emission believed to arise fromaccretion disks, eight AGNs are Compton thick.
| 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.
| 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.
| 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.
| 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.
| Emission Line Properties of Active Galactic Nuclei from a Post-COSTAR Hubble Space Telescope Faint Object Spectrograph Spectral Atlas
We present consistent emission-line measurements for active galacticnuclei (AGNs), useful for reliable statistical studies of emission lineproperties. This paper joins a series including similar measurements of993 spectra from the Large Bright Quasar Survey and 174 spectra of AGNsobtained from the Faint Object Spectrograph (FOS) on the Hubble SpaceTelescope (HST) prior to the installation of COSTAR. This time weconcentrate on 220 spectra obtained with the FOS after the installationof COSTAR, completing the emission line analysis of all FOS archivalspectra. We use the same automated technique as in previous papers,which accounts for Galactic extinction, models blended optical and UViron emission, includes Galactic and intrinsic absorption lines, andmodels emission lines using multiple Gaussians. We present UV andoptical emission line parameters (equivalent widths, fluxes, FWHM, andline positions) for a large number (28) of emission lines includingupper limits for undetected lines. Further scientific analyses will bepresented in subsequent papers.
| A Complete Atlas of Recalibrated Hubble Space Telescope Faint Object Spectrograph Spectra of Active Galactic Nuclei and Quasars. I. Pre-COSTAR Spectra
We have recalibrated all nonpolarimetric, pre-COSTAR, archival HubbleSpace Telescope Faint Object Spectrograph UV and opticalspectrophotometry of active galaxies and quasars in order to extractuniformly calibrated spectrophotometric data for further detailedscientific investigations. The raw archival spectra have beenrecalibrated using the latest algorithms and calibration data. Spectraldata contaminated by intermittent noisy diodes and cosmic-ray eventshave been identified manually and eliminated. Wherever possible we havecombined multiple observations of the same source to produce a singlespectrum per object with the highest possible signal-to-noise ratio andcovering the widest wavelength range, scaling the individual data setsto the same photometric scale where necessary. Detailed qualityassurance has been performed to ensure that the merged object spectraare of the highest quality consistent with the limitations of theindividual data sets and calibrations.The recalibrated, merged object spectra are available in electronicform.We use this data set to present statistics of the photometric accuracyin the wavelength overlap regions for observations spanning multiplegratings.
| Double-barred galaxies. I. A catalog of barred galaxies with stellar secondary bars and inner disks
I present a catalog of 67 barred galaxies which contain distinct,elliptical stellar structures inside their bars. Fifty of these aredouble-barred galaxies: a small-scale, inner or secondary bar isembedded within a large-scale, outer or primary bar. I providehomogenized measurements of the sizes, ellipticities, and orientationsof both inner and outer bars, along with global parameters for thegalaxies. The other 17 are classified as inner-disk galaxies, where alarge-scale bar harbors an inner elliptical structure which is alignedwith the galaxy's outer disk. Four of the double-barred galaxies alsopossess inner disks, located in between the inner and outer bars. Whilethe inner-disk classification is ad-hoc - and undoubtedly includes someinner bars with chance alignments (five such probable cases areidentified) - there is good evidence that inner disks form astatistically distinct population, and that at least some are indeeddisks rather than bars. In addition, I list 36 galaxies which may bedouble-barred, but for which current observations are ambiguous orincomplete, and another 23 galaxies which have been previously suggestedas potentially being double-barred, but which are probably not. Falsedouble-bar identifications are usually due to features such as nuclearrings and spirals being misclassified as bars; I provide someillustrated examples of how this can happen.A detailed statistical analysis of the general population of double-barand inner-disk galaxies, as represented by this catalog, will bepresented in a companion paper.Tables \ref{tab:measured} and \ref{tab:deproj} are only available inelectronic form at http://www.edpsciences.org
| FLASH redshift survey - I. Observations and catalogue
The FLAIR Shapley-Hydra (FLASH) redshift survey catalogue consists of4613 galaxies brighter than bJ= 16.7 (corrected for Galacticextinction) over a 700-deg2 region of sky in the generaldirection of the Local Group motion. The survey region is a70°× 10° strip spanning the sky from the ShapleySupercluster to the Hydra cluster, and contains 3141 galaxies withmeasured redshifts. Designed to explore the effect of the galaxyconcentrations in this direction (in particular the Supergalactic planeand the Shapley Supercluster) upon the Local Group motion, the 68 percent completeness allows us to sample the large-scale structure betterthan similar sparsely-sampled surveys. The survey region does notoverlap with the areas covered by ongoing wide-angle (Sloan or 2dF)complete redshift surveys. In this paper, the first in a series, wedescribe the observation and data reduction procedures, the analysis forthe redshift errors and survey completeness, and present the surveydata.
| 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 HST View of the Narrow Line Region of Seyfert Galaxies
HST observations provided us with the spatial resolution required toexplore in detail the properties of the Narrow Line Region of Seyfertgalaxies. In Seyferts with extended radio structures the HST imagesrevealed a very close connection between radio and optical lineemission. We interpret this result as strong evidence that theline-emitting gas is compressed by the shocks created by the passage ofthe radio-emitting outflow. The increase in the density due to theshocks causes the line emission to be highly enhanced in the regionwhere this interaction occurs. Spectroscopy of the NLR confirms thepresence of a strong interaction: in the region co-spatial with theradio-jets, the gas velocity field is highly perturbed and shows twovelocity systems separated by as much as 1700km s-1. In severallocations the split lines form an almost complete velocity ellipsoid,implying that we are seeing an expanding shell of gas. We conclude thatthe morphology and the kinematics of the NLR in these sources aredominated by the presence of radio outflows, which might also have asignificant role in the ionization balance. Observations of Seyfertgalaxies represent a unique tool to study energetics and evolution ofISM/jets systems.
| Comparisons of Infrared Colors and Emission-line Intensities between Two types of Seyfert 2 Galaxies
We study the relation between the infrared colors, [OIII] emissionlines, gaseous absorbing column density (NH),and thedetectability of the polarized (hidden) broad-line region (HBLR) in alarge sample of 75 Seyfert 2 galaxies (Sy2s). From the indicators ofstar-formation activity, f60/f100 andLFIR/LB, we find some evidence that the Sy2swithout HBLR show higher star-formation activities than those with HBLR,in agreement with previous prediction. Also, we confirm that the HBLRSy2s tend to have a larger luminosity ratio of the core to the hostgalaxy, suggesting that the HBLR Sy2s display more powerful AGNactivity. However, the level of obscuration found in previous papers isnearly indistinguishable between the two types of Sy2s. The resultssupport the statement that the non-HBLR Sy2s, with a weaker corecomponent and a stronger star-formation activity component, areintrinsically different from the HBLR Sy2s, which are Sy1 systems with ahidden powerful AGN core and a low star-formation activity. Theindications are that the non-HBLR Sy2s might be at an earlierevolutionary phase than the HBLR Sy2s.
| Emission Line Properties of Active Galactic Nuclei from a pre-COSTAR Faint Object Spectrograph Hubble Space Telescope Spectral Atlas
UV/optical emission lines offer some of the most detailed informationobtainable about the intrinsic properties of quasars. Studies of thedensity, ionization and metal abundance of gas near the accreting blackhole are probed through an intriguing but poorly understood complex ofcorrelations between emission lines and overall quasar spectral energydistributions that has long suffered from a lack of large, consistentlymeasured samples. As part of a broader effort to expand and systematizethe data upon which these studies are built, we present measurements ofthe UV/optical emission line parameters in a sample of 158 activegalactic nuclei observed with the Faint Object Spectrograph on theHubble Space Telescope (HST), prior to the installation of COSTAR. Weuse an automated technique that accounts for galactic reddening,includes iron emission blends, galactic and intrinsic absorption lines,and performs multicomponent fits to the emission line profiles. Wepresent measured line parameters (equivalent width and FWHM) for a largenumber (28) of different UV/optical lines, including upper limits forundetected lines. We also study the relations between the emission lineequivalent widths and luminosity (the Baldwin effect), as well asredshift (evolution). We compare results from this HST FOS sample withour previous measurements of 993 QSOs in the Large Bright Quasar Surveyusing the same analysis technique and sum the samples to achieve bettercoverage of the luminosity-redshift plane. We confirm a significantBaldwin effect for UV iron emission from Green et al. and find thatevolution dominates the effect for iron and for Si IV emission. Thevalues of the Baldwin effect slopes for all UV emission lines and thedependence of the slopes on the sample's luminosity range point to achange of the SED as the cause of the Baldwin effect in the FOS sample.
| A Classic Type 2 QSO
In the Chandra Deep Field-South 1 Ms exposure, we have found, atredshift 3.700+/-0.005, the most distant type 2 active galactic nucleusever detected. It is the source with the hardest X-ray spectrum withredshift z>3. The optical spectrum has no detected continuum emissionto a 3 σ detection limit of ~3×10-19 ergss-1 cm-2 Å-1 and shows narrowlines of Lyα, C IV, N V, He II, O VI, [O III], and C III]. TheirFWHM line widths have a range of ~700-2300 km s-1 with anaverage of approximately ~1500 km s-1. The emitting gas ismetal-rich (Z~=2.5-3Zsolar). In the X-ray spectrum of 130counts in the 0.5-7 keV band, there is evidence for intrinsic absorptionwith NH>~1024 cm-2. An iron Kαline with rest-frame energy and equivalent width of ~6.4 keV and ~1 keV,respectively, in agreement with the obscuration scenario, is detected ata 2 σ level. If confirmed by our forthcoming XMM-Newtonobservations, this would be the highest redshift detection of FeKα. Depending on the assumed cosmology and the X-ray transfermodel, the 2-10 keV rest frame luminosity corrected for absorption is~1045 +/- 0.5 ergs cm-2 s-1, whichmakes our source a classic example of the long-sought type 2 QSO. Fromstandard population synthesis models, these sources are expected toaccount for a relevant fraction of the black hole-powered QSOdistribution at high redshift.
| Hubble Space Telescope Imaging of Bipolar Nuclear Shells in the Disturbed Virgo Cluster Galaxy NGC 4438
We present broadband and narrowband Hubble Space Telescope (HST) imagesof the central region of the heavily disturbed Virgo cluster galaxy NGC4438 (Arp 120), whose nucleus has been described as a type 1 LINER ordwarf Seyfert galaxy. Narrowband Hα and [N II] HST images revealstriking bipolar shell features, 1 kpc in projected length from end toend, that are likely the result of an outflow from the nuclear regionexperiencing a strong interaction with the interstellar medium (ISM).While these outflow shells share similarities with those in some otherstarburst or active galactic nucleus (AGN) galaxies, those in NGC 4438are notable because NGC 4438 harbors neither a luminous circumnuclearstarburst nor a luminous AGN. The shells appear to be closed at theirouter ends, suggesting that the outflow in NGC 4438 is dynamicallyyounger than those in some other galaxies. The radio continuum emissionis strongly enhanced near the outer ends of the shells, suggestingworking surfaces arising from collimated nuclear outflows that haveimpacted and shocked the surrounding ISM. The two shells are quitedifferent, as the northwestern shell is luminous and compact, while thesoutheastern shell is 2.5 times longer and much fainter in both opticalemission lines and the nonthermal radio continuum. The differencesbetween the two shells may be attributed to a difference in ISM densityon the two sides of the nuclear disk. Such an ISM asymmetry exists onlarger scales in this heavily disturbed galaxy. At the base of theoutflow is a nuclear source, which is the highest surface brightnesssource in the galaxy at optical wavelengths. This source is resolvedwith a FWHM=0.3"=25 pc and has modest luminosities, uncorrected forextinction, of 5×1038 ergs s-1 in Hαand MB=-13. We discuss whether the outflow is powered by alow-luminosity AGN or a compact nuclear starburst. The kinetic energyassociated with the ionized gas in the shells is ~1053 ergs,which could be produced by either massive star formation or an AGN.While the northwestern shell, which contributes most of the flux in mostground-based apertures centered on the nucleus, exhibits LINER-type lineratios, the nucleus has an Hα/[N II] ratio consistent with an H IIregion. Although there appears to be very little massive star formationoccurring in the central kpc, the nucleus may contain a partiallyobscured, young nuclear star cluster. On the other hand, the collimationof the southeastern shell and the strongly enhanced radio continuumemission at the outer ends of the shells are more easily explained byjets associated with a nuclear black hole than by a compact nuclearstarburst, although there is no direct evidence of a jet. The 2000 kms-1 broad-line component could be due to an AGN broad-lineregion but might also be due to high-velocity gas in the outflow. SinceNGC 4438 has a large bulge, a large nuclear black hole might beexpected. Based on observations with the Hubble Space Telescope, whichis operated by AURA, Inc., under NASA contract NAS 5-26555.
| Analysis of the distribution of HII regions in external galaxies. IV. The new galaxy sample. Position and inclination angles
We have compiled a new sample of galaxies with published catalogs of HIIregion coordinates. This sample, together with the former catalog ofGarcía-Gómez & Athanassoula (\cite{gga1}), will formthe basis for subsequent studies of the spiral structure in discgalaxies. In this paper we address the problem of the deprojection ofthe galaxy images. For this purpose we use two deprojection methodsbased on the HII region distribution and compare the results with thevalues found in the literature using other deprojection methods. Takinginto account the results of all the methods, we propose optimum valuesfor the position and inclination angles of all the galaxies in oursample. Tables 2 and 3 are only available in electronic form athttp://www.edpsciences.org
| Seyfert Galaxies with Circumnuclear/Nuclear Starbursts
In this paper, we present our preliminary results on Seyfert galaxieswith circumnuclear/nuclear starburst (SB) activity. We have searched therecent available literature and found 76 active galaxies with clearevidence of nuclear SB activity, among which 16 are Seyfert 1s, 51Seyfert 2s, and 9 LINERs. After studying the 51 Seyfert 2s, we find thatthose Seyfert 2s with hidden Seyfert 1 nuclei, have similarInfrared-Radio properties as Seyfert 1 galaxies, and are different from``real'' Seyfert 2s without a hidden Seyfert 1 nucleus. The later aresimilar to starburst galaxies.
| Jet cocoons in rotating Seyfert galaxies: adaptive three-dimensional hydrodynamics
The narrow-line regions of some Seyfert galaxies show evidence fornuclear jets interacting with the rotating interstellar gas; this isshown by point-symmetric emission-line structures in, for example, Mrk573 and NGC 3393. We study this situation with numerical simulations ofa jet in a sidewind of uniform density but linearly increasing velocityas one moves from the source. We use a new three-dimensionalhydrodynamic code on a binary adaptive grid. We consider two differentmodels, one with a cocoon expansion speed higher and one with expansionspeed lower than the ISM speed. We find that the model with high cocoonexpansion speed is similar to results from previous calculations withouta sidewind, except for minor asymmetries. However, model B with the slowexpansion speed and fast wind speed shows considerable qualitativedifferences. The jet hits and bounces off the dense cooling envelope,which is dragged by the sidewind into the straight path of the jet. Thepath of the jet within the cocoon is straight as long as the extendedhot cocoon acts as a shield. Once the jet hits the cold envelope of thecocoon it is bent directly by the ram pressure of the ambient medium andfollows a parabola of the third degree, which we derive as an analyticalapproximation for the path. The region where the jet hits the envelopeis the start of strong radio emission. This point moves towards thesource with age of the jet and its bending angle. We therefore find apossible observable correlation between the distance of the first strongradio knot and the overall bending of jets in Seyfert galaxies. Acomparison of our results with observations of Mrk 573 shows that theessential structural and spectral features can be reproduced by choosingan appropriate viewing angle and evolutionary stage. Lookingapproximately along the original jet direction a structure is foundwhich strongly resembles an ionization cone. Hence caution shouldprevail when interpreting these sorts of structure within thenarrow-line region of Seyfert galaxies.
| Integral Field Near-Infrared Spectroscopy of a Sample of Seyfert and LINER Galaxies. I. The Data
We present near-IR integral field spectroscopy of a sample of 31 Seyfertand LINER galaxies which were selected both to span a wide range ofnuclear magnitudes and to possess roughly equal numbers of Seyfert type1 and 2 nuclei. Moderate resolution (R~1000 R~2000 for three cases)integral field K-band spectra were obtained for all 31 galaxies in oursample and for 18 galaxies (R~1000 R~2000 for four cases) H-bandintegral field spectra were also obtained. In each case, we presentnuclear, larger aperture, and difference spectra with correspondinginformation about emission line wavelengths, fluxes, and widths.Line-free H- and K-band continuum images as well as [Fe II]λ1.644 μm, Brγ, and H2 1-0 S(1) emissionlines are also presented. In addition, we provide extensive informationabout each galaxy obtained from the literature that will be usefulsubsequently for characterizing the sample and for comparison with ournear-IR data. Based on observations obtained with the Anglo-AustralianTelescope, Siding Spring, Australia, the European Southern Observatory,La Silla, the William Herschel Telescope operated on the island of LaPalma by the Isaac Newton Group in the Spanish Observatorio del Roque delos Muchachos of the Instituto de Astrofísica de Canarias, andthe Calar Alto 3.5 m, which is operated by the Max-Planck-Institutfür Astronomie, Heidelberg, Germany, jointly with the SpanishNational Commission for Astronomy.
| Jet Directions in Seyfert Galaxies: Radio Continuum Imaging Data
We present the results of VLA A-array 8.46 GHz continuum imaging of 55Seyfert galaxies (19 Seyfert 1's and 36 Seyfert 2's). These galaxies arepart of a larger sample of 88 Seyfert galaxies, selected from mostlyisotropic properties, the flux at 60 μm, and warm infrared 25-60μm colors. These images are used to study the structure of the radiocontinuum emission of these galaxies and their position angles, in thecase of extended sources. These data, combined with information frombroadband B and I observations, have been used to study the orientationof radio jets relative to the plane of their host galaxies (Kinney etal.).
| 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 Multitude of Unresolved Continuum Sources at 1.6 Microns in Hubble Space Telescope Images of Seyfert Galaxies
We examine 112 Seyfert galaxies observed by the Hubble Space Telescopeat 1.6 μm. We find that ~50% of the Seyfert 2.0 galaxies which arepart of the Revised Shapely-Ames (RSA) Catalog or the CfA redshiftsample contain unresolved continuum sources at 1.6 μm. All but acouple of the Seyfert 1.0-1.9 galaxies display unresolved continuumsources. The unresolved sources have fluxes of order 1 mJy,near-infrared luminosities of order 1041 ergs s-1,and absolute magnitudes MH~-16. Comparison non-Seyfertgalaxies from the RSA Catalog display significantly fewer (~20%),somewhat lower luminosity nuclear sources, which could be due to compactstar clusters. We find that the luminosities of the unresolved Seyfert1.0-1.9 sources at 1.6 μm are correlated with [O III] λ5007and hard X-ray luminosities, implying that these sources are nonstellar.Assuming a spectral energy distribution similar to that of a Seyfert 2galaxy, we estimate that a few percent of local spiral galaxies containblack holes emitting as Seyferts at a moderate fraction,~10-1-10-4, of their Eddington luminosities. Wefind no strong correlation between 1.6 μm fluxes and hard X-ray or [OIII] λ5007 fluxes for the pure Seyfert 2.0 galaxies. Thesegalaxies also tend to have lower 1.6 μm luminosities compared to theSeyfert 1.0-1.9 galaxies of similar [O III] luminosity. Either largeextinctions (AV~20-40) are present toward theircontinuum-emitting regions or some fraction of the unresolved sources at1.6 μm are compact star clusters. With increasing Seyfert type thefraction of unresolved sources detected at 1.6 μm and the ratio of1.6 μm to [O III] fluxes tend to decrease. These trends areconsistent with the unification model for Seyfert 1 and 2 galaxies.
| 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
| Nuclear obscuration and scattering in Seyfert 2 galaxies
We study the relationship between gaseous absorbing column density (N_Ha), infrared colors and detectability of the broad lines in a largesample of Seyfert 2 galaxies(Sy2s). We confirm that Sy2s withoutpolarized broad lines tend to have cooler 60 mu m/25 mu m colors; thiscorrelation was previously ascribed to the effect of obscuration towardsthe nuclear region. We find some evidence that Sy2s without polarizedbroad lines have larger absorbing column density (N_H a) and that afraction of them are characterized by dust lanes crossing their nuclei.However, we find that the IR colors do not correlate with N_H a, indisagreement with the obscuration scenario. Also, Sy2s without polarizedbroad lines follow the same radio-FIR relation as normal and starburstgalaxies, at variance with Sy2s with polarized broad lines. Theseresults indicate that the lack of broad lines in the polarized spectrumof Sy2s is mostly due to the contribution/dilution from the host galaxyor from a circumnuclear starburst, though a lesser extent to theobscuration toward the nuclear region also plays a role.
| The Narrow-Line Region in the Seyfert 2 Galaxy NGC 3393
The narrow-line region (NLR) of the Seyfert 2 galaxy NGC 3393 isdominated by a symmetric structure which appears as S-shaped arms inHubble Space Telescope (HST) images. These arms, which occupy thecentral few arcseconds of the nucleus, border a linear, triple-lobedradio source. We use HST imaging and spectra, ground-based opticalimages, long-slit spectra, Fabry-Perot imaging spectroscopy, and VLAradio data to perform a detailed investigation of the kinematics andionization of the line-emitting gas in NGC 3393 and of its relationshipwith the relativistic gas responsible for the radio emission. Theexcitation map [O III] λ5007/(Hα+[N II]λλ6548, 6584) shows a biconical structure, consistent withthe anisotropic nuclear ionizing radiation expected in the unifiedscheme. Extrapolation to ionizing frequencies of our upper limit to the2100 Å flux of the nuclear source provides a factor>=3×104 too few ionizing photons to account for therecombination line emission, which also suggests that the nuclearionizing source radiates anisotropically. However, the kinetic energy ofthe outflow is sufficient to power the line emission via photoionizingshocks, and a tentative detection of extended UV emission is consistentwith this model. Furthermore, the broad component of the emission lineshas a similar orientation and spatial extent as the triple radio source.Nevertheless, other tests are inconsistent with the photoionizing shockmodel-there is no correlation between local velocity dispersion, surfacebrightness, and excitation, and the gaseous abundances of [Ca II], AlII], and Mg II are much lower than expected if these species have beenliberated into the gas phase through grain destruction by shocks. Weconclude that the radio lobes appear to have created denser regions ofgas on their leading edges, thus forming the S-shaped arms, but that theionization is most likely due to photoionization by an obscured centralsource.
| 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.
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