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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.
| Neutral hydrogen in radio galaxies: Results from nearby, importance for far away
The study of neutral hydrogen emission and absorption in radio galaxiesis giving new and important insights on a variety of phenomena observedin these objects. Such observations are helping to understand the originof the host galaxy, the effects of the interaction between the radio jetand the ISM, the presence of fast gaseous outflows as well asjet-induced star formation. Recent results obtained on these phenomenaare summarized in this review. Although the {H I observationsconcentrate on nearby radio galaxies, the results also have relevancefor the high-z objects as all these phenomena are important, and likelyeven more common, in high-redshift radio sources.
| Size and properties of the narrow-line region in Seyfert-1 galaxies from spatially-resolved optical spectroscopy
Context: .Spatially resolved emission-line spectroscopy is a powerfultool for determining the physical conditions in the narrow-line region(NLR) of active galactic nuclei (AGNs). We recently used opticallong-slit spectroscopy to study the NLRs of a sample of six Seyfert-2galaxies. We have shown that such an approach, in comparison to thecommonly used [O III] narrow-band imaging alone, allows us to probe thesize of the NLR in terms of AGN photoionisation. Moreover, severalphysical parameters of the NLR can be directly accessed. Aims: .Wehere apply the same methods to study the NLR of six Seyfert-1 galaxiesand compare our results to those of Seyfert-2 galaxies. Methods:.We employ diagnostically valuable emission-line ratios to determine thephysical properties of the NLR, including the core values and radialdependencies of density, ionisation parameter, and reddening. Trackingthe radial change of emission-line ratios in diagnostic diagrams allowsus to measure the transition between AGN-like and H II-like lineexcitation, and thus to measure the size of the NLR. Results: .Inthe diagnostic diagrams, we find a transition between line ratiosfalling in the AGN regime and those typical of H II regions in twoSeyfert-1 galaxies, thereby determining the size of the NLR. The centralelectron temperature and ionisation parameter are, in general, higher intype-1 Seyferts than in type 2s. In almost all cases, both electrondensity and ionisation parameter decrease with radius and the decreaseis faster in Seyfert-1 galaxies than in Sy 2s. In several objects, thegaseous velocity distribution is characteristic of rotational motion inan (inclined) emission-line disk in the centre. We give estimates of theblack-hole masses and discuss our findings in detail for each object.
| The infrared emission from the narrow line region
We present models for the mid- and far- infrared emission from theNarrow Line Region (NLR) of Active Galactic Nuclei (AGN). Using theMAPPINGS iii code we explore the effect of typical NLR parameters on thespectral characteristics of the IR emission. These include useful IRemission line ratio diagnostic diagrams for the determination of theseparameters, as well as Star formation-AGN mixing diagnostics. We alsoexamine emission line to continuum correlations which would assist inseparating the IR emission arising from the NLR from that coming fromthe inner torus. We find for AGN like NGC 1068 and NGC 4151 that the NLRonly contributes ~10% to the total IRAS 25 mum flux, and that othercomponents such as a dusty torus are necessary to explain the total AGNIR emission.
| The small dispersion of the mid IR - hard X-ray correlation in active galactic nuclei
Context: .We investigate mid-infrared and X-ray properties of the dustytorus in unification scenarios for active galactic nuclei. Aims:.We use the relation between mid IR and hard X-ray luminosities toconstrain AGN unification scenarios. Methods: .With VISIR at theVLT, we have obtained the currently highest angular resolution (0.35arcsec FWHM) narrow-band mid infrared images of the nuclei of 8 nearbySeyfert galaxies. Combining these observations with X-ray data from theliterature we study the correlation between their mid IR and hard X-rayluminosities. Results: .We find that the rest frame 12.3 μm(LMIR) and 2-10 keV (LX) luminosities arecorrelated at a highly significant level. The best fit power-law to ourdata is log{LMIR} ∝ (1.60 ± 0.22 )log{LX}, showing a much smaller dispersion than earlierstudies. Conclusions: .The similarity in thelog{LMIR}/log{LX} ratio between Sy1s and Sy2s evenusing high angular resolution MIR data implies that the similarity isintrinsic to AGN and not caused by contamination from extra-nuclearemission. This supports clumpy torus models. The exponent of thecorrelation constrains the inner geometry of the torus.
| Optical spectroscopy for a sample of southern binary galaxies
Aims.This work is part of a joint observational program aiming to getphotometric and spectroscopic information on southern pairs of galaxies.We present optical long-slit spectroscopic data on 80 probablecomponents of pairs, 61 of them collected with a spectral resolution of3.4 Å, and 19 with 12 Å. Nevertheless, our analysis takesinto account 53 components of pairs with better spectral resolution, as8 of these target galaxies actually belong to optical pairs. For thesample with better resolution, the covered wavelength range is 5724≤ λ ≤ 7036 Å. The spectroscopic and photometricinformation is gathered for an analysis relating galaxy morphologies totheir spectra. Methods: .We use Hα +[NII] and Hα equivalent widths as star formationtracers for the central region of our sample galaxies, and we classifythe spectra according to the emission lines' relative strength bylooking at their behavior. Results: . Some of our sample galaxiesexhibit high central star formation, most of them belonging to closepairs. However, not all galaxies' components of close pairs show thisbehavior. This may be a clue that besides interaction, other agents canstimulate central emission in binary galaxies. We suggest an enhancementin the number of galaxies with peculiar spectra (probably Seyferts) inour binary sample, when compared to isolated galaxies. Our dataindicates that the morphological types of interacting galaxies arerelated to their spectral characteristics, as almost all early-typegalaxies of our sample do not exhibit central optical emission. We notethat the star formation activity is most likely to take place in bothpairs' components, with a slightly higher mean strength for the lessbright component of the pair. It is interesting to point out that mostspirals exhibiting a strong HII emission line spectra present either abar or a peculiarity, but on a general basis we do not find anenhancement of star formation in our interaction sample.
| Photometric study of a sample of southern binary galaxies
Aims.This work exhibits the basic optical photometric data for a sampleof 50 probable southern binary galaxies. Our sample covers a broad rangeof pair separations, stages of interaction, and morphologies. From theinitial list of selected pairs, using spectroscopic data from theliterature and our own data, we conclude that 84% of these systems aretrue binary galaxies. Methods: .We present residual and asymmetricmaps, R major semi-axis profiles of surface brightness, ellipticity,position-angle, harmonic Fourier coefficients of third and fourth order(b3 and b_4) for 50 probable pairs, and B-R color maps for 47of these pairs. For most galaxies, we present the profiles in twodifferent ways, aiming to verify the influence of random errors onthem. Results: . We note that random errors in position-angleprofiles are at least 2°, but a more significant result must takeinto account a variation larger than 11° for this distribution.Barred galaxies usually show a typical behavior in ellipticity andposition-angle profiles: these profiles display variations when changingfrom a bar to a disk region. In some cases, the variations also occuralong the bar. Some galaxies show distribution profiles that are commonfor their morphological type, and the interaction signature is onlyevidenced by their residual maps. Bars are usually redder and rings arebluer, when compared with the galaxy outskirts. Conclusions: .Ourdata indicates that there is a connection between interaction strengthand morphological distortions in binary galaxies. If we consider theprojected separation of a pair as an indication of interaction strength,distortions such as displaced centers, anomalous shapes of spiral arms,and twistings of external regions are easily detected in some closepairs, although not all components of close pairs show this behavior.Our data suggests that besides interaction, other parameters, likeorbital geometry and internal properties of galaxies, can stimulatebinary galaxies' peculiarities.
| 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.
| Orientation and size of the `Z' in X-shaped radio galaxies
Some X-shaped radio galaxies show a Z-symmetric morphology in the lessluminous secondary lobes. Within the scenario of a merger between twogalaxies, each hosting a supermassive black hole in its centre, thisstructure has been explained before. As the smaller galaxy spiralstowards the common centre, it releases gas to the interstellar medium ofthe larger active galaxy. The ram pressure of this streaming gas willbend the lobes of the pre-merger jet into a Z-shape. After the blackholes have merged, the jet propagates in a new direction that is alignedwith the angular momentum of the binary black hole. In this paper wedeproject the pre- and post-merger jets. Taking into account theexpected angles between the jet pairs and with the assumption that theirdirections are uncorrelated, we show that one of three possibleorientations of the jets with respect to the line of sight is morelikely than the others. This actually depends on the distance where thebending occurs. Another result of our deprojection is that the streaminggas bends the jet into a Z-shape in a range between about 30 and 100 kpcdistance to the centre of the primary galaxy. We confirm this finding bycomparing our predictions for the properties of the rotational velocityfield and its radius with observations and numerical simulations ofmerging galaxies. Thus, our results support the merger scenario asexplanation for X- and Z-shaped radio galaxies with the jet pointingalong the former axis of orbital angular momentum of the binary.
| A jet-induced outflow of warm gas in 3C293
Using long-slit emission-line spectra we detect a fast outflow ofionized gas, with velocities up to 1000 km s-1, in the nearbypowerful radio galaxy 3C293 (z= 0.045). The fast outflow is locatedabout 1 kpc east of the nucleus, in a region of enhanced radio emissiondue to the presence of a distorted radio jet. We present results thatindicate that this fast outflow is caused by a jet-ISM interaction. Thekinematics of the outflowing ionized gas are very similar to those of afast outflow of neutral hydrogen gas in this galaxy, suggesting thatboth outflows are the result of the same driving mechanism. While themass of the outflowing ionized gas is about 1 ×105Msolar, the total HI mass involved in theneutral outflow is about 100 times higher(107Msolar). This shows that, despite the highenergies that must be involved in driving the outflow, most of the gasremains, or again becomes, neutral. Other outflows of ionized gas,although not as pronounced as in the region of the enhanced radioemission, are also seen in various other regions along the axis of theinner radio jets. The regular kinematics of the emission-line gas alongthe major axis of the host galaxy reveals a rotating ionized gas disc 30kpc in extent.
| Mass-to-light ratio gradients in early-type galaxy haloes
Owing to the fact that the near future should see a rapidly expandingset of probes of the halo masses of individual early-type galaxies, weintroduce a convenient parameter for characterizing the halo masses fromboth observational and theoretical results:∇lΥ, the logarithmic radial gradient of themass-to-light ratio. Using halo density profiles from Λ-cold darkmatter (CDM) simulations, we derive predictions for this gradient forvarious galaxy luminosities and star formation efficienciesɛSF. As a pilot study, we assemble the available∇lΥ data from kinematics in early-type galaxies- representing the first unbiased study of halo masses in a wide rangeof early-type galaxy luminosities - and find a correlation betweenluminosity and ∇lΥ, such that the brightestgalaxies appear the most dark-matter dominated. We find that thegradients in most of the brightest galaxies may fit in well with theΛCDM predictions, but that there is also a population of faintergalaxies whose gradients are so low as to imply an unreasonably highstar formation efficiency ɛSF > 1. This difficultyis eased if dark haloes are not assumed to have the standard ΛCDMprofiles, but lower central concentrations.
| The Relationship of Hard X-Ray and Optical Line Emission in Low-Redshift Active Galactic Nuclei
In this paper we assess the relationship of the population of activegalactic nuclei (AGNs) selected by hard X-rays to the traditionalpopulation of AGNs with strong optical emission lines. First, we studythe emission-line properties of a new hard-X-ray-selected sample of 47local AGNs (classified optically as Type 1 and 2 AGNs). We find that thehard X-ray (3-20 keV) and [O III] λ5007 optical emission-lineluminosities are well-correlated over a range of about 4 orders ofmagnitude in luminosity (mean luminosity ratio 2.15 dex with a standarddeviation of σ=0.51 dex). Second, we study the hard X-rayproperties of a sample of 55 local AGNs selected from the literature onthe basis of the flux in the [O III] line. The correlation between thehard X-ray (2-10 keV) and [O III] luminosity for the Type 1 AGNs isconsistent with what is seen in the hard-X-ray-selected sample. However,the Type 2 AGNs have a much larger range in the luminosity ratio, andmany are very weak in hard X-rays (as expected for heavily absorbedAGNs). We then compare the hard X-ray (3-20 keV) and [O III] luminosityfunctions of AGNs in the local universe. These have similar faint-endslopes, with a luminosity ratio of 1.60 dex (0.55 dex smaller than themean value for individual hard-X-ray-selected AGNs). We conclude that atlow redshift, selection by narrow optical emission lines will recovermost AGNs selected by hard X-rays (with the exception of BL Lacobjects). However, selection by hard X-rays misses a significantfraction of the local AGN population with strong emission lines.
| The Swift/BAT High-Latitude Survey: First Results
We present preliminary results from the first 3 months of the SwiftBurst Alert Telescope (BAT) high Galactic latitude survey in the 14-195keV band. The survey reaches a flux of ~10-11 ergscm-2 s-1 and has ~2.7 arcmin (90% confidence)positional uncertainties for the faintest sources. This represents themost sensitive survey to date in this energy band. These data confirmthe conjectures that a high-energy-selected active galactic nucleus(AGN) sample would have very different properties from those selected inother bands and that it represents a ``true'' sample of the AGNpopulation. We have identified 86% of the 66 high-latitude sources.Twelve are Galactic-type sources, and 44 can be identified withpreviously known AGNs. All but five of the AGNs have archival X-rayspectra, enabling us to estimate the line-of-sight column densities andother spectral properties. Both of the z>0.11 objects are blazars.The median redshift of the others (excluding radio-loud objects) is0.012. We find that the column density distribution of these AGNs isbimodal, with 64% of the nonblazar sources having column densitiesNH>=1022 cm-2. None of the sourceswith logLX>43.5 (cgs units) show high column densities,and very few of the lower LX sources have low columndensities. Based on these data, we expect the final BAT catalog to have>200 AGNs and reach fluxes of less than ~10-11 ergscm-2 s-1 over the entire sky.
| The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.
| Dust Morphology of Hidden Broad-Line Region and Non-Hidden Broad-Line Region Seyfert 2 Galaxies
We investigate the nuclear dust properties of hidden broad-line region(HBLR) and non-HBLR Seyfert 2 galaxies. Optical images obtained from theHubble Space Telescope for a selected sample of HBLR and non-HBLRSeyfert 2 galaxies are fitted with the Galfit package to probe the innerstructures of these galaxies within the central 1 kpc regions. Most ofthe galaxies show complicated dust features in these regions. However,the dust morphology shows no significant difference between the HBLR andnon-HBLR Seyfert 2 galaxies. Dust masses inside the 1 kpc nuclearregions (M1kpc) are estimated from the obscuration levels inthe central regions of these galaxies. We compare our results with otherobserved properties, including [O III], far-infrared, and radioemission. We find that the HBLR and non-HBLR Seyfert 2 galaxies showdifferent near-infrared colors and M1kpc-FIR correlations,indicating that these two classes of Seyfert 2 galaxies are dominated bydifferent emission mechanisms. We suggest that they are intrinsicallydifferent and cannot be explained by the standard unification model.
| Fast neutral outflows in powerful radio galaxies: a major source of feedback in massive galaxies
We report the detection of fast (~1000 km s-1), massiveoutflows of neutral gas observed - using the WSRT - as 21-cm H Iabsorption against the strong radio continuum of seven radio sources.The neutral outflows occur, in at least somes cases, at kpc distancefrom the nucleus, and they are most likely driven by the interactionsbetween the expanding radio jets and the gaseous medium enshrouding thecentral regions. We estimate that the associated mass outflow rates areup to ~50 Mȯ yr-1, comparable (although atthe lower end of the distribution) to the outflow rates found forstarburst-driven superwinds in Ultra Luminous IR Galaxies (ULIRG). Thissuggests that massive, jet-driven outflows of neutral gas in radio-loudAGN can have as large an impact on the evolution of the host galaxies asthe outflows associated with starbursts. A radio-loud phase of the AGNis likely a relatively common, albeit short, phase in the life of many(or even all) massive ellipticals. Jet-driven neutral outflows mayrepresent one of the main feedback mechanisms in these galaxies.
| The location of the broad H i absorption in 3C 305: clear evidence for a jet-accelerated neutral outflow
We present high-spatial resolution 21-cm H i VLA observations of theradio galaxy 3C 305 (z=0.041). These new high-resolution data show thatthe 1000 km s-1 broad H I absorption, earlier detectedin low-resolution WSRT observations, is occurring against the bright,eastern radio lobe, about 1.6 kpc from the nucleus. We use new opticalspectra taken with the WHT to make a detailed comparison of thekinematics of the neutral hydrogen with that of the ionised gas. Thestriking similarity between the complex kinematics of the two gas phasessuggests that both the ionised gas and the neutral gas are part of thesame outflow. Earlier studies of the ionised gas had already foundevidence for a strong interaction between the radio jet and theinterstellar medium at the location of the eastern radio lobe. Ourresults show that the fast outflow produced by this interaction alsocontains a component of neutral atomic hydrogen. The most likelyinterpretation is that the radio jet ionises the ISM and accelerates itto the high outflow velocities observed. Our observations demonstratethat, following this strong jet-cloud interaction, not all gas cloudsare destroyed and that part of the gas can cool and become neutral. Themass outflow rate measured in 3C 305 is comparable, although at thelower end of the distribution, to that found in Ultra-Luminous IRgalaxies. This suggests that AGN-driven outflows, and in particularjet-driven outflows, can have a similar impact on the evolution of agalaxy as starburst-driven superwinds.
| Nearby early-type galaxies with ionized gas. I. Line-strength indices of the underlying stellar population
With the aim of building a data-set of spectral properties of wellstudied early-type galaxies showing emission lines, we presentintermediate resolution spectra of 50 galaxies in the nearby Universe.The sample, which covers several of the E and S0 morphologicalsub-classes, is biased toward objects that might be expected to haveongoing and recent star formation, at least in small amounts, because ofthe presence of the emission lines. The emission is expected to comefrom the combination of active galactic nuclei and star formationregions within the galaxies. Sample galaxies are located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. Our long-slit spectra coverthe 3700-7250 Å wavelength range with a spectral resolution of≈7.6 Å at 5550 Å. The specific aim of this paper, and ourfirst step in the investigation, is to map the underlying galaxy stellarpopulation by measuring, along the slit positioned along the galaxymajor axis, line-strength indices at several, homogeneousgalacto-centric distances. For each object we extracted 7luminosity-weighted apertures (with radii 1.5´´,2.5´´, 10´´, r_e/10, r_e/8, r_e/4 and r_e/2)corrected for the galaxy ellipticity and 4 gradients (0 ≤ r ≤r_e/16, r_e/16 ≤ r ≤ r_e/8, r_e/8 ≤ r ≤ r_e/4 and r_e/4≤ r ≤ r_e/2). For each aperture and gradient we measured 25line-strength indices: 21 of the set defined by the Lick-IDS“standard” system (Trager et al. [CITE], ApJS, 116, 1) and 4introduced by Worthey & Ottaviani ([CITE], ApJS, 111, 377).Line-strength indices have been transformed to the Lick-IDS system.Indices derived then include Hβ, Mg1, Mg2, Mgb, MgFe, Fe5270,Fe5335 commonly used in classic index-index diagrams. The paperintroduces the sample, presents the observations, describes the datareduction procedures, the extraction of apertures and gradients, thedetermination and correction of the line-strength indices, the procedureadopted to transform them into the Lick-IDS System and the proceduresadopted for the emission correction. We finally discuss the comparisonsbetween our dataset and line-strength indices available in theliterature. A significant fraction, about 60%, of galaxies in thepresent sample has one previous measurement in the Lick-IDS system butbasically restricted within the r_e/8 region. Line-strength measuresobtained both from apertures and gradients outside this area and withinthe r_e/8 region, with the present radial mapping, are completely new.Full appendix and Figs. 8 to 13 are only available in electronic form athttp://www.edpsciences.org Full Tables 6, 7, 9 and 10 are only availableat 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/433/497 Based onobservations obtained at the European Southern Observatory, La Silla,Chile (Programs Nr. 60.A-0647 and 61.A-0406).
| Disks, tori, and cocoons: emission and absorption diagnostics of AGN environments
One of the most important problems in the study of active galaxies isunderstanding the detailed geometry, physics, and evolution of thecentral engines and their environments. The leading models involve anaccretion disk and torus structure around a central dense object,thought to be a supermassive black hole. Gas found in the environment ofactive galactic nuclei (AGN) is associated with different structures:molecular accretion disks, larger scale atomic tori, ionized and neutral“cocoons” in which the nuclear regions can be embedded. Allof them can be studied at radio wavelengths by various means. Here, wesummarize the work that has been done to date in the radio band tocharacterize these structures. Much has been learned about the centralfew parsecs of AGN in the last few decades with contemporary instrumentsbut the picture remains incomplete. In order to be able to define a moreaccurate model of this region, significant advances in sensitivity,spectral and angular resolution, and bandpass stability are required.The necessary advances will only be provided by the Square KilometerArray and we discuss the possibilities that these dramatic improvementswill open for the study of the gas in the central region of 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.
| MERLIN observations of Stephan's Quintet
We present MERLIN L-band images of the compact galaxy group, Stephan'sQuintet (SQ). The Seyfert 2 galaxy, NGC 7319, the brightest member ofthe compact group, is seen to have a triple radio structure typical ofmany extra-galactic radio sources that have a flat spectrum core and twosteep spectrum lobes with hot spots. The two lobes are asymmetricallydistributed on opposite sides of the core along the minor axis of thegalaxy. Ultraviolet (UV) emission revealed in a high-resolution channel(HRC)/ACS Hubble Space Telescope (HST) image is strongly aligned withthe radio plasma and we interpret the intense star formation in the coreand north lobe as an event induced by the collision of the north radiojet with over-dense ambient material. In addition, a remapping ofarchive Very Large Array (VLA) L-band observations reveals more extendedemission along the major axis of the galaxy, which is aligned with theoptical axis. Images formed from the combined MERLIN and archive VLAdata reveal more detailed structure of the two lobes and hot spots.
| Black hole mass estimation from X-ray variability measurements in active galactic nuclei
We propose a new method of estimation of the black hole masses inactive galactic nuclei (AGN) based on the normalized excess variance,σ2nxs. We derive a relation betweenσ2nxs, the length of the observation, T, thelight-curve bin size, Δt, and the black hole mass, assuming that(i) the power spectrum above the high-frequency break,νbf, has a slope of -2, (ii) the high-frequency breakscales with black hole mass, (iii) the power-spectrum amplitude (infrequency-power space) is universal and (iv)σ2nxs is calculated from observations oflength T < 1/νbf. Values of black hole masses in AGNobtained with this method are consistent with estimates based on othertechniques such as reverberation mapping or the MBH-stellarvelocity dispersion relation. The method is formally equivalent tomethods based on power spectrum scaling with mass, but the use ofσ2nxs has the big advantage of beingapplicable to relatively low-quality data.
| Spectropolarimetry of Compton-thin Seyfert 2 galaxies
We present new spectropolarimetry of a sample of nearby Compton-thinSeyfert 2 galaxies (i.e. those with NH < 1023cm-2). We show that the detection rate of scattered broadHα in this sample is considerably higher than in Seyfert 2galaxies as a whole. Our results also show that in this low obscurationset it is possible to find scattered broad Hα even when the globalproperties of the galaxy are largely dominated by the host galaxy andnot the active galactic nucleus. These results argue against theexistence of a population of `pure' Seyfert 2 galaxies.
| Gas Outflows in Radio Galaxies
We present a summary of our recent results on gas outflows in radiogalaxies. Fast outflows (up to 2000 km s-1) have beendetected both in ionized and neutral gas. The latter is particularlysurprising as it shows that, despite the extremely energetic phenomenaoccurring near an AGN, some of the outflowing gas remains, or becomesagain, neutral. These results are giving new and important insights onthe physical conditions of the gaseous medium around an AGN.
| 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.
| Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation?
Infrared (IR) emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 μmare generally attributed to IR fluorescence from (mainly)far-ultraviolet (FUV) pumped large polycyclic aromatic hydrocarbon (PAH)molecules. As such, these features trace the FUV stellar flux and arethus a measure of star formation. We examined the IR spectralcharacteristics of Galactic massive star-forming regions and of normaland starburst galaxies, as well as active galactic nuclei (AGNs) andultraluminous infrared galaxies (ULIRGs). The goal of this study is toanalyze whether PAH features are a good qualitative and/or quantitativetracer of star formation, and hence to evaluate the application of PAHemission as a diagnostic tool in order to identify the dominantprocesses contributing to the infrared emission from Seyfert galaxiesand ULIRGs. We develop a new mid-infrared (MIR)/far-infrared (FIR)diagnostic diagram based on our Galactic sample and compare it to thediagnostic tools of Genzel and coworkers and Laurent and coworkers, withthese diagnostic tools also applied to our Galactic sample. This MIR/FIRdiagnostic is derived from the FIR normalized 6.2 μm PAH flux and theFIR normalized 6.2 μm continuum flux. Within this diagram, theGalactic sources form a sequence spanning a range of 3 orders ofmagnitude in these ratios, ranging from embedded compact H II regions toexposed photodissociation regions (PDRs) and the (diffuse) interstellarmedium (ISM). However, the variation in the 6.2 μm PAHfeature-to-continuum ratio is relative small. Comparison of ourextragalactic sample with our Galactic sources revealed an excellentresemblance of normal and starburst galaxies to exposed PDRs. WhileSeyfert 2 galaxies coincide with the starburst trend, Seyfert 1 galaxiesare displaced by at least a factor of 10 in 6.2 μm continuum flux, inaccordance with general orientation-dependent unification schemes forAGNs. ULIRGs show a diverse spectral appearance. Some show a typical AGNhot dust continuum. More, however, either are starburst-like or showsigns of strong dust obscuration in the nucleus. One characteristic ofthe ULIRGs also seems to be the presence of more prominent FIR emissionthan either starburst galaxies or AGNs. We discuss the observedvariation in the Galactic sample in view of the evolutionary state andthe PAH/dust abundance and discuss the use of PAHs as quantitativetracers of star formation activity. Based on these investigations, wefind that PAHs may be better suited as a tracer of B stars, whichdominate the Galactic stellar energy budget, than as a tracer of massivestar formation (O stars).
| Breaking All the Rules: The Compact Symmetric Object 0402+379
We present results of multifrequency Very Long Baseline Array (VLBA)observations of the compact symmetric object (CSO) 0402+379. Theparsec-scale morphology of 0402+379 allows us to confirm it as a CSO,while VLA data clearly show the presence of kiloparsec-scale structure.Thus, 0402+379 is only the second known CSO to possess large-scalestructure. Another puzzling morphological characteristic found from ourobservations is the presence of two central, compact, flat-spectrumcomponents, which we identify as possible active nuclei. We also presentthe discovery of neutral hydrogen absorption along the southern hot spotof 0402+379 with a central velocity ~1000 km s-1 greater thanthe systemic velocity. Multiepoch observations from the VLA archive, theCaltech-Jodrell Bank Survey, and the VLBA Calibrator Survey allow us tofurther analyze these anomalous features. Results of this analysisreveal significant motion in the northern hot spot, as well asappreciable variability in both of the core candidates. We consider thepossibility that 0402+379 was formed during a recent merger. In thiscase, the two candidate cores could be interpreted as binarysupermassive black holes that have not yet coalesced, whereas thelarge-scale radio emission could be attributed to interactions directlylinked to the merger or to previous activity associated with one of thecores.
| Statistical properties of local active galactic nuclei inferred from the RXTE 3-20 keV all-sky survey
We compiled a sample of 95 AGNs serendipitously detected in the 3-20 keVband at Galactic latitude |b|>10o during the RXTE slewsurvey (XSS, Revnivtsev et al. 2004), and utilize it to study thestatistical properties of the local population of AGNs, including theX-ray luminosity function and absorption distribution. We find thatamong low X-ray luminosity (L3-20< 1043.5 ergs-1) AGNs, the ratio of absorbed (characterized by intrinsicabsorption in the range 1022cm-21041 erg s-1 estimated here issmaller than the earlier estimated total X-ray volume emissivity in thelocal Universe, suggesting that a comparable X-ray flux may be producedtogether by lower luminosity AGNs, non-active galaxies and clusters ofgalaxies. Finally, we present a sample of 35 AGN candidates, composed ofunidentified XSS sources.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/469
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