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Ýlgili Makaleler
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.
| Size and properties of the narrow-line region in Seyfert-2 galaxies from spatially-resolved optical spectroscopy
Context: .While [O III] narrow-band imaging is commonly used to measurethe size of the narrow-line regions (NLRs) in active galactic nuclei(AGNs), it can be contaminated by emission from surrounding starbursts.Recently, we have shown that long-slit spectroscopy provides a valuablealternative approach to probe the size in terms of AGN photoionisation.Moreover, several parameters of the NLR can be directly accessed.Aims: .We here apply the same methods developed and described for theSeyfert-2 galaxy NGC 1386 to study the NLR of five other Seyfert-2galaxies by using high-sensitivity spatially-resolved opticalspectroscopy obtained at the VLT and the NTT. Methods: .We probethe AGN-photoionisation of the NLR and thus, its "real" size usingdiagnostic line-ratio diagrams. We derive physical properties of the NLRsuch as reddening, ionisation parameter, electron density, and velocityas a function of distance from the nucleus. Results: .For NGC5643, the diagnostic diagrams unveil a similar transition between lineratios falling in the AGN regime and those typical for H II regions asfound for NGC 1386, thus determining the size of the NLR. For the otherfour objects, all measured line ratios fall in the AGN regime. In almostall cases, both electron density and ionisation parameter decrease withradius. Deviations from this general behaviour (such as a secondarypeak) seen in both the ionisation parameter and electron density can beinterpreted as signs of shocks from the interaction of a radio jet andthe NLR gas. In several objects, the gaseous velocity distribution ischaracteristic for rotational motion in an (inclined) emission-line diskin the centre. We compare our results to those of NGC 1386 and show thatthe latter can be considered as prototypical also for this largersample. We discuss our findings in detail for each object.
| The K-band properties of Seyfert 2 galaxies
Aims. It is well known that the [O iii]λ5007 emission line andhard X-ray (2-10 keV) luminosities are good indicators of AGN activitiesand that the near and mid-infrared emission of AGN originates fromre-radiation of dusty clouds heated by the UV/optical radiation from theaccretion disk. In this paper we present a study of the near-infraredK-band (2.2 μm) properties for a sample of 65 Seyfert 2 galaxies. Methods: .By using the AGN/Bulge/Disk decomposition technique, weanalyzed the 2MASS K_S-band images for Seyfert 2 galaxies in order toderive the K_S-band magnitudes for the central engine, bulge, and diskcomponents. Results: .We find that the K_S-band magnitudes of thecentral AGN component in Seyfert 2 galaxies are tightly correlated withthe [O iii]λ5007 and the hard X-ray luminosities, which suggeststhat the AGN K-band emission is also an excellent indicator of thenuclear activities at least for Seyfert 2 galaxies. We also confirm thegood relation between the central black hole masses and bulge's K-bandmagnitudes for Seyfert 2s.
| The connection between shear and star formation in spiral galaxies
We present a sample of 33 galaxies for which we have calculated (i) theaverage rate of shear from published rotation curves, (ii) thefar-infrared luminosity from IRAS fluxes, and (iii) theKs-band luminosity from the Two Micron All Sky Survey(2MASS). We show that a correlation exists between the shear rate andthe ratio of the far-infrared to Ks-band luminosity. Thisratio is essentially a measure of the star formation rate per unit mass,or the specific star formation rate. From this correlation we show thata critical shear rate exists, above which star formation would turn offin the discs of spiral galaxies. Using the correlation between shearrate and spiral arm pitch angle, this shear rate corresponds to thelowest pitch angles typically measured in near-infrared images of spiralgalaxies.
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
| The 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.
| Connecting the cosmic infrared background to the X-ray background
We estimate the contribution of active galactic nuclei (AGN) and oftheir host galaxies to the infrared background. We use the luminosityfunction and evolution of AGN recently determined by the hard X-raysurveys, and new spectral energy distributions connecting the X-ray andthe infrared emission, divided in intervals of absorption. These twoingredients allow us to determine the contribution of AGN to theinfrared background by using mostly observed quantities, with only minorassumptions. We find that AGN emission contributes little to theinfrared background (<5 per cent over most of the infrared bands),implying that the latter is dominated by star formation. However, AGNhost galaxies may contribute significantly to the infrared background,and more specifically 10-20 per cent in the 1-20 μm range and ~5 percent at λ < 60μm. We also give the contribution of AGN andof their host galaxies to the source number counts in various infraredbands, focusing on those which will be observed with Spitzer. We alsoreport a significant discrepancy between the expected contribution ofAGN hosts to the submillimetre background and bright submillimetrenumber counts with the observational constraints. We discuss the causesand implications of this discrepancy and the possible effects on theSpitzer far-infrared bands.
| 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.
| The evolution of stars and gas in starburst galaxies
In systems undergoing starbursts the evolution of the young stellarpopulation is expected to drive changes in the emission-line properties.This evolution is usually studied theoretically, with a combination ofevolutionary synthesis models for the spectral energy distribution ofstarbursts and photoionization calculations. In this paper we present amore empirical approach to this issue. We apply empirical populationsynthesis techniques to samples of starburst and HII galaxies in orderto measure their evolutionary state and correlate the results with theiremission-line properties. A couple of useful tools are introduced thatgreatly facilitate the interpretation of the synthesis: (1) anevolutionary diagram, the axes of which are the strengths of the young,intermediate age and old components of the stellar population mix; and(2) the mean age of stars associated with the starburst, . These toolsare tested with grids of theoretical galaxy spectra and found to workvery well even when only a small number of observed properties(absorption-line equivalent widths and continuum colours) is used in thesynthesis.Starburst nuclei and HII galaxies are found to lie on a well-definedsequence in the evolutionary diagram. Using the empirically defined meanstarburst age in conjunction with emission-line data, we have verifiedthat the equivalent widths of Hβ and [OIII] decrease for increasing. The same evolutionary trend was identified for line ratios indicativeof the gas excitation, although no clear trend was identified formetal-rich systems. All these results are in excellent agreement withlong-known, but little tested, theoretical expectations.
| The mid-IR emission of Seyfert galaxies: Relevance for CANARICAM
New results on the mid-IR emission of Seyfert galaxies are presented.The observations were performed using the TIMMI-2 instrument at the 3.6m ESO telescope. We have obtained diffraction-limited images. In somecases the emission can be resolved and varies with wavelength. Therelevance for these studies for CANARICAM is briefly discussed.
| 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.
| A Hubble Space Telescope Survey of Extended [O III] λ5007 Å Emission in a Far-Infrared-Selected Sample of Seyfert Galaxies: Results
We present the results of a Hubble Space Telescope (HST) survey ofextended [O III] emission in a sample of 60 nearby Seyfert galaxies (22Seyfert 1's and 38 Seyfert 2's), selected by mostly isotropicproperties. The comparison between the semimajor axis size of their [OIII] emitting regions (Rmaj) shows that Seyfert 1's andSeyfert 2's have similar distributions, which seems to contradictunified model predictions. We discuss possible ways to explain thisresult, which could be due to either observational limitations or themodels used for the comparison with our data. We show that Seyfert 1narrow-line regions (NLRs) are more circular and concentrated thanSeyfert 2's, which can be attributed to foreshortening in the former. Wefind a good correlation between the NLR size and luminosity, followingthe relation Rmaj~L([O III])0.33 +/- 0.04, whichis flatter than a previous one found for QSOs and Seyfert 2's. Wediscuss possible reasons for the different results, and theirimplications to photoionization models. We confirm previous results thatshow that the [O III] and radio emission are well aligned and also findno correlation between the orientation of the extended [O III] emissionand the host galaxy major axis. This agrees with results showing thatthe torus axis and radio jet are not aligned with the host galaxyrotation axis, indicating that the orientation of the gas in the torus,and not the spin of the black hole, determines the orientation of theaccretion disk, and consequently, the orientation of the radio jet.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 Lack of Broad-Line Regions in Low Accretion Rate Active Galactic Nuclei as Evidence of Their Origin in the Accretion Disk
In this Letter, we present evidence suggesting that the absence orpresence of hidden broad-line regions (HBLRs) in Seyfert 2 galaxies isregulated by the rate at which matter accretes onto a centralsupermassive black hole, in units of the Eddington rate. Evidence isbased on data from a subsample of type 2 active galactic nucleiextracted from the Tran spectropolarimetric sample and made up of allthose sources that also have good-quality X-ray spectra available andfor which a bulge luminosity can be estimated. We use the intrinsic(i.e., unabsorbed) X-ray luminosities of these sources and their blackhole masses (estimated by using the well-known relationship betweennuclear mass and bulge luminosity in galaxies) to derive the nuclearaccretion rate in Eddington units. We find that virtually all HBLRsources have accretion rates larger than a threshold value ofmthres~=10-3 (in Eddington units), while non-HBLRsources lie at m<~mthres. These data nicely fitpredictions from a model proposed by Nicastro in which the broad-lineregions (BLRs) are formed by accretion disk instabilities occurring inproximity of the critical radius at which the disk changes from gaspressure dominated to radiation pressure dominated. This radiusdiminishes with decreasing m for low enough accretion rates (andtherefore luminosities), the critical radius becomes smaller than theinnermost stable orbit and BLRs cannot form.
| The Infrared Spectral Energy Distribution of the Seyfert 2 Prototype NGC 5252
The complete mid- to far-infrared continuum energy distributioncollected with the Infrared Space Observatory of the Seyfert 2 prototypeNGC 5252 is presented. ISOCAM images taken in the 3-15 μm show aresolved central source that is consistent at all bands with a region ofabout 1.3 kpc in size. Because of the lack of ongoing star formation inthe disk of the galaxy, this resolved emission is associated with eitherdust heated in the nuclear active region or bremsstrahlung emission fromthe nuclear and extended ionized gas. The size of the mid-IR emissioncontrasts with the standard unification scenario envisaging a compactdusty structure surrounding and hiding the active nucleus and thebroad-line region. The mid-IR data are complemented by ISOPHOT aperturephotometry in the 25-200 μm range. The overall IR spectral energydistribution is dominated by a well-defined component peaking at ~100μm, a characteristic temperature of T~=20 K, and an associated dustmass of 2.5×107 Msolar, which greatlydominates the total dust mass content of the galaxy. The heatingmechanism of this dust is probably the interstellar radiation field.After the contribution of this cold dust component is subtracted, thebulk of the residual emission is attributed to dust heated within thenuclear environment. Its luminosity consistently accounts for thereprocessing of the X-ray to UV emission derived for the nucleus of thisgalaxy. The comparison of NGC 5252's spectral energy distribution withcurrent torus models favors large nuclear disk structure on thekiloparsec scale.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| Iron Is Not Depleted in High-Ionization Nuclear Emission-Line Regions of Active Galactic Nuclei
To examine whether or not high-ionization nuclear emission-line regions(HINERs) in narrow-line regions of active galactic nuclei are dusty, wefocus on two high-ionization forbidden emission lines, [Fe VII]λ6087 and [Ne V] λ3426. We perform photoionization modelcalculations to investigate possible dependences of the flux ratio of[Fe VII] λ6087/[Ne V] λ3426 on various gas properties, inorder to investigate how useful this flux ratio is to explore the dustabundances in HINERs. Based on our photoionization model calculations,we show that the observed range of the flux ratio of [Fe VII]λ6087/[Ne V] λ3426 is consistent with the dust-freemodels, while it cannot easily be explained by the dusty models. Thissuggests that iron is not depleted in HINERs, which implies that theHINERs are not dusty. This result is consistent with the idea that theHINERs are located closer than the dust-sublimation radius (i.e., theinner radius of dusty tori) and thus can be hidden by dusty tori whenseen from a edge-on view toward the tori, which is also suggested by theAGN-type dependence of the visibility of high-ionization emission lines.
| High-energy sources before INTEGRAL. INTEGRAL reference catalog
We describe the INTEGRAL reference catalog which classifies previouslyknown bright X-ray and gamma-ray sources before the launch of INTEGRAL.These sources are, or have been at least once, brighter than ~ 1 mCrababove 3 keV, and are expected to be detected by INTEGRAL. This catalogis being used in the INTEGRAL Quick Look Analysis to discover newsources or significantly variable sources. We compiled several publishedX-ray and gamma-ray catalogs, and surveyed recent publications for newsources. Consequently, there are 1122 sources in our INTEGRAL referencecatalog. In addition to the source positions, we show an approximatespectral model and expected flux for each source, based on which wederive expected INTEGRAL counting rates. Assuming the default instrumentperformances and at least ~ 105 s exposure time for anypart of the sky, we expect that INTEGRAL will detect at least ~ 700sources below 10 keV and ~ 400 sources above 20 keV over the missionlife.The Catalog is available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/411/L59
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| High-resolution X-ray imaging and spectroscopy of the core of NGC 4945 with XMM-Newton and Chandra
We utilize the complementary capabilities of XMM-Newton and Chandra toconduct a detailed imaging and spectral study of the nearby galaxy NGC4945 focusing on its nucleus and immediate surroundings (within ~ kpc ofthe nucleus). A complex morphology is revealed including a predominantlyhard, but partially resolved, nuclear source plus a spectrally soft,conically shaped X-ray `plume', which extends 30 arcsec (500 pc) to thenorth-west. In NGC 4945 our direct view of the active galactic nucleus(AGN) is blocked below ~10 keV by extremely heavy line-of-sightabsorption, and the observed X-ray spectrum is dominated bymulti-temperature thermal emission associated with the nuclear starburstand the X-ray plume. Nevertheless the signature of the AGN is present inthe form of a neutral Compton reflection component and a 6.4 keVfluorescent iron Kα line. We conjecture that the site of thecontinuum reprocessing is the far wall of a highly inclined moleculartorus, a geometry which is consistent with the presence ofH2O megamaser emission in this source. The soft spectrum(~0.6 keV) and limb-brightened appearance of the X-ray plume suggest aninterpretation in terms of a mass-loaded superwind emanating from thenuclear starburst.
| 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.
| Seyfert 2 Galaxies with Spectropolarimetric Observations
We present a compilation of radio, infrared, optical, and hard X-ray(2-10 keV) data for a sample of 90 Seyfert 2 galaxies (Sy2s) withspectropolarimetric observations (41 Sy2s with detection of polarizedbroad lines [PBLs] and 49 without PBLs). Compared to Sy2s without PBLs,Sy2s with PBLs tend to be earlier type spirals and show warmermidinfrared color and significant excess of emissions (including thehard X-ray [2-10 keV], [O III] λ5007, infrared [25 μm], andradio). Our analyses indicate that the majority of Sy2s without PBLs arethose sources having less powerful active galactic nucleus (AGN)activities, most likely caused by a low accretion rate. It implies thatthe detectability of the polarized broad emission lines in Sy2s maydepend on their central AGN activities in most cases. Based on theavailable data, we find no compelling evidence for the presence of twotypes of Sy2s; one of which has been proposed to be intrinsicallydifferent from Sy2s claimed in the unification model.
| A Search for H2O Maser Emission in Southern Active Galactic Nuclei and Star-forming Galaxies: Discovery of a Maser in the Edge-on Galaxy IRAS F01063-8034
We report the cumulative results of five surveys for H2Omaser emission at 1.35 cm wavelength in 131 active galactic nuclei(AGNs) and star-forming galaxies, conducted at the Parkes Observatorybetween 1993 and 1998. We detected one new maser, in the edge-on galaxyIRAS F01063-8034, which exhibits a single ~0.1 Jy spectral feature at4282+/-6 km s-1 (heliocentric) with an unusually large54+/-16 km s-1 half-power full width. The centroid velocityof the emission increased to 4319.6+/-0.6 km s-1 (38+/-2 kms-1 width) over the 13 days between discovery andconfirmation of the detection. A similarly broad-line width and largechange in velocity has been noted for the maser in NGC 1052, wherein jetactivity excites the emission. Neither optical spectroscopy,radio-infrared correlations, nor infrared colors provide compellingevidence of unusual activity in the nucleus of IRAS F01063-8034. Sincethe galaxy appears to be outwardly normal at optical and infraredwavelengths, detection of an H2O maser therein is unique. Themaser emission is evidence that the galaxy harbors an AGN that isprobably obscured by the edge-on galactic disk. The detection highlightsthe possibility that undetected AGNs could be hidden in other relativelynearby galaxies. No other maser emission features have been identifiedat velocities between 3084 and 6181 km s-1.
| Position-velocity diagrams of ionized gas in the inner regions of disk galaxies
We use long-slit spectroscopy along the major axis of a sample of 23nearby disk galaxies to study the kinematic properties of theionized-gas component in their inner regions. For each galaxy, we derivethe position-velocity diagram of the ionized gas from its emissionlines. We discuss the variety of shapes observed in suchposition-velocity diagrams by comparing the gas velocity gradient,velocity dispersion and integrated flux measured in the inner (r =~+/-1'') and outer regions (r =~ +/-4''). This kind of analysis allowsthe identification of galaxies which are good candidates to host acircumnuclear Keplerian gaseous disk rotating around a central massconcentration, and to follow up with Hubble Space Telescopeobservations. Based on observations carried out at European SouthernObservatory (ESO N.58, A-0564), at the Multiple Mirror Telescope, whichis a joint facility of the Smithsonian Institution and the University ofArizona, and at the Isaac Newton Telescope operated by the Isaac Newtongroup at the La Palma island at the Spanish Observatorio del Roque delos Muchachos of the Instituto de Astrofisica de Canarias.
| Disclosing the true nature of the Sy 2 galaxy NGC 3281: One more Compton-thick source
We present the BeppoSAX broad-band X-ray spectrum of the Seyfert 2galaxy NGC 3281. The source high-energy spectrum is characterized by thenuclear transmitted component, with an absorbing column density of~2x1024 cm-2, while the MECS spectrum isreflection-dominated, with a prominent (EW~0.5-1.2 keV) iron Kαemission line. The source is detected at only the 5sigma significancelevel in the LECS band, because of the strong obscuration which hampersat low energies the direct view of the active nucleus harbored in NGC3281. BeppoSAX results are consistent with the scenario where NGC 3281is inclined more than 60deg with respect to theline-of-sight. Combining the NH value obtained from thepresent X-ray analysis with the AV measurement, aNH/AV about 50 times the Galactic value isderived.
| High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - II. The properties of compact radio components
We discuss the properties of compact nuclear radio components in Seyfertgalaxies from the extended 12-μm AGN sample of Rush et al. Our mainresults can be summarized as follows. Type 1 and type 2 Seyferts producecompact radio components which are indistinguishable in strength andaspect, indicating that their central engines are alike, as proposed bythe unification model. Infrared IRAS fluxes are more closely correlatedwith low-resolution radio fluxes than high-resolution radio fluxes,suggesting that they are dominated by kiloparsec-scale, extranuclearemission regions; extranuclear emission may be stronger in type 2Seyferts. Early-type Seyfert galaxies tend to have stronger nuclearradio emission than late-type Seyfert galaxies. V-shaped extendedemission-line regions, indicative of `ionization cones', are usuallyfound in sources with large, collimated radio outflows. Hidden broadlines are most likely to be found in sources with powerful nuclear radiosources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm fluxdensities have well-matched properties.
| 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.).
| N-Band Imaging of Seyfert Nuclei and the Mid-Infrared-X-Ray Correlation
We present new mid-infrared (N-band) images of a sample of eight nearbySeyfert galaxies. In all of our targets, we detect a central unresolvedsource, which in some cases has been identified for the first time. Inparticular, we have detected the mid-infrared emission from the activenucleus of NGC 4945, which previously remained undetected at anywavelength but hard X-rays. We also detect circumnuclear extendedemission in the Circinus galaxy along its major axis and find marginalevidence for extended circumnuclear emission in NGC 3281. The highspatial resolution (1.7") of our data allows us to separate the flux ofthe nuclear point sources from the extended circumnuclear starburst (ifpresent). We complement our sample with literature data for a number ofnonactive starburst galaxies and relate the nuclear N-band flux topublished hard (2-10 keV) X-ray fluxes. We find tight and well-separatedcorrelations between the nuclear N-band flux and X-ray flux for bothSeyfert and starburst nuclei that span over 3 orders of magnitude inluminosity. We demonstrate that these correlations can be used as apowerful classification tool for galactic nuclei. For example, we findstrong evidence against NGC 1808 currently harboring an active Seyfertnucleus based on its position in the mid-infrared-X-ray diagram. On theother hand, we confirm that NGC 4945 is in fact a Seyfert 2 galaxy.
| A Composite Seyfert 2 X-Ray Spectrum: Implications for the Origin of the Cosmic X-Ray Background
We present a composite 1-10 keV Seyfert 2 X-ray spectrum derived fromASCA observations of a distance-limited sample of nearby galaxies. All29 observed objects were detected. Above ~3 keV, the composite spectrumis inverted, confirming that Seyfert 2 galaxies as a class have thespectral properties necessary to explain the flat shape of the cosmicX-ray background spectrum. Integrating the composite spectrum overredshift, we find that the total emission from Seyfert 2 galaxies,combined with the expected contribution from unabsorbed type 1 objects,provides an excellent match to the spectrum and intensity of the hardX-ray background. The principal uncertainty in this procedure is thecosmic evolution of the Seyfert 2 X-ray luminosity function. Separatecomposite spectra for objects in our sample with and without polarizedbroad optical emission lines are also presented.
| 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.
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