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Lifetime of nuclear velocity dispersion drops in barred galaxies
We have made hydro/N-body simulations with and without star formation toshed some light on the conditions under which a central kinematicallycold stellar component (characterized by a velocity dispersion drop orσ-drop) could be created in a hot medium (e.g. a bulge) andsurvive enough time to be observed. We found that the time-scale for aσ-drop formation could be short (less than 500 Myr), whereas itslifetime could be long (more than 1 Gyr) provided that the centralregion is continuously or regularly fed by fresh gas which leads to acontinuous star formation activity. Star formation in the centralregion, even at a low rate as 1Msolaryr-1, ismandatory to sustain a permanent σ-drop by replacing heatedparticles by new low-σ ones. We moreover show that as soon as starformation is switched off, the σ-drop begins to disappear.
| A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
Seyfert galaxies commonly host compact jets spanning 10-100 pc scales,but larger structures are resolved out in long-baseline aperturesynthesis surveys. Previous, targeted studies showed thatkiloparsec-scale radio structures (KSRs) may be a common feature ofSeyfert and LINER galaxies, and the origin of KSRs may be starbursts oractive galactic nuclei (AGNs). We report a new Very Large Array surveyof a complete sample of Seyfert and LINER galaxies. Out of all of thesurveyed radio-quiet sources, we find that 44% (19 out of 43) showextended radio structures at least 1 kpc in total extent that do notmatch the morphology of the disk or its associated star-forming regions.The detection rate is a lower limit owing to the combined effects ofprojection and resolution. The infrared colors of the KSR host galaxiesare unremarkable compared to other Seyfert galaxies, and the large-scaleoutflows orient randomly with respect to the host galaxy axes. The KSRSeyfert galaxies instead stand out by deviating significantly from thefar-infrared-radio correlation for star-forming galaxies, with tendencytoward radio excess, and they are more likely to have a relativelyluminous, compact radio source in the nucleus; these results argue thatKSRs are powered by the AGNs rather than starbursts. The high detectionrate indicates that Seyfert galaxies generate radio outflows over asignificant fraction of their lifetime, which is much longer than thedynamical timescale of an AGN-powered jet but is comparable instead tothe buoyancy timescale. The likely explanation is that the KSRsoriginate from jet plasma that has been decelerated by interaction withthe nuclear interstellar medium (ISM). Based on a simple ram pressureargument, the kinetic power of the jet on kiloparsec scales is about 3orders of magnitude weaker than the power of the jet on 10-100 pcscales. This result is consistent with the interaction model, in whichcase virtually all of the jet power must be lost to the ISM within theinner kiloparsec.
| The Host Galaxies of Narrow-Line Seyfert 1 Galaxies: Nuclear Dust Morphology and Starburst Rings
We present a study of the nuclear morphology of a sample of narrow- andbroad-line Seyfert 1 galaxies (NLS1s and BLS1s, respectively) based onbroadband images in the Hubble Space Telescope archives. In our previousstudy we found that large-scale stellar bars at >1 kpc from thenucleus are more common in NLS1s than BLS1s. In this paper we find thatNLS1s preferentially have grand-design dust spirals within ~1 kpc oftheir centers. We also find that NLS1s have a higher fraction of nuclearstar-forming rings than BLS1s. We find that many of the morphologicaldifferences are due to the presence or absence of a large-scale stellarbar within the spiral host galaxy. In general, barred Seyfert 1 galaxiestend to have grand-design dust spirals at their centers, confirming theresults of other researchers. The high fraction of grand-design nucleardust spirals and stellar nuclear rings observed in NLS1s' host galaxiessuggests a means for efficient fueling of their nuclei to support theirhigh Eddington ratios.
| Low-Luminosity Active Galaxies and Their Central Black Holes
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
| On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.
| The radio structure of radio-quiet quasars
Aims.We investigate the radio emitting structures of radio-quiet activegalactic nuclei with an emphasis on radio-quiet quasars to study theirconnection to Seyfert galaxies. Methods: .We present and analysehigh-sensitivity VLA radio continuum images of 14 radio-quiet quasarsand six Seyfert galaxies. Results: .Many of the low redshiftradio-quiet quasars show radio structures that can be interpreted asjet-like outflows. However, the detection rate of extended radiostructures on arcsecond scales among our sample decreases withincreasing redshift and luminosity, most likely due to a lack ofresolution. The morphologies of the detected radio emission indicatestrong interactions of the jets with the surrounding medium. We alsocompare the radio data of seven quasars with corresponding HST images ofthe [O III] emitting narrow-line region (NLR). We find that the scenarioof interaction between the radio jet and the NLR gas is confirmed in twosources by structures in the NLR gas distribution as previously knownfor Seyfert galaxies. The extended radio structures of radio-quietquasars at sub-arcsecond resolution are by no means different from thatof Seyferts. Among the luminosities studied here, the morphologicalfeatures found are similar in both types of objects while the overallsize of the radio structures increases with luminosity. This supportsthe picture where radio-quiet quasars are the scaled-up versions ofSeyfert galaxies. In addition to known luminosity relations we find acorrelation of the NLR size and the radio size shared by quasars andSeyferts.
| 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.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| 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.
| Mega-Masers and Galaxies
In the Galaxy, microwave radiation can be amplified in the interstellarmedium in the immediate neighborhood of young stellar objects, orcircumstellar envelopes around evolved stars, resulting in cosmic maseremission. Cosmic masers exist because, in contrast to terrestrialconditions, the interstellar gas density is very low so that levelpopulation in molecules is typically not in thermal equilibrium, andsometimes inverted. In the nuclear regions of external galaxies, thereexist very powerful OH ( 18 cm) and H2O ( 1.35 cm) cosmicmasers with line luminosities of 102 104Lȯ, 106 times more luminous than typicalGalactic maser sources. These are the "mega-masers," found inhigh-density molecular gas located within parsecs of active galacticnuclei in the case of H2O mega-masers, or within the central100 pc of nuclear star-burst regions in the case of OH mega-masers.H2O mega-masers are most frequently found in galactic nucleiwith Seyfert2 or LINER spectral characteristics, in spiral and someelliptical galaxies. OH mega-masers are found in ultra-luminous IRgalaxies (ULIRG) with the warmest IR colors, and importantly, the OHluminosity is observed to increase with the IR luminosity:LOH L1.2IR. Because of the extremelyhigh-surface brightness, H2O mega-maser emission can bemapped at sub-milli-arc-second resolution by Very Long BaselineInterferometry (VLBI), providing a powerful tool to probe spatial andkinematic distributions of molecular gas in distant galactic nuclei atscales below one parsec. An excellent example is the active galaxy, NGC4258, in which mapping of the H2O mega-maser emission hasprovided the first direct evidence in an active galactic nucleus for theexistence of a thin Keplerian accretion disk with turbulence, as well ashighly compelling evidence for the existence of a massive black hole.The NGC 4258 mega-maser has also provided a geometric distancedetermination of extremely high precision. H2O mega-maseremission is also found to arise from postshocked gas from the impact ofnuclear jets or outflows on the surrounding molecular clouds.High-resolution observations have shown that OH mega-masers originatefrom the molecular gas medium in 100-pc scale nuclear star-burstregions. It is proposed that such extreme star-burst regions, withextensive high-density gas bathed in a very high far-IR radiation field,are conducive to the formation of a very large number of OH masersources that collectively produce the OH mega-maser emission. In theearly Universe, galaxies or mergers could go through a very luminousphase, powered by intensive star-bursts and AGN formation, and couldhave extremely large OH and H2O maser luminosities, possiblyproducing giga-masers. With the increasing sensitivity of new telescopesand receivers, surveys and high-resolution studies of mega-masers andgiga-masers will be very important tracers and high-resolution probes ofactive galactic nuclei, dust embedded star-bursts in the earliestgalaxies and galaxy mergers in the epoch of very active star formationat z 2 and beyond. Distance determination of giga-masers at z 1 2can provide on independent measure of how fast the universe isexpanding.
| On the Relation between Circular Velocity and Central Velocity Dispersion in High and Low Surface Brightness Galaxies
In order to investigate the correlation between the circular velocityVc and the central velocity dispersion of the spheroidalcomponent σc, we analyzed these quantities for a sampleof 40 high surface brightness (HSB) disk galaxies, eight giant lowsurface brightness (LSB) spiral galaxies, and 24 elliptical galaxiescharacterized by flat rotation curves. Galaxies have been selected tohave a velocity gradient <=2 km s-1 kpc-1 forR>=0.35R25. We used these data to better define theprevious Vc-σc correlation for spiralgalaxies (which turned out to be HSB) and elliptical galaxies,especially at the lower end of the σc values. We findthat the Vc-σc relation is described by alinear law out to velocity dispersions as low as σc~50km s-1, while in previous works a power law was adopted forgalaxies with σc>80 km s-1. Ellipticalgalaxies with Vc based on dynamical models or directlyderived from the H I rotation curves follow the same relation as the HSBgalaxies in the Vc-σc plane. On the otherhand, the LSB galaxies follow a different relation, since most of themshow either higher Vc or lower σc withrespect to the HSB galaxies. This argues against the relevance of baryoncollapse to the radial density profile of the dark matter halos of LSBgalaxies. Moreover, if the Vc-σc relation isequivalent to one between the mass of the dark matter halo and that ofthe supermassive black hole, then these results suggest that the LSBgalaxies host a supermassive black hole (SMBH) with a smaller masscompared to HSB galaxies with an equal dark matter halo. On the otherhand, if the fundamental correlation of SMBH mass is with the halocircular velocity, then LSB galaxies should have larger black holemasses for a given bulge dispersion. Elliptical galaxies withVc derived from H I data and LSB galaxies were not consideredin previous studies.Based on observations made with European Southern Observatory telescopesat the Paranal Observatory under programs 67.B-0283, 69.B-0573, and70.B-0171.
| The Size of the Radio-Emitting Region in Low-Luminosity Active Galactic Nuclei
We have used the VLA to study radio variability among a sample of 18low-luminosity active galactic nuclei (LLAGNs) on timescales of a fewhours to 10 days. The goal was to measure or limit the sizes of theLLAGN radio-emitting regions in order to use the size measurements asinput to models of the radio emission mechanisms in LLAGNs. We detectvariability on typical timescales of a few days at a confidence level of99% in half the target galaxies. Either variability that is intrinsic tothe radio-emitting regions or that is caused by scintillation in theGalactic interstellar medium is consistent with the data. For eitherinterpretation, the brightness temperature of the emission is below theinverse Compton limit for all our LLAGNs and has a mean value of about1010 K. The variability measurements plus VLBI upper limitsimply that the typical angular size of the LLAGN radio cores at 8.5 GHzis 0.2 mas, plus or minus a factor of 2. The ~1010 Kbrightness temperature strongly suggests that a population ofhigh-energy nonthermal electrons must be present, in addition to ahypothesized thermal population in an accretion flow, in order toproduce the observed radio emission.
| The Discovery of a High-Redshift X-Ray-Emitting QSO Very Close to the Nucleus of NGC 7319
A strong X-ray source only 8" from the nucleus of the Seyfert 2 galaxyNGC 7319 in Stephan's Quintet has been discovered by Chandra. We haveidentified the optical counterpart and show that it is a QSO withze=2.114. It is also an ultraluminous X-ray source withLX=1.5×1040 ergs s-1. From theoptical spectra of the QSO and the interstellar gas of NGC 7319together, we show that it is very likely that the QSO is interactingwith the interstellar gas.
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| Water-Vapor Maser Disk at the Nucleus of the Seyfert 2 Galaxy NGC 3079
We observed a Seyfert 2/LINER galaxy NGC 3079 using the NRO 45-mtelescope from 1992 to 2002, and the Very Long Baseline Array (VLBA) in2001 December, at 22GHz. The 45-m telescope detected weak red-shiftedand systemic features as well as strong blue-shifted features. Highspatial resolution observations with the VLBA indicated that theH2O maser spots are distributed along the north-southdirection, showing blue-shifted velocities in the north and red-shiftedin the south. We propose a nearly edge-on molecular gas disk in thenuclear region of NGC 3079, whose radius varies from 0.4 to 1.2pc andwhose thickness is 0.3pc. The disk is rotating at Vrot = 200100 km s-1, having a large velocity dispersion of ΔV≍ 50 km s-1. The binding mass within the disk is 106 Msolar. The disk is inclined such that theeast side is nearest to us. The large luminosity of the maser emissionand exceptionally strong blue-shifted features of NGC 3079 are probablythe result of local shocks in the disk. Three continuum components areinterpreted as jets emitted from the nucleus. The velocities of thecomponents relative to the dynamical center of the masing disk are (0.090.17) c, < 0.005 c, and < 0.05 c.
| QSOs and Active Galactic Nuclei Associated with NGC 2639
NGC 2639 is a bright, active LINER galaxy that has previously been foundto have a pair of bright optical, X-ray quasars paired across it(z=0.305 and 0.323). In addition, PSPC X-ray observations reveal a lineof seven X-ray sources lying along the northeast minor axis. Four ofthese have now been measured spectroscopically and are reported here tobe QSO/active galactic nuclei with redshifts ranging between z=0.337 andz=2.63. FIRST radio sources are found to be aligned along this sameminor axis, some of which are coincident with the optical objects.Redshifts of all of the known QSOs in the field lie close to thewell-known periodicity peaks.
| Comparison of Nuclear Starburst Luminosities between Seyfert 1 and 2 Galaxies Based on Near-Infrared Spectroscopy
We report on infrared K- (2-2.5 μm) and L-band (2.8-4.1 μm) slitspectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 μm samples. Apolycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 μm inthe L band is primarily used to investigate nuclear star-formingactivity in these galaxies. The 3.3 μm PAH emission is detected in 10sources (=43%), demonstrating that detection of nuclear star formationin a significant fraction of Seyfert 1 galaxies is now feasible. For thePAH-detected nuclei, the surface brightness values of the PAH emissionare as high as those of typical starbursts, suggesting that the PAHemission probes the putative nuclear starbursts in the dusty tori aroundthe central active galactic nuclei (AGNs). The magnitudes of the nuclearstarbursts are quantitatively estimated from the observed 3.3 μm PAHemission luminosities. The estimated starburst luminosities relative tosome indicators of AGN powers in these Seyfert 1 galaxies are comparedwith 32 Seyfert 2 galaxies in the same samples that we have previouslyobserved. We find that there is no significant difference in nuclearstarburst to AGN luminosity ratios of Seyfert 1 and 2 galaxies and thatnuclear starburst luminosity positively correlates with AGN power inboth types. Our results favor a slightly modified AGN unification model,which predicts that nuclear starbursts occurring in the dusty tori ofSeyfert galaxies are physically connected to the central AGNs, ratherthan the classical unification paradigm, in which the dusty tori simplyhide the central AGNs of Seyfert 2 galaxies and reprocess AGN radiationas infrared dust emission in Seyfert galaxies. No significantdifferences in nuclear star formation properties are recognizablebetween Seyfert 1 galaxies in the CfA and 12 μm samples.
| 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.
| 10 Micron Imaging of Seyfert Galaxies from the 12 Micron Sample
We present small-aperture (1.5") photometry and new high-resolutionimages at 10 μm (N band) for 87 Seyfert galaxies from the Extended 12μm Sample drawn from the IRAS database. With this data we hope totest the predictions of the unified model for active galactic nuclei andto search for bright, extended circumnuclear 10 μm emission. Wedetected 62 Seyfert galaxies, 18 of which have no previously publishedsmall-aperture photometry. All the detected sources, both Seyfert 1'sand Seyfert 2's, show a central point source. The 31 Seyfert 1's and 31Seyfert 2's that we detected have similar luminosity distributions.Except for previously known bright extended 10 μm structure aroundArp 220, NGC 1068, and NGC 7469, we see definitive evidence for brightextended emission around only one new object: Mrk 1239. Four otherSeyfert 1's and six other Seyfert 2's show evidence of faint, low-levelextended emission. One Seyfert 1 and two Seyfert 2's show evidence ofsignificantly increased flux over previously published small-aperturevalues. We also compared the N-band data with the J-Ks colorthat we derived from the Two Micron All Sky Survey (2MASS). There is adistinct trend of redder central bulge J-Ks colorscorresponding to brighter absolute N-band magnitudes. In color-magnitudespace there is a definite grouping of Seyfert 1's and Seyfert 2's, withtwo sets of outliers.
| Stacking Searches for Gamma-Ray Emission above 100 MeV from Radio and Seyfert Galaxies
The EGRET telescope on board Compton Gamma Ray Observatory detected morethan 60 sources of high-energy gamma radiation associated with activegalactic nuclei (AGNs). All but one of those belong to the blazarsubclass; the only exception is the nearby radio galaxy Centaurus A.Since there is no obvious reason other than proximity to expect Cen A tobe the only nonblazar AGN emitting in high-energy gamma rays, we haveutilized the ``stacking'' technique to search for emission above 100 MeVfrom two nonblazar AGN subclasses, radio galaxies and Seyfert galaxies.Maps of gamma-ray counts, exposure, and diffuse background have beencreated, then co-added in varying numbers based on sorts by redshift, 5GHz flux density, and optical brightness, and finally tested forgamma-ray emission. No detection significance greater than 2 σ hasbeen found for any subclass, sorting parameter, or number of objectsco-added. Monte Carlo simulations have also been performed to validatethe technique and estimate the significance of the results.
| Milliarcsec-scale radio structure of a matched sample of Seyfert 1 and Seyfert 2 galaxies
We have obtained mas-scale resolution very long baseline interferometry({VLBI}) images of a sample of Seyfert 1 and Seyfert 2 galaxies at 5 GHz(wavelength, λ = 6 cm). The Seyferts of the two types were chosento be matched in several orientation-independent properties, primarilyin order to rigorously test predictions of the unified scheme. Wedetected all the 15 objects that we observed. In this paper we describethe observations and data reduction procedures, and present the {VLBI}radio images as well as simultaneous Very Large Array images that weobtained for these 15 Seyferts.Full Fig. \ref{maps1} and Tables \ref{vla_param}, \ref{vlbi_param} areonly available in electronic form at http://www.edpsciences.orgTables \ref{vlbi_res} and \ref{vla_res} are also available in electronicform the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/425/99
| New optical spectra and general discussion on the nature of ULXs
We present optical spectroscopic observations of three Ultra LuminousX-ray sources (ULXs). Two of them are very close to the active galaxyNGC 720 and the other is near NGC 1073. The two around NGC 720 turn outto be quasars at z= 2.216 and z= 0.959, the one near NGC 1073 seems tobe associated to an HII region at the redshift of NGC 1073. Weconcentrate our analysis on the two quasars and analyze them inconjunction with a set of 22 additional X-ray sources close to nearbygalaxies which also fit the criteria of ULXs and which also have beenidentified as quasars of medium to high redshift. This sample shows anunusually large fraction of rare BL Lac type objects. The high redshiftsof these ULXs and their close proximity to their low redshift,supposedly parent galaxies is a surprising result in the light ofstandard models. We describe the main properties of each of theseobjects and their parent galaxy, and briefly discuss possibleinterpretations.
| Motion and properties of nuclear radio components in Seyfert galaxies seen with VLBI
We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm ofthe Seyfert galaxies NGC 7674, NGC5506, NGC 2110 and Mrk1210 to study their structure and proper motions on pc scalesand to add some constraints on the many possible causes of theradio-quietness of Seyferts. The component configurations inNGC 7674 and NGC 2110 are simple,linear structures, whereas the configurations in NGC5506 and Mrk 1210 have multiple componentswith no clear axis of symmetry. We suggest that NGC7674 is a low-luminosity compact symmetric object. Comparingthe images at different epochs, we find a proper motion in NGC7674 of (0.92±0.07) c between the two centralcomponents separated by 282 pc and, in NGC 5506, wefind a 3 σ upper limit of 0.50 c for the components separated by3.8 pc. Our results confirm and extend earlier work showing that theoutward motion of radio components in Seyfert galaxies isnon-relativistic on pc scales. We briefly discuss whether thisnon-relativistic motion is intrinsic to the jet-formation process orresults from deceleration of an initially relativistic jet byinteraction with the pc or sub-pc scale interstellar medium. We combinedour sample with a list compiled from the literature of VLBI observationsmade of Seyfert galaxies, and found that most Seyfert nuclei have atleast one flat-spectrum component on the VLBI scale, which was not seenin the spectral indices measured at arcsec resolution. We found alsothat the bimodal alignment of pc and kpc radio structures displayed byradio galaxies and quasars is not displayed by this sample of Seyferts,which shows a uniform distribution of misalignment between0° and 90°. The frequent misalignmentcould result from jet precession or from deflection of the jet byinteraction with gas in the interstellar medium.
| Long slit spectroscopy of a sample of isolated spirals with and without an AGN
We present the kinematical data obtained for a sample of active(Seyfert) and non active isolated spiral galaxies, based on long slitspectra along several position angles in the Hα line region and,in some cases, in the Ca triplet region as well. Gas velocitydistributions are presented, together with a simple circular rotationmodel that allows us to determine the kinematical major axes. Stellarvelocity distributions are also shown. The main result is that activeand control galaxies seem to be equivalent in all kinematical aspects.For both subsamples, the departure from pure circular rotation in somegalaxies can be explained by the presence of a bar and/or of a spiralarm. They also present the same kind of peculiarities, in particular,S-shape structures are quite common near the nuclear regions. Theydefine very similar Tully-Fisher relations. Emission line ratios aregiven for all the detected HII regions; the analysis of the[NII]/Hα metallicity indicator shows that active and non-activegalaxies have indistinguishable disk metallicities. These results arguein favour of active and non-active isolated spiral galaxies havingessentially the same properties, in agreement with our previous resultsbased on the analysis of near infrared images. It appears now necessaryto confirm these results on a larger sample.Based on observations made with WHT operated on the island of La Palmaby ING in the Spanish Observatorio del Roque de Los Muchachos of theInstituto de Astrofísica de Canarias, the European SouthernObservatory (La Silla), Calar Alto Observatory (Almería, Spain)and Las Campanas Observatories (Chile).Table 3 and Figs. \ref{res_cen_u1395}, 5, 7, 9, 11, 13, 15, 17, 19, 21,23, 25, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50 and 52 are onlyavailable in electronic form at http://www.edpsciences.orgTable 5 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/475
| Radio emission from AGN detected by the VLA FIRST survey
Using the most recent (April 2003) version of the VLA FIRST survey radiocatalog, we have searched for radio emission from >2800 AGN takenfrom the most recent (2001) version of the Veron-Cetty and Veron AGNcatalog. These AGN lie in the 9033 square degrees of sky alreadycovered by the VLA FIRST survey. Our work has resulted in positivedetection of radio emission from 775 AGN of which 214 are new detectionsat radio wavelengths.Tables 3 and 4 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/416/35
| A Survey for H2O Megamasers. III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalacticwater megamasers taken over a period of 9 years and a single epoch ofsensitive spectra for seven others. The primary motivation is a searchfor drifting line velocities analogous to those of the systemic featuresin NGC 4258, which are known to result from centripetal acceleration ofgas in an edge-on, subparsec molecular disk. We detect a velocity driftanalogous to that in NGC 4258 in only one source, NGC 2639. Another, themaser source in NGC 1052, exhibits erratic changes in its broad maserprofile over time. Narrow maser features in all of the other diskgalaxies discussed here either remain essentially constant in velocityover the monitoring period or are sufficiently weak or variable inintensity that individual features cannot be traced reliably from oneepoch to the next. In the context of a circumnuclear, molecular diskmodel, our results suggest that either (a) the maser lines seen aresystemic features subject to a much smaller acceleration than present inNGC 4258, presumably because the gas is farther from the nuclear blackhole, or (b) we are detecting ``satellite'' lines for which theacceleration is in the plane of the sky.Our data include the first K-band science observations taken with thenew 100 m Green Bank Telescope (GBT). The GBT data were taken duringtesting and commissioning of several new components and so are subjectto some limitations; nevertheless, they are in most cases the mostsensitive H2O spectra ever taken for each source and cover800 MHz (~=10,800 km s-1) of bandwidth. Many new maserfeatures are detected in these observations. Our data also include atentative and a clear detection of the megamaser in NGC 6240 at epochs ayear and a few months, respectively, prior to the detections reported byHagiwara et al. and Nakai et al.We also report a search for water vapor masers toward the nuclei of 58highly inclined (i>80deg), nearby galaxies. These sourceswere selected to investigate the tendency that H2O megamasersfavor inclined galaxies. None were detected, confirming that megamasersare associated exclusively with active galactic nuclei.
| QSOs Associated with M82
The starburst/AGN galaxy M82 was studied by Dahlem, Weaver, and Heckmanusing X-ray data from ROSAT and ASCA as part of their X-ray survey ofedge-on starburst galaxies. They found 17 unresolved hard X-ray sourcesaround M82, in addition to its strong nuclear source, and other X-rayswithin the main body of M82. We have measured optical point sources atthese positions and have obtained redshifts of six candidates at theKeck I 10 m telescope, using the low-resolution imaging spectrograph(LRIS). All six are highly compact optical and X-ray objects withredshifts ranging from 0.111 to 1.086. They all show emission lines. Thethree with the highest redshifts are clearly QSOs. The others with lowerredshifts may be either QSOs or compact emission-line galaxies. Inaddition to these six, there are nine QSOs lying very close to M82 whichwere discovered many years ago. There is no difference between opticalspectra of these latter QSOs, only two of which are known to be X-raysources, and the X-ray-emitting QSOs. The redshifts of all 15 rangebetween 0.111 and 2.05. The large number of QSOs and their apparentassociation with ejected matter from M82 suggest that they arephysically associated with the galaxy and have large intrinsic redshiftcomponents. If this is correct, the absolute magnitudes lie in the range-8
| On the Nature of Low-Luminosity Narrow-Line Active Galactic Nuclei
There is clear observational evidence that some narrow-line (type 2)active galactic nuclei (AGNs) have a hidden broad-line region (BLR) andare thus intrinsically broad-line (type 1) AGNs. Does this AGNunification apply for all type 2 AGNs? Indirect arguments suggest thatsome ``true'' type 2 AGNs, i.e., AGNs having no obscured BLR, do exist,but it is not clear why the BLR is missing in these AGNs. Here we pointout a possible natural explanation. The observed radius-luminosityrelation for the BLR implies an increasing line width with decreasingluminosity for a given black hole mass (MBH). In addition,there appears to be an upper limit to the observed width of broademission lines in AGNs of Δvmax~25,000 kms-1, which may reflect a physical limit above which the BLRmay not be able to survive. Thus, at a low enough luminosity the BLRradius shrinks below the Δvmax radius, leaving noregion where the BLR can exist, although the AGN may remain otherwise``normal.'' The implied minimum bolometric luminosity required tosustain a BLR with Δv<25,000 km s-1 isLmin~1041.8(MBH/108Msolar)2.All AGNs with L
| NGC 6212, 3C 345, and Other Quasi-stellar Objects Associated with Them
It is pointed out that the Seyfert 1 galaxy NGC 6212 is surrounded by alarge number of quasi-stellar objects (QSOs), including the very activeand rapidly varying radio QSO 3C 345, which lies only 4.7′ awayfrom the center of the galaxy. There are two pairs of QSOs with the sameredshifts, including 3C 345, very close to the nucleus of NGC 6212, andthe very high density of the QSOs falls off rapidly with distance fromthat galaxy. If 3C 345 lies at the distance of NGC 6212, the propermotions in the radio jet in 3C 345, which have been studied in detail,correspond to only mildly relativistic speeds, of about 0.33c.
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