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An Internet Database of Ultraviolet Continuum Light Curves for Seyfert Galaxies
Using the Multimission Archive at STScI (MAST), we have extractedspectra and determined continuum light curves for 175 Seyfert galaxiesthat have been observed with the International Ultraviolet Explorer andthe Faint Object Spectrograph on the Hubble Space Telescope. To obtainthe light curves as a function of Julian Date, we used fixed bins in theobject's rest frame and measured small regions (between 30 and 60Å) of each spectrum's continuum flux in the range 1150 to 3200Å. We provide access to the UV light curves and other basicinformation about the observations in tabular and graphical form via theInternet at http://www.chara.gsu.edu/PEGA/IUE.
| Flat-spectrum symmetric objects with ~1kpc sizes - I. The candidates
In order to understand the origin and evolution of radio galaxies,searches for the youngest such sources have been conducted. Compactsymmetric objects (CSOs)/medium symmetric objects (MSOs) are thought tobe the earliest stages of radio sources, with possible ages of<~103yr for CSOs (<1kpc in size) and104-105yr for MSOs (1-15kpc). From a literatureselection in heterogeneous surveys, we have established a sample of 37confirmed CSOs. In addition, we only found three confirmed flat-spectrumMSOs in the literature. The typical CSO resides on a z <~ 0.5 galaxy,has a flat radio spectrum (αthin < 0.5;Sν ~ ν-α), is <0.3kpc in size, hasan arm length ratio <= 2, and well-aligned (θ <= 20°)opposite lobes with a flux density ratio <=10. In order to populatethe 0.3-1kpc size range (large CSOs) and also in order to find moreflat-spectrum MSOs, we have built a sample of 157 radio sources withα4.851.40 < 0.5 that were resolved withthe Very Large Array A configuration (VLA-A) 8.4GHz. As first results,we have `rediscovered' nine of the known CSO/MSOs while identifying twonew ~14kpc MSOs and two candidate CSO/MSOs (which only lack redshiftsfor final classification). We were able to reject 61 of the remaining144 objects from literature information alone. In the series of papersthat starts with this one we plan to classify the remaining 83 CSO/MSOcandidates (thanks to radio and optical observations) as well ascharacterize the physical properties of the (likely) many 0.3-15kpcflat-spectrum CSO/MSOs to be found.
| Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| 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.
| Variability Study of Seyfert 2 Galaxies with XMM-Newton
We present the results of timing analysis of XMM-Newton observations ofSeyfert 2 galaxies in order to search for differences in the meanproperties of Seyfert 1 galaxies and Seyfert 2 galaxies. We selected 13Seyfert 2 galaxies from the XMM-Newton archive that have hard X-raycomponents in their spectra and calculated the excess variance(σ2rms) in the 2-10 keV band. We found thatsix Seyfert 2 galaxies (3C 98, IRAS 05189-2524, MCG -5-23-16, NGC 6300,UGC 4203, and PKS 1814-637) have buried luminous nuclei and that thenuclei have timing properties similar to those of Seyfert 1 nuclei. Thisindicates that these galaxies are candidates for having buried Seyfert 1nuclei as expected by the unified Seyfert model. The first five galaxiesshow significant time variability. The amplitude of the time variabilityof IRAS 05189-2524 is similar to that of narrow-line Seyfert 1 galaxies.In contrast, the amplitude of variability of the seven other galaxies isquite small, much smaller than that of Seyfert 1 galaxies with similarX-ray luminosity. The lack of short time variability in these objects isexplained by the dominance of the reflection component in three galaxies(Mrk 3, Mrk 463, and NGC 7582), and by the presence of very massiveblack holes and an inferred low accretion rate in the other threegalaxies (NGC 1052, NGC 4507, and NGC 7172). For Mrk 348, thesignificant time variability that is expected based on the estimate ofthe central black hole mass was not detected.
| Spatially Resolved Narrow-Line Region Kinematics in Active Galactic Nuclei
We have analyzed Hubble Space Telescope spectroscopy of 24 nearby activegalactic nuclei (AGNs) to investigate spatially resolved gas kinematicsin the narrow-line region (NLR). These observations effectively isolatethe nuclear line profiles on less than 100 pc scales and are used toinvestigate the origin of the substantial scatter between the widths ofstrong NLR lines and the stellar velocity dispersion σ*of the host galaxy, a quantity that relates with substantially lessscatter to the mass of the central, supermassive black hole and moregenerally characterize variations in the NLR velocity field with radius.We find that line widths measured with STIS at a range of spatial scalessystematically underestimate both σ* and the line widthmeasured from ground-based observations, although they do havecomparably large scatter to the relation between ground-based NLR linewidth and σ*. There are no obvious trends in theresiduals when compared with a range of host galaxy and nuclearproperties. The widths and asymmetries of [O III] λ5007 and [SII] λλ6716, 6731 as a function of radius exhibit a widerange of behavior. Some of the most common phenomena are substantialwidth increases from the STIS to the large-scale, ground-based apertureand almost no change in line profile between the unresolved nuclearspectrum and ground-based measurements. We identify asymmetries in asurprisingly large fraction of low-ionization [S II] line profiles andseveral examples of substantial red asymmetries in both [O III] and [SII]. These results underscore the complexity of the circumnuclearmaterial that constitutes the NLR and suggest that the scatter in theNLR width and σ* correlation cannot be substantiallyreduced with a simple set of empirical relations.
| 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.
| 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.
| Water-Vapor Maser Survey for Active Galactic Nuclei: A Megamaser in NGC 6926
We made a survey of water-vapor maser emission for 93 AGNs with theNobeyama 45-m and Mopra 22-m telescopes from 1999 to 2002. A megamaserwas detected in a Seyfert 2 galaxy, NGC 6926, at a distance of 80Mpc, in2002 June. [Greenhill et al. (2003a) have also reported a detection ofthe megamaser at the close date.] The peak flux density was 110mJy, andthe total isotropic luminosity was 340 Lȯ. The masershows triply peaked spectrum, suggesting an edge-on disk. A narrow-linefeature of the maser components at VLSR = 6001 kms-1 was strongly variable with a time scale of a few tens ofdays, and the variation should be of intrinsic origin. We also showed apossibility of variability of water-vapor maser emission of a megamaserpreviously detected in a Seyfert/ultraluminous FIR galaxy, NGC 6240.
| Molecular hydrogen and [FeII] in active galactic nuclei - II. Results for Seyfert 2 galaxies
Near-infrared spectroscopy is used to study the kinematics andexcitation mechanisms of H2 and [FeII] lines in a sampledominated by Seyfert 2 galaxies. The spectra simultaneously cover theJHK bands, allowing us to compare line fluxes emitted in the interval0.8-2.4 μm and avoiding aperture and seeing effects. TheH2 lines are systematically narrower than the narrow-lineregion lines, suggesting that, very likely, the H2 does notoriginate from the same parcel of gas that forms the narrow-line region.Emission-line ratios between H2 lines favour thermalexcitation mechanisms for the molecular gas in active galactic nuclei.It was found that non-thermal excitation contributes, at most, 30 percent of the observed H2. Thermal excitation is also confirmedby the rather similar vibrational and rotational temperatures in theobjects (~2000 K). The mass of hot H2 ranges from102 to 103Msolar, with nearly half ofobjects showing values of <500 Msolar. It shows that thefraction of molecular mass present in the nuclear region and emitting inthe near-infrared is a very small fraction of the warm molecular masspresent in the centre. A diagnostic diagram composed of the line ratiosH2/Brγ and [FeII]/Paβ proves to be a useful toolin the near-infrared for separating emission-line objects by theirdegree of nuclear activity. We found that active galactic nuclei arecharacterized by H2 2.121 μm/Brγ and [FeII] 1.257μm/Paβ flux ratios between 0.6 and 2. Starburst/HII galaxiesdisplay line ratios <0.6 while low-ionization nuclear emission-lineregions are characterized by values larger than 2 in either ratio.
| Unification in the low radio luminosity regime: evidence from optical line emission
We address the question of whether or not the properties of alllow-luminosity flat spectrum radio sources, not just the obvious BL Lacobjects, are consistent with them being the relativistically beamedcounterparts of the low radio luminosity radio galaxies (theFanaroff-Riley type 1, FR I). We have accumulated data on a well-definedsample of low redshift, core-dominated, radio sources all of which haveone-sided core-jet structures seen with very long baselineinterferometry, just like most BL Lac objects. We first compare theemission-line luminosities of the sample of core-dominated radio sourceswith a matched sample of FR I radio galaxies. The emission lines in thecore-dominated objects are on average significantly more luminous thanthose in the comparison sample, inconsistent with the simplest unifiedmodels in which there is no orientation dependence of the line emission.We then compare the properties of our core-dominated sample with thoseof a sample of radio-emitting UGC galaxies selected without bias to corestrength. The core-dominated objects fit well on the UGC correlationbetween line emission and radio core strength found by Verdoes Kleijn etal. The results are not consistent with all the objects participating ina simple unified model in which the observed line emission isorientation independent, though they could fit a single, unified modelprovided that some FR I radio galaxies have emission line regions thatbecome more visible when viewed along the jet axis. However, they areequally consistent with a scenario in which, for the majority ofobjects, beaming has minimal effect on the observed core luminosities ofa large fraction of the FR I population and that intrinsically strongercores simply give rise to stronger emission lines. We conclude that FR Iunification is much more complex than usually portrayed, and modelscombining beaming with an intrinsic relationship between core andemission line strengths need to be explored.
| Radio spectra of the low-luminosity active galactic nucleus NGC 266 at centimetre-to-submillimetre wavelengths
We report multi-frequency and multi-epoch radio continuum observationswith multi-spatial resolution for the low-luminosity active galacticnucleus (LLAGN) NGC 266. In the centimetre regime, we find diffusecomponents with Very Large Array (VLA) observations, and a variablecompact core with a rising spectrum with Very Long Baseline Array (VLBA)observations. Although the spectral index of the rising spectrum isconsistent with the prediction of the simple advection-dominatedaccretion flow (ADAF) model, the observed radio power is slightly highcompared with that of the model prediction. A spectral break atcentimetre-to-millimetre wavelengths is inferred from the upper limitsof flux densities from Nobeyama Millimetre Array (NMA) and James ClerkMaxwell Telescope (JCMT) data at millimetre and submillimetrewavelengths, respectively. More complicated considerations are requiredfor the theoretical model to interpret such observed radio properties.
| 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.
| An 8.4-GHz Long Baseline Array observation of the unusual Seyfert galaxy NGC 7213
We have observed the type 1.5 Seyfert galaxy NGC 7213 with theAustralian Long Baseline Array (LBA) at 8.4 GHz to discover whether thisobject has the high brightness temperature compact core suggested bylow-frequency variability. Confirmation would support the hypothesisthat radio-intermediate Seyfert galaxies have Doppler-boosted radiojets. Our observation confirms the existence of this core but with aflux density of almost a factor of 6 less than observed 12 yr earlier.Though few studies exist on the long-term radio variability of Seyferts,a decline of this magnitude does appear to be rare.
| 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.
| 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.
| Absorption-Line Study of Halo Gas in NGC 3067 toward the Background Quasar 3C 232
We present new H I 21 cm absorption data and ultraviolet spectroscopyfrom Hubble Space Telescope/Space Telescope Imaging Spectrograph of theQSO/galaxy pair 3C 232/NGC 3067. The QSO sight line lies near the minoraxis and 1&farcm;8 (11 h-170 kpc) above the planeof NGC 3067, a nearby luminous (cz=1465+/-5 km s-1,L=0.5L*) starburst galaxy with a moderate star formation rateof 1.4 Msolar yr-1. The UV spectra show that theSi IV and C IV doublets have the same three velocity components atcz=1369, 1417, and 1530 km s-1 found in Ca II H and K, Na ID, Mg I, Mg II, and Fe II, implying that the low- and high-ionizationgas are both found in three distinct absorbing clouds (only thestrongest component at 1420 km s-1 is detected in H I 21 cm).The new Lyα observation allows the first measurements of the spinand kinetic temperatures of halo gas: Ts=435+/-140 K andTk/Ts~1. However, while a standard photoionizationmodel can explain the low ions, the C IV and Si IV are explained moreeasily as collisionally ionized boundary layers of the photoionizedclouds. Because of their small inferred space velocity offsets(Δv=-260, -130, and +170 km s-1) relative to thenucleus of NGC 3067 and the spatial coincidence of low- andhigh-ionization gas, we propose that these absorbers are analogous toGalactic high-velocity clouds (HVCs). A comparison of the NGC 3067clouds and Galactic HVCs finds similar H I column densities, kinematics,metallicities, spin temperatures, and inferred sizes. We find nocompelling evidence that any halo gas along this sight line is escapingthe gravitational potential of NGC 3067, despite its modest starburst.
| Emergence of a Narrow H2O Maser Feature in NGC 1052
We report the emergence of a narrow H2O maser feature with anFWHM of 21 km s-1 in the LINER NGC 1052, which has been knownto show only a broad (FWHM>100 km s-1) maser line profilewith relatively bright continuum radio emission. The new narrow maserfeature with a peak flux density of 47 mJy at VLSR=1787 kms-1 is redshifted by 328 km s-1 with respect tothe systemic velocity. Broad features with peak velocities of 1510 and1704 km s-1, more redward than ever observed before, are alsodetected. The profile of the new narrow feature possibly showsbrightening by 16%+/-9% and narrowing by 30%+/-12% between 2003 May 30and June 2. During the same time, the continuum flux density hasincreased by 21%. Synchronous variation of maser and continuum fluxdensities on a timescale of days resembles that in Mrk 348, which isalso a broad megamaser source with a bright radio continuum. Continuumand maser brightening and narrowing indicate that an increase of thebackground seed photon and an increase of maser gain have occurredsimultaneously. A jet component running behind a mixture of ionizedregions and X-ray dissociation regions at a subrelativistic velocity canproduce such short-time variation. Another explanation is an interactionbetween the jet and molecular clouds.
| VLBA Identification of the Milliarcsecond Active Nucleus in the Seyfert Galaxy NGC 4151
The Seyfert galaxy NGC 4151 has been imaged at resolution better than0.1 pc using a VLBI array consisting of the Very Long Baseline Array andthree 100 m class telescopes. A flat-spectrum 3 mJy source with amonochromatic radio power of ~1037 ergs s-1 hasbeen detected, apparently at the location of the active galactic nucleus(AGN) and its central black hole. The radio source has a minimumbrightness temperature of 2.1×108 K and a size upperlimit of 0.035 pc, about 10 times the diameter of the broad-line regionand 15,000 times the diameter of the black hole's event horizon. Anadditional flat-spectrum component located within a parsec of theapparent nucleus is likely to be a knot in the inner radio jet. Thepresence of some steep-spectrum radio emission within 0.1 pc of thegalaxy nucleus limits the emission measure of a possible ionized torusto a maximum value of 108 cm-6 pc. If the hardX-ray source in NGC 4151 is associated with the radio AGN, itsradio-to-X-ray ratio is less than 10-5, putting NGC 4151securely in the radio-quiet class of AGNs. The radio image reveals a 0.2pc two-sided base to the well-known arcsecond radio jet. The apparentspeeds of the jet components relative to the radio AGN are less than0.050c and less than 0.028c at respective nuclear distances of 0.16 and6.8 pc. These are the lowest speed limits yet found for a Seyfert galaxyand indicate nonrelativistic jet motions, possibly due to thermalplasma, on a scale only an order of magnitude larger than the broad-lineregion.
| The Westerbork HI survey of spiral and irregular galaxies. III. HI observations of early-type disk galaxies
We present Hi observations of 68 early-type disk galaxies from the WHISPsurvey. They have morphological types between S0 and Sab and absoluteB-band magnitudes between -14 and -22. These galaxies form the massive,high surface-brightness extreme of the disk galaxy population, few ofwhich have been imaged in Hi before. The Hi properties of the galaxiesin our sample span a large range; the average values of MHI/LB and DH I/D25 are comparableto the ones found in later-type spirals, but the dispersions around themean are larger. No significant differences are found between the S0/S0aand the Sa/Sab galaxies. Our early-type disk galaxies follow the same Himass-diameter relation as later-type spiral galaxies, but theireffective Hi surface densities are slightly lower than those found inlater-type systems. In some galaxies, distinct rings of Hi emissioncoincide with regions of enhanced star formation, even though theaverage gas densities are far below the threshold of star formationderived by Kennicutt (1989, ApJ, 344, 685). Apparently, additionalmechanisms, as yet unknown, regulate star formation at low surfacedensities. Many of the galaxies in our sample have lopsided gasmorphologies; in most cases this can be linked to recent or ongoinginteractions or merger events. Asymmetries are rare in quiescentgalaxies. Kinematic lopsidedness is rare, both in interacting andisolated systems. In the appendix, we present an atlas of the Hiobservations: for all galaxies we show Hi surface density maps, globalprofiles, velocity fields and radial surface density profiles.
| Investigation of flat spectrum radio sources by the interplanetary scintillation method at 111 MHz
Interplanetary scintillation observations of 48 of the 55 Augusto et al.(1998) flat spectrum radio sources were carried out at 111 MHz using theinterplanetary scintillation method on the Large Phased Array (LPA) inRussia. Due to the large size of the LPA beam (1 ° × 0.5°) a careful inspection of all possible confusion sources was madeusing extant large radio surveys: 37 of the 48 sources are not confused.We were able to estimate the scintillating flux densities of 13 sources,getting upper limits for the remaining 35. Gathering more or improvingextant VLBI data on these sources might significantly improve ourresults. This proof-of-concept project tells us that compact (<1'')flat spectrum radio sources show strong enough scintillations at 111 MHzto establish/constrain their spectra (low-frequency end).
| 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.
| 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.
| 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 spectral energy distributions of the revised 200-mJy sample
We address the question of why low-luminosity radio sources withsimilar flat radio spectra show a range of optical activity. Theinvestigation is based on the spectral energy distributions (SEDs) ofobjects from the 200-mJy sample. We gathered new data from the VLA at 43GHz, from SCUBA in the JCMT at 2000, 1350 and 850 μm, and from theISOPHOT instrument on ISO at 170, 90, 60 and 25 μm. There isconsiderable diversity amongst the SEDs of the objects: there areobjects with steep broad-band spectra between centimetre and millimetrebands (14 per cent of the sample); there are those with flat broad-bandspectra over most of the spectral range (48 per cent of the sample); andthere are those which show pronounced submillimetre/infrared excesses(27 per cent of the sample). Some objects of the first group havetwo-sided radio morphology, indicating that their parsec-scale emissionis not dominated by beamed jet emission. Amongst the objects that havesmooth broad-band spectra from the radio to the infrared, there arepassive elliptical galaxies as well as the expected BL Lacertae objects.The most pronounced submillimetre/infrared excesses are shown by thebroad-emission-line objects.
| VLBA polarization observations of BL Lac objects and passive elliptical galaxies
We present Very Long Baseline Array (VLBA) 5-GHz polarimetricobservations of 23 BL Lacertae (BL Lac) objects and radio galaxiesselected from the 200-mJy sample. BL Lac objects have core polarizationvalues lower than those found in previous samples. The magnetic fieldgeometry in the jets is not unique: both parallel and perpendiculargeometries are observed, even in the same sources. The parsec-scalemorphology of radio galaxies is clearly divided in two classes:one-sided core-jet sources, which show polarized emission in the coreand/or jet, and two-sided symmetric objects which are not polarized. Wediscuss and compare the parsec-scale polarization properties of theradio cores and jets for the BL Lac objects and the radio galaxies inrelation to their parsec-scale morphology and high-frequency integratedspectral index.
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