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The relation between mergers and AGN activity. Results from radio galaxy and luminous infrared galaxy studies
There is morphological evidence that the activity in powerful radiogalaxies could be triggered by mergers and galaxy interactions. However,nothing is known about the timescales, order of events, and the type ofinteraction involved. It is not yet known whether there exists anevolutionary link between radio galaxies and other merger systems suchas very luminous and ultraluminous infrared galaxies (VLIRGs andULIRGs). Here, we report preliminary results obtained from the analysisof near-ultraviolet and optical spectroscopic observations of samples ofRadio Galaxies and VLIRGs-ULIRGs to investigate, through age-dating oftheir young stellar population, whether an evolutionary link existsbetween VLIRGs/ULIRGs-Radio Galaxies-normal elliptical galaxies. Theseresults will help to understand the genesis events that lead to theformation of radio jet and quasar activity, and they will allow us toplace radio galaxies in the context of hierarchical evolution models forthe population of giant elliptical galaxies.
| 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.
| 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.
| Molecular Gas at High Redshift
The Early Universe Molecular Emission Line Galaxies (EMGs) are apopulation of galaxies with only 36 examples that hold great promise forthe study of galaxy formation and evolution at high redshift. Theclassification, luminosity of molecular line emission, molecular mass,far-infrared (FIR) luminosity, star formation efficiency, morphology,and dynamical mass of the currently known sample are presented anddiscussed. The star formation rates derived from the FIR luminosityrange from about 300 to 5000 Mȯ year 1 andthe molecular mass from 4 × 109 to 1 ×1011 Mȯ. At the lower end, these starformation rates, gas masses, and diameters are similar to those of localultraluminous infrared galaxies and represent starbursts in centrallyconcentrated disks, sometimes, but not always, associated with activegalactic nuclei. The evidence for large (>5 kpc) molecular disks islimited. Morphology and several high angular resolution images suggestthat some EMGs are mergers with a massive molecular interstellar mediumin both components. A critical question is whether the EMGs, inparticular those at the higher end of the gas mass and luminositydistribution, represent the formation of massive, giant ellipticalgalaxies in the early Universe. The sample size is expected to growexplosively in the era of the Atacama Large Millimeter Array (ALMA).
| 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.
| 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.
| EGRET Upper Limits and Stacking Searches of Gamma-Ray Observations of Luminous and Ultraluminous Infrared Galaxies
We present a stacking analysis of EGRET γ-ray observations at thepositions of luminous and ultraluminous infrared galaxies. The latterwere selected from the recently presented HCN survey, which is thoughtto contain the most active star-forming regions of the universe.Different sorting criteria are used, and since there is no positivecollective detection of γ-ray emission from these objects, wedetermined both collective and individual upper limits. The uppermostexcess we find appears in the case of ULIRGs ordered by redshift, at avalue of 1.8 σ.
| Molecular gas properties of 12-μm Seyfert galaxies - I. The southern sample
We have used the 15-m Swedish European Southern Observatory (ESO)Submillimetre Telescope (SEST) to observe the J= 1 -> 0 and J= 2-> 1 transition lines of CO in 30 Southern hemisphere Seyfertgalaxies from the extended 12-μm sample of Rush, Malkan &Spinoglio. We detected CO J= 1 -> 0 in 16 out of the 30 Seyfertgalaxies and CO J= 2 -> 1 in 17 out of the 30 Seyfert galaxies. Fromthe observed spectra, various CO gas properties have been derivedincluding the luminosity of the CO gas and, using a standard conversionfactor, the H2 mass. The average H2 gas mass forSeyfert 1 galaxies was 3 × 109 Msolar for COJ= 1 -> 0 and 1 × 109 Msolar for CO J= 2-> 1, while in comparison the H2 gas mass for Seyfert 2type galaxies was 11 × 109 Msolar for CO J=1 -> 0 and 3 × 109 MsolarCO J= 2 -> 1.From this small sample of Seyfert galaxies, we tentatively support theconclusion that type 2 Seyfert galaxies contain more molecular gas thantheir type 1 counterparts.
| HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies
We report systematic HCN J=1-0 (and CO) observations of a sample of 53infrared (IR) and/or CO-bright and/or luminous galaxies, including sevenultraluminous infrared galaxies, nearly 20 luminous infrared galaxies,and more than a dozen of the nearest normal spiral galaxies. This is thelargest and most sensitive HCN survey of galaxies to date. All galaxiesobserved so far follow the tight correlation between the IR luminosityLIR and the HCN luminosity LHCN initially proposedby Solomon, Downes, & Radford, which is detailed in a companionpaper. We also address here the issue of HCN excitation. There is noparticularly strong correlation between LHCN and the 12 μmluminosity; in fact, of all the four IRAS bands, the 12 μm luminosityhas the weakest correlation with the HCN luminosity. There is also noevidence of stronger HCN emission or a higher ratio of HCN and COluminosities LHCN/LCO for galaxies with excess 12μm emission. This result implies that mid-IR radiative pumping, orpopulating, of the J=1 level of HCN by a mid-IR vibrational transitionis not important compared with the collisional excitation by densemolecular hydrogen. Furthermore, large velocity gradient calculationsjustify the use of HCN J=1-0 emission as a tracer of high-densitymolecular gas (>~3×104/τcm-3) andgive an estimate of the mass of dense molecular gas from HCNobservations. Therefore, LHCN may be used as a measure of thetotal mass of dense molecular gas, and the luminosity ratioLHCN/LCO may indicate the fraction of moleculargas that is dense.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Theoretical Modeling of the Diffuse Emission of Gamma Rays from Extreme Regions of Star Formation: The Case of ARP 220
Our current understanding of ultraluminous infrared galaxies suggeststhat they are recent galaxy mergers in which much of the gas in theformer spiral disks, particularly that located at distances less than 5kpc from each of the premerger nuclei, has fallen into a common center,triggering a huge starburst phenomenon. This large nuclear concentrationof molecular gas has been detected by many groups, and estimates ofmolecular mass and density have been made. Not surprisingly, theseestimates were found to be orders of magnitude larger than thecorresponding values found in our Galaxy. In this paper, aself-consistent model of the high-energy emission of the superstarburstgalaxy Arp 220 is presented. The model also provides an estimate of theradio emission from each of the components of the central region of thegalaxy (western and eastern extreme starbursts and molecular disk). Thepredicted radio spectrum is found as a result of the synchrotron andfree-free emission and absorption of the primary and secondary steadypopulation of electrons and positrons. The latter is the output ofcharged pion decay and knock-on leptonic production, subject to a fullset of losses in the interstellar medium. The resulting radio spectrumis in agreement with subarcsecond radio observations, which is whatallows us to estimate the magnetic field. In addition, the FIR emissionis modeled with dust emissivity, and the computed FIR photon density isused as a target for inverse Compton process as well as to give anaccount of losses in the γ-ray escape. Bremsstrahlung emission andneutral pion decay are also computed, and the γ-ray spectrum isfinally predicted. Future possible observations with GLAST and theground-based Cerenkov telescopes are discussed.
| 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.
| Luminous Infrared Galaxies as Plausible Gamma-Ray Sources for the Gamma-Ray Large Area Space Telescope and the Imaging Atmospheric Cerenkov Telescopes
We argue that luminous infrared galaxies may constitute a newlydetectable population of γ-ray sources for the next generation ofground- and space-based high-energy telescopes. In addition, we reportfor the first time upper limits on their fluxes using data obtained withthe EGRET telescope.
| The Star Formation Rate and Dense Molecular Gas in Galaxies
HCN luminosity is a tracer of dense molecular gas,n(H2)>~3×104cm-3, associatedwith star-forming giant molecular cloud (GMC) cores. We present theresults and analysis of our survey of HCN emission from 65 infraredgalaxies, including nine ultraluminous infrared galaxies (ULIGs,LIR>~1012Lsolar), 22 luminousinfrared galaxies (LIGs,1011Lsolar0.06 are LIGs or ULIGs. Normal spiralsall have similar and low dense gas fractionsLHCN/LCO=0.02-0.05. The global star formationefficiency depends on the fraction of the molecular gas in a densephase.
| Galaxy Interaction and the Starburst-Seyfert Connection
Galaxy interactions are studied in terms of the starburst-Seyfertconnection. The starburst requires a high rate of gas supply. Since theefficiency for supplying the gas is high in a galaxy interaction,although the companion is not necessarily discernible, Seyfert galaxieswith circumnuclear starbursts are expected to be interacting. Since thelarge amounts of circumnuclear gas and dust obscure the broad-lineregion, they are expected to be observed as Seyfert 2 galaxies. Theactive galactic nucleus itself does not require a high rate of gassupply. Seyfert galaxies without circumnuclear starbursts are notnecessarily expected to be interacting even at the highest luminosities.They are not necessarily expected to evolve from Seyfert galaxies withcircumnuclear starbursts. We derive these and other theoreticalexpectations and confirm them with statistics on observational data ofmagnitude-limited samples of Seyfert galaxies.
| An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample
The importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies.
| The relation between AGN hard X-ray emission and mid-infrared continuum from ISO spectra: Scatter and unification aspects
We use mid-infrared spectral decomposition to separate the 6 μmmid-infrared AGN continuum from the host emission in the ISO lowresolution spectra of 71 active galaxies and compare the results toobserved and intrinsic 2-10 keV hard X-ray fluxes from the literature.We find a correlation between mid-infrared luminosity and absorptioncorrected hard X-ray luminosity, but the scatter is about an order ofmagnitude, significantly larger than previously found with smallerstatistics. Main contributors to this scatter are likely variations inthe geometry of absorbing dust, and AGN variability in combination withnon-simultaneous observations. There is no significant differencebetween type 1 and type 2 objects in the average ratio of mid-infraredand hard X-ray emission, a result which is not consistent with the mostsimple version of a unified scheme in which an optically andgeometrically thick torus dominates the mid-infrared AGN continuum. Mostprobably, significant non-torus contributions to the AGN mid-IRcontinuum are masking the expected difference between the two types ofAGN.Table 1 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/418/465Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, TheNetherlands, and the UK) with the participation of ISAS and NASA.
| 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
| Compact Nuclear Starbursts in Seyfert 2 Galaxies from the CfA and 12 Micron Samples
We present infrared 2.8-4.1 μm slit spectra of 32 Seyfert 2 galaxiesin the CfA and 12 μm samples. The 3.3 μm polycyclic aromatichydrocarbon (PAH) emission feature was used to estimate the absolutemagnitude of a compact nuclear starburst (less than a few hundredparsecs in size) that is presumed to have occurred in the outer regionof an obscuring dusty molecular torus around a central supermassiveblack hole. We detected 3.3 μm PAH emission in 11 of the 32 Seyfert 2nuclei in our sample, providing evidence for the presence of compactnuclear starbursts in a significant fraction of Seyfert 2 nuclei.However, the rest-frame equivalent widths of the 3.3 μm PAH emissionand the 3.3 μm PAH-to-infrared luminosity ratios measured in thisstudy suggest that compact nuclear starbursts generally do notcontribute significantly to the observed 3-4 μm nuclear fluxes or tothe infrared luminosities of Seyfert 2 galaxies. Absorption features at3.4 μm from bare dust were clearly detected in only two of thenuclei, and features at 3.1 μm from ice-covered dust were detected inonly one nucleus. If the dust properties in the direction of theseSeyfert 2 nuclei do not differ significantly from the Galacticinterstellar medium, then these small absorption optical depths suggestthat dust extinction toward the 3-4 μm continuum emitting region inthe innermost part of the obscuring dusty torus is modest:AV<50-60 mag. Finally, the 3.3 μm PAH emissionluminosities measured in this study were found to be significantlycorrelated with IRAS 12 and 25 μm and nuclear N-band (10.6 μm)luminosities. If these three luminosities trace the power of the activegalactic nucleus (AGN), then the luminosities of compact nuclearstarbursts and AGNs are correlated. This correlation is in agreementwith theories predicting that the presence of a compact nuclearstarburst in the torus leads to an enhancement of the mass accretionrate onto the central supermassive black hole.
| Azimuthal and Kinematic Segregation of Neutral and Molecular Gas in Arp 118: The Yin-Yang Galaxy NGC 1144
We present new high-resolution H I observations of the disk of thecollisional infrared luminous(LIR=2.2×1011Lsolar) galaxy NGC1144, which reveal an apparent large-scale azimuthal and kinematicsegregation of neutral hydrogen relative to the molecular gasdistribution. Even among violently collisional galaxies, the CO/H Iasymmetry in NGC 1144 is unusual, both in the inner regions and in theouter disk. We suggest that we are observing Arp 118 at a specialmoment, shortly after a high-speed collision between NGC 1144 and itselliptical companion NGC 1143. H I emission with an average molecularfraction fmol<0.5 is observed on one side (northwest) ofthe rotating disk of NGC 1144, while the other side (southeast) isdominated by dense molecular complexes in which fmol isalmost unity. The interface region between the warm- and cool-clouddominated regions lies on a deep spiral-like dust lane that we identifyas a shock wave responsible for the relative shift in the dominance of HI and H2 gas. A strong shock being fed by diffuse H I cloudswith unusually large (>400 km s-1) rotational velocities canexplain (1) the CO/H I asymmetries, (2) a large velocity jump (185 kms-1) across the arm as measured by H I absorption against aradio bright continuum source that straddles the arm, and (3) theasymmetric distribution of star formation and off-nuclear molecular gasresulting from likely streaming motions associated with the strongshock. The new results provide for the first time a coherent picture ofArp 118's many peculiarities and underline the potentially complexchanges in the gas phase that can accompany large gravitationalperturbations of gas-rich galaxies.
| The Unified Model and Evolution of Active Galaxies: Implications from a Spectropolarimetric Study
We extend the analysis presented in Paper I of a spectropolarimetricsurvey of the CfA and 12 μm samples of Seyfert 2 galaxies (S2s). Weconfirm that polarized (hidden) broad-line region (HBLR) S2s tend tohave hotter circumnuclear dust temperatures, show mid-IR spectra morecharacteristic of Seyfert 1 galaxies (S1s), and are intrinsically moreluminous than non-HBLR S2s. The levels of obscuration and circumnuclearstar formation, however, appear to be similar between HBLR and non-HBLRS2 galaxies, based on an examination of various observationalindicators. HBLR S2s, on average, share many similar large-scale,presumably isotropic, characteristics with S1s, as would be expected ifthe unified model is correct, while non-HBLR S2s generally do not. Theactive nuclear engines of non-HBLR S2s, then, appear to be truly weakerthan HBLR S2s, which in turn are fully consistent with being S1s viewedfrom another direction. There is also evidence that the fraction ofdetected HBLRs increases with the radio power of the active galacticnucleus. Thus, all S2 galaxies may not be intrinsically similar innature, and we speculate that evolutionary processes may be at work.
| 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.
| Spectral Energy Distributions of Seyfert Nuclei
We present nuclear spectral energy distributions (SEDs) in the range0.4-16 μm for an expanded CfA sample of Seyfert galaxies. Thespectral indexes (fν~ν-αIR)from 1 to 16 μm range from αIR~0.9 to 3.8. Theshapes of the spectra are correlated with Seyfert type in the sense thatsteeper nuclear SEDs (νfν increasing with increasingwavelength) tend to be found in Seyfert 2's, and flatter SEDs(νfν is constant) in Seyfert 1-1.5's. The galaxiesoptically classified as Seyferts 1.8's and 1.9's display values ofαIR as in type 1 objects, or values intermediatebetween those of Seyfert 1's and Seyfert 2's. The intermediate SEDs ofmany Seyfert 1.8-1.9's may be consistent with the presence of a pureSeyfert 1 viewed through a moderate amount (AV<~5 mag) offoreground galaxy extinction. We find, however, that between 10% and 20%of galaxies with broad optical line components have steep infrared SEDs.Torus models usually adopt high equatorial opacities to reproduce theinfrared properties of Seyfert 1's and 2's, resulting in a dichotomy ofinfrared SEDs (flat for type 1's, and steep for type 2's). Such adichotomy, however, is not observed in our sample. The wide range ofspectral indexes observed in the type 2 objects, the lack of extremelysteep SEDs, and the large numbers of objects with intermediate spectralindexes cannot be reconciled with predictions from existing opticallythick torus models. We discuss possible modifications to improve torusmodels, including low optical depth tori, clumpy dusty tori, and highoptical depth tori with an extended optically thin component.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Multiwavelength Insights into Mixed-Morphology Binary Galaxies. I. ISOCAM, ISOPHOT, and Hα Imaging
We present Hα and ISO mid- and far-IR observations for a sample ofmixed-morphology galaxy pairs that reveal both the stellar andnonstellar signatures of the interaction process. A mixed-morphologypair is perhaps the simplest form of galaxy-galaxy interaction becauseit is expected to involve only a single rapidly rotating gas-richcomponent paired with a gas-poor elliptical or lenticular galaxy. Aprimary assumption that we address is whether spirals are the only IRemitter in these mixed (E+S) pairs. Our observations reveal that many ofthe early-type galaxies exhibit weak (low equivalent width) emission, asoften observed in field elliptical galaxies. These are the classicalmixed-morphology pairs. However, some of the early-type components,especially the lenticular galaxies, show evidence for significant starformation, with Hα equivalent widths and 15 μm luminositiescomparable to or exceeding those of their often much larger spiralcompanions. Our sample contains five Seyfert 2 nuclei, of which threecan be described as companions on the end of a spiral arm. The Seyfertnucleus is often accompanied by a starburst region, while other suchcompanions currently show only the starburst component. These pairs areamong the best candidates for direct interaction fuelling of bothstarbursts and active galactic nuclei.Based on observations with the Infrared Space Observatory (ISO), an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands and the United Kingdom) withthe participation of ISAS and NASA.
| On the relation between the coronal line emission and the infrared/X-ray emission in Seyfert galaxies
The relation between the X-ray, the coronal line and the infrared (IR)emissions in a sample of the brightest known Seyfert galaxies isanalysed. A close relationship between the absorption-corrected softX-ray emission and both the mid-IR and the coronal line emission isfound for the Seyfert type 2 objects in the sample. The coronal line andthe X-ray emissions are both main tracers of active galactic nuclei(AGN) activity and their relationship with the mid-IR emission points tothe heat of the carbuncle dust as the main energetic process associatedwith the AGN. On the other hand, the above relations do not seem to holdfor the type 1 Seyfert discussed in the sample, at least when thecomparisons are made in a flux diagram. This is partially because of thereduced number of objects of this type analysed in this work and alsothe fact that the measured soft X-ray emission in Seyfert 1s issystematically larger, by at least an order of magnitude, than that inthe Seyfert 2 counterparts. Finally, the hard X-ray emission in thestudied sample appears unrelated to either the mid-IR or the coronalline emission.
| 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.
| The WARPS Survey. VI. Galaxy Cluster and Source Identifications from Phase I
We present in catalog form the optical identifications for objects fromthe first phase of the Wide Angle ROSAT Pointed Survey (WARPS). WARPS isa serendipitous survey of relatively deep, pointed ROSAT observationsfor clusters of galaxies. The X-ray source detection algorithm used byWARPS is Voronoi Tessellation and Percolation (VTP), a technique whichis equally sensitive to point sources and extended sources of lowsurface brightness. WARPS-I is based on the central regions of 86 ROSATPSPC fields, covering an area of 16.2 square degrees. We describe herethe X-ray source screening and optical identification process forWARPS-I, which yielded 34 clusters at 0.06
| 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.
| The Far-Infrared Energy Distributions of Seyfert and Starburst Galaxies in the Local Universe: Infrared Space Observatory Photometry of the 12 Micron Active Galaxy Sample
New far-infrared photometry with ISOPHOT aboard the Infrared SpaceObservatory (ISO) is presented for 58 galaxies with homogeneouspublished data for another 32 galaxies, all belonging to the 12 μmgalaxy sample-in total, 29 Seyfert 1 galaxies, 35 Seyfert 2 galaxies,and 12 starburst galaxies, or about half of the 12 μm active galaxysample, plus 14 normal galaxies for comparison. ISO and InfraredAstronomical Satellite (IRAS) data are used to define color-colordiagrams and spectral energy distributions (SEDs). Thermal dust emissionat two temperatures (one cold at 15-30 K and one warm at 50-70 K) canfit the 60-200 μm SED, with a dust emissivity law proportional to theinverse square of the wavelength. Seyfert 1 galaxies and Seyfert 2galaxies are indistinguishable longward of 100 μm, while, as alreadyseen by IRAS, the former have flatter SEDs shortward of 60 μm. A mildanticorrelation is found between the [200-100] color and the ``60 μmexcess.'' We infer that this is due to the fact that galaxies with astrong starburst component and thus a strong 60 μm flux have asteeper far-infrared turnover. In non-Seyfert galaxies, increasing theluminosity corresponds to increasing the star formation rate, whichenhances the 25 and 60 μm emission. This shifts the peak emissionfrom around 150 μm in the most quiescent spirals to shorter than 60μm in the strongest starburst galaxies. To quantify these trendsfurther, we identified with the IRAS colors three idealized infraredSEDs: pure quiescent disk emission, pure starburst emission, and pureSeyfert nucleus emission. Even between 100 and 200 μm, the quiescentdisk emission remains much cooler than the starburst component. Seyfertgalaxies have 100-200 μm SEDs ranging from pure disks to purestarbursts, with no apparent contribution from their active nuclei atthose wavelengths. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, the Netherlands, and the United Kingdom) with theparticipation of ISAS and NASA.
| Hubble Space Telescope Imaging of the Circumnuclear Environments of the CfA Seyfert Galaxies: Nuclear Spirals and Fueling
We present archival Hubble Space Telescope (HST) images of the nuclearregions of 43 of the 46 Seyfert galaxies found in the volume-limited,spectroscopically complete CfA Redshift Survey sample. Using an improvedmethod of image contrast enhancement, we create detailed high-quality``structure maps'' that allow us to study the distributions of dust,star clusters, and emission-line gas in the circumnuclear regions(100-1000 pc scales) and in the associated host galaxy. Essentially allof these Seyfert galaxies have circumnuclear dust structures withmorphologies ranging from grand-design two-armed spirals to chaoticdusty disks. In most Seyfert galaxies there is a clear physicalconnection between the nuclear dust spirals on hundreds of parsec scalesand large-scale bars and spiral arms in the host galaxies proper. Theseconnections are particularly striking in the interacting and barredgalaxies. Such structures are predicted by numerical simulations of gasflows in barred and interacting galaxies and may be related to thefueling of active galactic nuclei by matter inflow from the host galaxydisks. We see no significant differences in the circumnuclear dustmorphologies of Seyfert 1s and 2s, and very few Seyfert 2 nuclei areobscured by large-scale dust structures in the host galaxies. If Seyfert2s are obscured Seyfert 1s, then the obscuration must occur on smallerscales than those probed by HST. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Science Institute. STScI is operated by the Associationof Universities for Research in Astronomy, Inc., under the NASA contractNAS 5-26555.
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