Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Local and Large-Scale Environment of Seyfert Galaxies
We present a three-dimensional study of the local (<=100h-1 kpc) and the large-scale (<=1 h-1 Mpc)environment of the two main types of Seyfert AGN galaxies. For thispurpose we use 48 Seyfert 1 galaxies (with redshifts in the range0.007<=z<=0.036) and 56 Seyfert 2 galaxies (with0.004<=z<=0.020), located at high galactic latitudes, as well astwo control samples of nonactive galaxies having the same morphological,redshift, and diameter size distributions as the corresponding Seyfertsamples. Using the Center for Astrophysics (CfA2) and Southern SkyRedshift Survey (SSRS) galaxy catalogs (mB~15.5) and our ownspectroscopic observations (mB~18.5), we find that within aprojected distance of 100 h-1 kpc and a radial velocityseparation of δv<~600 km s-1 around each of ourAGNs, the fraction of Seyfert 2 galaxies with a close neighbor issignificantly higher than that of their control (especially within 75h-1 kpc) and Seyfert 1 galaxy samples, confirming a previoustwo-dimensional analysis of Dultzin-Hacyan et al. We also find that thelarge-scale environment around the two types of Seyfert galaxies doesnot vary with respect to their control sample galaxies. However, theSeyfert 2 and control galaxy samples do differ significantly whencompared to the corresponding Seyfert 1 samples. Since the maindifference between these samples is their morphological typedistribution, we argue that the large-scale environmental differencecannot be attributed to differences in nuclear activity but rather totheir different type of host galaxies.
| 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.
| The K-band properties of Seyfert 2 galaxies
Aims. It is well known that the [O iii]λ5007 emission line andhard X-ray (2-10 keV) luminosities are good indicators of AGN activitiesand that the near and mid-infrared emission of AGN originates fromre-radiation of dusty clouds heated by the UV/optical radiation from theaccretion disk. In this paper we present a study of the near-infraredK-band (2.2 μm) properties for a sample of 65 Seyfert 2 galaxies. Methods: .By using the AGN/Bulge/Disk decomposition technique, weanalyzed the 2MASS K_S-band images for Seyfert 2 galaxies in order toderive the K_S-band magnitudes for the central engine, bulge, and diskcomponents. Results: .We find that the K_S-band magnitudes of thecentral AGN component in Seyfert 2 galaxies are tightly correlated withthe [O iii]λ5007 and the hard X-ray luminosities, which suggeststhat the AGN K-band emission is also an excellent indicator of thenuclear activities at least for Seyfert 2 galaxies. We also confirm thegood relation between the central black hole masses and bulge's K-bandmagnitudes for Seyfert 2s.
| NGC 7679: an anomalous, composite Seyfert 1 galaxy whose X-ray luminous AGN vanishes at optical wavelengths
Morphological disturbances and gas kinematics of the SB0 galaxy NGC 7679= Arp 216 are investigated to understand the history of this highlycomposite object, where AGN and starburst signatures dominate in theX-ray and optical/IR regime, respectively. Perturbations of the ionizedgas velocity field appear quite mild within 15'' (~5 kpc) of the center,so it can be straightforwardly modeled as a circularly rotating disk.Outside that radius, significant disturbances are seen. In particular,the eastern distorted arm as well as the huge neutral hydrogen bridgeconnecting NGC 7679 to the nearby Seyfert spiral NGC 7682 unambiguouslyrepresent the vestige of a close encounter of the two objects ~500 Myrago. The relationship of such a past event with the much more recent,centrally located starburst (not older than 20 Myr) cannot be easilyestablished. Together, the classification of NGC 7679 is less extremethan that proposed in the past, being simply a (disturbed) galaxy wherestarburst and AGN activity coexist with a starburst dominating thebolometric luminosity.
| 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.
| 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.
| The star formation history of Seyfert 2 nuclei
We present a study of the stellar populations in the central ~200 pc ofa large and homogeneous sample comprising 79 nearby galaxies, most ofwhich are Seyfert 2s. The star formation history of these nuclei isreconstructed by means of state-of-the-art population synthesismodelling of their spectra in the 3500-5200 Åinterval. Aquasar-like featureless continuum (FC) is added to the models to accountfor possible scattered light from a hidden active galactic nucleus(AGN).We find the following. (1) The star formation history of Seyfert 2nuclei is remarkably heterogeneous: young starbursts, intermediate-ageand old stellar populations all appear in significant and widely varyingproportions. (2) A significant fraction of the nuclei show a strong FCcomponent, but this FC is not always an indication of a hidden AGN: itcan also betray the presence of a young, dusty starburst. (3) We detectweak broad Hβ emission in several Seyfert 2s after cleaning theobserved spectrum by subtracting the synthesis model. These are mostlikely the weak scattered lines from the hidden broad-line regionenvisaged in the unified model, given that in most of these casesindependent spectropolarimetry data find a hidden Seyfert 1. (4) The FCstrengths obtained by the spectral decomposition are substantiallylarger for the Seyfert 2s which present evidence of broad lines,implying that the scattered non-stellar continuum is also detected. (5)There is no correlation between the star formation in the nucleus andeither the central or overall morphology of the parent galaxies.
| A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.
| 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.
| Near-infrared K-Band Spectroscopic Investigation of Seyfert 2 Nuclei in the CfA and 12 Micron Samples
We present near-infrared K-band slit spectra of the nuclei of 25 Seyfert2 galaxies in the CfA and 12 μm samples. The strength of the COabsorption features at 2.3-2.4 μm produced by stars is measured interms of a spectroscopic CO index. A clear anticorrelation between theobserved CO index and the nuclear K-L color is present, suggesting thata featureless hot dust continuum heated by an active galactic nucleus(AGN) contributes significantly to the observed K-band fluxes in thenuclei of Seyfert 2 galaxies. After correction for this AGNcontribution, we estimate nuclear stellar K-band luminosities for allsources and CO indices for sources with modestly large observed COindices. The corrected CO indices for 10 (=40%) Seyfert 2 nuclei arefound to be as high as those observed in star-forming or elliptical(=spheroidal) galaxies. We combine the K-band data with measurements ofthe L-band 3.3 μm polycyclic aromatic hydrocarbon (PAH) emissionfeature, another powerful indicator for star formation, and find thatthe 3.3 μm PAH to K-band stellar luminosity ratios are substantiallysmaller than those of starburst galaxies. Our results suggest that the3.3 μm PAH emission originates in the putative nuclear starbursts inthe dusty tori surrounding the AGNs, because of its high surfacebrightness, whereas the K-band CO absorption features detected at thenuclei are dominated by old bulge (=spheroid) stars and thus may not bea powerful indicator for the nuclear starbursts. We see no cleardifference in the strength of the CO absorption and PAH emissionfeatures between the CfA and 12 μm Seyfert 2 galaxies.
| 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
| 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.
| A Possible Signature of Connection between Blazars and Seyfert Galaxies
The accretion rates (dot{M}) and their correlation with cosmologicalredshifts for a sample of blazars and Seyfert galaxies are presented.The sample includes 77 blazars (28 FSRQs, 26 LBLs, and 23 HBLs) and 60Seyfert galaxies, of which the extended spectral energy distributioninformation and redshifts are available. Within the framework ofaccreting black holes, the accretion rates for these sources wereestimated based on their bolometric luminosities. The result shows thatthe accretion rates are significantly different for each subclass of theblazars and Seyfert galaxies. Their averages are, respectively, 50.2,17.0, 1.0, 0.1Modot yr-1 for the FSRQs, LBLs, HBLs, and theSeyfert galaxies, exhibiting a well descending sequence ofFSRQs-LBLs-HBLs-Seyfert galaxies. They are strongly correlated with theredshifts for both blazars and Seyfert galaxies. The linear correlationcoefficients are 0.81 and 0.68 with a chance probab ility of p <0.0001, respectively. A plot of dot{M} - z shows that the blazars andthe Seyfert galaxies distribute in a distinguishable regions with aconnection at z 0.7 and almost all the sources lie in a narrowregion of z1.40 ≤ dot{M} ≤ 250 z1.40,illustrating a strong correlation between the two quantities for thewhole sample. The regression line is dot{M} = (14.5 ± 1.2)z1.40±0.06 Modot yr-1 with a linearcoefficient of 0.93 and a chance probability of p < 0.0001,suggesting a connection between blazars and Seyfert galaxies. Thisconnection might imply that the two classes are on the same evolutionarysequence. Although the correlations of the data are formally solid, theconclusion may be affected by one source of considerable uncertainty atthe data level, which is also discussed.
| A Fundamental Plane of black hole activity
We examine the disc-jet connection in stellar mass and supermassiveblack holes by investigating the properties of their compact emission inthe X-ray and radio bands. We compile a sample of ~100 active galacticnuclei with measured masses, 5-GHz core emission, and 2-10 keVluminosities, together with eight galactic black holes with a total of~50 simultaneous observations in the radio and X-ray bands. Using thissample, we study the correlations between the radio (LR) andthe X-ray (LX) luminosity and the black hole mass (M). Wefind that the radio luminosity is correlated with bothM andLX, at a highly significant level. In particular, we showthat the sources define a `Fundamental Plane' in the three-dimensional(logLR, logLX, logM) space, given bylogLR= (0.60+0.11-0.11)logLX+ (0.78+0.11-0.09) logM+7.33+4.05-4.07, with a substantial scatter ofσR= 0.88. We compare our results to the theoreticalrelations between radio flux, black hole mass, and accretion ratederived by Heinz & Sunyaev. Such relations depend only on theassumed accretion model and on the observed radio spectral index.Therefore, we are able to show that the X-ray emission from black holesaccreting at less than a few per cent of the Eddington rate is unlikelyto be produced by radiatively efficient accretion, and is marginallyconsistent with optically thin synchrotron emission from the jet. On theother hand, models for radiatively inefficient accretion flows seem toagree well with the data.
| 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.
| 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.
| 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.
| 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.
| Active Galactic Nucleus Black Hole Masses and Bolometric Luminosities
Black hole mass, along with mass accretion rate, is a fundamentalproperty of active galactic nuclei (AGNs). Black hole mass sets anapproximate upper limit to AGN energetics via the Eddington limit. Wecollect and compare all AGN black hole mass estimates from theliterature; these 177 masses are mostly based on the virial assumptionfor the broad emission lines, with the broad-line region size determinedfrom either reverberation mapping or optical luminosity. We introduce200 additional black hole mass estimates based on properties of the hostgalaxy bulges, using either the observed stellar velocity dispersion orthe fundamental plane relation to infer σ these methods assumethat AGN hosts are normal galaxies. We compare 36 cases for which blackhole mass has been generated by different methods and find, forindividual objects, a scatter as high as a couple of orders ofmagnitude. The less direct the method, the larger the discrepancy withother estimates, probably due to the large scatter in the underlyingcorrelations assumed. Using published fluxes, we calculate bolometricluminosities for 234 AGNs and investigate the relation between blackhole mass and luminosity. In contrast to other studies, we find nosignificant correlation of black hole mass with luminosity, other thanthose induced by circular reasoning in the estimation of black holemass. The Eddington limit defines an approximate upper envelope to thedistribution of luminosities, but the lower envelope depends entirely onthe sample of AGNs included. For any given black hole mass, there is arange in Eddington ratio of up to 3 orders of magnitude.
| 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 Relation between Mid-Infrared Emission and Black Hole Mass in Active Galactic Nuclei: A Direct Way to Probe Black Hole Growth?
We use a large, heterogeneous sample of local active galactic nuclei(AGNs) that includes Seyfert 1 galaxies, Seyfert 2 galaxies, andPalomar-Green quasars to investigate for the first time the relationbetween black hole mass (MBH) and mid-infrared nuclearemission. We find a clear relation between MBH and 10 μmnuclear luminosity for these local AGNs. There are no significantdifferences between type 1 and type 2 objects, implying that thereprocessing of the 10 μm nuclear emission is not severely affectedby geometric and optical depth effects. We also confirm thatMBH is related to the 2-10 keV X-ray luminosity, but only forCompton-thin galaxies. We present a theoretical basis for theseempirical relations and discuss possible reasons for the observedscatter. Our results show that rest-frame 10 μm and hard X-rayluminosities (especially the former, which is applicable to all AGNtypes) can be powerful tools for conducting a census of BH masses athigh redshift and for probing their cosmological evolution.
| 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.
| Nested and Single Bars in Seyfert and Non-Seyfert Galaxies
We analyze the observed properties of nested and single stellar barsystems in disk galaxies. The 112 galaxies in our sample comprise thelargest matched Seyfert versus non-Seyfert galaxy sample of nearbygalaxies with complete near-infrared or optical imaging sensitive tolength scales ranging from tens of parsecs to tens of kiloparsecs. Thepresence of bars is deduced by fitting ellipses to isophotes in HubbleSpace Telescope (HST) H-band images up to 10" radius and in ground-basednear-infrared and optical images outside the H-band images. This is aconservative approach that is likely to result in an underestimate ofthe true bar fraction. We find that a significant fraction of the samplegalaxies, 17%+/-4%, have more than one bar, and that 28%+/-5% of barredgalaxies have nested bars. The bar fractions appear to be stableaccording to reasonable changes in our adopted bar criteria. For thenested bars, we detect a clear division in length between thelarge-scale (primary) bars and small-scale (secondary) bars, in bothabsolute and normalized (to the size of the galaxy) length. We arguethat this bimodal distribution can be understood within the framework ofdisk resonances, specifically the inner Lindblad resonances (ILRs),which are located where the gravitational potential of the innermostgalaxy switches effectively from three-dimensional to two-dimensional.This conclusion is further strengthened by the observed distribution ofthe sizes of nuclear rings which are dynamically associated with theILRs. While primary bar sizes are found to correlate with the hostgalaxy sizes, no such correlation is observed for the secondary bars.Moreover, we find that secondary bars differ morphologically from singlebars. Our matched Seyfert and non-Seyfert samples show a statisticallysignificant excess of bars among the Seyfert galaxies at practically alllength scales. We confirm our previous results that bars are moreabundant in Seyfert hosts than in non-Seyfert galaxies and that Seyfertgalaxies always show a preponderance of ``thick'' bars compared to thebars in non-Seyfert galaxies. Finally, no correlation is observedbetween the presence of a bar and that of companion galaxies, evenrelatively bright ones. Overall, since star formation and dustextinction can be significant even in the H band, the stellar dynamicsof the central kiloparsec cannot always be revealed reliably by the useof near-infrared surface photometry alone.
| Far-Infrared Census of Starburst-Seyfert Connection
Far-infrared flux densities are newly extracted from the IRAS databasefor the Revised Shapley-Ames and CfA complete samples of Seyfertgalaxies. These data are used to classify the Seyfert galaxies intothose where the far-infrared continuum emission is dominated by theactive galactic nucleus (AGN), circumnuclear starburst, or host galaxy.While AGN-dominant objects consist of comparable numbers of Seyfert 1and 2 galaxies, starburst- and host-dominant objects consistpreferentially of Seyfert 2 galaxies. Thus, in addition to the dustytorus, the circumnuclear starburst region and host galaxy are importantin hiding the broad-line region. Morphologically, starburst-dominantSeyfert galaxies are of later types and more strongly interacting thanAGN-dominant Seyfert galaxies. In a later type galaxy, the AGN centralengine has a lower Eddington luminosity, and the gaseous content ishigher. The gas is efficiently supplied to the starburst via agalaxy-galaxy interaction. Morphologies of host-dominant Seyfertgalaxies are of various types. Since starbursts in Seyfert galaxies areolder than those in classical starburst galaxies, we propose anevolution from starburst to starburst-dominant Seyfert to host-dominantSeyfert for a late-type galaxy. An evolution from AGN-dominant Seyfertto host-dominant Seyfert is proposed for an early-type galaxy. Thesesequences have durations of a few times 108 yr and occurrepeatedly within a galaxy during its evolution from a late type to anearly type.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| MERLIN radio observations of CfA Seyferts
We present new 1.6-GHz (18-cm) MERLIN maps of 15 Seyfert galaxies, withangular resolutions typically 0.1 to 0.3arcsec. These and previousobservations are used to investigate the properties of 19 of the 24 CfASeyfert galaxies brighter than 2mJy at 8.4GHz. This is the first time asignificant fraction of the CfA sample has been mapped at this frequencywith subarcsecond resolution, and our observations provide the highestresolution radio maps available for several sources. We use ourobservations to measure the two-point spectral indices of compact radiocomponents, and we investigate the correlation between infrared andradio emission shown by Seyfert galaxies. Our results can be summarizedas follows. Resolved structures as small as 20pc are found in threepreviously unresolved radio sources, and only four sources show single,unresolved radio components. The mean 1.6 to 8.4GHz spectral index of 31radio components is α=-0.6+/-0.1 (S~να), andapproximately 25 per cent of the components have a spectral indexflatter than α=-0.3. The spectral index distributions of type 1and type 2 Seyferts are statistically indistinguishable. The cores ofmultiple-component sources tend to have flatter radio spectra thansecondary components. The low-resolution infrared (IRAS) emission fromSeyfert galaxies is usually dominated by kiloparsec-scale, extranuclearemission regions.
| Circumnuclear kinematics in NGC 5248: the origin of nuclear spiral arms
We present for the first time a two-dimensional velocity field of thecentral region of the grand-design spiral galaxy NGC 5248, at 0.9-arcsecspatial resolution. The Hα velocity field is dominated by circularrotation. While no systematic streaming motions are seen in the area ofthe nuclear grand-design spiral or the circumnuclear ring, the amplitudeof residual velocities, after subtracting a model circular velocityfield, reaches 20kms-1 in projection. The rotation curvelevels out at around 140kms-1, after a well-resolved andrather shallow rise. We have generated an analytical model for thenuclear spiral and fitted it to our observations to obtain estimates ofthe pattern speed of the spiral and the speed of sound in the centralregion of NGC 5248. Our results are consistent with a low pattern speed,suggesting that the nuclear spiral rotates with the same rate as themain spiral structure in NGC 5248, and thus that the spiral structure iscoupled from scales of a few hundred parsecs to several kiloparsecs. Wehave also compared the observed structure and kinematics between thenuclear regions of NGC 5248 and M100. Several similarities anddifferences are discussed, including the location of the peak emissionregions on major and minor axes, and the spiral arm streaming motions.We find no kinematic evidence for the presence of a nuclear bar in NGC5248.
| Hubble Space Telescope Observations of the CfA Seyfert 2 Galaxies: Near-Infrared Surface Photometry and Nuclear Bars
We present near-infrared J and H surface photometry of 24 of the nearbySeyfert 1.8, 1.9, and 2 galaxies from the CfA Seyfert sample. Theexcellent angular resolution of the Hubble Space Telescope probesspatial scales as small as tens of parsecs in most of these activegalactic nuclei (AGNs) and is sensitive to the presence of nuclear barsand other potential signatures of the AGN fueling process that channelshost galaxy gas and dust to the nuclear region. We have used ellipticalisophote techniques to search for nuclear bars in all of these galaxiesand have employed a two-dimensional fitting technique to model thenuclear point source and surface brightness distribution of a brightsubsample of these galaxies in an attempt to alleviate the impact of thenuclear point source on our sensitivity to nuclear bars. We find stellarnuclear bar candidates in four of these galaxies: Mrk 471, Mrk 270, Mrk573, and NGC 5929, nearly 20% of the total sample. The percentage risesto ~30% when systems with disturbed morphologies or high inclinationsare excluded. The nuclear bars in Mrk 573 and Mrk 270 exhibit someevidence for dust lanes along their leading edges, analogous to thestructures seen in host galaxy bars, while the dust lanes in Mrk 471 andNGC 5929 exhibit a more complex morphology. The fact that most of theseAGNs do not appear to contain stellar nuclear bars suggests that theyare not the fueling mechanism for most low-luminosity AGNs. Based onobservations with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| Empirical Diagnostics of the Starburst-AGN Connection
We examine a representative sample of 35 Seyfert 2 nuclei. Previous workhas shown that nearly half (15) of these nuclei show the direct (butdifficult to detect) spectroscopic signature at optical/near-UVwavelengths of the hot massive stars that power circumnuclearstarbursts. In the present paper we examine a variety of more easilymeasured quantities for this sample, such as the equivalent widths ofstrong absorption features, continuum colors, emission line equivalentwidths, emission line ratios and profiles, far-IR luminosities, andnear-UV surface brightness. We compare the composite starburst+Seyfert 2nuclei to ``pure'' Seyfert 2 nuclei, Starburst galaxies, and normalgalactic nuclei. Our goals are to verify whether the easily measuredproperties of the composite nuclei are consistent with the expectedimpact of a starburst and to investigate alternative less demandingmethods to infer the presence of starbursts in Seyfert 2 nuclei,applicable to larger or more distant samples. We show that starbursts doindeed leave clear and easily quantifiable imprints on the near-UV tooptical continuum and emission line properties of Seyfert 2's. Compositestarburst+Seyfert 2 systems can be recognized by: (1) a strong``featureless continuum'' (FC), which dilutes the Ca II K line from oldstars in the host's bulge to an equivalent width WK<10Å (2) emission lines whose equivalent widths are intermediatebetween starburst galaxies and ``pure'' Seyfert 2's (3) relatively lowexcitation line ratios, which indicate that part of the gas ionizationin these Seyfert 2's (typically ~50% of Hβ) is due tophotoionization by OB stars; (4) large far-IR luminosities(>~1010 Lsolar) (5) high near-UV surfacebrightness (~103 Lsolar pc-2). Thesecharacteristics are all consistent with the expected impact ofcircumnuclear starbursts on the observed properties of Seyfert 2's.Furthermore, they offer alternative empirical diagnostics of thepresence of circumnuclear starbursts from a few easily measuredquantities.
| A Far-Infrared Study of the CfA Seyfert Sample. I. The Data
We present mid- and far-IR Infrared Space Observatory data of the CfASeyfert galaxy sample. These data allow a detailed study of the far-IRspectral energy distribution (SED) of these galaxies. A Bayesianinversion method has been used to invert the SED of these objects and toidentify three characteristic temperature ranges of dust emission. Themethod yields two fundamental results, namely, (1) that the mid- andfar-IR SED of Seyfert galaxies can be explained solely through thermalreradiation of higher energy photons by dust; and (2) that this thermalemission is made up of three different components, a warm, a cold, and avery cold dust component. These components are characterized by a peaktemperature and their emission dominated in each case by a singleastrophysical mechanism. These mechanisms have been readily explained asproduced, respectively, by warm dust heated by either the active nucleusor circumnuclear starbursts, cold dust heated by a star-forming regionin the galaxy disk, and very cold dust heated by the generalinterstellar radiation field. Comparisons between the parametersobtained from the analysis of the IR SEDs (fluxes, temperatures,luminosities) have been made. Our results suggest that the emission inthe mid-IR is anisotropic and the differences found between Seyfert 1and Seyfert 2 galaxies could be explained with thin molecular torimodels. Based on observations with the Infrared Space Observatory (ISO),an ESA project with instruments funded by ESA Member States (especiallythe PI countries: France, Germany, the Netherlands, and the UnitedKingdom) and with the participation of ISAS and NASA.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
Katalógusok és elnevezések:
|
