Contents
Images
Upload your image
DSS Images Other Images
Related articles
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.
| A Simple Test for the Existence of Two Accretion Modes in Active Galactic Nuclei
By analogy to the different accretion states observed in black holeX-ray binaries (BHXBs), it appears plausible that accretion disks inactive galactic nuclei (AGNs) undergo a state transition between aradiatively efficient and inefficient accretion flow. If the radiativeefficiency changes at some critical accretion rate, there will be achange in the distribution of black hole masses and bolometricluminosities at the corresponding transition luminosity. To test thisprediction, I consider the joint distribution of AGN black hole massesand bolometric luminosities for a sample taken from the literature. Thesmall number of objects with low Eddington-scaled accretion ratesm˙<0.01 and black hole massesMBH<109Msolar constitutes tentativeevidence for the existence of such a transition in AGNs. Selectioneffects, in particular those associated with flux-limited samples,systematically exclude objects in particular regions of the(MBH,Lbol) plane. Therefore, they requireparticular attention in the analysis of distributions of black holemass, bolometric luminosity, and derived quantities such as theaccretion rate. I suggest further observational tests of the BHXB-AGNunification scheme that are based on the jet domination of the energyoutput of BHXBs in the hard state, and of the possible equivalence ofBHXB in the very high (or steep power-law) state showing ejections andefficiently accreting quasars and radio galaxies with powerful radiojets.
| Structural parameters of nearby emission-line galaxies
We present the results of an investigation on the main structuralproperties derived from VRI and Hα surface photometry of galaxieshosting nuclear emission-line regions [including Seyfert 1, Seyfert 2,low-ionization nuclear emission region (LINER) and starburst galaxies]as compared with normal galaxies. Our original sample comprises 22active galaxies, four starbursts and one normal galaxy and has beenextended with several samples obtained from the literature. Bulge anddisc parameters, along with the bulge-to-disc luminosity ratio, havebeen derived applying an iterative procedure. The resulting parametershave been combined with additional data in order to reach astatistically significant sample. We find some differences in the bulgedistribution across the different nuclear types that could implyfamilies of bulges with different physical properties. Bulge and disccharacteristic colours have been defined and derived for our sample andcompared with a control sample of early-type objects. The resultssuggest that bulge and disc stellar populations are comparable in normaland active galaxies.
| Evidence for Interactions in H I Imaging of Seyfert Galaxies
We still do not know the primary trigger of local active galacticnuclei. Tidal torques and nuclear bars are often invoked to explain gasinfall onto the central supermassive black hole, but as yet neither hasdefinitive observational support. Here we exploit the sensitivity andendurance of H I to trace the strength and prevalence of tidalinteractions among Seyfert galaxies. To minimize selection biases, wesurvey the 27 northern Seyferts compiled in the comprehensive opticalcatalog of Véron-Cetty & Véron lying in the narrowredshift range 0.015-0.017. This paper is a detailed presentation of asubsample, including channel maps for all systems. In addition to theprogram objects, we also investigate galaxies imaged in the field.
| 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.
| 10 Micron Imaging of Seyfert Galaxies from the 12 Micron Sample
We present small-aperture (1.5") photometry and new high-resolutionimages at 10 μm (N band) for 87 Seyfert galaxies from the Extended 12μm Sample drawn from the IRAS database. With this data we hope totest the predictions of the unified model for active galactic nuclei andto search for bright, extended circumnuclear 10 μm emission. Wedetected 62 Seyfert galaxies, 18 of which have no previously publishedsmall-aperture photometry. All the detected sources, both Seyfert 1'sand Seyfert 2's, show a central point source. The 31 Seyfert 1's and 31Seyfert 2's that we detected have similar luminosity distributions.Except for previously known bright extended 10 μm structure aroundArp 220, NGC 1068, and NGC 7469, we see definitive evidence for brightextended emission around only one new object: Mrk 1239. Four otherSeyfert 1's and six other Seyfert 2's show evidence of faint, low-levelextended emission. One Seyfert 1 and two Seyfert 2's show evidence ofsignificantly increased flux over previously published small-aperturevalues. We also compared the N-band data with the J-Ks colorthat we derived from the Two Micron All Sky Survey (2MASS). There is adistinct trend of redder central bulge J-Ks colorscorresponding to brighter absolute N-band magnitudes. In color-magnitudespace there is a definite grouping of Seyfert 1's and Seyfert 2's, withtwo sets of outliers.
| Seyfert galaxies in UZC-Compact Groups
We present results concerning the occurrence of Seyfert galaxies in anew automatically selected sample of nearby Compact Groups of galaxies(UZC-CGs). Seventeen Seyferts are found, constituting 3% of theUZC-CG galaxy population. CGs hosting and non-hosting a Seyfert memberexhibit no significant differences, except that a relevant number of Sy2is found in unusual CGs, all presenting large velocity dispersion(σ>400 km s-1), many neighbours and a high number ofellipticals. We also find that the fraction of Seyferts in CGs is 3times as large as that among UZC-single-galaxies, and results from anexcess of Sy2s. CG-Seyferts are not more likely than other CG galaxiesto present major interaction patterns, nor to display a bar. Our resultsindirectly support the minor-merging fueling mechanism.
| 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.
| 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.
| 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 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.
| Compact groups in the UZC galaxy sample
Applying an automatic neighbour search algorithm to the 3D UZC galaxycatalogue (Falco et al. \cite{Falco}) we have identified 291 compactgroups (CGs) with radial velocity between 1000 and 10 000 kms-1. The sample is analysed to investigate whether Tripletsdisplay kinematical and morphological characteristics similar to higherorder CGs (Multiplets). It is found that Triplets constitute lowvelocity dispersion structures, have a gas-rich galaxy population andare typically retrieved in sparse environments. Conversely Multipletsshow higher velocity dispersion, include few gas-rich members and aregenerally embedded structures. Evidence hence emerges indicating thatTriplets and Multiplets, though sharing a common scale, correspond todifferent galaxy systems. Triplets are typically field structures whilstMultiplets are mainly subclumps (either temporarily projected orcollapsing) within larger structures. Simulations show that selectioneffects can only partially account for differences, but significantcontamination of Triplets by field galaxy interlopers could eventuallyinduce the observed dependences on multiplicity. Tables 1 and 2 are onlyavailable in electronic at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/35
| High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - II. The properties of compact radio components
We discuss the properties of compact nuclear radio components in Seyfertgalaxies from the extended 12-μm AGN sample of Rush et al. Our mainresults can be summarized as follows. Type 1 and type 2 Seyferts producecompact radio components which are indistinguishable in strength andaspect, indicating that their central engines are alike, as proposed bythe unification model. Infrared IRAS fluxes are more closely correlatedwith low-resolution radio fluxes than high-resolution radio fluxes,suggesting that they are dominated by kiloparsec-scale, extranuclearemission regions; extranuclear emission may be stronger in type 2Seyferts. Early-type Seyfert galaxies tend to have stronger nuclearradio emission than late-type Seyfert galaxies. V-shaped extendedemission-line regions, indicative of `ionization cones', are usuallyfound in sources with large, collimated radio outflows. Hidden broadlines are most likely to be found in sources with powerful nuclear radiosources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm fluxdensities have well-matched properties.
| Hidden Broad-Line Seyfert 2 Galaxies in the CFA and 12 μM Samples
We report the results of a spectropolarimetric survey of the CfA and 12μm samples of Seyfert 2 (S2) galaxies. Polarized (hidden) broad-lineregions (HBLRs) are confirmed in a number of galaxies, and several newcases (F02581-1136, MCG -3-58-7, NGC 5995, NGC 6552, NGC 7682) arereported. The 12 μm S2 galaxy sample shows a significantly higherincidence of HBLRs (50%) than its CfA counterpart (30%), suggesting thatthe latter may be incomplete in hidden active galactic nuclei. Comparedto the non-HBLR S2 galaxies, the HBLR S2 galaxies display distinctlyhigher radio power relative to their far-infrared output and hotter dusttemperature as indicated by the f25/f60 color.However, the level of obscuration is indistinguishable between the twotypes of S2 galaxies. These results strongly support the existence oftwo intrinsically different populations of S2 galaxies: one harboring anenergetic, hidden S1 nucleus with a broad-line region and the other a``pure'' S2 galaxy, with a weak or absent S1 nucleus and a strong,perhaps dominating starburst component. Thus, the simple purelyorientation-based unification model is not applicable to all Seyfertgalaxies.
| High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - I. The observations
We present 8.4-GHz VLA A-configuration observations of 87 sources fromthe mid-infrared-selected AGN sample of Rush et al. These0.25-arcsec-resolution observations allow elongated radio structurestens of pc in size to be resolved, and enable radio components smallerthan 3.5arcsec to be isolated from diffuse galactic disc emission. Whencombined with previous data, matched radio observations covering 90percent of the sample have been collected, and these represent the largestsubarcsecond-resolution radio imaging survey of a homogeneously selectedsample of Seyfert galaxies to date. We use our observations to identifyfive radio-loud AGN in the sample. The nature of the radio emission fromSeyfert nuclei will be discussed in subsequent papers.
| Broad-band and Hα surface photometry of the central regions of nearby active galaxies - I. Observations
Broad-band V, R and I and narrow-band Hα CCD surface photometry ofa sample of 24 active and three normal nearby galaxies is presented, inorder to study their morphological and structural properties. This firstpaper presents the observations, data reduction and processingtechniques used; the main characteristics of the central regions of thesample galaxies are derived from broad-band and Hα images;broad-band surface brightness and colour profiles are presented; basedon the inspection of individual images and the examination of the radialvariation of ellipticity and polar angle, the morphological class of thesample objects is revised: several objects previously classified asnon-barred or unclassified are found to be barred galaxies. A highfraction (~71per cent) of the objects that present any kind of nuclearactivity are barred. This proportion is even higher (~87per cent) ifonly Seyfert galaxies are considered. This result can reinforce theclues about the connection between bars and nuclear activity as afuelling mechanism that can trigger and sustain nuclear activity.
| Jet Directions in Seyfert Galaxies: B and I Imaging Data
We present the results of broadband B and I imaging observations for asample of 88 Seyfert galaxies (29 Seyfert 1s and 59 Seyfert 2s),selected from a mostly isotropic property, the flux at 60 μm. We alsopresent the B and I imaging results for an additional sample of 20Seyfert galaxies (7 Seyfert 1s and 13 Seyfert 2s), selected from theliterature and known to have extended radio emission. The I-band imagesare fitted with ellipses to determine the position angle and ellipticityof the host galaxy major axis. This information will be used in a futurepaper, combined with information from radio observations, to study theorientation of radio jets relative to the planes of their host galaxies.Here we present surface brightness profiles and magnitudes in the B andI bands, as well as mean ellipticities and major axis position angles.
| Jet Directions in Seyfert Galaxies
Here we present the study of the relative angle between the accretiondisk (or radio jet) and the galaxy disk for a sample of Seyfert galaxiesselected from a mostly isotropic property, the 60 μm flux, and warminfrared colors. We used VLA A-array 3.6 cm continuum data andground-based optical imaging, homogeneously observed and reduced tominimize selection effects. For parts of the analysis we enlarged thesample by including galaxies serendipitously selected from theliterature. For each galaxy we have a pair of points (i, δ), whichare the inclination of the galaxy relative to the line of sight and theangle between the jet projected into the plane of the sky and the hostgalaxy major axis, respectively. For some galaxies we also hadinformation about which side of the minor axis is closer to Earth. Thisdata is combined with a statistical technique, developed by us, todetermine the distribution of β angles in three dimensions, theangle between the jet and the host galaxy plane axis. We found from aninitial analysis of the data of the 60 μm sample, where Seyfert 1 and2 galaxies were not differentiated, that the observed distribution of iand δ values can be well represented either by a homogeneoussinβ distribution in the range0deg<=β<=90deg or in0deg<=β<=65deg, but not by anequatorial ring. A more general model, which tested β-distributionsin the range β1<=β<=β2, fordifferent ranges of β1 and β2 values,required β2 to be larger than 65° and gavepreference for β1 smaller than 40°-50°. Animportant result from our analysis was obtained when we determinedwhether the jet was projected against the near or the far side of thegalaxy and differentiated between Seyfert 1 and Seyfert 2 galaxies,which showed that the model could not represent Seyfert 1 galaxiesadequately. We found that the inclusion of viewing angle restrictionsfor Seyfert 1 galaxies, namely, that a galaxy can be recognized as aSeyfert 1 only if the angle between the jet and the line of sight(|φ|) is smaller than a given angle φc and that thegalaxy inclination i is smaller than an angle ic, gave riseto statistically acceptable models. This indication that there is adifference in viewing angle to the central engine between Seyfert 1galaxies and Seyfert 2 galaxies is a direct and independent confirmationof the underlying concepts of the unified model. We discuss possibleexplanations for the misalignment between the accretion disk and thehost galaxy disk: warping of the accretion disk by self-irradiationinstability, by the Bardeen-Petterson effect, or by a misalignedgravitational potential of a nuclear star cluster surrounding the blackhole, as well as feeding of the accretion disk by a misaligned inflow ofgas from minor mergers, capture of individual stars or gas from thenuclear star cluster, and the capture of individual molecular cloudsfrom the host galaxy.
| Molecular torus in Seyfert galaxies.
Not Available
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| Morphology of the 12 Micron Seyfert Galaxies. II. Optical and Near-Infrared Image Atlas
We present 263 optical and near-infrared (NIR) images for 42 1s and 48Seyfert 2s, selected from the Extended 12 μm Galaxy Sample.Elliptically averaged profiles are derived from the images, andisophotal radii and magnitudes are calculated from these. We also reportvirtual aperture photometry that, judging from comparison with previouswork, is accurate to roughly 0.05 mag in the optical, and 0.07 mag inthe NIR. Our B-band isophotal magnitude and radii, obtained from ellipsefitting, are in good agreement with those of Third Reference Catalogueof Bright Galaxies. When compared with the B band, V, I, J, and Kisophotal diameters show that the colors in the outer regions of Seyfertgalaxies are consistent with the colors of normal spirals. Differencesin the integrated isophotal colors and comparison with a simple modelshow that the active nucleus+bulge are stronger and redder in the NIRthan in the optical. Finally, roughly estimated Seyfert disk surfacebrightnesses are significantly brighter in B and K than those in normalspirals of similar morphological type.
| Radio Structures of Seyfert Galaxies. VIII. A Distance- and Magnitude-Limited Sample of Early-Type Galaxies
The VLA has been used at 3.6 and 20 cm to image a sample of about 50early-type Seyfert galaxies with recessional velocities less than 7000km s^-1 and total visual magnitude less than 14.5. Emission-line ([OIII] and Hα+[N II]) and continuum (green and red) imaging of thissample has been presented in a previous paper. In this paper, we presentthe radio results, discuss statistical relationships between the radioand other properties, and investigate these relationships within thecontext of unified models of Seyferts. The mean radio luminosities ofearly-type Seyfert 1's (i.e., Seyfert 1.0's, 1.2's, and 1.5's) and 2.0'sare found to be similar (consistent with the unified scheme), and theradio luminosity is independent of morphological type within thissample. The fraction of resolved radio sources is larger in the Seyfert2.0's (93%) than in the Seyfert 1's (64%). However, the mean radioextents of Seyfert 2.0's and 1's are not significantly different,although this result is limited by the small number of resolved Seyfert1's. The nuclear radio structures of Seyfert 2.0's in the early-typesample tend to be aligned with the [O III] and Hα+[N II]structures, even though the radio extents are smaller than the [O III]and Hα+[N II] extents by a factor of ~2 to more than 5. Thisalignment, previously known for individual Seyferts with ``linear''radio sources, is here shown to be characteristic of early-type Seyfertgalaxies as a class. Seyfert 2.0's in the early-type sample also show asignificant alignment between the emission-line ([O III] and Hα+[NII]) axes and the major axis of the host galaxy. These alignments areconsistent with a picture in which the ionized gas represents ambientgas predominantly coplanar with the galaxy stellar disk. This ambientgas is ionized by nuclear radiation that may escape preferentially alongand around the radio axis, and is compressed in shocks driven by theradio ejecta. We use this alignment to constrain the product of thevelocity of the radio ejecta and the period of any large-angleprecession of the inner accretion disk and jet: V_ejectaxP>=2 kpc. Aninvestigation of a larger sample of Seyferts reveals the unexpectedresult that the Seyfert 1's with the largest radio extent (>=1.5 kpc)are all of type Seyfert 1.2. It appears that classification as this typeof intermediate Seyfert depends on some factor other than the relativeorientation of the nuclear-obscuring torus to the line of sight. Amongall the other Seyferts, the distribution of radio extent with Seyfertintermediate type is consistent with the expectations of the unifiedscheme.
| The Distribution of Absorbing Column Densities among Seyfert 2 Galaxies
We use hard X-ray data for an ``optimal'' sample of Seyfert 2 galaxiesto derive the distribution of the gaseous absorbing column densitiesamong obscured active nuclei in the local universe. Of all Seyfert 2galaxies in the sample, 75% are heavily obscured (N_H>10^23 cm^-2),and about half are Compton thick (N_H>10^24 cm^-2). Intermediate type1.8-1.9 Seyfert galaxies are characterized by an average N_H much lowerthan ``strict'' Seyfert 2 galaxies. No correlation is found between N_Hand the intrinsic luminosity of the nuclear source. This N_Hdistribution has important consequences for the synthesis of the cosmicX-ray background. In addition, the large fraction of Compton-thickobjects implies that most of the obscuring gas is located within aradius of a few 10 pc from the nucleus.
| The Relative Orientation of Nuclear Accretion and Galaxy Stellar Disks in Seyfert Galaxies
We use the difference (delta) between the position angles of the nuclearradio emission and the host galaxy major axis to investigate thedistribution of the angle (beta) between the axes of the nuclearaccretion disk and the host galaxy disk in Seyfert galaxies. We providea critical appraisal of the quality of all measurements, and find thatthe data are limited by observational uncertainties and biases, such asthe well known deficiency of Seyfert galaxies of high inclination. Thereis weak evidence that the distribution of delta for Seyfert 2 galaxiesmay be different (at the 90% confidence level) from a uniformdistribution, while the Seyfert 1 delta distribution is notsignificantly different from a uniform distribution or from the Seyfert2 delta distribution. The cause of the possible nonuniformity in thedistribution of delta for Seyfert 2 galaxies is discussed. Seyfertnuclei in late-type spiral galaxies may favor large values of delta (atthe ~96% confidence level), while those in early-type galaxies show amore or less random distribution of delta. This may imply that thenuclear accretion disk in noninteracting late-type spirals tends toalign with the stellar disk, while that in early-type galaxies is morerandomly oriented, perhaps as a result of accretion following a galaxymerger. We point out that biases in the distribution of inclinationtranslate to biased estimates of beta in the context of the unifiedscheme. When this effect is taken into account, the distributions ofbeta for all Seyferts together, and of Seyfert 1s and 2s separately,agree with the hypothesis that the radio jets are randomly oriented withrespect to the galaxy disk. The data are consistent with theexpectations of the unified scheme, but do not demand it.
| Thomson scattering origin of the polarization of Seyfert spectra.
Not Available
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
