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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.
| Spitzer IRS Spectra of a Large Sample of Seyfert Galaxies: A Variety of Infrared Spectral Energy Distributions in the Local Active Galactic Nucleus Population
We are conducting a large observing program with the Spitzer SpaceTelescope to determine the mid- to far-IR spectral energy distributionsof a well-defined sample of 87 nearby, 12 μm-selected Seyfertgalaxies. In this paper we present the results of Spitzer IRSlow-resolution spectroscopy of a statistically representative subsampleof 51 of the galaxies (59%), with an analysis of the continuum shapesand a comparison of the Seyfert types. We find that the spectra clearlydivide into groups based on their continuum shapes and spectralfeatures. The largest group (47% of the sample of 51) shows a very redcontinuum suggestive of cool dust and strong emission featuresattributed to polycyclic aromatic hydrocarbons. Sixteen objects (31%)have a power-law continuum with spectral indices of α5-20μm=-2.3 to -0.9 that flatten to α20-35μm=-1.1 to 0.0 at ~20 μm. Clear silicate emission featuresat 10 and 18 μm are found in two of these objects (Mrk 6 and Mrk335). A further 16% of the sample show power-law continua withunchanging slopes of α5-35 μm=-1.7 to -1.1. Twoobjects are dominated by a broad silicate absorption feature. One objectin the sample shows an unusual spectrum dominated by emission features,which is unlike any of the other spectra. Some spectral features areclearly related to a starburst contribution to the IR spectrum, whilethe mechanisms producing observed power-law continuum shapes, attributedto an active galactic nucleus (AGN) component, may be dust or nonthermalemission. The IR spectral types appear to be related to the Seyferttypes. Principal component analysis results suggest that the relativecontribution of starburst emission may be the dominant cause of variancein the observed spectra. The derived starburst component of eachspectrum, however, contributes <40% of the total flux density. Wecompare the IR emission with the optically thin radio emissionassociated with the AGN and find that Seyfert 1 galaxies have higherratios of IR to radio emission than Seyfert 2 galaxies, as predicted bythe unified model if the torus is optically thick in the mid-IR.However, smooth-density torus models predict a much larger differencebetween Seyfert types 1 and 2 than the factor of 2 difference observedin our sample; the observed factor of ~2 difference between the type 1and type 2 galaxies in their IR-to-radio ratios above 15 μm requiresthe standard smooth-density torus models to be optically thin at thesewavelengths. However, the resulting low torus opacity requires that thehigh observed columns detected in X-ray absorption be produced in gaswith a very low dust-to-gas ratio (perhaps within the dust sublimationregion). On the other hand, our observations may be consistent withclumpy torus models containing a steep radial distribution of opticallythick dense clumps. The selection of our sample at 12 μm, where thetorus may be optically thick, implies that there may beorientation-dependent biases in the sample; however, we do not find thatthe sample is biased toward Seyfert 2 galaxies with more luminouscentral engines, as would be expected. We find that the Seyfert 2galaxies typically show stronger starburst contributions than theSeyfert 1 galaxies in the sample, contrary to what is expected based onthe unified scheme for AGNs. This may be due to the selection effectthat only those Seyfert 2 galaxies with strong starburst contributionshad high enough integrated 12 μm flux densities to fall above theflux limit of the sample.
| 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.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| Gas metallicity in the narrow-line regions of high-redshift active galactic nuclei
We analyze optical (UV rest-frame) spectra of X-ray selected narrow-lineQSOs at redshift 1.5 z 3.7 found in the Chandra Deep FieldSouth and of narrow-line radio galaxies at redshift 1.2 z 3.8 to investigate the gas metallicity of the narrow-line regions andtheir evolution in this redshift range. Such spectra are also comparedwith UV spectra of local Seyfert 2 galaxies. The observational data areinconsistent with the predictions of shock models, suggesting that thenarrow-line regions are mainly photoionized. The photoionization modelswith dust grains predict line flux ratios which are also in disagreementwith most of the observed values, suggesting that the high-ionizationpart of the narrow-line regions (which is sampled by the availablespectra) is dust-free. The photoionization dust-free models provide twopossible scenarios which are consistent with the observed data:low-density gas clouds (nH 103cm-3) with a sub-solar metallicity (0.2 Z_gas/Z_ȯ 1.0), or high-density gas clouds (nH 105 cm-3) with a wide range of gas metallicity(0.2 Z_gas/Z_ȯ 5.0). Regardless of the specificinterpretation, the observational data do not show any evidence for asignificant evolution of the gas metallicity in the narrow-line regionswithin the redshift range 1.2 z 3.8. Instead, we find atrend for more luminous active galactic nuclei to have more metal-richgas clouds (luminosity-metallicity relation), which is in agreement withthe same finding in the studies of the broad-line regions. The lack ofevolution for the gas metallicity of the narrow-line regions impliesthat the major epoch of star formation in the host galaxies of theseactive galactic nuclei is at z 4.
| 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.
| 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.
| Deconstructing NGC 7130
Observations of the Seyfert 2 and starburst galaxy NGC 7130 with theChandra X-Ray Observatory illustrate that both of these phenomenacontribute significantly to the galaxy's detectable X-ray emission. Theactive galactic nucleus (AGN) is strongly obscured, buried beneathcolumn density NH>1024cm-2, and itis most evident in a prominent Fe Kα emission line with equivalentwidth greater than 1 keV. The AGN accounts for most (60%) of theobserved X-rays at energy E>2 keV, with the remainder due tospatially extended star formation. The soft X-ray emission is strong andpredominantly thermal on both small and large scales. We attribute thethermal emission to stellar processes. In total, the AGN is responsiblefor only one-third of the observed 0.5-10 keV luminosity of3×1041ergss-1 of this galaxy and less thanhalf of its bolometric luminosity. Starburst/AGN composite galaxies likeNGC 7130 are truly common, and similar examples may contributesignificantly to the high-energy cosmic X-ray background while remaininghidden at lower energies, especially if they are distant.
| X-ray obscuration and obscured AGN in the local universe
We discuss the X-ray properties of 49 local (z<0.035) Seyfert 2galaxies with HST/WFC2 high-resolution optical coverage. It includes theresults of 26 still unpublished Chandra and XMM-Newton observations,which yield 25 (22) new X-ray detections in the 0.5-2 keV (2-10 keV)energy band. Our sample covers a range in the 2-10 keV observed flux,F2-10, from 3 × 10-11 to 6 ×10-15 erg cm-2 s-1. The percentage ofobjects that are likely obscured by Compton-thick matter (columndensity, NH ≥ σt-1 ≃1.6 × 1024 cm-2) is ≃50%, and reaches≃80% for log (F2-10) < 12.3. Hence, Kαfluorescent iron lines with large Equivalent Width ({EW} > 0.6 keV)are common in our sample (6 new detections at a confidence level≥2σ). They are explained as due to reflection off theilluminated side of optically thick material. We confirm a correlationbetween the presence of a 100-pc scale nuclear dust in the WFC2 imagesand Compton-thin obscuration. We interpret this correlation as due tothe large covering fraction of gas associated with the dust lanes. TheX-ray spectra of highly obscured AGN invariably present a prominent softexcess emission above the extrapolation of the hard X-ray component.This soft component can account for a very large fraction of the overallX-ray energy budget. As this component is generally unobscured - andtherefore likely produced in extended gas structures - it may lead to asevere underestimation of the nuclear obscuration in z 1 absorbedAGN, if standard X-ray colors are used to classify them. As a by-productof our study, we report the discovery of a soft X-ray, luminous(≃7 × 1040 erg s-1) halo embedding theinteracting galaxy pair Mkn 266.
| 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.
| The complex structure of low-luminosity active galactic nuclei: NGC 4579
We have modelled the low-luminosity active galactic nucleus (AGN) NGC4579 by explaining both the continuum and the line spectra observed withdifferent apertures. It was found that the nuclear emission is dominatedby an AGN such that the flux from the active centre (AC) is relativelylow compared with that of the narrow emission-line region (NLR) ofSeyfert galaxies. However, the contribution of a young starburst cannotbe neglected, as well as that of shock-dominated clouds with velocitiesof 100, 300 and 500kms-1. A small contribution from an olderstarburst with an age of 4.5 Myr, probably located in the externalnuclear region, is also found. HII regions appear in the extendedregions (~1 kpc), where radiation and shock-dominated clouds withVs= 100kms-1 prevail. The continuum SED of NGC4579 is characterized by the strong flux from an old stellar population.Emissions in the radio range show synchrotron radiation from the base ofthe jet outflowing from the accretion disc within 0.1 pc from the activecentre. Radio emission within intermediate distances (10-20 pc) isexplained by the bremsstrahlung from gas downstream of low-velocityshocks (Vs= 100kms-1) reached by a relatively lowradiation flux from the AC. In extended regions (>100 pc) the radioemission is synchrotron radiation created by the Fermi mechanism at theshock front. The shocks are created by collision of clouds with the jet.All types of emissions observed at different radius from the centre canbe reconciled with the presence of the jet.
| The infrared continuum of active galactic nuclei
We discuss the different physical processes contributing to the infraredcontinuum of active galactic nuclei (AGNs), assuming that bothphotoionization from the active centre and shocks ionize and heat thegas and dust contained in an ensemble of clouds surrounding the nucleus.In our model, radiation transfer of primary and secondary radiationthroughout a cloud is calculated consistently with collisional processesdue to the shock. We consider that the observed continuum corresponds toreprocessed radiation from both dust and gas in the clouds. Collisionalprocesses are important in the presence of shocks. The grains aresputtered crossing the shock front. The models are constrained bysputtering as well as by the far-infrared data. The model is applied tothe continuum of Seyfert galaxies from which the best estimate of thenuclear, stellar subtracted, emission is available. The results showthat radiation-dominated high-velocity clouds are more numerous inSeyfert 1-1.5 whereas shock-dominated low-velocity clouds are dominantin Seyfert type 2. This result is in full agreement with the unifiedmodel for AGNs, by which high-velocity clouds, placed deeper into thecentral region and therefore reached by a more intense radiation, shouldplay a more significant role in the spectra of broad-line objects. Wecould therefore conclude that in type 2 objects, radiation is partlysuppressed by a central dusty medium with a high dust-to-gas ratio. Oncethe model approach is tested, a grid of models is used to provide aphenomenological analysis of the observed infrared spectral energydistribution. This empirical method is a useful tool to rapidly accessthe physical conditions of the AGN emitting clouds. For this, analyticalforms are derived for the two processes contributing to the infraredemission: dust emission and thermal bremsstrahlung produced by thenarrow-line region clouds. Their relative contribution provides ameasurement of the dust-to-gas ratio.
| 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.
| Circumnuclear Structure and Black Hole Fueling: Hubble Space Telescope NICMOS Imaging of 250 Active and Normal Galaxies
Why are the nuclei of some galaxies more active than others? If mostgalaxies harbor a central massive black hole, the main difference isprobably in how well it is fueled by its surroundings. We investigatethe hypothesis that such a difference can be seen in the detailedcircumnuclear morphologies of galaxies using several quantitativelydefined features, including bars, isophotal twists, boxy and diskyisophotes, and strong nonaxisymmetric features in unsharp-masked images.These diagnostics are applied to 250 high-resolution images of galaxycenters obtained in the near-infrared with NICMOS on the Hubble SpaceTelescope. To guard against the influence of possible biases andselection effects, we have carefully matched samples of Seyfert 1,Seyfert 2, LINER, starburst, and normal galaxies in their basicproperties, taking particular care to ensure that each was observed witha similar average scale (10-15 pc pixel-1). Severalmorphological differences among our five different spectroscopicclassifications emerge from the analysis. The H II/starburst galaxiesshow the strongest deviations from smooth elliptical isophotes, whilethe normal galaxies and LINERs have the least disturbed morphology. TheSeyfert 2s have significantly more twisted isophotes than any othercategory, and the early-type Seyfert 2s are significantly more disturbedthan the early-type Seyfert 1s. The morphological differences betweenSeyfert 1s and Seyfert 2s suggest that more is at work than simply theviewing angle of the central engine. They may correspond to differentevolutionary stages.
| 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.
| A vigorous starburst in the SCUBA galaxy N2 850.4
We present optical and near-infrared (near-IR) spectroscopy of a z= 2.38hyperluminous IR galaxy, covering the rest-frame wavelength range from1000-5000 Å. It appears to comprise two components separated byless than 1 arcsec on the sky (<~8 kpc); one component (B) is blue,the other (P) is red in rest-frame ultraviolet(UV)-optical colours. Thecombined system has a rest-frame luminosity of ~8L*V and itsrest-frame optical spectrum is characteristic of a Seyfert activegalactic nucleus. However, its rest-frame UV spectrum exhibits strikingfeatures associated with young stars, including P-Cygni lines fromstellar winds and blueshifted interstellar absorption lines indicativeof a galactic outflow. Redshifts are derived from stellar photosphericlines in the UV and from narrow emission lines in the rest-frameoptical, and these are compared to those measured for the molecular gasrecently detected with the Institut de Radioastronomie Millimetrique(IRAM) interferometer. The offsets indicate that the far-IR emission ismost likely associated with the near-IR source P, which hosts theSeyfert nucleus, while the UV-bright component B is blueshifted by 400km s-1. This suggests that the two components are probablymerging and the resulting gravitational interactions have triggered thehyperluminous activity. Modelling of the UV spectral features impliesthat the starburst within the UV component of this system has been goingon for at least ~10 Myr. Assuming that the bolometrically-dominantobscured component has a similar lifetime, we estimate that it has sofar formed a total stellar mass of ~1011 Msolar.If this star formation continues at its present level for substantiallylonger, or if this activity is repeated, then the present-day descendantof N2 850.4 will be a very luminous galaxy.
| Extended gas in Seyfert 2 galaxies: implications for the nuclear source
We use long-slit spectroscopic optical data to derive the properties ofthe extended emitting gas and the nuclear luminosity of a sample of 18Seyfert 2 galaxies. From the emission-line luminosities and ratios wederive the density, reddening and mass of the ionized gas as a functionof distance up to 2-4 kpc from the nucleus. Taking into account thegeometric dilution of the nuclear radiation, we derive the radialdistribution of covering factors and the minimum rate of ionizingphotons emitted by the nuclear source. This number is an order ofmagnitude larger than that obtained from the rate of ionizing photons`intercepted' by the gas and measured from the Hα luminosity. Acalibration is proposed to recover this number from the observedluminosity. The HeIIλ4686/Hβ line ratio was used tocalculate the slope of the ionizing spectral energy distribution (SED),which in combination with the number of ionizing photons allows thecalculation of the hard X-ray luminosities. These luminosities areconsistent with those derived from X-ray spectra in the eight cases forwhich such data are available and recover the intrinsic X-ray emissionin Compton-thick cases. Our method can thus provide reliable estimatesof the X-ray fluxes in Seyfert 2 galaxies for the cases where it is notreadily available. We also use the ionizing SED and luminosity topredict the infrared luminosity under the assumption that it isdominated by reprocessed radiation from a dusty torus, and find a goodagreement with the observed IRAS luminosities.
| Stellar population gradients in Seyfert 2 galaxies: northern sample
We use high signal-to-noise ratio long-slit spectra in theλλ3600-4700 range of the 20 brightest northern Seyfert 2galaxies to study the variation of the stellar population properties asa function of distance from the nucleus. In order to characterize thestellar population and other continuum sources (e.g. featurelesscontinuum, FC) we have measured the equivalent width, W, of sixabsorption features, four continuum colours and their radial variations,and performed spectral population synthesis as a function of distancefrom the nucleus. About half of the sample has CaIIK and G band W valuessmaller at the nucleus than at 1 kpc from it, owing to a youngerpopulation and/or FC. The stellar population synthesis shows that, whileat the nucleus, 75 per cent of the galaxies present contribution >20per cent of ages <=100 Myr and/or of an FC, this proportion decreasesto 45 per cent at 3 kpc. In particular, 55 per cent of the galaxies havea contribution >10 per cent of the 3-Myr/FC component (a degeneratecomponent in which one cannot separate what is caused by an FC or by a3-Myr stellar population) at the nucleus, but only 25 per cent of themhave this contribution at 3 kpc. As a reference, the stellar populationof 10 non-Seyfert galaxies, spanning the Hubble types of the Seyfert(from S0 to Sc) was also studied. A comparison between the stellarpopulation of the Seyferts and that of the non-Seyferts shows systematicdifferences: the contribution of ages younger than 1 Gyr is in mostcases larger in the Seyfert galaxies than in non-Seyferts, not only atthe nucleus but up to 1 kpc from it.
| Nuclear Starburst Activity in the Seyfert 2 Galaxy NGC 2273
We present spectrophotometric results of the Seyfert 2 galaxy NGC 2273.The presence of high-order Balmer absorption lines (H8, H9, H10) andweak equivalent widths of CaII K λ3933, CN λ4200, G-bandλ4300 and MgIb λ5173 clearly indicate recent star-formingactivity in the nuclear region. Using a simple stellar populationsynthesis model, we find that for the best fit, the contributions of apower-law featureless continuum, an intermediate-age (108 yr) and an old (>109 yr) stellar populationto the total light at the reference normalization wavelength are 10.0%,33.4% and 56.6%, respectively. The existence of recent starburstactivity is also consistent with its high far-infrared luminosity (logLFIR/ Lȯ = 9.9), its infrared color indexes [α(25,60) = -1.81 and α(60, 100)= -0.79, typical values for Seyfertgalaxies with circumnuclear starburst], and its q-value (2.23, ratio ofinfrared to radio flux, very similar to that of normal spirals andstarburst galaxies). Byrd et al. have suggested that NGC 2273 might haveinteracted with NGC 2273B in less than 109 yr ago, so thestarburst activity in this galaxy could have been triggered by tidalinteraction, as indicated in recent numerical simulations.
| Circumnuclear Dust in Nearby Active and Inactive Galaxies. II. Bars, Nuclear Spirals, and the Fueling of Active Galactic Nuclei
We present a detailed study of the relation between circumnuclear dustmorphology, host-galaxy properties, and nuclear activity in nearbygalaxies. We use our sample of 123 nearby galaxies withvisible-near-infrared color maps from the Hubble Space Telescope tocreate well-matched, ``paired'' samples of 28 active and 28 inactivegalaxies, as well as 19 barred and 19 unbarred galaxies, that have thesame host-galaxy properties. Comparison of the barred and unbarredgalaxies shows that grand-design nuclear dust spirals are found only ingalaxies with a large-scale bar. These nuclear dust spirals, which arepresent in approximately one-third of all barred galaxies, also appearto be connected to the dust lanes along the leading edges of thelarge-scale bars. Grand-design nuclear spirals are more common thaninner rings, which are present in only a small minority of the barredgalaxies. Tightly wound nuclear dust spirals, in contrast, show a strongtendency to avoid galaxies with large-scale bars. Comparison of theactive galactic nuclei (AGNs)and inactive samples shows that nucleardust spirals, which may trace shocks and angular momentum dissipation inthe interstellar medium, occur with comparable frequency in both activeand inactive galaxies. The only difference between the active andinactive galaxies is that several inactive galaxies appear to completelylack dust structure in their circumnuclear region, while none of theAGNs lack this structure. The comparable frequency of nuclear spirals inactive and inactive galaxies, combined with previous work that finds nosignificant difference in the frequency of bars or interactions betweenwell-matched active and inactive galaxies, suggests that no universalfueling mechanism for low-luminosity AGNs operates at spatial scalesgreater than a ~100 pc radius from the galactic nuclei. The similaritiesof the circumnuclear environments of active and inactive galaxiessuggest that the lifetime of nuclear activity is less than thecharacteristic inflow time from these spatial scales. Anorder-of-magnitude estimate of this inflow time is the dynamicaltimescale. This sets an upper limit of several million years to thelifetime of an individual episode of nuclear activity.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 NAS5-26555.
| COLA. II. Radio and Spectroscopic Diagnostics of Nuclear Activity in Galaxies
We present optical spectroscopic observations of 93 galaxies taken fromthe infrared-selected COLA (compact objects in low-power AGNs) sample.These are all galaxies for which we have previously obtainedlow-resolution radio observations and high-resolution (<0.05")Australian Long Baseline Array snapshots. The sample spans the range offar-IR luminosities from normal galaxies to luminous infrared galaxiesand contains a significant number of galaxies involved in galaxy-galaxyinteractions. Of the galaxies observed, 78 (84%) exhibit emission linesindicating that they are either AGNs or actively forming stars(starburst galaxies). Using a newly developed, theoretically based,optical emission line scheme to classify the spectra, we find that 15%of the emission-line galaxies are Seyfert galaxies, 77% are starbursts,and the rest are either borderline AGN/starburst or show ambiguouscharacteristics. We find little evidence for an increase in the fractionof AGNs in the sample as a function of far-IR (FIR) luminosity, incontrast to previous studies, but our sample covers only a small rangein infrared luminosity(1010.5Lsolar<=LFIR<=1011.7 Lsolar), and thus a weak trend may be masked. Instead,as the infrared luminosity increases, so does the fraction of metal-richstarbursts, objects that on more traditional diagnostic diagrams wouldhave been classified as weak, low-ionization, narrow emission lineregions. As a whole the Seyfert galaxies exhibit a small, butstatistically significant, radio excess on the radio-FIR correlationcompared to the galaxies classified as starbursts. Compact (<0.05")radio cores are detected in 55% of the Seyfert galaxies, and thesegalaxies exhibit a significantly larger radio excess than the Seyfertgalaxies in which compact cores were not detected. Our results indicatethat there may be two distinct populations of Seyfert galaxies,``radio-excess'' Seyfert galaxies, which exhibit extended radiostructures and compact radio cores, and ``radio-quiet'' Seyfertgalaxies, in which the majority of the radio emission can be attributedto star formation in the host galaxy. No significant difference is seenbetween the IR and optical spectroscopic properties of Seyfert galaxieswith and without radio cores.
| 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 Seyfert Population in the Local Universe
The magnitude-limited catalog of the Southern Sky Redshift Survey(SSRS2) is used to characterize the properties of galaxies hostingactive galactic nuclei (AGNs). Using emission-line ratios, we identify atotal of 162 (3%) Seyfert galaxies out of the parent sample with 5399galaxies. The sample contains 121 Seyfert 2 galaxies and 41 Seyfert 1galaxies. The SSRS2 Seyfert galaxies are predominantly in spirals oftypes Sb and earlier or in galaxies with perturbed appearance as theresult of strong interactions or mergers. Seyfert galaxies in thissample are twice as common in barred hosts as the non-Seyfert galaxies.By assigning galaxies to groups using a percolation algorithm, we findthat the Seyfert galaxies in the SSRS2 are more likely to be found inbinary systems when compared with galaxies in the SSRS2 parent sample.However, there is no statistically significant difference between theSeyfert and SSRS2 parent sample when systems with more than two galaxiesare considered. The analysis of the present sample suggests that thereis a stronger correlation between the presence of the AGN phenomenonwith internal properties of galaxies (morphology, presence of bar,luminosity) than with environmental effects (local galaxy density, groupvelocity dispersion, nearest neighbor distance).Partly based on observations at European Southern Observatory (ESO),under the ESO-ON agreement to operate the 1.52 m telescope.
| 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.
| Iron Is Not Depleted in High-Ionization Nuclear Emission-Line Regions of Active Galactic Nuclei
To examine whether or not high-ionization nuclear emission-line regions(HINERs) in narrow-line regions of active galactic nuclei are dusty, wefocus on two high-ionization forbidden emission lines, [Fe VII]λ6087 and [Ne V] λ3426. We perform photoionization modelcalculations to investigate possible dependences of the flux ratio of[Fe VII] λ6087/[Ne V] λ3426 on various gas properties, inorder to investigate how useful this flux ratio is to explore the dustabundances in HINERs. Based on our photoionization model calculations,we show that the observed range of the flux ratio of [Fe VII]λ6087/[Ne V] λ3426 is consistent with the dust-freemodels, while it cannot easily be explained by the dusty models. Thissuggests that iron is not depleted in HINERs, which implies that theHINERs are not dusty. This result is consistent with the idea that theHINERs are located closer than the dust-sublimation radius (i.e., theinner radius of dusty tori) and thus can be hidden by dusty tori whenseen from a edge-on view toward the tori, which is also suggested by theAGN-type dependence of the visibility of high-ionization emission lines.
| The infrared supernova rate in starburst galaxies
We report the results of our ongoing search for extincted supernovae(SNe) at near-infrared wavelengths. We have monitored at 2.2 mu m asample of 46 Luminous Infrared Galaxies and detected 4 SNe. The numberof detections is still small but sufficient to provide the firstestimate of supernova rate at near-infrared wavelengths. We measure a SNrate of SNNIR_r=7.6+/- 3.8 SNu which is an order of magnitudelarger than observed in quiescent galaxies. On the other hand, theobserved near-infrared rate is still a factor 3-10 smaller than thatestimated from the far-infrared luminosity of the galaxies. Amongvarious possibilities, the most likely scenario is that dust extinctionis so high (AV>30) to obscure most SNe even in thenear-IR.The role of type Ia SNe is also discussed within this context. We derivethe type Ia SN rate as a function of the stellar mass of the galaxy andfind a sharp increase toward galaxies with higher activity of starformation. This suggests that a significant fraction of type Ia SNe areassociated with young stellar populations.Finally, as a by-product, we give the average K-band light curve ofcore-collapse SNe based on all the existing data, and review therelation between SN rate and far-infrared luminosity.Based on observations collected at the European Southern Observatory,Chile (proposal 66.B-0417), at the Italian Telescopio Nazionale Galileo(TNG) operated on the island of La Palma by the Centro Galileo Galileiof the INAF (Istituto Nazionale di Astrofisica), and at the StewardObservatory 61'' telescope.
| Study of the X-Ray Background Spectrum and Its Large-Scale Fluctuation with ASCA
We studied the energy spectrum and the large-scale fluctuation of theX-ray background with the ASCA GIS instrument based on the ASCA MediumSensitivity Survey and Large Sky Survey observations. A total of 91fields with Galactic latitude |b| > 10° were selected with a skycoverage of 50 deg2 and 4.2 Ms of exposure. For each field,non-X-ray events were carefully subtracted and sources brighter than ~ 2× 1013 erg cm-2 s-1 (2-10keV)were eliminated. Spectral fits with a single power-law model for theindividual 0.7-10 keV spectra showed a significant excess below ~ 2keV,which could be expressed by an additional thermal model with kT ~= 0.4keV or a steep power-law model with a photon index ofΓsoft ~= 6. The 0.5-2keV intensities of the softthermal component varied significantly from field to field by 1 σ= 52 +4-5%, and showed a maximum toward theGalactic Center. This component is considered to be entirely Galactic.As for the hard power-law component, an average photon index of 91fields was obtained to be Γhard = 1.412 +/- 0.007 +/-0.025 and the average 2-10keV intensity was calculated asFhardX = (6.38 +/- 0.04 +/- 0.64) ×10-8erg cm-2 s-1 sr-1 (1σ statistical and systematic errors). The Galactic component ismarginally detected in the hard band. The 2-10keV intensities show a 1σ deviation of 6.49+0.56-0.61%, whiledeviation due to the reproducibility of the particle background is 3.2%.The observed deviation can be explained by the Poisson noise of thesource count in the f.o.v. (~ 0.5 deg2), even assuming asingle N-logS relation on the whole s ky. Based on the observedfluctuation and the absolute intensity, an acceptable region of theN-logS relation was derived, showing a consistent feature with therecent Chandra and XMM-Newton results. The fluctuation of the spectralindex was also examined; it implied a large amount of hard sources and asubstantial variation in the intrinsic source spectra(Γs ~= 1.1 +/- 1.0).
| 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.
| Star Formation Rates in Interacting Starburst Galaxies
By narrowband imaging in Hα and in the adjacent red stellarcontinuum we have studied the rate and distribution of star formation in43 systems of luminous and ultraluminous IR galaxies currentlyundergoing interaction and merging. These galaxies are amongst the mostluminous at 60 μm and range in distance from ~50 up to 100 Mpc. Herewe present the Hα and the adjacent red-continuum narrowbandimages, and we compare the star formation rates derived from Hαwith those estimated from the IR luminosity. We find clear evidence forsubstantial extinction and obscuration of star-forming regions in theoptical. Without correction for reddening in the host galaxy orcorrection for [N II] contamination, the star formation rates derivedfor Hα are typically 0.5-1.0 dex lower than those estimated fromthe IR flux, and the scatter in the correlation is very large. However,an unexpected result is that when spectroscopic data are used toeliminate objects dominated by an active nucleus, to determine thegalaxian extinction, and to correct the Hα flux for both reddeningand for the contamination by the [N II] emission, a remarkably goodcorrelation emerges between the star formation rates estimated from theHα flux and those derived from the FIR continuum. In addition, astrong correlation is found between the extinction in the line-emittingregion, AHα, and the rate of star formation. Ourresults invalidate the use of Hα imaging as a reliable indicatorof star formation in starburst galaxies unless spectroscopic data arealso available. This has important implications for the determination ofstar formation rates in high-redshift galaxies. Finally, we find nocorrelation between the measured star formation rates, and theinteraction class, suggesting that the enhanced star formation ratestriggered by the interaction continue throughout the whole of themerging sequence.
| 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.
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