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The SAURON project - VI. Line strength maps of 48 elliptical and lenticular galaxies
We present absorption line strength maps of 48 representative ellipticaland lenticular galaxies obtained as part of a survey of nearby galaxiesusing our custom-built integral-field spectrograph, SAURON, operating onthe William Herschel Telescope. Using high-quality spectra, spatiallybinned to a constant signal-to-noise ratio, we measure four key age,metallicity and abundance ratio sensitive indices from the Lick/IDSsystem over a two-dimensional field extending up to approximately oneeffective radius. A discussion of calibrations and offsets is given,along with a description of error estimation and nebular emissioncorrection. We modify the classical Fe5270 index to define a new index,Fe5270S, which maximizes the useable spatial coverage ofSAURON. Maps of Hβ, Fe5015, Mgb and Fe5270S arepresented for each galaxy. We use the maps to compute average linestrengths integrated over circular apertures of one-eighth effectiveradius, and compare the resulting relations of index versus velocitydispersion with previous long-slit work. The metal line strength mapsshow generally negative gradients with increasing radius roughlyconsistent with the morphology of the light profiles. Remarkabledeviations from this general trend exist, particularly the Mgb isoindexcontours appear to be flatter than the isophotes of the surfacebrightness for about 40 per cent of our galaxies without significantdust features. Generally, these galaxies exhibit significant rotation.We infer from this that the fast-rotating component features a highermetallicity and/or an increased Mg/Fe ratio as compared to the galaxy asa whole. The Hβ maps are typically flat or show a mild positiveoutwards radial gradient, while a few galaxies show strong central peaksand/or elevated overall Hβ strength likely connected to recent starformation activity. For the most prominent post-starburst galaxies, eventhe metal line strength maps show a reversed gradient.
| The SAURON project - V. Integral-field emission-line kinematics of 48 elliptical and lenticular galaxies
We present the emission-line fluxes and kinematics of 48 representativeelliptical and lenticular galaxies obtained with our custom-builtintegral-field spectrograph, SAURON, operating on the William HerschelTelescope. Hβ, [OIII]λλ4959,5007 and[NI]λλ5198,5200 emission lines were measured using a newprocedure that simultaneously fits both the stellar spectrum and theemission lines. Using this technique we can detect emission lines downto an equivalent width of 0.1 Å set by the current limitations indescribing galaxy spectra with synthetic and real stellar templates,rather than by the quality of our spectra. Gas velocities and velocitydispersions are typically accurate to within 14 and 20 kms-1, respectively, and at worse to within 25 and 40 kms-1. The errors on the flux of the [OIII] and Hβ linesare on average 10 and 20 per cent, respectively, and never exceed 30 percent. Emission is clearly detected in 75 per cent of our samplegalaxies, and comes in a variety of resolved spatial distributions andkinematic behaviours. A mild dependence on the Hubble type and galacticenvironment is observed, with higher detection rates in lenticulargalaxies and field objects. More significant is the fact that only 55per cent of the galaxies in the Virgo cluster exhibit clearly detectedemission. The ionized-gas kinematics is rarely consistent with simplecoplanar circular motions. However, the gas almost never displayscompletely irregular kinematics, generally showing coherent motions withsmooth variations in angular momentum. In the majority of the cases, thegas kinematics is decoupled from the stellar kinematics, and in half ofthe objects this decoupling implies a recent acquisition of gaseousmaterial. Over the entire sample however, the distribution of the meanmisalignment values between stellar and gaseous angular momenta isinconsistent with a purely external origin. The distribution ofkinematic misalignment values is found to be strongly dependent on theapparent flattening and the level of rotational support of galaxies,with flatter, fast rotating objects hosting preferentially corotatinggaseous and stellar systems. In a third of the cases, the distributionand kinematics of the gas underscore the presence of non-axisymmetricperturbations of the gravitational potential. Consistent with previousstudies, the presence of dust features is always accompanied by gasemission while the converse is not always true. A considerable range ofvalues for the [OIII]/Hβ ratio is found both across the sample andwithin single galaxies. Despite the limitations of this ratio as anemission-line diagnostic, this finding suggests either that a variety ofmechanisms is responsible for the gas excitation in E and S0 galaxies orthat the metallicity of the interstellar material is quiteheterogeneous.
| On the origin of warps and the role of the intergalactic medium
There is still no consensus as to what causes galactic discs to becomewarped. Successful models should account for the frequent occurrence ofwarps in quite isolated galaxies, their amplitude as well as theobserved azimuthal and vertical distributions of the HI layer.Intergalactic accretion flows and intergalactic magnetic fields may bendthe outer parts of spiral galaxies. In this paper we consider theviability of these non-gravitational torques to take the gas off theplane. We show that magnetically generated warps are clearly flawedbecause they would wrap up into a spiral in less than two or threegalactic rotations. The inclusion of any magnetic diffusivity to dilutethe wrapping effect causes the amplitude of the warp to damp. We alsoconsider the observational consequences of the accretion of anintergalactic plane-parallel flow at infinity. We have computed theamplitude and warp asymmetry in the accretion model, for a disc embeddedin a flattened dark matter halo, including self-consistently thecontribution of the modes with azimuthal wavenumbers m= 0 and m= 1.Since the m= 0 component, giving a U-shaped profile, is not negligiblecompared to the m= 1 component, this model predicts quite asymmetricwarps, maximum gas displacements on the two sides in the ratio 3 : 2 forthe preferred Galactic parameters, and the presence of a fraction ~3.5per cent of U-shaped warps, at least. The azimuthal dependence of themoment transfer by the ram pressure would produce a strong asymmetry inthe thickness of the HI layer and asymmetric density distributions in z,in conflict with observational data for the warp in our Galaxy and inexternal galaxies. The amount of accretion that is required to explainthe Galactic warp would give gas scaleheights in the far outer disc thatare too small. We conclude that accretion of a flow with no net angularmomentum cannot be the main and only cause of warps.
| Stellar Populations in Nearby Lenticular Galaxies
We have obtained two-dimensional spectral data for a sample of 58 nearbyS0 galaxies with the Multi-Pupil Fiber/Field Spectrograph of the 6 mtelescope of the Special Astrophysical Observatory of the RussianAcademy of Sciences. The Lick indices Hβ, Mg b, and arecalculated separately for the nuclei and for the bulges taken as therings between R=4'' and 7", and the luminosity-weighted ages,metallicities, and Mg/Fe ratios of the stellar populations are estimatedby comparing the data to single stellar population (SSP) models. Fourtypes of galaxy environments are considered: clusters, centers ofgroups, other places in groups, and the field. The nuclei are found tobe on average slightly younger than the bulges in any type ofenvironment, and the bulges of S0 galaxies in sparse environments areyounger than those in dense environments. The effect can be partlyattributed to the well-known age correlation with the stellar velocitydispersion in early-type galaxies (in our sample the galaxies in sparseenvironments are on average less massive than those in denseenvironments), but for the most massive S0 galaxies, withσ*=170-220 km s-1, the age dependence on theenvironment is still significant at the confidence level of 1.5 σ.Based on observations collected with the 6 m telescope (BTA) at theSpecial Astrophysical Observatory (SAO) of the Russian Academy ofSciences (RAS).
| On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.
| Photometric survey of the polar ring galaxy NGC 6822
Context: .We have previously established, from a carbon star survey,that the Local Group dwarf irregular galaxy NGC 6822 is much larger thanits central bright core. Aims: . Four MegaCam fields are acquiredto survey a 2°× 2° area centred on NGC 6822 to fullydetermine its extent and map its stellar populations. Methods:.Photometry of over one million stars is obtained in the SDSS g', r', i'to three magnitudes below the TRGB. RGB stars, selected from theirmagnitudes and colours, are used to map the NGC 6822 stellardistribution up to a distance of 60 arcmin. Results: .We map thereddening over the whole area. We establish that the stellar outerstructure of NGC 6822 is elliptical in shape, with ɛ=0.36 and amajor-axis PA = 65°, contrasting with the orientation of the HIdisk. The density enhancement can be seen up to a semi-major axis of 36'making NGC 6822 as big as the Small Magellanic Cloud. We fit twoexponentials to the surface density profile of the spheroid, andidentify a bulge with a scale length of 3.85' and an outer spheroid witha scale length of 10.0'. We find intermediate-age C stars up to 40'while demonstrating that the SDSS filters are unsuitable to identifyextragalactic C stars. Conclusions: .NGC 6822 is a unique LocalGroup galaxy with shape and structure suggesting a polar ringconfiguration. Radial velocities of carbon stars have indeeddemonstrated that there are two kinematical systems in NGC 6822.
| X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies
Results obtained from an X-ray spectral survey of nearby Seyfertgalaxies using XMM-Newton are reported. The sample was opticallyselected, well defined, complete in B magnitude, and distance limited:it consists of the nearest (D 22 Mpc) 27 Seyfert galaxies (9 oftype 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315)sample. This is one of the largest atlases of hard X-ray spectra oflow-luminosity active galaxies ever assembled. All nuclear sourcesexcept two Seyfert 2s are detected between 2 and 10 keV, half for thefirst time ever, and average spectra are obtained for all of them.Nuclear luminosities reach values down to 1038 ergs-1. The shape of the distribution of X-ray parameters isaffected by the presence of Compton-thick objects (30% among type2s). The latter have been identified either directly from their intenseFeK line and flat X-ray spectra, or indirectly with flux diagnosticdiagrams which use isotropic indicators. After taking into account thesehighly absorbed sources, we find that (i) the intrinsic X-ray spectralproperties (i.e., spectral shapes and luminosities above 2 keV) areconsistent between type 1 and type 2 Seyferts, as expected from "unifiedmodels"; (ii) Seyfert galaxies as a whole are distributed fairlycontinuously over the entire range of N_H, between 1020 and1025 cm-2; and (iii) while Seyfert 1s tend to havelower NH and Seyfert 2s tend to have the highest, we find 30%and 10% exceptions, respectively. Overall the sample is of sufficientquality to well represent the average intrinsic X-ray spectralproperties of nearby active galactic nuclei, including a proper estimateof the distribution of their absorbing columns. Finally, we concludethat, with the exception of a few cases, the present study agrees withpredictions of unified models of Seyfert galaxies, and extends theirvalidity down to very low luminosities.
| How large are the bars in barred galaxies?
I present a study of the sizes (semimajor axes) of bars in discgalaxies, combining a detailed R-band study of 65 S0-Sb galaxies withthe B-band measurements of 70 Sb-Sd galaxies from Martin (1995). As hasbeen noted before with smaller samples, bars in early-type (S0-Sb)galaxies are clearly larger than bars in late-type (Sc-Sd) galaxies;this is true both for relative sizes (bar length as fraction ofisophotal radius R25 or exponential disc scalelength h) andabsolute sizes (kpc). S0-Sab bars extend to ~1-10 kpc (mean ~ 3.3 kpc),~0.2-0.8R25 (mean ~ 0.38R25) and ~0.5-2.5h (mean ~1.4h). Late-type bars extend to only ~0.5-3.5 kpc,~0.05-0.35R25 and 0.2-1.5h their mean sizes are ~1.5 kpc, ~0.14R25 and ~0.6h. Sb galaxies resemble earlier-type galaxiesin terms of bar size relative to h; their smallerR25-relative sizes may be a side effect of higher starformation, which increases R25 but not h. Sbc galaxies form atransition between the early- and late-type regimes. For S0-Sbcgalaxies, bar size correlates well with disc size (both R25and h); these correlations are stronger than the known correlation withMB. All correlations appear to be weaker or absent forlate-type galaxies; in particular, there seems to be no correlationbetween bar size and either h or MB for Sc-Sd galaxies.Because bar size scales with disc size and galaxy magnitude for mostHubble types, studies of bar evolution with redshift should selectsamples with similar distributions of disc size or magnitude(extrapolated to present-day values); otherwise, bar frequencies andsizes could be mis-estimated. Because early-type galaxies tend to havelarger bars, resolution-limited studies will preferentially find bars inearly-type galaxies (assuming no significant differential evolution inbar sizes). I show that the bars detected in Hubble Space Telescope(HST) near-infrared(IR) images at z~ 1 by Sheth et al. have absolutesizes consistent with those in bright, nearby S0-Sb galaxies. I alsocompare the sizes of real bars with those produced in simulations anddiscuss some possible implications for scenarios of secular evolutionalong the Hubble sequence. Simulations often produce bars as large as(or larger than) those seen in S0-Sb galaxies, but rarely any as smallas those in Sc-Sd galaxies.
| The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies
The Hubble Space Telescope has revealed a dichotomy in the centralsurface brightness profiles of early-type galaxies, which havesubsequently been grouped into two families: core, boxy, anisotropicsystems; and cuspy (`power-law'), discy, rotating ones. Here weinvestigate whether a dichotomy is also present in the X-ray propertiesof the two families. We consider both their total soft emission(LSX,tot), which is a measure of the galactic hot gascontent, and their nuclear hard emission (LHX,nuc), mostlycoming from Chandra observations, which is a measure of the nuclearactivity. At any optical luminosity, the highest LSX,totvalues are reached by core galaxies; this is explained by their beingthe central dominant galaxies of groups, subclusters or clusters, inmany of the logLSX,tot (ergs-1) >~ 41.5 cases.The highest LHX,nuc values, similar to those of classicalactive galactic nuclei (AGNs), in this sample are hosted only by core orintermediate galaxies; at low luminosity AGN levels, LHX,nucis independent of the central stellar profile shape. The presence ofoptical nuclei (also found by HST) is unrelated to the level ofLHX,nuc, even though the highest LHX,nuc are allassociated with optical nuclei. The implications of these findings forgalaxy evolution and accretion modalities at the present epoch arediscussed.
| The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties
In a recently completed survey of the stellar population properties oflow-ionization nuclear emission-line regions (LINERs) and LINER/HIItransition objects (TOs), we have identified a numerous class ofgalactic nuclei which stand out because of their conspicuous108-9 yr populations, traced by high-order Balmer absorptionlines and other stellar indices. These objects are called `young-TOs',because they all have TO-like emission-line ratios. In this paper weextend this previous work, which concentrated on the nuclear properties,by investigating the radial variations of spectral properties inlow-luminosity active galactic nuclei (LLAGNs). Our analysis is based onhigh signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distancesof typically up to 850 pc from the nucleus with a resolution of ~100 pc(~1 arcsec) and S/N ~ 30. Stellar population gradients are mapped by theradial profiles of absorption-line equivalent widths and continuumcolours along the slit. These variations are further analysed by meansof a decomposition of each spectrum in terms of template galaxiesrepresentative of very young (<=107 yr), intermediate age(108-9 yr) and old (1010 yr) stellar populations.This study reveals that young-TOs also differ from old-TOs andold-LINERs in terms of the spatial distributions of their stellarpopulations and dust. Specifically, our main findings are as follows.(i) Significant stellar population gradients are found almostexclusively in young-TOs. (ii) The intermediate age population ofyoung-TOs, although heavily concentrated in the nucleus, reachesdistances of up to a few hundred pc from the nucleus. Nevertheless, thehalf width at half-maximum of its brightness profile is more typically100 pc or less. (iii) Objects with predominantly old stellar populationspresent spatially homogeneous spectra, be they LINERs or TOs. (iv)Young-TOs have much more dust in their central regions than otherLLAGNs. (v) The B-band luminosities of the central <~1 Gyr populationin young-TOs are within an order of magnitude of MB=-15,implying masses of the order of ~107-108Msolar. This population was 10-100 times more luminous in itsformation epoch, at which time young massive stars would have completelyoutshone any active nucleus, unless the AGN too was brighter in thepast.
| A Spectroscopic and Photometric Study of NGC 2685, the Prototype Polar-Ring Galaxy
We present and analyze spectroscopic and photometric observations forNGC 2685, the prototype polar-ring galaxy. The spectroscopic data wereacquired using the 6 m telescope of the Special AstrophysicalObservatory of the Russian Academy of Sciences with the UAGSspectrograph and a scanning Fabry-Perot interferometer, while thephotometric analysis was based on images from the Hubble Space Telescopearchive. We demonstrate that the subsystem usually called the“inner polar ring” is a highly inhomogeneous gas and dustdisk rotating approximately in the polar plane relative to thegalaxy’s main body. When the self-absorption in the disk is takeninto account, a comparison of its color indices with those from modelcomputations of the color evolution of galaxies results in an age forthe disk of about 1.4 × 109 years, much lower than the previouslyaccepted estimate.
| The Westerbork HI survey of spiral and irregular galaxies. III. HI observations of early-type disk galaxies
We present Hi observations of 68 early-type disk galaxies from the WHISPsurvey. They have morphological types between S0 and Sab and absoluteB-band magnitudes between -14 and -22. These galaxies form the massive,high surface-brightness extreme of the disk galaxy population, few ofwhich have been imaged in Hi before. The Hi properties of the galaxiesin our sample span a large range; the average values of MHI/LB and DH I/D25 are comparableto the ones found in later-type spirals, but the dispersions around themean are larger. No significant differences are found between the S0/S0aand the Sa/Sab galaxies. Our early-type disk galaxies follow the same Himass-diameter relation as later-type spiral galaxies, but theireffective Hi surface densities are slightly lower than those found inlater-type systems. In some galaxies, distinct rings of Hi emissioncoincide with regions of enhanced star formation, even though theaverage gas densities are far below the threshold of star formationderived by Kennicutt (1989, ApJ, 344, 685). Apparently, additionalmechanisms, as yet unknown, regulate star formation at low surfacedensities. Many of the galaxies in our sample have lopsided gasmorphologies; in most cases this can be linked to recent or ongoinginteractions or merger events. Asymmetries are rare in quiescentgalaxies. Kinematic lopsidedness is rare, both in interacting andisolated systems. In the appendix, we present an atlas of the Hiobservations: for all galaxies we show Hi surface density maps, globalprofiles, velocity fields and radial surface density profiles.
| Mid-IR spectro-imaging observations with the ISOCAM CVF: Final reduction and archive
Mid-IR (5-17~μm) Spectro-Imaging observations towards several hundredsky positions were obtained with the Infrared Space Observatory Camera(ISOCAM), and a Circular Variable Filter (CVF) that provided spectralresolution R = λ/Δ λ ~ 45 over a 3' × 3' field(for 6, and 12´´ pixels, proportionally smaller for theobservations carried with the 3, and 1.5´´ pixels). Thewavelength range includes dust bands - in particular several of thearomatic carbon bands - fine structure lines from ionized gas, and H2rotational lines. The observed fields comprise nearby and distantgalaxies, Galactic and Extragalactic star forming regions, the Galacticdiffuse emission, nearby molecular clouds, infrared cirrus, youngstellar objects, evolved stars, and Solar System targets. We present thefinal data reduction procedure that improves on the standard pipelinereduction in several ways, mainly the subtraction of zodiacal light,that of stray light associated with the uniform, most often dominant,emission component and the correction of a pixel dependent wavelengthshift. We also propose a correction of the largest astrometry errorsintroduced by the optics. We have processed most of the ISOCAM CVFobservations and made the results available on the ISO archive forpublic use. The processed data may be routinely used over the fullsensitivity range of the instrument down to a brightness as faint as afew % of the zodiacal emission. For extended emission, the CVFobservations represent a data base with a lasting value which can servemany scientific goals and motivate follow-up observations in particularwith the Spitzer Space Telescope spectrometer.
| The SAURON project - III. Integral-field absorption-line kinematics of 48 elliptical and lenticular galaxies
We present the stellar kinematics of 48 representative elliptical andlenticular galaxies obtained with our custom-built integral-fieldspectrograph SAURON operating on the William Herschel Telescope. Thedata were homogeneously processed through a dedicated reduction andanalysis pipeline. All resulting SAURON data cubes were spatially binnedto a constant minimum signal-to-noise ratio. We have measured thestellar kinematics with an optimized (penalized pixel-fitting) routinewhich fits the spectra in pixel space, via the use of optimal templates,and prevents the presence of emission lines to affect the measurements.We have thus generated maps of the mean stellar velocity V, the velocitydispersion σ, and the Gauss-Hermite moments h3 andh4 of the line-of-sight velocity distributions. The mapsextend to approximately one effective radius. Many objects displaykinematic twists, kinematically decoupled components, central stellardiscs, and other peculiarities, the nature of which will be discussed infuture papers of this series.
| Are interactions the primary triggers of star formation in dwarf galaxies?
We investigate the assumption that the trigger of star formation indwarf galaxies is interactions with other galaxies, in the context of asearch for a `primary' trigger of a first generation of stars. This iscosmologically relevant because the galaxy formation process consistsnot only of the accumulation of gas in a gravitational potential wellbut also of the triggering of star formation in this gas mass, and alsobecause some high-z potentially primeval galaxy blocks look like nearbystar-forming dwarf galaxies. We review theoretical ideas proposed toaccount for the tidal interaction triggering mechanism and present aseries of observational tests of this assumption using published data.We also show results of a search in the vicinity of a composite sampleof 96 dwarf late-type galaxies for interaction candidates showing starformation. The small number of possible perturbing galaxies identifiedin the neighbourhood of our sample galaxies, along with similar findingsfrom other studies, supports the view that tidal interactions may not berelevant as primary triggers of star formation. We conclude thatinteractions between galaxies may explain some forms of star formationtriggering, perhaps in central regions of large galaxies, but they donot seem to be significant for dwarf galaxies and, by inference, forfirst-time galaxies forming at high redshifts. Intuitive reasoning,based on an analogy with stellar dynamics, shows that conditions forprimary star formation triggering may occur in gas masses oscillating ina dark-matter gravitational potential. We propose this mechanism as aplausible primary trigger scenario, which would be worth investigatingtheoretically.
| 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 Stellar Populations of Low-Luminosity Active Galactic Nuclei. II. Space Telescope Imaging Spectrograph Observations
We present a study of the stellar populations of low-luminosity activegalactic nuclei (LLAGNs). Our goal is to search for spectroscopicsignatures of young and intermediate-age stars and to investigate theirrelationship with the ionization mechanism in LLAGNs. The method used isbased on the stellar population synthesis of the optical continuum ofthe innermost (20-100 pc) regions in these galaxies. For this purpose,we have collected high spatial resolution optical (2900-5700 Å)STIS spectra of 28 nearby LLAGNs that are available in the Hubble SpaceTelescope archive. The analysis of these data is compared with a similaranalysis also presented here for 51 ground-based spectra of LLAGNs. Ourmain findings are as follows: (1) No features due to Wolf-Rayet starswere convincingly detected in the STIS spectra. (2) Young starscontribute very little to the optical continuum in the ground-basedaperture. However, the fraction of light provided by these stars ishigher than 10% in most of the weak-[O I] ([OI]/Hα<=0.25) LLAGNSTIS spectra. (3) Intermediate-age stars contribute significantly to theoptical continuum of these nuclei. This population is more frequent inobjects with weak than with strong [O I]. Weak-[O I] LLAGNs that haveyoung stars stand out for their intermediate-age population. (4) Most ofthe strong-[O I] LLAGNs have predominantly old stellar population. A fewof these objects also show a featureless continuum that contributessignificantly to the optical continuum. These results suggest that youngand intermediate-age stars do not play a significant role in theionization of LLAGNs with strong [O I]. However, the ionization inweak-[O I] LLAGNs with young and/or intermediate-age populations couldbe due to stellar processes. A comparison of the properties of theseobjects with Seyfert 2 galaxies that harbor a nuclear starburst suggeststhat weak-[O I] LLAGNs are the lower luminosity counterparts of theSeyfert 2 composite nuclei.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. Based on observations made with the Nordic OpticalTelescope (NOT), operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrofísica deCanarias.
| The Stellar Populations of Low-Luminosity Active Galactic Nuclei. I. Ground-based Observations
We present a spectroscopic study of the stellar populations oflow-luminosity active galactic nuclei (LLAGNs). Our main goal is todetermine whether the stars that live in the innermost (100 pc scale)regions of these galaxies are in some way related to the emission-lineproperties, which would imply a link between the stellar population andthe ionization mechanism. High signal-to-noise ratio, ground-basedlong-slit spectra in the 3500-5500 Å interval were collected for60 galaxies: 51 LINERs and LINER/H II transition objects, two starburstgalaxies, and seven nonactive galaxies. In this paper, the first of aseries, we (1) describe the sample; (2) present the nuclear spectra; (3)characterize the stellar populations of LLAGNs by means of an empiricalcomparison with normal galaxies; (4) measure a set of spectral indices,including several absorption-line equivalent widths and colorsindicative of stellar populations; and (5) correlate the stellar indiceswith emission-line ratios that may distinguish between possibleexcitation sources for the gas. Our main findings are as follows: (1)Few LLAGNs have a detectable young (<~107 yr) starburstcomponent, indicating that very massive stars do not contributesignificantly to the optical continuum. In particular, no features dueto Wolf-Rayet stars were convincingly detected. (2) High-order Balmerabsorption lines of H I (HOBLs), on the other hand, are detected in ~40%of LLAGNs. These features, which are strongest in108-109 yr intermediate-age stellar populations,are accompanied by diluted metal absorption lines and bluer colors thanother objects in the sample. (3) These intermediate-age populations arevery common (~50%) in LLAGNs with relatively weak [O I] emission([OI]/Hα<=0.25) but rare (~10%) in LLAGNs with stronger [O I].This is intriguing since LLAGNs with weak [O I] have been previouslyhypothesized to be ``transition objects'' in which both an AGN and youngstars contribute to the emission-line excitation. Massive stars, ifpresent, are completely outshone by intermediate-age and old stars inthe optical. This happens in at least a couple of objects whereindependent UV spectroscopy detects young starbursts not seen in theoptical. (4) Objects with predominantly old stars span the whole rangeof [O I]/Hα values, but (5) sources with significant young and/orintermediate-age populations are nearly all (~90%) weak-[O I] emitters.These new findings suggest a link between the stellar populations andthe gas ionization mechanism. The strong-[O I] objects are most likelytrue LLAGNs, with stellar processes being insignificant. However, theweak-[O I] objects may comprise two populations, one where theionization is dominated by stellar processes and another where it isgoverned by either an AGN or a more even mixture of stellar and AGNprocesses. Possible stellar sources for the ionization include weakstarbursts, supernova remnants, and evolved poststarburst populations.These scenarios are examined and constrained by means of complementaryobservations and detailed modeling of the stellar populations inforthcoming communications.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canárias.
| Radio Continuum Emission in Polar Ring Galaxies
We have used the Very Large Array aperture synthesis telescope toconduct a radio continuum survey of polar ring galaxies, at 20 cm and 6cm. Forty objects were observed at 20 cm with ~=5" resolution. Twenty(50%) of the program sources were detected at 20 cm, down to our 5σ limit of 0.5 mJy beam-1. This detection rate issimilar to those in surveys with comparable sensitivity for early-typegalaxies without polar rings. Sixteen of the objects we detected at 20cm were also observed at 6 cm. We show radio continuum maps for the fiveobjects in our sample that have noticeably extended emission. Ourspatial resolution was sufficient to distinguish emission originating inthe host galaxy from that in the polar ring. The radio morphology of theextended sources, as well as the ratio of radio to far-infrared flux andthe radio spectral indices of our detected sources, indicate that starformation, not nuclear activity, is the dominant source of the radiocontinuum emission in polar ring galaxies. However, the implied starformation rates are modest, and only one of our sample galaxies willconsume its supply of cool gas within 500 Myr.
| Inner-truncated Disks in Galaxies
We present an analysis of the disk brightness profiles of 218 spiral andlenticular galaxies. At least 28% of disk galaxies exhibit innertruncations in these profiles. There are no significant trends oftruncation incidence with Hubble type, but the incidence among barredsystems is 49%, more than 4 times that for nonbarred galaxies. However,not all barred systems have inner truncations, and not allinner-truncated systems are currently barred. Truncations represent areal dearth of disk stars in the inner regions and are not an artifactof our selection or fitting procedures nor the result of obscuration bydust. Disk surface brightness profiles in the outer regions are wellrepresented by simple exponentials for both truncated and nontruncateddisks. However, truncated and nontruncated systems have systematicallydifferent slopes and central surface brightness parameters for theirdisk brightness distributions. Truncation radii do not appear tocorrelate well with the sizes or brightnesses of the bulges. Thissuggests that the low angular momentum material apparently missing fromthe inner disk was not simply consumed in forming the bulge population.Disk parameters and the statistics of bar orientations in our sampleindicate that the missing stars of the inner disk have not simply beenredistributed azimuthally into bar structures. The sharpness of thebrightness truncations and their locations with respect to othergalactic structures suggest that resonances associated with diskkinematics, or tidal interactions with the mass of bulge stars, might beresponsible for this phenomenon.
| The Stellar Content of the Polar Rings in the Galaxies NGC 2685 and NGC 4650A
We present the results of stellar photometry of polar ring galaxies NGC2685 and NGC 4650A, using the archival data obtained with the HubbleSpace Telescope's Wide Field Planetary Camera 2. Polar rings of thesegalaxies were resolved into ~800 and ~430 stellar objects in the B, V,and IC bands, a considerable part of which are bluesupergiants located in the young stellar complexes. The stellar featuresin the CM diagrams are best represented by isochrones with metallicityZ=0.008. The process of star formation in the polar rings of bothgalaxies was continuous, and the age of the youngest detected stars isabout 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.Based on observations made with the NASA/ESO Hubble Space Telescope,obtained from the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS 5-26555.
| The stellar content of the ring in NGC 660
We present the results of stellar photometry of the polar-ring galaxyNGC 660 using the Hubble Space Telescope's archivaldata obtained with the Wide Field Planetary Camera 2. The final list ofthe resolved stars contains 550 objects, a considerable part of whichare blue and red supergiants belonging to the polar ring. The analysisof the Colour-Magnitude Diagram for polar ring stars shows that it isbest represented by the isochrones with metallicity Z = 0.008. Theprocess of star formation in the polar ring was continuous and the ageof the youngest detected stars is about 7 Myr.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS5-26555.
| Photometric structure of polar-ring galaxies
The results of B, V, R surface photometry of three polar-ring galaxies(PRGs) - A 0017+2212, UGC 1198, UGC 4385 - are presented. The data wereacquired at the 6-m telescope of the Special Astrophysical Observatoryof the Russian Academy of Sciences. It was shown that all three galaxiesare peculiar late-type spirals in the state of ongoing interaction ormerging. We discuss available photometric properties of the PRGs withspiral hosts and consider the Tully-Fisher relation for different typesof PRGs. In agreement with Iodice et al. (\cite{Iodice03}), we haveshown that true PRGs demonstrate 1/3 larger maximum rotationvelocities than spiral galaxies of the same luminosity. Peculiar objectswith forming polar structures satisfy, on average, the Tully-Fisherrelation for disk galaxies but with large scatter.
| Minor-axis velocity gradients in disk galaxies
We present the ionized-gas kinematics and photometry of a sample of 4spiral galaxies which are characterized by a zero-velocity plateau alongthe major axis and a velocity gradient along the minor axis,respectively. By combining these new kinematical data with thoseavailable in the literature for the ionized-gas component of the S0s andspirals listed in the Revised Shapley-Ames Catalog of Bright Galaxies werealized that about 50% of unbarred galaxies show a remarkable gasvelocity gradient along the optical minor axis. This fraction rises toabout 60% if we include unbarred galaxies with an irregular velocityprofile along the minor axis. This phenomenon is observed all along theHubble sequence of disk galaxies, and it is particularly frequent inearly-type spirals. Since minor-axis velocity gradients are unexpectedif the gas is moving onto circular orbits in a disk coplanar to thestellar one, we conclude that non-circular and off-plane gas motions arenot rare in the inner regions of disk galaxies.Based on observations carried out at the European Southern Observatoryin La Silla (Chile) (ESO 69.B-0706 and 70.B-0338), with the MultipleMirror Telescope which is a joint facility of the SmithsonianInstitution and the University of Arizona, and with the ItalianTelescopio Nazionale Galileo (AOT-5, 3-18) at the Observatorio del Roquede los Muchachos in La Palma (Spain).Table 1 is only available in electronic form athttp://www.edpsciences.org. Table 5 is only available in electronic format 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/416/507
| The stellar population study of the polar rings in the galaxies NGC 2685 and NGC 4650A
Polar ring galaxies (PRGs) are dynamically peculiar systems with a ringor annulus of gas, stars and dust orbiting in a plane nearlyperpendicular to the equatorial plane of the host galaxy (Withmore1990). According to the most popular point of view, PRGs are the resultof galaxy interaction, which ranges from simple gas accretion to acomplete merger. Alternatively, polar rings can represent the delayedinflows of primordial gas. Therefore, the problem of PR's age is actual(Eskridge & Podge 1997; Gallagher et al 2002). Deep single-starphotometry can directly identify stars in various evolutionary phasesthrough their positions on CMD. This can cast light on the origin andevolution of PRGs.
| Near-infrared imaging of ellipticals: surface brightness profiles and photometry
We present near-infrared K-band imaging of a large sample of candidatemerger remnant galaxies and Hickson Compact Group ellipticals. We derivelight profile indices, effective radii and surface brightnesses, as wellas total K-band magnitudes. We find that the light distributions of themerger remnant candidates are consistent with those of `normal'ellipticals, and scatter around a mean profile index of (1/n) = 0.20.Many of our sample galaxies have surface brightness profiles that arenot well described by a de Vaucouleurs law (1/n= 0.25), and we discussthe implications of this on the derived total magnitudes. Comparing thetotal K magnitudes calculated by extrapolating a de Vaucouleurs profileand those derived using a generalized Sérsic form, we find that asignificant bias is introduced if the de Vaucouleurs law is not a gooddescription of the actual light profile.
| An Imaging Survey of Early-Type Barred Galaxies
This paper presents the results of a high-resolution imaging survey,using both ground-based and Hubble Space Telescope images, of a completesample of nearby barred S0-Sa galaxies in the field, with a particularemphasis on identifying and measuring central structures within thebars: secondary bars, inner disks, nuclear rings and spirals, andoff-plane dust. A discussion of the frequency and statistical propertiesof the various types of inner structures has already been published.Here we present the data for the individual galaxies and measurements oftheir bars and inner structures. We set out the methods we use to findand measure these structures, and how we discriminate between them. Inparticular, we discuss some of the deficiencies of ellipse fitting ofthe isophotes, which by itself cannot always distinguish between bars,rings, spirals, and dust, and which can produce erroneous measurementsof bar sizes and orientations.
| Polar Ring Galaxies and the Tully-Fisher Relation: Implications for the Dark Halo Shape
We have investigated the Tully-Fisher relation for polar ring galaxies(PRGs), based on near-infrared, optical, and H I data available for asample of these peculiar objects. The total K-band luminosity, whichmainly comes from the central host galaxy, and the measured H I linewidth at 20% of the peak line flux density, which traces the potentialin the polar plane, place most polar rings in the sample far from theTully-Fisher relation defined for spiral galaxies, with many PRGsshowing larger H I line widths than expected for the observed K-bandluminosity. This result is confirmed by a larger sample of objects,based on B-band data. This observational evidence may be related to thedark halo shape and orientation in these systems, which we study bynumerical modeling of PRG formation and dynamics: the larger rotationvelocities observed in PRGs can be explained by a flattened polar halo,aligned with the polar ring.
| Lensing and the Centers of Distant Early-Type Galaxies
Gravitational lensing provides a unique probe of the inner 10-1000 pc ofdistant galaxies (z~0.2-1). Theoretical studies have predicted that eachstrong lens system should have a faint image near the center of the lensgalaxy, which should, in principle, be visible in radio lenses but hasnever been detected. We study the predicted ``core'' images using modelsderived from the stellar distributions in nearby early-type galaxies. Wefind that realistic lens galaxies produce a remarkably wide range ofcore images, with magnifications spanning some 6 orders of magnitude.More concentrated galaxies produce fainter core images, although notwith any model-independent relation between the galaxy properties andthe core images. Some real galaxies have diffuse cores that should yieldbright core images (magnification μcore>~0.1), but morecommon are galaxies that yield faint core images(μcore<~0.001). Thus, stellar mass distributions aloneare probably concentrated enough to explain the lack of observed coreimages. Observational sensitivity may need to improve by an order ofmagnitude before detections of core images become common. Two-imagelenses should tend to have brighter core images than four-image lenses,so they will be the better targets for finding core images andexploiting these tools for studying the central mass distributions ofdistant galaxies.
| Formation of polar ring galaxies
Polar ring galaxies are peculiar systems in which a gas-rich, nearlypolar ring surrounds an early-type or elliptical host galaxy. Twoformation scenarios for these objects have been proposed: they arethought to form either in major galaxy mergers or by tidal accretion ofthe polar material from a gas rich donor galaxy. Both scenarios arestudied through N-body simulations including gas dynamics and starformation. Constraints on physical parameters are drawn out, in order todetermine which scenario is the most likely to occur. Polar ringgalaxies from each scenario are compared with observations and wediscuss whether the accretion scenario and the merging scenario accountfor observational properties of polar ring galaxies. The conclusion ofthis study is that the accretion scenario is both the most likely andthe most supported by observations. Even if the merging scenario israther robust, most polar ring galaxies are shown to be the result oftidal gas accretion events.Appendices A and B are only available in electronic form athttp://www.edpsciences.org
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