Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
HI content in galaxies in loose groups
Gas deficiency in cluster spirals is well known and ram-pressurestripping is considered the main gas removal mechanism. In some compactgroups too gas deficiency is reported. However, gas deficiency in loosegroups is not yet well established. Lower dispersion of the membervelocities and the lower density of the intragroup medium in small loosegroups favour tidal stripping as the main gas removal process in them.Recent releases of data from the HI Parkes All-Sky Survey (HIPASS) andcatalogues of nearby loose groups with associated diffuse X-ray emissionhave allowed us to test this notion. In this paper, we address thefollowing questions: (i) do galaxies in groups with diffuse X-rayemission statistically have lower gas content compared to the ones ingroups without diffuse X-ray emission? (ii) does HI deficiency vary withthe X-ray luminosity, LX, of the loose group in a systematicway? We find that (i) galaxies in groups with diffuse X-ray emission, onaverage, are HI deficient, and have lost more gas compared to those ingroups without X-ray emission; the latter are found not to havesignificant HI deficiency; (ii) no systematic dependence of the HIdeficiency with LX is found. Ram-pressure-assisted tidalstripping and evaporation by thermal conduction are the two possiblemechanisms to account for this excess gas loss.
| Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Ultraviolet-to-Far-Infrared Properties of Local Star-forming Galaxies
We present the results of a multiwavelength study of nearby galaxiesaimed at understanding the relation between the ultraviolet andfar-infrared emission in star-forming galaxies. The data set comprisesnew ultraviolet (from HST STIS), ground-based Hα, and radiocontinuum observations, together with archival infrared data (from IRASand ISO). The local galaxies are used as benchmarks for comparison ofthe infrared-to-ultraviolet properties with two populations ofhigh-redshift galaxies: the submillimeter star-forming galaxies detectedby SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). Inaddition, the long wavelength baseline covered by the present dataenables us to compare the star formation rates (SFRs) derived from theobserved ultraviolet, Hα, infrared, and radio luminosities and togauge the impact of dust opacity in the local galaxies. We also derive anew calibration for the nonthermal part of the radio SFR estimator,based on the comparison of 1.4 GHz measurements with a new estimator ofthe bolometric luminosity of the star-forming regions. We find that moreactively star-forming galaxies show higher dust opacities, which is inline with previous results. We find that the local star-forming galaxieshave a lower Fλ(205 μm)/Fλ(UV)ratio by 2-3 orders of magnitude than the submillimeter-selectedgalaxies and may have a similar or somewhat higherFλ(205 μm)/Fλ(UV) ratio thanLBGs. The Fλ(205 μm)/Fλ(UV) ratioof the local galaxy population may be influenced by the cool dustemission in the far-infrared heated by nonionizing stellar populations,which may be reduced or absent in the LBGs.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| 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.
| 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.
| Dust properties of UV bright galaxies at z ~ 2
We investigate the properties of the extinction curve in the rest-frameUV for a sample of 34 UV-luminous galaxies at 2 < z < 2.5,selected from the FORS Deep Field (FDF) spectroscopic survey. A newparametric description of the rest-frame UV spectral energy distributionis adopted; its sensitivity to properties of the stellar populations orof dust attenuation is established with the use of models. The latterare computed by combining composite stellar population models andcalculations of radiative transfer of the stellar and scatteredradiation through the dusty interstellar medium (ISM) for a dust/starsconfiguration describing dust attenuation in local starbursts. In thefavoured configuration the stars are enveloped by a shell with atwo-phase, clumpy, dusty ISM. The distribution of the z 2UV-luminous FDF galaxies in several diagnostic diagrams shows that theirextinction curves range between those typical of the Small and LargeMagellanic Clouds (SMC and LMC, respectively). For the majority ofstrongly reddened objects having a UV continuum slope β > -0.4 asignificant 2175 Å absorption feature (or "UV bump") is inferred,indicating an LMC-like extinction curve. On the other hand, the UVcontinua of the least reddened objects are mostly consistent withSMC-like extinction curves, lacking a significant UV bump, as for thesample of local starbursts investigated by Calzetti and collaborators.Furthermore, the most opaque (β 0) and, thus (for ourmodels), dustiest UV-luminous FDF galaxies tend to be among the mostmetal-rich, most massive, and largest systems at z 2, indicating< Z > 0.5 {-} 1 Zȯ, < Mstars> 6 × 1010 Mȯ, and 4 kpc, respectively. The presence of the UVbump does not seem to depend on the total metallicity, as given by theequivalent width (EW) of the C IV doublet. Conversely, it seems to beassociated with a large average EW of the six most prominentinterstellar low-ionisation absorption lines falling in the FORSspectra. The average EW of these saturated lines offers a proxy for theISM topology. We interpret these results as the evidence for adifference in the properties of the dusty ISM among the most evolvedUV-luminous, massive galaxies at z 2.
| 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.
| 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.
| A Composite Extreme-Ultraviolet QSO Spectrum from FUSE
The Far Ultraviolet Spectroscopic Explorer (FUSE) has surveyed a largesample (>100) of active galactic nuclei (AGNs) in the low-redshiftuniverse (z<1). Its response at short wavelengths makes it possibleto measure directly the far-ultraviolet spectral properties ofquasi-stellar objects (QSOs) and Seyfert 1 galaxies at z<0.3. Usingarchival FUSE spectra, we form a composite extreme-ultraviolet (EUV)spectrum of QSOs at z<=0.67. After consideration of many possiblesources of systematic error in our analysis, we find that the spectralslope of the FUSE composite spectrum,α=-0.56+0.38-0.28 forFν~να, is significantly harder thanthe EUV (λ<~1200 Å) portion of the composite spectrum ofQSOs with z>0.33 formed from archival Hubble Space Telescope (HST)spectra, α=-1.76+/-0.12. We identify several prominent emissionlines in the FUSE composite and find that the high-ionization O VI andNe VIII emission lines are enhanced relative to the HST composite.Power-law continuum fits to the individual FUSE AGN spectra reveal acorrelation between EUV spectral slope and AGN luminosity in the FUSEand FUSE+HST samples, in the sense that lower luminosity AGNs showharder spectral slopes. We find an anticorrelation between the hardnessof the EUV spectral slope and AGN black hole mass, using estimates ofthis quantity found in the literature. We interpret these results in thecontext of the well-known anticorrelation between AGN luminosity andemission-line strength, the Baldwin effect, given that the medianluminosity of the FUSE AGN sample is an order of magnitude lower thanthat of the HST sample.
| The Spectral Energy Distribution and Emission-Line Properties of the Narrow-Line Seyfert 1 Galaxy Arakelian 564
We present the intrinsic spectral energy distribution (SED) of thenarrow-line Seyfert 1 galaxy (NLS1) Ark 564, constructed withcontemporaneous data obtained during a multiwavelength, multisatelliteobserving campaign in 2000 and 2001. We compare this SED with that ofthe NLS1 Ton S180 and with those obtained for broad-line Seyfert 1galaxies to infer how the relative accretion rates vary among theSeyfert 1 population. Although the peak of the SED is not wellconstrained, in our parameterization most of the energy of this objectis emitted in the 10-100 eV regime, constituting roughly half of theemitted energy in the optical/X-ray ranges. This is consistent with aprimary spectral component peaking in the extreme-UV/soft X-ray band andwith disk-corona models, hence high accretion rates. Indeed, we estimatethat m~1. We also address the issue of the energy budget in this sourceby examining the emission lines observed in its spectrum, and weconstrain the physical properties of the line-emitting gas throughphotoionization modeling. The available data suggest that theline-emitting gas is characterized by logn~11 and logU~0 and isstratified around logU~0. Our estimate of the radius of theHβ-emitting region RHβBLR~10+/-2 lt-daysis consistent with the RHβBLR-luminosityrelationships found for Seyfert 1 galaxies, which indicates that thenarrowness of the emission lines in this NLS1 is not due to thebroad-line region being relatively farther away from the central massthan in broad-line Seyfert 1 galaxies of comparable luminosity. We alsofind evidence for supersolar metallicity in this NLS1. We show that theemission lines are not good diagnostics for the underlying SEDs and thatthe absorption line studies offer a far more powerful tool to determinethe ionizing continuum of active galactic nuclei, especially if one iscomparing the lower and higher ionization lines.
| The evolution of stars and gas in starburst galaxies
In systems undergoing starbursts the evolution of the young stellarpopulation is expected to drive changes in the emission-line properties.This evolution is usually studied theoretically, with a combination ofevolutionary synthesis models for the spectral energy distribution ofstarbursts and photoionization calculations. In this paper we present amore empirical approach to this issue. We apply empirical populationsynthesis techniques to samples of starburst and HII galaxies in orderto measure their evolutionary state and correlate the results with theiremission-line properties. A couple of useful tools are introduced thatgreatly facilitate the interpretation of the synthesis: (1) anevolutionary diagram, the axes of which are the strengths of the young,intermediate age and old components of the stellar population mix; and(2) the mean age of stars associated with the starburst, . These toolsare tested with grids of theoretical galaxy spectra and found to workvery well even when only a small number of observed properties(absorption-line equivalent widths and continuum colours) is used in thesynthesis.Starburst nuclei and HII galaxies are found to lie on a well-definedsequence in the evolutionary diagram. Using the empirically defined meanstarburst age in conjunction with emission-line data, we have verifiedthat the equivalent widths of Hβ and [OIII] decrease for increasing. The same evolutionary trend was identified for line ratios indicativeof the gas excitation, although no clear trend was identified formetal-rich systems. All these results are in excellent agreement withlong-known, but little tested, theoretical expectations.
| 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.
| The Unified Model and Evolution of Active Galaxies: Implications from a Spectropolarimetric Study
We extend the analysis presented in Paper I of a spectropolarimetricsurvey of the CfA and 12 μm samples of Seyfert 2 galaxies (S2s). Weconfirm that polarized (hidden) broad-line region (HBLR) S2s tend tohave hotter circumnuclear dust temperatures, show mid-IR spectra morecharacteristic of Seyfert 1 galaxies (S1s), and are intrinsically moreluminous than non-HBLR S2s. The levels of obscuration and circumnuclearstar formation, however, appear to be similar between HBLR and non-HBLRS2 galaxies, based on an examination of various observationalindicators. HBLR S2s, on average, share many similar large-scale,presumably isotropic, characteristics with S1s, as would be expected ifthe unified model is correct, while non-HBLR S2s generally do not. Theactive nuclear engines of non-HBLR S2s, then, appear to be truly weakerthan HBLR S2s, which in turn are fully consistent with being S1s viewedfrom another direction. There is also evidence that the fraction ofdetected HBLRs increases with the radio power of the active galacticnucleus. Thus, all S2 galaxies may not be intrinsically similar innature, and we speculate that evolutionary processes may be at work.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| Do bulges of early- and late-type spirals have different morphology?
We study HST/NICMOS H-band images of bulges of two equal-sized samplesof early- (TRC3 <= 3) and late-type spiral (mainly Sbc-Sc)galaxies matched in outer disk axis ratio. We find that bulges oflate-type spirals are more elongated than their counterparts inearly-type spirals. Using a KS-test we find that the two distributionsare different at the 98.4% confidence level. We conclude that the twodata sets are different, i.e. late-type galaxies have a broaderellipticity distribution and contain more elongated features in theinner regions. We discuss the possibility that these would correspond tobars at a later evolutionary stage, i.e. secularly evolved bars.Consequent implications are raised, and we discuss relevant questionsregarding the formation and structure of bulges. Are bulges ofearly-type and late-type spirals different? Are their formationscenarios different? Can we talk about bulges in the same way fordifferent types of galaxies?
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| 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.
| The continuum variability of MCG-6-30-15: a detailed analysis of the long 1999 ASCA observation
We report on an analysis in the 3-10keV X-ray band of the long 1999 ASCAobservation of MCG-6-30-15. The time-averaged broad iron K line is welldescribed by disc emission near a Schwarzschild black hole, confirmingthe results of earlier analyses on the ASCA 1994 and 1997 data. Thetime-resolved iron-line profile is remarkably stable over a factor of 3change in source flux, and the line and continuum fluxes areuncorrelated. Detailed fits to the variable iron-line profile suggestthat the active region (parametrized by the best-fitting inner and outerradii of the accretion disc) responsible for iron-line emission actuallynarrows with increasing flux to a region around 4-5rg. Incontrast with the iron line, the power-law continuum exhibitssignificant variability during the 1999 observation. Time-resolvedspectral analysis reveals a new feature in the well-known photon index(Γ) versus flux correlation: Γ appears to approach alimiting value of Γ~2.1 at high flux. Two models are proposed toexplain both the new feature in the Γ versus flux correlation andthe uncorrelated iron-line flux: a phenomenological two power-law model,and the recently proposed `thundercloud' model of Merloni & Fabian.Both models are capable of reproducing the data well, but because theyare poorly constrained by the observed Γ versus flux relation,they cannot at present be tested meaningfully by the data. The variousimplications and the physical interpretation of these models arediscussed.
| 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.
| A New Database of Observed Spectral Energy Distributions of Nearby Starburst Galaxies from the Ultraviolet to the Far-Infrared
We present a database of UV-to-FIR data of 83 nearby starburst galaxies.The galaxies are selected based upon the availability of IUE data. Wehave recalibrated the IUE UV spectra for these galaxies by incorporatingthe most recent improvements. For 45 of these galaxies we useobservations by Storchi-Bergmann et al. and McQuade et al. for thespectra in the optical range. The NIR data are from new observationsobtained at the NASA/IRTF and the Mount Laguna Observatory, combinedwith the published results from observations at the Kitt Peak NationalObservatory. In addition, published calibrated ISO data are included toprovide mid-IR flux densities for some of the galaxies. Theoptical-to-IR data are matched as closely as possible to the IUE largeaperture. In conjunction with IRAS and ISO FIR flux densities, all thesedata form a set of observed spectral energy distributions (SEDs) of thenuclear regions of nearby starburst galaxies. The SEDs should be usefulin studying star formation and dust/gas attenuation in galaxies. We alsopresent the magnitudes in the standard BVRI and various HST/WFPC2bandpasses synthesized from the UV and optical wavelength ranges ofthese SEDs. For some of the galaxies, the HST/WFPC2 magnitudessynthesized from the SEDs are checked with those directly measured fromWFPC2 images to test the photometric errors of the optical data andtheir effective matching of apertures with the UV data. The implicationsof the new SEDs on the star formation rates and dust/gas attenuation arebriefly discussed.
| Optical Photometry and the Continuum of Active Galactic Nuclei
In this paper, the continuum emission of active galactic nuclei isstudied using broadband B, V, R, and I photometry. The nuclearcontribution is estimated from the observations using two differentapproaches. In the first one, the images are deconvolved by the seeingprofile and the corrected images are used to derive the nuclearcontribution. In the second method, in order to extract the stellarcontribution, a de Vaucouleurs brightness profile is assumed for thebulge. After subtraction of this component, the total nuclear emissionis obtained from the corrected image. Both methods indicate that thestellar contribution is dominant. The average contribution of thefeatureless continuum to the total observed continuum is 30% in the fourbands. We show that over 2 orders of magnitude the Hαemission-line luminosity correlates with the continuum emission in allthe observed bands. As a consequence, the galaxies in the sample musthave similar color index. This result provides a method to estimate thereddening correction for the nuclear continuum, which we found to belower than the emission-line extinction correction estimated from theobserved Hα/Hβ observed line ratio.
| Seyfert 2 Galaxies with Spectropolarimetric Observations
We present a compilation of radio, infrared, optical, and hard X-ray(2-10 keV) data for a sample of 90 Seyfert 2 galaxies (Sy2s) withspectropolarimetric observations (41 Sy2s with detection of polarizedbroad lines [PBLs] and 49 without PBLs). Compared to Sy2s without PBLs,Sy2s with PBLs tend to be earlier type spirals and show warmermidinfrared color and significant excess of emissions (including thehard X-ray [2-10 keV], [O III] λ5007, infrared [25 μm], andradio). Our analyses indicate that the majority of Sy2s without PBLs arethose sources having less powerful active galactic nucleus (AGN)activities, most likely caused by a low accretion rate. It implies thatthe detectability of the polarized broad emission lines in Sy2s maydepend on their central AGN activities in most cases. Based on theavailable data, we find no compelling evidence for the presence of twotypes of Sy2s; one of which has been proposed to be intrinsicallydifferent from Sy2s claimed in the unification model.
| The Far-Infrared Energy Distributions of Seyfert and Starburst Galaxies in the Local Universe: Infrared Space Observatory Photometry of the 12 Micron Active Galaxy Sample
New far-infrared photometry with ISOPHOT aboard the Infrared SpaceObservatory (ISO) is presented for 58 galaxies with homogeneouspublished data for another 32 galaxies, all belonging to the 12 μmgalaxy sample-in total, 29 Seyfert 1 galaxies, 35 Seyfert 2 galaxies,and 12 starburst galaxies, or about half of the 12 μm active galaxysample, plus 14 normal galaxies for comparison. ISO and InfraredAstronomical Satellite (IRAS) data are used to define color-colordiagrams and spectral energy distributions (SEDs). Thermal dust emissionat two temperatures (one cold at 15-30 K and one warm at 50-70 K) canfit the 60-200 μm SED, with a dust emissivity law proportional to theinverse square of the wavelength. Seyfert 1 galaxies and Seyfert 2galaxies are indistinguishable longward of 100 μm, while, as alreadyseen by IRAS, the former have flatter SEDs shortward of 60 μm. A mildanticorrelation is found between the [200-100] color and the ``60 μmexcess.'' We infer that this is due to the fact that galaxies with astrong starburst component and thus a strong 60 μm flux have asteeper far-infrared turnover. In non-Seyfert galaxies, increasing theluminosity corresponds to increasing the star formation rate, whichenhances the 25 and 60 μm emission. This shifts the peak emissionfrom around 150 μm in the most quiescent spirals to shorter than 60μm in the strongest starburst galaxies. To quantify these trendsfurther, we identified with the IRAS colors three idealized infraredSEDs: pure quiescent disk emission, pure starburst emission, and pureSeyfert nucleus emission. Even between 100 and 200 μm, the quiescentdisk emission remains much cooler than the starburst component. Seyfertgalaxies have 100-200 μm SEDs ranging from pure disks to purestarbursts, with no apparent contribution from their active nuclei atthose wavelengths. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, the Netherlands, and the United Kingdom) with theparticipation of ISAS and NASA.
| Subarcsecond Emission in Seyfert Galaxies: The Nuclear Component in the L and M Bands
We present deep L- (3.8 μm) and M- (4.7 μm) band imaging with theInfrared Spectrometer and Array Camera (ISAAC) on the ESO Very LargeTelescope (VLT) with unprecedented spatial resolution of the nearbySeyfert 2 galaxies NGC 7496 and NGC 7582 and the Seyfert 1 galaxy NGC7213, all at a comparable distance. The unresolved nuclear componentdominates the emission within the central 90 pc region, while the hostgalaxy accounts for up to 50% of the integrated emission at bothwavelengths within the detected sizes of ~1 kpc in the L band and ~0.5kpc in the M band. The overall morphology of the extended componentfollows the general isophote pattern defined by the near-infraredcontinuum of the galaxies. However, the central 300 pc regions show muchmore ordered elliptical isophotes than in the near-infrared. Inparticular, emission in the L and M bands shows well-defined centralpoint sources in the two Seyfert 2 galaxies. The brightness of theSeyfert nuclei in the LM bands indicates that the Seyfert 2 galaxy NGC7582 harbors as powerful a nucleus as that in the Seyfert 1 galaxy NGC7213, while the Seyfert 2 galaxy NGC 7496 is intrinsically weaker. InNGC 7582, we have detected the circumnuclear star-forming ring in the Lband.
| Nested and Single Bars in Seyfert and Non-Seyfert Galaxies
We analyze the observed properties of nested and single stellar barsystems in disk galaxies. The 112 galaxies in our sample comprise thelargest matched Seyfert versus non-Seyfert galaxy sample of nearbygalaxies with complete near-infrared or optical imaging sensitive tolength scales ranging from tens of parsecs to tens of kiloparsecs. Thepresence of bars is deduced by fitting ellipses to isophotes in HubbleSpace Telescope (HST) H-band images up to 10" radius and in ground-basednear-infrared and optical images outside the H-band images. This is aconservative approach that is likely to result in an underestimate ofthe true bar fraction. We find that a significant fraction of the samplegalaxies, 17%+/-4%, have more than one bar, and that 28%+/-5% of barredgalaxies have nested bars. The bar fractions appear to be stableaccording to reasonable changes in our adopted bar criteria. For thenested bars, we detect a clear division in length between thelarge-scale (primary) bars and small-scale (secondary) bars, in bothabsolute and normalized (to the size of the galaxy) length. We arguethat this bimodal distribution can be understood within the framework ofdisk resonances, specifically the inner Lindblad resonances (ILRs),which are located where the gravitational potential of the innermostgalaxy switches effectively from three-dimensional to two-dimensional.This conclusion is further strengthened by the observed distribution ofthe sizes of nuclear rings which are dynamically associated with theILRs. While primary bar sizes are found to correlate with the hostgalaxy sizes, no such correlation is observed for the secondary bars.Moreover, we find that secondary bars differ morphologically from singlebars. Our matched Seyfert and non-Seyfert samples show a statisticallysignificant excess of bars among the Seyfert galaxies at practically alllength scales. We confirm our previous results that bars are moreabundant in Seyfert hosts than in non-Seyfert galaxies and that Seyfertgalaxies always show a preponderance of ``thick'' bars compared to thebars in non-Seyfert galaxies. Finally, no correlation is observedbetween the presence of a bar and that of companion galaxies, evenrelatively bright ones. Overall, since star formation and dustextinction can be significant even in the H band, the stellar dynamicsof the central kiloparsec cannot always be revealed reliably by the useof near-infrared surface photometry alone.
| Spectropolarimetry of a complete infrared-selected sample of Seyfert 2 galaxies
We report the results of a spectropolarimetric survey of a completefar-infrared-selected sample of Seyfert 2 galaxies. We have foundpolarized broad Hα emission in one new source, NGC 5995. In thesample as a whole, there is a clear tendency for galaxies in which wehave detected broad Hα in polarized light to have warmmid-far-infrared colours(F60μm/F25μm<~4), in agreement with ourprevious results. However, a comparison of the optical, radio and hardX-ray properties of these systems leads us to conclude that this is asecondary consequence of the true mechanism governing our ability to seescattered light from the broad-line region. We find a strong trend forgalaxies showing such emission to lie above a critical value of therelative luminosity of the active core to the host galaxy (as measuredfrom the [OIII] 5007-Å equivalent width) which varies as afunction of the obscuring column density as measured from hard X-rayobservations. The warmth of the infrared colours is then largely due toa combination of the luminosity of the active core, the obscuring columnand the relative importance of the host galaxy in powering thefar-infrared emission, and not solely orientation as we inferred in ourprevious paper. Our data may also provide an explanation as to why themost highly polarized galaxies, which appear to have tori that arelargely edge-on, are also the most luminous and have the most easilydetectable scattered broad Hα.
| High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - II. The properties of compact radio components
We discuss the properties of compact nuclear radio components in Seyfertgalaxies from the extended 12-μm AGN sample of Rush et al. Our mainresults can be summarized as follows. Type 1 and type 2 Seyferts producecompact radio components which are indistinguishable in strength andaspect, indicating that their central engines are alike, as proposed bythe unification model. Infrared IRAS fluxes are more closely correlatedwith low-resolution radio fluxes than high-resolution radio fluxes,suggesting that they are dominated by kiloparsec-scale, extranuclearemission regions; extranuclear emission may be stronger in type 2Seyferts. Early-type Seyfert galaxies tend to have stronger nuclearradio emission than late-type Seyfert galaxies. V-shaped extendedemission-line regions, indicative of `ionization cones', are usuallyfound in sources with large, collimated radio outflows. Hidden broadlines are most likely to be found in sources with powerful nuclear radiosources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm fluxdensities have well-matched properties.
| The unified model and the Seyfert 2 infrared dichotomy
An optical spectropolarimetric study has shown that the detectability ofpolarized broad Hα in Seyfert 2 galaxies is correlated with theIRAS f60f25 flux ratio where only those Seyfert 2swith `warm' IRAS colours show polarized broad-line emission. It wassuggested that those Seyfert 2s with `cool' IRAS colours have highlyinclined tori which obscure the broad-line scattering screen. I presenthere hard X-ray observations inconsistent with this picture, showingthat the derived column densities of warm and cool Seyfert 2 galaxiesare statistically the same. I classify the Bright Galaxy Sample toproduce a non-Seyfert comparison. The analysis of the properties ofthese galaxies with the Seyfert 2s suggests that the IRASf60f25 flux ratio is consistent with implying therelative strength of galactic to Seyfert emission. I show that this newpicture can account for the absence of polarized broad Hα in thecool Seyfert 2s.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
Catálogos y designaciones:
|
