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|The Cool ISM in S0 Galaxies. II. A Survey of Atomic Gas|
The place of lenticular galaxies within the range of types of galaxiesremains unclear. We previously reported the mass of molecular hydrogenfor a volume-limited sample of lenticular galaxies, where we saw thatthe amount of gas was less than that predicted by the return of stellarmass to the interstellar medium. Here we report observations of atomichydrogen (H I) for the same sample. Detections in several galaxies makemore compelling the case presented in our earlier paper that the mass ofcool gas in S0 galaxies cuts off at ~10% of what is expected fromcurrent models of gas return from stellar evolution. The molecular andatomic phases of the gas in our sample galaxies appear to be separateand distinct, both spatially and in velocity space. We propose that themolecular gas arises mostly from the stellar mass returned to thegalaxy, while the atomic hydrogen is mainly accumulated from externalsources (infall, captured dwarfs, etc.). While this proposal fits mostof the observations, it makes the presence of the upper mass cutoff evenmore mysterious.
|Masses of the local group and of the M81 group estimated from distortions in the local velocity field|
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
|Weak redshift discretisation in the Local Group of galaxies?|
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
|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.
|Global Properties of Nearby Galaxies in Various Environments|
We analyze a sample of the Local Volume that contains 451 galaxieswithin 10 Mpc. We compare the various global parameters of thesegalaxies with their tidal index that characterizes the local density ofthe environment. The closest correlation is observed between the densityof the galaxy’s environment and its morphological type. Theabundance of neutral hydrogen in the members of close groups was foundto be, on average, a factor of 3 lower than that in isolated galaxies.However, much of this difference is attributable to differentmorphological composition for the group members and field galaxies. Thetotal mass-to-luminosity ratio is virtually independent of the tidalindex of the galaxy, which indirectly indicates a low percentage oftidal systems among dwarf galaxies. All of the galaxies with three ormore companions in the Local Volume are shown to have masses above thethreshold value of 1010 M ȯ.
|Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis|
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
|Detection of Radial Surface Brightness Fluctuations and Color Gradients in Elliptical Galaxies with the Advanced Camera for Surveys|
We study surface brightness fluctuations (SBFs) in a sample of eightelliptical galaxies using the Advanced Camera for Surveys (ACS) WideField Channel (WFC) data drawn from the Hubble Space Telescope archive.SBF magnitudes in the F814W bandpass and galaxy colors from F814W,F435W, and F606W images, when available, are presented. Galaxy surfacebrightness profiles are determined as well. We present the firstSBF-broadband color calibration for the ACS WFC F814W bandpass and(relative) distance moduli estimates for seven of our galaxies. Wedetect and study in detail the SBF variations within individual galaxiesas a probe of possible changes in the underlying stellar populations.Inspecting both the SBF and color gradients in comparison to modelpredictions, we argue that SBFs and SBF gradients can in principle beused for unraveling the different evolutionary paths taken by galaxies,although a more comprehensive study of this issue would be required. Weconfirm that the radial variation of galaxy stellar populationproperties is mainly connected to the presence of radial chemicalabundance gradients, with the outer galaxy regions being more metal-poorthan the inner ones.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. These observations areassociated with programs 9427, 9293, and 9399.
|Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch|
Using the Hubble Space Telescope (HST) Wide Field Camera (WFC) of theAdvanced Camera for Surveys (ACS), we have obtained deep (V, I)photometry of an outer halo field in NGC 5128, to a limiting magnitudeof I~=29. Our photometry directly reveals the core helium burningstellar population (the ``red clump'' or horizontal branch) in a giantE/S0 galaxy for the first time. The color-magnitude diagram displays avery wide red giant branch (RGB), an asymptotic giant branch (AGB) bump,and the red clump; no noticeable population of blue horizontal branchstars is present, confirming previous suggestions that old, verymetal-poor population is not ubiquitous in the halo of this galaxy. Fromthe upper RGB we derive the metallicity distribution, which we find tobe very broad and moderately metal-rich, with average [M/H]=-0.64 anddispersion 0.49 dex. The metallicity distribution function is virtuallyidentical to that found in other halo fields observed previously withHST, but with an enhanced metal-rich population that was partiallymissed in the previous surveys due to V-band incompleteness for thesevery red stars. Combining the metallicity-sensitive colors of the RGBstars with the metallicity- and age-sensitive features of the AGB bumpand the red clump, we infer the average age of the halo stars to be~8+3-3.5 Gyr. As part of our study, we present anempirical calibration of the ACS F606W and F814W filters to the standardV and I bands, achieved with ground-based observations of the same fieldmade from the EMMI camera of the New Technology Telescope of the ESO LaSilla Observatory.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprogram GO-9373. Also partially based on observations collected at theEuropean Southern Observatory, La Silla, Chile, in Observing Programme071.D-0560.
|The Hubble Space Telescope View of LINER Nuclei: Evidence for a Dual Population?|
We study a complete, distance-limited sample of 25 LINERs, 21 of whichhave been imaged with the Hubble Space Telescope. In nine objects wedetect an unresolved nucleus. To study their physical properties, wecompare the radio and optical properties of the nuclei of LINERs withthose of other samples of local active galactic nuclei (AGNs), namely,Seyfert galaxies and low-luminosity radio galaxies (LLRGs). Our resultsshow that the LINER population is not homogeneous, as there are twosubclasses: (1) the first class is similar to the LLRG class, as itextends the population of radio-loud nuclei to lower luminosities; (2)the second is similar to Seyfert galaxies and extends the properties ofradio-quiet nuclei toward the lowest luminosities. The objects areoptimally discriminated in the plane formed by the black hole massversus nuclear radio loudness: all radio-loud LINERs haveMBH>~108Msolar, while Seyfertgalaxies and radio-quiet LINERs haveMBH<~108Msolar. The different natureof the various classes of local AGNs are best understood when thefraction of the Eddington luminosity they irradiate,Lo/LEdd, is plotted against the nuclearradio-loudness parameter: Seyfert galaxies are associated withrelatively high radiative efficienciesLo/LEdd>~10-4 (and high accretionrates onto low-mass black holes); LLRGs are associated with lowradiative efficiencies (and low accretion rates onto high-mass blackholes); all LINERs have low radiative efficiency (and accretion rates)and can be radio-loud or radio-quiet, depending on their black holemass.Based on observations obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
|The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies|
LINER nuclei, which are present in many nearby galactic bulges, may bethe manifestation of low-rate or low-radiative-efficiency accretion ontosupermassive central black holes. However, it has been unclear whetherthe compact UV nuclear sources present in many LINERs are clusters ofmassive stars, rather than being directly related to the accretionprocess. We have used the Hubble Space Telescope to monitor the UVvariability of a sample of 17 galaxies with LINER nuclei and compactnuclear UV sources. Fifteen of the 17 galaxies were observed more thanonce, with two to five epochs per galaxy, spanning up to a year. Wedetect significant variability in most of the sample, with peak-to-peakamplitudes from a few percent to 50%. In most cases, correlatedvariations are seen in two independent bands (F250W and F330W).Comparison to previous UV measurements indicates, for many objects,long-term variations by factors of a few over decade timescales.Variability is detected in LINERs with and without detected compactradio cores, in LINERs that have broad Hα wings detected in theiroptical spectra (``LINER 1s''), and in those that do not (``LINER 2s'').This variability demonstrates the existence of a nonstellar component inthe UV continuum of all types and sets a lower limit to the luminosityof this component. Interestingly, all the LINERs that have detectedradio cores have variable UV nuclei, as one would expect from bona fideactive galactic nuclei. We note a trend in the UV color (F250W/F330W)with spectral type-LINER 1s tend to be bluer than LINER 2s. This trendmay indicate a link between the shape of the nonstellar continuum andthe presence or the visibility of a broad-line region. In one target,the poststarburst galaxy NGC 4736, we detect variability in a previouslynoted UV source that is offset by 2.5" (~60 pc in projection) from thenucleus. This may be the nearest example of a binary active nucleus andof the process leading to black hole merging.Based on observations with the Hubble Space Telescope, which is operatedby AURA, Inc., under NASA contract NAS 5-26555.
|Probing the Dust Obscuration in Seyfert Galaxies using Infrared Spectroscopy. II. Implication for the Unification of Seyfert Galaxies|
We report near-IR spectroscopic observations of 11 Seyfert galaxies (sixSeyfert 1s, one Seyfert 1.9, and four Seyfert 2s) and additionalgalaxies (four LINERs, two H II, and one type 2 transition) forcomparison, obtained using the Gemini twin-channel near-IR camera on theShane 3 m telescope at Lick Observatory. With the unique design of theGemini camera, full J and K spectra were taken simultaneously throughthe same slit. This produced accurate line ratios of hydrogenrecombination lines over a large wavelength baseline. For the Seyfert 1s(<=1.5), the line ratios of Paβ/Brγ are not onlycomparable in both broad- and narrow-line regions but also consistentwith case B recombination, indicating little or no reddening in bothnarrow- and broad-line regions. Seyfert 2 (>1.5) galaxies, however,show substantial reddening toward the narrow-line regions. We compareoptical reddening data from the literature and find significant supportfor the dichotomy between Seyfert 1s and Seyfert 2s, at least inlow-luminosity objects. Two different scenarios are explored to explainthe observed difference in reddening: a difference in reddening due toan extended dusty torus under active galactic nucleus unification, and adifference due to a different grain size distribution between the twoSeyfert types. We also discuss a similar potential difference found inthe strength of the 9.7 μm silicate line, along with a possiblecorrelation between the narrow-line reddening and the strength of thesilicate absorption line. We also analyzed CO band head absorptionfeatures longward of 2.3 μm to look for nonstellar contamination andevidence of recent star formation activity. The CO band head in Seyfert1s shows heavy contamination from nonstellar radiation, which iscorrelated with an H-K nuclear color excess. We confirm that the COspectroscopic indices in both Seyfert types do not show evidence ofrecent star formation. Taking the nonstellar contamination into account,there is little evidence from the CO index for a difference in starformation rates in the nuclei of Seyfert 1s and Seyfert 2s in ourlow-luminosity sample.
|A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion|
We present results from a high-resolution X-ray imaging study of nearbyLINERs observed by ACIS on board Chandra. This study complements andextends previous X-ray studies of LINERs, focusing on the underexploredpopulation of nearby dust-enshrouded infrared-bright LINERs. The sampleconsists of 15 IR-bright LINERs (LFIR/LB>3),with distances that range from 11 to 26 Mpc. Combining our sample withprevious Chandra studies, we find that ~51% (28/55) of the LINERsdisplay compact hard X-ray cores. The nuclear 2-10 keV luminosities ofthe galaxies in this expanded sample range from ~2×1038to ~2×1044 ergs s-1. We find that the mostextreme IR-faint LINERs are exclusively active galactic nuclei (AGNs).The fraction of LINERs containing AGNs appears to decrease with IRbrightness and increase again at the highest values ofLFIR/LB. We find that of the 24 LINERs showingcompact nuclear hard X-ray cores in the expanded sample that wereobserved at Hα wavelengths, only eight actually show evidence of abroad line. Similarly, of the 14 LINERs showing compact nuclear hardX-ray cores with corresponding radio observations, only eight display acompact flat spectrum radio core. These findings emphasize the need forhigh-resolution X-ray imaging observations in the study of IR-brightLINERs. Finally, we find an intriguing trend in the Eddington ratioversus LFIR and LFIR/LB for theAGN-LINERs in the expanded sample that extends over 7 orders ofmagnitude in L/LEdd. This correlation may imply a linkbetween black hole growth, as measured by the Eddington ratio, and thestar formation rate, as measured by the far-IR luminosity andIR-brightness ratio. If the far-IR luminosity is an indicator of themolecular gas content in our sample of LINERs, our results may furtherindicate that the mass accretion rate scales with the host galaxy's fuelsupply. We discuss the potential implications of our results in theframework of black hole growth and AGN fueling in low-luminosity AGNs.
|The Ultraluminous X-Ray Source Population from the Chandra Archive of Galaxies|
One hundred fifty-four discrete non-nuclear ultraluminous X-ray (ULX)sources, with spectroscopically determined intrinsic X-ray luminositiesgreater than 1039 ergs s-1, are identified in 82galaxies observed with Chandra's Advanced CCD Imaging Spectrometer.Source positions, X-ray luminosities, and spectral and timingcharacteristics are tabulated. Statistical comparisons between theseX-ray properties and those of the weaker discrete sources in the samefields (mainly neutron star and stellar-mass black hole binaries) aremade. Sources above ~1038 ergs s-1 display similarspatial, spectral, color, and variability distributions. In particular,there is no compelling evidence in the sample for a new and distinctclass of X-ray object such as the intermediate-mass black holes.Eighty-three percent of ULX candidates have spectra that can bedescribed as absorbed power laws with index <Γ>=1.74 andcolumn density =2.24×1021cm-2, or ~5 times the average Galactic column. About 20% ofthe ULXs have much steeper indices indicative of a soft, and likelythermal, spectrum. The locations of ULXs in their host galaxies arestrongly peaked toward their galaxy centers. The deprojected radialdistribution of the ULX candidates is somewhat steeper than anexponential disk, indistinguishable from that of the weaker sources.About 5%-15% of ULX candidates are variable during the Chandraobservations (which average 39.5 ks). Comparison of the cumulative X-rayluminosity functions of the ULXs to Chandra Deep Field results suggests~25% of the sources may be background objects, including 14% of the ULXcandidates in the sample of spiral galaxies and 44% of those inelliptical galaxies, implying the elliptical galaxy ULX population isseverely compromised by background active galactic nuclei. Correlationswith host galaxy properties confirm the number and total X-rayluminosity of the ULXs are associated with recent star formation andwith galaxy merging and interactions. The preponderance of ULXs instar-forming galaxies as well as their similarities to less-luminoussources suggest they originate in a young but short-lived populationsuch as the high-mass X-ray binaries with a smaller contribution (basedon spectral slope) from recent supernovae. The number of ULXs inelliptical galaxies scales with host galaxy mass and can be explainedmost simply as the high-luminosity end of the low-mass X-ray binarypopulation.
|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.
|Dust and Ionized Gas in Nine Nearby Early-Type Galaxies Imaged with the Hubble Space Telescope Advanced Camera for Surveys|
We present V and I continuum images and Hα+[N II] maps of nineearly-type galaxies observed with the Wide Field Channel of the AdvancedCamera for Surveys on the Hubble Space Telescope. Dust and ionized gasare detected in all galaxies. The optical nebulae are primarilyconcentrated on the nuclei and extend out to radii of a few hundredparsecs in compact clumps, filaments, or disks. Two galaxies, NGC 6166and NGC 6338, also possess diffuse, ionized filaments on kiloparsecscales. The ionized gas is entirely contained within the nuclear disksof ESO 208-G021, NGC 3078, and NGC 7720. In the radio-loud galaxy NGC6166, emission-line filaments are detected along the radio lobes,possibly as a result of shock ionization. A wide range of ionized gasmasses, Mg~7×102-3×106Msolar, are calculated from the observed fluxes. Even in thissmall sample, the orientation of the ionized material correlates wellwith the major or minor axis of the galaxies, consistent with anexternal origin for the dust and gas.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555.
|High-Resolution H I Observations of the Galaxy NGC 404: A Dwarf S0 with Abundant Interstellar Gas|
As part of a detailed study of the gas content in a sample of early-typegalaxies, we present 21 cm H I line maps of the S0 galaxy NGC 404 at aspatial resolution of 15.2"'×14.4"(α×δ) and a velocity resolution of 2.6 kms-1. The H I has been traced out to a radiusR~8R25 or 48 disk scale lengths, making it one of the largestH I extents reported (800" or 12.6 kpc at the assumed distance of 3.3Mpc). Approximately 75% of the H I resides in a doughnut, which is seenclose to face-on with inner and outer radii of ~R25 and~4R25, respectively. The optical galaxy fits snugly withinthe hole of the doughnut. The remaining 25% of the neutral gas is foundin an annulus concentric with the doughnut and with a somewhat largerellipticity, extending from a radius of ~4R25 to~8R25. A total H I mass of1.52+/-0.04×108Msolar is found, whichimplies an MHI/LB=0.22 in solar units. We arguethat most, if not all, of this gas is of external origin, most likelyfrom the merger of a dwarf irregular galaxy with MB~-15.5mag. The velocity field shows a steeply declining observed rotationcurve, compatible with Keplerian decline. However, because the galaxy isclose to face-on, there is a degeneracy in the determination of theintrinsic rotation curve and inclination. We therefore analyzed twoextreme cases, producing tilted-ring model fits forcing either aKeplerian or a flat rotation curve through the observations; bothapproaches result in equally plausible fits. In both model fits, theposition angle of the kinematical major axis of the annulus is distinctfrom that of the doughnut and ranges from 160° to 120° (for thedoughnut these values are 100° to 60°). Assuming a distance of3.3 Mpc, a total mass of 3×1010Msolar isfound on the basis of the Keplerian rotation curve. This implies aglobal MT/LB ratio of ~44 in solar units, which ishigh and likely a reflection of the low blue luminosity of the galaxy(~15 times lower than the average S0 luminosity). Values for a flatrotation curve are a factor of 4 higher.
|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.
|A Catalog of Neighboring Galaxies|
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
|BIMA CO (1-0) Observations of the Dwarf Elliptical Galaxy NGC 404|
We present high resolution observations of the CO emission in NGC 404, anearby dwarf elliptical (dE) galaxy (D = 3.3 Mpc). NGC 404 is only thethird dwarf elliptical to have its CO emission mapped by interferometricobservations, and is the first outside the Local Group. Our observationsshow a very concentrated, marginally resolved structure about 9 ×9 arcseconds in diameter. This corresponds to a very small cloud at thecenter of a much larger distribution of stars. NGC 404 is surrounded bya doughnut shaped distribution of HI gas centered on the stellarcomponent. The CO and HI appear to be kinematically distinct components,suggesting that the HI may be part of the galaxy's original gasdistribution, while the CO may be recycled from the products of stellarevolution.C.L.T. has been supported by CSU Sacramento via a Research and CreativeActivity Award. G.R.P. has been supported by the Laboratory forMillimeter-Wave Astronomy through NSF grant AST 99-81289
|The very local Hubble flow: Computer simulations of dynamical history|
The phenomenon of the very local (≤3 Mpc) Hubble flow is studied onthe basis of the data of recent precision observations. A set ofcomputer simulations is performed to trace the trajectories of the flowgalaxies back in time to the epoch of the formation of the Local Group.It is found that the ``initial conditions'' of the flow are drasticallydifferent from the linear velocity-distance relation. The simulationsenable one also to recognize the major trends of the flow evolution andidentify the dynamical role of universal antigravity produced by thecosmic vacuum.
|A joint mid-infrared spectroscopic and X-ray imaging investigation of LINER galaxies|
We present a comprehensive comparative high resolution mid-IRspectroscopic and X-ray imaging investigation of LINERs using archivalobservations from the ISO-SWS and the Chandra Advanced CCD ImagingSpectrometer. Although the sample is heterogenous and incomplete, thisis the first comprehensive study of the mid-infrared fine structure lineemission of LINERs. These results have been compared with similarobservations of starburst galaxies and AGN. We find that LINERs veryclearly fall between starbursts and AGN in their mid-IR fine structureline spectra, showing L[OIV]26 μm/LFIR andL[OIV]26 μm/L[NeII]12.8 μm ratios, bothmeasures of the dominant nuclear energy source in dust-enshroudedgalaxies, intermediate between those of AGN and starbursts. Chandraimaging observations of the LINERs reveal hard nuclear point sourcesmorphologically consistent with AGN in most (67%) of the sample, with aclear trend with IR-brightness. Most LINERs that show a single dominanthard compact X-ray core are IR-faint (LFIR/LB <1), whereas most LINERs that show scattered X-ray sources are IR-bright.A comparative X-ray/mid-IR spectroscopic investigation of LINERs revealssome puzzling results. Objects that display strong hard nuclear X-raycores should also display high excitation lines in the IR. However, wefind two LINERs disagree with this expectation. The galaxy NGC 404 showsweak soft X-ray emission consistent with a starburst but has the mostprominent highest excitation mid-IR spectrum of our entire sample. UsingIR emission line diagnostics alone, this galaxy would be classified ashosting a dominant AGN. Conversely, the IR luminous LINER NGC 6240 hasan extremely luminous binary AGN as revealed by the X-rays but showsweak IR emission lines. With the advent of SIRTF, and future IR missionssuch as Herschel and JWST, it is increasingly critical to determine theorigin of these multiwavelength anomalies.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (18.104.22.168) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/825Table 3 is only available in electronic form athttp://www.edpsciences.org
|Exploring the Resolved Stellar Contents of E/S0 Galaxies|
|Gas Content in Early Type Galaxies|
The interstellar medium of early type galaxies has attracted a greatdeal of interest ever since Faber & Gallagher (1976) showed that aconsiderable amount of mass ejected by evolved stars could pile up overthe lifetime of the galaxy (10^8 - 10^9 M[ scriptstyle sun ]).Surprisingly though, high resolution and sensitive HI observationsdemonstrated that in general much less gas is detected and that most ofit seems to have been accreted (van Gorkom et al 1986; Wiklind &Henkel 1990). We have started a project, mapping a few early-typegalaxies in detail, in order to shed light on the origin of the ISM inthese objects.
|The infrared-X-ray continuum correlation in active galactic nuclei|
The correlation between the soft X-ray and near-infrared emission fromactive galactic nuclei (AGNs) is analysed using composite models. Wefind new evidence for differences in the ranges of parameters thatcharacterize the narrow-line region (NLR) of Seyfert galaxies andlow-ionization nuclear emission regions (LINERs). Soft X-rays show lessvariability, so they are better fitted for this kind of analysis. In ourmodels, soft X-rays are emitted in the post-shock region of clouds withrelatively high shock velocities Vs > 250 kms-1. Consequently, dust emission peaks in the mid-infrared.On the other hand, in the photoionized zone, dust is at lowertemperature and usually does not contribute to the mid-infraredemission. The results are sensible enough to allow the same modellingmethod to be applied to different types of AGN. We found that shockvelocities are between 300 and 1000 km s-1, with the NLR oflow-luminosity active galactic nuclei (LLAGNs) and type 2 Seyfertgalaxies (Sy2s) showing lower velocities than type 1 Seyfert galaxies(Sy1s). The intensity of the ionizing radiation flux at the Lyman limitfrom the central source is low for LINERs and low-luminosity AGNs (logFh= 9 to 10), increasing towards Sy2s (log Fh~ 11)and Sy1s (11 <=Fh<= 12). Results obtained by modellingthe Einstein and the ROSAT samples of galaxies are in full agreement.Dust-to-gas ratios by number are >=10-14 in LINERs andLLAGNs, between 10-15 and 3 × 10-13 in Sy1sand up to 5 × 10-13 in Sy2s. In order to fit theinfrared and X-ray continua, an η factor is defined, which accountsfor the emitting area of the cloud. If the infrared emission is due tobremsstrahlung and comes from the same cloud that produces the softX-rays, the η values obtained from both emissions must be the same.Therefore, if (η)IR < (η)softX, theremust be a strong contribution of soft X-rays from the active centre.From the η values, we expect to identify the objects that couldpresent strong variability.
|CO (3-2) Observations of Early-Type Galaxies with the Heinrich Hertz Telescope|
We present Heinrich Hertz Telescope CO (3-2) observations of a sample of10 early-type galaxies detected both in far-infrared (IRAS) and in CO(1-0). Six of the objects (i.e., 60% of the sample) were detected in theCO (3-2) transition. Comparison of the beam-matched CO (3-2)/CO (1-0)and CO (2-1)/CO (1-0) intensity ratios with simple large velocitygradient and photodissociation region models reveals that early-typeobjects can be broadly classified into two categories. The majority ofobjects have a molecular interstellar medium of moderate density(nH2<=1000 cm-3) and temperature(T<=30 K). Two objects, NGC 3593 and NGC 4691, show indications ofquite denser and warmer environments, as well as gradients in theirphysical properties, compatible with their classification as starbursts.The heating source of the molecular gas and dust in all the objects inour sample appears to be ongoing star formation.
|The Cool Interstellar Medium in S0 Galaxies. I. A Survey of Molecular Gas|
Lenticular galaxies remain remarkably mysterious as a class.Observations to date have not led to any broad consensus about theirorigins, properties, and evolution, although they are often thought tohave formed in one big burst of star formation early in the history ofthe universe and to have evolved relatively passively since then. Inthat picture, current theory predicts that stellar evolution returnssubstantial quantities of gas to the interstellar medium; most isejected from the galaxy, but significant amounts of cool gas might beretained. Past searches for that material, though, have provided unclearresults. We present results from a survey of molecular gas in avolume-limited sample of field S0 galaxies selected from the NearbyGalaxies Catalog. CO emission is detected from 78% of the samplegalaxies. We find that the molecular gas is almost always located insidethe central few kiloparsecs of a lenticular galaxy, meaning that ingeneral it is more centrally concentrated than in spirals. We combineour data with H I observations from the literature to determine thetotal masses of cool and cold gas. Curiously, we find that, across awide range of luminosity, the most gas-rich galaxies have ~10% of thetotal amount of gas ever returned by their stars. That result isdifficult to understand within the context of either monolithic orhierarchical models of evolution of the interstellar medium.
|Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations|
To empirically calibrate the IR surface brightness fluctuation (SBF)distance scale and probe the properties of unresolved stellarpopulations, we measured fluctuations in 65 galaxies using NICMOS on theHubble Space Telescope. The early-type galaxies in this sample includeelliptical and S0 galaxies and spiral bulges in a variety ofenvironments. Absolute fluctuation magnitudes in the F160W (1.6 μm)filter (MF160W) were derived for each galaxy using previouslymeasured I-band SBF and Cepheid variable star distances. F160W SBFs canbe used to measure distances to early-type galaxies with a relativeaccuracy of ~10%, provided that the galaxy color is known to ~0.035 magor better. Near-IR fluctuations can also reveal the properties of themost luminous stellar populations in galaxies. Comparison of F160Wfluctuation magnitudes and optical colors to stellar population modelpredictions suggests that bluer elliptical and S0 galaxies havesignificantly younger populations than redder ones and may also be moremetal-rich. There are no galaxies in this sample with fluctuationmagnitudes consistent with old, metal-poor (t>5 Gyr, [Fe/H]<-0.7)stellar population models. Composite stellar population models implythat bright fluctuations in the bluer galaxies may be the result of anepisode of recent star formation in a fraction of the total mass of agalaxy. Age estimates from the F160W fluctuation magnitudes areconsistent with those measured using the Hβ Balmer-line index. Thetwo types of measurements make use of completely different techniquesand are sensitive to stars in different evolutionary phases. Bothtechniques reveal the presence of intermediate-age stars in theearly-type galaxies of this sample.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS 5-26555.
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