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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.
| The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions
The mean ages of early-type galaxies obtained from the analysis ofoptical spectra give a mean age of 8 Gyr at z=0, with 40% being youngerthan 6 Gyr. Independent age determinations are possible by usinginfrared spectra (5-21 μm), which we have obtained with the InfraredSpectrograph on Spitzer. This age indicator is based on the collectivemass-loss rate of stars, in which mass loss from AGB stars produces asilicate emission feature at 9-12 μm. This feature decreases morerapidly than the shorter wavelength continuum as a stellar populationages, providing an age indicator. From observations of 30 nearbyearly-type galaxies, 29 show a spectral energy distribution dominated bystars, and one has significant emission from the ISM and is excluded.The infrared age indicators for the 29 galaxies show them all to be old,with a mean age of about 10 Gyr and a standard deviation of only a fewGyr. This is consistent with the ages inferred from the values ofM/LB, but is inconsistent with the ages derived from theoptical line indices, which can be much younger. All of these ageindicators are luminosity weighted and should be correlated, even ifmultiple-age components are considered. The inconsistency indicates thatthere is a significant problem with either the infrared and theM/LB ages, which agree, or with the ages inferred from theoptical absorption lines.
| Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
| Star Formation Histories of Nearby Elliptical Galaxies. II. Merger Remnant Sample
This work presents high signal-to-noise spectroscopic observations of asample of six suspected merger remnants, selected primarily on the basisof H I tidal debris detections. Single stellar population analysis ofthese galaxies indicates that their ages, metallicities, andα-enhancement ratios are consistent with those of a representativesample of nearby elliptical galaxies. The expected stellar population ofa recent merger remnant, a young age combined with low [α/Fe], isnot seen in any H I-selected galaxy. However, one galaxy (NGC 2534) isfound to deviate from the Z-plane in the sense expected for a mergerremnant. Another galaxy (NGC 7332), selected by other criteria, bestmatches the merger remnant expectations.
| Peculiarities and populations in elliptical galaxies. III. Dating the last star formation event
Using 6 colours and 4 Lick line-indices we derive two-component modelsof the populations of ellipticals, involving a "primary" and a"juvenile" population. The first component is defined by the regressionsof indices against the central velocity dispersion found in Papers I andII for the {Nop} sample of non-peculiar objects. The second one isapproximated by an SSP, and the modeling derives its age A, metallicityZ and fractional V-luminosity q_V, the fractional mass qMbeing found therefrom. The model is designed for "blueish" peculiargalaxies, i.e. the {Pec} sample and NGC 2865 family in the terminologyof Paper I. The morphological peculiarities and the population anomalyare then believed to involve the same event, i.e. a merger plusstarburst. It is possible to improve the models in a few cases byintroducing diffuse dust (as suggested by far IR data), and/or by takinginto account the fact that Lick- and colour indices do not relate toidentical galaxy volumes. In most of the cases, the mass ratio of youngstars qM seems too small for the product of a recent majormerger: the events under consideration might be minor mergers bringing"the final touch" to the build-up of the structure of the E-type object.The same modeling has been successfully applied to blueish galaxies ofthe {Nop} sample, without morphological peculiarities however, tosupport the occurence of a distinct perturbing event. A few reddishobjects of the {Pec} sample (NGC 3923 family) and of the {Nop} sampleare also modeled, in terms of an excess of high metallicity stars, ordiffuse dust, or both, but the results are inconclusive.
| 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.
| Group, field and isolated early-type galaxies - II. Global trends from nuclear data
We have derived ages, metallicities and enhanced-element ratios[α/Fe] for a sample of 83 early-type galaxies essentially ingroups, the field or isolated objects. The stellar-population propertiesderived for each galaxy correspond to the nuclear re/8aperture extraction. The median age found for Es is 5.8+/-0.6 Gyr andthe average metallicity is +0.37+/-0.03 dex. For S0s, the median age is3.0+/-0.6 Gyr and [Z/H]= 0.53+/-0.04 dex. We compare the distribution ofour galaxies in the Hβ-[MgFe] diagram with Fornax galaxies. Ourelliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster.We find that the galaxies lie in a plane defined by [Z/H]= 0.99logσ0- 0.46 log(age) - 1.60, or in linear terms Z~σ0× (age) -0.5. More massive (largerσ0) and older galaxies present, on average, large[α/Fe] values, and therefore must have undergone shorterstar-formation time-scales. Comparing group against field/isolatedgalaxies, it is not clear that environment plays an important role indetermining their stellar-population history. In particular, ourisolated galaxies show ages differing by more than 8 Gyr. Finally weexplore our large spectral coverage to derive log(O/H) metallicity fromthe Hα and NIIλ6584 and compare it with model-dependent[Z/H]. We find that the O/H abundances are similar for all galaxies, andwe can interpret it as if most chemical evolution has already finishedin these galaxies.
| Group, field and isolated early-type galaxies - I. Observations and nuclear data
This is the first paper of a series on the investigation of stellarpopulation properties and galaxy evolution of an observationallyhomogeneous sample of early-type galaxies in groups, field and isolatedgalaxies.Here we present high signal-to-noise ratio (S/N) long-slit spectroscopyof 86 nearby elliptical and S0 galaxies. Eight of them are isolated,selected according to a rigorous criterion, which guarantees a genuinelow-density subsample. The present survey has the advantage of coveringa larger wavelength range than normally found in the literature, whichincludes [OIII]λ5007 and Hα, both lines important foremission correction. Among the 86 galaxies with S/N >= 15 (perresolution element, for re/8 central aperture), 57 have theirHβ-index corrected for emission (the average correction is 0.190Åin Hβ) and 42 galaxies reveal [OIII]λ5007 emission,of which 16 also show obvious Hα emission. Most of the galaxies inthe sample do not show obvious signs of disturbances nor tidal featuresin the morphologies, although 11 belong to the Arp catalogue of peculiargalaxies; only three of them (NGC 750, 751 and 3226) seem to be stronglyinteracting. We present the measurement of 25 central line-strengthindices calibrated to the Lick/IDS system. Kinematic information isobtained for the sample. We analyse the line-strength index versusvelocity dispersion relations for our sample of mainly low-densityenvironment galaxies, and compare the slope of the relations withcluster galaxies from the literature. Our main findings are that theindex-σ0 relations presented for low-density regionsare not significantly different from those of cluster E/S0s. The slopeof the index-σ0 relations does not seem to change forearly-type galaxies of different environmental densities, but thescatter of the relations seems larger for group, field and isolatedgalaxies than for cluster galaxies.
| Mid-Infrared Emission from Elliptical Galaxies: Sensitivity to Stellar Age
Mid-infrared observations (3.6-24 μm) of normal giant ellipticalgalaxies with the Spitzer Space Telescope are consistent with purepopulations of very old stars with no evidence of younger stars. Most ofthe stars in giant elliptical galaxies are old, but the mean stellar agedetermined from Balmer absorption in optical spectra can appear muchyounger due to a small admixture of younger stars. The mean stellar agecan also be determined from the spectral energy distribution in themid-infrared, which decreases with time relative to the optical emissionand shifts to shorter wavelengths. The observed flux ratiosF8μm/F3.6μm andF24μm/F3.6μm for elliptical galaxies withthe oldest Balmer line ages are lower than predicted by recent models ofsingle stellar populations. For elliptical galaxies with the youngestBalmer line ages in our sample, 3-5 Gyr, the flux ratiosF24μm/F3.6μm are identical to those of theoldest stars. When theoretical mid-IR spectra of old (12 Gyr) and youngstellar populations are combined, errors in theF24μm/F3.6μm observations are formallyinconsistent with a mass fraction of young stars that exceeds ~1%. Thisis less than the fraction of young stars expected in discussions ofrecent surveys of elliptical galaxies at higher redshifts. However, thisinconsistency between Balmer line ages and those inferred from mid-IRobservations must be regarded as provisional until more accurateobservations and theoretical spectra become available. Finally, there isno evidence to date that central disks or patches of dust commonlyvisible in optical images of elliptical galaxies contribute sensibly tothe mid-IR spectrum.
| Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities.
| Probing the Ionizing Continuum of Narrow-Line Seyfert 1 Galaxies. I. Observational Results
We present optical spectra and emission-line ratios of 12 narrow-lineSeyfert 1 (NLS1) galaxies that we observed to study the ionizing EUVcontinuum. A common feature in the EUV continuum of active galacticnuclei is the big blue bump (BBB), generally associated with thermalaccretion disk emission. While Galactic absorption prevents directaccess to the EUV range, it can be mapped by measuring the strength of avariety of forbidden optical emission lines that respond to differentEUV continuum regions. We find that narrow emission line ratiosinvolving [O II] λ3727, Hβ, [O III] λ5007, [O I]λ6300, Hα, [N II] λ6583, and [S II]λλ6716, 6731 indicate no significant difference betweenNLS1 galaxies and broad-line Seyfert 1 (BLS1) galaxies, which suggeststhat the spectral energy distributions of their ionizing EUV-soft X-raycontinua are similar. The relative strength of important forbiddenhigh-ionization lines like [Ne V] λ3426 compared to He IIλ4686 and the relative strength of [Fe X] λ6374 appear toshow the same range as in BLS1 galaxies. However, a trend of weaker F([OI] λ6300)/F(Hα) emission-line ratios is indicated for NLS1galaxies compared to BLS1 galaxies. To recover the broad emission lineprofiles, we used Gaussian components. This approach indicates that thebroad Hβ profile can be well described with a broad component(FWHM~=3275+/-800 km s-1) and an intermediate broad component(FWHM~=1200+/-300 km s-1). The width of the broad componentis in the typical range of normal BLS1 galaxies. The emission-line fluxthat is associated with the broad component in these NLS1 galaxiesamounts to at least 60% of the total flux. Thus, it dominates the totalline flux, similar to BLS1 galaxies.
| The Epochs of Early-Type Galaxy Formation as a Function of Environment
The aim of this paper is to set constraints on the epochs of early-typegalaxy formation through the ``archaeology'' of the stellar populationsin local galaxies. Using our models of absorption-line indices thataccount for variable abundance ratios, we derive ages, totalmetallicities, and element ratios of 124 early-type galaxies in high-and low-density environments. The data are analyzed by comparison withmock galaxy samples created through Monte Carlo simulations taking thetypical average observational errors into account, in order to eliminateartifacts caused by correlated errors. We find that all threeparameters, age, metallicity, and α/Fe ratio, are correlated withvelocity dispersion. We show that these results are robust againstrecent revisions of the local abundance pattern at high metallicities.To recover the observed scatter we need to assume an intrinsic scatterof about 20% in age, 0.08 dex in [Z/H], and 0.05 dex in [α/Fe].All low-mass objects withM*<~1010Msolar (σ<~130kms-1) show evidence for the presence of intermediate-agestellar populations with low α/Fe ratios. About 20% of theintermediate-mass objects with1010<~M*/Msolar<~1011[110<~σ/(kms-1)<~230 both elliptical andlenticular galaxies] must have either a young subpopulation or a bluehorizontal branch. On the basis of the above relationships, valid forthe bulk of the sample, we show that the Mg-σ relation is mainlydriven by metallicity, with similar contributions from the α/Feratio (23%) and age (17%). We further find evidence for an influence ofthe environment on the stellar population properties. Massive early-typegalaxies in low-density environments seem on average ~2 Gyr younger andslightly (~0.05-0.1 dex) more metal-rich than their counterparts inhigh-density environments. No offsets in the α/Fe ratios areinstead detected. With the aid of a simple chemical evolution model, wetranslate the derived ages and α/Fe ratios into star formationhistories. We show that most star formation activity in early-typegalaxies is expected to have happened between redshifts ~3 and 5 inhigh-density environments and between redshifts 1 and 2 in low-densityenvironments. We conclude that at least 50% of the total stellar massdensity must have already formed at z~1, in good agreement withobservational estimates of the total stellar mass density as a functionof redshift. Our results suggest that significant mass growth in theearly-type galaxy population below z~1 must be restricted to lessmassive objects, and a significant increase of the stellar mass densitybetween redshifts 1 and 2 should be present, caused mainly by the fieldgalaxy population. The results of this paper further imply the presenceof vigorous star formation episodes in massive objects at z~2-5 andevolved elliptical galaxies around z~1, both observationally identifiedas SCUBA galaxies and extremely red objects, respectively.
| Star Formation Histories of Nearby Elliptical Galaxies. I. Volume-Limited Sample
This work presents high signal-to-noise ratio spectroscopic observationsof a representative sample of nearby elliptical galaxies. Theseobservations provide a strong test of models for the formation ofelliptical galaxies and their star formation histories. Combining thesedata with the González data set, a volume-limited sample of 45galaxies has been defined. Results are in agreement with previousstudies: the existence of the metallicity hyperplane and the Z-plane ofTrager and coworkers is confirmed, and the distribution is clearly dueto physical variations in stellar population parameters and notmeasurement uncertainty. Trends between stellar population parametersand galaxy structural parameters suggest that angular momentum maydetermine the chemical abundance of a galaxy at a given mass.
| The Centers of Early-Type Galaxies with Hubble Space Telescope. V. New WFPC2 Photometry
We present observations of 77 early-type galaxies imaged with the PC1CCD of the Hubble Space Telescope (HST) WFPC2. ``Nuker-law'' parametricfits to the surface brightness profiles are used to classify the centralstructure into ``core'' or ``power-law'' forms. Core galaxies aretypically rounder than power-law galaxies. Nearly all power-law galaxieswith central ellipticities ɛ>=0.3 have stellar disks,implying that disks are present in power-law galaxies withɛ<0.3 but are not visible because of unfavorable geometry. Afew low-luminosity flattened core galaxies also have disks; these may betransition forms from power-law galaxies to more luminous core galaxies,which lack disks. Several core galaxies have strong isophote twistsinterior to their break radii, although power-law galaxies have interiortwists of similar physical significance when the photometricperturbations implied by the twists are evaluated. Central colorgradients are typically consistent with the envelope gradients; coregalaxies have somewhat weaker color gradients than power-law galaxies.Nuclei are found in 29% of the core galaxies and 60% of the power-lawgalaxies. Nuclei are typically bluer than the surrounding galaxy. Whilesome nuclei are associated with active galactic nuclei (AGNs), just asmany are not; conversely, not all galaxies known to have a low-level AGNexhibit detectable nuclei in the broadband filters. NGC 4073 and 4382are found to have central minima in their intrinsic starlightdistributions; NGC 4382 resembles the double nucleus of M31. In general,the peak brightness location is coincident with the photocenter of thecore to a typical physical scale of <1 pc. Five galaxies, however,have centers significantly displaced from their surrounding cores; thesemay be unresolved asymmetric double nuclei. Finally, as noted byprevious authors, central dust is visible in about half of the galaxies.The presence and strength of dust correlates with nuclear emission;thus, dust may outline gas that is falling into the central black hole.The prevalence of dust and its morphology suggest that dust clouds form,settle to the center, and disappear repeatedly on ~108 yrtimescales. We discuss the hypothesis that cores are created by thedecay of a massive black hole binary formed in a merger. Apart fromtheir brightness profiles, there are no strong differences between coregalaxies and power-law galaxies that demand this scenario; however, therounder shapes of core, their lack of disks, and their reduced colorgradients may be consistent with it.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 (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withGO and GTO proposals 5236, 5446, 5454, 5512, 5943, 5990, 5999, 6099,6386, 6554, 6587, 6633, 7468, 8683, and 9107.
| Near infra-red and optical colour gradients in E-type galaxies. Inferences on dust content
Colour gradients are considered for a sample of circa 50 E-type galaxiesin the Local Supercluster. The new data includes isophotal colourprofiles in J-H, J-K, V-J and V-K, measured using 2MASS frames mostlyfrom the Large Galaxies Atlas, V frames from previous work and Vprofiles from the literature. This is supplemented by U-B, B-V, B-R, V-Icolour gradients obtained anew from published photometric data. Colourgradients in E galaxies show remarkably large variations from object toobject and do not correlate with other properties. Metallicity gradientsare the primary cause as shown before. Age gradients with oppositeeffects are possibly needed to explain objects with small colourgradients. Some empirical evidence of such age effects has been foundfor a subset of objects with morphological peculiarities and youngerstars mixed. Dust has only modest effects on colour gradients, as shownby the fact that objects with zero IRAS 100 μ flux have the sameaverage values of the gradients, except in V-J and V-K, as those withnon zero flux (cf. Table 7). This last subsample however exhibits poorbut definite correlations between IRAS flux and gradients, which mightbe caused by the presence of a few relatively dusty galaxies in thesample. Given the absence of a correlation between any gradients andgalaxy velocity dispersion (and hence mass), the observations do notagree with the predictions of the monolithic scenario for the formationof E galaxies. Simulated datasets of “dummy” objectsmimicking the hierarchical scenario have been obtained, and used to testa technique for estimating the dust content of E-galaxies from thecomparison of the V-K (or V-J) colour gradients with the U-B (or B-V)ones: the contents of diffuse dust, gauged in terms of published models,are obtained for a dozen objects.
| Stellar Velocity Dispersion and Mass Estimation for Galactic Disks
Available velocity dispersion estimates for the old stellar populationof galactic disks at galactocentric distances r=2L (where L is thephotometric radial scale length of the disk) are used to determine thethreshold local surface density of disks that are stable againstgravitational perturbations. The mass of the disk Mdcalculated under the assumption of its marginal stability is comparedwith the total mass Mt and luminosity LB of thegalaxy within r=4L. We corroborate the conclusion that a substantialfraction of the mass in galaxies is probably located in their darkhalos. The ratio of the radial velocity dispersion to the circularvelocity increases along the sequence of galactic color indices anddecreases from the early to late morphological types. For most of thegalaxies with large color indices (B-V)0 > 0.75, whichmainly belong to the S0 type, the velocity dispersion exceedssignificantly the threshold value required for the disk to be stable.The reverse situation is true for spiral galaxies: the ratiosMd/LB for these agree well with those expected forevolving stellar systems with the observed color indices. This suggeststhat the disks of spiral galaxies underwent no significant dynamicalheating after they reached a quasi-equilibrium stable state.
| Companions to Isolated Elliptical Galaxies: Revisiting the Bothun-Sullivan Sample
We investigate the number of physical companion galaxies for a sample ofrelatively isolated elliptical galaxies. The NASA/IPAC ExtragalacticDatabase (NED) has been used to reinvestigate the incidence of satellitegalaxies for a sample of 34 elliptical galaxies, first investigated byBothun & Sullivan using a visual inspection of Palomar Sky Surveyprints out to a projected search radius of 75 kpc. We have repeatedtheir original investigation using data cataloged in NED. Nine of theseelliptical galaxies appear to be members of galaxy clusters; theremaining sample of 25 galaxies reveals an average of +1.0+/-0.5apparent companions per galaxy within a projected search radius of 75kpc, in excess of two equal-area comparison regions displaced by 150-300kpc. This is significantly larger than the +0.12+/-0.42companions/galaxy found by Bothun & Sullivan for the identicalsample. Making use of published radial velocities, mostly availablesince the completion of the Bothun-Sullivan study, identifies thephysical companions and gives a somewhat lower estimate of +0.4companions per elliptical galaxy. This is still 3 times larger than theoriginal statistical study, but given the incomplete and heterogeneousnature of the survey redshifts in NED, it still yields a firm lowerlimit on the number (and identity) of physical companions. An expansionof the search radius out to 300 kpc, again restricted to sampling onlythose objects with known redshifts in NED, gives another lower limit of4.5 physical companions per galaxy. (Excluding five elliptical galaxiesin the Fornax Cluster, this average drops to 3.5 companions perelliptical.) These physical companions are individually identified andlisted, and the ensemble-averaged radial density distribution of theseassociated galaxies is presented. For the ensemble, the radial densitydistribution is found to have a falloff consistent withρ~R-0.5 out to approximately 150 kpc. For non-FornaxCluster companions the falloff continues out to the 300 kpc limit of thesurvey. The velocity dispersion of these companions is found to reach amaximum of 350 km s-1 at around 120 kpc, after which theyfall at a rate consistent with Keplerian falloff. This falloff may thenindicate the detection of a cut-off in the mass-density distribution inthe elliptical galaxies' dark matter halo at ~100 kpc.
| A Correlation between Light Profile and [Mg/Fe] Abundance Ratio in Early-Type Galaxies
We explore possible correlations between light profile shapes, asparameterized by the Sérsic index n or the concentration indexCre(1/3), and relevant stellar populationparameters in early-type galaxies. Mean luminosity-weighted ages,metallicities, and abundance ratios were obtained from spectra of veryhigh signal-to-noise ratio and stellar population models that synthesizegalaxy spectra at the resolution given by their velocity dispersions,σ, in combination with an age indicator(Hγσ) that is virtually free of the effects ofmetallicity. We do not find any significant correlation between n [orCre(1/3)] and mean age or metallicity, but we dofind a strong positive correlation of the shape parameters with Mg/Feabundance ratio. This dependence is as strong as the Mg/Fe-σ andCre(1/3)-σ relations. We speculate thatearly-type galaxies settle up their structure on timescales in agreementwith those imposed by their Mg/Fe ratios. This suggests that the globalstructure of larger galaxies, with larger Mg/Fe ratios and shortertimescales, was already in place at high z, without experiencing asignificant time evolution.
| Peculiarities and populations in elliptical galaxies. I. An old question revisited
Morphological peculiarities, as defined from isophote asymmetries andnumber of detected shells, jets or similar features, have been estimatedin a sample of 117 E classified galaxies, and qualified by an ad hocΣ2 index. The overall frequency of ``peculiar'' objects(Pec subsample) is 32.5%. It decreases with the cosmic density of theenvironment, being minimal for the Virgo cluster, the densestenvironment in the sampled volume. This environmental effect is strongerfor galaxies with relatively large Σ2.The Pec subsample objects are compared with ``normal'' objects (Nopsubsample) as regards their basic properties. Firstly, theysystematically deviate from the Fundamental Plane and the Faber-Jacksonrelation derived for the Nop subsample, being too bright for their mass.Secondly, the dust content of galaxies, as estimated from IRAS fluxes,are similar in both subsamples. Third, the same is true of the frequencyof Kinematically Distinct cores (KDC), suggesting that KDC andmorphological peculiarities do not result from the same events in thehistory of E-galaxies.Using the Nop sample alone, we obtain very tight reference relationsbetween stellar population indicators (U-B, B-V, B-R, V-I,Mg2, Hβ, , Mgb) and the central velocitydispersion σ0. The discussion of the residuals of theserelations allows us to classify the Pec galaxies in two families i.e.the YP or NGC 2865 family, and the NP or NGC 3923 one. Galaxies in thefirst group show consistent evidence for a younger stellar populationmixed with the old one, in agreement with classical results (Schweizeret al. \cite{Schweizer1990}; Schweizer & Seitzer\cite{Schweizer1992}). The second group, however, has ``normal``, orreddish, populations. It is remarkable that a fraction (circa 40%) ofmorphologically perturbed objects do not display any signature of ayoung population, either because the event responsible for thepecularity is too ancient, or because it did not produce significantstar formation (or eventually that the young sub-population has highmetallicity).A preliminary attempt is made to interpret the populations of Pecobjects by combining a young Single Stellar Population with a Nopgalaxy, with only limited success, perhaps largely due to uncertaintiesin the SSP indices used.Based in part on observations collected at the Observatoire deHaute-Provence.Figures \ref{fig1}-\ref{fig3} are only available in electronic form athttp://www.edpsciences.orgTable 10 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/833
| SINGS: The SIRTF Nearby Galaxies Survey
The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging andspectroscopic survey of 75 nearby galaxies. Its primary goal is tocharacterize the infrared emission of galaxies and their principalinfrared-emitting components, across a broad range of galaxy propertiesand star formation environments. SINGS will provide new insights intothe physical processes connecting star formation to the interstellarmedium properties of galaxies and provide a vital foundation forunderstanding infrared observations of the distant universe andultraluminous and active galaxies. The galaxy sample and observingstrategy have been designed to maximize the scientific and archivalvalue of the data set for the SIRTF user community at large. The SIRTFimages and spectra will be supplemented by a comprehensivemultiwavelength library of ancillary and complementary observations,including radio continuum, H I, CO, submillimeter, BVRIJHK, Hα,Paα, ultraviolet, and X-ray data. This paper describes the mainastrophysical issues to be addressed by SINGS, the galaxy sample and theobserving strategy, and the SIRTF and other ancillary data products.
| Near-infrared imaging of ellipticals: surface brightness profiles and photometry
We present near-infrared K-band imaging of a large sample of candidatemerger remnant galaxies and Hickson Compact Group ellipticals. We derivelight profile indices, effective radii and surface brightnesses, as wellas total K-band magnitudes. We find that the light distributions of themerger remnant candidates are consistent with those of `normal'ellipticals, and scatter around a mean profile index of (1/n) = 0.20.Many of our sample galaxies have surface brightness profiles that arenot well described by a de Vaucouleurs law (1/n= 0.25), and we discussthe implications of this on the derived total magnitudes. Comparing thetotal K magnitudes calculated by extrapolating a de Vaucouleurs profileand those derived using a generalized Sérsic form, we find that asignificant bias is introduced if the de Vaucouleurs law is not a gooddescription of the actual light profile.
| An X-Ray Atlas of Groups of Galaxies
A search was conducted for a hot intragroup medium in 109 low-redshiftgalaxy groups observed with the ROSAT PSPC. Evidence for diffuse,extended X-ray emission is found in at least 61 groups. Approximatelyone-third of these detections have not been previously reported in theliterature. Most of the groups are detected out to less than half of thevirial radius with ROSAT. Although some spiral-rich groups do contain anintragroup medium, diffuse emission is restricted to groups that containat least one early-type galaxy.
| Metallicity distributions of globular cluster systems in galaxies
We collected a sample of 100 galaxies for which different observers havedetermined colour indices of globular cluster candidates. The sampleincludes representatives of galaxies of various morphological types anddifferent luminosities. Colour indices (in most cases (V-I), but also(B-I) and (C-T_1)) were transformed into metallicities [Fe/H] accordingto a relation by Kissler-Patig (1998). These data were analysed with theKMM software in order to estimate similarity of the distribution withuni- or bimodal Gaussian distribution. We found that 45 of 100 systemshave bimodal metallicity distributions. Mean metallicity of themetal-poor component for these galaxies is < [Fe/H]> = -1.40 +/-0.02, of the metal-rich component < [Fe/H]> = -0.69 +/- 0.03.Dispersions of the distributions are 0.15 and 0.18, respectively.Distribution of unimodal metallicities is rather wide. These data willbe analysed in a subsequent paper in order to find correlations withparameters of galaxies and galactic environment.
| Star Formation Histories of Early-Type Galaxies. I. Higher Order Balmer Lines as Age Indicators
We have obtained blue integrated spectra of 175 nearby early-typegalaxies, covering a wide range in galaxy velocity dispersion andemphasizing those with σ<100 km s-1. Galaxies havebeen observed both in the Virgo Cluster and in lower densityenvironments. The main goals are the evaluation of higher order Balmerlines as age indicators and differences in stellar populations as afunction of mass, environment, and morphology. In this first paper, ouremphasis is on presenting the methods used to characterize the behaviorof the Balmer lines through evolutionary population synthesis models.Lower σ galaxies exhibit a substantially greater intrinsicscatter, in a variety of line-strength indicators, than do higherσ galaxies, with the large intrinsic scatter setting in below aσ of 100 km s-1. Moreover, a greater contrast inscatter is present in the Balmer lines than in the lines of metalfeatures. Evolutionary synthesis modeling of the observed spectralindexes indicates that the strong Balmer lines found primarily among thelow-σ galaxies are caused by young age, rather than by lowmetallicity. Thus we find a trend between the population age and thecentral velocity dispersion, such that low-σ galaxies have youngerluminosity-weighted mean ages. We have repeated this analysis usingseveral different Balmer lines and find consistent results from onespectral indicator to another.
| Redshift-Distance Survey of Early-Type Galaxies: Circular-Aperture Photometry
We present R-band CCD photometry for 1332 early-type galaxies, observedas part of the ENEAR survey of peculiar motions using early-typegalaxies in the nearby universe. Circular apertures are used to tracethe surface brightness profiles, which are then fitted by atwo-component bulge-disk model. From the fits, we obtain the structuralparameters required to estimate galaxy distances using theDn-σ and fundamental plane relations. We find thatabout 12% of the galaxies are well represented by a pure r1/4law, while 87% are best fitted by a two-component model. There are 356repeated observations of 257 galaxies obtained during different runsthat are used to derive statistical corrections and bring the data to acommon system. We also use these repeated observations to estimate ourinternal errors. The accuracy of our measurements are tested by thecomparison of 354 galaxies in common with other authors. Typical errorsin our measurements are 0.011 dex for logDn, 0.064 dex forlogre, 0.086 mag arcsec-2 for<μe>, and 0.09 for mRC,comparable to those estimated by other authors. The photometric datareported here represent one of the largest high-quality and uniformall-sky samples currently available for early-type galaxies in thenearby universe, especially suitable for peculiar motion studies.Based on observations at Cerro Tololo Inter-American Observatory (CTIO),National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation (NSF);European Southern Observatory (ESO); Fred Lawrence Whipple Observatory(FLWO); and the MDM Observatory on Kitt Peak.
| 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
| A catalogue and analysis of local galaxy ages and metallicities
We have assembled a catalogue of relative ages, metallicities andabundance ratios for about 150 local galaxies in field, group andcluster environments. The galaxies span morphological types from cD andellipticals, to late-type spirals. Ages and metallicities were estimatedfrom high-quality published spectral line indices using Worthey &Ottaviani (1997) single stellar population evolutionary models. Theidentification of galaxy age as a fourth parameter in the fundamentalplane (Forbes, Ponman & Brown 1998) is confirmed by our largersample of ages. We investigate trends between age and metallicity, andwith other physical parameters of the galaxies, such as ellipticity,luminosity and kinematic anisotropy. We demonstrate the existence of agalaxy age-metallicity relation similar to that seen for local galacticdisc stars, whereby young galaxies have high metallicity, while oldgalaxies span a large range in metallicities. We also investigate theinfluence of environment and morphology on the galaxy age andmetallicity, especially the predictions made by semi-analytichierarchical clustering models (HCM). We confirm that non-clusterellipticals are indeed younger on average than cluster ellipticals aspredicted by the HCM models. However we also find a trend for the moreluminous galaxies to have a higher [Mg/Fe] ratio than the lowerluminosity galaxies, which is opposite to the expectation from HCMmodels.
| Globular Cluster Systems and the Missing Satellite Problem: Implications for Cold Dark Matter Models
We analyze the metallicity distributions of globular clusters belongingto 28 early-type galaxies in the survey of Kundu & Whitmore. A MonteCarlo algorithm that simulates the chemical evolution of galaxies thatgrow hierarchically via dissipationless mergers is used to determine themost probable protogalactic mass function for each galaxy. Contrary tothe claims of Kundu & Whitmore, we find that the observedmetallicity distributions are in close agreement with the predictions ofsuch hierarchical formation models. The mass spectrum of protogalacticfragments for the galaxies in our sample has a power-law behavior,n(M)~Mα, with an index of α~=-2. This spectrum isindistinguishable from the mass spectrum of dark matter halos predictedby cold dark matter models for structure formation. We argue that theseprotogalactic fragments-the likely sites of globular cluster formationin the early universe-are the disrupted remains of the ``missing''satellite galaxies predicted by cold dark matter models. Our findingssuggest that the solution to the missing satellite problem is throughthe suppression of gas accretion in low-mass halos after reionization,or via self-interacting dark matter, and argue against models withsuppressed small-scale power or warm dark matter.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
| Line-strength indices and velocity dispersions for 148 early-type galaxies in different environments
We have derived high quality line-strength indices and velocitydispersions for a sample of 148 early-type galaxies in differentenvironments. The wavelength region covered by the observations (lambda=~ 4600 to 6600 Å) includes the Lick/IDS indices Hβ,Mg1, Mg2, Mgb, Fe5015, Fe5270, Fe5335, Fe5406,Fe5709, Fe5782, NaD, TiO1 and TiO2. The data areintended to address possible differences of the stellar populations ofearly-type galaxies in low- and high-density environments. This paperdescribes the sample properties, explains the data reduction andpresents the complete list of all the measurements. Most galaxies of thesample (85%) had no previous measurements of any Lick/IDS indices andfor 30% of the galaxies we present first-time determinations of theirvelocity dispersions. Special care is taken to identify galaxies withemission lines. We found that 62 per cent of the galaxies in the samplehave emission lines, as measured by the equivalent width of the [OIII]5007Å line, EW[OIII] > 0.3 Å. Tables 5 and 6 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/ A+A/395/431. They are also available via ftp atftp.mpe.mpg.de in the directory people/dthomas/Beuing02 or via WWW atftp://ftp.mpe.mpg.de/people/dthomas/Beuing02.
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