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Gemini/GMOS spectra of globular clusters in the Virgo giant elliptical NGC 4649
NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We haveobtained Gemini/GMOS (Gemini North Multi-Object Spectrograph) spectrafor 38 globular clusters (GCs) associated with this galaxy. Applying themulti-index χ2 minimization technique of Proctor andSansom with the single stellar population models of Thomas, Maraston andKorn, we derive ages, metallicities and α-element abundanceratios. We find several young (2-3 Gyr old) supersolar metallicity GCs,while the majority are old (>10 Gyr), spanning a range ofmetallicities from solar to [Z/H]=-2. At least two of these young GCsare at large projected radii of 17-20 kpc. The galaxy itself shows noobvious signs of a recent starburst, interaction or merger. A trend ofdecreasing α-element ratio with increasing metallicity is found.
| Gemini/GMOS spectra of globular clusters in the Leo group elliptical NGC 3379
The Leo group elliptical NGC 3379 is one of the few normal ellipticalgalaxies close enough to make possible observations of resolved stellarpopulations, deep globular cluster (GC) photometry and highsignal-to-noise ratio GC spectra. We have obtained Gemini/GMOS spectrafor 22 GCs associated with NGC 3379. We derive ages, metallicities andα-element abundance ratios from simple stellar population modelsusing the recent multi-index χ2 minimization method ofProctor & Sansom. All of these GCs are found to be consistent withold ages, i.e. >~10Gyr, with a wide range of metallicities. This iscomparable to the ages and metallicities that Gregg et al. found acouple of years ago for resolved stellar populations in the outerregions of this elliptical. A trend of decreasing α-elementabundance ratio with increasing metallicity is indicated.The projected velocity dispersion of the GC system is consistent withbeing constant with radius. Non-parametric, isotropic models require asignificant increase in the mass-to-light ratio at large radii. Thisresult is in contrast to that of Romanowsky et al., who recently found adecrease in the velocity dispersion profile as determined from planetarynebulae (PN). Our constant dispersion requires a normal-sized dark halo,although without anisotropic models we cannot rigorously determine thedark halo mass.A two-sided χ2 test over all radii gives a 2σdifference between the mass profile derived from our GCs compared to thePN-derived mass model of Romanowsky et al. However, if we restrict ouranalysis to radii beyond one effective radius and test if the GCvelocity dispersion is consistently higher, we determine a > 3σdifference between the mass models, and hence we favour the conclusionthat NGC 3379 does indeed have dark matter at large radii in its halo.
| 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.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| The initial mass distribution of the M82 star cluster system
We explore whether we can constrain the shape of the initial massdistribution of the star cluster population in M82's ~1-Gyr-oldpost-starburst region `B', in which the present-day cluster massfunction (CMF) is closely approximated by a lognormal distribution. Weconclude that the M82 B initial CMF must have had a mean mass very closeto that of the `equilibrium' CMF of Vesperini. Consequently, if thepresently observed M82 B CMF has remained approximately constant sinceits formation, as predicted, then the initial CMF must have beencharacterized by a mean mass that was only slightly larger than thepresent mean mass. From our detailed analysis of the expected evolutionof CMFs, we conclude that our observations of the M82 B CMF areinconsistent with a scenario in which the 1-Gyr-old cluster populationoriginated from an initial power-law mass distribution. Our conclusionis supported by arguments related to the initial density in M82 B, whichwould have been unphysically high if the present cluster population werethe remains of an initial power-law distribution.
| 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.
| Evolutionary history of the elliptical galaxy NGC 1052
We have obtained Keck spectra for 16 globular clusters (GCs) associatedwith the merger remnant elliptical NGC 1052, as well as a long-slitspectrum of the galaxy. We derive ages, metallicities and abundanceratios from simple stellar population models using the recentlypublished methods of Proctor & Sansom, applied to extragalactic GCsfor the first time. A number of GCs indicate the presence of strong bluehorizontal branches that are not fully accounted for in the currentstellar population models. We find all of the GCs to be ~13 Gyr oldaccording to simple stellar populations, with a large range ofmetallicities. From the galaxy spectrum we find NGC 1052 to have aluminosity-weighted central age of ~2 Gyr and metallicity of[Fe/H]~+0.6. No strong gradients in either age or metallicity were foundto the maximum radius measured (0.3re~= 1 kpc). However, wedo find a strong radial gradient in α-element abundance, whichreaches a very high central value. The young central starburst age isconsistent with the age inferred from the HI tidal tails and infallinggas of ~1 Gyr. Thus, although NGC 1052 shows substantial evidence for arecent merger and an associated starburst, it appears that the mergerdid not induce the formation of new GCs, perhaps suggesting that littlerecent star formation occurred. This interpretation is consistent with`frosting' models for early-type galaxy formation.
| 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.
| 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.
| Extragalactic Globular Clusters: Old Spectroscopic Ages and New Views on Their Formation
We present the results of a meta-analysis of Keck spectra ofextragalactic globular clusters (GCs) in a sample of eight galaxies,ranging from dwarf galaxies to massive elliptical galaxies. We inferages for the metal-poor and metal-rich GCs in these galaxies throughcomparisons to Galactic GCs. Both subpopulations appear to be no youngerthan their Galactic counterparts, with ages >~10 Gyr. This is thelargest sample of galaxies for which ages have been constrainedspectroscopically. Our results support the formation of most GCs inmassive galaxies at high redshift. We propose a scenario for theformation of GC subpopulations that synthesizes aspects of bothaccretion and in situ approaches in the context of galaxy formationthrough hierarchical merging.
| 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.
| The Chemical Properties of Milky Way and M31 Globular Clusters. II. Stellar Population Model Predictions
We derive ages, metallicities, and abundance ratios ([α/Fe]) fromthe integrated spectra of 23 globular clusters in M31 by employingmultivariate fits to two different stellar population models. We alsoperform a parallel analysis on 21 Galactic globular clusters as aconsistency check and in order to facilitate a differential analysis.Our analysis shows that the M31 globular clusters separate into threedistinct components in age and metallicity; we identify an old,metal-poor group (seven clusters), an old, metal-rich group (10clusters), and an intermediate-age (3-6 Gyr), intermediate-metallicity([Z/H]~-1) group (six clusters). This third group is not identified inthe Galactic globular cluster sample. We also see evidence that the old,metal-rich Galactic globular clusters are 1-2 Gyr older than theircounterparts in M31. The majority of globular clusters in both samplesappear to be enhanced in α-elements, but the degree of enhancementis rather model-dependent. The intermediate-age globular clusters appearto be the most enhanced, with [α/Fe]~0.4. These clusters areclearly depressed in CN with respect to the models and the bulk of theM31 and Milky Way sample. Compared with the bulge of M31, M32, and NGC205, these clusters most resemble the stellar populations in NGC 205 interms of age, metallicity, and CN abundance. We infer horizontal branchmorphologies for the M31 clusters using the Rose Ca II index anddemonstrate that blue horizontal branches are not leading to erroneousage estimates in our analysis. We discuss and reject as unlikely thehypothesis that these objects are in fact foreground stars contaminatingthe optical catalogs. The intermediate-age clusters have generallyhigher velocities than the bulk of the M31 cluster population.Spatially, three of these clusters are projected onto the bulge region,and the remaining three are distributed at large radii. We discuss theseobjects within the context of the build-up of the M31 halo and suggestthat these clusters possibly originated in a gas-rich dwarf galaxy,which may or may not be presently observable in M31.
| 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.
| Radio sources in low-luminosity active galactic nuclei. IV. Radio luminosity function, importance of jet power, and radio properties of the complete Palomar sample
We present the completed results of a high resolution radio imagingsurvey of all ( 200) low-luminosity active galactic nuclei (LLAGNs) andAGNs in the Palomar Spectroscopic Sample of all ( 488) bright northerngalaxies. The high incidences of pc-scale radio nuclei, with impliedbrightness temperatures ≳107 K, and sub-parsec jetsargue for accreting black holes in ≳50% of all LINERs andlow-luminosity Seyferts; there is no evidence against all LLAGNs beingmini-AGNs. The detected parsec-scale radio nuclei are preferentiallyfound in massive ellipticals and in type 1 nuclei (i.e. nuclei withbroad Hα emission). The radio luminosity function (RLF) of PalomarSample LLAGNs and AGNs extends three orders of magnitude below, and iscontinuous with, that of “classical” AGNs. We find marginalevidence for a low-luminosity turnover in the RLF; nevertheless LLAGNsare responsible for a significant fraction of present day massaccretion. Adopting a model of a relativistic jet from Falcke &Biermann, we show that the accretion power output in LLAGNs is dominatedby the kinetic power in the observed jets rather than the radiatedbolometric luminosity. The Palomar LLAGNs and AGNs follow the samescaling between jet kinetic power and narrow line region (NLR)luminosity as the parsec to kilo-parsec jets in powerful radio galaxies.Eddington ratios {l_Edd} (=L_Emitted/L_Eddington) of≤10-1{-}10-5 are implied in jet models of theradio emission. We find evidence that, in analogy to Galactic black holecandidates, LINERs are in a “low/hard” state (gas poornuclei, low Eddington ratio, ability to launch collimated jets) whilelow-luminosity Seyferts are in a “high” state (gas richnuclei, higher Eddington ratio, less likely to launch collimated jets).In addition to dominating the radiated bolometric luminosity of thenucleus, the radio jets are energetically more significant thansupernovae in the host galaxies, and are potentially able to depositsufficient energy into the innermost parsecs to significantly slow thegas supply to the accretion disk.
| A dichotomy in the orientation of dust and radio jets in nearby low-power radio galaxies
We examine the properties of central dust in nearby quiescent and activeearly-type galaxies. The active galaxies are low-power radio galaxieswith Fanaroff & Riley type I or I/II radio jets. We focus on (a) thecomparison of the dust distributions in the active and quiescent galaxysamples; and (b) the relation between the radio jet and dustorientations. Our main observational conclusions are: (i) in line withprevious studies, the dust detection rate is higher in radio-jetgalaxies than in non radio-jet galaxies; (ii) radio galaxies contain ahigher fraction of regular dust “ellipses” compared toquiescent galaxies which contain more often irregular dustdistributions; (iii) the morphology, size and orientation of dustellipses and lanes in quiescent early-types and active early-types withkpc-scale radio jets is very similar; (iv) dust ellipses are alignedwith the major axis of the galaxy, dust lanes do not show a preferredalignment except for large (>kpc) dust lanes which are aligned withthe minor axis of the galaxy; and (v) as projected on the sky, jets donot show a preferred orientation relative to the galaxy major axis (andhence dust ellipses), but jets are preferentially perpendicular to dustlanes. We show that the dust ellipses are consistent with being nearlycircular thin disks viewed at random viewing angles. The lanes arelikely warped dust structures, which may be in the process of settlingdown to become regular disks or are being perturbed by anon-gravitational force. We use the observed dust-jet orientations toconstrain the three-dimensional angle θDJ between jetand dust. For dust-lane galaxies, the jet is approximately perpendicularto the dust structure, while for dust-ellipse galaxies there is a muchwider distribution of θDJ. We discuss two scenariosthat could explain the dust/jet/galaxy orientation dichotomy. If lanesare indeed settling, then the jet orientation apparently is roughlyaligned with the angular momentum of the dust before it settles. Iflanes are perturbed by a jet-related force, it appears that it causesthe dust to move out of its equilibrium plane in the galaxy into a planewhich is perpendicular to the jet.
| The star cluster population of M 51. II. Age distribution and relations among the derived parameters
We use archival Hubble Space Telescope observations of broad-band imagesfrom the ultraviolet (F255W-filter) through the near infrared (NICMOSF160W-filter) to study the star cluster population of the interactingspiral galaxy M 51. We obtain age, mass, extinction, and effectiveradius estimates for 1152 star clusters in a region of ~7.3 × 8.1kpc centered on the nucleus and extending into the outer spiral arms. Inthis paper we present the data set and exploit it to determine the agedistribution and relationships among the fundamental parameters (i.e.age, mass, effective radius). We show the critical dependence of the agedistribution on the sample selection, and confirm that using a constantmass cut-off, above which the sample is complete for the entire agerange of interest, is essential. In particular, in this sample we arecomplete only for masses above 5×104~Mȯ for the last 1 Gyr. Using this datasetwe find: i) that the cluster formation rate seems to have had a largeincrease ~50-70 Myr ago, which is coincident with the suggested secondpassage of its companion, NGC 5195; ii) a large number of extremelyyoung (<10 Myr) star clusters, which we interpret as a population ofunbound clusters of which a large majority will disrupt within the next~10 Myr; and iii) that the distribution of cluster sizes can be wellapproximated by a power-law with exponent, -η = -2.2 ± 0.2,which is very similar to that of Galactic globular clusters, indicatingthat cluster disruption is largely independent of cluster radius. Inaddition, we have used this dataset to search for correlations among thederived parameters. In particular, we do not find any strong trendsbetween the age and mass, mass and effective radius, nor between thegalactocentric distance and effective radius. There is, however, astrong correlation between the age of a cluster and its extinction, withyounger clusters being more heavily reddened than older clusters.
| Peculiarities and populations in elliptical galaxies. II. Visual-near IR colours as population indices
As a complement to the data collected and discussed in Paper I of thisseries, 2MASS near-IR images have been used, in connection withavailable V light aperture photometry, to derive the colours V-J, V-K,J-H and J-K within the effective aperture A_e: nearly the same completesample of 110 E-type galaxies is treated. In Paper I these wereclassified, based on morphological criteria, into the ``peculiar'' (orPec) and ``normal'' (or Nop) subsamples. For the Nop subsample, thederived colour indices are tightly related to the galaxy masses, asmeasured by the central velocity dispersion σ0,although with rather small slopes as regards J-H and J-K. For the Pecsubsample, the V-J and V-K colours behave as UBV and line-indices: partof the objects show blue residuals from the appropriatecolour-σ0 regression, which is evidence of a youngerpopulation mixed with the ``normal'' one traced by the Nop regressions;the other shows no deviations from the Nop subsample. The distinctionamong Pec objects between the YP family (NGC 2865 type), and the NP one(NGC 3923 type), is statistically supported, and generally confirmed inspecific cases.Based in part on observations collected at the Observatoire deHaute-Provence.Table 4 is only available in electonic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/819
| Nuclear activity and the dynamics of elliptical galaxies
This Letter looks for any correlation between the internal dynamics ofelliptical galaxies and the relatively mild nuclear activity found inmany such systems. We show that there is such a relation in the sensethat the active ellipticals tend to be significantly less rotationallysupported than their inactive cousins. The correlation can partly berelated to the galaxies' luminosities: the brightest galaxies tend to bemore active and less rotationally supported. However, even at lowerluminosities the active and inactive galaxies seem to havesystematically different dynamics. This variation suggests that thereare significant large-scale structural differences between active andinactive elliptical galaxies, and hence that the existence of both typesof system cannot just be the result of random sporadic nuclear activity.
| Star formation history in early-type galaxies - I. The line absorption indices diagnostics
To unravel the formation mechanism and the evolutionary history ofelliptical galaxies (EGs) is one of the goals of modern astrophysics. Ina simplified picture of the issue, the question to be answered iswhether they have formed by hierarchical merging of pre-existingsubstructures (maybe disc galaxies) made of stars and gas, with eachmerging event probably accompanied by strong star formation, orconversely, whether they originated from the early aggregation of lumpsof gas turned into stars in the remote past via a burst-like episodeever since followed by quiescence so as to mimic a sort of monolithicprocess. Even if the two alternatives seem to oppose each other,actually they may both contribute to shaping the final properties of EGsas seen today. Are there distinct signatures of the underlying dominantprocess in the observational data? To this aim we have examined the lineabsorption indices on the Lick system of the normal, field EGs of Tragerand the interacting EGs (pair- and shell-objects) of Longhetti et al.The data show that both normal, field and interacting galaxies have thesame scattered but smooth distribution in the Hβ versus [MgFe]plane even if the interacting ones show a more pronounced tail towardhigh Hβ values. This may suggest that a common physical cause is atthe origin of their distribution. There are two straightforwardinterpretations of increasing complexity. (i) EGs span true large rangesof ages and metallicities. A young age is the signature of theaggregation mechanism, each event accompanied by metal enrichment. Thissimple scheme cannot, however, explain other spectro-photometricproperties of EGs and has to be discarded. (ii) The bulk population ofstars is old but subsequent episodes of star formation scatter the EGsin the diagnostic planes. However, this scheme would predict anoutstanding clump at low Hβ values, contrary to what is observed.The model can be cured by supposing that the primary star formationactivity lasted for a significant fraction of the Hubble time (5<=T<= 13 Gyr) accompanied by global metal enrichment. The`younger' galaxies are more metal-rich. The later burst of starformation should be small otherwise too many high-Hβ objects wouldbe observed. Therefore, the distribution of normal, pair- andshell-galaxies in the Hβ versus [MgFe] plane is due to global metalenrichment. Even though the above schemes provide a formal explanation,they seem to be too demanding because of the many ad hoc ingredientsthat have to be introduced. Furthermore, they neglect theobservationally grounded hint that the stellar content of EGs is likelyto be enhanced in α-elements with [α/Fe] ranging from 0.1 to0.4 dex. Here we propose a new scheme, in which the bulk dispersion ofgalaxies in the Hβ versus [MgFe] plane is caused by a differentmean degree of enhancement. In this model, neither the large age rangesnor the universal enrichment law for the old component are required andthe observed distribution along Hβ is naturally recovered.Furthermore, later bursts of stellar activity are a rare event,involving only those galaxies with very high Hβ (roughly >2.5).Finally, simulations of the scatter in broad-band colours of EGs seem toconfirm that the bulk stars have formed in the remote past, and thatmergers and companion star formation in a recent past are not likely,unless the intensity of the secondary activity is very small.
| Cold Dust in Early-Type Galaxies. I. Observations
We describe far-infrared observations of early-type galaxies selectedfrom the Infrared Space Observatory (ISO) archive. This ratherinhomogeneous sample includes 39 giant elliptical galaxies and 14 S0 (orlater) galaxies. These galaxies were observed with the array photometerPHOT on-board the ISO satellite using a variety of different observingmodes-sparse maps, mini-maps, oversampled maps, and singlepointings-each of which requires different and often rather elaboratephotometric reduction procedures. The ISO background data agree wellwith the COBE-DIRBE results to which we have renormalized ourcalibrations. As a further check, the ISO fluxes from galaxies at 60 and100 μm agree very well with those previously observed with IRAS atthese wavelengths. The spatial resolution of ISO is several timesgreater than that of IRAS, and the ISO observations extend out to 200μm, which views a significantly greater mass of colder dust notassessable to IRAS. Most of the galaxies are essentially point sourcesat ISO resolution, but a few are clearly extended at FIR wavelengthswith image sizes that increase with FIR wavelength. The integratedfar-infrared luminosities do not correlate with optical luminosities,suggesting that the dust may have an external, merger-related origin. Ingeneral, the far-infrared spectral energy distributions can be modeledwith dust at two temperatures, ~43 and ~20 K, which probably representlimits of a continuous range of temperatures. The colder dust componentdominates the total mass of dust, 106-107Msolar, which is typically more than 10 times larger than thedust masses previously estimated for the same galaxies using IRASobservations. For S0 galaxies we find that the optically normalizedfar-infrared luminosity LFIR/LB correlatesstrongly with the mid-infrared luminosityL15μm/LB, but that correlation is weaker forelliptical galaxies.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and United Kingdom) and with the participation of ISAS andNASA.
| 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.
| The Globular Cluster System of NGC 5128. II. Ages, Metallicities, Kinematics, and Formation
We present a study of the nearby postmerger giant elliptical galaxy NGC5128 (Centaurus A), in which we use the properties of its globularcluster (GC) and planetary nebula (PN) systems to constrain itsevolution. Using photometric and spectroscopic data for 215 GCspresented in Paper I, we study trends in age, metallicity, andkinematics for the GC system. We confirm that the GC metallicitydistribution is bimodal and show that these two subpopulations havedifferent properties. Using spectral line index measurements of thebrightest clusters, we show that the metal-poor GCs have old ages likethe Milky Way globular clusters while the metal-rich GCs have Hβline strengths that could be interpreted as a mean age of~5+3-2 Gyr. Both populations appear to have[Mg/Fe] ratios consistent with that of the Galactic GC system, althoughthis quantity is not very well constrained. The kinematics of themetal-rich GCs are similar to those of the planetary nebulae, exhibitingsignificant rotation about a misaligned axis, while the metal-poor GCshave a higher velocity dispersion and show a weaker kinematiccorrelation with the field stars. The total gravitating mass of NGC 5128derived from the GCs is in excellent agreement with the value derivedfrom stellar (PN) kinematics. We suggest that these and other datasupport a picture in which the main body of NGC 5128 was formed 3-8 Gyrago by the dissipational merger of two unequal-mass disk galaxiessupplemented by the continual accretion of both gas-rich and gas-poorsatellites.
| Probing Spectral Line Gradients beyond One Effective Radius in NGC 3610
The outer region [(0.75-1.25)re in the B band] of the mergerremnant elliptical galaxy NGC 3610 is studied using extremely highsignal-to-noise ratio Keck spectra, with a supplementary spectrum of thegalaxy center. Stellar population parameters-age, [Z/H], and[α/Fe]-are measured in several apertures along the slit. Using themulti-index simultaneous fitting method of R. Proctor and coauthors, nosignificant stellar population gradients are detected in the outer partsof the galaxy. The overall gradients relative to the galaxy center areconsistent with those found in many other early-type galaxies, althoughthe metallicity gradient is much steeper than would be expected if NGC3610 formed in a major merger event. Standard analysis methods that usethe Hβ index are found to produce spurious radially variablegradients.
| A Principal Components Analysis of the Lick Indices of Galactic Globular Clusters
We present a principal components analysis (PCA) of high-qualityLick/IDS absorption-line measurements for 11 indices in the wavelengthrange 4100-5400 Å for 39 Galactic globular clusters (GCs). Onlythe first principal component appears to be physically significant. Wefind that there is a tight linear relationship between this firstcomponent (PC1) and GC metallicity over a wide range in [m/H](-1.8<=[m/H]<=0), suggesting that PC1 can be used to accuratelyestimate metallicities for old extragalactic GCs from their integratedspectra. (Metallicity calibrations for other sets of indices in thiswavelength range can be furnished upon request.) The classic GC``second-parameter effect'' is noticeable in the Balmer indices,although it does not appear in our PCA. We find little evidence forsubstantial differences in broad abundance patterns among Galactic GCs.One implication is that the metal-poor and metal-rich GC subpopulationsformed from very similar physical processes.
| Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921
Low-resolution, ultraviolet-to-visual spectra of two candidate globularclusters in the merger remnant NGC 3921 (czLG=6021 kms-1) are presented. These two clusters of apparent magnitudeV~22.2 (MV~-12.5) lie at projected distances of about 5 kpc(0.9 Reff) from the center and move with halo-type radialvelocities relative to the local galaxy background. Their spectra showstrong Balmer absorption lines [EW(Hβ-Hδ)=11-13 Å]indicative of main-sequence turnoffs dominated by A-type stars.Comparisons with model-cluster spectra computed by Bruzual & Charlotand others yield cluster ages in the range of 200-530 Myr, andmetallicities about solar to within a factor of three. Given their smallhalf-light radii (Reff<~5 pc) and ages corresponding to~102 core-crossing times, these clusters are gravitationallybound and hence indeed young globulars. Assuming that they hadChabrier-type initial mass functions, their estimated current masses are(2.3+/-0.1)×106Msolar and(1.5+/-0.1)×106Msolar, respectively, orroughly half the mass of ω Cen. Since NGC 3921 itself shows manysigns of being a 0.7+/-0.3 Gyr old protoelliptical, these two youngglobulars of roughly solar metallicity and their many counterpartsobserved with the Hubble Space Telescope provide supporting evidencethat, in the process of forming elliptical-like remnants, major mergersof gas-rich disks can also increase the number of metal-rich globularclusters.
| Revisiting the Globular Cluster System of the Merger Remnant Elliptical Galaxy NGC 3610
We have obtained Keck spectra of six candidate globular clusters (GCs)in the dynamically young elliptical galaxy NGC 3610, supplementing ourprevious Keck spectra of eight confirmed GCs. Five of our new candidatesare confirmed to be GCs. Of the 13 GCs, 11 are located within the K-bandeffective radius of 7 kpc. Two of these 13 clusters are found to beyoung (~2 Gyr) and very metal-rich ([Z/H]~+0.5), three are old andmetal-poor, and the remaining eight clusters are old and metal-rich. Theages of the young clusters are consistent with a recent spectroscopicage estimate of 1.6+/-0.5 Gyr for the galaxy itself and suggest thatthese clusters formed in the disk-disk merger that likely created NGC3610. Intriguingly, both young GCs have [α/Fe]~+0.3, while themajority of the old clusters are not α-enhanced, in contrast toGalactic and M31 GCs, and contrary to predictions of nucleosyntheticcalculations. The two old subpopulations of GCs can be attributed to themerger progenitors. The relative numbers of old and new metal-rich GCsare poorly constrained because of the expected differences in radialdistributions of the two subpopulations. However, based on ourspectroscopic results and a comparison of the Hubble Space Telescopecolor-magnitude diagram with stellar population models, we argue thatmore than half of the metal-rich GCs are likely to be old.
| 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
| VLT spectroscopy of globular cluster systems. I. The photometric and spectroscopic data set
We present Lick line-index measurements of extragalactic globularclusters in seven early-type galaxies (NGC 1380, 2434, 3115, 3379, 3585,5846, and 7192) with different morphological types (E-S0) located infield and group/cluster environments. High-quality spectra were takenwith the FORS2 instrument at ESO's Very Large Telescope. 50% of ourdata allows an age resolution Δ t/t≈0.3 and a metallicityresolution 0.25-0.4 dex, depending on the absolute metallicity.Globular cluster candidates are selected from deep B, V, R, I, KFORS2/ISAAC photometry with 80-100% success rate inside one effectiveradius. Using combined optical/near-infrared colour-colour diagrams wepresent a method to efficiently reduce fore-/background contaminationdown to 10%. We find clear signs for bi-modality in the globularcluster colour distributions of NGC 1380, 3115, and 3585. The colourdistributions of globular clusters in NGC 2434, 3379, 5846, and 7192 areconsistent with a broad single-peak distribution. For the analysedglobular cluster systems the slopes of projected radial surface densityprofiles, of the form Σ(R) R -Γ, varybetween 0.8 and 2.6. Blue and red globular cluster sub-populationsshow similar slopes in the clearly bi-modal systems. For galaxies withsingle-peak globular cluster colour distributions, there is a hint thatthe blue cluster system seems to have a more extended radialdistribution than the red one. Using globular clusters as a tracerpopulation we determine total dynamical masses of host galaxies out tolarge radii (1.6-4.8 R_eff). For the sample we find masses in therange 8.8×1010 M_ȯ up to1.2×1012 M_ȯ. The line index data presentedhere will be used in accompanying papers of this series to derive ages,metallicities and abundance ratios. A compilation of currently availablehigh-quality Lick index measurements for globular clusters inelliptical, lenticular, and late-type galaxies is provided and willserve to augment the current data set.Based on observations collected at the European Southern Observatory,Cerro Paranal, Chile under programme ID P65.N-0281, P66.B-0068, andP67.B-0034.Appendix A is only available in electronic form athttp://www.edpsciences.orgAppendices B and C are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/123
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