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|The SAURON project - VI. Line strength maps of 48 elliptical and lenticular galaxies|
We present absorption line strength maps of 48 representative ellipticaland lenticular galaxies obtained as part of a survey of nearby galaxiesusing our custom-built integral-field spectrograph, SAURON, operating onthe William Herschel Telescope. Using high-quality spectra, spatiallybinned to a constant signal-to-noise ratio, we measure four key age,metallicity and abundance ratio sensitive indices from the Lick/IDSsystem over a two-dimensional field extending up to approximately oneeffective radius. A discussion of calibrations and offsets is given,along with a description of error estimation and nebular emissioncorrection. We modify the classical Fe5270 index to define a new index,Fe5270S, which maximizes the useable spatial coverage ofSAURON. Maps of Hβ, Fe5015, Mgb and Fe5270S arepresented for each galaxy. We use the maps to compute average linestrengths integrated over circular apertures of one-eighth effectiveradius, and compare the resulting relations of index versus velocitydispersion with previous long-slit work. The metal line strength mapsshow generally negative gradients with increasing radius roughlyconsistent with the morphology of the light profiles. Remarkabledeviations from this general trend exist, particularly the Mgb isoindexcontours appear to be flatter than the isophotes of the surfacebrightness for about 40 per cent of our galaxies without significantdust features. Generally, these galaxies exhibit significant rotation.We infer from this that the fast-rotating component features a highermetallicity and/or an increased Mg/Fe ratio as compared to the galaxy asa whole. The Hβ maps are typically flat or show a mild positiveoutwards radial gradient, while a few galaxies show strong central peaksand/or elevated overall Hβ strength likely connected to recent starformation activity. For the most prominent post-starburst galaxies, eventhe metal line strength maps show a reversed gradient.
|The SAURON project - V. Integral-field emission-line kinematics of 48 elliptical and lenticular galaxies|
We present the emission-line fluxes and kinematics of 48 representativeelliptical and lenticular galaxies obtained with our custom-builtintegral-field spectrograph, SAURON, operating on the William HerschelTelescope. Hβ, [OIII]λλ4959,5007 and[NI]λλ5198,5200 emission lines were measured using a newprocedure that simultaneously fits both the stellar spectrum and theemission lines. Using this technique we can detect emission lines downto an equivalent width of 0.1 Å set by the current limitations indescribing galaxy spectra with synthetic and real stellar templates,rather than by the quality of our spectra. Gas velocities and velocitydispersions are typically accurate to within 14 and 20 kms-1, respectively, and at worse to within 25 and 40 kms-1. The errors on the flux of the [OIII] and Hβ linesare on average 10 and 20 per cent, respectively, and never exceed 30 percent. Emission is clearly detected in 75 per cent of our samplegalaxies, and comes in a variety of resolved spatial distributions andkinematic behaviours. A mild dependence on the Hubble type and galacticenvironment is observed, with higher detection rates in lenticulargalaxies and field objects. More significant is the fact that only 55per cent of the galaxies in the Virgo cluster exhibit clearly detectedemission. The ionized-gas kinematics is rarely consistent with simplecoplanar circular motions. However, the gas almost never displayscompletely irregular kinematics, generally showing coherent motions withsmooth variations in angular momentum. In the majority of the cases, thegas kinematics is decoupled from the stellar kinematics, and in half ofthe objects this decoupling implies a recent acquisition of gaseousmaterial. Over the entire sample however, the distribution of the meanmisalignment values between stellar and gaseous angular momenta isinconsistent with a purely external origin. The distribution ofkinematic misalignment values is found to be strongly dependent on theapparent flattening and the level of rotational support of galaxies,with flatter, fast rotating objects hosting preferentially corotatinggaseous and stellar systems. In a third of the cases, the distributionand kinematics of the gas underscore the presence of non-axisymmetricperturbations of the gravitational potential. Consistent with previousstudies, the presence of dust features is always accompanied by gasemission while the converse is not always true. A considerable range ofvalues for the [OIII]/Hβ ratio is found both across the sample andwithin single galaxies. Despite the limitations of this ratio as anemission-line diagnostic, this finding suggests either that a variety ofmechanisms is responsible for the gas excitation in E and S0 galaxies orthat the metallicity of the interstellar material is quiteheterogeneous.
|The SAURON project - IV. The mass-to-light ratio, the virial mass estimator and the Fundamental Plane of elliptical and lenticular galaxies|
We investigate the well-known correlations between the dynamicalmass-to-light ratio (M/L) and other global observables of elliptical (E)and lenticular (S0) galaxies. We construct two-integral Jeans andthree-integral Schwarzschild dynamical models for a sample of 25 E/S0galaxies with SAURON integral-field stellar kinematics to about oneeffective (half-light) radius Re. They have well-calibratedI-band Hubble Space Telescope WFPC2 and large-field ground-basedphotometry, accurate surface brightness fluctuation distances, and theirobserved kinematics is consistent with an axisymmetric intrinsic shape.All these factors result in an unprecedented accuracy in the M/Lmeasurements. We find a tight correlation of the form (M/L) = (3.80 +/-0.14) ×(σe/200kms-1)0.84+/-0.07 betweenthe M/L (in the I band) measured from the dynamical models and theluminosity-weighted second moment σe of the LOSVDwithin Re. The observed rms scatter in M/L for our sample is18 per cent, while the inferred intrinsic scatter is ~13 per cent. The(M/L)-σe relation can be included in the remarkableseries of tight correlations between σe and othergalaxy global observables. The comparison of the observed correlationswith the predictions of the Fundamental Plane (FP), and with simplevirial estimates, shows that the `tilt' of the FP of early-typegalaxies, describing the deviation of the FP from the virial relation,is almost exclusively due to a real M/L variation, while structural andorbital non-homology have a negligible effect. When the photometricparameters are determined in the `classic' way, using growth curves, andthe σe is measured in a large aperture, the virial massappears to be a reliable estimator of the mass in the central regions ofgalaxies, and can be safely used where more `expensive' models are notfeasible (e.g. in high-redshift studies). In this case the best-fittingvirial relation has the form (M/L)vir= (5.0 +/- 0.1)×Reσ2e/(LG), in reasonableagreement with simple theoretical predictions. We find no differencebetween the M/L of the galaxies in clusters and in the field. Thecomparison of the dynamical M/L with the (M/L)pop inferredfrom the analysis of the stellar population, indicates a median darkmatter fraction in early-type galaxies of ~30 per cent of the total massinside one Re, in broad agreement with previous studies, andit also shows that the stellar initial mass function varies little amongdifferent galaxies. Our results suggest a variation in M/L at constant(M/L)pop, which seems to be linked to the galaxy dynamics. Wespeculate that fast-rotating galaxies have lower dark matter fractionsthan the slow-rotating and generally more-massive ones. If correct, thiswould suggest a connection between the galaxy assembly history and thedark matter halo structure. The tightness of our correlation providessome evidence against cuspy nuclear dark matter profiles in galaxies.
|The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies|
The ACS Virgo Cluster Survey is a Hubble Space Telescope program toobtain high-resolution imaging in widely separated bandpasses (F475W~gand F850LP~z) for 100 early-type members of the Virgo Cluster, spanninga range of ~460 in blue luminosity. We use this large, homogenous dataset to examine the innermost structure of these galaxies and tocharacterize the properties of their compact central nuclei. We presenta sharp upward revision in the frequency of nucleation in early-typegalaxies brighter than MB~-15 (66%<~fn<~82%)and show that ground-based surveys underestimated the number of nucleidue to surface brightness selection effects, limited sensitivity andpoor spatial resolution. We speculate that previously reported claimsthat nucleated dwarfs are more concentrated toward the center of Virgothan their nonnucleated counterparts may be an artifact of theseselection effects. There is no clear evidence from the properties of thenuclei, or from the overall incidence of nucleation, for a change atMB~-17.6, the traditional dividing point between dwarf andgiant galaxies. There does, however, appear to be a fundamentaltransition at MB~-20.5, in the sense that the brighter,``core-Sérsic'' galaxies lack resolved (stellar) nuclei. A searchfor nuclei that may be offset from the photocenters of their hostgalaxies reveals only five candidates with displacements of more than0.5", all of which are in dwarf galaxies. In each case, however, theevidence suggests that these ``nuclei'' are, in fact, globular clustersprojected close to the galaxy photocenter. Working from a sample of 51galaxies with prominent nuclei, we find a median half-light radius of=4.2 pc, with the sizes of individual nucleiranging from 62 pc down to <=2 pc (i.e., unresolved in our images) inabout a half-dozen cases. Excluding these unresolved objects, the nucleisizes are found to depend on nuclear luminosity according to therelation rh L0.50+/-0.03. Because the largemajority of nuclei are resolved, we can rule out low-level AGNs as anexplanation for the central luminosity excess in almost all cases. Onaverage, the nuclei are ~3.5 mag brighter than a typical globularcluster. Based on their broadband colors, the nuclei appear to have oldto intermediate age stellar populations. The colors of the nuclei ingalaxies fainter than MB~-17.6 are tightly correlated withtheir luminosities, and less so with the luminosities of their hostgalaxies, suggesting that their chemical enrichment histories weregoverned by local or internal factors. Comparing the nuclei to the``nuclear clusters'' found in late-type spiral galaxies reveals a closematch in terms of size, luminosity, and overall frequency. A formationmechanism that is rather insensitive to the detailed properties of thehost galaxy properties is required to explain this ubiquity andhomogeneity. The mean of the frequency function for thenucleus-to-galaxy luminosity ratio in our nucleated galaxies,=-2.49+/-0.09 dex (σ=0.59+/-0.10), isindistinguishable from that of the SBH-to-bulge mass ratio,=-2.61+/-0.07dex (σ=0.45+/-0.09), calculated in 23 early-type galaxies withdetected supermassive black holes (SBHs). We argue that the compactstellar nuclei found in many of our program galaxies are the low-masscounterparts of the SBHs detected in the bright galaxies. If thisinterpretation is correct, then one should think in terms of ``centralmassive objects''-either SBHs or compact stellar nuclei-that accompanythe formation of almost all early-type galaxies and contain a meanfraction ~0.3% of the total bulge mass. In this view, SBHs would be thedominant formation mode above MB~-20.5.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
|The ACS Virgo Cluster Survey. IX. The Color Distributions of Globular Cluster Systems in Early-Type Galaxies|
We present the color distributions of globular cluster (GC) systems for100 early-type galaxies observed in the ACS Virgo Cluster Survey, thedeepest and most homogeneous survey of this kind to date. On average,galaxies at all luminosities in our study (-22
|Stellar Populations of Elliptical Galaxies in Virgo Cluster. I. The Data and Stellar Population Analysis|
We have determined spectroscopic ages of elliptical galaxies in theVirgo Cluster using spectra of very high signal-to-noise ratio(S/N>100 Å-1). We observed eight galaxies with theSubaru Telescope and have combined this sample with six galaxiespreviously observed with the WHT. To determine their ages, we have useda new method based on the Hγσ age indicator,which is virtually independent of the effects of metallicity. Apart fromages we have estimated abundances of various elements. In this paper wepresent the observations, the data reduction, and the reliability of theHγσ method. The results of this investigation arepresented in a companion paper.
|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.
|Density-potential pairs for spherical stellar systems with Sérsic light profiles and (optional) power-law cores|
Popular models for describing the luminosity-density profiles ofdynamically hot stellar systems (e.g. Jaffe, Hernquist, Dehnen) wereconstructed with the desire to match the deprojected form of anR1/4 light profile. Real galaxies, however, are now known tohave a range of different light-profile shapes that scale with mass.Consequently, although highly useful, the above models have implicitlimitations, and this is illustrated here through their application to anumber of real galaxy density profiles. On the other hand, theanalytical density profile given by Prugniel & Simien closelymatches the deprojected form of Sérsic R1/n lightprofiles - including deprojected exponential light profiles. It is thusapplicable for describing bulges in spiral galaxies, dwarf ellipticalgalaxies, and both ordinary and giant elliptical galaxies. Moreover, theobserved Sérsic quantities define the parameters of the densitymodel. Here we provide simple equations, in terms of elementary andspecial functions, for the gravitational potential and force associatedwith this density profile. Furthermore, to match galaxies with partiallydepleted cores, and better explore the supermassive black hole/galaxyconnection, we have added a power-law core to this density profile andderived similar expressions for the potential and force of this hybridprofile. Expressions for the mass and velocity dispersion, assumingisotropy, are also given. These spherical models may also proveappropriate for describing the dark matter distribution in haloes formedfrom ΛCDM cosmological simulations.
|The ACS Virgo Cluster Survey. X. Half-Light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation|
We have measured half-light radii, rh, for thousands ofglobular clusters (GCs) belonging to the 100 early-type galaxiesobserved in the ACS Virgo Cluster Survey and the elliptical galaxy NGC4697. An analysis of the dependencies of the measured half-light radiion both the properties of the GCs themselves and their host galaxiesreveals that, in analogy with GCs in the Galaxy but in a milder fashion,the average half-light radius increases with increasing galactocentricdistance or, alternatively, with decreasing galaxy surface brightness.For the first time, we find that the average half-light radius decreaseswith the host galaxy color. We also show that there is no evidence for avariation of rh with the luminosity of the GCs. Finally, wefind in agreement with previous observations that the averagerh depends on the color of GCs, with red GCs being ~17%smaller than their blue counterparts. We show that this difference isprobably a consequence of an intrinsic mechanism, rather than projectioneffects, and that it is in good agreement with the mechanism proposed byJordán. We discuss these findings in light of two simple picturesfor the origin of the rh of GCs and show that both lead to abehavior in rough agreement with the observations. After accounting forthe dependencies on galaxy color, galactocentric radius, and underlyingsurface brightness, we show that the average GC half-light radii can be successfully used as a standard ruler fordistance estimation. We outline the methodology, provide a calibrationfor its use, and discuss the prospects for this distance estimator withfuture observing facilities. We find =2.7+/-0.35 pcfor GCs with (g-z)=1.2 mag in a galaxy with color(g-z)gal=1.5 mag and at an underlying surface z-bandbrightness of μz=21 mag arcsec-2. Using thistechnique, we place an upper limit of 3.4 Mpc on the 1 σline-of-sight depth of the Virgo Cluster. Finally, we examine the formof the rh distribution for our sample galaxies and provide ananalytic expression that successfully describes this distribution.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|Diffuse Light in the Virgo Cluster|
We present deep optical imaging of the inner ~1.5d×1.5d of theVirgo Cluster to search for diffuse intracluster light (ICL). Our imagereaches a 1 σ depth of μV=28.5 magarcsec-2, which is 1.5 mag arcsec-2 deeper thanprevious surveys, and reveals an intricate web of diffuse ICL. We seeseveral long (>100 kpc) tidal streamers, as well as a myriad ofsmaller scale tidal tails and bridges between galaxies. The diffuse haloof M87 is traced out to nearly 200 kpc, appearing very irregular onthese scales, while significant diffuse light is also detected aroundthe M84/M86 pair. Several galaxies in the core are embedded in commonenvelopes, suggesting they are true physical subgroups. The complexsubstructure of Virgo's diffuse ICL reflects the hierarchical nature ofcluster assembly, rather than being the product of smooth accretionaround a central galaxy.
|UV Properties of Early-Type Galaxies in the Virgo Cluster|
We study the UV properties of a volume-limited sample of early-typegalaxies in the Virgo Cluster combining new GALEX far-ultraviolet (1530Å) and near-ultraviolet (2310 Å) data withspectrophotometric data available at other wavelengths. The sampleincludes 264 elliptical, lenticular, and dwarf galaxies spanning a largerange in luminosity (MB<=-15). While the NUV to optical ornear-IR color-magnitude relations (CMRs) are similar to those observedat optical wavelengths, with a monotonic reddening of the color indexwith increasing luminosity, the (FUV-V) and (FUV-H) CMRs show adiscontinuity between massive and dwarf objects. An even more pronounceddichotomy is observed in the (FUV-NUV) CMR. For elliptical galaxies, the(FUV-NUV) color becomes bluer with increasing luminosity and withincreasing reddening of the optical or near-IR color indices. For thedwarfs, the opposite trend is observed. These observational evidencesare consistent with the idea that the UV emission is dominated by hot,evolved stars in giant systems, while in dwarf ellipticals residual starformation activity is more common.
|The Nuclear Disk in the Dwarf Elliptical Galaxy NGC 4486A|
Many ellipticals contain nuclear disks of dust and gas. Some ellipticalscontain nuclear disks of stars that are distinct from the rest of thegalaxy. We show that the dwarf E2 galaxy NGC 4486A contains both-it is a``Rosetta stone'' object that tells us how nuclear disks evolve. Itsproperties suggest that, as accreted gas dissipates and settles towardthe center, it forms stars and builds a stellar disk. Secular growth mayexplain not only the most distinct nuclear disks such as the one in NGC4486A but also some of the disky distortions that are commonly seen inelliptical galaxies. That is, density distributions may grow secularlycuspier. This would result in chaotic mixing of stellar orbits in phasespace and would tend to make an elliptical galaxy evolve toward a morenearly axisymmetric shape.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555.
|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.
|Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles|
We present a study of the optical brightness profiles of early typegalaxies, using a number of samples of radio galaxies and opticallyselected elliptical galaxies. For the radio galaxy samples - B2 ofFanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined anumber of parameters that describe a "Nuker-law" profile, which werecompared with those already known for the optically selected objects. Wefind that radio active galaxies are always of the "core" type (i.e. aninner Nuker law slope γ < 0.3). However, there are core-typegalaxies which harbor no significant radio source and which areindistinguishable from the radio active galaxies. We do not find anyradio detected galaxy with a power law profile (γ > 0.5). Thisdifference is not due to any effect with absolute magnitude, since in aregion of overlap in magnitude the dichotomy between radio active andradio quiescent galaxies remains. We speculate that core-type objectsrepresent the galaxies that have been, are, or may become, radio activeat some stage in their lives; active and non-active core-type galaxiesare therefore identical in all respects except their eventualradio-activity: on HST scales we do not find any relationship betweenboxiness and radio-activity. There is a fundamental plane, defined bythe parameters of the core (break radius rb and breakbrightness μ_b), which is seen in the strong correlation betweenrb and μ_b. The break radius is also linearly proportionalto the optical Luminosity in the I band. Moreover, for the few galaxieswith an independently measured black hole mass, the break radius turnsout to be tightly correlated with MBH. The black hole masscorrelates even better with the combination of fundamental planeparameters rb and μ_b, which represents the centralvelocity dispersion.
|Nuclear stellar discs in low-luminosity elliptical galaxies: NGC 4458 and 4478|
We present the detection of nuclear stellar discs in the low-luminosityelliptical galaxies, NGC 4458 and 4478, which are known to host akinematically decoupled core. Using archival Hubble Space Telescopeimaging, and available absorption line-strength index data based onground-based spectroscopy, we investigate the photometric parameters andthe properties of the stellar populations of these central structures.Their scalelength, h, and face-on central surface brightness,μc0, fit on the μc0-hrelation for galaxy discs. For NGC 4458, these parameters are typicalfor nuclear discs, while the same quantities for NGC 4478 lie betweenthose of nuclear discs and the discs of discy ellipticals. We presentLick/Image Dissector Scanner (IDS) absorption line-strength measurementsof Hβ, Mgb and along the major and minor axes of thegalaxies. We model these data with simple stellar populations thataccount for the α/Fe overabundance. The counter-rotating centraldisc of NGC 4458 is found to have similar properties to the decoupledcores of bright ellipticals. This galaxy has been found to be uniformlyold despite being counter-rotating. In contrast, the cold central discof NGC 4478 is younger, richer in metals and less overabundant than themain body of the galaxy. This points to a prolonged star formationhistory, typical of an undisturbed disc-like, gas-rich (possiblypre-enriched) structure.
|Measuring shapes of galaxy images - II. Morphology of 2MASS galaxies|
We study a sample of 112 galaxies of various Hubble types imaged in theTwo Micron All Sky Survey (2MASS) in the near-infrared (NIR; 1-2 μm)J, H and Ks bands. The sample contains (optically classified)32 ellipticals, 16 lenticulars and 64 spirals acquired from the 2MASSExtended Source Catalogue (XSC).We use a set of non-parametric shape measures constructed from theMinkowski functionals (MFs) for galaxy shape analysis. We useellipticity (ɛ) and orientation angle (Φ) as shapediagnostics. With these parameters as functions of area within theisophotal contour, we note that the NIR elliptical galaxies withɛ > 0.2 show a trend of being centrally spherical andincreasingly flattened towards the edge, a trend similar to images inoptical wavelengths. The highly flattened elliptical galaxies showstrong change in ellipticity between the centre and the edge. Thelenticular galaxies show morphological properties resembling eitherellipticals or disc galaxies. Our analysis shows that almost half of thespiral galaxies appear to have bar-like features while the rest arelikely to be non-barred. Our results also indicate that almost one-thirdof spiral galaxies have optically hidden bars.The isophotal twist noted in the orientations of elliptical galaxiesdecreases with the flattening of these galaxies, indicating that twistand flattening are also anticorrelated in the NIR, as found in opticalwavelengths. The orientations of NIR lenticular and spiral galaxies showa wide range of twists.
|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.
|The SAURON project - III. Integral-field absorption-line kinematics of 48 elliptical and lenticular galaxies|
We present the stellar kinematics of 48 representative elliptical andlenticular galaxies obtained with our custom-built integral-fieldspectrograph SAURON operating on the William Herschel Telescope. Thedata were homogeneously processed through a dedicated reduction andanalysis pipeline. All resulting SAURON data cubes were spatially binnedto a constant minimum signal-to-noise ratio. We have measured thestellar kinematics with an optimized (penalized pixel-fitting) routinewhich fits the spectra in pixel space, via the use of optimal templates,and prevents the presence of emission lines to affect the measurements.We have thus generated maps of the mean stellar velocity V, the velocitydispersion σ, and the Gauss-Hermite moments h3 andh4 of the line-of-sight velocity distributions. The mapsextend to approximately one effective radius. Many objects displaykinematic twists, kinematically decoupled components, central stellardiscs, and other peculiarities, the nature of which will be discussed infuture papers of this series.
|Are interactions the primary triggers of star formation in dwarf galaxies?|
We investigate the assumption that the trigger of star formation indwarf galaxies is interactions with other galaxies, in the context of asearch for a `primary' trigger of a first generation of stars. This iscosmologically relevant because the galaxy formation process consistsnot only of the accumulation of gas in a gravitational potential wellbut also of the triggering of star formation in this gas mass, and alsobecause some high-z potentially primeval galaxy blocks look like nearbystar-forming dwarf galaxies. We review theoretical ideas proposed toaccount for the tidal interaction triggering mechanism and present aseries of observational tests of this assumption using published data.We also show results of a search in the vicinity of a composite sampleof 96 dwarf late-type galaxies for interaction candidates showing starformation. The small number of possible perturbing galaxies identifiedin the neighbourhood of our sample galaxies, along with similar findingsfrom other studies, supports the view that tidal interactions may not berelevant as primary triggers of star formation. We conclude thatinteractions between galaxies may explain some forms of star formationtriggering, perhaps in central regions of large galaxies, but they donot seem to be significant for dwarf galaxies and, by inference, forfirst-time galaxies forming at high redshifts. Intuitive reasoning,based on an analogy with stellar dynamics, shows that conditions forprimary star formation triggering may occur in gas masses oscillating ina dark-matter gravitational potential. We propose this mechanism as aplausible primary trigger scenario, which would be worth investigatingtheoretically.
|The relationship between the Sérsic law profiles measured along the major and minor axes of elliptical galaxies|
In this paper we discuss the reason why the parameters of theSérsic model best-fitting the major axis light profile ofelliptical galaxies can differ significantly from those derived for theminor axis profile. We show that this discrepancy is a naturalconsequence of the fact that the isophote eccentricity varies with theradius of the isophote and present a mathematical transformation thatallows the minor axis Sérsic model to be calculated from themajor axis model, provided that the elliptical isophotes are aligned andconcentric and that their eccentricity can be represented by a wellbehaved, though quite general, function of the radius. When there is novariation in eccentricity only the effective radius changes in theSérsic model, while for radial-dependent eccentricity thetransformation, which allows the minor axis Sérsic model to becalculated from the major axis model is given by the Lerch Φtranscendental function. The proposed transformation was tested usingphotometric data for 28 early-type galaxies.
|The ACS Virgo Cluster Survey. II. Data Reduction Procedures|
The ACS Virgo Cluster Survey is a large program to carry out multicolorimaging of 100 early-type members of the Virgo Cluster using theAdvanced Camera for Surveys (ACS) on the Hubble Space Telescope. DeepF475W and F850LP images (~SDSS g and z) are being used to study thecentral regions of the program galaxies, their globular cluster systems,and the three-dimensional structure of Virgo itself. In this paper, wedescribe in detail the data reduction procedures used for the survey,including image registration, drizzling strategies, the computation ofweight images, object detection, the identification of globular clustercandidates, and the measurement of their photometric and structuralparameters.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|The ACS Virgo Cluster Survey. I. Introduction to the Survey|
The Virgo Cluster is the dominant mass concentration in the LocalSupercluster and the largest collection of elliptical and lenticulargalaxies in the nearby universe. In this paper, we present anintroduction to the ACS Virgo Cluster Survey: a program to image, in theF475W and F850LP bandpasses (~Sloan g and z), 100 early-type galaxies inthe Virgo Cluster using the Advanced Camera for Surveys on the HubbleSpace Telescope. We describe the selection of the program galaxies andtheir ensemble properties, the choice of filters, the field placementand orientation, the limiting magnitudes of the survey, coordinatedparallel observations of 100 ``intergalactic'' fields with WFPC2, andsupporting ground-based spectroscopic observations of the programgalaxies. In terms of depth, spatial resolution, sample size, andhomogeneity, this represents the most comprehensive imaging survey todate of early-type galaxies in a cluster environment. We brieflydescribe the main scientific goals of the survey, which include themeasurement of luminosities, metallicities, ages, and structuralparameters for the many thousands of globular clusters associated withthese galaxies, a high-resolution isophotal analysis of galaxiesspanning a factor of ~450 in luminosity and sharing a commonenvironment, the measurement of accurate distances for the full sampleof galaxies using the method of surface brightness fluctuations, and adetermination of the three-dimensional structure of Virgo itself.ID="FN1"> 1Based on observations with the NASA/ESA Hubble SpaceTelescope obtained at the Space Telescope Science Institute, which isoperated by the association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
|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.
|Revised Rates of Stellar Disruption in Galactic Nuclei|
We compute rates of tidal disruption of stars by supermassive blackholes in galactic nuclei, using downwardly revised black hole massesfrom the MBH-σ relation. In galaxies with steep nucleardensity profiles, which dominate the overall event rate, the disruptionfrequency varies inversely with assumed black hole mass. We compute atotal rate for nondwarf galaxies of ~10-5 yr-1Mpc-3, about a factor of 10 higher than in earlier studies.Disruption rates are predicted to be highest in nucleated dwarfgalaxies, assuming that such galaxies contain black holes. Monitoring ofa rich galaxy cluster for a few years could rule out the existence ofintermediate-mass black holes in dwarf galaxies.
|Evidence for a New Elliptical-Galaxy Paradigm: Sérsic and Core Galaxies|
We fitted the surface-brightness profiles of 21 elliptical galaxiesusing both the Sérsic function and a new empirical model thatcombines an inner power law with an outer Sérsic function. Theprofiles are combinations of deconvolved Hubble Space Telescope (HST)profiles from the literature and ellipse fits to the full WFPC2 mosaicimages and thus span a radial range from ~0.02" to about twice thehalf-light radius. We are able to accurately fit the entire profilesusing either the Sérsic function or our new model. In doing so,we demonstrate that most, if not all, so-called ``power-law'' galaxiesare better described as ``Sérsic galaxies''-they are well modeledby the three-parameter Sérsic profile into the limits of HSTresolution-and that ``core'' galaxies are best understood as consistingof an outer Sérsic profile with an inner power-law cusp, which isa downward deviation from the inward extrapolation of the Sérsicprofile. This definition of cores resolves ambiguities that result whenthe popular ``Nuker law'' is fitted to the profiles of ellipticals andbulges, particularly at lower luminosities. We also find that using theNuker law to model core-galaxy nuclear profiles systematicallyoverestimates the core radii by factors of 1.5-4.5 and underestimatesthe inner power-law slope by ~20%-40% or more.
|The origin of H I-deficiency in galaxies on the outskirts of the Virgo cluster. II. Companions and uncertainties in distances and deficiencies|
The origin of the deficiency in neutral hydrogen of 13 spiral galaxieslying in the outskirts of the Virgo cluster is reassessed. If thesegalaxies have passed through the core of the cluster, their interstellargas should have been lost through ram pressure stripping by the hotX-ray emitting gas of the cluster. We analyze the positions of these HI-deficient and other spiral galaxies in velocity-distance plots, inwhich we include our compilation of velocity-distance data on 61elliptical galaxies, and compare with simulated velocity-distancediagrams obtained from cosmological N-body simulations. We find that20% relative Tully-Fisher distance errors are consistent with thegreat majority of the spirals, except for a small number of objectswhose positions in the velocity-distance diagram suggest grosslyincorrect distances, implying that the Tully-Fisher error distributionfunction has non-Gaussian wings. Moreover, we find that the distanceerrors may lead to an incorrect fitting of the Tolman-Bondi solutionthat can generate significant errors in the distance and especially themass estimates of the cluster. We suggest 4 possibilities for theoutlying H I-deficient spirals (in decreasing frequency): 1) they havelarge relative distance errors and are in fact close enough (atdistances between 12.7 and 20.9 Mpc from us) to the cluster to havepassed through its core and seen their gas removed by ram pressurestripping; 2) their gas is converted to stars by tidal interactions withother galaxies; 3) their gas is heated during recent mergers withsmaller galaxies; and 4) they are not truly H I-deficient (e.g. S0/amisclassified as Sa).Appendix A is only available in electronic form athttp://www.edpsciences.org
|Spectrophotometry of galaxies in the Virgo cluster. II. The data|
Drift-scan mode (3600-6800 Å) spectra with 500
|Stellar collisions in galactic centres: black hole growth and colour gradients|
We study the effects of stellar collisions, particularly on feedingmassive black holes (BHs) and colour gradients, in realistic galacticcentres. We find that the mass released by stellar collisions is notsufficient to account for the present BH mass in galactic centres,especially in bright galaxies. This study, together with the study byMagorrian & Tremaine on tidal disruption of stars by massive BHs,implies that the material for BH growth (especially in galaxies brighterthan ~109 Lsolar) can only come from othersources, for example, the mass released by stellar evolution in theinitial ~1 Gyr of the lifetime of the galaxy, or the gas that sinks tothe galactic centre in a galaxy merger. We also analyse how the colourof a stellar system is affected by collisions of stars. We find thatcollisions between main-sequence stars cannot cause observable colourgradients in the visible bands at projected radius R>~ 0.1 arcsec inM31, M32 and other nearby galactic centres. This result is consistentwith the lack of an observable colour gradient in M32 at R >~ 0.1arcsec. At even smaller radii, the colour differences caused bycollisions between main-sequence stars are at most 0.08 mag at R = 0.02arcsec. The averaged bluing caused by stellar collisions in the region R< 0.1 arcsec of M32 should not be larger than 0.06 mag in colourindex U - V and 0.02 mag in V - I. The observed bluing in the centre ofthe galaxy M31 (in a 0.14 × 0.14 arcsec2 box) must becaused by some mechanism other than collisions between main-sequencestars.
|Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations|
To empirically calibrate the IR surface brightness fluctuation (SBF)distance scale and probe the properties of unresolved stellarpopulations, we measured fluctuations in 65 galaxies using NICMOS on theHubble Space Telescope. The early-type galaxies in this sample includeelliptical and S0 galaxies and spiral bulges in a variety ofenvironments. Absolute fluctuation magnitudes in the F160W (1.6 μm)filter (MF160W) were derived for each galaxy using previouslymeasured I-band SBF and Cepheid variable star distances. F160W SBFs canbe used to measure distances to early-type galaxies with a relativeaccuracy of ~10%, provided that the galaxy color is known to ~0.035 magor better. Near-IR fluctuations can also reveal the properties of themost luminous stellar populations in galaxies. Comparison of F160Wfluctuation magnitudes and optical colors to stellar population modelpredictions suggests that bluer elliptical and S0 galaxies havesignificantly younger populations than redder ones and may also be moremetal-rich. There are no galaxies in this sample with fluctuationmagnitudes consistent with old, metal-poor (t>5 Gyr, [Fe/H]<-0.7)stellar population models. Composite stellar population models implythat bright fluctuations in the bluer galaxies may be the result of anepisode of recent star formation in a fraction of the total mass of agalaxy. Age estimates from the F160W fluctuation magnitudes areconsistent with those measured using the Hβ Balmer-line index. Thetwo types of measurements make use of completely different techniquesand are sensitive to stars in different evolutionary phases. Bothtechniques reveal the presence of intermediate-age stars in theearly-type galaxies of this sample.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS 5-26555.
|Lensing and the Centers of Distant Early-Type Galaxies|
Gravitational lensing provides a unique probe of the inner 10-1000 pc ofdistant galaxies (z~0.2-1). Theoretical studies have predicted that eachstrong lens system should have a faint image near the center of the lensgalaxy, which should, in principle, be visible in radio lenses but hasnever been detected. We study the predicted ``core'' images using modelsderived from the stellar distributions in nearby early-type galaxies. Wefind that realistic lens galaxies produce a remarkably wide range ofcore images, with magnifications spanning some 6 orders of magnitude.More concentrated galaxies produce fainter core images, although notwith any model-independent relation between the galaxy properties andthe core images. Some real galaxies have diffuse cores that should yieldbright core images (magnification μcore>~0.1), but morecommon are galaxies that yield faint core images(μcore<~0.001). Thus, stellar mass distributions aloneare probably concentrated enough to explain the lack of observed coreimages. Observational sensitivity may need to improve by an order ofmagnitude before detections of core images become common. Two-imagelenses should tend to have brighter core images than four-image lenses,so they will be the better targets for finding core images andexploiting these tools for studying the central mass distributions ofdistant galaxies.
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