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Stellar kinematics and populations of early-type galaxies with the SAURON and OASIS integral-field spectrographs
We summarise the results and achievements of integral-field spectroscopyof early-type galaxies, observed as part of a survey using both theSAURON and OASIS spectrographs. From the perspective of integral-fieldspectroscopy, these otherwise smooth and featureless objects show awealth of structure, both in their stellar kinematics and populations.We focus on the stellar content, and examine properties on bothkiloparsec scales with SAURON, and scales of 100’s of parsecs withOASIS. These complementary studies reveal two types of kinematicallydistinct components (KDCs), differing primarily in their intrinsicsizes. In previous studies, KDCs and their host galaxies have generallybeen found to be unremarkable in other aspects. We show that large KDCs,typical of the well-studied cases, indeed show little or no agedifferences with their host galaxy. The KDCs detected with the higherspatial-resolution of OASIS are intrinsically smaller and include, incontrast, a significant fraction of young stars. We speculate on therelationship between KDCs and their host galaxies, and the implicationsfor young populations in early-type galaxies.
| 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.
| Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.
| 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.
| A data-driven Bayesian approach for finding young stellar populations in early-type galaxies from their ultraviolet-optical spectra
Efficient predictive models and data analysis techniques for theanalysis of photometric and spectroscopic observations of galaxies arenot only desirable, but also required, in view of the overwhelmingquantities of data becoming available. We present the results of a novelapplication of Bayesian latent variable modelling techniques, where wehave formulated a data-driven algorithm that allows one to explore thestellar populations of a large sample of galaxies from their spectra,without the application of detailed physical models. Our only assumptionis that the galaxy spectrum can be expressed as a linear superpositionof a small number of independent factors, each a spectrum of a stellarsubpopulation that cannot be individually observed. A probabilisticlatent variable architecture that explicitly encodes this assumption isthen formulated, and a rigorous Bayesian methodology is employed forsolving the inverse modelling problem from the available data. Apowerful aspect of this method is that it formulates a density model ofthe spectra, based on which we can handle observational errors. Further,we can recover missing data both from the original set of spectra whichmight have incomplete spectral coverage of each galaxy, or frompreviously unseen spectra of the same kind.We apply this method to a sample of 21 ultraviolet-optical spectra ofwell-studied early-type galaxies, for which we also derive detailedphysical models of star formation history (i.e. age, metallicity andrelative mass fraction of the component stellar populations). We alsoapply it to synthetic spectra made up of two stellar populations,spanning a large range of parameters. We apply four different datamodels, starting from a formulation of principal component analysis(PCA), which has been widely used. We explore alternative factor models,relaxing the physically unrealistic assumption of Gaussian factors, aswell as constraining the possibility of negative flux values that areallowed in PCA, and show that other models perform equally well orbetter, while yielding more physically acceptable results. Inparticular, the more physically motivated assumptions of our rectifiedfactor analysis enable it to perform better than PCA, and to recoverphysically meaningful results.We find that our data-driven Bayesian modelling allows us to identifythose early-type galaxies that contain a significant stellar populationthat is <~1-Gyr old. This experiment also concludes that our sampleof early-type spectra showed no evidence of more than two major stellarpopulations differing significantly in age and metallicity. This methodwill help us to search for such young populations in a large ensemble ofspectra of early-type galaxies, without fitting detailed models, andthereby to study the underlying physical processes governing theformation and evolution of early-type galaxies, particularly thoseleading to the suppression of star formation in dense environments. Inparticular, this method would be a very useful tool for automaticallydiscovering various interesting subclasses of galaxies, for example,post-starburst or E+A 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.
| Spitzer IRS spectra of Virgo Early-Type Galaxies: Detection of Stellar Silicate Emission
We present high signal-to-noise ratio Spitzer Infrared Spectrographobservations of 17 Virgo early-type galaxies. The galaxies were selectedfrom those that define the color-magnitude relation of the cluster, withthe aim of detecting the silicate emission of their dusty, mass-losingevolved stars. To flux calibrate these extended sources, we have deviseda new procedure that allows us to obtain the intrinsic spectral energydistribution and to disentangle resolved and unresolved emission withinthe same object. We have found that 13 objects of the sample (76%) arepassively evolving galaxies with a pronounced broad silicate featurethat is spatially extended and likely of stellar origin, in agreementwith model predictions. The other four objects (24%) are characterizedby different levels of activity. In NGC 4486 (M87), the line emissionand the broad silicate emission are evidently unresolved, and, givenalso the typical shape of the continuum, they likely originate in thenuclear torus. NGC 4636 shows emission lines superposed on extended(i.e., stellar) silicate emission, thus pushing the percentage ofgalaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435are characterized by polycyclic aromatic hydrocarbon (PAH) and lineemission, arising from a central unresolved region. A more detailedanalysis of our sample, with updated models, will be presented in aforthcoming paper.
| The ACS Virgo Cluster Survey. XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies
We use HST ACS imaging of 100 early-type galaxies in the ACS VirgoCluster Survey to investigate the nature of diffuse star clusters(DSCs). Compared to globular clusters (GCs), these star clusters havelow luminosities (MV>-8) and a broad distribution of sizes(320 magarcsec-2). The median colors of diffuse star cluster systems(1.1
| A Chandra Survey of Early-Type Galaxies. I. Metal Enrichment in the Interstellar Medium
We present a Chandra study of the emission-weighted metal abundances in28 early-type galaxies, spanning ~3 orders of magnitude in X-rayluminosity (LX). We report constraints for Fe, O, Ne, Mg, Si,S, and Ni. We find no evidence of the very subsolar Fe abundance(ZFe) historically reported, confirming a trend in recentobservations of bright galaxies and groups, nor do we find anycorrelation between ZFe and luminosity. Excepting one case,the ISM is single-phase, indicating that multitemperature fits foundwith ASCA reflected temperature gradients that we resolve with Chandra.We find no evidence that ZFe (ISM) is substantially lowerthan the stellar metallicity estimated from simple stellar populationmodels. In general, these quantities are similar, which is inconsistentwith galactic wind models and recent hierarchical chemical enrichmentsimulations. Our abundance ratio constraints imply that 66%+/-11% of theISM Fe was produced in SNe Ia, similar to the solar neighborhood,indicating similar enrichment histories for elliptical galaxies and theMilky Way. Although these values are sensitive to the considerablesystematic uncertainty in the supernova yields, they are in agreementwith observations of more massive systems. This indicates considerablehomology in the enrichment process operating from cluster scales tolow-to-intermediate-LX galaxies. The data uniformly exhibitlow ZO/ZMg ratios, which have been reported insome clusters, groups, and galaxies. This is inconsistent with standardSN II metal yield calculations and may indicate an additional source ofenrichment, such as Population III hypernovae.
| 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.
| Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton
We investigated the dynamical structure of 53 elliptical galaxies usingthe Chandra archival X-ray data. In X-ray-luminous galaxies, temperatureincreases with radius and gas density is systematically higher at theoptical outskirts, indicating the presence of a significant amount ofthe group-scale hot gas. In contrast, X-ray-dim galaxies show a flat ordeclining temperature profile against radius and the gas density isrelatively lower at the optical outskirts. Thus, it is found thatX-ray-bright and faint elliptical galaxies are clearly distinguished bythe temperature and gas density profile. The mass profile is well scaledby a virial radius r200 rather than an optical half-radiusre, is quite similar at (0.001-0.03)r200 betweenX-ray-luminous and dim galaxies, and smoothly connects to those profilesof clusters of galaxies. At the inner region of(0.001-0.01)r200 or (0.1-1)re, the mass profilewell traces a stellar mass with a constant mass-to-light ratio ofM/LB=3-10 Msolar/Lsolar. TheM/LB ratio of X-ray-bright galaxies rises up steeply beyond0.01r200 and thus requires a presence of massive dark matterhalo. From the deprojection analysis combined with the XMM-Newton data,we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 alsohave a high M/LB ratio of 20-30 at 20 kpc, comparable to thatof X-ray-luminous galaxies. Therefore, dark matter is indicated to becommon in elliptical galaxies; their dark matter distribution, as wellas that of galaxy clusters, almost follows the NFW profile.
| Modelling the Mg[b] Spectroscopic Index with the Tunable Filters of OSIRIS
We present our first attempt to model a new metallicity indicator forold and intermediate aged stellar populations on the basis of the Mg[b]absorption index at λ 5175 Å. Model spectra ofstellar populations are convolved with the responses of the TunableFilters of OSIRIS-GTC to provide a photometric index matching thespectroscopic Mg[b]. The new index allows us to obtain galaxy images ofthis feature on the basis of a photometric approach, without the veryhigh signal-to-noise requirements of the spectroscopic methods.
| 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.
| O VI in Elliptical Galaxies: Indicators of Cooling Flows
Early-type galaxies often contain a hot X-ray-emitting interstellarmedium [(3-8)×106 K] with an apparent radiative coolingtime much less than a Hubble time. If unopposed by a heating mechanism,the gas will radiatively cool to temperatures <~104 K at arate proportional to LX/TX, typically 0.03-1Msolar yr-1. We can test whether gas is coolingthrough the 3×105 K range by observing the O VIdoublet, whose luminosity is proportional to the cooling rate. Here wereport on a study of an unbiased sample of 24 galaxies, obtaining FarUltraviolet Spectroscopic Explorer spectra to complement the X-ray dataof ROSAT and Chandra. The O VI line emission was detected in about 40%of the galaxies and at a luminosity level similar to the prediction fromthe cooling flow model. There is a correlation betweenM˙OVI and M˙X, although there issignificant dispersion about the relationship, where the O VI isbrighter or dimmer than expected by a factor of 3 or more. If thecooling flow picture is to be retained, then this dispersion requiresthat cooling flows be time-dependent, as might occur by the activity ofan AGN. However, of detected objects, those with the highest or lowestvalues of M˙OVI/M˙X are not systematicallyhot or cool, as one might predict from AGN heating.
| 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.
| A Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies
We report on the discovery of a new correlation between globalparameters of the hot interstellar gas in elliptical galaxies. Wereanalyze archival Chandra data for 30 normal early-type systems,removing the contributions of resolved and unresolved point sources toreveal the X-ray morphology of the hot gas. We determine the half-lightradius, RX, and the mean surface brightness, IX,from the gas surface brightness profiles. A spectral analysis determinesthe temperature, TX, of the gas within 3 optical effectiveradii. We find that the galaxies lie on an X-ray gas fundamental plane(XGFP) of the formTX~R0.28XI0.22X.This is close to, but distinct from, a simple luminosity-temperaturerelation. The intrinsic width of the XGFP is only 0.07 dex, nearlyidentical to that of the stellar (optical) fundamental plane (SFP). Thisis surprising since X-ray gas masses are typically ~10-2 ofthe stellar masses. We show that the XGFP is not a simple consequence ofthe virial theorem or hydrostatic equilibrium and that it is essentiallyindependent of the SFP. The XGFP thus represents a genuinely newconstraint on the hydrodynamical evolution of elliptical galaxies.
| The Birthplace of Low-Mass X-Ray Binaries: Field Versus Globular Cluster Populations
Recent Chandra studies of low-mass X-ray binaries (LMXBs) withinearly-type galaxies have found that LMXBs are commonly located withinglobular clusters of the galaxies. However, whether all LMXBs are formedwithin globular clusters has remained an open question. If all LMXBsformed within globular clusters, the summed X-ray luminosity of theLMXBs in a galaxy should be directly proportional to the number ofglobular clusters in the galaxy regardless of where the LMXBs currentlyreside. We have compared these two quantities over the same angular areafor a sample of 12 elliptical and S0 galaxies observed with Chandra andfound that the correlation between the two quantities is weaker thanexpected if all LMXBs formed within globular clusters. This indicatesthat a significant number of the LMXBs were formed in the field andnaturally accounts for the spread in field-to-cluster fractions of LMXBsfrom galaxy to galaxy. We also find that the ``pollution'' of globularcluster LMXBs into the field has been minimal within ellipticalgalaxies, but there is evidence that roughly half of the LMXBsoriginally in the globular clusters of S0 galaxies in our sample haveescaped into the field. This is likely due to higher globular clusterdisruption rates in S0s, resulting from stronger gravitational shockscaused by the passage of globular clusters through the disks of S0galaxies that are absent in elliptical 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.
| 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.
| 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.
| 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.
| 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.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| 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.
| 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.
| Formation and evolution of S0 galaxies: a SAURON case study of NGC 7332
We present SAURON integral-field observations of the S0 galaxy NGC7332. Existing broad-band ground-based and Hubble Space Telescope (HST)photometry reveals a double-disc structure and a boxy bulge interpretedas a bar viewed close to edge-on. The SAURON two-dimensional stellarkinematic maps confirm the existence of the bar and inner disc but alsouncover the presence of a cold counter-rotating stellar component withinthe central 250 pc. The Hβ and [O III] emission line maps show thatthe ionized gas has a complex morphology and kinematics, including botha component counter-rotating with respect to the stars and a faintercorotating one. Analysis of the absorption line-strength maps show thatNGC 7332 is young everywhere. The presence of a large-scale bar canexplain most of those properties, but the fact that we see a significantamount of unsettled gas, together with a few peculiar features in themaps, suggests that NGC 7332 is still evolving. Interactions as well asbar-driven processes must thus have played an important role in theformation and evolution of NGC 7332, and presumably of S0 galaxies ingeneral.
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