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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.
| 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.
| Photochemical evolution of elliptical galaxies - II. The impact of merging-induced starbursts
The effects of late gas accretion episodes and subsequent merger-inducedstarbursts on the photochemical evolution of elliptical galaxies arestudied and compared to the picture of galaxy formation occurring athigh redshift with a unique and intense starburst modulated by a veryshort infall, as suggested by Pipino and Matteucci in Paper I. By meansof the comparison with the colour-magnitude relations (CMRs) and the[V]-σ relation observed in ellipticals, weconclude that either bursts involving a gas mass comparable to the massalready transformed into stars during the first episode of starformation (SF) and occurring at any redshift, or bursts occurring at lowredshift (i.e. z<= 0.2) and with a large range of accreted mass, areruled out. These models fail in matching the above relations even if theinitial infalling hypothesis is relaxed, and the galaxies form either bymeans of more complicated SF histories or by means of the classicalmonolithic model. On the other hand, galaxies accreting a small amountof gas at high redshift (i.e. z>= 3) produce a spread in the modelresults, with respect to the best model of Paper I, which is consistentwith the observational scatter of the CMRs, although there is onlymarginal agreement with the [V]-σrelation. Therefore, only small perturbations to the standard scenarioseem to be allowed. We stress that the strongest constraints togalaxy-formation mechanisms are represented by the chemical abundances,whereas the colours can be reproduced under several differenthypotheses.
| The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions
The mean ages of early-type galaxies obtained from the analysis ofoptical spectra give a mean age of 8 Gyr at z=0, with 40% being youngerthan 6 Gyr. Independent age determinations are possible by usinginfrared spectra (5-21 μm), which we have obtained with the InfraredSpectrograph on Spitzer. This age indicator is based on the collectivemass-loss rate of stars, in which mass loss from AGB stars produces asilicate emission feature at 9-12 μm. This feature decreases morerapidly than the shorter wavelength continuum as a stellar populationages, providing an age indicator. From observations of 30 nearbyearly-type galaxies, 29 show a spectral energy distribution dominated bystars, and one has significant emission from the ISM and is excluded.The infrared age indicators for the 29 galaxies show them all to be old,with a mean age of about 10 Gyr and a standard deviation of only a fewGyr. This is consistent with the ages inferred from the values ofM/LB, but is inconsistent with the ages derived from theoptical line indices, which can be much younger. All of these ageindicators are luminosity weighted and should be correlated, even ifmultiple-age components are considered. The inconsistency indicates thatthere is a significant problem with either the infrared and theM/LB ages, which agree, or with the ages inferred from theoptical absorption lines.
| Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. II. Optical Study and Interpretation
Our X-ray study of the nuclear activity in a new sample of six quiescentearly-type galaxies, as well as in a larger sample from the literature,confirmed (Paper I) that the Bondi accretion rate of diffuse hot gas isnot a good indicator of the SMBH X-ray luminosity. Here we suggest thata more reliable estimate of the accretion rate must include the gasreleased by the stellar population inside the sphere of influence of theSMBH, in addition to the Bondi inflow of hot gas across that surface. Weuse optical surface brightness profiles to estimate the mass-loss ratefrom stars in the nuclear region: we show that for our sample ofgalaxies it is an order of magnitude higher (~10-4 to10-3 Msolar yr-1) than the Bondi inflowrate of hot gas, as estimated from Chandra (Paper I). Only by takinginto account both sources of fuel can we constrain the true accretionrate, the accretion efficiency, and the power budget. Radiativelyefficient accretion is ruled out, for quiescent SMBHs. For typicalradiatively inefficient flows, the observed X-ray luminosities of theSMBHs imply accretion fractions ~1%-10% (i.e., ~90%-99% of the availablegas does not reach the SMBH) for at least five of our six targetgalaxies and most of the other galaxies with known SMBH masses. Wediscuss the conditions for mass conservation inside the sphere ofinfluence, so that the total gas injection is balanced by accretion plusoutflows. We show that a fraction of the total accretion power(mechanical plus radiative) would be sufficient to sustain aself-regulating, slow outflow that removes from the nuclear region allthe gas that does not sink into the BH (``BH feedback''). The rest ofthe accretion power may be carried out in a jet or advected. We alsodiscuss scenarios that would lead to an intermittent nuclear activity.
| Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. I. X-Ray Study
We have studied the nuclear activity in a sample of six quiescentearly-type galaxies, with new Chandra data and archival HST opticalimages. Their nuclear sources have X-ray luminosities~1038-1039 ergs s-1(LX/LEdd~10-8 to 10-7) andcolors or spectra consistent with accreting supermassive black holes(SMBHs), except for the nucleus of NGC 4486B, which is softer thantypical AGN spectra. In a few cases, the X-ray morphology of the nuclearsources shows hints of marginally extended structures, in addition tothe surrounding diffuse thermal emission from hot gas, which isdetectable on scales >~1 kpc. In one case (NGC 5845), a dusty diskmay partially obstruct our direct view of the SMBH. We have estimatedthe temperature and density of the hot interstellar medium, which is onemajor source of fuel for the accreting SMBH; typical central densitiesare ne~(0.02+/-0.01) cm-3. Assuming that the hotgas is captured by the SMBH at the Bondi rate, we show that the observedX-ray luminosities are too faint to be consistent with standard diskaccretion, but brighter than predicted by radiatively inefficientsolutions (e.g., advection-dominated accretion flows [ADAFs]). In total,there are ~20 galaxies for which SMBH mass, hot gas density, and nuclearX-ray luminosity are simultaneously known. In some cases, the nuclearsources are brighter than predicted by the ADAF model; in other cases,they are consistent or fainter. We discuss the apparent lack ofcorrelations between Bondi rate and X-ray luminosity and suggest that,in order to understand the observed distribution, we need to know twoadditional parameters: the amount of gas supplied by the stellarpopulation inside the accretion radius, and the fraction (possibly<<1) of the total gas available that is accreted by the SMBH. Weleave a detailed study of these issues to a subsequent paper.
| The Outside-In Formation of Elliptical Galaxies
In this paper we compare the predictions of a detailed multizonechemical evolution model for elliptical galaxies with the very recentobservations of the galaxy NGC 4697. In particular, the model allows foran initial gas infall and a subsequent galactic wind; it takes intoaccount detailed nucleosynthesis prescriptions of both Types II and Iasupernovae and reproduces the main photochemical properties of normalelliptical galaxies. As a consequence of the earlier development of thewind in the outer regions with respect to the inner ones, we predict anincrease of the mean stellar [] ratio with radius, in verygood agreement with the data for NGC 4697. This finding stronglysupports the proposed outside-in formation scenario for ellipticalgalaxies. We also calculate the theoretical ``G-dwarf'' distributions ofstars as functions of both metallicity ([Z/H]) and [Fe/H], showing thatthey are broad and asymmetric so that a SSP cannot correctly mimic themixture of stellar populations at any given radius. We also compute thestellar distribution as a function of the [Mg/Fe] ratio, which hasnegligible ``skewness'' and is narrower than functions of [Z/H] and[Fe/H] and hence can be better represented by a SSP with an abundanceratio given by the average [] ratio. Moreover, we computethe luminosity distributions of stars for a typical elliptical galaxy asfunctions of the [Z/H], [Fe/H], and [Mg/Fe] ratios. We find that thesedistributions differ from the G-dwarf distributions especially at largeradii, except for that as a function of [Mg/Fe]. Therefore, we concludethat in elliptical galaxies the [Mg/Fe] ratio is the most reliablequantity to be compared with observations and is the best estimator ofthe star formation timescale at each radius.
| Type I Ultraluminous Infrared Galaxies: Transition Stage from ULIRGs to QSOs
We examine whether the ultraluminous infrared galaxies that contain atype 1 Seyfert nucleus (a type I ULIRG) are in the transition stage fromULIRGs to quasi-stellar objects (QSOs). To investigate this issue, wecompare the black hole (BH) mass, the bulge luminosity, and thefar-infrared luminosity among type I ULIRGs, QSOs, and ellipticalgalaxies. As a result, we find the following results: (1) The type IULIRGs have systematically smaller BH masses in spite of the comparablebulge luminosity relative to QSOs and elliptical galaxies. (2) Thefar-infrared luminosity of most type I ULIRGs is larger than theEddington luminosity. We show that the above results do not changesignificantly for three type I ULIRGs for which we can estimate thevisual extinction from the column density. Also, for all eight type IULIRGs, we investigate the effect of uncertainties of BH massmeasurements and our sample bias to make sure that our results are notaltered even if we consider the above two effects. In addition, Anabukirecently revealed that their X-ray properties are similar to those ofthe narrow-line Seyfert 1 galaxies. These would indicate that activegalactic nuclei (AGNs) with a high mass accretion rate exist in type IULIRGs. On the basis of all of these findings, we conclude that it wouldbe a natural interpretation that type I ULIRGs are the early phase of BHgrowth, namely, the missing link between ULIRGs and QSOs. Moreover, bycomparing our results with a theoretical model of a coevolution scenarioof a QSO BH and a galactic bulge, we show clearly that this explanationcould be valid.
| On the Correlations of Massive Black Holes with Their Host Galaxies
We address the correlations of black hole (BH) mass with four differenthost-galaxy properties from 11 existing data sets. For the purpose ofguiding theoretical understanding, we first try to quantify thetightness of the intrinsic correlations. We assume that all of therelations are power laws and perform linear regressions that aresymmetric in the two variables on the logarithms of the data points.Given the estimated measurement errors, we evaluate the probabilitydistribution of the residual variance in excess of that expected fromthe measurement errors. Our central result is that the current data setsdo not allow definite conclusions regarding the quality of the truecorrelations because the obtained probability distributions for theresidual variance overlap for most quantities. Velocity dispersion ascollected by Merritt & Ferrarese (σMF) and galaxylight concentration as measured by Graham and coworkers (CRe)are consistent with zero residual variance. Taken at face value, thismeans that these two correlations are better than the others, but thisconclusion is highly sensitive to the assumed measurement errors andwould be undone if the present estimated errors were too large. We thenconsider which of the relations offer the best inferences of BH masswhen there is no direct measurement available. As with the residualvariances, we find that the probability distribution of expecteduncertainty in inferred BH masses overlaps significantly for most of therelations. Photometric methods would then be preferred because the dataare easier to obtain, as long as bulge-disk decomposition or detailedmodeling of the photometric profile (as studied by Graham and coworkers)do not present problems. Determining which correlation offers the bestinferences requires reducing the uncertainty in the expected error inthe inferred BH masses (the ``error on the error''). This uncertainty iscurrently limited by uncertainty in the residual variance for all of therelations. The only quantities for which BH mass inferences are limitedby measurement error are σMF and CRe.Therefore, if these relations are truly better than the others, thennew, improved measurements should allow improved inferences of BHmasses. If they do not, the conclusion must be that the present lowresidual variances for these two relations result from overestimatederror bars.
| Low-Luminosity Active Galaxies and Their Central Black Holes
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
| The AMIGA sample of isolated galaxies. II. Morphological refinement
We present a refinement of the optical morphologies for galaxies in theCatalog of Isolated Galaxies that forms the basis of the AMIGA (Analysisof the interstellar Medium of Isolated GAlaxies) project. Uniformreclassification using the digitized POSS II data benefited from thehigh resolution and dynamic range of that sky survey. Comparison withindependent classifications made for an SDSS overlap sample of more than200 galaxies confirms the reliability of the early vs. late-typediscrimination and the accuracy of spiral subtypes within Δ T =1-2. CCD images taken at the Observatorio de Sierra Nevada were alsoused to solve ambiguities in early versus late-type classifications. Aconsiderable number of galaxies in the catalog (n = 193) are flagged forthe presence of nearby companions or signs of distortion likely due tointeraction. This most isolated sample of galaxies in the local Universeis dominated by two populations: 1) 82% are spirals (Sa-Sd) with thebulk being luminous systems with small bulges (63% between types Sb-Sc)and 2) a significant population of early-type E-S0 galaxies (14%). Mostof the types later than Sd are low luminosity galaxies concentrated inthe local supercluster where isolation is difficult to evaluate. Thelate-type spiral majority of the sample spans a luminosity rangeMB-corr = -18 to -22 mag. Few of the E/S0 population are moreluminous than -21.0 marking the absence of the often-sought superL* merger (e.g. fossil elliptical) population. The rarity ofhigh luminosity systems results in a fainter derived M* forthis population compared to the spiral optical luminosity function(OLF). The E-S0 population is from 0.2 to 0.6 mag fainter depending onhow the sample is defined. This marks the AMIGA sample as unique amongsamples that compare early and late-type OLFs separately. In othersamples, which always involve galaxies in higher density environments,M^*_E/S0 is almost always 0.3-0.5 mag brighter than M^*_S, presumablyreflecting a stronger correlation between M* andenvironmental density for early-type galaxies.
| Peculiarities and populations in elliptical galaxies. III. Dating the last star formation event
Using 6 colours and 4 Lick line-indices we derive two-component modelsof the populations of ellipticals, involving a "primary" and a"juvenile" population. The first component is defined by the regressionsof indices against the central velocity dispersion found in Papers I andII for the {Nop} sample of non-peculiar objects. The second one isapproximated by an SSP, and the modeling derives its age A, metallicityZ and fractional V-luminosity q_V, the fractional mass qMbeing found therefrom. The model is designed for "blueish" peculiargalaxies, i.e. the {Pec} sample and NGC 2865 family in the terminologyof Paper I. The morphological peculiarities and the population anomalyare then believed to involve the same event, i.e. a merger plusstarburst. It is possible to improve the models in a few cases byintroducing diffuse dust (as suggested by far IR data), and/or by takinginto account the fact that Lick- and colour indices do not relate toidentical galaxy volumes. In most of the cases, the mass ratio of youngstars qM seems too small for the product of a recent majormerger: the events under consideration might be minor mergers bringing"the final touch" to the build-up of the structure of the E-type object.The same modeling has been successfully applied to blueish galaxies ofthe {Nop} sample, without morphological peculiarities however, tosupport the occurence of a distinct perturbing event. A few reddishobjects of the {Pec} sample (NGC 3923 family) and of the {Nop} sampleare also modeled, in terms of an excess of high metallicity stars, ordiffuse dust, or both, but the results are inconclusive.
| Wide-field kinematics of globular clusters in the Leo I group
We present wide-field spectroscopy of globular clusters around theLeo I group galaxies NGC 3379 andNGC 3384 using the FLAMES multi-fibre instrument atthe VLT. We obtain accurate radial velocities for 42 globular clusters(GCs) in total, 30 for GCs around the elliptical NGC 3379, eight aroundthe lenticular NGC 3384, and four which may be associated with eithergalaxy. These data are notable for their large radial range extendingfrom 0.7 arcmin to 14.6 arcmin (2 to 42 kpc) from the centre of NGC3379, and small velocity uncertainties of about 10 km s-1. Wecombine our sample of 30 radial velocities for globular clusters aroundNGC 3379 with 8 additional GC velocities from the literature, and find aprojected velocity dispersion of σp =175+24-22 km s-1 at R < 5' andσp = 147+44-39 at R > 5'.These velocity dispersions are consistent with a dark matter halo aroundNGC 3379 with a concentration in the range expected from a ΛCDMcosmological model and a total mass of ≈ 6 × 1011~Mȯ. Such a model is also consistent with the stellarvelocity dispersion at small radii and the rotation of the H I ring atlarge radii, and has a (M/L)B that increases by a factor offive from several kpc to 100 kpc. Our velocity dispersion for theglobular cluster system of NGC 3379 is somewhat higher than that foundfor the planetary nebulae (PNe) in the inner region covered by the PNdata, and we discuss possible reasons for this difference. For NGC 3384,we find the GC system has a rotation signature broadly similar to thatseen in other kinematic probes of this SB0 galaxy. This suggests thatsignificant rotation may not be unusual in the GC systems of discgalaxies.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| Lost and found dark matter in elliptical galaxies
There is strong evidence that the mass of the Universe is dominated bydark matter, which exerts gravitational attraction but whose exactnature is unknown. In particular, all galaxies are believed to beembedded in massive haloes of dark matter. This view has recently beenchallenged by the observation of surprisingly low random stellarvelocities in the outskirts of ordinary elliptical galaxies, which hasbeen interpreted as indicating a lack of dark matter. Here we show thatthe low velocities are in fact compatible with galaxy formation indark-matter haloes. Using numerical simulations of disk-galaxy mergers,we find that the stellar orbits in the outer regions of the resultingellipticals are very elongated. These stars were torn by tidal forcesfrom their original galaxies during the first close passage and put onoutgoing trajectories. The elongated orbits, combined with the steeplyfalling density profile of the observed tracers, explain the observedlow velocities even in the presence of large amounts of dark matter.Projection effects when viewing a triaxial elliptical can lead to evenlower observed velocities along certain lines of sight.
| 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.
| Dark matter in early-type galaxies: dynamical modelling of IC 1459, IC 3370, NGC 3379 and NGC 4105
We analyse long-slit spectra of four early-type galaxies which extendfrom ~1 to 3 effective radii: IC 1459; IC 3370; NGC 3379 and NGC 4105.We have extracted the full line-of-sight velocity distribution (in thecase of NGC 3379 we also used data from the literature), which we modelusing the two-integral approach. Using two-integral modelling, we findno strong evidence for dark haloes, but the fits suggest thatthree-integral modelling is necessary. We also find that the inferredconstant mass-to-light ratio in all the four cases is typical forearly-type galaxies. Finally, we also discuss the constraints on themass-to-light ratio, which can be obtained using X-ray haloes in thecase of IC 1459, NGC 3379 and NGC 4105, and compare the estimated valueswith the predictions from the dynamical modelling.
| Spatially resolved stellar populations in the isolated elliptical NGC 821
We present the analysis of Lick absorption-line indices from threeseparate long-slit spectroscopic observations of the nearby isolatedelliptical galaxy NGC 821. The three data sets present a consistentpicture of the stellar population within one effective radius, in whichstrong gradients are evident in both luminosity-weighted age andmetallicity. The central population exhibits a young age of ~4 Gyr and ametallicity ~3 times solar. At one effective radius the age has risen to~12 Gyr and the metallicity fallen to less than ~1/3 times solar. Thelow-metallicity population around one effective radius appears to havean exclusively red horizontal branch (RHB), with no significantcontribution from the blue horizontal branch evident in some globularclusters of the same age and metallicity. Despite the strong central agegradient, we demonstrate that only a small fraction (<=10 per cent)of the galaxy's stellar mass can have been created in recent starformation events. We consider possible star formation histories for NGC821 and find that the most likely cause of the young central populationwas a minor merger or tidal interaction that caused NGC 821 to consumeits own gas in a centrally concentrated burst of star formation 1-4 Gyrago.
| The correlation of metallicity gradient with galaxy mass
A number of previous studies have searched for a correlation betweenradial metallicity gradients and early-type galaxy mass - no convincingtrends have been found. Here we re-examine this issue with several keyenhancements: using total metallicity from studies that have broken theage-metallicity degeneracy, excluding galaxies with young stellar ages(i.e. those that have experienced a recent central starburst) and usingthe K band to derive galaxy luminosities. We find that Coma clustergalaxies have metallicity gradients that correlate with galaxy mass.Furthermore, gradients have values similar to those of monolithiccollapse models. The combination of dissipative formation and energyinjection from supernovae provides a mechanism for the trends withgalaxy mass; however, other explanations are possible. Additionalhigh-quality observational data are needed to constrain further the gasphysics involved in galaxy formation.
| The fundamental plane of isolated early-type galaxies
Here we present new measurements of effective radii, surfacebrightnesses and internal velocity dispersions for 23 isolatedearly-type galaxies. The photometric properties are derived from newmulticolour imaging of ten galaxies, whereas the central kinematics forseven galaxies are taken from forthcoming work by Hau & Forbes.These are supplemented with data from the literature. We reproduce thecolour-magnitude and Kormendy relations and strengthen the result of therecent work of Reda et al. that isolated galaxies follow the samephotometric relations as galaxies in high-density environments. We alsofind that some isolated galaxies reveal fine structure indicative of arecent merger, while others appear undisturbed. We examine theFundamental Plane both in the traditional Re,μe, σ space and also in κ space. Most isolatedgalaxies follow the same Fundamental Plane tilt and scatter for galaxiesin high-density environments. However, a few galaxies notably deviatefrom the Plane in the sense of having smaller M/L ratios. This can beunderstood in terms of their younger stellar populations, which arepresumably induced by a gaseous merger. Overall, isolated galaxies havesimilar properties to those in groups and clusters with a slightenhancement in the frequency of recent mergers/interactions.
| Group, field and isolated early-type galaxies - II. Global trends from nuclear data
We have derived ages, metallicities and enhanced-element ratios[α/Fe] for a sample of 83 early-type galaxies essentially ingroups, the field or isolated objects. The stellar-population propertiesderived for each galaxy correspond to the nuclear re/8aperture extraction. The median age found for Es is 5.8+/-0.6 Gyr andthe average metallicity is +0.37+/-0.03 dex. For S0s, the median age is3.0+/-0.6 Gyr and [Z/H]= 0.53+/-0.04 dex. We compare the distribution ofour galaxies in the Hβ-[MgFe] diagram with Fornax galaxies. Ourelliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster.We find that the galaxies lie in a plane defined by [Z/H]= 0.99logσ0- 0.46 log(age) - 1.60, or in linear terms Z~σ0× (age) -0.5. More massive (largerσ0) and older galaxies present, on average, large[α/Fe] values, and therefore must have undergone shorterstar-formation time-scales. Comparing group against field/isolatedgalaxies, it is not clear that environment plays an important role indetermining their stellar-population history. In particular, ourisolated galaxies show ages differing by more than 8 Gyr. Finally weexplore our large spectral coverage to derive log(O/H) metallicity fromthe Hα and NIIλ6584 and compare it with model-dependent[Z/H]. We find that the O/H abundances are similar for all galaxies, andwe can interpret it as if most chemical evolution has already finishedin these galaxies.
| Mass-to-light ratio gradients in early-type galaxy haloes
Owing to the fact that the near future should see a rapidly expandingset of probes of the halo masses of individual early-type galaxies, weintroduce a convenient parameter for characterizing the halo masses fromboth observational and theoretical results:∇lΥ, the logarithmic radial gradient of themass-to-light ratio. Using halo density profiles from Λ-cold darkmatter (CDM) simulations, we derive predictions for this gradient forvarious galaxy luminosities and star formation efficienciesɛSF. As a pilot study, we assemble the available∇lΥ data from kinematics in early-type galaxies- representing the first unbiased study of halo masses in a wide rangeof early-type galaxy luminosities - and find a correlation betweenluminosity and ∇lΥ, such that the brightestgalaxies appear the most dark-matter dominated. We find that thegradients in most of the brightest galaxies may fit in well with theΛCDM predictions, but that there is also a population of faintergalaxies whose gradients are so low as to imply an unreasonably highstar formation efficiency ɛSF > 1. This difficultyis eased if dark haloes are not assumed to have the standard ΛCDMprofiles, but lower central concentrations.
| Group, field and isolated early-type galaxies - I. Observations and nuclear data
This is the first paper of a series on the investigation of stellarpopulation properties and galaxy evolution of an observationallyhomogeneous sample of early-type galaxies in groups, field and isolatedgalaxies.Here we present high signal-to-noise ratio (S/N) long-slit spectroscopyof 86 nearby elliptical and S0 galaxies. Eight of them are isolated,selected according to a rigorous criterion, which guarantees a genuinelow-density subsample. The present survey has the advantage of coveringa larger wavelength range than normally found in the literature, whichincludes [OIII]λ5007 and Hα, both lines important foremission correction. Among the 86 galaxies with S/N >= 15 (perresolution element, for re/8 central aperture), 57 have theirHβ-index corrected for emission (the average correction is 0.190Åin Hβ) and 42 galaxies reveal [OIII]λ5007 emission,of which 16 also show obvious Hα emission. Most of the galaxies inthe sample do not show obvious signs of disturbances nor tidal featuresin the morphologies, although 11 belong to the Arp catalogue of peculiargalaxies; only three of them (NGC 750, 751 and 3226) seem to be stronglyinteracting. We present the measurement of 25 central line-strengthindices calibrated to the Lick/IDS system. Kinematic information isobtained for the sample. We analyse the line-strength index versusvelocity dispersion relations for our sample of mainly low-densityenvironment galaxies, and compare the slope of the relations withcluster galaxies from the literature. Our main findings are that theindex-σ0 relations presented for low-density regionsare not significantly different from those of cluster E/S0s. The slopeof the index-σ0 relations does not seem to change forearly-type galaxies of different environmental densities, but thescatter of the relations seems larger for group, field and isolatedgalaxies than for cluster galaxies.
| Planetary Nebulae and Stellar Kinematics in the Flattened Elliptical Galaxy NGC 1344
We present photometric and kinematic information obtained by measuring197 planetary nebulae (PNs) discovered in the flattened Fornaxelliptical galaxy NGC 1344 (also known as NGC 1340) with an on-band,off-band, and grism+on-band filter technique. We build the PN luminosityfunction (PNLF) and use it to derive a distance modulus m-M=31.4+/-0.18,which is slightly smaller than, but in good agreement with, the surfacebrightness fluctuation distance. The PNLF also provides an estimate ofthe specific PN formation rate: (6+/-3)×10-12 PNsyr-1 L-1solar. If we combine thepositional information from the on-band image with PN positions measuredon the grism+on-band image, we can measure the radial velocities of 195PNs, some of them distant more than three effective radii from thecenter of NGC 1344. We complement this data set with stellar kinematicsderived from integrated spectra along the major and minor axes andparallel to the major axis of NGC 1344. The line-of-sight velocitydispersion profile indicates the presence of a dark matter halo aroundthis galaxy.Some of the data presented herein were obtained at the European SouthernObservatory, Chile, in programs ESO 67.B-0231 and 68.B-0173A.
| A Possible Detection of M31* with Chandra
Two independent sets of Chandra and HST images of the nuclear region ofM31 allow registration of X-ray and optical images to ~0.1". Thisregistration shows that none of the bright (~1037 ergss-1) X-ray sources near the nucleus is coincident with thecentral supermassive black hole, M31*. A 50 ks Chandra HRC image shows2.5 σ evidence for a faint (~1036 ergs s-1)discrete source that is consistent with the position of M31*. The Bondiradius of M31* is 0.9", making it one of the few supermassive blackholes with a resolvable accretion flow. This large radius and theprevious detections of diffuse X-ray-emitting gas in the nuclear regionmake M31* one of the most secure cases for a radiatively inefficientaccretion flow and place some of the most severe constraints on theradiative processes in such a flow.
| Subarcsecond Structure and Velocity Field of Optical Line-emitting Gas in NGC 1052
We have obtained integral field spectra of the low-ionizationemission-line region in the galaxy NGC 1052 by using the KyotoTridimensional Spectrograph II mounted on the Subaru Telescope. Our highsignal-to-noise ratio data with precise template subtraction haverevealed weaker features at the nucleus, including the [Fe III] and HeII emission lines, as well as a broad component of the Hβ emission.The broad Hβ component suggests the existence of a broad-lineregion. The spatial structure and velocity field derived from the datacube suggest the existence of three main components: a high-velocitybipolar outflow, low-velocity disk rotation, and a spatially unresolvednuclear component. The outflow axis does not coincide with the diskrotation axis. The opening angle of the outflow decreases with velocityshift from the systemic velocity both in bluer and redder velocitychannels. This is explained only if the outflow has intrinsically highervelocity components inside, i.e., in regions closer to the outflow axis.At both sides of the bipolar outflow, we find that the highest velocitycomponents are detached from the nucleus. This gap can be explained byan acceleration of at least a part of the flow or the surroundingmatter, or by bow shocks that may be produced by even higher velocityoutflow components that are not yet detected. Along the edges of theoutflow and extending east-northeast and west-southwest, there existstrong [O III] emission ridges. These are closely related to the radiojet-counterjet structure. The abrupt change in the velocity field of theionized gas and a large [OIII]/Hβ line flux ratio in this regionsuggest a strong interaction of the jets, and possibly also of someridge components of the line-emitting gas, with the interstellar matter.
| Nuclear Accretion in Galaxies of the Local Universe: Clues from Chandra Observations
In order to find an explanation for the radiative quiescence ofsupermassive black holes in the local universe, the most accurateestimates for a sample of nearby galaxies are collected for the mass ofa central black hole (MBH), the nuclear X-ray luminosityLX,nuc, and the circumnuclear hot gas density andtemperature, by using Chandra data. The nuclear X-ray luminosityLX,nuc varies by ~3 orders of magnitude and does not show arelationship with MBH or with the Bondi mass accretion rateM˙B LX,nuc is always much lower than expectedif M˙B ends in a standard accretion disk with highradiative efficiency (this instead can be the case of the active nucleusof Cen A). Radiatively inefficient accretion as in the standardadvection-dominated accretion flow (ADAF) modeling may explain the lowluminosities of a few cases; for others, the predicted luminosity isstill too high, and, in terms of Eddington-scaled quantities, it isincreasingly higher than that observed for increasingM˙B. Variants of the simple radiatively inefficientscenario including outflow and convection may reproduce the low emissionlevels observed, since the amount of matter actually accreted is reducedconsiderably. However, the most promising scenario includes feedbackfrom accretion on the surrounding gas; this has the important advantagesof naturally explaining the observed lack of relationship amongLX,nuc, MBH, and M˙B, and evadingthe problem of the fate of the material accumulating in the centralgalactic regions over cosmological times.
| The Epochs of Early-Type Galaxy Formation as a Function of Environment
The aim of this paper is to set constraints on the epochs of early-typegalaxy formation through the ``archaeology'' of the stellar populationsin local galaxies. Using our models of absorption-line indices thataccount for variable abundance ratios, we derive ages, totalmetallicities, and element ratios of 124 early-type galaxies in high-and low-density environments. The data are analyzed by comparison withmock galaxy samples created through Monte Carlo simulations taking thetypical average observational errors into account, in order to eliminateartifacts caused by correlated errors. We find that all threeparameters, age, metallicity, and α/Fe ratio, are correlated withvelocity dispersion. We show that these results are robust againstrecent revisions of the local abundance pattern at high metallicities.To recover the observed scatter we need to assume an intrinsic scatterof about 20% in age, 0.08 dex in [Z/H], and 0.05 dex in [α/Fe].All low-mass objects withM*<~1010Msolar (σ<~130kms-1) show evidence for the presence of intermediate-agestellar populations with low α/Fe ratios. About 20% of theintermediate-mass objects with1010<~M*/Msolar<~1011[110<~σ/(kms-1)<~230 both elliptical andlenticular galaxies] must have either a young subpopulation or a bluehorizontal branch. On the basis of the above relationships, valid forthe bulk of the sample, we show that the Mg-σ relation is mainlydriven by metallicity, with similar contributions from the α/Feratio (23%) and age (17%). We further find evidence for an influence ofthe environment on the stellar population properties. Massive early-typegalaxies in low-density environments seem on average ~2 Gyr younger andslightly (~0.05-0.1 dex) more metal-rich than their counterparts inhigh-density environments. No offsets in the α/Fe ratios areinstead detected. With the aid of a simple chemical evolution model, wetranslate the derived ages and α/Fe ratios into star formationhistories. We show that most star formation activity in early-typegalaxies is expected to have happened between redshifts ~3 and 5 inhigh-density environments and between redshifts 1 and 2 in low-densityenvironments. We conclude that at least 50% of the total stellar massdensity must have already formed at z~1, in good agreement withobservational estimates of the total stellar mass density as a functionof redshift. Our results suggest that significant mass growth in theearly-type galaxy population below z~1 must be restricted to lessmassive objects, and a significant increase of the stellar mass densitybetween redshifts 1 and 2 should be present, caused mainly by the fieldgalaxy population. The results of this paper further imply the presenceof vigorous star formation episodes in massive objects at z~2-5 andevolved elliptical galaxies around z~1, both observationally identifiedas SCUBA galaxies and extremely red objects, respectively.
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