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|A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph|
We present a catalogue of positions, magnitudes and velocities for 3300emission-line objects found by the Planetary Nebula Spectrograph in asurvey of the Andromeda galaxy, M31. Of these objects, 2615 are foundlikely to be planetary nebulae (PNe) associated with M31. The surveyarea covers the whole of M31's disc out to a radius of . Beyond thisradius, observations have been made along the major and minor axes, andthe Northern Spur and Southern Stream regions. The calibrated data havebeen checked for internal consistency and compared with othercatalogues. With the exception of the very central, high surfacebrightness region of M31, this survey is complete to a magnitude limitof m5007 ~ 23.75, 3.5 mag into the PN luminosity function.We have identified emission-line objects associated with M31'ssatellites and other background galaxies. We have examined the data fromthe region tentatively identified as a new satellite galaxy, AndromedaVIII, comparing it to data in the other quadrants of the galaxy. We findthat the PNe in this region have velocities that appear to be consistentwith membership of M31 itself.The luminosity function of the surveyed PNe is well matched to the usualsmooth monotonic function. The only significant spatial variation in theluminosity function occurs in the vicinity of M31's molecular ring,where the luminosities of PNe on the near side of the galaxy aresystematically ~0.2 mag fainter than those on the far side. Thisdifference can be explained naturally by a modest amount of obscurationby the ring. The absence of any difference in luminosity functionbetween bulge and disc suggests that the sample of PNe is not stronglypopulated by objects whose progenitors are more massive stars. Thisconclusion is reinforced by the excellent agreement between the numbercounts of PNe and the R-band light.The number counts of kinematically selected PNe also allow us to probethe stellar distribution in M31 down to very faint limits. There is noindication of a cut-off in M31's disc out to beyond four scalelengths,and no signs of a spheroidal halo population in excess of the bulge outto 10 effective bulge radii.We have also carried out a preliminary analysis of the kinematics of thesurveyed PNe. The mean streaming velocity of the M31 disc PNe is foundto show a significant asymmetric drift out to large radii. Theirvelocity dispersion, although initially declining with radius, flattensout to a constant value in the outer parts of the galaxy. There are noindications that the disc velocity dispersion varies with PN luminosity,once again implying that the progenitors of PNe of all magnitudes form arelatively homogeneous old population. The dispersion profile andasymmetric drift results are shown to be mutually consistent, butrequire that the disc flares with radius if the shape of its velocityellipsoid remains invariant.
|Planetary nebulae as tracers of galaxy stellar populations|
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
|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.
|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.
|Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT HRI Observations. II. Statistical Properties|
The statistical properties of the nonnuclear X-ray point sources fromthe ROSAT HRI survey of nearby galaxies in Paper I are studied, withparticular attention to the contamination from background and/orforeground objects. This study reveals a statistical preference for theultraluminous X-ray sources (ULXs) to occur in late-type galaxies overearly-type galaxies, and in starburst/H II galaxies over nonstarburstgalaxies. There is a trend of greater occurrence frequencies and ULXrates for galaxies with increasing star formation rates, confirming theconnection between the ULX phenomenon and the star formation. Anonlinear correlation is found between the number of ULXs and the starformation rate, with significantly more ULXs at low star formation ratesthan the ULX population expected from the high-mass X-ray binaries(HMXBs) as an indicator of the star formation and the accompanying youngstellar population, suggestive of another population of ULXs associatedwith the low-mass X-ray binaries (LMXBs) and the old stellar population.There are no breaks around 1039 ergs s-1 in theluminosity functions of ULXs in all galaxies or in late-type galaxies,suggesting the regular ULXs below 1040 ergs s-1are a high-luminosity extension of the ordinary HMXB/LMXB populationsbelow 1039 ergs s-1. There is evidence that theextreme ULXs above 1040 ergs s-1 might be adifferent ULX class from the regular ULXs below 1040 ergss-1, although a larger sample with more ULXs is needed toestablish the statistical properties of the extreme ULXs as a class.
|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.
|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.
|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.
|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.
|XMM-Newton Observation of Diffuse Gas and Low-Mass X-Ray Binaries in the Elliptical Galaxy NGC 4649 (M60)|
We present an XMM-Newton X-ray observation of the X-ray-bright E2elliptical galaxy NGC 4649. In addition to bright diffuse emission, weresolve 158 discrete sources, ~50 of which are likely to be LMXBsassociated with NGC 4649. We find evidence for variability in threesources between this observation and a previous Chandra observation.Additionally, we detect five sources that were not detected with Chandradespite its better detection limit, suggesting that these sources havesince brightened. The total X-ray spectrum of the resolved sources iswell fit by a hard power law, while the diffuse spectrum requires a hardand a soft component, presumably due to the relatively soft diffuse gasand the harder unresolved sources. A deprojection of the diffuseemission revealed a radial temperature gradient that is hot in thecenter, drops to a minimum at about 20"-50" (1.6-4.1 kpc), and risesagain in the outer regions. The diffuse emission appears to require atwo-temperature model with heavy-element abundance ratios that differfrom the solar values. We have verified the existence of faint radialfeatures extending out from the core of NGC 4649 that had previouslybeen seen with Chandra. The fingers are morphologically similar toradial features seen in hydrodynamic simulations of cooling flows inelliptical galaxies, and although their other properties do not matchthe predictions of the particular simulations used, we conclude that theradial fingers might be due to convective motions of hot outflowing gasand cooler inflowing gas. We also find evidence for a longer, previouslyundetected filament that extends to the northeastern edge of NGC 4649.The diffuse gas in the region of the filament appears to have a lowertemperature and may also have a higher abundance as compared to nearbyregions. There also appears to be an excess of X-ray sources along thefilament, although the excess is not statistically significant. Weconclude that the filament may be the result of a tidal interaction,possibly with NGC 4647, although more work is necessary to verify thisconclusion.
|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.
|Kinematic Evidence for Different Planetary Nebula Populations in the Elliptical Galaxy NGC 4697|
We have analyzed the magnitudes, kinematics, and positions of a completesample of 320 planetary nebulae (PNs) in the elliptical galaxy NGC 4697.We show the following. (1) The PNs in NGC 4697 do not constitute asingle population that is a fair tracer of the distribution of allstars. The radial velocity distributions, mean velocities, anddispersions of bright and faint subsamples differ with high statisticalconfidence. (2) Using the combined data for PNs brighter than 26.2, wehave identified a subpopulation of PNs that is azimuthally unmixed andkinematically peculiar, and thus neither traces the distribution of allstars nor can be in dynamical equilibrium with the galaxy potential. (3)The planetary nebula luminosity functions (PNLFs) of two kinematicsubsamples in NGC 4697 differ with 99.7% confidence, ruling out auniversal PNLF. We estimate that the inferred secondary PN populationintroduces an uncertainty in the bright cutoff magnitude of ~0.15 magfor this galaxy. We argue that this secondary PN distribution may beassociated with a younger, >~1 Gyr old stellar population, perhapsformed in tidal structures that have now fallen back onto the galaxy, ashas previously been suggested for the X-ray point sources in thisgalaxy, or coming from a more recent merger/accretion with a red galaxy.The use of PNs for extragalactic distance determinations is notnecessarily compromised, but their use as dynamical tracers of darkhalos will require deep observations and careful analysis of large PNsamples.
|Nearby early-type galaxies with ionized gas. II. Line-strength indices for 18 additional galaxies|
We previously presented a data-set of line-strength indices for 50early-type galaxies in the nearby Universe. The galaxy sample is biasedtoward galaxies showing emission lines, located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. The present addendum enlargesthe above data-set of line-strength indices by analyzing 18 additionalearly-type galaxies (three galaxies, NGC 3607, NGC 5077 and NGC 5898were presented in the previous set). We measured 25 line-strengthindices, defined by the Lick IDS "standard" system (Trager et al. 1998,ApJS, 116, 1; Worthey & Ottaviani 1997, ApJS, 111, 377), for 7luminosity weighted apertures and 4 gradients of each galaxy. Thisaddendum presents the line-strength data-set and compares it with theavailable data in the literature.
|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.
|An explanation for long flares from extragalactic globular cluster X-ray sources|
Repeatedly flaring X-ray binaries have recently been discovered in NGC4697 by Sivakoff and collaborators. We show that these flares can beexplained as the result of eccentric binaries in globular clusters whichaccrete more rapidly at the periastron than during the rest of thebinary orbit. We show that theoretical time-scales for producingeccentricities and circularizing the binaries are consistent with whatis needed to produce the observed population of flaring sources,although the circularization time-scales are highly uncertain on bothobservational and theoretical grounds. This model makes two cleartheoretical predictions: (i) the flares should be seen to be strictlyperiodic if adequate sampling is provided and that periodicity should beof approximately 15 h; and (ii) this class of flaring behaviour shouldbe seen only in globular cluster sources and predominantly in thedensest globular clusters. We also test the model for producingeccentricities through fly-bys of a third star near the binary in aglobular cluster against a much larger data base of millisecond pulsarobservations than has been used in past work and find that thetheoretical cross-sections for producing eccentricity in binaries are inreasonable agreement with most of the data, provided that the pulsarages are about 4 × 109 yr.
|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.
|Star-forming accretion flows and the low-luminosity nuclei of giant elliptical galaxies|
The luminosities of the centres of nearby elliptical galaxies are verylow compared to models of thin disc accretion on to their black holes atthe Bondi rate, typically a few hundredths to a few tenths of a solarmass per year. This has motivated models of inefficiently radiatedaccretion that invoke weak electron-ion thermal coupling, and/orinhibited accretion rates due to convection or outflows. Here we pointout that, even if such processes are operating, a significant fractionof the accreting gas is prevented from reaching the central black holebecause it condenses into stars in a gravitationally unstable disc. Starformation occurs inside the Bondi radius (typically ~100 pc in giantellipticals), but still relatively far from the black hole in terms ofSchwarzschild radii. Star formation depletes and heats the gas disc,eventually leading to a marginally stable, but much reduced, accretionflow to the black hole. We predict the presence of cold (~100 K), dustygas discs, containing clustered Hα emission and occasional Type IIsupernovae, both resulting from the presence of massive stars. Starformation accounts for several features of the M87 system: a thin disc,traced by Hα emission, is observed on scales of about 100 pc, withfeatures reminiscent of spiral arms and dust lanes; the star formationrate inferred from the intensity of Hα emission is consistent withthe Bondi accretion rate of the system. Star formation may thereforehelp to suppress accretion on to the central engines of massiveellipticals. We also discuss some implications for the fuelling of theGalactic Centre and quasars.
|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.
|Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis|
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
|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.
|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.
|Baryonically Closed Galaxy Groups|
Elliptical galaxies and their groups having the largestLX/LB lie close to the locus LX = 4.3× 1043(LB/1011LB,solar)1.75 expected for closed systems havingbaryon fractions equal to the cosmic mean value, fb~0.16. Theestimated baryon fractions for several of these galaxies/groups are alsoclose to fb=0.16 when the gas density is extrapolated to thevirial radius. Evidently they are the least massive baryonically closedsystems. Gas retention in these groups implies that nongravitationalheating cannot exceed about 1 keV per particle, consistent with theheating required to produce the deviation of groups from theLX-T correlation for more massive clusters. Isolatedgalaxies/groups with X-ray luminosities significantly lower thanbaryonically closed groups may have undermassive dark halos, overactivecentral AGNs, or higher star formation efficiencies. The virial mass andhot gas temperatures of nearly or completely closed groups correlatewith the group X-ray luminosities and the optical luminosities of thegroup-centered elliptical galaxy, i.e.,Mvir~L1.33B, an expected consequence oftheir merging history. The ratio of halo mass to the mass of the centralgalaxy for X-ray-luminous galaxies/groups isMvir/M*~80.
|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 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.
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