Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
A log-quadratic relation between the nuclear black hole masses and velocity dispersions of galaxies
We demonstrate that a log-linear relation does not provide an adequatedescription of the correlation between the masses of super massive blackholes (SMBHs, Mbh) and the velocity dispersions of their hostspheroid (σ). An unknown relation between logMbh andlogσ may be expanded to second order to obtain a log-quadraticrelation of the form log(Mbh)=α+βlog(σ/200kms-1)+β2[log(σ/200kms-1)]2. Weperform a Bayesian analysis using the local sample described in Tremaineet al., and solve for β, β2 and α, inaddition to the intrinsic scatter (δ). We find unbiased parameterestimates of β= 4.2 +/- 0.37, β2= 1.6 +/- 1.3 andδ= 0.275 +/- 0.05. At the 90 per cent level theMbh-σ relation does not follow a uniform power law.Indeed, over the velocity range 70 <~σ<~ 380kms-1 the logarithmic slope d logMbh/d logσof the best-fitting relation varies between 2.7 and 5.1, which should becompared with a power-law estimate of 4.02 +/- 0.33. The addition of the14 galaxies with reverberation SMBH masses and measured velocitydispersions to the local SMBH sample leads to a log-quadratic relationwith the same best fit as the local sample. However, the addition of thereverberation masses increases the significance of the log-quadraticcontribution, yielding a value of β2 that is non-zero atthe 5σ level. Furthermore, assuming no systematic offset, singleepoch virial SMBH masses estimated for active galactic nuclei (AGNs)follow the same log-quadratic Mbh-σ relation as thelocal sample, but extend it downward in mass by an order of magnitude.The log-quadratic term in the Mbh-σ relation has asignificant effect on estimates of the local SMBH mass function atMbh>~ 109Msolar, leading todensities of SMBHs with Mbh>~1010Msolar that are several orders of magnitudelarger than inferred from a log-linear Mbh-σ relation.We also estimate unbiased parameters for the SMBH-bulge mass relationusing the sample assembled by Häring and Rix. With aparametrization log(Mbh)=αbulge+βbulgelog(Mbulge/1011Msolar)+β2,bulge[log(Mbulge/1011Msolar)]2,we find βbulge= 1.15 +/- 0.18 andβ2,bulge= 0.12 +/- 0.14. We determined an intrinsicscatter δbulge= 0.41 +/- 0.07 which is ~50 per centlarger than the scatter in the Mbh-σ relation.
| A quasar in every galaxy ?
Not Available
| The internal dynamics of the Local Group dwarf elliptical galaxies NGC 147, 185 and 205
We present three-integral dynamical models for the three Local Groupdwarf elliptical galaxies: NGC 147, 185 and 205. These models are fittedto the Two-Micron All-Sky Survey (2MASS) J-band surface-brightnessdistribution and the major-axis kinematics (mean streaming velocity andvelocity dispersion) and, in the case of NGC 205, also to the minor-axiskinematics. The kinematical information extends out to 2Re inthe case of NGC 205 and out to about 1Re in the case of NGC147 and 185. It is the first time models are constructed for the LocalGroup dwarf ellipticals (dEs) that allow for the presence of dark matterat large radii and that are constrained by kinematics out to at leastone half-light radius. The B-band mass-to-light ratios of all the threegalaxies are rather similar, (M/L)B ~3-4Msolar/Lsolar,B. Within the innertwo half-light radii, about 40-50 per cent of the mass is in the form ofdark matter, so dEs contain about as much dark matter as brightellipticals.Based on their appreciable apparent flattening, we modelled NGC 205 and147 as being viewed edge-on. For NGC 185, having a much rounderappearance on the sky, we produced models for different inclinations.NGC 205 and 147 have a relatively isotropic velocity dispersion tensorwithin the region where the internal dynamics are strongly constrainedby the data. Our estimated inclination for NGC 185 is i ~ 50°because in that case the model has an intrinsic flattening close to thepeak of the intrinsic shape distribution of dEs and it, like thebest-fitting models for NGC 147 and 205, is nearly isotropic. We alsoshow that the dynamical properties of the bright nucleus of NGC 205 arenot unlike those of a massive globular cluster.Based on observations collected at the Observatoire de Haute-Provence.E-mail: sven.derijcke@UGent.be (SDR) ‡Postdoctoral Fellow of the Fund for Scientific Research - Flanders(Belgium)(F.W.O).
| Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas
We perform high-resolution N-body+SPH (smoothed particle hydrodynamics)simulations of gas-rich dwarf galaxy satellites orbiting within a MilkyWay-sized halo and study for the first time the combined effects oftides and ram pressure. The structure of the galaxy models and theorbital configurations are chosen in accordance with those expected in aLambda cold dark matter (ΛCDM) universe. While tidal stirring ofdisky dwarfs produces objects whose stellar structure and kinematicsresembles that of dwarf spheroidals after a few orbits, ram pressurestripping is needed to entirely remove their gas component.Gravitational tides can aid ram pressure stripping by diminishing theoverall potential of the dwarf, but tides also induce bar formationwhich funnels gas inwards making subsequent stripping more difficult.This inflow is particularly effective when the gas can cool radiatively.Assuming a low density of the hot Galactic corona consistent withobservational constraints, dwarfs with Vpeak < 30 kms-1 can be completely stripped of their gas content on orbitswith pericenters of 50 kpc or less. Instead, dwarfs with more massivedark haloes and Vpeak > 30 km s-1 lose most orall of their gas content only if a heating source keeps the gasextended, partially counteracting the bar-driven inflow. We show thatthe ionizing radiation from the cosmic ultraviolet (UV) background at z> 2 can provide the required heating. In these objects, most of thegas is removed or becomes ionized at the first pericenter passage,explaining the early truncation of the star formation observed in Dracoand Ursa Minor. Galaxies on orbits with larger pericenters and/orfalling into the Milky Way halo at lower redshift can retain significantamounts of the centrally concentrated gas. These dwarfs would continueto form stars over a longer period of time, especially close topericenter passages, as observed in Fornax and other dwarf spheroidalgalaxies (dSphs) of the Local Group. The stripped gas breaks up intoindividual clouds pressure confined by the outer gaseous medium thathave masses, sizes and densities comparable to the HI clouds recentlydiscovered around M31.
| Classical novae from the POINT-AGAPE microlensing survey of M31 - II. Rate and statistical characteristics of the nova population
The POINT-AGAPE (Pixel-lensing Observations with the Isaac NewtonTelescope-Andromeda Galaxy Amplified Pixels Experiment) survey is anoptical search for gravitational microlensing events towards theAndromeda galaxy (M31). As well as microlensing, the survey is sensitiveto many different classes of variable stars and transients. In our firstpaper of this series, we reported the detection of 20 classical novae(CNe) observed in Sloan r' and i' passbands.An analysis of the maximum magnitude versus rate of decline (MMRD)relationship in M31 is performed using the resulting POINT-AGAPE CNcatalogue. Within the limits of the uncertainties of extinction internalto M31, good fits are produced to the MMRD in two filters. The MMRDcalibration is the first to be performed for Sloan r' and i' filters.However, we are unable to verify that novae have the same absolutemagnitude 15 d after peak (the t15 relationship), nor anysimilar relationship for either Sloan filter.The subsequent analysis of the automated pipeline has provided us withthe most thorough knowledge of the completeness of a CN survey to date.In addition, the large field of view of the survey has permitted us toprobe the outburst rate well into the galactic disc, unlike previous CCDimaging surveys. Using this analysis, we are able to probe the CNdistribution of M31 and evaluate the global nova rate. Using models ofthe galactic surface brightness of M31, we show that the observed CNdistribution consists of a separate bulge and disc population. We alsoshow that the M31 bulge CN eruption rate per unit r' flux is more thanfive times greater than that of the disc.Through a combination of the completeness, M31 surface brightness modeland our M31 CN eruption model, we deduce a global M31 CN rate of65+16-15 yr-1, a value much higher thanfound by previous surveys. Using the global rate, we derive a M31 bulgerate of 38+15-12 yr-1 and a disc rateof 27+19-15 yr-1. Given ourunderstanding of the completeness and an analysis of other sources oferror, we conclude that the true global nova rate of M31 is at least 50per cent higher than was previously thought and this has consequentimplications for the presumed CN rate in the Milky Way. We deduce aGalactic bulge rate of 14+6-5 yr-1, adisc rate of 20+14-11 yr-1 and a globalGalactic rate of 34+15-12 yr-1,consistent with the Galactic global rate derived elsewhere byindependent methods.
| 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 nebula velocities in the disc and bulge of M31
We present radial velocities for a sample of 723 planetary nebulae inthe disc and bulge of M31, measured using the WYFFOS fibre spectrographon the William Herschel Telescope. Velocities are determined using the[OIII]λ5007 emission line. Rotation and velocity dispersion aremeasured to a radius of 50arcmin (11.5kpc), the first stellar rotationcurve and velocity dispersion profile for M31 to such a radius. Ourkinematics are consistent with rotational support at radii well beyondthe bulge effective radius of 1.4kpc, although our data beyond a radiusof 5kpc are limited. We present tentative evidence for kinematicsubstructure in the bulge of M31 to be studied fully in a later work.This paper is part of an ongoing project to constrain the total mass,mass distribution and velocity anisotropy of the disc, bulge and halo ofM31.
| Upper limits on the central black hole masses of 47Tuc and NGC 6397 from radio continuum emission
We present upper limits on the masses of the putative centralintermediate-mass black holes in two nearby Galactic globular clusters:47Tuc (NGC 104), the second brightest Galactic globular cluster, and NGC6397, a core-collapse globular cluster and, with a distance of 2.7 kpc,quite possibly the nearest globular cluster. These upper limits areobtained using a technique suggested by T. Maccarone. These massestimates have been derived from 3σ upper limits on the radiocontinuum flux at 1.4 GHz, assuming that the putative central black holeaccretes the surrounding matter at a rate of between 0.1 and 1 per centof the Bondi accretion rate. For 47Tuc, we find a 3σ upper limitof 2060-670 Msolar, depending on the actual accretion rate ofthe black hole and the distance to 47Tuc. For NGC 6397, which is closerto us, we derive a 3σ upper limit of 1290-390 Msolar.While estimating mass upper limits based on radio continuum observationsrequires making assumptions about the gas density and the accretion rateof the black hole, their derivation does not require complex andtime-consuming dynamical modelling. Thus, this method offers anindependent way of estimating black hole masses in nearby globularclusters. If, generally, central black holes in stellar systems accretematter faster than 0.1 per cent of the Bondi accretion rate, then theseresults indicate the absence of black holes in these globular clusterswith masses as predicted by the extrapolatedM•-σc relation.
| The dipole anisotropy of the 2 Micron All-Sky Redshift Survey
We estimate the acceleration on the Local Group (LG) from the 2 MicronAll-Sky Redshift Survey (2MRS). The sample used includes about 23200galaxies with extinction-corrected magnitudes brighter thanKs= 11.25 and it allows us to calculate the flux-weighteddipole. The near-infrared flux-weighted dipoles are very robust becausethey closely approximate a mass-weighted dipole, bypassing the effectsof redshift distortions and require no preferred reference frame. Thisis combined with the redshift information to determine the change indipole with distance. The misalignment angle between the LG and thecosmic microwave background (CMB) dipole drops to 12°+/- 7° ataround 50h-1Mpc, but then increases at larger distances,reaching 21°+/- 8° at around 130h-1Mpc. Exclusion ofthe galaxies Maffei 1, Maffei 2, Dwingeloo 1, IC342 and M87 brings theresultant flux dipole to 14°+/- 7° away from the CMB velocitydipole. In both cases, the dipole seemingly converges by60h-1Mpc. Assuming convergence, the comparison of the 2MRSflux dipole and the CMB dipole provides a value for the combination ofthe mass density and luminosity bias parametersΩ0.6m/bL= 0.40 +/- 0.09.
| 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.
| Morphological classification of nearby galaxies based on asymmetry and luminosity concentration
We investigate the behaviour of the asymmetry parameter A as amorphological parameter using a `volume-limited' sample of 349 galaxies(distance <=25Mpc,MV<=-18.5mag) and a largermagnitude-limited sample of 707 nearby galaxies. We confirm thecorrelation of A with morphological type. The late-type galaxies (Sdm,Sm and Im) have larger A than early-type galaxies, and they tend to havelarger A than spiral galaxies. We investigate the usefulness of the Aversus concentration index Cin diagram as a tool for theregular-irregular and early-late classification. The diagram is not veryuseful to the regular versus late-type irregular classification, asinferred previously, but it is found to be useful to the early-lateclassification.
| Investigating the Andromeda stream - II. Orbital fits and properties of the progenitor
We construct test-particle orbits and simple N-body models that matchthe properties of the giant stellar stream observed to the south of M31,using the model of M31's potential derived in the companion paper byGeehan et al. We introduce a simple approximation to account for thedifference in position between the stream and the orbit of theprogenitor; this significantly affects the best-fitting orbits. Theprogenitor orbits we derive have orbital apocentre ~60kpc and pericentre~3kpc, though these quantities vary somewhat with the current orbitalphase of the progenitor which is as yet unknown. Our best combined fitto the stream and galaxy properties implies a mass within 125 kpc of M31of (7.4 +/- 1.2) × 1011Msolar. Based on itslength, width, luminosity, and velocity dispersion, we conclude that thestream originates from a progenitor satellite with mass Ms~109Msolar, and at most modest amounts of darkmatter; the estimate of Ms is again correlated with the phaseof the progenitor. M31 displays a large number of faint features in itsinner halo which may be progenitors or continuations of the stream.While the orbital fits are not constrained enough for us to conclusivelyidentify the progenitor, we can identify several plausible candidates,of which a feature in the planetary nebula distribution found by Merrettet al. is the most plausible, and rule out several others. We makepredictions for the kinematic properties of the successful candidates.These may aid in observational identification of the progenitor object,which would greatly constrain the allowed models of the stream.
| Investigating the Andromeda stream - I. Simple analytic bulge-disc-halo model for M31
This paper is the first in a series which studies interactions betweenM31 and its satellites, including the origin of the giant southernstream. We construct accurate yet simple analytic models for thepotential of the M31 galaxy to provide an easy basis for the calculationof orbits in M31's halo. We use a Navarro, Frenk and White (NFW) darkhalo, an exponential disc, a Hernquist bulge, and a central black holepoint mass to describe the galaxy potential. We constrain the parametersof these functions by comparing to existing surface-brightness,velocity-dispersion, and rotation-curve measurements of M31. Ourdescription provides a good fit to the observations, and agrees wellwith more sophisticated modelling of M31. While in many respects theparameter set is well constrained, there is substantial uncertainty inthe outer halo potential and a near-degeneracy between the disc and halocomponents, producing a large, nearly two-dimensional allowed region inparameter space. We limit the allowed region using theoreticalexpectations for the halo concentration, baryonic content, and stellarmass-to-light ratio (M/LR), finding a smaller region wherethe parameters are physically plausible. Our proposed mass model for M31has Mbulge= 3.2 × 1010Msolar,Mdisc= 7.2 × 1010Msolar, andM200= 7.1 × 1011Msolar, withuncorrected (for internal and foreground extinction) mass-to-lightratios of M/LR= 3.9 and 3.3 for the bulge and disc,respectively. We present some illustrative test-particle orbits for theprogenitor of the stellar stream in our galaxy potential, highlightingthe effects of the remaining uncertainty in the disc and halo masses.
| A data-driven Bayesian approach for finding young stellar populations in early-type galaxies from their ultraviolet-optical spectra
Efficient predictive models and data analysis techniques for theanalysis of photometric and spectroscopic observations of galaxies arenot only desirable, but also required, in view of the overwhelmingquantities of data becoming available. We present the results of a novelapplication of Bayesian latent variable modelling techniques, where wehave formulated a data-driven algorithm that allows one to explore thestellar populations of a large sample of galaxies from their spectra,without the application of detailed physical models. Our only assumptionis that the galaxy spectrum can be expressed as a linear superpositionof a small number of independent factors, each a spectrum of a stellarsubpopulation that cannot be individually observed. A probabilisticlatent variable architecture that explicitly encodes this assumption isthen formulated, and a rigorous Bayesian methodology is employed forsolving the inverse modelling problem from the available data. Apowerful aspect of this method is that it formulates a density model ofthe spectra, based on which we can handle observational errors. Further,we can recover missing data both from the original set of spectra whichmight have incomplete spectral coverage of each galaxy, or frompreviously unseen spectra of the same kind.We apply this method to a sample of 21 ultraviolet-optical spectra ofwell-studied early-type galaxies, for which we also derive detailedphysical models of star formation history (i.e. age, metallicity andrelative mass fraction of the component stellar populations). We alsoapply it to synthetic spectra made up of two stellar populations,spanning a large range of parameters. We apply four different datamodels, starting from a formulation of principal component analysis(PCA), which has been widely used. We explore alternative factor models,relaxing the physically unrealistic assumption of Gaussian factors, aswell as constraining the possibility of negative flux values that areallowed in PCA, and show that other models perform equally well orbetter, while yielding more physically acceptable results. Inparticular, the more physically motivated assumptions of our rectifiedfactor analysis enable it to perform better than PCA, and to recoverphysically meaningful results.We find that our data-driven Bayesian modelling allows us to identifythose early-type galaxies that contain a significant stellar populationthat is <~1-Gyr old. This experiment also concludes that our sampleof early-type spectra showed no evidence of more than two major stellarpopulations differing significantly in age and metallicity. This methodwill help us to search for such young populations in a large ensemble ofspectra of early-type galaxies, without fitting detailed models, andthereby to study the underlying physical processes governing theformation and evolution of early-type galaxies, particularly thoseleading to the suppression of star formation in dense environments. Inparticular, this method would be a very useful tool for automaticallydiscovering various interesting subclasses of galaxies, for example,post-starburst or E+A galaxies.
| The satellite distribution of M31
The spatial distribution of the Galactic satellite system plays animportant role in Galactic dynamics and cosmology, where its successfulreproduction is a key test of simulations of galaxy halo formation.Here, we examine its representative nature by conducting an analysis ofthe three-dimensional spatial distribution of the M31 subgroup ofgalaxies, the next closest system to our own. We begin by a discussionof distance estimates and incompleteness concerns, before revisiting thequestion of membership of the M31 subgroup. We constrain this byconsideration of the spatial and kinematic properties of the putativesatellites. Comparison of the distribution of M31 and Galacticsatellites relative to the galactic discs suggests that the Galacticsystem is probably modestly incomplete at low latitudes by ~=20 percent. We find that the radial distribution of satellites around M31 ismore extended than the Galactic subgroup; 50 per cent of the Galacticsatellites are found within ~100 kpc of the Galaxy, compared to ~200 kpcfor M31. We search for `ghostly streams' of satellites around M31, inthe same way others have done for the Galaxy, and find several,including some that contain many of the dwarf spheroidal satellites. Thelack of M31-centric kinematic data, however, means that we are unable toprobe whether these streams represent real physical associations.Finally, we find that the M31 satellites are asymmetrically distributedwith respect to our line of sight to this object, so that the majorityof its satellites are on its near side with respect to our line ofsight. We quantify this result in terms of the offset between M31 andthe centre of its satellite distribution, and find it to be significantat the ~ 3σ level. We discuss possible explanations for thisfinding, and suggest that many of the M31 satellites may have beenaccreted only relatively recently. Alternatively, this anisotropy may berelated to a similar result recently reported for the 2dFGRS, whichwould imply that the halo of M31 is not yet virialized. Until such timeas a satisfactory explanation for this finding is presented, however,our results warn against treating the M31 subgroup as complete, unbiasedand relaxed.
| Beyond the big Galaxy: the structure of the stellar system 1900 - 1952
Not Available
| Caroline Herschel's catalogue of nebulae
Not Available
| The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies
The ACS Virgo Cluster Survey is a Hubble Space Telescope program toobtain high-resolution imaging in widely separated bandpasses (F475W~gand F850LP~z) for 100 early-type members of the Virgo Cluster, spanninga range of ~460 in blue luminosity. We use this large, homogenous dataset to examine the innermost structure of these galaxies and tocharacterize the properties of their compact central nuclei. We presenta sharp upward revision in the frequency of nucleation in early-typegalaxies brighter than MB~-15 (66%<~fn<~82%)and show that ground-based surveys underestimated the number of nucleidue to surface brightness selection effects, limited sensitivity andpoor spatial resolution. We speculate that previously reported claimsthat nucleated dwarfs are more concentrated toward the center of Virgothan their nonnucleated counterparts may be an artifact of theseselection effects. There is no clear evidence from the properties of thenuclei, or from the overall incidence of nucleation, for a change atMB~-17.6, the traditional dividing point between dwarf andgiant galaxies. There does, however, appear to be a fundamentaltransition at MB~-20.5, in the sense that the brighter,``core-Sérsic'' galaxies lack resolved (stellar) nuclei. A searchfor nuclei that may be offset from the photocenters of their hostgalaxies reveals only five candidates with displacements of more than0.5", all of which are in dwarf galaxies. In each case, however, theevidence suggests that these ``nuclei'' are, in fact, globular clustersprojected close to the galaxy photocenter. Working from a sample of 51galaxies with prominent nuclei, we find a median half-light radius of=4.2 pc, with the sizes of individual nucleiranging from 62 pc down to <=2 pc (i.e., unresolved in our images) inabout a half-dozen cases. Excluding these unresolved objects, the nucleisizes are found to depend on nuclear luminosity according to therelation rh L0.50+/-0.03. Because the largemajority of nuclei are resolved, we can rule out low-level AGNs as anexplanation for the central luminosity excess in almost all cases. Onaverage, the nuclei are ~3.5 mag brighter than a typical globularcluster. Based on their broadband colors, the nuclei appear to have oldto intermediate age stellar populations. The colors of the nuclei ingalaxies fainter than MB~-17.6 are tightly correlated withtheir luminosities, and less so with the luminosities of their hostgalaxies, suggesting that their chemical enrichment histories weregoverned by local or internal factors. Comparing the nuclei to the``nuclear clusters'' found in late-type spiral galaxies reveals a closematch in terms of size, luminosity, and overall frequency. A formationmechanism that is rather insensitive to the detailed properties of thehost galaxy properties is required to explain this ubiquity andhomogeneity. The mean of the frequency function for thenucleus-to-galaxy luminosity ratio in our nucleated galaxies,=-2.49+/-0.09 dex (σ=0.59+/-0.10), isindistinguishable from that of the SBH-to-bulge mass ratio,=-2.61+/-0.07dex (σ=0.45+/-0.09), calculated in 23 early-type galaxies withdetected supermassive black holes (SBHs). We argue that the compactstellar nuclei found in many of our program galaxies are the low-masscounterparts of the SBHs detected in the bright galaxies. If thisinterpretation is correct, then one should think in terms of ``centralmassive objects''-either SBHs or compact stellar nuclei-that accompanythe formation of almost all early-type galaxies and contain a meanfraction ~0.3% of the total bulge mass. In this view, SBHs would be thedominant formation mode above MB~-20.5.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Mapping and Mass Measurement of the Cold Dust in NGC 205 with Spitzer
We present observations at 3.6, 4.5, 5.8, 8, 24, 70, and 160 μm ofNGC 205, the dwarf elliptical companion of M31, obtained with theSpitzer Space Telescope. The point-source-subtracted images at 8 and 24μm display a complex and fragmented infrared emission coming fromboth very small dust particles and larger grains. The extended dustemission is spatially concentrated in three main emission regions, seenat all wavelengths from 8 to 160 μm. These regions lie approximatelyalong NGC 205's semimajor axis and range from ~100 to 300 pc in size. Onthe basis of our mid-/far-infrared flux density measurements alone, wederive a total dust mass estimate on the order of3.2×104 Msolar, mainly at a temperature of~20 K. The gas mass associated with this component matches the predictedmass returned by the dying stars from the last burst of star formationin NGC 205 (~0.5 Gyr ago). Analysis of the Spitzer data combined withprevious 1.1 mm observations over a small central or ``Core'' region(18" diameter) suggests the presence of very cold (T~12 K) dust and adust mass about 16 times higher than is estimated from the Spitzermeasurements alone. Assuming a gas-to-dust mass ratio of 100, these twodata sets, i.e., with and without the millimeter observations, suggest atotal gas mass in the range from 3.2×106 to5×107 Msolar.
| A Fundamental Relation between Compact Stellar Nuclei, Supermassive Black Holes, and Their Host Galaxies
Imaging surveys with the Hubble Space Telescope (HST) have shown that~50%-80% of low- and intermediate-luminosity galaxies contain a compactstellar nucleus at their center, regardless of host galaxy morphologicaltype. We combine HST imaging for early-type galaxies from the ACS VirgoCluster Survey with ground-based long-slit spectra from KPNO to showthat the masses of compact stellar nuclei in Virgo Cluster galaxies obeya tight correlation with the masses of the host galaxies. The samecorrelation is obeyed by the supermassive black holes (SBHs) found inpredominantly massive galaxies. The compact stellar nuclei in the LocalGroup galaxies M33 and NGC 205 are also found to fall along this samescaling relation. These results indicate that a generic by-product ofgalaxy formation is the creation of a central massive object(CMO)-either an SBH or a compact stellar nucleus-that contains a meanfraction, ~0.2%, of the total galactic mass. In galaxies with massesgreater than Mgal ~ a few × 1010Msolar, SBHs appear to be the dominant mode of CMOformation.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 (AURA), Inc.,under NASA contract NAS 5-26555.
| Local Group Dwarf Galaxies and the Fundamental Manifold of Spheroids
The fundamental manifold (FM), an extension of the fundamental planeformalism, incorporates all spheroid-dominated stellar systems fromdwarf ellipticals up to the intracluster stellar populations of galaxyclusters by accounting for the continuous variation of the mass-to-lightratio within the effective radius re with scale. Here we findthat Local Group dwarf spheroidal and dwarf elliptical galaxies, whichprobe the FM relationship roughly one decade lower in re thanprevious work, lie on the extrapolation of the FM. When combined withthe earlier data, these Local Group dwarfs demonstrate the validity ofthe empirical manifold over nearly 4 orders of magnitude inre. The continuity of the galaxy locus on the manifold and,more specifically, the overlap on the FM of dwarf ellipticals like M32and dwarf spheroidals like Leo II, imply that dwarf spheroidals belongto the same family of spheroids as their more massive counterparts. Theonly significant outliers are Ursa Minor and Draco. We explore whetherthe deviation of these two galaxies from the manifold reflects abreakdown in the coherence of the empirical relationship at lowluminosities or rather the individual dynamical peculiarities of thesetwo objects. We discuss some implications of our results for how thelowest mass galaxies form.
| Luminous Compact Blue Galaxies up to z~1 in the Hubble Space Telescope Ultra Deep Field. I. Small Galaxies or Blue Centers of Massive Disks?
We analyze 26 luminous compact blue galaxies (LCBGs) in the Hubble SpaceTelescope ACS Ultra Deep Field (UDF) at z~0.2-1.3, to determine whetherthese truly are small galaxies or, rather, bright central starburstswithin existing or forming large disk galaxies. Surface brightnessprofiles from UDF images reach fainter than rest-frame 26.5 B magarcsec-2 even for compact objects at z~1. Most LCBGs show asmaller, brighter component that is likely star-forming, and anextended, roughly exponential component with colors suggesting stellarages from >~100 Myr to a few gigayears. Scale lengths of the extendedcomponents are mostly <~2 kpc, more than 1.5-2 times smaller thanthose of nearby large disk galaxies like the Milky Way. Larger, very lowsurface brightness disks can be excluded down to faint rest-framesurface brightnesses (>~26 B mag arcsec-2). However, oneor two of the LCBGs are large, disklike galaxies that meet LCBGselection criteria because of a bright central nucleus, possibly aforming bulge. These results indicate that >~90% of high-z LCBGs aresmall galaxies that will evolve into small disk galaxies, or low-massspheroidal or irregular galaxies in the local universe, assuming passiveevolution and no significant disk growth. The data do not reveal signsof disk formation around small, H II galaxy-like LCBGs, nor do theysuggest a simple inside-out growth scenario for larger LCBGs with adisklike morphology. Irregular blue emission in distant LCBGs isrelatively extended, suggesting that nebular emission lines fromstar-forming regions sample a major fraction of an LCBG's velocityfield.
| Spitzer MIPS Infrared Imaging of M31: Further Evidence for a Spiral-Ring Composite Structure
New images of M31 at 24, 70, and 160 μm taken with the MultibandImaging Photometer for Spitzer (MIPS) reveal the morphology of the dustin this galaxy. This morphology is well represented by a composite oftwo logarithmic spiral arms and a circular ring (radius ~10 kpc) of starformation offset from the nucleus. The two spiral arms appear to startat the ends of a bar in the nuclear region and extend beyond thestar-forming ring. As has been found in previous work, the spiral armsare not continuous, but composed of spiral segments. The star-formingring is very circular except for a region near M32 where it splits. Thelack of well-defined spiral arms and the prominence of the nearlycircular ring suggest that M31 has been distorted by interactions withits satellite galaxies. Using new dynamical simulations of M31interacting with M32 and NGC 205, we find that, qualitatively, suchinteractions can produce an offset, split ring like that seen in theMIPS images.
| A Local Group Polar Ring Galaxy: NGC 6822
Star counts obtained from a 2° × 2° area centered on NGC6822 have revealed an optical image of this galaxy composed of twocomponents: in addition to the well-known H I disk with its youngstellar component, there is a spheroidal stellar structure as extensiveas its H I disk, but with its major axis at roughly right angles to it,that we traced to at least 36'. Radial velocities of over 100intermediate-age carbon stars found within this structure displaykinematics contrasting strongly with those of the H I disk. These Cstars belong to the spheroid. Although devoid of gas, the spheroidrotation is consistent with the I-band Tully-Fisher relation. Theorientation of the rotation axis that minimizes the stellar velocitydispersion coincides with the minor axis of the stellar populationellipsoid, lying very nearly in the plane of the H I disk. We concludethat the H I disk is a polar ring and that the spheroidal component isan erstwhile disk, a fossil remainder of a past close encounter episode.Based on observations obtained with MegaPrime/MegaCam, a joint projectof CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT),which is operated by the National Research Council (NRC) of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique (CNRS) of France, and the University of Hawaii.Based on observations acquired at the du Pont Telescope, from theObservatories of the Carnegie Institution of Washington.
| Luminous Compact Blue Galaxies in Intermediate-Redshift Galaxy Clusters: A Significant but Extreme Butcher-Oemler Population
We identify a population of luminous compact blue galaxies (LCBGs) intwo galaxy clusters: MS 0451.6-0305 (z=0.54) and Cl 1604+4304 (z=0.9).LCBGs are identified via photometric characteristics and photometricredshifts derived from broad- and narrowband images taken with the WIYNtelescope and the Hubble Space Telescope (HST). We analyze their surfacedensities and clustering properties to find that they compose astatistically significant portion (42% and 53%, respectively) of theButcher-Oemler (BO) galaxies in both clusters and that their spatialdistributions are best characterized by a shell model. The enhancementof the projected space density of LCBGs with MB<-18.5 inthe clusters relative to the field is 3-10 times higher than the BOpopulation as a whole but 2 times lower than the red population, exceptin the core where LCBGs are absent. Assuming some fading, a naturaldescendant would be small, low-luminosity galaxies found preferentiallyin today's clusters, such as dE galaxies.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| Weak redshift discretisation in the Local Group of galaxies?
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
| Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. I. On the Possible Disk Nature of Bright Early-Type Dwarfs
We present a systematic search for disk features in 476 Virgo Clusterearly-type dwarf (dE) galaxies. This is the first such study of analmost-complete, statistically significant dE sample, which includes allcertain or possible cluster members with mB<=18 that arecovered by the optical imaging data of the Sloan Digital Sky Survey DataRelease 4. Disk features (spiral arms, edge-on disks, or bars) wereidentified by applying unsharp masks to a combined image from threebands (g, r, and i), as well as by subtracting the axisymmetric lightdistribution of each galaxy from that image. Fourteen objects areunambiguous identifications of disks, 10 objects show ``probable disk''features, and 17 objects show ``possible disk'' features. The numberfraction of these galaxies, for which we introduce the term ``dEdi,''reaches more than 50% at the bright end of the dE population anddecreases to less than 5% for magnitudes mB>16. Althoughpart of this observed decline might be due to the lower signal-to-noiseratio at fainter magnitudes, we show that it cannot be caused solely bythe limitations of our detection method. The luminosity function of ourfull dE sample can be explained by a superposition of dEdis and ordinarydEs, strongly suggesting that dEdis are a distinct type of galaxy. Thisis supported by the projected spatial distribution: dEdis show basicallyno clustering and roughly follow the spatial distribution of spirals andirregulars, whereas ordinary dEs are distributed similarly to thestrongly clustered E/S0 galaxies. While the flattening distribution ofordinary dEs is typical for spheroidal objects, the distribution ofdEdis is significantly different and agrees with their being flat oblateobjects. We therefore conclude that the dEdis are not spheroidalgalaxies that just have an embedded disk component but are instead apopulation of genuine disk galaxies. Several dEdis display well-definedspiral arms with grand-design features that clearly differ from theflocculent, open arms typical for late-type spirals that have frequentlybeen proposed as progenitors of dEs. This raises the question of whatprocess is able to create such spiral arms-with pitch angles like thoseof Sab/Sb galaxies-in bulgeless dwarf galaxies.
| Tramp Classical Novae as Tracers of Intergalactic Stars
Simulations predict that collisions between galaxies must liberate starsinto intergalactic space. The stripping of a galaxy's stars by thepotential of a cluster in which it resides must also occur. Thisprediction is verified by the detections of classical novae, red giants,and planetary nebulae between the galaxies of the Virgo and FornaxClusters. These tracers suggest a tramp stellar component of 10%-40% ofthe cluster baryonic mass. I point out that classical novae can usefullyextend these results to the 250,000 Mpc3 of intergalacticspace outside of galaxy clusters surrounding the Local Group. This isbecause individual novae are well-understood standard candles, withlight curves and spectra that are distinct from all other astrophysicalphenomena. In addition, the frequency of nova outbursts in any givengalaxy is measured to be directly proportional to that galaxy's K-bandluminosity (and independent of its Hubble type). Thus, intergalacticnovae should be excellent tracers of the fraction of stars liberatedfrom galaxies over the past 13 Gyr. Pan-STARRS, the Large SynopticSurvey Telescope (LSST), and other large-area synoptic survey telescopeswill begin to regularly discover tramp classical novae out to 20-40 Mpcin the coming decade. I estimate the expected discovery rates withLSST-like surveys to be hundreds of intergalactic tramp novae per year,and suggest survey parameters to optimize detections of these tramps.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
INHABITED SKY
