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|A quasar in every galaxy ?|
|OSIRIS: AO-assisted integral-field spectroscopy at the Keck Observatory|
OSIRIS (OH-Suppressing Infra-Red Integral-field Spectrograph) is a newfacility instrument for the Keck Observatory. After seeing first lightin February 2005, OSIRIS is currently undergoing commissioning. OSIRISprovides the capability of performing three-dimensional spectroscopy inthe near-infrared z, J, H, and K bands at the resolution limit of theKeck II telescope, which is equipped with adaptive optics and a laserguide star. The science case for OSIRIS is summarized, and theinstrument and associated data reduction software are described.
|A universal density slope Velocity anisotropy relation for relaxed structures|
We identify a universal relation between the radial density slopeα(r) and the velocity anisotropy β(r) for equilibratedstructures. This relation holds for a variety of systems, including diskgalaxy mergers, spherical collapses, and cold dark matter (CDM) halosboth with and without cooling. We argue that the shape of the relationis reasonable from fundamental principles when the dark matter or starsare assumed to obey Tsallis statistics, and in that case we fit theα β relation with just one free parameter. One can use thisresult to close the Jeans equations, for example to construct massmodels of elliptical galaxies from observational data or to tune darkmatter structures directly to detection experiments. We also predict theasymptotic central slope and anisotropy of CDM halos to be approximately‑1 and 0.
|Dwarf galaxies in the dynamically evolved NGC 1407 Group|
The NGC 1407 Group stands out among nearby structures by its propertiesthat suggest it is massive and evolved. It shares properties withentities that have been called fossil groups: the 1.4mdifferential between the dominant elliptical galaxy and the secondbrightest galaxy comes close to satisfying the definition that has beenused to define the fossil class. There are few intermediate-luminositygalaxies, but a large number of dwarfs in the group. We estimate thereare 250 group members to the depth of our survey. The slope of the faintend of the luminosity function (reaching MR = -12) is α= -1.35. Velocities for 35 galaxies demonstrate that this group with onedominant galaxy has a mass of 7 × 1013Msolarand M/LR = 340Msolar/Lsolar. Twogalaxies in close proximity to NGC 1407 have very large blueshifts. Themost notable is the second brightest galaxy, NGC 1400, with a velocityof -1072 km s-1 with respect to the group mean. We report thedetection of X-ray emission from this galaxy and from the group.
|Magnetic jets from swirling discs|
A broad swathe of astrophysical phenomena, ranging from tubularplanetary nebulae through Herbig-Haro objects, radio galaxy and quasaremissions to gamma-ray bursts and perhaps high-energy cosmic rays, maybe driven by magnetically dominated jets emanating from accretion discs.We give a self-contained account of the analytic theory ofnon-relativistic magnetically dominated jets wound up by a swirling discand making a magnetic cavity in a background medium of any prescribedpressure, p(z). We solve the time-dependent problem for any specifieddistribution of magnetic flux P(R, 0) emerging from the disc at z = 0,with any specified disc angular velocity Ωd(R). Thephysics required to do this involves only the freezing of the lines offorce to the conducting medium and the principle of minimum energy.In a constant pressure environment, the magnetically dominated cavity ishighly collimated and advances along the axis at a constant speedclosely related to the maximum circular velocity of the accretion disc.Even within the cavity the field is strongly concentrated towards theaxis. The twist in the jet field/<|Bz|> is close to and thewidth of the jet decreases upwards. By contrast, when the backgroundpressure falls off with height with powers approaching z-4,the head of the jet accelerates strongly and the twist of the jet ismuch smaller. The width increases to give an almost conical magneticcavity with apex at the source. Such a regime may be responsible forsome of the longest strongly collimated jets. When the backgroundpressure falls off faster than z-4, there are no quasi-staticconfigurations of well-twisted fields and the pressure confinement isreplaced by a dynamic effective pressure or a relativistic expansion. Inthe regimes with rapid acceleration, the outgoing and incoming fieldslinking the twist back to the source are almost anti-parallel so thereis a possibility that magnetic reconnections may break up the jet into aseries of magnetic `smoke-rings' travelling out along the axis.
|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.
|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.
|Constraints on alternatives to supermassive black holes|
Observations of the centres of galaxies continue to evolve, and it isuseful to take a fresh look at the constraints that exist onalternatives to supermassive black holes at their centres. We discussconstraints complementary to those of Maoz and demonstrate that anextremely wide range of other possibilities can be excluded. Inparticular, we present the new argument that for the velocitydispersions inferred for many galactic nuclei, even binaries made ofpoint masses cannot stave off core collapse because hard binaries are sotight that they merge via emission of gravitational radiation beforethey can engage in three-body or four-body interactions. We also showthat under these conditions core collapse leads inevitably to runawaygrowth of a central black hole with a significant fraction of theinitial mass, regardless of the masses of the individual stars. Forclusters of non-interacting low-mass objects (from low-mass stars toelementary particles), the relaxation of stars and compact objects thatpass inside the dark region will be accelerated by interactions with thedark mass. If the dark region is instead a self-supported object, suchas a fermion ball, then if stellar-mass black holes exist they willcollide with the object, settle, and consume it. The net result is thatthe keyhole through which alternatives to supermassive black holes mustpass is substantially smaller and more contrived than it was even a fewyears ago.
|Correlations of near-infrared, optical and X-ray luminosity for early-type galaxies|
The relation between X-ray luminosity and near-infrared (NIR) luminosityfor early-type galaxies has been examined. NIR luminosities shouldprovide a superior measure of stellar mass compared to opticalluminosities used in previous studies, especially if there issignificant star formation or dust present in the galaxies. However, weshow that the X-ray-NIR relations are remarkably consistent with theX-ray-optical relations. This indicates that the large scatter of therelations is dominated by scatter in the X-ray properties of early-typegalaxies, and is consistent with early-types consisting of old,quiescent stellar populations.We have investigated scatter in terms of environment, surface brightnessprofile, Mg2, Hβ, Hγ line strength indices,spectroscopic age and nuclear Hα emission. We found that galaxieswith high Mg2 index, low Hβ and Hγ indices or a`core' profile have a large scatter in LX, whereas galaxieswith low Mg2, high Hβ and Hγ indices or`power-law' profiles generally have LX < 1041erg s-1. There is no clear trend in the scatter withenvironment or nuclear Hα emission.
|The SAURON project - IV. The mass-to-light ratio, the virial mass estimator and the Fundamental Plane of elliptical and lenticular galaxies|
We investigate the well-known correlations between the dynamicalmass-to-light ratio (M/L) and other global observables of elliptical (E)and lenticular (S0) galaxies. We construct two-integral Jeans andthree-integral Schwarzschild dynamical models for a sample of 25 E/S0galaxies with SAURON integral-field stellar kinematics to about oneeffective (half-light) radius Re. They have well-calibratedI-band Hubble Space Telescope WFPC2 and large-field ground-basedphotometry, accurate surface brightness fluctuation distances, and theirobserved kinematics is consistent with an axisymmetric intrinsic shape.All these factors result in an unprecedented accuracy in the M/Lmeasurements. We find a tight correlation of the form (M/L) = (3.80 +/-0.14) ×(σe/200kms-1)0.84+/-0.07 betweenthe M/L (in the I band) measured from the dynamical models and theluminosity-weighted second moment σe of the LOSVDwithin Re. The observed rms scatter in M/L for our sample is18 per cent, while the inferred intrinsic scatter is ~13 per cent. The(M/L)-σe relation can be included in the remarkableseries of tight correlations between σe and othergalaxy global observables. The comparison of the observed correlationswith the predictions of the Fundamental Plane (FP), and with simplevirial estimates, shows that the `tilt' of the FP of early-typegalaxies, describing the deviation of the FP from the virial relation,is almost exclusively due to a real M/L variation, while structural andorbital non-homology have a negligible effect. When the photometricparameters are determined in the `classic' way, using growth curves, andthe σe is measured in a large aperture, the virial massappears to be a reliable estimator of the mass in the central regions ofgalaxies, and can be safely used where more `expensive' models are notfeasible (e.g. in high-redshift studies). In this case the best-fittingvirial relation has the form (M/L)vir= (5.0 +/- 0.1)×Reσ2e/(LG), in reasonableagreement with simple theoretical predictions. We find no differencebetween the M/L of the galaxies in clusters and in the field. Thecomparison of the dynamical M/L with the (M/L)pop inferredfrom the analysis of the stellar population, indicates a median darkmatter fraction in early-type galaxies of ~30 per cent of the total massinside one Re, in broad agreement with previous studies, andit also shows that the stellar initial mass function varies little amongdifferent galaxies. Our results suggest a variation in M/L at constant(M/L)pop, which seems to be linked to the galaxy dynamics. Wespeculate that fast-rotating galaxies have lower dark matter fractionsthan the slow-rotating and generally more-massive ones. If correct, thiswould suggest a connection between the galaxy assembly history and thedark matter halo structure. The tightness of our correlation providessome evidence against cuspy nuclear dark matter profiles in galaxies.
|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.
|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|
|Hot Subdwarfs in the Galactic Bulge|
Recent observations and theories suggest that extreme horizontal branch(EHB) stars and their progeny should be the cause of the UV excess thatmany elliptical galaxies show in their spectra. Since the Galactic bulgeis the closest representation of an old, metal-rich spheroid in which weare able to study the EHB scenario in detail, we obtained spectra ofbulge EHB star candidates and we confirm their status as hot evolvedstars. It is the first time that such stars are unambiguously observedin the Galactic bulge.
|Uv Upturn of Elliptical Galaxies|
We investigate the UV upturn phenomenon of elliptical galaxies byapplying the binary model of hot subdwarfs of Han et al. (2002, 2003).Preliminary results show that the model provides a natural explanationfor the UV upturn phenomenon and that the model could be used to detectlow level recent star formation.
|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.
|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.
|From Supermassive Black Holes to Dwarf Elliptical Nuclei: A Mass Continuum|
Considerable evidence suggests that supermassive black holes reside atthe centers of massive galactic bulges. At a lower galactic mass range,many dwarf galaxies contain extremely compact nuclei that structurallyresemble massive globular clusters. We show that both these types ofcentral massive objects (CMOs) define a single unbroken relation betweenCMO mass and the luminosity of their host galaxy spheroid. Equivalently,MCMO is directly proportional to the host spheroid mass over4 orders of magnitude. We therefore suggest that the dE,N nuclei may bethe low-mass analogs of supermassive black holes and that these twotypes of CMOs may have both developed starting from similar initialformation processes. The overlap mass interval between the two types ofCMOs is small and suggests that for MCMO>107Msolar, the formation of a black hole was strongly favored,perhaps because the initial gas infall to the center was too rapid andviolent for star formation to occur efficiently.
|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.
|Kinematics and Metallicity of M31 Red Giants: The Giant Southern Stream and Discovery of a Second Cold Component at R=20 kpc|
We present spectroscopic observations of red giant branch (RGB) stars inthe Andromeda spiral galaxy (M31), acquired with the DEIMOS instrumenton the Keck II 10 m telescope. The three fields targeted in this studyare in the M31 spheroid, outer disk, and giant southern stream. In thispaper, we focus on the kinematics and chemical composition of RGB starsin the stream field located at a projected distance of R=20 kpc fromM31's center. A mix of stellar populations is found in this field. M31RGB stars are isolated from Milky Way dwarf star contaminants using avariety of spectral and photometric diagnostics. The radial velocitydistribution of RGB stars displays a clear bimodality-a primary peakcentered at v¯1=-513 km s-1 and a secondaryone at v¯2=-417 km s-1-along with anunderlying broad component that is presumably representative of thesmooth spheroid of M31. Both peaks are found to be dynamically cold withintrinsic velocity dispersions of σ(v)~16 km s-1. Themean metallicity and metallicity dispersion of stars in the two peaks isalso found to be similar: <[Fe/H]>~-0.45 and σ([Fe/H])=0.2.The observed velocity of the primary peak is consistent with thatpredicted by dynamical models for the stream, but there is no obviousexplanation for the secondary peak. The nature of the secondary coldpopulation is unclear: it may represent (1) tidal debris from asatellite merger event that is superimposed on, but unrelated to, thegiant southern stream; (2) a wrapped around component of the giantsouthern stream; or (3) a warp or overdensity in M31's disk atRdisk>50 kpc (this component is well above the outwardextrapolation of the smooth exponential disk brightness profile).Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.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 Science de l'Univers of the Centre National de laRecherche Scientifique (CNRS) of France, and the University of Hawaii.
|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.
|Spitzer IRS spectra of Virgo Early-Type Galaxies: Detection of Stellar Silicate Emission|
We present high signal-to-noise ratio Spitzer Infrared Spectrographobservations of 17 Virgo early-type galaxies. The galaxies were selectedfrom those that define the color-magnitude relation of the cluster, withthe aim of detecting the silicate emission of their dusty, mass-losingevolved stars. To flux calibrate these extended sources, we have deviseda new procedure that allows us to obtain the intrinsic spectral energydistribution and to disentangle resolved and unresolved emission withinthe same object. We have found that 13 objects of the sample (76%) arepassively evolving galaxies with a pronounced broad silicate featurethat is spatially extended and likely of stellar origin, in agreementwith model predictions. The other four objects (24%) are characterizedby different levels of activity. In NGC 4486 (M87), the line emissionand the broad silicate emission are evidently unresolved, and, givenalso the typical shape of the continuum, they likely originate in thenuclear torus. NGC 4636 shows emission lines superposed on extended(i.e., stellar) silicate emission, thus pushing the percentage ofgalaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435are characterized by polycyclic aromatic hydrocarbon (PAH) and lineemission, arising from a central unresolved region. A more detailedanalysis of our sample, with updated models, will be presented in aforthcoming paper.
|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.
|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.
|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.
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