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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.
| Does the Fornax dwarf spheroidal have a central cusp or core?
The dark matter dominated Fornax dwarf spheroidal has five globularclusters orbiting at ~1kpc from its centre. In a cuspy cold dark matterhalo the globulars would sink to the centre from their current positionswithin a few Gyr, presenting a puzzle as to why they survive undigestedat the present epoch. We show that a solution to this timing problem isto adopt a cored dark matter halo. We use numerical simulations andanalytic calculations to show that, under these conditions, the sinkingtime becomes many Hubble times; the globulars effectively stall at thedark matter core radius. We conclude that the Fornax dwarf spheroidalhas a shallow inner density profile with a core radius constrained bythe observed positions of its globular clusters. If the phase spacedensity of the core is primordial then it implies a warm dark matterparticle and gives an upper limit to its mass of ~0.5keV, consistentwith that required to significantly alleviate the substructure problem.
| Comptonization of cosmic microwave background photons in dwarf spheroidal galaxies
We present theoretical modelling of the electron distribution producedby annihilating neutralino dark matter in dwarf spheroidal galaxies(dSphs). In particular, we follow up the idea of Colafrancesco and findthat such electrons distort the cosmic microwave background (CMB) by theSunyaev-Zeldovich (SZ) effect. For an assumed neutralino mass of 10 GeVand beam size of 1 arcsec, the SZ temperature decrement is of the orderof nano-Kelvin for dSph models with a soft core. By contrast, it is ofthe order of micro-Kelvin for the strongly cusped dSph models favouredby some cosmological simulations. Although this is out of reach ofcurrent instruments, it may well be detectable by future mm telescopes,such as the Atacama Large Millimetre Array. We also show that theupscattered CMB photons have energies within reach of upcoming X-rayobservatories, but that the flux of such photons is too small to bedetectable now. None the less, we conclude that searching for the darkmatter induced SZ effect is a promising way of constraining the darkdistribution in dSphs, especially if the particles are light.
| Globular clusters, satellite galaxies and stellar haloes from early dark matter peaks
The Milky Way contains several distinct old stellar components thatprovide a fossil record of its formation. We can understand theirspatial distribution and kinematics in a hierarchical formation scenarioby associating the protogalactic fragments envisaged by Searle &Zinn (1978) with the rare peaks able to cool gas in the cold dark matterdensity field collapsing at redshift z > 10. We use hierarchicalstructure formation simulations to explore the kinematics and spatialdistribution of these early star-forming structures in galaxy haloestoday. Most of the protogalaxies rapidly merge, their stellar contentsand dark matter becoming smoothly distributed and forming the innerGalactic halo. The metal-poor globular clusters and old halo starsbecome tracers of this early evolutionary phase, centrally biased andnaturally reproducing the observed steep fall off with radius. The mostoutlying peaks fall in late and survive to the present day as satellitegalaxies. The observed radial velocity dispersion profile and the localradial velocity anisotropy of Milky Way halo stars are successfullyreproduced in this model. If this epoch of structure formation coincideswith a suppression of further cooling into lower sigma peaks then we canreproduce the rarity, kinematics and spatial distribution of satellitegalaxies as suggested by Bullock, Kravtsov & Weinberg (2000).Reionization at z= 12 +/- 2 provides a natural solution to the missingsatellites problem. Measuring the distribution of globular clusters andhalo light on scales from galaxies to clusters could be used toconstrain global versus local reionization models. If reionizationoccurs contemporary, our model predicts a constant frequency of blueglobulars relative to the host halo mass, except for dwarf galaxieswhere the average relative frequencies become smaller.
| Probing dark matter with X-ray binaries
Low-mass X-ray binaries (LMXBs), which occur in old stellar populations,have velocities exceeding those of their parent distribution by at least20 km s-1. This makes them ideal probes for dark matter, inparticular in dwarf spheroidals (dSph), where the LMXBs should penetratewell outside the visible galaxy. We argue that the most likelyexplanation of the observation of LMXBs in the Sculptor dSph byMaccarone et al. is the presence of a dark matter halo of>~109Msolar, corresponding to a total-mass tolight ratio of >~600 (M/LV)solar. In this casethere should be an extended halo of LMXBs which may be observable.
| The importance of tides for the Local Group dwarf spheroidals
There are two main tidal effects that can act on the Local Group dwarfspheroidals (dSphs): tidal stripping and tidal shocking. Using N-bodysimulations, we show that tidal stripping always leads to flat or risingprojected velocity dispersions beyond a critical radius; it is ~5 timesmore likely, when averaging over all possible projection angles, thatthe cylindrically averaged projected dispersion will rise, rather thanbe flat. In contrast, the Local Group dSphs, as a class, show flat orfalling projected velocity dispersions interior to ~1kpc. This arguesfor tidal stripping being unimportant interior to ~1kpc for most of theLocal Group dSphs observed so far. We show that tidal shocking may stillbe important, however, even when tidal stripping is not. This couldexplain the observed correlation for the Local Group dSphs betweencentral surface brightness and distance from the nearest large galaxy.These results have important implications for the formation of the dSphsand for cosmology. As a result of the existence of cold stars at largeradii in several dSphs, a tidal origin for the formation of these LocalGroup dSphs (in which they contain no dark matter) is stronglydisfavoured. In the cosmological context, a naive solution to themissing satellites problem is to allow only the most massivesubstructure dark matter haloes around the Milky Way to form stars. Itis possible for dSphs to reside within these haloes(~1010Msolar) and have their velocity dispersionslowered through the action of tidal shocks, but only if they have acentral density core in their dark matter, rather than a cusp. A centraldensity cusp persists even after unrealistically extreme tidal shockingand leads to central velocity dispersions which are too high to beconsistent with data from the Local Group dSphs. dSphs can reside withincuspy dark matter haloes if their haloes are less massive(~109Msolar) and therefore have smaller centralvelocity dispersions initially.
| Isolated and non-isolated dark matter haloes and the Navarro, Frenk and White profile
We compare the conclusions reached using the coarse-graining techniqueemployed by Henriksen for a one degree of freedom (per particle)collisionless system to those presented in a paper by Binney based on anexact one degree of freedom model. We find agreement in detail, but inaddition we show that the isolated 1D system is self-similar andtherefore unrelaxed. Fine graining of this system recovers much lessprominent wave-like structure than in a spherically symmetric isotropic3D system. The rate of central flattening is also reduced in the 1Dsystem. We take this to be evidence that relaxation of collisionlesssystems proceeds ultimately by way of short wavelength Landau damping.N-body systems, both real and simulated, can be trapped in anincompletely relaxed state because of a break in the cascade of energyto small scales. This may be due to the rapid dissipation of thesmall-scale oscillations in an isolated system to the existence ofconserved quantities such as angular momentum, or to the failure insimulations to resolve sub-Jeans length scales. Such a partially relaxedstate appears to be the Navarro, Frenk and White (NFW) state and is tobe expected especially in young systems. The NFW core is shown to beisolated. In non-isolated systems, continuing coarse-grained relaxationshould be towards a density core in solid body rotation.
| Structural properties of the M31 dwarf spheroidal galaxies
The projected structures and integrated properties of the Andromeda I,II, III, V, VI, VII and Cetus dwarf spheroidal galaxies are analysedbased upon resolved counts of red giant branch stars. The observationswere taken as part of the Isaac Newton Telescope Wide Field Survey ofM31 and its environs. For each object, we have derived isopleth maps,surface brightness profiles, intensity-weighted centres, positionangles, ellipticities, tidal radii, core radii, concentrationparameters, exponential scalelengths, Plummer scalelengths, half-lightradii, absolute magnitudes and central surface brightnesses. Ouranalysis probes into larger radius and fainter surface brightnesses thanmost previous studies, and as a result we find that the galaxies aregenerally larger and brighter than has previously been recognized. Inparticular, the luminosity of Andromeda V is found to be consistent withthe higher metallicity value which has been derived for it. We find thatexponential and Plummer profiles provide adequate fits to the surfacebrightness profiles, although the more general King models provide thebest formal fits. Andromeda I shows strong evidence of tidal disruptionand S-shaped tidal tails are clearly visible. On the other hand, Cetusdoes not show any evidence of tidal truncation, let alone disruption,which is perhaps unsurprising given its isolated location. Andromeda IIshows compelling evidence of a large excess of stars at small radius andsuggests that this galaxy consists of a secondary core component, inanalogy with recent results for Sculptor and Sextans. Comparing the M31dwarf spheroidal population with the Galactic population, we find thatthe scaleradii of the M31 population are larger than those for theGalactic population by at least a factor of 2, for all absolutemagnitudes. This difference is either due to environmental factors ordue to orbital properties, suggesting that the ensemble average tidalfield experienced by the M31 dwarf spheroidals is weaker than thatexperienced by the Galactic dwarf spheroidals. We find that the twopopulations are offset from one another in the central surfacebrightness - luminosity relation, which is probably related to thisdifference in their scale sizes. Finally, we find that the M31 dwarfspheroidals show the same correlation with distance from host as shownby the Galactic population, such that dwarf spheroidals with a highercentral surface brightness are found further from their host. This againsuggests that environment plays a significant role in dwarf galaxyevolution, and requires detailed modelling to explain the origin of thisresult.
| 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.
| The evolution of barium and europium in local dwarf spheroidal galaxies
By means of a detailed chemical evolution model, we follow the evolutionof barium (Ba) and europium (Eu) in four Local Group Dwarf Spheroidal(dSph) galaxies, in order to set constraints on the nucleosynthesis ofthese elements and on the evolution of this type of galaxies comparedwith the Milky Way. The model, which is able to reproduce severalobserved abundance ratios and the present-day total mass and gas masscontent of these galaxies, adopts up-to-date nucleosynthesis and takesinto account the role played by supernovae (SNe) of different types (II,Ia) allowing us to follow in detail the evolution of several chemicalelements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba and Eu). By assumingthat Ba is a neutron-capture element produced in low-mass asymptoticgiant branch stars by s-process but also in massive stars (in the massrange 10-30 Msolar) by r-process, during the explosive eventof SNe of Type II, and that Eu is a pure r-process element synthesizedin massive stars also in the range of masses 10-30 Msolar, weare able to reproduce the observed [Ba/Fe] and [Eu/Fe] as functions of[Fe/H] in all four galaxies studied. We confirm also the important roleplayed by the very low star formation (SF) efficiencies (ν= 0.005-0.5Gyr-1) and by the intense galactic winds (6-13 times the starformation rate) in the evolution of these galaxies. These low SFefficiencies (compared to the one for the Milky Way disc) adopted forthe dSph galaxies are the main reason for the differences between thetrends of [Ba/Fe] and [Eu/Fe] predicted and observed in these galaxiesand in the metal-poor stars of our Galaxy. Finally, we providepredictions for Sagittarius galaxy for which data of only two stars areavailable.
| Cosmological Implications of Dwarf Spheroidal Chemical Evolution
The chemical properties of dwarf spheroidals in the local group areshown to be inconsistent with star formation being truncated after thereionization epoch (z~8). Enhanced levels of [Ba/Y] in stars in dwarfspheroidals like Sculptor indicate strong s-process production fromlow-mass stars whose lifetimes are comparable with the duration of thepre-reionization epoch. The chemical evolution of Sculptor is followedusing a model with SN II and SN Ia feedback and mass- andmetallicity-dependent nucleosynthetic yields for elements from H to Pb.We are unable to reproduce the Ba/Y ratio unless stars formed over aninterval long enough for the low-mass stars to pollute the interstellarmedium with s-elements. This robust result challenges the suggestionthat most of the local group dwarf spheroidals are fossils ofreionization and supports the case for large initial dark matter halos.
| Fossils of Reionization in the Local Group
We use a combination of high-resolution gas dynamics simulations ofhigh-redshift dwarf galaxies and dissipationless simulations of a MilkyWay-sized halo to estimate the expected abundance and spatialdistribution of the dwarf satellite galaxies that formed most of theirstars around z~8, evolving only little since then. Such galaxies can beconsidered ``fossils'' of the reionization era, and studying theirproperties could provide a direct window into the early,pre-reionization stages of galaxy formation. We show that ~5%-15% of theobjects existing at z~8 do indeed survive until the present in a MilkyWay-like environment without significant evolution. This implies that itis plausible that the fossil dwarf galaxies do exist in the Local Group.Because such galaxies form their stellar systems early during the periodof active merging and accretion, they should have a spheroidalmorphology regardless of their current distance from the host galaxy.Their observed counterparts should therefore be identified among thedwarf spheroidal galaxies. We show that both the expected luminosityfunction and the spatial distribution of dark matter halos that arelikely to host fossil galaxies agree reasonably well with the observeddistributions of the luminous (LV>~106Lsolar) Local Group fossil candidates near the host galaxy(d<~200 kpc). However, the predicted abundance is substantiallylarger (by a factor of 2-3) for fainter galaxies(LV<106 Lsolar) at larger distances(d>~300 kpc). We discuss several possible explanations for thisdiscrepancy.
| On Kinematic Substructure in the Sextans Dwarf Spheroidal Galaxy
We present multifiber echelle radial velocity results for 551 stars inthe Sextans dwarf spheroidal galaxy and identify 294 stars as probableSextans members. The projected velocity dispersion profile of the binneddata remains flat to a maximum angular radius of 30'. We introduce anonparametric technique for estimating the projected velocity dispersionsurface and use this to search for kinematic substructure. Our data donot confirm previous reports of a kinematically distinct stellarpopulation at the Sextans center. Instead we detect a region near theSextans core radius that is kinematically colder than the overallSextans sample with 95% confidence.Based on observations using the Magellan telescopes.
| 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.
| A Successful Targeted Search for Hypervelocity Stars
Hypervelocity stars (HVSs) travel with velocities so extreme thatdynamical ejection from a massive black hole is their only suggestedorigin. Following the discovery of the first HVS by Brown andcollaborators, we have undertaken a dedicated survey for more HVSs inthe Galactic halo and present here the resulting discovery of two newHVSs: SDSS J091301.0+305120 and SDSS J091759.5+672238, traveling withGalactic rest-frame velocities of at least +558 +/- 12 and +638 +/- 12km s-1, respectively. Assuming the HVSs are B8 main-sequencestars, they are at distances of ~75 and ~55 kpc, respectively, and havetravel times from the Galactic center consistent with their lifetimes.The existence of two B8 HVSs in our 1900 deg2 survey,combined with the Yu & Tremaine HVS rate estimates, is consistentwith HVSs drawn from a standard initial mass function but inconsistentwith HVS drawn from a truncated mass function like the one in thetop-heavy Arches cluster. The travel times of the five currently knownHVSs provide no evidence for a burst of HVSs from a major infall eventat the Galactic center in the last ~160 Myr.
| The Cosmological Significance of High-Velocity Cloud Complex H
We have used new and archival infrared and radio observations to searchfor a dwarf galaxy associated with the high-velocity cloud (HVC) knownas `complex H.' Complex H is a large (Ω>~400 deg2)and probably nearby (d=27 kpc) HVC whose location in the Galactic planehas hampered previous investigations of its stellar content. The H Imass of the cloud is 2.0×107(d/27 kpc)2Msolar, making complex H one of the most massive HVCs if itsdistance is more than ~20 kpc. Virtually all similar H I clouds in othergalaxy groups are associated with low surface brightness dwarf galaxies.We selected mid-infrared sources observed by the MSX satellite in thedirection of complex H that appeared likely to be star-forming regionsand observed them at the wavelength of the CO J=1-->0 rotationaltransition in order to determine their velocities. Of the 60 observedsources, 59 show emission at Milky Way velocities, and we detected noemission at velocities consistent with that of complex H. We use theseobservations to set an upper limit on the ongoing star formation rate inthe HVC of <~5×10-4 Msolaryr-1. We also searched the 2MASS database for evidence of anydwarf-galaxy-like stellar population in the direction of the HVC andfound no trace of a distant red giant population, with an upper limit onthe stellar mass of ~106 Msolar. Given the lack ofevidence for either current star formation or an evolved population, weconclude that complex H cannot be a dwarf galaxy with properties similarto those of known dwarfs. Complex H is therefore one of the most massiveknown H I clouds that does not contain any stars. If complex H isself-gravitating, then this object is one of the few known dark galaxycandidates. These findings may offer observational support for the ideathat the cold dark matter substructure problem is related to thedifficulty of forming stars in low-mass dark matter halos;alternatively, complex H could be an example of a cold accretion flowonto the Milky Way.
| Constraining Global Properties of the Draco Dwarf Spheroidal Galaxy
By fitting a flexible stellar anisotropy model to the observed surfacebrightness and line-of-sight velocity dispersion profiles of Draco wederive a sequence of cosmologically plausible two-component (stars +dark matter) models for this galaxy. The models are consistent with allthe available observations and can have either cuspy Navarro-Frenk-Whiteor flat-cored dark matter density profiles. The dark matter halos eitherformed relatively recently (at z~2-7) and are massive (up to~5×109 Msolar), or formed before the end ofthe reionization of the universe (z~7-11) and are less massive (down to~7×107 Msolar). Our results thus supporteither of the two popular solutions of the ``missing satellites''problem of Λ cold dark matter cosmology-that dwarf spheroidalsare either very massive or very old. We carry out high-resolutionsimulations of the tidal evolution of our two-component Draco models inthe potential of the Milky Way. The results of our simulations suggestthat the observable properties of Draco have not been appreciablyaffected by the Galactic tides after 10 Gyr of evolution. We rule outDraco being a ``tidal dwarf''-a tidally disrupted dwarf galaxy. Almostradial Draco orbits (with the pericentric distance <~15 kpc) are alsoruled out by our analysis. The case of a harmonic dark matter core canbe consistent with observations only for a very limited choice of Dracoorbits (with the apocentric-to-pericentric distances ratio of <~2.5).
| Further Evidence of a Merger Origin for the Thick Disk: Galactic Stars along Lines of Sight to Dwarf Spheroidal Galaxies
The history of the Milky Way is written in the properties of its stellarpopulations. Here we analyze stars observed as part of surveys of localdwarf spheroidal galaxies, but which from their kinematics are highlylikely to be nonmembers. The selection function-designed to targetmetal-poor giants in the dwarf galaxies, at distances of ~100kpc-includes F-M dwarfs in the Milky Way, at distances of up to severalkiloparsecs. The stars whose motions are analyzed here lie in thecardinal directions of Galactic longitude l~270° and l~90°,where the radial velocity is sensitive to the orbital rotationalvelocity. We demonstrate that the faint F and G stars contain asignificant population with Vφ~100 km s-1,similar to that found by a targeted, but limited in areal coverage,survey of thick disk and halo stars by Gilmore et al. This value of meanorbital rotation does not match either the canonical thick disk or thestellar halo. We argue that this population, detected at both l~270°and l~90°, has the expected properties of ``satellite debris'' inthe thick disk-halo interface, which we interpret as remnants of themerger that heated a preexisting thin disk to form the thick disk.
| Spectroscopy of QUEST RR Lyrae Variables: The New Virgo Stellar Stream
Eighteen RR Lyrae variables (RRLs) that lie in the ``12.4h clump''identified by the Quasar Equatorial Survey Team (QUEST) have beenobserved spectroscopically to measure their radial velocities and metalabundances. Ten blue horizontal branch (BHB) stars identified by theSloan Digital Sky Survey (SDSS) were added to this sample. Six of thenine stars in the densest region of the clump have a mean radialvelocity in the Galactic rest frame (Vgsr) of 99.8 andσ=17.3 km s-1, which is slightly smaller than theaverage error of the measurements. The whole sample contains eight RRLsand five BHB stars that have values of Vgsr suggestingmembership in this stream. For seven of these RRLs, the measurements of[Fe/H], which have an internal precision of 0.08 dex, yield<[Fe/H]>=-1.86 and σ=0.40. These values suggest that thestream is a tidally disrupted dwarf spheroidal galaxy of low luminosity.Photometry from the database of the SDSS indicates that this streamcovers at least 106 deg2 of the sky in the constellationVirgo. The name Virgo stellar stream is suggested.
| 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%.
| The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog
The first catalog of the RR Lyrae stars (RRLSs) in the Galactic halo bythe Quasar Equatorial Survey Team (QUEST) has been searched forsignificant overdensities that may be debris from disrupted dwarfgalaxies or globular clusters. These RRLSs are contained in a band ~2.3dwide in declination that spans ~165° in right ascension and lie ~4to ~60 kpc from the Sun. Away from the major overdensities, thedistribution of these stars is adequately fitted by a smooth halo model,in which the flattening of the halo decreases with increasinggalactocentric distance (as reported by Preston et al.). This model wasused to estimate the ``background'' of RRLSs on which the halooverdensities are overlaid. A procedure was developed for recognizinggroups of stars that constitute significant overdensities with respectto this background. To test this procedure, a Monte Carlo routine wasused to make artificial RRLS surveys that follow the smooth halo modelbut with Poisson-distributed noise in the numbers of RRLSs and, withinlimits, random variations in the positions and magnitudes of theartificial stars. The 104 artificial surveys created by thisroutine were examined for significant groups in exactly the same way asthe QUEST survey. These calculations provided estimates of thefrequencies with which random fluctuations produce significant groups.In the QUEST survey there are six significant overdensities that containsix or more stars and several smaller ones. The small ones and possiblyone or two of the larger ones may be artifacts of statisticalfluctuations, and they need to be confirmed by measurements of radialvelocity and/or proper motion. The most prominent groups are thenorthern stream from the Sagittarius dwarf spheroidal galaxy and a largegroup in Virgo, formerly known as the ``12.4 hr clump,'' which Duffauand coworkers have recently shown to contain a stellar stream (the Virgostellar stream). Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Anothergroup is related to the globular cluster Palomar 5.
| Neutral Hydrogen Clouds Near Early-Type Dwarf Galaxies of the Local Group
Parkes neutral hydrogen 21 cm line (H I) observations of thesurroundings of nine early-type Local Group dwarfs are presented. Wedetected numerous H I clouds in the general direction of those dwarfs,and these clouds are often offset from the optical center of thegalaxies. Although all the observed dwarfs, except Antlia, occupyphase-space regions where the high-velocity cloud (HVC) density is wellabove average, the measured offsets are smaller than one would expectfrom a fully random cloud distribution. Possible association is detectedfor 11 of the 16 investigated clouds, while for two galaxies, Sextansand Leo I, no H I was detected. The galaxies in which H I clouds werefound not to coincide with the optical yet have a significantprobability of being associated are the Sculptor dwarf, Tucana, LGS 3,Cetus, and Fornax. If the clouds are indeed associated, these galaxieshave H I masses of MHI=2×105,2×106, 7×105, 7×105,and 1×105 Msolar, respectively. However,neither ram pressure nor tidal stripping can easily explain the offsets.In some cases, large offsets are found where ram pressure should be theleast effective.
| Internal Kinematics of the Fornax Dwarf Spheroidal Galaxy
We present new radial velocity results for 176 stars in the Fornax dwarfspheroidal galaxy, of which at least 156 are probable Fornax members. Wecombine with previously published data to obtain a radial velocitysample with 206 stars, of which at least 176 are probable Fornaxmembers. We detect the hint of rotation about an axis near Fornax'smorphological minor axis, although the significance of the rotationsignal in the galactic rest frame is sensitive to the adopted value ofFornax's proper motion. Regardless, the observed stellar kinematics isdominated by random motions, and we do not find kinematic evidence oftidal disruption. The projected velocity dispersion profile of thebinned data set remains flat over the sampled region, which reaches amaximum angular radius of 65'. Single-component King models in whichmass follows light fail to reproduce the observed flatness of thevelocity dispersion profile. Two-component (luminous plus dark matter)models can reproduce the data, provided that the dark component extendssufficiently beyond the luminous component and the central dark matterdensity is of the same order as the central luminous density. Theserequirements suggest a more massive, darker Fornax than standardcore-fitting analyses have previously concluded, with M/LVover the sampled region reaching 10-40 times the M/LV of theluminous component. We also apply a nonparametric mass estimationtechnique, introduced in a companion paper. Although it is designed tooperate on data sets containing velocities for >1000 stars, theestimation yields preliminary results suggesting M/LV~15inside r<1.5 kpc.
| The Proper Motion of the Large Magellanic Cloud: A Reanalysis
We have determined the proper motion (PM) of the Large Magellanic Cloud(LMC) relative to four background quasi-stellar objects, combining datafrom two previous studies made by our group and new observations carriedout in three epochs not included in the original investigations. The newobservations provided a significant increase in the time base and thenumber of frames, relative to what was available in our previousstudies. We have derived a total LMC PM of μ=(+2.0+/-0.1) masyr-1, with a position angle of θ=62.4d+/-3.1d. Our newvalues agree well with most results obtained by other authors, and webelieve we have clarified the large discrepancy between previous resultsfrom our group. Using published values of the radial velocity for thecenter of the LMC, in combination with the transverse velocity vectorderived from our measured PM, we have calculated the absolute spacevelocity of the LMC. This value, along with some assumptions regardingthe mass distribution of the Galaxy, has in turn been used to calculatethe mass of the Milky Way. Our measured PM also indicates that the LMCis not a member of a proposed stream of galaxies with similar orbitsaround our Galaxy.
| Proper Motions of Dwarf Spheroidal Galaxies from Hubble Space Telescope Imaging. IV. Measurement for Sculptor
This article presents a measurement of the proper motion of the Sculptordwarf spheroidal galaxy determined from images taken with the HubbleSpace Telescope using the Space Telescope Imaging Spectrograph in theimaging mode. Each of two distinct fields contains a quasi-stellarobject that serves as the ``reference point.'' The measured propermotion of Sculptor, expressed in the equatorial coordinate system, is(μα, μδ)=(9+/-13, 2+/-13) mascentury-1. Removing the contributions from the motion of theSun and the motion of the local standard of rest produces the propermotion in the Galactic rest frame:(μGrfα,μGrfδ)=(-23+/-13, 45+/-13) mascentury-1. The implied space velocity with respect to theGalactic center has a radial component of Vr=79+/-6 kms-1 and a tangential component of Vt=198+/-50 kms-1. Integrating the motion of Sculptor in a realisticpotential for the Milky Way produces orbital elements. Theperigalacticon and apogalacticon are 68 (31, 83) and 122 (97, 313) kpc,respectively, where the values in the parentheses represent the 95%confidence interval derived from Monte Carlo experiments. Theeccentricity of the orbit is 0.29 (0.26, 0.60), and the orbital periodis 2.2 (1.5, 4.9) Gyr. Sculptor is on a polar orbit around the MilkyWay: the angle of inclination is 86° (83°, 90°).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.
| The Anisotropic Distribution of M31 Satellite Galaxies: A Polar Great Plane of Early-type Companions
The highly anisotropic distribution and apparent alignment of theGalactic satellites in polar great planes begs the question of howcommon such distributions are. The satellite system of M31 is the onlynearby system for which we currently have sufficiently accuratedistances to study the three-dimensional satellite distribution. Wepresent the spatial distribution of the 15 currently known M31companions in a coordinate system centered on M31 and aligned with itsdisk. Through a detailed statistical analysis we show that the fullsatellite sample describes a plane that is inclined by -56° withrespect to the poles of M31 and has an rms height of 100 kpc. At 88% thestatistical significance of this plane is low, and it is unlikely tohave a physical meaning. We note that the great stellar stream foundnear Andromeda is inclined to this plane by 7°. Most of the M31satellites are found within <+/-40° of M31's disk; i.e., there islittle evidence for a Holmberg effect. If we confine our analysis toearly-type dwarfs, we find a best-fit polar plane within 5°-7°from the pole of M31. This polar great plane has a statisticalsignificance of 99.7% and includes all dSphs (except for And II), M32,NGC 147, and PegDIG. The rms distance of these galaxies from the polarplane is 16 kpc. The nearby spiral M33 has a distance of only ~3 kpcfrom this plane, which points toward the M81 group. We discuss theanisotropic distribution of M31's early-type companions in the frameworkof three scenarios, namely, as remnants of the breakup of a largerprogenitor, as a tracer of a prolate dark matter halo, and as a tracerof collapse along large-scale filaments. The first scenario requiresthat the breakup must have occurred at very early times and that thedwarfs continued to form stars thereafter to account for their stellarpopulation content and luminosity-metallicity relation. The thirdscenario seems to be plausible, especially when considering the apparentalignment of our potential satellite filament with several nearbygroups. The current data do not permit us to rule out any of thescenarios. Orbit information is needed to test the physical reality ofthe polar plane and of the different scenarios in more detail.
| The Nature of the Density Clump in the Fornax Dwarf Spheroidal Galaxy
We have imaged the recently discovered stellar overdensity locatedapproximately one core radius from the center of the Fornax dwarfspheroidal galaxy using the Magellan Clay 6.5 m telescope with theMagellan Instant Camera. Superb seeing conditions allowed us to probethe stellar populations of this overdensity and of a control fieldwithin Fornax to a limiting magnitude of R=26. The color-magnitudediagram of the overdensity field is virtually identical to that of thecontrol field, with the exception of the presence of a populationarising from a very short (less than 300 Myr in duration) burst of starformation 1.4 Gyr ago. Coleman et al. have argued that this overdensitymight be related to a shell structure in Fornax that was created whenFornax captured a smaller galaxy. Our results are consistent with thismodel, but we argue that the metallicity of this young component favorsa scenario in which the gas was part of Fornax itself.
| Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy
The Carina dwarf spheroidal galaxy is the only galaxy of this type thatshows clearly episodic star formation separated by long pauses. Here wepresent metallicities for 437 radial velocity members of this Galacticsatellite. The metallicities and radial velocities were measured as partof a Large Programme with the Very Large Telescope at the EuropeanSouthern Observatory, Chile. We obtained medium-resolution spectroscopywith the multiobject spectrograph FLAMES. Our target red giants coverthe entire projected surface area of Carina. Our spectra are centered atthe near-infrared Ca II triplet, which is a well-established metallicityindicator for old and intermediate-age red giants. The resulting datasample provides the largest collection of spectroscopically derivedmetallicities for a Local Group dwarf spheroidal galaxy to date. Four ofour likely radial velocity members of Carina lie outside this galaxy'snominal tidal radius, supporting earlier claims of the possibleexistence of such stars beyond the main body of Carina. We find a meanmetallicity of [Fe/H]~-1.7 dex on the 1997 metallicity scale of Carrettaand Gratton for Carina. The formal FWHM of the metallicity distributionfunction is 0.92 dex, while the full range of metallicities is found tospan approximately -3.0 dex<[Fe/H]<0.0 dex. The metallicitydistribution function might be indicative of several subpopulationsdistinct in metallicity. There appears to be a mild radial gradient suchthat more metal-rich populations are more centrally concentrated,matching a similar trend for an increasing fraction of intermediate-agestars (see the 2001 work of Harbeck and coworkers). This, as well as thephotometric colors of the more metal-rich red giants, suggests thatCarina exhibits an age-metallicity relation. Indeed, the age-metallicitydegeneracy seems to conspire to form a narrow red giant branch despitethe considerable spread in metallicity and wide range of ages. Themetallicity distribution function is not well matched by a simpleclosed-box model of chemical evolution. Qualitatively better matches areobtained by chemical models that also take into account infall andoutflows. A G dwarf problem remains for all these models.Based on observations collected at the European Southern Observatory atParanal, Chile; proposal 171.B-0520(A).
| Exploring Halo Substructure with Giant Stars. VIII. The Extended Structure of the Sculptor Dwarf Spheroidal Galaxy
We explore the spatial distribution of stars in the Sculptor dwarfspheroidal (dSph) galaxy over an area of 7.82 deg2, includingcoverage of the central region but extending mostly south and east ofthe dSph core. Two methods are used to identify stars that are mostlikely associated with the dSph, and these filtered samples of stars areused to map its spatial structure. First, following the method ofprevious contributions in this series, we utilize Washington M,T2+DDO51 photometry to identify red giant branch (RGB) starcandidates with approximately the same distance and metallicity as theSculptor dSph. Second, a prominent blue horizontal branch (BHB)population provides a fairly populous and pure sample of Sculptor starshaving broadband colors unlike the bulk of the Galactic field starpopulation. A spectroscopically observed subset of Sculptor candidatestars (147 total stars: ~5% of all Sculptor candidates and ~10% ofSculptor giant candidates) yields a systemic heliocentric velocity forthe system of vhel=110.43+/-0.79 km s-1, in goodagreement with previous studies. We also find a global velocitydispersion of σv=8.8+/-0.6 km s-1, withslight indications of a rise in the velocity dispersion past ~0.4rlim.These spectra also provide a check on the reliability of our candidateSculptor giant sample to M~19 94% of the photometrically selectedSculptor giant star candidates with follow-up spectroscopy are found tobe kinematically associated with Sculptor, while 4 out of 10 starsoutside of our Sculptor giant star selection criteria that we testedspectroscopically appear to be velocity members of Sculptor. Thesepercentages are in agreement with results for an additional 22 Sculptorfield stars with radial velocities in the literature. All availablevelocities show that our methodology for picking Sculptor giants is bothreliable and conservative. Thus, these giant star samples should providea reliable means to explore the structure of the Sculptor dSph.Nevertheless, considerable care has been taken to assess the level ofbackground contamination in our photometric sample to ensure anaccurately derived density profile of the Sculptor dSph to large radii.Multiple background assessments verify that we detect a considerablestellar density of Sculptor stars to the limits of our main survey areafor both the RGB and BHB candidate samples. While we find that a Kingprofile of limiting radius rlim=79.6 arcmin fits the densityprofile of Sculptor well to ~60', beyond this, we identify a ``break''in the profile and a clearly detected population of Sculptor starsfollowing a Σ~r-2 decline to more than 2rlim. Thisbreak population must signify either the presence of an extremely broaddistribution of bound ``halo stars'' around the Sculptor dSph or thepresence of unbound tidal debris. If the latter is true, we determine afractional mass-loss rate of approximately 0.042 Gyr-1 forthe Sculptor dSph. Additional support for the notion that there is tidaldisruption comes from the two-dimensional distribution of our Sculptorcandidate stars; both the RGB and BHB samples show increasinglyelongated isodensity contours with radius that point to an apparentstretching reminiscent of what is seen in models of disrupting satellitegalaxies. Finally, we find that RGB stars that are more likely to bemetal-poor (based on their color and magnitude) are significantly lesscentrally concentrated and therefore constitute the primary contributingstellar population to the likely tidally stripped parts of the dSph.
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