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|DDO 44 and UGC 4998: Distances, Metallicities, and Star Formation Histories|
We have obtained deep HST imaging of two candidate dwarf systems in thenearby M81 Group, DDO 44 and UGC 4998. Both are isolated, low surfacebrightness systems, but with likely very different star formationhistories based on their mean colors and color fluctuations. Thecolor-magnitude diagrams of these galaxies have been used to estimatetheir distances using the tip of the red giant branch (RGB), and theirmean metallicities from the colors of the RGB. For DDO 44 we find adistance of D=3.01+/-0.18 Mpc-confirming it to be a member of the M81Group-and a mean metallicity of [Fe/H]=-1.54+/-0.14. We also find fromthe properties and numbers of a population of luminous asymptotic giantbranch (AGB) stars that about 20% of the luminous population of DDO 44consists of ``intermediate-age'' stars (with ages between about 2 and 8Gyr) that give rise to the observed AGB. There is no difference in thespatial distribution of the RGB and AGB stars in this galaxy. For UGC4998 we derive a metallicity of [Fe/H]=-1.58+/-0.21 and a distance ofD=8.24+/-0.43 Mpc. The latter puts UGC 4998 well behind the M81 Group,confirming results from previous measurements. We find this galaxy tocontain a population of young (<50 Myr old) stars, and there isevidence of older RGB stars. The young stars are considerably morecentrally concentrated than the older stars. The current star formationrate in UGC 4998 is ~9×10-4 Msolaryr-1 over the entire galaxy, or ~5×10-4Msolar yr-1 kpc-2. Given its H I mass(1×107 Msolar), this galaxy can continue toform stars at this rate for another ~11 Gyr.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations are associated with proposalGO-8137.
|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.
|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.
|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.
|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.
|Origin of Two Distinct Populations in Dwarf Spheroidal Galaxies|
We study the chemical and kinematic properties of the first galaxiesthat formed at high redshift, using high-resolution cosmologicalnumerical simulations, and compare them with the recent observationalresults for the Sculptor dwarf spheroidal galaxy of Tolstoy et al., whofound two distinct stellar populations: the lower metallicity stars aremore spatially extended and possess a higher velocity dispersion thanthe higher metallicity stars. Our calculations reproduce theseobservations as the result of a steep metallicity gradient within asingle population, induced by dissipative collapse of the gas component.We also predict strong [N/O] enhancements in the lowest metallicitystars in dwarf spheroidals, due to the preferential retention of ejectedgas from intermediate-mass stars, compared to Type II supernovae.
|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.
|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.
|Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images|
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
|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.
|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.
|The Star Clusters of the Small Magellanic Cloud: Structural Parameters|
We present structural parameters for 204 stellar clusters in the SmallMagellanic Cloud derived from fitting King and Elson, Fall, and Freeman(EFF) model profiles to the V-band surface brightness profiles asmeasured from the Magellanic Clouds Photometric Survey images. Both Kingand EFF profiles are satisfactory fits to the majority of the profiles,although King profiles are generally slightly superior to the softenedpower-law profiles of EFF and provide statistically acceptable fits to~90% of the sample. We find no correlation between the preferred modeland cluster age. The only systematic deviation in the surface brightnessprofiles that we identify is a lack of a central concentration in asubsample of clusters, which we designate as ``ring'' clusters. Inagreement with previous studies, we find that the clusters in the SMCare significantly more elliptical than those in the Milky Way. However,given the mean age difference and the rapid destruction of thesesystems, the comparison between the SMC and the Milky Way should notdirectly be interpreted as a difference in either the initial clusterproperties or their subsequent evolution. We find that clusterellipticity correlates with cluster mass more strongly than with clusterage. We identify several other correlations (central surface brightnessvs. local background density, core radius vs. tidal force, and size vs.distance) that can be used to constrain models of cluster evolution inthe SMC.
|Globular Clusters in Dwarf Galaxies|
Data are currently available on the luminosities and half-light radii of101 globular clusters associated with low-luminosity parent galaxies.The luminosity distribution of globular clusters embedded in dwarfgalaxies having Mv>-16 is found to differ dramaticallyfrom that for globular clusters surrounding giant host galaxies withMv<-16. The luminosity distribution of globular clustersin giant galaxies peaks at Mv~-7.5, whereas that for dwarfgalaxies is found to increase monotonically down to the completenesslimit of the cluster data at Mv~-5.0. Unexpectedly, thepower-law distribution of the luminosities of globular clusters hostedby dwarf galaxies is seen to be much flatter than that of the bright,unevolved part of the luminosity distribution of globular clustersassociated with giant galaxies. The specific frequency of globularclusters fainter than Mv=-7.5 is found to be particularlyhigh in dwarf galaxies. The luminosity distribution of the LMC globularclusters is similar to that found in giant galaxies and differs fromthose of globular clusters in dwarf galaxies. The present data appear toshow no strong dependence of globular cluster luminosity on themorphological type of their parent galaxies. No attempt is made toexplain the unexpected discovery that the luminosity distribution ofglobular clusters is critically dependent on parent galaxy luminosity(or mass) but insensitive to the morphological type of the host galaxy.
|VLT/UVES spectroscopy of individual stars in three globular clusters in the Fornax dwarf spheroidal galaxy|
We present a high resolution (R ~ 43 000) abundance analysis of a totalof nine stars in three of the five globular clusters associated with thenearby Fornax dwarf spheroidal galaxy. These three clusters (1, 2 and 3)trace the oldest, most metal-poor stellar populations in Fornax. Wedetermine abundances of O, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba, Nd andEu in most of these stars, and for some stars also Mn and La. Wedemonstrate that classical indirect methods (isochrone fitting andintegrated spectra) of metallicity determination lead to values of[Fe/H] which are 0.3 to 0.5 dex too high, and that this is primarily dueto the underlying reference calibration typically used by these studies.We show that Cluster 1, with [Fe /H] = -2.5, now holds the record forthe lowest metallicity globular cluster. We also measure anover-abundance of Eu in Cluster 3 stars that has only been previouslydetected in a subgroup of stars in M 15. We find that the Fornaxglobular cluster properties are a global match to what is found in theirGalactic counterparts; including deep mixing abundance patterns in twostars. We conclude that at the epoch of formation of globular clustersboth the Milky Way and the Fornax dwarf spheroidal galaxy shared thesame initial conditions, presumably pre-enriched by the same processes,with identical nucleosynthesis patterns.
|Globular cluster system and Milky Way properties revisited|
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
|Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452|
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
|Implications of elemental abundances in dwarf spheroidal galaxies|
Unusual elemental abundance patterns observed for stars belonging tonearby dwarf spheroidal (dSph) galaxies are discussed. Analysis of the [α/H] vs. [Fe/H] diagrams where α represents Mg or an averageof α-elements reveals that Fe from type Ia supernovae (SNe Ia)does not contribute to the stellar abundances in the dSph galaxies wherethe member stars exhibit low α/Fe ratios except for the mostmassive dSph galaxy, Sagitarrius. The more massive dwarf (irregular)galaxy, the Large Magellanic Cloud, also has an SNe Ia signature in thestellar abundances. These findings suggest that whether SNe Iacontribute to chemical evolution in dwarf galaxies is likely to dependon the mass scale of galaxies. Unusual Mg abundances in some dSph starsare also found to be the origin of the large scatter in the [Mg/Fe]ratios and are responsible for a seemingly decreasing [Mg/Fe] featurewith increasing [Fe/H]. In addition, the lack of massive stars in thedSph galaxies does not satisfactorily account for the low-αsignature. Considering the assembly of deficient elements (O, Mg, Si,Ca, Ti and Zn), all of which are synthesized in pre-SN massive stars andin SN explosions, the low-α signature appears to reflect theheavy-element yields of massive stars with less rotation compared tosolar neighborhood stars.
|Homogeneous Photometry. IV. On the Standard Sequence in the Globular Cluster NGC 2419|
I present a new analysis of CCD-based BVRI broadband photometry for theglobular cluster NGC 2419, based on 340 CCD images either donated bycolleagues or retrieved from public archives. The calibrated resultshave been made available through my Web site. I compare the results ofmy analysis with those of an independent analysis of a subset of thesedata by Saha et al. (2005, PASP, 117, 37), who have found acolor-dependent discrepancy of up to 0.05 mag between their I-bandphotometry and mine for stars in this cluster. I conclude that a majorpart of the discrepancy appears to be associated with small shuttertiming errors (a few hundredths of a second) in the Mini-Mosaic (MIMO)camera on the WIYN 3.5 m telescope. Smaller contributions to the anomalylikely come from (1) a color-scale error with a maximum amplitude of~+/-0.02 mag in my secondary standard list as of 2004 September, and (2)statistical effects arising from the previous study's use of arelatively small number of standard-star observations to determine acomparatively large number of fitting parameters in the photometrictransformations.Based in part on observations obtained at the 3.5 m and 0.9 m WIYNTelescopes. The WIYN Observatory is a joint facility of the Universityof Wisconsin-Madison, Indiana University, Yale University, and theNational Optical Astronomy Observatory (NOAO).
|Astrophysics in 2004|
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
|A Dynamical Model for the Orbit of the Andromeda Galaxy M31 and the Origin of the Local Group of Galaxies|
We propose a new model for the origin and evolution of the Local Groupof Galaxies (LGG) that naturally explains the formation of theMagellanic Clouds and their large orbital angular momenta around theGalaxy. The basic idea is that an off-center hydrodynamical collisionoccurred some 10Gyr ago between the primordial Andromeda galaxy (M31)and a similar Galaxy, and compressed the halo gas to form the LGG dwarfgalaxies, including the Magellanic Clouds. New-born dwarf galaxies canbe expected to locate on the orbital plane of these two massivegalaxies. We reexamined the two-dimensional sky distribution of the LGGmembers, and confirmed an early idea that they align along two similargreat circles. The planes of these circles are approximately normal tothe line joining the present position of the Sun and the galacticcenter. We made a distribution map of these objects, and found awell-defined plane of finite thickness. Thus we could determine theorbital elements of M31 relative to the Galaxy by reproducing thewell-studied dynamics of the LMC and the SMC around the Galaxy. Theexpected proper motion of M31 is (μl, μb) =(38 ± 16 μas yr-1, -49 ± 5 μasyr-1).
|Wide field imaging from space: Galaxy formation from nearby stellar populations [review article]|
A wide field, high resolution imaging facility in space would enablebreakthrough science in the study of stellar populations. Optical highresolution imaging at the diffraction limit of a 2 m telescope is aminimum requirement. In the Galaxy and Local Group, proper motionstudies could settle the contribution of cool white dwarfs to the halodark matter mass and define the dynamics of the Galactic bulge. Thewhite dwarf cooling age method could be applied to a larger number ofglobular clusters and could include measurements covering the whole of agiven cluster. This powerful method can constrain the age of theUniverse as well as the early star formation history of globularclusters. A wide field imager could derive main sequence turnoff agesfor the interaction streams and multiple globular clusters in the M31halo, as well as in other Local Group galaxies. Imaging in the halos ofnearby galaxies to 10 Mpc, including the Virgo cluster, could map outtidal streams and debris tail and would help to define the stellarpopulations (and therefore the assembly history and nature of) Galactichalos.
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