Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
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.
| The stellar content of the isolated transition dwarf galaxy DDO210
We use Subaru Suprime-Cam and VLT FORS1 photometry of the dwarf galaxyDDO210 to study the global stellar content and structural properties ofa transition-type galaxy (with properties intermediate between dwarfirregular and dwarf spheroidal systems). This galaxy is sufficientlyisolated that tidal interactions are not likely to have affected itsevolution in any way. The colour-magnitude diagrams of DDO210 show a redgiant branch (RGB) population (with an RGB bump), a bright asymptoticgiant branch population, a red clump, young main-sequence stars andblue-loop stars. The youngest stars formed within the last 60Myr andhave a distinct radial distribution compared to the main population.Whereas the overall stellar spatial distribution and HI spatialdistribution are concentric, the young stars are offset from the centreof DDO210 and are coincident with a `dent' in the HI distribution. Theimplied recent star formation rate required to form the young populationis significantly higher than the derived current star formation rate, bya factor of >10.Most of the stars in DDO210 are found in a red clump, and its meanI-band magnitude suggests that the majority of stars in DDO210 have anaverage age of 4+2-1Gyr. Given this age, thecolour of the RGB implies a mean metallicity of [Fe/H] ~= -1.3. Bycomparing the shape of the red clump with models for a variety of starformation histories, we estimate that an old (>10 Gyr) stellarpopulation can contribute ~20-30 per cent of the stars in DDO210 atmost. The unusual star formation history of DDO210, its low-massestimate and its isolated nature, provide insight into how starformation proceeds in the lowest mass, unperturbed, dwarf galaxy haloes.Based in part on data collected at Subaru Telescope, which is operatedby the National Astronomical Observatory of JapanE-mail: alan@uvic.ca
| Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy
We present the discovery of three faint dwarf galaxies and a globularcluster in the halo of the Andromeda galaxy (M31), found in our MegaCamsurvey that spans the southern quadrant of M31, from a projecteddistance of ~50 to ~150kpc. Though the survey covers 57 deg2,the four satellites lie within 2° of one another. From the tip ofthe red giant branch (RGB), we estimate that the globular cluster liesat a distance of 631 +/- 58kpc from the Milky Way and along with a~100kpc projected distance from M31 we derive a total distance of 175+/- 55kpc from its host, making it the farthest M31 globular clusterknown. It also shows the typical characteristics of a bright globularcluster, with a half-light radius of 2.3 +/- 0.2pc and an absolutemagnitude in the V band of MV,0 = -8.5 +/- 0.3. Isochronefitting reveals that it is dominated by a very old population with ametallicity of [Fe/H] ~ -1.3. The three dwarf galaxies are revealed asoverdensities of stars that are aligned along the RGB tracks in theircolour-magnitude diagrams. These satellites are all very faint, withabsolute magnitudes in the range -7.3 <~ MV,0 <~ -6.4,and show strikingly similar characteristics with metallicities of [Fe/H]~ -1.4 and half-light radii of ~120 +/- 45pc, making these dwarfgalaxies two to three times smaller than the smallest previously knownsatellites of M31. Given their faintness, their distance is difficult toconstrain, but we estimate them to be between 740 and 955kpc whichplaces them well within the virial radius of the host galaxy. Thepanoramic view of the MegaCam survey can provide an unbiased view of thesatellite distribution of the Andromeda galaxy and, extrapolating fromits coverage of the halo, we estimate that up to 45 +/- 20 satellitesbrighter than MV ~ -6.5 should be orbiting M31. Hence faintdwarf galaxies cannot alone account for the missing satellites that arepredicted by Λ cold dark matter models, unless they reside indark matter minihaloes that are more massive than the typical masses of107Msolar currently inferred from their centralradial velocity dispersion.Based on observations obtained with MegaPrime/MegaCam, a joint projectof CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT)which is operated by the National Research Council (NRC) of Canada, theInstitut National des Science de l'Univers of the Centre National de laRecherche Scientifique (CNRS) of France and the University of Hawaii.E-mail: martin@astro.u-strasbg.fr
| 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.
| A radial velocity survey of low Galactic latitude structures - III. The Monoceros Ring in front of the Carina and Andromeda galaxies
As part of our radial velocity survey of low Galactic latitudestructures that surround the Galactic disc, we report the detection ofthe so-called Monoceros Ring in the foreground of the Carina dwarfgalaxy at Galactic coordinates (l, b) = (260°, -22°) based onVery Large Telescope/Fibre Large Array Multi Element Spectrographobservations of the dwarf galaxy. At this location, 20° in longitudegreater than previous detections, the Ring has a mean radial velocity of145 +/- 5kms-1 and a velocity dispersion of only 17 +/-5kms-1. Based on Keck/Deep Imaging Multi-Object Spectrographobservations, we also determine that the Ring has a mean radial velocityof -75 +/- 4kms-1 in the foreground of the Andromeda galaxyat (l, b) ~ (122°, -22°), along with a velocity dispersion of 26+/- 3kms-1. These two kinematic detections are both highlycompatible with known characteristics of the structure and, along withprevious detections, provide radial velocity values of the Ring over the120° < l < 260° range. This should place strongconstraints on numerical models of the accretion of the dwarf galaxythat is believed to be the progenitor of the Ring.
| 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.
| Carbon stars in local group dwarf galaxies: C and O abundances
We present abundances of carbon and oxygen as well as abundance ratios12C/13C for a sample of carbon stars in the LMC,SMC, Carina, Sculptor and Fornax dwarf galaxies. The overallmetallicities in these dwarf galaxies are lower than in the galacticdisc. The observations cover most of the AGB and we discuss theabundance patterns in different regions along the AGB. The abundancesare determined from infrared spectra obtained with the ISAACspectrometer on VLT (R=1500) and the Phoenix Spectrometer on GeminiSouth (R=50000). The synthetic spectra used in the analysis werecomputed with MARCS model atmospheres. We find that the oxygen abundanceis decreasing with decreasing overall metallicity of the system whilethe C/O ratio at a given evolutionary phase is increasing withdecreasing oxygen abundance.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO Programme 70.D-0414 & 072.D-0501)
| Search for variable stars in the Carina dSph galaxy.
We discuss methods and preliminary results in a search for variablestars in the Carina dSph galaxy. This galaxy presents a complex starformation history and hosts a unique sample of variable stars. Wecollected B,V time series data during different runs, from 1999 to 2004,with the MOSAICII camera available at 4m CTIO telescope. The methodologyis based on correlations between a version of the Welch-Stetsonvariability index, the variability probability of the Lomb-Scargleperiodogram, and general statistical parameters associated with theindividual light curves.
| Stellar populations in the Carina dSph galaxy.
The small and elusive Carina dSph still presents many open questionsconcerning its stellar populations and dynamical evolution. It presentsa unique star formation history, with well separated episodes over atime of 10 Gyr. However, the complex star formation history doesnot seem to have caused a strong chemical evolution of its stellarcontent. Moreover, the occurrence of extra-tidal stars has not beenfirmly established. These might be the signature of ongoing tidalinteraction with the Galaxy, or belong to a diffuse halo as suggested byrecent N-body simulations. We undertook a photometric and spectroscopicinvestigation over a substantial fraction of the Carina body, to fullycharacterize its stellar content.
| A large and homogeneous sample of SX Phe stars in the Fornax dwarf spheroidal galaxy .
We report on the detection of sixty-one SX Phe stars in the Fornax dwarfspheroidal galaxy using the Wide-Field Imager at the 2.2m ESO-MPItelescope. In spite of their extreme faintness (22.0
| The Millimeter- and Submillimeter-Wave Spectrum of Iso-Propanol [(CH3)2CHOH]
Iso-propanol [(CH3)2CHOH], an isomer ofn-propanol, has been studied in the millimeter- and submillimeter-waveregion of the electromagnetic spectrum with our FASSST spectrometerthrough 360 GHz. Spectra arising from the ground vibrational state ofall three hydroxyl torsional substates, given the labels symmetricgauche, antisymmetric gauche, and trans in order of increasing energy,have been observed. We have successfully assigned ~7600 pure rotationaltransitions within the torsional substates as well as ~4700torsional-rotational transitions between the symmetric and antisymmetricgauche substates through the lower rotational quantum numberJ''=68. Spectral lines involving one or both of the twogauche forms have been simultaneously analyzed with a 2×2effective torsional-rotational Hamiltonian, which includes terms throughfifth order in the torsional-rotational interaction. Excluding perturbedtransitions, the assigned transitions were fitted to a root mean squaredeviation of 76 kHz. The trans substate was analyzed as a semirigidrotor, and its unperturbed transitions fitted to a root mean squaredeviation of 63 kHz. A perturbation was seen at transitions withJ''>50 in the trans substate. The torsional excitationenergy for the trans substate above ground was estimated from intensityratios to be about 120 K.
| The Local Group Stellar Populations Archive from the Hubble Space Telescope WFPC2
We present a database (LOGPHOT) of stellar photometry of Local Groupgalaxies obtained with the Hubble Space Telescope using the Wide FieldPlanetary Camera 2 (WFPC2). The database includes photometry from allWFPC2 observations taken through 2003 with long exposures (>500 s) inF555W and F814W, and many observations in which long exposures weretaken in at least two broadband filters. We have attempted to derive anduse techniques that produce the best photometry; the database has beenfully populated using the HSTphot photometry package. To test theeffects of different techniques, independent reductions were made for afew fields, and the comparison of these highlights some important issuesand gives an estimate of plausible errors; these tests also led to someminor modifications and improvements to HSTphot. We provide bothpoint-spread function photometry and subtracted-frame aperturephotometry and discuss the merits of each. The database is availableelectronically. In addition to discussing the techniques used toconstruct the database, we present color-magnitude diagrams from singlefields in each of the Local Group galaxies that have been observed;these provide an educational and visual display of the variety of starformation histories observed in Local Group galaxies.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| A New View of the Dwarf Spheroidal Satellites of the Milky Way from VLT FLAMES: Where Are the Very Metal-poor Stars?
As part of the Dwarf galaxies Abundances and Radial-velocities Team(DART) program, we have measured the metallicities of a large sample ofstars in four nearby dwarf spheroidal galaxies (dSph's): Sculptor,Sextans, Fornax, and Carina. The low mean metal abundances and thepresence of very old stellar populations in these galaxies havesupported the view that they are fossils from the early universe.However, contrary to naive expectations, we find a significant lack ofstars with metallicities below [Fe/H]~-3 dex in all four systems. Thissuggests that the gas that made up the stars in these systems had beenuniformly enriched prior to their formation. Furthermore, the metal-poortail of the dSph metallicity distribution is significantly differentfrom that of the Galactic halo. These findings show that the progenitorsof nearby dSph's appear to have been fundamentally different from thebuilding blocks of the Milky Way, even at the earliest epochs.Based on FLAMES/GIRAFFE observations collected at the European SouthernObservatory, proposal 171.B-0588.
| Exploring Halo Substructure with Giant Stars: The Dynamics and Metallicity of the Dwarf Spheroidal in Boötes
We report the results of a spectroscopic study of the Boötes (Boo)dwarf spheroidal (dSph) galaxy carried out with the WIYN telescope andthe Hydra multifiber spectrograph. Radial velocities have been measuredfor 58 Boo candidate stars selected to have magnitudes and colorsconsistent with its red and asymptotic giant branches. Within the 13'half-light radius, seven members of Boo yield a systemic velocity ofVr=95.6+/-3.4 km s-1 and a velocity dispersion ofσ0=6.6+/-2.3 km s-1. This implies a mass onthe order of 1×107 Msolar, similar to theinferred masses of other Galactic dSphs. Adopting a total Boo luminosityof L=1.8×104 to 8.6×104Lsolar implies M/L~610-130, making Boo, the most distortedknown Milky Way dwarf galaxy, potentially also the darkest. From thespectra of Boo member stars we estimate its metallicity to be [Fe/H]~-2.5, which would make it the most metal-poor dSph known to date.
| Exploring Halo Substructure with Giant Stars. XI. The Tidal Tails of the Carina Dwarf Spheroidal Galaxy and the Discovery of Magellanic Cloud Stars in the Carina Foreground
A new large-area Washington M, T2+DDO51 filter survey of morethan 10 deg2 around the Carina dSph galaxy reveals aspectroscopically confirmed power-law radial density ``break''population of Carina giant stars extending several degrees beyond thecentral King profile. Magellan telescope MIKE spectroscopy establishesthe existence of Carina stars to at least 4.5 times its central Kinglimiting radius, rlim, and primarily along Carina's majoraxis. To keep these stars bound to the dSph would require a globalCarina mass-to-light ratio of M/L>=6300 (M/L)solar. TheMIKE velocities, supplemented with ~950 additional Carina fieldvelocities from archived VLT+GIRAFFE spectra with r<~rlim,demonstrate a nearly constant Carina velocity dispersion(σv) to just beyond r=rlim and both a risingσv and a velocity shear at still larger radii.Together, the observational evidence suggests that the discoveredextended Carina population represents tidal debris from the dSph. Of 65giant candidates at large angular radii from the Carina center for whichMIKE spectra have been obtained, 94% are associated with either Carinaor a second, newly discovered diffuse, but strongly radialvelocity-coherent (σv=9.8 km s-1),foreground halo system. The 15 stars in this second, retrograde velocitypopulation have (1) a mean metallicity ~1 dex higher than that of Carinaand (2) colors and magnitudes consistent with the red clump of the LMC.Additional spectroscopy of giant star candidates in fields linkingCarina and the LMC shows a smooth velocity gradient between the LMC andthe retrograde Carina moving group. We conclude that we have foundMagellanic stars almost twice as far (22°) from the LMC center thanpreviously known.
| 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.
| 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.
| 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.
| 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.
| 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%.
| VI Photometry of Globular Clusters NGC 6293 and NGC 6541: The Formation of the Metal-poor Inner Halo Globular Clusters
We present VI photometry of the metal-poor inner halo globular clustersNGC 6293 and NGC 6541 using the WFPC2 on board the Hubble SpaceTelescope (HST). Our color-magnitude diagrams of the clusters showwell-defined blue horizontal-branch populations, consistent with theirlow metallicities and old ages. NGC 6293 appears to have blue stragglerstars in the cluster's central region. We discuss the interstellarreddening and the distance modulus of NGC 6293 and NGC 6541 and obtainE(B-V)=0.40 and (m-M)0=14.61 for NGC 6293 and E(B-V)=0.14 and(m-M)0=14.19 for NGC 6541. Our results confirm that NGC 6293and NGC 6541 are clearly located in the Galaxy's central regions(RGC<=3 kpc). We also discuss the differential reddeningacross NGC 6293. The interstellar reddening value of NGC 6293 appears tovary by ΔE(B-V)~0.02-0.04 mag within our small field of view. Themost notable result of our study is that the inner halo clusters NGC6293 and NGC 6541 essentially have the same ages as M92, confirming theprevious result from the HST NIC3 observations of NGC 6287.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| The Dwarf Galaxy Population in Nearby Groups: The Data
We used V and I CCD photometry to search for low surface brightnessdwarf galaxies in the central (<0.5 h-1 Mpc) region of thegroups NGC 6868, NGC 5846, HCG 42, and the poor cluster IC 4765. We usedthe exponential profile fit parameters to identify 80 low surfacebrightness dwarf galaxy candidates with 17 magMV>-11.4), V-I<1.5 mag,μ0>22.5 V mag arcsec-2, h>1.5", anddiameters larger than 1.2 h-1 kpc. Twenty of these galaxiesare extended low surface brightness galaxies that were detected only onsmoothed images, after masking all high surface brightness objects. Thecompleteness in the sample detection is ~80% for V<~20 and 22.5 V magarcsec-2<μ0<24.5 V magarcsec-2. For galaxies with V>20 mag andμ0>24.5 V mag arcsec-2, the completeness isbelow 50% but increases to ~80% when we search for galaxies in smoothedimages. The detected galaxies are highly concentrated toward the centerof the groups (inner 250 h-1 kpc). The best-fit power-lawslope of the surface density distribution is, on average, β~-1.5,in agreement with the values found for satellite dwarfs around isolatedE/S0 galaxies and in X-ray-bright groups. The selected dwarf galaxies donot show a clear correlation in the MV-μ0plane, suggesting that the correlation noted by other studies could beproduced by selection effects. We also find that these galaxies follow awell-defined color-magnitude relation, extending for more than 10 mag(from bright elliptical galaxies to faint dwarfs). A similar relation isknown to be valid for galaxy clusters, but it is the first time that ithas been demonstrated in the sparse environments of groups. Thespectroscopic follow-up shows that only 78 of the 409 galaxies withmeasured velocities are group members. Of these, five are low surfacebrightness dwarf galaxies. In addition, new structures along the groups'lines of sight were discovered. These new structures are groups and poorclusters that extend to ~0.3 in redshift space.Based on data collected with the 1.3 m Warsaw telescope and 2.5 m DuPont telescope at Las Campanas Observatory, Chile.
| The Dwarf Satellites of M31 and the Galaxy
The satellite systems of M31 and the Galaxy are compared. It is notedthat all five of the suspected stripped dwarf spheroidal (dSph) cores ofM31's companions are located within a projected distance of 40 kpc fromthe nucleus of this galaxy, whereas the normal dSph companions to thisobject have distances >40 kpc from the center of M31. All companionswithin 200 kpc25 kpc) satellites.
| 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.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
Catálogos y designaciones:
|
