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RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| A Double-Mode RR Lyrae Star with a Strong Fundamental-Mode Component
NSVS 5222076, a 13th magnitude star in the Northern Sky VariabilitySurvey, was identified by L. Oaster as a possible new double-mode RRLyrae star. We confirm the double-mode nature of NSVS 5222076,supplementing the survey data with new V-band photometry. NSVS 5222076has a fundamental-mode period (P0) of 0.4940 days and afirst-overtone period (P1) of about 0.3668 days, giving aperiod ratio of P1/P0=0.743. In most double-modeRR Lyrae stars, the amplitude of the first-overtone mode's pulsation isgreater than that of the fundamental-mode pulsation. That is not truefor this star. Its fundamental-mode light curve has an amplitude twiceas large as that of the first-overtone mode, a ratio very rarely seeneven among the double-mode RR Lyrae stars that have relatively strongfundamental-mode pulsation. Data from the literature are used to discussthe location in the Petersen diagram of double-mode RR Lyrae starshaving strong fundamental-mode pulsation. Such stars tend to occurtoward the short-period end of the Petersen diagram, and NSVS 5222076 isno exception to this rule.
| 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.
| 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.
| 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)
| BVR photometry of the Sculptor dwarf spheroidal galaxy .
We present accurate B, V, R photometry of the Sculptor dwarf spheroidalgalaxy. The data have been collected with the Wide Field Imageravailable at 2.2 m ESO/MPI telescope and cover an area of about100×40 arcmin2 around the Sculptor center. The finalcatalogue includes 100,000 stars. We found that blue and red HorizontalBranch stars present different radial distributions. This findingsupports previous results. Moreover, we present a preliminary comparisonbetween theoretical predictions for H and He burning phases andobservational data.
| 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.
| Color-Magnitude Diagrams of Resolved Stars in Virgo Cluster Dwarf Galaxies
The Advanced Camera for Surveys (ACS) on the Hubble Space Telescope(HST) has been used to image two fields in the core of the Virgo Clusterthat contain a number of dwarf elliptical galaxies. The combined F555Wand F814W images have resolved red giant stars in these galaxies, downto 1 mag below the giant branch tip. Two of the galaxies were targetedbecause of their extremely low central surface brightnesses(Bo>27.0) thus, the successful resolution into starsconfirms the existence of such tenuous galaxies. Red giant stars werealso found that are not ostensibly associated with any galaxy.Color-magnitude diagrams in V and I have been derived for the fivedwarfs, as well as the halo of a nearby spiral galaxy and theintracluster stars in the two fields. These diagrams were used to derivedistances and metallicities via the magnitude of the red giant branchtip, and the mean color of the giant branch. The mean abundances ofstars in the dwarfs range from -1.2<[Fe/H]<-2.4, and fall alongthe relation between galaxy luminosity and metallicity found for LocalGroup and M81 group dwarf elliptical galaxies. [Fe/H] does not appear tobe well-correlated with galaxy surface brightness, as the two extremelylow surface brightness galaxies do not have extreme abundances. The meandistance modulus of the six Virgo galaxies is 31.0+/-0.05, or 16.1+/-0.4Mpc, whereas that for the intracluster stars in those fields is31.2+/-0.09 (17.4+/-0.7 Mpc).
| 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.
| 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.
| 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.
| 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 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.
| 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.
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