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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
| 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 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.
| 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.
| 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.
| 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.
| Exploring Halo Substructure with Giant Stars. VIII. The Extended Structure of the Sculptor Dwarf Spheroidal Galaxy
We explore the spatial distribution of stars in the Sculptor dwarfspheroidal (dSph) galaxy over an area of 7.82 deg2, includingcoverage of the central region but extending mostly south and east ofthe dSph core. Two methods are used to identify stars that are mostlikely associated with the dSph, and these filtered samples of stars areused to map its spatial structure. First, following the method ofprevious contributions in this series, we utilize Washington M,T2+DDO51 photometry to identify red giant branch (RGB) starcandidates with approximately the same distance and metallicity as theSculptor dSph. Second, a prominent blue horizontal branch (BHB)population provides a fairly populous and pure sample of Sculptor starshaving broadband colors unlike the bulk of the Galactic field starpopulation. A spectroscopically observed subset of Sculptor candidatestars (147 total stars: ~5% of all Sculptor candidates and ~10% ofSculptor giant candidates) yields a systemic heliocentric velocity forthe system of vhel=110.43+/-0.79 km s-1, in goodagreement with previous studies. We also find a global velocitydispersion of σv=8.8+/-0.6 km s-1, withslight indications of a rise in the velocity dispersion past ~0.4rlim.These spectra also provide a check on the reliability of our candidateSculptor giant sample to M~19 94% of the photometrically selectedSculptor giant star candidates with follow-up spectroscopy are found tobe kinematically associated with Sculptor, while 4 out of 10 starsoutside of our Sculptor giant star selection criteria that we testedspectroscopically appear to be velocity members of Sculptor. Thesepercentages are in agreement with results for an additional 22 Sculptorfield stars with radial velocities in the literature. All availablevelocities show that our methodology for picking Sculptor giants is bothreliable and conservative. Thus, these giant star samples should providea reliable means to explore the structure of the Sculptor dSph.Nevertheless, considerable care has been taken to assess the level ofbackground contamination in our photometric sample to ensure anaccurately derived density profile of the Sculptor dSph to large radii.Multiple background assessments verify that we detect a considerablestellar density of Sculptor stars to the limits of our main survey areafor both the RGB and BHB candidate samples. While we find that a Kingprofile of limiting radius rlim=79.6 arcmin fits the densityprofile of Sculptor well to ~60', beyond this, we identify a ``break''in the profile and a clearly detected population of Sculptor starsfollowing a Σ~r-2 decline to more than 2rlim. Thisbreak population must signify either the presence of an extremely broaddistribution of bound ``halo stars'' around the Sculptor dSph or thepresence of unbound tidal debris. If the latter is true, we determine afractional mass-loss rate of approximately 0.042 Gyr-1 forthe Sculptor dSph. Additional support for the notion that there is tidaldisruption comes from the two-dimensional distribution of our Sculptorcandidate stars; both the RGB and BHB samples show increasinglyelongated isodensity contours with radius that point to an apparentstretching reminiscent of what is seen in models of disrupting satellitegalaxies. Finally, we find that RGB stars that are more likely to bemetal-poor (based on their color and magnitude) are significantly lesscentrally concentrated and therefore constitute the primary contributingstellar population to the likely tidally stripped parts of the dSph.
| Towards a phylogenetic analysis of galaxy evolution: a case study with the dwarf galaxies of the Local Group
Context: .The Hubble tuning-fork diagram has always been the preferredscheme for classifying galaxies. It is based only on morphology. Incontrast, biologists have long taken the genealogical relatedness ofliving entities into account for classification purposes. Aims:.Assuming branching evolution of galaxies as a "descent withmodification", we show here that the concepts and tools of phylogeneticsystematics that are widely used in biology can be heuristicallytransposed to the case of galaxies. Methods: .This approach,which we call "astrocladistics", is applied to dwarf galaxies of theLocal Group and provides the first evolutionary tree for realgalaxies. Results: .The trees that we present here are solidenough to support the existence of a hierarchical organisation in thediversity of dwarf galaxies of the Local Group. They also show thatthese galaxies all stem from a common ancestral kind of object. We findthat some kinds of dIrrs are progenitors of both dSphs and other kindsof dIrrs. We also identify three evolutionary groups, each one with itsown characteristics and own evolution. Conclusions: .The presentwork opens a new way to analysing galaxy evolution and a path towards anew systematics of galaxies. Work on other galaxies in the Universe isin progress.
| 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).
| A new population of extended, luminous star clusters in the halo of M31
We present three new clusters discovered in the halo of M31 which,although having globular-like colours and luminosities, have unusuallylarge half-light radii, ~30 pc. They lie at projected galactocentricdistances of ~15 to ~35 kpc. These objects begin to fill the gap inparameter space between globular clusters and dwarf spheroidals, and areunlike any clusters found in the Milky Way, or elsewhere to date.Colour-magnitude diagrams, integrated photometric properties and derivedKing profile fit parameters are given, and we discuss possible originsof these clusters and their relationships to other populations.
| The galaxy luminosity function from MR=-25 to MR=-9
Redshift surveys such as the Sloan Digital Sky Survey (SDSS) have givena very precise measurement of the galaxy luminosity function down toabout MR=-17 (~MB=-16). Fainter absolutemagnitudes cannot be probed because of the flux limit required forspectroscopy. Wide-field surveys of nearby groups using mosaic CCDs onlarge telescopes are able to reach much fainter absolute magnitudes,about MR=-10. These diffuse, spiral-rich groups are thoughtto be typical environments for galaxies, so their luminosity functionsshould be the same as the field luminosity function. The luminosityfunction of the groups at the bright end (MR < -17) islimited by Poisson statistics and is far less precise than that derivedfrom redshift surveys. Here we combine the results of the SDSS and thesurveys of nearby groups, and we supplement the results with studies ofLocal Group galaxies in order to determine the galaxy luminosityfunction over the entire range -25 < MR < -9. Theaverage logarithmic slope of the field luminosity function betweenMR=-19 and MR=-9 is α=-1.26, although asingle power law is a poor fit to the data over the entire magnituderange. We also determine the luminosity function of galaxy clusters anddemonstrate that it is different from the field luminosity function at ahigh level of significance; there are many more dwarf galaxies inclusters than in the field, due to a rise in the cluster luminosityfunction of α~-1.6 between MR=-17 andMR=-14.
| Distances and metallicities for 17 Local Group galaxies
We have obtained Johnson V and Gunni photometry for a large number ofLocal Group galaxies using the Isaac Newton Telescope Wide Field Camera(INT WFC). The majority of these galaxies are members of the M31subgroup and the observations are deep enough to study the top fewmagnitudes of the red giant branch in each system. We previouslymeasured the location of the tip of the red giant branch (TRGB) forAndromeda I, Andromeda II and M33 to within systematic uncertainties oftypically <0.05 mag. As the TRGB acts as a standard candle in old,metal-poor stellar populations, we were able to derive distances to eachof these galaxies. Here we derive TRGB distances to the giant spiralgalaxy M31 and 13 additional dwarf galaxies - NGC 205, 185, 147,Pegasus, WLM, LGS3, Cetus, Aquarius, And III, V, VI, VII and the newlydiscovered dwarf spheroidal And IX. The observations for each of thedwarf galaxies were intentionally taken in photometric conditions. Inaddition to the distances, we also self-consistently derive the medianmetallicity of each system from the colour of their red giant branches.This allows us to take into account the small metallicity variation ofthe absolute I magnitude of the TRGB. The homogeneous nature of our dataand the identical analysis applied to each of the 17 Local Groupgalaxies ensures that these estimates form a reliable set of distanceand metallicity determinations that are ideal for comparative studies ofLocal Group galaxy properties.
| Formation Histories of Dwarf Galaxies in the Local Group
We compare the properties of dwarf galaxies in the Local Group with thesimulated galaxies formed before reionization in a cosmologicalsimulation of unprecedented spatial and mass resolution, includingradiative feedback effects. We find that a subset of the Local Groupdwarfs are already remarkably similar to the simulated dwarf galaxies inall their properties before reionization. On the basis of thissimilarity, we propose the hypothesis that Local Group dwarfs form in avariety of ways: some of them are ``true fossils'' of thepre-reionization era, some of them form most of their stars later, afterreionization (we call them ``survivors'' of the reionization era), andthe rest of them form an intermediate group of ``polluted fossils.'' Wealso identify a simple observational test that is able to test ourhypothesis.
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
| Extragalactic Globular Clusters: Old Spectroscopic Ages and New Views on Their Formation
We present the results of a meta-analysis of Keck spectra ofextragalactic globular clusters (GCs) in a sample of eight galaxies,ranging from dwarf galaxies to massive elliptical galaxies. We inferages for the metal-poor and metal-rich GCs in these galaxies throughcomparisons to Galactic GCs. Both subpopulations appear to be no youngerthan their Galactic counterparts, with ages >~10 Gyr. This is thelargest sample of galaxies for which ages have been constrainedspectroscopically. Our results support the formation of most GCs inmassive galaxies at high redshift. We propose a scenario for theformation of GC subpopulations that synthesizes aspects of bothaccretion and in situ approaches in the context of galaxy formationthrough hierarchical merging.
| The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda I and Andromeda III
We present the results of variable star searches of the M31 dwarfspheroidal (dSph) companions Andromeda I and Andromeda III using theHubble Space Telescope. A total of 100 variable stars were found in AndI, while 56 were found in And III. One variable found in And I andanother in And III may be type II Cepheids. In addition to this variablein And III, another four variables are anomalous Cepheids (ACs). So far,no definite ACs have been discovered in And I. We discuss the propertiesof these variables with respect to those found in the other dSphgalaxies and revisit the AC period-luminosity relations. We found 72fundamental mode RR Lyrae (RRab) stars and 26 first-overtone mode RRLyrae (RRc) stars in And I giving mean periods of 0.575 and 0.388 days,respectively. One likely RR Lyrae star in And I remains unclassifiedbecause of a lack of F555W data. For And III, 39 RR Lyrae stars arepulsating in the fundamental mode with a mean period of 0.657 days, and12 are in the first-overtone mode with a mean period of 0.402 days.Using the mean metal abundances derived from the red giant branchcolors, the mean RRab period for And I is consistent with the meanperiod-metallicity relation seen in the RR Lyrae populations of Galacticglobular clusters, while And III is not, having too large a mean RRabperiod for its abundance. In And I, we found two RR Lyrae stars that arenoticeably fainter than the horizontal branch. We discuss thepossibility that these stars are associated with the recently discoveredstellar stream in the halo of M31. Using various methods, we estimatethe mean metallicity of the RR Lyrae stars to be <[Fe/H]>~-1.5 forAnd I and ~-1.8 for And III. These estimates match well with other meanmetallicity estimates for the galaxies. Interestingly, a comparison ofthe period-amplitude diagrams for these two galaxies with other dSphgalaxies shows that And III is lacking in shorter period, higheramplitude RR Lyrae stars. This may be a consequence of the very redhorizontal branch morphology in this dSph. Not including the two faintRR Lyrae stars, we find =25.14+/-0.04 mag for AndI, resulting in a distance of 765+/-25 kpc. For And III,=25.01+/-0.04 mag, giving a distance of 740+/-20kpc. These distance estimates are consistent with those previously foundfor these galaxies. We discuss the relation between the specificfrequency of the ACs in dSph galaxies and the mean metallicity of thegalaxy, finding that the M31 dSph galaxies follow the same relations asthe Galactic dSph galaxies. We also find that the period-luminosityrelations of ACs and short-period Cepheids are distinct, with theshort-period Cepheids having higher luminosities at a given period.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute (STScI), which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| The Local Group and Other Neighboring Galaxy Groups
Over the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ``ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| The Stellar Populations of dE Galaxies in Nearby Groups
In this contribution Gemini-North NIRI J,K-observations are used toinvestigate the upper-Asymptotic Giant Branch (AGB) intermediate-agepopulation in the M81 Group dwarf elliptical (dE) F8D1. Hubble SpaceTelescope (HST) `snapshot' V,I-observations are also analysed toinvestigate the upper-AGB populations in two other M81 Group dEs, DDO 71and kk077. In all three dEs, significant intermediate-age populationsare found. Further, there are sizeable dE-to-dE differences in thesepopulations: F8D1 contains relatively more, and relatively moreluminous, upper-AGB stars. These results are compared with existinginformation for Local Group and Sculptor group dwarfs. It is suggestedthat `environmental harrassment' plays an important role in governingdwarf galaxy evolution.
| Using SKA to observe relativistic jets from X-ray binary systems
I briefly outline our current observational understanding of therelativistic jets observed from X-ray binary systems, and how theirstudy may shed light on analogous phenomena in active galactic nucleiand gamma ray bursts. How SKA may impact on this field is sketched,including the routine tracking of relativistic ejections to largedistances from the binaries, detecting and monitoring the radiocounterparts to ‘quiescent’ black holes, and detecting theradio counterparts of the brightest X-ray binaries throughout the localgroup of galaxies.
| Seeing double in the local group: extragalactic binaries
Eclipsing binaries, both inside and outside our Galaxy, are proving tobe powerful tools for studying a wide spectrum of astrophysicalproblems. They are also are extremely valuable for providing fundamentalquantities such as stellar masses, radii, luminosities, ages anddistances. Recently, eclipsing binaries are turning out to be accuratedistance indicators for star clusters inside our Galaxy and fordetermining accurate distances to nearby galaxies - such as theMagellanic Clouds and the Andromeda Galaxy. Also with eclipsingbinaries, it is possible to study the physical properties and determineevolution for a wide variety of objects that are lucky enough to bebinary members. These objects include pre-main sequence stars, mainsequence stars, giants, supergiants, various pulsating stars, whitedwarfs, black holes and even exosolar planets. At the present time over7000 eclipsing binaries have been discovered in Local Group galaxies.These systems are mostly members of the Magellanic Clouds and theAndromeda Galaxy. But also an increasing number of extragalacticbinaries are being found as members of dwarf elliptical galaxies and lowsurface density irregular galaxy members of the Local Group. It will beimportant to study the properties of eclipsing binaries that have formedin galaxies with vastly different dynamical, star formation, andchemical histories than our home Galaxy. The study of these binaries mayprovide clues about the star formation rates and dynamics of their hostgalaxies as well as the possible effects of varying chemical abundanceon stellar evolution and structure. An overview of eclipsing andinteracting binary star systems in exterior galaxies is presented thattraces the development of this emerging field of research. Alsodiscussed are some recent developments and future expectations for thestudy of extragalactic binaries.
| Determining the location of the tip of the red giant branch in old stellar populations: M33, Andromeda I and II
The absolute bolometric luminosity of the point of core helium ignitionin old, metal-poor, red giant stars is of roughly constant magnitude,varying only very slightly with mass or metallicity It can thus be usedas a standard candle. Here, we review the main difficulties in measuringthis location in any real data set and we develop an empirical approachto optimize it for tip of the red giant branch (TRGB) analysis. We go onto present a new algorithm for the identification of the TRGB in nearbymetal-poor stellar systems. Our method uses a least-squares fit of adata adaptive slope to the luminosity function in 1-mag windows. Thisfinds the region of the luminosity function that shows the mostsignificant decline in star counts as we go to brighter magnitudes; thebase of this decline is attributed as the location of the tip. Thistechnique then allows for the determination of realistic uncertaintieswhich reflect the quality of the luminosity function used, but which aretypically ~0.02 mag rms +~0.03 mag systematic, a significant improvementupon previous methods that have used the tip as a standard candle.Finally, we apply our technique to the Local Group spiral galaxy M33 andthe dwarf galaxies Andromeda I and II, and derive distance modulii of24.50 +/- 0.06 mag (794 +/- 23 kpc), 24.33 +/- 0.07 mag (735 +/- 23 kpc)and 24.05 +/- 0.06 mag (645 +/- 19 kpc) respectively. The result for M33is in excellent agreement with the Cepheid distances to this galaxy, andmakes the possibility of a significant amount of reddening in thisobject unlikely.
| The Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies
Cold dark matter models for galaxy formation predict that low-masssystems will be the first sites of star formation. As these objects haveshallow gravitational potential wells, the subsequent growth of theirstellar populations may be halted by heating and gas loss due toreionization. This effect has been suggested to have profoundlyinfluenced properties of present-day dwarf galaxies, including theirstellar populations and even survival as visible galaxies. In thisLetter we draw on results from quantitative studies of Local Group dwarfgalaxy star formation histories, especially for Milky Way satellites, toshow that no clear signature exists for a widespread evolutionary impactfrom reionization. All nearby dwarf galaxies studied in sufficientdetail contain ancient populations indistinguishable in age from theoldest Galactic globular clusters. Ancient star formation activityproceeded over several gigayears, and some dwarf spheroidal galaxieseven experienced fairly continuous star formation until just a fewgigayears ago. Despite their uniformly low masses, their star formationhistories differ considerably. The evolutionary histories of nearbydwarf galaxies appear to reflect influences from a variety of localprocesses rather than a dominant effect from reionization.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| New Mass Estimators For Tracer Populations
We introduce the ``tracer mass estimator.'' This is a new and simple wayto estimate the enclosed mass from the projected positions andline-of-sight velocities of a tracer population (such as globularclusters, halo stars, and planetary nebulae). Like the projected massestimator, it works by averaging(projecteddistance)(radialvelocity)2/G over the sample.However, it applies to the commonest case of all, in which the tracerpopulation does not follow the overall dark matter density. The methodis verified against simulated data sets drawn from Monte Carlorealizations of exact solutions of the collisionless Boltzmann equationand applied to the M31 globular cluster data set of Perrett andcoworkers, as well as to M31's satellite galaxies.
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