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|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.
|Hypervelocity Stars. I. The Spectroscopic Survey|
We discuss our targeted search for hypervelocity stars (HVSs), starstraveling with velocities so extreme that dynamical ejection from amassive black hole is their only suggested origin. Our survey, nowhalf-complete, has successfully identified a total of four probable HVSsplus a number of other unusual objects. Here we report the two mostrecently discovered HVSs: SDSS J110557.45+093439.5 and possibly SDSSJ113312.12+010824, traveling with Galactic rest-frame velocities atleast +508+/-12 and +418+/-10 km s-1, respectively. The otherlate B-type objects in our survey are consistent with a population ofpost-main-sequence stars or blue stragglers in the Galactic halo, withmean metallicity [Fe/H]Wk=-1.3 and velocitydispersion 108+/-5 km s-1. It is interesting to note that thevelocity distribution shows a tail of objects with large positivevelocities that may be a mix of low-velocity HVSs and high-velocityrunaway stars. Our survey also includes a number of DA white dwarfs withunusually red colors, possibly extremely low mass objects. Two of ourobjects are B supergiants in the Leo A dwarf, providing the firstspectroscopic evidence for star formation in this dwarf galaxy withinthe last ~30 Myr.
|Detection of a 60°-long Dwarf Galaxy Debris Stream|
We report on a 60°-long stream of stars, extending from Ursa Majorto Sextans, in the Sloan Digital Sky Survey. The stream is approximately2° wide and is clearly distinct from the northern tidal arm of theSagittarius dwarf galaxy. The apparent width of the stream indicates aprogenitor with a size and mass similar to that of a dwarf galaxy. Thestream is about 21 kpc distant and appears to be oriented almostperpendicular to our line of sight. The visible portion of the streamdoes not pass near any known dwarf galaxies, although we cannot rule outthat the stream may form the inner part of a known dwarf galaxy's orbit.The most likely explanation is that the stream constitutes the remainsof a dwarf galaxy that has been completely disrupted at some point inthe past. We also briefly report on the discovery of a diminutiveGalactic satellite that lies near the projected path of the new streambut is unlikely to be related to it.
|A New Milky Way Dwarf Satellite in Canes Venatici|
In this Letter, we announce the discovery of a new dwarf satellite ofthe Milky Way, located in the constellation Canes Venatici. It was foundas a stellar overdensity in the north Galactic cap using Sloan DigitalSky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitudediagram shows a well-defined red giant branch as well as a horizontalbranch. As judged from the tip of the red giant branch, it lies at adistance of ~220 kpc. Based on the SDSS data, we estimate an absolutemagnitude of MV~-7.9, a central surface brightness ofμ0,V~28 mag arcsec-2, and a half-light radiusof ~8.5 arcmin (~550 pc at the measured distance). The outer regions ofCanes Venatici appear extended and distorted. The discovery of such afaint galaxy in proximity to the Milky Way strongly suggests that moresuch objects remain to be found.
|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.
|Constraining Global Properties of the Draco Dwarf Spheroidal Galaxy|
By fitting a flexible stellar anisotropy model to the observed surfacebrightness and line-of-sight velocity dispersion profiles of Draco wederive a sequence of cosmologically plausible two-component (stars +dark matter) models for this galaxy. The models are consistent with allthe available observations and can have either cuspy Navarro-Frenk-Whiteor flat-cored dark matter density profiles. The dark matter halos eitherformed relatively recently (at z~2-7) and are massive (up to~5×109 Msolar), or formed before the end ofthe reionization of the universe (z~7-11) and are less massive (down to~7×107 Msolar). Our results thus supporteither of the two popular solutions of the ``missing satellites''problem of Λ cold dark matter cosmology-that dwarf spheroidalsare either very massive or very old. We carry out high-resolutionsimulations of the tidal evolution of our two-component Draco models inthe potential of the Milky Way. The results of our simulations suggestthat the observable properties of Draco have not been appreciablyaffected by the Galactic tides after 10 Gyr of evolution. We rule outDraco being a ``tidal dwarf''-a tidally disrupted dwarf galaxy. Almostradial Draco orbits (with the pericentric distance <~15 kpc) are alsoruled out by our analysis. The case of a harmonic dark matter core canbe consistent with observations only for a very limited choice of Dracoorbits (with the apocentric-to-pericentric distances ratio of <~2.5).
|Masses of the local group and of the M81 group estimated from distortions in the local velocity field|
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
|Weak redshift discretisation in the Local Group of galaxies?|
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
|The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog|
The first catalog of the RR Lyrae stars (RRLSs) in the Galactic halo bythe Quasar Equatorial Survey Team (QUEST) has been searched forsignificant overdensities that may be debris from disrupted dwarfgalaxies or globular clusters. These RRLSs are contained in a band ~2.3dwide in declination that spans ~165° in right ascension and lie ~4to ~60 kpc from the Sun. Away from the major overdensities, thedistribution of these stars is adequately fitted by a smooth halo model,in which the flattening of the halo decreases with increasinggalactocentric distance (as reported by Preston et al.). This model wasused to estimate the ``background'' of RRLSs on which the halooverdensities are overlaid. A procedure was developed for recognizinggroups of stars that constitute significant overdensities with respectto this background. To test this procedure, a Monte Carlo routine wasused to make artificial RRLS surveys that follow the smooth halo modelbut with Poisson-distributed noise in the numbers of RRLSs and, withinlimits, random variations in the positions and magnitudes of theartificial stars. The 104 artificial surveys created by thisroutine were examined for significant groups in exactly the same way asthe QUEST survey. These calculations provided estimates of thefrequencies with which random fluctuations produce significant groups.In the QUEST survey there are six significant overdensities that containsix or more stars and several smaller ones. The small ones and possiblyone or two of the larger ones may be artifacts of statisticalfluctuations, and they need to be confirmed by measurements of radialvelocity and/or proper motion. The most prominent groups are thenorthern stream from the Sagittarius dwarf spheroidal galaxy and a largegroup in Virgo, formerly known as the ``12.4 hr clump,'' which Duffauand coworkers have recently shown to contain a stellar stream (the Virgostellar stream). Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Anothergroup is related to the globular cluster Palomar 5.
|Tramp Classical Novae as Tracers of Intergalactic Stars|
Simulations predict that collisions between galaxies must liberate starsinto intergalactic space. The stripping of a galaxy's stars by thepotential of a cluster in which it resides must also occur. Thisprediction is verified by the detections of classical novae, red giants,and planetary nebulae between the galaxies of the Virgo and FornaxClusters. These tracers suggest a tramp stellar component of 10%-40% ofthe cluster baryonic mass. I point out that classical novae can usefullyextend these results to the 250,000 Mpc3 of intergalacticspace outside of galaxy clusters surrounding the Local Group. This isbecause individual novae are well-understood standard candles, withlight curves and spectra that are distinct from all other astrophysicalphenomena. In addition, the frequency of nova outbursts in any givengalaxy is measured to be directly proportional to that galaxy's K-bandluminosity (and independent of its Hubble type). Thus, intergalacticnovae should be excellent tracers of the fraction of stars liberatedfrom galaxies over the past 13 Gyr. Pan-STARRS, the Large SynopticSurvey Telescope (LSST), and other large-area synoptic survey telescopeswill begin to regularly discover tramp classical novae out to 20-40 Mpcin the coming decade. I estimate the expected discovery rates withLSST-like surveys to be hundreds of intergalactic tramp novae per year,and suggest survey parameters to optimize detections of these tramps.
|Neutral Hydrogen Clouds Near Early-Type Dwarf Galaxies of the Local Group|
Parkes neutral hydrogen 21 cm line (H I) observations of thesurroundings of nine early-type Local Group dwarfs are presented. Wedetected numerous H I clouds in the general direction of those dwarfs,and these clouds are often offset from the optical center of thegalaxies. Although all the observed dwarfs, except Antlia, occupyphase-space regions where the high-velocity cloud (HVC) density is wellabove average, the measured offsets are smaller than one would expectfrom a fully random cloud distribution. Possible association is detectedfor 11 of the 16 investigated clouds, while for two galaxies, Sextansand Leo I, no H I was detected. The galaxies in which H I clouds werefound not to coincide with the optical yet have a significantprobability of being associated are the Sculptor dwarf, Tucana, LGS 3,Cetus, and Fornax. If the clouds are indeed associated, these galaxieshave H I masses of MHI=2×105,2×106, 7×105, 7×105,and 1×105 Msolar, respectively. However,neither ram pressure nor tidal stripping can easily explain the offsets.In some cases, large offsets are found where ram pressure should be theleast effective.
|Internal Kinematics of the Fornax Dwarf Spheroidal Galaxy|
We present new radial velocity results for 176 stars in the Fornax dwarfspheroidal galaxy, of which at least 156 are probable Fornax members. Wecombine with previously published data to obtain a radial velocitysample with 206 stars, of which at least 176 are probable Fornaxmembers. We detect the hint of rotation about an axis near Fornax'smorphological minor axis, although the significance of the rotationsignal in the galactic rest frame is sensitive to the adopted value ofFornax's proper motion. Regardless, the observed stellar kinematics isdominated by random motions, and we do not find kinematic evidence oftidal disruption. The projected velocity dispersion profile of thebinned data set remains flat over the sampled region, which reaches amaximum angular radius of 65'. Single-component King models in whichmass follows light fail to reproduce the observed flatness of thevelocity dispersion profile. Two-component (luminous plus dark matter)models can reproduce the data, provided that the dark component extendssufficiently beyond the luminous component and the central dark matterdensity is of the same order as the central luminous density. Theserequirements suggest a more massive, darker Fornax than standardcore-fitting analyses have previously concluded, with M/LVover the sampled region reaching 10-40 times the M/LV of theluminous component. We also apply a nonparametric mass estimationtechnique, introduced in a companion paper. Although it is designed tooperate on data sets containing velocities for >1000 stars, theestimation yields preliminary results suggesting M/LV~15inside r<1.5 kpc.
|Proper Motions of Dwarf Spheroidal Galaxies from Hubble Space Telescope Imaging. IV. Measurement for Sculptor|
This article presents a measurement of the proper motion of the Sculptordwarf spheroidal galaxy determined from images taken with the HubbleSpace Telescope using the Space Telescope Imaging Spectrograph in theimaging mode. Each of two distinct fields contains a quasi-stellarobject that serves as the ``reference point.'' The measured propermotion of Sculptor, expressed in the equatorial coordinate system, is(μα, μδ)=(9+/-13, 2+/-13) mascentury-1. Removing the contributions from the motion of theSun and the motion of the local standard of rest produces the propermotion in the Galactic rest frame:(μGrfα,μGrfδ)=(-23+/-13, 45+/-13) mascentury-1. The implied space velocity with respect to theGalactic center has a radial component of Vr=79+/-6 kms-1 and a tangential component of Vt=198+/-50 kms-1. Integrating the motion of Sculptor in a realisticpotential for the Milky Way produces orbital elements. Theperigalacticon and apogalacticon are 68 (31, 83) and 122 (97, 313) kpc,respectively, where the values in the parentheses represent the 95%confidence interval derived from Monte Carlo experiments. Theeccentricity of the orbit is 0.29 (0.26, 0.60), and the orbital periodis 2.2 (1.5, 4.9) Gyr. Sculptor is on a polar orbit around the MilkyWay: the angle of inclination is 86° (83°, 90°).Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
|The Nature of the Density Clump in the Fornax Dwarf Spheroidal Galaxy|
We have imaged the recently discovered stellar overdensity locatedapproximately one core radius from the center of the Fornax dwarfspheroidal galaxy using the Magellan Clay 6.5 m telescope with theMagellan Instant Camera. Superb seeing conditions allowed us to probethe stellar populations of this overdensity and of a control fieldwithin Fornax to a limiting magnitude of R=26. The color-magnitudediagram of the overdensity field is virtually identical to that of thecontrol field, with the exception of the presence of a populationarising from a very short (less than 300 Myr in duration) burst of starformation 1.4 Gyr ago. Coleman et al. have argued that this overdensitymight be related to a shell structure in Fornax that was created whenFornax captured a smaller galaxy. Our results are consistent with thismodel, but we argue that the metallicity of this young component favorsa scenario in which the gas was part of Fornax itself.
|Galaxy Models with Tangentially Anisotropic Velocity Distributions|
This paper provides two families of flexible and simple galaxy models.Many representatives of these families possess important cosmologicalcusps, with the density behaving like r-1, r-4/3,or r-3/2 at small radii. The density falls off betweenr-3 and r-5 at large radii. We provide analyticand anisotropic distribution functions for all the models. Unlike manyexisting methods, our algorithm can yield tangentially anisotropicvelocity dispersions in the outer parts, and so is useful for modelingpopulations of satellite galaxies and substructure in host galaxy halos.As an application, we demonstrate the degeneracy between mass andanisotropy for the satellite galaxy population of the Milky Way. Thiscan introduce a factor of ~3 uncertainty in the mass of the Milky Way asinferred from the kinematics of the satellite population.
|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.
|Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452|
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
|Implications of elemental abundances in dwarf spheroidal galaxies|
Unusual elemental abundance patterns observed for stars belonging tonearby dwarf spheroidal (dSph) galaxies are discussed. Analysis of the [α/H] vs. [Fe/H] diagrams where α represents Mg or an averageof α-elements reveals that Fe from type Ia supernovae (SNe Ia)does not contribute to the stellar abundances in the dSph galaxies wherethe member stars exhibit low α/Fe ratios except for the mostmassive dSph galaxy, Sagitarrius. The more massive dwarf (irregular)galaxy, the Large Magellanic Cloud, also has an SNe Ia signature in thestellar abundances. These findings suggest that whether SNe Iacontribute to chemical evolution in dwarf galaxies is likely to dependon the mass scale of galaxies. Unusual Mg abundances in some dSph starsare also found to be the origin of the large scatter in the [Mg/Fe]ratios and are responsible for a seemingly decreasing [Mg/Fe] featurewith increasing [Fe/H]. In addition, the lack of massive stars in thedSph galaxies does not satisfactorily account for the low-αsignature. Considering the assembly of deficient elements (O, Mg, Si,Ca, Ti and Zn), all of which are synthesized in pre-SN massive stars andin SN explosions, the low-α signature appears to reflect theheavy-element yields of massive stars with less rotation compared tosolar neighborhood stars.
|Imaging resources for the GTC: the Local Group Census|
The Local Group Census is a narrowband imaging survey aimed atcataloguing the emission-line populations in the galaxies of the LocalGroup. Data, which were obtained using the Wide Field Camera of the 2.5mIsaac Newton Telescope, are available to the whole astronomicalcommunity, resulting in a valuable imaging resource for follow-upspectroscopy with the GTC.
|Reionization and the Fate of Dwarf Galaxies|
Our understanding of the origins and evolution of dwarf galaxies hasbeen changing very rapidly. New observations are giving better insightinto the relationship between the two main families of dwarf galaxies,the dwarf ellipticals and the dwarf irregulars. Theoretical simulationsappear to be both posing several problems and eliminating them. However,it is possible that we are beginning to observe some of the more robustresults of these simulations. For example, HST observations of LocalGroup galaxies show evidence of the impact of the ionizing backgroundradiation on their star formation histories. If this is the correctinterpretation, it greatly alters our understanding of dwarf galaxyevolution.
|Homogeneous Photometry. IV. On the Standard Sequence in the Globular Cluster NGC 2419|
I present a new analysis of CCD-based BVRI broadband photometry for theglobular cluster NGC 2419, based on 340 CCD images either donated bycolleagues or retrieved from public archives. The calibrated resultshave been made available through my Web site. I compare the results ofmy analysis with those of an independent analysis of a subset of thesedata by Saha et al. (2005, PASP, 117, 37), who have found acolor-dependent discrepancy of up to 0.05 mag between their I-bandphotometry and mine for stars in this cluster. I conclude that a majorpart of the discrepancy appears to be associated with small shuttertiming errors (a few hundredths of a second) in the Mini-Mosaic (MIMO)camera on the WIYN 3.5 m telescope. Smaller contributions to the anomalylikely come from (1) a color-scale error with a maximum amplitude of~+/-0.02 mag in my secondary standard list as of 2004 September, and (2)statistical effects arising from the previous study's use of arelatively small number of standard-star observations to determine acomparatively large number of fitting parameters in the photometrictransformations.Based in part on observations obtained at the 3.5 m and 0.9 m WIYNTelescopes. The WIYN Observatory is a joint facility of the Universityof Wisconsin-Madison, Indiana University, Yale University, and theNational Optical Astronomy Observatory (NOAO).
|Astrophysics in 2004|
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
|A Dynamical Model for the Orbit of the Andromeda Galaxy M31 and the Origin of the Local Group of Galaxies|
We propose a new model for the origin and evolution of the Local Groupof Galaxies (LGG) that naturally explains the formation of theMagellanic Clouds and their large orbital angular momenta around theGalaxy. The basic idea is that an off-center hydrodynamical collisionoccurred some 10Gyr ago between the primordial Andromeda galaxy (M31)and a similar Galaxy, and compressed the halo gas to form the LGG dwarfgalaxies, including the Magellanic Clouds. New-born dwarf galaxies canbe expected to locate on the orbital plane of these two massivegalaxies. We reexamined the two-dimensional sky distribution of the LGGmembers, and confirmed an early idea that they align along two similargreat circles. The planes of these circles are approximately normal tothe line joining the present position of the Sun and the galacticcenter. We made a distribution map of these objects, and found awell-defined plane of finite thickness. Thus we could determine theorbital elements of M31 relative to the Galaxy by reproducing thewell-studied dynamics of the LMC and the SMC around the Galaxy. Theexpected proper motion of M31 is (μl, μb) =(38 ± 16 μas yr-1, -49 ± 5 μasyr-1).
|Star formation histories in local group dwarf galaxies [review article]|
I review observations made with the Hubble Space Telescope which haveimproved our view of both recent (ages ⩽1 Gyr) and ancient (ages⩾1 Gyr) star formation histories in dwarf galaxies. The method ofreconstructing recent star formation histories has been well tested, andnow the major challenge is to build a large database of suitableobservations of nearby dwarf irregular galaxies. With the exception ofthe dSph companions of our Galaxy, questions concerning the ancient starformation histories of nearby galaxies are stymied by a lack of suitablydeep imaging observations. The few observations which do exist providetantalizing evidence of strong evolution in star formation rates. Thisevolution is likely due to environmental effects, and we may be seeingevidence of the effects of reionization on the star formation historiesof dwarf galaxies. Due to its wide field of view and its excellentimaging resolution, the proposed model for SNAP could solve theseproblems.
|The metal abundance distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy*|
Low-resolution spectroscopy obtained with FORS2 at the Very LargeTelescope (VLT) has been used for the measurement of individual metalabundances ([Fe/H]) for 110 variable stars, including 107 RR Lyrae starsand one anomalous Cepheid, and to trace the metal distribution of theoldest stellar component in the Sculptor dwarf spheroidal galaxy. The RRLyrae stars are spread over a 15 × 15 arcmin2 areaaround the galaxy centre. Their metallicities have an average value of[Fe/H]=-1.83 +/- 0.03 (rms = 0.26 dex) and cover the metallicity range-2.40 < [Fe/H] < -0.85 (on the scale of Zinn & West), butthere is only one variable that has [Fe/H] > -1.3. The star-to-starscatter is larger than typical errors on individual metallicities(+/-0.15-0.16 dex), indicating a real spread in metal abundances. Theradial velocities measured from the RR Lyrae spectra have a dispersionof 12.9kms-1. This value is consistent with the dispersionderived by Tolstoy et al. for metal-poor red giants associated with theblue horizontal branch stars in Sculptor. Along with the metallicitydistribution these results suggest that most of the RR Lyrae stars inSculptor arise from the same burst of stellar formation that producedthe metal-poor component, originating the galaxy blue horizontal branch.The metal-rich red horizontal branch population found to be centrallyconcentrated only produced a few (if any) of the RR Lyrae stars in oursample. The spectroscopic metallicities were used along with theapparent luminosities to study the luminosity-metallicity relationfollowed by the RR Lyrae stars in Sculptor, for which we derive ashallow slope of 0.09magdex-1. This result can be due to ahigh level of evolution off the zero-age horizontal branch of the RRLyrae stars in this galaxy, again in agreement with their origin fromthe blue horizontal branch population.
|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.
|Upper limits on central black hole masses of globular clusters from radio emission and a possible black hole detection in the Ursa Minor dwarf galaxy|
Intermediate-mass black holes (IMBHs) have been alternatively predictedto be quite common in the centres of globular clusters or nearlyimpossible to form and retain in the centres of globular clusters. As ithas been recently shown that radio observations are currently the mostsensitive observational technique for detecting such objects, we haveobtained new deep radio observations of Omega Cen, and have reanalyzedolder observations of M 15 in the hope of constraining the masses ofpossible black holes in their centres. In both cases, upper limits ofabout 100 μJy are found atGHz frequencies. We find that if theBondi-Hoyle accretion rate truly represents the spherical accretion rateonto a black hole, then the masses of the black holes in the centres ofthese two galaxies are severely constrained - with mass limits of lessthan about 100 solar masses in both cases. If more realistic assumptionsare made based on recent work showing the Bondi rate to be a severeoverestimate, then the data for Omega Cen are marginally consistent witha black hole of about 1/1000 of the mass of the cluster (i.e. about1000Msolar). The data for M 15 are then only marginallyconsistent with previous reports of a ~2000 solar mass black hole, andwe note that there is considerable hope for either detecting the blackhole or improving this upper limit with current instrumentation.Finally, we discuss the possibility that the radio source near the coreof the Ursa Minor dwarf spheroidal galaxy is a~104-Msolar black hole.
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