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The ACS LCID Project: RR Lyrae Stars as Tracers of Old Population Gradients in the Isolated Dwarf Spheroidal Galaxy Tucana
We present a study of the radial distribution of RR Lyrae variables,which present a range of photometric and pulsational properties, in thedwarf spheroidal galaxy Tucana. We find that the fainter RR Lyrae stars,having a shorter period, are more centrally concentrated than the moreluminous, longer period RR Lyrae variables. Through comparison with thepredictions of theoretical models of stellar evolution and stellarpulsation, we interpret the fainter RR Lyrae stars as a more metal-richsubsample. In addition, we show that they must be older than about 10Gyr. Therefore, the metallicity gradient must have appeared very earlyon in the history of this galaxy.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated with program10505.
| M31 Transverse Velocity and Local Group Mass from Satellite Kinematics
We present several different statistical methods to determine thetransverse velocity vector of M31. The underlying assumptions are thatthe M31 satellites on average follow the motion of M31 through space andthat the galaxies in the outer parts of the Local Group on averagefollow the motion of the Local Group barycenter through space. We applythe methods to the line-of-sight velocities of 17 M31 satellites, to theproper motions of the two satellites M33 and IC 10, and to theline-of-sight velocities of five galaxies near the Local Groupturnaround radius, respectively. This yields four independent butmutually consistent determinations of the heliocentric M31 transversevelocities in the west and north directions, with weighted averages=-78+/-41 km s-1 and=-38+/-34 km s-1. The uncertaintiescorrespond to proper motions of ~10 μas yr-1, which willbe difficult to verify observationally within the next decade. Thegalactocentric tangential velocity of M31 is 42 km s-1, with1 σ confidence interval Vtan<=56 km s-1.The implied M31-Milky Way orbit is bound if the total Local Group mass Mexceeds 1.72+0.26-0.25×1012Msolar. If the orbit is indeed bound, then the timingargument combined with the known age of the universe implies thatM=5.58+0.85-0.72×1012Msolar. This is on the high end of the allowed mass rangesuggested by cosmologically motivated models for the individualstructure and dynamics of M31 and the Milky Way, respectively. It istherefore possible that the timing mass is an overestimate of the truemass, especially if one takes into account recent results from theMillenium Simulation that show that there is also a theoreticaluncertainty of 41% (Gaussian dispersion) in timing mass estimates. TheM31 transverse velocity implies that M33 is in a tightly bound orbitaround M31. This may have led to some tidal deformation of M33. It willbe worthwhile to search for observational evidence of this.
| Cosmic ménage à trois: the origin of satellite galaxies on extreme orbits
We examine the orbits of satellite galaxies identified in a suite ofN-body/gasdynamical simulations of the formation of L*galaxies in a Lambda cold dark matter universe. The numerical resolutionof the simulations allows us to track in detail the orbits of the ~ 10brightest satellites around each primary. Most satellites followconventional orbits; after turning around, they accrete into their hosthalo and settle on orbits whose apocentric radii are steadily eroded bydynamical friction. As a result, satellites associated with the primaryare typically found within its virial radius, rvir, and havevelocities consistent with a Gaussian distribution with mild radialanisotropy. However, a number of outliers are also present. We find thata surprising number (about one-third) of satellites identified at z = 0are on unorthodox orbits, with apocentres that exceed their turnaroundradii. These include a number of objects with extreme velocities andapocentric radii at times exceeding ~3.5rvir (or, e.g. >~1Mpc when scaled to the Milky Way). This population of satellites onextreme orbits consists typically of the faint member of a satellitepair whose kinship is severed by the tidal field of the primary duringfirst approach. Under the right circumstances, the heavier member of thepair remains bound to the primary, whilst the lighter companion isejected on to a highly energetic orbit. Since the concurrent accretionof multiple satellite systems is a defining feature of hierarchicalmodels of galaxy formation, a fairly robust prediction of this scenariois that at least some of these extreme objects should be present in theLocal Group. We speculate that this three-body ejection mechanism may bethe origin of (i) some of the newly discovered high-speed satellitesaround M31 (such as Andromeda XIV); (ii) some of the distantfast-receding Local Group members, such as Leo I and (iii) the oddlyisolated dwarf spheroidals Cetus and Tucana in the outskirts of theLocal Group. Our results suggest that care must be exercised when usingthe orbits of the most weakly bound satellites to place constraints onthe total mass of the Local Group.
| SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets around Young Stars
We present a review of precursor observing programs for the SIMPlanetQuest Key Project devoted to detecting Jupiter-mass planets aroundyoung stars. In order to ensure that the stars in the sample are free ofvarious sources of astrometric noise that might impede the detection ofplanets, we have initiated programs to collect photometry, high-contrastimages, interferometric data, and radial velocities for stars in boththe northern and southern hemispheres. We have completed a high-contrastimaging survey of target stars in Taurus and the Pleiades and found nodefinitive common proper motion companions within 1" (140 AU) of the SIMtargets. Our radial velocity surveys have shown that many of the targetstars in Sco-Cen are fast rotators, and a few stars in Taurus and thePleiades may have substellar companions. Interferometric data of a fewstars in Taurus show no signs of stellar or substellar companions withseparations of 5-50 mas. The photometric survey suggests thatapproximately half of the stars initially selected for this program arevariable to a degree (1 σ > 0.1 mag) that would degrade theastrometric accuracy achievable for that star. While the precursorprograms are still a work in progress, we provide a comprehensive listof all targets and rank them according to their viability as a result ofthe observations taken to date. The observable that removes by far themost targets from the SIM young stellar object (YSO) program isphotometric variability.
| Effects of the galactic winds on the stellar metallicity distribution of dwarf spheroidal galaxies
Aims.To study the effects of galactic winds on the stellar metallicitydistributions and on the evolution of Draco and Ursa Minor dwarfspheroidal galaxies (dSphs), we compared the predictions of severalchemical evolution models, adopting different prescriptions for thegalactic winds (including a model with no wind), with thephotometrically-derived stellar metallicity distributions (SMDs) of bothgalaxies. Methods: We adopted chemical evolution models for Draco andUrsa Minor, which are able to reproduce several observational featuresof these two galaxies, such as the [ α/Fe] , [Eu/Fe], [Ba/Fe]ratios and the present gas mass. The models take up-to-datenucleosynthesis into account for intermediate-mass stars and supernovaeof both types, as well as the effect of these objects on the energeticsof the systems. The predictions were compared to the photometric SMDs,which are accurate enough for a global comparison with general aspectssuch as metallicity range, shape, position of the peak, andhigh-metallicity tail, leaving aside minor details of the distributions. Results: For both galaxies, the model that best fits the data containsan intense continuous galactic wind, occurring at a rate proportional tothe star formation rate. Models with a wind rate assumed to beproportional only to the supernova rate also reproduce the observed SMD,but do not match the gas mass, whereas the models with no galactic windsfail to reproduce the observed SMDs. In the case of Ursa Minor, the samemodel as in previous works reproduces the observed distribution verywell with no need to modify the main parameters of the model (starformation efficiency ν = 0.1 Gyr-1 and wind efficiencywi = 10). The model for Draco, on the other hand, is slightlymodified. The observed SMD requires a model with a lower supernova typeIa thermalization efficiency (η_SNeIa = 0.5 instead of η_SNeIa =1.0, as used in previous papers of this series) in order to delay thegalactic wind, whereas all the other parameters are kept the same (ν= 0.05 Gyr-1, wi = 4). Conclusions: The modelresults, compared to observations, strongly suggest that intense andcontinuous galactic winds play a very important role in the evolution oflocal dSphs. They not only remove a large fraction of the gas content ofthe galaxy, but also define the pattern of the abundance ratios and ofthe SMD, giving rise to low (below solar) values for [ α/Fe] ,[Eu/Fe], as well as high [Ba/Fe] at high metallicities, as observed, andpreventing metal-rich stars being formed, which is reflected in thestrong decrease in the high-metallicity tail of the stellardistributions in these galaxies.
| Inside the whale: the structure and dynamics of the isolated Cetus dwarf spheroidal
This paper presents a study of the Cetus dwarf, an isolated dwarf galaxywithin the Local Group. A matched-filter analysis of the INT/WFC imagingof this system reveals no evidence for significant tidal debris thatcould have been torn off the galaxy, bolstering the hypothesis thatCetus has never significantly interacted with either the Milky Way orM31. Additionally, Keck/Deimos spectroscopic observations identify thisgalaxy as a distinct kinematic population possessing a systematicvelocity of -87 +/- 2kms-1 and with a velocity dispersion of17 +/- 2kms-1 while tentative, these data also suggest thatCetus possesses a moderate rotational velocity of ~8kms-1.The population is confirmed to be relatively metal-poor, consistent with[Fe/H] ~ -1.9, and, assuming virial equilibrium, implies that the Cetusdwarf galaxy possesses a mass-to-light ratio of ~70. It appears,therefore, that Cetus may represent a primordial dwarf galaxy, retainingthe kinematic and structural properties lost by other members of thedwarf population of the Local Group in their interactions with the largegalaxies. An analysis of Cetus' orbit through the Local Group indicatesthat it is at apocentre; taken in conjunction with the general dwarfpopulation, this shows the mass of the Local Group to be >~2 ×1012Msolar.
| Planar distribution of the galaxies in the Local Group: a statistical and dynamical analysis
Aims.Basing on the projected distribution of the galaxies in the LocalGroup, Sawa & Fujimoto found that they all seem to distribute on arather thin plane containing the two mayor local galaxies of the LocalGroup, Milky Way and Andromeda, and all their dwarf satellites. As theirconclusion could be severely biased by projectional distortion effects,we re-analyse the whole issue using a different approach. In brief,adopting known data on positions and distances, we make use of theanalytical geometry and look for the plane that minimizes the distancesof all galaxies to it. A planar distribution is indeed found that,however, does not coincide with the plane found by Sawa & Fujimoto.Why? The second part of this study is devoted to answer this questionand to find a dynamical justification for the planar distribution.Methods: .To this aim, we apply the Hamilton Method (Minimum Action) toinvestigate the dynamics of the two major system of the Local Group,Milky Way and Andromeda, under the action of external forces exerted bynearby galaxies or groups external to the Local Group. Results:.We find that the planar distribution is fully compatible with theminimum action and that the external force field is likely parallel tothe plane. It pulls the galaxies of the Local Group without alteringtheir planar distribution. Special care is paid to evaluate therobustness of this result. Conclusions: .In this paper we haveexamined the spatial distribution of galaxies in the Local Group. Theyare confined to a plane that can be statistically and dynamicallyunderstood as the result of the Minimum Action. The planar distributionseems to be stable for a large fraction of the Hubble time. The externalforce field, that has likely been constant over the same time interval,does not alter the planar distribution as it is nearly parallel to it.Effects due to undetected halos of sole Dark Matter are brieflydiscussed. They could be a point of uncertainty of the present study.Appendix A is only available in elctronic form at http://www.aanda.org
| Minivoids in the Local Volume
We consider a sphere of 7.5 Mpc radius, which contains 355 galaxies withaccurately measured distances, to detect the nearest empty volumes.Using a simple void detection algorithm, we found six large (mini)voidsin Aquila, Eridanus, Leo, Vela, Cepheus, and Octans, each of more than30 Mpc3. In addition, 24 middle-sized ``bubbles'' of morethan 5 Mpc3 volume are detected, as well as 52 small``pores.'' The six largest minivoids occupy 58% of the consideredvolume. The addition of the bubbles and pores to them increases thetotal empty volume up to 75% and 81%, respectively. The detected localvoids look like oblong potatoes with typical axial ratios b/a=0.75 andc/a=0.62 (in the triaxial ellipsoid approximation). Being arranged bythe size of their volume, local voids follow a power law for volume-rankdependence. A correlation Gamma function of the Local Volume galaxiesfollows a power law with a formally calculated fractal dimension D=1.5.We found that galaxies surrounding the local minivoids do not differsignificantly from other nearby galaxies in their luminosity, but haveappreciably higher hydrogen mass-to-luminosity ratios and also higherstar formation rates. We recognize an effect of the local expansion of atypical minivoid to be ΔH/H0~(25+/-15)%.
| 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.
| 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.
| Associations of Dwarf Galaxies
The Hubble Space Telescope Advanced Camera for Surveys has been used todetermine accurate distances for 20 galaxies from measurements of theluminosity of the brightest red giant branch stars. Five associations ofdwarf galaxies that had originally been identified based on strongcorrelations on the plane of the sky and in velocity are shown to beequally well correlated in distance. Two more associations with similarproperties have been discovered. Another association is identified thatis suggested to be unbound through tidal disruption. The associationshave the spatial and kinematic properties expected of bound structureswith (1-10)×1011 Msolar. However, theseentities have little light, with the consequence that the mass-to-lightratios are in the range 100-1000 MsolarL-1solar. Within a well-surveyed volume extendingto a 3 Mpc radius, all but one known galaxy lie within one of the groupsor associations that have been identified.
| 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.
| 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.
| DDO 44 and UGC 4998: Distances, Metallicities, and Star Formation Histories
We have obtained deep HST imaging of two candidate dwarf systems in thenearby M81 Group, DDO 44 and UGC 4998. Both are isolated, low surfacebrightness systems, but with likely very different star formationhistories based on their mean colors and color fluctuations. Thecolor-magnitude diagrams of these galaxies have been used to estimatetheir distances using the tip of the red giant branch (RGB), and theirmean metallicities from the colors of the RGB. For DDO 44 we find adistance of D=3.01+/-0.18 Mpc-confirming it to be a member of the M81Group-and a mean metallicity of [Fe/H]=-1.54+/-0.14. We also find fromthe properties and numbers of a population of luminous asymptotic giantbranch (AGB) stars that about 20% of the luminous population of DDO 44consists of ``intermediate-age'' stars (with ages between about 2 and 8Gyr) that give rise to the observed AGB. There is no difference in thespatial distribution of the RGB and AGB stars in this galaxy. For UGC4998 we derive a metallicity of [Fe/H]=-1.58+/-0.21 and a distance ofD=8.24+/-0.43 Mpc. The latter puts UGC 4998 well behind the M81 Group,confirming results from previous measurements. We find this galaxy tocontain a population of young (<50 Myr old) stars, and there isevidence of older RGB stars. The young stars are considerably morecentrally concentrated than the older stars. The current star formationrate in UGC 4998 is ~9×10-4 Msolaryr-1 over the entire galaxy, or ~5×10-4Msolar yr-1 kpc-2. Given its H I mass(1×107 Msolar), this galaxy can continue toform stars at this rate for another ~11 Gyr.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations are associated with proposalGO-8137.
| 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.
| 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.
| 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 H I content of the recently discovered field dwarf galaxy APPLES 1
We present observations in a search for neutral hydrogen associated withthe recently detected field dwarf galaxy APPLES 1,performed with the Parkes radiotelescope. The observed radio spectrumshows no evident (>3σ rms) line emission indicating an upperlimit for the H i content of the galaxy of ~106Mȯ and providing an upper value for theMHI/LB ratio equal to 2.4 /. The low value of theH i content suggested by the observations, together with the galaxyoptical morphology, might indicate that APPLES 1 is adwarf spheroidal. This indication is in contrast with the evidence ofrecent star formation, which is typical for dwarf irregular galaxies.This may suggest that APPLES 1 belongs to the classof mixed dwarf irregular/spheroidal transition-type galaxies. We alsoconclude that the relatively low neutral gas mass in APPLES1 can be explained by an extended and inefficient starformation process, without the need for a dramatic event such asenhanced star formation or a past encounter with a massive galaxy orgalaxy group.
| Detection of Neutral Hydrogen in Early-Type Dwarf Galaxies of the Sculptor Group
We present results of deep 21 cm neutral hydrogen (H I) lineobservations of five early- and mixed-type dwarf galaxies in the nearbySculptor group using the Australia Telescope National Facility 64 mParkes Radio Telescope. Four of these objects, ESO 294-G010, 410-G005,540-G030, and 540-G032, were detected in H I with neutral hydrogenmasses in the range (2-9)×105 Msolar(MHI/LB=0.08, 0.13, 0.16, and 0.18Msolar L-1solar, respectively). These HI masses are consistent with the gas mass expected from stellar outflowsover a large period of time. Higher spatial resolution H I data from theAustralia Telescope Compact Array interferometer were further analyzedto measure more accurate positions and the distribution of the H I gas.In the cases of the dwarfs ESO 294-G010 and ESO 540-G030, we findsignificant offsets of 290 and 460 pc, respectively, between theposition of the H I peak flux and the center of the stellar component.These offsets are likely to have internal causes such as the winds fromstar-forming regions. The fifth object, the spatially isolated dwarfelliptical galaxy Scl-dE1, remains undetected at our 3 σ limit of22.5 mJy km s-1 and thus must contain less than105 Msolar of neutral hydrogen. This leavesScl-dE1 as the only Sculptor group galaxy known in which no interstellarmedium has been found to date. The object joins a list of similarsystems, including the Local Group dwarfs Tucana and Cetus, that do notfit into the global picture of the morphology-density relation in whichgas-rich dwarf irregular galaxies are in relative isolation andgas-deficient dwarf elliptical galaxies are satellites of more luminousgalaxies.
| 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).
| Global Properties of Nearby Galaxies in Various Environments
We analyze a sample of the Local Volume that contains 451 galaxieswithin 10 Mpc. We compare the various global parameters of thesegalaxies with their tidal index that characterizes the local density ofthe environment. The closest correlation is observed between the densityof the galaxy’s environment and its morphological type. Theabundance of neutral hydrogen in the members of close groups was foundto be, on average, a factor of 3 lower than that in isolated galaxies.However, much of this difference is attributable to differentmorphological composition for the group members and field galaxies. Thetotal mass-to-luminosity ratio is virtually independent of the tidalindex of the galaxy, which indirectly indicates a low percentage oftidal systems among dwarf galaxies. All of the galaxies with three ormore companions in the Local Volume are shown to have masses above thethreshold value of 1010 M ȯ.
| 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.
| 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.
| 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.
| Discovery of a Solitary Dwarf Galaxy in the APPLES Survey
During the APPLES parallel campaign, the Hubble Space Telescope (HST)Advanced Camera for Surveys has resolved a distant stellar system, whichappears to be an isolated dwarf galaxy. It is characterized by acircularly symmetric distribution of stars with an integrated magnitudemF775W=20.13+/-0.02, a central surface brightnessμF775W~=21.33+/-0.18 mag arcsec-2, and ahalf-light radius of ~=1.8". The ACS and VLT spectra show no evidence ofionized gas and appear to be dominated by a 3 Gyr old stellarpopulation. The OB spectral type derived for two resolved stars in thegrism data and the systemic radial velocity (Vhel~=670 kms-1) measured from the VLT data give a fiducial distance of~=9+/-2 Mpc. These findings, with the support of the spatial morphology,would classify the system among the dwarf spheroidal (dSph) galaxies.Following IAU rules, we have named this newly discovered galaxy APPLES1. An intriguing peculiarity of APPLES 1 is that the properties (age andmetallicity) of the stellar content so far detected are similar to thoseof dSph galaxies in the Local Group, where star formation is thought tobe driven by galaxy interactions and mergers. Yet, APPLES 1 seems not tobe associated with a major group or cluster of galaxies. Therefore,APPLES 1 could be the first example of a field dSph galaxy withself-sustained and regulated star formation, and therefore would make aninteresting test case for studies of the formation and evolution ofunperturbed dSph galaxies.
| 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.
| 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.
| The Tumultuous Lives of Galactic Dwarfs and the Missing Satellites Problem
Hierarchical cold dark matter (CDM) models predict that Milky Way-sizedhalos contain several hundred dense low-mass dark matter satellites (thesubstructure), an order of magnitude more than the number of observedsatellites in the Local Group. If the CDM paradigm is correct, thisprediction implies that the Milky Way and Andromeda are filled withnumerous dark halos. To understand why these halos failed to form starsand become galaxies, we need to understand their history. We analyze thedynamical evolution of the substructure halos in a high-resolutioncosmological simulation of Milky Way-sized halos in the ΛCDMcosmology. We find that about 10% of the substructure halos with thepresent masses <~108-109 Msolar(circular velocities Vm<~30 km s-1) hadconsiderably larger masses and circular velocities when they formed atredshifts z>~2. After the initial period of mass accretion inisolation, these objects experience dramatic mass loss because of tidalstripping. Our analysis shows that strong tidal interaction is oftencaused by actively merging massive neighboring halos, even before thesatellites are accreted by their host halo. These results can explainhow the smallest dwarf spheroidal galaxies of the Local Group were ableto build up a sizable stellar mass in their seemingly shallow potentialwells. We propose a new model in which all the luminous dwarfspheroidals in the Local Group are descendants of the relatively massive(>~109 Msolar) high-redshift systems, in whichthe gas could cool efficiently by atomic line emission, and which werenot significantly affected by the extragalactic ultraviolet radiation.We present a simple galaxy formation model based on the trajectoriesextracted from the simulation, which accounts for the bursts of starformation after strong tidal shocks and the inefficiency of gas coolingin halos with virial temperatures Tvir<~104 K.Our model reproduces the abundance, spatial distribution, andmorphological segregation of the observed Galactic satellites. Theresults are insensitive to the redshift of reionization.
| H I Observations of Nearby Dwarf Galaxies
We present H I detections towards several Local Group dSphs anddIrr/dSphs. The possibility that the detected emission can be due toHigh Velocity Clouds (HVC) is ruled out although it appears that most ofour targets are in HVC rich regions.
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