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|An Observational Limit on the Dwarf Galaxy Population of the Local Group|
We present the results of an all-sky, deep optical survey for faintLocal Group dwarf galaxies. Candidate objects were selected from thesecond Palomar Observatory Sky Survey and ESO/Science Research Councilsurvey plates, and follow-up observations were performed to determinewhether they were indeed overlooked members of the Local Group. Only twogalaxies (Antlia and Cetus) were discovered this way out of 206candidates. Based on internal and external comparisons, we estimate thatour visual survey is more than 77% complete for objects larger than 1'in size and with a surface brightness greater than an extremely faintlimit over the 72% of the sky not obstructed by the Milky Way. Our limitof sensitivity cannot be calculated exactly, but it is certainly fainterthan 25 mag arcsec-2 in R, probably 25.5 and possiblyapproaching 26. We conclude that there are at most one or two LocalGroup dwarf galaxies fitting our observational criteria stillundiscovered in the clear part of the sky, and roughly a dozen hiddenbehind the Milky Way. Our work places the ``missing satellite problem''on a firm quantitative observational basis. We present detailed data onall our candidates, including surface brightness measurements.
|Interstellar Medium Disruption in the Centaurus A Group|
We present the results of a 21 cm neutral hydrogen (H I) line detectionexperiment in the direction of 18 low-luminosity dwarf galaxies of theCentaurus A group, using the Australia Telescope National Facility 64 mParkes Radio Telescope and the Australia Telescope Compact Array. Fivedwarfs have H I masses between MHI=4×105 and2.1×107 Msolar and 0.04 MsolarL-1solar,B107 orMHI<106 Msolar. This gap may beexplained by the ram pressure stripping mechanism at work in this denseenvironment in which all galaxies with MHI<107Msolar have been stripped of their gas. The requiredintergalactic medium density to achieve this is ~10-3cm-3.
|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.
|Non-Friedmann cosmology for the Local Universe, significance of the universal Hubble constant, and short-distance indicators of dark energy|
Based on the increasing evidence of the cosmological relevance of thelocal Hubble flow, we consider a simple analytical cosmological modelfor the Local Universe. This is a non-Friedmann model with a non-uniformstatic space-time. The major dynamical factor controlling the localexpansion is the antigravity produced by the omnipresent and permanentdark energy of the cosmic vacuum (or the cosmological constant). Theantigravity dominates at larger distances than 1-2 Mpc from the centerof the Local group. The model gives a natural explanation of the two keyquantitative characteristics of the local expansion flow, which are thelocal Hubble constant and the velocity dispersion of the flow. Theobserved kinematical similarity of the local and global flows ofexpansion is clarified by the model. We analytically demonstrate theefficiency of the vacuum cooling mechanism that allows one to see theHubble law this close to the Local group. The "universal Hubbleconstant" HV (≈60 km s-1 Mpc-1),depending only on the vacuum density, has special significance locallyand globally. The model makes a number of verifiable predictions. Italso unexpectedly shows that the dwarf galaxies of the local flow withthe shortest distances and lowest redshifts may be the most sensitiveindicators of dark energy in our neighborhood.
|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.
|Hot Dust and Polycyclic Aromatic Hydrocarbon Emission at Low Metallicity: A Spitzer Survey of Local Group and Other Nearby Dwarf Galaxies|
We present Spitzer 4.5 and 8.0 μm imaging of 15 Local Group andnearby dwarf galaxies. We find that the diffuse 8 μm emission isspatially correlated with regions of active star formation. Our samplespans a range of >1 dex in nebular metallicity and 3 orders ofmagnitude in current star formation rate, allowing us to examine thedependence of emission from hot dust and PAHs on these parameters. Wedetect prominent diffuse 8 μm emission from the four most luminousgalaxies in the sample (IC 1613, IC 5152, NGC 55, and NGC 3109) and onlyvery low surface brightness emission from four others (DDO 216, SextansA, Sextans B, and WLM). These are the first spatially resolved images ofdiffuse 8 μm emission from such low-metallicity objects[12+log(O/H)~7.5]. We observe correlations of this emission with thecurrent star formation rate and the nebular metallicity of thesegalaxies. However, we also see evidence suggesting that other processesmay also have a significant effect on the generation of this emission.These systems all have evidence for old and intermediate-age starformation; thus, the lack of diffuse 8 μm emission cannot beattributed to low galaxy ages. Also, winds cannot explain the paucity ofthis emission, since high-resolution imaging of the neutral gas in theseobjects shows no evidence of blowout. We propose that the lack ofdiffuse 8 μm emission in low-metallicity systems may be due to thedestruction of dust grains by supernova shocks, assuming a longtimescale to regrow dust. It is likely that the observed weak emissionis at least partly due to a general absence of dust (including PAHs), inagreement with their low metallicities.
|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.
|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%.
|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.
|The chemistry of planetary nebulae and HII regions in the dwarf galaxies Sextans A and B from deep VLT spectra|
Spectroscopic observations obtained with the VLT of one planetary nebula(PN) in Sextans A and of five PNe in Sextans A and of several H iiregions in these two dwarf irregular galaxies are presented. Theextended spectral coverage, from 320.0 to 1000.0 nm, and the largetelescope aperture allowed us to detect a number of emission lines,covering more than one ionization stage for several elements (He, O, S,Ar). The electron temperature diagnostic O iii line at 436.3 nm wasmeasured in all six PNe and in several H ii regions allowing for anaccurate determination of the ionic and total chemical abundances bymeans of the Ionization Correction Factors method. For the time being,these PNe are the farthest ones where such a direct measurement of theelectron temperature is obtained. In addition, all PNe and H ii regionswere also modelled using the photoionization code CLOUDY (Ferland et al.1998, PASP, 110, 761). The physico-chemical properties of PNe and H iiregions are presented and discussed. A small dispersion in the oxygenabundance of H ii regions was found in both galaxies: 12 + log(O/H)=7.6± 0.2 in Sextans A, and 7.8± 0.2 in Sextans B.For the five PNe of Sextans B, we find that 12 + log (O/H)=8.0±0.3, with a mean abundance consistent with that of H ii regions. Theonly PN known in Sextans A appears to have been produced by a quitemassive progenitor, and has a significant nitrogen overabundance. Inaddition, its oxygen abundance is 0.4 dex larger than the mean abundanceof H ii regions, possibly indicating an efficient third dredge-up formassive, low-metallicity PN progenitors. The metal enrichment of bothgalaxies is analyzed using these new data.
|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).
|The Distance and Metallicity of the Newly Discovered, Nearby Irregular Galaxy HIZSS 3|
HIZSS 3 is an H I source in the Zone of Avoidance. Its radiocharacteristics are consistent with it being a previously unknown,nearby (~1.8 Mpc), low-mass dwarf irregular (dIrr) galaxy. Opticalobservations have shown that it contains a modest H II region, but theyfailed to reveal a resolved stellar population. New spectroscopicobservations of the H II region obtained at the MMT Observatory arepresented here. They are used to derive the line-of-sight extinction[E(B-V)=1.41+/-0.04] and gas metallicity (logO/H+12~7.8) of the H IIregion. New near-IR imaging observations obtained at the ESO Very LargeTelescope are also presented here. These images clearly reveal theresolved stellar population of HIZSS 3 for the first time. NarrowbandPaβ images of the H II region are used in combination withpreviously published Hα data to obtain an independentline-of-sight extinction estimate: E(B-V)=1.32+/-0.04. The adoptedforeground extinction is E(B-V)=1.36+/-0.06. Using the K-band luminosityfunction and K,J-K color-magnitude diagram, the apparent magnitude andcolor of the tip of the red giant branch are derived. In turn, theseparameters are combined with the adopted foreground extinction toestimate the distance (1.69+/-0.07 Mpc) and mean red giant branchmetallicity ([Fe/H]=-0.5+/-0.1). As an ensemble, these new observationssignificantly strengthen the conclusion that HIZSS 3 is a newlydiscovered low-mass dIrr galaxy lurking behind the Milky Way in theoutskirts of the Local Group.The optical spectroscopic observations reported here were obtained atthe MMT Observatory, a joint facility of the Smithsonian Institution andthe University of Arizona. The near-IR imaging observations reportedhere were collected at the European Southern Observatory, Cerro Paranal,Chile, within observing program 271.B-5047.
|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.
|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.
|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.
|First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A|
We present the abundance analyses of three isolated A-type supergiantstars in the dwarf irregular galaxy Sextans A (=DDO 75) fromhigh-resolution spectra obtained with the Ultraviolet-Visual EchelleSpectrograph (UVES) on the Kueyen telescope (UT2) of the ESO Very LargeTelescope (VLT). Detailed model atmosphere analyses have been used todetermine the stellar atmospheric parameters and the elementalabundances of the stars. The mean iron-group abundance was determinedfrom these three stars to be <[(Fe II, CrII)/H]>=-0.99+/-0.04+/-0.06.2 This isthe first determination of the present-day iron-group abundances inSextans A. These three stars now represent the most metal-poor massivestars for which detailed abundance analyses have been carried out. Themean stellar α-element abundance was determined from theα-element magnesium as <[α(MgI)/H]>=-1.09+/-0.02+/-0.19. This is in excellent agreement with thenebular α-element abundances as determined from oxygen in the H IIregions. These results are consistent from star to star, with nosignificant spatial variations over a length of 0.8 kpc in Sextans A.This supports the nebular abundance studies of dwarf irregular galaxies,where homogeneous oxygen abundances are found throughout, and arguesagainst in situ (``on the spot'') enrichment. The α/Fe abundanceratio is <[α(Mg I)/Fe II, Cr II]>=-0.11+/-0.02+/-0.10, whichis slightly lower but consistent with the solar ratio. This isconsistent with the results from A-supergiant analyses in other LocalGroup dwarf irregular galaxies, NGC 6822 and WLM. The results ofnear-solar [α/Fe] ratios in dwarf galaxies is in stark contrastwith the high [α/Fe] results from metal-poor stars in the Galaxy(which plateau at values near +0.4 dex) and is most clearly seen fromthese three stars in Sextans A because of their lower metallicities. Thelow [α/Fe] ratios are consistent with the slow chemical evolutionexpected for dwarf galaxies from analyses of their stellar populations.Based on UVES observations collected at the European SouthernObservatory at Paranal, Chile (proposals 68.D-0136, 69.D-0383,70.D-0473). Additional spectra were gathered with the ESI spectrographat the W. M. Keck Observatory.
|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.
|Stellar Spectroscopy of Individual Stars in Local Group Galaxies with the VLT|
The large collecting area and high-throughput multi-object instrumentson the VLT make it possible to carry out detailed studies of stellarproperties and distributions in environments well beyond our Galaxy.
|The Variable-Star Population in Phoenix: Coexistence of Anomalous and Short-Period Classical Cepheids and Detection of RR Lyrae Variables|
We present the results of a search for variable stars in the Local Groupdwarf galaxy Phoenix. Nineteen Cepheids, six candidate long-periodvariables, one candidate eclipsing binary, and a large number ofcandidate RR Lyrae stars have been identified. Periods and light curveshave been obtained for all the Cepheid variables. Their distribution inthe period-luminosity diagram reveals that both anomalous Cepheids (ACs)and short-period classical Cepheids (s-pCC's) are found in our sample.This is the first time that both types of variable stars are identifiedin the same system even though they likely coexist, but have goneunnoticed so far, in other low-metallicity galaxies such as Leo A andSextans A. We argue that the conditions for the existence of both typesof variable stars in the same galaxy are a low metallicity at all agesand the presence of both young and intermediate-age (or old, dependingon the nature of AC) stars. The RR Lyrae candidates trace, together withthe well-developed horizontal branch, the existence of an important oldpopulation in Phoenix. The different spatial distributions of s-pCC's,ACs, and RR Lyrae variables in the Phoenix field are consistent with thestellar population gradients found in Phoenix, in the sense that theyounger population is concentrated in the central part of the galaxy.The gradients in the distribution of the young population within thecentral part of Phoenix, which seem to indicate a propagation of therecent star formation, are also reflected in the spatial distribution ofthe s-pCC's.
|The very local Hubble flow: Computer simulations of dynamical history|
The phenomenon of the very local (≤3 Mpc) Hubble flow is studied onthe basis of the data of recent precision observations. A set ofcomputer simulations is performed to trace the trajectories of the flowgalaxies back in time to the epoch of the formation of the Local Group.It is found that the ``initial conditions'' of the flow are drasticallydifferent from the linear velocity-distance relation. The simulationsenable one also to recognize the major trends of the flow evolution andidentify the dynamical role of universal antigravity produced by thecosmic vacuum.
|A Neighboring Dwarf Irregular Galaxy Hidden by the Milky Way|
We have obtained VLA and optical follow-up observations of thelow-velocity H I source HIZSS 3 discovered by Henning et al. and Riversin a survey for nearby galaxies hidden by the disk of the Milky Way. Itsradio characteristics are consistent with this being a nearby (~1.8 Mpc)low-mass dwarf irregular galaxy (dIm). Our optical imaging failed toreveal a resolved stellar population but did detect an extended Hαemission region. The location of the Hα source is coincident witha partially resolved H I cloud in the 21 cm map. Spectroscopy confirmsthat the Hα source has a similar radial velocity to that of the HI emission at this location, and thus we have identified an opticalcounterpart. The Hα emission (100 pc in diameter and with aluminosity of 1.4×1038 ergs s-1) ischaracteristic of a single H II region containing a modest population ofOB stars. The galaxy's radial velocity and distance from the solar apexsuggests that it is not a Local Group member, although a more accuratedistance is needed to be certain. The properties of HIZSS 3 arecomparable to those of GR 8, a nearby dIm with a modest amount ofcurrent star formation. Further observations are needed to characterizeits stellar population, determine the chemical abundances, and obtain amore reliable distance estimate.
|Star Formation and the Interstellar Medium in Four Dwarf Irregular Galaxies|
We present new, high-sensitivity VLA observations of H I in four dwarfgalaxies (UGCA 292, GR 8, DDO 210, and DDO 216), and we use these datato study interactions between star formation and the interstellarmedium. H I velocity dispersions and line shapes in UGCA 292, GR 8, andDDO 210 show evidence that these three galaxies contain both warm andcool or cold H I phases. The presence of the cold neutral medium isindicated by a low-dispersion (3-6 km s-1) H I component orby the Gauss-Hermite shape parameter h4>0. Contrary toexpectations, we find no trend between the incidence of thelow-dispersion (colder) phase and the star formation rate in five dwarfgalaxies. The colder H I phase may be a necessary ingredient for starformation, but it is clearly not sufficient. However, there is a globaltrend between the star formation rate of a galaxy and the incidence ofasymmetric H I profiles. This trend probably reflects kinetic energyinput from young massive stars. Numerical simulations show that theeffects of rotational broadening (finite angular resolution) are minimalfor these galaxies. Simulations are also used to estimate the errors inthe column densities of the high-dispersion and the low-dispersion H Iphases.
|Deep Hubble Space Telescope Imaging of Sextans A. III. The Star Formation History|
We present a measurement of the star formation history of Sextans A,based on WFPC2 photometry that is 50% complete to V=27.5(MV~+1.9) and I=27.0. The star formation history and chemicalenrichment history have been measured through modeling of thecolor-magnitude diagram (CMD). We find evidence for increased reddeningin the youngest stellar populations and an intrinsic metallicity spreadat all ages. Sextans A has been actively forming stars at a high ratefor ~2.5 Gyr ago, with an increased rate beginning ~0.1 Gyr ago. We finda nonzero number of stars older than 2.5 Gyr, but because of the limiteddepth of the photometry, a detailed star formation history atintermediate and older ages has considerable uncertainties. The meanmetallicity was found to be [M/H]~-1.4 over the measured history of thegalaxy, with most of the enrichment happening at ages of at least 10Gyr. We also find that an rms metallicity spread of 0.15 dex at all agesallows the best fits to the observed CMD. We revisit our determinationof the recent star formation history (age<=0.7 Gyr) using bluehelium-burning (BHeB) stars and find good agreement for all but the last25 Myr, a discrepancy resulting primarily from different distances usedin the two analyses and the differential extinction in the youngestpopulations. This indicates that star formation histories determinedsolely from BHeB stars should be confined to CMD regions where nocontamination from reddened main-sequence stars is present.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 proposal ID7496.
|Halo-Disk vs. Shrinking Scenarios|
We discuss the possibility that the extended structures, lacking youngstars, routinely found in dwarf irregular galaxies (dIrr) are aconsequence of a progressive shrinking of the star forming region. Thiswould be an alternative scenario to the possible halo-disk structure ofdIrr. Although the final answer must wait until kinematical data of someaccuracy become available, we present two alternative approaches to thestudy of the extended structures in dIrrs. In the first one, the radialdistribution of stellar populations together with population synthesis,based on surface brightness fluctuations (SBF), has been applied to DDO165. This is a dIrr about 5 Mpc away from the Milky Way. Results showthat the resolved, young population is more concentrated than theintermediate-old, unresolved population. The later is 7 Gyr old, inaverage, and has a very low metallicity (about Z = 0.0007). Thisindicates that, together with a truly old stellar population, animportant intermediate-age population is present in the outer region ofthe galaxy. In our second approach, deep color-magnitude diagrams(CMDs), based on Hubble Space Telescope (HST) observations and reachingthe oldest turn-offs, are used to analyze the inner and outer stellarpopulations of the Phoenix dwarf galaxy. Results show that, togetherwith an old stellar population, the outer field contains also anintermediate-age population. These results are compatible with ascenario in which star forming regions are shrinking with time (theshrinking scenario). It seems more difficult to support a halo-diskscenario, which would require extended structures populated only byreally old stars.
|The Cepheid Distance to NGC 5236 (M83) with the ESO Very Large Telescope|
Cepheids have been observed in NGC 5236 (M83) using the Antu (UnitTelescope 1) 8.2 m telescope of the ESO Very Large Telescope with theFocal Reducer/Low Dispersion Spectrograph 1. Repeated imagingobservations have been made between 2000 January and 2001 July. Imageswere obtained in 34 epochs in the V band and in six epochs in the Iband. The photometry was made with the ROMAFOT reduction package andchecked independently with DoPHOT and a modified version of HSTPHOT.Twelve Cepheid candidates have periods ranging between 12 and 55 days.The dereddened distance modulus is adopted to be(m-M)0=28.25+/-0.15, which corresponds to a distance of4.5+/-0.3 Mpc. The Cepheid distance of NGC 5253 has been rediscussed andstrengthened by its SN 1972E. The mean distance of(m-M)0=28.01+/-0.15 (based on SN 1972E) shows the galaxy tobe a close neighbor of M83, suggesting that the two galaxies may haveinteracted in the past and thus possibly explaining the amorphousmorphology of NGC 5253. The distance difference between M83 and NGC 5253is only 0.5+/-0.4 Mpc. The projected distance is only ~ 0.15 Mpc. M83 isthe principal member of the nearby M83 group containing also, besidesNGC 5253, several dwarf members, for five of which tip of the red giantbranch distances are available. The adopted group distance of(m-M)0=28.28+/-0.10 (4.5+/-0.2 Mpc), together with its meanrecession velocity of vLG=249+/-42 km s-1, showsagain the extreme quietness of the local (1-10 Mpc) expansion field. M83fits onto the local mean Hubble flow line of the velocity-distancerelation (with H0~60) with no significant deviation,supporting the earlier conclusion that the local velocity expansionfield is remarkably cold on a scale of 10 Mpc, contrary to thepredictions of the simplest cold dark matter model for large-scalestructure. The role of a cosmological constant has been invoked as apossible solution in providing a nearly uniform force field everywherein the presence of a lumpy galaxy distribution.Based on observations collected with the UT1 of the Very LargeTelescope, which is operated by the European Southern Observatory.
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