Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Asymptotic giant branch stars in the Phoenix dwarf galaxy
JHKs near-infrared photometry of stars in the Phoenix dwarfgalaxy is presented and discussed. Combining these data with the opticalphotometry of Massey et al. allows a rather clean separation of fieldstars from Phoenix members. The discovery of a Mira variable (P = 425d), which is almost certainly a carbon star, leads to an estimate of thedistance modulus of 23.10 +/- 0.18 that is consistent with otherestimates and indicates the existence of a significant population of age~2 Gyr. The two carbon stars of Da Costa have Mbol = -3.8 andare consistent with belonging to a population of similar age; some otherpossible members of such a population are identified. A Da Costanon-carbon star is ΔKs ~ 0.3 mag brighter than thesetwo carbon stars. It may be an asymptotic giant branch star of thedominant old population. The nature of other stars lying close to it inthe Ks, (J - Ks) diagram needs studying.
| The Local Group dwarf Leo T: HI on the brink of star formation
We present Giant Meterwave Radio Telescope (GMRT) and WesterborkSynthesis Radio Telescope (WSRT) observations of the recently discoveredLocal Group dwarf galaxy, Leo T. The peak HI column density is measuredto be 7 × 1020cm-2, and the total HI mass is2.8 × 105Msolar, based on a distance of 420kpc. Leo T has both cold (~500 K) and warm (~6000 K) HI at its core,with a global velocity dispersion of 6.9 kms-1, from which wederive a dynamical mass within the HI radius of 3.3 ×106Msolar, and a mass-to-light ratio of >50. Wecalculate the Jeans mass from the radial profiles of the HI columndensity and velocity dispersion, and predict that the gas should beglobally stable against star formation. This finding is inconsistentwith the half light radius of Leo T, which extends to 170 pc, andindicates that local conditions must determine where star formationtakes place. Leo T is not only the lowest luminosity galaxy withon-going star formation discovered to date, but it is also the most darkmatter-dominated, gas-rich dwarf in the Local Group.
| Ram Pressure Stripping of an Isolated Local Group Dwarf Galaxy: Evidence for an Intragroup Medium
We compare the stellar structure of the isolated, Local Group dwarfgalaxy Pegasus (DDO 216) with low-resolution H I maps from L. M. Younget al. Our comparison reveals that Pegasus displays the characteristicmorphology of ram pressure stripping; in particular, the H I has a``cometary'' appearance that is not reflected in the regular, ellipticaldistribution of the stars. This is the first time this phenomenon hasbeen observed in an isolated Local Group galaxy. The density of themedium required to ram pressure strip Pegasus is at least10-5 to 10-6 cm-3. We conclude thatthis is strong evidence for an intergalactic medium associated with theLocal Group.
| A Survey of Local Group Galaxies Currently Forming Stars. III. A Search for Luminous Blue Variables and Other Hα Emission-Line Stars
We describe a search for Hα emission-line stars in M31, M33, andseven dwarfs in or near the Local Group (IC 10, NGC 6822, WLM, SextansB, Sextans A, Pegasus, and the Phoenix dwarf) using interference filterimaging with the KPNO and CTIO 4 m telescopes and Mosaic cameras. Thesurvey is aimed primarily at identifying new luminous blue variables(LBVs) from their spectroscopic similarity to known LBVs, avoiding thebias toward photometric variability, which may require centuries tomanifest itself if LBVs go through long quiescent periods. Follow-upspectroscopy with WIYN confirms that our survey detected a wealth ofstars whose spectra are similar to the known LBVs. We "classify" thespectra of known LBVs and compare these to the spectra of the new LBVcandidates. We demonstrate spectacular spectral variability for severalof the new LBV candidates, such as AM2, previously classified as aWolf-Rayet star (WR), which now shows Fe I, Fe II, and Balmer emissionlines but neither the N III λλ4634, 4642 nor the He IIλ4686 emission it did in 1982. Profound spectral changes are alsonoted for other suspected and known LBVs. Several of the LBV candidatesalso show >0.5 mag changes in V over the past 10-20 years. The numberof known or suspected LBVs is now 24 in M31, 37 in M33, 1 in NGC 6822,and 3 in IC 10. We estimate that the total number of LBVs in M31 and M33may be several hundred, in contrast to the eight known historicallythrough large-scale photometric variability. This has significantimplications for the timescale of the LBV phase. We also identify a fewnew WRs and peculiar emission-line objects.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.
| Breathing in Low-Mass Galaxies: A Study of Episodic Star Formation
We simulate the collapse of isolated dwarf galaxies using SPH + N-bodysimulations including a physically motivated description of the effectsof supernova feedback. As the gas collapses and stars form, thesupernova feedback disrupts enough gas to temporarily quench starformation. The gas flows outward into a hot halo, where it cools untilstar formation can continue once more and the cycle repeats. The starformation histories of isolated Local Group dwarf galaxies exhibitsimilar episodic bursts of star formation. We examine the massdependence of the stellar velocity dispersions and find that they are noless than half the velocity of the halos measured at the virial radius.
| Stellar Velocity Dispersion of the Leo A Dwarf Galaxy
We measure the first stellar velocity dispersion of the Leo A dwarfgalaxy, σ=9.3+/-1.3 km s-1. We derive the velocitydispersion from the radial velocities of 10 young B supergiants and twoH II regions in the central region of Leo A. We estimate a projectedmass of (8+/-2.7)×107 Msolar within a radiusof 2' and a mass-to-light ratio of at least 20+/-6Msolar/Lsolar. These results imply Leo A is atleast ~80% dark matter by mass.
| Stellar Content and Recent Star Formation History of the Local Group Dwarf Irregular Galaxy IC 1613
We present resolved-star VI photometry of the Local Group dwarfirregular galaxy IC 1613 reaching I ~ 23.5, obtained with the wide-fieldcamera at the 2.5 m Isaac Newton Telescope. A fit to the stellar densitydistribution shows an exponential profile of scale length 2.9' ±0.1' and gives a central surface brightness μV,0 = 22.7± 0.6. The significant number of red giant branch (RGB) starspresent in the outer part of our images (r > 16.5') indicates thatthe galaxy is actually more extended than previously estimated. Acomparison of the color-magnitude diagrams (CMDs) as a function ofgalactocentric distance shows a clear gradient in the age of itspopulation, the scale length increasing with age, while we find noevidence of a metallicity gradient from the width of the RGB. We presentquantitative results of the recent star formation history from asynthetic CMD analysis using IAC-STAR. We find a mean star formationrate of (1.6 ± 0.8) × 10-3 Modotyr-1 kpc-2 in the central r lesssim 2.5' for thelast 300 Myr.Based on observations made with the Isaac Newton Telescope, operated onthe island of La Palma by the Isaac Newton Group, in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.
| 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.
| A Survey of Local Group Galaxies Currently Forming Stars. II. UBVRI Photometry of Stars in Seven Dwarfs and a Comparison of the Entire Sample
We have obtained UBVRI images with the Kitt Peak National Observatoryand Cerro Tololo Inter-American Observatory 4 m telescopes and Mosaiccameras of seven dwarfs in (or near) the Local Group, all of which haveknown evidence of recent star formation: IC 10, NGC 6822, WLM, SextansB, Sextans A, Pegasus, and Phoenix. We construct color-magnitudediagrams (CMDs) of these systems, as well as neighboring regions thatcan be used to evaluate the degree of foreground contamination by starsin the Milky Way. Intercomparison of these CMDs with those of M31, M33,the LMC, and the SMC permits us to determine improved reddening valuesfor a typical OB star found within these galaxies. All of the CMDsreveal a strong or modest number of blue supergiants. All but Pegasusand Phoenix also show the clear presence of red supergiants in the CMD,although IC 10 appears to be deficient in these objects given its largeWolf-Rayet star population. The bright stars of intermediate color inthe CMD are badly contaminated by foreground stars (30%-100%), andconsiderable spectroscopy is needed before statistics on the yellowsupergiants in these systems will be known. This study is intended toserve as both the impetus and ``finding charts'' for further space-basedimaging and for many spectroscopic programs at large aperture.
| The Aptly Named Phoenix Dwarf Galaxy
The Local Group galaxy Phoenix has the properties of a dwarf spheroidalgalaxy, but an adjacent H I cloud has been recently found to be at thesame radial velocity as the stars. The proximity suggests that thiscloud is associated with the most recent (<=100 Myr) star formationin Phoenix. We have obtained relatively high sensitivity and highresolution H I imaging with the VLA with the goal of distinguishingbetween different processes for displacing the gas from the galaxy. Dueto the outer curvature of the H I cloud, it appears that expulsion fromthe galaxy by winds from supernovae is more likely than ram pressurestripping. The isolation of the galaxy makes tidal stripping highlyunlikely. Using a star formation history constructed from HST imaging,we construct a simple kinematic model that implies that the H I cloud isstill gravitationally bound to the galaxy. Gas that is expelled from thecenters of dwarf galaxies but remains gravitationally bound may explainthe episodic star formation observed in several dwarfs. In the specificcase of Phoenix, there may be future star formation in this currentlydSph-like galaxy.
| 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.
| Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way
We announce the discovery of a new dwarf galaxy, Leo T, in the LocalGroup. It was found as a stellar overdensity in the Sloan Digital SkySurvey Data Release 5 (SDSS DR5). The color-magnitude diagram of Leo Tshows two well-defined features, which we interpret as a red giantbranch and a sequence of young, massive stars. As judged from fits tothe color-magnitude diagram, it lies at a distance of ~420 kpc and hasan intermediate-age stellar population with a metallicity of [Fe/H] =-1.6, together with a young population of blue stars of age ~200 Myr.There is a compact cloud of neutral hydrogen with mass ~105Msolar and radial velocity +35 km s-1 coincidentwith the object visible in the HIPASS channel maps. Leo T is thesmallest, lowest luminosity galaxy found to date with recent starformation. It appears to be a transition object similar to, but muchlower luminosity than, the Phoenix dwarf.
| 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.
| Optical and NIR spectroscopy of Mrk 1210: constraints and physical conditions of the active nucleus
Aims.Mrk 1210 is an outstanding Seyfert 2 galaxy because it displayssignatures of recent circumnuclear star formation and a high level ofX-ray activity, in addition to the classical spectral characteristicstypical of an AGN. Here we investigate the extinction affecting thenuclear and extended emitting gas, the kinematics of the narrow-lineregion, and the physical properties and conditions of that gas. Methods: Near-infrared and optical spectra of the nuclear and extendedemission region of Mrk 1210 are presented, covering the interval 0.4-2.4μm. Emission and absorption lines were used to infer, respectively,the geometrical extension of the ionized gas and the contribution of theunderlying stellar population to the observed integrated continuum. Theemission line profiles were employed to study the kinematics in the NLR.The reddening and physical condition of the gas were investigated bymeans of flux ratios among permitted and forbidden lines. Results: TheNIR nuclear spectrum is dominated by H I and He I recombination lines,as well as [S II], [S III], and [Fe II] forbidden lines. Coronal linesof [S VIII], [S IX], [Si VI], [Si X], and [Ca VIII], in addition tomolecular H{2} lines, were also detected. The 12CO(6{-3)}1.618 μm overtone bandhead helped to estimate the contribution of thestellar population to the continuum. It was found that 83±8% ofthe H-band continuum has a stellar origin. It improves previousestimates, which claimed that at least 50% of the observed continuum wasattributed to the AGN. Analysis of the emission line profiles, bothallowed and forbidden, shows a narrower ({FWHM} ˜ 500 kms-1) line on top of a broad ({FWHM} > 1000 kms-1) blue-shifted component. This seems to be associated to anuclear outflow. This hypothesis is supported by 6 cm VLBI observations,which show a radio ejecta extending up to 30 pc from the nucleus. Thisresult does not require the presence of the hidden BLR claimed to bepresent in previous NIR observations of this object. Internalextinction, calculated by means of several indicators including Fe II]flux ratios not previously used before in AGNs, reveals a dusty AGN,while the extended regions are barely affected by dust, if at all. Thedensity and temperature are calculated for the NLR using optical and NIRlines as diagnostic ratios. The results show electronic temperaturesfrom 10 000 K up to 40 000 K and densities between 10^3-105cm-3. The higher temperatures show that shocks, most probablyrelated to the radio outflow, must contribute to the line emission.Based in part in observations collected at the Pico dos DiasObservatory/LNA, Brazil. Figures 1-3 are only available in electronicform at http://www.aanda.org
| 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
| Star formation history and evolution of gas-rich dwarf galaxies in the Centaurus A group
We analyse the properties of three unusual dwarf galaxies in theCentaurus A group discovered with the HIPASS survey. From their opticalmorphology, they appear to be low surface brightness dwarf spheroidals,yet they are gas rich (MHI/LB > 1) withgas-mass-to-stellar light ratios larger than typical dwarf irregulargalaxies. Therefore these systems appear different from any dwarfs ofthe Local Group. They should be favoured hosts for starburst, whereas wefind a faint star formation region in only one object. We have obtained21-cm data and Hubble Space Telescope photometry in V and I bands, andhave constructed colour magnitude diagrams (CMDs) to investigate theirstellar populations and to set a constraint on their age. From thecomparison of the observed and model CMDs, we infer that all threegalaxies are at least older than 2 Gyr (possibly even as old as 10 Gyr)and remain gas rich because their star formation rates have been verylow (<~10-3Msolaryr-1) throughout.In such systems, star formation appears to have been sporadic and local,though one object (HIPASS J1321-31) has a peculiar red plume in its CMDsuggesting that many of its stars were formed in a `miniburst' 300-500Myr ago. The question of why there are no similar dwarf galaxies in theLocal Group remains open.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociations of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555; the Australia Telescope Compact Arraywhich is part of the Australia Telescope, funded by the Commonwealth ofAustralia for operation as a National Facility managed by CSIRO.E-mail: marco.grossi@roma 1.infn.it ‡Visiting Astronomer, Kitt Peak National Observatory, National OpticalAstronomy Observatories, which is operated by the Association ofUnicersities for for Reasearch in Astronomy. Inc. (AURA) undercooperative agreement with the National Science Foundation.
| 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 CO content of the Local Group dwarf irregular galaxies IC5152, UGCA438 and the Phoenix dwarf
We present a search for CO(1 -> 0) emission in three Local Groupdwarf irregular galaxies: IC5152, the Phoenix dwarf and UGCA 438, usingthe ATNF Mopra radio telescope. Our scans largely cover the opticalextent of the galaxies and the stripped HI cloud west of the Phoenixdwarf. Apart from a tentative but non-significant emission peak at oneposition in the Phoenix dwarf, no significant emission was detected inthe CO spectra of these galaxies. For a velocity width of 6kms-1, we derive 4σ upper limits of 0.03, 0.04 and0.06Kkm s-1 for IC5152, the Phoenix dwarf and UGCA438,respectively. This is an improvement of over a factor of 10 comparedwith previous observations of IC5152 the other two galaxies had not yetbeen observed at millimetre wavelengths. Assuming a GalacticCO-to-H2 conversion factor, we derive upper limits on themolecular gas mass of 6.2 × 104,3.7 ×103 and 1.4 × 105Msolar forIC5152, the Phoenix dwarf and UGCA438, respectively. We investigate twopossible causes for the lack of CO emission in these galaxies. On theone hand, there may be a genuine lack of molecular gas in these systems,in spite of the presence of large amounts of neutral gas. However, inthe case of IC5152 which is actively forming stars, molecular gas is atleast expected to be present in the star-forming regions. On the otherhand, there may be a large increase in the CO-to-H2conversion factor in very low-metallicity dwarfs (-2 <= [Fe/H] <=-1), making CO a poor tracer of the molecular gas content in dwarfgalaxies.
| 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)%.
| Synthetic properties of bright metal-poor variables. I. "Anomalous" Cepheids
We present new grids of evolutionary models for the so-called"Anomalous" Cepheids (ACs), adopting Z = 0.0001 and various assumptionson the progenitor mass and mass-loss efficiency. These computations arecombined with the results of our previous set of pulsation models andused to build synthetic populations of the predicted pulsators as wellas to provide a Mass-Luminosity relation in the absence of mass-loss. Weinvestigate the effect of mass-loss on the predicted boundaries of theinstability strip and we find that the only significant dependenceoccurs in the Period-Magnitude plane, where the synthetic distributionof the pulsators is, on average, brighter by about 0.1 mag than the onein absence of mass-loss. Tight Period-Magnitude relations are derived inthe K band for both fundamental and first overtone pulsators, providinga useful tool for distance evaluations with an intrinsic uncertainty ofabout 0.15 mag, which decreases to ~0.04 mag if the mass term is takeninto account. The constraints provided by the evolutionary models areused to derive evolutionary (i.e., mass-independent)Period-Magnitude-Color relations which provide distance determinationswith a formal uncertainty of the order of ~0.1 mag, once the intrinsiccolors are well known. We also use model computations from theliterature to investigate the effect of metal content both on theinstability strip and on the evolutionary Period-Magnitude-Colorrelations. Finally, we compare our theoretical predictions with observedvariables and we confirm that a secure identification of actual ACsrequires simultaneous information on period, magnitude and color, thatalso provide constraints on the pulsation mode.
| 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.
| The Dwarf Satellites of M31 and the Galaxy
The satellite systems of M31 and the Galaxy are compared. It is notedthat all five of the suspected stripped dwarf spheroidal (dSph) cores ofM31's companions are located within a projected distance of 40 kpc fromthe nucleus of this galaxy, whereas the normal dSph companions to thisobject have distances >40 kpc from the center of M31. All companionswithin 200 kpc25 kpc) satellites.
| 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.
| Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas
We perform high-resolution N-body+SPH (smoothed particle hydrodynamics)simulations of gas-rich dwarf galaxy satellites orbiting within a MilkyWay-sized halo and study for the first time the combined effects oftides and ram pressure. The structure of the galaxy models and theorbital configurations are chosen in accordance with those expected in aLambda cold dark matter (ΛCDM) universe. While tidal stirring ofdisky dwarfs produces objects whose stellar structure and kinematicsresembles that of dwarf spheroidals after a few orbits, ram pressurestripping is needed to entirely remove their gas component.Gravitational tides can aid ram pressure stripping by diminishing theoverall potential of the dwarf, but tides also induce bar formationwhich funnels gas inwards making subsequent stripping more difficult.This inflow is particularly effective when the gas can cool radiatively.Assuming a low density of the hot Galactic corona consistent withobservational constraints, dwarfs with Vpeak < 30 kms-1 can be completely stripped of their gas content on orbitswith pericenters of 50 kpc or less. Instead, dwarfs with more massivedark haloes and Vpeak > 30 km s-1 lose most orall of their gas content only if a heating source keeps the gasextended, partially counteracting the bar-driven inflow. We show thatthe ionizing radiation from the cosmic ultraviolet (UV) background at z> 2 can provide the required heating. In these objects, most of thegas is removed or becomes ionized at the first pericenter passage,explaining the early truncation of the star formation observed in Dracoand Ursa Minor. Galaxies on orbits with larger pericenters and/orfalling into the Milky Way halo at lower redshift can retain significantamounts of the centrally concentrated gas. These dwarfs would continueto form stars over a longer period of time, especially close topericenter passages, as observed in Fornax and other dwarf spheroidalgalaxies (dSphs) of the Local Group. The stripped gas breaks up intoindividual clouds pressure confined by the outer gaseous medium thathave masses, sizes and densities comparable to the HI clouds recentlydiscovered around M31.
| 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.
| The planetary nebula population of the Sagittarius dwarf spheroidal galaxy
The identification of two new planetary nebulae (PNe) in the Sagittariusdwarf spheroidal galaxy (Sgr) is presented. This brings the total numberto four. Both new PNe were previously classified as Galactic objects.The first, StWr 2-21, belongs to the main body of Sgr, from its velocityand location. The second, the halo PN BoBn1, has a location, distanceand velocity in agreement with the leading tidal tail of Sgr. Weestimate that 10 per cent of the Galactic halo consists of Sgr debris.The specific frequency of PNe indicates a total luminosity of Sgr,including its tidal tails, of MV = -14.1. StWr 2-21 shows ahigh abundance of [O/H]= -0.23, which confirms the high-metallicitypopulation in Sgr uncovered by Bonifacio et al. The steepmetallicity-age gradient in Sgr is due to interstellar medium (ISM)removal during the Galactic plane passages, ISM reformation due tostellar mass-loss, and possibly accretion of metal-enriched gas from ourGalaxy. The ISM re-formation rate of Sgr, from stellar mass-loss, is 5× 10-4Msolaryr-1, amounting to~106Msolar per orbital period.Hubble Space Telescope images of three of the PNe reveal well-developedbipolar morphologies, and provide clear detections of the central stars.All three stars with deep spectra show WR lines, suggesting that theprogenitor mass and metallicity determines whether a PN central stardevelops a WR spectrum. One Sgr PN belongs to the class of IR-[WC]stars. Expansion velocities are determined for three nebulae. Comparisonwith hydrodynamical models indicates an initial density profile of ρ~ r-3. This is evidence for increasing mass-loss rates on theasymptotic giant branch. Peak mass-loss rates are indicated of~10-4Msolaryr-1.The IR-[WC] PN, He2-436, provides the sole direct detection of dust in adwarf spheroidal galaxy, to date.
| 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.
| A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33
We present UBVRI photometry obtained from Mosaic images of M31 and M33using the Kitt Peak National Observatory 4 m telescope. We describe ourdata reduction and automated photometry techniques in some detail, as wewill shortly perform a similar analysis of other Local Group galaxies.The present study covered 2.2 deg2 along the major axis ofM31 and 0.8 deg2 on M33, chosen so as to include all of theregions currently active in forming massive stars. We calibrated ourdata using photometry from the Lowell 1.1 m telescope, and this externalmethod resulted in millimagnitude differences in the photometry ofoverlapping fields, providing some assurance that our photometry isreliable. The final catalog contains 371,781 and 146,622 stars in M31and M33, respectively, where every star has a counterpart in (at least)the B, V, and R passbands. Our survey goes deep enough to achieve 1%-2%photometry at 21 mag (corresponding to stars more massive than 20Msolar) and achieves <10% errors at U~B~V~R~I~23 mag.Although our typical seeing was only modest (0.8"-1.4", with median1.0") by some standards, we find excellent correspondence between ourcatalog sources and those we see in our Hubble Space Telescope ACS datafor OB48, a crowded region in M31. We compare our final photometry withthat of others and find good agreement with the CCD catalog of M31 starsby Magnier et al., although our study covers twice the area and goesabout 2 mag deeper. There is also excellent agreement with the CCD``DIRECT'' surveys of M31 and M33. The photographic studies of othersfare less well, particularly at the faint end in V, where accuratebackground subtraction is needed for good photometry. We providecross-references to the stars confirmed as members by spectroscopy andcompare the locations of these to the complete set in color-magnitudediagrams. While follow-up spectroscopy is needed for many projects, wedemonstrate the success of our photometry in being able to distinguishM31/M33 members from foreground Galactic stars. Finally, we present theresults of a single night of spectroscopy on the WIYN 3.5 m telescope,examining the brightest likely members of M31. The spectra identify 34newly confirmed members, including B-A supergiants, the earliest O starknown in M31, and two new luminous blue variable candidates whosespectra are similar to that of P Cygni.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(AURA), Inc., under NASA contract NAS5-26555. These observations areassociated with program GO-9794.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
