Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Hubble Space Telescope Photometry for the Halo Stars in the Leo Elliptical NGC 3377
We have used the ACS camera on HST to obtain (V,I) photometry for 57,000red giant stars in the halo of the Leo elliptical NGC 3377, anintermediate-luminosity elliptical. We use this sample of stars toderive the metallicity distribution function (MDF) for its halo fieldstars and comment on its chemical evolution history compared with bothlarger and smaller E galaxies. Our ACS WFC field spans a radial rangeextending from 4 to 18 kpc projected distance from the center of NGC3377 and thus covers a significant portion of this galaxy's halo. Wefind that the MDF is broad, reaching a peak atlog(Z/Zsolar)~=-0.6, but containing virtually no stars moremetal-poor than log(Z/Zsolar)=-1.5. It may, in addition, haverelatively few stars more metal-rich thanlog(Z/Zsolar)~=-0.3, although interpretation of thehigh-metallicity end of the MDF is limited by photometric completenessthat affects the detection of the reddest, most metal-rich stars. NGC3377 appears to have an enrichment history intermediate between those ofnormal dwarf ellipticals and the much larger giants. As yet, we find noclear evidence that the halo of NGC 3377 contains a significantpopulation of ``young'' (<3 Gyr) stars.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 NAS 5-26555. These observations are associated withprogram 9811. Support for this work was provided in part by NASA throughgrant HST-GO-09811.01-A from the Space Telescope Science Institute,under NASA contract NAS 5-26555.
| Kinematic Decoupling of Globular Clusters with the Extended Horizontal Branch
About 25% of the Milky Way globular clusters (GCs) exhibit unusuallyextended color distribution of stars in the core helium-burninghorizontal-branch (HB) phase. This phenomenon is now best understood asdue to the presence of helium-enhanced second-generation subpopulations,which has raised the possibility that these peculiar GCs might have aunique origin. Here we show that these GCs with extended HB are clearlydistinct from other normal GCs in kinematics and mass. The GCs withextended HB are more massive than normal GCs and are dominated by randommotion with no correlation between kinematics and metallicity.Surprisingly, however, when they are excluded, most normal GCs in theinner halo show clear signs of dissipational collapse that apparentlyled to the formation of the disk. Normal GCs in the outer halo sharetheir kinematic properties with the extended HB GCs, which is consistentwith the accretion origin. Our result further suggests heterogeneousorigins of GCs, and we anticipate this to be a starting point for moredetailed investigations of Milky Way formation, including early mergers,collapse, and later accretion.
| Andromeda X, a New Dwarf Spheroidal Satellite of M31: Photometry
We report the discovery of Andromeda X, a new dwarf spheroidal satelliteof M31, based on stellar photometry from the Sloan Digital Sky Survey.Using follow-up imaging data we have estimated its distance and otherphysical properties. We find that Andromeda X has a dereddened centralsurface brightness of μV,0~26.7 mag arcsec-2and a total apparent magnitude of Vtot~16.1, which at thederived distance modulus, (m-M)0~24.12-24.34, yields anabsolute magnitude of MV~-8.1+/-0.5 these values are quitecomparable to those of Andromeda IX, a previously discoveredlow-luminosity M31 satellite. The discoveries of Andromeda X and ofnumerous other extremely faint satellites around M31 and the Milky Wayin the past few years suggest that such objects may be plentiful in theLocal Group.
| ACS Photometry of Newly Discovered Globular Clusters in the Outer Halo of M31
We report the first results from deep ACS imaging of 10 classicalglobular clusters in the far outer regions (15kpc<~Rp<~100 kpc) of M31. Eight of the clusters,including two of the most remote M31 globular clusters presently known,are described for the first time. Our F606W, F814W color-magnitudediagrams extend ~3 mag below the horizontal branch and clearlydemonstrate that the majority of these objects are old (>~10 Gyr),metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional fourhave -1.9 <~ [Fe/H] <~ -1.5. The remaining object is moremetal-rich, with [Fe/H] ~ -0.70. Several clusters exhibit thesecond-parameter effect. Using aperture photometry, we estimateintegrated luminosities and structural parameters for all clusters.Many, including all four clusters with projected radii greater than 45kpc, are compact and very luminous, with -8.9 <~ MV <~-8.3. These four outermost clusters are thus quite unlike their MilkyWay counterparts, which are typically diffuse, subluminous (-6.0 <~MV <~ -4.7), and more metal-rich (-1.8 <~ [Fe/H] <~-1.3).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 program10394.
| The Discovery of Cepheids and a Distance to NGC 5128
We discuss a new distance to NGC 5128 (Centaurus A) based on Cepheidvariables observed with the Hubble Space Telescope. Twelve F555W (V) andsix F814W (I) epochs of cosmic-ray-split WFPC2 observations wereobtained. A total of 56 bona fide Cepheids were discovered, with periodsranging from 5 to ~50 days; five of these are likely Population IICepheids of the W Virginis class, associated with the bulge or halo ofNGC 5128. Based on the period and V- and I-band luminosities of asubsample of 42 classical (Population I) Cepheids, and adopting an LMCdistance modulus and extinction of 18.50+/-0.10 mag and E(B-V)=0.10 mag,respectively, the true reddening-corrected distance modulus to NGC 5128is μ0=27.67+/-0.12 (random)+/-0.16 (systematic) mag,corresponding to a distance of 3.42+/-0.18 (random)+/-0.25 (systematic)Mpc. The random uncertainty in the distance is dominated by the error onthe assumed value for the ratio of total to selective absorption,RV, in NGC 5128, and by the possible metallicity dependenceof the Cepheid period-luminosity relation at V and I. This representsthe first determination of a Cepheid distance to an early-type galaxy.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.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| The MODEST questions: Challenges and future directions in stellar cluster research
We present a review of some of the current major challenges in stellarcluster research, including young clusters, globular clusters, andgalactic nuclei. Topics considered include: primordial mass segregationand runaway mergers, expulsion of gas from clusters, the production ofstellar exotica seen in some clusters (e.g., blue stragglers and extremehorizontal-branch stars), binary populations within clusters, theblack-hole population within stellar clusters, the final parsec problem,stellar dynamics around a massive black hole, and stellar collisions.The Modest Questions posed here are the outcome of discussions whichtook place at the Modest-6A workshop held in Lund, Sweden, in December,2005. Modest-6A was organised as part of the activities of the ModestCollaboration (see www.manybody.org for further details).
| Empirical color transformations between SDSS photometry and other photometric systems
Aims.We present empirical color transformations between the SloanDigital Sky Survey (SDSS) ugriz photometry and the Johnson-Cousins UBVRIsystem and Becker's RGU system, respectively. Owing to the magnitude ofdata that is becoming available in the SDSS photometric system it isparticularly important to be able to convert between this new system andtraditional photometric systems. Unlike earlier publishedtransformations we based our calculations on stars actually measured bythe SDSS with the SDSS 2.5-m telescope. The photometric database of theSDSS provides in a sense a single-epoch set of "tertiary standards"covering more than one quarter of the sky. Our transformations shouldfacilitate their use to easily and reliably derive the correspondingapproximate Johnson-Cousins or RGU magnitudes. Methods: .The SDSSsurvey covers a number of areas that were previously established asstandard fields in the Johnson-Cousins system, in particular, fieldsestablished by Landolt and by Stetson. We used these overlapping fieldsto create well-photometered star samples on which our calculatedtransformations are based. For the RGU photometry we used fieldsobserved in the framework of the new Basel high-latitude field starsurvey. Results: .We calculated empirical color transformationsbetween SDSS photometry and Johnson-Cousins UBVRI and Becker's RGUsystem. For all transformations we found linear relations to besufficient. Furthermore we showed that the transformations between theJohnson-Cousins and the SDSS system have a slight dependence onmetallicity.
| Multicolour CCD photometry of the variable stars in globular cluster M3
We present time-series data on the variable stars of the galacticglobular cluster Messier 3 (M3). We give BVIC light curvesfor 226 RR Lyrae, 2 SX Phe and 1 W Vir type variables, along withestimated fundamental photometric parameters such as intensity andmagnitude-averaged brightness and pulsation periods. In some cases, theperiods we have found significantly differ from the previously publishedones. This is the first published light curve and period determinationfor variable V266. The I-band light curve has not been observedpreviously for numerous (76) variables. Three new RR Lyrae variableshave been discovered. Groups of RR Lyrae variables that belong todifferent evolutionary stages and have been separated previously on thebasis of V data were found here for all colours and colour indices bycluster analysis. The I-band period-luminosity relation is alsodiscussed. From the 66 modulated (Blazhko type) RR Lyrae stars weinvestigated, six are newly identified and two of them are firstovertone pulsators. In the case of 13 RR Lyrae, the period of Blazhkocycle has been estimated for the first time. V252 is identified as a newRRd variable. Amplitude ratios of RRd stars have been investigated tosearch possible mode content changes. In contrast to previouspublications no changes have been found. Problems with the sampling ofthe time-series of typical cluster variability surveys are demonstrated.
| Deep HST Photometry of NGC 6388: Age and Horizontal-Branch Luminosity
Using the Hubble Space Telescope, we have obtained the firstcolor-magnitude diagram (CMD) to reach the main-sequence turnoff of theGalactic globular cluster NGC 6388. From a comparison between thecluster CMD and that of 47 Tucanae, we find that the bulk of the starsin these two clusters have nearly the same age and chemical composition.On the other hand, our results indicate that the blue horizontal branchand RR Lyrae components in NGC 6388 are intrinsically overluminous,which must be due to one or more, still undetermined, noncanonicalsecond parameter(s) affecting a relatively minor fraction of the starsin NGC 6388.Based on observations 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 (AURA), Inc.,under NASA contract NAS 5-26555, and on observations retrieved from theESO ST-ECF Archive.
| VI Photometry of Globular Clusters NGC 6293 and NGC 6541: The Formation of the Metal-poor Inner Halo Globular Clusters
We present VI photometry of the metal-poor inner halo globular clustersNGC 6293 and NGC 6541 using the WFPC2 on board the Hubble SpaceTelescope (HST). Our color-magnitude diagrams of the clusters showwell-defined blue horizontal-branch populations, consistent with theirlow metallicities and old ages. NGC 6293 appears to have blue stragglerstars in the cluster's central region. We discuss the interstellarreddening and the distance modulus of NGC 6293 and NGC 6541 and obtainE(B-V)=0.40 and (m-M)0=14.61 for NGC 6293 and E(B-V)=0.14 and(m-M)0=14.19 for NGC 6541. Our results confirm that NGC 6293and NGC 6541 are clearly located in the Galaxy's central regions(RGC<=3 kpc). We also discuss the differential reddeningacross NGC 6293. The interstellar reddening value of NGC 6293 appears tovary by ΔE(B-V)~0.02-0.04 mag within our small field of view. Themost notable result of our study is that the inner halo clusters NGC6293 and NGC 6541 essentially have the same ages as M92, confirming theprevious result from the HST NIC3 observations of NGC 6287.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.
| Hubble Space Telescope ACS Wide-Field Photometry of the Sombrero Galaxy Globular Cluster System
A detailed imaging analysis of the globular cluster (GC) system of theSombrero galaxy (NGC 4594) has been accomplished using a six-imagemosaic from the Hubble Space Telescope Advanced Camera for Surveys. Thequality of the data is such that contamination by foreground stars andbackground galaxies is negligible for all but the faintest 5% of the GCluminosity function. This enables the study of an effectively puresample of 659 GCs until ~2 mag fainter than the turnover magnitude,which occurs at MTOMV=-7.60+/-0.06 for an assumedm-M=29.77. Two GC metallicity subpopulations are easily distinguishable,with the metal-poor subpopulation exhibiting a smaller intrinsicdispersion in color compared to the metal-rich subpopulation. There arethree new discoveries. (1) A metal-poor GC color-magnitude trend hasbeen observed. (2) The fact that the metal-rich GCs are ~17% smallerthan the metal-poor ones for small projected galactocentric radii (lessthan ~2') has been confirmed. However, the median half-light radii ofthe two subpopulations become identical at ~3' from the center. This ismost easily explained if the size difference is the result of projectioneffects. (3) The brightest (MV<-9.0) members of the GCsystem show a size-magnitude upturn, where the average GC size increaseswith increasing luminosity. Evidence is presented that supports anintrinsic origin for this feature rather than being a result of accreteddwarf elliptical nuclei. In addition, the metal-rich GCs show ashallower positive size-magnitude trend, similar to what is found inprevious studies of young star clusters.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained (from the data archive) at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555. Theseobservations are associated with program 9714.
| 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.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| Detection of a 60°-long Dwarf Galaxy Debris Stream
We report on a 60°-long stream of stars, extending from Ursa Majorto Sextans, in the Sloan Digital Sky Survey. The stream is approximately2° wide and is clearly distinct from the northern tidal arm of theSagittarius dwarf galaxy. The apparent width of the stream indicates aprogenitor with a size and mass similar to that of a dwarf galaxy. Thestream is about 21 kpc distant and appears to be oriented almostperpendicular to our line of sight. The visible portion of the streamdoes not pass near any known dwarf galaxies, although we cannot rule outthat the stream may form the inner part of a known dwarf galaxy's orbit.The most likely explanation is that the stream constitutes the remainsof a dwarf galaxy that has been completely disrupted at some point inthe past. We also briefly report on the discovery of a diminutiveGalactic satellite that lies near the projected path of the new streambut is unlikely to be related to it.
| Atmospheres, Chemical Compositions, and Evolutionary Histories of Very Metal-Poor Red Horizontal-Branch Stars in the Galactic Field and in NGC 7078 (M15)
We have conducted spectrum analyses of 24 field metal-poor([Fe/H]<-2) red horizontal-branch (RHB) stars identified in the HKobjective-prism survey and 6 such stars in the globular cluster M15,based on high-quality spectra (R~40,000, S/N~100) obtained with theMagellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 mtelescope at Las Campanas Observatory. The atmospheric parameters of theRHB stars provide interesting bridges between turnoff stars of similartemperature and red giant branch (RGB) stars of similar gravity, andthey permit investigations of abundance trends [X/Fe] versus [Fe/H] in arelatively unexplored region of the temperature-gravity plane. We findthat the Teff, logg, vt, and [Fe/H] valuesdetermined from our spectra are consistent with expectations fromliterature spectroscopic studies of other evolved metal-poor stellarclasses. We show that the RHB stars have abundance distributions thatare consistent with typical halo stars of similar metallicities. Thephotometric and spectroscopic gravities of the M15 stars differ byamounts that grow with declining temperature. We use a regressionderived from these differences to calculate photometric gravities forthe field RHB stars. Then we use the locations of the field RHB starsamong the evolutionary tracks of Cassisi et al. in the logg versuslogTeff plane to estimate their masses and lifetimes as RHBstars. We use these lifetimes to estimate the size of the metal-poor HBpopulation from which they arise. Then, using counts of HB and RGB starsin metal-poor globular clusters, we conclude that the number ofmetal-poor RGB stars at high latitudes (|b|>30deg)brighter than V=15 exceeds those identified in extant objective-prismsurveys by more than an order of magnitude. Finally, we deduce theeffective temperature of the fundamental red edge of the metal-poor RRLyrae instability strip, logTeff(FRE)=3.80+/-0.01, from theinterface between the temperature distributions of metal-poor field RHBstars and the RR Lyrae stars of similar [Fe/H] in five metal-poorglobular clusters.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center, California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation. This paper includes data gatheredwith the 6.5 m Magellan telescopes located at Las Campanas Observatory,Chile.
| The new sample of giant radio sources. II. Update of optical counterparts, further spectroscopy of identified faint host galaxies, high-frequency radio maps, and polarisation properties of the sources
Our sample of giant radio-source candidates, published in Paper I, isupdated and supplemented with further radio and optical data. In thispaper we present: (i) newly detected host galaxies, their photometricmagnitude, and redshift estimate for the sample sources not identifiedyet, (ii) optical spectra and spectroscopic redshift for the hostgalaxies fainter than about R≈ 18.5 mag taken with the Apache PointObservatory 3.5 m telescope, and (iii) the VLA 4.9 GHz total-intensityand polarised-intensity radio maps of the sample members. In a few casesthey reveal extremely faint radio cores undetected before, which confirmthe previously uncertain optical identifications. The radio maps areanalysed and the polarisation properties of the sample sourcessummarised. A comparison of our updated sample with three samplespublished by other authors implies that all these four samples probe thesame part of the population of extragalactic radio sources. There is nosignificant difference between the distributions of intrinsic size andradio power among these samples. The median redshift of 0.38±0.07in our sample is the highest among the corresponding values in the foursamples, indicating that the angular size and flux-density limits in oursample, lower than those for the other three samples, result ineffective detections of more distant, giant-size galaxies compared tothose detected in the other samples. This sample and a comparison sampleof "normal"-size radio galaxies will be used in Paper III (this volume)to investigate of a number of trends and correlations in the entiredata.
| Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - II. The unique LMC cluster ESO 121-SC03
We present the results of photometric measurements from images of theLarge Magellanic Cloud (LMC) cluster ESO 121-SC03 taken with theAdvanced Camera for Surveys (ACS) on the Hubble Space Telescope. Ourresulting colour-magnitude diagram (CMD) reaches 3 mag below themain-sequence turn-off, and represents by far the deepest observation ofthis cluster to date. We also present similar photometry from ACSimaging of the accreted Sagittarius dSph cluster Palomar 12, used inthis work as a comparison cluster. From analysis of its CMD, we obtainestimates for the metallicity and reddening of ESO 121-SC03: [Fe/H] = -0.97 +/- 0.10 and E(V - I) = 0.04 +/- 0.02, in excellent agreement withprevious studies. The observed horizontal branch (HB) level in ESO121-SC03 suggests this cluster may lie 20 per cent closer to us thandoes the centre of the LMC. ESO 121-SC03 also possesses a significantpopulation of blue stragglers, which we briefly discuss. Our newphotometry allows us to undertake a detailed study of the age of ESO121-SC03 relative to Palomar 12 and the Galactic globular cluster 47Tuc. We employ both vertical and horizontal differential indicators onthe CMD, calibrated against isochrones from the Victoria-Regina stellarmodels. These models allow us to account for the differentα-element abundances in Palomar 12 and 47 Tuc, as well as theunknown run of α-elements in ESO 121-SC03. Taking a straighterror-weighted mean of our set of age measurements yields ESO 121-SC03to be 73 +/- 4 per cent the age of 47 Tuc, and 91 +/- 5 per cent the ageof Palomar 12. Palomar 12 is 79 +/- 6 per cent as old as 47 Tuc,consistent with previous work. Our result corresponds to an absolute agefor ESO 121-SC03 in the range 8.3-9.8 Gyr, depending on the age assumedfor 47 Tuc, therefore confirming ESO 121-SC03 as the only known clusterto lie squarely within the LMC age gap. We briefly discuss a suggestionfrom earlier work that ESO 121-SC03 may have been accreted into the LMCsystem.
| 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.
| A New Milky Way Dwarf Satellite in Canes Venatici
In this Letter, we announce the discovery of a new dwarf satellite ofthe Milky Way, located in the constellation Canes Venatici. It was foundas a stellar overdensity in the north Galactic cap using Sloan DigitalSky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitudediagram shows a well-defined red giant branch as well as a horizontalbranch. As judged from the tip of the red giant branch, it lies at adistance of ~220 kpc. Based on the SDSS data, we estimate an absolutemagnitude of MV~-7.9, a central surface brightness ofμ0,V~28 mag arcsec-2, and a half-light radiusof ~8.5 arcmin (~550 pc at the measured distance). The outer regions ofCanes Venatici appear extended and distorted. The discovery of such afaint galaxy in proximity to the Milky Way strongly suggests that moresuch objects remain to be found.
| A `super' star cluster grown old: the most massive star cluster in the Local Group
We independently redetermine the reddening and age of the globularcluster (GC) 037-B327 in M31 by comparing independently obtainedmulticolour photometry with theoretical stellar population synthesismodels. 037-B327 has long been known to have a very large reddeningvalue, which we confirm to be E(B - V) = 1.360 +/- 0.013, in goodagreement with the previous results. We redetermine its most likely ageat 12.4 +/- 3.2 Gyr.037-B327 is a prime example of an unusually bright early counterpart tothe ubiquitous `super' star clusters presently observed in mosthigh-intensity star-forming regions in the local Universe. In order tohave survived for a Hubble time, we conclude that its stellar initialmass function (IMF) cannot have been top-heavy. Using this constraint,and a variety of simple stellar population (SSP) models, we determine aphotometric mass of , somewhat depending on the SSP models used, themetallicity and age adopted and the IMF representation. This mass, andits relatively small uncertainties, makes this object the most massivestar cluster of any age in the Local Group. Assuming that thephotometric mass estimate thus derived is fairly close to its dynamicalmass, we predict that this GC has a (one-dimensional) velocitydispersion of the order of (72 +/- 13) km s-1. As a surviving`super' star cluster, this object is of prime importance for theoriesaimed at describing massive star cluster evolution.
| Detection of a 63° Cold Stellar Stream in the Sloan Digital Sky Survey
We report on the detection in Sloan Digital Sky Survey data of a63°-long tidal stream of stars, extending from Ursa Major to Cancer.The stream has no obvious association with the orbit of any knowncluster or galaxy. The contrast of the detected stream is greatest whenusing a star count filter that is matched to the color-magnitudedistribution of stars in M13, which suggests that the stars making upthe stream are old and metal-poor. The visible portion of the stream isvery narrow and about 8.5 kpc above the Galactic disk, suggesting thatthe progenitor is or was a globular cluster. While the surface densityof the stream varies considerably along its length, its path on the skyis very smooth and uniform, showing no evidence of perturbations bylarge mass concentrations in the nearby halo. While definitiveconstraints cannot be established without radial velocity information,the stream's projected path and estimates of its distance suggest thatwe are observing the stream near the perigalacticon of its orbit.
| Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.
| The ultra-cool white dwarf companion of PSR J0751+1807
We present optical and near-infrared observations with Keck of thebinary millisecond pulsar PSR J0751+1807. We detect a faint, red object- with R=25.08 ± 0.07, B-R=2.5 ± 0.3, and R-I=0.90± 0.10 - at the celestial position of the pulsar and argue thatit is the white dwarf companion of the pulsar. The colours are thereddest among all known white dwarfs, and indicate a very lowtemperature, T_eff≈4000 K. This implies that the white dwarf cannothave the relatively thick hydrogen envelope that is expected onevolutionary grounds. Our observations pose two puzzles. First, whilethe atmosphere was expected to be pure hydrogen, the colours areinconsistent with this composition. Second, given the low temperature,irradiation by the pulsar should be important, but we see no evidencefor it. We discuss possible solutions to these puzzles.
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
| Structural parameters from ground-based observations of newly discovered globular clusters in NGC 5128
We have investigated a number of globular cluster candidates from arecent wide-field study by Harris et al. (2004a, AJ, 128, 712) of thegiant elliptical galaxy NGC 5128. We used theMagellan I telescope + MagIC camera under excellent seeing conditions(0.3 arcsec-0.6 arcsec) and obtained very high resolution images for asample of 44 candidates. Of these, 15 appear to be bonafide globularclusters in NGC 5128 while the rest are eitherforeground stars or background galaxies. We also serendipitouslydiscovered 18 new cluster candidates in the same fields. Our imagesallow us to study the light profiles of the likely clusters, all ofwhich are well resolved. This is the first ground-based study ofstructural parameters for globular clusters outside the Local Group. Wecompare the psf-deconvolved profiles with King models and derivestructural parameters, ellipticities and surface brightnesses. Wecompare the derived structural properties with those of otherwell-studied globular cluster systems. In general, our clusters aresimilar in size, ellipticity, core radius and central surface brightnessto their counterparts in other galaxies, in particular those inNGC 5128 observed with HST by Harris et al. (2002,AJ, 124, 1435). However, our clusters extend to higher ellipticities andlarger half-light radii than their Galactic counterparts, as do theHarris et al. sample. Combining our results with those of Harris et al.fills in the gaps previously existing in rh - MVparameter space and indicates that any substantial difference betweenpresumed distinct cluster types in this diagram, including for examplethe Faint Fuzzies of Larsen & Brodie (2000, AJ, 120, 2938) and the"extended, luminous" M 31 clusters of Huxor et al. (2005, MNRAS, 360,1007) is now removed and that clusters form a continuum in this diagram.Indeed, this continuum now extends to the realm of the Ultra CompactDwarfs. The metal-rich clusters in our sample have half-light radii thatare almost twice as large in the mean as their metal-poor counterparts,at odds with the generally accepted trend. The possibility exists thatthis result could be due in part to contamination by backgroundgalaxies. We have carried out additional analysis to quantify thiscontamination. This shows that, although galaxies cannot be easily toldapart from clusters in some of the structural diagrams, the combinationof excellent image quality and Washington photometry should limit thecontamination to roughly 10% of the population of cluster candidates.Finally, our discovery of a substantial number of new cluster candidatesin the relatively distant regions of the NGC 5128halo suggests that current values of the total number of globularclusters may be underestimates.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| The Discovery of Tidal Tails around the Globular Cluster NGC 5466
We report the discovery of tidal tails around the high-latitude Galacticglobular cluster NGC 5466 in Sloan Digital Sky Survey data. Neuralnetworks are used to reconstruct the probability distribution of clusterstars in ugriz space. The tails are clearly visible once extragalacticcontaminants and field stars have been eliminated. They extend ~4°on the sky, corresponding to ~1 kpc in projected length. The orientationof the tails is in good agreement with the cluster's Galactic orbit, asjudged from the proper-motion data.
| 037-B327 in M31: Luminous Globular Cluster or Core of a Former Dwarf Spheroidal Companion to M31?
The cluster 037-B327 is of interest because it is both the most luminousand the most highly reddened cluster known in M31. Deep observationswith the Advanced Camera for Surveys on the Hubble Space Telescopeprovide photometric data in the F606W band and also show that thiscluster is crossed by a dust lane. We have determined the structuralparameters of 037-B327 by fitting the observed surface brightnessdistribution to a King model with rc=0.72" (=2.69 pc),rt=5.87" (=21.93 pc), and a concentration indexc=log(rt/rc)=0.91. The surface brightness profileappears to be essentially flat within 0.25" of the center and shows nosigns of core collapse. Although the dust lane affects the photometry,the King model fits the surface brightness profile well except for theregions badly affected by the dust lane. We also calculate thehalf-light radius, rh=1.11" (=4.15 pc). Combined withprevious photometry, we find that this object falls in the same regionof the MV versus logRh diagram as do ωCentauri, M54, and NGC 2419 in the Milky Way and the massive cluster G1in M31. All four of these objects have been claimed to be the strippedcores of former dwarf galaxies. This suggests that 037-B327 may also bethe stripped core of a former dwarf companion to M31.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 NAS 5-26555. These observations are associated withproposal 9453.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
Katalógusok és elnevezések:
|
