תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
CCD Photometry of the Globular Cluster M15: RR Lyrae Fourier Decomposition and Physical Parameters
Results of CCD photometry using V and R filters are reported for 33 RRLyrae stars in M15. The periodicities of some variables have beenrevised and new ephemerides are given. The Blazhko effect, previouslyreported in V12, was not detected. Applying the approach of Fourierdecomposition of the light curves, the physical parameters of the typeRRab and RRc variables were estimated. The cluster is Oosterhoff type IIand the values for the iron content and distance are:[Fe/H]=-1.98+/-0.24 and d=8.67+/-0.41 kpc, respectively. The mean valuesof the physical parameters determined for the RR Lyrae stars place thecluster precisely into the sequences Oosterhoff type --metallicity andmetallicity-- effective temperature, valid for globular clusters.Evidences of evolution from the ZAHB are found for the RRc but not forthe RRab stars.
| DDO 44 and UGC 4998: Distances, Metallicities, and Star Formation Histories
We have obtained deep HST imaging of two candidate dwarf systems in thenearby M81 Group, DDO 44 and UGC 4998. Both are isolated, low surfacebrightness systems, but with likely very different star formationhistories based on their mean colors and color fluctuations. Thecolor-magnitude diagrams of these galaxies have been used to estimatetheir distances using the tip of the red giant branch (RGB), and theirmean metallicities from the colors of the RGB. For DDO 44 we find adistance of D=3.01+/-0.18 Mpc-confirming it to be a member of the M81Group-and a mean metallicity of [Fe/H]=-1.54+/-0.14. We also find fromthe properties and numbers of a population of luminous asymptotic giantbranch (AGB) stars that about 20% of the luminous population of DDO 44consists of ``intermediate-age'' stars (with ages between about 2 and 8Gyr) that give rise to the observed AGB. There is no difference in thespatial distribution of the RGB and AGB stars in this galaxy. For UGC4998 we derive a metallicity of [Fe/H]=-1.58+/-0.21 and a distance ofD=8.24+/-0.43 Mpc. The latter puts UGC 4998 well behind the M81 Group,confirming results from previous measurements. We find this galaxy tocontain a population of young (<50 Myr old) stars, and there isevidence of older RGB stars. The young stars are considerably morecentrally concentrated than the older stars. The current star formationrate in UGC 4998 is ~9×10-4 Msolaryr-1 over the entire galaxy, or ~5×10-4Msolar yr-1 kpc-2. Given its H I mass(1×107 Msolar), this galaxy can continue toform stars at this rate for another ~11 Gyr.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations are associated with proposalGO-8137.
| CCD photometry of the globular cluster M2: RR Lyrae physical parameters and new variables
We report the results of CCD V and R photometry of the RR Lyrae stars inM2. The periodicities of most variables are revised and new ephemeridesare calculated. Light-curve decomposition of the RR Lyrae stars wascarried out and the corresponding mean physical parameters [Fe/H] =-1.47, Teff = 6276K, logL = 1.63Lsolar andMV = 0.71 from nine RRab and [Fe/H] = -1.61, M =0.54Msolar, Teff = 7215K, logL =1.74Lsolar and MV = 0.71 from two RRc stars werecalculated. A comparison of the radii obtained from the above luminosityand temperature with predicted radii from non-linear convective modelsis discussed. The estimated mean distance to the cluster is 10.49 +/-0.15kpc. These results place M2 correctly in the general globularcluster sequences for Oosterhoff type, mass, luminosity and temperature,all as a function of the metallicity. Mean relationships for M,logL/Lsolar, Teff and MV as a functionof [Fe/H] for a family of globular clusters are offered. These trendsare consistent with evolutionary and structural notions on thehorizontal branch. Eight new variables are reported.
| Stellar mass loss and the intracluster medium in Galactic globular clusters: a deep radio survey for HI and OH
We present the results of a survey, the deepest to date, for HI emissionat 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720MHz) in the direction towards the Galactic globular clusters M15, M2,NGC6934, NGC7006 and Pal13. The aim is to measure the amount of hydrogenin the intracluster medium, and to find OH masers in the circumstellarenvelopes of globular cluster red giants. We present a tentativedetection of 0.3Msolar of neutral hydrogen in M15 andpossible detections of neutral hydrogen in M2 and Pal13. We derive upperlimits to the neutral hydrogen content of NGC6934 and 7006. No OHemission is detected. We also present deep HI data of the northern tipof the Magellanic Stream behind Pal13.
| Caroline Herschel's catalogue of nebulae
Not Available
| 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.
| 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.
| 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.
| Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441
We have applied Alard's image-subtraction method (ISIS ver. 2.1) to theobservations of the globular clusters NGC 6388 and NGC 6441 previouslyanalyzed using standard photometric techniques (DAOPHOT and ALLFRAME).In this reanalysis of observations obtained at the Cerro TololoInter-American Observatory, besides recovering the variables previouslydetected on the basis of our ground-based images, we have also been ableto recover most of the RR Lyrae variables previously detected only inthe analysis of Hubble Space Telescope WFPC2 observations of the innerregion of NGC 6441. In addition, we report five possible new variablesnot found in the analysis of the Hubble Space Telescope observations ofNGC 6441. This dramatically illustrates the capabilities ofimage-subtraction techniques applied to ground-based data to recovervariables in extremely crowded fields. We have also detected 12 newvariables and 6 possible variables in NGC 6388 not found in our previousground-based studies. The revised mean period for RRab stars in NGC 6388is 0.676 days, while the mean period of RRab stars in NGC 6441 isunchanged at 0.759 days. These values are among the largest known forany Galactic globular cluster. Additional probable type II Cepheids wereidentified in NGC 6388, confirming its status as a metal-rich globularcluster rich in Cepheids.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
| On the Predicted and Observed Color Boundaries of the RR Lyrae Instability Strip as a Function of Metallicity
The purpose of this paper is to predict the temperature at thefundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL)variables from the pulsation equation that relates temperature toperiod, luminosity, and mass. Modern data for the correlations betweenperiod, luminosity, and metallicity at the FBE for field and cluster RRLstars are used for the temperature calculation. The predictedtemperatures are changed to B-V colors using an adopted colortransformation. The predicted temperatures at the FBE become hotter as[Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicitydecreases to -2.5. The temperature range over this interval ofmetallicity is ΔlogTe=0.04, or 640 K at 6900 K. Thepredicted color variation is at the level of 0.03 mag in B-V over mostof this range. The predictions are compared with the observed RRL colorsat the FBE for both the field and cluster variables, showing generalagreement at the level of 0.02 mag in (B-V)0, which, however,is the uncertainty of the reddening corrections. The focus of theproblem is then reversed by fitting a better envelope to the observedFBE relation between color and metallicity for metallicities smallerthan -1.8, which, when inserted in the pulsation equation, gives anonlinear calibration of the absolute magnitude of the average evolvedlevel of the horizontal branch (HB) ofMV=1.109+0.600([Fe/H])+0.140([Fe/H])2, where thezero point has been set by the observed RR Lyrae stars in the LMC at0=19.064 by Clementini et al. for [Fe/H]=-1.5, andusing an adopted LMC modulus of (m-M)0=18.54 from Tammann etal. that is independent of the LMC Cepheids. This equation givesMV=0.52 at [Fe/H]=-1.5.However, the calibration fails for the extreme second-parameter clustersNGC 6388 and NGC 6441. Proof that the MV absolute magnitudesfor their RR Lyrae variables are ~0.4 mag brighter than the calibrationequation predicts is from the unusually long periods for givenamplitudes at their high metallicities of [Fe/H]~-0.5. Allsecond-parameter clusters are believed to violate the equation, but lessseverely than these two extreme examples. An additional complication inusing RRL stars as distance indicators at the 0.1 mag level is shown bythe difference of ΔlogP=0.029+/-0.007 in the position of theenvelope locus at the shortest periods for the observedperiod-metallicity correlation between the field and cluster variables.The field variables have shorter periods than cluster variables at theenvelope. This requires the cluster RRL stars to be 0.09 mag brighterthan the field variables at the same temperature and mass, or to have atemperature difference of ΔlogTe=0.008 at fixedluminosity and mass. The field and cluster variables also differ in thenear absence of cluster RR Lyrae stars in the -1.7>[Fe/H]>-2.0metallicity interval, whereas the field variables show no such gap. Atest is proposed for different origins for the field and the clustervariables by comparing the morphology of the HBs in the local dwarfspheroidal galaxies with that in the Galactic globular clusters in theinner halo and by relating the differences with the relevantsecond-parameter indicators.
| 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.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| 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.
| Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
| Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.
| The metal abundance distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy*
Low-resolution spectroscopy obtained with FORS2 at the Very LargeTelescope (VLT) has been used for the measurement of individual metalabundances ([Fe/H]) for 110 variable stars, including 107 RR Lyrae starsand one anomalous Cepheid, and to trace the metal distribution of theoldest stellar component in the Sculptor dwarf spheroidal galaxy. The RRLyrae stars are spread over a 15 × 15 arcmin2 areaaround the galaxy centre. Their metallicities have an average value of[Fe/H]=-1.83 +/- 0.03 (rms = 0.26 dex) and cover the metallicity range-2.40 < [Fe/H] < -0.85 (on the scale of Zinn & West), butthere is only one variable that has [Fe/H] > -1.3. The star-to-starscatter is larger than typical errors on individual metallicities(+/-0.15-0.16 dex), indicating a real spread in metal abundances. Theradial velocities measured from the RR Lyrae spectra have a dispersionof 12.9kms-1. This value is consistent with the dispersionderived by Tolstoy et al. for metal-poor red giants associated with theblue horizontal branch stars in Sculptor. Along with the metallicitydistribution these results suggest that most of the RR Lyrae stars inSculptor arise from the same burst of stellar formation that producedthe metal-poor component, originating the galaxy blue horizontal branch.The metal-rich red horizontal branch population found to be centrallyconcentrated only produced a few (if any) of the RR Lyrae stars in oursample. The spectroscopic metallicities were used along with theapparent luminosities to study the luminosity-metallicity relationfollowed by the RR Lyrae stars in Sculptor, for which we derive ashallow slope of 0.09magdex-1. This result can be due to ahigh level of evolution off the zero-age horizontal branch of the RRLyrae stars in this galaxy, again in agreement with their origin fromthe blue horizontal branch population.
| A new population of extended, luminous star clusters in the halo of M31
We present three new clusters discovered in the halo of M31 which,although having globular-like colours and luminosities, have unusuallylarge half-light radii, ~30 pc. They lie at projected galactocentricdistances of ~15 to ~35 kpc. These objects begin to fill the gap inparameter space between globular clusters and dwarf spheroidals, and areunlike any clusters found in the Milky Way, or elsewhere to date.Colour-magnitude diagrams, integrated photometric properties and derivedKing profile fit parameters are given, and we discuss possible originsof these clusters and their relationships to other populations.
| On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
| The Sloan Digital Sky Survey u-band Galaxy Survey: luminosity functions and evolution
We construct and analyse a u-band selected galaxy sample from the SloanDigital Sky Survey (SDSS) Southern Survey, which covers275deg2. The sample includes 43223 galaxies withspectroscopic redshifts in the range 0.005 < z < 0.3 and with 14.5< u < 20.5. The signal-to-noise (S/N) ratio in the u-bandPetrosian aperture is improved by co-adding multiple epochs of imagingdata and by including sky-subtraction corrections. Luminosity functionsfor the near-UV 0.1u band (λ~ 322 +/- 26nm) aredetermined in redshift slices of width 0.02, which show a highlysignificant evolution in M* of -0.8 +/- 0.1 mag between z= 0 and 0.3;with M*-5 logh70=-18.84 +/- 0.05 (AB mag), logφ*=-2.06+/- 0.03 (h370Mpc-3) andlogρL= 19.11 +/- 0.02(h70WHz-1Mpc-3) at z= 0.1. Thefaint-end slope determined for z < 0.06 is given by α=-1.05 +/-0.08. This is in agreement with recent determinations from the GalaxyEvolution Explorer at shorter wavelengths. Comparing our z < 0.3luminosity density measurements with 0.2 < z < 1.2 fromClassifying Objects by Medium Band Observations in 17 Filters(COMBO-17), we find that the 280-nm density evolves asρL~ (1 +z)β with β= 2.1 +/- 0.2; andfind no evidence for any change in slope over this redshift range. Bycomparing with other measurements of cosmic star formation history, weestimate that the effective dust attenuation at 280nm has increased by0.8 +/- 0.3mag between z= 0 and 1.
| Caroline Herschel as observer
Not Available
| Washington Photometry of Red Giants in Five Galactic Globular Clusters
Surface metal abundances and effective temperatures of 165 stars in theGalactic globular clusters NGC 288, NGC 6397, NGC 6656, NGC 7078 and NGC7089 are determined from photoelectric photometry in the Washingtonsystem. Mean cluster metal abundances are determined by the two-colordiagram (TCD) technique described by Geisler et al. (1991). Theresulting metallicities are compared with those derived by the standardgiant branches (SGBs) method developed by Geisler and Sarajedini (1999).Although both methods provide metallicities in reasonably goodagreement, we find that the SGBs technique yields metallicities in abetter agreement with the existing metallicity scales. This is due tothe higher metallicity sensitivity of the SGBs method as well as to itslower sensitivity to reddening and photometric errors.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
| Halos of Spiral Galaxies. III. Metallicity Distributions
We report results of a campaign to image the stellar populations in thehalos of highly inclined spiral galaxies, with the fields roughly 10 kpc(projected) from the nuclei. We use the F814W (I) and F606W (V) filtersin the Wide Field Planetary Camera 2 on board the Hubble SpaceTelescope. We unambiguously resolve the stellar halos 1 to 2 mag fainterthan the tip of the red giant branch. Extended halo populations aredetected in all galaxies. The color-magnitude diagrams appear to becompletely dominated by giant branch stars, with no evidence for thepresence of young stellar populations in any of the fields. Themetallicity distribution function for the galaxy sample is derived frominterpolation within an extensive grid of red giant branch loci. Theseloci are derived from theoretical sequences that are calibrated usingthe Galactic globular clusters and from empirical sequences formetal-rich stellar populations. We find that the metallicitydistribution functions are dominated by metal-rich populations, with atail extending toward the metal-poor end. To first order, the overallshapes of the metallicity distribution functions are similar to what ispredicted by a simple, single-component model of chemical evolution withthe effective yields increasing with galaxy luminosity. However,metallicity distributions significantly narrower than the simple modelare observed for a few of the most luminous galaxies in the sample. Thediscrepancies are similar to those previously observed for NGC 5128, thehalo of M31, and the Galactic bulge. Our observations can be used tohelp distinguish between models for the formation of spiral galaxies. Itappears that more luminous spiral galaxies also have more metal-richstellar halos. The increasingly significant departures from theclosed-box model for the more luminous galaxies indicate that aparameter in addition to a single yield is required to describe chemicalevolution. This parameter, which could be related to gas infall oroutflow either in situ or in progenitor dwarf galaxies that later mergeto form the stellar halo, tends to make the metallicity distributionsnarrower at high metallicity.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.
| Metal Abundances of RR Lyrae Stars in the Metal-rich Globular Cluster NGC 6441
Low-resolution spectra have been used to measure individual metalabundances of RR Lyrae stars in NGC 6441, a Galactic globular clusterknown to have very unusual horizontal-branch morphology and periods ofthe RR Lyrae stars for its high metallicity. We find an average metalabundance of [Fe/H]=-0.69+/-0.06 (rms=0.33 dex) and [Fe/H]=-0.41+/-0.06(rms=0.36 dex) on Zinn & West and Carretta & Gratton metallicityscales, respectively, consistent with the cluster metal abundancederived by Armandroff & Zinn. Most of the metallicities wereextrapolated from calibration relations defined for [Fe/H]<=-1however, they are clearly high and contrast with the rather long periodsof the NGC 6441 variables, thus confirming that the cluster does not fitin the general Oosterhoff classification scheme. The rms scatter of theaverage is larger than observational errors (0.15-0.16 dex), possiblyindicating some spread in metallicity. However, even the metal-poorvariables, if confirmed to be cluster members, are still more metal-richthan those commonly found in the Oosterhoff type II globular clusters.Based on data collected at the Very Large Telescope of the EuropeanSouthern Observatory, Paranal, Chile, program number 71.B-0621.
| A HST Study of the Stellar Populations in the Cometary Dwarf Irregular Galaxy NGC 2366
We present V and I photometry of the resolved stars in the cometarydwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2images obtained with the Hubble Space Telescope. The resultingcolor-magnitude diagram reaches down to I~26.0 mag. It reveals not onlya young population of blue main-sequence stars (age <~30 Myr) butalso an intermediate-age population of blue and red supergiants (20 Myr<~ age <~100 Myr) and older evolved populations of asymptoticgiant branch (AGB) stars (age >~100 Myr) and red giant branch (RGB)stars (age >~1 Gyr). The measured magnitude I=23.65+/-0.10 mag of theRGB tip results in a distance modulus m-M=27.67+/-0.10, whichcorresponds to a distance of 3.42+/-0.15 Mpc, in agreement with previousdistance determinations. The youngest stars are associated with thebright complex of H II regions NGC 2363 (=Mrk 71) in the southwestextremity of the galaxy. As a consequence of the diffusion andrelaxation processes of stellar ensembles, the older the stellarpopulation is, the smoother and more extended is its spatialdistribution. An underlying population of older stars is foundthroughout the body of NGC 2366. The most notable feature of this olderpopulation is the presence of numerous relatively bright AGB stars. Thenumber ratio of AGB to RGB stars and the average absolute brightness ofAGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403,indicating a younger age of the AGB stars in NGC 2366. In addition tothe present burst of age <~100 Myr, there has been strong starformation activity in the past of NGC 2366, from ~100 Myr to <~3 Gyrago.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS5-26555.
| Andromeda IX: Properties of the Faintest M31 Dwarf Satellite Galaxy
We present WIYN observations of the recently discovered And IX dwarfspheroidal (dSph) satellite galaxy of M31. Our data, obtained at anatural seeing of 0.5" and just reaching the horizontal-branch level,confirm that And IX is a dSph galaxy with a distance similar to M31. Asurvey for carbon stars shows no evidence for an intermediate-age (1-10Gyr) stellar population in And IX. From the red giant branch we estimatea metallicity of roughly -2 dex. Combined with the tip of the red giantbranch luminosity, this results in a distance of 735 kpc, placing And IXapproximately 45 kpc from M31. This faint dSph follows the relationsbetween luminosity and metallicity and between luminosity and surfacebrightness defined by other Local Group dSph galaxies. The core andtidal radii are found to be 1.35 arcmin and 5.9 arcmin, respectively. Weconclude that, despite its low luminosity, And IX might be an ordinaryLocal Group dSph and discuss implications for its formation from a oncemore massive but stripped progenitor or from an intrinsically low-massseed.
| Galaxy Evolution Explorer Ultraviolet Photometry of Globular Clusters in M31
We present ultraviolet photometry for globular clusters (GCs) in M31from 15 deg2 of imaging using the Galaxy Evolution Explorer(GALEX). We detect 200 and 94 GCs with certainty in the near-ultraviolet(NUV; 1750-2750 Å) and far-ultraviolet (FUV; 1350-1750 Å)bandpasses, respectively. Our rate of detection is about 50% in the NUVand 23% in the FUV, to an approximate limiting V magnitude of 19. Out ofsix clusters with [Fe/H]>-1 seen in the NUV, none is detected in theFUV bandpass. Furthermore, we find no candidate metal-rich clusters withsignificant FUV flux because of the contribution of bluehorizontal-branch (HB) stars, such as NGC 6388 and NGC 6441, which aremetal-rich Galactic GCs with hot HB stars. We show that our GALEXphotometry follows the general color trends established in previous UVstudies of GCs in M31 and the Galaxy. Comparing our data with GalacticGCs in the UV and with population synthesis models, we suggest that theage ranges of M31 and Galactic halo GCs are similar.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
קטלוגים וכינוים:
|
NEWS@SKY (Science&Space News)
