目录
图像
上传图像
DSS Images Other Images
相关文章
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.
| The satellite distribution of M31
The spatial distribution of the Galactic satellite system plays animportant role in Galactic dynamics and cosmology, where its successfulreproduction is a key test of simulations of galaxy halo formation.Here, we examine its representative nature by conducting an analysis ofthe three-dimensional spatial distribution of the M31 subgroup ofgalaxies, the next closest system to our own. We begin by a discussionof distance estimates and incompleteness concerns, before revisiting thequestion of membership of the M31 subgroup. We constrain this byconsideration of the spatial and kinematic properties of the putativesatellites. Comparison of the distribution of M31 and Galacticsatellites relative to the galactic discs suggests that the Galacticsystem is probably modestly incomplete at low latitudes by ~=20 percent. We find that the radial distribution of satellites around M31 ismore extended than the Galactic subgroup; 50 per cent of the Galacticsatellites are found within ~100 kpc of the Galaxy, compared to ~200 kpcfor M31. We search for `ghostly streams' of satellites around M31, inthe same way others have done for the Galaxy, and find several,including some that contain many of the dwarf spheroidal satellites. Thelack of M31-centric kinematic data, however, means that we are unable toprobe whether these streams represent real physical associations.Finally, we find that the M31 satellites are asymmetrically distributedwith respect to our line of sight to this object, so that the majorityof its satellites are on its near side with respect to our line ofsight. We quantify this result in terms of the offset between M31 andthe centre of its satellite distribution, and find it to be significantat the ~ 3σ level. We discuss possible explanations for thisfinding, and suggest that many of the M31 satellites may have beenaccreted only relatively recently. Alternatively, this anisotropy may berelated to a similar result recently reported for the 2dFGRS, whichwould imply that the halo of M31 is not yet virialized. Until such timeas a satisfactory explanation for this finding is presented, however,our results warn against treating the M31 subgroup as complete, unbiasedand relaxed.
| The Araucaria Project .
Results from a long-term observational project called the AraucariaProject are presented. Based on Wide Field optical monitoring of 8nearby galaxies, covering a large range of metallicities, more than 500Cepheids and a few hundred Blue Supergiant candidates were identified.From the analysis of Cepheid P-L relations of outstanding qualityderived from our data we conclude that the slope of these relations inthe I band and Wesenheit index are not dependent on metallicity.Comparing the I-band magnitudes of Cepheids of a period of ten days, ascomputed from our P-L relations, to the I-band magnitudes of the tip ofthe RGB, which is widely believed to be independent of populationeffects, we cannot see any obvious dependence of the zero point of theI-band P-L relation on metallicity. A preliminary analysis of IRfollow-up observations of sub-samples of the identified Cepheids invarious galaxies of the project show that the distances obtained fromthese data are systematically shorter by about of 0.1 mag than thosederived from the optical photometry. It is likely that this effect canbe attributed to the internal reddening in the program galaxies. Theselected Blue Supergiant candidates were observed spectroscopically with8m-class telescopes to determine their element abundances, and theirluminosities from the Flux-weighted Gravity-Luminosity Relationship.Results on this aspect of the Araucaria Project are presented in thereview of Kudritzki presented during this conference.
| On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
| 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.
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| A Local Group Polar Ring Galaxy: NGC 6822
Star counts obtained from a 2° × 2° area centered on NGC6822 have revealed an optical image of this galaxy composed of twocomponents: in addition to the well-known H I disk with its youngstellar component, there is a spheroidal stellar structure as extensiveas its H I disk, but with its major axis at roughly right angles to it,that we traced to at least 36'. Radial velocities of over 100intermediate-age carbon stars found within this structure displaykinematics contrasting strongly with those of the H I disk. These Cstars belong to the spheroid. Although devoid of gas, the spheroidrotation is consistent with the I-band Tully-Fisher relation. Theorientation of the rotation axis that minimizes the stellar velocitydispersion coincides with the minor axis of the stellar populationellipsoid, lying very nearly in the plane of the H I disk. We concludethat the H I disk is a polar ring and that the spheroidal component isan erstwhile disk, a fossil remainder of a past close encounter episode.Based on observations obtained with MegaPrime/MegaCam, a joint projectof CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT),which is operated by the National Research Council (NRC) of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique (CNRS) of France, and the University of Hawaii.Based on observations acquired at the du Pont Telescope, from theObservatories of the Carnegie Institution of Washington.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| Weak redshift discretisation in the Local Group of galaxies?
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
| 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.
| WLM-1: An old, Nonrotating, Gravitationally Unperturbed, Highly Elliptical Extragalactic Globular Cluster
Globular clusters have long been known for presenting (at times)significant deviations from spherical symmetry. While rotation has beenthe main proposed explanation, other complicating factors such as theirconstant interaction with the strong gravitational potential of theirhost galaxy have made it difficult for a consensus to be reached. Toaddress this question we have obtained high-resolution spectra of WLM-1,the lone, old globular cluster associated with the isolated, low-massdwarf irregular galaxy WLM. Using archival HST WFPC2 data, we measurethe radial ellipticity profile of WLM-1, finding it to be highlyelliptical, with a mean value of 0.17 in the region 0.5"-5", which iscomparable to what is found in our Galaxy for the most ellipticalglobular clusters. There is no evidence of isophote twisting, except forthe innermost regions of the cluster (r<0.5"). To investigate whetherthe observed flattening can be ascribed to rotation, we have obtainedlong-slit high-resolution Very Large Telescope UVES spectra of thiscluster along and perpendicular to the axis of flattening. Usingcross-correlation we find that the velocity profile of the cluster isconsistent with zero rotation along either axis. Thus, neither clusterrotation nor galactic tides can be responsible for the flattenedmorphology of WLM-1. We argue that the required velocity dispersionanisotropy between the semimajor and semiminor axes that would berequired to account for the observed flattening is relatively small, oforder 1 km s-1. Even though our errors preclude us fromconclusively establishing that such a difference indeed exists, velocityanisotropy remains at present the most plausible explanation for theshape of this cluster.
| The mid- and far-infrared colours of AGB and post-AGB stars
With the advent of space missions, like SPITZER and ASTRO-F, withsensitive detectors in the near- and mid-infra red covering a reasonablefield-of-view and having a good spatial resolution, it will be possibleto detect individual AGB stars in Local Group galaxies. The filters usedby these missions are non-standard and different from mission tomission. In this paper, the colours of mass-losing AGB and post-AGBstars are calculated in the broad-band filters of the SPITZER andASTRO-F missions, as well as Bessell V,I and 2MASS J,H,K to connectthese results to existing ground-based data. The models are calculatedfor carbon- and oxygen-rich chemistry and cover different effectivetemperatures and dust compositions.
| Imaging resources for the GTC: the Local Group Census
The Local Group Census is a narrowband imaging survey aimed atcataloguing the emission-line populations in the galaxies of the LocalGroup. Data, which were obtained using the Wide Field Camera of the 2.5mIsaac Newton Telescope, are available to the whole astronomicalcommunity, resulting in a valuable imaging resource for follow-upspectroscopy with the GTC.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| A Dynamical Model for the Orbit of the Andromeda Galaxy M31 and the Origin of the Local Group of Galaxies
We propose a new model for the origin and evolution of the Local Groupof Galaxies (LGG) that naturally explains the formation of theMagellanic Clouds and their large orbital angular momenta around theGalaxy. The basic idea is that an off-center hydrodynamical collisionoccurred some 10Gyr ago between the primordial Andromeda galaxy (M31)and a similar Galaxy, and compressed the halo gas to form the LGG dwarfgalaxies, including the Magellanic Clouds. New-born dwarf galaxies canbe expected to locate on the orbital plane of these two massivegalaxies. We reexamined the two-dimensional sky distribution of the LGGmembers, and confirmed an early idea that they align along two similargreat circles. The planes of these circles are approximately normal tothe line joining the present position of the Sun and the galacticcenter. We made a distribution map of these objects, and found awell-defined plane of finite thickness. Thus we could determine theorbital elements of M31 relative to the Galaxy by reproducing thewell-studied dynamics of the LMC and the SMC around the Galaxy. Theexpected proper motion of M31 is (μl, μb) =(38 ± 16 μas yr-1, -49 ± 5 μasyr-1).
| Measuring improved distances to nearby galaxies: Thae Araucaria project.
Not Available
| The Local Group Census: searching for planetary nebulae in IC 1613, WLM and GR8
In the framework of the Local Group Census (LGC), a survey of the LocalGroup (LG) galaxies above Dec =-30° aimed at surveying thepopulations that have strong emission lines, we have searched forplanetary nebulae (PNe) in the low-metallicity dwarf irregular galaxiesIC 1613, WLM, GR 8. Two new candidate PNe have been found in IC 1613,one in WLM and none in GR 8. The observations presented in this paper,together with the previous results from the LGC, represent the firststep in the study of the PN population in low-metallicity, dwarfirregular galaxies of the Local Group. These observations will befollowed by deep spectroscopy to confirm the nature of these objects andto study their physical-chemical properties. We use the observed numberof PNe in each LG galaxy to estimate a lower limit to the mass of theintermediate-age population, which is compared with the star formationrate (SFR) of LG dwarf galaxies. These results are in agreement withthose from accurate star formation history (SFH) analyses for thesesmall galaxy systems.
| The galaxy luminosity function from MR=-25 to MR=-9
Redshift surveys such as the Sloan Digital Sky Survey (SDSS) have givena very precise measurement of the galaxy luminosity function down toabout MR=-17 (~MB=-16). Fainter absolutemagnitudes cannot be probed because of the flux limit required forspectroscopy. Wide-field surveys of nearby groups using mosaic CCDs onlarge telescopes are able to reach much fainter absolute magnitudes,about MR=-10. These diffuse, spiral-rich groups are thoughtto be typical environments for galaxies, so their luminosity functionsshould be the same as the field luminosity function. The luminosityfunction of the groups at the bright end (MR < -17) islimited by Poisson statistics and is far less precise than that derivedfrom redshift surveys. Here we combine the results of the SDSS and thesurveys of nearby groups, and we supplement the results with studies ofLocal Group galaxies in order to determine the galaxy luminosityfunction over the entire range -25 < MR < -9. Theaverage logarithmic slope of the field luminosity function betweenMR=-19 and MR=-9 is α=-1.26, although asingle power law is a poor fit to the data over the entire magnituderange. We also determine the luminosity function of galaxy clusters anddemonstrate that it is different from the field luminosity function at ahigh level of significance; there are many more dwarf galaxies inclusters than in the field, due to a rise in the cluster luminosityfunction of α~-1.6 between MR=-17 andMR=-14.
| Distances and metallicities for 17 Local Group galaxies
We have obtained Johnson V and Gunni photometry for a large number ofLocal Group galaxies using the Isaac Newton Telescope Wide Field Camera(INT WFC). The majority of these galaxies are members of the M31subgroup and the observations are deep enough to study the top fewmagnitudes of the red giant branch in each system. We previouslymeasured the location of the tip of the red giant branch (TRGB) forAndromeda I, Andromeda II and M33 to within systematic uncertainties oftypically <0.05 mag. As the TRGB acts as a standard candle in old,metal-poor stellar populations, we were able to derive distances to eachof these galaxies. Here we derive TRGB distances to the giant spiralgalaxy M31 and 13 additional dwarf galaxies - NGC 205, 185, 147,Pegasus, WLM, LGS3, Cetus, Aquarius, And III, V, VI, VII and the newlydiscovered dwarf spheroidal And IX. The observations for each of thedwarf galaxies were intentionally taken in photometric conditions. Inaddition to the distances, we also self-consistently derive the medianmetallicity of each system from the colour of their red giant branches.This allows us to take into account the small metallicity variation ofthe absolute I magnitude of the TRGB. The homogeneous nature of our dataand the identical analysis applied to each of the 17 Local Groupgalaxies ensures that these estimates form a reliable set of distanceand metallicity determinations that are ideal for comparative studies ofLocal Group galaxy properties.
| The Stellar Structure of Irregular Galaxies. Face-on Galaxies
Stellar photometry of nearby irregular galaxies of the Local Group isused to identify and study the young and old stellar populations ofthese galaxies. An analysis of the spatial distributions of stars ofdifferent ages in face-on galaxies shows that the young stellarpopulations in irregular galaxies are concentrated toward the center,and form local inhomogeneities in star-forming regions, while the oldstellar populationsred giantsform extended structuresaround the irregular galaxies. The sizes of these structures exceed thevisible sizes of the galaxies at the 25m2 isophote by a factor of two tothree. The surface density of the red giants decreases exponentiallyfrom the center toward the edge, similar to the disk components inspiral galaxies.
| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| Chemical Composition of Two H II Regions in NGC 6822 Based on VLT Spectroscopy
We present long-slit spectrophotometry of regions V and X of the LocalGroup irregular galaxy NGC 6822. The data consist of VLT FORSobservations in the 3450-7500 Å range. We have obtained electrontemperatures and densities using different line intensity ratios. Wehave derived the He, C, and O abundances relative to H based onrecombination lines; the abundance ratios among these elements arealmost independent of the temperature structure of the nebulae. We havealso determined the N, O, Ne, S, Cl, and Ar abundances based oncollisionally excited lines; the ratios of these abundances relative tothat of H depend strongly on the temperature structure of the nebulae.The chemical composition of NGC 6822 V is compared with those of theSun, the Orion Nebula, NGC 346 in the SMC, and 30 Doradus in the LMC.The value of O/H derived from recombination lines is in good agreementwith that derived by Venn and coworkers from two A-type supergiants inNGC 6822.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 69.C-0203(A).
| The Baryonic Tully-Fisher Relation of Galaxies with Extended Rotation Curves and the Stellar Mass of Rotating Galaxies
I investigate the baryonic Tully-Fisher relation for a sample ofgalaxies with extended 21 cm rotation curves spanning the range 20 kms-1<~Vf<=300 km s-1. A variety ofscalings of the stellar mass-to-light ratio Υ* areconsidered. For each prescription for Υ*, I give fitsof the form Md=AVxf.Presumably, the prescription that comes closest to the correct valuewill minimize the scatter in the relation. The fit with minimum scatterhas A=50 Msolar km-4 s4 andx=4. This relation holds over five decades in mass. Galaxy color,stellar fraction, and Υ* are correlated with eachother and with Md, in the sense that more massivegalaxies tend to be more evolved. There is a systematic dependence ofthe degree of maximality of disks on surface brightness. High surfacebrightness galaxies typically have Υ*~3/4 of themaximum disk value, while low surface brightness galaxies typicallyattain ~1/4 of this amount.
| Formation Histories of Dwarf Galaxies in the Local Group
We compare the properties of dwarf galaxies in the Local Group with thesimulated galaxies formed before reionization in a cosmologicalsimulation of unprecedented spatial and mass resolution, includingradiative feedback effects. We find that a subset of the Local Groupdwarfs are already remarkably similar to the simulated dwarf galaxies inall their properties before reionization. On the basis of thissimilarity, we propose the hypothesis that Local Group dwarfs form in avariety of ways: some of them are ``true fossils'' of thepre-reionization era, some of them form most of their stars later, afterreionization (we call them ``survivors'' of the reionization era), andthe rest of them form an intermediate group of ``polluted fossils.'' Wealso identify a simple observational test that is able to test ourhypothesis.
| The Distance and Metallicity of the Newly Discovered, Nearby Irregular Galaxy HIZSS 3
HIZSS 3 is an H I source in the Zone of Avoidance. Its radiocharacteristics are consistent with it being a previously unknown,nearby (~1.8 Mpc), low-mass dwarf irregular (dIrr) galaxy. Opticalobservations have shown that it contains a modest H II region, but theyfailed to reveal a resolved stellar population. New spectroscopicobservations of the H II region obtained at the MMT Observatory arepresented here. They are used to derive the line-of-sight extinction[E(B-V)=1.41+/-0.04] and gas metallicity (logO/H+12~7.8) of the H IIregion. New near-IR imaging observations obtained at the ESO Very LargeTelescope are also presented here. These images clearly reveal theresolved stellar population of HIZSS 3 for the first time. NarrowbandPaβ images of the H II region are used in combination withpreviously published Hα data to obtain an independentline-of-sight extinction estimate: E(B-V)=1.32+/-0.04. The adoptedforeground extinction is E(B-V)=1.36+/-0.06. Using the K-band luminosityfunction and K,J-K color-magnitude diagram, the apparent magnitude andcolor of the tip of the red giant branch are derived. In turn, theseparameters are combined with the adopted foreground extinction toestimate the distance (1.69+/-0.07 Mpc) and mean red giant branchmetallicity ([Fe/H]=-0.5+/-0.1). As an ensemble, these new observationssignificantly strengthen the conclusion that HIZSS 3 is a newlydiscovered low-mass dIrr galaxy lurking behind the Milky Way in theoutskirts of the Local Group.The optical spectroscopic observations reported here were obtained atthe MMT Observatory, a joint facility of the Smithsonian Institution andthe University of Arizona. The near-IR imaging observations reportedhere were collected at the European Southern Observatory, Cerro Paranal,Chile, within observing program 271.B-5047.
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
| Investigating the Possible Anomaly between Nebular and Stellar Oxygen Abundances in the Dwarf Irregular Galaxy WLM
We obtained new optical spectra of 13 H II regions in WLM with EFOSC2;oxygen abundances are derived for nine H II regions. Thetemperature-sensitive [O III] λ4363 emission line was measured intwo bright H II regions, HM 7 and HM 9. The direct oxygen abundances forHM 7 and HM 9 are 12+log(O/H)=7.72+/-0.04 and 7.91+/-0.04, respectively.We adopt a mean oxygen abundance of 12+log(O/H)=7.83+/-0.06. Thiscorresponds to [O/H]=-0.83 dex, or 15% of the solar value. In H IIregions where [O III] λ4363 was not measured, oxygen abundancesderived with bright-line methods are in general agreement with directvalues of the oxygen abundance to an accuracy of about 0.2 dex. Ingeneral, the present measurements show that the H II region oxygenabundances agree with previous values in the literature. The nebularoxygen abundances are marginally consistent with the mean stellarmagnesium abundance ([Mg/H]=-0.62). However, there is still a 0.62 dexdiscrepancy in oxygen abundance between the nebular result and theA-type supergiant star WLM 15 ([O/H]=-0.21). Nonzero reddening valuesderived from Balmer line ratios were found in H II regions near a secondH I peak. There may be a connection between the location of the second HI peak, regions of higher extinction, and the position of WLM 15 on theeastern side of the galaxy.Based on EFOSC2 observations collected at the European SouthernObservatory, Chile; proposal No. 71.D-0491(B).
| On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback from Momentum-driven Winds
We investigate large-scale galactic winds driven by momentum deposition.Momentum injection is provided by (1) radiation pressure produced by thecontinuum absorption and scattering of photons on dust grains and (2)supernovae (momentum injection by supernovae is important even if thesupernova energy is radiated away). Radiation can be produced by astarburst or active galactic nucleus (AGN) activity. We argue thatmomentum-driven winds are an efficient mechanism for feedback during theformation of galaxies. We show that above a limiting luminosity,momentum deposition from star formation can expel a significant fractionof the gas in a galaxy. The limiting, Eddington-like luminosity isLM~=(4fgc/G)σ4, where σ isthe galaxy velocity dispersion and fg is the gas fraction;the subscript M refers to momentum driving. A starburst that attainsLM moderates its star formation rate and its luminosity doesnot increase significantly further. We argue that elliptical galaxiesattain this limit during their growth at z>~1 and that this is theorigin of the Faber-Jackson relation. We show that Lyman break galaxiesand ultraluminous infrared galaxies have luminosities nearLM. Since these starbursting galaxies account for asignificant fraction of the star formation at z>~1, this supports ourhypothesis that much of the observed stellar mass in early-type galaxieswas formed during Eddington-limited star formation. Star formation isunlikely to efficiently remove gas from very small scales in galacticnuclei, i.e., scales much smaller than that of a nuclear starburst. Thisgas is available to fuel a central black hole (BH). We argue that a BHclears gas out of its galactic nucleus when the luminosity of the BHitself reaches ~LM. This shuts off the fuel supply to the BHand may also terminate star formation in the surrounding galaxy. As aresult, the BH mass is fixed to beMBH~=(fgκes/πG2)σ4,where κes is the electron scattering opacity. Thislimit is in accord with the observed MBH-σ relation.
| Spectrophotometry of Sextans A and B: Chemical Abundances of H II Regions and Planetary Nebulae
We present the results of high-quality long-slit spectroscopy ofplanetary nebulae (PNe) and H II regions in the two dwarf irregular(dIrr) galaxies Sextans A and B, which belong to a small group ofgalaxies just outside the Local Group. The observations were obtainedwith the New Technology Telescope ESO Multi-Mode Instrument. In SextansA we obtained the element abundances in its only known PN and in three HII regions with the classical Te method. The oxygenabundances in these three H II regions of Sextans A are all consistentwithin the individual rms uncertainties, with an average12+log(O/H)=7.54+/-0.06. The oxygen abundance of the PN in Sextans A is,however, significantly higher: 12+log(O/H)=8.02+/-0.05. This PN is evenmore enriched in nitrogen and helium, suggesting a classification as aPN of type I. The PN abundances of S and Ar, which are presumablyunaffected by nucleosynthesis in the progenitor star, are well belowthose in the H II regions, indicating lower metallicity at the epoch ofthe PN progenitor formation (~1.5 Gyr ago, according to our estimatesbased on the PN parameters). In Sextans B we obtained spectra of one PNand six H II regions. Element abundances with the Te methodcould be derived for the PN and three of the H II regions. For two ofthese H II regions, which have a separation of only ~70 pc inprojection, the oxygen abundances do not differ within the rmsuncertainties, with a mean of 12+log(O/H)=7.53+/-0.05. The third H IIregion, which is about 0.6 kpc northeast of the first two, is twice asmetal-rich, with 12+log(O/H)=7.84+/-0.05. This suggests considerableinhomogeneity in the present-day metallicity distribution in Sextans B.Whether this implies a general chemical inhomogeneity among populationsof comparable age in Sextans B, and thus a metallicity spread at a givenage, or whether we happen to see the short-lived effects of freshlyejected nucleosynthesis products prior to their dispersal and mixingwith the ambient interstellar medium will require further study. For thePN we measured an O/H ratio of 12+log(O/H)=7.47+/-0.16, consistent withthat of the low-metallicity H II regions. We discuss the new metallicitydata for the H II regions and PNe in the context of the published starformation histories and published abundances of the two dIrr galaxies.Both dIrrs show generally similar star formation histories in the senseof continuous star formation with amplitude variations but differ intheir detailed enrichment timescales and star formation rates as afunction of time. If we combine the photometrically derived estimatesfor the mean metallicity of the old red giant branch population in bothdIrrs with the present-day metallicity of the H II regions, both dIrrshave experienced chemical enrichment by at least 0.8 dex (lower limit)throughout their history.Based on observations obtained at the European Southern Observatory, LaSilla, Chile [072.A-0087(B)].
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
