目录
图像
上传图像
DSS Images Other Images
相关文章
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.
| 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%.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| The Compression of Dark Matter Halos by Baryonic Infall
The initial radial density profiles of dark matter halos are laid downby gravitational collapse in hierarchical structure formation scenariosand are subject to further compression as baryons cool and settle to thehalo centers. Here we describe an explicit implementation of thealgorithm, originally developed by Young, to calculate changes to thedensity profile as the result of adiabatic infall in a spherical halomodel. Halos with random motion are more resistant to compression thanare those in which random motions are neglected, which is a key weaknessof the simple method widely employed. Young's algorithm results indensity profiles in excellent agreement with those from N-bodysimulations. We show how the algorithm can be applied to determine theoriginal uncompressed halos of real galaxies, a step that must becomputed with care in order to enable a confrontation with theoreticalpredictions from theories such as ΛCDM.
| 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.
| The Molecular Interstellar Medium of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies
We present a new survey for CO in dwarf galaxies using the ARO Kitt Peak12 m telescope. This survey consists of observations of the centralregions of 121 northern dwarfs with IRAS detections and no known COemission. We detect CO in 28 of these galaxies and marginally detectanother 16, increasing by about 50% the number of such galaxies known tohave significant CO emission. The galaxies we detect are comparable instellar and dynamical mass to the Large Magellanic Cloud, althoughsomewhat brighter in CO and fainter in the far-IR. Within dwarfs, wefind that the CO luminosity LCO is most strongly correlatedwith the K-band and the far-infrared luminosities. There are also strongcorrelations with the radio continuum (RC) and B-band luminosities andlinear diameter. Conversely, we find that far-IR dust temperature is apoor predictor of CO emission within the dwarfs alone, although a goodpredictor of normalized CO content among a larger sample of galaxies. Wesuggest that LCO and LK correlate well because thestellar component of a galaxy dominates the midplane gravitational fieldand thus sets the pressure and density of the atomic gas, which controlthe formation of H2 from H I. We compare our sample with moremassive galaxies and find that dwarfs and large galaxies obey the samerelationship between CO and the 1.4 GHz RC surface brightness. Thisrelationship is well described by a Schmidt law withΣRC~Σ1.3CO. Therefore,dwarf galaxies and large spirals exhibit the same relationship betweenmolecular gas and star formation rate (SFR). We find that this result isrobust to moderate changes in the RC-to-SFR and CO-to-H2conversion factors. Our data appear to be inconsistent with large (orderof magnitude) variations in the CO-to-H2 conversion factor inthe star-forming molecular gas.
| 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.
| Detections of CO in Late-Type, Low Surface Brightness Spiral Galaxies
Using the IRAM 30 m telescope, we have obtained 12CO J=1-0and 2-1 spectral line observations toward the nuclear regions of 15edge-on, low surface brightness (LSB) spiral galaxies. Our samplecomprises extreme late-type LSB spirals with disk-dominated morphologiesand rotational velocities Vrot<~120 km s-1. Wereport detections of four galaxies in at least one transition (>~5σ) for the remainder of the sample we provide upper limits on thenuclear CO content. Adopting a standard GalacticICO-to-H2 conversion factor implies molecular gasmasses of (3.3-9.8)×106 Msolar in thenuclear regions (inner 1.1-1.8 kpc) of the detected galaxies. Combiningour new data with samples of late-type spirals from the literature, wefind that CO-detected LSB spirals adhere to the sameMH2-far-infrared correlation as more luminous andhigher surface brightness galaxies. The amount of CO in the centralregions of late-type spirals appears to depend more strongly on massthan on central optical surface brightness, and CO detectabilitydeclines significantly for moderate to low surface brightness spiralswith Vrot<~90 km s-1 no LSB spirals have so farbeen detected in CO below this threshold. Metallicity effects alone areunlikely to account for this trend, and we speculate that we are seeingthe effects of a decrease in the mean fraction of a galaxy disk able tosupport giant molecular cloud formation with decreasing galaxy mass.
| Red Thick Disks of Nearby Galaxies
Edge-on systems reveal the properties of disk galaxies as a function ofheight, z, above the plane. Four local edge-on galaxies that are closeenough to have been resolved into stars by the Hubble Space Telescopeshow thick disks composed of a red stellar population that is old andrelatively metal rich. Color gradients, Δ(V-I)/Δz, are zeroor slightly positive. Favored models may have an explicit thick diskformation phase.
| The Local Group and Other Neighboring Galaxy Groups
Over the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ``ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.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,Inc., under NASA contract NAS 5-26555.
| The inner structure of ΛCDM haloes - II. Halo mass profiles and low surface brightness galaxy rotation curves
We use a set of high-resolution cosmological N-body simulations toinvestigate the inner mass profile of galaxy-sized cold dark matter(CDM) haloes. These simulations extend the numerical convergence studypresented in Paper I of this series, and demonstrate that the massprofile of CDM galaxy haloes can be robustly estimated beyond a minimumconverged radius of order rconv~ 1h-1 kpc in ourhighest-resolution runs. The density profiles of simulated haloes becomeprogressively shallower from the virial radius inwards, and show no signof approaching a well-defined power law near the centre. Atrconv, the density profile is steeper than expected from theformula proposed by Navarro, Frenk & White, which has aρ~r-1 cusp, but significantly shallower than the steeplydivergent ρ~r-1.5 cusp proposed by Moore et al. Weperform a direct comparison of the spherically averaged dark mattercircular velocity profiles with Hα rotation curves of a sample oflow surface brightness (LSB) galaxies. We find that most galaxies in thesample (about 70 per cent) have rotation curves that are consistent withthe structure of CDM haloes. Of the remainder, 20 per cent have rotationcurves which cannot be fit by any smooth fitting function with few freeparameters, and 10 per cent are inconsistent with CDM haloes. However,the latter consist mostly of rotation curves that do not extend to largeenough radii to accurately determine their shapes and maximumvelocities. We conclude that the inner structure of CDM haloes is notmanifestly inconsistent with the rotation curves of LSB galaxies.
| Cores of dark matter haloes correlate with stellar scalelengths
We investigate in detail the mass distribution obtained by means ofhigh-resolution rotation curves of 25 galaxies of differentmorphological types. The dark matter contribution to the circularrotation velocity is well-described by resorting to a dark component,the density of which shows an inner core, i.e. a central constantdensity region. We find a very strong correlation between the coreradius size RC and the stellar exponential scalelengthRD: RC~=13[RD/(5kpc)]1.05kpc, and between RCand the galaxy dynamical mass at this distance,Mdyn(RC). These relationships would not beexpected if the core radii were the product of an incorrectdecomposition procedure, or the biased result of wrong or misunderstoodobservational data. The very strong correlation between the dark andluminous scalelengths found here seems to hold also for different Hubbletypes and opens new scenarios for the nature of the dark matter ingalaxies.
| The Mass Discrepancy-Acceleration Relation: Disk Mass and the Dark Matter Distribution
The mass discrepancy in disk galaxies is shown to be well correlatedwith acceleration, increasing systematically with decreasingacceleration below a critical scalea0~3700km2s-2kpc-1=1.2×10-10ms-2.For each galaxy, there is an optimal choice of stellar mass-to-lightratio that minimizes the scatter in this mass discrepancy-accelerationrelation. The same mass-to-light ratios also minimize the scatter in thebaryonic Tully-Fisher relation and are in excellent agreement with theexpectations of stellar population synthesis. Once the disk mass isdetermined in this fashion, the dark matter distribution is specified.The circular velocity attributable to the dark matter can be expressedas a simple equation that depends only on the observed distribution ofbaryonic mass. It is a challenge to understand how this very fine-tunedcoupling between mass and light comes about.
| Constraining the Magnetic Effects on H I Rotation Curves and the Need for Dark Halos
The density profiles of dark halos are usually inferred from therotation curves of disk galaxies based on the assumption that the gas isa good tracer of the gravitational potential of the galaxies. Someauthors have suggested that magnetic pinching forces could altersignificantly the rotation curves of spiral galaxies. In contrast toother studies that have concentrated on the vertical structure of thedisk, here we focus on the problem of magnetic confinement in the radialdirection to determine the magnetic effects on the H I rotation curves.It is shown that azimuthal magnetic fields hardly speed up H I disks ofgalaxies as a whole. In fact, based on virial constraints we show thatthe contribution of galactic magnetic fields to the rotation curvescannot be larger than ~10 km s-1 at the outermost point of HI detection, if the galaxies did not contain dark matter at all, and isup to 20 km s-1 in the conventional dark halo scenario. Theprocedure to estimate the maximum effect of magnetic fields is generaland applicable to any particular galaxy disk. The inclusion of thesurface terms, namely, the intergalactic (thermal, magnetic, or ram)pressure, does not change our conclusions. Other problems related to themagnetic alternative to dark halos are highlighted. The relevance ofmagnetic fields in the cuspy problem of dark halos is also discussed.
| Investigating the Origins of Dark Matter Halo Density Profiles
Although high-resolution N-body simulations make robust empiricalpredictions of the density distribution within cold dark matter halos,these studies have yielded little physical insight into the origins ofthe distribution. We therefore attempt to investigate the problem usinganalytic and semianalytic approaches. Simple analytic considerationssuggest that the inner slope of the central cusps in dark matter haloscannot be steeper than α=2 (where ρ~r-α),with α=1.5-1.7 being a more realistic upper limit. Moreover, ouranalysis suggests that any number of effects, whether real (e.g.,angular momentum imparted by tidal torques and secondary perturbations)or artificial (e.g., two-body interactions, the accuracy of thenumerical integrator, round-off errors) will result in shallower slopes.We also find that the halos should exhibit a well-defined relationshipbetween rperi/rapo andjθ/jr. We derive this relationshipanalytically and speculate that it may be ``universal.'' Using asemianalytic scheme based on Ryden & Gunn, we further explore therelationship between the specific angular momentum distribution in ahalo and its density profile. For present purposes, we restrictourselves to halos that form primarily via the nearly smooth accretionof matter, and consider only the specific angular momentum generated bysecondary perturbations associated with the cold dark matter spectrum ofdensity fluctuations. Compared to those formed in N-body simulations,our ``semianalytic'' halos are more extended, have flatter rotationcurves, and have a higher specific angular momentum, even though we havenot yet taken into account the effects of tidal torques. Whether thedensity profile of numerical halos is indeed the result of loss inangular momentum outside the central region, and whether this loss is afeature of hierarchical merging and major mergers in particular, isunder investigation.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties
We performed a compilation of more than 1000 published spectra of H IIregions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. Theradial distributions of oxygen and nitrogen abundances were derived. Thecorrelations between oxygen abundance and macroscopic properties areexamined. We found that the oxygen abundance in spiral galaxiescorrelates with its luminosity, rotation velocity, and morphologicaltype: the correlation with the rotation velocity may be slightlytighter. There is a significant difference between theluminosity-metallicity relationship obtained here and that based on theoxygen abundances determined through the R23-calibrations.The oxygen abundance of NGC 5457 recently determined using directmeasurements of Te (Kennicutt et al. \cite{Kennicutt2003})agrees with the luminosity-metallicity relationship derived in thispaper, but is in conflict with the luminosity-metallicity relationshipderived with the R23-based oxygen abundances. The obtainedluminosity-metallicity relation for spiral galaxies is compared to thatfor irregular galaxies. Our sample of galaxies shows evidence that theslope of the O/H - MB relationship for spirals (-0.079± 0.018) is slightly more shallow than that for irregulargalaxies (-0.139 ± 0.011). The effective oxygen yields wereestimated for spiral and irregular galaxies. The effective oxygen yieldincreases with increasing luminosity from MB -11 toMB -18 (or with increasing rotation velocity fromVrot 10 km s-1 to Vrot 100km s-1) and then remains approximately constant. Irregulargalaxies from our sample have effective oxygen yields lowered by afactor of 3 at maximum, i.e. irregular galaxies usually keep at least1/3 of the oxygen they manufactured during their evolution.Appendix, Tables \ref{table:refero}, \ref{table:referV}, and Figs.\ref{figure:sample2}-\ref{figure:sample5} are only available inelectronic form at http://www.edpsciences.org}
| The very local Hubble flow: Computer simulations of dynamical history
The phenomenon of the very local (≤3 Mpc) Hubble flow is studied onthe basis of the data of recent precision observations. A set ofcomputer simulations is performed to trace the trajectories of the flowgalaxies back in time to the epoch of the formation of the Local Group.It is found that the ``initial conditions'' of the flow are drasticallydifferent from the linear velocity-distance relation. The simulationsenable one also to recognize the major trends of the flow evolution andidentify the dynamical role of universal antigravity produced by thecosmic vacuum.
| Chemically consistent evolution of galaxies. II. Spectrophotometric evolution from zero to high redshift
The composite stellar populations of galaxies comprise stars of a widerange of metallicities. Subsolar metallicities become increasinglyimportant, both in the local universe when going from early towardslater galaxy types as well as for dwarf galaxies and for all types ofgalaxies towards higher redshifts.We present a new generation of chemically consistent evolutionarysynthesis models for galaxies of various spectral types from E throughSd. The models follow the chemical enrichment of the ISM and take intoaccount the increasing initial metallicity of successive stellargenerations using recently published metallicity dependent stellarevolutionary isochrones, spectra and yields.Our first set of closed-box 1-zone models does not include any spatialresolution or dynamics. For a Salpeter initial mass function (IMF) thestar formation rate (SFR) and its time evolution are shown tosuccessfully parameterise spectral galaxy types E, ..., Sd. We show howthe stellar metallicity distribution in various galaxy types build upwith time to yield after 12 Gyr agreement with stellar metallicitydistributions observed in our and other local galaxies.The models give integrated galaxy spectra over a wide wavelength range(90.9 Å-160 μm), which for ages of 12 Gyr are in goodagreement not only with observed broad band colours but also withtemplate spectra for the respective galaxy types.Using filter functions for Johnson-Cousins U, B, V, RC,IC, as well as for HST broad band filters in the optical andBessel & Brett's NIR J, H, K filter system, we calculate theluminosity and colour evolution of model galaxies over a Hubble time.Including a standard cosmological model (H0 = 65,Ω0 = 0.1) and the attenuation by intergalactic hydrogenwe present evolutionary and cosmological corrections as well as apparentluminosities in various filters over the redshift range from z 5to the present for our galaxy types and compare to earlier models usingsingle (=solar) metallicity input physics only. We also resent a firstcomparison of our cc models to HDF data. A more detailed comparison withHubble Deep Field (HDF) and other deep field data and an analysis andinterpretation of high redshift galaxies in terms of ages,metallicities, star formation histories and, galaxy types will be thesubject of a forthcoming paper.
| Simulating observations of dark matter dominated galaxies: towards the optimal halo profile
Low surface brightness galaxies are dominated by dark matter, and theirrotation curves thus reflect their dark matter distribution. Recenthigh-resolution rotation curves suggest that their dark mattermass-density distributions are dominated by a constant-density core.This seems inconsistent with the predictions of cold dark matter (CDM)models which produce haloes with compact density cusps and steepmass-density profiles. However, the observationally determined massprofiles may be affected by non-circular motions, asymmetries andoffsets between optical and dynamical centres, all of which tend tolower the observed slopes. Here we determine the impact of each of theseeffects on a variety of halo models, and we compare the results withobserved mass-density profiles. Our simulations suggest that no singlesystematic effect can reconcile the data with the cuspy CDM haloes. Thedata are best described by a model with a soft core with an innerpower-law mass-density slope α=-0.2 +/- 0.2. However, no singleuniversal halo profile provides a completely adequate description of thedata.
| A Survey for H2O Megamasers. III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalacticwater megamasers taken over a period of 9 years and a single epoch ofsensitive spectra for seven others. The primary motivation is a searchfor drifting line velocities analogous to those of the systemic featuresin NGC 4258, which are known to result from centripetal acceleration ofgas in an edge-on, subparsec molecular disk. We detect a velocity driftanalogous to that in NGC 4258 in only one source, NGC 2639. Another, themaser source in NGC 1052, exhibits erratic changes in its broad maserprofile over time. Narrow maser features in all of the other diskgalaxies discussed here either remain essentially constant in velocityover the monitoring period or are sufficiently weak or variable inintensity that individual features cannot be traced reliably from oneepoch to the next. In the context of a circumnuclear, molecular diskmodel, our results suggest that either (a) the maser lines seen aresystemic features subject to a much smaller acceleration than present inNGC 4258, presumably because the gas is farther from the nuclear blackhole, or (b) we are detecting ``satellite'' lines for which theacceleration is in the plane of the sky.Our data include the first K-band science observations taken with thenew 100 m Green Bank Telescope (GBT). The GBT data were taken duringtesting and commissioning of several new components and so are subjectto some limitations; nevertheless, they are in most cases the mostsensitive H2O spectra ever taken for each source and cover800 MHz (~=10,800 km s-1) of bandwidth. Many new maserfeatures are detected in these observations. Our data also include atentative and a clear detection of the megamaser in NGC 6240 at epochs ayear and a few months, respectively, prior to the detections reported byHagiwara et al. and Nakai et al.We also report a search for water vapor masers toward the nuclei of 58highly inclined (i>80deg), nearby galaxies. These sourceswere selected to investigate the tendency that H2O megamasersfavor inclined galaxies. None were detected, confirming that megamasersare associated exclusively with active galactic nuclei.
| Modified Newtonian Dynamics and the ``Dearth of Dark Matter in Ordinary Elliptical Galaxies''
The recent findings of Romanowsky et al., of an ``unexpectedly'' smallmass discrepancy within 5 effective radii in several ellipticalgalaxies, are not surprising in the context of modified Newtoniandynamics (MOND). As we show here, they are, in fact, in full concordancewith its predictions. One is dealing with high surface density galaxieswith mean accelerations rather larger than the acceleration constant ofMOND. These findings continue, and are now the extreme examples of, thetrend predicted by MOND: the mass discrepancy sets in at larger andlarger scaled radii in galaxies with larger and larger mean surfacedensities or, equivalently, mean accelerations.
| High-Resolution Measurements of the Dark Matter Halo of NGC 2976: Evidence for a Shallow Density Profile
We have obtained two-dimensional velocity fields of the dwarf spiralgalaxy NGC 2976 in Hα and CO. The high spatial (~75 pc) andspectral (13 and 2 km s-1, respectively) resolution of theseobservations, along with our multicolor optical and near-infraredimaging, allows us to measure the shape of the density profile of thedark matter halo with good precision. We find that the total (baryonicplus dark matter) mass distribution of NGC 2976 follows aρtot~r-0.27+/-0.09 power law out to a radiusof 1.8 kpc, assuming that the observed radial motions provide nosupport. The density profile attributed to the dark halo is evenshallower, consistent with a nearly constant density of dark matter overthe entire observed region. A maximal disk fit yields an upper limit tothe K-band stellar mass-to-light ratio (M*/LK) of0.09+0.15-0.08Msolar/LsolarK(including systematic uncertainties), with the caveat that forM*/LK>0.19Msolar/LsolarKthe dark matter density increases with radius, which is unphysical.Assuming0.10Msolar/LsolarK<~M*/LK<=0.19Msolar/LsolarK,the dark matter density profile lies betweenρDM~r-0.17 andρDM~r-0.01. Therefore, independent of anyassumptions about the stellar disk or the functional form of the densityprofile, NGC 2976 does not contain a cuspy dark matter halo. We alsoinvestigate some of the systematic effects that can hamper rotationcurve studies and show that (1) long-slit rotation curves are far morevulnerable to systematic errors than two-dimensional velocity fields,(2) NGC 2976 contains radial motions that are as large as 90% of therotational velocities at small radii, and (3) the Hα and COvelocity fields of NGC 2976 agree within their uncertainties, with atypical scatter between the two velocities of 5.3 km s-1 atany position in the galaxy.Based on observations carried out at the WIYN Observatory. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory.
| The Kinematic State of the Local Volume
The kinematics of galaxies within 10 Mpc of the Milky Way isinvestigated using published distances and radial velocities. Withrespect to the average Hubble flow (isotropic or simple anisotropic),there is no systematic relation between peculiar velocity dispersion andabsolute magnitude over a range of 10 mag; neither is there any apparentvariation with galaxy type or between field and cluster members. Thereare several possible explanations for the lack of variation, though allhave difficulties: either there is no relationship between light andmass on these scales, the peculiar velocities are not produced bygravitational interaction, or the background dynamical picture is wrongin some systematic way. The extremely cold local flow of 40-60 kms-1 dispersion reported by some authors is shown to be anartifact of sparse data, a velocity dispersion of over 100 kms-1 being closer to the actual value. Galaxies with a high(positive) radial velocity have been selected against in studies of thisvolume, biasing numerical results.
| The Contribution of H I-rich Galaxies to the Damped Lyα Absorber Population at z = 0
We present a study of the expected properties of the low-redshift dampedLyα absorber population determined from a sample of H I-selectedgalaxies in the local universe. Because of a tight correlation betweenthe H I mass and H I cross section, which we demonstrate spans allgalaxy types, we can use our H I-selected sample to predict theproperties of the absorption-line systems. We use measurements of thenumber density and H I cross section of galaxies to show that the totalH I cross section at column densities sufficient to produce dampedLyα absorption is consistent with no evolution of the absorberpopulation. We also find that the dN/dz distribution is dominated bygalaxies with H I masses near 109 Msolar. However,because of the large dispersion in the correlation between H I mass andstellar luminosity, we find that the distribution of dN/dz as a functionof LJ is fairly flat. In addition, we examine the line widthsof the H I-selected galaxies and show that there may be evolution in thekinematics of H I-rich galaxies, but it is not necessary for the higherredshift population to contain a greater proportion of high-massgalaxies than we find locally.
| Uncovering Additional Clues to Galaxy Evolution. II. The Environmental Impact of the Virgo Cluster on the Evolution of Dwarf Irregular Galaxies
The impact of the cluster environment on the evolution of dwarf galaxiesis investigated by comparing the properties of a sample of dwarfirregular galaxies (dI's) in the Virgo Cluster with a control sample ofnearby (``field'') dI's having oxygen abundances derived from [O III]λ4363 measurements and measured distances from resolved stellarconstituents. Spectroscopic data are obtained for H II regions in 11Virgo dI's distributed in the central and outer regions of the cluster.To ensure that oxygen abundances are derived in a homogeneous manner,oxygen abundances for field and Virgo dI's are computed using thebright-line method and compared with abundances directly obtained from[O III] λ4363, where available. They are found to agree to withinabout 0.2 dex, with no systematic offset. At a given optical luminosity,there is no systematic difference in oxygen abundance between the sampleof Virgo dI's and the sample of nearby dI's. However, five of the 11Virgo dI's exhibit much lower baryonic gas fractions than field dI's atcomparable oxygen abundances. Using field dI's as a reference, agas-deficiency index for dI's is constructed, making it possiblequantitatively to identify which galaxies have lost gas. For the Virgosample, some of the dwarfs are gas-deficient by a factor of 30. The gasdeficiency correlates roughly with the X-ray surface brightness of theintracluster gas. Ram pressure stripping can best explain the observedgas-poor dI's in the cluster sample. Together with the lack ofsignificant fading and reddening of the gas-poor dI's compared withgas-normal dI's, these observations suggest that the gas-poor dI's inVirgo have recently encountered the intracluster medium for the firsttime. Faded remnants of gas-poor dI's in Virgo will resemble brightdwarf elliptical galaxies currently seen in the cluster core.
| Uncovering Additional Clues to Galaxy Evolution. I. Dwarf Irregular Galaxies in the Field
In order to recognize environmental effects on the evolution of dwarfgalaxies in clusters of galaxies, it is first necessary to quantify theproperties of objects that have evolved in relative isolation. Withoxygen abundance as the gauge of metallicity, two key diagnostics of theevolution of dwarf irregular galaxies in the field are reexamined: themetallicity-luminosity relationship and the metallicity-gas fractionrelationship. Gas fractions are evaluated from the masses of luminouscomponents only, i.e., constituents of the nucleogenetic pool. Resultsfrom new optical spectroscopy obtained for H II regions in five dwarfirregular galaxies in the local volume are incorporated into a newanalysis of field dwarfs with [O III] λ4363 detections and gooddistances. The updated fit to the metallicity-luminosity relationship isconsistent with results reported in the literature. The fit to themetallicity-gas fraction relation shows an excellent correlationconsistent with expectations of the simple ``closed box'' model ofchemical evolution. The simplest explanation consistent with the data isthat flow rates are zero, although the observations allow for thepossibility of modest flows. The derived oxygen yield is one-quarter ofthe value for the solar neighborhood.
| Distances to nearby galaxies around IC 342
We present an analysis of Hubble Space Telescope/WFPC2 images of sixnearby galaxies in the projected vicinity of IC 342: Cas dSph, KK 35,UGCA 86, Cam A, NGC 1560, and Cam B. We derive distances to five of themfrom the luminosity of the tip of the red giant branch stars with atypical accuracy of ~ 10%. The galaxy distances are 0.79 Mpc (CasdSph), 3.16 Mpc (KK 35), 3.93 Mpc (Cam A), 3.45 Mpc (NGC 1560), and 3.34Mpc (Cam B). Two other observed galaxies, MB 2 and Cam C = KK 26, turnout to be Galactic cirrus and a Galactic H II region, respectively. CasdSph belongs to the Local Group and is a companion of M 31. Combiningour data with literature results, we find that there are seven dwarfgalaxies associated with the giant spiral galaxy IC 342. This group ischaracterized by an average distance of (3.28 +/- 0.15) Mpc, an averageradial velocity of (229 +/- 23) km s-1, a projected radius of322 kpc, a radial velocity dispersion of 60 km s-1, and atotal blue luminosity of 3.43x 1010 Lsun. Thederived virial and orbital mass-to-luminosity ratios are 20 and 28Msun/Lsun, respectively. The galaxy group aroundMaffei 1 has so far a less reliable distance of ~ 3.0 Mpc and anaverage radial velocity of (309 +/- 22) km s-1. This groupconsists of eight galaxies and is characterized by a projected radius of112 kpc, a radial velocity dispersion of 59 km s-1, and atotal blue luminosity of 2.97x 1010 Lsun. For theMaffei group we estimate mass-to-light ratios ofMvir/LB = 16 and Morb/LB = 5in solar units. The sum of the virial (119x 1010Msun) and orbital (109x 1010 Msun)masses of both groups agree well with their total mass, (107 +/- 33)1010 Msun, derived from the radius of the ``zerovelocity surface'', R0 = (0.9 +/- 0.1) Mpc, which separatesthe IC 342/Maffei complex from the Hubble flow.Based on observations made with the NASA/ESA Hubble Space Telescope. TheSpace Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. under NASA contract NAS5-26555.Figure A.1 is only available in electronic form athttp://www.edpsciences.org
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
|
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:
|
