Contents
Images
Upload your image
DSS Images Other Images
Related articles
A log-quadratic relation between the nuclear black hole masses and velocity dispersions of galaxies
We demonstrate that a log-linear relation does not provide an adequatedescription of the correlation between the masses of super massive blackholes (SMBHs, Mbh) and the velocity dispersions of their hostspheroid (σ). An unknown relation between logMbh andlogσ may be expanded to second order to obtain a log-quadraticrelation of the form log(Mbh)=α+βlog(σ/200kms-1)+β2[log(σ/200kms-1)]2. Weperform a Bayesian analysis using the local sample described in Tremaineet al., and solve for β, β2 and α, inaddition to the intrinsic scatter (δ). We find unbiased parameterestimates of β= 4.2 +/- 0.37, β2= 1.6 +/- 1.3 andδ= 0.275 +/- 0.05. At the 90 per cent level theMbh-σ relation does not follow a uniform power law.Indeed, over the velocity range 70 <~σ<~ 380kms-1 the logarithmic slope d logMbh/d logσof the best-fitting relation varies between 2.7 and 5.1, which should becompared with a power-law estimate of 4.02 +/- 0.33. The addition of the14 galaxies with reverberation SMBH masses and measured velocitydispersions to the local SMBH sample leads to a log-quadratic relationwith the same best fit as the local sample. However, the addition of thereverberation masses increases the significance of the log-quadraticcontribution, yielding a value of β2 that is non-zero atthe 5σ level. Furthermore, assuming no systematic offset, singleepoch virial SMBH masses estimated for active galactic nuclei (AGNs)follow the same log-quadratic Mbh-σ relation as thelocal sample, but extend it downward in mass by an order of magnitude.The log-quadratic term in the Mbh-σ relation has asignificant effect on estimates of the local SMBH mass function atMbh>~ 109Msolar, leading todensities of SMBHs with Mbh>~1010Msolar that are several orders of magnitudelarger than inferred from a log-linear Mbh-σ relation.We also estimate unbiased parameters for the SMBH-bulge mass relationusing the sample assembled by Häring and Rix. With aparametrization log(Mbh)=αbulge+βbulgelog(Mbulge/1011Msolar)+β2,bulge[log(Mbulge/1011Msolar)]2,we find βbulge= 1.15 +/- 0.18 andβ2,bulge= 0.12 +/- 0.14. We determined an intrinsicscatter δbulge= 0.41 +/- 0.07 which is ~50 per centlarger than the scatter in the Mbh-σ relation.
| A quasar in every galaxy ?
Not Available
| GALEX : seeing starbirth, near and far.
Not Available
| Bias-free Measurement of Giant Molecular Cloud Properties
We review methods for measuring the sizes, line widths, and luminositiesof giant molecular clouds (GMCs) in molecular-line data cubes with lowresolution and sensitivity. We find that moment methods are robust andsensitive, making full use of both position and intensity information,and we recommend a standard method to measure the position angle, majorand minor axis sizes, line width, and luminosity using moment methods.Without corrections for the effects of beam convolution and sensitivityto GMC properties, the resulting properties may be severely biased. Thisis particularly true for extragalactic observations, where resolutionand sensitivity effects often bias measured values by 40% or more. Wecorrect for finite spatial and spectral resolutions with a simpledeconvolution, and we correct for sensitivity biases by extrapolatingproperties of a GMC to those we would expect to measure with perfectsensitivity (i.e., the 0 K isosurface). The resulting method recoversthe properties of a GMC to within 10% over a large range of resolutionsand sensitivities, provided the clouds are marginally resolved with apeak signal-to-noise ratio greater than 10. We note that interferometerssystematically underestimate cloud properties, particularly the fluxfrom a cloud. The degree of bias depends on the sensitivity of theobservations and the (u,v) coverage of the observations. In an Appendixto the paper we present a conservative, new decomposition algorithm foridentifying GMCs in molecular-line observations. This algorithm treatsthe data in physical rather than observational units (i.e., parsecsrather than beams or arcseconds), does not produce spurious clouds inthe presence of noise, and is sensitive to a range of morphologies. As aresult, the output of this decomposition should be directly comparableamong disparate data sets.
| Spectral Energy Distributions of M81 Globular Clusters in the BATC Multicolor Survey
In this paper, we give the spectral energy distributions (SEDs) of 42M81 globular clusters in 13 intermediate-band filters from 4000 to 10000Å using the CCD images of M81, observed as part of theBeijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey. TheBATC multicolor filter system is specifically designed to exclude mostof the bright and variable night-sky emission lines, including the OHforest. Hence, it can present accurate SEDs of the observed objects.These SEDs are low-resolution spectra and can reflect the stellarpopulations of the globular clusters. This paper confirms theconclusions of Schroder et al., that M81 contains clusters as young as afew Gyr, which were also observed in both M31 and M33.
| Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
| Long-term V-band monitoring of the bright stars of M33 at the Wise Observatory
We have conducted a long-term V-band photometric monitoring of M33 on 95nights during four observing seasons (2000-2003). A total number of 6418light curves of bright objects in the range of 14-21mag have beenobtained. All measurements are publicly available. A total of 127 newvariables were detected, of which 28 are periodic. 10 previously knownnon-periodic variables were identified as periodic, three of which areCepheids, and another previously known periodic variable was identifiedas an eclipsing binary. Our derived periods range from 2.11 to almost300d. For 50 variables we have combined our observations with those ofthe DIRECT project, obtaining light curves of up to 500 measurements,with a time-span of ~7yr. We have detected a few interesting variables,including a 99.3d periodic variable with a 0.04mag amplitude, at theposition of supernova remnant (SNR) 19.
| SN 2004A: another Type II-P supernova with a red supergiant progenitor
We present a monitoring study of SN 2004A and probable discovery of aprogenitor star in pre-explosion Hubble Space Telescope (HST) images.The photometric and spectroscopic monitoring of SN 2004A show that itwas a normal Type II-P which was discovered in NGC 6207 about two weeksafter explosion. We compare SN 2004A to the similar Type II-P SN 1999emand estimate an explosion epoch of 2004 January 6. We also calculatethree new distances to NGC 6207 of 21.0 +/- 4.3,21.4 +/- 3.5 and 25.1+/- 1.7Mpc. The former was calculated using the Standard Candle Method(SCM) for SNeII-P, and the latter two from the brightest supergiantsmethod (BSM). We combine these three distances with existing kinematicdistances, to derive a mean value of 20.3 +/- 3.4Mpc. Using thisdistance, we estimate that the ejected nickel mass in the explosion is0.046+0.031-0.017Msolar. The progenitorof SN 2004A is identified in pre-explosion WFPC2 F814W images with amagnitude of mF814W = 24.3 +/- 0.3, but is below thedetection limit of the F606W images. We show that this was likely a redsupergiant (RSG) with a mass of9+3-2Msolar. The object is detected at4.7σ above the background noise. Even if this detection isspurious, the 5σ upper limit would give a robust upper mass limitof 12Msolar for a RSG progenitor. These initial masses arevery similar to those of two previously identified RSG progenitors ofthe Type II-P SNe 2004gd (8+4-2Msolar)and 2005cs (9+3-2Msolar).
| A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph
We present a catalogue of positions, magnitudes and velocities for 3300emission-line objects found by the Planetary Nebula Spectrograph in asurvey of the Andromeda galaxy, M31. Of these objects, 2615 are foundlikely to be planetary nebulae (PNe) associated with M31. The surveyarea covers the whole of M31's disc out to a radius of . Beyond thisradius, observations have been made along the major and minor axes, andthe Northern Spur and Southern Stream regions. The calibrated data havebeen checked for internal consistency and compared with othercatalogues. With the exception of the very central, high surfacebrightness region of M31, this survey is complete to a magnitude limitof m5007 ~ 23.75, 3.5 mag into the PN luminosity function.We have identified emission-line objects associated with M31'ssatellites and other background galaxies. We have examined the data fromthe region tentatively identified as a new satellite galaxy, AndromedaVIII, comparing it to data in the other quadrants of the galaxy. We findthat the PNe in this region have velocities that appear to be consistentwith membership of M31 itself.The luminosity function of the surveyed PNe is well matched to the usualsmooth monotonic function. The only significant spatial variation in theluminosity function occurs in the vicinity of M31's molecular ring,where the luminosities of PNe on the near side of the galaxy aresystematically ~0.2 mag fainter than those on the far side. Thisdifference can be explained naturally by a modest amount of obscurationby the ring. The absence of any difference in luminosity functionbetween bulge and disc suggests that the sample of PNe is not stronglypopulated by objects whose progenitors are more massive stars. Thisconclusion is reinforced by the excellent agreement between the numbercounts of PNe and the R-band light.The number counts of kinematically selected PNe also allow us to probethe stellar distribution in M31 down to very faint limits. There is noindication of a cut-off in M31's disc out to beyond four scalelengths,and no signs of a spheroidal halo population in excess of the bulge outto 10 effective bulge radii.We have also carried out a preliminary analysis of the kinematics of thesurveyed PNe. The mean streaming velocity of the M31 disc PNe is foundto show a significant asymmetric drift out to large radii. Theirvelocity dispersion, although initially declining with radius, flattensout to a constant value in the outer parts of the galaxy. There are noindications that the disc velocity dispersion varies with PN luminosity,once again implying that the progenitors of PNe of all magnitudes form arelatively homogeneous old population. The dispersion profile andasymmetric drift results are shown to be mutually consistent, butrequire that the disc flares with radius if the shape of its velocityellipsoid remains invariant.
| Upper limits on the central black hole masses of 47Tuc and NGC 6397 from radio continuum emission
We present upper limits on the masses of the putative centralintermediate-mass black holes in two nearby Galactic globular clusters:47Tuc (NGC 104), the second brightest Galactic globular cluster, and NGC6397, a core-collapse globular cluster and, with a distance of 2.7 kpc,quite possibly the nearest globular cluster. These upper limits areobtained using a technique suggested by T. Maccarone. These massestimates have been derived from 3σ upper limits on the radiocontinuum flux at 1.4 GHz, assuming that the putative central black holeaccretes the surrounding matter at a rate of between 0.1 and 1 per centof the Bondi accretion rate. For 47Tuc, we find a 3σ upper limitof 2060-670 Msolar, depending on the actual accretion rate ofthe black hole and the distance to 47Tuc. For NGC 6397, which is closerto us, we derive a 3σ upper limit of 1290-390 Msolar.While estimating mass upper limits based on radio continuum observationsrequires making assumptions about the gas density and the accretion rateof the black hole, their derivation does not require complex andtime-consuming dynamical modelling. Thus, this method offers anindependent way of estimating black hole masses in nearby globularclusters. If, generally, central black holes in stellar systems accretematter faster than 0.1 per cent of the Bondi accretion rate, then theseresults indicate the absence of black holes in these globular clusterswith masses as predicted by the extrapolatedM•-σc relation.
| An ultraluminous X-ray microquasar in NGC5408?
We studied the radio source associated with the ultraluminous X-raysource in NGC5408 (LX ~ 1040ergs-1).The radio spectrum is steep (index ~ -1), consistent with optically thinsynchrotron emission, not with flat-spectrum core emission. Its fluxdensity (~0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same inthe March 2000 and December 2004 observations, suggesting steadyemission rather than a transient outburst. However, it is orders ofmagnitude higher than expected from steady jets in stellar-massmicroquasar. Based on its radio flux and spectral index, we suggest thatthe radio source is either an unusually bright supernova remnant, or,more likely, a radio lobe powered by a jet from the black hole (BH).Moreover, there is speculative evidence that the source is marginallyresolved with a radius ~30 pc. A faint HII region of similar sizeappears to coincide with the radio and X-ray sources, but its ionizationmechanism remains unclear. Using a self-similar solution for theexpansion of a jet-powered electron-positron plasma bubble, in theminimum-energy approximation, we show that the observed flux and(speculative) size are consistent with an average jet power ~ 7 ×1038ergs-1 ~ 0.1LX ~0.1LEdd, an age ~105 yr, a current velocity ofexpansion ~80 km s-1. We briefly discuss the importance ofthis source as a key to understand the balance between luminosity andjet power in accreting BHs.
| The dipole anisotropy of the 2 Micron All-Sky Redshift Survey
We estimate the acceleration on the Local Group (LG) from the 2 MicronAll-Sky Redshift Survey (2MRS). The sample used includes about 23200galaxies with extinction-corrected magnitudes brighter thanKs= 11.25 and it allows us to calculate the flux-weighteddipole. The near-infrared flux-weighted dipoles are very robust becausethey closely approximate a mass-weighted dipole, bypassing the effectsof redshift distortions and require no preferred reference frame. Thisis combined with the redshift information to determine the change indipole with distance. The misalignment angle between the LG and thecosmic microwave background (CMB) dipole drops to 12°+/- 7° ataround 50h-1Mpc, but then increases at larger distances,reaching 21°+/- 8° at around 130h-1Mpc. Exclusion ofthe galaxies Maffei 1, Maffei 2, Dwingeloo 1, IC342 and M87 brings theresultant flux dipole to 14°+/- 7° away from the CMB velocitydipole. In both cases, the dipole seemingly converges by60h-1Mpc. Assuming convergence, the comparison of the 2MRSflux dipole and the CMB dipole provides a value for the combination ofthe mass density and luminosity bias parametersΩ0.6m/bL= 0.40 +/- 0.09.
| 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.
| Planetary nebulae as tracers of galaxy stellar populations
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
| Morphological classification of nearby galaxies based on asymmetry and luminosity concentration
We investigate the behaviour of the asymmetry parameter A as amorphological parameter using a `volume-limited' sample of 349 galaxies(distance <=25Mpc,MV<=-18.5mag) and a largermagnitude-limited sample of 707 nearby galaxies. We confirm thecorrelation of A with morphological type. The late-type galaxies (Sdm,Sm and Im) have larger A than early-type galaxies, and they tend to havelarger A than spiral galaxies. We investigate the usefulness of the Aversus concentration index Cin diagram as a tool for theregular-irregular and early-late classification. The diagram is not veryuseful to the regular versus late-type irregular classification, asinferred previously, but it is found to be useful to the early-lateclassification.
| Self-consistent response of a galactic disc to vertical perturbations
We study the self-consistent, linear response of a galactic disc tovertical perturbations, as induced, say, by a tidal interaction. Wecalculate the self-gravitational potential corresponding to anon-axisymmetric, self-consistent density response of the disc using theGreen's function method. The response potential is shown to oppose theperturbation potential because the self-gravity of the disc resists theimposed potential, and this resistance is stronger in the inner parts ofa galactic disc. For the m= 1 azimuthal wavenumber, the disc responseopposes the imposed perturbation up to a radius that spans a range of4-6 disc scalelengths, so that the disc shows a net warp only beyondthis region. This physically explains the well known but so farunexplained observation that warps typically set in beyond this range ofradii. We show that the inclusion of a dark matter halo in thecalculation only marginally changes (by ~10 per cent) the radius for theonset of warps. For perturbations with higher azimuthal wavenumbers, thenet signature of the vertical perturbations can only be seen at largerradii - for example, beyond 7 exponential disc scalelengths for m= 10.Also, for the high-m cases, the magnitude of the negative disc responsedue to the disc self-gravity is much smaller. This is shown to result incorrugations of the mid-plane density, which explains the puzzlingscalloping with m= 10 detected in HI in the outermost regions ~30 kpc inthe Galaxy.
| Investigating the Andromeda stream - I. Simple analytic bulge-disc-halo model for M31
This paper is the first in a series which studies interactions betweenM31 and its satellites, including the origin of the giant southernstream. We construct accurate yet simple analytic models for thepotential of the M31 galaxy to provide an easy basis for the calculationof orbits in M31's halo. We use a Navarro, Frenk and White (NFW) darkhalo, an exponential disc, a Hernquist bulge, and a central black holepoint mass to describe the galaxy potential. We constrain the parametersof these functions by comparing to existing surface-brightness,velocity-dispersion, and rotation-curve measurements of M31. Ourdescription provides a good fit to the observations, and agrees wellwith more sophisticated modelling of M31. While in many respects theparameter set is well constrained, there is substantial uncertainty inthe outer halo potential and a near-degeneracy between the disc and halocomponents, producing a large, nearly two-dimensional allowed region inparameter space. We limit the allowed region using theoreticalexpectations for the halo concentration, baryonic content, and stellarmass-to-light ratio (M/LR), finding a smaller region wherethe parameters are physically plausible. Our proposed mass model for M31has Mbulge= 3.2 × 1010Msolar,Mdisc= 7.2 × 1010Msolar, andM200= 7.1 × 1011Msolar, withuncorrected (for internal and foreground extinction) mass-to-lightratios of M/LR= 3.9 and 3.3 for the bulge and disc,respectively. We present some illustrative test-particle orbits for theprogenitor of the stellar stream in our galaxy potential, highlightingthe effects of the remaining uncertainty in the disc and halo masses.
| A dynamical model for the extraplanar gas in spiral galaxies
Recent HI observations reveal that the discs of spiral galaxies aresurrounded by extended gaseous haloes. This extraplanar gas reacheslarge distances (several kiloparsecs) from the disc and shows peculiarkinematics (low rotation and inflow). We have modelled the extraplanargas as a continuous flow of material from the disc of a spiral galaxyinto its halo region. The output of our models is pseudo data cubes thatcan be directly compared to the HI data. We have applied these models totwo spiral galaxies (NGC 891 and NGC 2403) known to have a substantialamount of extraplanar gas. Our models are able to reproduce accuratelythe vertical distribution of extraplanar gas for an energy inputcorresponding to a small fraction (<4 per cent) of the energyreleased by supernovae. However, they fail in two important aspects: (1)they do not reproduce the right gradient in rotation velocity; (2) theypredict a general outflow of the extraplanar gas, contrary to what isobserved. We show that neither of these difficulties can be removed ifclouds are ionized and invisible at 21cm as they leave the disc butbecome visible at some point on their orbits. We speculate that thesefailures indicate the need for accreted material from the intergalacticmedium that could provide the low angular momentum and inflow required.
| 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.
| On the origin of warps and the role of the intergalactic medium
There is still no consensus as to what causes galactic discs to becomewarped. Successful models should account for the frequent occurrence ofwarps in quite isolated galaxies, their amplitude as well as theobserved azimuthal and vertical distributions of the HI layer.Intergalactic accretion flows and intergalactic magnetic fields may bendthe outer parts of spiral galaxies. In this paper we consider theviability of these non-gravitational torques to take the gas off theplane. We show that magnetically generated warps are clearly flawedbecause they would wrap up into a spiral in less than two or threegalactic rotations. The inclusion of any magnetic diffusivity to dilutethe wrapping effect causes the amplitude of the warp to damp. We alsoconsider the observational consequences of the accretion of anintergalactic plane-parallel flow at infinity. We have computed theamplitude and warp asymmetry in the accretion model, for a disc embeddedin a flattened dark matter halo, including self-consistently thecontribution of the modes with azimuthal wavenumbers m= 0 and m= 1.Since the m= 0 component, giving a U-shaped profile, is not negligiblecompared to the m= 1 component, this model predicts quite asymmetricwarps, maximum gas displacements on the two sides in the ratio 3 : 2 forthe preferred Galactic parameters, and the presence of a fraction ~3.5per cent of U-shaped warps, at least. The azimuthal dependence of themoment transfer by the ram pressure would produce a strong asymmetry inthe thickness of the HI layer and asymmetric density distributions in z,in conflict with observational data for the warp in our Galaxy and inexternal galaxies. The amount of accretion that is required to explainthe Galactic warp would give gas scaleheights in the far outer disc thatare too small. We conclude that accretion of a flow with no net angularmomentum cannot be the main and only cause of warps.
| Georges Lemaitre, fondateur de la cosmologie moderne.
Not Available
| Beyond the big Galaxy: the structure of the stellar system 1900 - 1952
Not Available
| Scale Heights of Non-Edge-on Spiral Galaxies
We present a method of calculating the scale height of non-edge-onspiral galaxies, together with a formula for errors. The method is basedon solving Poisson's equation for a logarithmic disturbance of matterdensity in spiral galaxies. We show that the spiral arms can not extendto inside the ``forbidden radius'' r0, due to the effect ofthe finite thickness of the disk. The method is tested by re-calculatingthe scale heights of 71 northern spiral galaxies previously calculatedby Ma, Peng & Gu. Our results differ from theirs by less than 9%. Wealso present the scale heights of a further 23 non-edge-on spiralgalaxies.
| Near-Infrared [Fe II] Emission in Starburst Galaxies. I. Measured Properties
We used the near-infrared [Fe II] emission line signature to detectsupernova remnants (SNRs) in the nearby starburst galaxies NGC 1569, NGC3738, and NGC 5253. The near-infrared narrowband imaging program has ledto the detection of 10 SNR candidates in NGC 1569, 7 in NGC 5253, andnone in NGC 3738. The luminosity of the SNRs candidates varies from 72to 780 Lsolar and from 69 to 331 Lsolar for NGC1569 and NGC 5253, respectively. Also, a spatially extended component tothe [Fe II] line emission is observed in NGC 1569 and NGC 5253. Thiscomponent dominates the integrated [Fe II] luminosity in both galaxies,the compact sources accounting for 14% and 7% of the total [Fe II]luminosity of NGC 1569 and NGC 5253, respectively.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| A FUSE Survey of High-Latitude Galactic Molecular Hydrogen
Measurements of molecular hydrogen (H2) column densities arepresented for the first six rotational levels (J=0-5) for 73extragalactic targets observed with the Far Ultraviolet SpectroscopicExplorer (FUSE). All of these have a final signal-to-noise ratio largerthan 10 and are located at Galactic latitude |b|>20deg.The individual observations were calibrated with the FUSE calibrationpipeline CalFUSE version 2.1 or higher and then carefully aligned invelocity. The final velocity shifts for all the FUSE segments arelisted. H2 column densities or limits are determined for thesix lowest rotational (J) levels for each H I component in the line ofsight, using a curve-of-growth approach at low column densities(<16.5) and Voigt-profile fitting at higher column densities.Detections include 65 measurements of low-velocity H2 in theGalactic disk and lower halo. Eight sight lines yield nondetections forGalactic H2. The measured column densities range fromlogN(H2)=14 to 20. Strong correlations are found betweenlogN(H2) and T01, the excitation temperature ofthe H2, as well as between logN(H2) and the levelpopulation ratios (log[N(J')/N(J)]). The average fraction ofnuclei in molecular hydrogen [f(H2)] in each sight line iscalculated; however, because there are many H I clouds in each sightline, the physics of the transition from H I to H2 cannot bestudied. Detections also include H2 in 16intermediate-velocity clouds in the Galactic halo (out of 35 IVCs).Molecular hydrogen is seen in one high-velocity cloud (the Leading Armof the Magellanic Stream), although 19 high-velocity clouds areintersected; this strongly suggests that dust is rare or absent in theseobjects. Finally, there are five detections of H2 in externalgalaxies.
| Understanding Radio-selected Thermal Sources in M33: Ultraviolet, Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations
We present ultraviolet, optical, near-infrared, Spitzer mid-infrared,and radio images of 14 radio-selected objects in M33. These objects arethought to represent the youngest phase of star cluster formation. Wehave detected the majority of cluster candidates in M33 at allwavelengths. From the near-IR images, we derived ages 2-10 Myr,KS-band extinctions (AKS) of 0-1 mag,and stellar masses of 103-104 Msolar.We have generated spectral energy distributions (SEDs) of each clusterfrom 0.1 to 160 μm. From these SEDs, we have modeled the dustemission around these star clusters to determine the dust masses(1-103 Msolar) and temperatures (40-90 K) of theclusters' local interstellar medium. Extinctions derived from theJHKS, Hα, and UV images are similar to within a factorof 2 or 3. These results suggest that 11 of the 14 radio-selectedobjects are optically visible young star clusters with a surrounding HII region, that 2 are background objects, possibly active galacticnuclei (AGNs), and that 1 is a Wolf-Rayet star with a surrounding H IIregion.
| The Stellar Halo and Outer Disk of M33
We present first results from a Keck DEIMOS spectroscopic survey of redgiant branch (RGB) stars in M33. The radial velocity distributions ofthe stars in our fields are well described by three Gaussian components,corresponding to a candidate halo component with an uncorrected radialvelocity dispersion of σ~=50 km s-1, a candidate diskcomponent with a dispersion σ~=16 km s-1, and a thirdcomponent offset from the disk by ~50 km s-1, but for whichthe dispersion is not well constrained. By comparing our data to a modelof M33 based on its H I rotation curve, we find that the stellar disk isoffset in velocity by ~25 km s-1 from the H I disk,consistent with the warping that exists between these components. Thespectroscopic metallicity of the halo component is [Fe/H]~=-1.5,significantly more metal-poor than the implied metallicity of the diskpopulation ([Fe/H]~=-0.9), which also has a broader color dispersionthan the halo population. These data represent the first detections ofindividual stars in the halo of M33 and, despite being ~10 times lessmassive than M31 or the Milky Way, all three of these disk galaxies havestellar halo components with a similar metallicity. The colordistribution of the third component is different from the disk and thehalo but is similar to that expected for a single, coeval, stellarpopulation, and could represent a stellar stream. More observations arerequired to determine the true nature of this intriguing third kinematiccomponent in M33.
| Modeling the Pan-Spectral Energy Distribution of Starburst Galaxies. II. Control of the H II Region Parameters
We examine, from a theoretical viewpoint, how the physical parameters ofH II regions are controlled in both normal galaxies and in starburstenvironments. These parameters are the H II region luminosity function,the time-dependent size, the covering fraction of molecular clouds, thepressure in the ionized gas, and the ionization parameter. The factorsthat control them are the initial mass function (IMF) of the excitingstars, the cluster mass function, the metallicity, and the mean pressurein the surrounding interstellar medium. We investigate the sensitivityof the Hα luminosity to the IMF, and find that this can translateto more than a factor 2 variation in derived star formation rates. Themolecular cloud dissipation timescale is estimated from a case study ofM17 to be ~1 Myr for this object. Based on H II luminosity functionfitting for nearby galaxies, we suggest that the H II region clustermass function is fitted by a lognormal form peaking at ~100Msolar. The cluster mass function continues the stellar IMFto a higher mass regime. The pressure in the H II regions is controlledby the mechanical luminosity flux from the central cluster. Since thisis closely related to the ionizing photon flux, we show that theionization parameter is not a free variable, and that the diffuseionized medium may be composed of many large, faint, and old H IIregions. Finally, we derive theoretical probability distributions forthe ionization parameter as a function of metallicity and compare theseto those derived for SDSS galaxies.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
