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|The X-ray properties of the dwarf Magellanic-type galaxy NGC 55|
We present an analysis of the X-ray properties of the Magellanic-typegalaxy NGC 55 based on two contiguous XMM-Newton observations. We detecta total of 137 X-ray sources in the field of view, down to a flux of ~5× 10-15ergcm-2s-1 (0.3-6 keV), 42of which are located within the optical confines of the galaxy. On thebasis of X-ray colour classification and after correcting for backgroundobjects, we conclude that our source sample includes ~20 X-ray binaries(XRBs), five supernova remnants and seven very soft sources (includingtwo good candidate supersoft sources) associated with NGC 55. We alsodetect an X-ray source coincident with a previously identified globularcluster in NGC 55. Detailed spectral and timing analysis was carried outon four of the brightest X-ray sources (excluding the brightest source,which was the subject of a previous paper). One of these objects isidentified with a Galactic foreground star and is a possible new RS CVnsystem. The other three are persistent X-ray sources with X-ray spectrawell described by either a single absorbed power law (Γ ~ 2) or amulticolour disc blackbody (kTin ~ 1 keV) model. While theobserved luminosities of these sources (LX ~ 1-2 ×1038ergs-1) and their X-ray spectra are consistentwith accreting XRBs, further evidence of short-term variability isrequired to confirm this. Although the observed X-ray emission from NGC55 is dominated by point sources, we do find evidence of an underlyingcomponent, which is concentrated on the bar region but has an extent ofat least 6 arcmin (3 kpc) in the plane of the galaxy and +/-1 arcmin(+/-500 pc) perpendicular to it. This emission is best fitted by athermal plasma (MEKAL) (kT ~ 0.2 keV) plus power-law (Γ ~ 2) modelbut with high intrinsic absorption consistent with its location in thecentral disc of the galaxy. We interpret the soft component as diffusethermal emission linked to regions of current star formation, whilst thehard power-law component may originate in unresolved X-ray binarysources. The intrinsic luminosity of this residual disc emission mayexceed LX ~ 6 × 1038ergs-1 (0.3-6keV). A comparison with other Magellanic systems confirms that, in termsof both its discrete X-ray source population and its extended emission,NGC 55 has X-ray properties which are typical of its class.
|XMM-Newton observations of the brightest ultraluminous X-ray sources|
We present an analysis of 13 of the best quality ultraluminous X-raysource (ULX) data sets available from XMM-Newton European Photon ImagingCamera (EPIC) observations. We utilize the high signal-to-noise in theseULX spectra to investigate the best descriptions of their spectral shapein the 0.3-10keV range. Simple models of an absorbed power law ormulticolour disc blackbody prove inadequate at describing the spectra.Better fits are found using a combination of these two components, withboth variants of this model - a cool (~0.2keV) disc blackbody plus hardpower-law continuum, and a soft power-law continuum, dominant at lowenergies, plus a warm (~1.7keV) disc blackbody - providing good fits to8/13 ULX spectra. However, by examining the data above 2keV, we findevidence for curvature in the majority of data sets (8/13 with at leastmarginal detections), inconsistent with the dominance of a power law inthis regime. In fact, the most successful empirical description of thespectra proved to be a combination of a cool (~0.2keV) classic blackbodyspectrum, plus a warm disc blackbody that fits acceptably to 10/13 ULXs.The best overall fits are provided by a physically self-consistentaccretion disc plus Comptonized corona model (DISKPN + EQPAIR), whichfits acceptably to 11/13 ULXs. This model provides a physicalexplanation for the spectral curvature, namely that it originates in anoptically thick corona, though the accretion disc photons seeding thiscorona still originate in an apparently cool disc. We note similaritiesbetween this fit and models of Galactic black hole binaries at highaccretion rates, most notably the model of Done & Kubota. In thisscenario the inner disc and corona become energetically coupled at highaccretion rates, resulting in a cooled accretion disc and opticallythick corona. We conclude that this analysis of the best spectral datafor ULXs shows it to be plausible that the majority of the populationare high accretion rate stellar-mass (perhaps up to 80Msolar)black holes, though we cannot categorically rule out the presence oflarger, ~1000-Msolar intermediate-mass black holes (IMBHs) inindividual sources with the current X-ray data.
|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.
|The Disk and Extraplanar Environment of NGC 247|
The stellar content of the spiral galaxy NGC 247 is investigated usingdeep visible and near-infrared images. The main-sequence turnoff (MSTO)in the inner 12 kpc of the disk corresponds to an age of ~6 Myr. A meanstar formation rate (SFR) of 0.1 Msolar yr-1during the past 16 Myr is computed from star counts. The color of thered supergiant plume does not change with radius, suggesting that themean metallicity of young stars does not vary by more than ~0.1 dex. Thenumber of bright main-sequence stars per local stellar mass densityclimbs toward larger radii out to a distance of 12 kpc; the scalelengths that characterize the radial distributions of young and oldstars in the disk thus differ. The density of bright main-sequence starswith respect to projected H I mass gradually drops with increasingradius. The population of very young stars disappears in the outer disk;the MSTO at galactocentric radii between 12 and 15 kpc corresponds to~16 Myr, while between 15 and 18 kpc the age is >=40 Myr. Red giantbranch (RGB) stars are resolved at a projected minor-axis galactocentricdistance of ~12 kpc. There is a broad spread in metallicity among theRGB stars, with a mean [M/H]~-1.2. The RGB tip occurs ati'=24.5+/-0.1, indicating that the distance modulus is27.9+/-0.1. Luminous AGB stars with an age ~3 Gyr are also seen in thisfield.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council of Canada (Canada), CONICYT (Chile), theAustralian Research Council (Australia), CNPq (Brazil), and CONICET(Argentina).This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
|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.
|Associations of Dwarf Galaxies|
The Hubble Space Telescope Advanced Camera for Surveys has been used todetermine accurate distances for 20 galaxies from measurements of theluminosity of the brightest red giant branch stars. Five associations ofdwarf galaxies that had originally been identified based on strongcorrelations on the plane of the sky and in velocity are shown to beequally well correlated in distance. Two more associations with similarproperties have been discovered. Another association is identified thatis suggested to be unbound through tidal disruption. The associationshave the spatial and kinematic properties expected of bound structureswith (1-10)×1011 Msolar. However, theseentities have little light, with the consequence that the mass-to-lightratios are in the range 100-1000 MsolarL-1solar. Within a well-surveyed volume extendingto a 3 Mpc radius, all but one known galaxy lie within one of the groupsor associations that have been identified.
|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
|Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius|
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
|Nearby Spiral Globular Cluster Systems. I. Luminosity Functions|
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
|Remarkable Disk and Off-Nuclear Starburst Activity in the Tadpole Galaxy as revealed by the Spitzer Space Telescope|
We present ground-based optical and Spitzer Space Telescope infraredimaging observations of the interacting galaxy UGC 10214, the Tadpolegalaxy (z=0.0310), focusing on the star formation activity in thenuclear, disk, spiral arms, and tidal tail regions. The ground-basedoptical data set spans a wavelength range between 0.3 and 0.8 μm, thenear-IR data set spans 1-2.2 μm, and the Spitzer IR data set spans3-70 μm. The major findings of this study are that the Tadpole isactively forming stars in the main disk outside of the nucleus and inthe tidal plume, with an estimated mean star formation rate of ~2-4Msolar yr-1. The most prominent sites of mid-IRemission define a ``ring'' morphology that, combined with the overallmorphology of the system, suggests the interaction may belong to therare class of off-center collisional ring systems that form bothshock-induced rings of star formation and tidal plumes. In starkcontrast to the disk star formation, the nuclear emission is solelypowered by older stars, with little evidence for ongoing star formationat the center of the Tadpole. Extranuclear star formation accounts for>50% of the total star formation in the disk and spiral arms,featuring infrared-bright ``hot spots'' that exhibit strong polycyclicaromatic hydrocarbon (PAH) emission, the band strength of which iscomparable to that of late-type star-forming disk galaxies. The tidaltail, which extends 2' (~75 kpc) into the intergalactic medium, ispopulated by supermassive star clusters, M~106Msolar, likely triggered by the galaxy-galaxy interactionthat has distorted UGC 10214 into its current ``tadpole'' shape. TheTadpole is therefore an example of an off-nuclear or tidal-tailstarburst, with several large sites of massive star formation in thedisk and in the plume, including the most prominent Hubble SpaceTelescope-revealed cluster, J160616.85+552640.6. The clusters exhibitremarkable IR properties, including exceptionally strong 24 μmemission relative to the underlying starlight, hot dust continuum, andPAH emission, with an estimated current star formation rate of ~0.1-0.4Msolar yr-1, representing >10% of the totalstar formation in the system. We estimate the mass of the largestcluster to be ~(1.4-1.6)×106 Msolar based onthe g'-band (0.5 μm) and near-IR (2.2 μm) integrated fluxes incombination with an assumed mass-to-light ratio appropriate to youngclusters, or large enough to be classified as a nascent dwarf galaxy orglobular cluster.
|Distance determination to NGC 55 from the planetary nebula luminosity function|
We analysed [O III], Hα, and continuum images of the SculptorGroup spiral galaxy NGC 55 obtained with the WFI instrument at the 2.2-mtelescope of ESO. We identified 21 new planetary nebula candidates. Weconstructed the [O III]λ5007 Planetary Nebula Luminosity Function(PNLF) and determined a most likely distance of 2.30 ± 0.35 Mpc.The distance to NGC 55 is a bit larger than previously determineddistances, which means that the Sculptor Group is further away from theLocal Group than previously thought. The PNLF distance to NGC 55 iscomparable to the PNLF distance of NGC 300 (Soffner et al. 1996,A&A, 306, 9), adding support to the suggestion that these galaxiesform a bound pair. There doesn't seem to be a shortage of planetarynebula candidates in this metal poor galaxy.
|The multi-phase gaseous halos of star forming late-type galaxies. I. XMM-Newton observations of the hot ionized medium|
This study presents first results from an X-ray mini-survey carried outwith XMM-Newton to investigate the diffuse Hot Ionized Medium in thehalos of nine nearby star-forming edge-on spiral galaxies. Diffusegaseous X-ray halos are detected in eight of our targets, covering awide range of star formation rates from quiescent to starburst cases.For four edge-on spiral galaxies, namely NGC 3044, NGC 3221, NGC 4634,and NGC 5775, we present the first published high resolution/sensitivitydetections of extended soft X-ray halos. EPIC X-ray contour mapsoverlaid onto Hα imaging data reveals that in all cases thepresence of X-ray halos is correlated with extraplanar Diffuse IonizedGas. Moreover, these halos are also associated with non-thermal cosmicray halos, as evidenced by radio continuum observations. SupplementalUV-data obtained with the OM-telescope at 210 nm show Diffuse IonizedGas to be well associated with UV emission originating in the underlyingdisk. Beside NGC 891, NGC 4634 is the second non-starburst galaxy with adiffuse soft X-ray halo (|z|≤ 4 kpc). In case of NGC 3877, for whichwe also present the first high resolution X-ray imaging data, no haloemission is detectable. EPIC pn spectra (0.3-12 keV) of the diffuseX-ray emission are extracted at different offset positions from thedisk, giving evidence to a significant decrease of gas temperatures,electron densities, and gas masses with increasing distance to theplane. A comparison between dynamical and radiative cooling time scalesimplies that the outflow in all targets is likely to be sustained. Wefind very strong indications that spatially correlated multi-phasegaseous halos are created by star forming activity in the disk plane. Ina forthcoming paper, we will present multi-frequency luminosityrelations and evaluate key parameters which might trigger the formationof multi-phase galaxy halos.
|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.|
|XMM-Newton EPIC observations of the ultraluminous X-ray source NGC 5204 X-1|
We present the results of two XMM-Newton observations of theultraluminous X-ray source NGC 5204 X-1. The EPIC spectra are wellfitted by the standard spectral model of a black hole X-ray binary,comprising a soft multicolour disc blackbody component plus a harderpower-law continuum. The cool (kTin~ 0.2 keV) inner-disctemperature required by this model favours the presence of anintermediate-mass black hole in this system, though we highlight apossible anomaly in the slope of the power-law continuum in such fits.We discuss the interpretation of this and other, non-standard spectralmodelling of the data.
|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.
|Halos of Spiral Galaxies. III. Metallicity Distributions|
We report results of a campaign to image the stellar populations in thehalos of highly inclined spiral galaxies, with the fields roughly 10 kpc(projected) from the nuclei. We use the F814W (I) and F606W (V) filtersin the Wide Field Planetary Camera 2 on board the Hubble SpaceTelescope. We unambiguously resolve the stellar halos 1 to 2 mag fainterthan the tip of the red giant branch. Extended halo populations aredetected in all galaxies. The color-magnitude diagrams appear to becompletely dominated by giant branch stars, with no evidence for thepresence of young stellar populations in any of the fields. Themetallicity distribution function for the galaxy sample is derived frominterpolation within an extensive grid of red giant branch loci. Theseloci are derived from theoretical sequences that are calibrated usingthe Galactic globular clusters and from empirical sequences formetal-rich stellar populations. We find that the metallicitydistribution functions are dominated by metal-rich populations, with atail extending toward the metal-poor end. To first order, the overallshapes of the metallicity distribution functions are similar to what ispredicted by a simple, single-component model of chemical evolution withthe effective yields increasing with galaxy luminosity. However,metallicity distributions significantly narrower than the simple modelare observed for a few of the most luminous galaxies in the sample. Thediscrepancies are similar to those previously observed for NGC 5128, thehalo of M31, and the Galactic bulge. Our observations can be used tohelp distinguish between models for the formation of spiral galaxies. Itappears that more luminous spiral galaxies also have more metal-richstellar halos. The increasingly significant departures from theclosed-box model for the more luminous galaxies indicate that aparameter in addition to a single yield is required to describe chemicalevolution. This parameter, which could be related to gas infall oroutflow either in situ or in progenitor dwarf galaxies that later mergeto form the stellar halo, tends to make the metallicity distributionsnarrower at high metallicity.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|Halos of Spiral Galaxies. II. Halo Metallicity-Luminosity Relation|
Using the Hubble Space Telescope, we have resolved individual red giantbranch stars in the halos of eight nearby spiral galaxies. The fieldslie at projected distances between 2 and 13 kpc along the galaxies'minor axes. The data set allows a first look at the systematic trends inhalo stellar populations. We have found that bright galaxies tend tohave broad red giant branch star color distributions with redder meancolors, suggesting that the heavy-element abundance spread increaseswith the parent galaxy luminosity. The mean metallicity of the stellarhalo, estimated using the mean colors of red giant branch stars,correlates with the parent galaxy luminosity. The metallicity of theMilky Way halo falls nearly 1 dex below this luminosity-metallicityrelation, suggesting that the halo of the Galaxy is more the exceptionthan the rule for spiral galaxies; i.e., massive spirals with metal-poorhalos are unusual. The luminosity-halo stellar abundance relation isconsistent with the scaling relation expected for stellar systemsembedded in dominant halos, suggesting that the bulk of the halo stellarpopulation may have formed in situ.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|Halos of Spiral Galaxies. I. The Tip of the Red Giant Branch as a Distance Indicator|
We have imaged the halo populations of a sample of nearby spiralgalaxies using the Wide Field Planetary Camera 2 on board the HubbleSpace Telescope with the aim of studying the stellar populationproperties and relating them to those of the host galaxies. In fourgalaxies, the red giant branch is sufficiently well populated to measurethe magnitude of the tip of the red giant branch (TRGB), a well-knowndistance indicator. Using both the Sobel edge-detection technique andmaximum likelihood analysis to measure the I-band magnitude of the TRGB,we determine distances to four nearby galaxies: NGC 253, NGC 4244, NGC4945, and NGC 4258. For the first three galaxies, the TRGB distance ishere determined more directly, and is likely to be more accurate, thanprevious distance estimates. In the case of NGC 4258, our TRGB distanceis in good agreement with the geometrical maser distance, supporting theLarge Magellanic Cloud distance modulus (m-M)0=18.50 that isgenerally adopted in recent estimates of the Hubble constant.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|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 Emergence of the Thick Disk in a CDM Universe. II. Colors and Abundance Patterns|
The recently emerging conviction that thick disks are prevalent in diskgalaxies, and their seemingly ubiquitous old ages, means that theformation of the thick disk, perhaps more than any other component,holds the key to unravelling the evolution of the Milky Way, and indeedall disk galaxies. In Paper I, we proposed that the thick disk wasformed in an epoch of gas-rich mergers at high redshift. This hypothesiswas based on comparing N-body SPH simulations to a variety of Galacticand extragalactic observations, including stellar kinematics, ages, andchemical properties. Here we examine our thick-disk formation scenarioin light of the most recent observations of extragalactic thick disks.In agreement, our simulated thick disks are old and relatively metalrich, with V-I colors that do not vary significantly with distance fromthe plane. Furthermore, we show that our proposal results in anenhancement of α-elements in thick-disk stars as compared withthin-disk stars, consistent with observations of the relevantpopulations of the Milky Way. We also find that our scenario naturallyleads to the formation of an old, metal-weak, stellar halo populationwith high α-element abundances.
|Metallicity Effects on Mid-Infrared Colors and the 8 μm PAH Emission in Galaxies|
We examine colors from 3.6 to 24 μm as a function of metallicity(O/H) for a sample of 34 galaxies. The galaxies range over 2 orders ofmagnitude in metallicity. They display an abrupt shift in the 8μm-to-24 μm color for metallicities between one-third andone-fifth of the solar value. The mean 8-to-24 μm flux density ratiobelow and above 12+log(O/H)=8.2 is 0.08+/-0.04 and 0.70+/-0.53,respectively. We use mid-IR colors and spectroscopy to demonstrate thatthe shift is primarily due to a decrease in the 8 μm flux density, asopposed to an increase in the 24 μm flux density. This result is mostsimply interpreted as being due to a weakening at low metallicity of themid-IR emission bands usually attributed to PAHs (polycyclic aromatichydrocarbons) relative to the small-grain dust emission. However,existing empirical spectral energy distribution models cannot accountfor the observed short-wavelength (below 8 μm) colors of thelow-metallicity galaxies merely by reducing the strength of the PAHfeatures; some other emission source (e.g., hot dust) is required.
|The Disk and Extraplanar Regions of NGC 55|
The stellar content of the nearby SB(s)m galaxy NGC 55 is investigatedusing images obtained with the Gemini South and Canada-France-Hawaiitelescopes. The (K, H-K) and (K, J-K) color-magnitude diagrams (CMDs) ofstars near the plane of the disk reveal signatures of large-scale starformation during recent and intermediate epochs in the form of redsupergiants (RSGs) with MK=-11.5, and an asymptotic giantbranch (AGB) that peaks near MK=-10. Comparisons with stellarevolution models suggest that the brightest RSGs have an age near 8 Myr.A well-defined plume, which stellar evolution models suggest containsstars with masses near the RSG-AGB transition, is detected in CMDsconstructed both from infrared and visible wavelength observations. Itis concluded that star formation in the thin disk of NGC 55 has occurredat a significant rate for at least the past 0.1-0.2 Gyr, and this isconsistent with other indicators. The near-infrared spectral energydistribution of the integrated light near the center of the galaxy isconsistent with that in other Magellanic irregular galaxies, indicatingthat the star-forming history of NGC 55, when averaged over timescalesof 0.1-1 Gyr, has likely not been peculiar when compared with otherlate-type systems. Evidence is also presented that the disk contains alarge population of old [log(tyr)~10] stars, and it is arguedthat a stable disk has been in place in NGC 55 for a significantfraction of the age of the universe. At projected distances in excess of2 kpc off of the disk plane, the brightest AGB stars have ages10+3-2 Gyr. Thus, despite indications that dustand gas are present in the envelope surrounding the NGC 55 disk, the AGBcontent suggests that recently formed stars do not occur in largenumbers in the extraplanar region. The (r'-i')colors of the RGB in the extraplanar region are consistent with [Fe/H]between -2.2 and -0.7, with the majority of stars having [Fe/H]>-1.2,and the mean metallicity inferred from the RGB color does not changewith distance above the disk plane. Thus, the stellar component in theextraplanar envelope is well mixed, at least in terms of metallicity.The mean metallicity of RGB stars is in excellent agreement with thatmeasured in the extraplanar H II regions EHR 1 and 2, suggesting thatthe age-metallicity relation in this part of NGC 55 has been flat for atleast a few Gyr. Finally, the RGB tip occurs near i'=23.1 inthe extraplanar region, and a distance modulus of 26.5 is computed fromthis feature.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council of Canada (Canada), CONICYT (Chile), theAustralian Research Council (Australia), CNPq (Brazil), and CONICET(Argentina).This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
|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.
|Mapping Large-Scale Gaseous Outflows in Ultraluminous Galaxies with Keck II ESI Spectra: Variations in Outflow Velocity with Galactic Mass|
Measurements of interstellar Na I λλ5890, 5896 absorptionlines in 18 ultraluminous infrared galaxies (ULIGs) have been combinedwith published Na I data, to reassess the dependence of galactic outflowspeeds on starburst luminosity and galactic mass. The Doppler shiftsreveal outflows of relatively cool gas in 15 of 18 ULIGs with an averageoutflow speed at the line center of 330+/-100 km s-1. Therelation between outflow speed and star formation rate (SFR), defined bythe distribution's upper envelope over 4 orders of magnitude in SFR,demonstrates that winds from more luminous starbursts accelerateinterstellar gas to higher speeds roughly as v~SFR0.35. Thisresult is surprising since, in the traditional model forstarburst-driven winds, these relatively cool gas clouds are acceleratedby the ram pressure of a hot, supernova-heated wind that exhibits weak(if any) X-ray temperature variation with increasing galactic mass. Thelack of evidence for much hotter winds is partly a sensitivity issue,but the Na I velocities in ultraluminous starbursts actually areconsistent with acceleration by the tepid wind, indicating that a hottercomponent is unlikely to dominate the momentum flux. The Na I velocitiesin the dwarf starburst winds do not reach the terminal velocity of a hotwind at the measured temperature of kT~0.73 keV, a result that could beinterpreted simply as evidence that the hot superbubbles are tooconfined in dwarf starbursts to generate a free-flowing wind. Adynamically motivated scenario, however, is that the dwarf starburstwinds simply lack enough momentum to accelerate the clouds to thevelocity of the hot wind. Among the subsample of starbursts withwell-constrained dynamical masses, the terminal outflow velocities arealways found to approach the galactic escape velocity. Motivated by asimilar scaling relation for stellar winds, the galactic Eddingtonluminosity for dusty starbursts is shown to be within the range measuredfor ULIGs. If radiation pressure on dust grains, coupled to the coolwind, is indeed important for galactic wind dynamics, then feedback isstronger in massive galaxies than previously thought, helping shape thehigh-mass end of the galaxy luminosity function. Regardless of thenature of the acceleration mechanism in ULIGs, the mass flux of cool gasestimated from these data demonstrates that starburst-driven windstransport significant gas during the assembly stage of field ellipticalgalaxies, a factor that helps explain the rapid decline in SFR in thesesystems inferred from elemental abundance ratios.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
|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.
|Light-to-Mass Variations with Environment|
Large and well-defined variations exist between the distribution of massand the light of stars on extragalactic scales. Mass concentrations inthe range 1012-1013 Msolar manifest themost light per unit mass. Group halos in this range are typically thehosts of spiral and irregular galaxies with ongoing star formation. Onaverage M/LB~90 Msolar/Lsolar in thesegroups . More massive halos have less light per unit mass. Within agiven mass range, halos that are dynamically old as measured by crossingtimes and galaxy morphologies have distinctly less light per unit mass.At the other end of the mass spectrum, below 1012Msolar, there is a cutoff in the manifestation of light.Group halos in the range 1011-1012Msolar can host dwarf galaxies but with such low luminositiesthat M/LB values can range from several hundred to severalthousand. It is suspected that there must be completely dark halos atlower masses. Given the form of the halo mass function, the low relativeluminosities of the high-mass halos have the greatest cosmologicalimplications. Of order half the clustered mass may reside in halos withgreater than 1014 Msolar. By contrast, only 5%-10%of clustered mass would lie in entities with less than 1012Msolar.
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