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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.
| Broadband Imaging of a Large Sample of Irregular Galaxies
We present the results of UBV imaging of a large sample of irregulargalaxies: 94 Im systems, 24 blue compact dwarfs (BCDs), and 18 Smgalaxies. We also include JHK imaging of 42 of these galaxies. Thesample spans a large range in galactic parameters. Ellipse fit axialratios, inclinations, and position angles are derived, integratedphotometry and azimuthally averaged surface photometry profiles aredetermined, and exponential fits give the central surface brightnesses,scale lengths, and isophotal and half-power radii. These data are usedto address the shapes of Im galaxies, look for clues to pastinteractions in large-scale peculiarities, examine the nature andconsequences of bars, study color gradients and large-scale colorvariations, and compare the exponential disk profiles of the young andold stellar components. For example, color gradients exhibit a greatvariety and not all passbands are correlated. Bars are associated withhigher star formation rates. Many irregulars show a double-exponentialradial light profile that is steeper in the outer parts, and these arereproduced by a new model of star formation that is discussed in acompanion paper. Some galaxies, primarily BCDs, have double exponentialsthat are steeper (and bluer) in the inner parts, presumably fromcentralized star formation. Im-type galaxies have thicker, lessprominent dust layers than spiral galaxies because of their loweraverage surface densities and midplane extinctions.
| On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
| Hot Dust and Polycyclic Aromatic Hydrocarbon Emission at Low Metallicity: A Spitzer Survey of Local Group and Other Nearby Dwarf Galaxies
We present Spitzer 4.5 and 8.0 μm imaging of 15 Local Group andnearby dwarf galaxies. We find that the diffuse 8 μm emission isspatially correlated with regions of active star formation. Our samplespans a range of >1 dex in nebular metallicity and 3 orders ofmagnitude in current star formation rate, allowing us to examine thedependence of emission from hot dust and PAHs on these parameters. Wedetect prominent diffuse 8 μm emission from the four most luminousgalaxies in the sample (IC 1613, IC 5152, NGC 55, and NGC 3109) and onlyvery low surface brightness emission from four others (DDO 216, SextansA, Sextans B, and WLM). These are the first spatially resolved images ofdiffuse 8 μm emission from such low-metallicity objects[12+log(O/H)~7.5]. We observe correlations of this emission with thecurrent star formation rate and the nebular metallicity of thesegalaxies. However, we also see evidence suggesting that other processesmay also have a significant effect on the generation of this emission.These systems all have evidence for old and intermediate-age starformation; thus, the lack of diffuse 8 μm emission cannot beattributed to low galaxy ages. Also, winds cannot explain the paucity ofthis emission, since high-resolution imaging of the neutral gas in theseobjects shows no evidence of blowout. We propose that the lack ofdiffuse 8 μm emission in low-metallicity systems may be due to thedestruction of dust grains by supernova shocks, assuming a longtimescale to regrow dust. It is likely that the observed weak emissionis at least partly due to a general absence of dust (including PAHs), inagreement with their low metallicities.
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| A Local Group Polar Ring Galaxy: NGC 6822
Star counts obtained from a 2° × 2° area centered on NGC6822 have revealed an optical image of this galaxy composed of twocomponents: in addition to the well-known H I disk with its youngstellar component, there is a spheroidal stellar structure as extensiveas its H I disk, but with its major axis at roughly right angles to it,that we traced to at least 36'. Radial velocities of over 100intermediate-age carbon stars found within this structure displaykinematics contrasting strongly with those of the H I disk. These Cstars belong to the spheroid. Although devoid of gas, the spheroidrotation is consistent with the I-band Tully-Fisher relation. Theorientation of the rotation axis that minimizes the stellar velocitydispersion coincides with the minor axis of the stellar populationellipsoid, lying very nearly in the plane of the H I disk. We concludethat the H I disk is a polar ring and that the spheroidal component isan erstwhile disk, a fossil remainder of a past close encounter episode.Based on observations obtained with MegaPrime/MegaCam, a joint projectof CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT),which is operated by the National Research Council (NRC) of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique (CNRS) of France, and the University of Hawaii.Based on observations acquired at the du Pont Telescope, from theObservatories of the Carnegie Institution of Washington.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| Weak redshift discretisation in the Local Group of galaxies?
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
| Mid-Infrared Images of Stars and Dust in Irregular Galaxies
We present mid-IR to optical properties of 22 representative irregulargalaxies: 18 irregular (Im) galaxies, 3 blue compact dwarfs, and 1Magellanic-type spiral galaxy. The mid-IR is based on images from theSpitzer Space Telescope archives. The 3.6 and 4.5 μm bands and theUBVJHK images are used to examine disk morphology and the integrated andazimuthally averaged magnitudes and colors of stars. The nonstellarcontribution to the 4.5 μm images is used to trace hot dust. The 5.8and 8.0 μm images reveal emission from hot dust and polycyclicaromatic hydrocarbons (PAHs), and both may contribute to thesepassbands, although we refer to the nonstellar emission as PAH emission.We compare the 8.0 μm images to Hα. Im galaxies have no hiddenbars, and those with double-exponential optical light profiles have thesame at mid-IR. Most galaxies have similar optical and mid-IR scalelengths. Four galaxies have super star clusters that are not visible atoptical bands. Galaxies with higher area-normalized star formation rateshave more dust and PAH emission relative to starlight. Hot dust and PAHemission is found mostly in high surface brightness H II regions,implying that massive stars are the primary source of heating. Galaxieswith intense, widespread star formation have more extended PAH emission.The ratio of PAH to Hα emission is not constant on small scales.PAHs are associated with shells and giant filaments, so they are notdestroyed during shell formation.This work is based in part on archival data obtained with the SpitzerSpace Telescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under a contract with NASA.
| 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
| Neutral Hydrogen Clouds Near Early-Type Dwarf Galaxies of the Local Group
Parkes neutral hydrogen 21 cm line (H I) observations of thesurroundings of nine early-type Local Group dwarfs are presented. Wedetected numerous H I clouds in the general direction of those dwarfs,and these clouds are often offset from the optical center of thegalaxies. Although all the observed dwarfs, except Antlia, occupyphase-space regions where the high-velocity cloud (HVC) density is wellabove average, the measured offsets are smaller than one would expectfrom a fully random cloud distribution. Possible association is detectedfor 11 of the 16 investigated clouds, while for two galaxies, Sextansand Leo I, no H I was detected. The galaxies in which H I clouds werefound not to coincide with the optical yet have a significantprobability of being associated are the Sculptor dwarf, Tucana, LGS 3,Cetus, and Fornax. If the clouds are indeed associated, these galaxieshave H I masses of MHI=2×105,2×106, 7×105, 7×105,and 1×105 Msolar, respectively. However,neither ram pressure nor tidal stripping can easily explain the offsets.In some cases, large offsets are found where ram pressure should be theleast effective.
| 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.
| The C star population of DDO 190
We have carried out deep R, I, CN, TiO observations of the dwarfirregular galaxy DDO 190. We confirm the existence of anintermediate-age population around this galaxy. The identification of 47carbon stars seen up to 5 arcmin from the centre of the galaxy impliesthat the population distribution of DDO 190 is similar to those found insome other Local Group dIrr galaxies. An estimate of the metallicity,[Fe/H] = -1.55 ± 0.12, is obtained based on the observed C/Mratio. From the analysis of star counts, corrected for the radialvariation of the incompleteness level, we determine a scale-lengthα = 40 ± 5'', in agreement with the recent literature.
| Some astronomical niches with 3D spectroscopy
An overview of some of the most interesting results obtained with theuse of 3D spectrometers working in 4m-class telescopes is given with thepurpose of taking advantage of those experiences in the definition ofscientific programs for telescopes of larger diameter as the GTC.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| A Dynamical Model for the Orbit of the Andromeda Galaxy M31 and the Origin of the Local Group of Galaxies
We propose a new model for the origin and evolution of the Local Groupof Galaxies (LGG) that naturally explains the formation of theMagellanic Clouds and their large orbital angular momenta around theGalaxy. The basic idea is that an off-center hydrodynamical collisionoccurred some 10Gyr ago between the primordial Andromeda galaxy (M31)and a similar Galaxy, and compressed the halo gas to form the LGG dwarfgalaxies, including the Magellanic Clouds. New-born dwarf galaxies canbe expected to locate on the orbital plane of these two massivegalaxies. We reexamined the two-dimensional sky distribution of the LGGmembers, and confirmed an early idea that they align along two similargreat circles. The planes of these circles are approximately normal tothe line joining the present position of the Sun and the galacticcenter. We made a distribution map of these objects, and found awell-defined plane of finite thickness. Thus we could determine theorbital elements of M31 relative to the Galaxy by reproducing thewell-studied dynamics of the LMC and the SMC around the Galaxy. Theexpected proper motion of M31 is (μl, μb) =(38 ± 16 μas yr-1, -49 ± 5 μasyr-1).
| Measuring improved distances to nearby galaxies: Thae Araucaria project.
Not Available
| The Local Group Census: searching for planetary nebulae in IC 1613, WLM and GR8
In the framework of the Local Group Census (LGC), a survey of the LocalGroup (LG) galaxies above Dec =-30° aimed at surveying thepopulations that have strong emission lines, we have searched forplanetary nebulae (PNe) in the low-metallicity dwarf irregular galaxiesIC 1613, WLM, GR 8. Two new candidate PNe have been found in IC 1613,one in WLM and none in GR 8. The observations presented in this paper,together with the previous results from the LGC, represent the firststep in the study of the PN population in low-metallicity, dwarfirregular galaxies of the Local Group. These observations will befollowed by deep spectroscopy to confirm the nature of these objects andto study their physical-chemical properties. We use the observed numberof PNe in each LG galaxy to estimate a lower limit to the mass of theintermediate-age population, which is compared with the star formationrate (SFR) of LG dwarf galaxies. These results are in agreement withthose from accurate star formation history (SFH) analyses for thesesmall galaxy systems.
| The dwarf low surface brightness galaxy population of the Virgo Cluster - II. Colours and HI line observations
In order to investigate the nature of dwarf low surface brightness (LSB)galaxies we have undertaken a deep B- and I-band CCD survey of a14-deg2 strip in the Virgo Cluster and applied a Fourierconvolution technique to explore its dwarf galaxy population down to acentral surface brightness of ~26 B magarcsec-2 and a totalabsolute B mag of ~-10. In this paper we carry out an analysis of theirmorphology, (B-I) colours and atomic hydrogen content. We compare theseproperties with those of dwarf galaxies in other environments to try andassess how the cluster environment has influenced their evolution. Fielddwarfs are generally of a more irregular morphology, are bluer andcontain relatively more gas. We assess the importance that variousphysical processes have on the evolution of cluster dwarf galaxies(ram-pressure stripping, tidal interactions, supernova-driven gas loss).We suggest that enhanced star formation triggered by tidal interactionsis the major reason for the very different general properties of clusterdwarfs: they have undergone accelerated evolution.
| The galaxy luminosity function from MR=-25 to MR=-9
Redshift surveys such as the Sloan Digital Sky Survey (SDSS) have givena very precise measurement of the galaxy luminosity function down toabout MR=-17 (~MB=-16). Fainter absolutemagnitudes cannot be probed because of the flux limit required forspectroscopy. Wide-field surveys of nearby groups using mosaic CCDs onlarge telescopes are able to reach much fainter absolute magnitudes,about MR=-10. These diffuse, spiral-rich groups are thoughtto be typical environments for galaxies, so their luminosity functionsshould be the same as the field luminosity function. The luminosityfunction of the groups at the bright end (MR < -17) islimited by Poisson statistics and is far less precise than that derivedfrom redshift surveys. Here we combine the results of the SDSS and thesurveys of nearby groups, and we supplement the results with studies ofLocal Group galaxies in order to determine the galaxy luminosityfunction over the entire range -25 < MR < -9. Theaverage logarithmic slope of the field luminosity function betweenMR=-19 and MR=-9 is α=-1.26, although asingle power law is a poor fit to the data over the entire magnituderange. We also determine the luminosity function of galaxy clusters anddemonstrate that it is different from the field luminosity function at ahigh level of significance; there are many more dwarf galaxies inclusters than in the field, due to a rise in the cluster luminosityfunction of α~-1.6 between MR=-17 andMR=-14.
| Mass-to-light ratio gradients in early-type galaxy haloes
Owing to the fact that the near future should see a rapidly expandingset of probes of the halo masses of individual early-type galaxies, weintroduce a convenient parameter for characterizing the halo masses fromboth observational and theoretical results:∇lΥ, the logarithmic radial gradient of themass-to-light ratio. Using halo density profiles from Λ-cold darkmatter (CDM) simulations, we derive predictions for this gradient forvarious galaxy luminosities and star formation efficienciesɛSF. As a pilot study, we assemble the available∇lΥ data from kinematics in early-type galaxies- representing the first unbiased study of halo masses in a wide rangeof early-type galaxy luminosities - and find a correlation betweenluminosity and ∇lΥ, such that the brightestgalaxies appear the most dark-matter dominated. We find that thegradients in most of the brightest galaxies may fit in well with theΛCDM predictions, but that there is also a population of faintergalaxies whose gradients are so low as to imply an unreasonably highstar formation efficiency ɛSF > 1. This difficultyis eased if dark haloes are not assumed to have the standard ΛCDMprofiles, but lower central concentrations.
| Formation and evolution of the Magellanic Clouds - I. Origin of structural, kinematic and chemical properties of the Large Magellanic Cloud
We investigate the dynamical and chemical evolution of the LargeMagellanic Cloud (LMC) interacting with the Galaxy and the SmallMagellanic Cloud (SMC) based on a series of self-consistentchemodynamical simulations. Our numerical models are aimed at explainingthe entire properties of the LMC, i.e. the observed structure andkinematics of its stellar halo and disc components as well as thepopulations of the field stars and star clusters. The main results ofthe present simulations are as follows.Tidal interaction between the Clouds and the Galaxy during the last 9Gyr has transformed the initially thin, non-barred LMC disc into threedifferent components: central bar, thick disc and kinematically hotstellar halo. The central bar is composed of both old field stars andnewly formed ones, with the two fractions being equal in its innermostpart. The final thick disc has central velocity dispersion of ~30 kms-1 and shows rotationally supported kinematics withVm/σ0~ 2.3.The stellar halo is formed during the interaction, and consists mainlyof old stars originating from the outer part of the initially thin LMCdisc. The outer halo shows velocity dispersion of ~40 km s-1at a distance of 7.5 kpc from the LMC centre and has a somewhatinhomogeneous distribution of stars. The stellar halo containsrelatively young, metal-rich stars with a mass fraction of 2 per cent.Repetitive interaction between the Clouds and the Galaxy has moderatelyenhanced the star formation rate to ~0.4 Msolaryr-1 in the LMC disc. Most of the new stars (~90 per cent)are formed within the central 3 kpc of the disc, in particular, withinthe central bar for the last 9 Gyr. Consequently, the half-mass radiusis different by a factor of 2.3 between old field stars and newly formedones.Efficient globular cluster formation does not occur until the LMC startsinteracting violently and closely with the SMC (~3 Gyr ago). The newlyformed globular cluster system has a disc-like distribution withrotational kinematics, and its mean metallicity is ~1.2 higher than thatof new field stars because of pre-enrichment by the formation of fieldstars prior to cluster formation.The LMC evolution depends on its initial mass and orbit with respect tothe Galaxy and the SMC. In particular, the epoch of the bar and thickdisc formation and the mass fraction of the stellar halo depend on theinitial mass of the LMC.Based on these results, we discuss the entire formation history of theLMC, the possible fossil records of past interaction between the Cloudsand the Galaxy, and the star formation history of the SMC for the pastseveral Gyr.
| The Stellar Structure of Irregular Galaxies. Face-on Galaxies
Stellar photometry of nearby irregular galaxies of the Local Group isused to identify and study the young and old stellar populations ofthese galaxies. An analysis of the spatial distributions of stars ofdifferent ages in face-on galaxies shows that the young stellarpopulations in irregular galaxies are concentrated toward the center,and form local inhomogeneities in star-forming regions, while the oldstellar populationsred giantsform extended structuresaround the irregular galaxies. The sizes of these structures exceed thevisible sizes of the galaxies at the 25m2 isophote by a factor of two tothree. The surface density of the red giants decreases exponentiallyfrom the center toward the edge, similar to the disk components inspiral galaxies.
| The Baryonic Tully-Fisher Relation of Galaxies with Extended Rotation Curves and the Stellar Mass of Rotating Galaxies
I investigate the baryonic Tully-Fisher relation for a sample ofgalaxies with extended 21 cm rotation curves spanning the range 20 kms-1<~Vf<=300 km s-1. A variety ofscalings of the stellar mass-to-light ratio Υ* areconsidered. For each prescription for Υ*, I give fitsof the form Md=AVxf.Presumably, the prescription that comes closest to the correct valuewill minimize the scatter in the relation. The fit with minimum scatterhas A=50 Msolar km-4 s4 andx=4. This relation holds over five decades in mass. Galaxy color,stellar fraction, and Υ* are correlated with eachother and with Md, in the sense that more massivegalaxies tend to be more evolved. There is a systematic dependence ofthe degree of maximality of disks on surface brightness. High surfacebrightness galaxies typically have Υ*~3/4 of themaximum disk value, while low surface brightness galaxies typicallyattain ~1/4 of this amount.
| Formation Histories of Dwarf Galaxies in the Local Group
We compare the properties of dwarf galaxies in the Local Group with thesimulated galaxies formed before reionization in a cosmologicalsimulation of unprecedented spatial and mass resolution, includingradiative feedback effects. We find that a subset of the Local Groupdwarfs are already remarkably similar to the simulated dwarf galaxies inall their properties before reionization. On the basis of thissimilarity, we propose the hypothesis that Local Group dwarfs form in avariety of ways: some of them are ``true fossils'' of thepre-reionization era, some of them form most of their stars later, afterreionization (we call them ``survivors'' of the reionization era), andthe rest of them form an intermediate group of ``polluted fossils.'' Wealso identify a simple observational test that is able to test ourhypothesis.
| The Distance and Metallicity of the Newly Discovered, Nearby Irregular Galaxy HIZSS 3
HIZSS 3 is an H I source in the Zone of Avoidance. Its radiocharacteristics are consistent with it being a previously unknown,nearby (~1.8 Mpc), low-mass dwarf irregular (dIrr) galaxy. Opticalobservations have shown that it contains a modest H II region, but theyfailed to reveal a resolved stellar population. New spectroscopicobservations of the H II region obtained at the MMT Observatory arepresented here. They are used to derive the line-of-sight extinction[E(B-V)=1.41+/-0.04] and gas metallicity (logO/H+12~7.8) of the H IIregion. New near-IR imaging observations obtained at the ESO Very LargeTelescope are also presented here. These images clearly reveal theresolved stellar population of HIZSS 3 for the first time. NarrowbandPaβ images of the H II region are used in combination withpreviously published Hα data to obtain an independentline-of-sight extinction estimate: E(B-V)=1.32+/-0.04. The adoptedforeground extinction is E(B-V)=1.36+/-0.06. Using the K-band luminosityfunction and K,J-K color-magnitude diagram, the apparent magnitude andcolor of the tip of the red giant branch are derived. In turn, theseparameters are combined with the adopted foreground extinction toestimate the distance (1.69+/-0.07 Mpc) and mean red giant branchmetallicity ([Fe/H]=-0.5+/-0.1). As an ensemble, these new observationssignificantly strengthen the conclusion that HIZSS 3 is a newlydiscovered low-mass dIrr galaxy lurking behind the Milky Way in theoutskirts of the Local Group.The optical spectroscopic observations reported here were obtained atthe MMT Observatory, a joint facility of the Smithsonian Institution andthe University of Arizona. The near-IR imaging observations reportedhere were collected at the European Southern Observatory, Cerro Paranal,Chile, within observing program 271.B-5047.
| Mass Modeling of Disk Galaxies: Degeneracies, Constraints, and Adiabatic Contraction
This paper addresses available constraints on mass models fitted torotation curves. Mass models of disk galaxies have well-knowndegeneracies that prevent a unique mass decomposition. The most notableis due to the unknown value of the stellar mass-to-light ratio (thedisk-halo degeneracy); even with this known, degeneracies between thehalo parameters themselves may prevent an unambiguous determination ofthe shape of the dark halo profile, which includes the inner densityslope of the dark matter halo. The latter is often referred to as the``cusp-core degeneracy.'' We explore constraints on the disk and haloparameters and apply these to four mock and six observed disk galaxieswith high resolution and extended rotation curves. Our full set ofconstraints consists of mass-to-light (M/L) ratios from stellarpopulation synthesis models based on B-R colors, constraints on haloparameters from N-body simulations, and constraining the halo virialvelocity to be less than the maximum observed velocity. Theseconstraints are only partially successful in lifting the cusp-coredegeneracy. The effect of adiabatic contraction of the halo by the diskis to steepen cores into cusps and reduce the best-fit haloconcentration and M/L values (often significantly). We also discuss theeffect of disk thickness, halo flattening, distance errors, and rotationcurve error values on mass modeling. Increasing the imposed minimumrotation curve error from typically low, underestimated values to morerealistic estimates decreases the χ2 substantially andmakes distinguishing between a cuspy or cored halo profile even moredifficult. In spite of the degeneracies and uncertainties present, ourconstrained mass modeling favors submaximal disks (i.e., a dominanthalo) at 2.2 disk scale lengths, withVdisk/Vtot<~0.6. This result holds for both theunbarred and weakly barred galaxies in our sample.
| 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.
| 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.
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