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Period-colour and amplitude-colour relations in classical Cepheid variables - III. The Large Magellanic Cloud Cepheid models
Period-colour (PC) and amplitude-colour (AC) relations are studied forthe Large Magellanic Cloud (LMC) Cepheids under the theoreticalframework of the hydrogen ionization front (HIF)-photosphereinteraction. LMC models are constructed with pulsation codes thatinclude turbulent convection, and the properties of these models arestudied at maximum, mean and minimum light. As with Galactic models, atmaximum light the photosphere is located next to the HIF for the LMCmodels. However, very different behaviour is found at minimum light. Thelong-period (P > 10 d) LMC models imply that the photosphere isdisengaged from the HIF at minimum light, similar to the Galacticmodels, but there are some indications that the photosphere is locatednear the HIF for the short-period (P < 10 d) LMC models. We also usethe updated LMC data to derive empirical PC and AC relations at thesephases. Our numerical models are broadly consistent with our theory andthe observed data, though we discuss some caveats in the paper. We applythe idea of the HIF-photosphere interaction to explain recentsuggestions that the LMC period-luminosity (PL) and PC relations arenon-linear with a break at a period close to 10 d. Our empirical LMC PCand PL relations are also found to be non-linear with the F-test. Ourexplanation relies on the properties of the Saha ionization equation,the HIF-photosphere interaction and the way this interaction changeswith the phase of pulsation and metallicity to produce the observedchanges in the LMC PC and PL relations.
| 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.
| 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.
| Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.
| Morphology and star formation in nearby low surface brightness galaxies
We present observations (B, R, K, Hα and HI) of six nearby lowsurface brightness galaxies (LSBGs). They show an astonishing amount ofvariety; while some systems appear smooth and featureless, othersresolve into loose assemblies of gas clouds. We have derived rotationcurves, gas surface density profiles and star formation thresholds forthree of the galaxies.The results have been used to test two ideas describing their starformation: one in which star formation depends solely on the HI gassurface density, and one that depends on differential rotation. We findthat a critical HI surface density criterion in the range 2.6-12.6× 1020cm-2(2.1-10.1Msolarpc-2) best describes thestar-forming ability of these galaxies on local and global scales. Acritical gas surface density based on the rotation of the gas is alsoable to describe the results on a global scale for two of the threegalaxies for which we were able to derive rotation curves.
| The P-L relation in the BVRI bands for Cepheids in IC 1613 .
A set of six BVRI observations collected with the WFI at the ESO 2.2 mtelescope have been used to derive multicolor data of Cepheids in IC1613 identified in previous surveys. The method of Freedman has beenapplied to get reliable mean intensity values of Cepheid magnitudes inthe various bands. The resulting slopes of the relations in the BVIbands are similar, within the uncertainties, to those previouslyobtained by other authors for the LMC.
| 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.
| Georges Lemaitre, fondateur de la cosmologie moderne.
Not Available
| 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.
| The Araucaria Project: An Accurate Distance to the Local Group Galaxy NGC 6822 from Near-Infrared Photometry of Cepheid Variables
We have measured near-infrared magnitudes in the J and K bands for 56Cepheid variables in the Local Group galaxy NGC 6822 withwell-determined periods and optical light curves in the V and I bands.Using the template light-curve approach of Soszyński andcoworkers, accurate mean magnitudes were obtained from these data, whichallowed us to determine with unprecedented accuracy the distance to NGC6822 from a multiwavelength period-luminosity solution in the VIJKbands. From our data, we obtain a distance to NGC 6822 of(m-M)0=23.312+/-0.021 (random error) mag, with an additionalsystematic uncertainty of ~3%. This distance value is tied to an assumedLMC distance modulus of 18.50. From our multiwavelength approach, wefind for the total (average) reddening to the NGC 6822 CepheidsE(B-V)=0.356+/-0.013 mag, which is in excellent agreement with aprevious determination by McGonegal and coworkers from near-infraredphotometry and implies significant internal reddening of the Cepheids inNGC 6822. Our present, definitive distance determination of NGC 6822from Cepheids agrees within 2% with the previous distance we had derivedfrom optical photometry alone, but has significantly reduced error bars.Our Cepheid distance to NGC 6822 is in excellent agreement with therecent independent determination of Cioni & Habing from the I-bandmagnitude of the tip of the red giant branch. It also agrees well,within the errors, with the early determination of McGonegal et al.(1983) from random-phase H-band photometry of nine Cepheids.Based on observations obtained with the European Southern Observatory(ESO) New Technology Telescope (NTT) for Large Program 171.D-0004, andwith the Magellan Telescope of Las Campanas Observatory.
| 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.
| Detection of a 60°-long Dwarf Galaxy Debris Stream
We report on a 60°-long stream of stars, extending from Ursa Majorto Sextans, in the Sloan Digital Sky Survey. The stream is approximately2° wide and is clearly distinct from the northern tidal arm of theSagittarius dwarf galaxy. The apparent width of the stream indicates aprogenitor with a size and mass similar to that of a dwarf galaxy. Thestream is about 21 kpc distant and appears to be oriented almostperpendicular to our line of sight. The visible portion of the streamdoes not pass near any known dwarf galaxies, although we cannot rule outthat the stream may form the inner part of a known dwarf galaxy's orbit.The most likely explanation is that the stream constitutes the remainsof a dwarf galaxy that has been completely disrupted at some point inthe past. We also briefly report on the discovery of a diminutiveGalactic satellite that lies near the projected path of the new streambut is unlikely to be related to it.
| Chemical and Photometric Evolution of the Local Group Galaxy NGC 6822 in a Cosmological Context
Based on the photometric properties of NGC 6822, we derive a robust starformation history. Adopting this history, we compute 15 models ofgalactic chemical evolution. All of them match present-day photometricproperties. The dark halo mass in all models evolves according to themass assembly history predicted by a ΛCDM cosmology. We model theevolution of the baryonic mass aggregation history in this cosmologicalcontext assuming that part of the gas available for accretion neverfalls into the system due to two different physical processes. For sevenmodels we assume that during accretion the universal baryon fraction isreduced by reionization only. The best model of this first group, acomplex model with an early outflow, fits the observed gaseous mass andthe O/H, C/O, and Fe/O present-day values. This model requires a lowerupper mass limit for the IMF than that of the solar vicinity, inagreement with recent results by other authors. We have also computedeight models where, in addition to reionization, the accreted baryonfraction is reduced by large-scale shock heating. The best model of thisseries, which also requires an early outflow and a lower upper masslimit for the IMF, can marginally fit the gaseous mass and the O/H andC/O observed values.
| The Vertical Structure of the Outer Milky Way H I Disk
We examine the outer Galactic H I disk for deviations from theb=0deg plane by constructing maps of disk surface density,mean height, and thickness. We find that the Galactic warp is welldescribed by a vertical offset plus two Fourier modes of frequency m=1and 2, all of which grow with galactocentric radius. Adding the m=2 modeaccounts for the large asymmetry between the northern and southernwarps. We use a Morlet wavelet transform to investigate the spatial andfrequency localization of higher frequency modes; these modes are oftenreferred to as ``scalloping.'' We find that the m=10 and 15 scallopingmodes are well above the noise, but localized; this suggests that thescalloping does not pervade the whole disk, but only local regions.
| Molecular Gas in the Low-Metallicity, Star-forming Dwarf IC 10
We present a complete survey of CO (1-->0) emission in the LocalGroup dwarf irregular IC 10. The survey, conducted with the BIMAinterferometer, covers the stellar disk and a large fraction of theextended H I envelope with the sensitivity and resolution necessary todetect individual giant molecular clouds (GMCs) at the distance of IC 10(950 kpc). We find 16 clouds with a total CO luminosity of1×106 K km s-1 pc2, equivalent to4×106 Msolar of molecular gas using theGalactic CO-to-H2 conversion factor. Observations with theARO 12 m find that BIMA may resolve out as much as 50% of the COemission, and we estimate the total CO luminosity as~2.2×106 K km s-1 pc2. We measurethe properties of 14 GMCs from high-resolution OVRO data. These cloudsare very similar to Galactic GMCs in their sizes, line widths,luminosities, and CO-to-H2 conversion factors, despite thelow metallicity of IC 10 (Z~1/5 Zsolar). Comparing the BIMAsurvey to the atomic gas and stellar content of IC 10, we find that mostof the CO emission is coincident with high surface density H I. IC 10displays a much higher star formation rate per unit molecular(H2) or total (H I+H2) gas than most galaxies.This could be a real difference or may be an evolutionary effect-thestar formation rate may have been higher in the recent past.
| The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822
To test the existence of a possible radial gradient in oxygen abundanceswithin the Local Group dwarf irregular galaxy NGC 6822, we have obtainedoptical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6 mtelescope at ESO La Silla. The extent of the measured nebulae spansgalactocentric radii in the range between 0.05 and 2 kpc (over 4exponential scale lengths). In five H II regions (Hubble I, Hubble V,Kα, Kβ, KD 28e), the temperature-sensitive [O III]λ4363 emission line was detected, and direct oxygen abundanceswere derived. Oxygen abundances for the remaining H II regions werederived using bright-line methods. The oxygen abundances for threeA-type supergiant stars are slightly higher than nebular values atcomparable radii. Linear least-square fits to various subsets ofabundance data were obtained. When all of the measured nebulae areincluded, no clear signature is found for an abundance gradient. A fitto only newly observed H II regions with [O III] λ4363 detectionsyields an oxygen abundance gradient of -0.14+/-0.07 dexkpc-1. The gradient becomes slightly more significant(-0.16+/-0.05 dex kpc-1) when three additional H II regionswith [O III] λ4363 measurements from the literature are added.Assuming no abundance gradient, we derive a mean nebular oxygenabundance 12+log(O/H)=8.11+/-0.10 from [O III] λ4363 detectionsin the five H II regions from our present data; this mean valuecorresponds to [O/H]=-0.55.Based on EFOSC2 observations collected at the European SouthernObservatory, Chile: proposal 71.B-0549(A).
| The Araucaria Project: The Distance to the Local Group Galaxy IC 1613 from Near-Infrared Photometry of Cepheid Variables
We have measured accurate near-infrared magnitudes in the J and K bandsof 39 Cepheid variables in the irregular Local Group galaxy IC 1613 withwell-determined periods and optical VI light curves. Using the templatelight curve approach of Soszyński, Gieren, &Pietrzyński, accurate mean magnitudes were obtained from thesedata, which allowed us to determine the distance to IC 1613 relative tothe LMC from a multiwavelength period-luminosity solution in the opticalVI and near-IR JK bands with an unprecedented accuracy. Our result forthe IC 1613 distance is (m-M)0=24.291+/-0.035 (random error)mag, with an additional systematic uncertainty smaller than 2%. From ourmultiwavelength approach, we find for the total (average) reddening tothe IC 1613 Cepheids E(B-V)=0.090+/-0.019 mag, which is significantlyhigher than the foreground reddening of about 0.03 mag, showing thepresence of appreciable dust extinction inside the galaxy. Our datasuggest that the extinction law in IC 1613 is very similar to theGalactic one. Our distance result agrees, within the uncertainties, withtwo earlier infrared Cepheid studies in this galaxy, of Macri et al.(from HST data on 4 Cepheids) and McAlary et al. (from ground-basedH-band photometry of 10 Cepheids), but our result has reduced the totaluncertainty on the distance to IC 1613 (relative to the LMC) to lessthan 3%. With distances to nearby galaxies from Cepheid infraredphotometry at this level of accuracy, which are currently being obtainedin our Araucaria Project, it seems possible to significantly reduce thesystematic uncertainty of the Hubble constant, as derived from the HSTKey Project approach, by improving the calibration of the metallicityeffect on PL relation zero points and by improving the distancedetermination to the LMC.Based on observations obtained with the New Technology Telescope (NNT)at ESO La Silla for programs 074.D-0318(B) and 074.D-0505(B).
| The Cosmological Significance of High-Velocity Cloud Complex H
We have used new and archival infrared and radio observations to searchfor a dwarf galaxy associated with the high-velocity cloud (HVC) knownas `complex H.' Complex H is a large (Ω>~400 deg2)and probably nearby (d=27 kpc) HVC whose location in the Galactic planehas hampered previous investigations of its stellar content. The H Imass of the cloud is 2.0×107(d/27 kpc)2Msolar, making complex H one of the most massive HVCs if itsdistance is more than ~20 kpc. Virtually all similar H I clouds in othergalaxy groups are associated with low surface brightness dwarf galaxies.We selected mid-infrared sources observed by the MSX satellite in thedirection of complex H that appeared likely to be star-forming regionsand observed them at the wavelength of the CO J=1-->0 rotationaltransition in order to determine their velocities. Of the 60 observedsources, 59 show emission at Milky Way velocities, and we detected noemission at velocities consistent with that of complex H. We use theseobservations to set an upper limit on the ongoing star formation rate inthe HVC of <~5×10-4 Msolaryr-1. We also searched the 2MASS database for evidence of anydwarf-galaxy-like stellar population in the direction of the HVC andfound no trace of a distant red giant population, with an upper limit onthe stellar mass of ~106 Msolar. Given the lack ofevidence for either current star formation or an evolved population, weconclude that complex H cannot be a dwarf galaxy with properties similarto those of known dwarfs. Complex H is therefore one of the most massiveknown H I clouds that does not contain any stars. If complex H isself-gravitating, then this object is one of the few known dark galaxycandidates. These findings may offer observational support for the ideathat the cold dark matter substructure problem is related to thedifficulty of forming stars in low-mass dark matter halos;alternatively, complex H could be an example of a cold accretion flowonto the Milky Way.
| The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium
We investigate the relationship between the velocity dispersion of thegas and the supernova (SN) rate and feedback efficiency withthree-dimensional numerical simulations of SN-driven turbulence in theinterstellar medium (ISM). Our simulations aim to explore the constancyof the velocity dispersion profiles in the outer parts of galactic disksat ~6-8 km s-1 and the transition to the starburst regime,i.e., high star formation rates (SFRs) associated with high velocitydispersions. With our fiducial value of the SN feedback efficiency(i.e., ε=0.25, corresponding to an injected energy per SN of0.25×1051 ergs), our results show that (1) SN drivingleads to constant velocity dispersions of σ~6 km s-1for the total gas and σHI~3 km s-1 for the HI gas, independent of the SN rate, for values of the rate between 0.01and 0.5 the Galactic value (ηG) (2) the position of thetransition to the starburst regime (i.e., location of sharp increase inthe velocity dispersion) at around SFR/area~=5×10-3 to10-2 Msolar yr-1 kpc-2observed in the simulations is in good agreement with the transition tothe starburst regime in the observations (e.g., NGC 628 and NGC 6949);(3) for the high SN rates, no H I gas is present in the simulations box;however, for the total gas velocity dispersion, there is good agreementbetween the models and the observations; (4) at the intermediate SNrates (η/ηG~0.5-1), taking into account the thermalbroadening of the H I line helps reach a good agreement in that regimebetween the models and the observations; and (5) forη/ηG<0.5, σ and σHI fallbelow the observed values by a factor of ~2. However, a set ofsimulations with different values of ɛ indicates that, forlarger values of the SN feedback efficiencies, velocity dispersions ofthe H I gas of the order of 5-6 km s-1 can be obtained, incloser agreement with the observations. The fact that forη/ηG<0.5, the H I gas velocity dispersions are afactor of ~2 smaller than the observed values could result from the factthat we might have underestimated the SN feedback efficiency. On theother hand, it might also be an indication that other physical processescouple to the stellar feedback in order to produce the observed level ofturbulence in galactic disks.
| Impact of Dark Matter Subhalos on Extended H I Disks of Galaxies: Possible Formation of H I Fine Structures and Stars
Recent observations have discovered star formation activities in theextreme outer regions of disk galaxies. However, it remains unclear whatphysical mechanisms are responsible for triggering star formation insuch low-density gaseous environments of galaxies. In order tounderstand the origin of these outer star-forming regions, wenumerically investigate how the impact of dark matter subhalos orbitinga gas-rich disk galaxy embedded in a massive dark matter halo influencesthe dynamical evolution of the outer H I gas disk of the galaxy. We findthat if the masses of the subhalos (Msb) in a galaxy with anextended H I gas disk are as large as 10-3Mh,where Mh is the total mass of the galaxy's dark halo, localfine structures can be formed in the extended H I disk. We also findthat the gas densities of some apparently filamentary structures canexceed a threshold gas density for star formation and thus be likely tobe converted into new stars in the outer part of the H I disk in somemodels with larger Msb. These results thus imply that theimpact of dark matter subhalos (``dark impact'') can be important forbetter understanding the origin of recent star formation discovered inthe extreme outer regions of disk galaxies. We also suggest thatcharacteristic morphologies of local gaseous structures formed by thedark impact can indirectly prove the existence of dark matter subhalosin galaxies. We discuss the origin of giant H I holes observed in somegas-rich galaxies (e.g., NGC 6822) in the context of the dark impact.
| Neon and Oxygen Abundances in M33
We present new spectroscopic observations of 13 H II regions in theLocal Group spiral galaxy M33. The regions observed range from 1 to 7kpc in distance from the nucleus. Of the 13 H II regions observed, the[O III] λ4363 line was detected in six regions. Electrontemperatures were thus able to be determined directly from the spectrausing the [O III] λλ4959, 5007/λ4363 line ratio.Based on these temperature measurements, oxygen and neon abundances andtheir radial gradients were calculated. For neon, a gradient of-0.016+/-0.017 dex kpc-1 was computed, which agrees with theNe/H gradient derived previously from ISO spectra. A gradient of-0.012+/-0.011 dex kpc-1 was computed for O/H, much shallowerthan was derived in previous studies. The newly calculated O/H and Ne/Hgradients are in much better agreement with each other, as expected frompredictions of stellar nucleosynthesis. We examine the correlationbetween the WC/WN ratio and metallicity, and find that the new M33abundances do not impact the observed correlation significantly. We alsoidentify two new He II-emitting H II regions in M33, the first to bediscovered in a spiral galaxy other than the Milky Way. In both casesthe nebular He II emission is not associated with Wolf-Rayet stars.Therefore, caution is warranted in interpreting the relationship betweennebular He II emission and Wolf-Rayet stars when both are observed inthe integrated spectrum of an H II region.
| 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%.
| Chandra Observation of NGC 6822
Chandra observed the nearby dwarf galaxy NGC 6822. There are 70 sourcesin the Chandra field, two of which are fairly extended and likelybackground clusters of galaxies. A third source was resolved by Chandrathat has the same size and position as a known supernova remnant. Thesuspected variability detected from this source in previous missions isshown to be in error. The majority of the remaining 67 sources areconsistent with background sources. Of the 61 sources detected above acompleteness limit of 10 events, we estimate that 9+/-4+/-8 areassociated with the galaxy, including both systematic and statisticalerrors. We compare the X-ray positions of all sources with variouscatalogs and Hubble Space Telescope data and offer tentativeidentifications for several. Based on the mass and star formation rateof NGC 6822, we expect only about 10 sources, mostly high-mass systems.
| Massive Star Cluster Populations in Irregular Galaxies as Probable Younger Counterparts of Old Metal-rich Globular Cluster Populations in Spheroids
Peak metallicities of metal-rich populations of globular clusters(MRGCs) belonging to early-type galaxies and spheroidal subsystems ofspiral galaxies (spheroids) of different mass fall within the somewhatconservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible ageeffects are taken into account, this metallicity range might becomesmaller. Irregular galaxies such as the Large Magellanic Cloud (LMC),with longer timescales of formation and lower star formation (SF)efficiency, do not contain old MRGCs with [Fe/H]>-1.0, but they areobserved to form populations of young/intermediate-age massive starclusters (MSCs) with masses exceeding 104 Msolar.Their formation is widely believed to be an accidental process fullydependent on external factors. From the analysis of available data onthe populations and their hosts, including intermediate-age populousstar clusters in the LMC, we find that their most probable meanmetallicities fall within -0.7<=[Fe/H]<=-0.3, as the peakmetallicities of MRGCs do, irrespective of signs of interaction.Moreover, both the disk giant metallicity distribution function (MDF) inthe LMC and the MDFs for old giants in the halos of massive spheroidsexhibit a significant increase toward [Fe/H]~-0.5. That is in agreementwith a correlation found between SF activity in galaxies and theirmetallicity. The formation of both the old MRGCs in spheroids and MSCpopulations in irregular galaxies probably occurs at approximately thesame stage of the host galaxies' chemical evolution and is related tothe essentially increased SF activity in the hosts around the samemetallicity that is achieved very early in massive spheroids, later inlower mass spheroids, and much later in irregular galaxies. Changes inthe interstellar dust, particularly in elemental abundances in dustgrains and in the mass distribution function of the grains, may be amongthe factors regulating star and MSC formation activity in galaxies.Strong interactions and mergers affecting the MSC formation presumablyplay an additional role, although they can substantially intensify theinternally regulated MSC formation process. Several implications of oursuggestions are briefly discussed.
| 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.
| 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
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