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Comment regarding the functional form of the Schmidt law
Star formation rates on the galactic scale are describedphenomenologically by two distinct relationships, as emphasized recentlyby Elmegreen [Elmegreen, B.G., 2002. ApJ, 577, 206. astro-ph/0207114.].The first of these is the Schmidt law, which is a power-law relationbetween the star formation rate SFR and the column density Σ. Theother relationship is that there is a cutoff in the gas density belowwhich star formation shuts off. The purpose of this paper is to arguethat: (1) these two relationships can be accommodated by a singlefunctional form of the Schmidt law, (2) this functional form ismotivated by the hypothesis that star formation is a criticalphenomenon, and that as a corollary, (3) the existence of a sharp cutoffmay thus be an emergent property of galaxies, as was argued by Seiden[Seiden, P.E.,1983. ApJ, 266, 555.], as opposed to the classical viewthat this cutoff is due to an instability criterion.
| Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
| Investigating the Andromeda stream - I. Simple analytic bulge-disc-halo model for M31
This paper is the first in a series which studies interactions betweenM31 and its satellites, including the origin of the giant southernstream. We construct accurate yet simple analytic models for thepotential of the M31 galaxy to provide an easy basis for the calculationof orbits in M31's halo. We use a Navarro, Frenk and White (NFW) darkhalo, an exponential disc, a Hernquist bulge, and a central black holepoint mass to describe the galaxy potential. We constrain the parametersof these functions by comparing to existing surface-brightness,velocity-dispersion, and rotation-curve measurements of M31. Ourdescription provides a good fit to the observations, and agrees wellwith more sophisticated modelling of M31. While in many respects theparameter set is well constrained, there is substantial uncertainty inthe outer halo potential and a near-degeneracy between the disc and halocomponents, producing a large, nearly two-dimensional allowed region inparameter space. We limit the allowed region using theoreticalexpectations for the halo concentration, baryonic content, and stellarmass-to-light ratio (M/LR), finding a smaller region wherethe parameters are physically plausible. Our proposed mass model for M31has Mbulge= 3.2 × 1010Msolar,Mdisc= 7.2 × 1010Msolar, andM200= 7.1 × 1011Msolar, withuncorrected (for internal and foreground extinction) mass-to-lightratios of M/LR= 3.9 and 3.3 for the bulge and disc,respectively. We present some illustrative test-particle orbits for theprogenitor of the stellar stream in our galaxy potential, highlightingthe effects of the remaining uncertainty in the disc and halo masses.
| The satellite distribution of M31
The spatial distribution of the Galactic satellite system plays animportant role in Galactic dynamics and cosmology, where its successfulreproduction is a key test of simulations of galaxy halo formation.Here, we examine its representative nature by conducting an analysis ofthe three-dimensional spatial distribution of the M31 subgroup ofgalaxies, the next closest system to our own. We begin by a discussionof distance estimates and incompleteness concerns, before revisiting thequestion of membership of the M31 subgroup. We constrain this byconsideration of the spatial and kinematic properties of the putativesatellites. Comparison of the distribution of M31 and Galacticsatellites relative to the galactic discs suggests that the Galacticsystem is probably modestly incomplete at low latitudes by ~=20 percent. We find that the radial distribution of satellites around M31 ismore extended than the Galactic subgroup; 50 per cent of the Galacticsatellites are found within ~100 kpc of the Galaxy, compared to ~200 kpcfor M31. We search for `ghostly streams' of satellites around M31, inthe same way others have done for the Galaxy, and find several,including some that contain many of the dwarf spheroidal satellites. Thelack of M31-centric kinematic data, however, means that we are unable toprobe whether these streams represent real physical associations.Finally, we find that the M31 satellites are asymmetrically distributedwith respect to our line of sight to this object, so that the majorityof its satellites are on its near side with respect to our line ofsight. We quantify this result in terms of the offset between M31 andthe centre of its satellite distribution, and find it to be significantat the ~ 3σ level. We discuss possible explanations for thisfinding, and suggest that many of the M31 satellites may have beenaccreted only relatively recently. Alternatively, this anisotropy may berelated to a similar result recently reported for the 2dFGRS, whichwould imply that the halo of M31 is not yet virialized. Until such timeas a satisfactory explanation for this finding is presented, however,our results warn against treating the M31 subgroup as complete, unbiasedand relaxed.
| 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.
| Evidence for a universal slope of the period-luminosity relation from direct distances to Cepheids in the LMC.
We have applied the infrared surface brightness (ISB) technique toderive distances to 13 Cepheid variables in the LMC which have periodsfrom 3-42 days. The corresponding absolute magnitudes define PLrelations in VIWJK bands which agree exceedingly well with thecorresponding Milky Way relations obtained from the same technique, andare in significant disagreement with the observed LMC Cepheid PLrelations, by OGLE-II and Persson et al., in these bands. Our datauncover a systematic error in the p-factor law which transforms Cepheidradial velocities into pulsational velocities. We correct the p-factorlaw by requiring that all LMC Cepheids share the same distance.Re-calculating all Milky Way and LMC Cepheid distances with the revisedp-factor law, we find that the PL relations from the ISB technique bothin LMC and in the Milky Way agree with the OGLE-II and Persson et al.LMC PL relations, supporting the conclusion of no metallicity effect onthe slope of the Cepheid PL relation in optical/near infrared bands.
| How good are RR Lyrae and Cepheids really as distance indicators? . The observational approach
A number of recent technical developments, including the Hipparcossatellite, the Hubble Space Telescope fine guidance sensors and longbase line near-IR interferometry has made it possible to employ severallargely geometrical methods to determine direct distances to RR Lyraestars and Cepheids. The distance scale now rests on a much firmer basisand the significant differences between the distances based on RR Lyraestars (short) and Cepheids (long) to the LMC have been largelyeliminated. The effects of metallicity on the RR Lyrae period-luminosity(PL) relation in the K-band as well as on the Cepheid PL relationappears to be the main remaining issues but even here empirical resultsare beginning to show convergence. I review here some of these recentdevelopments seen from the perspective of the near-IR surface brightnessmethod.
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| 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 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.
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| Weak redshift discretisation in the Local Group of galaxies?
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
| 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.
| A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33
We present UBVRI photometry obtained from Mosaic images of M31 and M33using the Kitt Peak National Observatory 4 m telescope. We describe ourdata reduction and automated photometry techniques in some detail, as wewill shortly perform a similar analysis of other Local Group galaxies.The present study covered 2.2 deg2 along the major axis ofM31 and 0.8 deg2 on M33, chosen so as to include all of theregions currently active in forming massive stars. We calibrated ourdata using photometry from the Lowell 1.1 m telescope, and this externalmethod resulted in millimagnitude differences in the photometry ofoverlapping fields, providing some assurance that our photometry isreliable. The final catalog contains 371,781 and 146,622 stars in M31and M33, respectively, where every star has a counterpart in (at least)the B, V, and R passbands. Our survey goes deep enough to achieve 1%-2%photometry at 21 mag (corresponding to stars more massive than 20Msolar) and achieves <10% errors at U~B~V~R~I~23 mag.Although our typical seeing was only modest (0.8"-1.4", with median1.0") by some standards, we find excellent correspondence between ourcatalog sources and those we see in our Hubble Space Telescope ACS datafor OB48, a crowded region in M31. We compare our final photometry withthat of others and find good agreement with the CCD catalog of M31 starsby Magnier et al., although our study covers twice the area and goesabout 2 mag deeper. There is also excellent agreement with the CCD``DIRECT'' surveys of M31 and M33. The photographic studies of othersfare less well, particularly at the faint end in V, where accuratebackground subtraction is needed for good photometry. We providecross-references to the stars confirmed as members by spectroscopy andcompare the locations of these to the complete set in color-magnitudediagrams. While follow-up spectroscopy is needed for many projects, wedemonstrate the success of our photometry in being able to distinguishM31/M33 members from foreground Galactic stars. Finally, we present theresults of a single night of spectroscopy on the WIYN 3.5 m telescope,examining the brightest likely members of M31. The spectra identify 34newly confirmed members, including B-A supergiants, the earliest O starknown in M31, and two new luminous blue variable candidates whosespectra are similar to that of P Cygni.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(AURA), Inc., under NASA contract NAS5-26555. These observations areassociated with program GO-9794.
| The Anisotropic Distribution of M31 Satellite Galaxies: A Polar Great Plane of Early-type Companions
The highly anisotropic distribution and apparent alignment of theGalactic satellites in polar great planes begs the question of howcommon such distributions are. The satellite system of M31 is the onlynearby system for which we currently have sufficiently accuratedistances to study the three-dimensional satellite distribution. Wepresent the spatial distribution of the 15 currently known M31companions in a coordinate system centered on M31 and aligned with itsdisk. Through a detailed statistical analysis we show that the fullsatellite sample describes a plane that is inclined by -56° withrespect to the poles of M31 and has an rms height of 100 kpc. At 88% thestatistical significance of this plane is low, and it is unlikely tohave a physical meaning. We note that the great stellar stream foundnear Andromeda is inclined to this plane by 7°. Most of the M31satellites are found within <+/-40° of M31's disk; i.e., there islittle evidence for a Holmberg effect. If we confine our analysis toearly-type dwarfs, we find a best-fit polar plane within 5°-7°from the pole of M31. This polar great plane has a statisticalsignificance of 99.7% and includes all dSphs (except for And II), M32,NGC 147, and PegDIG. The rms distance of these galaxies from the polarplane is 16 kpc. The nearby spiral M33 has a distance of only ~3 kpcfrom this plane, which points toward the M81 group. We discuss theanisotropic distribution of M31's early-type companions in the frameworkof three scenarios, namely, as remnants of the breakup of a largerprogenitor, as a tracer of a prolate dark matter halo, and as a tracerof collapse along large-scale filaments. The first scenario requiresthat the breakup must have occurred at very early times and that thedwarfs continued to form stars thereafter to account for their stellarpopulation content and luminosity-metallicity relation. The thirdscenario seems to be plausible, especially when considering the apparentalignment of our potential satellite filament with several nearbygroups. The current data do not permit us to rule out any of thescenarios. Orbit information is needed to test the physical reality ofthe polar plane and of the different scenarios in more detail.
| Globular Clusters in Dwarf Galaxies
Data are currently available on the luminosities and half-light radii of101 globular clusters associated with low-luminosity parent galaxies.The luminosity distribution of globular clusters embedded in dwarfgalaxies having Mv>-16 is found to differ dramaticallyfrom that for globular clusters surrounding giant host galaxies withMv<-16. The luminosity distribution of globular clustersin giant galaxies peaks at Mv~-7.5, whereas that for dwarfgalaxies is found to increase monotonically down to the completenesslimit of the cluster data at Mv~-5.0. Unexpectedly, thepower-law distribution of the luminosities of globular clusters hostedby dwarf galaxies is seen to be much flatter than that of the bright,unevolved part of the luminosity distribution of globular clustersassociated with giant galaxies. The specific frequency of globularclusters fainter than Mv=-7.5 is found to be particularlyhigh in dwarf galaxies. The luminosity distribution of the LMC globularclusters is similar to that found in giant galaxies and differs fromthose of globular clusters in dwarf galaxies. The present data appear toshow no strong dependence of globular cluster luminosity on themorphological type of their parent galaxies. No attempt is made toexplain the unexpected discovery that the luminosity distribution ofglobular clusters is critically dependent on parent galaxy luminosity(or mass) but insensitive to the morphological type of the host galaxy.
| Reduced Wolf-Rayet line luminosities at low metallicity
New NTT/EMMI spectrophotometry of single WN2-5 stars in the Small andLarge Magellanic Clouds are presented, from which He ii λ4686line luminosities have been derived, and compared with observations ofother Magellanic Cloud Wolf-Rayet stars. SMC WN3-4 stars possess lineluminosities which are a factor of 4 times lower than LMC counterparts,incorporating several binary SMC WN3-4 stars from the literature.Similar results are found for WN5-6 stars, despite reduced statistics,incorporating observations of single LMC WN5-9 stars from theliterature. C iv λ5808 line luminosities of carbon sequence WRstars in the SMC and IC 1613 (both WO subtypes) from the recentliterature are a factor of 3 lower than LMC WC stars from Mt Stromlo/DBSspectrophotometry, although similar results are also obtained for thesole LMC WO star. We demonstrate how reduced line luminosities at lowmetallicity follow naturally if WR winds are metallicity-dependent, asrecent empirical and theoretical results suggest. We apply massloss-metallicity scalings to atmospheric non-LTE models of Milky Way andLMC WR stars to predict the wind signatures of WR stars in themetal-poor star forming WR galaxy I Zw 18. WN He ii λ4686 lineluminosities are 7-20 times lower than in metal-rich counterparts ofidentical bolometric luminosity, whilst WC C iv λ5808 lineluminosities are 3-6 times lower. Significant He+ Lymancontinuum fluxes are predicted for metal-poor early-type WR stars.Consequently, our results suggest a larger population of WR stars in IZw 18 than is presently assumed, particularly for WN stars, potentiallyposing a severe challenge to evolutionary models at very lowmetallicity. Finally, reduced wind strengths from WR stars at lowmetallicities impacts upon the immediate circumstellar environment oflong duration GRB afterglows, particularly since the host galaxies ofhigh-redshift GRBs tend to be metal-poor.
| On the light curve shape of Cepheids in IC 1613 and NGC 6822
Comparison of the Fourier parameters of fundamental mode Cepheids withperiod near 10 d in galaxies of the Local Group (IC 1613, NGC 6822,Milky Way, Magellanic Clouds) confirms the previous indication of thelack of a spread of φ21 values in some dwarf irregulargalaxies. It is not yet clear whether this is a real effect or if it isjust due to the low number of Cepheids in these galaxies. We suspecthowever that in this period range the Cepheids of IC 1613 and NGC 6822behave differently from those in the Milky Way and the MagellanicClouds. The main effect of the different metallicity on the Fourierparameters is confirmed to be the larger R21 values ofshorter period Cepheids in metal-poorer galaxies. However themetallicity alone should not be enough to explain the variousdifferences among the Cepheids of the four galaxies. The differencebetween the spread of φ21 values near 10 d in Milky Wayand Magellanic clouds is pointed out.
| Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
| On the neutral gas distribution and kinematics in the dwarf irregular galaxy IC 1613
Aims.We study the neutral hydrogen distribution and kinematics in theLocal Group dwarf irregular galaxy IC 1613 and compare them with theionized gas distribution and stellar content of the galaxy. We discussseveral mechanisms which may be responsible for the origin of theobserved complicated HI morphology and compare parameters of the mostprominent kpc-scale HI structure with the multiple SNescenario.Methods.The observations were performed with the Vary LargeArray of NRAO with a linear resolution 23 pc at the adopted distanceof 725 kpc and the spectral channel width of 2.57 km s-1. Thenumerical calculations have been provided with our 2.5D Lagrangianscheme based on the thin layer approximation.Results.We found that theISM of the galaxy is highly inhomogeneous and identified a number ofintermediate-scale (200 pc-300 pc in diameter) HI arcs and shells havingexpansion velocities of 10 to 20 km s-1. Besides theseshells, several giant holes and arc-shaped structures have beenrevealed, whose radii exceed several hundred parsecs. We found thatparameters of the most prominent (M_HI = 2.8 × 107{M}ȯ) kpc-scale structure and the level of the detectedstar formation activity are inconsistent with the multiple SNehypothesis.
| 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.
| Variable stars in nearby galaxies. VII. P-L relation in the BVRI bands of 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. Since part of the previously knowndata were obtained only in VI filters or without filter (Wh) bands, themethod of Freedman has been applied to get reliable mean intensityvalues of Cepheid magnitudes in the various bands. The resulting slopesof the relations in the BVI bands are similar, within the uncertainties,to those previously obtained by other authors for the LMC. Thedistribution of the Cepheids in the period-color diagrams is compatiblewith a change near P 10 d as observed in LMC. The distribution inthe color-color diagrams is more similar to that in SMC, and this shouldbe related to the very low metallicity of the galaxy.
| 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.
| Imaging resources for the GTC: the Local Group Census
The Local Group Census is a narrowband imaging survey aimed atcataloguing the emission-line populations in the galaxies of the LocalGroup. Data, which were obtained using the Wide Field Camera of the 2.5mIsaac Newton Telescope, are available to the whole astronomicalcommunity, resulting in a valuable imaging resource for follow-upspectroscopy with the GTC.
| Reionization and the Fate of Dwarf Galaxies
Our understanding of the origins and evolution of dwarf galaxies hasbeen changing very rapidly. New observations are giving better insightinto the relationship between the two main families of dwarf galaxies,the dwarf ellipticals and the dwarf irregulars. Theoretical simulationsappear to be both posing several problems and eliminating them. However,it is possible that we are beginning to observe some of the more robustresults of these simulations. For example, HST observations of LocalGroup galaxies show evidence of the impact of the ionizing backgroundradiation on their star formation histories. If this is the correctinterpretation, it greatly alters our understanding of dwarf galaxyevolution.
| Mean JHK Magnitudes of Fundamental-Mode Cepheids from Single-Epoch Observations
We present an empirical method for converting single-point near-infraredJ, H, and K measurements of fundamental-mode Cepheids to meanmagnitudes, using complete light curves in V or I bands. The algorithmis based on the template light curves in the near-infrared bandpasses.The mean uncertainty of the method is estimated to about 0.03 mag, whichis smaller than the uncertainties obtained in other approaches to theproblem in the literature.
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