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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.
| Gas distribution, kinematics and star formation in faint dwarf galaxies
We compare the gas distribution, kinematics and the current starformation in a sample of 10 very faint (-13.37 < MB <-9.55) dwarf galaxies. For five of these galaxies we present fresh,high-sensitivity, Giant Metrewave Radio Telescope HI 21-cm observations.We find that the large-scale HI distribution in the galaxies istypically irregular and clumpy, with the peak gas density rarelyoccurring at the geometric centre. We also find that the velocity fieldsfor all the galaxies have an ordered component, although in general, thepatterns seen do not fit that expected from a rotating disc. For all ourgalaxies we construct maps of the HI column density at a constant linearresolution of ~300 pc; this forms an excellent data set to check for thepresence of a threshold column density for star formation. We find thatwhile current star formation (as traced by Hα emission) isconfined to regions with relatively large [NHI > (0.4-1.7)× 1021cm-2] HI column density, themorphology of the Hα emission is in general not correlated withthat of the high HI column density gas. Thus, while high column densitygas may be necessary for star formation, in this sample at least, it isnot sufficient to ensure that star formation does in fact occur. Weexamine the line profiles of the HI emission, but do not find a simplerelation between regions with complex line profiles and those withongoing star formation. Our sample includes examples of regions wherethere is ongoing star formation, but the profiles are well fitted by asingle Gaussian, as well as regions where there is no star formation butthe line profiles are complex. Finally, we examine the very fine scale(~20-100 pc) distribution of the HI gas, and find that at these scalesthe emission exhibits a variety of shell-like, clumpy and filamentaryfeatures. The Hα emission is sometimes associated withhigh-density HI clumps, sometimes the Hα emission lies inside ahigh-density shell, and sometimes there is no correspondence between theHα emission and the HI clumps. In summary, the interplay betweenstar formation and gas density in these galaxies does not seem to showthe simple large-scale patterns observed in brighter galaxies.
| 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.
| Toward a census of variable stars in northern local group dwarf irregular galaxies .
Dwarf galaxies in the local group provide a unique astrophysicallaboratory. In particular, they allow us to probe pulsating (and other)variable stars in low-metallicity environments with abundances belowthat of the SMC. Our observing program, described in detail by C.Gössl's contribution, yields a large number of intrinsically brightvariable stars that can serve as probes of the stellar population andstar formation history of these galaxies. Most prominent are pulsationvariables like Miras (LPVs) and delta Cep stars, but we also find othertypes of variable stars, e.g. RV Tauri stars, irregular red variablesetc. We present a preliminary census for the three galaxies DDO 216, LeoA and GR8.
| Using delta Cep stars to study northern dwarf irregular galaxies of the Local Group .
Dwarf galaxies in the Local Group provide a unique astrophysicallaboratory. Despite their proximity some of these systems still lack areliable distance determination as well as studies of their stellarcontent and star formation history. We present first results of oursurvey of variable stars in a sample of six Local Group dwarf irregulargalaxies. We describe observational strategies and data reduction, anddiscuss the lightcurves of newly found and rediscovered delta Cep starsin DDO 216, Leo A and GR8. Based on these data, we present newly derivedindependent Cepheid distances. Other variable stars found in our surveyare discussed in a related article of this volume (Snigula et al.).
| Synthetic evolutionary models and pulsational constraints for metal-poor Cepheids. .
In order to investigate the occurence of a Period-Luminosity relationfor Anomalous Cepheids, we present synthetic evolutionary models topopulate the pulsation Instability Strip discussed in \citep{mfc}. Wefind that the evolutionary effects decrease as the wavelength increases,from the V to the K band. In particular the Period-Luminosity relationin the K band appears to be a robust distance indicator, as also happensfor RR Lyrae stars.
| The Millimeter- and Submillimeter-Wave Spectrum of Iso-Propanol [(CH3)2CHOH]
Iso-propanol [(CH3)2CHOH], an isomer ofn-propanol, has been studied in the millimeter- and submillimeter-waveregion of the electromagnetic spectrum with our FASSST spectrometerthrough 360 GHz. Spectra arising from the ground vibrational state ofall three hydroxyl torsional substates, given the labels symmetricgauche, antisymmetric gauche, and trans in order of increasing energy,have been observed. We have successfully assigned ~7600 pure rotationaltransitions within the torsional substates as well as ~4700torsional-rotational transitions between the symmetric and antisymmetricgauche substates through the lower rotational quantum numberJ''=68. Spectral lines involving one or both of the twogauche forms have been simultaneously analyzed with a 2×2effective torsional-rotational Hamiltonian, which includes terms throughfifth order in the torsional-rotational interaction. Excluding perturbedtransitions, the assigned transitions were fitted to a root mean squaredeviation of 76 kHz. The trans substate was analyzed as a semirigidrotor, and its unperturbed transitions fitted to a root mean squaredeviation of 63 kHz. A perturbation was seen at transitions withJ''>50 in the trans substate. The torsional excitationenergy for the trans substate above ground was estimated from intensityratios to be about 120 K.
| The Local Group Stellar Populations Archive from the Hubble Space Telescope WFPC2
We present a database (LOGPHOT) of stellar photometry of Local Groupgalaxies obtained with the Hubble Space Telescope using the Wide FieldPlanetary Camera 2 (WFPC2). The database includes photometry from allWFPC2 observations taken through 2003 with long exposures (>500 s) inF555W and F814W, and many observations in which long exposures weretaken in at least two broadband filters. We have attempted to derive anduse techniques that produce the best photometry; the database has beenfully populated using the HSTphot photometry package. To test theeffects of different techniques, independent reductions were made for afew fields, and the comparison of these highlights some important issuesand gives an estimate of plausible errors; these tests also led to someminor modifications and improvements to HSTphot. We provide bothpoint-spread function photometry and subtracted-frame aperturephotometry and discuss the merits of each. The database is availableelectronically. In addition to discussing the techniques used toconstruct the database, we present color-magnitude diagrams from singlefields in each of the Local Group galaxies that have been observed;these provide an educational and visual display of the variety of starformation histories observed in Local Group galaxies.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| 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.
| Hypervelocity Stars. I. The Spectroscopic Survey
We discuss our targeted search for hypervelocity stars (HVSs), starstraveling with velocities so extreme that dynamical ejection from amassive black hole is their only suggested origin. Our survey, nowhalf-complete, has successfully identified a total of four probable HVSsplus a number of other unusual objects. Here we report the two mostrecently discovered HVSs: SDSS J110557.45+093439.5 and possibly SDSSJ113312.12+010824, traveling with Galactic rest-frame velocities atleast +508+/-12 and +418+/-10 km s-1, respectively. The otherlate B-type objects in our survey are consistent with a population ofpost-main-sequence stars or blue stragglers in the Galactic halo, withmean metallicity [Fe/H]Wk=-1.3 and velocitydispersion 108+/-5 km s-1. It is interesting to note that thevelocity distribution shows a tail of objects with large positivevelocities that may be a mix of low-velocity HVSs and high-velocityrunaway stars. Our survey also includes a number of DA white dwarfs withunusually red colors, possibly extremely low mass objects. Two of ourobjects are B supergiants in the Leo A dwarf, providing the firstspectroscopic evidence for star formation in this dwarf galaxy withinthe last ~30 Myr.
| 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.
| A Successful Targeted Search for Hypervelocity Stars
Hypervelocity stars (HVSs) travel with velocities so extreme thatdynamical ejection from a massive black hole is their only suggestedorigin. Following the discovery of the first HVS by Brown andcollaborators, we have undertaken a dedicated survey for more HVSs inthe Galactic halo and present here the resulting discovery of two newHVSs: SDSS J091301.0+305120 and SDSS J091759.5+672238, traveling withGalactic rest-frame velocities of at least +558 +/- 12 and +638 +/- 12km s-1, respectively. Assuming the HVSs are B8 main-sequencestars, they are at distances of ~75 and ~55 kpc, respectively, and havetravel times from the Galactic center consistent with their lifetimes.The existence of two B8 HVSs in our 1900 deg2 survey,combined with the Yu & Tremaine HVS rate estimates, is consistentwith HVSs drawn from a standard initial mass function but inconsistentwith HVS drawn from a truncated mass function like the one in thetop-heavy Arches cluster. The travel times of the five currently knownHVSs provide no evidence for a burst of HVSs from a major infall eventat the Galactic center in the last ~160 Myr.
| 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.
| Oxygen and Nitrogen in Isolated Dwarf Irregular Galaxies
We present long-slit optical spectroscopy of 67 H II regions in 21 dwarfirregular galaxies to investigate the enrichment of oxygen, nitrogen,neon, sulfur, and argon in low-mass galaxies. Oxygen abundances areobtained via direct detection of the temperature-sensitive emissionlines for 25 H II regions; for the remainder of the sample, oxygenabundances are estimated from strong-line calibrations. The directabundance determinations are compared to the strong-line abundancecalibrations of both McGaugh and Pilyugin. While the McGaugh calibrationyields a statistical offset of 0.07 dex, the photoionization model gridtraces the appropriate isometallicity contour shape in theR23-O32 diagnostic diagram. In contrast, while thePilyugin calibration yields a negligible statistical offset, theresiduals in this strong-line calibration method are correlated withionization parameter. Thus, these observations indicate that oxygenabundances will be overestimated by the p-method for H II regions withlow-ionization parameters. Global oxygen and nitrogen abundances forthis sample of dwarf irregular galaxies are examined in the context ofopen- and closed-box chemical evolution models. While several galaxiesare consistent with closed-box chemical evolution, the majority of thissample has an effective yield ~1/4 of the expected yield for a constantstar formation rate and Salpeter IMF, indicating that either outflow ofenriched gas or inflow of pristine gas has occurred. The effective yieldstrongly correlates with MH/LB in the sense thatgas-rich galaxies are more likely to be closed systems. However, theeffective yield does not appear to correlate with other globalparameters such as dynamical mass, absolute magnitude, star formationrate, or surface brightness. In addition, open and closed systems arenot identified easily in other global abundance measures; for example,the observed correlation between luminosity and metallicity isconsistent with other recent results in the literature. A correlation isfound between the observed nitrogen-to-oxygen ratio and the color of theunderlying stellar population; redder dwarf irregular galaxies havehigher N/O ratios than blue dwarf irregular galaxies. The relativeabundance ratios are interpreted in the context of delayed release ofnitrogen and varied star formation histories.
| 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.
| Towards a phylogenetic analysis of galaxy evolution: a case study with the dwarf galaxies of the Local Group
Context: .The Hubble tuning-fork diagram has always been the preferredscheme for classifying galaxies. It is based only on morphology. Incontrast, biologists have long taken the genealogical relatedness ofliving entities into account for classification purposes. Aims:.Assuming branching evolution of galaxies as a "descent withmodification", we show here that the concepts and tools of phylogeneticsystematics that are widely used in biology can be heuristicallytransposed to the case of galaxies. Methods: .This approach,which we call "astrocladistics", is applied to dwarf galaxies of theLocal Group and provides the first evolutionary tree for realgalaxies. Results: .The trees that we present here are solidenough to support the existence of a hierarchical organisation in thediversity of dwarf galaxies of the Local Group. They also show thatthese galaxies all stem from a common ancestral kind of object. We findthat some kinds of dIrrs are progenitors of both dSphs and other kindsof dIrrs. We also identify three evolutionary groups, each one with itsown characteristics and own evolution. Conclusions: .The presentwork opens a new way to analysing galaxy evolution and a path towards anew systematics of galaxies. Work on other galaxies in the Universe isin progress.
| Photometric survey of the polar ring galaxy NGC 6822
Context: .We have previously established, from a carbon star survey,that the Local Group dwarf irregular galaxy NGC 6822 is much larger thanits central bright core. Aims: . Four MegaCam fields are acquiredto survey a 2°× 2° area centred on NGC 6822 to fullydetermine its extent and map its stellar populations. Methods:.Photometry of over one million stars is obtained in the SDSS g', r', i'to three magnitudes below the TRGB. RGB stars, selected from theirmagnitudes and colours, are used to map the NGC 6822 stellardistribution up to a distance of 60 arcmin. Results: .We map thereddening over the whole area. We establish that the stellar outerstructure of NGC 6822 is elliptical in shape, with ɛ=0.36 and amajor-axis PA = 65°, contrasting with the orientation of the HIdisk. The density enhancement can be seen up to a semi-major axis of 36'making NGC 6822 as big as the Small Magellanic Cloud. We fit twoexponentials to the surface density profile of the spheroid, andidentify a bulge with a scale length of 3.85' and an outer spheroid witha scale length of 10.0'. We find intermediate-age C stars up to 40'while demonstrating that the SDSS filters are unsuitable to identifyextragalactic C stars. Conclusions: .NGC 6822 is a unique LocalGroup galaxy with shape and structure suggesting a polar ringconfiguration. Radial velocities of carbon stars have indeeddemonstrated that there are two kinematical systems in NGC 6822.
| 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.
| 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).
| Star formation histories in local group dwarf galaxies [review article]
I review observations made with the Hubble Space Telescope which haveimproved our view of both recent (ages ⩽1 Gyr) and ancient (ages⩾1 Gyr) star formation histories in dwarf galaxies. The method ofreconstructing recent star formation histories has been well tested, andnow the major challenge is to build a large database of suitableobservations of nearby dwarf irregular galaxies. With the exception ofthe dSph companions of our Galaxy, questions concerning the ancient starformation histories of nearby galaxies are stymied by a lack of suitablydeep imaging observations. The few observations which do exist providetantalizing evidence of strong evolution in star formation rates. Thisevolution is likely due to environmental effects, and we may be seeingevidence of the effects of reionization on the star formation historiesof dwarf galaxies. Due to its wide field of view and its excellentimaging resolution, the proposed model for SNAP could solve theseproblems.
| 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 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.
| Do Young Galaxies Exist in the Local Universe? Red Giant Branch Detection in the Metal-poor Dwarf Galaxy SBS 1415+437
We present results from an HST/ACS imaging study of the metal-poor bluecompact dwarf galaxy SBS 1415+437. It has been argued previously thatthis is a very young galaxy that started to form stars only <~100 Myrago. However, we find that the optical color-magnitude diagramprominently reveals asymptotic giant branch and red giant branch (RGB)stars. The brightness of the RGB tip yields a distance D~13.6 Mpc. Thecolor of the RGB implies that its stars must be older than ~1.3 Gyr,with the exact age depending on the assumed metallicity and dustextinction. The number of RGB stars implies that most of the stellarmass resides in this evolved population. In view of these and other HSTresults for metal-poor galaxies, it seems that the local universe simplymay not contain any galaxies that are currently undergoing their firstburst of star formation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., for NASA under contract NAS5-26555.
| 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.
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
| FUSE Observations of Interstellar and Intergalactic Absorption toward the X-Ray-bright BL Lacertae Object Markarian 421
High-quality Far Ultraviolet Spectroscopic Explorer (FUSE) observationsat 20 km s-1 resolution of interstellar and intergalacticabsorption from 910 to 1187 Å are presented for the X-ray-brightBL Lac object Mrk 421. These observations are supplemented with FUSEdata for the distant halo stars BD +38°2182 and HD 93521 near theMrk 421 line of sight, in order to obtain information about the distanceto absorbing structures in the Milky Way toward Mrk 421. The FUSE ISMobservations provide measures of absorption by O VI and many otherspecies commonly found in warm neutral and warm ionized gas, including HI, C II, C III, O I, N I, N II, Fe II, and Fe III. In this study weconsider the O VI absorption between -140 and 165 km s-1 andits relationship to the lower ionization absorption and strongabsorption produced by O VII and O VIII at X-ray wavelengths. The O VIabsorption extending from -140 to 60 km s-1 is associatedwith strong low-ionization gas absorption and originates in the Galacticthick disk/halo. This O VI appears to be produced by a combination ofprocesses, including conductive interfaces between warm and hot gas andpossibly cooling Galactic fountain gas and hot halo gas bubbles. The OVI absorption extending from 60 to 165 km s-1 has unusualionization properties in that there is very little associatedlow-ionization absorption, with the exception of C III. This absorptionis not observed toward the two halo stars, implying that it occurs ingas more distant than 3.5 kpc from the Galactic disk. Over the 60-165 kms-1 velocity range, O VI and C III absorption have the samekinematic behavior. The ratio N(OVI)/N(CIII)=10+/-3 over the 60-120 kms-1 velocity range. Given the association of O VI with C III,it is unlikely that the high-velocity O VI coexists with the hotter gasresponsible for the O VII and O VIII absorption. The O VI positivevelocity absorption wing might be tracing cooler gas entrained in a hotGalactic fountain outflow. The O VII and O VIII absorption observed byChandra and XMM-Newton may trace the hot gas in a highly extended (~100kpc) Galactic corona or hot gas in the Local Group. The low resolutionof the current X-ray observations (~750-900 km s-1) and thekinematical complexity of the O VI absorption along typical lines ofsight through the Milky Way halo make it difficult to clearly associatethe O VI absorption with that produced by O VII and O VIII. A search formetal lines associated with the Lyα absorber at z=0.01, which issituated in a galactic void, was unsuccessful.
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