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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.
| 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.
| 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 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%.
| Associations of Dwarf Galaxies
The Hubble Space Telescope Advanced Camera for Surveys has been used todetermine accurate distances for 20 galaxies from measurements of theluminosity of the brightest red giant branch stars. Five associations ofdwarf galaxies that had originally been identified based on strongcorrelations on the plane of the sky and in velocity are shown to beequally well correlated in distance. Two more associations with similarproperties have been discovered. Another association is identified thatis suggested to be unbound through tidal disruption. The associationshave the spatial and kinematic properties expected of bound structureswith (1-10)×1011 Msolar. However, theseentities have little light, with the consequence that the mass-to-lightratios are in the range 100-1000 MsolarL-1solar. Within a well-surveyed volume extendingto a 3 Mpc radius, all but one known galaxy lie within one of the groupsor associations that have been identified.
| 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.
| 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.
| On the geometrical evolution of the ionized gas in HII galaxies
Aims.In this paper, we investigate the behaviour of the number of Lymancontinuum ionizing photons as compared to the actual number of hydrogenrecombinations in HII galaxies. Methods: .We evaluate the numberof ionizing photons from the population synthesis of spectra observed inthe visible, extrapolating the spectra to the extreme ultraviolet (EUV),beyond the Lyman limit. We check for possible systematic deviations ofthe predicted ionizing spectra in the EUV by comparing the ratio of thepredicted number of ionizing photons to the number of recombinations, asmeasured in Hβ, {Δ log Q(H^0)}, with the metallicity. We findthat, as far as the number of ionizing photons is concerned, nosystematic tendency can be detected. The Hβ equivalent width can beunderstood as a nebular age indicator, decreasing with age, although theobserved Hβ equivalent width can also be affected by thecontribution to the continuum by the accumulation of previous,non-ionizing stellar populations. Results: .We attribute theincrease of {Δ log Q(H^0)} with the age of the burst to the factthat more and more ionizing photons escape the nebulae when the nebulaeget older, because of their increasing, expansion-inducedsubfragmentation.
| The AMIGA sample of isolated galaxies. II. Morphological refinement
We present a refinement of the optical morphologies for galaxies in theCatalog of Isolated Galaxies that forms the basis of the AMIGA (Analysisof the interstellar Medium of Isolated GAlaxies) project. Uniformreclassification using the digitized POSS II data benefited from thehigh resolution and dynamic range of that sky survey. Comparison withindependent classifications made for an SDSS overlap sample of more than200 galaxies confirms the reliability of the early vs. late-typediscrimination and the accuracy of spiral subtypes within Δ T =1-2. CCD images taken at the Observatorio de Sierra Nevada were alsoused to solve ambiguities in early versus late-type classifications. Aconsiderable number of galaxies in the catalog (n = 193) are flagged forthe presence of nearby companions or signs of distortion likely due tointeraction. This most isolated sample of galaxies in the local Universeis dominated by two populations: 1) 82% are spirals (Sa-Sd) with thebulk being luminous systems with small bulges (63% between types Sb-Sc)and 2) a significant population of early-type E-S0 galaxies (14%). Mostof the types later than Sd are low luminosity galaxies concentrated inthe local supercluster where isolation is difficult to evaluate. Thelate-type spiral majority of the sample spans a luminosity rangeMB-corr = -18 to -22 mag. Few of the E/S0 population are moreluminous than -21.0 marking the absence of the often-sought superL* merger (e.g. fossil elliptical) population. The rarity ofhigh luminosity systems results in a fainter derived M* forthis population compared to the spiral optical luminosity function(OLF). The E-S0 population is from 0.2 to 0.6 mag fainter depending onhow the sample is defined. This marks the AMIGA sample as unique amongsamples that compare early and late-type OLFs separately. In othersamples, which always involve galaxies in higher density environments,M^*_E/S0 is almost always 0.3-0.5 mag brighter than M^*_S, presumablyreflecting a stronger correlation between M* andenvironmental density for early-type galaxies.
| Dynamical and chemical evolution of NGC 1569
Blue Compact Dwarf and Dwarf Irregular galaxies are generally believedto be unevolved objects, due to their blue colors, compact appearanceand large gas fractions. Many of these objects show an ongoing intenseburst of star formation or have experienced it in the recent past. Bymeans of 2-D hydrodynamical simulations, coupled with detailed chemicalyields originating from SNeII, SNeIa, and intermediate-mass stars, westudy the dynamical and chemical evolution of model galaxies withstructural parameters similar to NGC 1569, a prototypicalstarburst galaxy. A burst of star formation with short duration is notable to account for the chemical and morphological properties of thisgalaxy. The best way to reproduce the chemical composition of thisobject is by assuming long-lasting episodes of star formation and a morerecent burst, separated from the previous episodes by a short quiescentperiod. The last burst of star formation, in most of the explored cases,does not affect the chemical composition of the galaxy, since theenriched gas produced by young stars is in a too hot phase to bedetectable with the optical spectroscopy. Models assuming the infall ofa big cloud towards the center of the galaxy reproduce the chemicalcomposition of the NGC 1569, but the pressure exercised by thecloud hampers the expansion of the galactic wind, at variance with whatobserved in NGC 1569.
| 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.
| 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).
| 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.
| The Distance and Metallicity of the Newly Discovered, Nearby Irregular Galaxy HIZSS 3
HIZSS 3 is an H I source in the Zone of Avoidance. Its radiocharacteristics are consistent with it being a previously unknown,nearby (~1.8 Mpc), low-mass dwarf irregular (dIrr) galaxy. Opticalobservations have shown that it contains a modest H II region, but theyfailed to reveal a resolved stellar population. New spectroscopicobservations of the H II region obtained at the MMT Observatory arepresented here. They are used to derive the line-of-sight extinction[E(B-V)=1.41+/-0.04] and gas metallicity (logO/H+12~7.8) of the H IIregion. New near-IR imaging observations obtained at the ESO Very LargeTelescope are also presented here. These images clearly reveal theresolved stellar population of HIZSS 3 for the first time. NarrowbandPaβ images of the H II region are used in combination withpreviously published Hα data to obtain an independentline-of-sight extinction estimate: E(B-V)=1.32+/-0.04. The adoptedforeground extinction is E(B-V)=1.36+/-0.06. Using the K-band luminosityfunction and K,J-K color-magnitude diagram, the apparent magnitude andcolor of the tip of the red giant branch are derived. In turn, theseparameters are combined with the adopted foreground extinction toestimate the distance (1.69+/-0.07 Mpc) and mean red giant branchmetallicity ([Fe/H]=-0.5+/-0.1). As an ensemble, these new observationssignificantly strengthen the conclusion that HIZSS 3 is a newlydiscovered low-mass dIrr galaxy lurking behind the Milky Way in theoutskirts of the Local Group.The optical spectroscopic observations reported here were obtained atthe MMT Observatory, a joint facility of the Smithsonian Institution andthe University of Arizona. The near-IR imaging observations reportedhere were collected at the European Southern Observatory, Cerro Paranal,Chile, within observing program 271.B-5047.
| 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.
| Infrared Properties of Star-forming Dwarf Galaxies. I. Dwarf Irregular Galaxies in the Local Volume
A sample of 34 dwarf irregular galaxies (dIs) in the Local Volume, mostnearer than 5 Mpc, has been imaged in the near-infrared (NIR) in J andKs at the Canada-France-Hawaii Telescope (CFHT) in Hawaii andthe Observatorio Astronómico Nacional in the Sierra San PedroMártir, in Mexico. Absolute magnitudes in Ks rangefrom -14 to -18. In the CFHT images, stars brighter thanMKs~-7.5 were resolved. We show that the resolvedcomponent comprises more than 50% of the light from star formationbursts within the last 3 Gyr. In most cases, the resolved populationdown to MKs=-7.5 represents less than 5% of thetotal NIR flux in Ks, with fractions in J being 1.5-2 timeslarger. Thus, the NIR light of dIs can be considered to be predominantlycontributed by stars older than about 4 Gyr. Although exponential atlarge radii, surface brightness profiles for the unresolved componentflatten in the centers. They can be fitted across the whole range ofradii with a hyperbolic secant (sech) defined as a function of twoparameters: the central surface brightness and the scale length of theexponential. With respect to this model, only two galaxies (NGC 1569 andNGC 3738) show an excess of flux in the center, both of which arehosting starbursts. Isophotal, total, and fitted sech magnitudes havebeen calculated for all galaxies for which the unresolved component wasdetected, along with semimajor axes at μJ=23 magarcsec-2 and μKs=22 magarcsec-2. The scale length and the semimajor axes correlatelinearly with absolute isophotal magnitude. The same is true for colorsand the central brightness. More luminous dIs tend to be larger, redder,and brighter in the center. The fraction of light contributed by youngstars is independent of both luminosity and central surface brightness.The Tully-Fisher relation shows considerable scatter, but residuals aretied to surface brightness. The galaxies appear to lie in a``fundamental plane'' defined by the sech absolute magnitude, the sechcentral surface brightness, and the H I line width. The rms of residualsin MK is only 0.4 mag, which implies that the plane can beused to evaluate the distances of star-forming dwarfs. Corrections fortilt do not reduce the residuals, so line widths must be governedpredominantly by random motions. Color-magnitude diagrams (CMDs) arepresented for 29 galaxies in which stars were resolved. Most show afinger centered around J-Ks=1 mag. In some cases, there is ared tail extending to J-Ks=2.5 mag. Most color profilesconstructed for the unresolved component show a remarkably constantJ-Ks=0.8-1.0 mag, matching the color of the finger in theCMDs.Based on observations obtained at the Canada-France-Hawaii Telescope,which is operated by the National Research Council of Canada, the CentreNational de la Recherche Scientifique de France, and the University ofHawaii; also based on data acquired at OAN-SPM in Mexico.
| Spectrophotometry of Sextans A and B: Chemical Abundances of H II Regions and Planetary Nebulae
We present the results of high-quality long-slit spectroscopy ofplanetary nebulae (PNe) and H II regions in the two dwarf irregular(dIrr) galaxies Sextans A and B, which belong to a small group ofgalaxies just outside the Local Group. The observations were obtainedwith the New Technology Telescope ESO Multi-Mode Instrument. In SextansA we obtained the element abundances in its only known PN and in three HII regions with the classical Te method. The oxygenabundances in these three H II regions of Sextans A are all consistentwithin the individual rms uncertainties, with an average12+log(O/H)=7.54+/-0.06. The oxygen abundance of the PN in Sextans A is,however, significantly higher: 12+log(O/H)=8.02+/-0.05. This PN is evenmore enriched in nitrogen and helium, suggesting a classification as aPN of type I. The PN abundances of S and Ar, which are presumablyunaffected by nucleosynthesis in the progenitor star, are well belowthose in the H II regions, indicating lower metallicity at the epoch ofthe PN progenitor formation (~1.5 Gyr ago, according to our estimatesbased on the PN parameters). In Sextans B we obtained spectra of one PNand six H II regions. Element abundances with the Te methodcould be derived for the PN and three of the H II regions. For two ofthese H II regions, which have a separation of only ~70 pc inprojection, the oxygen abundances do not differ within the rmsuncertainties, with a mean of 12+log(O/H)=7.53+/-0.05. The third H IIregion, which is about 0.6 kpc northeast of the first two, is twice asmetal-rich, with 12+log(O/H)=7.84+/-0.05. This suggests considerableinhomogeneity in the present-day metallicity distribution in Sextans B.Whether this implies a general chemical inhomogeneity among populationsof comparable age in Sextans B, and thus a metallicity spread at a givenage, or whether we happen to see the short-lived effects of freshlyejected nucleosynthesis products prior to their dispersal and mixingwith the ambient interstellar medium will require further study. For thePN we measured an O/H ratio of 12+log(O/H)=7.47+/-0.16, consistent withthat of the low-metallicity H II regions. We discuss the new metallicitydata for the H II regions and PNe in the context of the published starformation histories and published abundances of the two dIrr galaxies.Both dIrrs show generally similar star formation histories in the senseof continuous star formation with amplitude variations but differ intheir detailed enrichment timescales and star formation rates as afunction of time. If we combine the photometrically derived estimatesfor the mean metallicity of the old red giant branch population in bothdIrrs with the present-day metallicity of the H II regions, both dIrrshave experienced chemical enrichment by at least 0.8 dex (lower limit)throughout their history.Based on observations obtained at the European Southern Observatory, LaSilla, Chile [072.A-0087(B)].
| The Local Group and Other Neighboring Galaxy Groups
Over the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ``ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| The chemistry of planetary nebulae and HII regions in the dwarf galaxies Sextans A and B from deep VLT spectra
Spectroscopic observations obtained with the VLT of one planetary nebula(PN) in Sextans A and of five PNe in Sextans A and of several H iiregions in these two dwarf irregular galaxies are presented. Theextended spectral coverage, from 320.0 to 1000.0 nm, and the largetelescope aperture allowed us to detect a number of emission lines,covering more than one ionization stage for several elements (He, O, S,Ar). The electron temperature diagnostic O iii line at 436.3 nm wasmeasured in all six PNe and in several H ii regions allowing for anaccurate determination of the ionic and total chemical abundances bymeans of the Ionization Correction Factors method. For the time being,these PNe are the farthest ones where such a direct measurement of theelectron temperature is obtained. In addition, all PNe and H ii regionswere also modelled using the photoionization code CLOUDY (Ferland et al.1998, PASP, 110, 761). The physico-chemical properties of PNe and H iiregions are presented and discussed. A small dispersion in the oxygenabundance of H ii regions was found in both galaxies: 12 + log(O/H)=7.6± 0.2 in Sextans A, and 7.8± 0.2 in Sextans B.For the five PNe of Sextans B, we find that 12 + log (O/H)=8.0±0.3, with a mean abundance consistent with that of H ii regions. Theonly PN known in Sextans A appears to have been produced by a quitemassive progenitor, and has a significant nitrogen overabundance. Inaddition, its oxygen abundance is 0.4 dex larger than the mean abundanceof H ii regions, possibly indicating an efficient third dredge-up formassive, low-metallicity PN progenitors. The metal enrichment of bothgalaxies is analyzed using these new data.
| Study of DDO 68: nearest candidate for a young galaxy?
We present the results of optical spectroscopy and imaging with the SAO6 m telescope for the dwarf galaxy DDO 68 (UGC 5340 = VV 542), fallinginto the region of very low density of luminous (L > L*)galaxies (Lynx-Cancer void). Its deep images in V,R bands and in thenarrow Hα-filter show that this galaxy has the very irregularmorphology, with a long curved tail on the South and a ring-likestructure at the Northern edge. The latter consists of 5 separateregions, in three of which we could measure O/H by the classicalTe method. Their weighted mean oxygen abundance correspondsto 12+log (O/H)=7.21 ± 0.03, coincident within uncertainties withthose for I Zw 18. The (V-R) colour of DDO 68 is rather blue all overthe galaxy, indicating the youth of its stellar populations. Comparingthe (V-R)0 colour of the underlying exponential disk of0.12m±0.04 with the PEGASE.2 models for the evolving stellarclusters, we give the first estimate of the ages of the oldest stellarpopulation, which needs confirmation by the other colours and thephotometry of resolved stars. These ages are in the range of 200-900 Myrfor continuous star formation law, and ~100-115 Myr for theinstantaneous starburst. We discuss the properties and the possibleyouth of this nearby object (~2.3 times closer than the famous younggalaxy I Zw 18) in the context of its atypical environment.
| Using SKA to observe relativistic jets from X-ray binary systems
I briefly outline our current observational understanding of therelativistic jets observed from X-ray binary systems, and how theirstudy may shed light on analogous phenomena in active galactic nucleiand gamma ray bursts. How SKA may impact on this field is sketched,including the routine tracking of relativistic ejections to largedistances from the binaries, detecting and monitoring the radiocounterparts to ‘quiescent’ black holes, and detecting theradio counterparts of the brightest X-ray binaries throughout the localgroup of galaxies.
| The Effect of Metallicity on Cepheid-based Distances
We have used the Wide Field Planetary Camera 2 on board the Hubble SpaceTelescope to obtain V and I images of seven nearby galaxies. For each,we have measured a distance using the tip of the red giant branch (TRGB)method. By comparing the TRGB distances with published Cepheiddistances, we investigate the metallicity dependence of the Cepheidperiod-luminosity relation. Our sample is supplemented by 10 additionalgalaxies for which both TRGB and Cepheid distances are available in theliterature, thus providing a uniform coverage in Cepheid abundancesbetween 1/20 and 2 (O/H)solar. We find that the differencebetween Cepheid and TRGB distances decreases monotonically withincreasing Cepheid abundance, consistent with a mean metallicitydependence of the Cepheid distance moduli ofδ(m-M)/δ[O/H]=-0.24+/-0.05 mag dex-1.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated with programGO-8584.
| Classical Cepheids and the Distances of HST Program Galaxies
The distances of HST program galaxies are revised using the PL-relationswe have obtained previously along with a different method from thatemployed by Freedman et al. On the average, the resulting distances tothese galaxies have higher internal accuracies than those obtainedbefore by others. In addition, we have used no corrections formetallicity or for the incompleteness of the samples of classicalcepheids in deriving these distances. Despite this, our distance moduli,with a dispersion of ±0m.395, agree with those of Freedman et al.This indicates that these two effects have little or even no effect forthe samples of classical cepheids in the HST program galaxies.
| Star Formation Properties of a Large Sample of Irregular Galaxies
We present the results of Hα imaging of a large sample ofirregular galaxies. Our sample includes 94 galaxies with morphologicalclassifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems.The sample spans a large range in galactic parameters, includingintegrated absolute magnitude (MV of -9 to -19), averagesurface brightness (20-27 mag arcsec-2), current starformation activity (0-1.3 Msolar yr-1kpc-2), and relative gas content(0.02-5Msolar/LB). The Hα images were usedto measure the integrated star formation rates, determine the extents ofstar formation in the disks, and compare azimuthally averaged radialprofiles of current star formation to older starlight. The integratedstar formation rates of Im galaxies normalized to the physical size ofthe galaxy span a range of a factor of 104 with 10% Imgalaxies and one Sm system having no measurable star formation at thepresent time. The BCDs fall, on average, at the high star formation rateend of the range. We find no correlation between star formation activityand proximity to other cataloged galaxies. Two galaxies located in voidsare similar in properties to the Sm group in our sample. The H IIregions in these galaxies are most often found within the Holmbergradius RH, although in a few systems H II regions are tracedas far as 1.7RH. Similarly, most of the star formation isfound within three disk scale lengths RD, but in somegalaxies H II regions are traced as far as 6RD. A comparisonof Hα surface photometry with V-band surface photometry shows thatthe two approximately follow each other with radius in Sm galaxies, butin most BCDs there is an excess of Hα emission in the centers thatdrops with radius. In approximately half of the Im galaxies Hα andV correspond well, and in the rest there are small to large differencesin the relative rate of falloff with radius. The cases with stronggradients in the LHα/LV ratios and with highcentral star formation rate densities, which include most of the BCDs,require a significant fraction of their gas to migrate to the center inthe last gigayear. We discuss possible torques that could have causedthis without leaving an obvious signature, including dark matter barsand past interactions or mergers with small galaxies or H I clouds.There is now a substantial amount of evidence for these processes amongmany surveys of BCDs. We note that such gas migration will also increasethe local pressure and possibly enhance the formation of massive denseclusters but conclude that the star formation process itself does notappear to differ much among BCD, Im, and Sm types. In particular, thereis evidence in the distribution function for Hα surface brightnessthat the turbulent Mach numbers are all about the same in these systems.This follows from the Hα distribution functions corrected forexponential disk gradients, which are log-normal with a nearly constantdispersion. Thus, the influence of shock-triggered star formation isapparently no greater in BCDs than in Im and Sm types.
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