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A new RR Lyrae Survey of the Galactic Halo using QUEST Collaboration Data
This thesis project intends to study the known substructures in thegalactic halo and search for new ones using data from the QUESTcollaboration. These substructures are believed to be remnants ofglobular clusters or accreted dwarf galaxies, similar to the Sagittariusdwarf spheroidal galaxy. RR Lyrae stars will be used to trace the halobecause they are relatively easy to detect and because they areexcellent standard candles whose distances are better determined thanalternative tracers,e.g., M giants. Part of the project involvesspectroscopic observation of RR Lyrae stars in the leading stream fromthe Sgr dSph galaxy, which will better determine its spatial dimensions,structure, radial velocity and velocity dispersion. These propertiesconstrain models of the stream that assume different shapes for the darkmatter halo.
| 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.
| 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 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).
| Resolving the mystery of the dwarf galaxy HIZSS003
The nearby galaxy HIZSS003 was recently discovered during a blind HIsurvey of the Zone of Avoidance. Follow-up Very Large Array (VLA)observations as well as optical and near-infrared imaging andspectroscopy confirm that it is a low-metallicity dwarf irregulargalaxy. However, there are two puzzling aspects of these observations:(i) current star formation, as traced by Hα emission, is confinedto a small region at the edge of the VLA HI image; and (ii) themetallicity of the older red giant branch stars is higher than that ofthe gas in the HII region. We present high spatial and velocityresolution Giant Metrewave Radio Telescope observations which resolvethese puzzles by showing that HIZSS003 is actually a galaxy pair andthat the HII region lies at the centre of a much smaller companiongalaxy (HIZSS003B) to the main galaxy (HIZSS003A). The HI emission fromthese two galaxies overlaps in projection, but can be separated invelocity space. HIZSS003B has an HI mass of 2.6 ×106Msolar, and a highly disturbed velocity field.Since the velocity field is disturbed, an accurate rotation curve cannotbe derived; however, the dynamical mass indicated is ~5 ×107Msolar. For the bigger galaxy HIZSS003A wederive an HI mass of 1.4 × 107Msolar. Thevelocity field of this galaxy is quite regular, and from its rotationcurve we derive a total dynamical mass of ~6.5 ×108Msolar.
| 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 structure of our stellar system.
Not Available
| The Stellar Structure of Irregular Galaxies. Face-on Galaxies
Stellar photometry of nearby irregular galaxies of the Local Group isused to identify and study the young and old stellar populations ofthese galaxies. An analysis of the spatial distributions of stars ofdifferent ages in face-on galaxies shows that the young stellarpopulations in irregular galaxies are concentrated toward the center,and form local inhomogeneities in star-forming regions, while the oldstellar populationsred giantsform extended structuresaround the irregular galaxies. The sizes of these structures exceed thevisible sizes of the galaxies at the 25m2 isophote by a factor of two tothree. The surface density of the red giants decreases exponentiallyfrom the center toward the edge, similar to the disk components inspiral galaxies.
| 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.
| DDO 43: A Prototypical Dwarf Irregular Galaxy?
We present sensitive and high-resolution 21 cm observations of the dwarfirregular (Im) galaxy DDO 43, in conjunction with optical broadband andnarrowband images in U, B, V, and Hα. The observations are used toexamine the relationship of its H I morphology and kinematics to pastand present star formation. Optically, it is a small (R25=990pc), faint (MB of -14.0) dwarf Im with a slightly boxy shape.In H I, DDO 43 has an extended (RHI/RH=2.8) gasenvelope. There is a high-density ridge associated with the optical bodyof the galaxy containing several higher density knots and lower densityholes. The largest hole is ~850×530 pc. No expansion is detected,so it must be relatively old. The largest and potentially oldest (7-70Myr) of the six identified star clusters is located at the western edgeof the hole. Four of the other clusters are located near high-densitypeaks. There are several H II regions, most (but not all) of which areassociated with peaks in the H I surface density. The overall starformation rate is average for its type. In many ways, DDO 43 is a verytypical dwarf Im galaxy. Its H I morphology is consistent with a historyof episodes of localized star formation that create holes and shells inthe interstellar medium, some of which can overlap. These features arelocated within the area of solid-body rotation in the galaxy; the lackof shear in these small systems allows such structures to persist forlong periods of time.
| 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 standard candle aspect of carbon stars
This paper represents the second part of our effort to summarise variousaspects of our homogeneous survey of carbon stars in nearby galaxies.Here we investigate the relationships between the size of the C starpopulations and the luminosity of the parent galaxies. We also explorethe constancy of < M_I> for various metallicities and propertiesof the parent galaxies. We conclude that C stars are a viable standardcandle for galaxies large enough to contain one hundred or more C stars.The application of narrow-band filters limits, however, the use of Cstars as standard candles to no more than ~2 Mpc with currentlyavailable ground-based telescopes. Near-IR photometry from spaceobservations can significantly push the limit of detectability of Cstars thus making them an interesting alternative to the TRGB method.
| HST/ACS observations of the old and metal-poor Sagittarius dwarf irregular galaxy
We have obtained deep color-magnitude diagrams (CMDs) of the Sagittariusdwarf irregular galaxy with the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope. The new diagrams show for the firsttime the star formation history of SagDIG. The young main-sequence istraced down to mF606W ≃27.5, well separated from theolder He burning blue loop stars. The wide color range spanned by theblue and red super-giants is consistent with Padua evolutionary modelswith Z0.0004. A well-defined red giant branch indicates that thegalaxy is dominated by a stellar population older than 1 Gyr, for whichwe have estimated a metallicity in the range [ Fe/H] = -2.2 to -1.9depending on the adopted reddening. The identification of several Cstars on a luminous asymptotic giant branch, as well as the presence ofa conspicuous red clump with an elongated structure, are indicative ofan extended star formation epoch between 1 and 10 Gyr ago. Perhaps mostimportantly, we have identified a small population of stars on agenuinely old red horizontal branch at mF606W=25.70±0.05, which sets the first epoch of star formation inSagDIG back to 9{-}10 Gyr ago. This shows that SagDIG has been able tomaintain a very low mean metallicity over a 10 Gyr life time, and thatall Local Group dwarf galaxies share an ancient (≥10 Gyr) initialepisode of star formation. Thus, at the moment, I Zw 18 remains the onlyexception to this general trend, favoring the so-called "young galaxy"scenario where a very low metallicity galaxy is not necessarily old.However, a re-analysis of ACS observations of I Zw 18 suggests that anolder RGB population may be present also in I Zw 18. In order to addressthe interplay between stars and Inter-Stellar Medium, we compare thedistribution of atomic hydrogen with that of stellar populations ofvarious ages. The youngest stars are found in clumps located: (i) nearto, but not coincident with, the highest column densities of H I, and(ii) in tails reminiscent of propagating star formation. We find,however, little correlation between stellar populations older than 1Gyr and the H I. In particular we fail to find any generation of starsthat preferentially lie within the large H I hole. We therefore excludethe possibility that the ring-like H I distribution is the result ofmultiple supernova events. Alternative scenarios have to be explored forthe ring-like H I morphology of SagDIG, which amongst dwarf irregulargalaxies, is not unusual.
| The calibration of the metallicity versus C/M relation
This paper presents a critical reassessment of the C/M ratio in theLocal Group galaxies that have been surveyed by us in the last fewyears. We update distances, reddenings and metallicities with particularcare to obtain coherent values for each galaxy. A new equation for thebolometric correction of M stars in terms of their (R-I) colours isobtained from Bessell et al. (1998) data. We present a criticaldiscussion on the colour - spectral type relation for M stars whichreveals a dramatic lack of reliable data for late M stars. Finally, weshow that - when homogeneous data are used - the log(C/M0+) is wellcorrelated to the [Fe/H] of the parent galaxy.
| Subaru/HDS Abundances in Three Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy
With the HDS (High Dispersion Spectrograph) on the Subaru Telescope, weobtained high-resolution optical region spectra of three red giant stars(COS 4, COS 82, and COS 347) in the Ursa Minor dwarf spheroidal galaxy.The chemical abundances in these stars were analyzed for 26 elements,including α-, iron-peak, and neutron-capture elements. All threestars show low abundances of α-elements (Mg, Si, and Ca), and twostars (COS 82 and COS 347) show high abundances of Mn compared toGalactic halo stars of similar metallicity. One star (COS 4) has beenconfirmed to be very metal deficient ([Fe/H] = ‑2.7) and found toshow anomalously low abundances of Mn, Cu, and Ba. In another star, COS82 ([Fe/H] = ‑1.5), we have found a large excess of heavyneutron-capture elements with a general abundance pattern similar to thescaled solar system r-process abundance curve. These observationalresults are rather puzzling: low abundances of α-elements and highabundance of Mn seem to suggest a significant contribution of SNe Ia atlow metallicity, while there is no hint of an s-process (i.e., AGBstars) contribution, even at [Fe/H] = ‑1.5, suggesting a peculiarnucleosynthetic history of the UMi dSph galaxy.
| 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.
| Trails of solar system minor bodies on WFC/ACS images
In this paper, we analyse very short arcs of minor bodies of the SolarSystem detected on Hubble Space Telescope (HST) Wide Field Channel ACSimages. In particular, we address how to constrain the Keplerian orbitalelements for minor body detections, illustrating the method for twoobjects. One of the minor bodies left 13 successive trails, making itthe most well-sampled object yet identified in the HST archive. Mostinterestingly, we also address the problem of ephemeris prediction andshow that in the particular case of HST very short arcs the confinementwindow for subsequent recovery is significantly reduced to a narrowlinear region, that would facilitate successive observations.
| The baryonic mass-velocity relation: clues to feedback processes during structure formation and the cosmic baryon inventory
We show that a global relation between baryonic mass and virial velocitycan be constructed from the scale of dwarf galaxies up to that of richgalaxy clusters. The slope of this relation is close to that expected ifdark matter haloes form in the standard hierarchical cosmogony andcapture a universal baryon fraction, once the details of halo structureand the adiabatic contraction of haloes due to cooling gas are takeninto account. The scatter and deficiency of baryons within low-masshaloes (Vvir < 50 km s-1) are consistent withthe expected suppression of gas accretion by photoevaporation due to thecosmic UV background at high redshift. The data are not consistent withsignificant gas removal from strong supernovae winds unless thevelocities of galaxies measured from their gas kinematics aresignificantly lower than the true halo velocities for objects withVvir < 100 km s-1. Thus models such as Λcold dark matter (ΛCDM) with a steep mass function of haloes mayfind it difficult to reproduce the baryonic mass-velocity relationpresented here whilst at the same time reproducing the flatluminosity/HI function of galaxies. Galaxies hold about 10 per cent ofthe baryons in the Universe, which is close to the collapsed massfraction expected within hierarchical models on these scales, suggestinga high efficiency for galaxy formation. Most of the baryons are expectedto be evenly distributed between diffuse intergalactic gas inlow-density environments and the intragalactic medium within galaxygroups.
| On the highly reddened members in six young galactic star clusters - a multiwavelength study
The spectral and reddening properties of 211 highly reddenedproper-motion members with V < 15 mag in six young galactic starclusters are investigated using low-resolution spectroscopic, broad-bandUBVRIJHK and mid-infrared (IR) data. We report emission features in CaIIHK and HI lines for a sample of 29 stars including 11 stars reported forthe first time and also provide either a new or more reliable spectralclass for a sample of 24 stars. CaII triplet width measurements are usedto indicate the presence of an accretion disc for a dozen stars and toindicate luminosity for a couple of stars. On the basis of spectralfeatures, near-IR excesses, dereddened colour-colour diagrams and mid-IRspectral indices we identify a group of 28 pre-main-sequence clustermembers including five highly probable Herbig Ae/Be and six classical TTauri stars. A total of 25 non-emission main-sequence (MS) stars,amounting to ~10 per cent early-type MS members, appears to showVega-like characteristics or are precursors to such a phenomenon. Thevarious membership indicators suggest that ~16 per cent of theproper-motion members are non-members. A significant fraction (>70per cent) of programme stars in NGC 1976, NGC 2244, NGC 6530 and NGC6611 show anomalous reddening with RV= 4.78 +/- 0.10, 3.54+/- 0.04, 3.87 +/- 0.05 and 3.56 +/- 0.02, respectively, indicating thepresence of grain size dust larger than that typical of the diffusemedium. A small number of stars in NGC 1976, NGC 2244 and NGC 6611 alsoshow normal behaviour while the cluster NGC 6823 appears to have normalreddening. Three highly luminous late-type giants, one in NGC 2244 andtwo in NGC 6530, appear to be members and are inpost-hydrogen-core-burning stages, suggesting a prolonged duration (~25Myr) of star formation.
| Photoionized Gas in Dark Matter Minihalos in the Galactic Halo and Local Group
We present computations of the metals photoionization structures ofpressure-supported gas clouds in gravitationally dominant dark matterminihalos in the extended Galactic halo or Local Group environment. Weconsider low-metallicity (0.1-0.3 Zsolar) clouds that arephotoionized by the present-day metagalactic radiation field and arealso pressure confined by a hot external medium. We study the propertiesof ΛCDM Burkert (and also NFW) minihalos with characteristiccircular velocities from 12 to 30 km s-1(108-2×109 Msolar). We presentresults for the volume densities and projected column densities oflow-ionization metal atoms and ions, C II, N I, O I, Si II, and S II,produced in the neutral cloud cores, and high ions, C IV, N V, O VI, SiIII, Si IV, and S III, produced in the ionized shielding envelopes. Weexamine the possible relationships between the compact H I high-velocityclouds (CHVCs), dwarf galaxies, and high-velocity ionized C IVabsorbers, within the context of photoionized minihalo models for suchobjects. In pressure-confined (P/k~50 cm-3 K) minihalo modelsfor the CHVCs the photoionization states are much lower than in the C IVabsorbers. However, for lower bounding pressures <~1 cm-3K, in more massive ~2×109 Msolar dwarfgalaxy-scale halos, the photoionization states in the outer envelopesresemble those observed in the C IV HVCs. An important exception is OVI, for which our photoionization models underpredict the relativeabundances by about an order of magnitude, suggesting that additionalprocesses are at play as concluded by Sembach and coworkers. For thecloud size scales expected in median ΛCDM Burkert halos, a gasmetallicity of 0.3 Zsolar is required to produceabsorption-line strengths comparable to those in the C IV HVCs. We arguethat fully ionized and starless ``dark galaxies'' could be detectable inthe local universe as UV metal line absorbers with ionization statessimilar to the C IV absorbers.
| 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.
| WIYN Survey for Carbon Stars in the M31 and Cetus Dwarf Spheroidal Galaxies: Evolutionary Implications
We report results of a photometric survey with the WIYN telescope forcarbon stars in the M31 dwarf spheroidal (dSph) companions And III, V,VI, and VII, as well as in the relatively isolated Local Group dSphCetus. We find three carbon star candidates in And VII and one carbonstar in each And VI and Cetus. Comparing the carbon star content withother Local Group dwarf galaxies, we argue against the presence ofsubstantial intermediate-age stellar populations in the all of thegalaxies surveyed with the exception of And VII. We discuss theseresults in the context of the origin of the Andromeda dSph's andconclude that these are ancient galaxies, most of which ceased starformation long before the main merger events in M31. The M31 dSph'stherefore show less diversity in star formation histories than theGalactic dSph companions, or the M31 dE companions, as illustrated byNGC 147, which was surveyed as a calibration object. All of our dSphtargets except And V have candidate carbon stars below the tip of thered giant branch, which resemble CH stars found in globular clusters. Weestimate that 0.3% of stars in the dSph's are CH stars, presumably as aresult of C pollution from a binary companion. Comparisons with CH starfrequencies in globular clusters could constrain the impact of denseenvironments on the frequency of this form of binary star evolution.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties
We performed a compilation of more than 1000 published spectra of H IIregions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. Theradial distributions of oxygen and nitrogen abundances were derived. Thecorrelations between oxygen abundance and macroscopic properties areexamined. We found that the oxygen abundance in spiral galaxiescorrelates with its luminosity, rotation velocity, and morphologicaltype: the correlation with the rotation velocity may be slightlytighter. There is a significant difference between theluminosity-metallicity relationship obtained here and that based on theoxygen abundances determined through the R23-calibrations.The oxygen abundance of NGC 5457 recently determined using directmeasurements of Te (Kennicutt et al. \cite{Kennicutt2003})agrees with the luminosity-metallicity relationship derived in thispaper, but is in conflict with the luminosity-metallicity relationshipderived with the R23-based oxygen abundances. The obtainedluminosity-metallicity relation for spiral galaxies is compared to thatfor irregular galaxies. Our sample of galaxies shows evidence that theslope of the O/H - MB relationship for spirals (-0.079± 0.018) is slightly more shallow than that for irregulargalaxies (-0.139 ± 0.011). The effective oxygen yields wereestimated for spiral and irregular galaxies. The effective oxygen yieldincreases with increasing luminosity from MB -11 toMB -18 (or with increasing rotation velocity fromVrot 10 km s-1 to Vrot 100km s-1) and then remains approximately constant. Irregulargalaxies from our sample have effective oxygen yields lowered by afactor of 3 at maximum, i.e. irregular galaxies usually keep at least1/3 of the oxygen they manufactured during their evolution.Appendix, Tables \ref{table:refero}, \ref{table:referV}, and Figs.\ref{figure:sample2}-\ref{figure:sample5} are only available inelectronic form at http://www.edpsciences.org}
| The very local Hubble flow: Computer simulations of dynamical history
The phenomenon of the very local (≤3 Mpc) Hubble flow is studied onthe basis of the data of recent precision observations. A set ofcomputer simulations is performed to trace the trajectories of the flowgalaxies back in time to the epoch of the formation of the Local Group.It is found that the ``initial conditions'' of the flow are drasticallydifferent from the linear velocity-distance relation. The simulationsenable one also to recognize the major trends of the flow evolution andidentify the dynamical role of universal antigravity produced by thecosmic vacuum.
| Evidence for a warm interstellar medium in the Fornax dwarf ellipticals FCC046 and FCC207
We present Hα+[NII] narrow-band imaging of FCC046 and FCC207, twodwarf ellipticals (dES) in the Fornax cluster. B-R and B-I colour mapsclearly show signs of dust absorption in FCC207. FCC046 has a verybright blue nucleus, offset by about 1.1 arcsec with respect to theouter isophotes. FCC046 was classified as a non-nucleated dE4 so thepresence of its nucleus came as a surprise. Moreover, FCC046 shows apronounced lopsided shape. Given that FCC046 is an isolated galaxy, itis unlikely that an encounter caused this asymmetry. The emittingregions differ considerably between the two galaxies. Whereas FCC207 hasonly one central emission region, FCC046 also contains fainter emissionregions. Based on broad-band colours, its disturbed shape and its verybright nucleus, FCC046 is akin to the class of amorphous dwarfs. Thecentral emission regions of both galaxies are barely resolved underseeing conditions of FWHM ~0.8 arcsec and we estimate their diameters atabout 60 pc. Their Hα luminosities can be explained asphotoionization by post asymptotic giant branch stars in an oldpopulation. Some of the fainter extended emission regions in FCC046 areresolved and have diameters of the order of 50-150 pc and Hαluminosities of the order of 1030 W, comparable to supernovaremnants or nebulae around Wolf-Rayet stars. Hence, FCC046 is clearlyundergoing star formation while for FCC207 the case is not as clear-cut.We estimate the mass of the HII gas in FCC046 at MHII= 40-150Msolar (for T= 104 K, Ne= 1000cm-3). The ionized-gas content of FCC207 is somewhat higher:MHII= 60-190 Msolar.
| The Padova Survey of Local Group galaxies
The main results of an ongoing survey of the resolved stellarpopulations of Local Group dwarf galaxies are presented. Based both ondeep optical color-magnitude diagrams and wide field data, the evolutionof the star formation and its spatial variations are investigated bymeans of synthetic stellar populations. Radial age gradients andextremely varied star formation histories are found among the galaxiesin our sample.Based on data collected at ESO
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