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The Survey for Ionization in Neutral Gas Galaxies. I. Description and Initial Results
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG),a census of star formation in H I-selected galaxies. The survey consistsof Hα and R-band imaging of a sample of 468 galaxies selected fromthe H I Parkes All Sky Survey (HIPASS). The sample spans three decadesin H I mass and is free of many of the biases that affect otherstar-forming galaxy samples. We present the criteria for sampleselection, list the entire sample, discuss our observational techniques,and describe the data reduction and calibration methods. This paperfocuses on 93 SINGG targets whose observations have been fully reducedand analyzed to date. The majority of these show a single emission linegalaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs ina single field. All of the targets in this sample are detected inHα, indicating that dormant (non-star-forming) galaxies withMHI>~3×107 Msolar are veryrare. A database of the measured global properties of the ELGs ispresented. The ELG sample spans 4 orders of magnitude in luminosity(Hα and R band), and Hα surface brightness, nearly 3 ordersof magnitude in R surface brightness and nearly 2 orders of magnitude inHα equivalent width (EW). The surface brightness distribution ofour sample is broader than that of the Sloan Digital Sky Survey (SDSS)spectroscopic sample, the EW distribution is broader than prism-selectedsamples, and the morphologies found include all common types ofstar-forming galaxies (e.g., irregular, spiral, blue compact dwarf,starbursts, merging and colliding systems, and even residual starformation in S0 and Sa spirals). Thus, SINGG presents a superior censusof star formation in the local universe suitable for further studiesranging from the analysis of H II regions to determination of the localcosmic star formation rate density.
| A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| Galactic Winds
Galactic winds are the primary mechanism by which energy and metals arerecycled in galaxies and are deposited into the intergalactic medium.New observations are revealing the ubiquity of this process,particularly at high redshift. We describe the physics behind thesewinds, discuss the observational evidence for them in nearbystar-forming and active galaxies and in the high-redshift universe, andconsider the implications of energetic winds for the formation andevolution of galaxies and the intergalactic medium. To inspire futureresearch, we conclude with a set of observational and theoreticalchallenges.
| Multiphase High-Velocity Clouds toward HE 0226-4110 and PG 0953+414
We study the physical conditions, elemental abundances, and kinematicsof the high-velocity clouds (HVCs) along the sight lines toward activegalaxies HE 0226-4110 and PG 0953+414 using Hubble Space Telescope SpaceTelescope Imaging Spectrograph and Far Ultraviolet SpectroscopicExplorer data. No 21 cm H I emission is detected in these clouds, butour observations reveal multiple components of HVC absorption in linesof H I, C II, C III, C IV, O VI, Si II, Si III, and Si IV in bothdirections. We investigate whether photoionization by the extragalacticbackground radiation or by escaping Milky Way radiation can explain theobserved ionization pattern. We find that photoionization is a goodexplanation for the C II, C III, Si II, and Si III features but not forthe O VI or C IV associated with the HVCs, suggesting that two principalphases exist: a warm (T~104 K), photoionized phase and ahotter (T=1-3×105 K), collisionally ionized phase; thebroader line widths of the high ions are consistent with this multiphasehypothesis. The warm HVCs toward HE 0226-4110 have high levels ofionization (97%-99%) and metallicities ([Z/H] between -0.9 and -0.4)close to those in the Magellanic Stream, which lies 11° away on thesky at similar velocities. These HVCs may well be stripped fragments ofthe Stream that have been ionized by the pervading radiation field; theyhave thermal pressures that would place them close to equilibrium in afully ionized 106 K Galactic corona withnH=4-9×10-5 cm-3 at 50 kpc. Thewarm HVCs seen at -146 and 125 km s-1 toward PG 0953+414 have[Z/H]=-0.6+/-0.2 and -0.8+/-0.2, respectively, suggesting they are notformed from purely Galactic material. A minisurvey of the hot,collisionally ionized HVC components seen here and in five other sightlines finds that in 11/12 cases, the high ions have kinematics and ionicratios that are consistent with an origin in conductive interfaces,where energy flows into the HVCs from a hot surrounding medium andproduces O VI- and C IV-bearing boundary layers. However, the broadabsorption wing on the O VI profile toward PG 0953+414 is not completelyexplained by the interface scenario. This feature may be tracing theoutflow of hot gas into the Milky Way halo as part of a Galacticfountain or wind.Based on observations from the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer mission, operated by Johns Hopkins University,supported by NASA contract NAS5-32985, and from the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| Probing the Multiphase Interstellar Medium of the Dwarf Starburst Galaxy NGC 625 with Far Ultraviolet Spectroscopic Explorer Spectroscopy
We present new Far Ultraviolet Spectroscopic Explorer (FUSE)spectroscopy of the dwarf starburst galaxy NGC 625. These observationsprobe multiple phases of the interstellar medium (ISM), including thecoronal, ionized, neutral, and molecular gas. This nearby (D=3.9+/-0.2Mpc) system shows a clear detection of outflowing coronal gas as tracedby O VI λ1032 absorption. The centroid of the O VI profile isblueshifted with respect to the galaxy systemic velocity by ~30 kms-1, suggesting a low-velocity outflow. The implied O VIvelocity extent is found to be 100+/-20 km s-1, which isfully consistent with the detected H I outflow velocity found in radiosynthesis observations. We detect multiple lines of diffuseH2 absorption from the ISM of NGC 625; this is one of only afew extragalactic systems with FUSE detections of H2 lines inthe Lyman and Werner bands. We find a potential abundance offset betweenthe neutral and nebular gas that exceeds the errors on the derivedcolumn densities. Since such an offset has been found in multiple dwarfgalaxies, we discuss the implications of a lower-metallicity halosurrounding the central star-forming regions of dwarf galaxies. Theapparent offset may be due to saturation of the observed O I line, andhigher signal-to-noise ratio (S/N) observations are required to resolvethis issue.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985.
| 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 radio continuum of the extremely metal-poor blue compact dwarf galaxy I Zw 18
We present 1.4, 4.8 and 8.4 GHz Very Large Array observations of thelowest metallicity blue compact dwarf (BCD) galaxy known, I Zw 18, witha heavy element abundance of ~2% that of the Sun. The 1.4 and 4.8 GHzimages reveal a halo of mostly non-thermal extended emission, withasymmetric lobes extending laterally in the east-west direction. Weinterpret the radio halo as produced by a superbubble carved out in theinterstellar medium (ISM) by supernovae, with a bipolar outflow orientedalong the galaxy's rotation axis. The overall spectral index is -0.39from 1.4 Ghz to 4.8 Ghz and -0.13 from 4.8 to 8.4 GHz. The radioluminosity of I Zw 18 has a thermal to total emission fraction of 0.30at 1.4 GHz. This fraction increases to 0.41 at 4.8 GHz and to 0.47 at8.4 GHz. The thermal radio luminosity gives a total of 1200 O7 V starsand a star formation rate of 0.1 M_ȯ yr-1. Unlike theBCD SBS 0335-052 which has a similar metallicity and forms stars with ahigh rate in regions which are dense and compact, I Zw 18 makes stars ata smaller rate in complexes which are diffuse and extended. Starformation in BCDs thus appears to occur in a bimodal fashion,independently of the metallicity of the interstellar medium.
| Imaging and photometry of nearby dwarf galaxies. II. Southern dwarfs
We carried out CCD photometry in the Johnson-Cousins B and R bands of 23dwarf galaxies: SDIG, ESO 410-17, KK11, ESO 245-05, KKs3, KK27, KK38,KK40, IC 4662, KK244, KK246, KK247, KK248, KK249, KK253, KK255, KK256,KK257, KK258, KK259, UGCA 438, ESO 347-17, and UGCA 442. Both isolatedgalaxies and members of the Sculptor group and the NGC 1313 group wereobserved. The galaxy sample is characterized by a median distance of 9.3Mpc, and median absolute magnitude of -14.8 mag. The central surfacebrightnesses are in the range from 22.2 to 24.4 mag arcsec-2in B.Based on observations obtained with CTIO 1.5-m telescope, which isoperated by the Association of Universities for Research in AstronomyInc. (AURA), under a cooperative agreement with the National ScienceFoundation as part of the National Optical Astronomy Observatories.Tables 1 and 2, complete Figs. 1 and 2 are only available in electronicform at http://www.edpsciences.org
| Dwarf and Normal Spiral Galaxies: are they Self-Similar?
The investigation presented here was focused on clarifying the existenceof dwarf spiral galaxies as a separate group from classical spirals.First, a list of spiral galaxies with small sizes was obtained.Information on colors, luminosities, morphologies and chemical contentwas searched for in the literature for these galaxies. Using thisinformation, it can be concluded that dwarf spirals are not likely to bethe tail of the distribution of classical galaxies. On the contrary,significant differences in some of the most important properties ofspiral galaxies, such as the metallicity gradient and the bar frecuency,were found. In any case, further and more accurate observations areneeded for a definitive answer.
| The Chandra view of NGC1800 and the X-ray scaling properties of dwarf starbursts
The superb spatial resolution of Chandra is utilized to study the X-raymorphology of the dwarf starburst galaxy NGC1800 embedded in a smallgroup of galaxies. Diffuse galactic emission is detected, extendingseveral kiloparsec above the galactic plane, with an overall morphologysimilar to the galactic winds seen in nearby X-ray-bright starburstgalaxies. This makes NGC1800 the most distant dwarf starburst with aclear detection of diffuse X-ray emission. The diffuse X-ray luminosityof 1.3 +/- 0.3 × 1038ergs-1 accounts for atleast 60 per cent of the total soft X-ray output of the galaxy. A hotgas temperature of kT= 0.25 keV and metallicity Z~ 0.05Zsolarare derived, the latter being consistent with results from opticalspectroscopy of the interstellar medium. Our failure to detect any hotgas associated with the embedding galaxy group translates into an upperlimit to the group X-ray luminosity of LX <1041ergs-1. There is no convincing evidence thatthe outflowing wind of NGC1800 is currently interacting with anyintragroup gas, and mechanical considerations indicate that the wind canescape the galaxy and its surrounding HI halo, eventually deliveringenergy and metals to the intragroup gas. Properties of NGC1800 arecompared to those of other dwarf starburst galaxies, and a firstdetailed discussion of the X-ray scaling properties of this populationof objects is given, set against the equivalent results obtained fornormal starburst galaxies. Results indicate that dwarf starbursts to alarge degree behave as down-scaled versions of normal starburstgalaxies.
| Chemical Abundances of H II Regions in the Starburst Galaxy NGC 1705
We report optical spectroscopy of 16 H II regions in NGC 1705 and [OIII] λ4363 detections for the first time in five H II regions.The resulting mean oxygen abundance derived directly from measuredelectron temperatures is 12+log(O/H)=8.21+/-0.05, which corresponds to[O/H]=-0.45, or 35% of the solar value. There are no significant spatialinhomogeneities in [O III] λ4363 oxygen abundances from H IIregions at a radius approximately 10" from the super star cluster. In HII regions where [O III] λ4363 was not measured, oxygenabundances derived with bright-line methods (accurate only to 0.2 dex)are in agreement with direct values of the oxygen abundance. Faint,narrow He II λ4686 emission is found in two H II regions, but theimplied contribution from O+3 to the total oxygen abundanceis only 0.01 dex. The mean argon-, neon-, and nitrogen-to-oxygenabundance ratios are consistent with mean values for other dwarfirregular galaxies, blue compact dwarf galaxies, and H II galaxies atcomparable oxygen abundances. Interestingly, the nitrogen-to-oxygenabundance ratio in the ionized H II gas agrees with the value for theneutral H I, even though the metallicity of the neutral gas may be afactor of 6 lower than that of the ionized gas. This may be indicativeof low-metallicity gas in the halo of the galaxy. Extinction values,AV, derived from observed Balmer line ratios along lines ofsight to H II regions are in the range between 0 and 0.9 mag.Significant and variable extinction may have important effects on theinterpretation of resolved stellar populations and derived starformation histories. With respect to the metallicity-luminosity andmetallicity-gas fraction diagnostics, the measured oxygen abundance forNGC 1705 is comparable to those of Local Group dwarf irregular galaxiesat a given luminosity and gas fraction. Simple chemical evolution modelssuggest that the galaxy is quickly evolving into a gas-poor dwarfgalaxy.Based on EFOSC2 observations collected at the European SouthernObservatory, Chile.
| The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625
We present new multifrequency radio continuum imaging of the dwarfstarburst galaxy NGC 625 obtained with the Very Large Array. Data at 20,6, and 3.6 cm reveal global continuum emission dominated by free-freeemission, with only mild synchrotron components. Each of the major H IIregions is detected; the individual spectral indices are thermal for theyoungest regions (showing largest Hα emission) and nonthermal forthe oldest. We do not detect any sources that appear to be associatedwith deeply embedded, dense, young clusters, although we have discoveredone low-luminosity, obscured source that has no luminous opticalcounterpart and resides in the region of highest optical extinction.Since NGC 625 is a Wolf-Rayet galaxy with strong recent star formation,these radio properties suggest that the youngest star formationcomplexes have not yet evolved to the point where their thermal spectraare significantly contaminated by synchrotron emission. The nonthermalcomponents are associated with regions of older star formation that havesmaller ionized gas components. These results imply a range of ages forthe H II regions and radio components that agrees with our previousresolved stellar population analysis, where an extended burst of starformation has pervaded the disk of NGC 625 over the last ~50 Myr. Wecompare the nature of radio continuum emission in selected nearby dwarfstarburst and Wolf-Rayet galaxies, demonstrating that thermal radiocontinuum emission appears to be more common in these systems than intypical H II galaxies with less recent star formation and more evolvedstellar clusters.
| The Complex Neutral Gas Dynamics of the Dwarf Starburst Galaxy NGC 625
We present new multiconfiguration H I aperture synthesis imaging of thenearby dwarf starburst galaxy NGC 625 obtained with the AustraliaTelescope Compact Array. Total H I column density images show gas wellaligned with the optical major axis and low column density H I extendingto greater than 6 optical scale lengths. The H I velocity field, on theother hand, is highly disturbed, with neutral gas at nearly all detectedvelocities within the central region. After considering variousinterpretations, we find that a blowout scenario most accuratelydescribes the data. Since at our resolution we do not detect any largeevacuated holes in the H I disk, we interpret this blowout to be theresult of the extended (both spatially and temporally) star formationevent that NGC 625 has undergone in the last 100 Myr. This is one of theclearest examples of H I outflow detected in a dwarf galaxy. We find noobvious external trigger for this extended star formation event. Wedetect strong radio continuum emission from the largest H II regions;comparing to our Hubble Space Telescope and ground-based Hα fluxessuggests either appreciable amounts of extinction toward the starformation regions or the contribution of nonthermal sources to the radiocontinuum luminosity.
| H I Observations of the Local Group Dwarf WLM
We present Australia Telescope Compact Array mosaic H I imaging of theLocal Group dwarf irregular galaxy WLM. We find an integrated flux of149 Jy km s-1 and a total H I mass of(3.2+/-0.3)×107Msolar. The major axis of theH I is aligned with the stellar component of the galaxy. The overall H Idistribution is relatively smooth at our resolution and has adouble-peaked central core. One of these peaks is aligned with a regionfound to have extinction that is internal to WLM, and we take this aspossible evidence of a large molecular gas complex in the southern halfof the galaxy. The other H I peak is in close proximity to the brightestH II regions. WLM's overall velocity field is consistent with rigid-bodyrotation. A rotation curve is derived, and we find a total dynamicalmass of (3.00+/-0.80)×108Msolar. We alsoperformed a wide-field search, 38' in radius, for H I companions orevidence of recent interactions (e.g., tidal tails) and found nodetections to an H I mass limit ofMHI>8.4×105Msolar.
| Rotationally Supported Virgo Cluster Dwarf Elliptical Galaxies: Stripped Dwarf Irregular Galaxies?
New observations of 16 dwarf elliptical galaxies (dEs) in the VirgoCluster indicate that at least seven dEs have significant velocitygradients along their optical major axis, with typical rotationamplitudes of 20-30 km s-1. Of the remaining nine galaxies inthis sample, six have velocity gradients of less than 20 kms-1 kpc-1, while the other three observations hadtoo low a signal-to-noise ratio to determine an accurate velocitygradient. Typical velocity dispersions for these galaxies are ~44+/-5 kms-1, indicating that rotation can be a significant componentof the stellar dynamics of Virgo dEs. When corrected for the limitedspatial extent of the spectral data, the rotation amplitudes of therotating dEs are comparable to those of similar-brightness dwarfirregular galaxies (dIs). Evidence of a relationship between therotation amplitude and galaxy luminosity is found and, in fact, agreeswell with the Tully-Fisher relation. The similarity in the scalingrelations of dIs and dEs implies that it is unlikely that dEs evolvefrom significantly more luminous galaxies. These observations reaffirmthe possibility that some cluster dEs may be formed when the neutralgaseous medium is stripped from dIs in the cluster environment. Wehypothesize that several different mechanisms are involved in thecreation of the overall population of dEs and that stripping ofinfalling dIs may be the dominant process in the creation of dEs inclusters like Virgo.
| 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.
| The Stellar Content of the Southern Tail of NGC 4038/4039 and a Revised Distance
We have used the Hubble Space Telescope and Wide Field Planetary Camera2 to image the putative tidal dwarf galaxy located at the tip of theSouthern tidal tail of NGC 4038/4039, the Antennae. We resolveindividual stars and identify two stellar populations. Hundreds ofmassive stars are present, concentrated into tight OB associations onscales of 200 pc, with ages ranging from 2 to 100 Myr. An older stellarpopulation is distributed roughly following the outer contours of theneutral hydrogen in the tidal tail; we associate these stars withmaterial ejected from the outer disks of the two spirals. The olderstellar population has a red giant branch tip at I=26.5+/-0.2 from whichwe derive a distance modulus (m-M)0=30.7+/-0.25. The implieddistance of 13.8+/-1.7 Mpc is significantly smaller than commonly quoteddistances for NGC 4038/4039. In contrast to the previously studied coreof the merger, we find no super-star clusters (SSCs). One might concludethat SSCs require the higher pressures found in the central regions inorder to form, while spontaneous star formation in the tail produces thekind of OB star associations seen in dwarf irregular galaxies. Theyoungest population in the putative tidal dwarf has a total stellar massof ~2×105 Msolar, while the old populationhas a stellar mass of ~7×107 Msolar. If oursmaller distance modulus is correct, it has far-reaching consequencesfor this prototypical merger. Specifically, the luminous to dynamicalmass limits for the tidal dwarf candidates are significantly less than1, the central SSCs have sizes typical of Galactic globular clusters,rather than being 1.5 times as large, and the unusually luminous X-raypopulation becomes both less luminous and less populous.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, Inc., under NASAcontract NAS 5-26555. These observations were made in connection withproposal GO-6669.
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
| Weak Gravitational Lensing by a Sample of X-Ray-luminous Clusters of Galaxies. III. Serendipitous Weak Lensing Detections of Dark and Luminous Mass Concentrations
In the course of a weak gravitational lensing survey of 39 clusters ofgalaxies, covering a total sky area of ~1 deg2, we haveserendipitously discovered mass concentrations in the fields of A1705and A1722 that are most probably not associated with the main clustertarget. By combining weak lensing information with two-color galaxyphotometry in fields centered on our sample clusters, we identify a newcluster candidate at z~0.5 in the field of A1705. This cluster candidatealso displays strong lensing in the form of a giant luminous arc. Thenew mass concentration in the field of A1722 also seems to be associatedwith an optically luminous cluster of galaxies at z~0.5, but in thiscase there is some evidence for additional structures along the line ofsight that may contribute to the lensing signal. A third cluster, A959,has a dark subclump that shows interesting morphological evidence in themass map for being associated with the main cluster. This is the firstcase where there is any significant evidence for a physical associationbetween a dark subclump (discovered from weak lensing) and a normalcluster. Analysis of archival X-ray data shows that the three new massconcentrations are not firmly detected in X-rays and that they areX-ray-underluminous.
| The Recent Evolution of the Dwarf Starburst Galaxy NGC 625 from Hubble Space Telescope Imaging
New Hubble Space Telescope WFPC2 imaging of the dwarf starburst galaxyNGC 625 is presented. These data, which are 80% complete to V and Imagnitudes of 26.0 and 25.3, respectively, allow us to study the recentstar formation history of NGC 625. Using outlying red giant stars, wederive a tip of the red giant branch (TRGB) distance modulus of27.95+/-0.07. This corresponds to a distance of 3.89+/-0.22 Mpc, placingthis system on the far side of the Sculptor Group. NGC 625 has awell-defined radial stellar population gradient, evidenced by a centralconcentration of young main-sequence stars and a red giant branch(RGB)/asymptotic giant branch (AGB) ratio that increases withgalactocentric distance. The prominent AGB is very red, similar to thepopulation found in the Local Group dwarf irregular galaxy NGC 6822. TheRGB stars can be detected out far from the central star-forming activityand show an elliptical distribution in agreement with the galaxy's outerH I distribution. Using Hα and Hβ narrowband imaging andprevious optical spectroscopy, we identify substantial and varyinginternal extinction associated with the central active star formationregions. This extinction, which varies from AV=0.0 to 0.6mag, hampers efforts to derive a detailed recent star formation history.To better understand the effects of internal extinction on the analysisof young stellar populations, synthetic models are presented that, forthe first time, examine and account for this effect. Using the luminousblue helium-burning stars, we construct a simple model of the recent(<100 Myr) star formation in which an elevated but declining starformation rate has been present over this entire period. This is at oddswith the presence of spectroscopic Wolf-Rayet (W-R) features in themajor star formation region, which implies a short duration (<=5 Myr)for the recent starburst. This suggests that starbursts displaying W-Rfeatures are not necessarily all of a short duration. Finally, wespeculate on the possible causes of the present burst of star formationin this apparently isolated galaxy and compare it with other nearby,well-studied dwarf starburst systems.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.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| Interstellar Medium Abundances in Sculptor Group Dwarf Irregular Galaxies
Using the Cerro Tololo Inter-American Observatory 4 m telescope, we haveobtained optical spectra of H II regions in five Sculptor group dwarfirregular galaxies. We derive oxygen, nitrogen, and sulfur abundancesfrom the H II region spectra. Oxygen abundances are derived via threedifferent methods (the ``direct'' method, the empirical method guided byphotoionization modeling of McGaugh [published in 1991], and the purelyempirical method of Pilyugin, published in 2000) and are compared.Significant systematic differences are found between the three methods,and we suggest that a recalibration of the empirical abundance scale isrequired. Until differences between these three methods are betterunderstood, the issue of the degree of uniformity of the interstellarmedium abundances in a dwarf galaxy cannot be properly addressed. TheN/O ratio for the metal-poor dI ESO 473-G24 of log(N/O)=-1.43+/-0.03lies well above the plateau of log(N/O)=-1.60+/-0.02 found by Izotov& Thuan for a collection of metal-poor, blue compact galaxies. Thisshows that not all galaxies with 12+log(O/H)<=7.6 have identicalelemental abundance ratios, and this implies that the Izotov & Thuanscenario for low-metallicity galaxies is not universal. Measurements ofthe H II regions in NGC 625 yield log(N/O)~-1.25. Assuming N productionby intermediate-mass stars, this relatively high N/O ratio may beindicative of a long quiescent period prior to the recent active burstof star formation. The oxygen abundances in the Sculptor group dI's arein good agreement with the relationship between metallicity andluminosity observed in the Local Group dI's. Taken together, theobservations show a better relationship between metallicity andluminosity than between metallicity and galaxy central surfacebrightness. The Sculptor group dI's, in general, lie closer to thesimple closed-box model evolutionary path than the Local Group dI's. Thehigher gas contents, lower average star formation rates, and closerresemblance to closed-box evolution could all be indicative of evolutionin a relatively low density environment.
| Star Formation in Sculptor Group Dwarf Irregular Galaxies and the Nature of ``Transition'' Galaxies
We present new Hα narrowband imaging of the H II regions in eightSculptor group dwarf irregular (dI) galaxies. The Hα luminositiesof the detected H II regions range from some of the faintest detected inextragalactic H II regions (~1035 ergs s-1 in SC24) to some of the most luminous (~1040 ergs s-1in NGC 625). The total Hα luminosities are converted into currentstar formation rates (SFRs). Comparing the Sculptor group dI's to theLocal Group dI's, we find that the Sculptor group dI's have, on average,lower values of SFR when normalized to either galaxy luminosity or gasmass (although there is considerable overlap between the two samples).The range for both the Sculptor group and Local Group samples is largewhen compared with that seen for the sample of gas-rich, quiescent, lowsurface brightness (LSB) dI's from van Zee et al. (published in 1997)and the sample of isolated dI's from van Zee (from 2000 and 2001). Thisis probably best understood as a selection effect since the nearby groupsamples have a much larger fraction of extremely low luminosity galaxiesand the smaller galaxies are much more liable to large relativevariations in current SFRs. The Sculptor group and LSB samples are verysimilar with regard to mean values of both τgas andτform, and the Local Group and isolated dI samples arealso similar to each other in these two quantities. Currently, theSculptor group lacks dI galaxies with elevated normalized current SFRsas high as the Local Group dI's IC 10 and GR 8. The properties of``transition'' (dSph/dIrr) galaxies in Sculptor and the Local Group arealso compared and found to be similar. The transition galaxies aretypically among the lowest luminosities of the gas-rich dwarf galaxies.Relative to the dwarf irregular galaxies, the transition galaxies arefound preferentially nearer to spiral galaxies and are found nearer tothe center of the mass distribution in the local cloud. While most ofthese systems are consistent with normal dI galaxies, exhibitingtemporarily interrupted star formation, the observed density-morphologyrelationship (which is weaker than that observed for the dwarfspheroidal galaxies) indicates that environmental processes such as``tidal stirring'' may play a role in causing their lower SFRs.
| Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies. III. SBS 1415+437
We present a detailed optical spectroscopic and B, V, I, Hαphotometric study of the metal-deficient cometary blue compact dwarf(BCD) galaxy SBS 1415+437. We derive an oxygen abundance 12 + log (O/H)= 7.61+/-0.01 and 7.62+/-0.03 (Z = Zsun/20) in the twobrightest H II regions, among the lowest in BCDs. The helium massfractions in these regions are Y = 0.246 +/-0.003 and 0.243+/-0.010.Four techniques based on the equivalent widths of the hydrogen emissionand absorption lines, the spectral energy distribution and the coloursof the galaxy are used to put constraints on the age of the stellarpopulation in the low-surface-brightness (LSB) component of the galaxy,assuming two limiting cases of star formation (SF), the case of aninstantaneous burst and that of a continuous SF with a constant or avariable star formation rate (SFR). The spectroscopic and photometricdata for different regions of the LSB component are well reproduced by ayoung stellar population with an age t <= 250 Myr, assuming a smallextinction in the range AV = 0-0.6 mag. Assuming noextinction, we find that the upper limit for the mass of the old stellarpopulation, formed between 2.5 Gyr and 10 Gyr, is not greater than ~(1/20-1) of that of the stellar population formed during the last ~ 250Myr. Depending on the region considered, this also implies that the SFRin the most recent SF period must be 20 to 1000 times greater than theSFR at ages ga 2.5 Gyr. We compare the photometric and spectroscopicproperties of SBS 1415+437 with those of a sample of 26 low-metallicitydwarf irregular and BCD galaxies. We show that there is a clear trendfor the stellar LSB component of lower-metallicity galaxies to be bluer.This trend cannot be explained only by metallicity effects. There mustbe also a change in the age of the stellar populations. The mostmetal-deficient galaxies have also smaller luminosity-weighted ages.12+log (O/H)sun = 8.92 (Anders & Grevesse\cite{Anders89}).
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| Distances to nearby galaxies in Sculptor
We present an analysis of Hubble Space Telescope/WFPC2 images of ninenearby galaxies in Sculptor. We derive their distances from theluminosity of the tip of the red giant branch stars with a typicalaccuracy of ~ 12%. Their distances are 4.21 Mpc (Sc 22), 4.92 Mpc (DDO226), 3.94 Mpc (NGC 253), 3.40 Mpc (KDG 2), 3.34 Mpc (DDO 6), 3.42 Mpc(ESO 540-030), 4.43 Mpc (ESO 245-05), 4.27 Mpc (UGCA 442), and 3.91 Mpc(NGC 7793). The galaxies are concentrated in several spatially separatedloose groups around NGC 300, NGC 253, and NGC 7793. The Sculptor galaxycomplex together with the CVn I cloud and the Local Group form a 10 Mpcfilament, apparently driven by the free Hubble flow.Based on observations made with the NASA/ESA Hubble Space Telescope. TheSpace Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. under NASA contract NAS5-26555.
| A Search for H2O Maser Emission in Southern Active Galactic Nuclei and Star-forming Galaxies: Discovery of a Maser in the Edge-on Galaxy IRAS F01063-8034
We report the cumulative results of five surveys for H2Omaser emission at 1.35 cm wavelength in 131 active galactic nuclei(AGNs) and star-forming galaxies, conducted at the Parkes Observatorybetween 1993 and 1998. We detected one new maser, in the edge-on galaxyIRAS F01063-8034, which exhibits a single ~0.1 Jy spectral feature at4282+/-6 km s-1 (heliocentric) with an unusually large54+/-16 km s-1 half-power full width. The centroid velocityof the emission increased to 4319.6+/-0.6 km s-1 (38+/-2 kms-1 width) over the 13 days between discovery andconfirmation of the detection. A similarly broad-line width and largechange in velocity has been noted for the maser in NGC 1052, wherein jetactivity excites the emission. Neither optical spectroscopy,radio-infrared correlations, nor infrared colors provide compellingevidence of unusual activity in the nucleus of IRAS F01063-8034. Sincethe galaxy appears to be outwardly normal at optical and infraredwavelengths, detection of an H2O maser therein is unique. Themaser emission is evidence that the galaxy harbors an AGN that isprobably obscured by the edge-on galactic disk. The detection highlightsthe possibility that undetected AGNs could be hidden in other relativelynearby galaxies. No other maser emission features have been identifiedat velocities between 3084 and 6181 km s-1.
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