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Broadband Imaging of a Large Sample of Irregular Galaxies
We present the results of UBV imaging of a large sample of irregulargalaxies: 94 Im systems, 24 blue compact dwarfs (BCDs), and 18 Smgalaxies. We also include JHK imaging of 42 of these galaxies. Thesample spans a large range in galactic parameters. Ellipse fit axialratios, inclinations, and position angles are derived, integratedphotometry and azimuthally averaged surface photometry profiles aredetermined, and exponential fits give the central surface brightnesses,scale lengths, and isophotal and half-power radii. These data are usedto address the shapes of Im galaxies, look for clues to pastinteractions in large-scale peculiarities, examine the nature andconsequences of bars, study color gradients and large-scale colorvariations, and compare the exponential disk profiles of the young andold stellar components. For example, color gradients exhibit a greatvariety and not all passbands are correlated. Bars are associated withhigher star formation rates. Many irregulars show a double-exponentialradial light profile that is steeper in the outer parts, and these arereproduced by a new model of star formation that is discussed in acompanion paper. Some galaxies, primarily BCDs, have double exponentialsthat are steeper (and bluer) in the inner parts, presumably fromcentralized star formation. Im-type galaxies have thicker, lessprominent dust layers than spiral galaxies because of their loweraverage surface densities and midplane extinctions.
| On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
| Modeling the Pan-Spectral Energy Distribution of Starburst Galaxies. II. Control of the H II Region Parameters
We examine, from a theoretical viewpoint, how the physical parameters ofH II regions are controlled in both normal galaxies and in starburstenvironments. These parameters are the H II region luminosity function,the time-dependent size, the covering fraction of molecular clouds, thepressure in the ionized gas, and the ionization parameter. The factorsthat control them are the initial mass function (IMF) of the excitingstars, the cluster mass function, the metallicity, and the mean pressurein the surrounding interstellar medium. We investigate the sensitivityof the Hα luminosity to the IMF, and find that this can translateto more than a factor 2 variation in derived star formation rates. Themolecular cloud dissipation timescale is estimated from a case study ofM17 to be ~1 Myr for this object. Based on H II luminosity functionfitting for nearby galaxies, we suggest that the H II region clustermass function is fitted by a lognormal form peaking at ~100Msolar. The cluster mass function continues the stellar IMFto a higher mass regime. The pressure in the H II regions is controlledby the mechanical luminosity flux from the central cluster. Since thisis closely related to the ionizing photon flux, we show that theionization parameter is not a free variable, and that the diffuseionized medium may be composed of many large, faint, and old H IIregions. Finally, we derive theoretical probability distributions forthe ionization parameter as a function of metallicity and compare theseto those derived for SDSS galaxies.
| On the Determination of N and O Abundances in Low-Metallicity Systems
We show that in order to minimize the uncertainties in the N and Oabundances of low-mass, low-metallicity (O/H<=1/5 solar)emission-line galaxies, it is necessary to employ separateparameterizations for inferring Te(N+) andTe(O+) from Te(O+2). Inaddition, we show that for the above systems, the ionization correctionfactor (ICF) for obtaining N/O from N+/O+, wherethe latter is derived from optical emission-line flux ratios, is=1.08+/-0.09. These findings are based on state-of-the-art single-star HII region simulations, employing our own modeled stellar spectra asinput. Our models offer the advantage of having matching stellar andnebular abundances. In addition, they have O/H as low as 1/50 solar(lower than any past work), as well as log(N/O) and log(C/O) fixed atcharacteristic values of -1.46 and -0.7, respectively. The above resultswere used to rederive N and O abundances for a sample of 68 systems with12+log(O/H)<=8.1, whose dereddened emission-line strengths werecollected from the literature. The analysis of the log(N/O) versus12+log(O/H) diagram of the above systems shows that (1) the largestgroup of objects forms the well-known N/O plateau with a value for themean (and its statistical error) of-1.43+0.0084-0.0085, (2) the objects aredistributed within a range in log(N/O) of -1.54 to -1.27 in Gaussianfashion around the mean with a standard deviation ofσ=+0.071-0.084, and (3) a χ2analysis suggests that only a small amount of the observed scatter inlog(N/O) is intrinsic.
| Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies
We have used the Balmer and Paschen jumps to determine the temperaturesof the H+ zones of a total sample of 47 H II regions. TheBalmer jump was used on MMT spectrophotometric data of 22low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies andof one H II region in the spiral galaxy M101. The Paschen jump was usedon spectra of 24 H II emission-line galaxies selected from the DataRelease 3 of the Sloan Digital Sky Survey (SDSS). To derive thetemperatures, we have used a Monte Carlo technique varying the electrontemperature in the H+ zone, the extinction of the ionized gasand that of the stellar population, the relative contribution of theionized gas to the total emission, and the star formation history to fitthe spectral energy distribution of the galaxies. For the MMT spectra,the fit was done in the wavelength range 3200-5200 Å, whichincludes the Balmer discontinuity, and for the SDSS spectra, in thewavelength range 3900-9200 Å, which includes the Paschendiscontinuity. We find for our sample of H II regions that thetemperatures of the O2+ zones determined from thenebular-to-auroral line intensity ratio of doubly ionized oxygen [O III]λλ(4959+5007)/λ4363 do not differ, in a statisticalsense, from the temperatures of the H+ zones determined fromfitting the Balmer and Paschen jumps and the spectral energydistributions (SEDs). We cannot rule out small temperature differencesof the order of 3%-5%.
| 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.
| Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
| The evolution of actively star-forming galaxies in the mid-infrared
In this paper we analyze the evolution of actively star-forming galaxiesin the mid-infrared (MIR). This spectral region, characterized bycontinuum emission by hot dust and by the presence of strong emissionfeatures generally ascribed to polycyclic aromatic hydrocarbon (PAH)molecules, is the most strongly affected by the heating processesassociated with star formation and/or active galactic nuclei (AGNs).Following the detailed observational characterization of galaxies in theMIR by the Infrared Space Observatory (ISO), we have updated themodelling of this spectral region in our spectrophotometric modelGRASIL. In the diffuse component we have updated the treatment of PAHsaccording to the model by Li & Draine. As for the dense phase of theinterstellar medium associated with the star-forming regions, themolecular clouds, we strongly decrease the abundance of PAHs as comparedto that in the cirrus, based on the observational evidence of the lackor weakness of PAH bands close to the newly formed stars, possibly dueto the destruction of the molecules in strong ultraviolet fields. Therobustness of the model is checked by fitting near-infrared to radiobroad-band spectra and the corresponding detailed MIR spectra of a largesample of galaxies, at once. With this model, we have analyzed thelarger sample of actively star-forming galaxies by Dale et al. We showthat the observed trends of galaxies in the ISO-IRAS-radio colour-colourplots can be interpreted in terms of the different evolutionary phasesof star formation activity, and the consequent different dominance inthe spectral energy distribution of the diffuse or dense phase of theISM. We find that the observed colours indicate a surprising homogeneityof the starburst phenomenon, allowing only a limited variation of themost important physical parameters, such as the optical depth of themolecular clouds, the time-scale of the escape of young stars from theirfor mation sites, and the gas consumption time-scale. In this paper wedo not attempt to reproduce the far-infrared coolest region in thecolour-colour plots, as we concentrate on models meant to reproduceactive star-forming galaxies, but we discuss possible requirements of amore complex modelling for the coldest objects.
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - IV. 44 new velocity fields. Extension, shape and asymmetry of Hα rotation curves
We present Fabry-Perot observations obtained in the frame of the GHASPsurvey (Gassendi HAlpha survey of SPirals). We have derived the Hαmap, the velocity field and the rotation curve for a new set of 44galaxies. The data presented in this paper are combined with the datapublished in the three previous papers providing a total number of 85 ofthe 96 galaxies observed up to now. This sample of kinematical data hasbeen divided into two groups: isolated (ISO) and softly interacting(SOFT) galaxies. In this paper, the extension of the Hα discs, theshape of the rotation curves, the kinematical asymmetry and theTully-Fisher relation have been investigated for both ISO and SOFTgalaxies. The Hα extension is roughly proportional toR25 for ISO as well as for SOFT galaxies. The smallestextensions of the ionized disc are found for ISO galaxies. The innerslope of the rotation curves is found to be correlated with the centralconcentration of light more clearly than with the type or thekinematical asymmetry, for ISO as well as for SOFT galaxies. The outerslope of the rotation curves increases with the type and with thekinematical asymmetry for ISO galaxies but shows no special trend forSOFT galaxies. No decreasing rotation curve is found for SOFT galaxies.The asymmetry of the rotation curves is correlated with themorphological type, the luminosity, the (B-V) colour and the maximalrotational velocity of galaxies. Our results show that the brightest,the most massive and the reddest galaxies, which are fast rotators, arethe least asymmetric, meaning that they are the most efficient withwhich to average the mass distribution on the whole disc. Asymmetry inthe rotation curves seems to be linked with local star formation,betraying disturbances of the gravitational potential. The Tully-Fisherrelation has a smaller slope for ISO than for SOFT galaxies.
| Radio Identifications of Markarian Galaxies and the Correlation between Radio and Far-Infrared Properties
By checking DSS optical images and NVSS radio images, 782 Markariangalaxies were identified to be NVSS radio sources. A comparison of theradio luminosity at 1.4 GHz and the far-infrared (FIR) luminosity for468 ``normal" galaxies shows a tight correlation. Most of the Seyfertgalaxies and quasars follow the radio-FIR relation deduced from the``normal" galaxy sample, but with a somewhat larger scatter. A total 167Markarian galaxies, comprising 100 ``normal" galaxies, 66 Seyfertgalaxies and one quasar, have either excess radio emission or much lowerFIR spectral index α (25μm, 60μm). These galaxies may beclassified as ``AGN-powered". For ``normal" galaxies, the average qvalue (defined as the log ratio between FIR and radio luminosities) is2.3. There seems a trend for q to slightly decrease with increasingradio luminosity. This may imply that the ongoing active star formationin galaxies with higher radio luminosities is more efficient in heatingthe cosmic-ray electrons.
| First Results from THINGS: The HI Nearby Galaxy Survey
We describe The HI Nearby Galaxy Survey (THINGS), the largestprogramever undertaken at the Very Large Array to perform 21-cm HIobservations of thehighest quality ( 7'', ≤ 5 km s^{-1}resolution) ofnearby galaxies. The goal of THINGS is to investigatekeycharacteristics related to galaxy morphology, star formation andmassdistribution across the Hubble sequence. A sample of 34 objectswithdistances between 3 and 10 Mpc will be observed, covering a widerangeof evolutionary stages and properties. Data from THINGSwillcomplement SINGS, the Spitzer Infrared Nearby Galaxy Survey. Forthe THINGS sample, high-quality observations at comparable resolutionwillthus be available from the X-ray regime through to the radio partofthe spectrum. THINGS data can be used to investigate issues such asthesmall-scale structure of the ISM, its three-dimensional structure,the(dark) matter distribution and processes leading to starformation. Todemonstrate the quality of the THINGS data products, wepresent someprelimary HI maps here of four galaxies from the THINGSsample.
| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| High-Ionization Emission in Metal-deficient Blue Compact Dwarf Galaxies
Primordial stars are expected to be very massive and hot, producingcopious amounts of hard ionizing radiation. The best place to study hardionizing radiation in the local universe is in very metal-deficient bluecompact dwarf (BCD) galaxies. We have carried out a MMT spectroscopicsearch for [Ne V] λ3426 (ionization potential of 7.1 ryd), [Fe V]λ4227 (ionization potential of 4 ryd), and He II λ4686(ionization potential of 4 ryd) emission in a sample of 18 BCDs. We haveadded data from previous work and from the Data Release 3 of the SloanDigital Sky Survey. In total, we have assembled a BCD high-ionizationsample with [Ne V] emission in four galaxies, [Fe V] emission in 15galaxies, and He II emission in 465 galaxies. With this large sample, wehave reached the following conclusions. There is a general trend ofhigher [Ne V], [Fe V], and He II emission at lower metallicities.However, metallicity is not the only factor that controls the hardnessof the radiation. High-mass X-ray binaries and main-sequence stars areprobably excluded as the main sources of the very hard ionizingradiation responsible for [Ne V] emission. The most likely source of [NeV] emission is probably fast radiative shocks moving with velocities>~450 km s-1 through a dense interstellar medium with anelectron number density of several hundreds cm-3 andassociated with supernova explosions of the most massive stars. Thesehave masses of ~50-100 Msolar and are formed in very compactsuper-star clusters (SSCs). The softer ionizing radiation required forHe II emission is likely associated with less massive evolved starsand/or radiative shocks moving through a less dense interstellar medium.The observations reported here were obtained at the MMT Observatory, ajoint facility of the Smithsonian Institution and the University ofArizona.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| A HST Study of the Stellar Populations in the Cometary Dwarf Irregular Galaxy NGC 2366
We present V and I photometry of the resolved stars in the cometarydwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2images obtained with the Hubble Space Telescope. The resultingcolor-magnitude diagram reaches down to I~26.0 mag. It reveals not onlya young population of blue main-sequence stars (age <~30 Myr) butalso an intermediate-age population of blue and red supergiants (20 Myr<~ age <~100 Myr) and older evolved populations of asymptoticgiant branch (AGB) stars (age >~100 Myr) and red giant branch (RGB)stars (age >~1 Gyr). The measured magnitude I=23.65+/-0.10 mag of theRGB tip results in a distance modulus m-M=27.67+/-0.10, whichcorresponds to a distance of 3.42+/-0.15 Mpc, in agreement with previousdistance determinations. The youngest stars are associated with thebright complex of H II regions NGC 2363 (=Mrk 71) in the southwestextremity of the galaxy. As a consequence of the diffusion andrelaxation processes of stellar ensembles, the older the stellarpopulation is, the smoother and more extended is its spatialdistribution. An underlying population of older stars is foundthroughout the body of NGC 2366. The most notable feature of this olderpopulation is the presence of numerous relatively bright AGB stars. Thenumber ratio of AGB to RGB stars and the average absolute brightness ofAGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403,indicating a younger age of the AGB stars in NGC 2366. In addition tothe present burst of age <~100 Myr, there has been strong starformation activity in the past of NGC 2366, from ~100 Myr to <~3 Gyrago.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS5-26555.
| The Molecular Interstellar Medium of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies
We present a new survey for CO in dwarf galaxies using the ARO Kitt Peak12 m telescope. This survey consists of observations of the centralregions of 121 northern dwarfs with IRAS detections and no known COemission. We detect CO in 28 of these galaxies and marginally detectanother 16, increasing by about 50% the number of such galaxies known tohave significant CO emission. The galaxies we detect are comparable instellar and dynamical mass to the Large Magellanic Cloud, althoughsomewhat brighter in CO and fainter in the far-IR. Within dwarfs, wefind that the CO luminosity LCO is most strongly correlatedwith the K-band and the far-infrared luminosities. There are also strongcorrelations with the radio continuum (RC) and B-band luminosities andlinear diameter. Conversely, we find that far-IR dust temperature is apoor predictor of CO emission within the dwarfs alone, although a goodpredictor of normalized CO content among a larger sample of galaxies. Wesuggest that LCO and LK correlate well because thestellar component of a galaxy dominates the midplane gravitational fieldand thus sets the pressure and density of the atomic gas, which controlthe formation of H2 from H I. We compare our sample with moremassive galaxies and find that dwarfs and large galaxies obey the samerelationship between CO and the 1.4 GHz RC surface brightness. Thisrelationship is well described by a Schmidt law withΣRC~Σ1.3CO. Therefore,dwarf galaxies and large spirals exhibit the same relationship betweenmolecular gas and star formation rate (SFR). We find that this result isrobust to moderate changes in the RC-to-SFR and CO-to-H2conversion factors. Our data appear to be inconsistent with large (orderof magnitude) variations in the CO-to-H2 conversion factor inthe star-forming molecular gas.
| 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.
| Power Spectra in V band and Hα of Nine Irregular Galaxies
Fourier transform power spectra of major axis cuts in V and Hαimages were made for a sample of nine irregular galaxies. These powerspectra reveal structure over a wide range of scales. For six of thegalaxies the power spectrum slopes at intermediate scales (10-400 pc) inthe V-band images range from -1.3 to -1.5. The similarity of slopessuggests that the same processes are structuring these systems. Theseslopes are slightly shallower than those observed in other galaxies in HI, molecular emission, dust extinction, and optical light. Three of thegalaxies have flat power spectra like sky noise; these three galaxiesare relatively indistinct in the direct images. The power spectrum slopefor Hα steepens with increasing star formation rate, ranging froma shallow value comparable to the noise at low rates to a steep valuewith a slope of ~-1.5 at high rates. This change reflects the increasingareal filling factor of Hα emission with increasing star formationrate and an apparently universal slope inside the Hα regions thatis comparable to that for Kolmogorov turbulence. The power spectrum of HI in one galaxy has a steeper power law, with a slope of ~-2.9. The factthat the power laws of star formation are about the same for dwarfgalaxies and giant spiral galaxies suggests the microscopic processesare the same, independent of spiral density waves and galaxy size.
| 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.
| DDO 88: A Galaxy-sized Hole in the Interstellar Medium
We present an H I and optical study of the gas-rich dwarf irregulargalaxy DDO 88. Although the global optical and H I parameters of DDO 88are normal for its morphological type, it hosts a large (3 kpc diameter)and unusually complete ring of enhanced H I emission. The normalappearance of this galaxy in the optical and the outer regions of the HI give no hint of the presence of the striking H I ring in the innerregions. The gas ring is located at approximately one-third of the totalH I radius and one-half the optically defined Holmberg radius, andcontains 30% of the total H I of the galaxy. The ring surrounds acentral depression in the H I distribution. If the H I ring and centraldepression in the gas were formed by the energy input from winds andsupernova explosions of massive stars formed in a starburst, as isthought often to be the case, the star-forming event would have formed0.1%-1% of the total stellar mass of the galaxy. However, the UBV colorsin the H I hole are not bluer than the rest of the galaxy, as would beexpected if an unusual star-forming event had taken place thererecently, although there is an old (~1-3 Gyr), red cluster near thecenter of the hole that is massive enough to have produced the hole inthe H I. An age estimate for the ring is uncertain, however, because itis not observed to be expanding. An expansion model produces a lowerestimate of 0.5 Gyr, but the presence of faint star formation regionsassociated with the ring indicates a much younger age. We also estimatethat the ring could have dispersed by now if it is older than 0.5 Gyr.This implies that the ring is younger than 0.5 Gyr. A younger age wouldindicate that the red cluster did not produce the hole and ring.Therefore, uncertainties prevent us from concluding that the cluster andthe H I hole are definitely related. If this ring and the depression inthe gas that it surrounds were not formed by stellar winds andsupernovae, this would indicate that some other, currently unidentified,mechanism is operating.
| SN 2002kg - the brightening of LBV V37 in NGC 2403
SN 2002kg is a type IIn supernova, detected in October 2002 in thenearby spiral galaxy NGC 2403. We show that the position of SN 2002kgagrees within the errors with the position of the LBV V37. Ground basedand HST ACS images however show that V37 is still present after the SN2002kg event. We compiled a lightcurve of V37 which underlines thevariablity of the object, and shows that SN 2002kg was the brighteningof V37 and not a supernova. The recent brightening is not a gianteruption, but more likely part of an S Dor phase. V37 shows strongHα +[N II] emission in recent images and in the SN2002kg spectrum, which we interprete as the signature of the presence ofan LBV nebula. A historic spectrum lacks emission, which may hint thatwe are witnessing the formation of an LBV nebula.
| The inner structure of ΛCDM haloes - II. Halo mass profiles and low surface brightness galaxy rotation curves
We use a set of high-resolution cosmological N-body simulations toinvestigate the inner mass profile of galaxy-sized cold dark matter(CDM) haloes. These simulations extend the numerical convergence studypresented in Paper I of this series, and demonstrate that the massprofile of CDM galaxy haloes can be robustly estimated beyond a minimumconverged radius of order rconv~ 1h-1 kpc in ourhighest-resolution runs. The density profiles of simulated haloes becomeprogressively shallower from the virial radius inwards, and show no signof approaching a well-defined power law near the centre. Atrconv, the density profile is steeper than expected from theformula proposed by Navarro, Frenk & White, which has aρ~r-1 cusp, but significantly shallower than the steeplydivergent ρ~r-1.5 cusp proposed by Moore et al. Weperform a direct comparison of the spherically averaged dark mattercircular velocity profiles with Hα rotation curves of a sample oflow surface brightness (LSB) galaxies. We find that most galaxies in thesample (about 70 per cent) have rotation curves that are consistent withthe structure of CDM haloes. Of the remainder, 20 per cent have rotationcurves which cannot be fit by any smooth fitting function with few freeparameters, and 10 per cent are inconsistent with CDM haloes. However,the latter consist mostly of rotation curves that do not extend to largeenough radii to accurately determine their shapes and maximumvelocities. We conclude that the inner structure of CDM haloes is notmanifestly inconsistent with the rotation curves of LSB galaxies.
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - III. 15 new velocity fields and study of 46 rotation curves
We present Fabry-Pérot observations obtained in the frame of theGHASP survey (Gassendi Hα survey of SPirals). We have derived theHα maps, the velocity fields and the rotation curves for a set of15 galaxies. The data presented in this paper are combined with the datapublished in our two previous papers in order to make a preliminaryanalysis of the rotation curves obtained for 46 galaxies. We check theconsistency of our data with the Tully-Fisher relationship and concludethat our Hα rotation curves reach the maximum velocity in most ofthe cases, even with solid-body rotating galaxies. We find that ourrotation curves, on average, almost reach the isophotal radiusR25. We confirm the trend, already mentioned by Rubin,Waterman & Kenney and Márquez et al., that the maximumextension of the Hα rotation curves increases with the type of thespiral galaxy, up to t~ 7-8 and we find that it decreases for magellanicand irregular galaxies. We also confirm the trend seen by Márquezet al. that later types tend to have lower values of the internal slopeof the rotation curve, in agreement with Rubin et al.
| Deep Hubble Space Telescope ACS Observations of I Zw 18: a Young Galaxy in Formation
We present V and I photometry of the resolved stars in the mostmetal-deficient blue compact dwarf galaxy known, I Zw 18(Zsolar/50), using Hubble Space Telescope/ACS images, thedeepest ones ever obtained for this galaxy. The resulting I versus V-Icolor-magnitude diagram (CMD) reaches limiting magnitudes V=I=29 mag. Itreveals a young stellar population of blue main-sequence (MS) stars(age<~30 Myr) and blue and red supergiants (10Myr<~age<~100Myr), but also an older evolved population of asymptotic giant branch(AGB) stars (100Myr<~age<~500 Myr). We derive a distance to I Zw18 in the range 12.6-15 Mpc from the brightness of its AGB stars, withpreferred values in the higher range. Red giant branch (RGB) stars areconspicuous by their absence, although, for a distance of I Zw 18<=15 Mpc, our imaging data go ~1-2 mag below the tip of the RGB.Thus, the most evolved stars in the galaxy are not older than 500 Myrand I Zw 18 is a bona fide young galaxy. Several star formation episodescan be inferred from the CMDs of the main body and the C component.There have been, respectively, three and two episodes in these twoparts, separated by periods of ~100-200 Myr. In the main body, theyounger MS and massive post-MS stars are distributed over a larger areathan the older AGB stars, suggesting that I Zw 18 is still forming fromthe inside out. In the C component, different star formation episodesare spatially distinct, with stellar population ages decreasing from thenorthwest to the southeast, also suggesting the ongoing buildup of ayoung galaxy.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated byAURA, Inc. under NASA contract NAS5-26555.
| 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.
| Systematic Effects and a New Determination of the Primordial Abundance of 4He and dY/dZ from Observations of Blue Compact Galaxies
We use spectroscopic observations of a sample of 82 H II regions in 76blue compact galaxies to determine the primordial helium abundanceYp and the slope dY/dZ from the Y-O/H linear regression. Toimprove the accuracy of the dY/dZ measurement, we have included newspectrophotometric observations of 33 H II regions that span a largemetallicity range, with oxygen abundance 12+log(O/H) varying between7.43 and 8.30 (Zsolar/30<=Z<=Zsolar/4). Mostof the new galaxies were selected from the First Byurakan, theHamburg/SAO, and the University of Michigan objective prism surveys. Fora subsample of seven H II regions, we derive the He mass fraction takinginto account known systematic effects, including collisional andfluorescent enhancements of He I emission lines, collisional excitationof hydrogen emission, underlying stellar He I absorption, and thedifference between the temperatures Te(He II) in theHe+ zone and Te(O III) derived from thecollisionally excited [O III] lines. We find that the net result of allthe systematic effects combined is small, changing the He mass fractionby less than 0.6%. By extrapolating the Y versus O/H linear regressionto O/H=0 for seven H II regions of this subsample, we obtainYp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds todY/dZ=3.7+/-1.2, assuming the oxygen mass fraction to be O=0.66Z. In theframework of the standard big bang nucleosynthesis theory, thisYp corresponds toΩbh2=0.012+0.003-0.002,where h is the Hubble constant in units of 100 km s-1Mpc-1. This is smaller at the 2 σ level than the valueobtained from recent deuterium abundance and microwave backgroundradiation measurements. The linear regression slope dY/dO=4.3+/-0.7(corresponding to dY/dZ=2.8+/-0.5) for the whole sample of 82 H IIregions is similar to that derived for the subsample of seven H IIregions, although it has a considerably smaller uncertainty.
| 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.
| The H I Kinematics and Distribution of Four Blue Compact Dwarf Galaxies
We present VLA H I observations of the blue compact dwarf (BCD) galaxiesNGC 2366, NGC 4861, VII Zw 403, and Haro 2. These galaxies span a rangeof BCD morphological types. The cometary-like BCDs NGC 2366 and NGC 4861have regular rotational kinematics with a V/σ of 8.7 and 6.4,respectively. On the other hand, the velocity fields of the iE BCD VIIZw 403 and of the nE BCD Haro 2 lack regularity, and their rotationalmotion is around the major, not the minor, axis. The H I distribution iscentrally peaked in VII Zw 403 and Haro 2, a general feature of all iEand nE-type BCDs, the most common ones. In contrast, cometary-type BCDshave multiple H I peaks that are scattered over the disk. The activeregions of star formation are associated with regions of high H I columndensities, with slight displacements between the H I and stellar peaks.NGC 2366 shows many H I minima, resulting from the disruptive influenceof massive star formation and supernovae on the interstellar medium(ISM). In NGC 2366 and NGC 4861, there is a tendency for H I gas with ahigher velocity dispersion to be associated with regions of lower H Icolumn density. This anticorrelation can be understood in the context ofa two-phase model of the ISM. In all BCDs, the radio continuum emissionis associated with the star-forming regions and is predominantly thermalin nature. H I clouds with no optical counterparts have been found inthe vicinity of NGC 4861 and Haro 2.Based on observations obtained at the National Radio AstronomyObservatory, a facility of the National Science Foundation, operatedunder cooperative agreement by Associated Universities, Inc.
| 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.
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