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Magnetic fields and ionized gas in nearby late type galaxies
Aims.In order to analyze the importance of the star formation rate ingenerating and amplifying magnetic fields in the interstellar medium weperform a deep continuum polarization study of three angularly large,late type spiral galaxies. Methods: . We obtained deep total powerand polarization maps at 4.85 GHz of NGC 4236, NGC 4656 and IC 2574using the 100-m Effelsberg radio telescope. This was accompanied byimaging in the Hα line. We also observed these objects at 1.4 GHzto obtain their integrated fluxes at this frequency and to determinetheir radio spectra. Results: . All galaxies were found to possessweak but detectable total power emission at 4.85 GHz, coincident withregions of recent star formation as traced by bright Hα regions.The surface brightness of the radio-strongest object of our sample (NGC4656) is comparable to the radio-weakest objects in a sample of morethan 50 normally star-forming spiral galaxies for which measurements at4.8 GHz with the Effelsberg radio telescope are available. The surfacebrightness of the two other objects is even three times smaller. Thefractional polarization of the 2 galaxies of our sample is less than 2%,significantly lower than for spiral galaxies of intermediate types,suggesting that the magnetic fields are not only weaker, but also lessordered than in spiral galaxies. The radio spectra of galaxies in oursmall sample are indicative of a substantial fraction of thermalemission, with a higher thermal fraction than in spirals with high starformation rates (SFR), while the nonthermal emission in our sample isrelatively weak compared to spiral galaxies. We propose an equipartitionmodel where the nonthermal emission increases ∝{SFR}≈1.4 and the ratio of nonthermal to thermal emission increases∝{SFR}≈ 0.4. The objects of our sample still followthe radio-FIR correlation of surface brightness of the total emission,extending it towards the lowest values measured so far. Based on the observations with the 100-m telescope at Effelsbergoperated by the Max-Planck-Institut für Radioastronomie (MPIfR) onbehalf of the Max-Planck-Gesellschaft.
| On the origin of warps and the role of the intergalactic medium
There is still no consensus as to what causes galactic discs to becomewarped. Successful models should account for the frequent occurrence ofwarps in quite isolated galaxies, their amplitude as well as theobserved azimuthal and vertical distributions of the HI layer.Intergalactic accretion flows and intergalactic magnetic fields may bendthe outer parts of spiral galaxies. In this paper we consider theviability of these non-gravitational torques to take the gas off theplane. We show that magnetically generated warps are clearly flawedbecause they would wrap up into a spiral in less than two or threegalactic rotations. The inclusion of any magnetic diffusivity to dilutethe wrapping effect causes the amplitude of the warp to damp. We alsoconsider the observational consequences of the accretion of anintergalactic plane-parallel flow at infinity. We have computed theamplitude and warp asymmetry in the accretion model, for a disc embeddedin a flattened dark matter halo, including self-consistently thecontribution of the modes with azimuthal wavenumbers m= 0 and m= 1.Since the m= 0 component, giving a U-shaped profile, is not negligiblecompared to the m= 1 component, this model predicts quite asymmetricwarps, maximum gas displacements on the two sides in the ratio 3 : 2 forthe preferred Galactic parameters, and the presence of a fraction ~3.5per cent of U-shaped warps, at least. The azimuthal dependence of themoment transfer by the ram pressure would produce a strong asymmetry inthe thickness of the HI layer and asymmetric density distributions in z,in conflict with observational data for the warp in our Galaxy and inexternal galaxies. The amount of accretion that is required to explainthe Galactic warp would give gas scaleheights in the far outer disc thatare too small. We conclude that accretion of a flow with no net angularmomentum cannot be the main and only cause of warps.
| The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 as Revealed by Spitzer
We present Spitzer imaging of the metal-deficient (Z~=30%Zsolar) Local Group dwarf galaxy NGC 6822. On spatial scalesof ~130 pc, we study the nature of IR, Hα, H I, and radiocontinuum emission. Nebular emission strength correlates with IR surfacebrightness; however, roughly half of the IR emission is associated withdiffuse regions not luminous at Hα (as found in previous studies).The global ratio of dust to H I gas in the ISM, while uncertain at thefactor of ~2 level, is ~25 times lower than the global values derivedfor spiral galaxies using similar modeling techniques; localized ratiosof dust to H I gas are about a factor of 5 higher than the global valuein NGC 6822. There are strong variations (factors of ~10) in therelative ratios of Hα and IR flux throughout the central disk; thelow dust content of NGC 6822 is likely responsible for the differentHα/IR ratios compared to those found in more metal-richenvironments. The Hα and IR emission is associated with highcolumn density (>~1021 cm-2) neutral gas.Increases in IR surface brightness appear to be affected by bothincreased radiation field strength and increased local gas density.Individual regions and the galaxy as a whole fall within the observedscatter of recent high-resolution studies of the radio-far-IRcorrelation in nearby spiral galaxies; this is likely the result ofdepleted radio and far-IR emission strengths in the ISM of this dwarfgalaxy.
| XMM-Newton Archival Study of the Ultraluminous X-Ray Population in Nearby Galaxies
We present the results of an archival XMM-Newton study of the brightX-ray point sources (LX>1038 ergss-1) in 32 nearby galaxies. From our list of approximately100 point sources, we attempt to determine if there is a low-statecounterpart to the ultraluminous X-ray (ULX) population, searching for asoft-hard state dichotomy similar to that known for Galactic X-raybinaries and testing the specific predictions of the intermediate-massblack hole (IMBH) hypothesis. To this end, we searched for ``low-state''objects, which we defined as objects within our sample that had aspectrum well fitted by a simple absorbed power law, and ``high-state''objects, which we defined as objects better fitted by a combinedblackbody and a power law. Assuming that low-state objects accrete atapproximately 10% of the Eddington luminosity (as found by Done &Gierlinski) and that high-state objects accrete near the Eddingtonluminosity, we further divided our sample of sources into low- andhigh-state ULX sources. We classify 16 sources as low-state ULXs and 26objects as high-state ULXs. As in Galactic BH systems, the spectralindices, Γ, of the low-state objects, as well as the luminosities,tend to be lower than those of the high-state objects. The observedrange of blackbody temperatures for the high state is 0.1-1 keV, withthe most luminous systems tending toward the lowest temperatures. Wetherefore divide our high-state ULXs into candidate IMBHs (withblackbody temperatures of approximately 0.1 keV) and candidate stellarmass BHs (with blackbody temperatures of approximately 1.0 keV). Asubset of the candidate stellar mass BHs have spectra that are wellfitted by a Comptonization model, a property similar to Galactic BHsradiating in the ``very high'' state near the Eddington limit.
| 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.
| 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.
| Mid-Infrared Images of Stars and Dust in Irregular Galaxies
We present mid-IR to optical properties of 22 representative irregulargalaxies: 18 irregular (Im) galaxies, 3 blue compact dwarfs, and 1Magellanic-type spiral galaxy. The mid-IR is based on images from theSpitzer Space Telescope archives. The 3.6 and 4.5 μm bands and theUBVJHK images are used to examine disk morphology and the integrated andazimuthally averaged magnitudes and colors of stars. The nonstellarcontribution to the 4.5 μm images is used to trace hot dust. The 5.8and 8.0 μm images reveal emission from hot dust and polycyclicaromatic hydrocarbons (PAHs), and both may contribute to thesepassbands, although we refer to the nonstellar emission as PAH emission.We compare the 8.0 μm images to Hα. Im galaxies have no hiddenbars, and those with double-exponential optical light profiles have thesame at mid-IR. Most galaxies have similar optical and mid-IR scalelengths. Four galaxies have super star clusters that are not visible atoptical bands. Galaxies with higher area-normalized star formation rateshave more dust and PAH emission relative to starlight. Hot dust and PAHemission is found mostly in high surface brightness H II regions,implying that massive stars are the primary source of heating. Galaxieswith intense, widespread star formation have more extended PAH emission.The ratio of PAH to Hα emission is not constant on small scales.PAHs are associated with shells and giant filaments, so they are notdestroyed during shell formation.This work is based in part on archival data obtained with the SpitzerSpace Telescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under a contract with NASA.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| The Stellar Population and Interstellar Medium in NGC 6822
We present a comprehensive study of the stellar population and theinterstellar medium in NGC 6822 using high-quality H I data (obtainedwith the Australia Telescope Compact Array) and optical broadband andnarrowband data (obtained with Subaru and the Isaac Newton Telescope).Our Hα observations are an order of magnitude deeper than previousstudies and reveal a complex filamentary network covering almost theentire central disk of NGC 6822. We find hitherto unknown H II regionsin the outskirts of NGC 6822 and the companion galaxy. The old andintermediate-age stellar population can be traced out to radii of over0.6d (>5 kpc), significantly more extended than the H I disk. Insharp contrast, the distribution of the young, blue stars closelyfollows the distribution of the H I disk and displays a highlystructured morphology. We find evidence for an older stellar populationin the companion galaxy; the current star formation activity, althoughlikely to have been triggered by the interaction with NGC 6822, is notthe first star formation episode in this object. We show that theproperties of the giant kiloparsec-sized hole in the outer H I disk ofNGC 6822 are consistent with it being formed by the effects of stellarevolution.
| Star formation histories in local group dwarf galaxies [review article]
I review observations made with the Hubble Space Telescope which haveimproved our view of both recent (ages ⩽1 Gyr) and ancient (ages⩾1 Gyr) star formation histories in dwarf galaxies. The method ofreconstructing recent star formation histories has been well tested, andnow the major challenge is to build a large database of suitableobservations of nearby dwarf irregular galaxies. With the exception ofthe dSph companions of our Galaxy, questions concerning the ancient starformation histories of nearby galaxies are stymied by a lack of suitablydeep imaging observations. The few observations which do exist providetantalizing evidence of strong evolution in star formation rates. Thisevolution is likely due to environmental effects, and we may be seeingevidence of the effects of reionization on the star formation historiesof dwarf galaxies. Due to its wide field of view and its excellentimaging resolution, the proposed model for SNAP could solve theseproblems.
| On the properties of HI shells in the Small Magellanic Cloud
There are 509 expanding neutral hydrogen shells catalogued in the SmallMagellanic Cloud (SMC), all apparently very young, with dynamical agesof a few Myr. To examine their relationship with young stellar objects,we cross-correlated the shell catalogue with various catalogues of OBassociations, supergiants, Cepheids, Wolf-Rayet stars, supernovaremnants and star clusters. The incidence of chance line-ups wasestimated via Monte Carlo simulations, and found to be high. However, itis important that there are 1.5 times more shells that are not spatiallycorrelated to an OB association, than shells that are. Moreover, 59 ofthe 509 shells lie mainly in low stellar density fields and have noyoung stellar objects associated with them, and therefore no obviousenergy source. It is shown that, on the whole, the properties of these`empty' shells are very similar to the properties of the rest of theshells, once selection biases are taken into account. In both cases, theshell radius and expansion velocity distribution functions areconsistent with the standard model, according to which shells arecreated by stellar winds and supernova explosions, as long as all shellswere created in a single burst and with a power-law distribution of theinput mechanical luminosity. This would indicate a burst of starformation. This interpretation, however, cannot explain why the 59shells, with no young stellar counterparts, show almost exactly the samebehaviour as shells with OB associations within their radius. Gamma-raybursts could account for some but certainly not for the majority of the`empty' shells. Many `empty' shells, including most of thehigh-luminosity ones, are located in the north-western outer regions ofthe SMC, and may be associated with a chimney-like feature that is knownto exist in that area. Finally, it is noted that turbulence is apromising mechanism for the formation of the shell-like structures, butdirect comparison with the observations was not possible at this stage,due to lack of detailed models.
| Mass-to-light ratio gradients in early-type galaxy haloes
Owing to the fact that the near future should see a rapidly expandingset of probes of the halo masses of individual early-type galaxies, weintroduce a convenient parameter for characterizing the halo masses fromboth observational and theoretical results:∇lΥ, the logarithmic radial gradient of themass-to-light ratio. Using halo density profiles from Λ-cold darkmatter (CDM) simulations, we derive predictions for this gradient forvarious galaxy luminosities and star formation efficienciesɛSF. As a pilot study, we assemble the available∇lΥ data from kinematics in early-type galaxies- representing the first unbiased study of halo masses in a wide rangeof early-type galaxy luminosities - and find a correlation betweenluminosity and ∇lΥ, such that the brightestgalaxies appear the most dark-matter dominated. We find that thegradients in most of the brightest galaxies may fit in well with theΛCDM predictions, but that there is also a population of faintergalaxies whose gradients are so low as to imply an unreasonably highstar formation efficiency ɛSF > 1. This difficultyis eased if dark haloes are not assumed to have the standard ΛCDMprofiles, but lower central concentrations.
| 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."
| 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.
| Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities.
| The Baryonic Tully-Fisher Relation of Galaxies with Extended Rotation Curves and the Stellar Mass of Rotating Galaxies
I investigate the baryonic Tully-Fisher relation for a sample ofgalaxies with extended 21 cm rotation curves spanning the range 20 kms-1<~Vf<=300 km s-1. A variety ofscalings of the stellar mass-to-light ratio Υ* areconsidered. For each prescription for Υ*, I give fitsof the form Md=AVxf.Presumably, the prescription that comes closest to the correct valuewill minimize the scatter in the relation. The fit with minimum scatterhas A=50 Msolar km-4 s4 andx=4. This relation holds over five decades in mass. Galaxy color,stellar fraction, and Υ* are correlated with eachother and with Md, in the sense that more massivegalaxies tend to be more evolved. There is a systematic dependence ofthe degree of maximality of disks on surface brightness. High surfacebrightness galaxies typically have Υ*~3/4 of themaximum disk value, while low surface brightness galaxies typicallyattain ~1/4 of this amount.
| Spitzer Observations of the Supergiant Shell Region in IC 2574
We present spatially resolved Spitzer Space Telescope imaging of thesupergiant shell region of the M81 group dwarf galaxy IC 2574 obtainedas part of the Spitzer Infrared Nearby Galaxies Survey. This regionharbors one of the best nearby examples of a kinematically distinct H Ishell, with an associated remnant stellar cluster; the shell isinitiating sequential star formation as it interacts with thesurrounding interstellar medium. This region dominates the infraredluminosity of IC 2574 and is spatially resolved in all Spitzer imagingbands. We study the differences in dust temperature as a function oflocal environment and compare local star formation rates as inferredfrom Hα and total infrared luminosities. We find that the strongHα sources are associated with regions of warm dust; however, themost luminous infrared and Hα sources are not necessarilycospatial. The coolest dust is found in the regions farthest from therim of the shell; these regions show the best agreement between starformation rates derived from Hα and from total infraredluminosities (although discrepancies at the factor of 3-4 level stillexist). There is considerable variation in the radio-far-infraredcorrelation in different regions surrounding the shell. The low dustcontent of the region may influence the scatter seen in these relations;these data demonstrate that the expanding shell is dramaticallyaffecting its surroundings by triggering star formation and altering thedust temperature.
| On the Origin of the H I Holes in the Interstellar Medium of Dwarf Irregular Galaxies
We suggest that large H I holes observed in the interstellar medium(ISM) of galaxies such as the Large Magellanic Cloud (LMC), the SmallMagellanic Cloud (SMC), and Holmberg II (Ho II, DDO 50, UGC 4305) canform as the combined result of turbulence coupled with thermal andgravitational instabilities. We investigate this problem withthree-dimensional hydrodynamic simulations, taking into account coolingand heating processes and the action of the self-gravity of the gas. Weconstruct an algorithm for radiative transfer to postprocess thesimulated data and build emission maps in the 21 cm neutral hydrogenline. With this approach, we are able to reproduce the structure of theshells and holes as observed in regions of the ISM where no stellaractivity is detected. In order to quantify the comparison of oursynthetic maps to the observations, we calculate the physicalscale-autocorrelation length relation (L-Lcr relation) bothon the synthetic H I maps and the H I map of Ho II. The L-Lcrrelation shows a linear increase of the autocorrelation length with thephysical scale up to the scale of energy injection and flattens forlarger scales. The comparison of the L-Lcr relation betweenthe observations and the synthetic maps suggests that turbulence isdriven in the ISM of Ho II on large scales (~6 kpc). The slope of theL-Lcr relation in the linear regime in Ho II is betterreproduced by models where turbulence is coupled with a low-efficiencycooling of the gas. These results demonstrate the importance of theinterplay between turbulence and the thermodynamics of the gas forstructure formation in the ISM. Our analysis can be used to determinethe scale on which kinetic energy is injected into the ISM of dwarfirregular galaxies and to derive, in a first approximation, the coolingrate of the gas.
| VLA Imaging of the Intriguing H I Cloud HIJASS J1021+6842 in the M81 Group
We present VLA H I 21 cm observations of HIJASS J1021+6842, which hasbeen discovered in the direction of the M81 Group. Our synthesis imagingreveals that the H I is distributed over a larger angular extent andvelocity range than the single-dish discovery observations. Assumingthat HIJASS J1021+6842 is at the distance of the M81 Group, we detect1.5×108 Msolar of H I distributed over asmuch as 30 kpc, i.e., substantially larger than the biggest dwarfgalaxies in the same group. At the depth of our imaging, the H I appearsto be confined to at least seven clouds. Peak H I column densities are~1.8×1020 atoms cm-2, which is well belowthe canonical star formation threshold of ~1021 atomscm-2 and therefore consistent with the fact that no opticalcounterpart has as yet been identified. A gradient in velocity isobserved across the extent of the detected H I; assuming that the objectis gravitationally bound we derive a dynamical mass of7×109 Msolar and a dark-to-luminous massratio of >10. Alternatively, a tidal origin may also result in theobserved velocity gradient, which would lead to a considerably lowerdynamical mass. Given the above properties and the absence of evidenceof a stellar population, HIJASS J1021+6842 is unique amongst the othersystems in the M81 Group.
| 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.
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
| Mass Modeling of Disk Galaxies: Degeneracies, Constraints, and Adiabatic Contraction
This paper addresses available constraints on mass models fitted torotation curves. Mass models of disk galaxies have well-knowndegeneracies that prevent a unique mass decomposition. The most notableis due to the unknown value of the stellar mass-to-light ratio (thedisk-halo degeneracy); even with this known, degeneracies between thehalo parameters themselves may prevent an unambiguous determination ofthe shape of the dark halo profile, which includes the inner densityslope of the dark matter halo. The latter is often referred to as the``cusp-core degeneracy.'' We explore constraints on the disk and haloparameters and apply these to four mock and six observed disk galaxieswith high resolution and extended rotation curves. Our full set ofconstraints consists of mass-to-light (M/L) ratios from stellarpopulation synthesis models based on B-R colors, constraints on haloparameters from N-body simulations, and constraining the halo virialvelocity to be less than the maximum observed velocity. Theseconstraints are only partially successful in lifting the cusp-coredegeneracy. The effect of adiabatic contraction of the halo by the diskis to steepen cores into cusps and reduce the best-fit haloconcentration and M/L values (often significantly). We also discuss theeffect of disk thickness, halo flattening, distance errors, and rotationcurve error values on mass modeling. Increasing the imposed minimumrotation curve error from typically low, underestimated values to morerealistic estimates decreases the χ2 substantially andmakes distinguishing between a cuspy or cored halo profile even moredifficult. In spite of the degeneracies and uncertainties present, ourconstrained mass modeling favors submaximal disks (i.e., a dominanthalo) at 2.2 disk scale lengths, withVdisk/Vtot<~0.6. This result holds for both theunbarred and weakly barred galaxies in our sample.
| Dust and gas in nearby galaxies: first results from SINGS and THINGS.
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| 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.
| Triggering and Feedback: The Relation between the H I Gas and the Starburst in the Dwarf Galaxy NGC 1569
As part of our study on the impact of violent star formation on theinterstellar medium (ISM) of dwarf galaxies, we report observations ofneutral atomic hydrogen (H I) in the starburst dwarf galaxy NGC 1569.High-resolution measurements with the Very Large Array (B, C, and Dconfiguration) are aimed at identifying morphological and kinematicalsignatures in H I caused by the starburst. Our kinematical data suggesta huge hole in the H I distribution, probably due to the large number ofsupernovae explosions in the center of the galaxy over the past 20 Myr.Investigating the large-scale H I structure, we confirm the existence ofa possible H I companion and a so-called H I bridge east of NGC 1569.Furthermore, we report the detection of additional low-intensity H Ihalo emission, which leads us to suggest a revised halo structure. Onthe basis of our new picture, we discuss the origin of the halo gas andpossible implications for the evolution of the starburst in NGC 1569.
| 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.
| H I shells in the outer Milky Way
We present results of a method for an automatic search for HI shells in3D data cubes and apply it to the Leiden-Dwingeloo HI survey of thenorthern Milky Way. In the 2nd Galactic quadrant, where identificationsof structures are not substantially influenced by overlapping, we findnearly 300 structures. The Galactic distribution of shells has anexponential profile in the radial direction with a scale length ofσgsh = 3 kpc. In the z direction, one half of theshells are found at distances smaller than 500 pc. We also calculate theenergies necessary to create the shells: there are several structureswith energies greater than 10ESN but only one with an energyexceeding 100ESN. Their size distribution, corrected fordistance effects, is approximated by a power-law with an index α =2.1. Our identifications provide a lower limit to the filling factor ofshells in the outer Milky Way: f2D = 0.4 and f3D =0.05.
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