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Dwarf and Normal Spiral Galaxies: are they Self-Similar?
The investigation presented here was focused on clarifying the existenceof dwarf spiral galaxies as a separate group from classical spirals.First, a list of spiral galaxies with small sizes was obtained.Information on colors, luminosities, morphologies and chemical contentwas searched for in the literature for these galaxies. Using thisinformation, it can be concluded that dwarf spirals are not likely to bethe tail of the distribution of classical galaxies. On the contrary,significant differences in some of the most important properties ofspiral galaxies, such as the metallicity gradient and the bar frecuency,were found. In any case, further and more accurate observations areneeded for a definitive answer.
| The Wisconsin Hα Mapper Northern Sky Survey
The Wisconsin Hα Mapper (WHAM) has surveyed the distribution andkinematics of ionized gas in the Galaxy above declination -30°. TheWHAM Northern Sky Survey (WHAM-NSS) has an angular resolution of 1°and provides the first absolutely calibrated, kinematically resolved mapof the Hα emission from the warm ionized medium (WIM) within~+/-100 km s-1 of the local standard of rest. LeveragingWHAM's 12 km s-1 spectral resolution, we have modeled andremoved atmospheric emission and zodiacal absorption features from eachof the 37,565 spectra. The resulting Hα profiles reveal ionizedgas detected in nearly every direction on the sky with a sensitivity of0.15 R (3 σ). Complex distributions of ionized gas are revealed inthe nearby spiral arms up to 1-2 kpc away from the Galactic plane.Toward the inner Galaxy, the WHAM-NSS provides information about the WIMout to the tangent point down to a few degrees from the plane. Ionizedgas is also detected toward many intermediate velocity clouds at highlatitudes. Several new H II regions are revealed around early B starsand evolved stellar cores (sdB/O). This work presents the details of theinstrument, the survey, and the data reduction techniques. The WHAM-NSSis also presented and analyzed for its gross properties. Finally, somegeneral conclusions are presented about the nature of the WIM asrevealed by the WHAM-NSS.
| Extraplanar Emission-Line Gas in Edge-On Spiral Galaxies. I. Deep Emission-Line Imaging
The extraplanar diffuse ionized gas (eDIG) in 17 nearby, edge-on diskgalaxies is studied using deep Taurus Tunable Filter Hα and [N II]λ6583 images and conventional interference filter Hα+[N II]λλ6548, 6583 images that reach flux levels generally below~1×10-17 ergs s-1 cm-2arcsec-2. [N II] λ6583/Hα excitation maps areavailable for 10 of these objects. All but one galaxy in the sampleexhibit eDIG. The contribution of the eDIG to the total Hαluminosity is relatively constant, on the order of 12%+/-4%. TheHα scale height of the eDIG derived from a two-exponential fit tothe vertical emission profile ranges from 0.4 to 17.9 kpc, with anaverage of 4.3 kpc. This average value is noticeably larger than theeDIG scale height measured in our Galaxy and other galaxies. Thisdifference in scale height is probably due in part to the lower fluxlimits of our observations. The ionized mass of the extraplanarcomponent inferred by assuming a constant filling factor of 0.2 and aconstant path length through the disk of 5 kpc ranges from1.4×107 to 2.4×108 Msolar,with an average value of 1.2×108 Msolar.Under these same assumptions, the recombination rate required to keepthe eDIG ionized ranges from 0.44×106 to13×106 s-1 cm-2 of the disk, orabout 10%-325% of the Galactic value. A quantitative analysis of thetopology of the eDIG confirms that several galaxies in the sample have ahighly structured eDIG morphology. The distribution of the eDIG emissionis often correlated with the locations of the H II regions in the disk,supporting the hypothesis that the predominant source of ionization ofthe eDIG is photoionization from OB stars located in the H II regions. Astrong correlation is found between the IR (or far-IR) luminosities perunit disk area (basically a measure of the star formation rate per unitdisk area) and the extraplanar ionized mass, further providing supportfor a strong connection between the disk and eDIG components in thesegalaxies. The excitation maps confirm that the [N II]/Hα ratiosare systematically higher in the eDIG than in the disk. Althoughphotoionization by disk OB stars is generally able to explain theseelevated [N II]/Hα ratios, a secondary source of ionizationappears to be needed when one also takes into account other line ratios;more detail is given in a companion paper (our Paper II).
| A Survey for H2O Megamasers. III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalacticwater megamasers taken over a period of 9 years and a single epoch ofsensitive spectra for seven others. The primary motivation is a searchfor drifting line velocities analogous to those of the systemic featuresin NGC 4258, which are known to result from centripetal acceleration ofgas in an edge-on, subparsec molecular disk. We detect a velocity driftanalogous to that in NGC 4258 in only one source, NGC 2639. Another, themaser source in NGC 1052, exhibits erratic changes in its broad maserprofile over time. Narrow maser features in all of the other diskgalaxies discussed here either remain essentially constant in velocityover the monitoring period or are sufficiently weak or variable inintensity that individual features cannot be traced reliably from oneepoch to the next. In the context of a circumnuclear, molecular diskmodel, our results suggest that either (a) the maser lines seen aresystemic features subject to a much smaller acceleration than present inNGC 4258, presumably because the gas is farther from the nuclear blackhole, or (b) we are detecting ``satellite'' lines for which theacceleration is in the plane of the sky.Our data include the first K-band science observations taken with thenew 100 m Green Bank Telescope (GBT). The GBT data were taken duringtesting and commissioning of several new components and so are subjectto some limitations; nevertheless, they are in most cases the mostsensitive H2O spectra ever taken for each source and cover800 MHz (~=10,800 km s-1) of bandwidth. Many new maserfeatures are detected in these observations. Our data also include atentative and a clear detection of the megamaser in NGC 6240 at epochs ayear and a few months, respectively, prior to the detections reported byHagiwara et al. and Nakai et al.We also report a search for water vapor masers toward the nuclei of 58highly inclined (i>80deg), nearby galaxies. These sourceswere selected to investigate the tendency that H2O megamasersfavor inclined galaxies. None were detected, confirming that megamasersare associated exclusively with active galactic nuclei.
| An Hα survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. II. The Hα survey atlas and catalog
In this second paper on the investigation of extraplanar diffuse ionizedgas in nearby edge-on spiral galaxies we present the actual results ofthe individual galaxies of our Hα imaging survey. A grand totalof 74 galaxies have been studied, including the 9 galaxies of a recentlystudied sub-sample \citep{Ro00}. 40.5% of all studied galaxies revealextraplanar diffuse ionized gas, whereas in 59.5% of the survey galaxiesno extraplanar diffuse ionized gas could be detected. The averagedistances of this extended emission above the galactic midplane rangefrom 1-2 kpc, while individual filaments in a few galaxies reachdistances of up to |z| ~ 6 kpc. In several cases a pervasive layer ofionized gas was detected, similar to the Reynolds layer in our MilkyWay, while other galaxies reveal only extended emission locally. Themorphology of the diffuse ionized gas is discussed for each galaxy andis compared with observations of other important ISM constituents in thecontext of the disk-halo connection, in those cases where publishedresults were available. Furthermore, we present the distribution ofextraplanar dust in these galaxies, based on an analysis of theunsharp-masked R-band images. The results are compared with thedistribution of the diffuse ionized gas.Based on observations collected at the European Southern Observatory,Chile (ESO No. 63.N-0070, ESO No. 64.N-0034, ESO No. 65.N.-0002).\ref{fig22}-\ref{fig54} are only available in electronic form athttp://www.edpsciences.org
| An Hα survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
In a series of two papers we present results of a new Hα imagingsurvey, aiming at the detection of extraplanar diffuse ionized gas inhalos of late-type spiral galaxies. We have investigated a sample of 74nearby edge-on spirals, covering the northern and southern hemisphere.In 30 galaxies we detected extraplanar diffuse emission at meandistances of |z| ~ 1-2 kpc. Individual filaments can be traced out to|z|<=6 kpc in a few cases. We find a good correlation between the FIRflux ratio (S60/S100) and the SFR per unit area(LFIR/D225), based on thedetections/non-detections. This is actually valid for starburst, normaland for quiescent galaxies. A minimal SFR per unit area for the lowestS60/S100 values, at which extended emission hasbeen detected, was derived, which amounts to dotEA25thres = (3.2+/-0.5)*E40ergs-1 kpc-2. There are galaxies where extraplanaremission was detected at smaller values ofLFIR/D225, however, only in combinationwith a significantly enhanced dust temperature. The results corroboratethe general view that the gaseous halos are a direct consequence of SFactivity in the underlying galactic disk.Based on observations collected at the European Southern Observatory,Chile (ESO No. 63.N-0070, ESO No. 64.N-0034, ESO No. 65.N.-0002).
| Properties of tidally-triggered vertical disk perturbations
We present a detailed analysis of the properties of warps andtidally-triggered perturbations perpendicular to the plane of 47interacting/merging edge-on spiral galaxies. The derived parameters arecompared with those obtained for a sample of 61 non-interacting edge-onspirals. The entire optical (R-band) sample used for this study waspresented in two previous papers. We find that the scale height of disksin the interacting/merging sample is characterized by perturbations onboth large ( =~ disk cut-off radius) and short ( =~ z0)scales, with amplitudes of the order of 280 pc and 130 pc on average,respectively. The size of these large (short) -scale instabilitiescorresponds to 14% (6%) of the mean disk scale height. This is a factorof 2 (1.5) larger than the value found for non-interacting galaxies. Ahallmark of nearly all tidally distorted disks is a scale height thatincreases systematically with radial distance. The frequent occurrenceand the significantly larger size of these gradients indicate that diskasymmetries on large scales are a common and persistent phenomenon,while local disturbances and bending instabilities decline on shortertimescales. Nearly all (93%) of the interacting/merging and 45% of thenon-interacting galaxies studied are noticeably warped. Warps ofinteracting/merging galaxies are ~ 2.5 times larger on average thanthose observed in the non-interacting sample, with sizes of the order of340 pc and 140 pc, respectively. This indicates that tidal distortionsdo considerably contribute to the formation and size of warps. However,they cannot entirely explain the frequent occurrence of warped disks.Based on observations obtained at the European Southern Observatory(ESO, La Silla, Chile), Calar Alto Observatory operated by the MPIA(DSAZ, Spain), Lowell Observatory (Flagstaff,AZ, USA), and Hoher ListObservatory (Germany).
| The Wisconsin Hα Mapper: A New Look at the Warm Ionized Medium
The Wisconsin Hα Mapper (WHAM) has surveyed the entire northernsky in Hα from Kitt Peak, Arizona. Using a high-throughput, 15-cmdiameter double-etalon Fabry-Perot spectrometer and a sensitive CCDdetector, the WHAM survey provides the first calibrated,velocity-resolved map of Hα emission in our Galaxy. A largeportion of the Galaxy, which samples regions of the Local (Orion) spiralarm and the more distant Perseus arm, has been also been observed withthe WHAM in lines of [S II] and [N II]. These new data directly probethe physical conditions of the Warm Ionized Medium (WIM) on a globalscale for the first time. Trends in these line ratios over this largeregion of the sky suggest that temperature variations are traced by the[N II]/Hα and [S II]/Hα maps. Since these ratios increasedramatically away from the Galactic plane, they reveal a substantialtemperature rise in WIM halo gas. In addition to this striking newresult, the data set also reveals new information about the ionizationin the WIM through the [S II]/[N II] ratio, uncovers a previouslyundiscovered B-star H II region, and provides an accurate measurement ofthe electron scale height of the WIM.
| Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| The influence of interactions and minor mergers on the structure of galactic disks I. Observations and disk models
This paper is the first part in our series on the influence of tidalinteractions and minor mergers on the radial and vertical disk structureof spiral galaxies. We report on the sample selection, our observations,and data reduction. Surface photometry of the optical and near infrareddata of a sample of 110 highly-inclined/edge-on disk galaxies arepresented. This sample consists of two subsamples of 61 non-interactinggalaxies (control sample) and of 49 interacting galaxies/minor mergingcandidates. Additionally, 41 of these galaxies were observed in the nearinfrared. We show that the distribution of morphological types of bothsubsamples is almost indistinguishable, covering the range between 0<= T <= 9. An improved, 3-dimensional disk modelling- and fittingprocedure is described in order to analyze and to compare the diskstructure of our sample galaxies by using characteristic parameters. Wefind that the vertical brightness profiles of galactic disks respondvery sensitive even to small deviations from the perfect edge-onorientation. Hence, projection effects of slightly inclined disks maycause substantial changes in the value of the disk scale height and musttherefore be considered in the subsequent study. Based on observationsobtained at the European Southern Observatory (ESO, La Silla, Chile),Calar Alto Observatory operated by the MPIA (DSAZ, Spain), LowellObservatory (Flagstaff/AZ, USA), and Hoher List Observatory (Germany).
| Spectroscopy of diffuse ionized gas in halos of selected edge-on galaxies
In order to examine the excitation and ionization mechanism ofextraplanar diffuse ionized gas (DIG) we have obtained optical longslitspectra of seven edge-on spiral galaxies. In four objects the brightestemission lines can be traced out to distances of typically 1.5 kpc abovethe disk. For NGC 1963 and NGC 3044 line ratios such as [N Ii]lambda6583 /Hα or [S Ii] lambda6717 /Hα as well as [ion{O{iii}] lambda5007 }/Hβ could be measured for the halo DIG. Thisallows us to discuss the DIG in the halo of these objects in theframework of diagnostic diagrams. For these two objects, the line ratiosof [ion {O{iii}] lambda5007 }/Hβ decrease with increasing |z|,different from the recently reported trend in NGC 891 (Rand\cite{rand}). We find that emission lines from the DIG in the disks arein good agreement with photoionization models using a dilute radiationfield. However, with increasing |z| these models fail to predict themeasured [ion {Oi] lambda 6300}/Hα \ and ion {Hei lambda5876}/Hα line ratios for NGC 1963. Diagnostic diagrams reveal forNGC 1963 the need for a second ionization mechanism of the halo DIG(besides photoionization). This additional source could be shockionization. The same diagrams demonstrate an intermediate classificationfor NGC 3044. Plots of [ion {S}{ii}]/[ion {N}{ii}] vs. emission measurereveal significant changes towards the halo and seem to trace localsmall scale density fluctuations of the extraplanar DIG. Based onobservations obtained at ESO/La Silla (Chile)
| Extraplanar diffuse ionized gas in a small sample of nearby edge-on galaxies
We present narrowband Hα imaging data of a small survey of nearbyedge-on spiral galaxies, aiming at the detection of `extraplanar'diffuse ionized gas (DIG). A few of our studied edge-on spirals showsigns of disk-halo interaction (DHI), where extended line emission farabove the galactic plane of these galaxies is detected. In some cases anextraplanar diffuse ionized gas (eDIG) layer is discovered, e.g.,NGC4634, NGC 3044, while othergalaxies show only filamentary features reaching into the halo (e.g.,IC 2531) and some galaxies show no sign of eDIG atall. The extraplanar distances of the DIG layer in our narrowbandHα images reach values of z<= 2 kpc above the galactic plane.The derived star formation rates (SFRs) from the Hα flux of thestudied galaxies range from 0.05-0.7 Msun yr-1,neglecting a correction for internal absorption. The variation of theSFR values among our sample galaxies reflects the diversity of starformation within this sample. A diagnostic diagram is introduced, whichallows to predict the existence of gas halos in `quiescent' galaxiesbased on the ratio S60 /S100 versusLFIR / D225 in this diagram. We comparethe positions of the non-starburst galaxies with starburst galaxies,since these galaxies populate distinct positions in these diagrams.Based on observations collected at the European Southern Observatory, LaSilla, Chile
| Spectral classification of emission-line galaxies
The main goal of this work is to further investigate the classificationof emission-line galaxies from the ``Spectrophotometric Catalogue of HII galaxies'' by Terlevich et al. (1991) in a homogeneous and objectiveway, using the three line-ratio diagrams, called diagnostic diagrams, ofVeilleux & Osterbrock (1987). On the basis of the resultingcatalogue, we critically discuss the classification methods in theoptical range. In particular we compare our classification scheme to theone done by Rola et al. (1997) which is efficient for the classificationof redshifted galaxies. We also propose a new diagnostic diagraminvolving the known intensity ratio R23=([O II],l 3727+[OIII] l 4959+{[O III] l 5007)/Hb which appears to be a very goodcriterion allowing to discriminate the Seyfert 2 from H ii galaxies. Therevised catalogue including 314 narrow-emission-line galaxies contains HII galaxies, Seyfert 2 galaxies, Low Ionization Nuclear Emission-LineRegions (hereafter LINERs) galaxies and some particular types ofgalaxies with the most intriguing ones, called ``ambiguous'', due to theambiguity of their location in the diagnostic diagrams. These galaxiesappear as H II galaxies and as active galactic nuclei (hereafter AGNs)in different diagrams of Veilleux & Osterbrock and constitutecertainly a sample of particularly interesting candidates for a thoroughstudy of connections between starbursts and AGNs. Available inelectronic form only via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Long slit spectroscopy of diffuse ionized gas in NGC 55
Spectroscopic measurements of emission line ratios and velocities arepresented for ionized gas across the central plane of NGC 55, alate-type galaxy in the Sculptor group. The low metallicity in NGC 55leads to relatively low line ratios of [S Ii]/Hα and [NIi]/Hα for H Ii regions as well as the diffuse ionized gas. Theseare the first spectroscopical measurements of line ratios in ionized gasexterior to the stellar disc of NGC 55. Analysis of the line ratios andthe relative velocities of different features suggests thatphotoionization is a plausible explanation for the ionization of thisdiffuse gas. The observed shell structures and the correspondingvelocities support the idea of diffuse gas being pushed into the halo bysupernova explosions and stellar winds. Based on observations obtainedat ESO/La Silla
| Diffuse ionized gas in edge-on galaxies
We review observations of diffuse ionized gas (DIG) in edge-on spiralgalaxies. Deep imaging has revealed a variety of morphologies for`extraplanar' (above the HII region layer) or `halo' DIG: some showwidespread prominent layers of truly diffuse gas and filamentarystructures, others show just one or a few patches of extraplanaremission, while still others show no detectable extraplanar DIG at all.The key galactic property which appears to govern the prominence of theDIG layer is the star formation activity. Smaller scale connectionsbetween DIG and local star formation activity can be seen in some of thegalaxies. DIG halos are also related to radio continuum, X-ray and HIhalos. Those galaxies with the most prominent DIG layers also have themost prominent radio halos. In NGC 891, there are spatial correlationsbetween extraplanar DIG, radio emission, X-ray emission and HI, andthese in turn are related to the underlying disk star formationactivity. Progress on identifying the sources of ionization and heatingof the gas has come from spectroscopy, imaging in different emissionlines, and Fabry-Perot observations. By comparing with theoreticalpredictions, much evidence is found for photo-ionization by disk starsas the primary ionization and heating mechanism, but recent observationsare beginning to point towards the need for additional sources ofionization and/or heating.
| Catalogue of HI maps of galaxies. I.
A catalogue is presented of galaxies having large-scale observations inthe HI line. This catalogue collects from the literature the informationthat characterizes the observations in the 21-cm line and the way thatthese data were presented by means of maps, graphics and tables, forshowing the distribution and kinematics of the gas. It containsfurthermore a measure of the HI extension that is detected at the levelof the maximum sensitivity reached in the observations. This catalogueis intended as a guide for references on the HI maps published in theliterature from 1953 to 1995 and is the basis for the analysis of thedata presented in Paper II. The catalogue is only available inelectronic form at the CDS via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
| A catalogue of spatially resolved kinematics of galaxies: Bibliography
We present a catalogue of galaxies for which spatially resolved data ontheir internal kinematics have been published; there is no a priorirestriction regarding their morphological type. The catalogue lists thereferences to the articles where the data are published, as well as acoded description of these data: observed emission or absorption lines,velocity or velocity dispersion, radial profile or 2D field, positionangle. Tables 1, 2, and 3 are proposed in electronic form only, and areavailable from the CDS, via anonymous ftp to cdsarc.u-strasbg.fr (to130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Gaseous Halos of Late-Type Spiral Galaxies
This article reviews the most recent observational results on disk-halointeractions in nearby galaxies. The implications of these results onour understanding of the structure of the interstellar medium (ISM) inour Galaxy and external spiral galaxies, with particular emphasis on thehalo ISM, are discussed. Disk-halo interactions occur only above thebrightest \hii\ regions, which are found almost exclusively in late-typespirals and dwarf and irregular galaxies. Circumnuclear starbursts arethe most energetic flavor of this phenomenon. The existence, the shapes,and the properties of gaseous halos depend on the level of energy inputinto the disk ISM per unit surface area, i.e. on the feedback of mass,momentum, and energy produced by vigorous star formation. Thus, currenttheoretical models of the ISM take into account that gaseous halos arenatural extensions of disk ISMs and are heated by the winds and ionizingradiation of massive stars, and by the shock waves of supernovae andtheir remnants. Observationally, this is reflected by the fact that allphases of the ISM known to exist in galaxy disks have also been detectedin halos. Disk-halo interactions are a very effective way ofredistributing energy, metals, and magnetic fields in galaxies and ofexpelling them into intergalactic space when outflows reach escapevelocit. (SECTION: Invited Review Articles)
| On the ionization of the diffuse ionized gas: spectroscopy of NGC 2188.
NGC 2188 is a highly inclined irregular galaxy which shows spectacularfilaments of ionized gas emerging from an HII region complex into thehalo of the galaxy. Here we present Hα imaging and opticalspectroscopy of NGC 2188. Since the galaxy is low in metallicity anddust poor, the spectroscopic observations open up a parameter range forthe extraplanar DIG which is not yet explored. Using empiricaldiagnostic diagrams we find that the ionized gas above the plane is mostlikely photoionized. Differences in line ratios between the DIG inspiral galaxies and in NGC 2188 can fully be explained by differences inmetallicities. We argue that dust is not of high importance in alteringthe line ratios measured for the DIG either by scattering orphotoelectric heating.
| On the Size and Formation Mechanism of Star Complexes in Sm, Im, and BCD Galaxies
The diameters D_c_ of the largest star-forming complexes in 67Magellanic spiral and irregular galaxies and 16 blue compact dwarf (BCD)galaxies are found to scale approximately with the square root of thegalaxy luminosity for each type, i.e., smaller galaxies haveproportionately smaller star-forming regions. This is the same relationas for the largest complexes in bright spiral galaxies found previously,although Sm/Im galaxies have complexes that, on average, are a factor of2 larger than the extrapolation for spiral galaxies at the same absolutemagnitude, and the BCD galaxies have complexes that are ~2 times largerthan those typical of the Sm/Im galaxies at the same absolute magnitude.These results are consistent with the interpretation that the largestcomplexes form at the gravitational length scale in a marginally stableinterstellar medium with a nearly constant velocity dispersion c ~ 5-10km s^-1^. The luminosity scaling is then the result of higher averagetotal densities in smaller galaxies compared with the outer regions ofgiant spirals. This total density correlation is shown using published HI line widths and optical galaxy sizes. The implication of these resultsis that star formation begins when the ratio of the gas density ρ tothe total density (gas + stars + dark matter) exceeds several tenths. Ifstar formation lasts for a time scaling with (Grho_)^-1/2^ ~D_c_/c, then the main morphological differences between star formationin galaxies of various sizes can be explained: large galaxies have largestar complexes that form groups of OB associations slowly for up to 50Myr; small galaxies have small complexes (in terms of absolute size)that form dense associations quickly, in bursts spanning less than 5Myr.
| The Absence of X-Ray Flashes from Nearby Galaxies and the Gamma-Ray Burst Distance Scale
If typical gamma-ray bursts (GRBs) have X-ray counterparts similar tothose detected by Ginga, then sensitive-focusing X-ray telescopes willbe able to detect GRBs 3 orders of magnitude fainter than the detectionlimit of the Burst and Transient Source Experiment (BATSE). If asubstantial portion of the burst population detected by BATSE originatesin a Galactic halo at distances greater than or equal to 150 kpc,existing X-ray telescopes will be able to detect GRBs in externalgalaxies out to a distance of at least 4.5 Mpc. As reported in Gotthelf,Hamilton, & Helfand, the imaging proportional counter (IPC) on boardthe Einstein Observatory detected 42 transient events with pointlikespatial characteristics and timescales of less than 10 s. These eventsare distributed isotropically on the sky; in particular, they are notconcentrated in the directions of nearby external galaxies. For halomodels of the BATSE bursts with radii of 150 kpc or greater, we wouldexpect to see several burst events in observations pointed toward nearbygalaxies. We see none. We therefore conclude that if the Gingadetections are representative of the population of GRBs sampled byBATSE, GRBs cannot originate in a Galactic halo population with limitingradii between 150 and 400 kpc. Inasmuch as halos with limiting radiioutside of this range have been excluded by the BATSE isotropymeasurements, our result indicates that all halo models are excluded.This result is independent of whether the flashes we do detect have anastronomical origin.
| What perturbs NGC2188?
VLA HI observations of the dwarf irregular galaxy NGC2188 show that thegas and stars have spatial distributions which are substantiallydifferent. One end of the optical disk is strongly gas deficient, whileneutral gas extends into the halo over distances of more than 2kpc fromthe midplane. This and the peculiar velocity field suggest that NGC2188is a perturbed system, although it is not obviously an interactinggalaxy. In addition, NGC2188 is remarkable for its interstellardisk-halo connection. An Hα image of NGC2188 shows the presence ofspectacular features of ionized gas extending from a large star-formingcomplex up to 500pc into the halo. Also, peculiar HI filaments and atleast one superbubble are present in NGC2188.
| Star formation activity in spiral galaxy disks and the properties of radio halos: Observational evidence for a direct dependence
In this article we address observationally the questions: how does starformation (SF) in the disks of galaxies lead to the creation of radiohalos, and what minimum energy input into the interstellar medium (ISM)is needed to facilitate this? For the investigation we use a sample offive edge-on galaxies exhibiting radio continuum emmission in theirhalos and enhanced SF spread over large parts of their disks. In adetailed study of the two galaxies in our sample for which we have thebest data, NGC 891 and NGC 4631, we show that the radio halos cut offabruptly at galactocentric radii smaller than those of the underlyingthin radio disks. Our most important result is that the halo cutoffs arespatially coincident with the radii where the SF activity in theunderlying disks drops sharply. The difference in radius of the emissiondistributions tracing ongoing SF in the disks (IRAS 50 micrometers, Halpha) versus that of the nonthermal radio continuum thin disks (tracingthe distribution of cosmic-ray (CR) electrons) is typically a few kpc.This difference in extent is caused by CR diffusion. We have measuredthe CR diffusion coefficients in the thin disks of both NGC 891 and NGC4631. For radial diffusion of CR electrons within the galactic disks thevalues are Dr = 1.1-2.5 x 10 29 sq cm/s (NGC 4631)and Dr = 1.2 x 1029 sq cm/s (NGC 891). For motionsin the z-direction in areas within the thin disks where no outflowsoccur, we derive a firm upper limit of Dz less than or equalto 0.2 x 1028 sq cm/s for NGC 891. The value for NGC 4631 isDz = 1.4 x 10 28 sq cm/s. The other three galaxies inour sample, NGC 3044, NGC 4666, and NGC 5775 show (at the sensitivity ofour data less extended, more filamentary radio halos. Isolatesspurs or filaments of nonthermal radio continuum emission in their halosare traced only above the most actively star-forming regions in thedisks. This, in conjuction with the results obtained for NGC 891 and NGC4631, suggests a direct connection between the shapes and radial extentsof radio halos of normal galaxies and the level and the spatialdistribution of the current SF in their disks. The existence offilamentary structures implies that expansion and convection ofCR-heated plasma plays an important role even at low z-distances abovethe plane. Based on radio data we derive the disk-averaged mean energyinput rates into the ISM due to supernova explosions per unit surfacearea in nine galaxies, including the ones in our sample. We find a cleartrend in the sense that the galaxies with the highest mean energy inputrates in the underlying disks have the most extended and pervasive radiohalos. Below a certain threshold, outflows are inhibited. As a lowerlimit for this threshhold we derive an energy input rate ofapproximately 10 -4 erg/s sq cm. The fact that radio haloemission is found only above the most luminous giant H 2 regions inlate-type 'normal' galaxies (of type Sbc or later) indicates that onlythese are capable of initiating disk-halo interactions by locallyexceeding the threshold.
| A Preliminary Classification Scheme for the Central Regions of Late-Type Galaxies
The large-scale prints in The Carnegie Atlas of Galaxies have been usedto formulate a classification scheme for the central regions oflate-type galaxies. Systems that exhibit small bright central bulges ordisks (type CB) are found to be of earlier Hubble type and of higherluminosity than galaxies that do not contain nuclei (type NN). Galaxiescontaining nuclear bars, or exhibiting central regions that are resolvedinto individual stars and knots, and galaxies with semistellar nuclei,are seen to have characteristics that are intermediate between those oftypes CB and NN. The presence or absence of a nucleus appears to be auseful criterion for distinguishing between spiral galaxies andmagellanic irregulars.
| Integrated photoelectric magnitudes and color indices of bright galaxies in the Johnson UBV system
The photoelectric total magnitudes and color indices published in theThird Reference Catalogue of Bright Galaxies (RC3) are based on ananalysis of approximately equals 26,000 B, 25,000 B-V, and 17,000 U-Bmultiaperture measurements available up to mid 1987 from nearly 350sources. This paper provides the full details of the analysis andestimates of internal and external errors in the parameters. Thederivation of the parameters is based on techniques described by theVaucouleurs & Corwin (1977) whereby photoelectric multiaperture dataare fitted by mean Hubble-type-dependent curves which describe theintegral of the B-band flux and the typical B-V and U-B integrated colorgradients. A sophisticated analysis of the residuals of thesemeasurements from the curves was made to allow for the random andsystematic errors that effect such data. The result is a homogeneous setof total magnitudes BTA total colors(B-V)T and (U-B)T, and effective colors(B-V)e and (U-B)e for more than 3000 brightgalaxies in RC3.
| The ISM in NGC 2188: a case study for disk-halo interaction.
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| CO observations of 25 dwarf galaxies.
Out of 25 dwarf galaxies searched for J=1-0 ^12^CO emission, four (NGC55, Haro 2, NGC 6822 and NGC 7793) were unambiguously detected. Fourmore (DDO 68, 83, 99 and 165) showed doubtful emission at 2-5 times therms noise level. These and previously obtained data yield 24 dwarfgalaxies both (almost) completely covered by the CO observing beam andhaving known optical colours. The two detected galaxies NGC 1569 andHaro 2 are blue with (B-V)_0_=0.3; the uncertain detections also haveblue colours. The galaxies not detected cover the full range(B-V)_0_=0.0-0.8. Inferred CO-to-FIR flux ratios of the generallymetal-poor dwarf galaxies fall between those of high-metallicity centersof bright spiral galaxies and Galactic HII regions. This suggests thatthe CO emissivity of dwarf galaxies is more determined by ambientradiation field strengths than by metallicity. Except for Haro 2 and NGC7793, inferred mass ratios M(H_2_)/M(HI) are less than a few per cent,more than an order of magnitude lower than those of late-type spiralgalaxies. This does not necessarily reflect a real lack of H_2_ in dwarfgalaxies. Instead, in these objects the CO to H_2_ conversion factor maywell be higher than the standard Galactic factor, thus allowing thepresence of potentially large amounts of undetected molecular hydrogengas.
| Element Abundances in Blue Compact Galaxies
Models of the chemical evolution of blue compact galaxies, where thestar formation is assumed to proceed in short, intense bursts ofactivity, have been computed, with special attention to the evolution ofhelium, nitrogen, oxygen and iron. The contribution to the chemicalenrichment by supernovae (SNe) of the various types (Ia, lb, II) istaken into account, following the most up-to-date prescriptions.Galactic winds powered by SNe are also considered and confirmed as beingable to explain the observed element distributions. In particular, windsreleasing metals in different proportions can explain the high ratio of{DELTA}(He/H)/{DELTA}(O/H) and the distribution of N/O with O/H observedin extragalactic HII regions. No anomalous assumption on the initialmass function (IMF) slope and/or mass cut-offs seems to be required toobtain model predictions in agreement with the data. In particular,Salpeter`s initial mass function is applicable to all these galaxies,although some slight differences cannot be excluded. The [O/Fe] ratiosresulting from these models are always positive, except when many starformation episodes and differential galactic winds are assumed, thussuggesting the latter scenario for systems like the Magellanic Clouds,where the observed [O/Fe] ratios are negative.
| On the size and formation mechanism of the largest star-forming complexes in spiral and irregular galaxies
The average diameters of the largest star complexes in most of thespiral and irregular galaxies in the Sandage and Bedke Atlas of Galaxieswere measured from the Atlas photographs. The complex diametersDc correlate with galaxy magnitude as Dc = 0.18 -0.14MB, which has about the same slope as the correlation forthe largest H II regions studied by Kennicutt. There is no obviouscorrelation between Dc and either Hubble type or spiral armclass at a given magnitude. The variation of Dc withMB closely matches the expected variation in thecharacteristic length of the gaseous gravitational instabilityconsidering that the rotation curve varies with MB and thatthe stability parameter Q is about 1 in the outer regions of the disk.This match corresponds to an effective velocity dispersion of 6.1 km/sthat is about the same for all spiral and irregular galaxies.
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