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The Survey for Ionization in Neutral Gas Galaxies. I. Description and Initial Results
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG),a census of star formation in H I-selected galaxies. The survey consistsof Hα and R-band imaging of a sample of 468 galaxies selected fromthe H I Parkes All Sky Survey (HIPASS). The sample spans three decadesin H I mass and is free of many of the biases that affect otherstar-forming galaxy samples. We present the criteria for sampleselection, list the entire sample, discuss our observational techniques,and describe the data reduction and calibration methods. This paperfocuses on 93 SINGG targets whose observations have been fully reducedand analyzed to date. The majority of these show a single emission linegalaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs ina single field. All of the targets in this sample are detected inHα, indicating that dormant (non-star-forming) galaxies withMHI>~3×107 Msolar are veryrare. A database of the measured global properties of the ELGs ispresented. The ELG sample spans 4 orders of magnitude in luminosity(Hα and R band), and Hα surface brightness, nearly 3 ordersof magnitude in R surface brightness and nearly 2 orders of magnitude inHα equivalent width (EW). The surface brightness distribution ofour sample is broader than that of the Sloan Digital Sky Survey (SDSS)spectroscopic sample, the EW distribution is broader than prism-selectedsamples, and the morphologies found include all common types ofstar-forming galaxies (e.g., irregular, spiral, blue compact dwarf,starbursts, merging and colliding systems, and even residual starformation in S0 and Sa spirals). Thus, SINGG presents a superior censusof star formation in the local universe suitable for further studiesranging from the analysis of H II regions to determination of the localcosmic star formation rate density.
| On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback from Momentum-driven Winds
We investigate large-scale galactic winds driven by momentum deposition.Momentum injection is provided by (1) radiation pressure produced by thecontinuum absorption and scattering of photons on dust grains and (2)supernovae (momentum injection by supernovae is important even if thesupernova energy is radiated away). Radiation can be produced by astarburst or active galactic nucleus (AGN) activity. We argue thatmomentum-driven winds are an efficient mechanism for feedback during theformation of galaxies. We show that above a limiting luminosity,momentum deposition from star formation can expel a significant fractionof the gas in a galaxy. The limiting, Eddington-like luminosity isLM~=(4fgc/G)σ4, where σ isthe galaxy velocity dispersion and fg is the gas fraction;the subscript M refers to momentum driving. A starburst that attainsLM moderates its star formation rate and its luminosity doesnot increase significantly further. We argue that elliptical galaxiesattain this limit during their growth at z>~1 and that this is theorigin of the Faber-Jackson relation. We show that Lyman break galaxiesand ultraluminous infrared galaxies have luminosities nearLM. Since these starbursting galaxies account for asignificant fraction of the star formation at z>~1, this supports ourhypothesis that much of the observed stellar mass in early-type galaxieswas formed during Eddington-limited star formation. Star formation isunlikely to efficiently remove gas from very small scales in galacticnuclei, i.e., scales much smaller than that of a nuclear starburst. Thisgas is available to fuel a central black hole (BH). We argue that a BHclears gas out of its galactic nucleus when the luminosity of the BHitself reaches ~LM. This shuts off the fuel supply to the BHand may also terminate star formation in the surrounding galaxy. As aresult, the BH mass is fixed to beMBH~=(fgκes/πG2)σ4,where κes is the electron scattering opacity. Thislimit is in accord with the observed MBH-σ relation.
| Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.
| Kinematics of the local universe . XII. 21-cm line measurements of 586 galaxies with the new Nançay receiver
This paper presents 586 new 21-cm neutral hydrogen line measurementscarried out with the FORT receiver of the meridian transit Nançayradiotelescope in the period July 2000-March 2003. This observationalprogramme is part of a larger project aiming at collecting an exhaustiveand magnitude-complete HI extragalactic catalogue for Tully-Fisherapplications. It is associated with the building of the MIGALEspectroscopic archive and database.Tables 2, 3 and HI-profiles and corresponding comments are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/373, or directly atour web site http://klun.obs-nancay.fr
| The Chandra view of NGC1800 and the X-ray scaling properties of dwarf starbursts
The superb spatial resolution of Chandra is utilized to study the X-raymorphology of the dwarf starburst galaxy NGC1800 embedded in a smallgroup of galaxies. Diffuse galactic emission is detected, extendingseveral kiloparsec above the galactic plane, with an overall morphologysimilar to the galactic winds seen in nearby X-ray-bright starburstgalaxies. This makes NGC1800 the most distant dwarf starburst with aclear detection of diffuse X-ray emission. The diffuse X-ray luminosityof 1.3 +/- 0.3 × 1038ergs-1 accounts for atleast 60 per cent of the total soft X-ray output of the galaxy. A hotgas temperature of kT= 0.25 keV and metallicity Z~ 0.05Zsolarare derived, the latter being consistent with results from opticalspectroscopy of the interstellar medium. Our failure to detect any hotgas associated with the embedding galaxy group translates into an upperlimit to the group X-ray luminosity of LX <1041ergs-1. There is no convincing evidence thatthe outflowing wind of NGC1800 is currently interacting with anyintragroup gas, and mechanical considerations indicate that the wind canescape the galaxy and its surrounding HI halo, eventually deliveringenergy and metals to the intragroup gas. Properties of NGC1800 arecompared to those of other dwarf starburst galaxies, and a firstdetailed discussion of the X-ray scaling properties of this populationof objects is given, set against the equivalent results obtained fornormal starburst galaxies. Results indicate that dwarf starbursts to alarge degree behave as down-scaled versions of normal starburstgalaxies.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| The ISOPHOT 170 μm Serendipity Survey II. The catalog of optically identified galaxies%
The ISOPHOT Serendipity Sky Survey strip-scanning measurements covering≈15% of the far-infrared (FIR) sky at 170 μm were searched forcompact sources associated with optically identified galaxies. CompactSerendipity Survey sources with a high signal-to-noise ratio in at leasttwo ISOPHOT C200 detector pixels were selected that have a positionalassociation with a galaxy identification in the NED and/or Simbaddatabases and a galaxy counterpart visible on the Digitized Sky Surveyplates. A catalog with 170 μm fluxes for more than 1900 galaxies hasbeen established, 200 of which were measured several times. The faintest170 μm fluxes reach values just below 0.5 Jy, while the brightest,already somewhat extended galaxies have fluxes up to ≈600 Jy. For thevast majority of listed galaxies, the 170 μm fluxes were measured forthe first time. While most of the galaxies are spirals, about 70 of thesources are classified as ellipticals or lenticulars. This is the onlycurrently available large-scale galaxy catalog containing a sufficientnumber of sources with 170 μm fluxes to allow further statisticalstudies of various FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) areMPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena,Imperial College London.Full Table 4 and Table 6 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/39
| The evolution of stars and gas in starburst galaxies
In systems undergoing starbursts the evolution of the young stellarpopulation is expected to drive changes in the emission-line properties.This evolution is usually studied theoretically, with a combination ofevolutionary synthesis models for the spectral energy distribution ofstarbursts and photoionization calculations. In this paper we present amore empirical approach to this issue. We apply empirical populationsynthesis techniques to samples of starburst and HII galaxies in orderto measure their evolutionary state and correlate the results with theiremission-line properties. A couple of useful tools are introduced thatgreatly facilitate the interpretation of the synthesis: (1) anevolutionary diagram, the axes of which are the strengths of the young,intermediate age and old components of the stellar population mix; and(2) the mean age of stars associated with the starburst, . These toolsare tested with grids of theoretical galaxy spectra and found to workvery well even when only a small number of observed properties(absorption-line equivalent widths and continuum colours) is used in thesynthesis.Starburst nuclei and HII galaxies are found to lie on a well-definedsequence in the evolutionary diagram. Using the empirically defined meanstarburst age in conjunction with emission-line data, we have verifiedthat the equivalent widths of Hβ and [OIII] decrease for increasing. The same evolutionary trend was identified for line ratios indicativeof the gas excitation, although no clear trend was identified formetal-rich systems. All these results are in excellent agreement withlong-known, but little tested, theoretical expectations.
| The column density distribution function at z= 0 from HI selected galaxies
We have measured the column density distribution function, f(NHI), at z= 0 using 21-cm HI emission from galaxies selected from ablind HI survey. f(NH I) is found to be smaller and flatterat z= 0 than indicated by high-redshift measurements of damped Lymanα (DLA) systems, consistent with the predictions of hierarchicalgalaxy formation. The derived DLA number density per unit redshift,dNDLA/dz= 0.058, is in moderate agreement with valuescalculated from low-redshift QSO absorption line studies. We use twodifferent methods to determine the types of galaxies which contributemost to the DLA cross-section: comparing the power-law slope off(NH I) to theoretical predictions and analysingcontributions to dNDLA/dz. We find that comparison of thepower-law slope cannot rule out spiral discs as the dominant galaxy typeresponsible for DLA systems. Analysis of dNDLA/dz however, ismuch more discriminating. We find that galaxies with log MHI< 9.0 make up 34 per cent of dNDLA/dz Irregular andMagellanic types contribute 25 per cent; galaxies with surfacebrightness account for 22 per cent and sub-L* galaxiescontribute 45 per cent to dNDLA/dz. We conclude that a largerange of galaxy types give rise to DLA systems, not just large spiralgalaxies as previously speculated.
| Do bulges of early- and late-type spirals have different morphology?
We study HST/NICMOS H-band images of bulges of two equal-sized samplesof early- (TRC3 <= 3) and late-type spiral (mainly Sbc-Sc)galaxies matched in outer disk axis ratio. We find that bulges oflate-type spirals are more elongated than their counterparts inearly-type spirals. Using a KS-test we find that the two distributionsare different at the 98.4% confidence level. We conclude that the twodata sets are different, i.e. late-type galaxies have a broaderellipticity distribution and contain more elongated features in theinner regions. We discuss the possibility that these would correspond tobars at a later evolutionary stage, i.e. secularly evolved bars.Consequent implications are raised, and we discuss relevant questionsregarding the formation and structure of bulges. Are bulges ofearly-type and late-type spirals different? Are their formationscenarios different? Can we talk about bulges in the same way fordifferent types of galaxies?
| Distribution of star-forming complexes in dwarf irregular galaxies
We study the distribution of bright star-forming complexes in ahomogeneous sample of 72 late-type (``irregular'') dwarf galaxieslocated within the 10 Mpc volume. Star-forming complexes are identifiedas bright lumps in B-band galaxy images and isolated by means of theunsharp-masking method. For the sample as a whole the radial numberdistribution of bright lumps largely traces the underlyingexponential-disk light profiles, but peaks at a 10 percent smaller scalelength. Moreover, the presence of a tail of star forming regions out toat least six optical scale lengths provides evidence against asystematic star formation truncation within that galaxy extension.Considering these findings, we apply a scale length-independentconcentration index, taking into account the implied non-uniform randomspread of star formation regions throughout the disk. The numberprofiles frequently manifest a second, minor peak at about two scalelengths. Relying on a two-dimensional stochastic self-propagating starformation model, we show these secondary peaks to be consistent withtriggered star formation; for a few of the brighter galaxies a peculiarpeak distribution is observed that is conceivably due to the onset ofshear provided by differential rotation. On scales between 100 and 1000pc, and by taking into account exponential-disk structure, bright lumpsreveal cluster dimensions between 1.3 and 2, with a weak trend to higherdimensions for brighter galaxies. Cluster dimension weaklyanticorrelates with the lumpiness index (the fraction of the totalgalaxy light due to the light contributed by the lumps), the latterindex showing no dependence on luminosity. Lump spreading within thedisk, as measured by the concentration index, and lump clustering, asgiven by the cluster dimension, are not linked to each other.Interpreting cluster dimension in terms of porosity of a self-similarintragalactic medium, we derive a relation between current starformation rate, scale length, and porosity.
| Hα, SCUBA and MERLIN imaging of NGC 4490
We describe Hα, SCUBA and MERLIN imaging of the interacting galaxypair NGC 4490 and 4485. We detect an Hα filament emerging from thedisc of NGC 4490 to a projected distance of 3kpc which has counterpartsin both the radio continuum and HI. The HI counterpart extends to aprojected distance of ~30kpc from NGC 4490 and we argue that this isevidence that the giant HI envelope in this system has its origins instar formation. We use SCUBA and radio continuum data to attempt toplace constraints on the distribution of dust with respect to the starforming regions. This analysis is limited by the lack of an independentestimate of the dust temperature, something that both `SIRTF' and`SOFIA' will be able to provide, however we find some evidence that mostobscuring dust is not located within HII regions themselves.
| An empirical calibration of star formation rate estimators
The observational determination of the behaviour of the star formationrate (SFR) with look-back time or redshift has two main weaknesses: (i)the large uncertainty of the dust/extinction corrections, and (ii) thatsystematic errors may be introduced by the fact that the SFR isestimated using different methods at different redshifts. Mostfrequently, the luminosity of the Hα emission line, that of theforbidden line [O II] λ3727 and that of the far-ultravioletcontinuum are used with low-, intermediate- and high-redshift galaxies,respectively. To assess the possible systematic differences among thedifferent SFR estimators and the role of dust, we have compared SFRestimates using Hα, [O II] λ3727 Å, ultraviolet (UV)and far-infrared (FIR) luminosities [SFR(Hα), SFR(O II), SFR(UV)and SFR(FIR), respectively of a sample comprising the 31 nearbystar-forming galaxies that have high-quality photometric data in the UV,optical and FIR. We review the different `standard' methods for theestimation of the SFR and find that while the standard method providesgood agreement between SFR(Hα) and SFR(FIR), both SFR(O II) andSFR(UV) are systematically higher than SFR(FIR), irrespective of theextinction law. We show that the excess in the SFR(O II) and SFR(UV) ismainly due to an overestimation of the extinction resulting from theeffect of underlying stellar Balmer absorptions in the measured emissionline fluxes. Taking this effect into consideration in the determinationof the extinction brings the SFR(O II) and SFR(UV) in line with theSFR(FIR), and simultaneously reduces the internal scatter of the SFRestimations. Based on these results, we have derived `unbiased' SFRexpressions for the SFR(UV), SFR(OII) and SFR(Hα). We have usedthese estimators to recompute the SFR history of the Universe using theresults of published surveys. The main results are that the use of theunbiased SFR estimators brings into agreement the results of allsurveys. Particularly important is the agreement achieved for the SFRderived from the FIR/millimetre and optical/UV surveys. The `unbiased'star formation history of the Universe shows a steep rise in the SFRfrom z =0 to z =1 with SFR ~(1+z)4.5, followed by a declinefor z>2 where SFR ~(1+z)-1.5. Galaxy formation models tendto have a much flatter slope from z=0 to z=1
| A New Database of Observed Spectral Energy Distributions of Nearby Starburst Galaxies from the Ultraviolet to the Far-Infrared
We present a database of UV-to-FIR data of 83 nearby starburst galaxies.The galaxies are selected based upon the availability of IUE data. Wehave recalibrated the IUE UV spectra for these galaxies by incorporatingthe most recent improvements. For 45 of these galaxies we useobservations by Storchi-Bergmann et al. and McQuade et al. for thespectra in the optical range. The NIR data are from new observationsobtained at the NASA/IRTF and the Mount Laguna Observatory, combinedwith the published results from observations at the Kitt Peak NationalObservatory. In addition, published calibrated ISO data are included toprovide mid-IR flux densities for some of the galaxies. Theoptical-to-IR data are matched as closely as possible to the IUE largeaperture. In conjunction with IRAS and ISO FIR flux densities, all thesedata form a set of observed spectral energy distributions (SEDs) of thenuclear regions of nearby starburst galaxies. The SEDs should be usefulin studying star formation and dust/gas attenuation in galaxies. We alsopresent the magnitudes in the standard BVRI and various HST/WFPC2bandpasses synthesized from the UV and optical wavelength ranges ofthese SEDs. For some of the galaxies, the HST/WFPC2 magnitudessynthesized from the SEDs are checked with those directly measured fromWFPC2 images to test the photometric errors of the optical data andtheir effective matching of apertures with the UV data. The implicationsof the new SEDs on the star formation rates and dust/gas attenuation arebriefly discussed.
| Spiral Galaxies with HST/NICMOS. I. Nuclear Morphologies, Color Maps, and Distinct Nuclei
This is the first of two papers where we present the analysis of anHST/NICMOS2 near-infrared (NIR) snapshot survey in the F160W (H) filterfor a sample of 78 spiral galaxies selected from the UGC and ESOLVcatalogs. For 69 of these objects we provide nuclear color informationderived by combining the H data either with additional NICMOS F110W (J)images or with V WFPC2/HST data. Here we present the NIR images and theoptical-NIR color maps. We focus our attention on the properties of thephotometrically distinct ``nuclei'' which are found embedded in most ofthe galaxies and provide measurements of their half-light radii andmagnitudes in the H (and when available in the J) band. We find that (1)in the NIR the nuclei embedded in the bright early- to intermediate-typegalaxies span a much larger range in brightness than the nuclei whichare typically found embedded in bulgeless late-type disks: the nucleiembedded in the early- to intermediate-type galaxies reach, on thebright end, values up to HAB~-17.7 mag; (2) nuclei are foundin both nonbarred and barred hosts, in large-scale (>~1 kpc) as wellas in nuclear (up to a few 100 pc) bars; (3) there is a significantincrease in half-light radius with increasing luminosity of the nucleusin the early/intermediate types (a decade in radius for ~8 magbrightening), a correlation which was found in the V band and which isalso seen in the NIR data; (4) the nuclei of early/intermediate-typespirals cover a large range of optical-NIR colors, from V-H~-0.5 to 3.Some nuclei are bluer and others redder than the surroundinggalaxy,indicating the presence of activity or reddening by dust in many ofthese systems; (5) someearly/intermediate nuclei are elongated and/orslightly offset from the isophotal center of the host galaxy. Onaverage, however, these nuclei appear as centered, star-cluster-likestructures similar to those whichare found in the late-type disks. Basedon observations with the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated by Associationof Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
| The blue compact dwarf galaxy I Zw 18: A comparative study of its low-surface-brightness component
Using HST and ground-based optical and NIR imaging data we investigatewhether the blue compact dwarf (BCD) galaxy I Zw 18 possesses anextended low-surface-brightness (LSB) old stellar population underlyingits star-forming regions, as is the case in the majority of BCDs. Thisquestion is central to the long-standing debate on the evolutionarystate of I Zw 18. We show that the exponential intensity decreaseobserved in the filamentary LSB envelope of the BCD out to ga 18 arcsec(ga 1.3 kpc assuming a distance of 15 Mpc) is not due to an evolvedstellar disc underlying its star-forming regions, but rather, due toextended ionized gas emission. Ionized gas accounts for more than 80% ofthe line-of-sight emission at a galactocentric distance of ~ 0.65 kpc (~ 3 effective radii), and for ga 30% to 50% of the R light of the mainbody of I Zw 18. Broad-band images reveal, after subtraction of nebularline emission, a relatively smooth stellar host extending slightlybeyond the star-forming regions. This unresolved stellar component,though very compact, is not exceptional for intrinsically faint dwarfswith respect to its structural properties. However, being blue over aradius range of ~ 5 exponential scale lengths and showing little colourcontrast to the star-forming regions, it differs strikingly from the redLSB host of standard BCDs. This fact, together with the comparably bluecolours of the faint C component, ~ 1.6 kpc away from the main body of IZw 18, suggests that the formation of I Zw 18 as a whole has occurredwithin the last 0.5 Gyr, making it a young BCD candidate. Furthermore,we show that the ionized envelope of I Zw 18 is not exceptional amongstar-forming dwarf galaxies, neither by its exponential intensityfall-off nor by its scale length. However, contrary to evolved BCDs, thestellar LSB component of I Zw 18 is much more compact than the ionizedgas envelope. In the absence of an appreciable underlying stellarpopulation, extended ionized gas emission dominates in the outer partsof I Zw 18, mimicking an exponential stellar disc on optical surfacebrightness profiles. Based on observations with the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by AURA, Inc., under NASA contract No. NAS 5-26555.Obtained at the German-Spanish Astronomical Center, Calar Alto, operatedby the Max-Planck-Institute for Astronomy, Heidelberg, jointly with theSpanish National Commission for Astronomy. Obtained at the Kitt PeakNational Observatory, operated by the Association of Universities forResearch in Astronomy, Inc., under cooperative agreement with theNational Science Foundation.
| Structure and stellar content of dwarf galaxies. VII. B and R photometry of 25 southern field dwarfs and a disk parameter analysis of the complete sample of nearby irregulars
We present B and R band surface photometry of 25 Southern field dwarfgalaxies within a distance of 10 Mpc. For each galaxy we give theessential model-free photometric parameters and, by fitting exponentialsto the surface brightness profiles, the central extrapolated surfacebrightness and the exponential scale length, in both colour bands.Surface brightness and colour profiles are shown. One of the objects, avery faint dwarf elliptical in the vicinity of NGC 2784, has beendiscovered in the course of this work. Drawing on the data from this andall previous papers of this series, we construct a complete sample of 72late-type (``irregular'') dwarf galaxies in nearby groups and the fieldwithin the 10 Mpc volume, to study the exponential-disk parameterrelations of these galaxies with respect to galaxy environment. Weconfirm our previous finding of statistically lower scale lengths/highercentral surface brightnesses for field and group galaxies as compared tocluster galaxies. However, using a clear-cut definition of ``group''versus ``field'' environment, we find no significant difference in thephotometric structure of group and field irregulars. A difference in thestar formation history may partly account for this structure-environmentrelation: for a given luminosity cluster dwarfs are on average redderthan field and group galaxies. We also report evidence for the colourgradients of dwarf irregulars being roughly inversely proportional tothe disk scale lengths. Supplementing our photometric data withkinematic data from the literature, we study possible relations withkinematic properties of the inner disk. Applying the dark matter scalingrelations for a Burkert halo we show that for field and group galaxiesof a given luminosity faster-than-mean disk rotational velocities at aradius of about two scale lengths are correlated with larger-than-meandisk scale lengths. Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile. Table 3 containing ``BRphotometry and kinematic data for the 72 irregular dwarf galaxies of ourcomplete sample'' is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/388/29
| Star formation and dust extinction in nearby star-forming and starburst galaxies
We study the star formation rate and dust extinction properties of asample of nearby star-forming galaxies as derived from Hα and UV (~ 2000 Å) observations and we compare them to those of a sample ofstarburst galaxies. The dust extinction in Hα is estimated fromthe Balmer decrement and the extinction in UV using the FIR to UV fluxratio or the attenuation law for starburst galaxies of Calzetti et al.(\cite{calzetti5}). The Hα and UV emissions are stronglycorrelated with a very low scatter for the star-forming objects and witha much higher scatter for the starburst galaxies. The Hα to UVflux ratio is found to be larger by a factor ~ 2 for the starburstgalaxies. We compare both samples with a purely UV selected sample ofgalaxies and we conclude that the mean Hα and UV properties ofnearby star-forming galaxies are more representative of UV-selectedgalaxies than starburst galaxies. We emphasize that the Hα to UVflux ratio is strongly dependent on the dust extinction: the positivecorrelation found between FHα/FUV andFFIR/FUV vanishes when the Hα and UV fluxare corrected for dust extinction. The Hα to UV flux ratiosconverted into star formation rate and combined with the Balmerdecrement measurements are tentatively used to estimate the dustextinction in UV.
| Strömgren Photometry from z=0 to z~1. I. The Method
We use rest-frame Strömgren photometry to observe clusters ofgalaxies in a self-consistent manner from z=0 to z=0.8. Strömgrenphotometry of galaxies is intended as a compromise between standardbroadband photometry and spectroscopy, in the sense that it is moresensitive to subtle variations in spectral energy distributions than theformer, yet much less time-consuming than the latter. principalcomponent analysis is used to facilitate extraction of information fromthe Strömgren data. By calibrating the principal components usingwell-studied galaxies, as well as models of stellar populations, wedevelop a purely empirical method to detect, and subsequently classify,cluster galaxies at all redshifts smaller than 0.8. Interlopers arediscarded with unprecedented efficiency (up to 100%). The firstprincipal component essentially reproduces the Hubble sequence and canthus be used to determine the global star formation history of clustermembers. The (PC2, PC3) plane allows us to identify Seyfert galaxies(and distinguish them from starbursts) based on photometric colorsalone. In the case of E/S0 galaxies with known redshift, we are able toresolve the age-dust-metallicity degeneracy, albeit at the accuracylimit of our present observations. We use this technique in later papersto probe galaxy clusters well beyond their cores and to faintermagnitudes than spectroscopy can achieve, because the faint end of theluminosity function as well as the outer cluster regions seem to exhibitthe strongest evolutionary trends. We are able to directly compare thesedata over the entire redshift range without a priori assumptions becauseour observations do not require first-order k-corrections. Thecompilation of such data for different cluster types over a wideredshift range is likely to set important constraints on the evolutionof galaxies and on the clustering process.
| 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 Taxonomy of Blue Amorphous Galaxies. II. Structure and Evolution
Dwarf galaxies play an important role in our understanding of galaxyformation and evolution, and starbursts are believed to affect thestructure and evolution of dwarf galaxies strongly. We have thereforeembarked on a systematic study of 12 of the nearest dwarf galaxiesthought to be undergoing bursts of star formation. These were selectedprimarily by their morphological type (blue ``amorphous'' galaxies). Weshow that these blue amorphous galaxies are not physicallydistinguishable from dwarfs selected as starbursting by other methods,such as blue compact dwarfs (BCDs) and H II galaxies. All these classesexhibit surface brightness profiles that are exponential in the outerregions (r>~1.5r_e) but often have a predominantly central blueexcess, suggesting a young burst in an older, redder galaxy. Typically,the starbursting ``cores'' are young (~10^7-10^8 yr) events compared tothe older (~10^9-10^10 yr) underlying galaxy (the ``envelope''). Theratio of the core to envelope in blue light ranges from essentially zeroto about 2. These starbursts are therefore modest events involving onlya few percent of the stellar mass. The envelopes have surfacebrightnesses that are much higher than typical dwarf irregular (dI)galaxies, so it is unlikely that there is a straightforward evolutionaryrelation between typical dIs and dwarf starburst galaxies. Instead wesuggest that amorphous galaxies may repeatedly cycle through starburstand quiescent phases, corresponding to the galaxies with strong andweak/absent cores, respectively. Once amorphous galaxies use up theavailable gas (either through star formation or galactic winds) so thatstar formation is shut off, the faded remnants would strongly resembledwarf elliptical galaxies. However, in the current cosmological epoch,this is evidently a slow process that is the aftermath of a series ofmany weak, recurring bursts. Present-day dE's must have experienced morerapid and intense evolution than this in the distant past.
| Dust Absorption and the Ultraviolet Luminosity Density at Z~3 as Calibrated by Local Starburst Galaxies
We refine a technique to measure the absorption-corrected ultraviolet(UV) luminosity of starburst galaxies using rest-frame UV quantitiesalone and apply it to Lyman-limit U dropouts at z~3 found in the HubbleDeep Field (HDF). The method is based on an observed correlation betweenthe ratio of far-infrared (FIR) to UV fluxes with spectral slope beta (aUV color). A simple fit to this relation allows the UV flux absorbed bydust and reprocessed to the FIR to be calculated, and hence thedust-free UV luminosity to be determined. International UltravioletExplorer spectra and Infrared Astronomical Satellite fluxes of localstarbursts are used to calibrate the F_FIR/F_1600 versus beta relationin terms of A_1600 (the dust absorption at 1600 Å) and thetransformation from broadband photometric color to beta. Bothcalibrations are almost completely independent of theoreticalstellar-population models. We show that the recent marginal andnondetections of HDF U dropouts at radio and submillimeter wavelengthsare consistent with their assumed starburst nature and our calculatedA_1600. This is also true of recent observations of the ratio of opticalemission-line flux to UV flux density in the brightest U dropouts. Thislatter ratio turns out not to be a good indicator of dust extinction. InU dropouts, absolute magnitude M_1600,0 correlates with beta: brightergalaxies are redder, as is observed to be the case for local starburstgalaxies. This suggests that a mass-metallicity relationship is alreadyin place at z~3. The absorption-corrected UV luminosity function of Udropouts extends up to M_1600,0~-24 AB mag, corresponding to a starformation rate ~200 M_solar yr^-1 (H_0=50 km s^-1 Mpc^-3 and q_0=0.5 areassumed throughout). The absorption-corrected UV luminosity density atz~3 is rho_1600,0>=1.4x10^27 ergs^-1 Hz^-1 Mpc^-1. It is still alower limit since completeness corrections have not been done andbecause only galaxies with A_1600<~3.6 mag are blue enough in the UVto be selected as U dropouts. The luminosity-weighted meandust-absorption factor of our sample is 5.4+/-0.9 at 1600 Å.
| High-Velocity Clouds: Building Blocks of the Local Group
We suggest that the high-velocity clouds (HVCs) are large clouds, withtypical diameters of 25 kpc, containing 3x10^7 M_solar of neutral gasand 3x10^8 M_solar of dark matter, falling onto the Local Group;altogether the HVCs contain 10^10 M_solar of neutral gas. Ourreexamination of the Local Group hypothesis for the HVCs connects theirproperties to the hierarchical structure formation scenario and to thegas seen in absorption toward quasars. We show that at least one HVCcomplex (besides the Magellanic Stream) must be extragalactic at adistance of more than 40 kpc from the Galactic center, with a diametergreater than 20 kpc and a mass of more than 10^8 M_solar. We discuss anumber of other clouds that are positionally associated with the LocalGroup galaxies, and we show that the entire ensemble of HVCs isinconsistent with a Galactic origin. The observed kinematics implyrather that the HVCs are falling toward the Local Group barycenter. Wesimulate the dynamical evolution of the Local Group and find thatmaterial falling onto the Local Group reproduces the location of two ofthe three most significant groupings of clouds and the kinematics of theentire cloud ensemble (excluding the Magellanic Stream). We interpretthe third grouping (the A, C, and M complexes) as the nearest HVC. It istidally unstable and is falling onto the Galactic disk. We interpret themore distant HVCs as gas contained within dark matter ``minihalos''moving along filaments toward the Local Group. Most poor galaxy groupsshould contain similar H I clouds bound to the group at large distancesfrom the individual galaxies. We suggest that the HVCs are local analogsof the Lyman limit absorbing clouds observed against distant quasars.Our picture implies that the chemical evolution of the Galactic disk isgoverned by episodic infall of metal-poor HVC gas that only slowly mixeswith the rest of the interstellar medium. We argue that there is aGalactic fountain in the Milky Way, but that the fountain does notexplain the origin of the HVCs. Our analysis of the H I data leads tothe detection of a vertical infall of low-velocity gas toward the planeand implies that the H I disk is not in hydrostatic equilibrium. Wesuggest that the fountain is manifested mainly by relatively localneutral gas with characteristic velocities of 6 km s^-1 rather than 100km s^-1. The Local Group infall hypothesis makes a number of testablepredictions. The HVCs should have subsolar metallicities. Their Hαemission should be less than that seen from the Magellanic Stream. Theclouds should not be seen in absorption against nearby stars. The cloudsshould be detectable in both emission and absorption around other galaxygroups. We show that current observations are consistent with thesepredictions and discuss future tests.
| On Measuring Nebular Chemical Abundances in Distant Galaxies Using Global Emission-Line Spectra
The advent of 8-10 m class telescopes enables direct measurement of thechemical properties in the ionized gas of cosmologically distantgalaxies with the same nebular analysis techniques used in local H IIregions. We show that spatially unresolved (i.e., global) emission-linespectra can reliably indicate the chemical properties of distantstar-forming galaxies. However, standard nebular chemical abundancemeasurement methods (those with a measured electron temperature from [OIII] lambda4363) may be subject to small systematic errors when theobserved volume includes a mixture of gas with diverse temperatures,ionization parameters, and metallicities. To characterize thesesystematic effects, we compare physical conditions derived fromspectroscopy of individual H II regions with results from global galaxyspectroscopy. We consider both low-mass, metal-poor galaxies withuniform abundances and larger galaxies with internal chemical gradients.For low-mass galaxies, standard chemical analyses using global spectraproduce small systematic errors in that the derived electrontemperatures are 1000-3000 K too high due to nonuniform electrontemperatures and large variations in the ionization parameter. As aresult, the oxygen abundances derived from direct measurements of theelectron temperatures are too low, but it is possible to compensate forthis effect by applying a correction of Delta(O/H)<=+0.1 dex to theoxygen abundances derived from global spectra. For more massivemetal-rich galaxies like local spiral galaxies, direct measurements ofelectron temperatures are seldom possible from global spectra.Well-established empirical calibrations using strong-line ratios canserve as reliable (+/-0.2 dex) indicators of the overall systemic oxygenabundance even when the signal to noise of the Hβ and [O III]emission lines is as low as 8:1. We present prescriptions, directedtoward high-redshift observers, for using global emission-line spectrato trace the chemical properties of star-forming galaxies in the distantuniverse.
| Deep Hα Images of the Wolf-Rayet Galaxy HE 2-10
We present continuum-free Hα images of the remarkable Wolf-Rayetgalaxy He 2-10. There are extensive low surface brightness structures ofionized gas around He 2-10, including several shells, a possiblesupershell, and a small, isolated Hα clump. The clump is welloutside the body of the galaxy and coincides with a CO feature at theend of the galaxy's tidal tail. We believe the clump to be a small starcluster, containing about 100 O stars. This cluster may be the analog ona smaller scale of the dwarf galaxies formed at the end of the tidaltails in the NGC 4038/4039 merger.
| HI properties of nearby galaxies from a volume-limited sample
We consider global HI and optical properties of about three hundrednearby galaxies with V_0 < 500 km s(-1) . The majority of them haveindividual photometric distance estimates. The galaxy sample parametersshow some known and some new correlations implying a meaningful dynamicexplanation: 1) In the whole range of diameters, 1 - 40 Kpc, the galaxystandard diameter and rotational velocity follows a nearly linearTully-Fisher relation, lg A25~(0.99+/-0.06)lg V_m. 2) The HImass-to-luminosity ratio and the HI mass-to-``total" mass (inside thestandard optical diameter) ratio increase systematically from giantgalaxies towards dwarfs, reaching maximum values 5 ;M_ȯ/L_ȯand 3, respectively. 3) For all the Local Volume galaxies their totalmass-to-luminosity ratio lies within a range of [0.2-16]M_ȯ/L_ȯ with a median of 3.0 ;M_ȯ/L_ȯ. TheM25/L ratio decreases slightly from giant towards dwarfgalaxies. 4) The M_HI/L and M25/L ratios for the samplegalaxies correlate with their mean optical surface brightness, which maybe caused by star formation activity in the galaxies. 5) The M_HI/L andM25/L ratios are practically independent of the local massdensity of surrounding galaxies within the range of densities of aboutsix orders of magnitude. 6) For the LV galaxies their HI mass andangular momentum follow a nearly linear relation: lgM_HI~(0.99+/-0.04)lg (V_m* A25), expected for rotatinggaseous disks being near the threshold of gravitational instability,favourable for active star formation. Table in the Appendix is availableonly in electronic form at the CDS via anonymous ftp 130.79.128.5 orhttp//cdsweb.u-strasbg.fr/Abstract.html
| Ultraviolet spectral properties of magellanic and non-magellanic irregulars, H BT II and starburst galaxies
This paper presents the results of a stellar population analysisperformed on nearby (V_R<=5 000 km s^{-1}) star-forming galaxies,comprising magellanic and non-magellanic irregulars, H Ii and starburstgalaxies observed with the IUE satellite. Before any comparison ofgalaxy spectra, we have formed subsets according to absolute magnitudeand morphological classification. Subsequently, we have coadded thespectra within each subset into groups of similar spectral properties inthe UV. As a consequence, high signal-to-noise ratio templates have beenobtained, and information on spectral features can now be extracted andanalysed. Seven groups resulted from this procedure: the magellanicirregulars (including H Ii galaxies) produced two different bluespectral groups; the non-magellanic irregulars could be grouped into twospectral groups with rather peculiar properties; and the luminousstarbursts produced one flat and two blue template spectra. Theirstellar populations are analysed by means of a population synthesisalgorithm based on star cluster spectral components. The syntheticspectra reproduce the observed ones successfully (except thenon-magellanic irregular groups) both in terms of continuum distributionand spectral features. The synthesis flux fractions of different agegroups were transformed into mass fractions, allowing inferences on thestar formation histories. Young stellar populations (age <500 Myrs)are the main flux contributors; in a few cases the intermediate agepopulation (age~1 M_B-2 Myrs) is important, while the old bulgepopulation contributes at most with ~2 % of the lambda2646 Angstromsflux in the case of starburst galaxies, and is negligible in themagellanic irregulars. We also study the reddening values and theextinction law: an SMC-like extinction law is appropriate for all cases.Based upon data collected with the International Ultraviolet Explorer(IUE) Satellite, supported by NASA, SERC and ESA.
| Accurate Positions for MCG Galaxies
We have measured accurate celestial coordinates for 4741 extragalacticobjects, primarily drawn from a list of MCG galaxies with no recentlypublished accurate positions. The standard deviations in the newpositions depend slightly on the measurement method but are on the orderof 1.0" to 1.2". Standard deviations in the original MCG positions areconfirmed to be at the 1.5′-2.0′ level. These new positionswere integrated into NED in 1997 December.
| Morphology of star formation regions in irregular galaxies
The location of HII regions, which indicates the locus of present starformation in galaxies, is analysed for a large collection of 110irregular galaxies (Irr) imaged in Hα and nearby continuum. Theanalysis is primarily by visual inspection, although a two-dimensionalquantitative measure is also employed. The two different analyses yieldessentially identical results. HII regions appear preferentially at theedges of the light distribution, predominantly on one side of thegalaxy, contrary to what is expected from stochastic self-propagatingstar formation scenarios. This peculiar distribution of star-formingregions cannot be explained by a scenario of star formation triggered byan interaction with extragalactic gas, or by a strong one-armed spiralpattern.
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