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An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| The Polycyclic Aromatic Hydrocarbon Emission Deficit in Low-Metallicity Galaxies-A Spitzer View
Archival observations of 18 starburst galaxies that span a wide range inmetallicity reveal for the first time a correlation between the ratio ofemission-line fluxes of [Fe II] at 26 μm and [Ne II] at 12.8 μmand the 7.7 μm PAH strength, with the [Fe II]/[Ne II] flux ratiodecreasing with increasing PAH strength. We also find a strongcorrelation between the [Fe II]/[Ne II] flux ratio and the host galaxymetallicity, with the flux ratio decreasing with increasing metallicity.Since [Fe II] emission has been linked primarily to supernova shocks, weattribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies toenhanced supernova activity. We consider this to be a dominant mechanismfor PAH destruction, rather than grain destruction in photoionizedregions surrounding young massive stars. We also consider whether theextreme youth of the low-metallicity galaxies is responsible for thelack of PAH emission.
| A Spectroscopic Study of the Star-Forming Properties of the Center of NGC 4194
We have obtained Space Telescope Imaging Spectrograph long-slit spectraof the central region of the advanced merger NGC 4194. The spectra coverthe wavelength ranges 1150-1750 Å in the UV and 2900-10270 Åin the visible. Results from the study of the properties of 14star-forming regions (knots) are presented. If the [N II] contributionis 40% of the combined Hα + [N II] flux, then the averageE(B-V)=0.7 mag. The metal abundances are approximately solar, withindividual knot abundances ranging from log(O/H)+12=8.1+/-0.5 to8.9+/-0.4. The Hα luminosities of the 14 observed knots yield atotal star formation rate (SFR) of ~46 Msolaryr-1. The sizes of the H II regions associated with the knotswere determined from L(Hβ) and range from ~28 to ~119 pc when afilling factor of 0.1 is assumed. The sizes are a factor of ~2.15smaller for a filling factor of 1. Using Starburst99, the EW(Hα +[N II]), and EW(Hβ), we estimate the ages of the star-formingregions to be 5.5-10.5 Myr. From ground-based spectra the effectivetemperatures of the H II regions are found to be ~11,000 K, and theelectron densities are determined to be ~530 cm-3. We findthat eight of the knots probably formed with a Salpeter initial massfunction truncated at an upper mass of 30 Msolar, and one ofthe knots likely formed with a standard Salpeter initial mass function.We suggest that the knots in our sample are the precursors of globularclusters. Two of the knots are in a region of flowing gas and are amongthe most massive, are the largest in radius, have the highest SFR, andare among the youngest of the knots.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute (STScI), which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| A Survey of Merger Remnants. II. The Emerging Kinematic and Photometric Correlations
This paper is the second in a series exploring the properties of 51optically selected, single-nuclei merger remnants. Spectroscopic datahave been obtained for a subsample of 38 mergers and combined withpreviously obtained infrared photometry to test whether mergers exhibitthe same correlations as elliptical galaxies among parameters such asstellar luminosity and distribution, central stellar velocity dispersion(σ0), and metallicity. Paramount to the study is totest whether mergers lie on the fundamental plane. Measurements ofσ0 have been made using the Ca triplet absorption lineat 8500 Å for all 38 mergers in the subsample. Additionalmeasurements of σ0 were made for two of the mergers inthe subsample using the CO absorption line at 2.29 μm. The resultsindicate that mergers show a strong correlation among the parameters ofthe fundamental plane but fail to show a strong correlation betweenσ0 and metallicity (Mg2). In contrast toearlier studies, the σ0 values of the mergers areconsistent with objects that lie somewhere between intermediate-mass andluminous giant elliptical galaxies. However, the discrepancies withearlier studies appear to correlate with whether the Ca triplet or COabsorption lines are used to derive σ0, with the latteralmost always producing smaller values. Finally, the photometric andkinematic data are used to demonstrate for the first time that thecentral phase-space densities of mergers are equivalent to those inelliptical galaxies. This resolves a long-standing criticism of themerger hypothesis.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| Spectroscopic properties and dynamical evolution of the merging system AM 1003-435
Aims.We study the system AM 1003-435, which iscomposed of two strong interacting galaxies. Methods: .We obtainedlong-slit optical spectra of twelve zones of the system, and performednumerical simulations of the encounter between the components followingthe evolution of their stellar and gaseous contents. Results: .Thespectrum of the NW nucleus is typical of a starburst, while that of theSE one shows weak emission lines. The highest values of the oxygen andnitrogen abundances are at the NW nucleus. Its derived Hα andHα + [N II] equivalent widths indicate very intense starformation, in accord to its starburst nature. Indicative ages of thestarbursts in the nuclei were obtained. The morphological types of bothcomponents derived from their spectral characteristics are in agreementwith previous determinations based on photometric parameters. About 70%of the measured Hα luminosity would correspond to the NW componentcontribution. The IR luminosity of the system is not high (L_IR <1011~Lȯ). The estimated star formation ratefor AM 1003-435 indicates that its activity is alsomoderate. The IR radiation, if it has the same origin as the Hαemission, would arise mostly from the NW component. The resultingabundances, burst ages, and masses suggest that the starburst in the SEcomponent, the minor one, started earlier than that of the NW one, anddid so in a medium poorer in weighted elements. On the other hand, froma set of N-body simulations of the encounter between both components itwas found the time of the perigalacticum, to be comparable to the burstage derived for the SE component, and the star formation in bothgalaxies would have begun after the perigalacticum. It was alsoestimated that the central bodies will merge in about 1 Gyr.
| The Second Byurakan Survey. General Catalogue
The Second Byurakan Survey (SBS) General Catalogue is presented. TheSBS, a continuation of the Markarian survey reaching fainter limitingmagnitudes, is the first survey which combines the search of galaxiesand QSOs. A total area of 991OS#square;degrees of the Northern sky wascovered with the use of three objective prisms in combination withSchott filters. The limited magnitude on the best plates reached B ~19.5.The General Catalogue consists of 3563 objects presented in two parts: aCatalogue of galaxies (1863 objects) and one of stellar objects (1700objects). The Catalogue of SBS AGN consists of 761 objects (155 SyG, 596QSOs, and 10 BLLac). Multi-wavelength data are presented for 1438 SBSobjects identified with X-ray, IRAS and FIRST sources.Spectrophotometric observations obtained over 26 years are available for3132 objects. Redshifts were measured for ~ 2100 extragalactic objects.Spectral classification is presented for ~ 2970 objects. The majority ofthe data is presented here for the first time. The Catalogue presentsnew large homogeneous deep representative complete samples of brightQSOs, AGNs, and faint UVX galaxies in the Northern sky. The SBS sampleis found to be complete at 70% for galaxies and ~ 85% for AGN/QSOs withB ≤ 17.5.
| The evolution of actively star-forming galaxies in the mid-infrared
In this paper we analyze the evolution of actively star-forming galaxiesin the mid-infrared (MIR). This spectral region, characterized bycontinuum emission by hot dust and by the presence of strong emissionfeatures generally ascribed to polycyclic aromatic hydrocarbon (PAH)molecules, is the most strongly affected by the heating processesassociated with star formation and/or active galactic nuclei (AGNs).Following the detailed observational characterization of galaxies in theMIR by the Infrared Space Observatory (ISO), we have updated themodelling of this spectral region in our spectrophotometric modelGRASIL. In the diffuse component we have updated the treatment of PAHsaccording to the model by Li & Draine. As for the dense phase of theinterstellar medium associated with the star-forming regions, themolecular clouds, we strongly decrease the abundance of PAHs as comparedto that in the cirrus, based on the observational evidence of the lackor weakness of PAH bands close to the newly formed stars, possibly dueto the destruction of the molecules in strong ultraviolet fields. Therobustness of the model is checked by fitting near-infrared to radiobroad-band spectra and the corresponding detailed MIR spectra of a largesample of galaxies, at once. With this model, we have analyzed thelarger sample of actively star-forming galaxies by Dale et al. We showthat the observed trends of galaxies in the ISO-IRAS-radio colour-colourplots can be interpreted in terms of the different evolutionary phasesof star formation activity, and the consequent different dominance inthe spectral energy distribution of the diffuse or dense phase of theISM. We find that the observed colours indicate a surprising homogeneityof the starburst phenomenon, allowing only a limited variation of themost important physical parameters, such as the optical depth of themolecular clouds, the time-scale of the escape of young stars from theirfor mation sites, and the gas consumption time-scale. In this paper wedo not attempt to reproduce the far-infrared coolest region in thecolour-colour plots, as we concentrate on models meant to reproduceactive star-forming galaxies, but we discuss possible requirements of amore complex modelling for the coldest objects.
| Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope
Full low-resolution (65
| Metallicity Effects on Mid-Infrared Colors and the 8 μm PAH Emission in Galaxies
We examine colors from 3.6 to 24 μm as a function of metallicity(O/H) for a sample of 34 galaxies. The galaxies range over 2 orders ofmagnitude in metallicity. They display an abrupt shift in the 8μm-to-24 μm color for metallicities between one-third andone-fifth of the solar value. The mean 8-to-24 μm flux density ratiobelow and above 12+log(O/H)=8.2 is 0.08+/-0.04 and 0.70+/-0.53,respectively. We use mid-IR colors and spectroscopy to demonstrate thatthe shift is primarily due to a decrease in the 8 μm flux density, asopposed to an increase in the 24 μm flux density. This result is mostsimply interpreted as being due to a weakening at low metallicity of themid-IR emission bands usually attributed to PAHs (polycyclic aromatichydrocarbons) relative to the small-grain dust emission. However,existing empirical spectral energy distribution models cannot accountfor the observed short-wavelength (below 8 μm) colors of thelow-metallicity galaxies merely by reducing the strength of the PAHfeatures; some other emission source (e.g., hot dust) is required.
| Simulating the Spitzer Mid-Infrared Color-Color Diagrams
We use a simple parameterization of the mid-IR spectra of a wide rangeof galaxy types in order to predict their distribution in the InfraredArray Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 μm and MultibandPhotometer for Spitzer 24 μm color-color diagrams. We distinguishthree basic spectral types by the energetically dominant component inthe 3-12 μm regime: stellar-dominated, polycyclic aromatichydrocarbon (PAH)-dominated, and continuum-dominated. We use a Markovchain Monte Carlo approach to arrive at a more systematic and robustrepresentation of the mid-IR spectra of galaxies than do moretraditional approaches. We find that IRAC color-color plots are wellsuited to distinguishing the above spectral types, while the addition of24 μm data allows us to suggest practical three-color cuts thatpreferentially select higher redshift sources of a specific type. Wecompare our simulations with the color-color plot obtained by theSpitzer First Look Survey and find reasonable agreement. Lastly, wediscuss other applications as well as future directions for this work.
| Multiwavelength observations of two S+E merger candidates: the Medusa and NGC 4441.
Not Available
| Sub-arcsecond imaging of the radio continuum and neutral hydrogen in the Medusa merger
We present sub-arcsecond, Multi-Element Radio Linked Interferometer(MERLIN) observations of the decimetre radio continuum structure andneutral hydrogen (Hi) absorption from the nuclear region of thestarburst galaxy NGC 4194 (the Medusa Merger). The continuum structureof the central kiloparsec of the Medusa has been imaged, revealing apair of compact radio components surrounded by more diffuse, weak radioemission. Using the constraints provided by these observations and thosewithin the literature we conclude that the majority of this radioemission is related to the ongoing star-formation in this merger system.With these observations we also trace deep Hi absorption across thedetected radio continuum structure. The absorbing Hi gas structureexhibits large variations in column densities. The largest columndensities are found toward the south of the nuclear radio continuum,co-spatial with both a nuclear dust lane and peaks in 12CO(1→ 0) emission. The dynamics of the Hi absorption, which areconsistent with lower resolution 12CO emission observations,trace a shallow north-south velocity gradient of ~320 km s-1kpc-1. This gradient is interpreted as part of a rotating gasstructure within the nuclear region. The Hi and CO velocity structure,in conjunction with the observed gas column densities and distribution,is further discussed in the context of the fuelling and gas physics ofthe ongoing starburst within the centre of this merger.
| Molecular gas in compact galaxies
New observations of eleven compact galaxies in the 12CO J =2{-}1 and J = 3{-}2 transitions are presented. From these observationsand literature data accurate line ratios in matched beams have beenconstructed, allowing the modelling of physical parameters. Matching asingle gas component to observed line ratios tends to produce physicallyunrealistic results, and is often not possible at all. Much betterresults are obtained by modelling two distinct gas components. In mostobserved galaxies, the molecular gas is warm (Tk = 50{-}150K) and at least partially dense (n(H2) ≥ 3000cm-3). Most of the gas-phase carbon in these galaxies is inatomic form; only a small fraction ( 5%) is in carbon monoxide.Beam-averaged CO column densities are low (of the order of1016 cm-2). However, molecular hydrogen columndensities are high (of the order of 1022 cm-2)confirming large CO-to- H2 conversion factors (typically X =1021{-}1022 cm-2/ {K kms-1}) found for low-metallicity environments by othermethods. From CO spectroscopy, three different types of molecularenvironment may be distinguished in compact galaxies. Type I (highrotational and isotopic ratios) corresponds to hot and dense molecularclouds dominated by star-forming regions. Type II has lower ratios,similar to the mean found for infrared-luminous galaxies in general, andcorresponds to environments engaged in, but not dominated by,star-forming activity. Type III, characterized by low 12CO(2-1)/(1-0) ratios, corresponds to mostly inactive environments ofrelatively low density.
| Spectroscopic study of blue compact galaxies. V. Oxygen abundance and the metallicity-luminosity relation
This is the fifth paper in a series studying the stellar components,star formation histories, star formation rates and metallicities of ablue compact galaxy (BCG) sample. Based on our high-quality ground-basedspectroscopic observations, we have determined the electrontemperatures, electron densities, nitrogen abundances and oxygenabundances for 72 star-forming BCGs in our sample, using differentoxygen abundance indicators. The oxygen abundance covers the range 7.15< 12 + log (O/H)< 9.0, and nitrogen is found to be mostly aproduct of secondary nucleosynthesis for 12 + log (O/H)>8.2 andapparently a product of primary nucleosynthesis for 12 + log (O/H)<8.2. To assess the possible systematic differences among differentoxygen abundance indicators, we have compared oxygen abundances of BCGsobtained with the Te method, R23 method, P method,N2 method and O3N2 method. The oxygen abundances derived from theTe method are systematically lower by 0.1-0.25 dex than thosederived from the strong line empirical abundance indicators, consistentwith previous studies based on region samples. We confirm the existenceof the metallicity-luminosity relation in BCGs over a large range ofabundances and luminosities. Our sample of galaxies shows that the slopeof the metallicity-luminosity relation for the luminous galaxies(~-0.05) is slightly shallower than that for the dwarf galaxies(~-0.17). An offset was found in the metallicity-luminosity relation ofthe local galaxies and that of the intermediate redshift galaxies. Itshows that the metallicity-luminosity relation for the emission linegalaxies at high redshift is displaced to lower abundances, higherluminosities, or both.
| Cold and warm dust along a merging galaxy sequence
We investigate the cold and warm dust properties during galaxyinteractions using a merging galaxy sample ordered into a chronologicalsequence from pre- to post-mergers. Our sample comprises a total of 29merging systems selected to have far-infrared and submillimetreobservations. The submillimetre data are mainly culled from theliterature, while for five galaxies (NGC 3597, 3690, 6090, 6670 and7252) the submillimetre observations are presented here for the firsttime. We use the 100- to 850-μm flux density ratio,f100/f850, as a proxy for the mass fraction of thewarm and cold dust in these systems. We find evidence for an increase inf100/f850 along the merging sequence from early toadvanced mergers, and interpret this trend as an increase of the warmrelative to the cold dust mass. We argue that the two key parametersaffecting the f100/f850 flux ratio is the starformation rate and the dust content of individual systems relative tothe stars. Using a sophisticated model for the absorption andre-emission of the stellar ultraviolet radiation by dust, we show thatthese parameters can indeed explain both the increase and the observedscatter in f100/f850 along the merging galaxysequence. We also discuss our results under the hypothesis thatelliptical galaxies are formed via disc galaxy mergers.
| Infrared mergers and infrared quasi-stellar objects with galactic winds - I. NGC 2623: nuclear outflow in a proto-elliptical candidate
We present the first results of a study of the morphology, kinematicsand ionization structure of infrared (IR) mergers/quasi-stellar objects(QSOs) with galactic winds. This study is based mainly on INTEGRALtwo-dimensional (2D) fibre spectroscopy [obtained on the 4.2-m WilliamHerschel Telescope (WHT), La Palma] combined with high-resolution HubbleSpace Telescope (HST) observations.Clear evidence of outflow (OF) from the nucleus of the luminous infraredmerger NGC 2623 is reported. Specifically: (i) the INTEGRAL 2D Hα,[N II] and [S II] emission line maps depict a cone-shaped extendednebula that emerges from the nucleus, with an aperture angle θ=100°+/- 5° and reaching a distance of ~3.2 kpc from the nucleus;(ii) inside the nebula and in the central region, all the emission-lineWHT spectra show low velocity blue/OF components, with= (-405 +/- 35) km s-1 and (iii) in theOF nebula, the emission line ratios are consistent with ionization by adusty nuclear starburst plus shock heating. These results are consistentwith a galactic wind process powered mainly by a nuclear starburst.The INTEGRAL 2D Hα and [N II]λ6583 velocity field (VF) mapsfor the main body of NGC 2623 (16.4 × 12.3 arcsec2;~5.9 × 4.4 kpc2) show outflow motion in the nuclear andthe Hα+[N II] nebular regions superposed on a general circularmotion. This circular motion prevails inside r~ 1.5 kpc, and for largerradii we detected non-circular motions. In the central region, theaverage observed rotation curve was fitted with a model corresponding toa single-component Plummer spherical potential. After the subtraction ofthe Plummer and an axisymmetric polynomial model, the residues of the VFin both cases indicate ejection as the origin of the cone nebula. Thefitted Plummer model implies a total mass of MT= 1.5 ×1010 Msolar and a spherical distribution of matterin the central region.The high-resolution HST WFPC2 F555W (~V) and F814W (~I) broad-bandimages display a strongly obscured nucleus in the apex of a smallnuclear cone, an asymmetrical clumpy spiral arm located to the east ofthe nucleus, a ring plus an arc to the west and several large-scalefilaments of dust. A good r1/4-law fit to the HST WFPC2 Iband luminosity profile was found.In 85 per cent of the INTEGRAL 2D field we measure very high values(>1) of the [N II]λ6583/Hα and [S II]λ6717 +31/Hα ratios, suggesting that shocks are important on large scales(in almost all the main body). Furthermore, the 2D full width at halfmaximum FWHM-[N II] and VF residual maps show a good spatialcorrelation, suggesting that the OF shocks ionize the gas and broadenthe emission lines. However, close to the nucleus, the OF nebula showslow values of the [N II]λ6583/Hα ratio (in the range0.1-0.4), indicating that photoionization by a dusty nuclear starburstalso plays a significant role in the excitation of the nebula. Thecharacteristics of the nucleus of NGC 2623 could be associated with astarburst-related LINER.The properties found in IR mergers/QSOs with galactic winds mainlyunderline the importance of studying the possible link between IRmergers with starburst + galactic wind -> IR QSOs with compositenature + galactic wind, and elliptical galaxies.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation?
Infrared (IR) emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 μmare generally attributed to IR fluorescence from (mainly)far-ultraviolet (FUV) pumped large polycyclic aromatic hydrocarbon (PAH)molecules. As such, these features trace the FUV stellar flux and arethus a measure of star formation. We examined the IR spectralcharacteristics of Galactic massive star-forming regions and of normaland starburst galaxies, as well as active galactic nuclei (AGNs) andultraluminous infrared galaxies (ULIRGs). The goal of this study is toanalyze whether PAH features are a good qualitative and/or quantitativetracer of star formation, and hence to evaluate the application of PAHemission as a diagnostic tool in order to identify the dominantprocesses contributing to the infrared emission from Seyfert galaxiesand ULIRGs. We develop a new mid-infrared (MIR)/far-infrared (FIR)diagnostic diagram based on our Galactic sample and compare it to thediagnostic tools of Genzel and coworkers and Laurent and coworkers, withthese diagnostic tools also applied to our Galactic sample. This MIR/FIRdiagnostic is derived from the FIR normalized 6.2 μm PAH flux and theFIR normalized 6.2 μm continuum flux. Within this diagram, theGalactic sources form a sequence spanning a range of 3 orders ofmagnitude in these ratios, ranging from embedded compact H II regions toexposed photodissociation regions (PDRs) and the (diffuse) interstellarmedium (ISM). However, the variation in the 6.2 μm PAHfeature-to-continuum ratio is relative small. Comparison of ourextragalactic sample with our Galactic sources revealed an excellentresemblance of normal and starburst galaxies to exposed PDRs. WhileSeyfert 2 galaxies coincide with the starburst trend, Seyfert 1 galaxiesare displaced by at least a factor of 10 in 6.2 μm continuum flux, inaccordance with general orientation-dependent unification schemes forAGNs. ULIRGs show a diverse spectral appearance. Some show a typical AGNhot dust continuum. More, however, either are starburst-like or showsigns of strong dust obscuration in the nucleus. One characteristic ofthe ULIRGs also seems to be the presence of more prominent FIR emissionthan either starburst galaxies or AGNs. We discuss the observedvariation in the Galactic sample in view of the evolutionary state andthe PAH/dust abundance and discuss the use of PAHs as quantitativetracers of star formation activity. Based on these investigations, wefind that PAHs may be better suited as a tracer of B stars, whichdominate the Galactic stellar energy budget, than as a tracer of massivestar formation (O stars).
| Star Formation History and Extinction in the Central Kiloparsec of M82-like Starbursts
We report on the star formation histories and extinction in the centralkiloparsec region of a sample of starburst galaxies that have similarfar-infrared (FIR), 10 μm, and K-band luminosities as those of thearchetype starburst M82. Our study is based on new optical spectra andpreviously published K-band photometric data, both sampling the samearea around the nucleus. Model starburst spectra were synthesized as acombination of stellar populations of distinct ages formed over theHubble time and were fitted to the observed optical spectra and K-bandflux. The model is able to reproduce simultaneously the equivalentwidths of emission and absorption lines, the continuum fluxes between3500 and 7000 Å, and the K-band and FIR flux. A good fit requiresa minimum of three populations: (1) a young population of age <=8Myr, with its corresponding nebular emission, (2) an intermediate-agepopulation (age <500 Myr), and (3) an old population that forms partof the underlying disk or/and bulge population. The birthrate parameter,which is defined as the ratio of the current star formation rate to theaverage past rate, is found to be in the range 1-12. The contribution ofthe old population to the K-band luminosity depends on the birthrateparameter and remains above 60% in the majority of the sample galaxies.Even in the blue band, the intermediate-age and old populationscontribute more than 40% of the total flux in all the cases. Arelatively high contribution from the old stars to the K-band nuclearflux is also apparent from the strength of the 4000 Å break andthe Ca II K line. The extinction of the old population is found to bearound half that of the young population. The contribution to thecontinuum from the relatively old stars has the effect of diluting theemission equivalent widths below the values expected for young bursts.The mean dilution factors are found to be 5 and 3 for the Hα andHβ lines, respectively.
| A Deep K-Band Photometric Survey of Merger Remnants
We present K-band photometry for 51 candidate merger remnants to assessthe viability of whether spiral-spiral mergers can produce bona fideelliptical galaxies. Using both the de Vaucouleurs r1/4 andSérsic r1/n fitting laws, it is found that the stellarcomponent in a majority of the galaxies in the sample has undergoneviolent relaxation. However, the sample shows evidence for incompletephase mixing. The analysis also indicates the presence of ``excesslight'' in the surface brightness profiles of nearly one-third of themerger remnants. Circumstantial evidence suggests that this is due tothe effects of a starburst induced by the dissipative collapse of thegas. The integrated light of the galaxies also shows that mergers canmake L* elliptical galaxies, in contrast to earlier infrared studies.The isophotal shapes and related structural parameters are alsodiscussed, including the fact that 70% of the sample show evidence fordisky isophotes. The data and results presented are part of a largerphotometric and spectroscopic campaign to thoroughly investigate a largesample of mergers in the local universe.
| Looking Closely at Medusa: Star-forming Knots at the Center of NGC 4194
We report high-resolution ultraviolet and visible-wavelength imaging ofthe blue compact galaxy NGC 4194 (the Medusa) using the Hubble SpaceTelescope. A complete sample of 38 UV-bright knots is identified. Theseknots produce 20% of the ultraviolet emission from the area of thegalaxy observed. The bright end of the knot flux distribution can befitted by a power law with exponent α=-1.47+/-0.27. The completeflux distribution is Gaussian. Comparison with starburst evolutionarytracks indicates that two-thirds of the knots are younger than 20 Myr.There is one concentration of very young knots less than 5 Myr old. Theknot masses are large, comparable to those found in other major mergergalaxies. The mass distribution is a power law with exponent-1.85+/-0.24. The difference between the far-ultraviolet fluxdistribution and the mass distribution can be understood by the rapiddecrease in a knot's far-ultraviolet flux due to stellar evolution. Thestar formation rate calculated from knots younger than 20 Myr is ~6Msolar yr-1, suggesting that the star formationrate throughout the galaxy may be as high as 30 Msolaryr-1. Estimates of knot stability suggest that at least halfthe knots are bound protoclusters and will become globular clusters inseveral gigayears. The location and masses of individual star-formingregions in NGC 4194 and the rate of star formation are typical of majormerger galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| A Study of the Distribution of Star-forming Regions in Luminous Infrared Galaxies by Means of Hα Imaging Observations
We performed Hα imaging observations of 22 luminous infraredgalaxies to investigate how the distribution of star-forming regions inthese galaxies is related to galaxy interactions. Based on correlationdiagrams between Hα flux and continuum emission for individualgalaxies, a sequence for the distribution of star-forming regions wasfound: very compact (~100 pc) nuclear starbursts with almost nostar-forming activity in the outer regions (type 1), dominant nuclearstarbursts <~1 kpc in size with a negligible contribution from theouter regions (type 2), nuclear starbursts >~1 kpc in size with asignificant contribution from the outer regions (type 3), and extendedstarbursts with relatively faint nuclei (type 4). These classes ofstar-forming regions were found to be strongly related to globalstar-forming properties, such as star formation efficiency, far-infraredcolor, and dust extinction. There was a clear tendency for the objectswith more compact distributions of star-forming regions to show a higherstar formation efficiency and hotter far-infrared color. An appreciablefraction of the sample objects were dominated by extended starbursts(type 4), which is unexpected in the standard scenario ofinteraction-induced starburst galaxies. We also found that thedistribution of star-forming regions was weakly but clearly related togalaxy morphology: severely disturbed objects had a more concentrateddistribution of star-forming regions. This suggests that the propertiesof galaxy interactions, such as dynamical phase and orbital parameters,play a more important role than the internal properties of progenitorgalaxies, such as dynamical structure or gas mass fraction. We alsodiscuss the evolution of the distribution of star-forming regions ininteracting galaxies.
| Star Formation in the Center of the Advanced Merger NGC 4194
We have obtained STIS longslit spectra of the central region of theadvanced merger NGC 4194. The spectra cover the wavelength range1150-1750 angstroms in the FUV and 2900-10270 angstroms in the visible.Results from the study of the properties of 14 star forming regions(knots) are presented. We determine the average E(B-V)=0.98 mag bycomparing the Halpha+[N II] flux to the Hbeta flux. If the [N II]contribution is 40% then the average E(B-V)=0.50 mag. The presence ofstrong stellar winds, indicated by blue shifted FUV absorption lines,and the absence of emission lines lead us to believe that knot 6 is abubble. The combined Halpha+[N II] luminosity of the 14 knots yield atotal SFR of 6 solar masses per year and the combined [O II] luminosityof those knots yield a total SFR of 8 solar masses per year. UsingStarburst99, and the EW(Halpha+[N II]) and EW(Hbeta), we estimate theages of the star forming regions to be 6-11 Myr. The sizes of the knotswere determined from L(Hbeta) and range from 7 pc to 22 pc when afilling factor of 1 is assumed. The sizes are a factor of 2.15 largerfor a filling factor of 0.1.
| Spectroscopic study of blue compact galaxies. IV. Star formation rates and gas depletion timescales
This is the fourth paper in a series studying star formation rates,stellar components, metallicities, and star formation histories of ablue compact galaxy (BCG) sample. Using Hα, [O II]λ3727,infrared (IR), radio (1.4 GHz) luminosities and neutral hydrogen (H I)gas masses, we estimated star formation rates and gas depletiontimescales of 72 star-forming BCGs. The star formation rates of the BCGsin our sample span nearly four orders of magnitude, from approximately10-2 to 102 Mȯ yr-1,with a median star formation rate of about 3 Mȯyr-1. The typical gas depletion timescale of BCGs is aboutone billion years. Star formation could be sustained at the currentlevel only on a timescale significantly lower than the age of theuniverse before their neutral gas reservoir is completely depleted. Toassess the possible systematic differences among different starformation rate indicators, we compared the star formation rates derivedfrom Hα, [O II]λ3727, IR, and radio luminosities, andinvestigated the effects from underlying stellar absorption and dustextinction. We found that subtracting underlying stellar absorption isvery important to calculate both dust extinction and star formation rateof galaxies. Otherwise, the intrinsic extinction will be overestimated,the star formation rates derived from [O II]λ3727 and Hαwill be underestimated (if the underlying stellar absorption and theinternal extinction were not corrected from the observed luminosity) oroverestimated (if an overestimated internal extinction were used forextinction correction). After both the underlying stellar absorption andthe dust extinction were corrected, a remarkably good correlationemerges among Hα, [O II]λ3727, IR and radio star formationrate indicators. Finally, we find a good correlation between themeasured star formation rate and the absolute blue magnitude,metallicity, interstellar extinction of BCGs. Our results indicate thatfaint, low-mass BCGs have lower star formation rates.Star formation rates and gas depletion timescales of BCGs are availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/425/417
| Starbursts in barred spiral galaxies. VI. HI observations and the K-band Tully-Fisher relation
This paper reports a study of the effect of a bar on the neutralhydrogen (HI) content of starburst and Seyfert galaxies. We also makecomparisons with a sample of ``normal'' galaxies and investigate howwell starburst and Seyfert galaxies follow the fundamental scalingTully-Fisher (TF) relation defined for normal galaxies. 111 Markarian(Mrk) IRAS galaxies were observed with the Nançay radiotelescope,and HI data were obtained for 80 galaxies, of which 64 are newdetections. We determined the (20 and 50%) linewidths, the maximumvelocity of rotation and total HI flux for each galaxy. Thesemeasurements are complemented by data from the literature to form asample of Mrk IRAS (74% starburst, 23% Seyfert and 3% unknown) galaxiescontaining 105 unbarred and 113 barred ones. Barred galaxies have lowertotal and bias-corrected HI masses than unbarred galaxies, and this istrue for both Mrk IRAS and normal galaxies. This robust result suggeststhat bars funnel the HI gas toward the center of the galaxy where itbecomes molecular before forming new stars. The Mrk IRAS galaxies havehigher bias-corrected HI masses than normal galaxies. They also showsignificant departures from the TF relation, both in the B and K bands.The most deviant points from the TF relation tend to have a strongfar-infrared luminosity and a low oxygen abundance. These resultssuggest that a fraction of our Mrk IRAS galaxies are still in theprocess of formation, and that their neutral HI gas, partly of externalorigin, has not yet reached a stationary state.Based on observations obtained at the large radiotelescope ofObservatoire de Nançay, operated by Observatoire de Paris.Tables 5 and 6 are only (and Table 4 also) available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/515
| Radio emission from AGN detected by the VLA FIRST survey
Using the most recent (April 2003) version of the VLA FIRST survey radiocatalog, we have searched for radio emission from >2800 AGN takenfrom the most recent (2001) version of the Veron-Cetty and Veron AGNcatalog. These AGN lie in the 9033 square degrees of sky alreadycovered by the VLA FIRST survey. Our work has resulted in positivedetection of radio emission from 775 AGN of which 214 are new detectionsat radio wavelengths.Tables 3 and 4 are 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/416/35
| Cold dust and molecular gas towards the centers of Magellanic type galaxies and irregulars. I. The data
We present 1300 μm continuum emission measurements and observationsof the 12CO (1-0) and (2-1) transition towards the centers of64 Magellanic type galaxies (Sdm/Sm) and irregulars (Im/I0/Irr). Thesources are selected to have IRAS flux densities S100 μm≥1000 mJy and optical diameters mainly below 180 arcsec. We wereable to detect 12CO towards 41 and the continuum emissiontowards 28 galaxies. In addition, we obtained the corresponding data fora set of 6 complementary galaxies of different morphological type.Based on observations collected at ESO, La Silla, Chile and IRAM, PicoVeleta, Spain.The full version of Figs. \ref{spec1.fig} and \ref{spec2.fig} is onlyavailable in electronic form at http://www.edpsciences.org
| 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 PDS versus Markarian starburst galaxies: comparing strong and weak IRAS emitter at 12 and 25 μm in the nearby Universe
The characteristics of the starburst galaxies from the Pico dos Diassurvey (PDS) are compared with those of the nearby ultraviolet (UV)bright Markarian starburst galaxies, having the same limit in redshift(vh < 7500 km s-1) and absolute B magnitude(MB < -18). An important difference is found: theMarkarian galaxies are generally undetected at 12 and 25 μm in IRAS.This is consistent with the UV excess shown by these galaxies andsuggests that the youngest star-forming regions dominating thesegalaxies are relatively free of dust.The far-infrared selection criteria for the PDS are shown to introduce astrong bias towards massive (luminous) and large size late-type spiralgalaxies. This is contrary to the Markarian galaxies, which are found tobe remarkably rich in smaller size early-type galaxies. These resultssuggest that only late-type spirals with a large and massive disc arestrong emitters at 12 and 25 μm in IRAS in the nearby Universe.The Markarian and PDS starburst galaxies are shown to share the sameenvironment. This rules out an explanation of the differences observedin terms of external parameters. These differences may be explained byassuming two different levels of evolution, the Markarian being lessevolved than the PDS galaxies. This interpretation is fully consistentwith the disc formation hypothesis proposed by Coziol et al. to explainthe special properties of the Markarian SBNG.
| Quantifying dust and the ultraviolet radiation density in the local Universe
A sample of local galaxies for which far-infrared and ultraviolet fluxesare available is used to estimate the characteristic dust extinction ingalaxies and to test whether standard dust properties are plausible.Assuming galaxies can be characterized by a single dust optical depth(certainly not valid for galaxies with a dominant starburst component),the infrared excess and ultraviolet colours of local galaxies are foundto be consistent with normal Milky Way dust, with a mean value forE(B-V) of 0.16. A significant fraction of the dust heating is caused byolder, lower-mass stars, and this fraction increases towards earliergalaxy types.Analysis of (FFIR/FUV) versus ultraviolet colourdiagrams for starburst galaxies in terms of a simple screen dust modeldoes not support a Calzetti (1997) rather than a Milky Way extinctionlaw, though the absence of the expected strong 2200-Å feature inseveral galaxies with IUE spectra does show that more detailed radiativetransfer models are needed - probably with nonspherical geometry.A simple treatment in which the 100/60-μm flux ratio is used tosubtract the optically thick starburst contribution to the far-infraredradiation results in lower extinction estimates for the optically thincirrus component, with a mean E(B-V) of 0.10.The ultraviolet luminosity density, corrected for dust extinction, isderived and a value for the local mean star formation rate inferred.This is consistent with previous estimates from ultraviolet surveys andfrom Hα surveys.
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