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Extragalactic integral field spectroscopy on the Gemini telescopes
We have undertaken a programme on the Gemini 8-m telescopes todemonstrate the power of integral field spectroscopy, using the CIRPASSinstrument in the near-infrared. Here, we present some of our resultsfrom 3D spectroscopy of extra-galactic objects: mapping the Hαvelocity field in a z ≈ 1 disc galaxy; exploring dark mattersub-structure through observations of an Einstein cross gravitationallens; and the star formation time-scales of young massive clusters in astarburst galaxy.
| Near-infrared integral-field spectroscopy of violent starburst environments
Near-infrared (NIR) integral-field spectroscopy (IFS) of violentstarburst environments at high spatial (and spectral) resolution has thepotential to revolutionise our ideas regarding the local interactionsbetween the newly formed massive stars and the interstellar medium (ISM)of their host galaxies. To illustrate this point, I present NIR IFSanalysis of the central starburst region of NGC 1140, obtained withCIRPASS on Gemini-South. While strong [FeII] emission is foundthroughout the galaxy, higher-order Brackett emission is predominantlyassociated with the northern starburst region. Based on the spatialdistributions of the [FeII] versus Brackett line emission, I concludethat a galaxy-wide starburst was induced several ×107yr ago, with more recent starburst activity concentrated around thenorthern starburst region. I look forward and discuss the excitingprospects that IFS at higher spatial (and spectral) resolution willallow us trace (i) the massive outflows (“superwinds”)expected to originate in the dense, young massive star clusters commonlyfound in intense starburst environments, and (ii) their impact on thegalaxy’s ISM.
| Primordial Helium Abundance: A Reanalysis of the Izotov-Thuan Spectroscopic Sample
A reanalysis is made for the helium abundance determination for theIzotov-Thuan spectroscopic sample of extragalactic H II regions. We findthat the effect of underlying stellar absorption of the He I lines,which is more important for metal-poor systems, affects significantlythe inferred primordial helium abundance Yp obtained in thezero-metallicity limit and the slope of linear extrapolation, dY/dZ.This brings Yp from 0.234+/-0.004 to 0.250+/-0.004 anddY/dZ=4.7+/-1.0 to 1.1+/-1.4. Conservatively, this indicates theimportance of the proper understanding of underlying stellar absorptionfor accurate determinations of the primordial helium abundance to theerror of δYp~=0.002-0.004.
| Mid-Infrared Properties of Low-Metallicity Blue Compact Dwarf Galaxies from the Spitzer Infrared Spectrograph
We present a Spitzer-based mid-infrared (MIR) study of a large sample ofblue compact dwarfs (BCDs) using the Infrared Spectrograph (IRS),including the first MIR spectrum of I Zw 18, the archetype for the BCDclass and among the most metal-poor galaxies known. We show the spectraof polycyclic aromatic hydrocarbon (PAH) emission in a low-metallicityenvironment. We find that the equivalent widths (EWs) of PAHs at 6.2,7.7, 8.6, and 11.2 μm are generally weaker in BCDs than in typicalstarburst galaxies and that the fine-structure line ratio, [Ne III]/[NeII], has a weak anticorrelation with the PAH EW. A much strongeranticorrelation is shown between the PAH EW and the product of the [NeIII]/[Ne II] ratio and the UV luminosity density divided by themetallicity. We conclude that the PAH EW in metal-poor high-excitationenvironments is determined by a combination of PAH formation anddestruction effects.
| Massive Star Cluster Populations in Irregular Galaxies as Probable Younger Counterparts of Old Metal-rich Globular Cluster Populations in Spheroids
Peak metallicities of metal-rich populations of globular clusters(MRGCs) belonging to early-type galaxies and spheroidal subsystems ofspiral galaxies (spheroids) of different mass fall within the somewhatconservative -0.7<=[Fe/H]<=-0.3 range. Indeed, if possible ageeffects are taken into account, this metallicity range might becomesmaller. Irregular galaxies such as the Large Magellanic Cloud (LMC),with longer timescales of formation and lower star formation (SF)efficiency, do not contain old MRGCs with [Fe/H]>-1.0, but they areobserved to form populations of young/intermediate-age massive starclusters (MSCs) with masses exceeding 104 Msolar.Their formation is widely believed to be an accidental process fullydependent on external factors. From the analysis of available data onthe populations and their hosts, including intermediate-age populousstar clusters in the LMC, we find that their most probable meanmetallicities fall within -0.7<=[Fe/H]<=-0.3, as the peakmetallicities of MRGCs do, irrespective of signs of interaction.Moreover, both the disk giant metallicity distribution function (MDF) inthe LMC and the MDFs for old giants in the halos of massive spheroidsexhibit a significant increase toward [Fe/H]~-0.5. That is in agreementwith a correlation found between SF activity in galaxies and theirmetallicity. The formation of both the old MRGCs in spheroids and MSCpopulations in irregular galaxies probably occurs at approximately thesame stage of the host galaxies' chemical evolution and is related tothe essentially increased SF activity in the hosts around the samemetallicity that is achieved very early in massive spheroids, later inlower mass spheroids, and much later in irregular galaxies. Changes inthe interstellar dust, particularly in elemental abundances in dustgrains and in the mass distribution function of the grains, may be amongthe factors regulating star and MSC formation activity in galaxies.Strong interactions and mergers affecting the MSC formation presumablyplay an additional role, although they can substantially intensify theinternally regulated MSC formation process. Several implications of oursuggestions are briefly discussed.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| ISM properties in low-metallicity environments
We present new ISOCAM mid-infrared spectra of three starbursting nearbydwarf galaxies, NGC 1569, NGC 1140 and II Zw 40 and the 30 Dor region ofthe LMC and explore the properties of the ISM in low-metallicityenvironments, also using additional sources from the literature. Weanalyse the various components of the ISM probed by the mid-infraredobservations and compare them with other Galactic and extragalacticobjects. The MIR spectra of the low-metallicity starburst sources aredominated by the [ Ne iii] λ 15.56~μ m and [ S iv] λ10.51~μ m lines, as well as a steeply rising dust continuum. PAHbands are generaly faint, both locally and averaged over the fullgalaxy, in stark contrast to dustier starburst galaxies, where the PAHfeatures are very prominant and even dominate on global scales. Thehardness of the modeled interstellar radiation fields for the dwarfgalaxies increases as the presence of PAH band emission becomes lesspronounced. The [ Ne iii] /[ Ne ii] ratios averaged over the full galaxyare strikingly high, often >10. Thus, the hard radiation fields arepronounced and pervasive. We find a prominent correlation between thePAHs/VSGs and the [ Ne iii] /[ Ne ii] ratios for a wide range ofobjects, including the low metallicity galaxies as well as Galactic H iiregions and other metal-rich galaxies. This effect is consistent withthe hardness of the interstellar radiation field playing a major role inthe destruction of PAHs in the low metallicity ISM. We see a PAHs/VSGsand metallicity correlation, also found by Engelbracht et al. (2005,ApJ, 628, 29) for a larger survey. Combined effects of metallicity andradiation field seem to be playing important roles in the observedbehavior of PAHs in the low metallicity systems.
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
| The HI Content of the Eridanus Group of Galaxies
The HI content of galaxies in the Eridanus group is studied using theGMRT observations and the HIPASS data. A significant HI deficiency up toa factor of 2-3 is observed in galaxies in the high galaxy densityregions. The HI deficiency in galaxies is observed to be directlycorrelated to the local projected galaxy density, and inverselycorrelated to the lineof-sight radial velocity. Furthermore, galaxieswith larger optical diameters are predominantly in the lower galaxydensity regions. It is suggested that the HI deficiency in Eridanus isdue to tidal interactions. In some galaxies, evidences of tidalinteractions are seen. An important implication is that significantevolution of galaxies can take place in the group environment. In thehierarchical way of formation of clusters via mergers of groups, afraction of the observed HI deficiency in clusters could have originatedin groups. The co-existence of S0s and severely HI deficient galaxies inthe Eridanus group suggests that tidal interaction is likely to be aneffective mechanism for transforming spirals to S0s.
| 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.
| ISM properties in low-metallicity environments. III. The spectral energy distributions of II Zw 40, He 2-10 and NGC 1140
We present new 450 and 850 μm SCUBA data and 1.3 mm MAMBO data of thedwarf galaxies II Zw 40, He 2-10 and NGC 1140. Additional ISOCAM, IRASas well as ground based data are used to construct the observedmid-infrared to millimeter spectral energy distribution of thesegalaxies. These spectral energy distributions are modeled in aself-consistent way, as was achieved with NGC 1569 (Galliano et al.2003, A&A, 407, 159), synthesizing both the global stellar radiationfield and the dust emission, with further constraints provided by thephotoionisation of the gas. Our study shows that low-metallicitygalaxies have very different dust properties compared to the Galaxy. Ourmain results are: (i) a paucity of PAHs which are likely destroyed bythe hard penetrating radiation field; (ii) a very small ( 3{-}4 nm)average size of grains, consistent with the fragmentation and erosion ofdust particles by the numerous shocks; (iii) a significant millimetreexcess in the dust spectral energy distribution which can be explainedby the presence of ubiquitous very cold dust (T = 5{-}9 K) accountingfor 40 to 80% of the total dust mass, probably distributed in smallclumps. We derive a range of gas-to-dust mass ratios between 300 and2000, larger than the Galactic values and dust-to-metals ratios of 1/30to 1/2. The modeled dust size distributions are used to synthesize anextinction curve for each galaxy. The UV slopes of the extinction curvesresemble that observed in some regions in the Large Magellanic Cloud.The 2175 Å bumps of the modeled extinction curves are weaker thanthat of the Galaxy, except in the case of II Zw 40 where we are unableto accurately constrain the 2175 Å bump carrier.
| CIRPASS near-infrared integral-field spectroscopy of massive star clusters in the starburst galaxy NGC 1140
We analyse near-infrared integral field spectroscopy of the centralstarburst region of NGC 1140, obtained at the Gemini-South telescopeequipped with CIRPASS. Our ~1.45-1.67μm wavelength coverage includesthe bright [FeII]λ 1.64-μm emission line, as well ashigh-order Brackett (hydrogen) lines. While strong [FeII] emission,thought to originate in the thermal shocks associated with supernovaremnants, is found throughout the galaxy, both Br 12-4 and Br 14-4emission, and weak CO(6,3) absorption, is predominantly associated withthe northern starburst region. The Brackett lines originate fromrecombination processes occurring on smaller scales in (young) HIIregions. The time-scale associated with strong [FeII] emission impliesthat most of the recent star-formation activity in NGC 1140 was inducedin the past ~35-55Myr. Based on the spatial distributions of the [FeII]versus Brackett line emission, we conclude that a galaxy-wide starburstwas induced several tens of Myr ago, with more recent starburst activityconcentrated around the northern starburst region.This scenario is (provisionally) confirmed by our analysis of thespectral energy distributions of the compact, young massive starclusters (YMCs) detected in new and archival broad-band Hubble SpaceTelescope images. The YMC ages in NGC 1140 are all <~20Myr,consistent with independently determined estimates of the starburst ageof the galaxy, while there appears to be an age difference between thenorthern and southern YMC complexes in the sense expected from ourCIRPASS analysis. Our photometric mass estimates of the NGC 1140 YMCs,likely upper limits, are comparable to those of the highest-massGalactic globular clusters and to spectroscopically confirmed masses of(compact) YMCs in other starburst galaxies. Our detection of similarlymassive YMCs in NGC 1140 supports the scenario that such objects formpreferentially in the extreme environments of interacting and starburstgalaxies.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Systematic Effects and a New Determination of the Primordial Abundance of 4He and dY/dZ from Observations of Blue Compact Galaxies
We use spectroscopic observations of a sample of 82 H II regions in 76blue compact galaxies to determine the primordial helium abundanceYp and the slope dY/dZ from the Y-O/H linear regression. Toimprove the accuracy of the dY/dZ measurement, we have included newspectrophotometric observations of 33 H II regions that span a largemetallicity range, with oxygen abundance 12+log(O/H) varying between7.43 and 8.30 (Zsolar/30<=Z<=Zsolar/4). Mostof the new galaxies were selected from the First Byurakan, theHamburg/SAO, and the University of Michigan objective prism surveys. Fora subsample of seven H II regions, we derive the He mass fraction takinginto account known systematic effects, including collisional andfluorescent enhancements of He I emission lines, collisional excitationof hydrogen emission, underlying stellar He I absorption, and thedifference between the temperatures Te(He II) in theHe+ zone and Te(O III) derived from thecollisionally excited [O III] lines. We find that the net result of allthe systematic effects combined is small, changing the He mass fractionby less than 0.6%. By extrapolating the Y versus O/H linear regressionto O/H=0 for seven H II regions of this subsample, we obtainYp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds todY/dZ=3.7+/-1.2, assuming the oxygen mass fraction to be O=0.66Z. In theframework of the standard big bang nucleosynthesis theory, thisYp corresponds toΩbh2=0.012+0.003-0.002,where h is the Hubble constant in units of 100 km s-1Mpc-1. This is smaller at the 2 σ level than the valueobtained from recent deuterium abundance and microwave backgroundradiation measurements. The linear regression slope dY/dO=4.3+/-0.7(corresponding to dY/dZ=2.8+/-0.5) for the whole sample of 82 H IIregions is similar to that derived for the subsample of seven H IIregions, although it has a considerably smaller uncertainty.
| Extragalactic integral field spectroscopy on the Gemini telescopes
We have been undertaking a programme on the Gemini 8-m telescopes todemonstrate the power of integral field spectroscopy, using the opticalGMOS spectrograph, and the new CIRPASS instrument in the near-infrared.Here we present some preliminary results from 3D spectroscopy ofextra-galactic objects, mapping the emission lines in a 3CR radio galaxyand in a gravitationally lensed arc, exploring dark matter sub-structurethrough observations of an Einstein Cross gravitational lens, and thestar formation time-scales of young massive clusters in the starburstgalaxy NGC 1140.
| Catalog of Double Nucleus Disk Galaxies
We have compiled a catalog of disk galaxies that have a double nucleus,through systematic examination of existing catalogs and publications.The Catalog of Double Nucleus Disk Galaxies includes 107 objects,together with their basic data. The aim of the catalog is to provide amore systematic and homogeneous basis for the study of the relevance ofgalaxy interactions and minor mergers in the formation of these doublenuclei. We have also investigated possible correlations betweengeometric and photometric parameters of the double nuclei and their hostgalaxies. The preliminary results indicate the presence of severalsignificant correlations that should be considered in any theoreticalscenario describing minor mergers and disk galaxy evolution.
| Mid-IR emission of galaxies in the Virgo cluster and in the Coma supercluster. IV. The nature of the dust heating sources
We study the relationship between the mid-IR (5-18 μm) emission oflate-type galaxies and various other star formation tracers in order toinvestigate the nature of the dust heating sources in this spectraldomain. The analysis is carried out using a sample of 123 normal,late-type, nearby galaxies with available data at several frequencies.The mid-IR luminosity (normalized to the H-band luminosity) correlatesbetter with the far-IR luminosity than with more direct tracers of theyoung stellar population such as the Hα and the UV luminosity. Thecomparison of resolved images reveals a remarkable similarity in theHα and mid-IR morphologies, with prominent HII regions at bothfrequencies. The mid-IR images, however, show in addition a diffuseemission not associated with HII regions nor with the diffuse Hαemission. This evidence indicates that the stellar populationresponsible for the heating of dust emitting in the mid-IR is similar tothat heating big grains emitting in the far-IR, including relativelyevolved stars responsible for the non-ionizing radiation. The scatter inthe mid-IR vs. Hα, UV and far-IR luminosity relation is mostly dueto metallicity effects, with metal-poor objects having a lower mid-IRemission per unit star formation rate than metal-rich galaxies. Ouranalysis indicates that the mid-IR luminosity is not an optimal starformation tracer in normal, late-type galaxies.
| 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
| 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.
| Evolutionary spectral energy distribution diagnostics of starburst galaxies: signature of bimodality
We construct an evolutionary spectral energy distribution (SED) model ofa starburst region, from the ultraviolet to submillimetre wavelengths.This model allows us to derive the star formation rate, optical depth bydust and apparent effective radius of starburst regions at variouswavelengths; as a result, the intrinsic surface brightness of starburstregions can be derived. Using this SED model, we analyse 16ultraviolet-selected starburst galaxies and 10 ultraluminous infraredgalaxies. The derived star formation rates and optical depths arecompared with emission-line measurements and are found to be consistent.The derived apparent effective radii are also consistent withobservations. From the SED analysis, we find a bimodal property of thestar formation rate with the optical depth and the compactness ofstellar distributions. While mild starbursts have a limiting intrinsicsurface brightnessLbolr-2e~= 1012Lsolar kpc-2, intense starbursts tend to be moreheavily obscured and concentrated within a characteristic scale ofre~= 0.3 kpc. We suggest that the mild starbursts can betriggered by a self-gravitating disc instability in which feedback iseffective in the shallow gravitational potential. On the other hand, theintense starbursts can be induced via an external dynamical perturbationsuch as galaxy merging, in which feedback is less effective owing to thedeep gravitational potential attained by the large gas concentrationwithin the central starburst region.
| The Contribution of H I-rich Galaxies to the Damped Lyα Absorber Population at z = 0
We present a study of the expected properties of the low-redshift dampedLyα absorber population determined from a sample of H I-selectedgalaxies in the local universe. Because of a tight correlation betweenthe H I mass and H I cross section, which we demonstrate spans allgalaxy types, we can use our H I-selected sample to predict theproperties of the absorption-line systems. We use measurements of thenumber density and H I cross section of galaxies to show that the totalH I cross section at column densities sufficient to produce dampedLyα absorption is consistent with no evolution of the absorberpopulation. We also find that the dN/dz distribution is dominated bygalaxies with H I masses near 109 Msolar. However,because of the large dispersion in the correlation between H I mass andstellar luminosity, we find that the distribution of dN/dz as a functionof LJ is fairly flat. In addition, we examine the line widthsof the H I-selected galaxies and show that there may be evolution in thekinematics of H I-rich galaxies, but it is not necessary for the higherredshift population to contain a greater proportion of high-massgalaxies than we find locally.
| Star formation rate in galaxies from UV, IR, and Hα estimators
Infrared (IR) luminosity of galaxies originating from dust thermalemission can be used as an indicator of the star formation rate (SFR).Inoue et al. (\cite{inoue00}, IHK) have derived a formula for theconversion from dust IR luminosity to SFR by using the following threequantities: the fraction of Lyman continuum luminosity absorbed by gas(f), the fraction of UV luminosity absorbed by dust (epsilon ), and thefraction of dust heating from old (ga 108 yr) stellarpopulations (eta ). We develop a method to estimate those threequantities based on the idea that the various way of SFR estimates fromultraviolet (UV) luminosity (2000 Å luminosity), Hαluminosity, and dust IR luminosity should return the same SFR. Afterapplying our method to samples of galaxies, the following results areobtained in our framework. First, our method is applied to a sample ofstar-forming galaxies, finding that f ~ 0.6, epsilon ~ 0.5, and eta ~0.4 as representative values. Next, we apply the method to a starburstsample, which shows larger extinction than the star-forming galaxysample. With the aid of f, epsilon , and eta , we are able to estimatereliable SFRs from UV and/or IR luminosities. Moreover, the Hαluminosity, if the Hα extinction is corrected by using the Balmerdecrement, is suitable for a statistical analysis of SFR, because thesame {correction factor for the Lyman continuum extinction (i.e. 1/f)}is applicable to both normal and starburst galaxies over all the rangeof SFR. The metallicity dependence of f and epsilon is also tested:Only the latter proves to have a correlation with metallicity. As anextension of our result, the local (z=0) comoving density of SFR can beestimated with our dust extinction corrections. We show that all UV,Hα , and IR comoving luminosity densities at z=0 give a consistentSFR per comoving volume ( ~ 3x 10-2h M_sun yr-1Mpc-3). Useful formulae for SFR estimate are listed.Tables 1 and 2, and Appendix A are only available in electronic form athttp://www.edpsciences.org
| 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.
| A Hubble Space Telescope Survey of the Mid-Ultraviolet Morphology of Nearby Galaxies
We present a systematic imaging survey of 37 nearby galaxies observedwith the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2(WFPC2) in the mid-UV F300W filter, centered at 2930 Å, as well asin the I-band (F814W) filter at 8230 Å. Eleven of these galaxieswere also imaged in the F255W filter, centered at 2550 Å. Oursample is carefully selected to include galaxies of sufficiently smallradius and high predicted mid-UV surface brightness to be detectablewith WFPC2 in one orbit and covers a wide range of Hubble types andinclinations. The mid-UV (2000-3200 Å) spans the gap betweenground-based UBVR(IJHK) images, which are available or were acquired forthe current study, and far-UV images available from the Astro/UITmissions for 15 galaxies in our sample. The first qualitative resultsfrom our study are as follows:1. Early-type galaxies show a significantdecrease in surface brightness going from the red to the mid-UV,reflecting the absence of a dominant young stellar population and insome cases the presence of significant (central) dust lanes. Galaxiesthat are early types in the optical show a variety of morphologies inthe mid-UV that can lead to a different morphological classification,although not necessarily as later type. Some early-type galaxies becomedominated by a blue nuclear feature or a point source in the mid-UV,e.g., as a result of the presence of a Seyfert nucleus or a LINER. Thisis in part due to our mid-UV surface brightness selection, but it alsosuggests that part of the strong apparent evolution of weak AGNs inearly-type galaxies may be due to surface brightness dimming of theirUV-faint stellar population, which renders the early-type host galaxiesinvisible at intermediate to higher redshifts.2. About half of themid-type spiral and star-forming galaxies appear as a latermorphological type in the mid-UV, as Astro/UIT also found primarily inthe far-UV. Sometimes these differences are dramatic (e.g., NGC 6782shows a spectacular ring of hot stars in the mid-UV). However, not allmid-type spiral galaxies look significantly different in the mid-UV.Their mid-UV images show a considerable range in the scale and surfacebrightness of individual star-forming regions. Almost without exception,the mid-type spirals in our sample have their small bulges bisected by adust lane, which often appears to be connected to the inner spiral armstructure.3. The majority of the heterogeneous subset of late-type,irregular, peculiar, and merging galaxies display F300W morphologiesthat are similar to those seen in F814W, but with important differencesdue to recognizable dust features absorbing the bluer light and to hotstars, star clusters, and star formation ``ridges'' that are bright inthe mid-UV. Less than one-third of the galaxies classified as late typein the optical appear sufficiently different in the mid-UV to result ina different classification.Our HST mid-UV survey of nearby galaxiesshows that, when observed in the rest-frame mid-UV, early- to mid-typegalaxies are more likely to be misclassified as later types thanlate-type galaxies are to be misclassified as earlier types. This isbecause the later type galaxies are dominated by the same young and hotstars in all filters from the mid-UV to the red and so have a smaller``morphological K-correction'' than true earlier type galaxies. Themorphological K-correction can thus explain part, but certainly not all,of the excess faint blue late-type galaxies seen in deep HST fields.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS 5-26555. Also based in part onobservations made with the Vatican Advanced Technology Telescope: theAlice P. Lennon Telescope and the Thomas J. Bannan AstrophysicsFacility.
| The Low Metallicity ISM of Dwarf Galaxies
The properties of the low metallicity environments of dwarf galaxies arestudied through dust observations in conjunction with the FIRfine-structure cooling lines. There is a striking enhancement of theI[CII]/I(CO) in dwarf galaxies that is explained by the decreasedattenuation of the UV light in molecular clouds. An importantconsequence is that a significant mass of the molecular gas massinventory can be missed through CO observations alone. Modeling theinfrared spectral energy distribution into submillimeter wavelengths indwarf galaxies reveals the presence of very cold ( 8K) dust,whichaccounts for a large fraction of the dust mass, until now missed bymodels using IRAS observations alone. In spite of the strikingdefficiency of the mid-infrared aromatic band carriers, cooling in thephotodissociation regions, via [CII] line emission is a very efficientprocess.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
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