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Is HCN a True Tracer of Dense Molecular Gas in Luminous and Ultraluminous Infrared Galaxies?
We present the results of the first HCO+ survey probing thedense molecular gas content of a sample of 16 luminous and ultraluminousinfrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN (1-0)observations, had shown that LIRGs and ULIRGs possess a significantlyhigher fraction of dense molecular gas compared to normal galaxies.While the picture issued from HCO+ partly confirms thisresult, we have discovered an intriguing correlation between the HCN(1-0)/HCO+ (1-0) luminosity ratio and the IR luminosity ofthe galaxy (LIR). This trend casts doubts on the use of HCNas an unbiased quantitative tracer of the dense molecular gas content inLIRGs and ULIRGs. A plausible scenario explaining the observed trendimplies that X-rays coming from an embedded active galactic nucleus mayplay a dominant role in the chemistry of molecular gas atLIR>=1012 Lsolar. We discuss theimplications of this result for the understanding of LIRGs, ULIRGs, andhigh-redshift gas-rich galaxies.
| The relation between mergers and AGN activity. Results from radio galaxy and luminous infrared galaxy studies
There is morphological evidence that the activity in powerful radiogalaxies could be triggered by mergers and galaxy interactions. However,nothing is known about the timescales, order of events, and the type ofinteraction involved. It is not yet known whether there exists anevolutionary link between radio galaxies and other merger systems suchas very luminous and ultraluminous infrared galaxies (VLIRGs andULIRGs). Here, we report preliminary results obtained from the analysisof near-ultraviolet and optical spectroscopic observations of samples ofRadio Galaxies and VLIRGs-ULIRGs to investigate, through age-dating oftheir young stellar population, whether an evolutionary link existsbetween VLIRGs/ULIRGs-Radio Galaxies-normal elliptical galaxies. Theseresults will help to understand the genesis events that lead to theformation of radio jet and quasar activity, and they will allow us toplace radio galaxies in the context of hierarchical evolution models forthe population of giant elliptical galaxies.
| SINFONI adaptive optics integral field spectroscopy of the Circinus Galaxy
Aims.We investigate the star formation activity and the gas and stellardynamics on scales of a few parsecs in the nucleus of the CircinusGalaxy. Methods: .Using the adaptive optics near infrared integralfield spectrometer SINFONI on the VLT, we have obtained observations ofthe Circinus galaxy on scales of a few parsecs and at a spectralresolution of 70 km s-1 FWHM. The physical properties of thenucleus are analyzed by means of line and velocity maps extracted fromthe SINFONI datacube. Starburst models are constrained using theBrγ flux, stellar continuum (as traced via the CO absorptionbandheads longward of 2.3 μm), and radio continuum. Results:.The similarity of the morphologies of the H2 1-0 S(1) 2.12 μm andBrγ 2.17 μm lines to the stellar continuum and also theirkinematics, suggest a common origin in star formation. Within 8 pc ofthe AGN we find there has been a recent starburst in the last 100 Myr,which currently accounts for 1.4% of the galaxy's bolometric luminosity.The similarity of the spatial scales over which the stars and gas existindicates that this star formation is occuring within the torus; andcomparison of the gas column density through the torus to the maximumpossible optical depth to the stars implies the torus is a clumpymedium. The coronal lines show asymmetric profiles with a spatiallycompact narrow component and a spatially extended blue wing. Thesecharacteristics are consistent with some of the emission arising inclouds gravitationally bound to the AGN, and some outflowing incloudlets which have been eroded away from the bound clouds.
| 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.
| Outflows in Infrared-Luminous Starbursts at z < 0.5. II. Analysis and Discussion1,
We have performed an absorption-line survey of outflowing gas in 78starburst-dominated, infrared-luminous galaxies. This is the largeststudy of superwinds at z<~3. Superwinds are found in almost allinfrared-luminous galaxies, and changes in detection rate with SFR-windsare found twice as often in ultraluminous infrared galaxies (ULIRGs) asin less-luminous galaxies-reflect different wind geometries. The maximumvelocities we measure are 600 km s-1, though most of theoutflowing gas has lower velocities (100-200 km s-1). (Onegalaxy has velocities exceeding 1000 km s-1.) Velocities inLINERs are higher than in H II galaxies, and outflowing ionized gasoften has higher velocities than the neutral gas. Wind properties(velocity, mass, momentum, and energy) scale with galaxy properties(SFR, luminosity, and galaxy mass), consistent with ram-pressure drivingof the wind. Wind properties increase strongly with increasing galacticmass, contrary to expectation. These correlations flatten at high SFR(>~10-100 Msolar yr-1), luminosities, andmasses. This saturation is due to a lack of gas remaining in the wind'spath, a common neutral gas terminal velocity, and/or a decrease in theefficiency of thermalization of the supernovae energy. It means thatmass entrainment efficiency, rather than remaining constant, declines ingalaxies with SFR>10 Msolar yr-1 andMK<-24. Half of our sample consists of ULIRGs, which hostas much as half of the star formation in the universe at z>~1. Thepowerful, ubiquitous winds we observe in these galaxies imply thatsuperwinds in massive galaxies at redshifts above unity play animportant role in the evolution of galaxies and the intergalacticmedium.Some of the observations reported here were obtained at the MMTObservatory, which is a joint facility of the Smithsonian Institutionand the University of Arizona.Some of the observations reported here were obtained at the Kitt PeakNational Observatory, National Optical Astronomy Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc. (AURA), under cooperative agreement with the National ScienceFoundation.
| Outflows in Infrared-Luminous Starbursts at z < 0.5. I. Sample, Na I D Spectra, and Profile Fitting1,
We have conducted a spectroscopic survey of 78 starburstinginfrared-luminous galaxies at redshifts up to z=0.5. We usemoderate-resolution spectroscopy of the Na I D interstellar absorptionfeature to directly probe the neutral phase of outflowing gas in thesegalaxies. Over half of our sample are ultraluminous infrared galaxiesthat are classified as starbursts; the rest have infrared luminositiesin the range log(LIR/Lsolar)=10.2-12.0. The sampleselection, observations, and data reduction are described here. Theabsorption-line spectra of each galaxy are presented. We also discussthe theory behind absorption-line fitting in the case of a partiallycovered, blended absorption doublet observed at moderate-to-highresolution, a topic neglected in the literature. A detailed analysis ofthese data is presented in a companion paper.Some of the observations reported here were obtained at the MMTObservatory, a joint facility of the Smithsonian Institution and theUniversity of Arizona.Some of the observations reported here were obtained at the Kitt PeakNational Observatory, National Optical Astronomy Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc. (AURA), under cooperative agreement with the National ScienceFoundation.
| EGRET Upper Limits and Stacking Searches of Gamma-Ray Observations of Luminous and Ultraluminous Infrared Galaxies
We present a stacking analysis of EGRET γ-ray observations at thepositions of luminous and ultraluminous infrared galaxies. The latterwere selected from the recently presented HCN survey, which is thoughtto contain the most active star-forming regions of the universe.Different sorting criteria are used, and since there is no positivecollective detection of γ-ray emission from these objects, wedetermined both collective and individual upper limits. The uppermostexcess we find appears in the case of ULIRGs ordered by redshift, at avalue of 1.8 σ.
| HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies
We report systematic HCN J=1-0 (and CO) observations of a sample of 53infrared (IR) and/or CO-bright and/or luminous galaxies, including sevenultraluminous infrared galaxies, nearly 20 luminous infrared galaxies,and more than a dozen of the nearest normal spiral galaxies. This is thelargest and most sensitive HCN survey of galaxies to date. All galaxiesobserved so far follow the tight correlation between the IR luminosityLIR and the HCN luminosity LHCN initially proposedby Solomon, Downes, & Radford, which is detailed in a companionpaper. We also address here the issue of HCN excitation. There is noparticularly strong correlation between LHCN and the 12 μmluminosity; in fact, of all the four IRAS bands, the 12 μm luminosityhas the weakest correlation with the HCN luminosity. There is also noevidence of stronger HCN emission or a higher ratio of HCN and COluminosities LHCN/LCO for galaxies with excess 12μm emission. This result implies that mid-IR radiative pumping, orpopulating, of the J=1 level of HCN by a mid-IR vibrational transitionis not important compared with the collisional excitation by densemolecular hydrogen. Furthermore, large velocity gradient calculationsjustify the use of HCN J=1-0 emission as a tracer of high-densitymolecular gas (>~3×104/τcm-3) andgive an estimate of the mass of dense molecular gas from HCNobservations. Therefore, LHCN may be used as a measure of thetotal mass of dense molecular gas, and the luminosity ratioLHCN/LCO may indicate the fraction of moleculargas that is dense.
| 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).
| 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).
| The Star Formation Rate and Dense Molecular Gas in Galaxies
HCN luminosity is a tracer of dense molecular gas,n(H2)>~3×104cm-3, associatedwith star-forming giant molecular cloud (GMC) cores. We present theresults and analysis of our survey of HCN emission from 65 infraredgalaxies, including nine ultraluminous infrared galaxies (ULIGs,LIR>~1012Lsolar), 22 luminousinfrared galaxies (LIGs,1011Lsolar0.06 are LIGs or ULIGs. Normal spiralsall have similar and low dense gas fractionsLHCN/LCO=0.02-0.05. The global star formationefficiency depends on the fraction of the molecular gas in a densephase.
| [O II] as a Star Formation Rate Indicator
We investigate the [O II] emission line as a star formation rate (SFR)indicator using integrated spectra of 97 galaxies from the Nearby FieldGalaxies Survey (NFGS). The sample includes all Hubble types andcontains SFRs ranging from 0.01 to 100 Msolaryr-1. We compare the Kennicutt [O II] and Hα SFRcalibrations and show that there are two significant effects thatproduce disagreement between SFR([O II]) and SFR(Hα): reddeningand metallicity. Differences in the ionization state of the interstellarmedium do not contribute significantly to the observed differencebetween SFR([O II]) and SFR(Hα) for the NFGS galaxies withmetallicities log(O/H)+12>~8.5. The Kennicutt [O II]-SFR relationassumes a typical reddening for nearby galaxies; in practice, thereddening differs significantly from sample to sample. We derive a newSFR([O II]) calibration that does not contain a reddening assumption.Our new SFR([O II]) calibration also provides an optional correction formetallicity. Our SFRs derived from [O II] agree with those derived fromHα to within 0.03-0.05 dex. We show that the reddening, E(B-V),increases with intrinsic (i.e., reddening-corrected) [O II] luminosityfor the NFGS sample. We apply our SFR([O II]) calibration withmetallicity correction to two samples: high-redshift 0.8
| 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.
| The Second Kiso Survey for ultraviolet-excess galaxies. IV
The catalogue list and the identification chart of ultraviolet(UV)-excess galaxies which have been detected on two-color Kiso Schmidtplates are presented for 10 Schmidt fields. Catalogued are 189 objects,down to the photographic magnitude ~18.0 in the sky area of some 300square degrees. The total number of KUGs newly detected in the secondsurvey reaches 1,829.
| Objects from a Cross-Identification of the IRAS and Low-Frequency Texas Radio Catalogs
A cross-identification of objects in the low-frequency (365 MHz) Texasradio catalog and in IRAS catalogs at four infrared wavelengths hasyielded a list of 715 objects for further studies. Objects with steepspectra for which the difference in the centers of gravity of the radioand infrared sources was less than 3″ were selected from thislist. Seventeen of the objects have been observed at six wavelengthsusing the RATAN-600 radio telescope. Spectra of nine objects from theinitial list for which there were candidate optical counterparts wereobtained using the 2.1-m telescope of the INAOE. The results of theseobservations are discussed. The presence of steep spectral indices forthe radio sources is confirmed. The possible optical counterpartsinclude interacting galaxies, an infrared galaxy, two emission-linegalaxies, and a candidate BL Lac object. Optical images of the opticalcounterparts are presented together with radio and optical spectra.
| Infrared Emission of Normal Galaxies from 2.5 to 12 Micron: Infrared Space Observatory Spectra, Near-Infrared Continuum, and Mid-Infrared Emission Features
We present ISOPHOT spectra of the regions 2.5-4.9 μm and 5.8-11.6μm for a sample of 45 disk galaxies from the US Infrared SpaceObservatory Key Project on Normal Galaxies. The galaxies were selectedto span the range in global properties of normal, star-forming diskgalaxies in the local universe. The spectra can be decomposed into threespectral components: (1) continuum emission from stellar photospheres,which dominates the near-infrared (NIR; 2.5-4.9 μm) spectral region;(2) a weak NIR excess continuum, which has a color temperature of~103 K, carries a luminosity of a few percent of the totalfar-infrared (FIR) dust luminosity LFIR and most likelyarises from the interstellar medium (ISM); and (3) the well-known broademission features at 6.2, 7.7, 8.6, and 11.3 μm, which are generallyattributed to aromatic carbon particles. These aromatic features inemission (AFEs) dominate the mid-infrared (MIR; 5.8-11.6 μm) part ofthe spectrum and resemble the so-called type A spectra observed in manynonstellar sources and the diffuse ISM in our own Galaxy. The fewnotable exceptions include NGC 4418, where a dust continuum replaces theAFEs in MIR, and NGC 1569, where the AFEs are weak and the strongestemission feature is [S IV] 10.51 μm. The relative strengths of theAFEs vary by 15%-25% among the galaxies. However, little correlation isseen between these variations and either IRAS 60 μm/100 μm fluxdensity ratio R(60/100) or the FIR/blue luminosity ratioLFIR/LB, two widely used indicators of the currentstar formation activity, suggesting that the observed variations are nota consequence of the radiation field differences among the galaxies. Wedemonstrate that the NIR excess continuum and AFE emission arecorrelated, suggesting that they are produced by similar mechanisms andsimilar (or the same) material. On the other hand, as the current starformation activity increases, the overall strengths of the AFEs and theNIR excess continuum drop significantly with respect to that of the FIRemission from large dust grains. In particular, the summed luminosity ofthe AFEs falls from ~0.2 LFIR for the most ``IR-quiescent''galaxies to ~0.1 LFIR for the most ``IR-active'' galaxies.This is likely a consequence of the preferential destruction in intenseradiation fields of the small carriers responsible for the NIR/AFEemission.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries, France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| The Hα and Infrared Star Formation Rates for the Nearby Field Galaxy Survey
We investigate the Hα and infrared star formation rate (SFR)diagnostics for galaxies in the Nearby Field Galaxy Survey (NFGS). Forthe 81 galaxies in our sample, we derive Hα fluxes (included here)from integrated spectra. There is a strong correlation between the ratioof far-infrared to optical luminosities L(FIR)/L(Hα) and theextinction E(B-V) measured with the Balmer decrement. Before reddeningcorrection, the SFR(IR) and SFR(Hα) are related to each other by apower law: SFR(IR)=(2.7+/-0.3)SFR(Hα)1.30+/-0.06.Correction of the SFR(Hα) for extinction using the Balmerdecrement and a classical reddening curve both reduces the scatter inthe SFR(IR)-SFR(Hα) correlation and results in a much closeragreement between the two SFR indicators;SFR(IR)=(0.91+/-0.04)SFR(Hαcorr)1.07+/-0.03.SFR(IR) and SFR(Hα) agree to ~10%. This SFR relationship spans 4orders of magnitude and holds for all Hubble types with IRAS detectionsin the NFGS. A constant ratio between the SFR(IR) and SFR(Hα) forall Hubble types, including early types (S0-Sab), suggests that the IRemission in all these objects results from a young stellar population.
| Hot dust in normal star-forming galaxies: JHKL' photometry of the ISO Key Project sample
We present JHK and 3.8 mu m (L') photometry of 26 galaxies in theInfrared Space Observatory (ISO) Normal Galaxy Key Project (KP) sampleand of seven normal ellipticals with the aim of investigating the originof the 4 mu m emission. The majority of the KP galaxies, and all theellipticals, have K-L<~ 1.0, consistent with stellar photospheresplus moderate dust extinction. Ten of the 26 KP galaxies have K-L>~1.0, corresponding to a flat or rising 4 mu m continuum, consistent withsignificant emission from hot dust at 600-1000 K. K-L is anticorrelatedwith ISO flux ratio F6.75/F15, weakly correlatedwith line ratio [O I]/[C II], but not with [C II]/FIR or IRAS ratioF60/F100. Photodissociation-region models forthese galaxies show that the hot dust responsible for red K-L resides inregions of high pressure and intense far-ultraviolet radiation field.Taken together, these results suggest that star formation in normalstar-forming galaxies can assume two basic forms: an ``active'',relatively rare, mode characterized by hot dust, suppressed AromaticFeatures in Emission (AFEs), high pressure, and intense radiation field;and the more common ``passive'' mode that occurs under more quiescentphysical conditions, with AFEs, and without hot dust. The occurrence ofthese modes appears to only weakly depend on the star-formation rate perunit area. Passive star formation over large scales makes up the bulk ofstar-forming activity locally, while the ``active'' regime may dominateat high redshifts. Based on data obtained at TIRGO, Gornergrat,Switzerland.
| Far-Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium
The most important cooling lines of the neutral interstellar medium(ISM) lie in the far-infrared (FIR). We present measurements by theInfrared Space Observatory Long Wavelength Spectrometer of seven linesfrom neutral and ionized ISM of 60 normal, star-forming galaxies. Thegalaxy sample spans a range in properties such as morphology, FIR colors(indicating dust temperature), and FIR/blue ratios (indicating starformation activity and optical depth). In two-thirds of the galaxies inthis sample, the [C II] line flux is proportional to FIR dust continuum.The other one-third show a smooth decline inL[CII]/LFIR with increasing Fν(60μm)/Fν(100 μm) and LFIR/LB,spanning a range of a factor of more than 50. Two galaxies at the warmand active extreme of the range haveL[CII]/LFIR<2×10-4 (3 σupper limit). This is due to increased positive grain charge in thewarmer and more active galaxies, which leads to less efficient heatingby photoelectrons from dust grains. The ratio of the two principalphotodissociation region (PDR) cooling linesL[OI]/L[CII] shows a tight correlation withFν(60 μm)/Fν(100 μm), indicating thatboth gas and dust temperatures increase together. We derive atheoretical scaling between [N II] (122 μm) and [C II] from ionizedgas and use it to separate [C II] emission from neutral PDRs and ionizedgas. Comparison of PDR models of Kaufman et al. with observed ratios of(1) L[OI]/L[CII] and(L[CII]+L[OI])/LFIR and (2)L[OI]/LFIR and Fν(60μm)/Fν(100 μm) yields far-UV flux G0 andgas density n. The G0 and n values estimated from the twomethods agree to better than a factor of 2 and 1.5, respectively, inmore than half the sources. The derived G0 and n correlatewith each other, and G0 increases with n asG0~nα, where α~1.4 . We interpret thiscorrelation as arising from Strömgren sphere scalings if much ofthe line and continuum luminosity arises near star-forming regions. Thehigh values of PDR surface temperature (270-900 K) and pressure(6×104-1.5×107 K cm-3)derived also support the view that a significant part of grain and gasheating in the galaxies occurs very close to star-forming regions. Thedifferences in G0 and n from galaxy to galaxy may be due todifferences in the physical properties of the star-forming clouds.Galaxies with higher G0 and n have larger and/or denserstar-forming clouds.
| Radio Properties of Infrared-selected Galaxies in the IRAS 2 Jy Sample
The radio counterparts to the IRAS Redshift Survey galaxies areidentified in the NRAO VLA Sky Survey (NVSS) catalog. Our new catalog ofthe IR flux-limited (S60μm>=2 Jy) complete sample of1809 galaxies lists accurate radio positions, redshifts, and 1.4 GHzradio and IRAS fluxes. This sample is 6 times larger in size and 5 timesdeeper in redshift coverage (to z~0.15) than those used in earlierstudies of the radio and far-IR (FIR) properties of galaxies in thelocal volume. The well-known radio-FIR correlation is obeyed by theoverwhelming majority (>=98%) of the IR-selected galaxies, and theradio AGNs identified by their excess radio emission constitute onlyabout 1% of the sample, independent of the IR luminosity. TheseFIR-selected galaxies can account for the entire population of late-typefield galaxies in the local volume, and their radio continuum may beused directly to infer the extinction-free star formation rate in mostcases. Both the 1.4 GHz radio and 60 μm IR luminosity functions arereasonably well described by linear sums of two Schechter functions, onerepresenting normal, late-type field galaxies and the other representingstarbursts and other luminous IR galaxies. The integrated FIR luminositydensity for the local volume is (4.8+/-0.5)×107Lsolar Mpc-3, less than 10% of which iscontributed by the luminous IR galaxies withLFIR>=1011 Lsolar. The inferredextinction-free star formation density for the local volume is0.015+/-0.005 Msolar yr-1 Mpc-3.
| Starbursts versus Truncated Star Formation in Nearby Clusters of Galaxies
We present long-slit spectroscopy, B- and R-bandpass imaging, and 21 cmobservations of a sample of early-type galaxies in nearby clusters,which are known to be either in a star-forming phase or to have had starformation that recently terminated. From the long-slit spectra, obtainedwith the Blanco 4 m telescope, we find that emission lines in thestar-forming cluster galaxies are significantly more centrallyconcentrated than in a sample of field galaxies. The broadband imagingreveals that two currently star-forming early-type galaxies in thePegasus I cluster have blue nuclei, again indicating that recent starformation has been concentrated. In contrast, the two galaxies for whichstar formation has already ended show no central color gradient. ThePegasus I galaxy with the most evident signs of ongoing star formation(NGC 7648), exhibits signatures of a tidal encounter. Neutral hydrogenobservations of that galaxy with the Arecibo radio telescope reveal thepresence of ~4×108 Msolar of H I. Areciboobservations of other current or recent star-forming early-type galaxiesin Pegasus I indicate smaller amounts of gas in one of them, and onlyupper limits in others. These observations indicate that NGC 7648 in thePegasus I cluster owes its present star formation episode to some formof tidal interaction. The same may be true for the other galaxies withcentralized star formation, but we cannot rule out the possibility thattheir outer disks have been removed via ram pressure stripping, followedby rapid quenching of star formation in the central region.
| Infrared to millimetre photometry of ultra-luminous IR galaxies: New evidence favouring a 3-stage dust model
Infrared to millimetre spectral energy distributions (SEDs) have beenobtained for 41 bright ultra-luminous infrared galaxies (ULIRGs). Theobservations were carried out with ISOPHOT between 10 and 200 mu m andsupplemented for 16 sources with JCMT/SCUBA at 450 and 850 mu m and withSEST at 1.3 mm. In addition, seven sources were observed at 1.2 and 2.2mu m with the 2.2 m telescope on Calar Alto. These new SEDs representthe most complete set of infrared photometric templates obtained so faron ULIRGs in the local universe. The SEDs peak at 60-100 mu m and showoften a quite shallow Rayleigh-Jeans tail. Fits with one single modifiedblackbody yield a high FIR opacity and small dust emissivity exponentbeta < 2. However, this concept leads to conflicts with several otherobservational constraints, like the low PAH extinction or the extendedfilamentary optical morphology. A more consistent picture is obtainedusing several dust components with beta = 2, low to moderate FIR opacityand cool (50 K > T > 30 K) to cold (30 K > T > 10 K)temperatures. This provides evidence for two dust stages, the coolstarburst dominated one and the cold cirrus-like one. The third stagewith several hundred Kelvin warm dust is identified in the AGN dominatedULIRGs, showing up as a NIR-MIR power-law flux increase. While AGNs andSBs appear indistinguishable at FIR and submm wavelengths, they differin the NIR-MIR. This suggests that the cool FIR emitting dust is notrelated to the AGN, and that the AGN only powers the warm and hot dust.In comparison with optical and MIR spectroscopy, a criterion based onthe SED shapes and the NIR colours is established to reveal AGNs amongULIRGs. Also the possibility of recognising evolutionary trends amongthe ULIRGs via the relative amounts of cold, cool and warm dustcomponents is investigated. Based on observations with the InfraredSpace Observatory ISO, the James Clerk Maxwell Telescope JCMT, theSwedish ESO Submillimetre Telescope SEST and at the Calar AltoObservatory. ISO is an ESA project with instruments funded by ESA MemberStates (especially the PI countries France, Germany, The Netherlands andthe UK) and with the participation of ISAS and NASA. Appendices A and Bare only available in electronic form at http://www.edpsciences.com
| The SCUBA Local Universe Galaxy Survey - I. First measurements of the submillimetre luminosity and dust mass functions
This is the first of a series of papers presenting results from theSCUBA Local Universe Galaxy Survey (SLUGS), the first statistical surveyof the submillimetre properties of the local Universe. As the initialpart of this survey, we have used the SCUBA camera on the James ClerkMaxwell Telescope to observe 104 galaxies from the IRAS Bright GalaxySample. We present here the 850-μm flux measurements. The 60-, 100-,and 850-μm flux densities are well fitted by single-temperature dustspectral energy distributions, with the sample mean and standarddeviation for the best-fitting temperature beingTd=35.6+/-4.9K and for the dust emissivity indexβ=1.3+/-0.2. The dust temperature was found to correlate with60-μm luminosity. The low value of β may simply mean that thesegalaxies contain a significant amount of dust that is colder than thesetemperatures. We have estimated dust masses from the 850-μm fluxesand from the fitted temperature, although if a colder component ataround 20K is present (assuming a β of 2), then the estimated dustmasses are a factor of 1.5-3 too low. We have made the first directmeasurements of the submillimetre luminosity function (LF) and of thedust mass function. Unlike the IRAS 60-μm LF, these are well fittedby Schechter functions. The slope of the 850-μm LF at lowluminosities is steeper than -2, implying that the LF must flatten atluminosities lower than we probe here. We show that extrapolating the60-μm LF to 850μm using a single temperature and β does notreproduce the measured submillimetre LF. A population of `cold' galaxies(Td<25K) emitting strongly at submillimetre wavelengthswould have been excluded from the 60-μm-selected sample. If suchgalaxies do exist, then this estimate of the 850-μm flux is biased(it is underestimated). Whether such a population does exist is unknownat present. We correlate many of the global galaxy properties with theFIR/submillimetre properties. We find that there is a tendency for lessluminous galaxies to contain hotter dust and to have a greater starformation efficiency (cf. Young). The average gas-to-dust ratio for thesample is 581+/-43 (using both the atomic and molecular hydrogen), whichis significantly higher than the Galactic value of 160. We believe thatthis discrepancy is probably due to a `cold dust' component atTd<=20K in our galaxies. There is a surprisingly tightcorrelation between dust mass and the mass of molecular hydrogen,estimated from CO measurements, with an intrinsic scatter of ~=50percent.
| Spectrophotometry of Nearby Field Galaxies: The Data
We have obtained integrated and nuclear spectra as well as U, B, Rsurface photometry for a representative sample of 196 nearby galaxies.These galaxies span the entire Hubble sequence in morphological type, aswell as a wide range of luminosities (MB=-14 to -22). Here wepresent the spectrophotometry for these galaxies. The selection of thesample and the U, B, R surface photometry is described in a companionpaper. Our goals for the project include measuring the current starformation rates and metallicities of these galaxies, and elucidatingtheir star formation histories, as a function of luminosity andmorphology. We thereby extend the work of Kennicutt to lower luminositysystems. We anticipate that our study will be useful as a benchmark forstudies of galaxies at high redshift. We describe the observing, datareduction, and calibration techniques and demonstrate that ourspectrophotometry agrees well with that of Kennicutt. The spectra spanthe range 3550-7250 Å at a resolution (FWHM) of ~6 Å andhave an overall relative spectrophotometric accuracy of ~+/-6%. Wepresent a spectrophotometric atlas of integrated and nuclear rest-framespectra as well as tables of equivalent widths and synthetic colors. Theatlas and tables of measurements will be made available electronically.We study the correlations of galaxy properties determined from thespectra and images. Our findings include: (1) galaxies of a givenmorphological class display a wide range of continuum shapes andemission-line strengths if a broad range of luminosities are considered,(2) emission-line strengths tend to increase and continua tend to getbluer as the luminosity decreases, and (3) the scatter on the generalcorrelation between nuclear and integrated Hα emission-linestrengths is large.
| Surface Photometry of Nearby Field Galaxies: The Data
We have obtained integrated spectra and multifilter photometry for arepresentative sample of ~200 nearby galaxies. These galaxies span theentire Hubble sequence in morphological type, as well as a wide range ofluminosities (MB=-14 to -22) and colors (B-R=0.4-1.8). Herewe describe the sample selection criteria and the U, B, R surfacephotometry for these galaxies. The spectrophotometric results will bepresented in a companion paper. Our goals for the project includemeasuring the current star formation rates and metallicity of thesegalaxies, and elucidating their star formation histories, as a functionof luminosity and morphology. We thereby extend the work of Kennicutt tolower luminosity systems. We anticipate that our study will be useful asa benchmark for studies of galaxies at high redshift. We discuss theobserving, data reduction, and calibration techniques and show that ourphotometry agrees well with previous work in those cases in whichearlier data are available. We present an atlas of images, radialsurface brightness profiles, and color profiles as well as tables ofderived parameters. The atlas and tables of measurements will be madeavailable electronically. We study the correlations of galaxy propertiesdetermined from the galaxy images. Our findings include the following:(1) colors determined within the effective radius correlate better withmorphological type than with MB and (2) 50% of thelow-luminosity galaxies are bluest in their centers.
| Optical Spectral Signatures of Dusty Starburst Galaxies
We analyze the optical spectral properties of the complete sample ofVery Luminous Infrared Galaxies presented by Wu et al., and we find ahigh fraction (~50%) of spectra showing both a strong Hδ line inabsorption and relatively modest [O II] emission [e(a) spectra]. Thee(a) signature has been proposed as an efficient method to identifydusty starburst galaxies, and we study the star formation activity andthe nature of these galaxies, as well as the effects of dust on theirobserved properties. We examine their emission-line characteristics, inparticular their [O II]/Hα ratio, and we find this to be greatlyaffected by reddening. A search for AGN spectral signatures reveals thatthe e(a) galaxies are typically H II/LINER galaxies. We compare the starformation rates derived from the FIR luminosities with the estimatesbased on the Hα line and find that the values obtained from theoptical emission lines are a factor of 10-70 (Hα) and 20-140 ([OII]) lower than the FIR estimates (50-300 Msolaryr-1). We then study the morphological properties of the e(a)galaxies, looking for a near companion or signs of a merger/interaction.In order to explore the evolution of the e(a) population, we present anoverview of the available observations of e(a) galaxies in differentenvironments both at low and high redshift. Finally, we discuss the roleof dust in determining the e(a) spectral properties and we propose ascenario of selective obscuration in which the extinction decreases withthe stellar age.
| ISO Mid-Infrared Observations of Normal Star-Forming Galaxies: The Key Project Sample
We present mid-infrared maps and preliminary analysis for 61 galaxiesobserved with the ISOCAM instrument aboard the Infrared SpaceObservatory. Many of the general features of galaxies observed atoptical wavelengths-spiral arms, disks, rings, and bright knots ofemission-are also seen in the mid-infrared, except the prominent opticalbulges are absent at 6.75 and 15 μm. In addition, the maps are quitesimilar at 6.75 and 15 μm, except for a few cases where a centralstarburst leads to lower Iν(6.75μm)/Iν(15 μm) ratios in the inner region. We alsopresent infrared flux densities and mid-infrared sizes for thesegalaxies. The mid-infrared color Iν(6.75μm)/Iν(15 μm) shows a distinct trend with thefar-infrared color Iν(60 μm)/Iν(100μm). The quiescent galaxies in our sample [Iν(60μm)/Iν(100 μm)<~0.6] show Iν(6.75μm)/Iν(15 μm) near unity, whereas this ratio dropssignificantly for galaxies with higher global heating intensity levels.Azimuthally averaged surface brightness profiles indicate the extent towhich the mid-infrared flux is centrally concentrated, and provideinformation on the radial dependence of mid-infrared colors. Thegalaxies are mostly well resolved in these maps: almost half of themhave <10% of their flux in the central resolution element. Acomparison of optical and mid-infrared isophotal profiles indicates thatthe flux at 4400 Å near the optical outskirts of the galaxies isapproximately 8 (7) times that at 6.75 μm (15 μm), comparable toobservations of the diffuse quiescent regions of the Milky Way. Thispaper is based on observations with the Infrared Space Observatory(ISO). ISO is an ESA project with instruments funded by ESA memberstates (especially the PI countries: France, Germany, The Netherlands,and the United Kingdom) and with the participation of ISAS and NASA.
| New 8-13mum spectroscopy of luminous and ultraluminous infrared galaxies
New moderate-resolution mid-infrared spectroscopy at 10mum of 27infrared galaxies is presented. The galaxies have been chosen from three60-μm selected and one 12-μm selected complete flux-limitedcatalogues of galaxies; 17 of these sources haveL_IR(8-1000mum)>=5x10^11L_solar. A high-resolution spectrum of thesource Arp 299B1 is also presented. Combining these new results withpreviously published results, a nearly complete 60-μm selectedflux-limited subsample, with L_IR(8-1000mum)>=1.6x10^11L_solar, of 25galaxies is defined. Within this subsample, it is found that thedominant power source of infrared galaxies in the luminosity range1.6x10^11=10^12L_solar] withinthis subsample show evidence that an active galactic nucleus provides anenergetically important power source based on the detection of silicateabsorption in their mid-infrared spectra. The physical basis of apossible anticorrelation between the 11.3-μm feature equivalent widthand infrared light to molecular gas mass ratios is discussed.
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