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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.
| Atomic and Molecular Gas in Colliding Galaxy Systems. I. The Data
We present H I and CO (1-0) interferometric observations of 10comparable-mass interacting systems obtained at the Very Large Array(VLA) and the Owens Valley Radio Observatory (OVRO) millimeter array.The primary intent of this study is to investigate the response of coldgas during the early stages of collision of massive disk galaxies. Thesample sources are selected based on their luminosity(MB<=-19), projected separation (5-40 kpc), andsingle-dish CO (1-0) content (SCO>=20 Jy kms-1). These selection criteria result in a sample thatprimarily consists of systems in the early stages of an interaction or amerger. Despite this sample selection, 50% of the systems show long H Itidal tails indicative of a tidal disruption in a prograde orbit. Inaddition, all (4/4) of the infrared luminous pairs (LIRGs) in the sampleshow long H I tails, suggesting that the presence of a long H I tail canbe a possible signature of enhanced star formation activity in acollision of gas-rich galaxies. More than half of the groups show adisplacement of H I peaks from the stellar disks. The CO (1-0)distribution is generally clumpy and widely distributed, unlike in mostIR-selected late stage mergers-in fact, CO peaks are displaced from thestellar nucleus in 20% (4/18) of the galaxies with robust CO detection.H I and CO (1-0) position-velocity diagrams (PVDs) and rotation curvesare also presented, and their comparison with the numerical simulationanalyzed in Paper I show evidence for radial inflow and wide occurrencesof nuclear molecular rings. These results are further quantified byexamining physical and structural parameters derived in comparison withisolated systems in the BIMA SONG sample in our forthcoming paper.
| Simulating the Spitzer Mid-Infrared Color-Color Diagrams
We use a simple parameterization of the mid-IR spectra of a wide rangeof galaxy types in order to predict their distribution in the InfraredArray Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 μm and MultibandPhotometer for Spitzer 24 μm color-color diagrams. We distinguishthree basic spectral types by the energetically dominant component inthe 3-12 μm regime: stellar-dominated, polycyclic aromatichydrocarbon (PAH)-dominated, and continuum-dominated. We use a Markovchain Monte Carlo approach to arrive at a more systematic and robustrepresentation of the mid-IR spectra of galaxies than do moretraditional approaches. We find that IRAC color-color plots are wellsuited to distinguishing the above spectral types, while the addition of24 μm data allows us to suggest practical three-color cuts thatpreferentially select higher redshift sources of a specific type. Wecompare our simulations with the color-color plot obtained by theSpitzer First Look Survey and find reasonable agreement. Lastly, wediscuss other applications as well as future directions for this work.
| Cold and warm dust along a merging galaxy sequence
We investigate the cold and warm dust properties during galaxyinteractions using a merging galaxy sample ordered into a chronologicalsequence from pre- to post-mergers. Our sample comprises a total of 29merging systems selected to have far-infrared and submillimetreobservations. The submillimetre data are mainly culled from theliterature, while for five galaxies (NGC 3597, 3690, 6090, 6670 and7252) the submillimetre observations are presented here for the firsttime. We use the 100- to 850-μm flux density ratio,f100/f850, as a proxy for the mass fraction of thewarm and cold dust in these systems. We find evidence for an increase inf100/f850 along the merging sequence from early toadvanced mergers, and interpret this trend as an increase of the warmrelative to the cold dust mass. We argue that the two key parametersaffecting the f100/f850 flux ratio is the starformation rate and the dust content of individual systems relative tothe stars. Using a sophisticated model for the absorption andre-emission of the stellar ultraviolet radiation by dust, we show thatthese parameters can indeed explain both the increase and the observedscatter in f100/f850 along the merging galaxysequence. We also discuss our results under the hypothesis thatelliptical galaxies are formed via disc galaxy mergers.
| HST Observations of the Toomre Sequence of Merging Galaxies
We discuss our ongoing multi-instrument HST investigation of the nuclearregions of the 11 interacting and merging galaxies in the ToomreSequence. We are studying the nuclear kinematics using STIS (G750M)spectra, the nuclear stellar populations using STIS (G430L) spectra, andthe nuclear morphology using NICMOS and WFPC2 images. The results willprovide new insight into the physical processes that operate duringgalaxy interactions.
| 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).
| An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample
The importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies.
| 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
| The PDS versus Markarian starburst galaxies: comparing strong and weak IRAS emitter at 12 and 25 μm in the nearby Universe
The characteristics of the starburst galaxies from the Pico dos Diassurvey (PDS) are compared with those of the nearby ultraviolet (UV)bright Markarian starburst galaxies, having the same limit in redshift(vh < 7500 km s-1) and absolute B magnitude(MB < -18). An important difference is found: theMarkarian galaxies are generally undetected at 12 and 25 μm in IRAS.This is consistent with the UV excess shown by these galaxies andsuggests that the youngest star-forming regions dominating thesegalaxies are relatively free of dust.The far-infrared selection criteria for the PDS are shown to introduce astrong bias towards massive (luminous) and large size late-type spiralgalaxies. This is contrary to the Markarian galaxies, which are found tobe remarkably rich in smaller size early-type galaxies. These resultssuggest that only late-type spirals with a large and massive disc arestrong emitters at 12 and 25 μm in IRAS in the nearby Universe.The Markarian and PDS starburst galaxies are shown to share the sameenvironment. This rules out an explanation of the differences observedin terms of external parameters. These differences may be explained byassuming two different levels of evolution, the Markarian being lessevolved than the PDS galaxies. This interpretation is fully consistentwith the disc formation hypothesis proposed by Coziol et al. to explainthe special properties of the Markarian SBNG.
| A Hubble Space Telescope WFPC2 Investigation of the Nuclear Morphology in the Toomre Sequence of Merging Galaxies
We report on the properties of nuclear regions in the Toomre sequence ofmerging galaxies, based on imaging data gathered with the Hubble SpaceTelescope WFPC2 camera. We have imaged the 11 systems in the proposedevolutionary merger sequence in the F555W and F814W broadband filters,and in Hα+[N II] narrowband filters. The broadband morphology ofthe nuclear regions varies from nonnucleated starburst clumps throughdust-covered nuclei to a nucleated morphology. There is no unambiguoustrend in the morphology with merger stage. The emission-line morphologyis extended beyond the nucleus in most cases, but centrally concentrated(within 1 kpc) emission-line gas can be seen in the four latest-stagemerger systems. We have quantified the intrinsic luminosity densitiesand colors within the inner 100 pc and 1 kpc of each identified nucleus.We find little evidence for a clear trend in nuclear properties alongthe merger sequence other than a suggestive rise in the nuclearluminosity density in the most evolved members of the sequence. The lackof clear trends in nuclear properties is likely due both to the effectsof obscuration and geometry, as well as the physical variety of galaxiesincluded in the Toomre sequence.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations are associated with proposal8669.
| The Seyfert Population in the Local Universe
The magnitude-limited catalog of the Southern Sky Redshift Survey(SSRS2) is used to characterize the properties of galaxies hostingactive galactic nuclei (AGNs). Using emission-line ratios, we identify atotal of 162 (3%) Seyfert galaxies out of the parent sample with 5399galaxies. The sample contains 121 Seyfert 2 galaxies and 41 Seyfert 1galaxies. The SSRS2 Seyfert galaxies are predominantly in spirals oftypes Sb and earlier or in galaxies with perturbed appearance as theresult of strong interactions or mergers. Seyfert galaxies in thissample are twice as common in barred hosts as the non-Seyfert galaxies.By assigning galaxies to groups using a percolation algorithm, we findthat the Seyfert galaxies in the SSRS2 are more likely to be found inbinary systems when compared with galaxies in the SSRS2 parent sample.However, there is no statistically significant difference between theSeyfert and SSRS2 parent sample when systems with more than two galaxiesare considered. The analysis of the present sample suggests that thereis a stronger correlation between the presence of the AGN phenomenonwith internal properties of galaxies (morphology, presence of bar,luminosity) than with environmental effects (local galaxy density, groupvelocity dispersion, nearest neighbor distance).Partly based on observations at European Southern Observatory (ESO),under the ESO-ON agreement to operate the 1.52 m telescope.
| 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.
| The cold gas properties of Markarian galaxies
A sample of 61 Markarian galaxies detected in the CO line was compiled.Using available HI, element H2, optical and radio continuumdata, the analysis of the gas kinematics and the star formationproperties for this sample of galaxies was performed. The mainconclusion can be summarized as follows: (1) The HI and CO line widthsare well correlated. Interaction between galaxies has no influence onthe CO line broadening. A rapidly rotating nuclear disk in the galaxymight lead to the CO line broadening with less influence on the HI line.(2) The atomic and molecular gas surface densities are well correlatedwith the blue, FIR and radio continuum surface brightness; however, thecorrelation for molecular component is stronger.\ (3) In general, thegalaxies with UV-excess (Markarian galaxies) do not differ in their starformation properties from the non-UV galaxies.
| Tridimensional Spectroscopic Observation of the Interacting System NGC 7592
We performed a tridimensional spectroscopic study of NGC 7592, aninfrared-luminous interacting system of three galaxies, one of whichcontains a Seyfert 2 nucleus. Narrow-band images of Hα and [O III] were obtained using an imaging Fabry-Perot interferometer with atunable filter. An area-spectroscopic observation was also made by aslit scan in a direction perpendicular to the slit. The Seyfert nucleuswas found to be surrounded by a knotty star-forming region of disk- orring-shape. Along the axis of the disk or ring, a highly-ionized bipolargaseous region was identified. From kinematical analyses, it has beensuggested that this system is composed of two galaxies; one apparentnucleus is considered to be a giant H II region belonging to one galaxy.This result is supported by the morphological properties innear-infrared. Further, it has been suggested that one galaxy is anearly-type spiral, whereas the other is a late-type one, and the twogalaxies show a marked difference in the distribution of thestar-forming regions within each. The cause of the difference was arguedin relation with the dynamical perturbation between the two galaxies.
| Star Formation Rates in Interacting Starburst Galaxies
By narrowband imaging in Hα and in the adjacent red stellarcontinuum we have studied the rate and distribution of star formation in43 systems of luminous and ultraluminous IR galaxies currentlyundergoing interaction and merging. These galaxies are amongst the mostluminous at 60 μm and range in distance from ~50 up to 100 Mpc. Herewe present the Hα and the adjacent red-continuum narrowbandimages, and we compare the star formation rates derived from Hαwith those estimated from the IR luminosity. We find clear evidence forsubstantial extinction and obscuration of star-forming regions in theoptical. Without correction for reddening in the host galaxy orcorrection for [N II] contamination, the star formation rates derivedfor Hα are typically 0.5-1.0 dex lower than those estimated fromthe IR flux, and the scatter in the correlation is very large. However,an unexpected result is that when spectroscopic data are used toeliminate objects dominated by an active nucleus, to determine thegalaxian extinction, and to correct the Hα flux for both reddeningand for the contamination by the [N II] emission, a remarkably goodcorrelation emerges between the star formation rates estimated from theHα flux and those derived from the FIR continuum. In addition, astrong correlation is found between the extinction in the line-emittingregion, AHα, and the rate of star formation. Ourresults invalidate the use of Hα imaging as a reliable indicatorof star formation in starburst galaxies unless spectroscopic data arealso available. This has important implications for the determination ofstar formation rates in high-redshift galaxies. Finally, we find nocorrelation between the measured star formation rates, and theinteraction class, suggesting that the enhanced star formation ratestriggered by the interaction continue throughout the whole of themerging sequence.
| Seyfert Galaxies with Circumnuclear/Nuclear Starbursts
In this paper, we present our preliminary results on Seyfert galaxieswith circumnuclear/nuclear starburst (SB) activity. We have searched therecent available literature and found 76 active galaxies with clearevidence of nuclear SB activity, among which 16 are Seyfert 1s, 51Seyfert 2s, and 9 LINERs. After studying the 51 Seyfert 2s, we find thatthose Seyfert 2s with hidden Seyfert 1 nuclei, have similarInfrared-Radio properties as Seyfert 1 galaxies, and are different from``real'' Seyfert 2s without a hidden Seyfert 1 nucleus. The later aresimilar to starburst galaxies.
| The SCUBA Local Universe Galaxy Survey - II. 450-μm data: evidence for cold dust in bright IRAS galaxies
This is the second in a series of papers presenting results from theSCUBA Local Universe Galaxy Survey. In our first paper we provided850-μm flux densities for 104 galaxies selected from the IRAS BrightGalaxy Sample and we found that the 60-, 100-μm (IRAS) and 850-μm(SCUBA) fluxes could be adequately fitted by emission from dust at asingle temperature. In this paper we present 450-μm data for thegalaxies. With the new data, the spectral energy distributions of thegalaxies can no longer be fitted with an isothermal dust model - twotemperature components are now required. Using our 450-μm data andfluxes from the literature, we find that the 450/850-μm flux ratiofor the galaxies is remarkably constant, and this holds from objects inwhich the star formation rate is similar to our own Galaxy, toultraluminous infrared galaxies (ULIRGs) such as Arp 220. The onlypossible explanation for this is if the dust emissivity index for all ofthe galaxies is ~2 and the cold dust component has a similar temperaturein all galaxies [formmu3](Tc~20-21K). The 60-μmluminosities of the galaxies were found to depend on both the dust massand the relative amount of energy in the warm component, with a tendencyfor the temperature effects to dominate at the highest L60.The dust masses estimated using the new temperatures are higher by afactor of ~2 than those determined previously using a singletemperature. This brings the gas-to-dust ratios of the IRAS galaxiesinto agreement with those of the Milky Way and other spiral galaxieswhich have been intensively studied in the submm.
| Spectropolarimetry of a complete infrared-selected sample of Seyfert 2 galaxies
We report the results of a spectropolarimetric survey of a completefar-infrared-selected sample of Seyfert 2 galaxies. We have foundpolarized broad Hα emission in one new source, NGC 5995. In thesample as a whole, there is a clear tendency for galaxies in which wehave detected broad Hα in polarized light to have warmmid-far-infrared colours(F60μm/F25μm<~4), in agreement with ourprevious results. However, a comparison of the optical, radio and hardX-ray properties of these systems leads us to conclude that this is asecondary consequence of the true mechanism governing our ability to seescattered light from the broad-line region. We find a strong trend forgalaxies showing such emission to lie above a critical value of therelative luminosity of the active core to the host galaxy (as measuredfrom the [OIII] 5007-Å equivalent width) which varies as afunction of the obscuring column density as measured from hard X-rayobservations. The warmth of the infrared colours is then largely due toa combination of the luminosity of the active core, the obscuring columnand the relative importance of the host galaxy in powering thefar-infrared emission, and not solely orientation as we inferred in ourprevious paper. Our data may also provide an explanation as to why themost highly polarized galaxies, which appear to have tori that arelargely edge-on, are also the most luminous and have the most easilydetectable scattered broad Hα.
| Optical Classification of Southern Warm Infrared Galaxies
In this paper, we present high-resolution optical spectra and opticalclassifications from our large sample of 285 warm infrared galaxies108
| 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
| H I line observations of luminous infrared galaxy mergers
A total of 19 luminous infrared galaxy mergers, with LIR>~ 2 1011 Lsun, for H0=75 kms-1 Mpc-1, have been observed in the H I line atNançay and four of them were observed at Arecibo as well. Ofthese 19, ten had not been observed before. Six were clearly detected,one of which for the first time. The objective was to statisticallysample the H I gas mass in luminous infrared mergers along a starburstmerger sequence where the molecular CO gas content is already known. Wealso searched the literature for H I data and compared these with ourobservations.
| Cold gas and star formation in a merging galaxy sequence
We explore the evolution of the cold gas (molecular and neutralhydrogen) and star formation activity during galaxy interactions, usinga merging galaxy sequence comprising both pre- and post-mergercandidates. Data for this study come from the literature, but aresupplemented by some new radio observations presented here. First, weconfirm that the ratio of far-infrared luminosity to molecular hydrogenmass (LFIRM(H2); star formation efficiency)increases close to nuclear coalescence. After the merging of the twonuclei there is evidence that the star formation efficiency declinesagain to values typical of ellipticals. This trend can be attributed toM(H2) depletion arising from interaction induced starformation. However, there is significant scatter, likely to arise fromdifferences in the interaction details (e.g., disc-to-bulge ratio,geometry) of individual systems. Secondly, we find that the centralmolecular hydrogen surface density, ΣH2,increases close to the final stages of the merging of the two nuclei.Such a trend, indicating gas inflows caused by gravitationalinstabilities during the interaction, is also predicted by numericalsimulations. Furthermore, there is evidence for a decreasing fraction ofcold gas mass from early interacting systems to merger remnants,attributed to neutral hydrogen conversion into other forms (e.g., stars,hot gas) and molecular hydrogen depletion resulting from ongoing starformation. The evolution of the total-radio to blue-band luminosityratio, reflecting the total (disc and nucleus) star formation activity,is also investigated. Although this ratio is on average higher than thatfor isolated spirals, we find a marginal increase along the mergingsequence, attributed to the relative insensitivity of disc starformation to interactions. However, a similar result is also obtainedfor the nuclear radio emission, although galaxy interactions arebelieved to significantly affect the activity (star formation, AGN) inthe central galaxy regions. Nevertheless, the nuclear-radio to blue-bandluminosity ratio is significantly elevated compared with that forisolated spirals. Finally, we find that the FIR-radio flux ratiodistribution of interacting galaxies is consistent with star formationbeing the main energizing source.
| 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.
| Compact Radio Emission from Warm Infrared Galaxies
In this paper, we present a comparison between the optical spectroscopicdata and the incidence of compact radio emission for a sample of 60 warminfrared galaxies. We find that 80% of optically classified activegalactic nucleus (AGN)-type galaxies contain compact radio sources,while 37% of optically classified starburst galaxies contain compactradio sources. The compact radio luminosity shows a bimodaldistribution, indicating two populations in our sample. The majority ofthe higher radio luminosity class (L>104Lsolar) are AGNs, while the majority of the lower radioluminosity class (L<104 Lsolar) are starbursts.The compact radio emission in the starburst galaxies may be due toeither obscured AGNs or complexes of extremely luminous supernovae suchas that seen in Arp 220. The incidence of optically classified AGNsincreases with increasing far-infrared (FIR) luminosity. Using FIRcolor-color diagrams, we find that globally the energetics of 92% of thegalaxies in our sample are dominated by starburst activity, including60% of galaxies that we find to contain AGNs on the basis of theiroptical classification. The remainder are energetically dominated bytheir AGNs in the infrared. For starburst galaxies, electron densityincreases with dust temperature, consistent with the merger model forinfrared galaxies.
| Relationship between Infrared and Radio Emission of Seyfert Galaxies
The relationships between the monochromatic luminosity of Seyfertgalaxies at frequencies of 0.408, 1.49, and 4.85 GHz and the integratedluminosity in the far infrared (IR) range are investigated. At all radiofrequencies they are linear and equally close. Some Seyfert galaxies, ofmorphological types S0/a, E, and S0, have a far higher radio luminositythan Seyfert spiral galaxies with the same IR luminosity. Most of themare found to have compact central radio components. Seyfert spiralgalaxies follow the same relationship between radio and IR emission asnon-Seyfert spiral galaxies. The relationships between radio and IRluminosity for the individual groups of galaxies of spectral types Sy1-Sy 1.5 and Sy 1.8-Sy 2 are also linear.
| 2.5-11 micron spectroscopy and imaging of AGNs. Implication for unification schemes
We present low resolution spectrophotometric and imaging ISOobservations of a sample of 57 AGNs and one non-active SB galaxy overthe 2.5-11 mu m range. The sample is about equally divided into type I(<= 1.5; 28 sources) and type II (> 1.5; 29 sources) objects. Themid-IR (MIR) spectra of type I (Sf1) and type II (Sf2) objects arestatistically different: Sf1 spectra are characterized by a strongcontinuum well approximated by a power-law of average index < alpha> = -0.84+/-0.24 with only weak emission features from PolycyclicAromatic Hydrocarbon (PAH) bands at 3.3, 6.2, 7.7 and 8.6 mu m. In sharpcontrast to Sf1s, most Sf2s display a weak continuum but very strong PAHemission bands, with equivalent widths (EW) up to 7.2 mu m. On the otherhand, Sf1s and Sf2s do not have statistically different PAH luminositieswhile the 7 mu m continuum is on the average a factor ~ 8 less luminousin Sf2s than in Sf1s. Because the PAH emission is unrelated to thenuclear activity and arises in the interstellar medium of the underlyinggalactic bulge, its EW is a sensitive nuclear redenning indicator. Theseresults are consistent with unification schemes and imply that the MIRnuclear continuum source of Sf2s is, on the average, extinguished by92+/-37 visual magnitudes whereas it is directly visible in Sf1s. Thedispersion in Sf2's PAH EW is consistent with the expected spread inviewing angles. Those Sf2s with EW(PAH) > 5 μm suffer from anextinction Av > 125 magnitudes and are invariablyextremely weak X-ray sources. Such Sf2s presumably represent the highlyinclined objects where our line of sight intercepts the full extent ofthe molecular torus. Conversely, about a third of the Sf2s have PAH EW<= 2 mu m, in the range of Sf1s. Among them, those which have beenobserved in spectropolarimetry and/or in IR spectroscopy invariablydisplay ``hidden'' broad lines. As proposed by Heisler et al.(\cite{heisler}), such Sf2s are most likely seen at grazing incidencesuch that one has a direct view of both the ``reflecting screen'' andthe torus inner wall responsible for the near and mid-IR continuum. Ourobservations therefore constrain the screen and the torus inner wall tobe spatially co-located. Finally, the 9.7 mu m Silicate feature appearsweakly in emission in Sf1s, implying that the torus vertical opticalthickness cannot significantly exceed 1024 cm-2.Based on observations with ISO, an ESA projects with instruments fundedby ESA member states (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with the participation of ISASand NASA. Tables 2, 3, 4, 5 are only available in electronic form viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Molecular Gas Depletion and Starbursts in Luminous Infrared Galaxy Mergers
Most luminous infrared galaxies (LIGs) are closely interacting/mergingsystems that are rich in molecular gas. Here we study the relationshipbetween the stage of the galaxy-galaxy interactions, the molecular gasmass, and the star formation rate as deduced from the infraredluminosity L_IR in LIGs. We find a correlation between the CO (1-0)luminosity [a measure of molecular mass M(H_2)] and the projectedseparation of merger nuclei (the indicator of merging stages) in asample of 50 LIG mergers, which shows that the molecular gas contentdecreases as merging advances. The starburst is due to enhanced starformation in preexisting molecular clouds and not to the formation ofmore molecular clouds from atomic gas. Because of the starbursts, themolecular content is being rapidly depleted as merging progresses. Thisis further supported by an anticorrelation between L_IR/M(H_2), theglobal measure of the star formation rate per unit gas mass, and theprojected separation that implies an enhanced star formation``efficiency'' in late-stage mergers compared with that of earlymergers. This is the first evidence connecting the depletion ofmolecular gas with starbursts in interacting galaxies.
| Mid-Infrared Images of Luminous Infrared Galaxies in a Merging Sequence
We report mid-infrared observations of several luminous infraredgalaxies (LIGs) carried out with the Infrared Space Observatory. Oursample was chosen to represent different phases of a merger sequence ofgalaxy-galaxy interaction with special emphasis on early/intermediatestages of merging. The mid-infrared emission of these LIGs showsextended structures for the early and intermediate mergers, indicatingthat most of the mid-infrared luminosities are not from a central activegalactic nucleus. Both the infrared hardness (indicated by the IRAS 12,25, and 60 μm flux density ratios) and the peak-to-total flux densityratios of these LIGs increase as projected separation of theseinteracting galaxies become smaller, consistent with increasing starformation activities that are concentrated to a smaller area as themerging process advances. These observations provide among the firstobservational constraint of largely theoretically based scenarios.
| Very cold dust in galaxies
We present multi-filter far infrared photometry of active and inactivegalaxies obtained with ISOPHOT. We find that the far infrared andsubmillimeter spectrum of the active galaxies can be described by asingle modified black-body at a color temperature of 31.5 +/- 2.8 K. Theratio of infrared luminosity to gas mass, L_IR/M_gas, where the latterquantity has been obtained from 1.3 mm observations within the central11'' is about 90 L_sun/ M_sun. In contrast, the spectral energydistributions of inactive spirals require, apart from warm dust of 31.8+/- 2.8 K, an additional very cold component of at most 12.9+/- 1.7 K.Determining the gas mass from 1.3 mm dust continuum maps that cover theoptical extent of the inactive spirals we find L_IR/M_gas ~ 3 L_sun/M_sun , a factor ~ 30 lower than for the active galaxies. Based onobservations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) with the participation of ISAS andNASA.
| The Supernova Rate in Starburst Galaxies
We conducted an optical CCD search for supernovae in a sample of 142bright [m(B) <= 16 mag], nearby (z<=0.03) starburst galaxies overthe period 1988 December to 1991 June, to a limiting R-band magnitude of18. Five supernovae were found, in all cases outside the host galaxy'snucleus. We determine supernova rates (in supernova units or SNU) in theextranuclear regions to be 0.7 h^2 SNU for Type Ia, 0.7 h^2 SNU for TypeIb/c, and ~0.6 h^2 SNU for Type II, with large uncertainties but upperlimits of 2.2 h^2, 2.5 h^2, and 1.7 h^2 SNU, respectively. These ratesare similar to those measured in ``normal'' galaxies. We found noevidence for a supernova-induced brightening in any galactic nucleusand, with a few reasonable assumptions, can place upper limits of 9 h^2,12 h^2, and 7 h^2 SNU on the rates of unobscured supernovae Types Ia,Ib/c, and II, respectively, inside the nuclei.
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