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Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Infrared 3-4 μm Spectroscopy of Infrared Luminous Galaxies with Possible Signatures of Obscured Active Galactic Nuclei
We present the results of infrared 2.8-4.1 μm (L-band) spectroscopyof nearby infrared luminous galaxies with possible signatures ofdust-obscured active galactic nuclei (AGNs) in data at otherwavelengths. The samples are chosen to include sources with a radioexcess relative to far-infrared emission, strong absorption features inmid-infrared 5-11.5 μm spectra, unusually weak [C II] 158 μmemission relative to the far-infrared continuum, and radio galaxiesclassified optically as narrow-line objects. Our aim is to investigatewhether the signatures of possible obscured AGNs can be detected in ourL-band spectra based on the strengths of emission and absorptionfeatures. Six of nine observed sources clearly show 3.3 μm polycyclicaromatic hydrocarbon emission features, a good starburst indicator. Anabsorption feature at 3.1 μm due to ice-covered dust is detected inIRAS 04154+1755 and IRAS 17208-0014. The signature of a barecarbonaceous dust absorption feature at 3.4 μm is seen in NGC 1377.Our L-band spectra reveal strong signatures of obscured AGNs in allthree optical Seyfert 2 galaxies (IRAS 04154+1755, Cygnus A, and 3C 234)and two galaxies classified optically as non-Seyfert galaxies (NGC 828and NGC 1377). Among the remaining optical non-Seyfert galaxies, IRAS17208-0014 might also show a buried AGN signature, whereas no explicitAGN evidence is seen in the L-band spectra of the mid-infraredabsorption feature source IRAS 15250+3609 and two weak [C II] emitters,IC 860 and CGCG 1510.8+0725.Based in part on data collected at the Subaru Telescope, which isoperated by the National Astronomical Observatory of Japan.
| A Survey of Merger Remnants. II. The Emerging Kinematic and Photometric Correlations
This paper is the second in a series exploring the properties of 51optically selected, single-nuclei merger remnants. Spectroscopic datahave been obtained for a subsample of 38 mergers and combined withpreviously obtained infrared photometry to test whether mergers exhibitthe same correlations as elliptical galaxies among parameters such asstellar luminosity and distribution, central stellar velocity dispersion(σ0), and metallicity. Paramount to the study is totest whether mergers lie on the fundamental plane. Measurements ofσ0 have been made using the Ca triplet absorption lineat 8500 Å for all 38 mergers in the subsample. Additionalmeasurements of σ0 were made for two of the mergers inthe subsample using the CO absorption line at 2.29 μm. The resultsindicate that mergers show a strong correlation among the parameters ofthe fundamental plane but fail to show a strong correlation betweenσ0 and metallicity (Mg2). In contrast toearlier studies, the σ0 values of the mergers areconsistent with objects that lie somewhere between intermediate-mass andluminous giant elliptical galaxies. However, the discrepancies withearlier studies appear to correlate with whether the Ca triplet or COabsorption lines are used to derive σ0, with the latteralmost always producing smaller values. Finally, the photometric andkinematic data are used to demonstrate for the first time that thecentral phase-space densities of mergers are equivalent to those inelliptical galaxies. This resolves a long-standing criticism of themerger hypothesis.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| Molecular gas in the galaxy cluster Abell 262. CO observations of UGC 1347 and other galaxies of the cluster
We present millimeter CO line emission observations of 12 galaxieswithin the Abell 262 cluster, together with L_FIRdata, in the context of a possible molecular gas deficiency within theregion of the cluster center. Several indications of the presence ofsuch a deficiency are highlighted and connected to a model ofcirrus-like cloud stripping. The model predicts a drop in the average100 μm flux density of galaxies in the core of the cluster comparedto the average 100 μm flux density in the outer regions, which isactually indicated in the IRAS data of the cluster members. This drop isexplained by the decrease in the total hydrogen column density N(H) and,therefore, also includes a decrease in the molecular gas content. Inaddition to results for the global CO content of the galaxy sample,high-resolution interferometric CO(1-0) observations of one of thecluster members, UGC 1347, exemplify the spatial distribution of themolecular gas in a galaxy of the cluster. With these observations, itwas possible to confirm the existence of a bright off-nuclearCO-emission source and to derive molecular masses and line ratios forthis source and the nucleus.
| Warm, Dense Molecular Gas in the ISM of Starbursts, LIRGs, and ULIRGs
The role of star formation in luminous and ultraluminous infraredgalaxies (LIRGs, LIR>=1011 LsolarULIRGs, LIR>=1012 Lsolar) is a hotlydebated issue: while it is clear that starbursts play a large role inpowering the IR luminosity in these galaxies, the relative importance ofpossible enshrouded AGNs is unknown. It is therefore important to betterunderstand the role of star-forming gas in contributing to the infraredluminosity in IR-bright galaxies. The J=3 level of 12CO lies33 K above ground and has a critical density of~1.5×104 cm-3. The 12CO J=3-2line serves as an effective tracer for warm, dense molecular gas heatedby active star formation. Here we report on 12CO J=3-2observations of 17 starburst spiral galaxies, LIRGs, and ULIRGs, whichwe obtained with the Heinrich Hertz Submillimeter Telescope on MountGraham, Arizona. Our main results are as follows. (1) We find a nearlylinear relation between the infrared luminosity and warm, densemolecular gas such that the infrared luminosity increases as the warm,dense molecular gas to the power 0.92; we interpret this to be roughlyconsistent with the recent results of Gao & Solomon. (2) We findLIR/MH2warm,dense ratios ranging from~38 to ~482 Lsolar/Msolar using a modifiedCO-H2 conversion factor of 8.3×1019cm-2 (K km s-1)-1 derived in thispaper.
| 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.
| 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).
| Luminous Infrared Galaxies as Plausible Gamma-Ray Sources for the Gamma-Ray Large Area Space Telescope and the Imaging Atmospheric Cerenkov Telescopes
We argue that luminous infrared galaxies may constitute a newlydetectable population of γ-ray sources for the next generation ofground- and space-based high-energy telescopes. In addition, we reportfor the first time upper limits on their fluxes using data obtained withthe EGRET telescope.
| 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.
| A Deep K-Band Photometric Survey of Merger Remnants
We present K-band photometry for 51 candidate merger remnants to assessthe viability of whether spiral-spiral mergers can produce bona fideelliptical galaxies. Using both the de Vaucouleurs r1/4 andSérsic r1/n fitting laws, it is found that the stellarcomponent in a majority of the galaxies in the sample has undergoneviolent relaxation. However, the sample shows evidence for incompletephase mixing. The analysis also indicates the presence of ``excesslight'' in the surface brightness profiles of nearly one-third of themerger remnants. Circumstantial evidence suggests that this is due tothe effects of a starburst induced by the dissipative collapse of thegas. The integrated light of the galaxies also shows that mergers canmake L* elliptical galaxies, in contrast to earlier infrared studies.The isophotal shapes and related structural parameters are alsodiscussed, including the fact that 70% of the sample show evidence fordisky isophotes. The data and results presented are part of a largerphotometric and spectroscopic campaign to thoroughly investigate a largesample of mergers in the local universe.
| 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.
| Unifying B2 radio galaxies with BL Lacertae objects
In an earlier paper we presented nuclear X-ray flux densities, measuredwith ROSAT, for the B2 bright sample of nearby low-luminosity radiogalaxies. In this paper we construct a nuclear X-ray luminosity functionfor the B2 radio galaxies, and discuss the consequences of our resultsfor models in which such radio galaxies are the parent population of BLLacertae (BL Lac) objects. Based on our observations of the B2 sample,we use Monte Carlo techniques to simulate samples of beamed radiogalaxies, and use the selection criteria of existing samples of BL Lacobjects to compare our simulated results to what is observed. We findthat previous analytical results are not applicable since the BL Lacsamples are selected on beamed flux density. A simple model in which BLLacs are the moderately beamed (γ~ 3) counterparts of radiogalaxies, with some random dispersion (~0.4 decades) in the intrinsicradio-X-ray relationship, can reproduce many of the features of theradio-selected and X-ray-selected BL Lac samples, including their radioand X-ray luminosity functions and the distributions of theirradio-to-X-ray spectral indices. In contrast, models in which the X-rayand radio emission have systematically different beaming parameterscannot reproduce important features of the radio-galaxy and BL Lacpopulations, and recently proposed models in which the radio-to-X-rayspectral index is a function of source luminosity cannot in themselvesaccount for the differences in the slopes of the radio- andX-ray-selected BL Lac luminosity functions. The redshift distributionand number counts of the X-ray-selected Einstein Medium SensitivitySurvey (EMSS) sample are well reproduced by our best models, supportinga picture in which these objects are beamed Fanaroff-Riley type I radiogalaxies with intrinsic luminosities similar to those of the B2 sample.However, we cannot match the redshift distribution of the radio-selected1-Jy sample, and it is likely that a population of Fanaroff-Riley typeII radio galaxies is responsible for the high-redshift objects in thissample, in agreement with previously reported results on the sample'sradio and optical emission-line properties.
| 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
| GHASP: A 3-D Survey of Spiral and Irregular Galaxies at Hα
Not Available
| Redshifts for a Sample of Radio-selected Poor Clusters
Multifiber optical spectroscopy has been performed on galaxies in thevicinity of strong, nearby radio galaxies. These radio galaxies wereselected from the 3CR and B2 catalogs based on their exclusion from theAbell catalog, which is puzzling given the hypothesis that an externalmedium is required to confine the radio plasma of such galaxies.Velocities derived from the spectra were used to confirm the existenceof groups and poor clusters in the fields of most of the radio galaxies.We find that all radio galaxies with classical Fanaroff-Riley type Imorphologies prove to reside in clusters, whereas the other radiogalaxies often appear to be recent galaxy-galaxy mergers in regions oflow galaxy density. These findings confirm the earlier result that theexistence of extended X-ray emission combined with a statistical excessof neighboring galaxies can be used to identify poor clusters associatedwith radio galaxies.
| Rotation curves and metallicity gradients from HII regions in spiral galaxies
In this paper we study long slit spectra in the region of Hαemission line of a sample of 111 spiral galaxies with recognizable andwell defined spiral morphology and with a well determined environmentalstatus, ranging from isolation to non-disruptive interaction withsatellites or companions. The form and properties of the rotation curvesare considered as a function of the isolation degree, morphological typeand luminosity. The line ratios are used to estimate the metallicity ofall the detected HII regions, thus producing a composite metallicityprofile for different types of spirals. We have found that isolatedgalaxies tend to be of later types and lower luminosity than theinteracting galaxies. The outer parts of the rotation curves of isolatedgalaxies tend to be flatter than in interacting galaxies, but they showsimilar relations between global parameters. The scatter of theTully-Fisher relation defined by isolated galaxies is significantlylower than that of interacting galaxies. The [NII]/Hα ratios, usedas a metallicity indicator, show a clear trend between Z andmorphological type, t, with earlier spirals showing higher ratios; thistrend is tighter when instead of t the gradient of the inner rotationcurve, G, is used; no trend is found with the change in interactionstatus. The Z-gradient of the disks depends on the type, being almostflat for early spirals, and increasing for later types. The[NII]/Hα ratios measured for disk HII regions of interactinggalaxies are higher than for normal/isolated objects, even if all thegalaxy families present similar distributions of Hα EquivalentWidth. Tables 3 and 4 and Figs. 6, 7 and 21 are only available inelectronic form at http://www.edpsciences.org. Table 5 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/389 Based on dataobtained Asiago/Ekar Observatory. Also based on observations made withINT operated on the island of La Palma by ING in the SpanishObservatorio del Roque de Los Muchachos of the Instituto deAstrofísica de Canarias.
| Ice features in the mid-IR spectra of galactic nuclei
Mid infrared spectra provide a powerful probe of the conditions in dustygalactic nuclei. They variously contain emission features associatedwith star forming regions and absorptions by circumnuclear silicate dustplus ices in cold molecular cloud material. Here we report the detectionof 6-8 mu m water ice absorption in 18 galaxies observed by ISO. Whilethe mid-IR spectra of some of these galaxies show a strong resemblanceto the heavily absorbed spectrum of NGC 4418, other galaxies in thissample also show weak to strong PAH emission. The 18 ice galaxies arepart of a sample of 103 galaxies with good S/N mid-IR ISO spectra. Basedon our sample we find that ice is present in most of the ULIRGs, whereasit is weak or absent in the large majority of Seyferts and starburstgalaxies. This result is consistent with the presence of largerquantities of molecular material in ULIRGs as opposed to other galaxytypes. Like NGC 4418, several of our ice galaxy spectra show a maximumnear 8 mu m that is not or only partly due to PAH emission. While thisaffects only a small part of the galaxy population studied by ISO, itstresses the need for high S/N data and refined diagnostic methods, toproperly discriminate spectra dominated by PAH emission and spectradominated by heavy obscuration. The spectral variation from PAH emissionto absorbed continuum emission near 8 mu m shows strong similaritieswith Galactic star forming clouds. This leads us to believe that ourclassification of ice galaxy spectra might reflect an evolutionarysequence from strongly obscured beginnings of star formation (and AGNactivity) to a less enshrouded stage of advanced star formation (and AGNactivity), as the PAHs get stronger and the broad 8 mu m featureweakens. Based on observations with ISO, an ESA project with instrumentsfunded by ESA Member States (especially the PI countries: France,Germany, The Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA.
| Thermal Infrared 3-5 Micron Colors of Obscured and Unobscured Active Galactic Nuclei
Thermal infrared photometry in the L and M' bands and L-M' colors oftype 1 and type 2 active galactic nuclei (AGNs) are presented. Aftercombining our observations with photometric data at similar wavelengthstaken from the literature, we find that the excess of L-M' colors oftype 2 AGNs (37 sources, 50 data points) relative to type 1 AGNs (27sources, 36 data points), due to dust extinction, is statisticallydetectable but very small. We next investigate the L-M' colors of type 2AGNs by separating less dust obscured type 2 AGNs and highly dustobscured type 2 AGNs. In both cases, the L-M' colors are similar to theintrinsic L-M' color of unobscured AGNs, and the L-M' color excess ofthe latter highly dust obscured type 2 AGNs, due to dust extinction, ismuch smaller than that expected from the Galactic dust extinction curve.Contamination from starbursts and the time lag of flux variation areunlikely to explain this small L-M' color excess, which is bestexplained if the dust extinction curve in the close vicinity of AGNs isfairly flat at 3-5 μm, as a result of a size increase of theabsorbing dust grains through coagulation.
| 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.
| Surface photometry of radio loud elliptical galaxies from the B2 sample
V-band CCD imaging is presented for 72 galaxies from the B2 radio sample(Colla et al. \cite{colla}; Fanti et al. \cite{fanti78}), with redshiftsup to 0.2 and radio powersP408=1023-1026.5 W Hz-1.According to the morphology on the optical images 57 galaxies areclassified as ellipticals, 6 as spirals and 7 as irregular. Surfacephotometry of the sample of ellipticals was obtained fitting ellipses tothe light distribution. The light profile of these galaxies generallyfollows a de Vaucouleurs law, although in three cases the profiles showlarge excesses relative to the r1/4 law at large radii. Thefitted mu_e and r_e parameters for the de Vaucouleurs galaxies are givenin the paper. Three of the ellipticals show a bright nucleus. One ofthem is a known broad line radio galaxy (B2 1833+32) and the remainingtwo are Markarian galaxies, classified in the literature as BL Lacobjects (B2 1101+38 and B2 1652+39). The radial profiles forellipticity, position angle, and B_4 term of the Fourier analysis arepresented in the paper, and the morphological peculiarities of theellipticals are described, including the presence of shells, tails,nuclear dust, isophote twisting, off-centering, and boxiness or disknessof the isophotes. Only one of the galaxies in this work is included inthe subsample of B2 radio galaxies with well-defined jets (Parma et al.\cite{parma87}). In this sense the present sample complements the sampleof 24 radio galaxies with well-defined radio jets in Parma et al. forwhich a similar study was presented in González-Serrano et al.(\cite{gserrano93}). The irregular galaxy B2 0916+33 appears to bemisclassified, and we suggest that the right identification of the radiosource is a nearby point like object with V=18.45 mag. The spiral galaxyassociated with B2 1441+26 is also misclassified. A point-like opticalobject with V=18.88 mag, located at ~ 36 arcsec from the originalidentification and coincident with the radio core is the most probablecounterpart. Table 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Complete Figure 1 and Figure 2are only available at http://www.edpsciences.org
| Dense gas in the dust lane of Centaurus A
The interstellar medium of Centaurus A (NGC 5128) has been studiedextensively in recent years, using mostly molecular lines tracing low tomedium density gas (500 to several 103 cm-3). Theamount and distribution of the dense molecular gas was largely unknown.Here we present new millimeter data of the HCN(1-0), CS(2-1), andCS(3-2) rotational transitions towards the nearby radio galaxy CentaurusA observed with the SEST on La Silla, Chile. We obtained spectra of theHCN(1-0) emission which traces dense 104 cm-3molecular gas at the center and along the prominent dust lane at offsetpositions +/-60'' and +/-100''. We also obtained afew spectra of CS(2-1) and (3-2) tracing densities of ~ 105cm-3. The emission in these lines is weak and reaches a fewmK at the available angular resolutions of 54'' -36''. At the central position, the integrated intensity ratioI(HCN)/I(CO) peaks at 0.064, and decreases to ~ 0.02 to 0.04 in the dustlane. Using the new high density tracer data, we estimate the amount,distribution and physical conditions of the dense molecular gas in thedust lane of Centaurus A. We find that Cen A and the Milky Way arecomparable in their HCN(1-0) line luminosity. However, towards thenucleus the fraction of dense molecular gas measured via the lineluminosity ratio L(HCN)/L(CO) as well as the star formation efficiencyL_FIR/L_CO is comparable to ultra-luminous infrared galaxies (ULIRGs).Within the off-nuclear dust lane and for Cen A as a whole thesequantities are between those of ULIRGs and normal and infrared luminousgalaxies. This suggests that most of the FIR luminosity of Centaurus Aoriginates in regions of very dense molecular gas and high starformation efficiency. Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile
| Star formation in distant starburst galaxies
This paper discusses the stellar population content of distant (5 000 kms^{-1} <== V_R<=16 000 km s^{-1}) galaxies with enhancedstar-formation activity. Distinction is made between isolated galaxiesand galaxies morphologically disturbed, with clear signs of interactionsuch as mergers. In these galaxies the International UltravioletExplorer (M_BoxIUE) large aperture samples most of the galaxy's body.Consequently, the resulting integrated spectra arise primarily from bluestellar populations of different ages together with significantcontributions from intermediate and old age components, subject tovarying reddening amounts. Instead of analysing individual, usually lowSignal-to-Noise ratio (S/N) spectra, our approach is to coadd thespectra of objects with similar spectral properties in the UV,considering as well their properties in the visible/near-infraredranges. Consequently, the resulting high (S/N) template spectra containthe average properties of a rather uniform class of objects, andinformation on spectral features can now be analysed with moreprecision. Three groups have been found for the interacting galaxies,corresponding to a red, blue and very blue continuum. Isolated galaxieshave been separated into two groups, one with a flat/red continuum andthe other with a blue continuum. For comparison, we also include in thepresent analysis two groups of nearby disturbed galaxies. Stellarpopulations are analysed by means of a synthesis algorithm based on starcluster spectral components of different ages which fit the observedspectra both in terms of continuum distribution and spectral features.Flux fractions of the different age groups found in the synthesis havebeen transformed into mass fractions, allowing inferences on the starformation histories. Young stellar populations (age <500 Myr) are themain flux contributors, except for the groups with a red spectrum notdue to extinction, arising from the intermediate (age ~1 - 2 Gyr) andold age populations. We also study the reddening values and theextinction law: a Small Magellanic Cloud-like extinction law isappropriate for all cases. As compared to nearby galaxies with enhancedstar-formation, the distant starburst galaxy spectral groups exhibitlarger contributions from the intermediate and old age populations. Thiseffect is mainly accounted for by the larger spatial area sampled by theM_BoxIUE slit in the distant galaxies, including not only the entirebulge but also evolved disk populations. The present results provide aquantitative measure of the star-forming activity in interactinggalaxies, compared to isolated galaxies. Based upon data collected withthe International Ultraviolet Explorer (IUE) Satellite, supported byNASA, SERC and ESA.
| X-ray observations of low-power radio galaxies from the B2 catalogue
We present an analysis of X-ray data, taken with ROSAT, for awell-defined sample of low-power radio galaxies from the Bologna B2catalogue. Where possible, the HRI has been used in order to takeadvantage of the high spatial resolution provided by this instrument. Avariety of models are fitted to radial profiles in order to separate theresolved and unresolved X-ray emission from the galaxies. We demonstratea strong, approximately linear, correlation between the luminosities ofthe unresolved X-ray components and the 5-GHz luminosities of the radiocores in this sample. This suggests a physical relationship between thesoft X-ray emission of radio galaxies and the jet-generated radio coreemission. We infer a nuclear jet-related origin for at least some of theX-ray emission.
| 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.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| ^13CO (J=1-0) Depression in Luminous Starburst Mergers, Revisited
It is known that merging galaxies with luminous starbursts and highfar-infrared luminosities tend to have higher R_1-0=^12CO (J=1-0)/^13CO(J=1-0) integrated line intensity ratios (R_1-0~=20-50) than normalspiral galaxies (R_1-0~=5-15). Comparing far-infrared luminosities[L(FIR)] with those of ^12CO (J=1-0) and ^13CO (J=1-0) for a sample ofnormal and starburst galaxies, Taniguchi & Ohyama found that theobserved high R_1-0 values for the luminous starburst mergers areattributed to their lower (by a factor of 3 on average) ^13CO lineintensities. They suggested the following two possibilities: in theluminous starburst mergers (1) ^13CO is underabundant with respect to^12CO, or (2) exitation and/or optical depth effects are responsible forthe change in R_1-0. In this paper, we investigate the secondpossibility, using higher transition data of both ^12CO and ^13COemission lines. Applying the same method proposed by Taniguchi &Ohyama to both ^12CO (J=2-1) and ^13CO (J=2-1), we find that ^13CO(J=2-1) is also depressed with respect to ^12CO (J=2-1). This suggeststhat the ^13CO gas may be underabundant in the high-R_1-0 starburstmergers, although we cannot rule out the possibility that excitation andoptical depth effects are still affecting R_2-1, for example, as aresult of the large velocity widths in the CO emission lines. Additionalobservations of both ^12CO and ^13CO lines at J>=3 are required tobetter constrain the conditions of the molecular gas in luminousstarburst galaxies.
| High-Resolution CO Observations of Luminous Infrared Galaxies
We have performed a high-resolution imaging survey of the CO J=1-->0emission in seven galaxies with infrared (IR) luminosities exceeding3x10^11 L_solar-five of which are mergers. The resultant maps show thatthe molecular gas is very highly concentrated towards the cores of themergers, with gas surface densities approaching or exceeding 10^4M_ȯ pc^-2 within 300-400 pc of the nuclei in three cases. Thisresult supports earlier findings based on data from a smaller sample ofluminous mergers. In the two mergers that show closely spaced double IR(stellar) nuclei, CO emission peaks between the nuclei and shows anextent roughly equal to the nuclear separation. The gas cores of theindividual merging galaxies appear to be coalescing, while the stellarcores remain distinct. In the three single nucleus mergers, the CO peaksare coincident with the stellar nuclei, consistent with the hypothesisthat these are relatively evolved merger remnants. In two of the threemergers with the most compact CO emission (Mrk 231 and NGC 6240), theempirical Galactic conversion factor from CO luminosity to molecular gasmass appears to overestimate the nuclear gas mass by a factor of morethan 2 (3.6 in the case of Mrk 231). For Mrk 231, the high brightnesstemperature of the CO emission (T_b>34 K) is the likeliestexplanation for this overestimate. In the third such merger (NGC 2623),however, the geometry and kinematics suggest that the molecular gas massis within a factor of 2 of the value given by using the Galacticconversion factor. Nonetheless, in all three of these objects, themolecular gas probably dominates the nuclear gravitational potential. Wesuggest that the molecular gas in objects with such high gas masssurface densities (~10^4 M_ȯ pc^-2) is distributed in nuclear disks.These disks must be thin because of their self-gravity, with a fullwidth of 30-40 pc (compared to radii of 300-400 pc) for a verticalvelocity dispersion of 90 km s^-1. The mean volume density of molecularhydrogen in such disks must be over 10^4 cm^-3. The trend of increasingL_FIR/L_CO with increasing CO surface brightness is confirmed. The highconcentrations of molecular gas thus appear intimately related to thehigh luminosities of these systems and probably serve as the fuel.
| Multi-frequency study of the B3-VLA sample. II. The database
We present total flux densities of 1049 radio sources in the frequencyrange from 151 MHz to 10.6 GHz. These sources belong to the B3-VLAsample, which is complete down to 100 mJy at 408 MHz. The dataconstitute a homogeneous spectral database for a large sample of radiosources, 50 times fainter than the 3C catalogue, and will be used toperform a spectral ageing analysis, which is one of the critical pointsin understanding the physics and evolution of extragalactic radiosources. Table 2 is also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| What Controls the Star Formation in Luminous Starburst Mergers?
In order to understand what controls the star formation process inluminous starburst mergers (e.g., NGC 6240, Arp 220, and so on), weinvestigate observational properties of two samples of high-luminositystarburst galaxies mapped in CO J=1-0 independently using both the OwensValley Radio Observatory (Scoville et al.) and the IRAM interferometer(Downes & Solomon). We find that the surface density of far-infraredluminosity, Sigma(FIR), is proportional linearly to the H_2 surface massdensity, Sigma(H_2), for the two samples;Sigma(FIR)~Sigma(H_2)^1.01+/-0.06 with a correlation coefficient of0.96. It is often considered that Sigma(FIR) provides a good measure ofthe star formation rate per unit area, Sigma(SFR). It is also known thatmolecular gas is dominated in circumnuclear regions in the luminousstarburst mergers; i.e., Sigma(gas)~=Sigma(H_2). Therefore, the aboverelationship suggests a star formation law; Sigma(SFR)~Sigma(gas). Wesuggest that this star formation law favors the gravitationalinstability scenario rather than the cloud-cloud collision one.
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