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Unveiling the nature of Ultraluminous Infrared Galaxies with 3-4μm spectroscopy*
We present the results of L-band spectroscopical observations of localbright Ultraluminous Infrared Galaxies (ULIRGs), performed with theInfrared Spectrometer And Array Camera (ISAAC) at the Very LargeTelescope. The excellent sensitivity of the telescope and of theinstrument provided spectra of unprecedented quality for this class ofobjects, which allowed a detailed study of the active galactic nucleus(AGN)/starburst contribution to the energy output, and of thecomposition of the circumnuclear absorber. We discuss the L-bandspectral features of seven single sources, and the statisticalproperties of a complete sample of 15 sources obtained combining ourobservations with other published 3-4 μm spectra. Our main resultsare as follows. (i) When a spectral indicator suggesting the presenceof an AGN (low equivalent width of the 3.3-μm emission line, steepλ-fλ spectrum, presence of an absorptionfeature at 3.4 μm) is found, the AGN is always confirmed byindependent analysis at other wavelengths. Conversely, in all known AGNsat least one of the above indicators is present. (ii) Two newdiagnostic diagrams are proposed combining the above indicators, inwhich starbursts and AGNs are clearly and completely separated. (iii)The above diagnostic techniques are possible with spectra of relativelylow quality, which can be obtained for several tens of ULIRGs withcurrently available telescopes. This makes L-band spectroscopy thecurrent best tool to disentangle AGN and starburst contributions inULIRGs. (iv) The L-band properties of ULIRGs are heterogeneous.However, we show that all the spectral differences among ULIRGs can bereproduced starting from pure intrinsic AGN and starburst spectra andtwo varying parameters: the amount of dust extinction of the AGNcomponent, and the relative AGN/starburst contribution to the bolometricluminosity. (v) Using the above decomposition model, we show that AGNsin ULIRGs have a low dust-to-gas ratio and a dust extinction curvedifferent from Galactic. (vi) The estimate of the presence andcontribution of AGNs in a complete sample shows that AGNs are hosted byapproximately two-thirds of ULIRGs, but their energetic contribution isrelevant (>30 per cent of the bolometric luminosity) only in ~20 percent of the sample.
| Dynamical Masses in Luminous Infrared Galaxies
We have studied the dynamics and masses of a sample of 10 nearbyluminous and ultraluminous infrared galaxies (LIRGs and ULIRGs,respectively) using 2.3 μm 12CO-absorption linespectroscopy and near-infrared H- and Ks-band imaging. Bycombining velocity dispersions derived from the spectroscopy, disk scalelengths obtained from the imaging, and a set of likely model densityprofiles, we calculate dynamical masses for each LIRG. For the majorityof the sample, it is difficult to reconcile our mass estimates with thelarge amounts of gas derived from millimeter observations and from astandard conversion between 12CO emission and H2mass. Our results imply that LIRGs do not have huge amounts of moleculargas (1010-1011 Msolar) at their centersand support previous indications that the standard conversion of12CO to H2 probably overestimates the gas massesand cannot be used in these environments. This in turn suggests muchmore modest levels of extinction in the near-infrared for LIRGs thanpreviously predicted (AV~10-20 vs. AV~100-1000).The lower gas mass estimates indicated by our observations imply thatthe star formation efficiency in these systems is very high and istriggered by cloud-cloud collisions, shocks, and winds rather than bygravitational instabilities in circumnuclear gas disks.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| 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.
| 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.
| Radio properties of FIR-megamaser nuclei
Aims.Radio data on the nuclear emissions have been used to characterizethe dominant nuclear activity in a sample of FIR (ultra-) luminousgalaxies and the subgroup of known OH Megamasers. This study complementsan earlier study of the optical classification of these Megamasernuclei.Methods.Classification of the radio activity in the nuclei isbased on three critical parameters: the radio brightness temperature,the radio spectral index, and the ratio of FIR and radio fluxes. A firstmethod gives equal weight to the three parameters and a second methoduses a weighted function to classify the nuclei.Results.The presentsample shows that only 43% of the sample shows some - weak or strong -AGN characteristics. About 66% of the OH-MM sample and 81% of thenon-OH-MM sample can be actually classified as Starburst-dominatedsources. Radio diagnostic diagrams using these diagnostic parametersshow a continuous distribution ranging between AGN-dominated andSBN-dominated sources. The diagnostic diagrams also support the notionthat AGNs and starbursts coexist in the nuclei.Conclusions.A comparisonof the radio and optical classifications shows a consistency in theextreme cases of clear SBN and AGNs. A significant part of the sourceswith optical AGN-like activity have an SBN classification in the radio.The discrepant classifications are discussed in order to arrive at afinal classification of the dominant power source in the nucleus.
| Dense and Warm Molecular Gas between Double Nuclei of the Luminous Infrared Galaxy NGC 6240
High spatial resolution observations of the 12CO (1-0),HCN(1-0), HCO+(1-0), and 13CO(1-0) molecular linestoward the luminous infrared merger NGC 6240 have been performed usingthe Nobeyama Millimeter Array and the RAINBOW Interferometer. All of theobserved molecular emission lines are concentrated in the region betweenthe double nuclei of the galaxy. However, the distributions of both HCNand HCO+ emissions are more compact compared with that of12CO, and they are not coincident with the star-formingregions. The HCN/12CO line intensity ratio is 0.25; thissuggests that most of the molecular gas between the double nuclei isdense. A comparison of the observed high HCN/13CO intensityratio, 5.9, with large velocity gradient calculations suggests that themolecular gas is dense [n(H2) = 104--6cm-3] and warm (Tkin > 50 K). The observedstructure in NGC 6240 may be explained by time evolution of themolecular gas and star formation, which was induced by an almost head-oncollision or very close encounter of the two galactic nuclei accompaniedwith the dense gas and star-forming regions.
| Two-dimensional warm gas kinematics in interacting galaxy systems
Gas reservoirs, internal or acquired, play an important role in thesecular evolution of interacting galaxies, since they are able toenhance/trigger star formation episodes and, probably, feed the activityof active galactic nuclei. Using Fabry-Perot observations, we havemapped, in the Hα line, the warm (T~ 104) gasdistribution and the velocity fields of the galaxy members of fiveinteracting, gas-rich galaxy systems. We investigated two M51-likesystems (Arp 70 and Arp 74), two systems containing highly disruptedmembers (WBL 366 and RR 24) and a case of merging in progress (Arp 299,one of the nearest luminous infrared objects).We detected gas motions following the elongated arm/tail of Arp 70b,while in the fainter member of the pair of galaxies, Arp 70a, the gasdistribution is off-centred with respect to the stellar isophotes,suggesting an external acquisition. Our kinematic data highlightednon-circular motions in the velocity field of one of the members of Arp74 (Arp 74a). The two galaxies of the RR 24 system are connected by onetidal tail, through which the kinematically disturbed component RR 24bseems to supply warm gas to RR 24a. In spite of the nearly irregular gasdistribution and perturbed morphology, WBL 366a (the star-forming galaxyVV-523) and WBL 366b have nearly regular velocity fields. The velocityfield in the Arp 299 system is irregular, and gas flow between the twonuclei is detected.The present observations, discussed in the light of model predictionsand complementary observations from the literature, suggest that allthese systems are still probably in an early phase of the encounter.However, the ionized gas distribution and kinematics are stronglyinfluenced by tidal forces. In particular, cross-fuelling mechanismsbetween galaxies are in action. In Arp 299 the warm and cold gaseouscomponents show similar kinematic properties, although the cold gasseems to maintain a still better organized motion with respect to thewarm gas.
| Mega-Masers and Galaxies
In the Galaxy, microwave radiation can be amplified in the interstellarmedium in the immediate neighborhood of young stellar objects, orcircumstellar envelopes around evolved stars, resulting in cosmic maseremission. Cosmic masers exist because, in contrast to terrestrialconditions, the interstellar gas density is very low so that levelpopulation in molecules is typically not in thermal equilibrium, andsometimes inverted. In the nuclear regions of external galaxies, thereexist very powerful OH ( 18 cm) and H2O ( 1.35 cm) cosmicmasers with line luminosities of 102 104Lȯ, 106 times more luminous than typicalGalactic maser sources. These are the "mega-masers," found inhigh-density molecular gas located within parsecs of active galacticnuclei in the case of H2O mega-masers, or within the central100 pc of nuclear star-burst regions in the case of OH mega-masers.H2O mega-masers are most frequently found in galactic nucleiwith Seyfert2 or LINER spectral characteristics, in spiral and someelliptical galaxies. OH mega-masers are found in ultra-luminous IRgalaxies (ULIRG) with the warmest IR colors, and importantly, the OHluminosity is observed to increase with the IR luminosity:LOH L1.2IR. Because of the extremelyhigh-surface brightness, H2O mega-maser emission can bemapped at sub-milli-arc-second resolution by Very Long BaselineInterferometry (VLBI), providing a powerful tool to probe spatial andkinematic distributions of molecular gas in distant galactic nuclei atscales below one parsec. An excellent example is the active galaxy, NGC4258, in which mapping of the H2O mega-maser emission hasprovided the first direct evidence in an active galactic nucleus for theexistence of a thin Keplerian accretion disk with turbulence, as well ashighly compelling evidence for the existence of a massive black hole.The NGC 4258 mega-maser has also provided a geometric distancedetermination of extremely high precision. H2O mega-maseremission is also found to arise from postshocked gas from the impact ofnuclear jets or outflows on the surrounding molecular clouds.High-resolution observations have shown that OH mega-masers originatefrom the molecular gas medium in 100-pc scale nuclear star-burstregions. It is proposed that such extreme star-burst regions, withextensive high-density gas bathed in a very high far-IR radiation field,are conducive to the formation of a very large number of OH masersources that collectively produce the OH mega-maser emission. In theearly Universe, galaxies or mergers could go through a very luminousphase, powered by intensive star-bursts and AGN formation, and couldhave extremely large OH and H2O maser luminosities, possiblyproducing giga-masers. With the increasing sensitivity of new telescopesand receivers, surveys and high-resolution studies of mega-masers andgiga-masers will be very important tracers and high-resolution probes ofactive galactic nuclei, dust embedded star-bursts in the earliestgalaxies and galaxy mergers in the epoch of very active star formationat z 2 and beyond. Distance determination of giga-masers at z 1 2can provide on independent measure of how fast the universe isexpanding.
| ISO observations of the interacting galaxy Markarian 297. with the powerful supernova remnant 1982aa
Markarian (Mkn) 297 is a complex system comprised of two interactinggalaxies that has been modelled with a variety of scenarios.Observations of this system were made with the Infrared SpaceObservatory (ISO) using the ISOCAM, ISOPHOT and LWS instruments. ISOCAMmaps at 6.7 μm, 7.7 μm, 12 μm and 14.3 μm are presentedwhich, together with PHT-S spectrometry of the central interactingregion, probe the dust obscured star formation and the properties of theorganic dust. The ISOCAM observations reveal that the strongest emissionin the four bands is at a location completely unremarkable at visibleand near-IR (e.g. 2MASS) wavelengths, and does not coincide with thenuclear region of either colliding galaxy. This striking characteristichas also been observed in the overlap region of the colliding galaxiesin the Antennae (NGC 4038/4039), the intragroup region of Stephan'sQuintet, and in IC 694 in the interacting system Arp 299, and againunderlines the importance of infrared observations in understanding starformation in colliding/merging systems. At 15 μm, the hidden sourcein Mkn 297 is, respectively, 14.6 and 3.8 times more luminous than thehidden sources in the Antennae (NGC 4038/4039) and Stephan's Quintet.Numerical simulations of the Mkn 297 system indicate that a co-planarradial penetration between two disk galaxies yielded the observed wingformation in the system about 1.5 × 108 years after thecollision. A complex emission pattern with knots and ridges of emissionwas detected with ISOCAM. The 7.7 μm map predominantly shows thegalaxy in emission from the 7.7 μm feature attributed to PAHs(Polycyclic Aromatic Hydrocarbons). The 14.3/7.7 μm ratio is greaterthan unity over most of the galaxy, implying widespread strong starformation. Strong emission features were detected in the ISOPHOTspectrum, while [O I], [O III] and [C II] emission lines were seen withLWS. Using data from the three instruments, luminosities and masses fortwo dust components were determined. The total infrared luminosity isapproximately 10<(11)> Lȯ, which(marginally) classifies the system as a luminous infrared galaxy (LIRG).A supernova that exploded in 1979 (SN 1982aa) gave rise to one of themost powerful known radio remnants which falls close to the strongestmid-infrared source and is identified with star forming region 14 in theoptical. This supernova explosion may have been accompanied by agamma-ray burst (GRB), consistent with the idea that GRBs are associatedwith supernovae in star forming regions, and a search for a GRBconsistent with the direction to Mkn 297, in satellite data from July toDecember 1979, is recommended.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) with the participation of ISAS andNASA
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| Radio recombination lines from the starburst galaxy NGC 3256
We have detected the radio recombination lines H91α and H92αwith rest frequencies of 8.6 GHz and 8.3 GHz from the starburst nucleusNGC 3256 at an angular resolution of 16.4'' × 9.6'' using theAustralia Telescope Compact Array and at an angular resolution of 12.0''× 2.9'' using the VLA. The line was detected at ~1 mJybeam-1 peak with a width of 160 km s-1 with theATCA and at ~0.5 mJy beam-1 peak with a width of 114 kms-1 with the VLA. Modelling the line emitting region as acollection of H II regions, we derive constraints on the required numberof H II regions, their temperature, density, and distribution. We findthat a collection of 10 to 300 H II regions with temperatures of 5000 K,densities of 1000 cm-3 to 5000 cm-3 and diametersof 15 pc produced good matches to the line and continuum emmission. TheLyman continuum production rate required to maintain the ionization is 2× 1052~s-1 to 6 ×1053~s-1, which requires 600 to 17 000 O5 stars tobe produced in the starburst.
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
| A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.
| A Supernova Factory in the Merger System Arp 299
We have imaged the nearby galaxy merger Arp 299 at arcsecond andmilliarcsecond resolution, using both the Very Large Array (VLA) and theVery Long Baseline Array. The large-scale radio emission from the mergercontains five bright, compact radio sources embedded in diffuseemission, with diameters less than 200 pc. Supernova rates of 0.1-1yr-1 are required to produce the VLA-detected radio emissionin these sources. Two of the compact VLA radio sources, designatedsource A and source D, have also been detected and imaged atmilliarcsecond scales. Source A, which is associated with the nucleus ofone of the merging galaxies, contains five milliarcsecond-scale sources,each with a radio power between 100 and 1000 times that of the Galacticsupernova remnant Cassiopeia A. Four of these have flat or invertedspectra and appear to be young supernovae. Three of the VLBI-scalesources are located within 10 pc (projected) of one another, and two areseparated by less than 3 pc, indicating that they all may be within thesame super-star cluster or complex of such clusters. The brightestVLBI-scale source, A0, has an extremely inverted spectrum, withα>~+2 at gigahertz frequencies. It seems to be the youngestsupernova, having not yet broken out of its circumstellar shell. Themilliarcsecond radio sources within source A appear to constitute asupernova factory, confirming the presence of an extreme starburst thatpeaked at least a few million years ago.
| 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.
| Missing Massive Stars in Starbursts: Stellar Temperature Diagnostics and the Initial Mass Function
Determining the properties of starbursts requires spectral diagnosticsof their ultraviolet radiation fields, to test whether very massivestars are present. We test several such diagnostics, using new models ofline ratio behavior combining CLOUDY, Starburst99, and up-to-datespectral atlases. For six galaxies we obtain new measurements of He I1.7 μm/Br10, a difficult to measure but physically simple (andtherefore reliable) diagnostic. We obtain new measurements of He I 2.06μm/Brγ in five galaxies. We find that He I 2.06 μm/Brγand [O III]/Hβ are generally unreliable diagnostics in starbursts.The heteronuclear and homonuclear mid-infrared line ratios (notably [NeIII] 15.6 μm/[Ne II] 12.8 μm) consistently agree with each otherand with He I 1.7 μm/Br10 this argues that the mid-infrared lineratios are reliable diagnostics of spectral hardness. In a sample of 27starbursts, [Ne III]/[Ne II] is significantly lower than modelpredictions for a Salpeter initial mass function (IMF) extending to 100Msolar. Plausible model alterations strengthen thisconclusion. By contrast, the low-mass and low-metallicity galaxies II Zw40 and NGC 5253 show relatively high neon line ratios, compatible with aSalpeter slope extending to at least ~40-60 Msolar. Onesolution for the low neon line ratios in the high-metallicity starburstswould be that they are deficient in >~40 Msolar starscompared to a Salpeter IMF. An alternative explanation, which we prefer,is that massive stars in high-metallicity starbursts spend much of theirlives embedded within ultracompact H II regions that prevent the near-and mid-infrared nebular lines from forming and escaping. Thishypothesis has important consequences for starburst modeling andinterpretation.
| A Bias in Optical Observations of High-Redshift Luminous Infrared Galaxies
We present evidence for the dramatically different morphology betweenthe rest-frame UV and 7 μm mid-IR emission of VV 114 and Arp 299, twonearby (z~0) violently interacting luminous infrared galaxies (LIRGs).Nearly all LIRGs are interacting systems, and it is currently acceptedthat they dominate the IR emission at z>1. LIRGs located at z=1-2could easily be detected as unresolved sources in deep optical/near-IRground-based surveys as well as in upcoming 24 μm surveys with theSpace Infrared Telescope Facility. We demonstrate that the spatialresolution of these surveys will result in blending of the emission fromunresolved interacting components. An increased scatter will thus beintroduced in the observed optical-to-mid-IR colors of these galaxies,leading to a systematic underestimation of their dust content.
| Arp 299: A Second Merging System with Two Active Nuclei?
Recent BeppoSAX observations of Arp 299, a powerful far-IR mergingstarburst system composed of IC 694 and NGC 3690, clearly unveiled forthe first time in this system the presence of a strongly absorbed activegalactic nucleus (AGN). However, the system was not spatially resolvedby BeppoSAX. Here we present the analysis of archival Chandra and (forthe first time) XMM-Newton observations, which allow us to disentanglethe X-ray emission of the two galaxies. The detection of a strong 6.4keV line in NGC 3690 clearly demonstrates the existence of an AGN inthis galaxy, while the presence of a strong 6.7 keV Fe-Kα line inthe spectrum of IC 694 suggests that this nucleus might also harbor anAGN. This would be the second discovery of two AGNs in a merging systemafter NGC 6240.
| Dust enshrouded star-forming activity in Arp 299
We present mid-infrared spectro-imaging (5-16 μm) observations of theinfrared luminous interacting system Arp 299 (= Mrk 171 = IC 694+NGC3690) obtained with the ISOCAM instrument aboard ISO. Our observationsshow that nearly 40% of the total emission at 7 and 15 μm is diffuse,originating from the interacting disks of the galaxies. Moreover, theyindicate the presence of large amounts of hot dust in the main infraredsources of the system and large extinctions toward the nuclei. While theobserved spectra have an overall similar shape, mainly composed ofUnidentified Infrared Bands (UIB) in the short wavelength domain, astrong continuum at 13 μm and a deep silicate absorption band at10 μm, their differences reveal the varying physical conditions ofeach component. For each source, the spectral energy distribution (SED)can be reproduced by a linear combination of a UIB ``canonical''spectral template and a hot dust continuum due to a 230-300 K blackbody, after independently applying an extinction correction to both ofthem. We find that the UIB extinction does not vary much throughout thesystem (AV 5 mag) suggesting that most UIBs originatefrom less enshrouded regions. IC 694 appears to dominate the infraredemission of the system and our observations support the interpretationof a deeply embedded nuclear starburst located behind an absorption ofabout 40 magnitudes. The central region of NGC 3690 displays a hardradiation field characterized by a [NeIII]/[NeII] ratio ≥1.8. Italso hosts a strong continuum from 5 to 16 μm which can be explainedas thermal emission from a deeply embedded (AV60 mag)compact source, consistent with the mid-infrared signature of an activegalactic nucleus (AGN), and in agreement with recent X-ray findings.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) with the participation of ISAS and NASA.
| Dust-to-gas ratios in the starburst regions of luminous infrared galaxies
We investigate the properties of dust and dust-to-gas ratios in variousstarburst regions of luminous infrared galaxies (LIGs). We refer to thesample of seven LIGs recently observed in the mid-infrared by Soifer etal. using the Keck Telescope with spatial resolution approaching thediffraction limit. These seven objects are among the closest LIGs andhave been classified as starburst galaxies from optical spectroscopy.Our goal consists in modelling the continuum spectral energydistribution of each galaxy, particularly in the infrared range. Modelsare further constrained by observed emission-line ratios in the opticalrange. Multicloud models consistently account for the coupled effects ofshock, photoionization by hot stars and diffuse secondary radiation fromthe shock-heated gas. Emission from clouds in the neighbourhood ofevolved starbursts and with high shock velocities (~500 kms-1) explains both the bremsstrahlung and reradiation fromdust in the mid-infrared. Clouds with lower velocity (100 kms-1), which correspond to younger starbursts, also contributeto both line and continuum spectra. Both low- and high-velocity cloudsare thus present in nearly all the sample galaxies. For all thegalaxies, an old stellar population is revealed by blackbody emission inthe optical/near-infrared range. Dust-to-gas ratios vary in differentregions of individual galaxies.
| Chandra Observations of the X-Ray-luminous Star-forming Galaxy Merger Arp 299
We report the results of a Chandra observation of the X-ray-luminousstar-forming galaxy Arp 299 (NGC 3690/IC 694). We detect 18 discreteX-ray sources with luminosities above ~1039 ergss-1 (0.5-8.0 keV band), which contribute ~40% of the totalgalactic emission in this band. The remaining emission is associatedwith a diffuse component spatially coincident with regions of widespreadstar formation. We detect X-ray emission from both nuclei. One of thediscrete sources within the complex nuclear region of NGC 3690 is foundto have a very hard spectrum, and therefore we associate it with theorigin of the active galactic nucleus-like spectrum that has also beendetected at high X-ray energies using BeppoSAX.
| Arrival Distribution of Ultra-High-Energy Cosmic Rays: Prospects for the Future
We predict the arrival distribution of ultra-high-energy cosmic rays(UHECRs) above 4×1019 eV with the event number expectedby future experiments in the next few years. We perform eventsimulations with the source model that is adopted in our recent studyand can explain the current Akeno Giant Air Shower Array (AGASA)observation. At first, we calculate the harmonic amplitude and thetwo-point correlation function for the simulated event sets. We findthat significant anisotropy on a large angle scale will be observed when~103 cosmic rays above 4×1019 eV aredetected by future experiments. The Auger array will detect cosmic rayswith this event number in a few years of operation. The statistics ofthe two-point correlation function will also increase. The angle scaleat which the events have strong correlation with each other correspondsto the deflection angle of UHECRs propagating in the extragalacticmagnetic field (EGMF), which in turn can be determined by the futureobservations. We further investigate the relation between the number ofevents clustered in a direction and the distance of their sources.Despite the limited amount of data, we find that the C2 triplet eventsobserved by the AGASA may originate from a source within 100 Mpc from usat 2 σ confidence level. Merger galaxy Arp 299 (NGC 3690 + IC 694)is the best candidate for their source. If data accumulate, the UHECRsources within ~100 Mpc can be identified significantly from observedevent clusterings. This will provide some kinds of information aboutpoorly known parameters that influence the propagation of UHECRs, suchas extragalactic and Galactic magnetic fields and the chemicalcomposition of observed cosmic rays. Also, we will reveal the origin ofUHECRs with our method of identifying their sources. Finally, we predictthe arrival distribution of UHECRs above 1020 eV that isexpected to be observed if the current HiRes spectrum is correct anddiscuss their statistical features and implications.
| The Collimated Wind in NGC 253
Near-infrared Fabry-Perot imaging has revealed H2 emissionextended to about 130 pc from the disk of NGC 253. It is closely relatedto the hot plasma observed in soft X-rays: filamentary H2features are found at the edges of the hot plasma. These are the placesof direct interaction between a superwind and its surrounding moleculargas. We suggest that the filamentary features actually trace a more orless conical shell-like structure, whose tangential line of sight to usis intensely observed. The H2 emission shell is most likelyfrom the molecular gas blown out or swept to the side by the hot plasmaoutflow. Dust is associated with this molecular gas structure. Theoutflow is tilted with respect to the disk, possibly suggesting theinhomogeneous nature of the interstellar medium in which the starbursttakes place.
| 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.
| ISO spectroscopy of star formation and active nuclei in the luminous infrared galaxy NGC 6240
We present Infrared Space Observatory mid- and far-infrared spectroscopyof the merging galaxy NGC 6240, an object presentingmany aspects of importance for the role of star formation and AGNactivity in [ultra]luminous infrared galaxies. The mid-infrared spectrumshows starburst indicators in the form of low excitation fine-structureline emission and aromatic ``PAH'' features. A strong high excitation [OIV] line is observed which most likely originates in the Narrow LineRegion of an optically obscured AGN. NGC 6240 showsextremely powerful emission in the pure rotational lines of molecularhydrogen. We argue that this emission is mainly due to shocks in itsturbulent central gas component and its starburst superwind. The totalshock cooling in infrared emission lines accounts for ~ 0.6% of thebolometric luminosity, mainly through rotational H2 emissionand the [O I] 63 mu m line. We analyse several ways of estimating theluminosities of the starburst and the AGN in NGC 6240 and suggest thatthe contributions to its bolometric luminosity are most likely in therange 50-75% starburst and 25-50% AGN.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, theNetherlands, and the UK) with the participation of ISAS and NASA.
| A Hubble Space Telescope Survey of the Mid-Ultraviolet Morphology of Nearby Galaxies
We present a systematic imaging survey of 37 nearby galaxies observedwith the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2(WFPC2) in the mid-UV F300W filter, centered at 2930 Å, as well asin the I-band (F814W) filter at 8230 Å. Eleven of these galaxieswere also imaged in the F255W filter, centered at 2550 Å. Oursample is carefully selected to include galaxies of sufficiently smallradius and high predicted mid-UV surface brightness to be detectablewith WFPC2 in one orbit and covers a wide range of Hubble types andinclinations. The mid-UV (2000-3200 Å) spans the gap betweenground-based UBVR(IJHK) images, which are available or were acquired forthe current study, and far-UV images available from the Astro/UITmissions for 15 galaxies in our sample. The first qualitative resultsfrom our study are as follows:1. Early-type galaxies show a significantdecrease in surface brightness going from the red to the mid-UV,reflecting the absence of a dominant young stellar population and insome cases the presence of significant (central) dust lanes. Galaxiesthat are early types in the optical show a variety of morphologies inthe mid-UV that can lead to a different morphological classification,although not necessarily as later type. Some early-type galaxies becomedominated by a blue nuclear feature or a point source in the mid-UV,e.g., as a result of the presence of a Seyfert nucleus or a LINER. Thisis in part due to our mid-UV surface brightness selection, but it alsosuggests that part of the strong apparent evolution of weak AGNs inearly-type galaxies may be due to surface brightness dimming of theirUV-faint stellar population, which renders the early-type host galaxiesinvisible at intermediate to higher redshifts.2. About half of themid-type spiral and star-forming galaxies appear as a latermorphological type in the mid-UV, as Astro/UIT also found primarily inthe far-UV. Sometimes these differences are dramatic (e.g., NGC 6782shows a spectacular ring of hot stars in the mid-UV). However, not allmid-type spiral galaxies look significantly different in the mid-UV.Their mid-UV images show a considerable range in the scale and surfacebrightness of individual star-forming regions. Almost without exception,the mid-type spirals in our sample have their small bulges bisected by adust lane, which often appears to be connected to the inner spiral armstructure.3. The majority of the heterogeneous subset of late-type,irregular, peculiar, and merging galaxies display F300W morphologiesthat are similar to those seen in F814W, but with important differencesdue to recognizable dust features absorbing the bluer light and to hotstars, star clusters, and star formation ``ridges'' that are bright inthe mid-UV. Less than one-third of the galaxies classified as late typein the optical appear sufficiently different in the mid-UV to result ina different classification.Our HST mid-UV survey of nearby galaxiesshows that, when observed in the rest-frame mid-UV, early- to mid-typegalaxies are more likely to be misclassified as later types thanlate-type galaxies are to be misclassified as earlier types. This isbecause the later type galaxies are dominated by the same young and hotstars in all filters from the mid-UV to the red and so have a smaller``morphological K-correction'' than true earlier type galaxies. Themorphological K-correction can thus explain part, but certainly not all,of the excess faint blue late-type galaxies seen in deep HST fields.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS 5-26555. Also based in part onobservations made with the Vatican Advanced Technology Telescope: theAlice P. Lennon Telescope and the Thomas J. Bannan AstrophysicsFacility.
| An Enshrouded Active Galactic Nucleus in the Merging Starburst System Arp 299 Revealed by BeppoSAX
Using a long (~=150 ks) broadband (0.1-40 keV) BeppoSAX observation ofthe merging starburst system Arp 299 (=IC 694+NGC 3690), we found thefirst unambiguous evidence of the presence of a deeply buried(NH~=2.5×1024 cm-2) activegalactic nucleus (AGN) having  an intrinsic luminosity ofL0.5-100keV~=1.9×1043 ergs s-1.The X-ray spectral properties of this AGN are discussed in detail, aswell as the thermal component detected at soft X-ray energies, which,most likely, is associated with the starburst.
| Resolving the Buried Starburst in Arp 299
We present new 37.7 μm far-infrared imaging of the infrared luminous(LIR~5.16×1011Lsolar) interactinggalaxy Arp 299 (= IC 694 + NGC 3690). We show that the 38 μm flux,like the 60 and 100 μm emission, traces the luminosity of starforming galaxies but at considerably higher spatial resolution. Our dataestablish that the major star formation activity of the galaxyoriginates from a point source in its eastern component, IC 694, whichis inconspicuous in the optical, becoming visible only at the near- andmid-infrared. We find that IC 694 is 2 times more luminous than NGC3690, contributing to more than 46% of the total energy output of thesystem at this wavelength. The spectral energy distribution of thedifferent components of the system clearly shows that IC 694 has 6 timesthe infrared luminosity of M82, and it is the primary source responsiblefor the bolometric luminosity of Arp 299.
| Massive Star Formation in Luminous Infrared Galaxies: Giant H II Regions and Their Relation to Super-Star Clusters
We have used Hubble Space Telescope Near-Infrared Camera andMulti-Object Spectrometer broadband (at 1.6 μm) and narrowbandPaα (λrest=1.87 μm) images to identify,respectively, star clusters and H II regions in a sample of eightluminous infrared galaxies (LIRGs). These observations have revealed thepresence of a large population of super-star clusters and bright H IIregions. A significant fraction of the H II regions shows Hαluminosities above that of 30 Doradus, the prototypical giant H IIregion. The excess of extremely luminous H II regions in LIRGs has beenconfirmed by comparison with normal galaxies observed at similar spatialresolutions. Despite the large numbers of identified star clusters and HII regions in LIRGs, we find only a small fraction of coincidences,between 4% and 30% of the total number of detected sources. Usingevolutionary synthesis models we have reproduced the relative fractionsof young H II regions and intermediate and old star clusters observed inArp 299 and the central region NGC 3256 by using a Salpeter initial massfunction and instantaneous star formation. H II regions with no detectednear-infrared cluster counterpart (25%-39% of the detected sources)represent the youngest sites of star formation, with ages up toapproximately 5 Myr and mostly intermediate mass (~=105Msolar) ionizing clusters. For these two galaxies, within thepresent detection threshold we can detect only coincidences (4%-10% ofthe detected sources) between an H II region and a near-infrared starcluster for the most massive star clusters (~=106Msolar) during the first 7 Myr of their evolution. If thereis significant extinction during the first million years, we may notdetect the youngest star-forming regions, and hence the observedfractions of H II regions and coincidences will be lower limits. Theidentified near-infrared super-star clusters with no detectablePaα emission represent the ``old'' population (53%-66% of thedetected sources), with ages between 7 and 20-40 Myr. Older clusterspossibly created in this or previous episodes of star formation arelikely to exist in these systems but cannot be identified with thepresent detection threshold. Our study demonstrates that Paαnarrowband imaging of LIRGs and interacting galaxies identifies theyoungest sites of star formation that could be otherwise missed bynear-infrared broadband continuum surveys. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained from the data archive atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| N-Band Observations of Henize 2-10: Unveiling the Dusty Engine of a Starburst Galaxy
As part of an ongoing program to better understand the early stages ofmassive star cluster evolution and the physical conditions for theirformation, we have obtained J, H, K', and N (10.8 μm) images of thenuclear region of the starburst galaxy He 2-10. The N-band images wereobtained with the Gemini North Telescope. In only 10 minutes ofon-source integration time with Gemini, we were able to detect four ofthe five enshrouded clusters, or ``ultradense H II regions'' (UD H IIregions) recently discovered in radio maps. None of these sourcesappears in either the optical Hubble Space Telescope images or thenear-infrared (J, H, and K') images. These sources make up about 60% ofthe total N-band flux from He 2-10 and, we suspect, a similar fractionof the total far-infrared flux measured by IRAS. The inferred spectra ofthe UD H II regions are strikingly similar to those of Galacticultracompact H II regions. We have modeled the radio and IR spectra ofthese UD H II regions under the assumption that they are ``scaled-up''Galactic ultracompact H II regions. From this model, the bolometricluminosity of the brightest cluster alone is estimated to be~2×109 Lsolar. The total mass of the dustand gas in this UD H II region is Mshell~107Msolar. We have also used the observed spectra to placeconstraints on the masses and ages of the stellar clusters enshroudedwithin the UD H II regions. For the brightest UD H II region, we findthat the stellar mass must beMcluster>~2.5×106 Msolar, andthe age must be <~4.8×106 yr, with the most probableage being <~3.6×106 yr. If we assume that the regionis pressure confined and enforce the requirement that the star formationefficiency must be less than ~90%, we find that the age of this stellarcluster must lie within a very narrow range,4×105<τ<5×106 yr. All theclusters within the UD H II regions in He 2-10 are estimated to haveages less than about 5×106 yr and masses greater thanabout 5×105 Msolar. We find that thelogarithmic ratio of the radio to far-infrared flux densities, q, forthe UD H II regions in He 2-10 is ~4 q~2.6 for both He 2-10 as a wholeand NGC 5253, another nearby starburst known to host UD H II regions.These values of q are significantly larger than the average q=2.35 foundfor normal galaxies but are comparable to the values of q found forultraluminous infrared galaxies. We suggest that large q values forstarburst galaxies may indicate that a significant fraction of thefar-infrared flux may arise from thermal dust emission from UD H IIregions. Finally, the possibility that all the far-infrared flux from He2-10 and other starburst galaxies may be produced by regions completelyobscured at wavelengths as long as K' suggests that the well-knowncorrelation between ultraviolet continuum slope andinfrared-to-ultraviolet flux ratio in starbursts cannot be due entirelyto reprocessing of ultraviolet radiation by dust in a foreground screengeometry. In fact, the dust that reddens the ultraviolet continuum slopemust be largely decoupled from the dust that produces the large infraredfluxes in some starbursts. Based on observations obtained at the GeminiObservatory, which is operated by the Association of Universities forResearch in Astronomy, Inc., under a cooperative agreement with theNational Science Foundation on behalf of the Gemini partnership: theNational Science Foundation (US), the Particle Physics and AstronomyResearch Council (UK), the National Research Council (Canada), CONICYT(Chile), the Australian Research Council (Australia), CNPq (Brazil), andCONICET (Argentina).
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