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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.
| The Opaque Nascent Starburst in NGC 1377: Spitzer SINGS Observations
We analyze extensive data on NGC 1377 from the Spitzer Infrared NearbyGalaxies Survey (SINGS). Within the category of nascent starbursts thatwe previously selected as having infrared-to-radio continuum ratios inlarge excess of the average and containing hot dust, NGC 1377 has thelargest infrared excess yet measured. Optical imaging reveals amorphological distortion suggestive of a recent accretion event.Infrared spectroscopy reveals a compact and opaque source dominated by ahot, self-absorbed continuum (τ~20 in the 10 μm silicate band).We provide physical evidence against nonstellar activity being theheating source. H II regions are detected through the single [Ne II]line, probing <1% of the ionizing radiation. Not only is the opticaldepth in different gas and dust phases very high, but >85% ofionizing photons are suppressed by dust. The only other detectedemission features are molecular hydrogen lines, arguably excited mainlyby shocks, besides photodissociation regions, and weak aromatic bands.The new observations support our interpretation in terms of an extremelyyoung starburst (<1 Myr). More generally, galaxies deficient in radiosynchrotron emission are likely observed within a few Myr of the onsetof a starburst and after a long quiescence, prior to the replenishmentof the interstellar medium with cosmic rays. The similar infrared-radioproperties of NGC 1377 and some infrared-luminous galaxies suggest thatNGC 1377 constitutes an archetype that will be useful to betterunderstand starburst evolution. Although rare locally because observedin a brief evolutionary stage, nascent starbursts may represent anonnegligible fraction of merger-induced starbursts that dominate deepinfrared counts. Since they differ dramatically from usual starbursttemplates, they have important consequences for the interpretation ofdeep surveys.
| The First INTEGRAL AGN Catalog
We present the first INTEGRAL AGN catalog, based on observationsperformed from launch of the mission in 2002 October until 2004 January.The catalog includes 42 AGNs, of which 10 are Seyfert 1, 17 are Seyfert2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rathersmall, with five detected objects, and only one galaxy cluster and nostarburst galaxies have been detected so far. A complete subset consistsof 32 AGNs with a significance limit of 7 σ in the INTEGRAL ISGRI20-40 keV data. Although the sample is not flux limited, thedistribution of sources shows a ratio of obscured to unobscured AGNs of1.5-2.0, consistent with luminosity-dependent unified models for AGNs.Only four Compton-thick AGNs are found in the sample. Based on theINTEGRAL data presented here, the Seyfert 2 spectra are slightly harder(Γ=1.95+/-0.01) than Seyfert 1.5 (Γ=2.10+/-0.02) and Seyfert1 (Γ=2.11+/-0.05).
| Infrared 3-4 μm Spectroscopic Investigations of a Large Sample of Nearby Ultraluminous Infrared Galaxies
We present infrared L-band (3-4 μm) nuclear spectra of a large sampleof nearby ultraluminous infrared galaxies (ULIRGs). ULIRGs classifiedoptically as non-Seyfert galaxies (LINERs, H II regions, andunclassified) are our main targets. Using the 3.3 μm polycyclicaromatic hydrocarbon (PAH) emission and absorption features at 3.1 μmdue to ice-covered dust and at 3.4 μm produced by bare carbonaceousdust, we search for signatures of powerful AGNs deeply buried alongvirtually all lines of sight. The 3.3 μm PAH emission, the signaturesof starbursts, is detected in all but two non-Seyfert ULIRGs, but theestimated starburst magnitudes can account for only a small fraction ofthe infrared luminosities. Three LINER ULIRGs show spectra typical ofalmost pure buried AGNs, namely, strong absorption features with verysmall equivalent width PAH emission. Besides these three sources, 14LINER and three H II ULIRGs' nuclei show strong absorption featureswhose absolute optical depths suggest an energy source more centrallyconcentrated than the surrounding dust, such as a buried AGN. In total,17 out of 27 (63%) LINER and 3 out of 13 (23%) H II ULIRGs' nuclei showsome degree of evidence for powerful buried AGNs, suggesting thatpowerful buried AGNs may be more common in LINER ULIRGs than in H IIULIRGs. The evidence of AGNs is found in non-Seyfert ULIRGs with bothwarm and cool far-infrared colors. These spectra are compared with thoseof 15 ULIRGs' nuclei with optical Seyfert signatures taken forcomparison. The overall spectral properties suggest that the totalamount of dust around buried AGNs in non-Seyfert ULIRGs issystematically larger than that around AGNs in Seyfert 2 ULIRGs. Weargue that the optical (non)detectability of Seyfert signatures inULIRGs is highly dependent on how deeply buried the AGNs are, and thatit is essential to properly evaluate the energetic importance of buriedAGNs in non-Seyfert ULIRGs.
| Millimeter Interferometric Investigations of the Energy Sources of Three Ultraluminous Infrared Galaxies, UGC 5101, Markarian 273, and IRAS 17208-0014, Based on HCN-to-HCO+ Ratios
We present interferometric observations of three ultraluminous infraredgalaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3 mmwavelength range, using the Nobeyama Millimeter Array. Both the HCN(J=1-0) and HCO+ (J=1-0) molecular lines were observedsimultaneously. HCN emission was clearly detected at the nuclearpositions of these ULIRGs, and HCO+ emission was detected atthe nuclear positions of UGC 5101 and IRAS 17208-0014. TheHCN-to-HCO+ brightness-temperature ratios toward the nucleiof the three ULIRGs were derived and compared with those of lowerluminosity galaxies known to be dominated by active galactic nuclei(AGNs) or starbursts. In UGC 5101 and Mrk 273, where there is evidencefor obscured AGNs from previous observations at other wavelengths, wefound high HCN/HCO+ ratios (>1.8) that are in the rangefound for AGN-dominated galaxies. In IRAS 17208-0014, where the presenceof a powerful obscured AGN has been unclear, the ratio (~1.7) is betweenthe observed values for starburst- and AGN-dominated galaxies. The highHCN/HCO+ brightness-temperature ratios in UGC 5101 and Mrk273 could be the consequence of an HCN abundance enhancement, which isexpected from chemical effects of the central X-ray-emitting AGN on thesurrounding dense molecular gas. Our proposed millimeter interferometricmethod based on HCN/HCO+ ratios may be an effective tool forunveiling elusive buried AGNs at the cores of ULIRGs, especially becauseof the negligible dust extinction at these wavelengths.
| 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.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| 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.
| The evolution of actively star-forming galaxies in the mid-infrared
In this paper we analyze the evolution of actively star-forming galaxiesin the mid-infrared (MIR). This spectral region, characterized bycontinuum emission by hot dust and by the presence of strong emissionfeatures generally ascribed to polycyclic aromatic hydrocarbon (PAH)molecules, is the most strongly affected by the heating processesassociated with star formation and/or active galactic nuclei (AGNs).Following the detailed observational characterization of galaxies in theMIR by the Infrared Space Observatory (ISO), we have updated themodelling of this spectral region in our spectrophotometric modelGRASIL. In the diffuse component we have updated the treatment of PAHsaccording to the model by Li & Draine. As for the dense phase of theinterstellar medium associated with the star-forming regions, themolecular clouds, we strongly decrease the abundance of PAHs as comparedto that in the cirrus, based on the observational evidence of the lackor weakness of PAH bands close to the newly formed stars, possibly dueto the destruction of the molecules in strong ultraviolet fields. Therobustness of the model is checked by fitting near-infrared to radiobroad-band spectra and the corresponding detailed MIR spectra of a largesample of galaxies, at once. With this model, we have analyzed thelarger sample of actively star-forming galaxies by Dale et al. We showthat the observed trends of galaxies in the ISO-IRAS-radio colour-colourplots can be interpreted in terms of the different evolutionary phasesof star formation activity, and the consequent different dominance inthe spectral energy distribution of the diffuse or dense phase of theISM. We find that the observed colours indicate a surprising homogeneityof the starburst phenomenon, allowing only a limited variation of themost important physical parameters, such as the optical depth of themolecular clouds, the time-scale of the escape of young stars from theirfor mation sites, and the gas consumption time-scale. In this paper wedo not attempt to reproduce the far-infrared coolest region in thecolour-colour plots, as we concentrate on models meant to reproduceactive star-forming galaxies, but we discuss possible requirements of amore complex modelling for the coldest objects.
| On the X-ray properties of OH megamaser sources: Chandra snapshot observations
We present Chandra snapshot observations for a sample of seven sourcesselected from the Arecibo OH megamaser (OHM) survey at z~ 0.13-0.22 andwith far-infrared (FIR) luminosities in excess of1011Lsolar. In contrast with the knownH2O megamasers, which are mostly associated with powerfulactive galactic nuclei (AGN), the situation is far less clear for OHMs,which have been poorly studied in the X-ray band thus far. All of theobserved sources are X-ray weak, with only one OHM, IRAS FSC 03521+0028(z= 0.15), being detected by Chandra (with five counts). The resultsfrom this pilot programme indicate that the X-ray emission, withluminosities of less than ~1042ergs-1, isconsistent with that from star formation (as also suggested in somecases by the optical spectra) and low-luminosity active galactic nuclei(LLAGN) emission. If an AGN is present, its contribution to thebroad-band emission of OHM galaxies is likely modest. Under reasonableassumptions about the intrinsic X-ray spectral shape, the observed countdistribution from stacking analysis suggests absorption of~1022cm-2.
| Fe K emission in the ultraluminous infrared galaxy Arp 220
Prominent Fe Kα line emission is detected in the XMM-Newtonspectrum of the ultraluminous infrared galaxy Arp 220. The centroid ofthe line is found at an energy of 6.7 keV and the equivalent width ofthe line is EW ~ 1.9 keV (at 3.5σ significance). A few otherspectral features are found at various degrees of significance in thelower energy range on a hard 2.5-10 keV continuum (Γ~ 1). Thelarge EW of the Fe K line poses a problem with interpreting the hardX-ray emission as integrated X-ray binary emission. A thermal emissionspectrum with a temperature of kT~ 7 keV modified by absorption ofNH~= 3 × 1022cm-2, can describethe 2.5-10 keV continuum shape and the Fe K emission. A hot bubble thatis shocked internally in a starburst region would have a similartemperature and gives a good explanation for the observed X-rayproperties with a high star formation rate. An ensemble of radiosupernovae in a dense environment, as suggested from VLBI imaging, couldbe another possibility, if such powerful supernovae are producedcontinuously at a high rate. However, the apparent lack of emission fromX-ray binaries is incompatible with the high supernova rate (~2 SNeyr-1) required by both interpretations. Highly photoionized,low-density gas illuminated by a hidden Compton-thick active galacticnucleus is a possible alternative for the hard X-ray emission, which canbe tested by examining whether radiative recombination continua fromhighly ionized Ca and Fe are present in better quality data from aforthcoming observation.
| Silicate-break galaxies: an efficient selection method for distant ultraluminous infrared galaxies
We present a photometric selection method for ultraluminous infraredgalaxies (ULIRGs) in the redshift range of z= 1-2. We utilize the mostprominent spectral feature of ULIRGs, i.e. the silicate absorptionfeature at 9.7 μm and an optimized filter system at mid-infraredwavelengths. These `silicate-break' galaxies could be selected by usingcolour anomalies arising from the silicate absorption feature around9.7(1 +z)μm. Such filter systems are available on the Spitzer SpaceTelescope but we suggest that the most promising selection criteriawould be given with mid-infrared bands of the ASTRO-F satellite due to amore comprehensive set of filter bands than that of Spitzer. We studythe selection method of silicate-break galaxies with the SED model ofboth starbursts and quiescent star-forming galaxies, and then verify themethod by using the observed spectra of nearby galaxies. We would expectthat about 1000 candidates of silicate-break galaxies could be detectedper square degree in current and future mid-infrared surveys. Thesilicate-break selection criteria will break the degeneracy betweenvarious galaxy evolution models for extragalactic source counts andplace strong limits on the star formation activity at z= 1-2. Applyingour silicate-break technique to initial Spitzer results we havetentatively identified the first candidate silicate-break galaxy at z=1.6.
| The Origin of Infrared Emission from the Infrared Luminous Galaxy NGC 4418
We present a study of the origin of infrared (IR) emission in theoptically normal, infrared luminous galaxy NGC 4418. By decomposing thestellar absorption features and continua in the range of 36008000Å from the Sloan Digital Sky Survey into a set of simple stellarpopulations, we derive the stellar properties for the nuclear region ofNGC 4418. We compare the observed infrared luminosity with the onederived from the starburst model, and find that star-forming activitycontributes only 7% to the total IR emission, that as the IR emissionregion is spatially very compact, the most possible source for thegreater part of the IR emission is a deeply embedded AGN, though an AGNcomponent is found to be unnecessary for fitting the optical spectrum.
| Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope
Full low-resolution (65
| Millimeter Observations of Obscured Spitzer 24 μm Sources
We present MAMBO 1.2 mm observations of 40 extragalactic sources fromthe Spitzer First Look Survey that are bright in the mid-IR(S24μm>1 mJy) but optically obscured(log[νFν(24 μm)/νFν(0.7μm)]>1). We use these observations to search for cold dustemission, probing the similarity of their spectral energy distributionsto star-forming IR galaxies or obscured AGNs. The sample as a whole iswell detected at mean S1.2mm=0.74+/-0.09 mJy andS1.2mm/S24μm=0.15+/-0.03. Seven (three) of thesources are individually detected at >3 σ (>5 σ)levels. Mean millimeter fluxes are higher for sources with the reddestmid-IR/optical colors. Optically faint but with relatively lowmillimeter-to-mid-IR ratios, the typical SEDs are inconsistent withredshifted SED shapes of local star-forming IR galaxies. They alsodiffer from SEDs of typical submillimeter-selected galaxies, with the 24μm sources that are individually detected by MAMBO possiblyrepresenting intermediate objects. Compared to star-forming galaxies, astronger but optically obscured mid-IR component without associatedstrong far-IR emission has to be included. This component may be due toluminous optically obscured AGN, which would represent a significantpart of the high-redshift AGN population.
| Simulating the Spitzer Mid-Infrared Color-Color Diagrams
We use a simple parameterization of the mid-IR spectra of a wide rangeof galaxy types in order to predict their distribution in the InfraredArray Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 μm and MultibandPhotometer for Spitzer 24 μm color-color diagrams. We distinguishthree basic spectral types by the energetically dominant component inthe 3-12 μm regime: stellar-dominated, polycyclic aromatichydrocarbon (PAH)-dominated, and continuum-dominated. We use a Markovchain Monte Carlo approach to arrive at a more systematic and robustrepresentation of the mid-IR spectra of galaxies than do moretraditional approaches. We find that IRAC color-color plots are wellsuited to distinguishing the above spectral types, while the addition of24 μm data allows us to suggest practical three-color cuts thatpreferentially select higher redshift sources of a specific type. Wecompare our simulations with the color-color plot obtained by theSpitzer First Look Survey and find reasonable agreement. Lastly, wediscuss other applications as well as future directions for this work.
| Some Features of the X-Ray and Radio Emission from OH Megamaser Galaxies
The properties of OH megamaser galaxies in the x-ray and radio rangesare discussed. Based on a sample of OH megamaser galaxies it is shownthat the x-ray emission and the width of the OH radio line are closelyrelated. The line width is also related to the radio emission frommegamasers, but it depends only weakly on the infrared emission and thisrelationship is statistically less significant. The OH line width andthe radio and x-ray emissions depend on the central mass of megamasers.There is a weak correlation between the radio and x-ray emissions owingto multiple scattering of synchrotron photons and bursts from thegalactic nucleus. These results indicate that an active nucleus can bedominant in OH megamaser galaxies.
| Properties of OH megamaser galaxies in the radio continuum. I. Observational data
Radio observations with the VLA-A radio telescope of 30 OH megamasergalaxies at a frequency of 1.49 GHz are discussed. Radio emission wasdetected from all 30 of these galaxies. Radio emission was detected from5 of the 30 objects for the first time. Important results were obtainedfor 12 galaxies that had previously been little studied in the radiocontinuum. Additional data at 1.49 GHz were obtained for the remaining13 objects. The core component of the continuum radio emissionpredominates in the OH megamaser galaxies.
| Warm and Cold Molecular Gas in Galaxies
New and archival interferometric 12CO (1-->0) data setsfrom six nearby galaxies are combined with H2 2.122 μm andHα maps to explore in detail the interstellar medium in differentstar-forming galaxies. We investigate the relation between warm(H2 at T~2000 K) and cold (CO at T~50 K) molecular gas from100 pc to 2 kpc scales. On these scales, the ratio of warm-to-coldmolecular hydrogen correlates with thefν(60μm)/fν(100μm) ratio, which tracksthe star formation activity level. This result also holds for the globalproperties of galaxies from a much larger sample drawn from theliterature. The trend persists for over 3 orders of magnitude in themass ratio, regardless of source nuclear activity.
| Observational evidence for the presence of PAHs in distant Luminous Infrared Galaxies using ISO and Spitzer
We present ISOCAM 15 μm and MIPS 24 μm photometry of a sample of16 distant Luminous Infrared Galaxies (LIRGs) characterized by a medianluminosity L_IR 2 × 1011~Lȯ andredshift z = 0.7 (distributed from z = 0.1 to 1.2). While some sourcesdisplay 24/15 μm flux ratios also consistent with a featurelesscontinuum dominating their mid-infrared (MIR) spectral energydistributions (SEDs), the presence of prominent emission features suchas the Polycyclic Aromatic Hydrocarbons is clearly required to explainthe observed colors for more than half of the sample. As a result, ageneral good agreement is observed between the data and predictions fromthe local starburst-dominated SEDs that have been used so far toconstrain IR galaxy evolution. This is consistent with the star-formingnature of LIRGs derived from previous works, even though our approachcannot rule out the dominance of an AGN in some cases. Our study alsosupports the possibility of tracing the total IR luminosity of distantgalaxies (up to z 1) from their MIR emission.
| Mid-infrared luminosity as an indicator of the total infrared luminosity of galaxies
The infrared (IR) emission plays a crucial role in understanding thestar formation in galaxies hidden by dust. We first examined fourestimators of the IR luminosity of galaxies, LFIR [CITE],LTIR [CITE], revised version of LTIR [CITE] (wedenote LTIR2), and LIR [CITE] by using theobserved SEDs of well-known galaxies. We found that LIRprovides excellent estimates of the total IR luminosity for a variety ofgalaxy SEDs. The performance of LTIR2 was also found to bevery good. Using LIR, we then statistically analyzed the IRASPSCz galaxy sample [CITE] and found useful formulae relating the MIRmonochromatic luminosities [ L(12 μm) and L(25 μm)] andLIR. For this purpose we constructed a subsample of 1420galaxies with all four IRAS band (12, 25, 60, and 100 μm) fluxdensities. We found linear relations between LIR and MIRluminosities, L(12 μm) and L(25 μm). The prediction error with a95% confidence level is a factor of 4-5. Hence, these formulae areuseful for the estimation of the total IR luminosity only from 12 μmor 25 μm observations. We further tried to make an“interpolation” formula for galaxies at 0
| The observations and studies of OH megamasers associated with external galaxies
During the thirty years since the first discovery of OH megamaserassociated with external galaxies, a great progress of observations andstudies for OH megamasers associated with external galaxies has beenproceeded. So far 106 OH megamasers associated with external galaxieshave been found, including 59 higher red-shifted ones. The observationsand studies of the OH megamasers associated with AGN and starburstgalaxies are the very efficient tools to investigate characteristics oftheir central sources and circumnuclear discs. A review on the currentprogress concerning surveys, observations and theoretical investigationson extragalactic OH megamaser sources is given in this paper.
| The distribution of atomic gas and dust in nearby galaxies - III. Radial distributions and metallicity gradients
The radial distribution of dust and gas in 38 nearby galaxies isinvestigated, using a sample of galaxies for which matched resolution(25 arcsec) neutral hydrogen (HI) and 850-μm images are available.Most of these radial profiles are fitted well by an exponential model,and the derived 850-μm scalelengths are proportional to the HIscalelengths. From this relation, it is found that the metallicitygradients of these galaxies are much shallower than previous studies,unless the dust temperature is constant within the disc, or asignificant component of molecular gas exists at large radii that is nottraced by CO observations.
| The impact of broad mid-infrared emission bands on extragalactic surveys - a case study: the unidentified 22-μm feature
The effect of unidentified dust emission features in the mid-infrared(MIR) spectral energy distributions of dusty star-forming galaxies isinvestigated. As a case study, we discuss the broad emission feature at2 μm of Chan & Onaka. By modelling a distribution of featurestrengths using a starburst galaxy template, we speculativelyinvestigate the influence of the feature on galaxy fluxes, colours anddetected numbers. We find that the fluxes and colours of galaxies aresystematically affected by the feature from redshifts 0-3, atwavelengths ranging from 20 to 70 μm by up to factors of ~2,resulting in bluer MIR/far-infrared (FIR) and FIR/MIR colours at localand high redshifts, respectively, as the feature is redshifted in andout of the various observation passbands. Furthermore, we find that ifthe feature is prolific, it will manifest itself in the same manner asevolution in the infrared (IR) galaxy population in a similar way to theunidentified IR bands at shorter wavelengths. We also consider theeffect of the feature on the future IR SIRTF (Spitzer) and ASTRO-F spacemissions. We conclude that although the bulk effects of the feature maybe small, careful consideration will have to be made when discussing thecolours of sources in the relevant bands and redshift ranges affected bythe feature emission.
| The distribution of atomic gas and dust in nearby galaxies - II. Further matched-resolution Very Large Array H I and SCUBA 850-μm images
We present Very Large Array (VLA) C-array 21-cm HI images of galaxiesfrom the SCUBA Local Universe Galaxy Survey which have been observed at850 μm with the James Clerk Maxwell Telescope. Matched-resolution (~25 arcsec) HI images of 17 galaxies are presented and compared with850-μm images. HI or 850-μm images of an additional six galaxieswhich were detected at only one wavelength are presented. Additionally,lower resolution H I observations of nine galaxies are presented. Theobservations of these galaxies, along with results previously presented,do not show any obvious trends in the HI/dust or H2/dust massratios with morphological type.
| Studies of Extragalactic Formaldehyde and Radio Recombination Lines
We present the most sensitive and extensive survey yet performed ofextragalactic H2CO 6 cm (4.829 GHz) emission/absorption.Sixty-two sources were observed with the C-band system of the AreciboTelescope to a 1 σ rms noise level of ~0.3 mJy. We report a newdetection of H2CO 6 cm absorption toward NGC 520 and theconfirmation of H2CO 6 cm absorption toward several sources.We report confirmation of H2CO 6 cm emission toward the OHmegamasers Arp 220, IC 860, and IRAS 15107+0724. At present these arethe only extragalactic H2CO 6 cm emitters independentlyconfirmed. A characterization of the properties of formaldehydeabsorbers and emitters based on infrared properties of the galaxies isdiscussed. We also conducted a simultaneous survey of the H110αhydrogen recombination line toward a sample of 53 objects. We report thedetection of H110α toward the giant extragalactic H II region NGC604 in M33.
| Fire and Ice: Spitzer Infrared Spectrograph (IRS) Mid-Infrared Spectroscopy of IRAS F00183-7111
We report the detection of strong absorption and weak emission featuresin the 4-27 μm Spitzer Infrared Spectrograph (IRS) spectrum of thedistant ultraluminous infrared galaxy IRAS F00183-7111 (z=0.327). Theabsorption features of CO2 and CO gas, water ice,hydrocarbons, and silicates are indicative of a strongly obscured(A9.6>=5.4 AV>=90) and complex line of sightthrough both the hot diffuse interstellar medium and shielded coldmolecular clouds toward the nuclear power source. From the profile ofthe 4.67 μm CO fundamental vibration mode, we deduce that theabsorbing gas is dense (n~106 cm-3) and warm (720K) and has a CO column density of ~1019.5 cm-2,equivalent to NH~1023.5 cm-2. The hightemperature and density, as well as the small inferred size (<0.03pc), locates this absorbing gas close to the power source of thisregion. Weak emission features of molecular hydrogen, polycyclicaromatic hydrocarbons (PAHs), and Ne+, likely associated withstar formation, are detected against the 9.7 μm silicate feature,indicating an origin away from the absorbing region. Based on the 11.2μm PAH flux, we estimate the star formation component to beresponsible for up to 30% of the IR luminosity of the system. While ourmid-infrared spectrum shows no telltale signs of active galactic nucleus(AGN) activity, the similarities to the mid-infrared spectra of deeplyobscured sources (e.g., NGC 4418) and AGN hot dust (e.g., NGC 1068), aswell as evidence from other wavelength regions, suggest that the powersource hiding behind the optically thick dust screen may well be aburied AGN.
| 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).
| Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation?
Infrared (IR) emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 μmare generally attributed to IR fluorescence from (mainly)far-ultraviolet (FUV) pumped large polycyclic aromatic hydrocarbon (PAH)molecules. As such, these features trace the FUV stellar flux and arethus a measure of star formation. We examined the IR spectralcharacteristics of Galactic massive star-forming regions and of normaland starburst galaxies, as well as active galactic nuclei (AGNs) andultraluminous infrared galaxies (ULIRGs). The goal of this study is toanalyze whether PAH features are a good qualitative and/or quantitativetracer of star formation, and hence to evaluate the application of PAHemission as a diagnostic tool in order to identify the dominantprocesses contributing to the infrared emission from Seyfert galaxiesand ULIRGs. We develop a new mid-infrared (MIR)/far-infrared (FIR)diagnostic diagram based on our Galactic sample and compare it to thediagnostic tools of Genzel and coworkers and Laurent and coworkers, withthese diagnostic tools also applied to our Galactic sample. This MIR/FIRdiagnostic is derived from the FIR normalized 6.2 μm PAH flux and theFIR normalized 6.2 μm continuum flux. Within this diagram, theGalactic sources form a sequence spanning a range of 3 orders ofmagnitude in these ratios, ranging from embedded compact H II regions toexposed photodissociation regions (PDRs) and the (diffuse) interstellarmedium (ISM). However, the variation in the 6.2 μm PAHfeature-to-continuum ratio is relative small. Comparison of ourextragalactic sample with our Galactic sources revealed an excellentresemblance of normal and starburst galaxies to exposed PDRs. WhileSeyfert 2 galaxies coincide with the starburst trend, Seyfert 1 galaxiesare displaced by at least a factor of 10 in 6.2 μm continuum flux, inaccordance with general orientation-dependent unification schemes forAGNs. ULIRGs show a diverse spectral appearance. Some show a typical AGNhot dust continuum. More, however, either are starburst-like or showsigns of strong dust obscuration in the nucleus. One characteristic ofthe ULIRGs also seems to be the presence of more prominent FIR emissionthan either starburst galaxies or AGNs. We discuss the observedvariation in the Galactic sample in view of the evolutionary state andthe PAH/dust abundance and discuss the use of PAHs as quantitativetracers of star formation activity. Based on these investigations, wefind that PAHs may be better suited as a tracer of B stars, whichdominate the Galactic stellar energy budget, than as a tracer of massivestar formation (O stars).
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