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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.
| 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.
| 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.
| An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample
The importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies.
| A study of the infrared characteristics of host IRAS sources with OH megamasers
OH megamasers are the most luminous cosmic maser sources known. So farpowerful OH maser emission has been discovered from 90 extragalacticobjects. An important observational characteristic of the OH megamasersources is their relationship between L(OH) and L(IR). We study therelationship between logL(OH) and logL(IR) for the 67 OH megamasersources for which there are data on L(OH) and L(IR). Accounting forMalmquist bias, the relationship L(OH) ~L(IR)1.41 isobtained. We use the largest sample currently available to study therelationship between L(OH) and L(IR) for OH megamaser sources. Ourresults agree with Kandalian's results within the uncertainties.The infrared properties of the host IRAS sources with OH megamasers arealso studied. The most striking features are the anticorrelation of S(12μm)/S(25 μm) versus S(60 μm)/S(100 μm) and the correlationof S(12 μm)/S(25 μm) versus S(25 μm)/S(60 μm). They areconsistent with Henkel, Wouterlooy & Bally's finding that S(12μm)/S(25 μm) is anticorrelated with S(60 μm)/S(100 μm), butare the opposite of Henkel et al.'s result that S(12 μm)/S(25 μm)is correlated with S(25 μm)/S(60 μm). This is an interestingdifference. Our colour-colour plots suggest that the peak of theinfrared spectra of our sample of OH sources is at a longer wavelengththan the peak in the sample of Henkel et al. This suggests that infraredradiation from our sample is dominated by emission from material atgreater separations from the central source.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| The infrared supernova rate in starburst galaxies
We report the results of our ongoing search for extincted supernovae(SNe) at near-infrared wavelengths. We have monitored at 2.2 mu m asample of 46 Luminous Infrared Galaxies and detected 4 SNe. The numberof detections is still small but sufficient to provide the firstestimate of supernova rate at near-infrared wavelengths. We measure a SNrate of SNNIR_r=7.6+/- 3.8 SNu which is an order of magnitudelarger than observed in quiescent galaxies. On the other hand, theobserved near-infrared rate is still a factor 3-10 smaller than thatestimated from the far-infrared luminosity of the galaxies. Amongvarious possibilities, the most likely scenario is that dust extinctionis so high (AV>30) to obscure most SNe even in thenear-IR.The role of type Ia SNe is also discussed within this context. We derivethe type Ia SN rate as a function of the stellar mass of the galaxy andfind a sharp increase toward galaxies with higher activity of starformation. This suggests that a significant fraction of type Ia SNe areassociated with young stellar populations.Finally, as a by-product, we give the average K-band light curve ofcore-collapse SNe based on all the existing data, and review therelation between SN rate and far-infrared luminosity.Based on observations collected at the European Southern Observatory,Chile (proposal 66.B-0417), at the Italian Telescopio Nazionale Galileo(TNG) operated on the island of La Palma by the Centro Galileo Galileiof the INAF (Istituto Nazionale di Astrofisica), and at the StewardObservatory 61'' telescope.
| A Search for H2O Maser Emission in Southern Active Galactic Nuclei and Star-forming Galaxies: Discovery of a Maser in the Edge-on Galaxy IRAS F01063-8034
We report the cumulative results of five surveys for H2Omaser emission at 1.35 cm wavelength in 131 active galactic nuclei(AGNs) and star-forming galaxies, conducted at the Parkes Observatorybetween 1993 and 1998. We detected one new maser, in the edge-on galaxyIRAS F01063-8034, which exhibits a single ~0.1 Jy spectral feature at4282+/-6 km s-1 (heliocentric) with an unusually large54+/-16 km s-1 half-power full width. The centroid velocityof the emission increased to 4319.6+/-0.6 km s-1 (38+/-2 kms-1 width) over the 13 days between discovery andconfirmation of the detection. A similarly broad-line width and largechange in velocity has been noted for the maser in NGC 1052, wherein jetactivity excites the emission. Neither optical spectroscopy,radio-infrared correlations, nor infrared colors provide compellingevidence of unusual activity in the nucleus of IRAS F01063-8034. Sincethe galaxy appears to be outwardly normal at optical and infraredwavelengths, detection of an H2O maser therein is unique. Themaser emission is evidence that the galaxy harbors an AGN that isprobably obscured by the edge-on galactic disk. The detection highlightsthe possibility that undetected AGNs could be hidden in other relativelynearby galaxies. No other maser emission features have been identifiedat velocities between 3084 and 6181 km s-1.
| Correlation between luminosity of OH megamasers and infrared luminosity of their host galaxies.
This paper has studied the correlation between luminosity of 66 OHmegamasers and infrared luminosity of their host galaxies. A relationlog L(OH)=1.71 log L(IR)-17.67, i.e. L(OH) ? [L(IR)]1.71 , is obtained.The results interposed between L(OH) ? [L(IR)]2 obtained by Baan (1989)and L(OH) ? [L(IR)]1.38 obtained by Kandalian (1996). because the samplenumber is the largest among the mentioned results, our result closes thepractical condition. Furthermore, the 66 OH megamasers can be divided intwo groups. First group is the OH megamasers having small luminosity ofL(OH)<102L?, which contains 14 OH megamasers. Second one is onehaving large luminosity of L(OH)?102L?, which contains 52 OH megamasers.Our results indicate, the correlation between luminosity of OHmegamasers having small luminosity and infrared luminosity of their hostgalaxies is L( OH) ? L[( IR)] 1.43, which closes one obtained byKandalia (1996), and the correlation between luminosity of OH megamaserhaving large luminosity and infrared luminosity of their host galaxiesis L(OH) ? [L(IR)] 2 , which is the same one obtained by Baan(1989).
| Radio Properties of Infrared-selected Galaxies in the IRAS 2 Jy Sample
The radio counterparts to the IRAS Redshift Survey galaxies areidentified in the NRAO VLA Sky Survey (NVSS) catalog. Our new catalog ofthe IR flux-limited (S60μm>=2 Jy) complete sample of1809 galaxies lists accurate radio positions, redshifts, and 1.4 GHzradio and IRAS fluxes. This sample is 6 times larger in size and 5 timesdeeper in redshift coverage (to z~0.15) than those used in earlierstudies of the radio and far-IR (FIR) properties of galaxies in thelocal volume. The well-known radio-FIR correlation is obeyed by theoverwhelming majority (>=98%) of the IR-selected galaxies, and theradio AGNs identified by their excess radio emission constitute onlyabout 1% of the sample, independent of the IR luminosity. TheseFIR-selected galaxies can account for the entire population of late-typefield galaxies in the local volume, and their radio continuum may beused directly to infer the extinction-free star formation rate in mostcases. Both the 1.4 GHz radio and 60 μm IR luminosity functions arereasonably well described by linear sums of two Schechter functions, onerepresenting normal, late-type field galaxies and the other representingstarbursts and other luminous IR galaxies. The integrated FIR luminositydensity for the local volume is (4.8+/-0.5)×107Lsolar Mpc-3, less than 10% of which iscontributed by the luminous IR galaxies withLFIR>=1011 Lsolar. The inferredextinction-free star formation density for the local volume is0.015+/-0.005 Msolar yr-1 Mpc-3.
| Starburst or Seyfert? Adding a Radio and Far-Infrared Perspective to the Investigation of Activity in Composite Galaxies
It was once common to regard Seyfert and starburst galaxies ascompletely different types of object, but there is growing recognitionthat these classifications refer to the extremes of a continuousspectrum of galaxy types. In a previous study we investigated a sampleof galaxies with ambiguous optical emission-line ratios and concludedfrom near-infrared spectroscopic observations that the sample consistedof composite galaxies, containing both a starburst and an activegalactic nucleus (AGN). We now extend our study using radio synthesisand long-baseline interferometer observations made with the AustraliaTelescope, together with far-infrared IRAS observations, to discuss therelative contribution of starburst and AGN components to the overallluminosity of the composite galaxies. We find that only a small fractionof the radio emission (<10%) can be attributed to an AGN and that themajority of the far-infrared emission (>90%) is probably due to thestarburst component. We also show that an AGN contribution to theoptical emission of as little as 10% is sufficient to account for theambiguous line-ratio diagnostics.
| H I line observations of luminous infrared galaxy mergers
A total of 19 luminous infrared galaxy mergers, with LIR>~ 2 1011 Lsun, for H0=75 kms-1 Mpc-1, have been observed in the H I line atNançay and four of them were observed at Arecibo as well. Ofthese 19, ten had not been observed before. Six were clearly detected,one of which for the first time. The objective was to statisticallysample the H I gas mass in luminous infrared mergers along a starburstmerger sequence where the molecular CO gas content is already known. Wealso searched the literature for H I data and compared these with ourobservations.
| Cold gas and star formation in a merging galaxy sequence
We explore the evolution of the cold gas (molecular and neutralhydrogen) and star formation activity during galaxy interactions, usinga merging galaxy sequence comprising both pre- and post-mergercandidates. Data for this study come from the literature, but aresupplemented by some new radio observations presented here. First, weconfirm that the ratio of far-infrared luminosity to molecular hydrogenmass (LFIRM(H2); star formation efficiency)increases close to nuclear coalescence. After the merging of the twonuclei there is evidence that the star formation efficiency declinesagain to values typical of ellipticals. This trend can be attributed toM(H2) depletion arising from interaction induced starformation. However, there is significant scatter, likely to arise fromdifferences in the interaction details (e.g., disc-to-bulge ratio,geometry) of individual systems. Secondly, we find that the centralmolecular hydrogen surface density, ΣH2,increases close to the final stages of the merging of the two nuclei.Such a trend, indicating gas inflows caused by gravitationalinstabilities during the interaction, is also predicted by numericalsimulations. Furthermore, there is evidence for a decreasing fraction ofcold gas mass from early interacting systems to merger remnants,attributed to neutral hydrogen conversion into other forms (e.g., stars,hot gas) and molecular hydrogen depletion resulting from ongoing starformation. The evolution of the total-radio to blue-band luminosityratio, reflecting the total (disc and nucleus) star formation activity,is also investigated. Although this ratio is on average higher than thatfor isolated spirals, we find a marginal increase along the mergingsequence, attributed to the relative insensitivity of disc starformation to interactions. However, a similar result is also obtainedfor the nuclear radio emission, although galaxy interactions arebelieved to significantly affect the activity (star formation, AGN) inthe central galaxy regions. Nevertheless, the nuclear-radio to blue-bandluminosity ratio is significantly elevated compared with that forisolated spirals. Finally, we find that the FIR-radio flux ratiodistribution of interacting galaxies is consistent with star formationbeing the main energizing source.
| Near-infrared probing of embedded structures in starburst and Seyfert galaxies
Surface photometry in the J and K' bands of 15 southernSeyfert or starburst galaxies is presented. The detailed centralmorphology and structural properties of these objects were analyzed byfitting ellipses to isophotes. New central peculiar structures have beenidentified like, for instance, three double-barred systems (ESO215-G031; ESO 320-G030; ESO 443-G017), one object with a nested nuclearspiral structure at the center of a primary bar (NGC 5135), one objectwith a nuclear bar without evidence of a large-scale bar (NGC 4941), andone galaxy with a likely dissolved secondary bar within a primary one(ESO 508-G005). The J-K' radial profile proved to bereasonably well linked with the presence of a starburst, but not withthe Seyfert activity. For significant starbursts, the centralJ-K' value is 0.3 - 1.5 magnitude larger than the disc one.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO programme 59.A--0773).
| Molecular Gas Depletion and Starbursts in Luminous Infrared Galaxy Mergers
Most luminous infrared galaxies (LIGs) are closely interacting/mergingsystems that are rich in molecular gas. Here we study the relationshipbetween the stage of the galaxy-galaxy interactions, the molecular gasmass, and the star formation rate as deduced from the infraredluminosity L_IR in LIGs. We find a correlation between the CO (1-0)luminosity [a measure of molecular mass M(H_2)] and the projectedseparation of merger nuclei (the indicator of merging stages) in asample of 50 LIG mergers, which shows that the molecular gas contentdecreases as merging advances. The starburst is due to enhanced starformation in preexisting molecular clouds and not to the formation ofmore molecular clouds from atomic gas. Because of the starbursts, themolecular content is being rapidly depleted as merging progresses. Thisis further supported by an anticorrelation between L_IR/M(H_2), theglobal measure of the star formation rate per unit gas mass, and theprojected separation that implies an enhanced star formation``efficiency'' in late-stage mergers compared with that of earlymergers. This is the first evidence connecting the depletion ofmolecular gas with starbursts in interacting galaxies.
| A comprehensive search for extragalactic 6.7-GHz methanol masers
We have used the Australia Telescope Compact Array (ATCA) to search for6.7-GHz methanol maser emission towards 87 galaxies. We chose the targetsources using several criteria, including far-IR luminosities and thepresence of known OH megamasers. In addition, we searched for methanolmasers in the nearby starburst galaxy NGC 253, making a fullspectral-line synthesis image. No emission was detected in any of thegalaxies, with detection limits ranging from 25 to 75 mJy. This issurprising, given the close association of OH and methanol masers inGalactic star formation regions, and significantly constrains models ofOH megamaser emission. This absence of maser emission may be a result oflow methanol abundances in molecular clouds in starburst galaxies.
| Optical Classification of Megamaser Galaxies
We have obtained spectroscopic observations of the nuclear regions of 42galaxies known to harbor strong OH masers. These megamaser galaxiesrepresent a subsample of FIR (ultra)luminous galaxies, which typicallyhave FIR luminosities in excess of 10^11 L_ȯ. The primary goal ofthis study is to investigate the nuclear activity sources of OHmegamaser galaxies. We are able to classify the nuclear emission-linespectra of all but one of our sample, and we find that this class ofgalaxies is dominated by active galactic nuclei (AGNs), althoughstarburst galaxies do make up an appreciable fraction of the megamasersas well. Fully 45% of the megamasers exhibit Seyfert or LINER spectra,predominantly of Seyfert 2 type, although two galaxies with broad linesare observed. This observation is consistent with the currentunification models for the two types of Seyfert activity, since theassumed geometry necessary to detect a megamaser places the Seyfertnucleus behind a high column density of molecular gas (i.e., themolecular torus). Starburst-nucleus galaxies comprise 32.5% of oursample, while 22.5% are classified as ``composite nuclear spectra''(CSN) sources, showing evidence of both AGN and starburst activity. Anumber of objects show unusual emission-line ratios, not surprising fora group of galaxies that are known a priori to possess substantialabsorbing material along the line of sight to their nuclei. Our resultsare compared to previous studies of FIR-selected galaxy samples, as wellas to radio-continuum observations of these galaxies. The activityclassification obtained from the radio data disagrees with the opticalclassifications in roughly 25% of the sources; we discuss possibleexplanations for these discrepancies.
| OH Zeeman measurements of the magnetic fields in four megamaser galaxies
We report accurate circular polarization measurements of four bright OHmegamasers (III Zw 35, 04454-4838, 10039-3338 and 20100-4156) with theAustralia Telescope Compact Array. No convincing Zeeman splitting wasobserved. We place 3sigma upper limits of approximately 3 mG on anyuniform line-of-sight magnetic field present in the emitting regions. Ifthe field is non-uniform, then we can place upper limits ofapproximately 5mG. These limits in turn imply constraints on thephysical conditions of the maser emission regions which lie in thecentral 100 pc of megamaser galaxies.
| A 1.425 GHz Atlas of the IRAS Bright Galaxy Sample, Part II
Galaxies with δ >= -45^deg^ and |b| >= 10^deg^ in the IRASBright Galaxy Sample, Part II, were observed at 1.425 GHz by the VeryLarge Array in its B, CnB, C, DnC, and D configurations. An atlas ofradio contour maps and a table listing the principal radio sourceparameters (position, flux density, angular size) are given. This atlasof 187 galaxies supplements the 1.49 GHz atlas of 313 galaxies in therevised Bright Galaxy Sample, Part I. Together, they are complete forextragalactic sources stronger than S = 5.24Jy at λ = 60 micronsin the area |b| > 10^deg^, δ > -45^deg^. To the extent thatthe far-infrared and radio brightness distributions overlap, these radiomaps provide the most accurate positions and high-resolution images ofthe brightest extragalactic far-infrared sources.
| Megamaser galaxies
An analysis of a sample of OH megamaser galaxies is presented. It isshown that the dependence of LOH on LFIR (far infrared) is notquadratic, as previously assumed, but closer to linear. In megamasergalaxies, LC (radio continuum) LFIR1. Analysis of the data also showsthat as the OH emission line width decreases, the absolute values of thepumping efficiency and maser amplification coefficient increase. Theradio luminosity of the central component in these galaxies increaseswith a decrease in the ratio LFIR/LC, whereas the FIR luminosity remainsconstant. These results will, in all probability, force a reexaminationof the questions of pumping and maser emission mechanisms in megamasergalaxies.
| The IRAS Bright Galaxy Survey - Part II: Extension to Southern Declinations (delta ~< -30), and Low Galactic Latitudes (f<|b|
Complete IRAS Observations and redshifts are reported for all sourcesidentified in the IRAS Bright Galaxy Survey-Part II (hereafter referredto as BGS_2_). Source positions, radial velocities, optical magnitudes,and total flux densities, peak flux densities, and spatial extents at12, 25, and 100 ,microns are reported for 288 sources having 60 micronflux densities > 5.24 Jy, the completeness limit of the originalBright Galaxy Survey [Soifer et al., AJ, 98,766(1989)], hereafterreferred to as BGS_1_. These new data represent the extension of theIRAS Bright Galaxy Survey to southern declinations,δ<~-30^deg^, and low Galactic latitudes,5^deg^<|b|<30^deg^. Although the sky coverage of the BGS_2_ (~19935 deg^2^) is 37% larger than the sky coverage of the BGS_1_, thenumber of sources is 8% smaller due primarily to large scale structurein the local distribution of galaxies. Otherwise, the sources in theBGS_2_ show similar relationships between number counts and flux densityas observed for the 313 sources in the BGS_1_. The BGS_2_ along with theearlier BGS, represents the best sample currently available for definingthe infrared properties of galaxies in the local (z <~ 0.1) Universe.
| The OH zone in OH megamaser galaxies.
The physical conditions of kinetic temperature T_K_, dust temperatureT_d_, OH number density n(OH), H_2_ number density n(H_2_), velocityfields and microturbulence, in OH-containing clouds in OH megamasergalaxies, have been investigated using Accelerated Lambda Iteration(ALI). Compact (3pc) clouds, containing dust with T_d_>T_K_>40K to50K, with n(H_2_) between a few times 10^3^ and a few times 10^4^cm^-3^and n(OH)~10^-4^cm^-3^, are radiatively pumped to yield dominant 1667MHzemission, with 1667/1665 flux ratios covering the full range of thoseobserved. Larger clouds, between 25-50pc and 300pc, tend to show weaker18cm emission or 18cm absorption for the same OH column densities of~10^15^cm^-2^. These clouds yield 6GHz absorption, as observed in Arp220. Velocity fields are influential in determining the emission andabsorption properties of clouds via effects of FIR line overlap.Microturbulence does not however play an important role in determiningOH megamaser characteristics. A failure of the present work is theinability to model absorption at 4765MHz, observed in a number ofmegamaser galaxies. Using the results of ~2000 models, it is possible toestimate physical conditions in several zones in Arp 220, IRAS17208-0014 and III Zw 35, using the observed 18cm line ratios, with datafor the satellite lines. Models also predict that OH-absorptiongalaxies, showing absorption in 18cm and 5cm main lines, may frequentlybe accompanied by weak 1720MHz emission. 1612MHz emission may bediagnostic of the presence of larger clouds (50-100pc). These and otherpredictions may be directly investigated by observation.Multi-frequency, phase referenced interferometric data, includingcontinuum maps, are essential for significant progress in this field.Our results are consistent with a model in which OH megamaser galaxiesmay evolve into OH absorption galaxies, through OH-containing cloudscoalescing, becoming depleted in OH and suffering a reduction in dusttemperature to equal or below a kinetic temperature of 40K to 45K.
| A Search for Extragalactic Methanol Masers
A sensitive search for 6.7-GHz methanol maser emission has been madetowards 10 galaxies which have yielded detectable microwave molecular-line transitions. These include several that show OH megamaser orsuperluminous H_2_O maser emission. Within the Galaxy, CH_3_OH and OHmasers often occur in the same star formation regions and, in mostcases, the CH_3_OH masers have greater peak flux densities than their OHcounterparts. Thus we might expect CH_3_OH masers to be associated withextragalactic OH maser sources. We failed to detect any emission orabsorption above our 60-mJy detection limit. We conclude that, if thephysical conditions exist to produce CH_3_OH megamaser emission, theyare incompatible with the conditions that produce OH megamaser emission.
| A search for OH emission from IRAS sources at high galactic latitudes
A survey of high galactic latitude IRAS point sources with colourstypical for ultracompact HII regions was carried out in the main linesof OH. Amongst 70 IRAS sources surveyed, one new OH maser emitting onlyin the 1667 MHz main line was discovered and 6 weaker candidate maserswere found. It is proposed that the masers with dominant 1667 MHzemission are separate from the common main line Class I OH masersemitting mostly at 1665 MHz. The Class I masers could be divided intosubclasses Ia and Ib for 1665 and 1667 MHz emitting sourcesrespectively. A correlation between infrared and OH line emission fluxesis confirmed by a lower detection rate of the present survey as comparedto the survey of stronger IRAS sources. More than one half of thedetected sources show thermal OH emission and are probably associatedwith nearby dust clouds.
| A southern OH megamaser survey
A search for OH emission from luminous IRAS galaxies with the Parkestelescope has been completed. Two new megamasers were found out of 122galaxies searched. Based on these and previous searches we discuss therequired conditions for OH megamaser emission, and derive the set ofvariables necessary to explain the observed correlations for megamasers.New redshifts for 47 of the candidates were obtained with the 1.9-m MtStromlo telescope and the Anglo-Australian Telescope. Detailedspectroscopy of two of the megamaser galaxies shows them to havelow-ionization emission-line nuclei, suggesting the presence of highlyobscured active nuclei. Radio continuum measurements have also been madeof known megamaser galaxies with the Compact Array of the AustraliaTelescope. The radio-infrared relation is tightly defined for megamasergalaxies. As a class they have somewhat lower 5-GHz to 60-micron fluxdensity ratios than spiral galaxies, consistent with megamasers beingfound in young starburst galaxies.
| IRAS 14070 + 0525 - an OH gigamaser at Z = 0.265
OH gigamaser emission has been detected in the luminous FIR galaxy IRAS14070 + 0525 at a redshift of 0.265. The FIR luminosity of IRAS 14070 +0525 is 10 exp 12.5 solar luminosities. The OH line luminosity is 14,000solar luminosities, making it the most luminous known OH emitter. The OHline emission covers a velocity range of approximately 2400 km/s. Twodistinct components in the OH emission reveal a powerful outflowcomponent and/or the interactive nature of the system.
| The far-infrared/H-alpha correlation of extreme IRAS galaxies
Using IRAS IR data and H-alpha CCD data for a sample of 38 FIR selectedstarburst galaxies with FIR luminosities ranging from 109 to 5 x 10 exp11 solar luminosities, we find that the FIR and H-alpha luminosities arecorrelated. The average value of log(L(FIR)/L exp 0(H-alpha)) equals 1.9+/- 0.1. The H-alpha luminosities have been corrected for reddeningusing H-alpha/H-beta line ratios. The relation between the FIR anduncorrected H-alpha luminosities of optically selected normal galaxiesand FIR selected starburst galaxies indicates that the dust absorptionin H-alpha increases smoothly with increasing luminosities. Applicationof a simple starburst model to the L(FIR/L exp 0(H-alpha)) correlationindicates that in a starburst stars are formed up to masses of 60 solarmasses. Star formation rates derived for these starburst galaxies rangefrom 0.3 to 300 solar masses/yr. Estimates of the available gas massesindicate that these star formation rates can be sustained for at least10 exp 8 yr.
| A study of southern extreme IRAS galaxies. I - Optical observations
Optical observations are presented and spectra, broadband, color andcontinuum-subtracted H-alpha CCD images are shown for more than 50galaxies in order to study the processes that enhance the star formationrate in some galaxies. The spectra are mostly HII-types, although some25 percent are AGN-types. The broadband images show, among other things,that this sample predominantly consists of of barred signals anddistorted interacting systems, while the H-alpha images show in mostcases small compact sources. Redshifts, line-ratios, equivalent widths,and extinction determinations from the H-beta/H-alpha line ratios arederived from the spectra. Integrated magnitudes, colors, H-alpha fluxes,diameters, and luminosities of the galaxies from the CCD images are alsoderived. Average luminosity profiles of all early-type galaxies andpossible merger candidates in this sample are determined, and it isfound that all these galaxies have exponential disks except two objectsthat follow a de Vaucouleurs r exp 1/4 law: a merger candidate and anelliptical galaxy in a close interacting pair.
| A study of southern extreme IRAS galaxies. II - Radio continuum observations
Results are reported of observations of 76 extreme IRAS galaxies withhigh far-infrared/blue luminosity ratios at 6-cm wavelength with the VLAin order to investigate the star-forming regions unimpeded by dust. CCDimages and spectra of these objects are presented, as well as CO(1-0)spectra of a subsample. Twenty radio sources were found to be clearlyresolved with intrinsic sizes, ranging from 0.6 to 2.2 kpc; the upperlimits for the intrinsic size of the unresolved sources are 0.2-1.3 kpc.The resolved sources have a surface brightness similar to that ofgalactic nuclei, but considerably higher than that of galactic disks. Onaverage, some 60 percent of the total flux density derived from thelow-resolution maps is found in the high-resolution data. About 40percent of the total radio power emitted by the galaxies at 6 cm has athermal origin. This ratio is consistent with published radio spectralindices of similar extreme IRAS galaxies.
| Violent motions in starburst galaxies - The OH megamaser in IRAS 10039-3338
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990A&A...237L...1K&db_key=AST
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