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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.
| 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.
| 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.
| Redshifts for Fainter Galaxies in the First CFA Slice. III. To the Zwicky Catalog Limit
We have measured redshifts for 241 galaxies selected from the Zwickycatalog with magnitudes 15.6 or 15.7 in the region (26.5^deg^ <=32.5^deg^, 8^h^<= α <= 17^h^, 1950). This region is theoriginal CfA "Slice of the Universe," which was complete to 15.5; nownearly all Zwicky-catalog galaxies in the region have measuredredshifts. These somewhat fainter galaxies outline the same structuresas the brighter ones. By examining a sample of 27 galaxies within themost prominent void we find (1) the fraction of absorption-line galaxiesis typical of regions outside the cores of rich clusters; and (2) thelocal morphology- density relation holds within the globally underdensevoid.
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