The H[TINF]2[/TINF]O Megamasers in NGC 2639—Evidence for an Accretion Disk in a LINER Nucleus

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Abstract
We report monitoring of the spectrum of the water vapor megamasers in the nucleus of the LINER galaxy NGC 2639. Our highest sensitivity spectra reveal a bright, narrow (4.4 km s-1 FWHM) line, with a broad (50 km s-1) feature extending blueward of it, and tentatively detected weaker narrow features. The recessional velocity of the brightest line is found to be increasing at a rate of 6.6 ± 0.4 km s-1 yr-1 over a period of 1.4 yr. A similar redward drift is observed for the maser features near systemic velocity in NGC 4258 and is believed to result from the centripetal acceleration of clumps of gas in an accretion disk as they move across our line of sight to the central core. The megamaser spectra of NGC 2639 and NGC 4258 are remarkably similar, though high-velocity "satellite lines" have yet to be found in NGC 2639. By analogy with NGC 4258, we suggest that the H2O feature in NGC 2639 may originate in a cool dense nuclear accretion disk.

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