Detection of H92 alpha Recombination Line from the Galaxies NGC 3628, IC 694, and NGC 1365
Abstract
Using the Very Large Array with an angular resolution of ∼3", we have detected the hydrogen recombination line H92α (νrest = 8309.38 MHz) from the starburst nuclei in the galaxies NGC 3628 and IC 694 and also from the Seyfert II nucleus in NGC t365. In each case the line-emitting region extends over a few hundred parsecs. The detected lines have peak intensities in the range 0.5-1.5 mJy and widths (FWHM) 200- 400 km s-1. The line was not detected in three other galaxies (NGC 262, NGC 1068, and NGC 3079) that were observed to a similar sensitivity level.
We present a model in which a collection of H II regions in the nuclear region accounts for the observed H92α line. The required number of H II regions, their temperature, electron density, and linear size are constrained by the observed line flux density, line width, continuum spectrum, and size of the line-emitting region. If the temperature of the H II regions is above 5000 K, then electron densities in the range 5-50 × 103 cm-3 are permitted by the available constraints. Several hundred H II regions of a few parsecs in size, with a total mass of a few times 105 Msun, are required to account for the observed line flux density. The rate of production of Lyman continuum photons required to maintain the ionization is a few times 1054 s-1. Much of the line emission comes from internal stimulated emission due to the continuum generated within the H II regions which account for 5%-30% of the observed total continuum at 5 GHz. Predictions are made for the expected recombination line and continuum flux density as a function of frequency from the nuclear region of the galaxies.- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 1993
- DOI:
- 10.1086/173510
- Bibcode:
- 1993ApJ...419..585A
- Keywords:
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- GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1365;
- GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3628;
- GALAXIES: INDIVIDUAL ALPHANUMERIC: IC 694;
- GALAXIES: NUCLEI;
- RADIO LINES: GALAXIES