An improved method to derive H I absorption spectra through the disks of galaxies - Application to the Sculptor Group

Details An improved method to derive H I absorption spectra through the disks of galaxies - Application to the Sculptor Group An improved method to derive H I absorption spectra through the disks of galaxies - Application to the Sculptor Group

Feb 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992apj...385..501d&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 385, Feb. 1, 1992, p. 501-511.

Astronomy and Astrophysics

Astrophysics

18

Absorption Spectra, Hydrogen, Milky Way Galaxy, Radio Galaxies, Radio Spectra, Galactic Nuclei, Optical Thickness, Synthetic Apertures

Scientific paper

The optimum technique for measuring 21-cm absorption spectra with an aperture synthesis instrument is investigated. Continuum sources behind galaxies in the Sculptor Group are used as examples to derive formulas for the best estimate for the optical depth and the error on this estimate. In this case the dominant source of error is variation in the foreground H I emission of the disks of the galaxies, and the best way to treat this problem is discussed. In addition to the absorption previously known toward the nucleus of NGC 253, new absorption is detected tentatively in one case in NGC 247. The emission spectra over the area of the continuum emission in the nucleus of NGC 253 is interpolated in order to estimate the spin temperature. The atomic gas is extremely cold, below 25 K at some velocities. It is argued that the H I observed in absorption is in the molecular clouds in the nuclear region.

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