Neutral hydrogen absorption by galaxies and implications for the soft X-ray background

Abstract

view Abstract Citations (17) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Neutral Hydrogen Absorption by Galaxies and Implications for the Soft X-Ray Background Corbelli, Edvige ; Schneider, Stephen E. Abstract We have searched for neutral hydrogen absorption in the 21 cm spectra of 59 radio continuum sources in the proximity of spiral and lenticular galaxies using the Arecibo radio telescope. Searches of this type are of importance not only for establishing the properties and the extents of halos and outer disks of galaxies but also for defining the intensity of the extragalactic ionizing flux. Five galaxies showed possible but uncertain absorption; three of these were reobserved at the VLA, and in the case of 3C 275.1/NGC 4651 the absorption appears to be confirmed. For the 54 other sources we find either that the column density of the neutral hydrogen in the intervening material is well below 2 x 10^19^ cm^-2^ or that the spin temperature is at least one order of magnitude above the microwave background temperature. For these cases we show that subthermal effects are not hiding appreciable amounts of neutral hydrogen. Upper limits for N_H I_ obtained from emission studies should therefore be reliable. Moreover, from good constraints on the spin temperature in the outer H I disk of M33 (NGC 598) we estimate that the soft X-ray background must be at least as strong as predicted by extrapolations of the hard X-ray spectrum down to 13.6 eV if no other beat sources are present. Publication: The Astrophysical Journal Pub Date: June 1990 DOI: 10.1086/168810 Bibcode: 1990ApJ...356...14C Keywords: Absorption Spectra; Hydrogen; Quasars; Spiral Galaxies; X Ray Astronomy; Background Radiation; Continuous Radiation; Galactic Structure; Radio Telescopes; Red Shift; Very Large Array (Vla); Astrophysics; COSMIC BACKGROUND RADIATION; GALAXIES: INTERSTELLAR MATTER; QUASARS; RADIO SOURCES: 21 CM RADIATION full text sources ADS | data products NED (83) SIMBAD (43)