Neutral Hydrogen Absorption and Emission in the Quasar/Galaxy Pair 3C275.1/\NGC4651

Abstract

3C275.1 and \NGC4651 make a particularly interesting quasar/galaxy pairing because of the alignment of such a strong radio emitter behind the outer H\I disk of a relatively undisturbed spiral galaxy. This provides an opportunity to study the spin-temperature characteristics of atomic hydrogen at low column densities, in an apparently star-free environment. We previously reported a tentative detection of absorption against the quasar based on VLA C--array observations; we have now made more sensitive maps of the H\I emission from \NGC4651 with the VLA D--array, and we have attempted to confirm the weak H\I absorption against the quasar at higher spatial and spectral resolution in VLA B--array. The possible absorption feature against this quasar appears to be weaker than we previously suspected, even though it seems fairly clear that H\I emission is present close to the line of sight to the quasar.The weakness of the possible absorption seems also to confirm, conversely, the trend found in previous observations that where strong absorption lines are seen, the galaxies show evidence of disturbance. The possible detection of (or limits on) absorption suggest that the neutral gas in the outer disk is quite warm. We use the absorption and emission measurements to set lower limits on the combination of heating inputs outside the star-forming regions of a disk galaxy and/or the intensity of the cosmic background radiation around 100 eV.