Optical Reddening, Integrated H I Optical Depth, and Total Hydrogen Column Density
Abstract
Despite the vastly different angular scales on which they are measured, the integrated λ21 cm H I optical depth measured interferometrically, {{{\Upsilon }}}{{H}{{I}}}, is a good proxy for the optical reddening derived from IR dust emission, with {{{\Upsilon }}}{{H}{{I}}} ∝ E(B-V)1.10 for 0.04 mag ≲ E(B-V) ≲ 4 mag. For E(B-V) ≲ 0.04 mag or {{{\Upsilon }}}{{H}{{I}}} < 0.7 {km} {{{s}}}-1, less-absorbent warm and ionized gases assert themselves and τ(H I) is a less reliable tracer of E(B-V). The {{{\Upsilon }}}{{H}{{I}}}-E(B-V) relationship can be inverted to give a broken power-law relationship between the total hydrogen column density N(H) and {{{\Upsilon }}}{{H}{{I}}} such that knowledge of {{{\Upsilon }}}{{H}{{I}}} alone predicts N(H) with an accuracy of a factor of 1.5 (±0.18 dex) across two orders of magnitude variation of {{{\Upsilon }}}{{H}{{I}}}. The {{{\Upsilon }}}{{H}{{I}}}-N(H) relation is invariant under a linear rescaling of the reddening measure used in the analysis and does not depend on knowing properties of the H I such as the spin temperature.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2019
- DOI:
- 10.3847/1538-4357/ab2d23
- arXiv:
- arXiv:1906.10629
- Bibcode:
- 2019ApJ...881...29L
- Keywords:
-
- Diffuse interstellar clouds;
- Interstellar atomic gas;
- Interstellar dust extinction;
- Interstellar line absorption;
- 380;
- 833;
- 837;
- 843;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted to Ap J