Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical Reddening, Integrated HI Optical Depth, Total Hydrogen Column
  Density   Despite the vastly different angular scales on which they are measured, the
integrated $\lambda$21 cm \HI\ optical depth measured interferometrically,
\WHI, is a good proxy for the optical reddening derived from IR dust emission,
with %\WHI\ $\equiv \int\tau({\rm \HI}) ~{\rm dv} =
(13.8\pm0.7)$\EBV$^{(1.10\pm0.03)}$ \kms\ \WHI\ $\propto$ \EBV$^{1.10}$ for
0.04 mag $\la$ \EBV\ $\la$ 4 mag. For \EBV\ $\la 0.04$ mag or \WHI\ $< 0.7$
\kms, less-absorbent warm and ionized gases assert themselves and $\tau$(HI) is
a less reliable tracer of \EBV. The \WHI-\EBV\ relationship can be inverted to
give a broken power-law relationship between the total hydrogen column density
N(H) and \WHI\ such that knowledge of \WHI\ alone predicts N(H) with an
accuracy of a factor 1.5 ($\pm 0.18$ dex) across two orders of magnitude
variation of \WHI. The \WHI-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 HI such as the spin temperature.

18
The categories of this paper are: econ
1
The categories of this paper are: cs
14
The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
34
The categories of this paper are: chem-ph
29
The categories of this paper are: funct-an
31
The categories of this paper are: ao-sci
15
The categories of this paper are: nucl-ex
27
The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
21
The categories of this paper are: adap-org
17
The categories of this paper are: q-fin
26
The categories of this paper are: bayes-an
20
The categories of this paper are: acc-phys
0
The categories of this paper are: hep-ph