Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The origin of molecular hydrogen emission in cooling-flow filaments   The optical filaments found in many cooling flows in galaxy clusters consist
of low density ($\sim 10^3 \pcc$) cool ($\sim 10^3$ K) gas surrounded by
significant amounts of cosmic-ray and magnetic-field energy. Their spectra show
anomalously strong low-ionization and molecular emission lines when compared
with galactic molecular clouds exposed to ionizing radiation such as the Orion
complex. Previous studies have shown that the spectra cannot be produced by
O-star photoionization. Here we calculate the physical conditions in dusty gas
that is well shielded from external sources of ionizing photons and is
energized either by cosmic rays or dissipative MHD waves. Strong molecular
hydrogen lines, with relative intensities similar to those observed, are
produced. Selection effects introduced by the microphysics produce a
correlation between the \htwo line upper level energy and the population
temperature. These selection effects allow a purely collisional gas to produce
\htwo emission that masquerades as starlight-pumped \htwo but with intensities
that are far stronger. This physics may find application to any environment
where a broad range of gas densities or heating rates occur.

23
The categories of this paper are: solv-int
8
The categories of this paper are: hep-ex
21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
32
The categories of this paper are: atom-ph
34
The categories of this paper are: chem-ph
29
The categories of this paper are: funct-an
5
The categories of this paper are: gr-qc
19
The categories of this paper are: eess
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
37
The categories of this paper are: supr-con
16
The categories of this paper are: stat
9
The categories of this paper are: nlin