Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: H_alpha Emission from High-Velocity Clouds and their Distances   We present deep Halpha spectroscopy towards several high-velocity clouds
(HVCs) which vary in structure from compact (CHVCs) to the Magellanic Stream.
The clouds range from being bright (~640 mR) to having upper limits on the
order of 30 to 70 mR. The Halpha measurements are discussed in relation to
their HI properties and distance constraints are given to each of the complexes
based on f_esc = 6% of the ionizing photons escaping normal to the Galactic
disk (f_escs = 1 - 2% when averaged over solid angle). The results suggest that
many HVCs and CHVCs are within a ~40 kpc radius from the Galaxy and are not
members of the Local Group at megaparsec distances. However, the Magellanic
Stream is inconsistent with this model and needs to be explained. It has bright
Halpha emission and little [NII] emission and appears to fall into a different
category than the currently detected HVCs. This may reflect the lower
metallicities of the Magellanic Clouds compared to the Galaxy, but the strength
of the Halpha emission cannot be explained solely by photoionization from the
Galaxy. The interaction of the Stream with halo gas or the presence of yet
unassociated young stars may assist in ionizing the Stream.

5
The categories of this paper are: gr-qc
14
The categories of this paper are: math-ph
13
The categories of this paper are: quant-ph
8
The categories of this paper are: hep-ex
30
The categories of this paper are: q-alg
21
The categories of this paper are: adap-org
27
The categories of this paper are: comp-gas
37
The categories of this paper are: supr-con
1
The categories of this paper are: cs
31
The categories of this paper are: ao-sci
26
The categories of this paper are: bayes-an
10
The categories of this paper are: q-bio
11
The categories of this paper are: hep-lat
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
18
The categories of this paper are: econ