Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: UV Continuum, Physical Conditions and Filling Factor in Active Galactic
  Nuclei   The narrow line region of active galaxies is formed by gas clouds surrounded
by a diluted gas. Standard one-dimensional photoionization models are usually
used to model this region in order to reproduce the observed emission lines.
Since the narrow line region is not homogeneous, two major types of models are
used: (a) those assuming a homogeneous gas distribution and a filling factor
less than unity to mimic the presence of the emitting clouds; (b) those based
on a composition of single-cloud models combined in order to obtain the
observed spectra. The first method is largely used but may induce to misleading
conclusions as shown in this paper. The second one is more appropriate, but
requires a large number of observed lines in order to limit the number of
single models used. After discussing the case of an extragalactic HII region,
for which the ionizing radiation spectrum is better known, we show that 1-D
models for the narrow line region with a filling factor less than unit do not
properly mimic the clumpiness, but just simulates an overall lower density.
Multi-cloud models lead to more reliable results. Both models are tested in
this paper, using the emission-line spectra of two well-known Seyfert galaxies,
NGC 4151 and NGC 1068. It is shown that ionizing radiation spectra with a blue
bump cannot be excluded by multi-cloud models, although excluded by Alexander
et al. (1999, 2000)using homogeneous models with a filling factor less than
unity.

24
The categories of this paper are: dg-ga
2
The categories of this paper are: math
9
The categories of this paper are: nlin
30
The categories of this paper are: q-alg
14
The categories of this paper are: math-ph
33
The categories of this paper are: plasm-ph
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
31
The categories of this paper are: ao-sci
32
The categories of this paper are: atom-ph
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio
21
The categories of this paper are: adap-org
13
The categories of this paper are: quant-ph