Issue |
A&A
Volume 425, Number 2, October II 2004
|
|
---|---|---|
Page(s) | 457 - 474 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20034285 | |
Published online | 21 September 2004 |
Molecular hydrogen and [Fe II] in Active Galactic Nuclei*
1
Laboratório Nacional de Astrofísica, Rua dos Estados Unidos 154, Bairro das Nações, CEP 37500-000, Itajubá, MG, Brazil e-mail: aardila@lna.br
2
Departamento de Astronomia, Instituto de Física, UFRGS, Av. Bento Gonçalves 9500, Porto Alegre, RS, Brazil e-mail: mgp@if.ufrgs.br
3
Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Rua do Matão 1226, CEP 05508-900, São Paulo, SP, Brazil e-mail: viegas@astro.iag.usp.br
4
Department of Physics and Astronomy, The University of Western Ontario, London, ON, N6A 3K7, Canada
5
Department of Astronomy, The Ohio State University, Columbus, OH 43210-1106, USA
Received:
6
September
2003
Accepted:
13
June
2004
Near-infrared spectroscopy is used to study the kinematics and
excitation mechanisms of H2 and [Fe ii] lines in a sample of mostly
Seyfert 1 galaxies. The spectral coverage allows simultaneous
observation of the JHK bands, thus eliminating the aperture and seeing
effects that have usually plagued previous works. The H2 lines are
unresolved in all objects in which they were detected while the
[Fe ii] lines have widths implying gas velocities of up to 650 km s-1. This
suggests that, very likely, the H2 and [Fe ii] emission does not
originate from the same parcel of gas. Molecular H2 lines were
detected in 90% of the sample, including PG objects, indicating detectable
amounts of molecular material even in objects with low levels of
circumnuclear starburst activity. Analysis of the observations favors
thermal excitation mechanisms for the H2 lines.
Indeed, in NGC 3227, Mrk 766, NGC 4051 and NGC 4151, the molecular
emission is found to be purely thermal but with heating processes that
vary between the objects. Thermal excitation is also confirmed by the
rather similar vibrational and rotational temperatures in the objects
for which data were available. [Fe ii] lines are detected in all
of the sample AGN. The [Fe ii] 1.254 μm/Paβ ratio is compatible
with excitation of the [Fe ii] lines by the active nucleus in most
Seyfert 1 galaxies, but in Mrk 766 the ratio implies a stellar
origin. A correlation between H2/Brγ and [Fe ii]/Paβ
is found for our sample objects supplemented by data from the
literature. The correlation of these line ratios is a useful
diagnostic tool in the NIR to separate emitting line objects by their
level of nuclear activity. X-ray excitation models are able to explain
the observed H2 and part of the [Fe ii] emission but fail to explain
the observations in Seyfert 2 galaxies. Most likely, a combination of
X-ray heating, shocks driven by the radio jet and circumnuclear star
formation contributes, in different proportions, to the H2 and
[Fe ii] lines observed. In most of our sample objects, the
[Fe ii] 1.257 μm/1.644 μm ratio is found to be 30% lower than the
intrinsic value based on current atomic data. This implies either that
the extinction towards the [Fe ii]-emitting clouds is very similar in
most objects or there are possible inaccuracies in the A-values
in the transitions.
Key words: galaxies: Seyfert / molecular processes / line: formation / line: profiles / galaxies: active / galaxies: nuclei
© ESO, 2004
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