Issue |
A&A
Volume 461, Number 3, January III 2007
|
|
---|---|---|
Page(s) | 793 - 811 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:20066130 | |
Published online | 16 October 2006 |
Diagnostics of irradiated dense gas in galaxy nuclei
II. A grid of XDR and PDR models
1
Sterrewacht Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands e-mail: rowin@astro.berkeley.edu
2
Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
Received:
28
July
2006
Accepted:
6
October
2006
Aims.The nuclei of active galaxies harbor massive young stars, an accreting central black hole, or both. In order to determine the physical conditions that pertain to molecular gas close to the sources of radiation, numerical models are constructed.
Methods.These models iteratively determine the thermal and chemical balance of molecular gas that is exposed to X-rays (1–100 keV) and far-ultraviolet radiation (6–13.6 eV), as a function of depth.
Results. We present a grid of XDR and PDR models that span ranges in
density ( cm-3), irradiation (
and
erg cm-2 s-1) and
column density (
cm-2). Predictions are made for the most important
atomic fine-structure lines, e.g., [CII], [OI], [CI], [SiII], and
for molecular species like HCO+, HCN, HNC, CS and SiO up to
, CO and 13CO up to
, and column densities for CN,
CH, CH+, HCO, HOC+, NO and N2H+. We find that surface
temperatures are higher (lower) in PDRs compared to XDRs for
densities >104 (<104) cm-3. For the atomic lines, we
find that, largely due to the different XDR ionization balance, the
fine-structure line ratios of [SiII] 35 μm/[CII] 158 μm,
[OI] 63 μm/[CII] 158 μm, [FeII] 26 μm/[CII] 158 μm
and [CI] 369 μm/[CI] 609 μm are larger in XDRs than in PDRs,
for a given density, column and irradiation strength. Similarly,
for the molecular lines, we find that the line ratios HCN/HCO+
and HNC/HCN, as well as the column density ratio CN/HCN,
discriminate between PDRs and XDRs. In particular, the HCN/HCO+
1–0 ratio is <1 (>1) for XDRs (PDRs) if the density exceeds
105 cm-3 and if the column density is larger than
1023 cm-2. For columns less than 1022.5 cm-2
the XDR HCN/HCO+ 1–0 ratio becomes larger than one, although the
individual HCN 1–0 and HCO+ 1–0 line intensities are weaker. For
modest densities,
cm-3, and strong radiation
fields (>100 erg s-1 cm-2), HCN/HCO+ ratios can
become larger in XDRs than PDRs as well. Also, the HCN/CO 1–0 ratio
is typically smaller in XDRs, and the HCN emission in XDRs is
boosted with respect to CO only for high (column) density gas, with
columns in excess of 1023 cm-2 and densities larger than
104 cm-3. Furthermore, CO is typically warmer in XDRs than
in PDRs, for the same total energy input. This leads to higher CO
/CO 1–0,
, line ratios in XDRs. In particular, lines
with
, like CO(16–15) and CO(10–9) observable with
HIFI/Herschel, discriminate very well between XDRs and PDRs. This
is crucial since the XDR/AGN contribution will typically be of a
much smaller (possibly beam diluted) angular scale and a 10–25%
PDR contribution can already suppress XDR distinguishing features
involving HCN/HCO+ and HNC/HCN. For possible future observations,
column density ratios indicate that CH, CH+, NO, HOC+ and HCO
are good PDR/XDR discriminators.
Key words: galaxies: ISM / galaxies: active, X-rays: ISM / X-rays: galaxies
© ESO, 2007
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