Mon. Not. R. Astron. Soc. 000, 1–6 (2008) Printed 1 August 2008 (MN L
A
T
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X style file v2.2)
Mapping low and high density clouds in astrophysical
nebulae by imaging forbidden line emission
⋆
J. E. Steiner
1
†, R. B. Menezes
1
, T.V. Ricci
1
and A. S. Oliveira
2
1
Instituto de Astronomia, Geof´ı sica e Ciˆenci as Atmosf´ericas, Universidade de S˜ao Paulo, 05508-900, S˜ao Paulo, SP, Brasil
2
IP&D, Uni versidade do Vale do Para´ıba, Av. Shishima Hifumi, 2911, CEP 12244-000, S˜ao Jos´e dos Campos, SP, Brasil
Accepted . Received
ABSTRACT
Emission line ratios have been essential for determining physical parameters such
as gas temperature and density in astrophysical gaseous nebulae. With the advent
of panoramic spectroscopic devices, images of regions with emission lines related to
these physical parameters can, in principle, also be produced. We show that, with
observations from modern instruments, it is possible to transform images taken from
density sensitive forbidden lines into images of emission from high and low-density
clouds by applying a transformation matrix. In order to achieve this, images of the
pairs of density sensitive lines as well as the adjacent continuum have to be observed
and combined.
We have computed the critical densities for a series of pairs of lines in the in-
frared, optical, ultraviolet and X-rays bands, calculated the pair line intensity ratios
in the high and low-density limit using a 4 and 5 level atom approximation; we have
applied the method to two galactic nuclei. We conclude that this method provides
new information of astrophysical interest, especially for mapping low and high-density
clouds.
Key words: Atomic processes – techniques: image processing – techniques: spectro-
scopic – ISM: clouds.
1 INTRODUCTION
Forbidden line intensity ratios from given species (O
+
, O
++
,
N
+
, S
+
) have been widely used in the literature to derive
average electron temperatures and densities in astrophysical
nebulae. The method for measuring the electron tempera-
ture was suggested by Menzel, Aller & Hebb (1941) while the
idea of using the [O ii] line intensity ratios to measure elec-
tron densities was suggested by Aller, Ufford & van Vleck
(1949) and worked out quantitatively by Seaton (1954) for
both [O ii] and [S ii] lines. An early review of these methods
is given by Seaton (1960). Since then, intensity ratios for
lines from many other species have been proposed and used.
Electron temperatures are sensitive to the intensity ra-
⋆
Based on observations obtained at the Gemini O bser vatory,
which is operated by the Association of Universities for Re-
search in Astronomy, Inc., under a cooperative agreement with
the NSF on behalf of the Gemini partnership: the National Sci-
ence Foundation (United States), the Science and Technology Fa-
cilities Council (United Kingdom), the National Research Council
(Canada), CONICYT (Chile), the Australian Research Council
(Australia), Minist´erio da Ciˆencia e Tecnologia (Brazil) and SE-
CYT (Argentina).
tio of the auroral to the nebu lar components, for example,
for t he Carbon-like 2p
2
and the Silicon-like 3p
2
ions. A clas-
sical intensity ratio is that of [O iii] Iλ 4363/Iλ5007. Aver-
age electron densities are obtained, for example, from the
Nitrogen-like 2p
3
and Phosphorus-like 3p
3
ions. Intensity
ratios, used very often, are those of [O ii] Iλ3726/Iλ3729
and [S ii] Iλ6716/Iλ6731 lines. With the development of in-
frared, ultraviolet and X-ray detectors, other pairs of lines
have also been used. A comprehensive review on the subject
is given in Osterbrock & Ferland ( 2006).
In the traditional single aperture spectroscopic ap-
proach, one obtains a single spectrum of a given object and
only one intensity ratio is measured. This has the obvious
disadvantage of providing a single average property (tem-
perature and/or d ensity) for the object at a time for a sin-
gle object or along the slit. With the development of two-
dimensional spectroscopic devices such as the Integral Field
Units (IFU) and Fabry-Perot instruments, a new approach is
possible as one can obtain simultaneously the average prop-
erty along the line of sight for each point of the object on
the sky, if the object is spatially resolved.
In this paper we present a method of transforming two
images of density-sensitive emission lines into two other im-
ages, of high and low-density cloud emission. We demon-
c
2008 RAS
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