A&A 367, 457-469 (2001)
DOI: 10.1051/0004-6361:20000462
S. J. Curran1,2 - L. E. B. Johansson1 - P. Bergman1 - A. Heikkilä1,3 - S. Aalto1
1 - Onsala Space Observatory,
Chalmers University of Technology,
439 92 Onsala,
Sweden
2 - European Southern Observatory,
Casilla 19001, Santiago 19,
Chile
3 - Observatory, PO Box 14,
00014 University of Helsinki,
Finland
Received 11 July 2000 / Accepted 5 December 2000
Abstract
We present results of a multi-transition study of the dense molecular
gas in the central part of the hybrid star-burst/Seyfert galaxies
NGC 4945 and the Circinus galaxy. From the results of radiative transfer
calculations, we estimate in NGC 4945
cm-3 and
K and in
Circinus
cm-3 and
K for the molecular hydrogen density and kinetic
temperature, respectively.
As well as density/temperature tracing molecules, we have observed
C17O and C18O in each galaxy and the value of
C18O/C
for the isotopic column density ratio
suggests that both have relatively high populations of massive stars.
Finally, although star formation is present, the radiative transfer
results combined with the high HCN/CO and (possibly) HCN/FIR,
radio/FIR ratios may suggest that, in comparison with Circinus, a
higher proportion of the dense gas emission in NGC 4945 may be
located in the hypothesised central nuclear disk as opposed to dense
star forming cloud cores. Contrary to the literature, which
assumes that all of the far-infrared emission arises from star
formation, our results suggest that in NGC 4945 some of this emission
could arise from an additional source, and so we believe
that a revision of the star formation rate estimates may be required
for these two galaxies.
Key words: galaxies: abundances - galaxies: individual: NGC 4945, Circinus - galaxies: nuclei - galaxies: Seyfert - galaxies: star-burst
Seyfert galaxies account for 10% of all galaxies
(Maiolino & Rieke 1995; Ho et al. 1997) and along
with LINERs
and radio quiet quasars constitute one
of the three main classes of active galactic nuclei (AGNs); the other
classes being the more luminous FRI radio galaxies/BL Lac objects and
the most luminous FRII radio galaxies/quasars. Although sometimes
associated with enhanced star-burst activity
(e.g. Whittle 1992; Gu et al. 1997; Roy et al. 1998;
Gu et al. 1999), a Seyfert nucleus exhibits
the tell-tale signs of an AGN via a non-stellar nuclear source
(Rigopoulou et al. 1997) and unlike star-burst galaxies, the nuclei are often
associated with radio or optical jets
(Hummel et al. 1983; de Grijp et al. 1985;
Cecil et al. 1992)
.
Recently, Curran et al. (2000) have shown, from
a sample of 20 Seyfert
galaxies, that the HCN/CO luminosity ratio is similar to that of
ultra-luminous infrared galaxies (ULIRGs), while being significantly
higher than that of normal spiral galaxies;
for the distant sources (
)
and
for the near-by sources (
)
(Curran et al. 2000),
cf.
for normal spirals
(Solomon et al. 1992). The fraction of the HCN brightness which arises from the
AGN (via the water maser disks
) or from star forming cores is still, however, uncertain
(e.g. Kohno et al. 1999).
NGC 4945 has its visual appearance dominated by strong and patchy extinction in the disk, which hides any distinct visible nucleus, although a relatively strong and compact (30'') non-thermal radio source, which is coincident with the peak of red and infra-red emission (Moorwood & Glass 1984), is visible in the central region. Also, wide absorption lines (OH, CH, H2CO) are found towards the radio-continuum nucleus (Whiteoak & Gardner 1975; Whiteoak & Gardner 1985; Ables et al. 1987; Whiteoak & Wilson 1990). Towards this position a very powerful H2O maser has been detected (dos Santos & Lépine 1979; Batchelor et al. 1982; Greenhill et al. 1997). The nucleus exhibits star-burst as well as Seyfert characteristics and has properties similar to that of Circinus although for some species the molecular line characteristics differ.
The Circinus galaxy also has
maser activity
(Gardner & Whiteoak 1982; Greenhill et al. 1995;
Greenhill et al. 1997; Greenhill 2000) the high luminosity of which suggests the
presence of an active galactic nucleus (AGN). Again, this galaxy is
host to nuclear star-burst activity (Harnett et al. 1990; Moorwood et al. 1996a) and the
presence of visible and near infra-red coronal lines (Oliva et al. 1994), an
X-ray reflection dominated spectrum (Matt et al. 1996) and broad polarised
H
(Oliva et al. 1998) suggest the presence of a hidden Seyfert
nucleus.
These almost edge-on near-by Southern galaxies (Freeman et al. 1977; de Vaucouleurs et al. 1981; Ables et al. 1987) are similar, in that:
Molecule | Transition |
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12CO |
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|
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|
13CO |
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|
C18O |
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|
C17O |
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|
CS |
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|
13CS |
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<2 |
SO |
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|
HCN |
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|
H13CN |
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HCO+ |
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|
H13CO+ |
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<4 | |
H2CO |
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|
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|
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<3 | |
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<3 | |
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<3 | |
CH3C2H |
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<2 |
Molecule | Transition |
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12CO |
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|
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|
13CO |
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|
C18O |
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C17O |
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CS |
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|
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|
SO |
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HCN |
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HNC |
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HCO+ |
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H2CO |
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The observations were carried out in June 1989, February 1993, June
1995, June 1998, December 1998, December 1999 and April 2000 with the
Swedish-ESO Sub-millimetre Telescope (SEST) at La Silla, Chile. The receiver
system consisted of a 3 mm Schottky mixer and cryogenic 1.3 mm, 2 mm and 3
mm SIS mixers. The mixers were tuned to single sideband mode
(resulting system temperatures, corrected for the atmosphere, were
typically
K), and connected to low
resolution acousto-optical spectrometers (1440 channels, total
bandwidth of 1 GHz). Dual beam-switching, with a beam-throw of
12
in azimuth, was used as observing mode. The
intensity calibration was done with the chopper-wheel method. Pointing
and focus checks were made towards stellar SiO masers as well as the
continuum source in Centaurus A. The pointing offsets were typically
3'' rms in each coordinate. Calibration uncertainties are
estimated to be
in the 3, 2 and 1.3 mm
bands, respectively.
In NGC 4945 most of the molecules have stronger emission in the
blue-shifted than in the red-shifted part of the line. An exception to
this is CS, whose emission is more evenly distributed over the full
spectral profile (this has previously been noted by Henkel et al. 1990).
This asymmetry in the spectra may indicate that the
chemistries differ in the two regions or that the excitation is
varying, e.g. due to differences in the gas density/temperature,
electron density or the background infrared radiation. Henkel et al. (1994)
have suggested that the differing emission strengths may be due to
differences in the exposure to the far-infrared (FIR) radiation. Yet
another possible explanation is self-absorption, i.e. some locations
suffering a greater degree of saturation (Henkel et al. 1990), perhaps due to
a temperature gradient across the molecular ring where the inner edge
is expected to be hotter than the outer.
Unlike those of NGC 4945 the
profiles in Circinus are fairly symmetric. Again checking against
previous observations, the lines are similar to those which have been
previously observed i.e. 12CO
(Johansson et al. 1991; Israel 1992; Elmouttie et al. 1997;
Elmouttie et al. 1998),
13CO
,
12CO
,
13CO
(Johansson et al. 1991) and HCO+,
HCN and HNC, in the
transition (Israel 1992).
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Figure 1:
The observed spectra in NGC 4945. Apart from H2CO
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Open with DEXTER |
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Figure 1:
continued. The H13CN and H13CO+
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Open with DEXTER |
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Figure 2:
The observed spectra in Circinus. A constant or first
order baseline has been subtracted from each spectra. The spectra have
been smoothed to a channel width corresponding to 10 km s-1(except C17O
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Open with DEXTER |
In the analysis we use velocity integrated line intensities,
,
which have been corrected for beam-dilution due to
finite source and beam sizes. This is carried out by assuming that the
source distribution on the sky is Gaussian and applying the full-width
half-maximum (FWHM) diameter (
)
of the
corresponding 12CO transition to the various CO isotopomers. The
other molecules have transitions of higher excitation requirements and
in those cases the CO
value of
is applied (Table 3). The resulting corrected intensity
ratios are shown in Table 4.
CO
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CO
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CO
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|
SEST beam | 45'' | 22'' | 15'' |
NGC 4945 | 29'' | 20'' | 15'' |
Circinus | 42'' | 31'' | 21'' |
Molecule(s) | Transitions | I ratio | |
NGC 4945 | Circinus | ||
12CO |
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|
13CO |
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12CO/13CO |
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|
C18O |
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- |
12CO/C18O |
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- | |
C17O |
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- |
12CO/C17O |
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- | |
C18O/C17O |
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- | |
CS |
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- | ![]() |
|
CS/13CS |
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>4 | - |
SO |
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- |
CS/SO |
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- | |
HCN |
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- |
HCN/H13CN |
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- |
HCN/HNC |
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- | ![]() |
HCO+ |
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- |
12CO/CS |
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|
12CO/HCN |
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- | |
12CO/HCO+ |
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- | |
HCN/HCO+ |
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- | |
HCO+/H13CO+ |
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- |
HCO+/13CO |
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HCO+/CS |
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- |
H13CN |
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>3 | - |
H2CO |
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- |
H2CO |
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- |
H2CO |
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- |
For both galaxies we find a 12CO
intensity ratio of
1 which is typical for star-burst/Seyfert
galaxies (Aalto et al. 1991;
Dahlem et al. 1993)
. Also, the CO
/HCN
intensity ratios are similar to those found in (other)
Seyfert galaxies; with NGC 4945 showing the same ratio as for the more
distant Seyferts and Circinus showing a similar ratio to the near-by
sample (see Sect. 1). In NGC 4945 our 12CO and 13CO
ratio agrees with that previously
determined (e.g. Dahlem et al. 1993; Henkel et al. 1994).
In order to estimate the prevailing physical conditions and column
densities in the molecular gas, we have performed radiative transfer
calculations. In order to complement this, in temperature and column
density estimates we have also applied local thermodynamic equilibrium
(LTE) calculations. The presented column density estimates are peak
values in the sense that the intensities have been corrected for
finite source and beam size. However, small-scale (i.e. structures
smaller than the assumed source size) beam-filling has not been taken into
account. Conversion to beam-averaged quantities is obtained by
multiplication by
(see Table 3 for the
definition of this).
The code used here is described in Jansen (1995): The excitation
problem involves statistical equilibrium of a multi-level system
(incorporating typically 12 rotational levels in the lowest
vibrational state of the molecule in question). The radiative transfer
is treated in the mean-escape probability (MEP) approximation: like
the large velocity gradient (LVG) method (e.g. Leung & Liszt 1976), this
uses a local source function in which the optical depth in each
transition determines the mean escape probability (Osterbrock 1989) of a
photon from a typical location within a cloud. The model gas cloud has
a spherical shape and a uniform density and kinetic temperature. The
gas density and the temperature are estimated by fitting the observed
line ratios of different transitions of CO, CS, HCN and HCO+ to
the excitation and radiative transfer model.
The intensity ratios for each species were constrained by a routine
which tested the goodness-of-fit by calculating the error of the MEP integrated intensity ratios for each of the observed
values. The 12CO and C18O intensity ratios gave limits for
the column density whereas 13CO was quite specific (e.g. using
in NGC 4945).
Examining the
results we selected, for example, the 12CO column density which
gave the observed 12CO
to 13CO
intensity ratio. This process was repeated for each
transition of each isotopomer.
For NGC 4945 this gave
for the relative column densities and solutions could
only be found for
K and
cm-3, i.e. as Henkel et al. (1994). Constraining the
HCN solutions using the
and the CO/HCN
and
line
ratios, we could obtain a solution for
cm-3, regardless of kinetic
temperature. For CS at the kinetic temperature defined by the CO, the
line ratios only permit a value of
cm-3. Note that solutions at this
molecular hydrogen density can be found over a range of kinetic
temperatures (e.g.
K,
cm
and
K,
cm
), but for densities higher than this (
cm-3), solutions can only be found for
K (for
cm-3 there are no
solutions). It is important to note that these results apply to the
observed intensity ratios (within errors) only
, and so still hold if the CS traces a
different gas component to the CO. The upper limit for the
transitions of H13CN together with
the HCN/H13CN
ratio gives the range
of column densities shown in Table 5, where the MEP results
are summarised. Note that, as for the CO isotopomers, we obtain a
HCN/H13CN column density ratio of
50-200.
Molecule |
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N [cm-2] |
12CO | 3 103 | ![]() |
13CO | 3 103 | 1.2-1.5 1017 |
C18O | 3 103 | ![]() |
HCN | 104 | 1.5-2.4 1016 |
105 | ![]() |
|
CS | 104 | 1.5-2.4 1015 |
HCO+ | 104 | 1.5-2.1 1015 |
H13CN | 104 | 1-3 1014 |
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Figure 3:
MEP solutions for the CO isotopomers per velocity interval at
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Open with DEXTER |
For the CO values in Circinus, solutions could only be found for
K and
cm-3,
i.e. as the warm gas solution of Curran et al. (1998), where we obtained
solutions of the CO isotopomers convolved to the 12CO
beam. The column densities give
.
Since we have only the
transition available in this galaxy, models for HCN and HCO+ are
not so easy to constrain. Assuming, however, a similar kinetic
temperature between the various tracers and using the 12CO/HCN
and 12CO/HCO+
ratios (i.e. the HCN and CO trace
similar regions), solutions may be found for
cm-3. From the
and
ratios of CS, we could only find a
solution at a molecular hydrogen density of
cm-3. Note that solutions using this molecule
could only be obtained over a similar temperature range as for
the CO. No solutions which satisfy the observed CO/CS ratios at
or 106 cm-3 could be found,
regardless of kinetic temperature
.
The results are summarised in Table 6.
Molecule |
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N [cm-2] |
12CO | 2 103 | ![]() |
13CO | 2 103 | 2.5-3 1016 |
HCN | 104 | ![]() |
105 | ![]() |
|
106 | <1 1014 | |
CS | 105 | ![]() |
HCO+ | 105 | ![]() |
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Figure 4:
MEP solutions for the CO isotopomers per velocity interval at
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Open with DEXTER |
Using the total observed line emission to
estimate an individual cloud velocity width (neglecting effects due to
cloud-cloud shielding) from
In order to complement the MEP results, we applied the LTE
model to the observed data. Since this method depends only upon the
excitation temperature (
)
and the column density, it is
the simplest way to analyse the observed line ratios. Here we have
assumed that the excitation temperature is the same for both 12CO
and 13CO.
Galaxy |
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NGC 4945 | 20 | 2 1019 | 40 |
Circinus | 14 | 2 1018 | 25 |
From the solutions (Table 7) we see that the LTE results
differ significantly from the solutions obtained from the radiative
transfer calculations, although the derived column densities are
reasonable, cf. 8 1019 cm-2 for the peak column
densities in NGC 4945, obtained by multiplying the 12CO column
density per unit line width (
2 1017 cm
)
over the full line width (Fig. 1). For
Circinus the value obtained is closer to that for an individual cloud
(see Table 6). Applying the temperatures and column
densities from the LTE solutions to the MEP code we can satisfy the
observed intensity ratios for
cm-2 and
>5 1019 cm-2 in NGC 4945 and Circinus,
respectively. We cannot, however, reproduce the observed 13CO
ratios: 13CO
for NGC 4945 and
<0.87 (or
0.9 for
cm-2)
for Circinus using the molecular hydrogen densities
obtained from the MEP solutions.
From the MEP analysis it appears that the molecular gas in the two
galaxies arises from a relatively dense phase, i.e.
cm-3 in NGC 4945 and
cm-3 in Circinus
. The observed line ratios may be solved for a
constant kinetic temperature in each galaxy, i.e.
K
and
K in NGC 4945 and Circinus, respectively. Our
results for NGC 4945 compare well with those of the LVG model of
Henkel et al. (1994), i.e.
K,
cm-3 and
for
the CO cloud component. Like Henkel et al. (1994), we find
the model of Bergman et al. (1992) insufficient in density, due to an
underestimate of the 12CO
ratio
(obtained from Whiteoak et al. 1990), in order to reproduce our value for
this intensity ratio (which is similar to that previously obtained by
Dahlem et al. 1993; Henkel et al. 1994).
In the remainder of this section we will
discuss the MEP results.
In NGC 4945 the star-burst is believed to have reached an advanced
stage of evolution (Koornneef 1993; Henkel et al. 1994) and the presence of strong
far-infrared and continuum radiation sources, as well as the water
masers, indicate current vigorous star-formation activity
(Moorwood & Glass 1984; Moorwood & Oliva 1994; Nakai et al. 1995;
Moorwood et al. 1996b). Recent enrichment of the interstellar
medium by massive stars is possibly indicated by a high
abundance ratio.
is
believed to be produced by helium burning (i.e. in massive (
)
stars, where the necessarily high temperatures of
108 K are found), whereas
(the result of hydrogen
burning,
108 K) originates in intermediate mass (
)
stars. Accordingly the
abundance ratio may yield information on the relative abundances of
these stars
. Estimating this
from the C18O/C17O intensity ratio, we obtain a value of
6 in both NGC 4945 and Circinus. This compares well with the
previously obtained value of Henkel et al. (1994);
.
Also we obtain
30-60 for
the C16O/C18O intensity ratio in NGC 4945 (cf. the value
of
40, Henkel et al. 1994) and the MEP
analysis supports a column density ratio of
200
(cf.
150, Henkel et al. 1994). For Circinus the intensity ratio is
40.
Looking at the CO/HCN ratios, we see that 12CO
/HCN
and
17 for
NGC 4945 and Circinus, respectively. Whereas the value for Circinus is
similar to that expected from
Seyfert galaxies, the value for NGC 4945 is closer to that expected
from
Seyferts
(Curran et al. 2000). Naturally, since these two galaxies are somewhat closer
than those of Curran et al. (2000), due to CO contamination from the disk, we
would expect a lower CO/HCN ratio provided that the HCN is more
concentrated towards the nucleus than the CO. The fact remains,
however, that the CO/HCN ratio in NGC 4945 is about half that in Circinus.
If the CO traces the dynamical mass, and since both galaxies have
a dynamical mass of
within the central
600 pc (Mauersberger et al. 1996; Curran et al. 1998), this suggests that there is twice as
much HCN in the nuclear region of NGC 4945
.
Examining the HCN/FIR luminosity ratios, we find that for NGC 4945,
and
and that for Circinus
(the HCN in both
galaxies within the HPBW of 57'') and
(calculated from
Lonsdale et al. 1985
). These
values give
, and this strongly suggests that the HCN
luminosity in NGC 4945 arises from an additional component to the star
forming cores, e.g. the dense gas component of the inner
circumnuclear disk (Kohno et al. 1999; Curran et al. 2000):
although the maser emission
occurs on very small (pc) scales, the inner molecular gas
structure
which has an
elevated (cf. the ring) HCN to CO intensity ratio
is still extended enough to be perfectly detectable with
beams of
2'' in near-by AGNs (Sternberg et al. 1994;
Helfer & Blitz 1995;
Tacconi et al. 1998). Note also
that from mid-IR spectroscopy of NGC 4945, Spoon et al. (2000) postulate
that compared to Circinus the narrow line region and/or UV radiation from
the AGN may be severly obscured. This could perhaps be due to the
central accumulation of dense gas and this hypothesis may be further
supported by:
In support of our excitation analysis, in Circinus we find
,
which is
typical of galaxies with
and
suggests the presence of warm gas (Hüttemeister et al. 1995). In NGC 4945, assuming
optically thin lines, we
calculate a formaldehyde ortho/para ratio of H2CO
;
or probably even slightly higher than this since the
line is blended with the HC3N
line
, Table 1.
From HCN observations of the star-burst galaxies M 82 and NGC 253,
which like NGC 4945 and Circinus share similar CO and FIR properties,
Jackson et al. (1995) find that the
intensity
ratio gives molecular hydrogen and column densities per unit line width:
We have observed a number of molecules, in several transitions in the central positions of NGC 4945 and the Circinus galaxy. From a mean-escape probability analysis of the data we conclude the following:
We also find that the kinetic temperature of the gas to be
K and
K in NGC 4945 and Circinus,
respectively (from H2 lines, Spoon et al. 2000 also find warm gas,
K, in NGC 4945). Could the higher temperature in
NGC 4945 be related to the asymmetric shapes of (the temperature
tracing) molecular profiles? A temperature gradient across the
observed region could cause the possible self-absorption: Molecular
rings (which have been insinuated in both these galaxies) are expected
to have higher temperatures in their inner radius due to heating from
the AGN and star formation, and so we may expect such asymmetric
profiles in both cases. The fact that this is not seen in the Circinus
spectra may simply imply a smaller gradient in accordance with the
lower temperature derived. We hope that future interferometric (ALMA)
studies of these galaxies will answer this through detailed mapping of
temperature tracing molecules.
We find column densities (per unit line-width) of
2 1017 cm
for 12CO
in both galaxies. Combined with the 13CO results, these values
suggest a 12C/13C ratio of
50 for NGC 4945 and
60-80 for Circinus. These values are typical of Galactic clouds
(
40-90, Goldsmith 1987), but higher than in the Galactic centre.
We also report the first detection of SO in these galaxies, thus increasing the number of extragalactic SO detections to six (Petuchowski & Bennett 1992; Johansson et al. 1994; Takano et al. 1995; Chin et al. 1998; Heikkilä et al. 1999).
Finally, worth noting is the fact that both the MEP and LTE methods suggest that the gas in NGC 4945 is warmer than in Circinus (by roughly the same factor), and so perhaps LTE methods could provide a useful simple means of determining the relative gas temperatures in a larger sample of Seyfert galaxies.
Acknowledgements
We wish to thank the referee Christian Henkel for his helpful comments, Francisco Azagra at SEST for locating some of the older data for us and the operators in general for their help with the observations. Thanks also to Michael Olberg, John Black and Silvana Nikolic at Onsala for their help with RADEX. Also at Onsala, Antonis Polatidis who made modifications (not possible from SEST) to Figs. 3 and 4. AH acknowledges financial support from the Academy of Finland through grant No. 1011055.