A&A 371, 25-36 (2001)
DOI: 10.1051/0004-6361:20010082
A. Schulz1,3 - R. Güsten2 - B. Köster4,5 - D. Krause4,6
1 - Institut f. Physik & ihre Didaktik, Universität zu
Köln, Gronewaldstr. 2, 50931 Köln, Germany
2 -
Max-Planck-Institut f. Radioastronomie, Auf dem Hügel 69,
53121 Bonn, Germany
3 -
Institut f. Astrophysik & Extraterr. Forschung, Universität
Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
4 -
I. Physikalisches Institut, Universität zu Köln,
Universitätsstr. 17, 50937 Köln, Germany
5 -
Pricewaterhouse Coopers, Hohenzollernring 21-23, 50672 Köln,
Germany
6 -
Pixelpark, Friesenplatz 25, 50672 Köln, Germany
Received 7 July 2000 / Accepted 4 January 2001
Abstract
To study the different components of the molecular gas in the nuclear region
of the nearby spiral galaxy IC 342,
(3-2),
(3-2) and (2-1), as well as HCN(1-0) through (4-3)
observations are presented and analyzed in conjunction with a variety of
line and continuum emission data of the literature. We find several giant
molecular clouds embedded in a medium with lower column and volume density.
The gas shows strong density and temperature gradients with a
very clumpy substructure; the lower-density gas is rather warm (
50 K)
and the temperature decreases with increasing density. While LVG
calculations are
contradicting our observations, we are able to explain all CO and
[CII] line data by applying a model of photon dominated
regions (PDR); for the gas seen in these lines we find no evidence for an
additional dominant heating mechanism.
On the other hand, for our HCN observations tracing the dense
gas clumps (
cm-3)
where the influence of UV photons is expected
to be less important than for less dense gas seen in CO (
cm-3), an LVG model approach should be applicable and, in fact,
yields results which fit well the data and support the outlined scenario.
For this gas component which is rather cool (
K),
heating processes involving photoelectrons should
be unimportant and other mechanisms like
turbulent energy dissipation or cosmic rays should be considered.
The structure of the nucleus of IC 342 with several giant molecular clouds
with sizes of
20 to 50 pc and masses of order 106
partly
associated with HII regions and embedded into a lower-density interstellar
medium shows striking similarities in terms of cloud distribution and their
physical behaviour with our Galactic Centre region.
Key words: galaxies: individual: IC 342 - galaxies: galactic nuclei - galaxies: spiral - galaxies: interstellar matter - radio lines: molecular
Adopting a distance for IC342 of 1.8 Mpc (1'' corresponds to 8.7 pc)
instead of 4.5 Mpc (see McCall 1989), it can be suspected that the
nucleus of the nearby spiral galaxy IC342 (type Scd, HI diameter 50 kpc) may show some general similarities to our own Galactic Centre (see
Downes et al. 1992 and references therein). For example, giant molecular
clouds are found in its central region of similar size to those of the
Galactic Centre
like SgrA and SgrB2, and the total near infrared and far infrared
luminosities of the inner 400 pc are comparable to the equivalent region of our
Galaxy.
Its proximity and its almost face-on orientation towards the observer
provides a unique possibility to study the nucleus of a nearby spiral galaxy
from a very favorable viewing angle (i = 25)
without interference
from the disc: it is small enough to observe its real morphology with only
little confusion due to different velocity components in the beam, but
it is large enough to allow a kinematic analysis.
All this is reflected in the variety of recent investigations of the
neutral interstellar matter up to very high angular resolution (
2'').
Molecular line emission from IC342 is quite concentrated towards the centre, and molecular line maps represent the inner kpc. Close inspection of the high-resolution maps in various molecular lines show interesting coincidences and differences: HCN(1-0) interferometer data (2.7'' resol.; Downes et al. 1992) revealed 5 dense clouds of 10 to 50 pc size along the central mini-spiral. 12CO(1-0) emission of Ishizuki et al. (1990, 2.4'' resol.) and Meier et al. (2000, 4.5'' res.) - which is usually interpreted to trace predominantly intercloud material - is very closely correlated to the HCN emission, whereas the map of 13CO(1-0) emission (5'' resol.) by Turner & Hurt (1992) does not show the prominent HCN cloud "B'' which is stated to be correlated to strong HII radio continuum emission. This picture gets even more complex regarding the NH3 map (5'' resol.) of Ho et al. (1990) where some of the HCN clouds are well identifiable but some are missing.
Several investigations have shown that large differences in excitation
conditions occur on small scales (see Downes et al. 1992; Güsten et al.
1993; Harris et al. 1992; Wall & Jaffe 1990; Eckart et al. 1990). Attempts
to model the observed line ratios by two or more emitting gas components
(Downes et al.; Güsten et al.) suggest a component of small dense
warm clouds (a few to 10 pc, cm-3,
50 K) and at least one
of less dense partly cooler interclump and spiral arm material. In order to
derive possibly unique sets of physical parameters for the various gas
components, it was particularly disadvantageous up to now that for HCN and CS,
respectively, only one transition was observed with sufficiently
high angular resolution.
It is the aim of this study to confine the properties of the different gas components resolving them spatially. As a tracer of warm gas of intermediate density, 12CO and 13CO(3-2) is well suited. To extend this analysis to very high density material, additionally several lines of HCN were measured covering a large range of excitation conditions (i.e. critical densities). Since the highest possible angular resolution is essential for this task, the IRAM 30 m-telescope (MRT) is the only instrument at this time where this research is possible.
The observations were performed during various winter seasons from Jan. 1993
to Feb. 1998 at the IRAM 30 m-telescope (MRT) in the beam switching mode using
the chopping secondary mirror with a throw of 200'' in azimuth
at a switching frequency of 0.5 Hz. The integration time for a single
measurement was typically 10;
each position was
repeatedly measured obtaining total integration times of up to 4 hours per
position.
With molecular cloud sizes of 50pc (
6''), a very good telescope
pointing is essential; this was carefully checked every 30 to 40
on the nearby source
W3(OH) using the 3 mm and 1 mm receivers; data with pointing offsets of more
than 1.5'' (
10% of the data)
were ignored. At the beginning of each observing
run, the parallel alignment of all used receivers (at 3 mm, 2 mm, 1 mm and 0.8 mm)
was achieved by observing planets.
We used the IRAM facility 1MHz-backends (filter banks and autocorrelator).
Figure 1 presents the CO(3-2) and 13CO(3-2) spectrum at the (0, 0)
position of our map (the same reference position was also used
by Eckart et al. 1990 and Downes et al. 1992). Figures 2, 3 and 4 depict the
70-point map of the integrated intensity
of the 12CO(3-2) and the 25-point maps of the 13CO(3-2) and
13CO(2-1) spectra, all measured on a 4'' grid (full velocity range
of -30 to 110
).
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Figure 1:
Spectra of
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Figure 2:
Map of integrated
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Our 13CO(3-2) emission is even more concentrated to the central region of IC342 (which may partly be an effect of the lower signal-to-noise ratio), but the structure of the map (i.e. the positions of peaks and the contrast within the map) is the same as in 12CO. Regarding its slightly lower angular resolution, the 13CO(2-1) map also reveals this same structure. For the emission maxima we keep the notation of Downes et al. (1992) calling them "clouds'' and extend it by cloud F, G and H.
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Figure 3:
Map of integrated
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If we compare our CO(3-2) map with that of Steppe et al. (1990) obtained at the same telescope but with a Schottky diode receiver with a 10-times lower sensitivity than now, we see generally a fair agreement concerning calibration and intensity distribution; a major discrepancy appears only at the position east of cloud C which could possibly be caused by a pointing offset in the older map where much larger integration times had to be applied. Our recent spectra are of much higher S/N-quality.
Due to the weak intensities of the HCN lines it was not possible to fully map the nuclear region of IC342 in any of the transitions, but we could take spectra at a few positions showing CO(3-2) maxima. For clump C, four HCN transitions are shown in Fig. 5. Table 3 contains all obtained line intensities scaled to a 10'' beam (using HCN source sizes of Downes et al. 1992).
First of all, obviously our HCN(1-0) spectrum at the (0, 0)-position agrees with that of Downes et al. within 10% whereas there are large differences to the spectra of Rieu et al. (1992, 27'' resol.) and Paglione et al. (1997, 20'' resol.). On the other hand, our H13CN(1-0) spectrum shown in Fig. 5, despite its weak intensity, agrees with that of Paglione et al. within 20%; our measured isotopic (1-0) line ratio is 24.
Concerning HCN(3-2), our observations are in accordance with Paglione et al. within 20% regarding the largely different beams (within their 27'' beam, Paglione et al. "see'' at least 3 of the clumps). Concerning HCN(4-3), our spectrum (8.3'' resol.) is by a factor of 2 less intense than that of Jackson et al. (1995, 20''resol.) at the comparable position.
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Figure 4:
Map of integrated
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"Cloud'' | G | D | C | F | A | B | E | H | |
offset pos.a) | (4, 12) | (4, 8) | (4, 4) | (0, -4) | (4, -4) | (0, 0) | (-4, -4) | (-4, -8) | |
CO(3-2) | 6 | <6 | 16 | 9 | - | - | <6 | <6 | ![]() |
HCN | (![]() |
7.7 | 8.3 | (![]() |
8.8 | 10.8 | 3.1 | (![]() |
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a)
In arcsec from our (0, 0) position within our observing grid;
to fit into our 4'' grid of spectra, some positions are not exactly identical with Downes et al. (1992). b) From this work. c) From Downes et al. (1992); values in brackets are estimated. |
We have also attempted to measure H13CN(3-2) with 12'' resolution at
the clump positions.
The result was a spectral line of order 100 mK for clumps C and F
which is about the same as for the main isotopic line.
Furthermore, the velocity offset of the 4 clump spectra contradicts
the position-velocity relation in IC342 observed in the other
molecular lines: cloud G would be at the lowest and cloud H is at the highest
velocity.
Hence, this line can not be H13CN(3-2) emission but is arising from
the image
side band of the receiver; it is centered at 267.042 GHz which is close to
267.045 GHz, the frequency of a pair of ethanol (98-88) transitions.
Although Millar et al. (1995) report a rather high ethanol abundance of
10-8 relative to hydrogen for the ultracompact galactic HII region
G34.3+0.15, in SgrB2 this abundance is found to be a factor of order 10
lower (Nummelin et al. 2000). Therefore,
due to the rareness of this molecule, this identification must appear to be
extremely questionable.
On the other hand,
all other transitions within 30MHz listed in molecular line catalogues
have energies of their lower state of several 100 K and are therefore
even more unlikely candidates for extragalactic emission from an
70 pc size area. This needs further investigation.
Within the observed line profiles we estimate an upper limit for the blended H13CN(3-2) emission of 10-20 mK for clouds C and F.
In a first approach to a physical interpretation, we attempted to model the molecular interstellar medium of the nucleus of IC342 with a standard radiative transfer analysis using a single-component LVG (Large Velocity Gradient) line escape probability code to model the CO(3-2) and (2-1) transitions (12CO(2-1) data from Eckart et al. 1990) at the different maxima of CO and HCN. As in the case of Güsten et al. (1993) for the centre position at 15'' resolution, we failed to obtain a solution fitting all available CO lines. Güsten et al. also demonstrated that the 13CO(3-2) line is a very sensitive probe for the physical gas conditions particularly in connection with (4-3) or higher excited CO lines.
This means that the observed line ratios require us to consider temperature
(and possibly also density) gradients. Furthermore, even an LVG line escape
probability model with 2 cloud components of different density and temperature
permitted only a very limited range of resulting cloud parameters, being
the same for regions close to the
centre of IC342 as also for regions further out along the mini-spiral
which might be not realistic. Moreover, the CO line LVG models as well as a
model with 3 cloud components by Downes et al. (1992) demand
that the densest gas component is rather warm (50 K).
cloud | offset |
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R12/13(3-2)2) | R12/13(2-1) |
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size |
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104 cm-3 | pc | 106 ![]() |
|||||
G | (4, 12) | 0.80 | 0.75 | 10.5 (18.0) | 8.5 | 1.5 | 25 | 1. |
D | (4, 8) | 0.80 | 0.75 | 8.0 (12.0) | 7.5 | 1.5 | <25 | <2. |
C | (4, 4) | 0.85 | 0.77 | 7.5 (8.0) | 6.5 | 2. | 35 | 4. |
F | (0, -4) | 0.65 | 0.65 | 6.5 (7.0) | 7.5 | 1 | 25 | 2. |
A | (4, -4) | 0.70 | 0.60 | 9.0 (16.0) | 7.5 | 1 | - | - |
B | (0, 0) | 0.45 | 0.55 | 8.0 | 9.5 | 0.5 | - | - |
E | (-4, -4) | 0.50 | 0.55 | 8.0 (10.5) | 8.0 | 0.5 | - | - |
H | (-4, -8) | 0.60 | 0.50 | 8.5 (15.5) | 7.0 | <0.5 | <20 | <0.6 |
In a second approach of interpretation, we applied a model of Photon Dominated Regions (PDR) (Köster et al. 1994) for the clouds within the central 400 pc of IC342.
This model calculates the thermal and chemical structure of a one-dimensional
and plane-parallel PDR of finite size illuminated from both sides
(model A of Köster et al.). The adopted chemistry includes a chemical
network of 32 molecular
species (244 reactions). Input parameters are the (fixed) gas density, the
UV radiation strength, the total extent, the line width as well as the
fractional abundances of H, He, C and O (incl. their abundant isotopes)
and dust particles (scattering and absorption by grains).
The main heating source is the interstellar UV radiation field between 6.2
and 13.6 eV with a strength relative to the local field
(Draine 1978).
Heating is achieved via photoelectrons and - important at higher densities -
collisional de-excitation of UV-pumped vibrationally excited H2; cooling
is achieved via line emission of atomic fine structure lines and molecular
lines.
Model results are abundances,
absolute line intensities and column densities of the different atomic and
molecular gas
species as a function of visual extinction which is a measure of gas
(and dust) column density. Additionally, the gas temperature is
calculated throughout each "sub-cloud'' (assumed velocity interval of
1.7
). The entire cloud then consists of a number of such sub-clouds at
different velocities according to the number
of velocity intervals within the total line width observed at the modelled
position.
The assumption of plane-parallel geometry (observed face-on in our case)
is not disturbing the results since the observed angular extent of the clumps
defines their lateral extent. Furthermore, we emphasize that we prefer this
rather simple model to minimize the number of free parameters which
we find more appropriate in our approach modelling an entire galactic nuclear
region where we observe averages of emission over
linear dimensions of 20 pc, rather than using a more sophisticated model
with more free parameters for "fine tuning'' which we can not yet confine
observationally (e.g. Störzer et al. 2000); the latter would be
definitely appropriate when analyzing nearby Galactic clouds with observed
fine structure.
![]() |
Figure 5: HCN spectra observed at position of cloud C (offset (4,4)) |
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As also resulting from other models, within a rather large temperature range
CO line ratios of different transitions (e.g. (3-2)/(2-1) ratios) are
mainly determined by the gas density
); high density
yields thermal excitation and line ratios close to 1.
But, different from other models, we consider gradients of the chemical
and temperature structure; this is important since, as density
increases, the hot outer cloud layers increasingly contribute to the emission,
and therefore the line temperatures slowly rise with J-level.
On the other hand, isotopic CO line ratios for a specific transition are significantly determined by the optical depth of the cloud, i.e. both the gas column and volume density; fortunately, such line ratios do not suffer from unknown source sizes (beam filling factors) because of almost equal beam sizes of the compared lines.
The PDR model is able to reproduce the 12C-13C fractionation
in molecules at intermediate gas densities via a charge-exchange reaction
(for example, 13C+ + 12CO
12C+
+ 13CO + 36 K): with an input of a 12C/13C abundance ratio
of 66 we obtain a
column density ratio of
/
= 33; Henkel et al. (1998, and
references therein) in fact derive a value of the
12C/13C ratio in molecules of 30 to 40 for IC342.
A PDR model appears also to be justified by the observation of strong
158 m [CII] fine structure line emission (Stacey et al. 1991).
The interstellar UV radiation field in the nuclear region of IC342 can
be determined within a factor of order 2 to be
= 10
(see Fig. 18 of Stacey et al.) Much higher values of
are found for sites of massive star formation like Orion A
(see Stacey et al. 1993) and are
unrealistic as an average over such large areas of
order 50 pc (equivalent
to our spatial resolution for CO(3-2)); lower
values are unlikely
because of the observed dust temperature of order 40 K (Becklin et al.
1980) over an area of 60''. Büttgenbach et al. (1992) obtain the same
value of 10
modelling their CI observations.
Adopting element abundances of the gas phase to be entirely solar,
one particular problem we are faced with in this PDR model is that 13CO
becomes optically thick on very short path lengths
which results in a rather unlikely small range of gas column densities
fitting the various observed
/
line ratios.
One possibility to overcome this problem is to assume
element abundances (compared to hydrogen) not to be fully solar;
if we reduce the element abundances of carbon and oxygen to
and
relative to H2, the range of
column densities reaches reasonable values. Downes et al. (1992) give
a value of
to 5 10-5.
Another possibility of "fine tuning'' of the model would be to increase the
strength of the UV radiation field by a factor of order 2. Nevertheless, we
emphasize that this affects only the resulting column densities but not
the gas density.
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Figure 6: "H13CN(3-2)'' spectrum at the position of cloud C; the identification as a pair of ethanol (98-88) transitions at a rest frequency of 267.045 GHz is still very questionable |
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Figure 7:
Excitation plot of
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Specific results for several observed positions which fit the observed
CO(3-2) and (2-1) line ratios are given in Table 2. Figure 7 shows the
excitation plot for cloud C. The average gas density
for the clouds must be in the range of 0.5 to 2 104
,
i.e. slightly
below the critical density for the CO(3-2) line which in all cases appears
to be not fully thermally excited. Lower densities are unlikely because
(3-2)/(2-1) line ratios as also the absolute
(3-2) line intensities
would drop below the observed values;
higher densities would imply
and
(3-2)/(2-1) ratios higher than observed. For cloud C we derive from its
column density (2 1023
)
and its linear extent
a mass of 4 10
which is comparable to Sgr B2. Ratios of observed
line temperatures to modelled brightness temperature values yield area
filling factors of order 0.2 for the cloud positions which would be reduced
to 0.1 if one
attributes half the emission to the denser clouds and the other half to the
underlying pedestal of intercloud gas. Because of the uncertainties of element
abundances, gas column densities are more difficult to confine. However,
the derived column densities agree with values by Eckart et al. (1990),
Turner & Hurt (1992) and Güsten et al. (1993).
For the pedestal zone of emission (offset from the clouds) we obtain
slightly lower values of the (3-2)/(2-1) line ratios but
significantly higher isotopic line ratios which indicates that the gas has
mainly lower column densities.
Regarding even higher excited CO emission we find that the (4-3)/(2-1) line
ratio observed for cloud C (CO(4-3) by Güsten et al. 1993)
is well represented by our modelling, as well as the observed (6-5)/(2-1)
ratio (Harris et al. 1991) when the data are scaled to a 12'' beam.
Nevertheless, density gradients should be expected since a fixed gas
density throughout a cloud of 30 pc size can only be a first-order
assumption; such
gradients are in fact indicated by our HCN observations (Sect. 4.3).
Large-scale [CII] emission necessarily implies the existence of a PDR
(Stacey et al. 1991). An important test to justify the PDR model for the
CO observations is whether we succeed in modelling the CO and [CII]
line observations at the same time.
At present, such an investigation comparing emitting source sizes and beam
filling factors must be preliminary because of the low angular resolution
of the [CII] 158 m line observations.
Our model gives a CO(3-2) line brightness temperature of 18 K whereas 2 K is
half the observed value (attributing half of the emission to the clouds
themselves and the other half to the underlying pedestal in our 8'' beam,
which appears to be justified as a first guess facing the observed pedestal
emission and a filling factor
considerably below 1,
in general accordance with Stacey et al. 1993);
hence,
(CO) = 0.13 and the corresponding source size would be
3.5''. Our modelled [CII] line brightness temperature is 25 K,
and 0.5 K is half of the
observed value within the 55'' beam; hence,
which
results in fact in the same 3.5'' source size if we adopt to find the
equivalent emission of five identical cloud
sources within the large beam. Therefore, CO and [CII] lines may in fact
be emitted from the same regions.
Keeping the H12CN/H13CN ratio to be 40 and varying the kinetic temperature, gas density, and gas column density as free parameters, we obtain sets of model clouds with HCN line brightness temperature ratios. To compare those with the observed line ratios, we first have to scale the line intensities (all observed with the same telescope) all to the same beam size (see Table 3). To do this we adopt for cloud C the size determined by Downes et al. (1992) from their interferometric HCN(1-0) map; we take into account that large beams could include more than one cloud source. Since this source size is expected to be an upper limit for the higher transitions, our excitation analysis yields a lower limit to the gas density. For clouds F, G and H we had to make assumptions from CO sizes (Table 1). Our derived column density is mainly determined by the H12CN/H13CN ratio of the (1-0) transition, whereas our observed limit of H13CN(3-2) is not sensitive enough to give useful constraints.
We now discuss the conditions in more detail for cloud C because this is the most prominent cloud unambigously observed in all molecular transitions. With our five observed HCN transitions we obtain a rather definite modelling solution; in particular, the combination of HCN(4-3) and H13CN(1-0) very sensitively determines the resulting parameters (under the assumption that no temperature and density gradients are present). An excitation plot is given in Fig. 8.
As an additional important check, our modelling solution yields
not only sensible line ratios but also sensible absolute line brightness
temperatures as expected when considering filling factors from
beam and source sizes. The best fitting solution yields
K,
cm-3 and
cm-2 .
Nevertheless, the parameter set (20 K, 2 106
,
1013
)
still
gives a fair fit to the observations (see Fig. 8, open circles).
Higher kinetic temperatures are unlikely because this would raise the
HCN(3-2)/(1-0) ratio beyond the observed value or, alternatively, would
decrease the (4-3) as well as the isotopic (1-0) line intensities too much
due to lower
values for both.
Higher gas densities are unrealistic because under the assumption of source
size of Table 1 the (3-2) line is subthermally excited,
lower densities would yield a too low (4-3) intensity.
We again emphasize the high importance of HCN(4-3) observations, because with
these only very subthermally excited spectra we are able to constrain the gas
density quite well.
The column density is confined by the isotopic (1-0) line ratio and in
accordance with all other line ratios.
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Figure 8:
Excitation plot of HCN line emission (modelled line brightness
temperature ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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For the other cloud positions (F, G, H) we find modelling results given in Table 3 which are less accurate due to uncertain source sizes.
To compare ours with other molecular line maps we predominantly consider
those with angular resolutions of 10'' or better.
CO has been mapped
with resolutions
10'' only in the lowest transitions. A close
inspection
of such maps of 12CO(1-0) (Ishizuki et al. 1990, 2.4'' res., Wright
et al. 1993, 4.3
res.; Meier et al. 2000, 4.5'' res.)
and 13CO(1-0) (Turner & Hurt 1992, 4.9
res., Wright
et al. 1993, 4.3
res.; Meier et al. 2000, 4.5'' res.)
emission confirmes the general morphology concerning the mini-spiral,
but there are interesting differences in the details. The exact positions
of the emission maxima are slightly
(but in some cases possibly not significantly) different for all the lines.
Almost coincident emission maxima are observed in all CO maps (including ours)
for clouds C, G and H; also their line centre velocities agree. This holds
also for cloud D which
in all these maps is not very prominent along the northern ridge of the
mini-spiral.
Cloud E is almost blended with emission from cloud B in the two
(1-0)
maps,
whereas in all three 13CO(1-0) maps E prominently appears but cloud B
does not. However, in both our isotopic CO(3-2) maps cloud
E along the ridge of the mini-spiral can be supposed in
some of our channel maps; but at the position of B the emission falls off
rather steeply and B is even not visible in any of our channel maps.
Cloud A is very clearly visible in all CO(1-0) maps of 5''
resolution, but
it can be identified only in our 10
wide channel
maps at 45 and 55
centre velocity;
this is also the case in the 7'' 12CO(1-0) map of Lo et al. (1984).
line | cloud | C | F | G | H | |
off pos. | (4, 4) | (0, -4) | (4, 12) | (-4, -8) | ||
HCN(1-0) | 0.33 | 0.33 | 0.30 | 0.30 | (in Kelvin) | |
H13CN(1-0) | 0.01(4) | 0.01(4) | ![]() |
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||
HCN(2-1) | 0.30 | 0.25 | - | - | ||
HCN(3-2) | 0.13 | 0.11 | 0.08(4) | 0.04(7) | ||
HCN(4-3) | 0.06 | 0.04(5) | 0.03(4) | ![]() |
||
parameters | ||||||
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1. | 0.6 | 0.3 | ![]() |
106
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|
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0.2 | (0.3) | (0.1) | (0.07) | 106
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Cloud A is very prominent in the HCN(1-0) and lowest CO transitions, but
rather weak in both isotopes of CO(3-2) emission
(also, HCO+ emission is weak here, see Q-Rieu et al. 1992) and absolutely
absent in NH3; furthermore, the centre velocity of HCN(1-0) and of our
CO spectra differ by >10
.
One possible explanation for the
absence of NH3 and prominent CO(3-2) emission is
that this cloud is very cold.
The case for cloud B is even more confusing; it is coincidently seen in 12CO(1-0) and in HCN(1-0); this is surprising because both lines should trace gas at different volume densities, and B does not appear in 13CO(1-0) nor in both CO(3-2) transitions. The lack of 13CO emission would imply low optical depth, i.e. low column density, the high HCN emission would mean high volume density; both arguments would point at gas with an extremely low volume filling factor.
Inspecting maps of free-free (ff)-emission from HII regions at 6cm by
Becklin et al. (1980, 2'' resol.) and Turner & Ho (1983, 1'' resol.),
some of the emission maxima appear to be closely associated with clouds
A through H.
The strongly clumped H
emission (H
image presented
in Meier et al. 2000)
does mostly not coincide with ff-emission maxima, naturally
explained by extinction
of clumpy dust; we expectedly find H
also anti-coinciding with
molecular line emission, but it is also not concentrated to cloud B.
Although the very details are difficult to judge, it looks as if a number of HII regions (ff-emission clouds) are surrounding the centre of IC342 like a chain of pearls which are associated with the molecular clouds situated in the central mini-spiral, the largest of those we observe as clouds A, B, C, E. Hence, not only cloud B - as stated by Turner & Hurt (1992) and several other investigations - but all of them near the centre of IC342 show moderate star formation activity.
Our CO analysis reveals several clouds of 20 to about 40 pc
deconvolved sizes with masses
of
4 106
distributed around the centre of
IC342 and along the mini- spiral. The central clouds are associated with
moderate ff-emission indicating star formation activity similar to our
Galactic Centre.
The gas of the clouds observed in 12CO has high optical depths
(
)
while 13CO is
moderately optically thick (
).
(3-2)/(2-1) CO line ratios are very similar for both
12CO and 13CO and therefore indicate that both 12CO and
13CO lines are emitted from gas at similar average density,
i.e. not from completely different regions. Also, our 13CO
maps show the same morphology as 12CO for the central area, we derive
similar beam filling factors for both isotopes, and the line
profiles are identical; that means that the gas components emitting
and
seem to be well mixed at the positions of the clouds, although the
main parts of the
and
emission flux stem from different
depths of the clouds.
Hence, for the dense clouds we see no evidence to imply an additional gas
component being observed in 12CO but not in 13CO, as Wall & Jaffe
(1990) do. We find, on the other hand, no evidence for any optically thin
emission (see Wall & Jaffe 1990). 12CO emission appears to
originate not only from extended ridges but also from the dense clouds.
Furthermore, single-dish as well as interferometer observations should not
trace completely different gas components in 12CO and
:
although 12CO/13CO ratios as low as 10 or even lower might
be blends of drastically different regions, such low ratios are found by
single dish measurements as well as by interferometer measurements (Wright
et al. 1993) down to a few arcsec resolution; that means that
only at
much higher resolution one should be able to observe the clouds
substructure, i.e. to distinguish sharply between very dense
clumps and interclump gas - or one has to use different tracers (see below).
Our results are in good agreement with column densities and masses found by
Güsten et al. (1993) (
and
for the central 15''), and Downes et al. (1992) (
106
for cloud C) and Wright et al. (1993) (clouds of
20 to 50 pc with
106
each). Turner & Hurt (1992) derive a slightly higher mass of
9 106
for the central 14'' (120 pc).
On larger scales, the intercloud gas contributes an increasing fraction to
the total gas mass (4 107
within 23'', Wall & Jaffe 1990,
4.2 107
within 56'', Turner & Hurt 1992,
2 108
for the total central region, Eckart et al. 1990); this is a factor of order 10 lower than the dynamical mass of the centre
of IC342 (1.2 109
within 1500 pc, Young & Scoville 1982)
and would imply a "normal'' mass ratio of interstellar matter compared to stars.
Although the PDR model intrinsically considers gradients in temperature
as the gas temperature is calculated locally, it operates with the input
of a fixed gas density. Our 1-component model with a
fixed
density (2 104
for
cloud C) reproduces our observed line ratios for the cloud positions
even including the CO(4-3) line by Güsten et al. (1993) as well as the
CO(6-5) line by Harris et al. (1992).
On the other hand, Güsten et al.
also state that most of the total column density and mass is contributed by
their denser model cloud component.
The clouds themselves must have a clumpy substructure (low volume filling
factor). This becomes obvious
because masses calculated from average densities and source sizes largely
exceed the values derived from column densities.
The clumpiness should allow the interstellar radiation
field to penetrate the clouds heating the clumps "from outside''. This idea is
in accordance with the [CII] emission reported in Stacey et al. (1991);
Hollenbach & McKee (1989) point out that the [CII] emission should be due
to ionization by UV radiation and should not be produced by shocks.
Their finding that CII generally is associated with CO emission
agrees with our result that CII and CO are likely to be emitted from the
same regions (see Sect. 4.2).
In fact, we can completely explain the CO and [CII] observations
with a PDR model which was queried previously (e.g. Wall & Jaffe 1990).
As an important consequence, this scenario implies that denser parts of
the clouds should be cooler than the less dense outer layers (in contradiction
to LVG model results, see Sect. 4.1.1).
Our HCN analysis nicely confirms this model: There exists a dense gas
component
(
,
see Sect. 4.3) contributing about 5% to the total
cloud mass (in rough accordance with Paglione et al. 1997), and this
component again is clumpy and has a temperature of mostly 30 K; that is just
the temperature we obtain for the interior of our PDR cloud model.
A further hint at cool
temperatures for the denser components is given by Eckart et al. (1990) who
remark that at a temperature for the bulk of the dust of 42 K the derived
dust mass falls short by a factor of 5, but this is compensated if one adopts
30 K instead. We find no evidence for a dense component which is as warm as
50 K or more. We think that the older findings of a warm dense component are
an artefact of the LVG models which are unable to consider temperature
gradients in the calculations. On the other hand, one seems to observe
gradients in temperature and density everywhere on all scales throughout the
interstellar medium, in our and in external galaxies, in particular when
observing large areas within the beam. Such gradients can change the optical
depths of molecular lines considerably compared to constant LVG conditions,
and this effect might be particularly large for the outer layers of clouds
when they are exposed to soft UV radiation which is the case in
the centre of IC342 by Stacey et al. (1991).
Temperature gradients may also play an important role for the
emission
(Ho et al. 1990) revealing temperatures of 70 to 100 K: the observed lines are
optically thin, and therefore the beam penetrates all interstellar
matter within it; it might stem from warm gas of intermediate densities,
either an extended component or the outer layers of the clouds,
and not from the dense regions of the small cores. The total mass of the
warm gas seen in
of only 106
(Ho et al. 1990) could
support this idea.
However, temperature gradients should be much less important for the innermost
cloud cores which could be shielded from UV in case they are dense enough (see
Köster et al. 1994; van Dishoeck & Black 1998). Hence, we expect the
results of our HCN analysis - which does not take gradients into
account - to deviate not largely from real conditions in case
the dense cores in our beam do not contain
interior strong UV sources.
Our HCN analysis is also in accordance with
Serabyn & Güsten (1991) who obtain an upper limit of the density of the
dense gas component of 5 105
from an upper limit of 10 mK for the
CS(7-6) line emission adopting
K; this density limit would rise
by at least a factor of 2 when adopting less than half the temperature.
Apart from the dense clouds, we observe a pedestal of more extended CO
emission. Interestingly, (3-2)/(2-1) line ratios of
do not drop
off as expected for much lower gas densities (like Wright et al.
1993, obtain); instead,
emission is considerably lower hinting at decreasing gas column
densities. It is difficult to check this result by observations with much
higher angular resolution than ours because interferometers lose much of the
flux from extended components.
On the other hand, most of the molecular gas at higher densities
(104 cm-3)
appears to be cooler
compared to the low-density gas (
100 K at
cm-3):
an SiO line analysis at 33 clump positions within the inner
200 pc of the
Galactic Centre region (see Hüttemeister et al. 1998 and references
therein) reveals evidence that this (dense) gas is now at
-30 K
only,
although SiO is thought to be released from grains into the gas phase by
supersonic collision processes (which heat the gas to high temperatures).
This is in accordance with the idea that the high-temperature phase
producing gaseous SiO has a short energy dissipation time scale
(see Güsten 1989 and references therein), and at present these clouds are
cool inside, their outer layers being heated to a large fraction from outside.
In comparison, in IC342 we infact seem to find a very similar situation:
a large fraction of the [CII]
emission is likely to be emitted from the outer layers of the central
molecular clouds observed in CO (see Sect. 4.2) which appear to be
partly correlated with HII regions (Sect. 5.1.2). Also, the dense
gas component observed in HCN (30 K) is rather cool (Paglione et al.
1997 remark that the observed HCN(1-0) luminosity is about the same as
that of the inner 1 kpc of our Galaxy).
Extrapolating the [CII] line luminosity of the Galactic Centre
(3 104 )
within 860 pc2 - just as a crude check -
to 185000 pc2 (the 55'' beam area of the [CII] data in
IC342) yields 6.4106
.
2.6106
is observed,
only a factor of
2 lower whereas the difference of areas is a factor of
200! Hence, the total [CII] line luminosity of the nucleus of IC342
and the Galactic Centre appear to be about the same, and therefore also the
strength of the interstellar UV radiation field should be comparable in both
galactic nuclei. Even the Galactic Centre observations carried out by the COBE
satellite with its large beam (70 equivalent to about 900 000 pc2 at a
distance of 8kpc) roughly agree with these numbers (Fixsen et al. 1999).
For the clouds in the centre of IC342 we are able to explain the heating of
the gas throughout the field of 0.5 kpc extent observed in CO entirely by
the interstellar soft UV radiation field which even heats the interior of the
clouds (at
104
)
to
25 K. Alternative heating processes
may contribute only to the densest component. But,
despite the destorted velocity field (Turner & Hurt 1992) indicating
non-circular motion around the centre of IC342, we see no evidence for
turbulent friction as a dominant heating source for the clouds at this
time, nor that enhanced starburst should play any important role.
There is striking evidence that the physical properties of the molecular clouds in the nucleus of IC342 and our Galactic Centre region are very similar, and therefore also the ongoing physical processes.
From our multi-line analysis probing the different components of molecular gas in the nucleus of IC342, we draw the following conclusions:
(1) Embedded within a pedestal of CO emission we observe several giant molecular clouds (such as Sgr B2 in our Galactic Centre) whose morphological structure is in general accordance with previous investigations, possibly except clouds A and B;
(2) From our data for the clouds, we see no evidence that
and
are emitted from different regions; there is also no evidence
for optically thin
emission of the clouds.
While an LVG model approach yields results apparently
contradicting our observations, we are able to model our CO
observations and complementing [CII] data entirely in terms of a PDR model.
While such an approach was successfully applied for the typical starburst
galaxy M82, (see Mao et al. 2000), it is remarkable that such a model is
also required for a "normal'' quiescent spiral galaxy. We even suspect that
a PDR scenario might be applicable not only to these galactic nuclear regions
but also to very large parts of the interstellar medium of galactic
spiral disks;
The bulk of the gas of the GMC's with sizes of
20 to
40 pc and masses of
106
has an average
density of order 104 cm-3; it is highly clumped;
(3) Embedded in these GMC's we find denser gas (106
)
of
up to 5% of their mass which itself is again highly clumped and cool
(
-30 K);
(4) There is convincing
evidence from our observations in accordance with other investigations that
the large density gradients are connected with large temperature gradients
where the thin gas is rather warm and temperature decreases with
increasing density, as should be expected in a PDR scenario.
The highly clumpy structure allows the soft UV radiation to penetrate well
through all parts of the region; [CII] should be emitted to a large fraction
(i.e. 50%) from the outer layers of the identified
clouds which in some cases show also moderate thermal ff-emission from
associated HII regions;
(5) While we see no need to invoke other heating sources for the gas seen
in [CII]/CO, this should in proncipal be considered for the dense gas
phase seen in HCN
(LVG models do not tell anything about heating mechanisms): cosmic ray heating
is only sufficient if the cosmic ray flux is very much higher than in the
Galactic disk. Turbulent friction of clouds can not be ruled out, although
the short energy dissipation times might cause problems with this mechanism.
On the other hand, our PDR model yields a gas temperature of
25 K for the interior of the clouds, as is determined for the dense cores
from the HCN data;
(6) We report a line detection at 267.042 GHz whose identification as a pair of ethanol(98-88) transitions is still uncertain;
(7) A comparison with our own Galactic Centre shows increasing similarities of these two galactic nuclear regions concerning the line as well as FIR continuum emission of the clouds and hence their physical conditions. Therefore, their evolution should be similar and it appears worth while to investigate in future this apparent "mirror image'' of our own Galactic Centre using probes of very different states of excitation at much higher spatial resolution.
Acknowledgements
We like to thank H. Rothermel, A. Karpov, H. Hein, S. Navarro, D. John for their excellent and engaged support of our sometimes very experimental project, particularly when observing in the submm range. H. Störzer assisted with the modelling. F. Bertoldi, C. Henkel, R. Launhardt, K. Menten and J. Stutzki, contributed useful discussions and comments.