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Astron. Astrophys. 336, 433-444 (1998)
4. Analysis and discussion
4.1. Continuum colours
As noted by HIGW, the stellar absorption spectrum of the nucleus at
m and the overall spectral shape of the near-IR
continuum suggest that out to m the radiation
from the centre of the galaxy is dominated by emission from late-type
(super)giants in the bulge. Near-infrared photometry was published by
Lawrence et al. (1985), Willner et al. (1985) and Forbes et al.
(1992). The colours derived by Forbes et al. (1992) suggest relatively
low extinction, but this must be interpreted as a lower limit because
these authors used fairly large apertures, and, more importantly,
first aligned the peaks of their J, H and K-band
images, while we have noted in Sect. 3.1 that these are actually
displaced from one another. The colours measured by Willner et al.
(1985) suggest that the central 6" of NGC 3079 suffers a high
extinction (their Figs. 2 and 3). HIGW analysed
the available data and concluded to a "best" value
for the central 6".
We have used our accurately aligned J, H and
K-band images to produce the two-colour diagram shown in
Fig. 5, and we analyse the colours of the various regions in NGC 3079
in the following sections. The curves marked "screen" and "mixed" in
this figure indicate the effects upon the observed colours of
extinction by respectively foreground dust and dust uniformly mixed
with the stellar population. In the former case,
, while in the latter case,
, where and
are respectively the observed and intrinsic
intensities. A Galactic extinction curve is assumed.
4.1.1. The outer bulge
The colour observed from the outer bulge
(i.e., positions 4" or more east of the disk) agrees well with that of
typical bulges, but the colour is about
bluer. These results are similar to those found
by HIGW in a aperture. From the observed
K-band absorption features, HIGW determined that the mean
spectral type of the bulge population dominating the K-band
light is M0III, reasonably consistent with the "bulges" zeropoint in
Fig. 5 and with the observed colour of the
eastern bulge in NGC 3079. This leaves the blueness of the
eastern bulge in to be explained. HIGW note
that blue colours also prevail at wavelengths shorter than
m and that these colours point to the
contribution of young stars at m and shorter
wavelengths. There are several possibilities.
-
The blueness of the eastern bulge could be due to a contribution of
scattered light from a nuclear source. As shown in Fig. 5, scattered
power law emission with around
m would require , and
would have to contribute about 35% of the J-band emission of
the eastern bulge. However, the power law nuclear emission found in
active galaxies typically has a much lower exponent around
1 µm, e.g., in the sample of 34
Seyferts of Kotilainen et al. (1992), where no object has
.
-
The bulge of NGC 3079 is peanut-shaped (Fig. 1). This is
usually taken as the signature of a stellar bar (Combes & Sanders
1981; Kuijken & Merrifield 1995). Bars frequently harbour enhanced
star formation, and the bar potential allows young stars in the bar to
migrate into the peanut-like features. If this explanation is correct,
these stars would have to contribute about 20% of the J-band
light on the assumption of a characteristic spectral type A0V.
-
Alternatively, the relative blueness of the the eastern bulge could
be due to a contribution by scattered starlight. For instance, a
reddening corresponding to = 2.7 mag (i.e. 50%
light loss at J) combined with a contribution of
scattered light at J will match the observed
colours to the typical bulge colour. If instead we assume extinction
by dust mixed with stars, (75% light loss at
J) and a slightly smaller contribution by scattered light (20% at J)
will also explain the observed colours. Scattering of blue light from
the young disk population, contributing about 20% of the observed
J-band light likewise is a viable alternative, and consistent
with the overall optical blueness, (RC2), of
this edge-on galaxy. For various reasons (see Kuchinski & Terndrup
1996) these are only crude estimates; nevertheless they all suggest
that scattered light contributing of the order of 20-30% to the
observed J-band emission of the outer bulge is required to explain its
near-infrared colours.
![[FIGURE]](img98.gif) |
Fig. 5. Two-colour diagram of NGC 3079 near-infrared emission. Circles of various sizes indicate colours in apertures along the disk of NGC 3079 (assumed position angle ), displaced by the indicated amounts from the nucleus, assumed to be at the dynamical centre of the H2 emission. Open circles represent the disk north of the nucleus, while filled circles denote positions south of the nucleus. In addition, rectangles indicate the colour ranges found in the eastern bulge well away ( ) from the disk (labeled "Bulge"), in the disk well north and south of the nucleus ("Disk") and in dust lane west of the nucleus ("Dust lane west"). For comparison, the colours of unreddened bulges (Kuchinski & Terndrup 1996) are indicated by the large open circle ("bulges"). Solid curves identify the observed colours of Galactic main sequence stars of approximately solar abundance (curve marked "V") and of (super)giants (curve marked "I-III") of the indicated spectral types (Koornneef 1983). Additional solid lines show the effects upon the observed colours caused by the presence of emission from hot (500 or 1000 K) dust contributing 30% of the observed K-band flux, extinction by foreground dust for to in steps of ("screen"), extinction by dust mixed with the stars with V-band opacities of 0, 1, 2.5, 5, 10, 15, 20, 25, 30, 40, 50 and ("mixed"), and the presence of a blue power law ( ) contributing 0, 20, 40, 60, 80 and 100% of the observed J-band emission.
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4.1.2. Disk and western dust lane
In the disk away from the central part of NGC 3079, the colours of
the stellar population must correspond to a mean spectral type earlier
than those indicated by the "bulges" zeropoint, i.e. close to those of
the "main-sequence line" in Fig. 5. If, for instance, a mean spectral
type A0 is assumed for the intrinsic "disk" colours, a "screen"
reddening corresponding to would be required;
the dust lane would require a mean spectral type of early B and
. Since recent star formation is associated
with dust, this result is reasonable. Alternatively, the "mixed"
extinction model may be a more adequate representation of the relative
distributions of stars and dust in the disk of the galaxy. This model
suggests a mean spectral type of late F or early G, but also rather
high values for the extinction optical depth.
In reality, a combination of "screen" and "mixed" extinction is
probably appropriate, and intermediate values for both spectral type
and visual light loss are obtained by varying the relative importance
of "mixed" and "screen" extinction. The present data do not allow us
to draw a firm conclusion as to which combination is preferred.
4.1.3. Stellar emission and K-band excess in the central kiloparsec
In Fig. 2a, we have shown the and
colours in 1" diameter apertures along the
major axis of NGC 3079. A number of these positions are also marked in
Fig. 5,. Towards the nucleus, the colours redden rapidly, reaching
peak values , at the
nuclear arcsec2 (87 87 pc). As Fig. 5
shows, the ( ) colour within
from the nucleus is far too red to be
explained by either the "screen" or the "mixed" extinction curves in
Fig. 5, so that excess emission must be present in the K-band.
The amount of excess K-band emission implied by the
near-infrared colours depends, however, on the choice of extinction
model. We argue here that the "screen" model is appropriate, for the
following reasons.
-
First and foremost, the fact that NGC 3079 is very nearly edge-on
( ) and displays prominent dust lanes implies
that the nuclear region must undergo a large amount of foreground
("screen") extinction.
-
The occurrence of fast outflows from the nuclear region requires a
significant central volume swept clear of gas and dust. Indeed, Sofue
& Irwin (1992) have noted the presence of such a "hole" with a
diameter of about in the CO distribution.
Stars in this central cavity will therefore undergo only foreground
extinction.
-
We may use the 0.8 mm continuum measurements by HIGW to estimate
the column density of the emitting dust, and hence its visual optical
depth and extinction. From Hildebrand (1983) and Savage & Mathis
(1979) we derive for a dust emissivity proportional to
the relation , with a
factor of about two uncertainty but independent of actual dust-to-gas
ratios. In a aperture, HIGW determined an
0.8 mm flux density of 0.35 Jy for the unresolved central source,
implying for a dust temperature
K (see e.g. Braine et al. 1997). Since the
emitting material surrounds the nucleus, only half of it will
contribute to the extinction. Thus, the submm result implies an
extinction or optical depth
. Fig. 5 shows that this value of
would not nearly produce the required reddening
if the dust were mixed with the stars. Hence a dominant foreground
extinction component is required.
-
With an observed K magnitude of in
the central arcsecond and a distance modulus ,
the absolute K magnitude becomes , not
corrected for extinction. Large reddening corrections, as would result
from the use of the "mixed" reddening curve, must thus be considered
unlikely.
-
If the blue ( ) colour of the eastern bulge is
due to a young stellar population, the reddening vectors in Fig. 5
should begin in the rectangle marked "bulge". The reddened points
along the disk then lie very closely to the screen extinction model,
indicating a gradually increasing foreground extinction, as expected
for a circular disk seen edge-on. In contrast, in this case the mixed
extinction model has great difficulty explaining the gradual reddening
towards the nucleus, producing colours that are too red in
( ).
-
Finally, Fig. 5 shows that for dust mixed with the stars, the
nuclear colours cannot be reproduced even in the limit of infinite
. Furthermore, the remaining colour difference
= ,
( ) = cannot be
reproduced by dust emission for any temperature below the dust
sublimation temperature. This problem is even exacerbated if the bulge
contains a young stellar population.
All of these arguments indicate that the extinction towards the
nuclear region is dominated by foreground material. We will therefore
in the following assume foreground extinction exclusively.
4.2. The central region of NGC 3079
4.2.1. Hot dust
The JHK colours alone are insufficient to uniquely separate
the effects of extinction and dust emission, as the observed points in
Fig. 5 may be reached by different tracks. They do constrain, however,
the range of admissible parameters.
On the one hand, Fig. 5 shows that the screen extinction limit for
the central position is at arbitrarily low
. For the other positions, the limit is even
lower. On the other hand, for a dust emissivity
, dust-mixing curves with temperatures
K yield colours too blue in
( ) to explain the observed colours of the
central position for any extinction, i.e. they pass to the left of the
central point in Fig. 5. As that figure shows, somewhat higher dust
temperatures are in fact allowed for the other positions, but in the
absence of starburst activity, we consider it unlikely that the
projected central 250 pc is cooler than its surroundings. We thus
find that K for the center of NGC 3079.
The photometry by Lawrence et al. (1985) in a 6" aperture provides an
additional constraint. Their observed ( ) and
( ) colours are very close to reddening lines
originating in the "bulge" point. A non-negligible dust contribution,
required by Fig. 5, is therefore possible only for relatively high
dust temperatures that likewise bring the dust-mixing line close to
the reddening line. Taking into account the fact that Lawrence et al.
(1985) were pointing off the true nucleus, their Fig. 1a suggests
that, were they properly centered, the L and M
magnitudes may be lower by at most and
respectively. The resulting colours,
( ) = and
( ) = , imply that the
temperature of dust contributing to the near-infrared emission of the
center must be between K. Lower temperatures
leave no room for a significant dust contribution at K, and
higher temperatures produce colours too blue in
( ) and ( ). We thus
conclude that K. A similar result was obtained
by Armus et al. (1994) who concluded from their
and m measurements to
the presence of significant amounts of hot dust with
K; in this respect we also note the absence of
cold dust emitting at far-infrared wavelengths throughout the inner
1.5 kpc (Braine et al. 1997). Finally, with radiating hot dust
contributing to the near-infrared emission, especially at K,
NGC 3079 exhibits characteristics similar to those of the Seyfert
1 galaxies in which Kotilainen & Ward (1994) found significant
contributions from dust radiating at temperatures of 600-1000 K.
Limits to the contribution of radiating dust to the emission at
m may be estimated from the deep CO-band
absorption evident in Fig. 2 of HIGW. Comparison with unreddened
late-type stellar spectra by Arnaud et al. (1989) and Lancon &
Rocca-Volmerange (1992) suggests that up to 25-30% of the emission
from the central may be caused by dust. By
assuming i. the intrinsic colours of the stellar population to
be those of the typical bulge in Fig. 5, ii. a constant dust
temperature K across the nuclear region,
iii. identical (foreground) extinction for both radiating dust
and stars and iv. no effect of scattering, we estimate from the
observed J, H and K-band fluxes a peak extinction
in the central pixel of
, with a dust
contribution in K. The inner molecular disk
region has somewhat lower mean values and a
dust contribution to observed K. Both values
are similar to the nuclear extinction
4 estimated from the Balmer decrement by
Veilleux et al. (1994), which provides further support for our
conclusion that the extinction occurs mostly or entirely in the
foreground. If the radiating dust suffers less extinction than the
stars, these conclusions remain unchanged, but the dust contribution
to the emitted (deredenned) radiation will be proportionally
lower.
The dereddened peak stellar surface brightness is of the
order of
W m m-1 sr-1.
Further analysis shows that the stellar light distribution has a
halfwidth of 2" perpendicular to the major axis, and 3" along the
major axis. Within the errors, the extent of the hot, radiating dust
emission is the same, but its centroid appears displaced from the
stellar centroid by to the west. The integrated
hot dust emission, corrected for extinction, is
W m m-1,
corresponding to a luminosity . This is
of the far-infrared luminosity of the nucleus
and of the mechanical luminosity of the
modelled nuclear wind (HIGW). If the dust extinction is lower, these
values decrease proportionally.
4.2.2. H2 kinematics
In Fig. 6 we show three position-velocity diagrams along lines
parallel to the major axis of the total H2 distribution at
, and offset from one another by
. This diagram indicates, again, the presence
of two H2 emission components: a bright central component
with a large velocity width superposed on weaker emission which
clearly shows rotation with approaching velocities north-northwest of
the nucleus. The bright H2 component can be identified with
component C1 in the and OH absorption maps
published by Baan & Irwin (1995), while the weaker emission
corresponds to their component C2. Their component C3 has no
counterpart in our images.
![[FIGURE]](img161.gif) |
Fig. 6. Position-velocity diagrams of H2 emission along lines parallel to the total H2 major axis ( ), integrated over 1" strips perpendicular to the major axis. South is at left (negative position offsets), north is at right (positive offsets); zero position is that of peak integrated H2 emission. The middle diagram is along the line passing through both the H2 and the 2.1 µm continuum peaks, i.e., through the midplane of the disk. The upper figure is along a line offset from the midplane towards the east by (projected distance 130 pc), and the lower figure along a line offset from the midplane to the west by an equal amount. H2 contour levels are 1, 2, 3, 4, 5, 6, 8, 10, 12.5, 15 and 17.5 in units of 10-8W m-2 sr- 1.
|
The bright central component has a FWHM diameter along the disk of
(225 pc) and a broad, flat-topped velocity
distribution which agrees well with the H2
S(1) spectrum obtained by HIGW, who found a
trapezoid shape for the line with . The
H2 distribution in the lower two panels of Fig. 6 does not
show evidence for the velocity gradient of
pc-1
( arcsec-1) assigned to component C1
by Baan & Irwin. If anything, the H2 data suggest a
much faster rotation of order pc-1
in the opposite sense, i.e. velocity higher in the north, lower
in the south. We suspect that Baan & Irwin may have been misled by
the blending of the bright component C1 with the weaker extended
component C2 (see their Fig. 3 and see also Fig. 2 of Veilleux et al.
(1994).
A qualitative estimate of the velocity width of the S(1) line as a
function of position can be obtained by dividing the total
H2 emission by the central H2 "channel" only.
This shows that the H2 velocity range is smallest in the
midplane of the disk, and increases away from the disk towards the
east (i.e. in the direction of the conical outflow), and also somewhat
to the west. The situation in NGC 3079 appears to be similar to that
in the central region of NGC 4945, where Moorwood et al. (1996) found
that the H2 emission covers the surface of a hollow outflow
cone coated on the inside with H emission, that
presumably plays a role in the collimation of this outflow.
The more extended H2 component, which is seen at upper
left and lower right in the central panel of Fig. 6, appears to rotate
in the regular sense, with a velocity gradient of
pc-1
( arcsec-1). This is somewhat
steeper than the gradient of the rigidly rotating CO component
(Fig. 5b in Sofue & Irwin 1992) which extends to about 10" from
the nucleus, but it is identical to the gradient determined by Baan
& Irwin (1995) for component C2. The H2
position-velocity diagram in the top panel of Fig. 6 (east of the
midplane) repeats this pattern for the now less bright extended
emission. In contrast, only very weak extended emission is present in
the position-velocity diagram offset to the west (Fig. 6, lower
panel), where extinction must be considerably higher, especially if
part of the emitting H2 is outside the midplane of the
galaxy. The rotation of the extended component implies a dynamical
mass of within pc from
the nucleus, or pc-3.
4.2.3. Structure of the central region
We are now in a position to combine the structural information from
various observations. In Table 2 we have collected the available
size information. In the data discussed so far, three significant
scale sizes can be identified.
![[TABLE]](img173.gif)
Table 2. Sizes of emitting components in the central region of NGC 3079
Notes:
corrected for extinction and finite resolution
approximate values
after subtraction of "ridge" component
-
The inner region appears to be a cavity filled with bulge stars and
edges traced by the bright H2 and hot dust emission. It
corresponds to the pc radius hole in the CO
emission noted by Sofue & Irwin (1992). As HIGW noted, such a
cavity is required by the model of Duric & Seaquist (1988), in
which it represents the central volume swept clear of molecular
material and dust by the strong outflow from the galaxy nucleus. At
the interface, rapid rotation prevails. Both the near-infrared images
and the H2 channel maps suggest that the inner outflow of
NGC 3079 contains excited molecular hydrogen and hot dust, presumably
swept away from the inner molecular disk by the impacting winds.
We propose that the relatively intense H2 and hot dust
emission both arise as the result of the impact of the nuclear outflow
on dense and dusty molecular material at the interface between the
central cavity and the molecular disk. The observed integrated
intensity of the bright S(1) H2
emission component alone is W m-2;
correction for extinction raises this value to
W m-2, i.e. to a luminosity
= , which in turn
suggests a luminosity in all H2 lines of about
. The more extended diffuse H2 has a
dereddened luminosity about half that of the bright emission region.
Thus, the molecular hydrogen luminosities we derive here are about a
quarter of those found by HIGW, partly because of our lower observed
value and partly because of a lower derived extinction. This relaxes
the already low efficiency requirements discussed by HIGW even
further, so that there can be no doubt that the impacting winds can
indeed easily explain the observed molecular hydrogen emission. As the
estimated total H2 luminosity of about
is forty times lower than the hot dust
luminosity of , the latter obviously poses a
more critical efficiency constraint than the H2 luminosity.
Although the dust efficiency requirement appears to be compatible with
models of the type proposed by Draine (1981) it is, however, difficult
to quantify this in the absence of further data.
-
The inner molecular disk extends to a radius of about 300 pc, where
its thickness has increased from pc to about
400 pc, suggesting an opening angle of about 110o. Excited
H2 and hot dust are found throughout the disk but at
intensities much reduced from those at the interface.
-
More extended emission from warm dust and molecular gas traces the
cooler outer parts of the molecular disk out to radii of about 1 kpc,
after which the emission merges with the low-level emission from the
main body of the galaxy (see Braine et al. 1997). This cooler material
extends to distances of about 400 pc from the plane of the galaxy.
4.3. Molecular gas in NGC 3079
4.3.1. Relation of CO emission to H2 column density
We may connect the total hydrogen column density
to reddening and CO intensity by the following
relations:
![[EQUATION]](img184.gif)
and:
![[EQUATION]](img185.gif)
where and are the
factors by which respectively the gas-to-dust and the CO intensity to
H2 column density ratios in the center of NGC 3079 differs
from the canonical values; is in mag,
is in K km s-1 and N is in
cm-2. Our choice of both and
is such that they will be less than unity in
environments with metallicities higher than those in the solar
neighbourhood.
Baan & Irwin (1995) derive for their extended component C1 an
absorption column density
, where is the unknown
spin temperature. For the same extended region,
we find a mean extinction corresponding to
. Considering that extinction and HI absorption
sample only half of the line of sight sampled by CO emission, we
obtain:
![[EQUATION]](img194.gif)
From Young et al. (1988) we find that the central 8" (695 pc)
yields a J=1-0 CO emission signal
K km s-1, so that:
![[EQUATION]](img196.gif)
First, we obtain from this equation an upper limit to the spin
temperature associated with the extended
component C1 by assuming zero H2 column density towards the
center: K. If the gas-to-dust ratio in the
centre of NGC 3079 is less than that in the solar neighbourhood
( ), the limit on
becomes more stringent. Second, since , we find
, i.e., even for a `normal' gas-to-dust ratio
( = 1) CO luminosities in the centre of
NGC 3079 correspond to at most a twentieth of the H2 column
density we would obtain by applying the `standard' Galactic conversion
factor. The conversion factor appropriate to NGC 3079 is thus
. This value is also an upper limit because the
value of used here applies to a larger area
than used for the extinction. For low gas-to-dust ratios (i.e.,
) and reasonable spin temperatures,
and consequently
rapidly become small. Conversely, CO-to-H2 conversion
factors similar to that of the Galactic disk ( )
are obtained only for very large gas-to-dust ratios
( ). Such large gas-to-dust ratios are more
characteristic for extremely metal-poor dwarf galaxies than for the
centres of spiral galaxies. These results are rather constrained and
point to low values of both the spin temperature
and the CO-to-H2 conversion factor X for a large
range of acceptable gas-to-dust ratios.
A low value for the CO-to-H2 conversion factor is
consistent with the `discrepancies' between H2 masses
derived from CO and from submillimetre observations, noted in Sect. 5
of HIGW. Moreover, a rather similar result has been derived by Braine
et al. (1997). On the basis of their
observations, they arrive at a conservative estimate
. There is some evidence that within the
cavity, the nucleus itself is surrounded by a high-density,
parsec-sized accretion disk. Baan & Irwin's (1995) component A
exhibits a high value
cm-2 K-1. Trotter et al. (1998) argue that this
component is part of an inner jet emanating from a heavily absorbed
nuclear engine, and that this nucleus is surrounded by a turbulent and
presumably dense disk, 2 pc in diameter and traced by H2O
maser emission. The nucleus may thus suffer a much higher extinction
than the extended region C1. This does not change our results, because
both our extinction and the HI absorption value used do not refer to
the nucleus, but to the material in front of the extended cavity. In
addition, the proposed circumnuclear disk has a very small filling
factor with respect to the 8" region sampled in CO emission.
Even if the actual extinction were to be higher than assumed by us,
we would still require the conversion factor Xto be
much lower than the one applicable to the solar neighbourhood,
although the constraints on spin temperature would be much relaxed.
Finally, we test our result by calculating the extinction towards the
extended central region required by Eq. (3) if
and are both set to unity (i.e., Galactic
values for the CO-to-H2 conversion factor and the
gas-to-dust ratio). In this case Eq. (3) changes to the expression
, so that and
. This value of the extinction, although
possibly appropriate to the very nucleus, is clearly ruled out for the
extended circumnuclear region sampled by our data. This underlines the
robustness of our conclusion that at least the CO-to-H2
conversion factor in the centre of NGC 3079 is substantially lower
than the Galactic value.
The nuclear activity in NGC 3079 may be responsible for the
apparent, extremely low [H2]/[CO] abundance. Theoretical
models by Neufeld & Dalgarno (1989) predict that this may occur in
regions exposed to dissociative shocks. Behind the shock, most of the
carbon will be incorporated into CO by gas-phase reactions, but the
catalytic formation of H2 is severely inhibited if the
essential dust grains are heated to temperatures of the level we
propose for the inner parts of NGC 3079. As a result,
[H2]/[CO] abundances may be depressed by one to two orders
of magnitude. These theoretical predictions at least provide a
consistent framework for the interpretation of the phenomena observed
in the central region of NGC 3079: the presence of fast, energetic
nuclear winds, shocked molecular hydrogen, hot dust and an
underabundance of molecular hydrogen with respect to carbon
monoxide.
4.3.2. Gaseous content of NGC 3079
Comparison of the interferometric and single dish CO data suggest
that of the order of 35 to 50% of the CO emission from NGC 3079 finds
its origin in the inner molecular disk within 500 pc from the nucleus
(Young et al. 1988). However, in the preceding we have presented
evidence for a relatively small amount of molecular hydrogen
associated with the bright central CO emission. As the main body of
NGC 3079 may well be characterized by more "normal"
CO-to-H2 conversion factors (e.g. Braine et al. 1997) we
cannot conclude that most of the molecular mass is concentrated in the
central region.
Scaling the molecular mass estimate by Young et al. (1988) to our
adopted distance and our estimated CO-to-H2 conversion
factor, we find for the molecular disk a mass ,
or of the dynamical mass. For
K and , we find a mean
ratio , characteristic for an ISM dominated by
molecular clouds. For the rest of the galaxy, Young et al. found a
mass of which reduces to
after scaling to our adopted distance and a
"normal" conversion factor of . This is 25
times the central molecular mass, and about a third of the
mass of NGC 3079 (Irwin & Seaquist 1991).
Including helium, the total mass of gas in NGC 3079 is
, or about 10 of its total
mass (cf. Irwin & Seaquist 1991). Such a fraction of the total
mass is characteristic for late-type galaxies, as is the mean ratio
. We note that our molecular gas estimates
again are very similar to those derived by Braine et al. (1997) under
different assumptions.
It thus appears that the molecular hydrogen content of NGC 3079 is
not exceptional compared to that of other late-type galaxies.
Rather, the emissivity of CO in the central molecular disk is
unusually high.
© European Southern Observatory (ESO) 1998
Online publication: July 20, 1998
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