Large-scale HI structure of the Circinus Galaxy
K.L. Jones,1 B.S. Koribalski,2 M. Elmouttie1 and R.F. Haynes2
1Department of Physics,
The University of Queensland 4072, Australia
2Australia Telescope National Facility,
P.O. Box 76, Epping 2121, Australia
Abstract:
HI observations with the Australia Telescope Compact
Array (ATCA) are used to study the neutral gas distribution and
kinematics of the Circinus galaxy. One of the most outstanding features
of Circinus is its enormous size (at least 80 arcmin or nearly 100 kpc,
assuming a distance of 4 Mpc) revealing a beautiful large-scale spiral
pattern. The fit of a tilted-ring model to the velocity field reveals
a strong kinematic warp of the outer spiral arms. The inner region
(radius 250 arcsec or 5 kpc) contains a highly elongated structure
with non-circular motions, suggesting that Circinus is a barred spiral
galaxy. At the centre of Circinus there is a 50-arcsec or 1-kpc-radius
HI ring or disk rotating with a speed of at least 200 kms-1.
Circinus, a nearby (
4 Mpc) spiral galaxy with a Seyfert2 nucleus
(Moorwood & Oliva 1990; Oliva et al. 1994), was discovered by Freeman et al.
(1977) on an optical plate. At a Galactic latitude of
it is
highly confused by foreground stars. A deep optical picture is displayed by
Elmouttie et al. (1995).
Optical observations show a galaxy diameter of several arcminutes (see
Table 1), although its location makes it difficult to determine exact
values (for a detailed discussion see Freeman et al. 1977). For the same
reason the galaxy's morphological type (e.g. SAS3, de Vaucouleurs et al.
1991) remains somewhat uncertain.
Circinus has an active core with strong X-ray, infra-red, and radio
emission. Matt et al. (1996) found an X-ray spectrum consistent with
Compton scattering and fluorescent emission from cold matter illuminated
by an obscured active nucleus. IRAS fluxes of 246 Jy at 60
and
314 Jy at 100
(Ghosh et al. 1992; Yamada et al. 1993) imply a
far-infrared luminosity of 6 109 L.
Radio continuum observations reveal a bright, compact source (Freeman et
al. 1977; Whiteoak & Bunton 1985; Harnett 1987; Davies et al. 1998) as
well as prominent lobes roughly perpendicular to the galaxy major
axis (Harnett et al. 1990; Elmouttie et al. 1995), similar to those in
NGC 3079 (Duric & Seaquist 1988). The nucleus of Circinus is known to
contain a highly luminous 22-GHz H2O maser (Gardner & Whiteoak 1982;
Whiteoak & Gardner 1986; Nakai et al. 1995; Greenhill et al. 1997), with
a spectrum similar to that of the maser in the galaxy NGC 4258, which
probably traces a molecular disk rotating around a supermassive object
(Greenhill et al. 1995). Not surprisingly, NGC 3079 is also a megamaser
galaxy (Baan & Haschick 1996).
Harnett et al. (1990) observed the Circinus galaxy in OH absorption using
the 64-m Parkes radio telescope at 1665 and 1667 MHz. The broad linewidths
(180 kms-1)
suggest a rapidly rotating ring of clouds surrounding the nucleus or several
independent clouds in the nuclear vicinity with inflowing and outflowing
motions. Similar linewidths are also observed in HI absorption
(Koribalski 1996). The detailed nuclear kinematics are being analysed by
Koribalski & Whiteoak (in prep.). A nuclear ring has also been detected
in H
and [OIII] as well as a funnel-shaped structure toward the NW in
[OIII] indicative of nuclear gas outflow (Marconi et al. 1994). Elmouttie
et al. (1998) find outflow velocities in H
of 150 kms-1.
Early HI observations with the Parkes 64-m radio telescope by Mebold et al.
(1976) and Freeman et al. (1977) revealed neutral gas extending over an area
more than one degree in diameter (limiting column density:
atoms cm-2) with a half width of about 32 arcmin x 15
arcmin, much larger than the optical extent. The rotation curve appears flat
at about 120 kms-1 out to a radius of 322 arcmin, giving a total
mass of
M.
The estimated hydrogen mass is about
7.2 109 M(see Table 1).
Table 1:
Parameters of the Circinus galaxy.
Table 1:
Parameters of the Circinus galaxy.
central position |
(J2000) |
1413102, -652020 |
(1) |
|
(B1950) |
1409180, -650618 |
|
|
l, b |
3113, -38 |
(1,2) |
type |
|
SAS3 |
(1) |
adopted distance |
D |
4 Mpc (-> 1 = 19.4 pc) |
(2) |
optical extent |
at B25 |
69 x 27
(PA = 220) |
(1) |
" |
at B25 |
70 x 35
( " ) |
(3) |
optical extent* |
at B24.5 |
119 x 46 |
(2) |
" |
at B26.6 |
17217
(extrapolation) |
(2) |
HI extent** |
|
32 x 15 |
(2) |
position angle |
|
210 5 |
(2) |
inclination |
|
65 2 |
(2) |
systemic velocity |
|
439 2 kms-1 (HI) |
(2) |
(heliocentric) |
|
380 47 kms-1 (optical) |
(1) |
velocity width |
|
290 10 kms-1 (HI) |
(2) |
rotation velocity |
|
152 7 kms-1 (HI) |
(2) |
far-infrared luminosity |
|
6 109 L |
(4,5,6) |
HI mass |
|
M |
(2) |
total mass |
|
M |
(2) |
*extinction-corrected;
**half width of the HI column density distribution
after Gaussian-correction.
References: (1) RC3: de Vaucouleurs et al. 1991, (2) Freeman et al. 1977,
(3) ESO-Uppsala: Lauberts & Valentijn 1989, (4) calculated from the 60
and 100m fluxes in the IRAS PSC 1988, (5) Ghosh et al. 1992,
(6) Lonsdale et al. 1985.
|
The Australia Telescope Compact Array
(ATCA)
observations presented here have a much higher angular resolution than
the single-dish data and reveal the detailed distribution and kinematics
of the HI gas in the disk and along the spiral arms of the Circinus
galaxy.
In Table 1 several parameters of the Circinus galaxy are summarized.
In Section 2 we summarize the observations, followed by the results in
Section 3, e.g. a detailed description of the HI gas distribution and
kinematics. The three-dimensional shape of the Circinus galaxy is described
and modelled in Section 4 and the case for Circinus being a barred spiral
galaxy is presented in Section 5. The discussion of the HI mass and the
total mass is presented in Section 6. In Section 7 the properties of the
Circinus HI disk are compared to those of other spiral galaxies.
The final section contains our conclusions.
The HI observations of the southern Circinus galaxy have been carried out
with three configurations (375, 750A and 1.5A) of the ATCA between 1995
February and August. Several observing parameters are summarized in Table 2.
The data reduction and analysis was carried out with the Miriad and AIPS
software packages using mostly standard procedures. The uv-data
were inspected to find those channels containing HI emission.
As Circinus is near the plane of the Milky Way, Galactic HI emission
is present in our observed frequency spectrum at radial velocities between
about -280 kms-1 and +50 kms-1. The HI emission from Circinus is observed
between about +240 kms-1 and +640 kms-1. The line-free channels were subtracted
from the uv-data to form 1) HI line data and 2) narrow-band 20-cm
continuum data. Both data sets were Fourier transformed and then
CLEANed (Högbom 1974; Clark 1980) as described below. Using `natural
weighting' for maximum sensitivity in the HI data resulted in an rms noise of 1.3 mJybeam-1 per channel and
an angular resolution of 59 arcsec x 52 arcsec; ATCA baselines
longer than 3 kilometres (see Table 2) were omitted.
CLEANing was facilitated by constraining the image to be non-negative.
Using `uniform weighting' for maximum angular resolution in
the continuum data resulted in an rms noise of 0.13 mJybeam-1
and an angular resolution of 5.2 arcsec x 4.9 arcsec.
To derive the moment maps a smoothing function (width = 5 velocity channels
and 3 pixels) and a cut-off level of 5 mJybeam-1 was applied. A
smaller cut-off level could have been used, but this resulted only in an
increased noise level outside the half width of the primary beam; no
primary beam correction was applied.
The HI line data also display a complex absorption structure against the
central continuum source which is not discussed here. For further analysis
see Koribalski (1996) and Koribalski & Whiteoak (in prep.).
The integrated HI spectrum of the Circinus galaxy is shown in Fig. 1
and looks similar to that of Freeman et al. (1977). It shows a
double-horn shape, indicating a rather regularly rotating disk, but
more gas than expected from a galaxy with a flat rotation curve is seen
between the HI peaks. Departure from a plane disk with a flat rotation
curve
is further indicated by the peculiar shape of the HI emission in the
individual channel maps (see Fig. 2). The HI emission extends over a
maximum velocity range of 400 kms-1, from about 240 kms-1 to
640 kms-1. The emission at the extreme velocities, mostly present near the
galaxy centre, is rather weak. At the 20-per cent level of the peak flux
in the HI spectrum the velocity width is
= 320 kms-1, at
the 50-per cent level we derive
= 285 kms-1.
The velocity field of a uniformly rotating, inclined disk (see, e.g.
Giovanelli & Haynes 1988) looks like a `spider'-diagram; lines of equal
velocity vary from straight (along the galaxy minor axis) to V-shaped
(near the major axis) with the apex at the centre of rotation. In Circinus,
the channel maps (see Fig. 2) reveal the typical signs of warped spiral
arms, i.e. loops and many systematically kinked lines instead of purely
V-shaped structures.
The narrow-band, 20-cm radio continuum image was convolved with a 20-arcsec
beam to emphasize the large-scale structure (see Fig. 3). As found by
Elmouttie et al. (1995; and references therein), there is a strong
central core (450 mJy) and extended plumes of emission toward the
south-east and north-west, these latter being more prominent than the
disk emission along the major axis.
Recent H
observations (Elmouttie et al. 1998) revealed a funnel-shaped
nuclear gas outflow from the central core into the same direction as the
north-western plume, indicating that the two radio plumes are most
likely associated with a large-scale bipolar flow of highly ionized gas.
Because no effects of this outflow can be seen in the HI data (see, e.g.
Fig. 4) we conclude that the plumes are perpendicular to the galaxy plane.
The moment maps (HI distribution, velocity field, and velocity dispersion)
and a median-filtered optical image of the Circinus galaxy are shown in
Fig. 4. The integrated HI distribution (0. moment; Fig. 4a) shows an
enormous galaxy. At the first contour level (0.2 Jybeam-1 kms-1 = 39.2
K kms-1) which corresponds to a column density of
atoms cm-2 or a surface density of 0.6 Mpc-2 we measure a
projected size of about 50 arcmin x 35 arcmin (or 58 kpc x
41 kpc). This is already larger than the ATCA primary beam of 33 arcmin
which clearly restricts our measurements of the outer regions of Circinus.
Starting from the centre of the galaxy and working outward, the following
regions are identified:
- The nuclear region (r < 50 arcsec). This is one of the most
interesting regions, but our low-resolution data do not allow a detailed
analysis. The central depression in Fig. 4a is caused by HI absorption
against the central continuum source (Koribalski 1996). It is not very
pronounced here because of the 60-arcsec beam and because it was
suppressed in CLEANing the channel maps. A fast-rotating nuclear ring
or disk of radius 50 arcsec or 1 kpc is found in emission and
is related to the broad HI and OH absorption lines.
It is most
evident in the position-velocity diagrams (Fig. 5), but also seen in the HI channel maps (Fig. 2) as well as the velocity field (Fig. 4c and Fig. 6).
The pv-diagram of Circinus is very similar to that of NGC 4945 (Ott 1995;
for a more detailed comparison of the two galaxies see Koribalski 1996).
- The inner disk or bar extends some 250 arcsec or 5 kpc
in radius along the major axis. It is very elongated with a major-to-minor
axis ratio of 4 (see Fig. 4a). Its nature and peculiar dynamics are
discussed in Section 5.
- The outer disk (r > 250 arcsec) is characterized by a
large-scale spiral arm pattern. Two great spiral arms appear to start
at the ends of the bar, wrap around in a clock-wise direction through
180-360, possibly overlap, warp and bifurcate in the outer
parts of the disk. Out to a radius of
arcsec the spiral arms
are tightly wrapped and very bright, whereas, beyond that radius, they
clearly are much weaker and open up. Our measurements of the outer regions
of Circinus and therefore its size are limited by the size of the ATCA
primary beam. Freeman et al. (1977) measure an overall extent of at least
80 arcmin (or 96 kpc). There is a strong contrast between the spiral arms
and the gas-poor inter-arm regions.
In Fig. 4a the major-to-minor axis ratio decreases from the centre outwards.
For the inner disk (r = 250 arcsec) this ratio is 4,
decreasing to 2 for the prominent inner parts of the great spiral
arms (r = 600 arcsec) and to 1.4 at the outer limits of the disk as
detected in Fig. 4a. This could either be a projection effect caused by a
changing inclination of the HI disk, i.e. a warp, or an intrinsic change
in the ellipticity of the orbits, i.e. a bar or oval distortion, or both.
The latter is most likely as we have already found independent evidence for
a bar in the inner region (see Section 5) and a warp of the outer spiral
arms (see Section 4).
Fig. 4b shows an optical image of the Circinus galaxy (taken from the
Digitized Sky Survey (DSS), and median filtered to subtract the foreground
stars) which is much smaller than the HI image. The optical extent (7
arcmin at B25, see Table 1) is about the same as the length of the
bar. The slightly extended feature near the bottom of the DSS image
is the star HD124197, here saturated.
Fig. 4c displays the mean velocity field (1. moment) of Circinus.
Although the field appears quite regular and symmetric, there are strong
deviations from plane circular orbits. The line-of-nodes twists in various
ways and resembles in general a big, open Z-shape where the major kinks
occur again at a radius of
arcsec. The largest asymmetry
occurs towards the western side of the galaxy, where the velocity contours
show a 90 twist which we interpret as two warped and overlapping
spiral arms. The minor wiggles (5 kms-1) in the velocity contours
which occur quite regularly over the whole velocity field are the signature
of a spiral density wave (see, e.g. Visser 1980; Canzian 1993; Lindblad,
Lindblad & Athanassoula 1996).
Fig. 4d shows the mean velocity dispersion (2. moment) of Circinus.
The highest dispersion measured close to the nucleus (up to 145 kms-1) is
caused by the fast-rotating nuclear ring. Note also an increase of
the velocity dispersion within the bar and near the western edge
where two spiral arms appear to overlap.
Fig. 5a shows the major axis pv-diagram (PA = 210) of Circinus.
Two kinematic components are clearly visible: 1) a quite spectacular, steeply
rising nuclear component (r < 50 arcsec) with maximum velocities of
200 kms-1 and 2) a more slowly rising component which reaches its
maximum velocity of 180 kms-1 at a radius of 350 arcsec and beyond
that gradually drops to 100 kms-1. The overall shape of the rotation
curve is quite similar to that of our own Galaxy. Both components are also
seen in the minor axis pv-diagram (Fig. 5b). We interpret the first
component as the fast-rotating nuclear ring or disk of radius
50 arcsec or 1 kpc (see also Figs. 5c and d). The second component is
quite typical for spiral galaxies of type Sb to Sd (see Giovanelli &
Haynes 1988). The strong decline of the measured rotation curve in the outer
parts of Circinus is very likely caused by projection effects due to a warp
(see Section 4). From the pv-diagrams we obtain a size of 55 arcmin x
40 arcmin or 64 kpc x 47 kpc of the HI disk. The extent along the
major and minor axes is quite asymmetric, 29 arcmin NE, 26 arcmin SW
(Fig. 5a) and 23 arcmin SE, 17 arcmin NW (Fig. 5b), respectively.
Intensity maxima occur at several intersections with the spiral arms.
The approximate radial flux density distribution (Fig. 7a) has been obtained
from the integrated HI distribution (Fig. 4a) assuming circular symmetry
as well as an average inclination (i = 55) and position angle (PA =
210). Please note that parts of the HI disk have non-circular orbits
(see Section 5) and both the inclination and position angle vary by more than
10 (see Section 4). The three regions previously identified from
the HI moment maps (Fig. 4) are briefly discussed in Fig. 7a, and as
follows:
- the nuclear region (r < 50 arcsec). This is indicated here by
a central depression and is dominated by HI absorption against the continuum
emission of the core;
- the inner disk or bar (r < 250 arcsec). The maximum
HI column or surface density reached in this region is 1.25 1021
atoms cm-2 or 10.0 Mpc-2, respectively. The minimum close
to r = 250 arcsec coincides with the end of the bar and the start
of the spiral arms;
- the outer disk (r > 250 arcsec). This region is characterized
by the two great spiral arms. The maximum of the HI column or surface
density in that region is 1.18 1021 atoms cm-2 or
9.4 Mpc-2, respectively, and coincides with the bright, tightly
wound beginning of the arms. Beyond
arcsec the spiral arms are
much fainter and the HI column density is more uniform. Please note that
the data are not primary beam corrected and that we have not detected the
full size of the Circinus galaxy as known from single-dish observations
(Freeman et al. 1977).
Fig. 7b shows a comparison of the total mass and the HI surface density.
The ratio is approximately constant, at least out to a radius of r = 1500
arcsec, beyond that the HI flux density represents only a lower limit.
One possible explanation of the constant ratio is that the dark matter is
distributed similar to the HI (see, e.g., Pfenniger & Combes 1994).
For a detailed comparison with other spiral galaxies see Bosma (1981) and
Broeils & van Woerden (1994).
Over most of the disk the HI spectra are single peaked with a FWHM of
20-30 kms-1 (3-5 channel widths). More complex spectra are seen only in a
few regions, e.g. within 150 arcsec of the galaxy centre (Fig. 3).
The narrow-band 20-cm continuum image shows which spectra may be affected
by HI absorption. The radio lobes toward the south-east and north-west are
projections of plumes directed out of the plane of the disk (Elmouttie et
al. 1995). The spectra at the position of the radio lobes do not show any
significant high-velocity wings or other peculiarities indicating that there
is no substantial amount of neutral hydrogen gas within the lobes and no
interaction of the lobes with the HI disk (supporting their orientation
out of the plane).
Double-peaked spectra are seen within the bar and in a few regions close to
the spiral arms, the latter possibly being a result of the warp and some
overlapping spiral arms. The separation of the peaks in the spectra is up
to 40 kms-1.
There are numerous features which hint at a significant warp of the outer
spiral arms in Circinus; the unusual loops and kinks in several channel
maps (Fig. 2), the changing major-to-minor axis ratio of the overall HI distribution (Fig. 4a), and the general appearance and, in particular, the
Z-shape of the velocity field (Fig. 4c). Any of these features alone could
possibly be explained by a different mechanism. For example, spiral density
waves produce a twisted velocity field (Walsh, Staveley-Smith & Oosterloo
1997), where individual channel maps might resemble those observed in
Circinus. And, in the inner disk, the changing major-to-minor axis ratio is
almost certainly caused by a change in orbit shape (see Section 5). But we
are quite confident that the outer disk of Circinus is warped. Assuming
circular orbits, which is not true for all parts of the disk, we fit a
tilted-ring model to the mean velocity field to derive the rotation curve,
and the radial variation of the inclination and position angle.
A tilted-ring model was fitted to the Circinus velocity field (Fig. 4c)
using the AIPS task Rocur according to the method described by
Begeman (1989). Within the central 10-arcmin radius we found a systemic
velocity of
= 441.51.5 kms-1 and a centre position of
(J2000) =
,
-652026, both consistent with previous data (see,
e.g. Table 1). The deviations are well within the velocity and angular
resolution of our data. Keeping those parameters fixed, we then derive the
inclination angle (i), the position angle (PA) and the rotation velocity
()
of Circinus as a function of radius (r). For the
tilted-ring fit we used a ring width of 30 arcsec or a half beam-width
(in the plane of the galaxy). Fig. 8 shows the best fit obtained for the
overall disk as well as for the approaching and receding sides separately.
The main results are as set out below.
- Beyond the nuclear ring the rotation curve (Fig. 8a) appears rather
flat at
= 15510 kms-1. The small radial variations
could be caused by spiral density waves, varying orbit shapes, and different
disk components.
- For various reasons the inclination angle (Fig. 8c) of Circinus changes
quite dramatically with radius.
Because the nuclear region (r < 100 arcsec) is not well resolved and
strongly affected by HI absorption, the fit values in that region should be
regarded with care. The pv-diagrams (Fig. 5) clearly show a fast-rotating
nuclear ring, but its inclination angle, and consequently, its speed are not
well defined here. If the inclination of the HI ring is similar to that of
the H
ring,
(Marconi et al. 1994; see also Fig. 11),
then its speed would be about 200 kms-1, much higher than that of the disk
(see also Elmouttie et al. 1998).
Furthermore, if the inner disk (r < 250 arcsec) does indeed contain a
bar as discussed in the next section, the assumption of circular orbits
is violated and the results obtained in those regions are incorrect. Beyond
the inner disk (r > 250 arcsec) the HI data are very good and the orbits
appear circular. The inclination angle reaches a maximum of nearly 70 at r = 400 arcsec and then rather steadily declines to a minimum of
40 at
arcsec. Further out the fit indicates another
rise of the inclination angle to 6010. The maximum
warp-angle is 30.
- The position angle (Fig. 8b) also varies dramatically from about
190 to 220. The first maximum of PA = 206 occurs very close
to the radius of the maximum inclination angle. This radius, r = 400 arcsec,
possibly indicates the onset of the warp. In Circinus a PA increase with
radius indicates the kinematic major axis advances in the sense of the galaxy
rotation. This is the case in two regions, the inner disk and outside
the Holmberg radius (
arcsec). At a radius of about 900 arcsec
the PA reaches a second maximum of 220 and beyond that decreases
again.
Out to a radius of about 900 arcsec the fits of the approaching and receding
sites of the galaxy agree quite well. Some deviations are found further out,
generally increasing with radius. This possibly indicates an asymmetry of the
Circinus disk in the outer regions. Beyond about 1700 arcsec the tilted-ring
fit is rather unreliable, mainly because of a lack of data along the major
axis. A future ATCA mosaic of the whole Circinus galaxy will allow us to study
its outer regions in more detail.
The model and the residual velocity fields obtained from the tilted-ring fit
of Circinus are shown in Figs. 9a and b, respectively. Whereas the model
nicely reproduces the main features of the observed velocity field, the
residual field indicates various deviations over the whole disk where the
tilted-ring model was less successful. The bar, asymmetries of the
disk and spiral density waves are likely to be the main reasons for the small
discrepancies.
The three-dimensional shape of the warped disk is illustrated in Fig. 10,
taking into account the slightly different results for the approaching and
receding side of Circinus.
The HI gas dynamics in the inner disk of Circinus (see Fig. 6) suggest
that it is a barred galaxy. All previous classifications described it as an
unbarred galaxy (see Table 1). The bar has a length of 500 arcsec or
roughly 10 kpc. Its position angle of
= 225 is similar
to that of the major axis (PA = 210). The two great spiral arms
originate at the ends of the bar as is commonly observed in barred galaxies.
The geometry of Circinus, its velocity field, and various other indicators
(e.g., the shape and polarization of the radio lobes and the dust lane)
show that it is rotating in an anti-clockwise sense and that the spiral arms
observed in HI are trailing. The orbits within the bar are elliptical as
can be shown by measuring the angle between the major and minor axes which
would be 90 for purely circular motion. Following the contour of the
systemic velocity in Fig. 6 we find a much smaller angle of 60
within the bar. A similar analysis is shown by Lindblad et al. (1996; their
Fig. 20) on the galaxy NGC 1365.
The bar pattern speed is about 15 kms-1kpc-1 assuming a co-rotation
radius of 600 arcsec, which corresponds to 1.2 x the bar radius
(Contopoulos & Grosbøl 1989; Athanassoula 1992).
The nuclear ring is very likely located close to the inner Lindblad resonance.
Although the ring is clearly seen in HI (e.g. Fig. 5), it is much better
resolved in H
images of Circinus (Marconi et al. 1994; Elmouttie et al.
1998). The H
ring (40 arcsec diameter) is about half the size of the
HI ring (100 arcsec diameter) and is inclined at an angle of
40. CO spectra from the inner part of Circinus also suggest a
circum-nuclear disk or torus (Johansson et al. 1991; Israel 1992; Curran
et al. 1998).
In addition to the nuclear ring the H
images clearly show two short,
symmetric extensions starting at the southern and northern part of the
nuclear ring and curving in an anti-clockwise direction (see Fig. 11; also
Marconi et al. 1994). They look very much like spiral arms, but are
curved in the opposite direction from the large-scale HI arms. Since the
HI and H
velocity fields do not indicate any counter-rotation, we have
to conclude that these features are two leading spiral arms. Thus, Circinus
has both a set of trailing and leading spiral arms. This is certainly a
rather unique phenomenon and difficult to explain. One possible explanation
has been presented by Yuan & Kuo (1997). They studied ``the non-linear
response to a periodic bar potential in a differentially rotating gaseous
disk'' and find that ``major morphological differences exist between spiral
waves excited at the three types of Lindblad resonance.'' In particular, they
find that ``spirals associated with the outer Lindblad resonance are tightly
wound, while those associated with the inner Lindblad resonances are relatively
open. In general spirals are trailing. However, those excited at the inner
inner Lindblad resonance are leading.'' Yuan & Kuo also examine the process
of fueling of AGNs and starburst rings in the various spiral wave modes.
The total HI flux density,
,
of Circinus detected by the ATCA
out to a radius of 30 arcmin is 1000 Jy kms-1. Using the expression
(Roberts 1962) we derive an HI mass of
M.
As expected, this value is considerably
smaller than the
M
derived from the single-dish
map by Freeman et al. (1977). The higher flux value is more realistic and
can be explained by (a) the enormous extent of Circinus which is much larger
than the primary beam of the 22-m dishes of the ATCA and (b) the nature of
the interferometer to filter diffuse extended emission (although (a) is very
likely the dominating reason).
The total mass of Circinus within the same radius (
= 35 kpc)
is
M.
Because the extent of Circinus is much larger
than measured here, this value for the total mass should be regarded as a
lower limit. Freeman et al. (1977) find
M;
this is smaller than our value because they used an inclination-corrected
rotational velocity of only 118 kms-1. The hydrogen to total mass ratio is
about 0.02-0.04.
Briggs (1990) gives the rules of behaviour for galactic warps from a detailed
study of 12 galaxies with extended warped disks as follows.
- The HI layer typically is planar within R25 (the radius, where
the surface brightness in B reaches 25 magarcsec-2), but warping
becomes detectable within the Holmberg radius
.
This
is true for Circinus although, because of the bar, it is difficult to
determine the exact radius of the onset of the warp. Figs. 8b and c show a
definite change of position and inclination angle between r = 300 and 500
arcsec or roughly R25 and
(see Table 1). In this region
the orbits change from elliptical to circular. The most likely onset of the
warp is at r = 400 arcsec, where the inclination angle reaches a maximum
of 68. Beyond that radius the inclination angle keeps decreasing until
it reaches a minimum about 40 between 1300 and 1500 arcsec. The
warp-angle is about 30.
- Warps change character at a transition radius near .
We
detect an abrupt change of the position angle at the Holmberg radius (see
Fig. 8b).
- For radii greater than
the kinematic major axis advances
in the sense of galaxy rotation for successively larger radii. This is true
in Circinus until nearly twice the Holmberg radius and then reverses.
We conclude that Circinus follows Brigg's rules for the behaviour of warps but
find that an additional change in the major axis position angle occurs at
nearly twice the Holmberg radius.
An HI mass of 7.2 109 M
places Circinus as greater than
average (2.5-4.0 109 M)
for type Sa-Sc spirals (Haynes & Giovanelli
1984). Circinus is one the largest and most gas-rich Seyfert galaxies. In a
single-dish survey of 91 Seyfert or Seyfert-like galaxies, Mirabel & Wilson
(1984), found seven very gas-rich galaxies. There is however a great range
of HI properties of Seyfert galaxies. Of the 91 galaxies, HI was detected
in only 39.
The Circinus HI extent is at least 80 arcmin or 96 kpc (at a distance of
4 Mpc), a factor of 5 larger than the estimated Holmberg diameter
(De26.6) of 17 arcmin (Freeman et al. 1977; see Table 1). Bosma
(1981), in a study of the HI distribution and kinematics of 20 spiral
galaxies, found typical HI diameters of 20-50 kpc at a limiting column
density of 1.82 1020 atoms cm-2. Three of them had diameters larger
then 96 kpc (M81: 100 kpc; M33: 130 kpc; and M31: 250 kpc). In a
Westerbork Synthesis Radio Telescope search for spiral galaxies with extended
HI disks (sensitivity 1020 atoms cm-2), Broeils & van Woerden
(1994) found 5 out of 49 spiral galaxies had an HI extent > 96 kpc (125 kpc
maximum). For a summary of high-sensitivity observations of extended HI galaxy envelopes see Huchtmeier & Richter (1982). At a sensitivity of
4-10 1018 atoms cm-2 they list eight examples of galaxies with HI extent 150-300 kpc. Some of the galaxies with the greatest HI diameters
are: Mkn 348 (Morris & Wannier 1980; Heckman et al. 1982), NGC 628 (Briggs
et al. 1980), M101 (Huchtmeier & Witzel 1979), and M83 (Huchtmeier &
Bohnenstengel 1981). We conclude that the Circinus HI disk is among the
most extensive spiral galaxies yet discovered.
Circinus is truly a remarkable galaxy. Its nuclear region is very active,
revealing a starburst and an AGN as well as a fast-rotating ring and gas
outflow; the prominent, large-scale radio lobes are possibly related to
the nuclear outflow. Further kinematic features are the 5-kpc bar and the
warp of the extended HI layer. Circinus is huge compared to most galaxies
and is probably the closest AGN spiral galaxy known. It also appears quite
isolated as its closest known neighbours are more than 15 away. No
signs of tidal interactions have been found. A survey of the southern sky
and in particular the ``zone-of-avoidance'' with the newly installed
multibeam system at the 64-m Parkes telescope (Staveley-Smith et al. 1996)
will provide more details about the large-scale environment of the Circinus
galaxy (Koribalski et al., in prep.). The galaxy ESO097-G012, whose
velocity is so far unknown, lies at a projected distance of only 21.6 arcmin
from the centre of Circinus (close to the bottom right of Fig. 4) but must
be much further away as it has not been detected within the observed
velocity range.
Previous single-dish HI observations by Freeman et al. (1977) showed that
the Circinus galaxy has an enormous extent of at least 80 arcmin or nearly
100 kpc, assuming a distance of 4 Mpc. This is much larger than the primary
beam of the ATCA antennas and our HI study of the outer parts of the disk
is therefore quite limited. To overcome this deficit we will mosaic the whole
Circinus galaxy with the ATCA, which will also enable us to combine the
interferometer data with the single-dish multibeam data described above.
The biggest advantage of the ATCA data presented here is their sensitivity and
high angular resolution, which allows us to analyse in detail the enormous
HI disk of Circinus. Because of its location near the Galactic Plane,
Circinus is very difficult to study in the optical regime, and its optical
size and type remain uncertain. Most other studies have concentrated on the
bright nuclear region, and the existing single-dish HI observations have
not revealed any significant structure. The most important results obtained
from the ATCA HI data are summarized in the following.
- Circinus reveals a beautiful, large-scale spiral pattern. The two
trailing spiral arms, which emanate from the ends of a bar, are very bright
and quite tightly wound for the first 180-270. Further out they
are much fainter, open up, and bifurcate.
- The HI layer of the Circinus galaxy is warped, generally following
the rules given by Briggs (1990). The kinematic warp is most clearly seen
in the velocity field, and a tilted-ring model gives a warp-angle of about
30. The spiral arms appear to overlap on the western side of the galaxy.
- The bar is 10 kpc long and has a pattern speed of about
15 kms-1kpc-1. The HI velocity field clearly shows that the orbits
within the bar are non-circular.
- Although the nuclear region is not well resolved here, a few important
results have been obtained. The HI position-velocity diagrams (Fig. 5)
distinctly show a fast-rotating nuclear ring of about 1 kpc radius. This
ring appears slightly larger than the H
ring which was discovered by
Marconi et al. (1994; see also Elmouttie et al. 1998). The location of the
nuclear ring is likely to be close to the inner Lindblad resonance.
- One incredibly interesting feature we found is a symmetric pair of
leading spiral arms emanating from the northern and southern side of the
H
ring. The leading arms are only seen in H
whereas the trailing spiral
arms, which begin at the two ends of the bar, are seen in HI. A possible
explanation for the formation of both types of spiral galaxies is described
by Yuan & Kuo (1997).
- Circinus has an active galactic nucleus as well as a starburst ring.
High resolution HI data are being analysed (Koribalski & Whiteoak, in
prep.) to study the gas flow induced by the bar
and in particular the fueling of the nuclear region.
- The Miriad data analysis system at the University of Queensland is
maintained and supported by the Australia Telescope National Facility (ATNF).
- The Digitized Sky Survey (DSS) was produced by the Space Telescope
Science Institute (STScI) and is based on photographic data from the UK
Schmidt Telescope, the Royal Observatory Edinburgh, the UK Science and
Engineering Research Council, and the Anglo-Australian Observatory.
- This research has made use of the NASA/IPAC Extragalactic Database (NED)
which is operated by the jet propulsion laboratory, Caltech, under contract
with the National Aeronautics and Space Administration.
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Figure 1:
The integrated HI spectrum of the Circinus galaxy shows a
double-horn shape, indicating a rather regularly rotating disk. The
emission extends over
kms-1, from about 240 kms-1 to
640 kms-1. The relative large amount of gas near the systemic velocity
(
kms-1) indicates a falling rotation curve, which
is caused by the warping of the outer spiral arms. -
Only those channels containing line emission have been CLEANed and are
displayed here; the velocity resolution is 6.6 kms-1.
|
Figure 2:
The HI channels maps of Circinus. These maps have been obtained
using `natural weighting' of the uv-data. The centre velocity of the
channel is indicated in each panel; the channel width is 6.6 kms-1. The first
and last channel maps are unCLEANed. The contour levels are -4.5 , 4.5
(3), 20, 50, and 100 mJybeam-1; negative contours
are dashed. The cross marks the galaxy centre (see Table 1).
|
Figure 2:
continued.
|
Figure 2:
continued.
|
Figure 2:
continued.
|
Figure 3:
HI spectra near the centre of Circinus. The position at which
the spectrum was taken corresponds to the bottom left corner of the
corresponding subplot. The overlaid narrow-band 20-cm continuum image
(contours) shows the strong central source, the weak disk and the
prominent radio lobes.
|
Figure 4:
Moment maps of the Circinus galaxy.
-(a) HI distribution. The contour levels are 0.2, 0.5, 1, 2, 3,
4, and 5 Jybeam-1 kms-1.
-(b) Median-filtered optical image from the Digitized Sky Survey
(DSS).
-(c) Mean HI velocity field. The contour levels range from
291 to 581 kms-1in steps of 10 kms-1.
-(d) HI velocity dispersion (contours) overlaid onto
the HI distribution (grey scale). The contour levels are
5, 10, 15, 20, 30, 40 and 50 kms-1.
The beam size is displayed in the bottom left corner.
|
Figure:
Position-velocity diagrams of the Circinus galaxy along
(a) the major axis (PA = 210, width = 150 arcsec) and
(b) the minor axis.
Figures (c) and (d) show the nuclear region only.
|
Figure 6:
The HI distribution (grey scale) and mean velocity field (contours)
of the inner disk of Circinus. The contour levels are nearly
the same as those in Fig. 4c; to emphasize the systemic velocity the
two contours next to it have been omitted. The beam size is displayed
in the bottom left corner.
|
Figure 7:
(a) The HI column density,
,
of Circinus as a
function of projected radius assuming circular orbits and an
average inclination and position angle.
(b) A comparison of the surface density of the total mass,
(solid line) and the HI mass,
,
(open circles).
|
Figure 8:
Results of a three parameter fit to the mean velocity field of the
Circinus galaxy as a function of radius:
-(a) the rotation velocity,
,
-(b) the position angle, PA(r) and
-(c) the inclination angle, i(r).
|
Figure 9:
(a) Model and (b) residual velocity field for Circinus using
the parameters of the tilted-ring model (see Fig. 8). Overlaid
onto the residual velocity field (-50 to +22 kms-1; grey scale)
is the observed velocity field (contours). The contour levels
in both maps are the same as in Fig. 4c.
The beam size is displayed in the bottom left corner.
|
Figure 10:
Projection of the 3-dimensional shape of the Circinus HI disk
derived from the tilted-ring model (see Fig. 8).
|
Figure 11:
H
image of the inner region of the Circinus galaxy. In addition
to the bright nuclear emission two short, symmetric extensions
are visible. They start at the southern and northern part of the
nuclear ring and curve in an anti-clockwise direction, opposite
to that of the main spiral arms seen in HI.
|
Large-scale HI structure of the Circinus Galaxy
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Footnotes
- ...
(ATCA)
- The Australia Telescope is funded by the Commonwealth of
Australia for operation as a National Facility managed by CSIRO.
Baerbel Koribalski
1998-09-01