Dynamics, Distribution, and Amount of Molecular Gas in Galaxies
with NIR Isophote Twists: NGC 2273 & NGC 5728
McMaster University
Background
The nuclei of barred spiral galaxies are often the setting for
extraordinary events such as starbursts, molecular rings, inflow and
even Seyfert activity. The need to understand the mechanisms driving
these phenomenon has inspired a great number of observations and
computer simulations. Models suggest that bars in galaxies can drive
molecular gas into the nucleus where it can fuel the vigorous star
formation activity that would otherwise exhaust the molecular gas
content on timescales much shorter than observed (e.g. Combes
1994). Bars can only drive molecular gas inward until it reaches the
Inner Lindblad Resonance (ILR; see Fig. 1),
where it will accumulate into a ring, halting the inflow. To overcome
this, Shlosman, Frank, & Begelman (1989) proposed that the ring may
become unstable and form a secondary bar inside the radius of the ILR,
allowing gas to reach much farther into the nucleus and possibly be the
driving mechanism behind Seyfert nuclei.
This poster presents observations taken this Spring at Owens Valley
Radio Observatory (OVRO) of two galaxies that show NIR isophote twists
which may be the signature of a ``bar within a bar'' (e.g.
Devereux et al. 1992).
Models
There are three mechanisms which can account for NIR isophotal twists
(Elmegreen et al. 1996);
- The first suggests that the twists may be the result of a
triaxial bulge (Kormendy 1979). This mechanism is a stellar phenomenon
and should not be visible in the CO maps.
- The second mechanism, proposed by Shaw et al. (1993),
suggests the isophote twists are the result of an inner stellar bar
misaligned from the main bar, triggered by a dissipative gaseous
component. Their numerical simulations suggest that gas dissipation can
steal angular momentum causing the inner part of the gaseous bar to lag
behind the main bar. This exerts a torque on the stellar component of
the bar, pulling it out of alignment also. The whole system would then
rotate with the same angular frequency, with the inner gaseous bar
trailing slightly behind the main bar.
- The third mechanism suggests that the twists are the result of a
kinematically distinct inner bar (Friedli & Martinet 1993). Their
N-body simulation (with stars and gas) suggest gas inflow along the bar
can accumulate enough mass that the inner part of the gas bar can
become nearly self-gravitating and decouple from the main bar. The
inner bar may rotate with a pattern speed of nearly 6 times that of the
main bar.
The first model is associated with the stellar bulge and would not show
up in CO maps because most molecular gas is confined to the galactic
disk. The second model would exhibit an inner gaseous bar that leads the
inner stellar bar slightly, but have the same rotation speed as the
main bar. The third model would show a gaseous inner bar that is
rotating at a different rotation speed than the main bar. Both the
second and third model require the galaxy contain about 5-10% gas (by
mass) in order to simulate these effects.
NGC 2273
Distribution
The NIR image (Fig. 2a) shows a small
inner bar (seen as isophote twists) in the inner 10x10" misaligned from
the main bar by about 90 degrees. This may be responsible for fueling
the nuclear activity in the Seyfert 2 galaxy.
The 12CO J=1-0 map (Fig. 2b)
shows only the inner bar of the NIR image, which immediately rules out
Model (1)'s triaxial bulge explanation of the NIR isophote twists. Also
visible are the indications of inflow along the leading edge of the
main bar (the fingers extending NW and SE from the top and bottom
(respectively) of the CO bar). This inflow could be supplying the inner
bar with enough material to sustain the Seyfert activity.
Dynamics
There is no clear evidence that the gaseous inner bar is leading
the stellar inner bar by any significant amount, as predicted by Model
(2). This does not rule out Model (2), since they predict deviation
angles between the gaseous and stellar bars as small as 5 degrees,
i.e. smaller than the maps will allow us to measure accurately.
The Position-Velocity diagram taken along the axis of the CO bar
shown in Fig. 3 indicates that the bar is
rotating as a solid body, at approximately 400 km/s/kpc. Since we have
no detections beyond the inner bar shown in Fig.
2b, and there are no rotation curves published for the inner 1', we
cannot determine yet if the CO inner bar is kinematically distinct as
predicted by Model (3), or if it is rotating at the same angular
frequency as the main bar as predicted by Model (2). It is not counter
rotating (see Fig. 4).
Amount
The total CO flux for the nuclear region of NGC 2273 is 4.1 Jy/km/s
(see Fig. 4), which indicates a molecular mass
of 3.3x107 Mo (e.g. Wilson 1995). This
constitutes 0.03% of the galaxy's total dynamical mass (van Driel &
Buta 1991). Of course, this is only a lower limit since the
interferometer is insensitive to the large scale structure that is
likely present in the barred galaxy, and our maps only cover the inner
1x1' of NGC 2273s optical radius of ~1.5'.
NGC 5728
Distribution
The NIR image of NGC 5728 (Fig. 5a) shows
isophote twists in the nuclear region similar to that of NGC 2273 which
may also indicate the inner bar responsible for transporting material
inside the Inner Lindblad Resonance and fueling the Seyfert 2 nucleus.
The 12CO J=1-0 map (Fig. 5b)
does not show the same structure as NGC 2273. It shows 3
(perhaps 4) individual Bright Lumps Or Beads (hereafter, BLOBs) of
emission. These BLOBs seem to form a partial ring with a radius of 6"
(0.7 kpc at 27 Mpc). A similar structure was seen in the HST images by
Wilson et al. (1993) who see a ring of radius of ~5"
surrounding ionization cones at the Seyfert nucleus. It is likely a
gaseous ring collecting at the ILR (determined to be at about r=10" by
Schommer et al. 1988).
Dynamics
There is no evidence in our CO maps for a bar interior to the ring
as reported by Wilson et al. (1993), so naturally we cannot
confirm the reports that this inner bar may be counter-rotating (Prada
& Gutièrrez 1999). The spectra (see Fig.
6) show no strong evidence for rotation, as you may expect if we
were looking at a torus of molecular gas nearly face-on.
Amount
The CO flux for the nuclear region of NGC 5728 is 1.8 Jy/km/s,
which indicates a molecular mass of 2.1x107 Mo
(see Fig. 6). The total dynamical mass of the
galaxy in the inner r=5" is ~1x1010 Mo (Rubin
1980), so the gas constitutes 0.2% by mass. Again, this is a lower
limit because the interferometer will miss large scale structure and
our 1x1' CO maps are only covering the inner regions of this galaxy
(the optical radius is ~1.5').
Summary
- In NGC 2273 we see an inner bar of molecular gas, with the same
orientation as the NIR isophote twists. This immediately rules out the
triaxial bulge explanation for the isophote twists. The lack of a
detailed rotation curve for the inner 1x1' of NGC 2273 does
not allow us to rule out either Model (2) or (3); both are consistent
with the current data. It seems that the NIR isophote twists are the
result of a inner bar misaligned from the main bar, without more data,
we cannot determine how this inner bar was formed.
- In NGC 5728 we see traces of a molecular ring (see Fig. 5b) which is likely a molecular ring
accumulating at the ILR. The CO observations do not support Model (2)
or (3) and cannot rule out the existence of a triaxial stellar bulge as
the explanation for the NIR isophote twists [Model (1)].
- Our CO maps indicate that the NGC 2273 is 0.02% molecular gas
(by mass) over the entire galaxy and NGC 5728 is 0.2% molecular gas in
the inner 5". This is substantially lower than the values of 5-10%
required by Models (2) and (3) in order to simulate the observed
isophote twists in the nuclei of these barred galaxies. These values
are only lower limits.
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