We present high spatial resolution, near-IR images in J,H, and K of the
nucleus of NGC 3227, obtained with the Adaptive Optics bonnette on
CFHT. The 
(17pc) resolution allows us to probe structures in the
very core region at unprecedented scales. We are able to identify
an inwards spiraling starburst in all three near-IR bands and
a counterpart in an HST V-band image.
Dust obscuration becomes significantly less pronounced at longer wavelengths,
revealing the true geometry of the core region.
The observed structures may help to ellucidate
how the
material needed to fuel the active nucleus is traveling from the
orbit just inside the Inner Linblad Resonance (ILR) to the smaller
50pc scale probed by our images, and further
down to where viscous forces may take over to
carry material to
the accretion disk of the central black hole.
Additional support for this scenerio is obtained from OASIS Integral Field
Spectroscopy with 0.8'' resolution.
The OASIS instrument allows spatially resolved spectroscopy to be obtained
at the resolution provided by the adaptive optics bonnette.
Spectral imagery was obtained in the 
and [SIII] lines
under fairly poor seeing conditions and only part of the data
obtained is usable since
guiding was lost intermitantly in some of the exposures. The
best corrected FWHM obtained is only 0.8'' using the
0.3''/pixel sampling.
The [SIII] lines are excited under similar conditions to the [OIII] line and
is typically 1/3 the peak intensity. The advantage for adaptive optics is
that the PSF correction is much better at 9000Angstroms (SIII) than 5000 (OIII).
The CFHT K- and HST V-band images are presented in Figure 1(a,b) on a magnitude (log)
scale, chosen because AO provides a high dynamic range (typically 1.3
104 at
K) and significant details are seen at all flux levels.
A diffuse, ellongated structure containing wispy spiral bands is seen
surrounding the nucleus in all wavelengths.
Subtraction of a smooth model reveals that this region is punctuated with bright
knotty structures tracing out a mini-spiral pattern within a region 3''x2''
(Figure 2(c,d)).
We explored several methods of removing the low frequency galactic component,
including various smoothing filters, a one-dimentional elliptical
isophote model, and a multi-component (bulge+disk+point source) elliptical
isophote model.
All methods consistently unveil the knotty spiral structure.
However, the core region of the galaxy has large departures from ellipticity and
subtracting
isophotal fitting models results in prominent artifacts which obscure structural
details over much of the region of interest.
 |
We form color maps by convolving the images to the worst resolution of a given
pair and taking the flux ratio (Figure 2(a,b)).
Any color
gradients in these images can result from several different
processes: 1) change in dust 2) change in stellar population 3) change in gas.
The most prominent feature is an irregular-shaped patch to the southwest. The
fact that this region appears clearly as a deficit in the V-band image, and takes on a
patchy morphology is strong evidence for dust obscuration as the source of the
color gradient. The region is therefore most pronounced the V-K color map
(figure 2a), since the
K image is least affected by dust. The J-K image
indicates that substantial dust still affects this region in the J-band.
The color maps reveal that the nucleus is very red, possibly as a result of
thermal dust emission
in the K band.
The red colors of the spiral starburst knots stand out from a region
slightly bluer than the larger scale bulge of the galaxy.
 |
The 1D profiles of the galaxies are similar at V, J, H and K
(figure 2c shows K profile), displaying a bump in ellipticity
between 1 and 2 arcseconds radius, while the PA twists by 
degrees.
This is coincident in radius and position angle (PA) with the
elongated region defined by the nuclear spiral.
There is also a sharp rise and fall in ellipticity within the
central 0.5 arcsecond radius, but here the PA twists significantly (
).
The feature is most clearly visible
in the J-K and V-K color maps, as it seems to be bluer than the
rest of the galaxy at K. The HST V-band
profile is distorted at the smallest scales as the image
is saturated within 0.2" radius and dominated by diffraction spikes, thus
identification of the feature is more difficult at this wavelength.
This feature may represent the warping of a gaseous disk, with the presence of a
starbursting component as it is coincident with the innermost ring of
knot structures seen in the model subtracted images. Such a warped
nuclear disk is thought to be rather common in active galaxies and
related to their central activity (Schreier et al. 1998). The twisted
isophotes make the interpretation of the elongated feature
as a nuclear bar potential somewhat dubious, but the profiles are
difficult to analyze with certainty as the feature is near the level
of our image resolution. Higher resolution images might reveal the
twist as even smaller-scale nested bars.
The images are also compared to the 6cm and 18cm MERLIN radio continuum emission, both of which align with the axis of the nuclear spiral as seen in Figure 2d. Previous explanations for the radio structure (Mundell et al 1995) invoked the standard unified AGN model to explain this emission as collimated outflow. However, there is an offset in orientation of the [OIII] ``cone" and the small-scale radio features. A projection effect would be possible, but this would necessitate that the NE side of the disc is closer to us than the SW side. This could only occur if the spiral arms were leading rather than trailing (Mundell et al 1995). Discussion - Fueling the Monster
Several possible scenerios emerge from these results. On the largest scales Gonzalez Delgado et al. (1997) noted that a large-scale bar appears to transport material towards an inner radius which corresponds to the calculated inner Linblad resonance (ILR) at roughly 7''. At this point, prominent dust and HII regions indicate substantial star formation. It is plausible that the apparent elongated region defined around the mini-spiral, and the small scale twisted feature, are in fact related to nested bar potentials, with the knotty structures representing starbursts formed by the shocked gas in the leading face of the bar. This would provide a straightforward explanation for transporting material down to the scales where viscous forces can take over (Sholsman et al. 1989), effectively fueling the active nucleus.
In the absense of any true bar potential, the fate of knots of star formation generated near the ILR will then be to drift inwards with time, being carried apart by differential rotation. This process could possibly mimic the loose spiral seen in the core region. In our own galaxy at roughly comparable scales, similar processes are thought to be at work (Morris et al. 1996). However, the Milky Way does not appear to have an active nucleus, and this in itself could not be an explanation for why some galaxy cores are fueled and some not.
Regan and Mulchaey (1998) claim to have found spiral dust lanes that appear to provide fuel for black holes at the centers of active galaxies, suggesting that that material does indeed spiral in towards the center, rather than being forced down in the presence of a strong bar potential. Our images would be consistent with this picture as the core elongated starburst region is seen as intersperced wispy spirals in the color maps of Figure 2. However, there is no theoretical or numerical model supporting such a scenerio, and it is not yet clear that such a mechanism could fuel the active nuclei.
Initial results from our OASIS data seem to be consistent with the
unified model for NGC3227 proposed in (Delgado et al 1997, Arribas et al 1994),
providing a higher resolution perspective on the HII region to the
southwest, as well as the extended [OIII] and H
to the northeast.
The dusty region in the V-K color map falls over the blank region
which separates the HII region from the
core (Figure 3a), likely explaining the disconnected morphology.
In the unified AGN picture, biconical emission line regions are thought to be the
result of collimated continuum emission via a nuclear dust torus.
Although with NGC3227 one would expect a counterpart to the
extended [OIII] emission ``cone'' on the opposite side of the nucleus (SW),
the dusty region would
obscure such emission in addition to any H that extended from the nucleus
to the separated HII region.
There is however some evidence for smaller-scale conical morphology
in our [NII]/H ratio map (Figure 3b), with [NII] being
stronger along the axis defined by the [OIII] emission.
However, if the extended [OIII] region to the northeast defines the collimation axis of a small-scale obscuring torus, then it is very difficult to interpret the radio feature as an outflow/jet since the two do not align. The major axis of the mini-spiral is well aligned with the radio feature, with the bright radio points apparently lying on the nucleus and one of the emission knots which is brighter at V-band than K. Our high resolution imaging along with OASIS IFU spectroscopy therefore suggest that the radio emission is unrelated to any kind of jet outflow, and instead is associated with the nuclear starburst. Also, Kotilainen et al. (1997) found a blue excess in their optical color maps lying in lobes on the NW-SE axis, roughly aligned with our spiral elongated feature, and consistent with our explanation.
On the other hand, if our observed small-scale elongation is some sort of twisted disk, its plane lies roughly perpendicular to the axis defined by the radio ``jet" observed at 6 and 18cm. For the radio emission to be interpreted as an outflow, the collimated [OIII] ionization picture would have to be abandoned. Given that the evidence for the single [OIII] extension taking on the shape of a cone is minimal, even with our improved resolution OASIS data, this scenerio is perhaps equally as plausible.
