Dynamically decoupled circumnuclear regions in barred galaxies:
the panoramic spectroscopy results
Moiseev A.V. (Special Astropysical Observatory, Russia)
Observations
A sample of barred galaxies was observed at the Russian
6m telescope with two different panoramic spectrographs:
MultiPupil Fiber Spectrograph (MPFS)
Field of view: 16x15''
Wavelength : 4700-6200 ÅÅ, 5800-7100 ÅÅ
Results:
-velocity fields and velocity dispersion maps of stars
-velocity fields of the ionized gas in the Hb, [OIII] and
[NII] emission lines
-continuum and emission-line images
Lenses/fibers unit of MPFS
Scanning interferometer Fabry-Perot (IFP)
in the focal reducer SCORPIO:
Field of view : 5x5 arcmin
Wavelength : around Ha and [NII]
Results:
- large-scale velocity fields of the ionized gas in the Ha
and/or [NII] lines
- continuum and emission-line images
Russian 6m telescope BTA
Multimode focal reducer
SCORPIO
Examples of MPFS maps: images in
continuum and line-of-sight velocity
fields of stars in observed galaxies
See this URL for description of our devices:
http://www.sao.ru/hq/lsfvo/devices.html
Circumnuclear morphology
Our sample contains 13 galaxies, which are candidates to double-barred
galaxy. We tried to collect the maximum information about observed
2D distributions of the surface brightness and kinematic properties.
For this goal, the rotation curves and radial variations of the dynamical axis
position angle (PAdyn) were calculated from the velocity fields. The analysis
of the morphology of the galaxies was done by means of isophote analysis
of ground-based and HST images.
We suggest that candidate double-barred galaxies are, in fact, galaxies
with very different circumnuclear structure. It is necessary to note that
the majority of the observed morphological and kinematic features in our
sample galaxies may be explained without the secondary bar hypothesis.
Three cases of inner polar disks, one counter-rotating gaseous disk
and six nuclear disks (with and without mini-spirals) nested in large-scale
bars were found in this work.
The analysis of morphology observed galaxies is
presented in the article by Moiseev et al. (2004).
Embedded disks
Inner disks ``images'' in NGC 470 (6m telescop)
and NGC 5566 (HST).
Left: residual brightness after subtraction of
2D model ( bulge+bar+global disks). The solid
line corresponds to the galactic major axis.
In a significant fraction of the galaxies inner disks nested in the
central regions of the large-scale bars are found from kinematical
and morphological analysis. Here we summarize the signatures
indicating these inner-disk structures:
Right:: deprojection of the residual brightness
to the galactic plane. The gray scale is in
magnitudes. The circles mark the radius of the
inner disks denoted in the Table.
- Photometry: an orientation (PAdyn) of the inner isophotes agrees
with the PA of the line of nodes of an outer disk.
- Kinematics: the central PA of the stellar velocity field lies near
the line of nodes PA, that corresponds to a regular circular rotation.
- Morphology: there are inner mini-spirals at distances corresponding
to the nuclear disk (r<1 kpc) in NGC 2273, NGC 2681, NGC 5566,
and NGC 6951. The mini-spirals also appear in the images of
NGC 4736 and NGC 7743, though we cannot determine inner
disks in these galaxies.
The characteristic radial dependences of PAs of the dynamical axis
(from velocity fields): cases of a pure bar and a circumnuclear disk:
- Resonances: In the case of NGC 2681 and NGC 6951 we have
shown that an inner disk region is kinematically decoupled
because it lies inside the ILR of a global bar.
PAdyn in stars and gas differ from the line of nodes. The difference between
Hb and [OIII] measurements is caused by the shock fronts at the bar edges
PAdyn coincides with line of nodes
NGC 3945
Gas/stars counter-rotation in NGC 3945
Velocity fields of stars and gas
(MPFS data)
The velocity field of the stars in the circumnuclear disk shows a
regular circular rotation. Inside the r<6'' (0.5 kpc), the line-of-sight
velocities of the gas are inverted with respect to the stellar velocities.
Thus, the ionized gas in the central kiloparsec of NGC 3945 rotates
in the opposite direction with respect to the stellar disk. The
direction of gas rotation becomes conterminous with the stellar rotation at
larger distances from the nucleus. The large-scale IFP velocity field
confirms the fact of normal gas rotation at large radii, up to 11kpc.
According to Kuijken et al.(1996), the gaseous disks in lenticular galaxies
demonstrate a counter-rotation phenomenon in 24% of all cases. This is
probably attributable to a merger of an accreted gaseous cloud with the
corresponding direction of angular momentum.
V-band image (SCORPIO)
Polar (orthogonal) inner disks
DSS2 image of NGC 5850
In the circumnuclear region of NGC5850 the stellar rotation coincides with
orientation of the global disk, while direction of PAdyn in the ionized gas
o
velocity field differs by more than 50-60 from the line of nodes PA. The
radial behavior of PAdyn is typical for a disk inclined to the galactic plane.
A more reasonable assumption is that the gas, at r < 7'', rotates in a
polar plane with respect to the global galactic disk. In this case, the
polar gaseous disk lies orthogonal to the major axis of the bar.
WFPC2/HST image of NGC 3368; two ellipses
delineate the orientation of the mini-bar and
the circumnuclear polar dust ring. For the area
outlined by a square the NIR (F110W-F160W)
color map is shown in the upper right corner.
(See Sil’chenko et al., 2003)
A similar disk in NGC 3368 is detected
on the basis of the HST morphology:
the dust lane corresponds to the edge
of the disk. In Sil'chenko et al. (2003)
we argue that a gaseous-dust mini-disk
is located in the polar plane of the bar.
Velocity fields of stars
and gas (MPFS data)
The origin of the inner polar disk:
-External: the disks could be formed by an accretion of external matter with specific orientation of the
angular moment, as in ``classical'' large-scale polar rings.
-Internal: the gas in the centers of these galaxies has been moved into polar orbits due to the dynamical effect of the bar, because the polar
nuclear disks in NGC 3368 and NGC 5850 are orthogonal to the major axes of their bars.
- It is possible that both mechanisms (internal as well as external ones) are at work.
For a detailed discussion on polar mini-disks see Sil'chenko & Afanasiev (2004) and Corsini et al. (2003).
Mini-spirals in the circumnuclear disks
NGC 6951 is a spectacular example of a mini-disk nested in the large-scale bar.
The HST image reveal a multi-armed flocculent spiral. This mini-spiral is associated
only with the distribution of gas and dust rather than with the stellar component. As
has been shown by Pérez et al. (2000), the nuclear spiral structure, clearly seen in the V
band, completely disappears in the H band, where the effect of dust absorption is much
weaker.
Based on their IFP observations, Rozas et al. (2002) tried to interpret the gas non-circular
motions in terms of an iner inclined disk or a dissipation of the secondary bar. However, we
contradict their opinion and suggest that non-circular motions are caused by the inner
spiral. Our arguments are following:
- Deviations of the observed line-of-sight velocities from model of pure circular rotation
reveal regular (``3-armed'') pattern in the circumnuclear disk.
Left - color image of NGC 6951 combined from SCORPIO
observations in three optical filters ; right - view on the
central region from HST: top - color image was constructed
from PC2+NICMOS frames; bottom - PC2 image after ellitical
isophotes removal
-Fourier analysis of the azimuthal velocity
distributions shows the prevalence of
the contribution of odd (1,3) harmonics
into dispersion in the model of circular
rotation. At the same time, in optical
brightness we see invert picture: the
domination of even (2,4) harmonics
into the total Fourier spectrum. The same
situation is also observed in spiral galaxies,
where 2th spiral harmonic in brightness
leads the 1th and 3th harmonics in line-ofsight velocities (see Fridman et al., 2001).
IFP observations of the circumnuclear disk in NGC 6951:
monochromatic Ha image, ionized gas velocity field; residual
velocities after removal of pure circular model.
References:
Corsini, E.M., et al., 2003, A&A,408, 873
Fridman A.M., et al., 2001, A&A , 371, 538
Kuijken, K., Fisher, D., Merrifield, M.R., 1996, MNRAS, 283, 543
Moiseev, A.V., Valdés, J.R., Chavushyan, V.H., 2004, A&A, 421, 433
NGC 6951
NGC 157
(Fridman et al., 2001)
The averaged contribution of the individual Fourier harmonics into
dispersion in the model of pure rotation (top) and into the deviation
of the brightness distribution from the azimuthal symmetry (bottom).
Left - our data for NGC 6951, right - grand-design galaxy NGC 157
Pérez, E., Márquez, I., et al., 2000, A&A, 353, 893
Rozas, M., Relano, M., Zurita, A., Beckman, J.E., 2002, A&A, 386, 42
Sil’chenko, O.K., Moiseev, A.V., et al., 2003, ApJ, 591, 185
Sil’chenko, O.K. & Afanasiev, V.L., 2004, AJ, 127, 2641