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According to the most popular cosmological paradigm, the greater part
of the galaxy mass is accumulated in the dark matter halo. The mass
value can be measured in terms of various model assumptions, analyzing
the available observational data, for example, the distribution of the
rotation velocity at large distances from the center (the rotation curve).
It is much more difficult to make an estimate of the shape of the dark
halo in a particular galaxy, i.e. to understand whether it is spherical,
flattened or triaxial. We possess more or less confident evidence that
the shape of the halo is different from being spherical only for our
own galaxy. In most of other cases, we must be content with indirect
estimates and statistical laws. At the same time, the theoretical
simulations show that the halo shape can give information on the
history of galaxy formation, and serve as a test for modern cosmological
models.
The polar ring galaxies (PRGs) reveal external rings or disks of gas,
dust and stars rotating in a plane roughly perpendicular to the disk of
the central galaxy. It is believed that PRGs in most cases are formed by
either the merger of galaxies with the specific spin orientation or the
accretion by the host galaxy of the companion matter or gas clouds from
the intergalactic medium. Owing to the unique structure of these galaxies,
we can try to understand the shape of the gravitational potential,
comparing the rotation curves simultaneously measured in two planes - in
the central disk and the polar ring. Researchers repeatedly suggested
using this opportunity. Moreover, a statistical comparison of the maximum
rotation velocities of polar rings and luminosities of the central galaxies
points to a significant flattening of dark haloes. However, accurate
estimates of this parameter in individual objects were obtained with
great uncertainty and often proved to be contradictory.
This is first of all due to the fact that for such measurements one must
not only be familiar with the geometry of the system (precise inclinations
of the disk and rings to the line of sight and to each other), but also
to have reliable data on the stellar population motions in the outer
regions of galaxies possessing a relatively low brightness.
The stellar kinematics in two PRGs from
the new SPRC
catalog (Moiseev et al. 2011) was studied at the 6-m telescope
of the SAO RAS. Radial velocity distributions and velocity dispersions
of stars in SPRC- 7 and SPRC- 33 (NGC 4262) were obtained by using the
multi-mode
SCORPIO and
SCORPIO-2
spectrographs.
Rotation of gas in the ring of SPRC-7 has also been studied at the
6-m telescope with the SCORPIO instrument (the velocity field obtained
by A.V. Moiseev, published in the paper by
Brosch et
al. 2010), or NGC 4262 the previously published data by
Oosterloo
et al. (2010), were used, who studied the neutral hydrogen rotation
at the
WSRT
radio telescope. Apart from the data on the motion of gas and
stars, the detailed model of galaxies has also included the information
about the distribution of the gas density and brightness of the stellar
component according to the SDSS survey
data. The shape of the dark halo in the model was chosen so as to
achieve the best fit to the observed rotation of both galaxies themselves
and their polar rings. SPRC-7 is one of the largest (the ring diameter of
about 50 kpc) and most distant among the confirmed PRGs, a massive ring
here consists of gas and stars and rotates not perpendicularly to the
plane of the galactic disk, but rather at an angle of 73 degrees to
it (Fig.1).
NGC 4262 belongs to the nearby Virgo cluster of galaxies, the ring of
about 30 kpc in diameter contains almost no stars, it is mainly composed
of gas and is oriented almost exactly orthogonally to the central galaxy
(Fig.2). The studied galaxies notably differ from each other. Not
surprisingly, the dark haloes in them are dramatically different too.
In SPRC- 7 the halo is noticeably flattened to the plane of the ring
(the axial ratio of 1.5-1.7 depending on the adopted model of the
density distribution in the halo). In NGC 4262 the situation is more
complicated, since the observed pattern can only be explained if the
axial ratio in the distribution of potential of the dark halo greatly
varies with distance from the center, amounting to about 0.4 in the inner
regions and 1.5-2.3 in the outer regions. This is the first galaxy (except
for Milky Way), where the variation of the shape of the dark halo with
radius is reliably defined, as it was predicted by some theoretical
models of galaxy formation.
Published:
Khoperskov S.A., Moiseev A.V., Khoperskov A.V., Saburova A.S.
2014, MNRAS,
accepted;
arXiv:1404.1247 [astro-ph.GA]
Contacts - Alexei Moiseev
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Fig.1.
The SPRC-7 galaxy. Left: a combination of images in the g, r, i filters
from the SDSS. The polar ring, surrounding the central lenticular galaxy
is distinguished by its blue color due to the significant contribution of
young stars. Middle: the SDSS image is combined with the velocity field
of ionized hydrogen from the observational data of the 6-m BTA telescope:
the colors correspond to the observed radial velocities (the blue dots are
approaching us, the red ones are moving away from the observer), the
intensity is scaled in accordance with the brightness distribution in
the H-beta emission line. Right: the distribution of the gravitational
potential of the dark halo according to the numerical calculations.
The central disk and the polar ring are shown schematically.
Fig.2.
The same as in Fig.1 for the case of the NGC 4262 galaxy. Here, the
polar ring is almost invisible in the optical images, as it mainly
consists of gas. We demonstrate the velocity field of neutral hydrogen
from the observations of Oosterloo et al. (2010) in the 21-cm line.
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