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Measurement of the dark halo shape in polar ring galaxies

Russian version

    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.
Khoperskov S.A., Moiseev A.V., Khoperskov A.V., Saburova A.S.
2014, MNRAS, accepted; arXiv:1404.1247 [astro-ph.GA]

Contacts - Alexei Moiseev

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.