Galaxies with Abnormally High Gas Content in the Disk

The content of gas in galaxies with an anomalously high relative mass of hydrogen $M_{HI}/M_*$ for a given mass of the stellar population $M_*$ (VHR-galaxies) is considered, using the available samples of such galaxies. It is shown that, within the optical diameter $D_{25}$, the mass of HI in VHR galaxies, as well as in galaxies with "normal" HI content, is limited by a value that depends on the specific angular momentum of the disk. Outer gaseous disks beyond $D_{25}$, which contain the main amount of HI in most of the galaxies we consider, are gravitationally stable, and, as a rule, they retain an approximately constant value of the stability parameter $Q_{gas}$ over a large range of radial distances. It allows to propose that the outer disks of VHR galaxies are not recently acquired, but are of great age, and their gravitational instability was the main regulator of star formation during their formation. In this case, the extended disks of galaxies should also include a low-brightness stellar components of old stars extending far beyond their optical diameter $D_{25}$.Comment: 11 pages, 5 figures, 1 tabl

NGC 6340: an old S0 galaxy with a young polar disc. Clues from morphology, internal kinematics and stellar populations

Lenticular galaxies are believed to form by a combination of environmental effects and secular evolution. We study the nearby disc-dominated S0 galaxy NGC 6340 photometrically and spectroscopically to understand the mechanisms of S0 formation and evolution in groups. We use SDSS images to build colour maps and light profile of NGC 6340 which we decompose using a three-component model including Sersic and two exponential profiles. We also use Spitzer images to study the morphology of regions containing warm ISM and dust. Then, we re-process and re-analyse deep long-slit spectroscopic data for NGC 6340 and recover its stellar and gas kinematics, distribution of age and metallicity with the NBursts full spectral fitting. We obtain the profiles of internal kinematics, age, and metallicity out to >2 half-light radii. The three structural components of NGC 6340 are found to have distinct kinematical and stellar population properties. We see a kinematical misalignment between inner and outer regions of the galaxy. We confirm the old metal-rich centre and a wrapped inner gaseous polar disc (r~1 kpc) having weak ongoing star formation, counter-rotating in projection with respect to the stars. The central compact pseudo-bulge of NGC 6340 looks very similar to compact elliptical galaxies. In accordance with the results of numerical simulations, we conclude that properties of NGC 6340 can be explained as the result of a major merger of early-type and spiral galaxies which occurred about 12 Gyr ago. The intermediate exponential structure might be a triaxial pseudo-bulge formed by a past bar structure. The inner compact bulge could be the result of a nuclear starburst triggered by the merger. The inner polar disc appeared recently, 1/3-1/2 Gyr ago as a result of another minor merger or cold gas accretion.Comment: 12 pages, 8 figures, accepted to A&

A Ring-like Zone of Strong Radial Gas Motions in the Disk of NGC 6181

The Sc galaxy NGC 6181 was observed at the 6m telescope of SAO RAS with the scanning Perot-Fabry interferometer in the H-alpha emission line and at the 1m telescope of SAO RAS in BVRI broadband filters with CCD. Subtraction of the mean circular rotation curve from the two-dimensional velocity field has revealed a ring-like zone with a diameter about of 2 kpc where strong radial gas motions are present. The form of the ring is almost perfectly circular in the plane of the galaxy. It is located closer to the center than the beginning of the well-defined spiral structure, but outside of the central bulge-dominated region. The detected radial velocity reduced to the plane of the galaxy is about 100 km/sec and probably is azimuthally dependent. The very inner region of the galaxy, r < 3 arcsec or 0.5 kpc, shows a turn of the dynamical major axis by about 30 deg. Central continuum isophotes are also twisted which suggests the presence of small nuclear bar.Comment: Latex, 9 pages, 6 Postscript figures, 6 GIF figures (+ Fig2. which must be requested by e-mail from [email protected]). Accepted for publication in the Astronomy and Astrophysics Supplement Serie