Leo II Group: decoupled cores of NGC 3607 and NGC3608

Abstract

The kinematics, structure, and stellar population properties in the centres of two brightest early-type galaxies of the Leo II group, NGC 3607 and NGC 3608, are studied by means of integral-field spectroscopy. The kinematically distinct areas in the centres of these galaxies, with radii of 6^′′ and 5^′′, respectively, are found also to be chemically distinct: they are characterized by enhanced magnesium-line strength. However, no stellar age differences have been found between the decoupled cores and their outskirts. An analysis of two-dimensional line-of-sight velocity fields reveals systematic turns of kinematical major axes near the nuclei of both galaxies; in NGC 3608 the ionized gas rotates perpendicular to the stellar component rotation. When taking into account some morphological features, it is concluded that both NGC 3607 and NGC 3608 have large triaxial stellar spheroids. it is argued that the magnesium-enhanced cores are not circumnuclear disks; instead they resemble rather compact triaxial structures that force the formation of polar disks around them – a gaseous one in NGC 3608 and a stellar-gaseous one in NGC 3607; in the latter, star formation is perhaps still proceeding.

Keywords:

• Galaxies
• individual
• NGC 3607 – Galaxies
• individual
• NGC 3608 – Galaxies
• nuclei – Galaxies
• stellar content – Galaxies
• kinematics & dynamics – Galaxies
• evolution

Acknowledgements

We thank Dr A. V. Moiseev of SAO RAS for supporting the observations at the 6 m telescope. The 6 m telescope is operated under the financial support of Science Ministry of Russia (registration number 01-43). During the data analysis we have used the Lyon-Meudon Extragalactic Database (LEDA) supplied by the LEDA team at the CRAL-Observatoire de Lyon (France) and the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The research is partly based on the data taken from the ING Archive of the UK Astronomy Data Centre and on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The study of the central evolution of the galaxies in groups was supported by a grant from the Russian Foundation for Basic Researches 04-02-16087 and the study of the evolution of galactic centres – by the Federal Scientific-Technical Program – contract of the Science Ministry of Russia no.40.022.1.1.1101.