NGC 6340: an old S0 galaxy with a young polar disc^*

Clues from morphology, internal kinematics, and stellar populations

¹ Observatoire de Paris-Meudon, LERMA, UMR 8112, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: igor.chilingarian@obspm.fr
² Sternberg Astronomical Institute, Moscow State University, 13 Universitetski prospect, 119992 Moscow, Russia
³ Observatoire de Paris-Meudon, LUTH, UMR 8102, 5 pl. Jules Janssen, 92195 Meudon, France
⁴ Observatoire de Paris-Meudon, GEPI, UMR 8111, 5 pl. Jules Janssen, 92195 Meudon, France

Received: 20 January 2009
Accepted: 30 April 2009

Abstract

Context. Lenticular galaxies are believed to form by a combination of environmental effects and secular evolution.

Aims. We study the nearby disc-dominated S0 galaxy NGC 6340 photometrically and spectroscopically to understand the mechanisms of S0 formation and evolution in groups.

Methods. We use SDSS images to build colour maps and the light profile of NGC 6340, which we decompose using a three-component model including Sérsic and two exponential profiles. We also use Spitzer Space Telescope archival near-infrared images to study the morphology of regions containing warm interstellar medium and dust. Then, we re-process and re-analyse deep long-slit spectroscopic data for NGC 6340, applying a novel sky subtraction technique and recover its stellar and gas kinematics, distribution of age and metallicity with the NBursts full spectral fitting.

Results. 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 kinematic and stellar population properties. We see a kinematic misalignment between inner and outer regions of the galaxy. We confirm the old metal-rich centre and a wrapped inner gaseous polar disc ( 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.

Conclusions. In accordance with the results of numerical simulations, we conclude that the properties of NGC 6340 can be explained as the result of a major merger of an early-type galaxy and a spiral galaxy that 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.