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

Abstract

<i>Context. <i/>Lenticular galaxies are believed to form by a combination of environmental effects and secular evolution.<i>Aims. <i/>We study the nearby disc-dominated S0 galaxy NGC 6340 photometrically and spectroscopically to understand the mechanisms of S0 formation and evolution in groups.<i>Methods. <i/>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.<i>Results. <i/>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.<i>Conclusions. <i/>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.