Deep Hα observations of NGC 253: a very extended and possibly declining rotation curve?

This study presents a deep H kinematical analysis of the Sculptor Group galaxy NGC 253. The Fabry-Perot data were taken with the 36-cm Marseille Telescope in La Silla, Chile, using an EMCCD detector. Typical emission measures of 0.1 cm -6 pc are reached. The observations allow the detection of the diffuse ionized gas component through [N II] emission at very large radii of 11.5, 12.8 and 19.0 arcmin, on the receding side of the galaxy. No H emission is observed at radii larger than the neutral component (11.5 arcmin). The very extended rotation curve confirms previous results and shows signs of a significant decline, on the order of 30 per cent v max . Using the rotation data, mass models are constructed with and without the outer [N II] data points, and similar results are found. The declining part of the rotation curve is very well modelled, and seems to be truly declining.