H I 21cm observations and dynamical modelling of the thinnest galaxy: FGC 2366

Abstract

Superthin galaxies are bulgeless low-surface brightness galaxies with unusually high major-to-minor axes ratio of the stellar disc, i.e. 10 < a/b < 20. We present Giant Metrewave Radio Telescope (GMRT) H I 21cm radio-synthesis observations of FGC 2366, the thinnest galaxy known with a/b = 21.6. Employing the 3D tilted-ring modelling using fully automated TiRiFiC (FAT), we determine the structure and kinematics of the H I gas disc, obtaining an asymptotic rotational velocity equal to 100 km s−1 and a total H I mass equal to 109M. Using z-band stellar photometry, we obtain a central surface brightness of 22.8 mag arcsec−2, a disc scale length of 2.6 kpc, and a scale height of 260 pc. Next, we determine the dark matter density profile by constructing a mass model and find that an Navarro–Frenk–White (NFW) dark matter halo best-fits the steeply rising rotation curve. With the above mass inventory in place, we finally construct the dynamical model of the stellar disc of FGC 2366 using the stellar dynamical code ‘AGAMA’. To identify the key physical mechanisms responsible for the superthin vertical structure, we carry out a Principal Component Analysis of the data corresponding to all the relevant dynamical parameters and a/b for a sample of superthin and extremely thin galaxies studied so far. We note that the first two principal components explain 80 per cent of the variation in the data, and the significant contribution is from the compactness of the mass distribution, which is fundamentally responsible for the existence of superthin stellar discs.