NGC765-a disturbed HI giant

Abstract

We present H I spectral line and radio-continuum Very Large Array (VLA) data of the galaxy NGC 765, complemented by optical and Chandra X-ray maps. NGC 765 has the largest H I-to-optical ratio known to date of any spiral galaxy and one of the largest known H I discs in absolute size with a diameter of 240 kpc measured at a surface density of 2 × 1019* atom cm−2. We derive a total H i mass of , a dynamical mass of Mdyn∼ 5.1 × 1011 M⊙ and an H i mass-to-luminosity ratio of MH I/LB= 1.6, making it the nearest and largest ‘crouching giant’. Optical images reveal evidence of a central bar with tightly wound low surface brightness spiral arms extending from it. Radio-continuum (L1.4 GHz= 1.3 × 1021 W Hz−1) and X-ray (LX≈ 1.7 × 1040 erg s−1) emission is found to coincide with the optical core of the galaxy, compatible with nuclear activity powered by a low-luminosity active galactic nucleus. We may be dealing with a galaxy that has retained in its current morphology traces of its formation history. In fact, it may still be undergoing some accretion, as evidenced by the presence of H i clumps the size (<10 kpc) and mass (108–109 M⊙) of small (dIrr) galaxies in the outskirts of its H i disc and by the presence of two similarly sized companions. *[see original online abstract for correct notation]