2010MNRAS.407.1674P

We present Hi spectral line and radio-continuum Very Large Array (VLA) data of the galaxy NGC765, complemented by optical and Chandra X-ray maps. NGC765 has the largest Hi-to-optical ratio known to date of any spiral galaxy and one of the largest known Hi discs in absolute size with a diameter of 240 kpc measured at a surface density of 2x10¹⁹ atom/cm2. We derive a total Hi mass of, a dynamical mass of M_dyn∼ 5.1x10¹¹M_☉ and an Hi mass-to-luminosity ratio of M_HI/L_B= 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 (L_1.4GHz= 1.3x10²¹ W/Hz) and X-ray (L_X~ 1.7x10⁴⁰ erg/s) 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 Hi clumps the size (<10 kpc) and mass (10⁸-10⁹M_☉) of small (dIrr) galaxies in the outskirts of its Hi disc and by the presence of two similarly sized companions.