NGC 765 - A disturbed H I giant

We present H i spectral line and radio–continuum VLA data of the galaxy NGC765, complemented by optical and Chandra X-ray maps. NGC765 has the largest H i-tooptical 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× 10 atoms cm −2. We derive a total H i mass of MH I = 4.7×10 M , a dynamical mass of Mdyn ∼ 5.1 × 10 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 (L1.4GHz = 1.3 × 10 W Hz−1) and X–ray (LX ≈ 1.7 × 10 erg s−1) emission is found to coincide with the optical core of the galaxy, compatible with nuclear activity powered by a low-luminosity AGN. 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 (10–10 M ) of small (dIrr) galaxies in the outskirts of its H i disc and by the presence of two similarly sized companions.