We present H i spectral line and radio–continuum 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 \times 10 ^ { 19 } atoms cm ^ { -2 } .
We derive a total H i mass of M _ { H I } = 4.7 \times 10 ^ { 10 } { M } _ { \odot } , a dynamical mass of M _ { dyn } \sim 5.1 \times 10 ^ { 11 } { M } _ { \odot } and an H i mass to luminosity ratio of M _ { H I } /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 _ { 1.4 GHz } = 1.3 \times 10 ^ { 21 } W Hz ^ { -1 } ) and X–ray ( L _ { X } \approx 1.7 \times 10 ^ { 40 } 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 ^ { 8 } –10 ^ { 9 } { M } _ { \odot } ) of small ( dIrr ) galaxies in the outskirts of its H i disc and by the presence of two similarly sized companions .