NGC 765 – a disturbed H i giant

• Published in: Monthly notices of the Royal Astronomical Society Vol. 407; no. 3; pp. 1674 - 1688
• Main Authors: Portas, A. M., Brinks, E., Filho, M. E., Usero, A., Dyke, E. M., Belles, P.-E.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 21-09-2010; Wiley-Blackwell; Oxford University Press
• Subjects: Arrays; Astronomy; Density; Discs; Disks; Earth, ocean, space; Exact sciences and technology; Galaxies; galaxies: evolution; galaxies: individual: NGC 765; galaxies: individual: UGC 1453; galaxies: interactions; galaxies: kinematics and dynamics; galaxies: structure; Radio astronomy; Spectrum analysis; Spirals; Stars & galaxies; X-rays; galaxies: structure; galaxies: individual: NGC 765; galaxies: kinematics and dynamics; galaxies: evolution; galaxies: interactions; galaxies: individual: UGC 1453; Galaxy evolution; Accretion; Mass to light ratio; Spiral arm; Luminosity; Continuum; Active galaxies; Galaxy structure; Dynamics; Morphology; Active galaxy nuclei; Surface brightness; Kinematics; Galaxy nuclei; H lines; Spiral galaxies; Diameter
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2010.16994.x

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.