NGC765 - A disturbed HI giant

A. M. P. Portas, E. Brinks, M. E. Filho, A. Usero, E. M. Dyke, P.-E. Belles

Abstract
We present H_I 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 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 M_HI = 4.7 × 10¹⁰ M_☉, a dynamical mass of M_dyn ∼ 5.1 × 10¹¹ M_☉ and an H_I 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_{1.4 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 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 (10⁸-10⁹ M_☉) of small (dIrr) galaxies in the outskirts of its H_I disc and by the presence of two similarly sized companions.

Keywords
galaxies: evolution - galaxies: individual: NGC 765 - galaxies: individual: UGC1453 - galaxies: interactions - galaxies: kinematics and dynamics - galaxies: structure

Monthly Notices of the Royal Astronomical Society
Volume 407, Page 1674
September 2010

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