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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 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 2 × 1019 atom cm -2. We derive a total HI mass of MHI = 4.7 × 1010 M, a dynamical mass of Mdyn ∼ 5.1 × 1011 M and an HI mass-to-luminosity ratio of MHI/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 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 HI clumps the size (< 10 kpc) and mass (108-109 M) of small (dIrr) galaxies in the outskirts of its HI 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, Issue 3, Page 1674
2010 September

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia