Journal Article

NGC 765 – a disturbed H <span class="smallCaps">i</span> giant

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

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 407, issue 3, pages 1674-1688
Published in print September 2010 | ISSN: 0035-8711
Published online September 2010 | e-ISSN: 1365-2966 | DOI:
NGC 765 – a disturbed H i giant

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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.

Keywords: galaxies: evolution; galaxies: individual: NGC 765; galaxies: individual: UGC 1453; galaxies: interactions; galaxies: kinematics and dynamics; galaxies: structure

Journal Article.  8969 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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