NGC 765 - A disturbed H I giant

TL;DR

This paper presents detailed multi-wavelength observations of NGC 765, revealing it as a giant, disturbed spiral galaxy with the largest known Hi-to-optical ratio, complex morphology, and signs of ongoing accretion and nuclear activity.

Contribution

It provides the first comprehensive multi-wavelength analysis of NGC 765, highlighting its unique size, mass, and disturbed features, and discusses its possible formation and accretion history.

Findings

NGC 765 has the largest Hi-to-optical ratio among spiral galaxies.

It hosts an extensive Hi disc with a diameter of 240 kpc.

Evidence of ongoing accretion and nuclear activity is observed.

Abstract

We present Hi 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 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 2e19 atoms/cm^2. We derive a total Hi mass of M_HI = 4.7e10 Msun, a dynamical mass of Mdyn - 5.1e11 Msun and an Hi 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 = 1.3e21 W/Hz) and X-ray (L_X ~ 1.7e40 erg/s) 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 (10e8-10e9 Msun) of small (dIrr) galaxies in the outskirts of its Hi disc and by the presence of two similarly sized companions.