2011MNRAS.413.2621F

We present a photometric study of UGC 4599, a low-luminosity galaxy superficially resembling Hoag's Object in that on sky survey images it appears to be a complete ring surrounding a roundish core. The nature of the outer ring of Hoag-type galaxies is still debated and may be related either to slow secular evolution or to environmental processes, such as galaxy–galaxy interactions. As the nearest of its kind, UGC 4599 is a perfect target for a detailed study of the peculiar structure of this type of objects.

Surface photometry of the central body of the ring and the ring itself was performed using GALEX, Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) photometric data available online. To identify bright clumps across the ring that could be H II regions associated with the ring we obtained images of UGC 4599 at the Wise Observatory through U-, B- and R-band and narrow-band rest-frame Hα filters.

Although classified in HyperLEDA as S0 with an external ring, we show that in UGC 4599 (a) the nearly round central body follows well an {img} light profile almost all the way to the centre, (b) the isophotes are strongly twisted with a sharp 45° transition at a radius of r ≃ 6 arcsec, (c) the blue ring seems to have reached near-equilibrium configuration with the central body, (d) the ring is actually composed of a one-and-a-half turn spiral feature and (e) one side of the spiral shows conspicuous star formation in the form of at least nine H II regions, revealed by their Hα emission.

Based on the photometric data, together with H I information from the literature, we characterize UGC 4599 as an elliptical-like object surrounded by a luminous ring and a massive, extremely extended H I disc. Given its observed properties, we rule out UGC 4599 as representing a late phase in barred early-type galaxies evolution. We discuss the origin of UGC 4599 and conclude that this galaxy could be the result of a major interaction between two gas-rich spiral galaxies that took place at least 5 Gyr ago. However, deep optical imaging and a detailed stellar population analysis are required to determine whether the large gas reservoir could have been accreted directly from the intergalactic medium on to a pre-existing elliptical galaxy in the early Universe. A detailed kinematical study will shed light on the exact nature of the central body and the ring of UGC 4599.