A wide-area view of the Phoenix dwarf galaxy from Very Large Telescope/FORS imaging

We present results from a wide-area photometric survey of the Phoenix dwarf galaxy, one of the rare dwarf irregular/dwarf spheroidal transition-type galaxies (dTs) of the Local Group (LG). These objects offer the opportunity to study the existence of possible evolutionary links between the late- and early-type LG dwarf galaxies, since the properties of dTs suggest that they may be dwarf irregulars in the process of transforming into dwarf spheroidals. Using FORS at the Very Large Telescope (VLT), we have acquired VI photometry of Phoenix. The data reach a signal-to-noise ratio (S/N) similar to 10 just below the horizontal branch of the system and consist of a mosaic of images that covers an area of 26 x 26 arcmin2 centred on the coordinates of the optical centre of the galaxy. Examination of the colourmagnitude diagram and luminosity function revealed the presence of a bump above the red clump, consistent with being a red giant branch bump. The deep photometry combined with the large area covered allows us to put on a secure ground the determination of the overall structural properties of the galaxy and to derive the spatial distribution of stars in different evolutionary phases and age ranges, from 0.1 Gyr to the oldest stars. The best-fitting profile to the overall stellar population is a Sersic profile of Sersic radius RS= 1.82 +/- 0.06 arcmin and m= 0.83 +/- 0.03. We confirm that the spatial distribution of stars is found to become more and more centrally concentrated the younger the stellar population, as reported in previous studies. This is similar to the stellar population gradients found for close-by Milky Way dwarf spheroidal galaxies. We quantify such spatial variations by analysing the surface number density profiles of stellar populations in different age ranges; the parameters of the best-fitting profiles are derived, and these can provide useful constraints to models exploring the evolution of dwarf galaxies in terms of their star formation. The disc-like distribution previously found in the central regions in Phoenix appears to be present mainly among stars younger than 1 Gyr, and absent for the stars >rsim5 Gyr old, which on the other hand show a regular distribution also in the centre of the galaxy. This argues against a dischalo structure of the type found in large spirals such as the Milky Way.