Based on FORS observations collected at the European Southern Observatory (ESO), proposal 083.B-0252.
A wide-area view of the Phoenix dwarf galaxy from Very Large Telescope/FORS imaging★
Article first published online: 26 JUN 2012
© 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS
Monthly Notices of the Royal Astronomical Society
Volume 424, Issue 2, pages 1113–1131, August 2012
How to Cite
Battaglia, G., Rejkuba, M., Tolstoy, E., Irwin, M. J. and Beccari, G. (2012), A wide-area view of the Phoenix dwarf galaxy from Very Large Telescope/FORS imaging. Monthly Notices of the Royal Astronomical Society, 424: 1113–1131. doi: 10.1111/j.1365-2966.2012.21286.x
- Issue published online: 13 JUL 2012
- Article first published online: 26 JUN 2012
- Accepted 2012 May 10. Received 2012 April 18; in original form 2012 January 25
- techniques: photometric;
- galaxies: evolution;
- galaxies: individual: Phoenix;
- Local Group;
- galaxies: stellar content;
- galaxies: structure
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) ∼ 10 just below the horizontal branch of the system and consist of a mosaic of images that covers an area of 26 × 26 arcmin2 centred on the coordinates of the optical centre of the galaxy.
Examination of the colour–magnitude 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 Sérsic profile of Sérsic 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 disc–halo structure of the type found in large spirals such as the Milky Way.