SIMBAD references

We present a study of the young population in the starburst galaxy NGC 253. In particular, we focused our attention on searching for young star groups, obtaining their main properties and studying their hierarchical organization. For this task, we used multiband images and their corresponding photometric data obtained with the Advanced Camera for Surveys of the Hubble Space Telescope (ACS/HST). We first derived the absorption affecting the different regions of the galaxy. Then, we applied an automatic and objective searching method over the corrected data in order to detect young star groups. We complemented this result with the construction of a stellar density map for the blue young population. A statistical procedure to decontaminate the photometric diagrams from field stars was applied over the detected groups and we estimated their fundamental parameters. As a result, we built a catalogue of 875 newly identified young groups with their main characteristics, including coordinates, sizes, estimated number of members, stellar densities, luminosity function (LF) slopes and galactocentric distances. We observed that these groups delineate different structures of the galaxy, and they are the last step in the hierarchical way in which the young population is organized. From their size distribution, we found they have typical radii of ∼40-50pc. These values are consistent with those found in other nearby galaxies. We estimated a mean value for the LF slope of 0.21 and an average density of 0.0006 star pc^–3 for the identified young groups, taking into account stars earlier than B6.