2001A&A...366..796C

We present a deep UBV survey in the direction of the halo of the starburst galaxy NGC 253 aimed at investigating the possible existence of recent star formation far from its disk, as traced by early-type stars. We discuss different classes of objects with blue colors that may contaminate the region of color-color and color-magnitude diagrams occupied by early Population I stars at the distance of NGC 253. Strong upper limits are found on their contribution to the measured object counts by means of models, of surveys of the halo of our Galaxy, and of observations of nearby control fields. A population of objects with (B-V)<0, V>23 is identified in the direction of the halo of NGC 253 that has no counterpart in the control fields to a high level of statistical confidence. The absolute magnitudes of these objects at the distance of NGC 253 is consistent with them being main sequence B0-B2 stars. The spatial distribution of the bluest objects in the halo of NGC 253 seems to cluster in two groups: one is closer to the disk of NGC 253, and may contain runaway stars expelled from its disk. The other group has projected distances to the plane of the galaxy ranging between 9 and 15kpc, and at least its base coincides with a peak in radio continuum due to synchrotron emission of cosmic rays escaping the galactic disk. We hypothesize that the distant group of blue stars in the halo of NGC 253 is a result of the interaction between the superwind produced at its nuclear starburst (and perhaps also in star forming regions in the disk) and cold gas in the halo, in a phenomenon similar to the star formation near Centaurus A induced by the interaction of its jet with a HI cloud. If this is the case, NGC 253 provides an example of the ability of less energetic galactic outflows to trigger star formation in haloes, a phenomenon that may also be responsible for the suspected existence of Population I stars at large distances from the disk of the Milky Way.