Far-ultraviolet imaging of the field star population in the Large Magellanic Cloud with the Hubble Space Telescope

We present an analysis of the deepest pure UV observations with the highest angular resolution ever performed, a set of 12 exposures with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 and F160BW filter obtained in parallel observing mode, which covers ∼12 arcmin 2 in the Large Magellanic Cloud (LMC), north of the bar, and in the "general field" regime of the LMC. The 341 independent measurements of 198 objects represent an accumulated exposure of ≥2 × 10⁴ s and reveal stars as faint as m_UV ≃ 22 mag. The observations show that about two-thirds of the UV emission from the LMC is emitted by our HST-detected UV stars in the field, that is, not in clusters or associations. We identified optical counterparts in the Royal Observatory Edinburgh/Naval Research Laboratory photometric catalog for about one-third of the objects. The results are used to discuss the nature of these UV sources, to estimate the diffuse UV emission from the LMC as a prototype of dwarf galaxies, and to evaluate the contamination by field stars of UV observations of globular and open clusters in the LMC. We find that the projected density of UV stars in the general field of the LMC is a few times higher than in the Galactic disk close to the Sun. Combining our data with observations by the Ultraviolet Imaging Telescope allows us to define the stellar UV luminosity function from m_UV = 8-18 mag and to confirm that the field regions in the LMC have been forming stars at a steady rate during the last 1 Gyr, with an initial mass function close to the Salpeter law.