Ongoing Massive Star Formation in the Bulge of M51^*

We present a study of Hubble Space Telescope Wide Field Planetary Camera 2 observations of the inner kiloparsec of the interacting galaxy M51 in six bands from 2550 to 8140 Å. The images show an oval-shaped area (which we call the "bulge") of about 11'' × 16'', or 450 × 650 pc, around the nucleus that is dominated by a smooth "yellow/reddish" background population with overimposed dust lanes. These dust lanes are the inner extensions of the spiral arms. The extinction properties, derived in four fields in and outside dust lanes, are similar to the Galactic extinction law. The reddish stellar population has an intrinsic color of (B-V)₀ ≃ 1.0, suggesting an age in excess of 5 Gyr. We found 30 bright pointlike sources in the bulge of M51, i.e., within 110-350 pc from the nucleus. The point sources have 21.4 < V < 24.3, many of which are blue with B-V < 0 and are bright in the UV with 19.8 < m₂₅₅₀ < 22.0. These objects appear to be located in elongated "strings" that follow the general pattern of the dust lanes around the nucleus. The spectral energy distributions of the pointlike sources are compared with those predicted for models of clusters or single stars. There are three reasons to conclude that most of these point sources are isolated massive stars (or very small groups of a few isolated massive stars) rather than clusters: