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 Angstrom. The images show an oval-shaped area (which we call the "bulge") of about 11" x 16", or 450 x 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) similar or equal to 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 < m(2550) < 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 : 1. The energy distributions of most objects are best fitted with models of single stars of M-V between -6.1 and -9.1, temperatures between 4000 and 50,000 K, and with 4.2 < log (L/L.) < 7.2 and 12 M. < M-* < 200 M.. 2. In the Hertzsprung-Russel diagram the sources follow the Humphreys-Davidson luminosity upper limit for massive stars. 3. The distribution of the sources in the Hertzsprung-Russel diagram shows a gap in the range of 20,000K < T-eff < 10,000 K, which agrees with the rapid crossing of the Hertzsprung-Russel diagram by stars, but not of clusters. We have derived upper limits to the total mass of lower mass stars (M-* < 10 M.) that could be "hiding" within the point sources. For the "bluest" sources, the upper limit is only a few hundred M.. We conclude that the formation of massive stars outside clusters (or in very low mass clusters) is occurring in the bulge of M51. The estimated star formation rate in the bulge of M51 is (1-2) x 10(-3) M. yr(-1), depending on the adopted initial mass function. With the observed total amount of gas in the bulge, similar to4 x 10(5) M., and the observed normal gas-to-dust ratio of similar to150, this star formation rate could be sustained for about (2-4) x 10(8) yr. This suggests that the ongoing massive star formation in the bulge of M51 is fed/triggered by the interaction with its companion about 4 x 10(8) yr ago. The star formation in the bulge of M51 is compared with that in bulges of other spirals. Theoretical predictions of star formation suggest that isolated massive stars might be formed in clouds in which H-2, [O I] 63 mum and [C II] 158 mum are the dominant coolants. This is expected to occur in regions of rather low optical depth, A(V) less than or equal to 1 with a hot source that can dissociate the CO molecules. These conditions are met in the bulge of M51, where the extinction is low and where CO can be destroyed by the radiation from the bright nuclear starburst cluster in the center. The mode of formation of massive stars in the bulge of M51 may resemble the star formation in the early universe, when the CO and dust contents were low because of the low metallicity.