2000ApJ...543..799O

Near-infrared observations of the central part of W51 have been carried out. We obtained J, H, and K' broadband and Brγ narrowband imaging data over an area of 15 arcmin², centered near W51 IRS 2, which covers the whole of an H II region complex G49.5-0.4. K-band spectroscopic observations were also carried out for selected objects in this area. More than 7000 point sources were detected from our imaging data, and about 160 sources were identified as physically associated with G49.5-0.4. We estimated absolute J magnitude for each source from our photometric data and inferred the ages of H II regions from the sizes of nebulosities on our Brγ image. Coupling information of the ages and the luminosities of exciting stars, we estimated their initial masses and the derived initial mass function (IMF) of massive stars in G49.5-0.4. In the mass range between 10 and 30 M_☉, the IMF slope is consistent with the value 1.8, which is derived for field stars. However, there is an excess of stellar population in the range above 30 M_☉; i.e., a top heavy-shaped IMF was derived. We defined four subgroups which are aligned parallel to the Galactic plane. The age of the oldest group was estimated to be 2.3 Myr and that of the youngest group to be younger than 1 Myr. The youngest group contains 22 O stars, and it seems that 17 out of 22 O stars were born in the last 0.8 Myr. The massive star formation activity of this group is more violent than any other visible Galactic H II region.