Near-infrared observations of W51

We present J, H, and K images of the luminous H II regions W51 IRS 1 and IRS 2. The appearances of the H II regions at 2.2 micron correspond closely to their appearances at radio wavelengths. Comparing a narrowband Br-gamma filter image to the broadband K image reveals that the extended emission visible throughout our images are consistent with the thermal emission expected from the ionized hydrogen and helium in the H II regions. By comparing the surface brightnesses of this diffuse emission at radio and near-infrared wavelengths, we derive an extinction to the H II regions of AV = 24 +/- 3 mag, with localized areas of higher obscuration. More than 300 stars are visible in our images. Almost all the stars seen at J have the blue colors of slightly reddened foreground stars; they are distributed uniformly across our images. Red stars visible only at H and K are concentrated atop the H II regions. Their colors are redder than can be accounted for by simple foreground extinction, suggesting the presence of emission from hot dust close to the photospheres of these young stars. We believe we have identified the mid-infrared peak of IRS 2 in our images; broadened photometry and narrowband spectrophotometry indicate it may be a highly embedded massive star or stars. We have discovered a bright, red object, which we call IRS 3, that also has the spectral characteristics of a dust-embedded massive star.