We present wide-field JHKs-band photometric observations of the three compact H II regions G48.9-0.3, G49.0-0.3, and G49.2-0.3 in the active star-forming region W51B. The star clusters inside the three compact H II regions show the excess number of stars in the J-Ks histograms compared with reference fields. While the mean color excess ratio (EJ-H/EH-Ks) of the three compact H II regions are similar to ~2.07 the visual extinctions toward them are somewhat different ~17 mag for G48.9-0.3 and G49.0-0.3; ~23 ≤ 2 Myr The inferred total stellar mass,~1.4x104M⊙, of W51B makes it one of the most active star forming regions in the Galaxy with the star formation efficiency of ~10%.

• HYOSUN KIM(Department of Physics and Astronomy, Seoul National University) Corresponding Author
• YASUSHI NAKAJIMA(National Astronomical Observatory of Japan)
• HWANKYUNG SUNG(Department of Astronomy and Space Science, Sejong University)
• DAE-SIK MOON(Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Department of Astronomy and Astrophysics, University of Toronto)
• BON-CHUL KOO(Department of Physics and Astronomy, Seoul National University)