YOUNG STAR-CLUSTERS IN THE DWARF IRREGULAR GALAXY, UGC-7636, INTERACTING WITH THE GIANT ELLIPTIC GALAXY NGC-4472

We present integrated Washington CT1 photometry of 18 bright blue obje cts discovered in the dwarf galaxy UGC 7636 which is located 5'.5 sout heast of the giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster. Several lines of evidence indicate that UGC 7636 i s interacting violently with NGC 4472. These objects are very blue wit h colors of -0.4<(C-T-1)< 0.6, and their magnitudes are in the range o f 20.6< T-1 < 22.9 mag which corresponds to absolute magnitudes of -10 .6<M (T?1) < -8.3 mag for a distance modulus of (m - M)(0) = 31.2. The se objects are grouped spatially in three regions: the central region of UGC 7636, the tidal tail region, and the H I cloud region. No such objects were found in the counter-tail region. It is concluded that th ese objects are probably young star clusters which formed < 10(8) yr a go during the interaction between UGC 7636 and NGC 4472. Surface photo metry of UGC 7636 (r<83 '') shows that there is a significant excess o f blue light along the tidal tail region compared with ether regions. The star clusters are bluer than the stellar light in the tidal tail r egion, indicating that these clusters might have formed later than mos t stars in the tidal tail region which were formed later than most sta rs in the main body of the galaxy. (C) 1997 American Astronomical Soci ety.