Red supergiant stars in NGC 4449, NGC 5055 (M63) and NGC 5457 (M101)

The photometric properties of red supergiants (RSGs) in three nearby galaxies NGC 4449, NGC 5055 and NGC 5457 were investigated using near-infrared (JHK) imaging data obtained fromWFCAM UKIRT. The resulting near-infrared photometric catalogue was combined with optical archive data to remove foreground Galactic stars. We found that the foreground stars can be separated from the supergiants in a (i - K, r - i) colour-colour diagram. The (J - K, K)0 colour-magnitude diagrams of the three galaxies are investigated and compared with theoretical isochrones. The majority of RSGs in the three galaxies have common age ranges from log (t_yr) = 6.9 to log (t_yr) = 7.3, and this indicates that these galaxies have experienced recent star formation within 20 Myr. For NGC 4449 and NGC 5457, the RSGs are spatially correlated with the HII regions, which, however, is not the case for NGC 5055. We were not able to identify a clear metallicity dependence on the colours and the peak absolute magnitudes of the RSGs in the three galaxies. The inaccuracy of our photometric data contributes to the unclear metallicity effect on the colours ofRSGs. Instead, we found a similar colour distribution and a constant peak magnitude of M_K =-11.9 for the RSGs in the three galaxies. Therefore, we note that additional spectroscopy data for our RSG candidates, including photometry, are necessary to examine whether the RSG population changes with metallicity.