We present ultraviolet photometry for globular clusters (GCs) in M31 from 15 deg2 of imaging using the Galaxy Evolution Explorer (GALEX). We detect 200 and 94 GCs with certainty in the near-ultraviolet (NUV; 1750-2750 A) and far-ultraviolet (FUV; 1350-1750 Å) bandpasses, respectively. Our rate of detection is about 50% in the NUV and 23% in the FUV, to an approximate limiting V magnitude of 19. Out of six clusters with [Fe/H] > -1 seen in the NUV, none is detected in the FUV bandpass. Furthermore, we find no candidate metal-rich clusters with significant FUV flux because of the contribution of blue horizontal-branch (HB) stars, such as NGC 6388 and NGC 6441, which are metal-rich Galactic GCs with hot HB stars. We show that our GALEX photometry follows the general color trends established in previous UV studies of GCs in M31 and the Galaxy. Comparing our data with Galactic GCs in the UV and with population synthesis models, we suggest that the age ranges of M31 and Galactic halo GCs are similar.
Bibliographical noteFunding Information:
The authors thank Pauline Barmby for kindly providing her catalog of M31 clusters. We are also grateful to Carla Cacciari for valuable comments. GALEX is a NASA Small Explorer, launched in 2003 April. We gratefully acknowledge NASA’s support for construction, operation, and science analysis for the GALEX mission. Yonsei University participation is funded by the Korean Ministry of Science and Technology, for which we are grateful.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science