Re-analysis of VLT Data for M83 with Image Subtraction — Nine-fold Increase in Number of Cepheids

We have applied the image subtraction method to re-analyze the ESO Very Large Telescope data on M83 (NGC 5236), obtained and analyzed by Thim et al. Whereas Thim et al. found 12 Cepheids with periods between 12-55 days, we found 112 Cepheids with periods ranging from 7-91 days, as well as ∼ 60 other variables. These include 2 candidate eclipsing binaries, which, if confirmed, would be the first optically discovered outside the Local Group. We thus demonstrate that the image subtraction method is much more powerful for detecting variability, especially in crowded fields. However, HST observations are necessary to obtain a Cepheid period-luminosity distance not dominated by blending and crowding. We propose a “hybrid” approach, where numerous Cepheids are discovered and characterized using large ground-based telescopes and then followed-up with the HST to obtain precise distances. Subject headings: Cepheids – distance scale – galaxies: individual (M83, NGC 5236)