The Recent Star Formation History of the M31 Disk

The star formation history of the northern and southern M31 disk is measuredusing samples of BV photometry for 4' X 4' regions taken from the KPNO/CTIOLocal Group Survey (Massey et al. 2001). The distances, mean reddening values,and age distributions of the stars in these regions were measured using theroutines of Dolphin (1997, 2002). Independent measurements of overlappingfields show that the results are stable for most samples. A slight distancegradient is seen across the major axis of the southern disk, and a meandistance of 24.47+/-0.03 is found by combining the results. Higher meanreddening values follow the spiral structure. The stellar age distributions areconsistent with episodic star formation confined mainly to the gas-rich armregions. If these episodes were caused by propagating density waves, the wavesdid not cause significant star formation episodes in the gas-poor interarmregions. Combination of all of the results provides the total star formationrate for 1.4 square degrees of the M31 disk for six epochs. These resultssuggest that star formation in the disk declined by ~50% from ~250 to ~50 Myrago. The lowest star formation rate occurred ~25 Myr ago followed by a ~20%increase to the present. The mean star formation rate for this large portion ofM31 over the past 60 Myr is 0.63+/-0.07 solar masses per year, suggesting atotal mean rate for the disk of ~1 solar masses per year.Comment: 36 pages (14 text), 10 figures, 2 tables; accepted for publication in A