The form of abundance gradients in three nearby spiral galaxies: M33, M81 and M101

Abstract

Available emission-line data have been compiled for giant H II regions located in three nearby, well observed spirals M33, M81, and M101 in order to study the form of interstellar abundance patterns in the disks of these galaxies. Numerous line strength ratios are shown to vary systematically with galactocentric distance. Gradients for stellar effective temperature, ionization parameter, and the abundance ratios O/H, N/O, and S/O were determined using a sequential photoionization model analysis procedure. The observed behavior of line ratios across the disks of these galaxies is found to be consistent with abundance gradients which are exponential in form and have constant slopes, thereby challenging recent suggestions that gradients flatten in the outer disk. Additionally, the N/O gradient in M101 is found to be markedly steeper than in M33 and M81, suggesting that in M101 secondary nitrogen production is relatively more efficient than primary nitrogen production.