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Session 89 - Star-forming Galaxies.
Oral session, Friday, January 09
International Ballroom East,
We present the oxygen and nitrogen abundances of individual star burst knots in a sample of northern ring galaxies. In addition, we compare the reddening corrected broad-band colors for these knots with recent stellar evolution models.
These star burst knots have approximately half-solar oxygen abundances, independent of the linear diameter of the rings. This suggests that the oxygen is produced in massive, short lived stars within the ring. The nitrogen abundances show a correspondence with the diameter of the rings. N/H spans a range from roughly one-fifth solar for the smallest rings up to half-solar for the largest rings. The increase of N/H with the diameter of the rings might be due to the smaller rings inhabiting smaller, less chemically evolved galaxies whereas the larger rings inhabit larger, more chemically evolved systems. Material generally takes a few \times 10^7 years to pass in and out of a ring. The timescales for the release of freshly synthesized nitrogen is generally \geq 5 \times 10^8 years. Thus, it is likely that the nitrogen we observe was produced in stars in the pre-existing disk before the collision.
We have compared the colors of individual star burst knots with recent numerical models of stellar evolution and have determined that their ages lie in a range from 3 to 80 Myr. These ages are less than the dynamical ages of the rings, which are generally a few \times 10^8 years. The fact that the knots from individual ring galaxies cluster around the same (very young) age argues that their formation is recently triggered by a coherent event all around the ring. This strongly supports the triggering as being due to the propagation of a radial density wave. As such, it is unlikely that the knots existed in the pre-collisional disk of the target galaxy and are newly forming star clusters.
