A Metallicity Map of M33

TL;DR

This study provides a detailed metallicity map of M33 using spectroscopic data from ~200 HII regions, revealing a shallow oxygen abundance gradient and significant local metallicity variations.

Contribution

It offers the first extensive electron-temperature based metallicity measurements across M33, including at small galactocentric radii, and presents a two-dimensional abundance map.

Findings

Oxygen abundance gradient of -0.027 dex/kpc

Substantial intrinsic scatter of 0.11 dex in metallicity

Evidence for deviations from axisymmetry in M33

Abstract

We present initial results from the M33 Metallicity Project. Out of the thousands of cataloged HII regions in M33, only ~30 have electron-temperature based abundances in the literature. We have obtained Keck spectroscopy of a sample of ~200 HII regions in M33, with 61 detections of the [O III] 4363 A line that can be used for determining electron temperatures, including measurements at small galactocentric radii where auroral lines are generally difficult to detect. We find an oxygen abundance gradient of -0.027 +/- 0.012 dex/kpc, in agreement with infrared measurements of the neon abundance gradient but much shallower than most previous oxygen gradient measurements. There is substantial intrinsic scatter of 0.11 dex in the metallicity at any given radius in M33, which imposes a fundamental limit on the accuracy of gradient measurements that rely on small samples of objects. Finally, we present a two-dimensional map of oxygen abundances across the southern half of M33 and discuss the evidence for deviations from axisymmetry.