# Spectroscopic Observation and Analysis of HII regions in M33 with MMT: Temperatures and Oxygen Abundances

## TL;DR

This study used MMT spectroscopy to analyze 413 HII regions in M33, measuring physical parameters and oxygen abundances, revealing a consistent metallicity gradient and local variations linked to spiral arms.

## Contribution

It provides a comprehensive, homogeneous spectroscopic analysis of M33's HII regions, comparing different oxygen abundance calibration methods and mapping metallicity distribution.

## Key findings

- Confirmed axisymmetric metallicity distribution in M33.
- Identified local metallicity variations associated with spiral arms.
- Derived consistent oxygen abundance gradients across methods.

## Abstract

The spectra of 413 star-forming (or HII) regions in M33 (NGC 598) were observed by using the multifiber spectrograph of Hectospec at the 6.5-m Multiple Mirror Telescope (MMT). By using this homogeneous spectra sample, we measured the intensities of emission lines and some physical parameters, such as electron temperatures, electron densities, and metallicities. Oxygen abundances were derived via the direct method (when available) and two empirical strong-line methods, namely, O3N2 and N2. In the high-metallicity end, oxygen abundances derived from O3N2 calibration were higher than those derived from N2 index, indicating an inconsistency between O3N2 and N2 calibrations. We presented a detailed analysis of the spatial distribution of gas-phase oxygen abundances in M33 and confirmed the existence of the axisymmetric global metallicity distribution widely assumed in literature. Local variations were also observed and subsequently associated with spiral structures to provide evidence of radial migration driven by arms. Our O/H gradient fitted out to 1.1 $R_{25}$ resulted in slopes of $-0.17\pm0.03$, $-0.19\pm0.01$, and $-0.16\pm0.17$ dex $R_{25}^{-1}$ utilizing abundances from O3N2, N2 diagnostics, and direct method, respectively.