The effect of metallicity on Cepheid magnitudes and the distance to M33

TL;DR

This study investigates how metallicity influences Cepheid brightness and distance measurements to M33, revealing a metallicity effect that adjusts the galaxy's distance estimate.

Contribution

It provides the first detailed measurement of the metallicity dependence of Cepheid period-luminosity relation in M33 using multi-region observations.

Findings

Metallicity gradient in M33 affects Cepheid distance estimates.

Measured metallicity parameter gamma = -0.29 mag/dex.

Metallicity correction yields a refined distance modulus of 24.53 mag.

Abstract

We present the results from a multi-epoch survey of two regions of M33 using the 3.5m WIYN telescope. The inner field is located close to the centre of the galaxy, with the outer region situated about 5.1 kpc away in the southern spiral arm, allowing us to sample a large metallicity range. We have data for 167 fundamental mode Cepheids in the two regions. The reddening-free Wesenheit magnitude Wvi period-luminosity relations were used to establish the distance modulus of each region, with mu_{inner} = 24.37 +- 0.02 mag and mu_{outer} = 24.54 +- 0.03 mag. The apparent discrepancy between these two results can be explained by the significant metallicity gradient of the galaxy. We determine a value for the metallicity parameter of the Period--Luminosity relation gamma = d(m-M)/d log(Z) = -0.29 +- 0.11 mag/dex, consistent with previous measurements. This leads to a metallicity corrected distance modulus to M33 of 24.53 +- 0.11 mag.