A New Distance to M33 Using Blue Supergiants and the FGLR Method

The quantitative spectral analysis of medium resolution optical spectra of A and B supergiants obtained with DEIMOS and ESI at the Keck Telescopes is used to determine a distance modulus of 24.93 +/- 0.11 mag for the Triangulum Galaxy M33. The analysis yields stellar effective temperatures, gravities, interstellar reddening, and extinction, the combination of which provides a distance estimate via the Flux-weighted Gravity--Luminosity Relationship (FGLR). This result is based on an FGLR calibration that is continually being polished. An average reddening of <E(B-V)> ~ 0.08 mag is found, with a large variation ranging from 0.01 to 0.16 mag however, demonstrating the importance of accurate individual reddening measurements for stellar distance indicators in galaxies with evident signatures of interstellar absorption. The large distance modulus found is in good agreement with recent work on eclipsing binaries, planetary nebulae, long period variables, RR Lyrae stars, and also with HST observations of Cepheids, if reasonable reddening assumptions are made for the Cepheids. Since distances based on the tip of the red giant branch (TRGB) method found in the literature give conflicting results, we have used HST ACS V- and I-band images of outer regions of M 33 to determine a TRGB distance of 24.84 +/- 0.10 mag, in basic agreement with the FGLR result. We have also determined stellar metallicities and discussed the metallicity gradient in the disk of M33. We find metallicity of $Z_\odot$ at the center and 0.3 $Z_\odot$ in the outskirts at a distance of one isophotal radius. The average logarithmic metallicity gradient is -0.07 +/- 0.01 dex kpc^-1. However, there is a large scatter around this average value, very similar to what has been found for the HII regions in M33.