Stellar Metallicity of the Extended Disk and Distance of the Spiral Galaxy NGC 3621

Abstract: Low resolution ESO VLT/FORS spectra of blue supergiant stars are analyzed to determine stellar metallicities (based on elements such as Fe, Ti, Mg) in the extended disk of the spiral galaxy NGC3621. Mildly subsolar metallicity (-0.30 dex) is found for the outer objects beyond 7 kpc independent of galactocentric radius and compatible with the absence of a metallicity gradient confirming the results of a recent investigation of interstellar medium HII region gas oxygen abundances. The stellar metallicities are slightly higher than those from the HII regions when based on measurements of the weak forbidden auroral oxygen line at 4363 AE but lower than the ones obtained with the R23 strong line method. It is shown that the present level of metallicity in the extended disk cannot be the result of chemical evolution over the age of the disk with the present rate of in situ star formation. Additional mechanisms must be involved. In addition to metallicity, stellar effective temperatures, gravities, interstellar reddening, and bolometric magnitudes are determined. After application of individual reddening corrections for each target the flux-weighted gravity-luminosity relationship of blue supergiant stars is used to obtain a distance modulus of 29.07+/-0.09 mag (distance D=6.52+/-0.28 Mpc). This new distance is discussed in relation to Cepheid and tip of the red giant branch distances.