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[11.14] A New Determination of the Metallicity Dependence of the Cepheid Distance Scale

R.C. Kennicutt (U. Arizona), S. Sakai (UCLA), L. Ferrarese (Rutgers U.), A. Saha (NOAO)

We have obtained HST imaging of 7 nearby galaxies, and measured their distances using the tip of the red giant branch (TRGB) method. We have combined these data with published TRGB and Cepheid distances for a total of 17 galaxies, to measure the metallicity dependence of the Cepheid period-luminosity zeropoint. Our sample covers a range in Cepheid abundance of 1/20 -- 2 (O/H)_\odot, with good sampling over this entire metallicity range.

The residuals in \delta{{(m - M)}_{Ceph - TRGB}} show a monotonic decrease with increasing Cepheid abundance, and we can rule out a null dependence with greater than 99%\ confidence. Preliminary fits yield a (V, I) distance modulus dependence of -0.25 ±0.05 mag/dex, depending on the Cepheid samples and abundance scale used. Our results are consistent with those found earlier in the HST Extragalactic Distance Scale Key Project (Kennicutt et al. 1998, ApJ, 498, 181), and with those adopted in the Key Project Hubble constant determination (Mould et al.\ 2000, ApJ, 529, 786; Freedman et al.\ 2001, ApJ, 553, 47). These results reduce one of the primary extant uncertainties in the Cepheid-based extragalactic distance ladder.

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.