Chicago–Carnegie Hubble Program: A Multiwavelength Search for the Effects of Metallicity on the Cepheid Distance Scale. I. Empirical Evidence

Abstract Understanding the effects of metallicity on the multiwavelength properties of Cepheids is of continuing importance for their effective use as extragalactic distance indicators and an accurate measure of the Hubble constant, H 0 . Quantifying the magnitude, and even the sign, of a metallicity effect, at one or more wavelengths, has been and continues to be challenging, and has not yet converged. Here we present the results of a multipronged investigation looking for empirical evidence of correlations linking the metallicities of Cepheids to the zero-point of the their multiwavelength period–luminosity relations. The empirical evidence leads to a self-consistent conclusion based on six tests, across many galaxies and in up to nine different bandpasses, from the optical to the mid-infrared. The major result is that for classical Cepheids, in none of the cases examined is there a metallicity effect, of any statistical significance, detected at any of the wavelengths examined. However, theoretical models of stellar atmospheres clearly show that metallicity will impact each of the bandpasses differently: the level of this effect is largest in the ultraviolet (where line blanketing is most intense), reversing sign in the optical (due to flux redistribution from the UV), and then asymptotically falling back to zero from the red to the far infrared. Close examination of these models reveals infrared regions in Cepheid spectral energy distributions that allow for the simultaneous correction for extinction and metallicity differences by the judicious combination of infrared filters alone, one centered at 1.2 μ m and the other at 3.6 μ m. Both of these bands can be simultaneously observed in single exposures using the dual-channel imager NIRCam on JWST.