On the form of the Spitzer Leavitt Law and its dependence on metallicity

Abstract

The form and metallicity dependence of Spitzer mid-infrared Cepheid relations are a source of debate. Consequently, Spitzer 3.6 and 4.5 [mu]m period–magnitude and period–color diagrams were re-examined via robust routines, thus providing an alternative interpretation to consider. The relations (nearly mean-magnitude) appear nonlinear over an extensive baseline (0.45 < log P[subscript 0] < 2.0), particularly the period–color trend, which to first order follows constant (3.6–4.5) color for shorter-period Cepheids and may transition into a bluer convex trough at longer periods. The period–magnitude functions can be described by polynomials (e.g., [3.6 [mu]m] = K[subscript 0] − (3.071 [plus or minus] 0.059) log P[subscript 0] − (0.120 [plus or minus] 0.032) log P[subscript 0] [superscript 2]), and Cepheid distances computed using 3.6 and 4.5 [mu]m relations agree with each other and the latter provides a first-order consistency check (CO sampled at 4.5 [mu]m does not seriously compromise those distances). The period–magnitude relations appear relatively insensitive to metallicity variations ([Fe/H] ∼ 0 to −0.75 |[gamma] | < 0.1 mag dex[superscript −1]), a conclusion inferred partly from comparing galaxy distances established from those relations and NED-D (n > 700), yet a solid conclusion awaits comprehensive mid-infrared observations for metal-poor Cepheids in IC 1613 ([Fe/H] ∼ −1). The Cepheid-based distances were corrected for dust obscuration using a new ratio (i.e., A[subscript 3.6]/E[subscript B−V] = 0.18 [plus or minus] 0.06) deduced from GLIMPSE (Spitzer) data.