The transparency of galaxy clusters

If galaxy clusters contain intracluster dust, the spectra of galaxies lying behind clusters should show attenuation by dust absorption. We compare the optical (3500-7200 Å) spectra of 60,267 luminous, early-type galaxies selected from the Sloan Digital Sky Survey to search for the signatures of intracluster dust in z - 0.05 clusters. We select massive, quiescent (i.e., non-star-forming) galaxies using an EW(Hα) ≤ 2 Å cut and consider galaxies in three bins of velocity dispersion ranging from 150 to 300 km s ^-1. The uniformity of early-type galaxy spectra in the optical allows us to construct inverse-variance-weighted composite spectra with high signal-to-noise ratios (ranging from 10² to 10³). We compare the composite spectra of galaxies that lie behind and adjacent to galaxy clusters and find no convincing evidence of dust attenuation on scales of ∼0.15-2 Mpc; we derive a generic limit of E(B - V) < 3 × 10 ^-3 mag on scales of ∼1-2 Mpc at the 99% confidence level, using conservative jackknife error bars, corresponding to a dust mass ≲10 ⁸ M_⊙. On scales smaller than 1 Mpc, this limit is slightly weaker, E(B- F) < 8 × 10^-3 mag.