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. The uniformity of early-type galaxy spectra in the optical allows us to construct inverse-variance-weighted composite spectra with high signal-to-noise ratio (ranging from 10 − 10). 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 ∼ 0.15 − 2 Mpc; we derive a generic limit of E(B − V ) < 3 × 10 mag on scales ∼ 1 − 2 Mpc at the 99% confidence level, using conservative jackknife error bars, corresponding to a dust mass . 10 M⊙. On scales smaller than 1 Mpc this limit is slightly weaker, E(B − V ) < 8× 10 mag. Subject headings: dust, extinction — galaxies: clusters: general — intergalactic medium — methods: statistical