Effective Opacity of the Intergalactic Medium from Galaxy Spectra Analysis

We measure the effective opacity ($\tau_{eff}$) of the Intergalactic Medium (IGM) from the composite spectra of 281 Lyman-Break Galaxies (LBGs) in the redshift range $2 \lesssim z \lesssim 3$. Our spectra are taken from the COSMOS Lyman-Alpha Mapping And Tomographic Observations (CLAMATO) survey derived from the Low Resolution Imaging Spectrometer (LRIS) on the W.M. Keck I telescope. We generate composite spectra in two redshift intervals and fit them with spectral energy distribution (SED) models composed of simple stellar populations. Extrapolating these SED models into the Ly$\alpha$ forest, we measure the effective Ly$\alpha$ opacity ($\tau_{eff}$) in the $2.02 \leq z \leq 2.44$ range. At $z = 2.22$, we estimate $\tau_{eff} = 0.159 \pm 0.001$ from a power-law fit to the data. These measurements are consistent with estimates from quasar analyses at $z<2.5$ indicating that the systematic errors associated with normalizing quasar continua are not substantial. We provide a Gaussian Processes model of our results and previous $\tau_{eff}$ measurements that describes the steep redshift evolution in $\tau_{eff}$ from $z = 1.5 - 4$.