Effective Opacity of the Intergalactic Medium from Galaxy Spectra Analysis

We measure the effective opacity ( ${\tau }_{\mathrm{eff}}$ ) of the intergalactic medium from the composite spectra of 281 Lyman-break galaxies in the redshift range 2 ≲ z ≲ 3. Our spectra are taken from the COSMOS Lyα Mapping And Tomographic Observations survey derived from the Low Resolution Imaging Spectrometer 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 Lyα forest, we measure the effective Lyα opacity ( ${\tau }_{\mathrm{eff}}$ ) in the 2.02 ≤ z ≤ 2.44 range. At z = 2.22, we estimate ${\tau }_{\mathrm{eff}}\,=0.159\pm 0.001$ 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 ${\tau }_{\mathrm{eff}}$ measurements that describes the steep redshift evolution in ${\tau }_{\mathrm{eff}}$ from z = 1.5-4.