The COSMOS Lyman-Alpha Mapping And Tomography Observations (CLAMATO) survey aims to map the cosmic web at z=2.3, at over 11 billion light years away when the Universe was less than a quarter its current age, using the LRIS spectrograph on the Keck-I telescope at Maunakea, Hawai’i. We create a map using Lyα forest tomography, which expands upon the HI Lyα forest analysis of IGM absorption that is traditionally carried out along relatively sparse quasar lines-of-sight (e.g. the BOSS Lyα forest sample). We target ubiquitous star-forming Lyman-Break galaxies (LBGs) and faint quasars at z ∼ 2 − 3 as background sources at a much higher source density than surveys such as BOSS. The small separation between sources allows us to tomographically interpolate across the adjacent lines-of-sight to create 3D maps of the IGM HI absorption, a nonlinear tracer of the underlying dark matter cosmic web. The resulting maps will enable the first analysis of the relationship between galaxy properties and the cosmic web at z=2.3. Similar studies have previously been restricted to z ≤ 0.6, where sufficiently high-density galaxy samples exist to trace the cosmic web. CLAMATO will provide the first analysis of the relationship between galaxies and their environment at the epoch of peak star formation.
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Due to the long (∼ 400 h−1 Mpc) path length in the Lyα forest probed by each background source, Lyα forest tomography allows us to map large volumes at z > 2. CLAMATO will make the first characterization of sheets and filaments in the high-redshift cosmic web and carry out a volume-limited search for voids and protoclusters. Additionally, we can use the local IGM absorption as a proxy for the local dark matter density (and thus halo mass) of individual galaxies within the map volume.CLAMATO aims to eventually cover the central 0.5-0.8 deg2 of the COSMOS field.