First Data Release of the COSMOS Lyman-Alpha Mapping And Tomography Observations: 3D Lyman-$α$ Forest Tomography at 2.05 < z < 2.55 (1710.02894v3)

Abstract: Faint star-forming galaxies at $z\sim 2-3$ can be used as alternative background sources to probe the Lyman-$\alpha$ forest in addition to quasars, yielding high sightline densities that enable 3D tomographic reconstruction of the foreground absorption field. Here, we present the first data release from the COSMOS Lyman-Alpha Mapping And Mapping Observations (CLAMATO) Survey, which was conducted with the LRIS spectrograph on the Keck-I telescope. Over an observational footprint of 0.157$\mathrm{deg}^2$ within the COSMOS field, we used 240 galaxies and quasars at $2.17<z<3.00$, with a mean comoving transverse separation of $2.37\,h^{-1}\,\mathrm{Mpc^3}$, as background sources probing the foreground Lyman-$\alpha$ forest absorption at $2.05<z<2.55$. The Lyman-$\alpha$ forest data was then used to create a Wiener-filtered tomographic reconstruction over a comoving volume of $3.15\,\times 10^5\,h^{-3}\,\mathrm{Mpc^3}$ with an effective smoothing scale of $2.5\,h^{-1}\,\mathrm{Mpc}$. In addition to traditional figures, this map is also presented as a virtual-reality YouTube360 video visualization and manipulable interactive figure. We see large overdensities and underdensities that visually agree with the distribution of coeval galaxies from spectroscopic redshift surveys in the same field, including overdensities associated with several recently-discovered galaxy protoclusters in the volume. This data release includes the redshift catalog, reduced spectra, extracted Lyman-$\alpha$ forest pixel data, and tomographic map of the absorption.

Overview of the CLAMATO IGM Tomography Survey and Its First Data Release

The paper "First Data Release of the COSMOS Lyman-Alpha Mapping And Tomography Observations: 3D Lyman-$\alpha$ Forest Tomography at $2.05 < z < 2.55$" presents the initial data release from the CLAMATO Survey. It focuses on the innovative use of high-redshift star-forming galaxies as alternative background sources to quasars for probing the Lyman-$\alpha$ (Ly$\alpha$) forest. This method enables enhanced sightline densities for reconstructing the foreground intergalactic medium (IGM) absorption field in three dimensions.

This release encompasses observational data acquired with the LRIS spectrograph on the Keck-I telescope over a footprint of 0.157 square degrees within the COSMOS field. Utilizing 240 galaxies and quasars as background probes, with a mean comoving transverse separation of $2.37\,h^{-1}\mathrm{Mpc}$, the survey explores the absorption at $2.05$3.15 \times 10^5\,h^{-3}\,\mathrm{Mpc^3}$, achieving an effective smoothing scale of $2.5\,h^{-1}\,\mathrm{Mpc}$.

Methodological Insights

The employment of faint star-forming galaxies as background sources is a significant methodological shift from traditional quasar-based studies. This approach enables a remarkable increase in sightline density, facilitating detailed three-dimensional structural mapping of the IGM. The survey leverages the well-studied COSMOS field, optimizing multi-wavelength photometric and spectroscopic resources to target UV-emitting sources efficiently.

The data analysis leverages Wiener filtering, a statistical approach to reconstruct the Ly$\alpha$ absorption field, accounting for noise and data covariances. The reconstruction produces volumes reflective of the large-scale cosmic web comprising overdensities and underdensities that correlate with pre-identified galaxy clusters and voids. The survey achieves a substantial $S/N^{wiener} \approx 3.4$ over a $3\,h^{-1}\mathrm{Mpc}$ kernel, suggesting robust map quality enabling detailed structural studies.

Implications and Future Research

This data release has significant implications for studying the large-scale structure and cosmic environments during an epoch that is otherwise challenging to explore. The tomographic map serves as a critical resource for examining galaxy protoclusters and cosmic voids, with potential applications in refining photometric redshifts based on IGM structure. Future expansions of the survey, targeting larger footprints and finer scales, could provide deeper insights into cosmic evolution theories and galaxy-IGM interactions.

In the cosmological context, CLAMATO's detailed mapping could enhance our understanding of dark matter distributions and baryonic matter interaction at intermediate redshifts. The integration of IGM tomography with forthcoming spectroscopic surveys from facilities like the Subaru PFS could unify understanding across different observational scales and epochs.

Overall, the CLAMATO Survey's first data release is a substantive step toward more comprehensive three-dimensional cosmological mappings, providing foundational data for subsequent research in galaxy evolution, large-scale cosmic structure, and potentially, novel gravitational lensing studies. The survey is positioned as a pathfinding endeavor for future campaigns leveraging next-generation telescopes and instruments.