We present a new method of finding protoclusters using tomographic maps of Ly α forest flux. We review our method of creating tomographic flux maps and discuss our new highperformance implementation, which makes large reconstructions computationally feasible. Using a large N-body simulation, we illustrate how protoclusters create large-scale flux decrements, roughly 10 h-1 Mpc across, and how we can use this signal to find them in flux maps. We test the performance of our protocluster finding method by running it on the ideal, noiseless map and tomographic reconstructions from mock surveys, and comparing to the halo catalogue. Using the noiseless map, we find protocluster candidates with about 90 per cent purity, and recover about 75 per cent of the protoclusters that form massive clusters (>3 × 1014 h-1M⊙). We construct mock surveys similar to the ongoing COSMOS Lyman-Alpha Mapping And Tomography Observations survey. While the existing data have an average sightline separation of 2.3 h-1 Mpc, we test separations of 2-6 h-1 Mpc to see what can be tolerated for our application. Using reconstructed maps from small separation mock surveys, the protocluster candidate purity and completeness are very close to what was found in the noiseless case. As the sightline separation increases, the purity and completeness decrease, although they remain much higher than we initially expected. We extended our test cases to mock surveys with an average separation of 15 h-1 Mpc, meant to reproduce high source density areas of the Baryon Oscillation Spectroscopic Survey. We find that even with such a large sightline separation, the method can still be used to find some of the largest protoclusters.

We study the statistics of the Lyα forest in a flat Λ cold dark matter cosmology with the N-body + Eulerian hydrodynamics code NYX.We produce a suite of simulations, covering the observationally relevant redshift range 2 ≤ z ≤ 4. We find that a grid resolution of 20 h-1 kpc is required to produce 1 per cent convergence of Lyα forest flux statistics, up to k = 10 h-1 Mpc. In addition to establishing resolution requirements, we study the effects of missing modes in these simulations, and find that box sizes of L > 40h-1 Mpc are needed to suppress numerical errors to a sub-per cent level. Our optically thin simulations with the ionizing background prescription of Haardt & Madau reproduce an intergalactic medium density-temperature relation with T0 ≈ 104 K and γ ≈ 1.55 at z = 2, with a mean transmitted flux close to the observed values. When using the ionizing background prescription of Faucher-Giguère et al., the mean flux is 10-15 per cent below observed values at z = 2, and a factor of 2 too small at z = 4. We show the effects of the common practice of rescaling optical depths to the observed mean flux and how it affects convergence rates. We also investigate the practice of 'splicing' results from a number of different simulations to estimate the 1D flux power spectrum and show it is accurate at the 10 per cent level. Finally, we find that collisional heating of the gas from dark matter particles is negligible in modern cosmological simulations.

We present a new method of finding cosmic voids using tomographic maps of Lyα forest flux. We identify cosmological voids with radii of 2-12 h-1 Mpc in a large N-body simulation at z=2.5, and characterize the signal of the high-redshift voids in density and Lyα forest flux. The void properties are similar to what has been found at lower redshifts, but they are smaller and have steeper radial density profiles. Similarly towhat has been found for low-redshift voids, the radial velocity profiles have little scatter and agree very well with the linear theory prediction. We run the same void finder on an ideal Lyα flux field and tomographic reconstructions at various spatial samplings. We compare the tomographic map void catalogues to the density void catalogue and find good agreement even with modest-sized voids (r > 6 h-1 Mpc). Using our simple void-finding method, the configuration of the ongoing COSMOS Lyman Alpha Mapping And Tomography Observations (CLAMATO) survey covering 1 deg2 would provide a sample of about 100 high-redshift voids. We also provide void-finding forecasts for larger area surveys, and discuss how these void samples can be used to test modified gravity models, study high-redshift void galaxies, and to make an Alcock-Paczynski measurement. To aid future work in this area, we provide public access to our simulation products, catalogues, and sample tomographic flux maps.

We present the most distant detection of cosmic voids (z ∼ 2.3) and the first detection of three-dimensional voids in the Lyα forest. We used a 3D tomographic map of the absorption with an effective comoving spatial resolution of 2.5 h -1 Mpc and a volume of 3.15 105 h -3 Mpc3, which was reconstructed from moderate-resolution Keck I/LRIS spectra of 240 background Lyman-break galaxies and quasars in a 0.16 deg2 footprint in the COSMOS field. Voids were detected using a spherical overdensity finder calibrated from hydrodynamical simulations of the intergalactic medium (IGM). This allows us to identify voids in the IGM corresponding to voids in the underlying matter density field, yielding a consistent volume fraction of voids in both data (19.5%) and simulations (18.2%). We fit excursion set models to the void radius function and compare the radially averaged stacked profiles of large voids (r > 5 h -1 Mpc) to stacked voids in mock observations and the simulated density field. Comparing with 432 coeval galaxies with spectroscopic redshifts in the same volume as the tomographic map, we find that the tomography-identified voids are underdense in galaxies by 5.95σ compared to random cells.

Using moderate-resolution optical spectra from 58 background Lyman-break galaxies and quasars at z ∼ 2.3-3 within a 11′.5 × 13′.5 area of the COSMOS field (∼1200 deg-2 projected area density or ∼2.4 h-1 Mpc mean transverse separation), we reconstruct a 3D tomographic map of the foreground Lyα forest absorption at 2.2 < z < 2.5 with an effective smoothing scale of ∈3D ≈ 2.5 h-1 Mpc comoving. Comparing with 61 coeval galaxies with spectroscopic redshifts in the same volume, we find that the galaxy positions are clearly biased toward regions with enhanced intergalactic medium (IGM) absorption in the tomographic map. We find an extended IGM overdensity with deep absorption troughs at z = 2.45 associated with a recently discovered galaxy protocluster at the same redshift. Based on simulations matched to our data, we estimate the enclosed dark matter mass within this IGM overdensity to be Mdm (z= 2.45) = (1.1 ± 0.6) ×1014 h-1 M⊙, and argue based on this mass and absorption strength that it will form at least one z ∼ 0 galaxy cluster with M(z = 0) = (3 ± 1.5) ×1014 h-1 M⊙, although its elongated nature suggests that it will likely collapse into two separate clusters. We also point out a compact overdensity of six MOSDEF galaxies at z = 2.30 within a r ∼ 1 h-1 Mpc radius and Δz ∼ 0.006, which does not appear to have a large associated IGM overdensity. These results demonstrate the potential of Lyα forest tomography on larger volumes to study galaxy properties as a function of environment, as well as revealing the large-scale IGM overdensities associated with protoclusters or other features of large-scale structure.