Simulating MOS science on the ELT: Ly<i>α</i> forest tomography

Abstract

Mapping the large-scale structure through cosmic time has numerous applications in studies of cosmology and galaxy evolution. At z ≳ 2, the structure can be traced by the neutral intergalactic medium (IGM) by way of observing the Ly α forest towards densely sampled lines of sight of bright background sources, such as quasars and star-forming galaxies. We investigate the scientific potential of MOSAIC, a planned multi-object spectrograph on the European Extremely Large Telescope (ELT), for the 3D mapping of the IGM at z ≳ 3. We simulated a survey of 3 ≲ z ≲ 4 galaxies down to a limiting magnitude of m r ∼ 25.5 mag in an area of 1 degree 2 in the sky. Galaxies and their spectra (including the line-of-sight Ly α absorption) were taken from the lightcone extracted from the Horizon-AGN cosmological hydrodynamical simulation. The quality of the reconstruction of the original density field was studied for different spectral resolutions ( R = 1000 and R = 2000, corresponding to the transverse typical scales of 2.5 and 4 Mpc) and signal-to-noise ratios (S/N) of the spectra. We demonstrate that the minimum S/N (per resolution element) of the faintest galaxies that a survey like this has to reach is S / N = 4. We show that a survey with this sensitivity enables a robust extraction of cosmic filaments and the detection of the theoretically predicted galaxy stellar mass and star-formation rate gradients towards filaments. By simulating the realistic performance of MOSAIC, we obtain S / N ( T obs , R , m r ) scaling relations. We estimate that ≲35 (65) nights of observation time are required to carry out the survey with the instrument’s high multiplex mode and with a spectral resolution of R = 1000 (2000). A survey with a MOSAIC-concept instrument on the ELT is found to enable the mapping of the IGM at z &gt; 3 on Mpc scales, and as such will be complementary to and competitive with other planned IGM tomography surveys.