Constraining the Fluctuating Gunn–Peterson Approximation using Lyα Forest Tomography at z = 2

Abstract

The fluctuating Gunn–Peterson approximation (FGPA) is a commonly used method to generate mock Lyα forest absorption skewers at Cosmic Noon (z ≳ 2) from the matter density field of N-body simulations without running expensive hydrodynamical simulations. Motivated by recent developments in 3D intergalactic medium (IGM) tomography observations as well as matter density field reconstruction techniques applied to galaxy redshift samples at z ∼ 2, we examine the possibility of observationally testing FGPA by directly examining the relationship between Lyα transmission and the underlying matter density field. Specifically, we analyze the EAGLE, Illustris, IllustrisTNG, and Nyx cosmological hydrodynamic simulations that were run with different codes and sub-grid models. While the FGPA is an excellent description of the IGM in lower-density regions, the slope of the transmission–density distribution at higher densities is significantly affected by feedback processes causing the FGPA to break down in that regime. Even without added feedback, we find significant deviations caused by hydrodynamical effects arising from nonlinear structure growth. We then proceed to make comparisons using realistic mock data assuming the sightline sampling and spectral properties of the recent CLAMATO survey, and find that it would be challenging to discern between the FGPA and hydrodynamical models with current data sets. However, the improved sightline sampling from future extremely large telescopes or large volumes from multiplexed spectroscopic surveys such as Subaru PFS should allow for stringent tests of the FGPA, and make it possible to detect the effect of galaxy feedback on the IGM.

Observational Evidence for Large-scale Gas Heating in a Galaxy Protocluster at z = 2.30

“…Around COSTCO-I, we do not find a notable trend between the transmission and density; the δ F values remain almost constant. This is reminiscent of the flat transmission-density relations that have high levels of preheating, as studied by Kooistra et al (2022b). However, more quantitative studies involving the transmission-density relationship would require large observational samples and more careful modeling of systematics.…”

“…Their results showed that a significant entropy floor of K floor  50 keV cm −3 would be required to cause increased Lyα transmission to δ F ∼ 0 in z ∼ 2 protoclusters. Kooistra et al (2022b) were agnostic on the possible mechanisms that could cause such preheating. Over the years, however, there has been 3. of F pc d derived from the method in Section 3.2.…”

“…They found that for a uniform UV background, the effect of collisional ionization is negligible on the smoothed 3D Lyα forest signal associated with protoclusters. Kooistra et al (2022b), on the other hand, performed zoom-in hydrodynamical simulations on a set of galaxy protoclusters with various phenomenological preheating prescriptions of the protocluster gas. They did, however, perform fiducial runs where gas hydrodynamics was in effect, but no feedback or preheating was applied.…”

“…The second possibility for the large-scale heating is that feedback processes from protocluster galaxies or active galactic nuclei (AGN) are responsible for the reduced absorption in COSTCO-I. Kooistra et al (2022b) applied a simple phenomenological preheating model to the protocluster gas, which imposed an entropy floor, K floor , such that the protocluster gas cells at z = 3 have internal gas entropy values of T n K e 2 3 floor > -, where T is the gas temperature and n e is the electron density (see also Borgani & Viel 2009). Their results showed that a significant entropy floor of K floor  50 keV cm −3 would be required to cause increased Lyα transmission to δ F ∼ 0 in z ∼ 2 protoclusters.…”

Second Data Release of the COSMOS Lyα Mapping and Tomography Observations: The First 3D Maps of the Detailed Cosmic Web at 2.05 < z < 2.55

“…TARDIS is a fundamentally different approach than WF reconstruction since the resulting density fields are constrained to evolve from gravitational evolution and the hydrodynamical properties are determined via the fluctuating Gunn-Peterson approximation; see Section 4.3. Substantial differences in the field reconstructed between these two methods could be an indication of strong deviations from simple analytical approximations for the IGM flux-density relationship due to strong feedback within cluster environments (see, e.g., Kooistra et al 2022), or, for example, possible galaxy quenching in overdense environments (Newman et al 2022). In the case of mock catalogs which include hydrodynamical effects, TARDIS has shown to be still highly accurate in classification of cosmic structures at the resolution available to the CLAMATO survey (Horowitz et al 2021); see also Horowitz et al (2019) for a discussion of TARDIS reconstruction versus WF in the absence of hydrodynamical effects.…”