Radiative transfer distortions of Lyman α emitters: a new Fingers-of-God damping in the clustering in redshift space

Abstract

Complex radiative transfer (RT) of the Lyman-α photons poses a theoretical challenge to galaxy surveys which infer the large-scale structure with Lyman-α emitters (LAEs). Guided by RT simulations, prior studies investigated the impact of RT on the largescale LAE clustering, and claimed that RT induces a selection effect which results in an anisotropic distortion even in real space but in an otherwise negligible effect in redshift space. However, our previous study, which relies on a full radiative transfer code run on the Illustris simulations, shows that the anisotropic selection effect was drastically reduced with higher spatial resolution. Adopting the same simulation framework, we further study the impact of RT on the LAE clustering in redshift space. Since we measure LAE's radial position through a spectral peak of a Lyman-α emission, the frequency shift due to RT contaminates the redshift measurement and hence the inferred radial position in redshift space. We demonstrate that this additional RT offset suppresses the LAE clustering along the line of sight, which can be interpreted as a novel Fingers-of-God (FoG) effect. To assess the FoG effect, we develop a theoretical framework with particular emphasis on its connection with the underlying one-point velocity distribution which simultaneously takes into account the RT offset as well as the peculiar velocity that is commonly studied in the context of the Redshift Space Distortion (RSD). Although our findings strongly encourage a more careful RSD modeling in LAE surveys, we also seek a method to mitigate the additional FoG effect due to RT by making use of other information in a Lyman-α spectrum.

Determining the systemic redshift of Lyman α emitters with neural networks and improving the measured large-scale clustering

“…the Ly α wavelength is absorbed (e.g. Laursen, Sommer-Larsen & Razoumov 2011;Zheng et al 2011;Byrohl, Saito & Behrens 2019;Gurung-López et al 2020).…”

“…Along a similar line, Byrohl et al (2019) studied how the incorrect systemic redshift determination distorts the clustering of LAEs along the line of sight. In detail, an inaccurate Ly α wavelength determination is translated into an imprecise redshift, thus into an uncertainty in its radial position in redshift space.…”

“…In fact, this shuffling in the LAE radial position can be interpreted as a extra random radial velocity dispersion component, which translates into an additional 'Fingerof-God' suppressing the apparent clustering along the line of sight. Furthermore, Byrohl et al (2019) found that the clustering distortion was mitigated after correcting the shift of the peak with different recipes, such as the those in Verhamme et al (2018). However, the developed recipes in the literature (Verhamme et al 2018;Byrohl et al 2019;Muzahid et al 2019) to estimate the Ly α wavelength have limitations as the dispersion of the estimated Ly α wavelength was around 1 Å (in rest frame).…”

“…Furthermore, Byrohl et al (2019) found that the clustering distortion was mitigated after correcting the shift of the peak with different recipes, such as the those in Verhamme et al (2018). However, the developed recipes in the literature (Verhamme et al 2018;Byrohl et al 2019;Muzahid et al 2019) to estimate the Ly α wavelength have limitations as the dispersion of the estimated Ly α wavelength was around 1 Å (in rest frame). Such a large scatter introduces significant distortions to the apparent clustering of LAEs on scales ∼ 5 cMpc h −1 in the monopole and up to k 0.1 h cMpc −1 in Fourier space, as we will explicitly show in this paper.…”

Variations in shape among observed Lyman-α spectra due to intergalactic absorption

“…While recent studies have focused mainly on the intragalactic Lyα transfer (e.g., Smith et al 2019), less attention has been devoted to the effect of the IGM on the Lyα spectral shape. Even large-scale studies that include the IGM have focused primarily on global statistics, such as the Lyα emitter clustering or luminosity function (e.g., Iliev et al 2008;Zheng et al 2010;Behrens et al 2018;Byrohl et al 2019). In this paper, we seek to clarify the IGM's impact on the Lyα spectra using using a recent cosmological simulation in the redshift range of z = 0−5.…”