Exploring 21 cm-Lyα Emitter Synergies for SKA

Abstract

We study the signatures of reionization and ionizing properties of the early galaxies in the cross-correlations between the 21cm emission from the spin-flip transition of neutral hydrogen (H I ) and the underlying galaxy population, in particular a sub-population of galaxies visible as Lyman Alpha Emitters (LAEs). With both observables simultaneously derived from a z 6.6 hydrodynamical simulation (GADGET-2) snapshot post-processed with a radiative transfer code (pCRASH) and a dust model, we perform a parameter study and aim to constrain both the average intergalactic medium (IGM) ionization state (1 − χ HI ) and the reionization topology (outside-in versus inside-out). We find that in our model LAEs occupy the densest and most-ionized regions resulting in a very strong anti-correlation between the LAEs and the 21cm emission. A 1000 h SKA-LOW1 -Subaru Hyper Suprime Cam experiment can provide constraints on χ HI , allowing us to distinguish between IGM ionization levels of 50%, 25%, 10% and fully ionized at scales r < ∼ 10 comoving Mpc (assuming foreground avoidance for SKA). Our results support the inside-out reionization scenario where the densest knots (under-dense voids) are ionized first (last) for χ HI > ∼ 0.1. Further, 1000 h SKA-LOW1 observations should be able to confirm the inside-out scenario by detecting a lower 21cm brightness temperature (by about 2-10 mK) in the densest regions ( > ∼ 2 arcminute scales) hosting LAEs compared to lower-density regions devoid of them.

Survey parameters for detecting 21-cm-Ly α emitter cross-correlations with the Square Kilometre Array

“…The latter decreases the contrast between δT b at LAE locations and δT b , leading to a weaker anti-correlation. However, the anti-correlation strength also depends on the residual H I fraction within the ionized regions around LAEs (Hutter et al 2017). With decreasing fesc, the photoionization rate (ΓHI) drops and the residual H I fraction increases, which causes a slightly weaker anticorrelation for fesc = 0.05 than for 0.5.…”

“…Given that the reionization state and topology will be hard to interpret from either dataset alone, recent efforts have focused on investigating the power of cross correlations between the 21cm signal and LAEs (Wyithe & Loeb 2007;Vrbanec et al 2016;Sobacchi et al 2016;Hutter et al 2017;Heneka et al 2017;Wiersma et al 2013). Indeed, at a given χHI the amplitude of the 21cm-LAE cross correlation function on small scales is very similar for different reionization and LAE models (cf.…”

Probing the Intergalactic Medium with Lyα and 21 cm Fluctuations

“…For example, as shown in Hutter et al (2017), when coupling N-body/SPH simulations (Springel et al 2001;Springel 2005) with radiative transfer code (Partl et al 2011), a negative cross-correlation shows up when cross-correlating 21cm and Lyα fluctuations that breaks the parameter degeneracies present in reionization models for power spectra alone. Also, the cross-correlation of 21cm emission and Lyα emitters improves constraints on the mean ionized fraction (Sobacchi et al 2016).…”

Fundamental physics with the Square Kilometre Array

“…Combining data on the population of source galaxies with the global brightness temperature signal measured with SKA at these epochs, the fraction of reionisation that is caused by the galaxies can be constrained (Cohen et al 2017). Cross-correlation of the SKA brightness temperatures with the LAE/LBG samples from galaxy surveys provides additional constraints on the reionisation history, for example, to what extent different galaxy populations contribute to reionisation, the evolution of the ionisation fraction, and the topology of reionisation (Hutter et al 2017). The brightness temperature can further be correlated with the properties of galaxies directly, for example, from the Euclid or LSST wide+deep surveys (Bacon et al 2015).…”