Galaxy formation and evolution

Abstract:

We present the survey design and initial results from the Parallel Ionizing Emissivity (PIE) survey. PIE is a large Hubble Space Telescope survey designed to detect Lyman continuum (LyC) emitting galaxies at 3.1 < z < 3.5 and stack their images in order to measure average LyC escape fractions as a function of galaxy properties. PIE has imaged 37 independent fields in three filters (F336W, F625W, and F814W), of which 18 are observed with a fourth band (F475W) from the accompanying PIE+ program. We use photometric colors to select candidate Lyman break galaxies (LBGs) at 3.1 < z < 3.5, which can be followed up using ground-based spectrographs to confirm their redshifts. Unlike previous surveys, we use many independent fields to remove biases caused by correlated absorption in the intergalactic medium (IGM). In this paper, we describe the survey design, photometric measurements, and the use of mock galaxy samples to optimize our color selection. With three filters, we can select a galaxy sample of which ≈90% are LBGs and over 30% lie in the 3.1 < z < 3.5 range for which we can detect uncontaminated LyC emission in F336W. We also use mock IGM sight lines to measure the expected transmission of the IGM, which will allow us to determine escape fractions from our stacked galaxies. We color-select ≈1400 galaxies, and predict that this includes ≈80 LyC-emitting galaxies and ≈500 that we can use in stacking. Finally, we present the Keck/LRIS spectrum of a galaxy at z ≈ 2.99, demonstrating that we can confirm the redshifts of z ∼ 3 galaxies from the ground.

Abstract:

Cosmic ray (CR) feedback has been proposed as a powerful mechanism for driving warm gas outflows in galaxies. We use cosmological magnetohydrodynamic simulations to investigate the impact of CR feedback on neutral hydrogen (H_I) in a 10¹¹M⊙ dark matter halo at 2<z<4. To this end, we post-process the simulations with ionizing radiative transfer and perform Monte Carlo Lyman-α (Lyα) transfer calculations. CR feedback reduces H_I column densities around young stars, thereby allowing more Lyα photons to escape and consequently offering a better match to the Lyα luminosities of observed Lyα emitters. Although galaxies with CR-driven outflows have more extended H_I in the circumgalactic medium, two Lyα line properties sensitive to optical depth and gas kinematics - the location of the red peak in velocity space (v_red) and relative strength of the blue-to-red peaks (B/R) - cannot distinguish between the CR-driven and non-CR simulations. This is because Lyα photons propagate preferentially along low HI density channels created by the ionizing radiation, thereby limiting the scattering with volume-filling H_I. In contrast, the observed low flux ratios between the valley and peak and the surface brightness profiles are better reproduced in the model with CR-driven outflows because the Lyα photons interact more before escaping, rather than being destroyed by dust as is the case in the non-CR simulation. We discuss the potential cause of the paucity of sightlines in simulations that exhibit prominent red peaks and large v_red, which may require the presence of more volume-filling H_I.