Lyα Intensity Mapping in HETDEX: Galaxy-Lyα Intensity Cross-Power Spectrum

TL;DR

This study presents the first Lyα LIM cross-power spectrum between Lyα emitters and Lyα intensity in HETDEX over z ≈ 2–3.5, using undetected Lyα intensity and a comprehensive forward-model with lognormal mocks to capture nonlinear redshift-space distortions, sky-subtraction transfer losses, and survey masks. The authors detect the cross-power and constrain the product b_g b_I ⟨I⟩ F_RSD / F_RSD^fid across three redshift bins, finding results broadly consistent with Lyα radiative-transfer simulations but smaller than QSO-based cross-correlations, implying a substantial role for star-forming galaxies in Lyα emission. They demonstrate the statistical power of HETDEX for LIM while highlighting data-processing and modeling improvements needed for a more detailed physical interpretation, including Lyα absorption around LAEs and diffuse CGM/IGM emission. The work provides a valuable data point for constraining Lyα luminosity density and offers a framework to calibrate Lyα emission in galaxy-formation simulations and radiative-transfer studies.

Abstract

We present a measurement of the Lyman-$α$ (Ly$α$) intensity mapping power spectrum from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). We measure the cross-power spectrum of the Ly$α$ intensity and Ly$α$-emitting galaxies (LAEs) in a redshift range of $1.9 < z < 3.5$. We calculate the intensity from HETDEX spectra that do not contain any detected LAEs above a signal-to-noise ratio of $5.5$. To produce a power spectrum model and its covariance matrix, we simulate the data using lognormal mocks for the LAE catalog and Ly$α$ intensity in redshift space. The simulations include the HETDEX sensitivity, selection function, and mask. The measurements yield the product of the LAE bias, the intensity bias, the mean intensity of undetected sources, and the ratio of the actual and fiducial redshift-space distortion parameters, $b_\mathrm{g} b_I \langle I \rangle \bar{F}_{\rm RSD} / \bar{F}^{\rm fid}_{\rm RSD}= (6.7 \pm 3.1)$, $(11.7 \pm 1.4)$, and $(8.3 \pm 1.5) \times 10^{-22} \, \text{erg}\, \text{s}^{-1} \, \text{cm}^{-2} \, \text{arcsec}^{-2} \, \text{Å}^{-1}$ in three redshift bins centered at $\bar z=2.1$, 2.6, and 3.2, respectively. The results are reasonably consistent with cosmological hydrodynamical simulations that include Ly$α$ radiative transfer. They are, however, significantly smaller than previous results from cross-correlations of quasars with Ly$α$ intensity. These results demonstrate the statistical power of HETDEX for Ly$α$ intensity mapping and pave the way for a more comprehensive analysis. They will also be useful for constraining models of Ly$α$ emission from galaxies used in modern cosmological simulations of galaxy formation and evolution.

Figures (13)

• Figure 1: IFU coordinates in the Fall (top), Spring (middle), and NEP (bottom) fields. The fields are divided into $\ang{5.6;;}$ wide regions for the power spectrum, shown as black squares (see Section \ref{['sec:map_creation']}). Masked maps are shown as hatched regions (see Section \ref{['subsec:covariance_and_weights']}). The backward-facing (forward-facing) diagonal hatchings represent boxes masked in the low-$z$ (medium-$z$) bin, and the dotted hatchings indicate those masked in the high-$z$ bin.
• Figure 2: The normalized distributions of redshifts (left), luminosities (middle), and the best-fit Gaussian $\sigma$ line widths in observed wavelength units (right) of HETDEX LAEs in the Fall (blue), Spring (orange), and NEP (green) fields. The black dashed lines on the left panel show the boundaries of the three redshift bins used to calculate the power spectra.
• Figure 3: First seven weight vectors, $\hat{\mathbf{e}}^\mathrm{PCA}_j$ with $j=1-7$, of the Fall field are displayed as functions of wavelength. All $\hat{\mathbf{e}}^\mathrm{PCA}_j$ are normalized vectors. For comparison, we also show an example 'full-field' sky spectrum in counts (third panel on the right). Masked wavelength regions are shown in gray. The bottom right panel shows the first $200$ eigenvalues, which are proportional to the variance along the corresponding weight vectors.
• Figure 4: Standard deviation of the IFU spectra in each wavelength slice in the Fall (blue), Spring (orange), and NEP (green) fields compared to the pure-noise expectation inferred from Figure 18 of hill/etal:2021 (black). The dashed colored lines show the standard deviation of the IFU spectra without PCA ($N_\mathrm{PC} = 0$). The solid colored lines are the standard deviation of the IFU spectra after PCA cleaning, where $N_\mathrm{PC} = 170$ in the low-$z$ bin, $N_\mathrm{PC} = 130$ in the medium-$z$ bin, and $N_\mathrm{PC} = 100$ in the high-$z$ bin. The vertical black dashed lines indicate the boundaries of the three redshift bins used for power spectrum measurements. The gray shaded areas show masked wavelength values.
• Figure 5: (Left) Suppression of the power spectrum monopole due to PCA cleaning. We show the mean power spectrum monopole obtained from the fiducial mocks in the Spring field in the medium-$z$ bin. The different colors correspond to different values of $N_\mathrm{PC}$, as indicated in the color bar. Here, $u_I = 10^{-18}\,\mathrm{erg\, s^{-1}\, cm^{-2}\, arcsec^{-2}\,}\hbox{\normalfont\AA}^{-1}$. (Right) The ratio of the power spectrum monopole with and without $N_\mathrm{PC}$ PCs removed as a function of $k$ and $N_\mathrm{PC}$.