Lyα Nebulae in HETDEX: The Largest Statistical Census Bridging Lyα Halos and Blobs across Cosmic Noon

Abstract

The Hobby-Eberly Dark Energy Experiment (HETDEX) is an untargeted ~540 deg^2 spectroscopic survey of Lyα emission in the 1.9 < z < 3.5 Universe. In surface brightness, this survey reaches 1σ Lyα sensitivities of approximately 2-5 x 10^-18 erg s^-1 cm^-2 arcsec^-2, allowing large samples of extended Lyα nebulae (LAN) to be studied. We selected a sample of 70,691 Lyα-emitting galaxies (LAEs) with an emission-line signal-to-noise ratio greater than 6 and modeled the Lyα emission as a point-source component with an optional exponential envelope. Half (~47.5%) of the LAE sample (33,612 objects) exhibits significant extended emission and is best fit by the two-component model. The fraction of resolved sources increases with Lyα flux and luminosity. Their isophotal areas range from 10-130 arcsec^2 (median 15 arcsec^2), with integrated Lyα fluxes from 6-2000 x 10^-17 erg s^-1 cm^-2 (median 20 x 10^-17 erg s^-1 cm^-2). Comparison between point-spread-function-weighted and isophotal flux measurements shows that the HETDEX pipeline underestimates the total Lyα flux by ~30% on average, reflecting the substantial halo contribution in extended sources. Approximately 420 LANs are found per deg^2 over 79.5 deg^2 of non-contiguous sky. About 12% of resolved sources show active galactic nuclei signatures and are bright in Lyα and continuum. The remaining 88% span a wide range of morphologies and often lack continuum counterparts. Exponential scale lengths show no strong correlation with Lyα flux or luminosity (median 11.6 +/- 1.9 kpc). Only 2.9% of the full S/N > 6 LAE population with ancillary data have radio counterparts, but 64% of those are found to be extended, with the radio fraction increasing with Lyα size.

Figures (14)

• Figure 1: Sky distribution of HETDEX IFU observations and Ly$\alpha$ Nebula (LAN) sources across six HETDEX fields. The average sky number density of resolved LAEs is 420 deg$^{-2}$ (for the redshift range $1.9<z<3.5$ covered by HETDEX). Each panel shows the HETDEX IFU footprints in light gray (with each tile corresponding to a $51\hbox{$^{\prime\prime}$} \times 51\hbox{$^{\prime\prime}$}$ IFU), overlaid with spectroscopically confirmed LANs color coded by redshift (from $\texttt{z\_hetdex}=1.9$ to $\texttt{z\_hetdex}=3.5$). The top two panels display the wide-area Spring (dex-spring) and Fall fields (dex-fall), with rectangular red outlines marking the approximate survey boundaries; these two regions encompass approximately 390 and 150 deg$^2$ of sky area, respectively. The bottom four panels show targeted deep fields: COSMOS, GOODS-N, SA22, and the North Ecliptic Pole (NEP). The scale bars in the lower left of each panel indicate angular size.
• Figure 2: $1\sigma$ surface–brightness sensitivity measured from the pixel variance in the continuum-subtracted line-flux maps and converted into intrinsic luminosity surface–density limits for individual HETDEX IFU observations. Left: the observed--frame $1\sigma$ Ly$\alpha$ surface brightness, $\mathrm{SB}_{1\sigma}^{\mathrm{Ly}\alpha}$ (in units of $10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$), as a function of observed wavelength. Each point represents an IFU exposure and is color coded by the image quality (FWHM) in arcseconds. The solid line traces the median trend in 50 Å bins, showing increased sensitivity toward the blue due to higher instrumental throughput and lower sky background. Middle: distribution of $\mathrm{SB}_{1\sigma}^{\mathrm{Ly}\alpha}$ values across all exposures, typically ranging from 2–10$\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. Right: histogram of the intrinsic luminosity surface–density limit, $\Sigma_{L,\mathrm{lim}}^{\mathrm{Ly}\alpha}$, derived from each exposure using the luminosity distance and physical scale at its redshift. These limits correspond to $\log_{10}\Sigma_{L,\mathrm{lim}}^{\mathrm{Ly}\alpha}\sim39.5$–40.1 erg s$^{-1}$ kpc$^{-2}$, illustrating the range of Ly$\alpha$ sensitivity achieved across the survey.
• Figure 3: Multi-wavelength view of HLAN 4025592924 at $\texttt{z\_hetdex}=2.57$. Situated in the COSMOS Deep Field, this LAN is among the largest in the HETDEX sample, with an isophotal radius of 45.9 kpc -- placing it in the top 2% of the distribution. Despite its size, the structure's Ly$\alpha$ luminosity is moderate at log_L_lya = 43.8 erg s$^{-1}$, and it is not identified as an AGN. Top: DESI DR1 (TARGETID 39627829524040740; desi2desidr1) 1D spectrum on the central bright source (white '+' symbol). The lack of C$\;$ suggests the source is not AGN dominated. Main:$30\hbox{$^{\prime\prime}$}\times30\hbox{$^{\prime\prime}$}$ JWST/NIRCam three-colour composite (blue = $\tfrac{1}{2}$(F115W + F150W), green = F277W, red = F444W) from the COSMOS-Web DR1 mosaics cosmos-webNIRCAM2025. The Ly$\alpha$ line-flux map (see Section \ref{['sec:nbimages']}) from the HETDEX data cube is over-plotted as contours (levels $3$–$15\,\sigma$). The top-left inset shows 1D Ly$\alpha$ profiles extracted along the positions marked by “+’’ symbols in the image. A $30$ kpc scale bar is indicated at lower left. Right column:$30\hbox{$^{\prime\prime}$}\times30\hbox{$^{\prime\prime}$}$ postage stamp images (top to bottom) from Subaru/HSC-$r$ band, and JWST filters F115W, F150W, F277W and F444W reveal that multiple low-mass galaxies accompany the LAN.
• Figure 4: Examples of some of the largest extended Ly$\alpha$-emitters. Photometric imaging from HSC-$r$ is shown on the left with the 2$\sigma$ boundary of the Ly$\alpha$ emission shown as a dashed red contour. The second column displays the Ly$\alpha$ line-flux map centered on the wavelength listed in white text. The third column presents the radial surface-brightness profile in blue and our best-fit two-component model (PSF core + 2D exponential) in red. The dashed green line is the measured PSF from stars in the same observation as the LAN. The HETDEX/VIRUS spectrum for the central HETDEX detection is given in the rightmost panel. This spectrum is the PSF-weighted spectrum from the HETDEX pipeline. The spectral width of the line-flux map is highlighted in yellow on the spectrum. The line shapes are asymmetrical, and some appear to have multiple associated continuum counterparts in HSC-$r$ images.
• Figure 5: Top four panels: examples of extended Ly$\alpha$-emitters with a strong central AGN counterpart. The panels have the same format as described in Figure \ref{['fig:ex1']}. Unlike the sources in Figure \ref{['fig:ex1']}, the peak Ly$\alpha$-emission coincides with peak continuum emission in accompanying photometric data. Bottom Two Panels: examples of bright AGN that prefer a single-component, point-source model. These are not considered extended by our method.