Diffuse Lyman Alpha Emitting Halos: A Generic Property of High Redshift Star Forming Galaxies

Abstract: Using a sample of 92 UV continuum-selected, spectroscopically identified galaxies with <z> = 2.65, all of which have been imaged in the Ly-a line with extremely deep narrow-band imaging, we examine galaxy Ly-a emission profiles to very faint surface brightness limits. The galaxies are representative of spectroscopic samples of LBGs at similar redshifts in terms of apparent magnitude, UV luminosity, inferred extinction, and star formation rate, and were selected without regard to Ly-a emission properties. We use extremely deep stacks of UV continuum and Ly-a emission line images to show that all sub-samples exhibit diffuse Ly-a emission to radii of at least 10" (80 physical kpc), including galaxies whose spectra exhibit Ly-a in net absorption. The intensity scaling, but not the surface brightness distribution, is strongly correlated with the emission observed in the central ~1". The characteristic scale length for Ly-a line emission exceeds that of the UV continuum light for the same galaxies by factors of at least 5-10, regardless of the spectral morphology of Ly-a. Including the extended Ly-a halos increases the total Ly-a flux [and rest equivalent width W_0(Lya)] by an average factor of 5. We argue that most, if not all, of the observed Ly-a emission in the diffuse halos originates in the galaxy H II regions and is scattered in our direction by H I gas in the galaxy's circum-galactic medium (CGM). We show that whether or not a galaxy is classified as a giant "Lyman-a Blob" (LAB) depends sensitively on the Ly-a surface brightness threshold reached by an observation. Accounting for diffuse Ly-a halos, all LBGs would be LABs if surveys were routinely sensitive to 10 times lower surface brightness thresholds; also, essentially all LBGs would qualify as LAEs (W_0(Lya) > 20 A).

• The paper demonstrates that nearly all high-redshift LBGs exhibit diffuse Lyα emission extending to at least 80 kpc.
• It employs deep narrow-band imaging and spectroscopy to show that Lyα scale lengths are 5–10 times larger than UV continuum.
• The findings underscore the role of CGM scattering in shaping Lyα profiles, calling for improved detection and modeling techniques.

Overview of Diffuse Lyman Alpha Emitting Halos in High-Redshift Galaxies

The paper by Steidel et al. presents an exhaustive investigation into the presence and characteristics of diffuse Lyman Alpha (Lyα) emitting halos in star-forming galaxies at high redshifts. Utilizing a sample of 92 Lyman Break Galaxies (LBGs) at a mean redshift of 2.65, the research employs deep narrow-band imaging combined with spectroscopic data to explore the nature of Lyα emission at substantial physical extents. The findings uniformly indicate that diffuse Lyα emission is a pervasive attribute of such galaxies, irrespective of their spectral classifications.

The analysis reveals that Lyα halos extend to physical radii of at least 80 kpc, significantly surpassing the typical spatial distribution of UV continuum light, which demonstrates a marked compactness. This discrepancy is quantitatively supported by the observation that the exponential scale lengths of Lyα emission are 5-10 times greater compared to the UV continuum. Notably, this extended emission manifests even in galaxies where Lyα typically appears in absorption within spectroscopic measurements.

These findings have profound implications for the classification of galaxies based on Lyα properties. If observational surveys were sensitive enough to detect Lyα emission at lower surface brightness thresholds, essentially all LBGs would qualify as Lyman Alpha Emitters (LAEs), and most would be categorized as Lyman Alpha Blobs (LABs). This suggests that current detections potentially underestimate the prevalence of such phenomena due to limitations in sensitivity.

The study proposes that this widespread Lyα emission primarily originates from HII regions within the galaxies, subsequently being diffusely scattered by neutral hydrogen in the circum-galactic medium (CGM). This hypothesis is bolstered by the observation that the intensity of Lyα halos correlates with the spectral morphology of the galaxies, implying that structural and kinematic factors of the CGM are influential in shaping the observed emission profiles.

Furthermore, the consideration of dust and gas-phase kinematics substantially impacts the emergent Lyα surface brightness. The research indicates that spatial diffusion effects, resulting from resonant scattering, may strongly modulate the emergent emission, reinforcing the need for comprehensive radiative transfer modeling to fully encapsulate these processes.

This work suggests an intrinsic link between Lyα emission properties and the interstellar medium (ISM), with both being sensitive to the existing gas-phase geometry and kinematics. Consideration of dust extinction, albeit significant, may ultimately be secondary to these geometric configurations in influencing the detectability of Lyα.

The implications of these findings are multifaceted, affecting both practical observation strategies and theoretical modeling of galaxy formation and evolution. For observational astronomy, they underline the necessity of enhanced detection thresholds in narrow-band imaging to accurately assess the prevalence of Lyα emission. Theoretically, they call for refined simulations that integrate accurate models of CGM gas dynamics, allowing for a nuanced understanding of Lyα scattering and absorption processes.

In summary, this paper deepens our comprehension of high-redshift star-forming galaxies by elucidating the commonplace nature of diffuse Lyα halos, thereby opening new avenues for characterizing these primordial structures and their evolving interstellar environments. Future research may leverage these insights to unravel the complex interactions between stellar activity, ISM, and the broader cosmic web during critical epochs of galaxy formation.