First Identification of 10-kpc Scale [CII] 158um Halos around Star-Forming Galaxies at z=5-7

Abstract: We report the discovery of 10-kpc scale [CII] 158um halos surrounding star-forming galaxies in the early Universe. We choose deep ALMA data of 18 galaxies each with a star-formation rate of ~ 10-70 Msun with no signature of AGN whose [CII] lines are individually detected at z=5.153-7.142, and conduct stacking of the [CII] lines and dust-continuum in the uv-visibility plane. The radial profiles of the surface brightnesses show a 10-kpc scale [CII] halo at the 9.2sigma level significantly extended more than the HST stellar continuum data by a factor of ~5 on the exponential-profile basis, as well as the dust continuum. We also compare the radial profiles of [CII] and Lya halos universally found in star-forming galaxies at this epoch, and find that the scale lengths agree within the 1sigma level. While two independent hydrodynamical zoom-in simulations match the dust and stellar continuum properties, the simulations cannot reproduce the extended [CII] line emission. The existence of the extended [CII] halo is the evidence of outflow remnants in the early galaxies and suggest that the outflows may be dominated by cold-mode outflows expelling the neutral gas.

Overview of Extended [CII] Halo Study Surrounding Star-Forming Galaxies at z=5-7

The study by Fujimoto and colleagues presents the discovery of extended 10-kpc [CII] 158μm halos around star-forming galaxies at redshifts between 5 and 7. Utilizing deep ALMA observations, the research investigates the spatial distribution of [CII] emission compared to rest-frame UV and FIR continuum emissions. The findings elucidate significant implications for understanding galaxy formation and evolution in the early universe, highlighting discrepancies with current simulations and theoretical models.

Key Results

1. Detection of Extended [CII] Halos:
   The study reports the presence of 10-kpc scale [CII] halos around star-forming galaxies with high confidence (9.2σ level) through stacking of ALMA data from 18 galaxies at redshifts z=5.153-7.142. The [CII] emission is notably more extended than the corresponding UV and FIR continuum emissions.

2. Comparison with Simulations:
   Despite consistency in the total extent of the [CII], FIR, and UV emissions, two state-of-the-art cosmological hydrodynamical simulations fail to replicate the observed extended distribution of [CII] emission. This mismatch suggests that current models may not fully incorporate all relevant physical processes, such as environmental effects or detailed feedback mechanisms in the CGM.

3. Surface Brightness Profiles:
   Radial surface brightness profiles derived from the stacked data reveal that the [CII] halos extend approximately five times beyond the stellar continuum in the galaxy's core, suggesting a significant contribution from the circumgalactic medium (CGM) or other galaxy interactions.

4. Implications on Metal Enrichment:
   The presence of extended [CII] halos implies an early enrichment of the CGM with metals, likely from starburst-driven outflows. The study posits that cold-mode outflows may be predominant in expelling neutral gas from early star-forming galaxies, enriching the CGM with carbon even at these early cosmic times.

5. [CII] and Lyα Line Comparisons:
   Comparisons between [CII] and Lyα halos show comparable scale lengths, suggesting potential physical connections. The Lyα halos are typically associated with scattering off neutral hydrogen in the CGM, and the [CII] results may indicate similar diffuse structures contributing to halo formation.

Theoretical and Observational Challenges

The study points out the need for refinement in simulations addressing early galaxy evolution and CGM characteristics. The current inability of simulations to fully capture the extent and properties of [CII] emission underscores potential gaps in our understanding of feedback processes and the thermal state of the early universe's gaseous components.

Future Directions:
- Improving simulation fidelity by incorporating complex feedback and metal cooling processes could provide better alignment with observational data.
- Further observational deep field studies, combining multi-wavelength data, are necessary to continue unraveling the conditions and processes governing CG gas and metal distributions.

The study sets foundational knowledge for further inquiries into the nature of early universe galaxies, their evolution, and the mechanisms driving metal enrichment on cosmological scales. Continued advances in ALMA capabilities and theoretical modeling promise to refine these early insights into more comprehensive understandings.