- The paper demonstrates that deep WFC3 imaging in the HUDF identifies 11 primary z'-drop candidates consistent with galaxies at z≈7–9.
- The paper finds a star formation rate density significantly lower than at z≈6, suggesting that observed galaxies alone cannot fully reionize the IGM without high ionizing photon escape fractions.
- The paper highlights challenges in detecting faint galaxies and underscores the need for refined models to fully account for cosmic reionization processes.
Contribution of High Redshift Galaxies to Cosmic Reionization: Insights from the Hubble Ultra Deep Field
The paper by Bunker et al. examines the role of high redshift galaxies in the context of cosmic reionization, leveraging deep imaging data acquired through the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). This research explores the identification of candidate star-forming galaxies in the redshift range z≈7−10 using the Lyman-break galaxy (LBG) technique applied to the Hubble Ultra Deep Field (HUDF), with a focus on the Y-, J-, and H-band images.
Identification of High-Redshift Candidates
The paper identifies galaxies with prominent Lyman-break features between the z′-band (ACS) and the Y-band (WFC3), which is consistent with galaxies at z≈6.7−8.8. By imposing a color selection criterion of (z′−Y)AB>1.3 down to limiting magnitudes of YAB<28.5, the team identified 12 candidate z′-drop galaxies. Detailed examination revealed that one candidate is likely a T-dwarf star, reducing the primary sample of interest to 11 candidates. Notably, these z′-drop candidates display bluer spectral slopes than their lower-redshift counterparts.
The star formation rate density (SFRD) calculated for these z′-drop candidates is approximately 0.0017−0.004M⊙yr−1Mpc−3, after accounting for various corrections. This observed SFRD is less than half of that at z≈6 and an order of magnitude lower than the SFRD observed for LBGs at z=3−4, suggesting significant evolution in star-formation activity from z=7 to lower redshifts.
Contribution to Cosmic Reionization
The research further assesses whether the star formation rate density is sufficient to account for cosmic reionization. To ionize the intergalactic medium (IGM) at z≈7, a specific density of Lyman continuum photons must escape from galaxies. The inferred SFRD for the z′-drop galaxies in this paper appears inadequate for reionization purposes, unless several conditions are met—namely, a high escape fraction of ionizing photons (fesc>0.5), a low clumping factor for neutral hydrogen in the IGM, and potential contributions from a large population of faint, undetected galaxies. Alternative explanations could involve stellar populations with low metallicity or a top-heavy initial mass function, both of which could boost the ionizing photon output.
Future Prospects and Challenges
The detection of Y-band drop-outs potentially corresponding to z≈8 galaxies demonstrates progress in pushing LBG detections to higher redshifts, although no J-drops were found at z≈10. This absence implies either a rapid evolution in the luminosity function beyond z≈7 or challenges in faint galaxy detection. These findings compel researchers to refine models of early star formation and cosmic reionization.
In conclusion, Bunker et al. present compelling evidence that high-redshift galaxies identified in the HUDF contribute to our understanding of the early universe and cosmic reionization processes. However, the findings also underscore current limitations in our ability to comprehensively account for reionization solely through observable galaxies, directing future explorations towards understanding the faint end of the luminosity function and alternative astrophysical processes.