The Contribution of High Redshift Galaxies to Cosmic Reionization: New Results from Deep WFC3 Imaging of the Hubble Ultra Deep Field (0909.2255v3)

Abstract: We have searched for star-forming galaxies at z~7 by applying the Lyman-break technique to newly-released 1.1micron Y-band images from WFC3 on HST. By comparing these images of the Hubble Ultra Deep Field with the ACS z'-band (0.85micron), we identify objects with red colours, (z'-Y)_AB>1.3), consistent with the Ly-alpha forest absorption at z~6.7-8.8. We identify 12 of these z'-drops down to a limiting magnitude Y_AB<28.5 (equivalent to a star formation rate of 1.3M_sun/yr at z=7.1), which are undetected in the other ACS filters. We use the WFC3 J-band image to eliminate contaminant low mass Galactic stars, which typically have redder colours than z~7 galaxies. One of our z'-drops is a probably a T-dwarf star. The z~7 z'-drops have much bluer spectral slopes than Lyman-break galaxies at lower redshift. Our brightest z'-drop is not present in the NICMOS J-band image of the same field taken 5 years before, and is a possible transient object. From the 10 remaining z~7 candidates we determine a lower limit on the star formation rate density of 0.0017M_sun/yr/Mpc^3 for a Salpeter initial mass function, which rises to 0.0025-0.0034M_sun/yr/Mpc^3 after correction for luminosity bias. The star formation rate density is a factor of ~10 less than that at z=3-4, and is about half the value at z~6. While based on a single deep field, our results suggest that this star formation rate density would produce insufficient Lyman continuum photons to reionize the Universe unless the escape fraction of these photons is extremely high (f_esc>0.5), and the clumping factor of the Universe is low. Even then, we need to invoke a large contribution from galaxies below our detection limit. The apparent shortfall in ionizing photons might be alleviated if stellar populations at high redshift are low metallicity or have a top-heavy IMF.

• 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 $z\approx 7-10$ 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 $z\approx 6.7-8.8$. By imposing a color selection criterion of $(z'-Y)_{AB}>1.3$ down to limiting magnitudes of $Y_{AB}<28.5$, 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.

Implications for Star Formation Rate Density

The star formation rate density (SFRD) calculated for these $z'$-drop candidates is approximately $$0.0017-0.004\,M_{\odot}\,{\mathrm yr}^{-1}\,{\mathrm Mpc}^{-3}$$, after accounting for various corrections. This observed SFRD is less than half of that at $z\approx 6$ 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 $z\approx 7$, 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 ($f_{\mathrm esc}>0.5$), 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 $z\approx 8$ galaxies demonstrates progress in pushing LBG detections to higher redshifts, although no $J$-drops were found at $z\approx 10$. This absence implies either a rapid evolution in the luminosity function beyond $z\approx 7$ 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.