The Abundance of Star-Forming Galaxies in the Redshift Range 8.5 to 12: New Results from the 2012 Hubble Ultra Deep Field Campaign

Abstract: We present the results of the deepest search to date for star-forming galaxies beyond a redshift z~8.5 utilizing a new sequence of near-infrared Wide Field Camera 3 images of the Hubble Ultra Deep Field. This `UDF12' campaign completed in September 2012 doubles the earlier exposures with WFC3/IR in this field and quadruples the exposure in the key F105W filter used to locate such distant galaxies. Combined with additional imaging in the F140W filter, the fidelity of high redshift candidates is greatly improved. Using spectral energy distribution fitting techniques on objects selected from a deep multi-band near-infrared stack we find 7 promising z>8.5 candidates. As none of the previously claimed UDF candidates with 8.5<z\<10 is confirmed by our deeper multi-band imaging, our campaign has transformed the measured abundance of galaxies in this redshift range. Although we recover the candidate UDFj-39546284 (previously proposed at z=10.3), it is undetected in the newly added F140W image, implying it lies at z=11.9 or is an intense emission line galaxy at z~2.4. Although no physically-plausible model can explain the required line intensity given the lack of Lyman alpha or broad-band UV signal, without an infrared spectrum we cannot rule out an exotic interloper. Regardless, our robust z ~ 8.5 - 10 sample demonstrates a luminosity density that continues the smooth decline observed over 6 < z < 8. Such continuity has important implications for models of cosmic reionization and future searches for z\>10 galaxies with JWST.

• The paper demonstrates that deep HST imaging with the WFC3 successfully identifies seven promising high-redshift galaxy candidates.
• The study employs composite SED fitting to enhance candidate fidelity and revises previous identifications in the z 8.5–10 range.
• The findings challenge earlier galaxy formation models and provide key insights for future observations with next-generation telescopes like JWST.

The Abundance of Star-Forming Galaxies in the Redshift Range 8.5 to 12

The investigation concerning the abundance of star-forming galaxies within the redshift range of 8.5 to 12 includes data from the Hubble Ultra Deep Field 2012 campaign (UDF12). This study primarily leverages near-infrared imaging captured via the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The campaign seeks to elucidate the evolving landscape of early galaxy formation and its implications for cosmic reionization.

Study Overview

This research represents the deepest observational exploration to detect galaxies at high redshifts, aiming beyond $z \simeq 8.5$. Utilizing enhanced exposure-depths, particularly in the F105W filter, the study increases the fidelity of identifying high-redshift candidates. The approach involves composite spectral energy distribution (SED) fitting across multi-band near-infrared stacks to pinpoint potential $z > 8.5$ candidates. A notable outcome of the campaign was the identification of seven promising candidates at these extreme distances.

Markedly, prior candidate galaxies within the redshift range of $8.5 < z < 10$, identified in earlier campaigns, could not be corroborated with the enhanced data quality. However, the study confirms an earlier proposed candidate UDFj-39546284, potentially moving its inferred redshift to $z \simeq 11.9$, barring alternative plausible interpretations as an emission line galaxy at $z \simeq 2.4$.

Numerical Results and Claims

The UDF12 campaign presents a measured abundance of galaxies within the defined redshift parameter, notably transforming the understanding of galaxy formation density in this epoch. The luminosity density trend derived from the sample is consistent with a decline observed from $6 < z < 8$, holding significance for developing cosmic reionization models and enhancing search strategies for even more distant systems using facilities like the James Webb Space Telescope (JWST).

Implications and Future Directions

The implications of these findings for theoretical models lie in reinforcing or revising the expected continuity of galaxy formation and decline in luminosity density across redshifts. The presence of $z \simeq 12$ galaxy candidates challenges previous constraints and potentially extends the known timeline of significant star-forming activity. However, the reliability of high-redshift inferences fundamentally depends on spectroscopic confirmation, which remains challenging with current observational capabilities.

The study's contribution is pivotal in bridging observational gaps and refining the methods for a more complete census of the earliest galaxies. Continuous advancements in infrared observational technology and data processing methodologies are expected to significantly enhance our comprehension of high-redshift galaxy formation. As the observational field approaches the limits of current telescopic capabilities, the research sets a compelling precedent for anticipatory studies utilizing next-generation telescopic initiatives.