The UV Luminosity Function of star-forming galaxies via dropout selection at redshifts z ~ 7 and 8 from the 2012 Ultra Deep Field campaign

Abstract: We present a catalog of high redshift star-forming galaxies selected to lie within the redshift range z ~ 7-8 using the Ultra Deep Field 2012 (UDF12), the deepest near-infrared (near-IR) exposures yet taken with the Hubble Space Telescope. As a result of the increased near-infrared exposure time compared to previous HST imaging in this field, we probe 0.65 (0.25) mag fainter in absolute UV magnitude, at z ~ 7 (8), which increases confidence in a measurement of the faint end slope of the galaxy luminosity function. Through a 0.7 mag deeper limit in the key F105W filter that encompasses or lies just longward of the Lyman break, we also achieve a much-refined color-color selection that balances high redshift completeness and a low expected contamination fraction. We improve the number of drop-out selected UDF sources to 47 at z ~ 7 and 27 at z ~ 8. Incorporating brighter archival and ground-based samples, we measure the z ~ 7 UV luminosity function to an absolute magnitude limit of M_UV = -17 and find a faint end Schechter slope of \alpha = -1.87+/- 0.18. Using a similar color-color selection at z ~ 8 that takes account of our newly-added imaging in the F140W filter, and incorporating archival data from the HIPPIES and BoRG campaigns, we provide a robust estimate of the faint end slope at z ~ 8, \alpha = -1.94 +/- 0.23. We briefly discuss our results in the context of earlier work and that derived using the same UDF12 data but with an independent photometric redshift technique (McLure et al 2012).

The UV Luminosity Function of Star-Forming Galaxies at High Redshifts

The paper, "The UV Luminosity Function of Star-Forming Galaxies via Dropout Selection at Redshifts z ~ 7 and 8 from the 2012 Ultra Deep Field Campaign," provides a meticulous analysis of high-redshift galaxies using data obtained from the Ultra Deep Field 2012 (UDF12) campaign with the Hubble Space Telescope (HST). This study aims to refine our understanding of the UV luminosity function of star-forming galaxies at redshifts approximately z ~ 7 and z ~ 8, offering insights into the early universe during these epochs.

Methodology and Findings

The researchers employed dropout techniques and robust photometric criteria to improve identification accuracy of star-forming galaxies at these high redshifts. The enhanced UDF12 data, which included deeper near-infrared exposures due to increased observation time, allowed the team to probe fainter galaxies than previous HST imaging campaigns, importantly contributing to more precise constraints on the faint-end slope of the galaxy luminosity function.

A well-defined color-color selection criterion was established, balancing high redshift completeness with minimal low-redshift contamination. The researchers conducted simulations to estimate contamination levels and developed a thorough analysis incorporating both observational datasets and simulation predictions.

Their dropout selection technique identified 47 galaxies at redshift z ~ 7 and 27 galaxies at redshift z ~ 8, which were subsequently used to calculate the galaxy luminosity functions. They estimated the faint-end slopes for these functions to be α = -1.87 at z ~ 7 and α = -1.94 at z ~ 8, based on the Schechter function fits. This suggested an evolution in the faint-end slope across these redshifts, indicating an increasingly abundant population of sub-luminous galaxies at higher redshifts.

Implications

These findings have significant implications for our understanding of cosmic reionization and the epoch of galaxy formation. The steeper faint-end slopes observed infer that a larger fraction of UV light in the early universe was being emitted by numerous low-luminosity galaxies, potentially playing a crucial role in the reionization of the intergalactic medium.

Additionally, the paper's rigorous methodological approach highlights the importance of using both space-based observations and simulations to uncover the distributions and characteristics of high-redshift galaxies, enhancing the reliability of the derived luminosity functions.

Speculations on Future Developments

Continuous advancements in observational technologies and methodologies are expected to refine these analyses further. Future campaigns, such as the HST Frontier Fields, are anticipated to provide even deeper datasets, extending the depth and breadth of high-redshift observations. This will enable more accurate measurements of the luminosity functions and a better understanding of the galaxy formation processes during the first billion years of cosmic history.

In conclusion, the study offers a substantial contribution to our understanding of high-redshift star-forming galaxies through refined UV luminosity functions, which serve as essential tools in exploring and understanding the early universe's evolutionary dynamics.