A new multi-field determination of the galaxy luminosity function at z=7-9 incorporating the 2012 Hubble Ultra Deep Field imaging

Abstract: We present a new determination of the UV galaxy luminosity function (LF) at redshift z ~ 7 and z ~ 8, and a first estimate at z ~ 9. An accurate determination of the form and evolution of the LF at high z is crucial for improving our knowledge of early galaxy evolution and cosmic reionization. Our analysis exploits fully the new, deepest WFC3/IR imaging from our HST UDF12 campaign, and includes a new, consistent analysis of all appropriate, shallower/wider-area HST data. Our new measurement of the evolving LF at z ~ 7-8 is based on a final catalogue of ~600 galaxies, and involves a step-wise maximum likelihood determination based on the redshift probability distribution for each object; this makes full use of the 11-band imaging now available in the HUDF, including the new UDF12 F140W data, and the deep Spitzer IRAC imaging. The final result is a determination of the z ~ 7 LF extending down to M_UV = -16.75, and the z ~ 8 LF down to M_UV = -17.00. Fitting a Schechter function, we find M* = -19.90 (+0.23/-0.28), log phi* = -2.96 (+0.18/-0.23), and a faint-end slope alpha=-1.90 (+0.14/-0.15) at z~7, and M* = -20.12 (+0.37/-0.48), log phi* = -3.35 (+0.28/-0.47), alpha=-2.02 (+0.22/-0.23) at z~8. These results strengthen suggestions that the evolution at z > 7 is more akin to density evolution' than the apparentluminosity evolution' seen at z ~ 5-7. We also provide the first meaningful information on the LF at z ~ 9, explore alternative extrapolations to higher z, and consider the implications for the evolution of UV luminosity density. Finally, we provide catalogues (including z_phot, M_UV and all photometry) for the 100 most robust z~6.5-11.9 galaxies in the HUDF used in this analysis. We discuss our results in the context of earlier work and the results of an independent analysis of the UDF12 data based on colour-colour selection (Schenker et al. 2013).

• The paper refines the UV galaxy luminosity function at z≈7–9 using combined ultra-deep and wide-area Hubble imaging.
• It employs a step-wise maximum likelihood method on a sample of about 600 galaxies, determining Schechter function parameters at z≈7 and z≈8.
• The findings support density evolution over luminosity evolution, offering fresh insights into early galaxy formation and cosmic reionization.

Analysis of the Galaxy Luminosity Function at Redshift $z \approx 7-9$

The study, "A new multi-field determination of the galaxy luminosity function at $z=7-9$ incorporating the 2012 Hubble Ultra Deep Field imaging" by R.J. McLure et al., presents a valuable advancement in understanding the early phases of galaxy formation by detailing the UV galaxy luminosity functions at redshifts $z \approx 7$, $z \approx 8$, and providing an initial exploration at $z \approx 9$. This paper utilizes the deepest imaging data available from the Hubble Space Telescope's Ultra Deep Field 2012 (HUDF12) campaign and applies statistical methods to refine the galaxy luminosity functions.

Key Findings

• Data and Methodology: The research integrates data from multiple Hubble surveys, enhancing the dynamic range of galaxy luminosity function (LF) analysis by including both ultra-deep and wider-area imaging data. The dataset encompasses approximately 600 galaxies, analyzed using the step-wise maximum likelihood approach leveraging photometric-redshift probability distributions across 11 imaging bands.
• Luminosity Function at $z \approx 7 - 8$: The analysis extends the measurement of the galaxy luminosity function to UV absolute magnitudes $M_{1500}$ as faint as -16.75 at $z \approx 7$ and -17.00 at $z \approx 8$. A Schechter function fit reveals parameters at $z \approx 7$: $M_{1500}^{*} = -19.90^{+0.23}_{-0.28}$, $\log \phi^* = -2.96^{+0.18}_{-0.23}$, and a faint-end slope $\alpha = -1.90^{+0.14}_{-0.15}$. At $z \approx 8$, the corresponding values are $M_{1500}^{*} = -20.12^{+0.37}_{-0.48}$, $\log \phi^* = -3.35^{+0.28}_{-0.47}$, and $\alpha = -2.02^{+0.22}_{-0.23}$.
• Evidence of Evolution Patterns: The results suggest a transition in the evolutionary pattern beyond $z \approx 7$, favoring a 'density evolution' model over 'luminosity evolution' observed between $z \approx 5 - 7$. The faint-end slope remains remarkably steep, indicating a substantial population of faint galaxies potentially contributing to cosmic reionization.
• Initial Exploration at $z \approx 9$: The study also illuminates the luminosity function at $z \approx 9$ within the faint magnitude range, showing no drastic decline in faint galaxy density. This suggests continuity in galaxy formation dynamics as the universe transitions into the reionization epoch.

Implications and Future Prospects

The findings significantly improve our understanding of the high-redshift universe, contributing to models that describe early galaxy evolution and the reionization process. These insights imply that the UV luminosity density steadily rises from $z \approx 10$ to $z \approx 6$, with little to suggest abrupt changes over these epochs. Such results have profound implications for the nature of early galactic structures and star formation histories.

Future exploration in this domain may benefit from deeper infrared observations, potentially with instruments such as the James Webb Space Telescope, to further probe the early universe and trace galactic evolution beyond redshift $z=9$. These advancements could significantly elucidate the transition dynamics during cosmic reionization and refine theoretical models predicting the behavior and properties of early universe galaxies.

Conclusion

The study by McLure et al. represents a rigorous analysis leveraging enhanced data coverage and advanced statistical techniques to refine our understanding of galaxy formation during the critical high-redshift epochs. The evidence supports existing models of cosmic evolution while paving the way for further investigation into the faint end of the luminosity function and the underlying processes governing early galaxy evolution.