Directly Observing the Galaxies Likely Responsible for Reionization (1604.06799v2)

Abstract: We report a new analysis of the Hubble Frontier Fields clusters Abell 2744 and MACS 0416 using wavelet decomposition to remove the cluster light, enabling the detection of highly magnified (>50x) galaxies a factor of 10x fainter in luminosity than previous studies. We find 167 galaxies at z > 6, and with this sample we are able to characterize the UV luminosity function to M_UV = -12.5 at z ~ 6, -14 at z ~ 7 and -15 at z ~ 8. We find a steep faint-end slope (alpha <-2), and with our improved statistics at the faint end we reduce the fractional uncertainty on alpha to <2% at z ~ 6 - 7 and 4% at z ~ 8. We also investigate the systematic uncertainty due to the lens modelling by using every available lens model individually and comparing the results; this systematic fractional uncertainty on alpha is <4% at all redshifts. We now directly observe galaxies in the luminosity regime where simulations predict a change in the faint-end slope of the luminosity function, yet our results provide statistically very strong evidence against any turnover in the luminosity range probed, more consistent with simulations in which stars form in lower-mass halos. Thus we find strong support for the extension of the steep luminosity function to M_UV = -13 at z > 6, consistent with the number of faint galaxies needed to reionize the Universe under standard assumptions.

• The paper directly observes faint galaxies at high redshifts using Hubble Frontier Fields and wavelet decomposition to study their role in reionization.
• Analysis reveals a steep faint-end slope (alpha < -2) in the UV luminosity function down to M_UV = -12.5, suggesting abundant faint galaxies drive reionization.
• These findings support standard reionization models where faint galaxies contribute significantly and highlight the need for future, deeper observations with telescopes like JWST.

Analyzing the Galaxies Contributing to Reionization: A Detailed Study

Reionization represents a pivotal event in the Universe's history, marking the end of the cosmic Dark Ages, during which the first light from stars and galaxies ionized hydrogen that filled the intergalactic medium. Understanding the sources that powered this reionization requires direct observation of early galaxies. The paper by Livermore, Finkelstein, and Lotz offers valuable insights into this epoch by presenting detailed detections and analyses of faint, high-redshift galaxies in the Hubble Frontier Fields (HFF) clusters Abell 2744 and MACS 0416.

Methodology and Findings

The authors employ an innovative method of wavelet decomposition to subtract foreground cluster light from the HFF data, allowing for the detection of background galaxies that are highly magnified through gravitational lensing. This technique enhances the sensitivity of detection to galaxies over 50 times fainter in luminosity than previous HST capabilities permitted, with up to tenfold fainter luminosities observed. The data, comprising 167 galaxies at redshifts $z \gtrsim 6$, enable the characterization of the UV luminosity function down to $M_{\rm{UV}} = -12.5$ at $z \sim 6$, $M_{\rm{UV}} = -14$ at $z \sim 7$, and $M_{\rm{UV}} = -15$ at $z \sim 8$.

The analysis reveals a steep faint-end slope ($\alpha < -2$) for the UV luminosity function, which exhibits no observable turnover in the luminosity range probed. This observation suggests that a significant population of faint galaxies could contribute to the ionizing photon budget required for reionization, supporting models where stars form in low-mass halos. Importantly, the fractional uncertainty on the faint-end slope, $\alpha$, is constrained to within 2% at $z \sim 6 -7$, and 4% at $z \sim 8$, highlighting the robustness of statistical inferences derived from the data.

Implications for Reionization and Future Research Directions

The implication that the UV luminosity function continues its steep rise faintward argues against a turnover, implying an abundance of faint galaxies sufficient to sustain reionization, consistent with theoretical predictions of star formation in less massive halos. Observations of high-redshift galaxies as faint as $M_{\rm{UV}} = -13$ play a critical role in validating standard reionization models, suggesting galaxies contribute a significant portion of reionizing photons.

These findings beckon further examinations using upcoming telescopes like the James Webb Space Telescope (JWST), which will explore even fainter magnitudes and potentially higher redshifts. Coupled with refined lens models, future work will better constrain the nature and distribution of these faint galaxies, refining our understanding of cosmic dawn.

Conclusion

This paper provides a comprehensive probe into the reionization process, leveraging the natural magnification of HFF galaxy clusters. While exploring wavelet decomposition techniques to innovate beyond the existing limits, the research substantiates the hypothesis of abundant faint galaxies' contributions to early Universe ionization. It encourages further comparative studies between observed data and simulations, helping to refine our cosmological models and expand our comprehension of the Universe's early stages. The steady progress in understanding the faint galaxy regime hints at richer insights awaiting discovery, paving the way for future empirical constraints on reionization theories.