Great Optically Luminous Dropout Research Using Subaru HSC (GOLDRUSH). I. UV Luminosity Functions at $z \sim 4-7$ Derived with the Half-Million Dropouts on the 100 deg$^2$ Sky

Abstract: We study the UV luminosity functions (LFs) at $z\sim 4$, $5$, $6,$ and $7$ based on the deep large-area optical images taken by the Hyper Suprime-Cam (HSC) Subaru strategic program (SSP). On the 100 deg$^2$ sky of the HSC SSP data available to date, we make enormous samples consisting of a total of 579,565 dropout candidates at $z\sim 4-7$ by the standard color selection technique, 358 out of which are spectroscopically confirmed by our follow-up spectroscopy and other studies. We obtain UV LFs at $z \sim 4-7$ that span a very wide UV luminosity range of $\sim 0.002 - 100 \, L_{\rm UV}^\ast$ ($-26 < M_{\rm UV} < -14$ mag) by combining LFs from our program and the ultra-deep Hubble Space Telescope legacy surveys. We derive three parameters of the best-fit Schechter function, $\phi^\ast$, $M_{\rm UV}^\ast$, and $\alpha$, of the UV LFs in the magnitude range where the AGN contribution is negligible, and find that $\alpha$ and $\phi^\ast$ decrease from $z\sim 4$ to $7$ with no significant evolution of $M_{\rm UV}^\ast$. Because our HSC SSP data bridge the LFs of galaxies and AGNs with great statistical accuracy, we carefully investigate the bright end of the galaxy UV LFs that are estimated by the subtraction of the AGN contribution either aided with spectroscopy or the best-fit AGN UV LFs. We find that the bright end of the galaxy UV LFs cannot be explained by the Schechter function fits at $> 2 \sigma$ significance, and require either double power-law functions or modified Schechter functions that consider a magnification bias due to gravitational lensing.

• The paper presents robust UV luminosity functions derived from 579,565 dropout candidates across 100 deg² using precise color selection and follow-up spectroscopy.
• The paper identifies bright-end deviations from standard Schechter fits, suggesting contributions from AGNs or gravitational lensing effects.
• The paper’s findings refine our understanding of galaxy evolution by linking UV brightness trends to star formation processes and AGN activity at high redshifts.

Overview of "Great Optically Luminous Dropout Research Using Subaru HSC (GOLDRUSH). I. UV Luminosity Functions at $z \sim 4-7$"

The "Great Optically Luminous Dropout Research Using Subaru HSC (GOLDRUSH)" paper represents a comprehensive investigation into the UV luminosity functions (LFs) of galaxies across a significant cosmological timeframe, encompassing redshifts from $z \sim 4$ to $z \sim 7$. Utilizing the Hyper Suprime-Cam (HSC) Subaru strategic program data, the authors have explored a vast sky area of 100 deg$^2$, capturing a substantial dataset of 579,565 dropout candidates. This sample has allowed the researchers to construct robust UV LFs, spanning a wide range of luminosities from $\sim 0.002$ to 100 $L_{\rm UV}^\ast$ (corresponding to magnitudes between $-26$ and $-14$).

Methodology

The study employs the standard color selection technique to identify galaxies at different epochs based on their distinct dropout characteristics in the optical spectrum. Specifically, the analysis is underpinned by spectral data from the Subaru Telescope's HSC, which effectively isolates high-redshift galaxies (@ $z \sim 4-7$) through dropout identification. Follow-up spectroscopy has been conducted on 358 candidates, confirming their dropout status and associating them with specific redshifts. The study integrates both spectroscopic analysis and numerical modeling using the best-fit Schechter functions.

Key Findings

The derived UV LFs elucidate significant trends in galaxy formation and evolution during this epoch. Notably, the paper identifies:

1. LF Characterization Without Significant AGN Contribution: For the faint end where AGN contamination is minimal, the Schechter function parameters show a decline in $\alpha$ and $\phi^\ast$ as redshift increases, with almost no variation in $M_{\rm UV}^\ast$.
2. Bright-End Deviations from Schechter Fits: Beyond $2 \sigma$ confidence levels, the bright end of the galaxy UV LFs defies simple Schechter function fits. This suggests potential explanations including the necessity for the application of double power-law functions or modified Schechter functions accounting for magnification biases introduced by gravitational lensing.
3. AGN Contribution: The data reveal consistent bright-end hump features likely attributable to AGNs rather than typical galaxy assemblies. This insight bridges the gap between galaxy LFs and AGN LFs, showcasing how AGNs contribute significantly to UV LFs at high luminosities.

Implications

The findings have profound implications for understanding the cosmic history of star formation and the interplay between galaxies and AGNs. The bright-end upturn in UV LFs raises questions about the role of AGNs in galaxy evolution at high redshifts, potentially indicating inefficient feedback mechanisms in the early universe or gravitational interactions influencing luminosity perceptions. The study calls for further high-resolution investigations to differentiate between star forming galaxies and AGNs with precision, as well as to better understand the gravitational lensing effects on our observations.

Future Directions

Continued advancements in computational and observational techniques will enable further refinement of these findings. High-resolution space-based observations can better distinguish morphological characteristics, potentially validating the blending and magnification scenarios suggested. These developments will aid in constructing a more cohesive theory of galactic and black hole co-evolution, especially in the context of early universe conditions.

The GOLDRUSH paper serves as an exemplar of leveraging modern optical survey technology to deepen the understanding of high-redshift universe dynamics, setting a precedent for subsequent studies in cosmology and extragalactic astronomy.