The mass evolution of the first galaxies: stellar mass functions and star formation rates at $4 < z < 7$ in the CANDELS GOODS-South field (1408.2527v1)

Abstract: We measure new estimates for the galaxy stellar mass function and star formation rates for samples of galaxies at $z \sim 4,~5,~6~&~7$ using data in the CANDELS GOODS South field. The deep near-infrared observations allow us to construct the stellar mass function at $z \geq 6$ directly for the first time. We estimate stellar masses for our sample by fitting the observed spectral energy distributions with synthetic stellar populations, including nebular line and continuum emission. The observed UV luminosity functions for the samples are consistent with previous observations, however we find that the observed $M_{UV}$ - M${*}$ relation has a shallow slope more consistent with a constant mass to light ratio and a normalisation which evolves with redshift. Our stellar mass functions have steep low-mass slopes ($\alpha \approx -1.9$), steeper than previously observed at these redshifts and closer to that of the UV luminosity function. Integrating our new mass functions, we find the observed stellar mass density evolves from $\log{10} \rho_{*} = 6.64^{+0.58}_{-0.89}$ at $z \sim 7$ to $7.36\pm0.06$ $\text{M}_{\odot} \text{Mpc}^{-3}$ at $z \sim 4$. Finally, combining the measured UV continuum slopes ($\beta$) with their rest-frame UV luminosities, we calculate dust corrected star-formation rates (SFR) for our sample. We find the specific star-formation rate for a fixed stellar mass increases with redshift whilst the global SFR density falls rapidly over this period. Our new SFR density estimates are higher than previously observed at this redshift.

• The paper refines the galaxy stellar mass function at 4 < z < 7, revealing a steep low-mass slope (α ≈ -1.9) that aligns with the UV luminosity function.
• It employs advanced SED fitting with nebular emission to derive stable mass-to-light ratios and robust stellar mass estimates despite photometric uncertainties.
• It finds that although the specific star formation rate increases with redshift, the overall star formation rate density declines rapidly by up to 40× from z ≈ 7 to z ≈ 4.

Overview of "The mass evolution of the first galaxies: stellar mass functions and star formation rates at $4 < z < 7$ in the CANDELS GOODS-South field"

This paper presents an in-depth paper of the mass evolution of galaxies at high redshifts ($4 < z < 7$), utilizing the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) data from the GOODS-South field. The research undertaken offers fresh insights into the galaxy stellar mass function (SMF) and star formation rates (SFRs) during the early epochs of the universe, which are crucial for understanding the formation and evolution of galaxies.

Key Contributions

The paper provides new estimates for the galaxy SMF and SFR, significantly enhancing our understanding of galaxy formation and evolution in the early universe. Some of the principal contributions of the paper are:

1. Stellar Mass Functions: The paper elaborates the SMF for redshifts $z \sim 4, 5, 6, \text{and } 7$, focusing on the steep low-mass slopes ($\alpha \approx -1.9$). This finding contrasts with previous observations, indicating a steeper distribution and a more consistent alignment with the UV luminosity function.
2. Mass-to-Light Ratios: By examining the $M_{UV}$ - M$_{*}$ relationship, the paper finds a shallow slope, more consistent with a constant mass-to-light ratio rather than evolving. This result suggests a stable ratio over the redshift range explored, although the normalization evolves with redshift, indicating substantial changes in galaxy formation processes over time.
3. Star Formation Rates: The research analyzes the SFRs by combining measured UV slopes ($\beta$) with UV luminosities to obtain dust-corrected SFRs, observing an increase in specific star formation rate (sSFR) for a fixed stellar mass with redshift. Despite the increase, the global SFR density decreases rapidly over the examined period.
4. Stellar Mass Density: The evolution of the observed stellar mass density from high redshift shows a factor of $\sim 4$ to 40 reduction from $z \sim 7$ to $z \sim 4$, marking a significant increase in stellar mass assembly within $\sim 1$ Gyr.

Methodological Approach

The paper employs a robust methodological framework, estimating stellar masses by fitting observed spectral energy distributions (SEDs) with advanced synthetic stellar populations, including nebular emission components. This approach allows for accurate stellar mass estimation even at high redshifts, a challenging endeavor due to the dominance of UV light in observations. The corrective methods are refined to ensure accuracy despite the inherent uncertainties in photometric redshifts.

Implications and Future Work

The findings significantly impact our theoretical understanding of galaxy formation, challenging existing models, and urging modifications to account for the steeper slopes in the SMF observed at higher redshifts. The results advocate for further explorations into the processes contributing to early galaxy formation and evolution, specifically the role of low-mass galaxies in the cosmic history of star formation.

For future investigations, this paper lays the groundwork for expanded studies using newer and more comprehensive datasets from future observatories like the James Webb Space Telescope (JWST). Further, there is an opportunity for improved simulations and model refinements to reconcile with the robust observational data presented here.

Overall, this research marks a pivotal advancement in understanding the mass evolution of galaxies during a fundamental epoch in cosmic history, offering crucial insights and directing future avenues in astrophysical research.