Galaxy Stellar Mass Functions from ZFOURGE/CANDELS: An Excess of Low-Mass Galaxies Since z=2 and the Rapid Buildup of Quiescent Galaxies (1309.5972v2)

Abstract: Using observations from the FourStar Galaxy Evolution Survey (ZFOURGE), we obtain the deepest measurements to date of the galaxy stellar mass function at 0.5 < z < 2.5. ZFOURGE provides well-constrained photometric redshifts made possible through deep medium-bandwidth imaging at 1-2um . We combine this with HST imaging from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS), allowing for the efficient selection of both blue and red galaxies down to stellar masses ~10^9.5 Msol at z ~ 2.5. The total surveyed area is 316 arcmin^2 distributed over three independent fields. We supplement these data with the wider and shallower NEWFIRM Medium-Band Survey (NMBS) to provide stronger constraints at high masses. Several studies at z<=1 have revealed a steepening of the slope at the low-mass end of the stellar mass function (SMF), leading to an upturn at masses <10^10 Msol that is not well-described by a standard single-Schechter function. We find evidence that this feature extends to at least z ~ 2, and that it can be found in both the star-forming and quiescent populations individually. The characteristic mass (M*) and slope at the lowest masses (alpha) of a double-Schechter function fit to the SMF stay roughly constant at Log(M/Msol) ~ 10.65 and ~-1.5 respectively. The SMF of star-forming galaxies has evolved primarily in normalization, while the change in shape is relatively minor. This is not the case for quiescent galaxies: the depth of our imaging allows us to show for the first time significantly more evolution at Log(M/Msol) < 10.5 than at higher masses. We find that the total mass density (down to 10^9 Msol) in star-forming galaxies has increased by a factor of ~2.2 since z ~ 2.5, whereas in quiescent galaxies it has increased by a factor of ~12 .

• The paper presents the most detailed SMF measurements using deep medium-band imaging from ZFOURGE/CANDELS across 0.2 < z < 3.
• The paper reveals a 12-fold increase in quiescent galaxy stellar mass density since z = 2.5 compared to a modest rise for star-forming galaxies.
• The paper demonstrates that SMFs at z < 2 are best modeled by a double-Schechter function, with consistent parameters (M* ~10.65, α ~ -1.5) indicating a low-mass excess.

Overview of the Stellar Mass Functions from ZFOURGE/CANDELS

This paper presents the most detailed measurements of the galaxy stellar mass function (SMF) to date, using data from the FourStar Galaxy Evolution Survey (ZFOURGE) combined with imaging from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). The data cover a redshift range of $0.2 < z < 3$ and provide insights into the distribution and evolution of galaxies with varying masses over cosmic time. The paper explores the low-mass excess of galaxies since $z = 2$ and the significant increase in the number of quiescent galaxies.

Methodology and Data

The analysis uses deep medium-bandwidth imaging to obtain precise photometric redshifts and stellar mass estimates for thousands of galaxies. The total survey area of the paper is 316 arcmin² across three independent fields. The authors utilize additional data from the NEWFIRM Medium-Band Survey (NMBS) to enhance constraints on the high-mass end of the SMF.

Key Findings

1. Low-Mass Excess: There is compelling evidence for a steepening of the SMF at masses below $10^{10}$, extending to $z \sim 2$. This feature has been observed across star-forming and quiescent galaxy populations.
2. Quiescent Galaxy Buildup: The paper reports a rapid increase in the stellar mass density of quiescent galaxies by a factor of 12 since $z = 2.5$. In contrast, the density of star-forming galaxies increases only by a factor of about 2.2.
3. Schechter Function Parameters: The SMFs are better described by a double-Schechter function at $z < 2$. The characteristic mass ($M^*$) and the slope ($\alpha$) at the low-mass end of the SMF remain consistent—with $M^*$ around $10.65$ and $\alpha$ approximately $-1.5$.
4. Stellar Mass Evolution: The star-forming SMF primarily evolves in normalization, with minor changes in shape. There is a notable buildup at the low-mass end for quiescent galaxies, which underscores a differential growth between the two populations.

Implications and Future Directions

The results suggest that the processes governing the evolution of low-mass galaxies have undergone significant change since $z = 2.5$. The differential growth between star-forming and quiescent populations could imply variations in feedback mechanisms or environmental effects acting on different galaxy populations. The upturn in the SMF highlights the importance of accounting for a detailed range of masses when modeling galaxy formation and evolution.

The findings might have implications for future theoretical models exploring the connection between stellar mass and quenching processes. The capability to reach deeper redshifts and a broader range of stellar masses will continue to refine our understanding of the SMF. Further spectroscopic confirmation, particularly at the low-mass end, would benefit subsequent studies by reducing uncertainties in photometric redshift and mass estimates.

In summary, this paper provides valuable insights into the SMF evolution and its impact on our understanding of galaxy growth and quiescence over cosmic time. The detailed analysis challenges previous models and sets a foundation for future research to explore the complexities of galaxy evolution.