The Evolution of the Stellar Mass Functions of Star-Forming and Quiescent Galaxies to z = 4 from the COSMOS/UltraVISTA Survey

Abstract: We present measurements of the stellar mass functions (SMFs) of star-forming and quiescent galaxies to z = 4 using a sample of 95 675 galaxies in the COSMOS/UltraVISTA field. Sources have been selected from the DR1 UltraVISTA K_{s}-band imaging which covers a unique combination of a wide area (1.62 deg^2), to a significant depth (K_{s,tot} = 23.4). The SMFs of the combined population are in good agreement with previous measurements and show that the stellar mass density of the universe was only 50%, 10% and 1% of its current value at z ~ 0.75, 2.0, and 3.5, respectively. The quiescent population drives most of the overall growth, with the stellar mass density of these galaxies increasing by 2.71^{+0.93}_{-0.22} dex since z = 3.5. At z > 2.5, star-forming galaxies dominate the total SMF at all stellar masses, although a nonzero population of quiescent galaxies persists to z = 4. Comparisons of the K_{s}-selected star-forming galaxy SMFs to UV-selected SMFs at 2.5 < z < 4 show reasonable agreement and suggests UV-selected samples are representative of the majority of the stellar mass density at z > 3.5. We estimate the average mass growth of individual galaxies by selecting galaxies at fixed cumulative number density. The average galaxy with Log(M_{}/M_{sun}) = 11.5 at z = 0.3 has grown in mass by only 0.2 dex (0.3 dex) since z = 2.0(3.5), whereas those with Log(M_{}/M_{sun}) = 10.5 have grown by > 1.0 dex since z = 2. At z < 2, the time derivatives of the mass growth are always larger for lower-mass galaxies, which demonstrates that the mass growth in galaxies since that redshift is mass-dependent and primarily bottom-up. Lastly, we examine potential sources of systematic uncertainties on the SMFs and find that those from photo-z templates, SPS modeling, and the definition of quiescent galaxies dominate the total error budget in the SMFs.

• The paper examines the evolution of stellar mass functions for both star-forming and quiescent galaxies using a robust sample from the COSMOS/UltraVISTA Survey.
• It demonstrates that quiescent galaxies drive rapid mass density growth, with rates of ρstar ∝ (1+z)⁻⁴.7 compared to ρstar ∝ (1+z)⁻².3 for star-forming ones.
• The study reveals mass-dependent growth, where lower-mass galaxies grow over 1.0 dex since z=2, highlighting complex, bottom-up evolutionary processes.

An Analysis of Stellar Mass Function Evolution from COSMOS/UltraVISTA

The paper "The Evolution of the Stellar Mass Functions of Star-Forming and Quiescent Galaxies to $z = 4$ from the COSMOS/UltraVISTA Survey" presents a comprehensive examination of the evolution of stellar mass functions (SMFs) for both star-forming and quiescent galaxies up to a redshift of 4. The study utilizes a robust dataset from the COSMOS/UltraVISTA field, which is notable for its extensive coverage (1.62 deg$^2$) and depth (K$_{s, \text{tot}}$ = 23.4, 90% completeness).

The research establishes stellar mass functions from a substantial sample of 95,675 galaxies, revealing critical insights into cosmic stellar mass density evolution. The findings suggest that the stellar mass density of the universe was considerably lower in the past, standing at approximately 50%, 10%, and 1% of its current value at redshifts around 0.75, 2.0, and 3.5, respectively. This mass evolution is predominantly driven by the quiescent galaxy population, which shows greater mass density growth compared to star-forming galaxies. The paper quantifies this growth as $\rho_{star}$ $\propto$ (1 + $z$)$^{-4.7\pm0.4}$ for quiescent galaxies since $z = 3.5$, contrasting with $\rho_{star}$ $\propto$ (1 + $z$)$^{-2.3\pm0.2}$ for star-forming galaxies.

One of the study's essential conclusions is that star-forming galaxies dominate the total SMF at $z > 2.5$. However, there remains a non-negligible population of quiescent galaxies up to $z = 4$. This persistence of quiescent galaxies at high redshifts suggests a significant role in early galaxy evolution models.

A comparative analysis is conducted with previous UV-selected SMFs, indicating reasonable agreement and affirming that UV-selected samples effectively represent the majority of stellar mass density at $z > 3.5$. These findings underscore the efficacy of K$_{s}$-band imaging in the comprehensive capture of high-mass galaxies across substantial cosmic timescales.

Furthermore, the study assesses the average mass growth of galaxies via fixed cumulative number density. It finds that massive galaxies (Log(M$_{*}$/M$_{\odot}$) = 11.5 at $z = 0.3$) have undergone minimal mass growth (0.2 dex) since $z = 2.0$, while lower-mass galaxies (Log(M$_{*}$/M$_{\odot}$) = 10.5) exhibited more than 1.0 dex growth since $z = 2.0$. This pattern highlights a mass-dependent, bottom-up growth mechanism for galaxies, particularly at $z < 2$.

The paper also meticulously examines potential systematic uncertainties in the derived SMFs, attributing significant errors to photo-$z$ templates, stellar population synthesis (SPS) modeling, and the quiescent galaxy definition. This underscores the complexity and importance of accurately accounting for these factors in modeling galaxy evolution.

Overall, this research provides substantial evidence supporting existing theories of galaxy evolution while simultaneously highlighting areas that require further investigation. The intricate analysis of SMFs and the derived results present an illuminating perspective on the interplay of galaxy populations over a significant fraction of the universe's history. Future research, perhaps catalyzed by enhanced observational technologies and methodologies, may further refine these findings, offering deeper insights into the mechanisms governing cosmic structure formation and evolution.