Mass assembly in quiescent and star-forming galaxies since z=4 from UltraVISTA

Abstract: We estimate the galaxy stellar mass function and stellar mass density for star-forming and quiescent galaxies with 0.2<z\<4. We construct a deep K\<24 sample of 220000 galaxies selected using the UltraVISTA DR1 data release. Our analysis is based on precise 30-band photometric redshifts. By comparing these photometric redshifts with 10800 spectroscopic redshifts from the zCOSMOS bright and faint surveys, we find a precision of sigma(dz/(1+z))=0.008 at i\<22.5 and sigma(dz/(1+zs))=0.03 at 1.5<z\<4. We derive the stellar mass function and correct for the Eddington bias. We find a mass-dependent evolution of the global and star-forming populations. This mass-dependent evolution is a direct consequence of the star formation being quenched in galaxies more massive than M\>10^10.7Msun. For the mass function of the quiescent galaxies, we do not find any significant evolution of the high-mass end at z<1; however we observe a clear flattening of the faint-end slope. From z~3 to z~1, the density of quiescent galaxies increases over the entire mass range. Their comoving stellar mass density increases by 1.6 dex between z~3 and z~1 and by less than 0.2dex at z<1. We infer the star formation history from the mass density evolution and we find an excellent agreement with instantaneous star formation rate measurements at z<1.5, while we find differences of 0.2dex at z>1.5 consistent with the expected uncertainties. We also develop a new method to infer the specific star formation rate from the mass function of star-forming galaxies. We find that the specific star formation rate of 10^10Msun galaxies increases continuously in the redshift range 1<z<4. Finally, we compare our results with a semi-analytical model and find that these models overestimate the density of low mass quiescent galaxies by an order of magnitude, while the density of low-mass star-forming galaxies is successfully reproduced.

• The paper quantifies mass-dependent evolution, showing low-mass galaxies evolve rapidly while massive ones experience quenching near 10^(10.7-10.9) M☉.
• It finds a dramatic 1.6 dex increase in the density of quiescent galaxies from z≈3 to z≈1, with minimal change at lower redshifts.
• The paper introduces an innovative method to infer star formation rates from mass functions, aligning with direct measurements up to z<1.5 and challenging existing models.

Overview of Galaxy Mass Assembly in Quiescent and Star-Forming Galaxies

This paper presents a detailed analysis of the mass assembly process in quiescent and star-forming galaxies from redshifts $z\approx 4$ to $z=0.2$, leveraging data from the UltraVISTA survey. The research endeavors to understand the stellar mass function (MF) and the stellar mass density of these galaxies, offering insights into the cosmic processes affecting galaxy formation and evolution.

The dataset comprises 220,000 galaxies within COSMOS fields, $K_s<24$, harnessing the depth and breadth of 30-band photometric redshifts. The precision of these redshift estimates derives from a comparison with approximately 10,800 spectroscopic redshifts, revealing accuracy metrics of $\sigma_{\Delta z / (1+z)}=0.008$ at $i^+<22.5$ and $\sigma_{\Delta z / (1+z)}=0.03$ for $1.5

Key Findings

1. Mass-Dependent Evolution: The study finds significant variations in the evolution of galaxy mass functions based on mass. Low-mass galaxies evolve more dynamically across the observed redshift range than their more massive counterparts. Specifically, the MF evolution was shown to be mass-dependent, particularly due to the quenching of star formation in galaxies with masses greater than approximately $10^{10.7-10.9} M_{\sun}$.
2. Quiescent Galaxy Mass Function: For quiescent galaxies, the high-mass end of the mass function at $z<1$ shows little evolution, while the faint-end slope noticeably flattens. From $z\sim3$ to $z\sim1$, the density of quiescent galaxies increased significantly by 1.6 dex, but this slowed to less than 0.2 dex at lower redshifts. This underlines a transition period around $z\sim1$ for massive quiescent galaxies.
3. Inferred Star Formation Rates: An innovative method for inferring the star formation rate from MFs shows consistency with direct measurements up to $z<1.5$, although there are discrepancies of 0.2 dex at higher redshifts. This method uncovers a continuous increase in specific star formation rates (sSFR) for galaxies between $10^{10-10.5} M_{\sun}$ in the redshift range $1
4. Model Comparisons: The paper's MF results challenge existing semi-analytical models, which tend to overestimate the density of low-mass quiescent galaxies by an order of magnitude, despite accurately capturing the population of low-mass star-forming galaxies.

Implications and Future Directions

The findings support the notion of mass-dependent evolution driven by efficient mass quenching. This particularly affects massive galaxies and may be driven by either AGN activity, major mergers, or dynamical processes in dark matter haloes. The lesser evolution observed in the less massive galaxies suggests environmental quenching mechanisms linked to local environmental densities and dynamical processes. Conversely, these results underline the nuanced interplay between cosmic star formation history and galaxy evolution.

This study highlights the importance of increasingly accurate MF estimates to refine theoretical models related to galaxy evolution. As observational techniques advance and larger datasets become available, further research may explore more fine-grained stratifications in galaxy populations and their respective evolutionary paths.

As a next step, further application of this method in conjunction with increasingly accurate and expansive datasets could help resolve discrepancies between empirical observations and theoretical models, particularly in reconciling the predicted and observed population of low and high-mass galaxies at various redshifts.