[Paper Review] The COSMOS2015 galaxy stellar mass function: 13 billion years of stellar mass assembly in 10 snapshots

I. Davidzon, O. Ilbert+22 more|arXiv (Cornell University)|Jan 10, 2017

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy228 references114 citations

TL;DR

This paper derives the galaxy stellar mass function and stellar mass density in the COSMOS field from z ~ 0.1 to 6 using the COSMOS2015 catalogue with updated photometric redshifts and SED fitting for z > 2.5, and compares to the halo mass function from ΛCDM simulations.

ABSTRACT

We measure the stellar mass function (SMF) of galaxies in the COSMOS field up to $z\sim6$. We select them in the near-IR bands of the COSMOS2015 catalogue, which includes ultra-deep photometry from UltraVISTA-DR2, SPLASH, and Subaru/Hyper-SuprimeCam. At $z>2.5$ we use new precise photometric redshifts with error $σ_z=0.03(1+z)$ and an outlier fraction of $12\%$, estimated by means of the unique spectroscopic sample of COSMOS. The increased exposure time in the DR2, along with our panchromatic detection strategy, allow us to improve the stellar mass completeness at high $z$ with respect to previous UltraVISTA catalogues. We also identify passive galaxies through a robust colour-colour selection, extending their SMF estimate up to $z=4$. Our work provides a comprehensive view of galaxy stellar mass assembly between $z=0.1$ and 6, for the first time using consistent estimates across the entire redshift range. We fit these measurements with a Schechter function, correcting for Eddington bias. We compare the SMF fit with the halo mass function predicted from $Λ$CDM simulations. We find that at $z>3$ both functions decline with a similar slope in the high-mass end. This feature could be explained assuming that the mechanisms that quench star formation in massive haloes become less effective at high redshift; however further work needs to be done to confirm this scenario. Concerning the SMF low-mass end, it shows a progressive steepening as moving towards higher redshifts, with $α$ decreasing from $-1.47_{-0.02}^{+0.02}$ at $z\simeq0.1$ to $-2.11_{-0.13}^{+0.30}$ at $z\simeq5$. This slope depends on the characterisation of the observational uncertainties, which is crucial to properly remove the Eddington bias. We show that there is currently no consensus on the method to quantify such errors: different error models result in different best-fit Schechter parameters. [Abridged]

Motivation & Objective

• Provide a coherent, mass-complete measurement of the galaxy stellar mass function (SMF) from z ~ 0.1 to 6 in the COSMOS field.
• Improve high-redshift completeness and stellar mass estimates using the COSMOS2015 catalogue and updated SED fitting.
• Identify passive and active galaxy populations and track their mass assembly across cosmic time.
• Assess how the observed SMF evolves and compare with ΛCDM halo mass functions to interpret quenching and mass growth mechanisms.

Proposed method

• Use the COSMOS2015 catalogue with UltraVISTA-DR2, SPLASH, and Subaru/Hyper Suprime-Cam photometry.
• Recompute photometric redshifts for z > 2.5 with an expanded template library and updated SED fitting tuned for 3 < z < 6.
• Incorporate nebular emission lines in SED templates to improve z_phot and mass estimates.
• Apply a flux-limited sample selection with [3.6] < [3.6]_lim to ensure completeness up to z ~ 6, with completeness assessed via CANDELS CANDELS data cross-checks.
• Classify stars vs galaxies using multi-criteria diagnostics and remove AGN-dominated sources to avoid biased masses.

Experimental results

Research questions

• RQ1 What is the form and evolution of the galaxy stellar mass function (SMF) in the COSMOS field from z ~ 0.1 to 6?
• RQ2 How does the high-redshift (z > 2.5) SMF compare to lower-redshift measurements when using consistent methodologies?
• RQ3 What is the role of passive vs. star-forming galaxies in shaping the SMF across cosmic time?
• RQ4 How does the observed SMF slope and normalization compare with the halo mass function predicted by ΛCDM, and what does this imply about quenching efficiency at high redshift?
• RQ5 What are the systematic impacts of photometric redshift uncertainties and Eddington bias on Schechter function fits to the SMF?

Key findings

• The SMF is measured up to z ~ 6 with improved completeness at high z due to deeper DR2 data and a panchromatic detection strategy.
• The high-z SMF shows a steepening low-mass end slope α evolving from ~ -1.47 at z ~ 0.1 to as steep as ~ -2.11 at z ~ 5, depending on error modeling.
• Passive galaxy populations are identified and their SMF extended up to z = 4.
• At z > 3, the high-mass end slopes of the SMF and the halo mass function decline similarly, suggesting quenching mechanisms may be less effective at high redshift.
• Nebular emission lines are included in SED templates, improving redshift and mass estimates for high-z galaxies.
• Completeness and selection effects (e.g., [3.6]-band limits) are carefully assessed using CANDELS data to ensure robust high-z measurements.