The COSMOS2015 galaxy stellar mass function: 13 billion years of stellar mass assembly in 10 snapshots

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 $\sigma_z=0.03(1+z)$ 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 $\Lambda$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 $\alpha$ 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]

• The paper presents a detailed measurement of the galaxy stellar mass function across 13 billion years using comprehensive multi-wavelength data.
• It refines photometric redshifts and mass estimates, achieving over 75% completeness for galaxies with masses above 10¹⁰ M☉ at z = 5.
• Findings reveal a steeper low-mass end and diminished quenching in massive halos at z > 3, aligning with ΛCDM predictions.

Analysis of the COSMOS2015 Galaxy Stellar Mass Function

The study presents a comprehensive analysis of the galaxy stellar mass function (SMF) across 13 billion years, utilizing snapshots across various redshifts (up to $z \sim 6$). Leveraging the COSMOS field data from the COSMOS2015 catalogue, the authors aim to decipher stellar mass assembly over cosmic time. This analysis makes use of multi-wavelength data from multiple surveys, including the UltraVISTA consortium and the Spitzer Space Telescope SPLASH data, to enhance the precision and depth of the characterization of the SMF, particularly at higher redshifts.

Methodological Framework

Data Utilization and Redshift Estimation: The study employs data from near-infrared surveys, focusing on photometric redshifts that are refined with a precision error of $\sigma_z = 0.03(1+z)$, and an outlier fraction of 12%. Notably, the introduction of UltraVISTA-DR2 improves completeness at $z > 2.5$, achieving more than 75% completeness for galaxies with stellar masses above $10^{10} M_\odot$ at $z = 5$. The study also uses robust photometric redshifts calculated across a broad range using various synthetic spectral templates tuned to high-redshift characteristics.

Sample Selection and Mass Estimation: The selection criterion differs above $z = 4$, where the study utilizes a $[3.6]$ IRAC selection due to the redshifted stellar light captured more effectively in this band, rather than a $K_s$-band limit, optimizing for mass-completeness. Stellar mass estimates are derived through SED fitting with BC03 models, considering factors of metallicity and different star formation histories, further ensuring robustness in the characterization of the stellar mass.

Results and Implications

Evolution of the SMF: The SMF reveals a distinctive evolution, showing a steeper low-mass end at higher redshifts. The study models the SMF using Schechter functions, observing that at $z > 3$ both the observed SMF and the predicted halo mass function decline similarly at the high-mass end. This could imply a diminished efficacy of star formation quenching mechanisms in massive halos at earlier cosmic times.

Assessing the Impact of Eddington Bias: The research addresses Eddington bias corrections, acknowledging how uncertainties in stellar mass estimates can bias SMF measurements. It details the convolution of the Schechter function with distributions derived from error analysis, highlighting varying impacts on high-mass and low-mass ends of the SMF.

Comparison with Halo Mass Function: A significant aspect of this study is comparing the galaxy SMF with the halo mass function predicted by $\Lambda$CDM models. It finds a closer alignment at $z > 3$ between the two, suggesting that efficient feedback mechanisms hindering star formation in massive halos are less effective or absent during these epochs.

Active and Passive Galaxy Populations: The study distinguishes between active and passive galaxies using the $NUVrJ$ color-color diagram, with the results indicating an accumulation of passive galaxies in the intermediate mass range over cosmic time, with substantial growth primarily between $z = 2.5$ and $0.1$.

Conclusion

The insight provided by this study enhances the understanding of stellar mass assembly history and the influence of feedback mechanisms on galaxy evolution. By integrating extensive photometric surveys and employing advanced methods to counteract observational biases, this research paves the way for future explorations in high-redshift galaxy behavior, particularly in the advent of upcoming facilities like JWST. This work underscores the adaptive techniques necessary for capturing the complexity of galaxy evolution, offering a robust toolset and informative results for ongoing and future research in the field of astrophysics.