Galaxy And Mass Assembly (GAMA): Deconstructing the galaxy stellar mass function by star formation and environment

Using the equatorial Galaxy and Mass Assembly (GAMA) dataset, we investigate how the low-redshift galaxy stellar mass function (GSMF) varies across different galaxy populations and as a function of halo mass. We find that: (i) The GSMF of passive and star-forming galaxies are well described by a double and a single Schechter function, respectively, although the inclusion of a second component for the star-forming population yields a more accurate description. Furthermore, star-forming galaxies dominate the low-mass end of the total GSMF, whereas passive galaxies mainly shape the intermediate-to-high-mass regime. (ii) The GSMF of central galaxies dominates the high-mass end, whereas satellites and ungrouped galaxies shape the intermediate-to-low-mass regime. Additionally, we find a relative increase in the abundance of low-mass galaxies moving from dense group environments to isolated systems. (iii) More massive halos host more massive galaxies, have a higher fraction of passive systems, and show a steeper decline in the number of intermediate-mass galaxies. Finally, our results reveal larger differences between passive and star-forming GSMFs than predicted by a phenomenological quenching model, but generally confirm the environmental quenching trends for centrals and satellites reported in other works.

💡 Research Summary

This paper presents a comprehensive dissection of the low‑redshift galaxy stellar mass function (GSMF) using the equatorial Galaxy And Mass Assembly (GAMA) survey. The authors exploit the high spectroscopic completeness (≈98 % to r = 19.8 mag) of the three equatorial fields (G09, G12, G15) and combine state‑of‑the‑art stellar masses derived with the PROSPECT SED fitting code with star‑formation rates obtained from multi‑wavelength UV–IR photometry. Galaxies are classified into star‑forming (SF) and passive (P) populations based on specific SFR, and further divided into centrals, satellites, and ungrouped (field) objects using the G³C group catalogue, which also provides halo mass estimates for each group.

The GSMF for each subsample is measured with a 1/V_max weighting scheme and the step‑wise maximum‑likelihood (SWML) estimator. Schechter function fits are performed using both χ² minimisation and Markov Chain Monte Carlo sampling to quantify uncertainties in the low‑mass slope (α), characteristic mass (M★), and normalisation (φ*). The main findings are:

1. Passive galaxies are best described by a double‑Schechter function, confirming the presence of two distinct components that correspond to mass‑driven quenching (high‑mass end) and environment‑driven quenching (low‑mass end). This agrees with the phenomenological model of Peng et al. (2010).

2. Star‑forming galaxies are adequately fitted by a single Schechter function, but adding a second shallow component at low masses (α₂ ≈ −1.6) significantly improves the fit, indicating that low‑mass SF galaxies are sensitive to environment.

3. Centrals dominate the high‑mass tail (M > 10¹¹ M⊙) of the total GSMF and follow an almost pure exponential cutoff, whereas satellites and ungrouped galaxies shape the intermediate‑to‑low‑mass regime and exhibit a pronounced secondary low‑mass slope.

4. Halo mass dependence: As halo mass increases from 10¹² to 10¹⁴ M⊙, the characteristic mass shifts upward, the high‑mass exponential decline becomes steeper, and the passive fraction rises sharply. In massive halos the number density of intermediate‑mass galaxies (10¹⁰–10¹¹ M⊙) drops, consistent with strong ram‑pressure stripping and strangulation acting on satellites.

5. Environmental trends: Isolated (low‑density) environments host an excess of low‑mass star‑forming galaxies compared with group/cluster environments, supporting the view that field galaxies retain their gas supply and continue forming stars.

6. Model comparison: When the observed GSMFs are compared with the Peng et al. (2010, 2012) quenching framework, the differences between passive and star‑forming mass functions are larger than predicted, suggesting that current models underestimate the efficiency of both mass‑ and environment‑driven quenching, especially in the low‑mass regime.

The authors emphasize that the high completeness and deep group catalogue of GAMA enable robust measurements down to low halo masses, improving on earlier SDSS and 2dFGRS studies. Their results reinforce the view that the GSMF’s shape is a direct imprint of two quenching channels that operate preferentially on different galaxy populations and halo mass scales. The paper concludes that future semi‑analytic and hydrodynamic models must explicitly incorporate central‑satellite distinctions and halo‑mass dependent quenching efficiencies to reproduce the observed GSMF across all environments.