[Paper Review] zCOSMOS - 10k-bright spectroscopic sample. The bimodality in the Galaxy Stellar Mass Function: exploring its evolution with redshift

L. Pozzetti, M. Bolzonella+57 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|Jul 30, 2009

Galaxies: Formation, Evolution, Phenomena148 references239 citations

TL;DR

This paper presents the galaxy stellar mass function (GSMF) from the zCOSMOS-bright 10k spectroscopic sample, revealing a bimodal distribution best described by two Schechter functions for early- and late-type galaxies. It finds mass-dependent evolution (downsizing) with red galaxies forming earlier at high masses, driven primarily by gradual star formation quenching rather than major mergers, and suggests morphological transformation lags behind colour change by 1–2 Gyr.

ABSTRACT

We present the Galaxy Stellar Mass Function (MF) up to z~1 from the zCOSMOS-bright 10k spectroscopic sample. We investigate the total MF and the contribution of ETGs and LTGs, defined by different criteria (SED, morphology or star formation). We unveil a galaxy bimodality in the global MF, better represented by 2 Schechter functions dominated by ETGs and LTGs, respectively. For the global population we confirm that low-mass galaxies number density increases later and faster than for massive galaxies. We find that the MF evolution at intermediate-low values of Mstar (logM<10.6) is mostly explained by the growth in stellar mass driven by smoothly decreasing star formation activities. The low residual evolution is consistent with ~0.16 merger per galaxy per Gyr (of which fewer than 0.1 are major). We find that ETGs increase in number density with cosmic time faster for decreasing Mstar, with a median "building redshift" increasing with mass, in contrast with hierarchical models. For LTGs we find that the number density of blue or spiral galaxies remains almost constant from z~1. Instead, the most extreme population of active star forming galaxies is rapidly decreasing in number density. We suggest a transformation from blue active spirals of intermediate mass into blue quiescent and successively (1-2 Gyr after) into red passive types. The complete morphological transformation into red spheroidals, required longer time-scales or follows after 1-2 Gyr. A continuous replacement of blue galaxies is expected by low-mass active spirals growing in stellar mass. We estimate that on average ~25% of blue galaxies is transforming into red per Gyr for logM<11. We conclude that the build-up of galaxies and ETGs follows the same downsizing trend with mass as the formation of their stars, converse to the trend predicted by current SAMs. We expect a negligible evolution of the global Galaxy Baryonic MF.

Motivation & Objective

• To measure the galaxy stellar mass function (GSMF) to redshift z ≈ 1 using a large, spectroscopically confirmed sample of 8,500 galaxies with I < 22.5.
• To investigate the origin of the bimodal GSMF by separating contributions from early-type (ETGs) and late-type galaxies (LTGs) using multiple classification criteria.
• To determine the evolutionary mechanisms behind the observed GSMF evolution, particularly the role of star formation quenching and merging processes.
• To test predictions of semi-analytic models (SAMs) by comparing observed evolution with simulated merger rates and star formation histories.
• To assess the relative importance of smooth mass growth, minor mergers, and major mergers in shaping the GSMF and GBMF (galaxy baryonic mass function) over cosmic time.

Proposed method

• The study uses a 1.4 deg² area with 8,500 galaxies from the zCOSMOS-bright 10k spectroscopic survey, selected via I-band magnitude (I < 22.5) and redshift coverage up to z ≈ 1.
• Galaxy types are classified using multiple independent criteria: spectral energy distribution (SED), morphology, spectral features (e.g., D_n4000), and star formation activity (SFR).
• The GSMF is measured in redshift and mass bins, and fitted with a sum of two Schechter functions to model the bimodal distribution of early- and late-type galaxies.
• Evolution of number density and mass density is quantified across redshifts, with emphasis on intermediate-to-low mass (log(M/M☉) ~ 10.5) and high-mass (log(M/M☉) > 11) populations.
• The rate of galaxy transformations is estimated by comparing the growth in red galaxy density with the decline in blue galaxy density, assuming quenching of SFR as the primary driver.
• Merger rates are inferred from residual evolution in the GSMF, assuming that unexplained growth is due to mergers, with a median rate of ~0.16 merger per galaxy per Gyr (of which <0.1 are major).

Experimental results

Research questions

• RQ1 What is the shape and evolution of the galaxy stellar mass function (GSMF) up to z ≈ 1, and is it best described by a single or bimodal function?
• RQ2 How do the number densities of early-type and late-type galaxies evolve with redshift, and what does this imply about their formation timescales?
• RQ3 To what extent is the observed GSMF evolution driven by gradual star formation quenching versus major merger events?
• RQ4 Does morphological transformation of galaxies (e.g., spiral to spheroid) occur before or after colour transformation (quenching)?
• RQ5 How do the observed evolution rates compare with predictions from current semi-analytic models (SAMs) of galaxy formation?

Key findings

• The global GSMF exhibits a clear bimodal shape, best fitted by two Schechter functions—one for early-type galaxies (ETGs) and one for late-type galaxies (LTGs)—confirming a fundamental dichotomy in galaxy populations.
• Galaxy number density increases by a factor of two from z = 1 to z = 0 for intermediate-to-low mass galaxies (log(M/M☉) ~ 10.5), but by less than 15% for high-mass galaxies (log(M/M☉) > 11), indicating mass-dependent evolution.
• The evolution of the GSMF at intermediate-to-low masses is primarily driven by a gradual decline in star formation activity, not by major mergers, with only ~0.16 mergers per galaxy per Gyr (fewer than 0.1 major) required to explain residual evolution.
• The number density of intermediate-mass ETGs increases significantly with cosmic time (by a factor of 4–2 from z = 0.7 to z = 0.2 for log(M/M☉) = 10.3–10.7), indicating a top-down building history inconsistent with hierarchical SAM predictions.
• Colour transformation (quenching) of blue galaxies precedes morphological transformation into red spheroids by 1–2 Gyr, suggesting that internal processes like disc instabilities drive morphological change after SFR quenching.
• The galaxy baryonic mass function (GBMF) shows negligible evolution for the global population and a decrease with cosmic time for the blue galaxy population at intermediate-to-high masses, consistent with a scenario dominated by smooth mass growth and quenching rather than major mergers.