[Paper Review] The VIMOS VLT Deep Survey. The Assembly History of the Stellar Mass in Galaxies: from the Young to the Old Universe

L. Pozzetti, M. Bolzonella+48 more|ArXiv.org|Apr 12, 2007

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy90 references156 citations

TL;DR

This paper presents a comprehensive analysis of the galaxy stellar mass function (GSMF) up to z=2.5 using the VIMOS VLT Deep Survey, combining optical and near-IR data with two stellar mass estimation methods. It finds that massive galaxies formed their stars early (z>1), with minimal evolution below z≈0.7, while low-mass systems evolved significantly, indicating mass-dependent formation histories and challenging standard merging-based models.

ABSTRACT

We present a detailed analysis of the Galaxy Stellar Mass Function of galaxies up to z=2.5 as obtained from the VVDS. We estimate the stellar mass from broad-band photometry using 2 different assumptions on the galaxy star formation history and show that the addition of secondary bursts to a continuous star formation history produces systematically higher (up to 40%) stellar masses. At low redshift (z=0.2) we find a substantial population of low-mass galaxies (<10^9 Msun) composed by faint blue galaxies (M_I-M_K=0.3). In general the stellar mass function evolves slowly up to z=0.9 and more significantly above this redshift. Conversely, a massive tail is present up to z=2.5 and have extremely red colours (M_I-M_K=0.7-0.8). We find a decline with redshift of the overall number density of galaxies for all masses (59+-5% for M>10^8 Msun at z=1), and a mild mass-dependent average evolution (`mass-downsizing'). In particular our data are consistent with mild/negligible (<30%) evolution up to z=0.7 for massive galaxies (>6x10^10 Msun). For less massive systems the no-evolution scenario is excluded. A large fraction (>=50%) of massive galaxies have been already assembled and converted most of their gas into stars at z=1, ruling out the `dry mergers' as the major mechanism of their assembly history below z=1. This fraction decreases to 33% at z=2. Low-mass systems have decreased continuously in number and mass density (by a factor up to 4) from the present age to z=2, consistently with a prolonged mass assembly also at z<1.

Motivation & Objective

• To understand the cosmic assembly history of stellar mass in galaxies across cosmic time.
• To measure the evolution of the galaxy stellar mass function (GSMF) from z≈0.2 to z≈2.5.
• To assess the impact of star formation history assumptions on photometric stellar mass estimates.
• To test whether galaxy mass assembly is driven by mergers or prolonged star formation.
• To compare observed GSMF evolution with theoretical models of galaxy formation.

Proposed method

• Uses a spectroscopic sample of ~6,500 galaxies (I-band selected, 17.5 < I_AB < 24) over 1,750 arcmin² from the VIMOS VLT Deep Survey (VVDS).
• Combines with a photometric K-band selected sample (~10,200 galaxies, K_AB < 22.34–22.84) over 610 arcmin², with redshifts calibrated on the spectroscopic sample.
• Applies two stellar mass estimation methods: one assuming continuous star formation and another including secondary bursts, to assess systematic biases.
• Derives the GSMF, number density, and stellar mass density evolution using non-parametric and parametric fitting techniques.
• Models the evolution of the GSMF with power laws in redshift, fitting β in (1+z)^β for different mass thresholds.
• Compares results with theoretical models, including those from De Lucia et al. (2006), to assess consistency with observed mass assembly histories.

Experimental results

Research questions

• RQ1 When and how were the stellar masses of galaxies assembled across cosmic time?
• RQ2 How does the stellar mass function evolve with redshift, and is this evolution mass-dependent?
• RQ3 To what extent do secondary star formation bursts affect photometric stellar mass estimates?
• RQ4 Is the observed evolution consistent with dry merger-driven mass assembly or prolonged star formation?
• RQ5 How do the observed GSMF and mass density trends compare with predictions from current galaxy formation models?

Key findings

• The number density of galaxies with M_stars > 10^8 M_sun decreases by 59±5% from z≈0.2 to z=1, with a mild mass-dependent evolution (‘mass-downsizing’).
• Massive galaxies (M_stars > 6×10^10 M_sun) show negligible evolution (≤30%) below z≈0.7, indicating they were largely in place by z≈1.
• At z≈1, ≥50% of massive galaxies had already assembled most of their stellar mass, ruling out dry mergers as the dominant assembly mechanism below z≈1.
• Low-mass systems (M_stars < 10^9 M_sun) show a continuous decline in number density by a factor of 4.1±0.9 from z≈0.2 to z=2, indicating prolonged mass assembly.
• The stellar mass density decreases by a factor of 2.3±0.1 at z≈1 and by 4.5±0.3 at z≈2.5, showing relatively slow evolution compared to earlier surveys.
• The low-mass end of the GSMF remains flat up to high redshift, a feature not reproduced by most theoretical models, which tend to overpredict low-mass galaxy counts.