[Paper Review] Evolution of the specific Star Formation Rate Function at z<1.4 - Dissecting the mass-SFR plane in COSMOS and GOODS

O. Ilbert, S. Arnouts+22 more|arXiv (Cornell University)|Oct 17, 2014

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study measures the evolution of the specific star formation rate (sSFR) function in star-forming galaxies across 0.2 < z < 1.4 using mid- and far-infrared data from the COSMOS and GOODS surveys. It reveals mass-dependent sSFR evolution, increasing as (1+z)^b with b rising from 2.88 to 3.78 from low to high masses, and finds increasing scatter in the sSFR function with stellar mass, indicating growing diversity in star formation histories in massive galaxies.

ABSTRACT

The relation between the stellar mass and the star formation rate characterizes how the instantaneous star formation is determined by the galaxy past star formation history and by the growth of the dark matter structures. We deconstruct the M-SFR plane by measuring the specific SFR functions in several stellar mass bins from z=0.2 out to z=1.4. Our analysis is primary based on a MIPS 24$μm$ selected catalogue combining the COSMOS and GOODS surveys. We estimate the SFR by combining mid- and far-infrared data for 20500 galaxies. The sSFR functions are derived in four stellar mass bins within the range 9.5

Motivation & Objective

• To measure the evolution of the specific star formation rate (sSFR) function across cosmic time (z < 1.4) in star-forming galaxies.
• To investigate how the sSFR function varies with stellar mass and redshift, particularly in the context of the mass-SFR plane.
• To assess the role of selection effects in interpreting the mass-SFR relation and to test predictions from semi-analytical models (SAMs).
• To determine whether the shape of the sSFR function remains invariant over time and how its scatter evolves with mass.
• To explore physical mechanisms that may explain the observed decline in sSFR with increasing stellar mass and increasing scatter in the sSFR function.

Proposed method

• Constructs a 24 µm-selected galaxy catalogue combining data from the COSMOS and GOODS surveys to ensure deep, unbiased coverage of star-forming galaxies.
• Estimates star formation rates (SFRs) using combined mid- and far-infrared photometry, which minimizes dust extinction bias and enables robust SFR measurements.
• Derives sSFR functions (sSFR = SFR / M⋆) in four stellar mass bins spanning 9.5 < log(M⋆/M☉) < 11.5 across redshifts z = 0.2 to z = 1.4.
• Applies statistical corrections for selection effects and completeness to ensure accurate sSFR function estimation, especially at low sSFR values.
• Models the sSFR function as a log-normal distribution to quantify its intrinsic scatter (σ) and assess its evolution with redshift and mass.
• Compares observed sSFR functions with predictions from a semi-analytical model (Wang et al. 2008) and with cosmological accretion rates to test physical consistency.

Experimental results

Research questions

• RQ1 How does the median sSFR evolve with redshift, and does this evolution depend on stellar mass?
• RQ2 Is the shape of the sSFR function invariant with time at z < 1.4, or does it evolve in response to cosmic time and mass?
• RQ3 Why does the observed sSFR function deviate from semi-analytical model predictions at high stellar masses?
• RQ4 What physical processes could explain the increasing scatter in the sSFR function with increasing stellar mass?
• RQ5 How does the sSFR depend on stellar mass at fixed redshift, and what does this imply about the star formation history of massive galaxies?

Key findings

• The median sSFR evolves as (1+z)^b with b increasing from 2.88 ± 0.12 at M⋆ ≈ 10^9.75 M☉ to 3.78 ± 0.60 at M⋆ ≈ 10^11.1 M☉, indicating stronger evolution in massive galaxies.
• The intrinsic scatter (σ) of the sSFR function increases from 0.28 dex at low masses (log(M⋆/M☉) ≈ 9.5–10) to 0.46 dex at high masses (log(M⋆/M☉) ≈ 11–11.5), indicating growing diversity in star formation histories with increasing mass.
• The sSFR function shape remains invariant with redshift at z < 1.4, but its scatter increases with stellar mass, suggesting that mass-dependent physical processes drive SFH diversity.
• The sSFR decreases with stellar mass as log₁₀(sSFR) ∝ -0.17 M⋆, a trend inconsistent with a linear relation and better explained by exponentially declining star formation histories with τ ∝ 1/M⋆.
• The observed sSFR evolution is consistent with cosmological accretion rates and semi-analytical model predictions only for M⋆ < 10^10 M☉; the agreement breaks down for more massive galaxies, indicating limitations in current models.
• Physical mechanisms such as hot halo gas exhaustion, declining star formation efficiency in massive, bulge-dominated systems, and secular evolution may explain the observed decline in sSFR and increasing scatter at high masses.