[Paper Review] The Nature of High-Redshift Galaxies

Rachel S. Somerville, Joel R. Primack+1 more|arXiv (Cornell University)|Jun 26, 2000

Astronomy and Astrophysical Research4 citations

TL;DR

This paper uses semi-analytic models within the Cold Dark Matter (CDM) framework to investigate the nature of high-redshift Lyman-break galaxies (LBGs). It finds that a 'collisional starburst' model—where mergers trigger intense star formation—best reproduces observed luminosity functions, number densities, and physical properties (e.g., star formation rates, sizes, colours) at z ~ 3–4, while quiescent models fail due to underproduction of bright galaxies and cold gas. The model implies high-redshift star formation must be significantly more efficient than locally, which the starburst model naturally explains.

ABSTRACT

Using semi-analytic models of galaxy formation set within the Cold Dark Matter (CDM) merging hierarchy, we investigate several scenarios for the nature of the high-redshift ($z \ga 2$) Lyman-break galaxies (LBGs). We consider a ``collisional starburst'' model in which bursts of star formation are triggered by galaxy-galaxy mergers, and find that a significant fraction of LBGs are predicted to be starbursts. This model reproduces the observed comoving number density of bright LBGs as a function of redshift and the observed luminosity function at $z\sim 3$ and $z\sim 4$, with a reasonable amount of dust extinction. Model galaxies at $z \sim 3$ have star formation rates, half-light radii, I-K colours, and internal velocity dispersions that are in good agreement with the data. Global quantities such as the star formation rate density and cold gas and metal content of the Universe as a function of redshift also agree well. Two ``quiescent'' models without starbursts are also investigated. In one, the star formation efficiency in galaxies remains constant with redshift, while in the other, it scales inversely with disc dynamical time, and thus increases rapidly with redshift. The first quiescent model is strongly ruled out as it does not produce enough high redshift galaxies once realistic dust extinction is accounted for. The second quiescent model fits marginally, but underproduces cold gas and very bright galaxies at high redshift. A general conclusion is that star formation at high redshift must be more efficient than locally. The collisional starburst model appears to accomplish this naturally without violating other observational constraints.

Motivation & Objective

• To determine the physical nature of high-redshift Lyman-break galaxies (LBGs) at z > 2 using theoretical modeling.
• To test whether LBGs are best explained by starburst or quiescent evolutionary scenarios.
• To reconcile observed LBG number densities, luminosity functions, and physical properties with cosmological constraints.
• To assess the role of dust extinction and star formation efficiency in shaping high-redshift galaxy evolution.
• To evaluate whether the observed star formation rate density evolution (Madau plot) can be explained by different galaxy formation models.

Proposed method

• Employing semi-analytic models of galaxy formation within the Cold Dark Matter (CDM) merging hierarchy.
• Simulating galaxy evolution using a 'collisional starburst' model where mergers trigger short-lived, intense star formation episodes.
• Comparing model predictions to observed comoving number densities and luminosity functions of LBGs at z ~ 3 and z ~ 4.
• Applying dust extinction corrections using a luminosity-dependent scaling (Wang & Heckman) to UV luminosities, with minimal (0.4L*) and maximal (all galaxies) corrections.
• Calculating star formation rate density (SFRD) via the Madau plot, integrating over the UV luminosity function with dust corrections.
• Testing two quiescent models: one with constant star formation efficiency, and one with efficiency scaling inversely with disc dynamical timescale.

Experimental results

Research questions

• RQ1 Can a collisional starburst model reproduce the observed comoving number density and luminosity function of high-redshift LBGs at z ~ 3 and z ~ 4?
• RQ2 How do dust extinction corrections affect the inferred star formation rate density and the consistency of models with observations?
• RQ3 Do quiescent models with constant or redshift-dependent star formation efficiency reproduce the observed properties of high-redshift galaxies?
• RQ4 What constraints do the observed star formation rate density, cold gas content, and metallicity evolution place on galaxy formation models?
• RQ5 Is the observed plateau in the star formation rate density at z ~ 2–4 consistent with theoretical models, or does it require enhanced star formation efficiency at high redshift?

Key findings

• The collisional starburst model successfully reproduces the observed comoving number density of bright LBGs and the luminosity function at z ~ 3 and z ~ 4, with a reasonable dust extinction level.
• Model galaxies at z ~ 3 match observed star formation rates, half-light radii, I – K colours, and internal velocity dispersions, indicating good agreement with data.
• The star formation rate density (SFRD) derived from the model shows a plateau from z ~ 2 to z ~ 4, consistent with observational data, unlike the traditional Madau plot with a peak at z ~ 2.
• The quiescent model with constant star formation efficiency is strongly ruled out due to insufficient galaxy counts after applying realistic dust extinction.
• The quiescent model with efficiency scaling inversely with disc dynamical time marginally fits the data but underproduces cold gas and very bright galaxies at high redshift.
• The study concludes that high-redshift star formation must be significantly more efficient than locally, a condition naturally satisfied by the collisional starburst model.