On the puzzling plateau in the specific star formation rate at z=2-7

TL;DR

This study investigates the persistent puzzle of a constant specific star formation rate (sSFR) at high redshifts (z=2-7), which conflicts with galaxy formation models, by exploring modifications to semi-analytic models to reconcile observations.

Contribution

The paper introduces novel modifications to galaxy formation models, such as delayed star formation and enhanced growth mechanisms, to explain the observed sSFR plateau at high redshifts.

Findings

Successful models require suppressed SFR at z>4

Delayed gas consumption allows later star formation in massive galaxies

Enhanced growth or starburst activity explains the sSFR plateau

Abstract

The observational indications for a constant specific star-formation rate (sSFR) in the redshift range z=2-7 are puzzling in the context of current galaxy-formation models. Despite the tentative nature of the data, their marked conflict with theory motivates a study of the possible implications. The plateau at sSFR ~ 2 Gyr^-1 is hard to reproduce because (a) its level is low compared to the cosmological specific accretion rate at z > 6, (b) it is higher than the latter at z ~ 2, (c) the natural correlation between SFR and stellar mass makes it difficult to manipulate their ratio, and (d) a low SFR at high z makes it hard to produce enough massive galaxies by z ~ 2. Using a flexible semi-analytic model, we explore ad-hoc modifications to the standard physical recipes trying to obey the puzzling observational constraints. Successful models involve non-trivial modifications, such as (a) a suppressed SFR at z > 4 in galaxies of all masses, by enhanced feedback or reduced SFR efficiency, following an initial active phase at z > 7, (b) a delayed gas consumption into stars, allowing the gas that was prohibited from forming stars or ejected at high z to form stars later in more massive galaxies, and (c) enhanced growth of massive galaxies, in terms of either faster assembly or more efficient starbursts in mergers, or by efficient star formation in massive haloes.