Revealing the nature of the starburst galaxies in the $z=2.4$ overdensity HATLAS J0849

TL;DR

This study investigates the properties of gas-rich starburst galaxies at z=2.4, revealing that many are rotation-dominated discs with high velocities, challenging current galaxy evolution models and linking these early systems to present-day ellipticals.

Contribution

It provides detailed observations of five galaxies in a high-redshift overdensity, showing that a significant fraction are rotation-dominated discs with high velocities, which is not yet reproduced by models.

Findings

Over 42% of gas-rich starbursts in overdensities are rotation-dominated discs.

High-velocity discs (>400 km/s) are mostly found in overdense environments.

These early discs resemble local massive ellipticals in halo mass and gas properties.

Abstract

Today's most massive ellipticals are proposed to originate from starbursting galaxies in $z\gtrsim2$ overdensities. To discern what triggers these starbursts, and their $z=0$ descendants, we performed a detailed case study of five gas-rich galaxies in the $z=2.41$ overdensity, HATLAS J084933.4+021443. Using 0.15" resolution CO(4-3), [C I] 1-0, and dust-continuum observations, we characterised their cold gas morphology and kinematics. We find two rotating discs, W and C, both exhibiting non-axisymmetric radial gas motions (consistent with bars). Of the two extreme starbursts, W is a lopsided, rotation-dominated disc with a rotation velocity of $\sim520$ km s$^{-1}$, whereas T is most likely a late-stage merger. Combined with recent studies, we find that $\gtrsim42\%$ of gas-rich, massive starbursts in overdensities are rotation-dominated discs, a fraction not yet systematically reproduced by galaxy evolution models. Beyond $z=1$, disc galaxies with rotation velocities of $>400$ km s$^{-1}$ reside almost exclusively in overdensities, consistent with early mass assembly in dense environments. By comparing to local early-type galaxies with cold gas discs, we confirm that these systems already reside in halos comparable to the most massive $z\sim0$ ellipticals at the centres of groups and clusters. Despite their extreme star-formation rates, these discs lie on the same $\sigma-$SFR locus as lower-SFR field galaxies, implying that stellar feedback remains the dominant turbulence driver. We postulate that this is because inflowing gas is effectively transported through ordered streaming, such that only a small fraction of kinetic energy feeds disc-wide turbulence.