Revealing the nature of the starburst galaxies in the <i>z</i> = 2.4 overdensity HATLAS J0849

Abstract Today’s most massive ellipticals are proposed to originate from starbursting galaxies in z ≳ 2 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 $0\rm{.\!\!^{\prime \prime }}15$ 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 of which exhibit non-axisymmetric gas motions. Of the two extreme starbursts, W is a lopsided, rotation-dominated disc with a rotation velocity of ~520 km s−1, whereas T is most likely a late-stage merger. Combined with recent studies, we find that ≳ 44 % of massive, gas-rich 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 &amp;gt;400 km s−1 frequently appear 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 ~ 0 ellipticals at the centres of groups and clusters. Despite their extreme star-formation rates, these discs lie on the same σ −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 internal streaming, such that only a small fraction of kinetic energy feeds disc-wide turbulence.