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

Today’s most massive ellipticals are proposed to originate from starbursting galaxies in $z\gtrsim 2$ 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 $0.15\,\mathrm{ arcsec}$ resolution CO(4−3), [C i]1−0, and dust-continuum observations, we characterized 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 $\sim 520$ km s$^{-1}$, whereas T is most likely a merger. Combined with recent studies, we find that $\gtrsim 44~{{\ \rm per\ cent}}$ 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 $\gt 400$ 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 haloes comparable to the most massive $z\sim 0$ ellipticals at the centres of groups and clusters. Despite their extreme star formation rates (SFRs), 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 internal streaming, such that only a small fraction of kinetic energy feeds disc-wide turbulence.