The assembly and fate of a giant disc galaxy in a protocluster at $z=3$

Abstract

Recent JWST observations revealed two massive ($M_{\star }\gtrsim 10^{11}{M}_{\odot }$), unexpectedly large spiral galaxies at $z\sim 3$, both in overdense environments. We focus on one of these, ADF22.1 at $z=3.09$, which hosts an active galactic nucleus (AGN), exploiting its extended [CII] emission ($\sim$30 kpc in diameter) with high-resolution observations from the Atacama Large Millimetre Array and JWST. We find a flat outer rotation curve reaching $\sim$530 km s${}^{-1}$, and perform, for the first time for a system of this type, a rotation-curve decomposition. We infer a dark-matter halo mass of $\log (M_{200}/M_{\odot })=12.9_{-0.3}^{+0.4}$, a baryon-to-halo mass ratio of $0.4_{-0.3}^{+0.6}$ in units of the cosmological baryon fraction, and a ratio between the baryonic and dark-matter halo specific angular momentum of $1.0_{-0.5}^{+0.7}$. Comparing these quantities with those of local galaxies, we find that ADF22.1 is indistinguishable from $z=0$ giant discs, pointing to the inefficiency of AGN feedback in halting disc growth. Using the Mapping Nearby Galaxies at Apache Point Observatory survey, we identify potential $z=0$ descendants of ADF22.1, suggesting it will evolve into an extreme (in either mass or angular momentum) early-type galaxy. Finally, we argue that cold-stream accretion, invoked to explain disc formation at $z>1$, cannot simultaneously account for its size, dynamical properties, high specific angular momentum, and baryon-to-halo mass ratio. Instead, sustained accretion from the hot circumgalactic medium, either via spontaneous or fountain-driven condensation, offers a more physically plausible formation pathway.

24 pages, 14 figures. Submitted to MNRAS; comments are welcome