Bright [CII]158$\hat{I}¼$m Streamers as a Beacon for Giant Galaxy Formation in SPT2349$-$56 at $z=4.3$

Abstract

Observations of extreme starbursts, often located in the cores of protoclusters, challenge the classical bottom-up galaxy formation paradigm. Giant elliptical galaxies at $z=0$ must have assembled rapidly, possibly within few 100 Myr through an extreme growth phase at high-redshift, characterized by elevated star-formation rates of several thousand solar masses per year distributed over concurrent, gas-rich mergers. We present a novel view of the $z=4.3$ protocluster core SPT2349$-$56 from sensitive multi-cycle ALMA dust continuum and [CII]158$\hat{I}¼$m line observations. Distributed across 60 kpc, a highly structured gas reservoir with a line luminosity of $L_{[\mathrm{CII}]}=3.0\pm 0.2\times 10^9$ $L_{\odot }$ and an inferred cold gas mass of $M_{gas}=8.9\pm 0.7\times 10^9$ $M_{\odot }$ is found surrounding the central massive galaxy triplet. Like ``beads on a string'', the newly-discovered [CII] streamers fragment into a few kpc-spaced and turbulent clumps that have a similar column density as local Universe spiral galaxy arms at $\hat{I}{\pounds }_{gas}=20$--$60$ $M_{\odot }$ pc${}^{-2}$. For a dust temperature of 30 K, the [CII] emission from the ejected clumps carry $\gtrsim$3% of the FIR luminosity, translating into an exceptionally low mass-to-light ratio of $\hat{I}{\pm }_{[\mathrm{CII}]}=2.95\pm 0.3$ $M_{\odot }$ $L_{\odot }^{-1}$, indicative of shock-heated molecular gas. In phase space, about half of the galaxies in the protocluster core populate the same caustic as the [CII] streamers ($r/r_{vir}\times |\hat{I}”v|/\ddot{I}{ƒ}_{vir}\approx 0.1$), suggesting angular momentum dissipation via tidal ejection while the brightest cluster galaxy (BCG) is assembling. Our findings provide new evidence for the importance of tidal ejections of [CII]-bright, shocked material following multiple major mergers that might represent a landmark phase in the $z\gtrsim 4$ co-evolution of BCGs with their hot, metal enriched atmospheres.

31 pages, 12 figures, 5 tables. Submitted to ApJ