Where infall meets outflows: turbulent dissipation probed by CH$^+$ and Ly$\alpha$ in the starburst/AGN galaxy group SMM J02399$-$0136 at z$\sim$2.8

TL;DR

This study compares CH$^+$ and Ly$ extalpha$ lines in a z~2.8 galaxy group, revealing turbulence-driven energy transfer, a massive turbulent reservoir, and inflowing gas fueling star formation and black hole activity.

Contribution

It provides the first detailed comparison of CH$^+$ and Ly$ extalpha$ lines in a high-redshift galaxy group, highlighting turbulence's role in gas dynamics and galaxy fueling processes.

Findings

Identification of a massive, turbulent molecular gas reservoir.

Detection of inflowing gas at ~400 km/s towards the galaxies.

Evidence of shocks at the interface of inflowing CGM and Ly$ extalpha$ emission.

Abstract

We present a comparative analysis of the $\rm CH^+$(1-0) and $\rm Ly \alpha$ lines, observed with the Atacama Large Millimeter Array (ALMA) and Keck telescope respectively, in the field of the submillimetre-selected galaxy (SMG) SMM\,J02399$-$0136 at $z\sim2.8$, which comprises a heavily obscured starburst galaxy and a broad absorption line quasar, immersed in a large $\rm Ly \alpha$ nebula. This comparison highlights the critical role played by turbulence in channeling the energy across gas phases and scales, splitting the energy trail between hot/thermal and cool/turbulent phases in the circum-galactic medium (CGM). The unique chemical and spectroscopic properties of $\rm CH^+$ are used to infer the existence of a massive ($\sim 3.5 \times 10^{10}$ ${\rm M}_\odot$), highly turbulent reservoir of diffuse molecular gas of radius $\sim 20\,$kpc coinciding with the core of the $\rm Ly \alpha$ nebula. The whole cool and cold CGM is shown to be inflowing towards the galaxies at a velocity $\sim$ 400 km$\,s^{-1}$. Several kpc-scale shocks are detected tentatively in $\rm CH^+$ emission. Their specific location in space and velocity with respect to the high-velocity $\rm Ly \alpha$ emission suggests that they lie at the interface of the inflowing CGM and the high-velocity $\rm Ly \alpha$ emission, and signpost the feeding of CGM turbulence by AGN- and stellar-driven outflows. The mass and energy budgets of the CGM require net mass accretion at a rate commensurate with the star formation rate (SFR). From this similarity, we infer that the merger-driven burst of star formation and black-hole growth are ultimately fuelled by large-scale gas accretion.