Merging gas-rich galaxies that harbor low-luminosity twin quasars at z = 6.05: a promising progenitor of the most luminous quasars

Abstract: We present ALMA [CII] 158 $\mu$m line and underlying far-infrared (FIR) continuum emission observations ($0''.57 \times 0''.46$ resolution) toward a quasar-quasar pair system recently discovered at $z = 6.05$ (Matsuoka et al. 2024). The quasar nuclei (C1 and C2) are faint ($M_{\rm 1450} \gtrsim -23$ mag), but we detect very bright [CII] emission bridging the 12 kpc between the two objects and extending beyond them (total luminosity $L_{\rm [CII]} \simeq 6 \times 10^9~L_\odot$). The [CII]-based total star formation rate of the system is $\sim 550~M_\odot$ yr$^{-1}$ (IR-based dust-obscured SFR is $\sim 100~M_\odot$ yr$^{-1}$), with a [CII]-based total gas mass of $\sim 10^{11}~M_\odot$. The dynamical masses of the two galaxies are large ($\sim 9 \times 10^{10}~M_\odot$ for C1 and $\sim 5 \times 10^{10}~M_\odot$ for C2). There is a smooth velocity gradient in [CII], indicating that these quasars are a tidally interacting system. We identified a dynamically distinct, fast [CII] component around C1: detailed inspection of the line spectrum there reveals the presence of a broad wing component, which we interpret as the indication of fast outflows with a velocity of $\sim 600$ km s$^{-1}$. The expected mass loading factor of the outflows, after accounting for multiphase gas, is $\gtrsim 2-3$, which is intermediate between AGN-driven and starburst-driven outflows. Hydrodynamic simulations in the literature predicted that this pair will evolve to a luminous ($M_{\rm 1450} \lesssim -26$ mag), starbursting ($\gtrsim 1000~M_\odot$ yr$^{-1}$) quasar after coalescence, one of the most extreme populations in the early universe.

• The paper reveals that ALMA observations expose an extended [CII] bridge spanning ~12 kpc with a high star formation rate of about 550 M☉/yr.
• The paper demonstrates that dynamical mass estimates and smooth velocity gradients confirm the system is in a pre-merger stage, indicating significant tidal interactions.
• The paper identifies fast outflows reaching up to 600 km/s near one nucleus, suggesting combined AGN and starburst feedback that may evolve into a hyper-luminous quasar.

Merging Gas-rich Galaxies with Twin Quasars at Redshift 6.05

The study conducted by T. Izumi and collaborators provides an in-depth analysis of a quasar-quasar pair system at redshift $z = 6.05$. Utilizing ALMA observations, the authors investigate the [CII] 158 µm line and associated far-infrared (FIR) continuum emissions to uncover the dynamics and star formation rates of this unique cosmic system. This research provides valuable insights into the characteristics and evolution of high-redshift quasar systems, presenting this quasar pair as a significant precursor to the formation of highly luminous quasars in the early universe.

Observational Findings

The quasar nuclei in this study, labeled as C1 and C2, are characterized by relatively low luminosities with $M_{1450} \gtrsim -23$ mag. Interestingly, the study detects a substantially bright [CII] emission that bridges the distance of approximately 12 kpc between C1 and C2, extending even beyond this gap. The total [CII] luminosity is calculated to be around $6 \times 10^9~L_{\odot}$, with the [CII]-inferred star formation rate (SFR) of about $550~M_{\odot}\,\text{yr}^{-1}$ and a gas mass close to $10^{11}~M_{\odot}$. These observations suggest a dynamic environment with significant star-forming activity.

Dynamics and Implications

A notable aspect of this research is the assessment of the dynamical masses of the galaxies involved, which are approximated to be $9 \times 10^{10}~M_{\odot}$ for C1 and $5 \times 10^{10}~M_{\odot}$ for C2. Additionally, a smooth velocity gradient in the [CII] emission highlights a tidally interacting system, key evidence supporting the notion of these galaxies being in a pre-merger stage. This interaction likely contributes to the creation of the observed [CII] bridge and influences the emission characteristics.

Around C1, the research identifies fast-moving [CII] components interpreted as outflows reaching velocities near 600 km/s. These outflows are postulated to reflect an intermediate mass-loading factor ($\eta \gtrsim 2-3$), a trait suggestive of combined AGN and starburst-driven outflows. This places the system's outflows between purely AGN-driven and starburst-driven phenomena, hinting at complex feedback mechanisms.

Future Trajectories and Theoretical Implications

Theoretical models anticipate that systems like this quasar pair will evolve into hyper-luminous quasars post-coalescence, with amplified star formation (>1000 $M_{\odot}\,\text{yr}^{-1}$), situating them among the most extreme objects in the early universe. Given the significant gas reservoirs and dynamical interactions, the pair is poised for significant evolutionary transformations, possibly leading to a single massive galaxy system with heightened quasar activity.

Given the findings, there is a clear implication that such high-redshift systems may be more frequent precursors to the luminous quasars seen at $z > 6$. The detection of dual AGN activity within such mergers further dialogues with emerging paradigms in galaxy evolution and the pivotal role of mergers in fueling early quasar growth.

This study not only underscores the power of high-resolution ALMA observations in unraveling the complexities of gas dynamics and interactions at great distances but also feeds into broader discussions on the mechanisms behind rapid star formation and quasar evolution in the epoch of reionization. Future observations leveraging tools such as JWST and expanded ALMA datasets could provide even greater insight into the structures and feedback processes within these early cosmic assemblies, enhancing models of galaxy and supermassive black hole co-evolution.