Rise of the Titans: A Dusty, Hyper-Luminous "870 micron Riser" Galaxy at z~6

Abstract: We report the detection of ADFS-27, a dusty, starbursting major merger at a redshift of z=5.655, using the Atacama Large Millimeter/submillimeter Array (ALMA). ADFS-27 was selected from Herschel/SPIRE and APEX/LABOCA data as an extremely red "870 micron riser" (i.e., S_250<S_350<S_500<S_870), demonstrating the utility of this technique to identify some of the highest-redshift dusty galaxies. A scan of the 3mm atmospheric window with ALMA yields detections of CO(5-4) and CO(6-5) emission, and a tentative detection of H2O(211-202) emission, which provides an unambiguous redshift measurement. The strength of the CO lines implies a large molecular gas reservoir with a mass of M_gas=2.5x10^11(alpha_CO/0.8)(0.39/r_51) Msun, sufficient to maintain its ~2400 Msun/yr starburst for at least ~100 Myr. The 870 micron dust continuum emission is resolved into two components, 1.8 and 2.1 kpc in diameter, separated by 9.0 kpc, with comparable dust luminosities, suggesting an ongoing major merger. The infrared luminosity of L_IR~=2.4x10^13Lsun implies that this system represents a binary hyper-luminous infrared galaxy, the most distant of its kind presently known. This also implies star formation rate surface densities of Sigma_SFR=730 and 750Msun/yr/kpc2, consistent with a binary "maximum starburst". The discovery of this rare system is consistent with a significantly higher space density than previously thought for the most luminous dusty starbursts within the first billion years of cosmic time, easing tensions regarding the space densities of z~6 quasars and massive quiescent galaxies at z>~3.

• The paper identifies ADFS-27 as one of the highest-redshift dusty galaxies via an 870 μm riser selection using far-IR imaging.
• The paper reports ALMA detection of strong CO emissions and tentative H2O, deriving a molecular gas reservoir of ~2.5×10^11 solar masses that fuels a ~2400 solar masses/yr starburst.
• The paper reveals a binary HyLIRG system with two merging components separated by 9 kpc, highlighting major mergers in early galaxy formation.

An Analysis of the Discovery of A Dusty Hyper-Luminous Galaxy at z~6

The paper by Riechers et al. presents the detection and detailed study of ADFS-27, a hyper-luminous infrared galaxy (HyLIRG) at a redshift of z = 5.655, using observational data obtained from ALMA and other telescopes. This galaxy is characterized by its extreme luminosity, dusty nature, and significant distance, providing insights into galaxy formation during the early universe.

Key Findings

1. Detection and Identification: ADFS-27 was identified as a candidate due to its extremely red color profile in the far-infrared (FIR) bands obtained from Herschel/SPIRE and APEX/LABOCA. Its selection as an "870 μm riser" allowed the researchers to distinguish it as one of the highest-redshift dusty galaxies.
2. Observations and Results: Utilizing ALMA, the researchers detected significant emissions of CO lines (\eco\ and \fco) and tentatively identified \water\ emission. These lines provided an unequivocal redshift determination and insights into the galaxy's molecular gas composition.
3. Molecular Gas and Star Formation: The \eco\ and \fco\ lines suggested a substantial molecular gas reservoir with a mass of about 2.5 x 10^11 \msol. The calculated star formation rate (SFR) was approximately 2400 \msol\,yr$^{-1}$, sustained for roughly 100 Myr. This indicates a remarkably high SFR surface density consistent with a "maximum starburst" scenario.
4. Morphology and Structure: High-resolution ALMA imaging resolved the 870 μm continuum into two components, separated by about 9.0 kpc, hinting at an ongoing major merger. Each component harbors considerable dust luminosities, establishing ADFS-27 as a binary HyLIRG, the most distant known of its kind currently.
5. Quantitative Analysis: The fitted spectral energy distribution (SED) implied a total infrared luminosity of approximately 2.4 x 10^13 \lsol. Dust mass was estimated to be around 4.4 x 10^9 \msol, and the gas-to-dust ratio was consistent with other known high-redshift starbursts.

Implications and Future Directions

• Galactic Evolution: The discovery of ADFS-27 suggests that highly luminous dusty galaxies were more prevalent in the universe's first billion years than previously assumed. This realization could resolve existing discrepancies regarding the space densities of high-redshift quasars and massive galaxies at z>3.
• Measuring Early Universe Starbursts: The findings support the notion that massive starbursts were critical contributors to early cosmic structure formation, potentially linked to the growth of supermassive black holes (SMBHs).
• Techniques and Observations: The study validates the use of color selection techniques in identifying distant DSFGs and highlights the importance of advanced facilities like ALMA for characterizing such galaxies.
• Future Studies: Continued efforts in identifying and analyzing more such galaxies could refine our understanding of early universe star formation, molecular gas dynamics, and the evolutionary path toward present-day massive quiescent galaxies.

The study of ADFS-27 underlines the importance of multi-wavelength observations and sophisticated data analysis methods in advancing our knowledge of galaxy formation and evolution in the early universe. It opens avenues for future explorations into the nature of high-redshift star-forming galaxies, offering a deeper comprehension of their contribution to cosmic history.