Serenity-18, the Rosetta Stone of galaxy formation

The amount of molecular gas has been measured for the first time in a primordial galaxy observed when the Universe was only one billion years old. The team, led by the Observatory’s researchers Valentina D’Odorico and Clara Feruglio, has also detected a clump/filament of very metal poor gas, probably feeding the galaxy’s growth. In particular, Serenity-18 is the first main sequence high redshift galaxy (z ~ 6)  for which the carbon-monoxide (CO) emission has been detected. Both the measurement of CO and the detection of the pristine gas filament are very important for our understanding of the formation of stars and galaxies, at the end of the reionization era (link to the paper).

The first measurement of the amount of molecular gas in a typical galaxy observed when the Universe was only 1 billion years old has been carried out by a team led by Valentina D’Odorico and Chiara Feruglio of the Trieste Observatory. Together with the primordial galaxy, dubbed Serenity-18, a very metal poor gas clump/filament which is probably feeding its growth, was also discovered. These results are published on a Letter to the Astrophysical Journal.

Theory and state-of-the-art cosmological hydrodynamical simulations predict that the first galaxies at cosmic dawn formed in over-dense regions, likely at the encounter point of cosmic gas filaments, from which they accrete pristine, metal poor gas (Fig. 1). Although many primordial galaxies are known today at very early epochs (z∼6-10) - mainly detected through HI Lyman alpha and [CII] emission - this assembly mechanism through feeding via cosmic filaments has never been directly observed.

Serenity-18 is located at z = 5.939, i.e. near the end of the epoch of reionisation, and the team of researchers could measure its molecular gas reservoir, M(H2) = 5 × 109 M⊙, traced by the CO molecular line emission. Molecular gas is a parameter of great relevance because it represents the fuel of star formation. This discovery was possible thanks to an extremely sensitive observation performed with the Atacama Large (Sub)Millimetre Array (ALMA) led by the same team (Fig. 2). The total IR luminosity of Serenity-18 was estimated into L(FIR) ≈ 1.2 × 1012 L⊙, corresponding to a star formation rate of ≈ 100 M⊙ yr−1. Serenity-18 is therefore the first typical main sequence galaxy at z∼6 for which CO emission was detected.

Even more interestingly, the team also detected a very metal poor (about 1/1000 of the solar metallicity) gas clump/filament which is probably feeding the galaxy’s growth. The metal poor, nearly pristine gas patch was detected through a damped Lyman-α absorber observed at the same redshift of the CO-emitting galaxy, and at a projected spatial separation of 40 kpc, along the line of sight to the quasar SDSS J2310+1855 (zem = 6.0025). The configuration of the system is sketched in Fig. 3. The chemical abundances measured for the DLA are in very good agreement with those measured for other DLAs discovered at similar redshifts, indicating an enrichment due to massive PopII stars. The preferred interpretation is that the metal poor gas traces a filament or clump, which is feeding the early growth of Serenity-18.

The CO-emitting galaxy/DLA system we discovered is a direct observational evidence of the assembly of a galaxy at Cosmic dawn, and, as such, it may be regarded as the Rosetta Stone of galaxy formation.

The quest for molecular gas in typical star forming galaxies at z∼6 is a highly competitive scientific topic. On the same hand, studies using DLAs as signposts of galaxies for ALMA CO and [CII] observations are becoming more and more numerous at redshifts z=2-4. The very high-z of Serenity-18 makes it a key science case for the calibration of the [CII]-CO relation in primordial galaxies, in order to support the design of future ALMA observations of the first galaxies.

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Figure 1. Maps of the H I column density (left panel) and metallicity (right panel) for a simulated galaxy at z∼6, which lies at the center of each panel (Pallottini et al. 2017). The maps are 80 kpc on a side. Serenity-18 has characteristics very similar to the simulated galaxy, in this framework the DLA could trace the gas in the filament or in the periphery of one of the small clumps/satellites that are embedded in the same filament as the galaxy and that will eventually feed its growth. Image credits Andrea Pallottini. 


Figure 2. [Left panel]: the velocity-integrated map of the CO(6-5) line of Serenity-18 (indicated by a white arrow), located about -6.3, +2.2 arcsec offset from the quasar position (red star), which was the primary target of our ALMA observation. Contours are 2, 3, 4, 5σ, 1σ = 5.1 μJy/beam. [Middle panel]: a zoomed view onto the CO-emitting galaxy. [Right panel]: the ALMA spectrum of Serenity-18. The red curve is the fit to the CO(6-5) emission, while the red vertical mark corresponds to the redshifted frequency of the CO(6-5) line expected at the redshift of the DLA, which was detected in the XSHOOTER spectrum (Fig.2). The dashed magenta lines show the range where the line emission has been integrated to produce the velocity-integrated map (left panel). Figure from D’Odorico et a. (2018).


Figure 3. [Left panel]: Sketch of the system configuration of Serenity-18 and its associated DLA. The 40 kpc angular separation between the CO-emitting galaxy and the DLA lead us to exclude that the absorption system is located in the outer disk of Serenity-18.  Image credits Valentina D’Odorico.  [Right panel]: Fit of the observed transitions for the proximate DLA in the spectrum of J2310. In each panel, the observed spectrum is shown in blue, the error in red and the Voigt fit in green. The top panel shows the fit of the H I Ly-α velocity profile. The red mark indicates the redshift z = 5.938646, which is also the zero-point of the velocity scale in all the other panels. Figure drom D’Odorico et a. (2018).