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Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have observed a dusty star-forming galaxy known as SPT 0538−50. Results of the observational campaign, Dec. 4 on the arXiv pre-print server, provide more insights into the structure of this galaxy.

The so-called dusty star-forming galaxies (DSFGs) are highly obscured galaxies undergoing a period of intense star formation, with star-formation rates reaching even 1,000 solar masses per year. However, although many DSFGs are known, their nuclear structure, which can be essential to better understand the evolution of these galaxies, is still not fully explored.

SPT 0538−50 is a DSFG at a redshift of 2.78, with an infrared luminosity of 3.4 trillion solar luminosities. The galaxy has a stellar mass of about 33 billion solar masses and a star-formation rate at a level of 760 solar masses per year.

Recently, a group of astronomers led by Hannah Stacey of the European Southern Observatory (ESO), performed long-baseline continuum observations of SPT 0538−50 with ALMA, with the aim of disclosing its nuclear structure.

ALMA observations found that the central region of SPT 0538−50 exhibits a dual spiral arm morphology and a potential nuclear bar. The researchers assume that the nuclear bar could facilitate gas inflow, feeding the nuclear starburst and the galaxy's supermassive black hole. They added that these findings suggest that SPT 0538−50 may be dynamically cold, like local nuclear disks.

The study found that SPT 0538−50 has an effective radius of approximately 1,950 light years, a Sersic index of 1.2, and a position angle of 4.0. These parameters are consistent with a compact disk, comparable to other known DSFGs.

Based on the collected data, the astronomers obtained an extreme star-formation rate density for the central region of SPT 0538−50, which was estimated to be about 2,000 solar masses/year/kiloparsec². Therefore, this is a super-Eddington rate when compared to the Eddington limit of nearby ultra-luminous infrared galaxies.

The authors of the paper noted that a high gas density is required to explain such high star-formation rate density in the central region of the investigated galaxy. They added that additional influence of secular dynamical processes would be required in order to pile a high density of gas within a few hundred light years from the center of SPT 0538−50.

"These features may indicate that secular dynamical processes play a role in accumulating a high concentration of cold gas that fuels the rapid formation of a compact stellar spheroid and black hole accretion," the scientists concluded.

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