Astronomers Discover an Ultra-Massive Grand-Design Spiral Galaxy

Astronomers using the James Webb Space Telescope have discovered Zhulong, the most distant grand-design spiral galaxy identified so far, located at a redshift of approximately 5.2. Phys.Org reports: The galaxy was named Zhulong, after a giant red solar dragon and god in Chinese mythology. Its mass was found to be comparable to that of the Milky Way, which is relatively high for a galaxy that formed within one billion years after the Big Bang, as the redshift indicates. The study found that Zhulong has a classical bulge and a large face-on stellar disk with spiral arms extending across 62,000 light years. The spectral energy distribution (SED) analysis points to a quiescent-like core and a star-forming stellar disk. Furthermore, it turned out that compared to the stellar disk, the center core of Zhulong is red and has the highest stellar mass surface densities measured among quiescent galaxies. The core is quiescent, which is consistent with the expectations of inside-out galaxy growth and quenching.

The study also found that although the disk is still forming stars, Zhulong has a relatively low overall star-formation rate -- at a level of 66 solar masses per year. The baryons-to-stars conversion efficiency was calculated to be approximately 0.3, which is about 1.5 times higher than even the most efficient galaxies at later epochs. These results suggest that Zhulong must have been forming stars very efficiently and is in the transformation phase from star-forming to quiescence. In concluding remarks, the authors of the paper note that Zhulong appears to be the most distant spiral galaxy discovered to date. The properties of this galaxy seem to suggest that mature galaxies emerged much earlier than expected in the first billion years after the Big Bang. The findings have been published on the pre-print server arXiv.

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