Astronomers discover an ultra-massive grand-design spiral galaxy

Using the James Webb Space Telescope (JWST), an international team of astronomers has detected a new grand-design spiral galaxy as part of the PANORAMIC survey. The newfound galaxy, named Zhúlóng, is extremely massive and appears to be the most distant spiral galaxy identified so far. The finding was detailed in a paper published December 17 on the pre-print server arXiv.

Grand-design spiral galaxies are characterized by their prominent, well-defined arms, which circle outwards from a clear core. It is assumed that the arms in such galaxies are actually overdense regions of the disk which trigger star formation as incoming material is compressed in that region.

It is still not well understood when and how spiral galaxies first emerged in the early universe and such galaxies are generally rare at high redshifts. To date, only a few individual spirals have been found at a redshift higher than 3.0.

Now, a group of astronomers led by Mengyuan Xiao of the University of Geneva in Switzerland, reports the serendipitous detection of a new high-redshift grand-design spiral galaxy using JWST. The galaxy was named Zhúlóng, after a giant red solar dragon and god in Chinese mythology.

“Zhúlóng (α, δ [J2000] = 150.124874, 2.092919) was serendipitously discovered in the field (association) named j100024p0208 in the recent data release of the JWST PANORAMIC survey, a pure parallel extragalactic NIRCam imaging program,” the researchers wrote.

According to the study, Zhúlóng was identified at a photometric redshift of approximately 5.2. 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 Zhúlóng 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 Zhúlóng 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, Zhúlóng 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 Zhúlóng 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 Zhúlóng 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.

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