An ancient stream of stars

Star Stream

Illustration of the phoenix star stream (Image: Geraint F. Lewis / S5 Collaboration)

The fringes of the Milky Way also include numerous globular clusters – old, dense collections of mostly almost the same age stars. But now astronomers have discovered the remainder of such a cluster, which has been frayed to form a star stream and which is so far unique in our galaxy. Because the stars contained in this “Phoenix Stream” baptized star band have an exceptionally low content of iron and other heavy elements. It is well below the lower limit for this so-called metallicity, which was previously the lower limit for star clusters. How this is possible and whether the Phoenix Stream is perhaps the first representative of an entire population remains open.

Many stars in our galaxy – probably including our sun – were once born in star clusters. These emerged from dense accumulations of gas and dust, which collapsed under their own gravity and thus formed whole “nests” of stars. It was only later that turbulence within the galaxy caused the clusters to be torn apart. In the outer area of ​​the Milky Way, in its halo, around 150 old globular clusters have been preserved. Typical for these clusters of around one hundred thousand to one million stars is the almost same age of their members and a relative poverty of elements heavier than helium – astronomers speak of a low metallicity. However, so far there seemed to be a lower limit: “Observations in the Milky Way and other galaxies have found evidence of a metallic soil,” explain Zhen Wan from the University of Sydney and his colleagues. “According to this, no globular star clusters with metal contents of less than 0.3 to 04 percent of the solar metallicity were found.” With regard to the ratio of iron to hydrogen, this assumed lower limit is -2.5.

“Impossible” metal poor

But now Wan and his team have discovered a surprising exception to this lower metallic limit. It is a small, narrow stream of stars about 60,000 light years from Earth that astronomers have dubbed the “Phoenix Stream”. “The length of the Phoenix Stream spans around eight arc minutes in the sky and was originally discovered by the Dark Sky Survey,” report Wan and his team. To find out more about the origin of this star stream, they analyzed the movements of the stars and their spectrum using the Anglo-Australian Telescope in New South Wales, Australia. From the data, the astronomers conclude that the Phoenix Current probably originated from a star cluster in the vicinity of the Milky Way that was captured and torn apart by our galaxy around two billion years ago. This predecessor of the Phoenix Current probably comprised hardly more than 30,000 solar masses, as the researchers report.

The researchers then determined the metallicity of the stars in the Phoenix Stream. The surprising result: “With one exception, all measured metallicities for these stars are substantially below Fe / H = -2.5”, report the astronomers. Both the star flux and its predecessor star cluster therefore include stars with an extremely low proportion of heavier elements – and a metallicity that, at -2.7, is significantly below the previously applicable lower limit for globular clusters. “This star stream comes from a star cluster that, according to our understanding, should not have existed at all,” emphasizes co-author Daniel Zucker from Macquarie University. Neither in the Milky Way nor in any other galaxy in the local universe is a globular cluster known with such a low level of metallicity.

Relic from the early days of the cosmos?

The star cluster from which the Phoenix Stream was created is thus so far unique – and its existence is difficult to explain. “One possible explanation would be that the Phoenix Stream is the last of its kind – the relic of a population of globular clusters that formed under radically different conditions than those found in our cosmic environment today,” speculates co-author Ting Li from the observatories the Carnegie Institution for Science in Pasadena. This could mean that there were significantly more such low-metal globular clusters in the early days of the Milky Way, which were then torn apart and destroyed in the course of the galactic evolution. “We may have discovered the remains of one of these clusters just in time before they are forever scattered in the halo of our galaxy,” says Wan.

Nevertheless, the astronomers do not rule out that there could be more such relics of ancient globular clusters in the halo of the Milky Way. “Should we find more such star streams, this could provide new insights into what was going on in the early universe,” says Li. Perhaps, the astronomers hope, NASA’s James Webb Space Telescope can help identify more remains to track down such extremely low-metal globular clusters. The launch of the space telescope is planned for the end of 2021. “Even if globular clusters like the predecessor of the Phoenix Stream no longer exist, their relics could continue to exist as thin star streams,” says co-author Jeffrey Simpson of the University of New South Wales.

Source: Zhen Wan (The University of Sydney) et al., Nature, doi: 10.1038 / s41586-020-2483-6

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