A puzzling tale of distant galaxy GN-z11: the vanishing and reappearing dust veil

[Posted on 14 April 2023]


Astronomers could soon be compelled to rethink the ways of formation and evolution of the earliest galaxies in our Universe. The latest spectroscopic results from GN-z11 -- identified as one of the distant and early galaxies-- has confirmed a complete absence of dust particles from its surroundings for an interim time period despite possessing a very high star formation rate.

The process of star formation and the subsequent stellar evolution inevitably generates massive amounts of dust and makes the host galaxy opaque to some extent due to the apparent view of a thick veil around it. This textbook-like phenomenon has been missing in GN-z11’s behaviour, leaving astronomers amazed beyond measure.

A compact galaxy containing dense matter, GN-z11, was first discovered in 2015 by the Hubble Space Telescope (HST). GN-z11 displays a high redshift (of the order z = 10.95). This implies that it is located approximately 32 billion light years away from the Earth. Even though it has been in existence since the Universe was barely 400 million years old post the Big Bang event, it is surprising that GN-z11 had already formed a billion solar mass worth of stars, an anomalous behaviour according to astronomers. Back then, GN-z11 already possessed approximately three percent of the total stellar mass of what our own galaxy, the Milky Way, holds today when the Universe is 14 billion years old.

Astronomers intrigued by the absence of dust in GN-z11 have tried to unravel the mystery behind it. Scientists of the Raman Research Institute, an autonomous institute of the Department of Science and Technology (DST), have inferred the possible physical characteristics of the violent dynamical events that could have resulted in the destruction and evacuation of dust out of the galaxy on a relatively short time, making in transparent.

The scientists have demonstrated that the energetics of the observed star formation rate was sufficient to tear apart the dusty veil on time-scales of 20–25 Myr. They have also shown that this was a temporary phenomenon and was followed soon after by the last of the supernovae explosions resulting in the shrinking of the expanding shell, thereby obscuring the galaxy on time-scales of around 5–8 Myr.

“It is simply mind-boggling to think when and how GN-z11 gathered so much of gas which ultimately collapsed to form massive stars and remained dust-free. And all this, while the galaxy came into existence when our Universe was very young, that is, around 420 million years,” remarked lead author of the newest paper Prof. Biman Nath, senior faculty at the Raman Research Institute (RRI), funded by the Department of Science and Technology.

Some of the possible reasons to explain this temporary disappearance of dust clouds, the study states, are – the suppression of dust by reverse shocks from a supernovae explosion, destruction of dust by supernovae-triggered shocks, evacuation of dust by gaseous outflow driven by other stellar activity. Likewise, the re-emergence of the dust veil could be linked with the massive gravitation force possessed by GN-z11.

Prof. Nath, along with his Russian collaborators Evgenii O Vasiliev, Sergey A. Drozdov, and Yuri A. Shchekinov from the Lebedev Physical Institute, used observations of the past five years obtained from the Keck telescopes located at Mauna Kea in the Hawaiian Islands. In the present study titled ‘Dust-free starburst galaxies at redshift z>10’ published recently in the Monthly Notices of the Royal Astronomical Society, the researchers developed specialised computer simulations to understand anomalous galactic behaviour displayed by GN-z11 to prove a new possibility that galaxies with high redshift could remain dust-free.

“Computing and simulating data of 25- 30 million years, during which the dust levels around this distant galaxy was negligible, making it appear nearly transparent, was challenging,” said Prof. Nath, who dubbed this phenomenon involving vanishing and re-emerging of dust clouds as a possibly an intermittent activity never known before.

Astronomers are now beginning to believe that many primeval galaxies could show similar opacity but have remained undetected. However, the study of high redshift galaxies could get more complex and exciting starting in the near future with better data generated by the recently launched James Webb Space Telescope.

Research Paper link - https://academic.oup.com/mnras/article/521/1/662/7049970