20 Times Larger Than the Milky Way: Large Atomic Gas Structure Discovered

Map of the Atomic Hydrogen 21 cm Line Emission

A map of the atomic hydrogen (HI) 21-cm line emission (shown as the red haze) in the vicinity of Stephan’s Quintet, a famous compact group of galaxies discovered in 1887, overlaid on a deep optical color image. Credit: NASA, ESA, CSA, and STScI

The atomic gas structure was discovered using the Five-hundred-meter Aperture Spherical Telescope.

Atomic gas is the fundamental material from which all galaxies are formed. The evolution of galaxies is primarily a process of accreting atomic gas from the intergalactic medium and transforming it into stars.

As a result, the observation and research of atomic gas in and around galaxies are critical to the study of galaxy formation and evolution models. Observing the 21-cm fine structure line emission of atomic hydrogen in the radio waveband is the most direct way to explore atomic gas.

Recent deep mapping observations of 21-cm line emission in the vicinity of the well-known compact group of galaxies “Stephan’s Quintet,” using the Five-hundred-meter Aperture Spherical Telescope’s (19-beam receiver) led by Xu Cong, a researcher from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), revealed a very large atomic gas structure with a length of about 2 million light-years (about 20 times the size of the Milky Way).

The discovery was recently published in the journal Nature

FAST is currently the largest and most sensitive single-dish radio telescope in the world, and its 19-beam receiver is the largest L-band multi-beam feed array for 21-cm line observations. The full commissioning of the FAST 19-beam receiver opened a new window on atomic gas in the Universe, particularly for low-density diffuse gas far away from galaxies.

“This is the largest atomic gas structure ever found around a galaxy group,” said XU. The observations reached a sensitivity of 1σ=4.2×1016 cm-2 per channel (Δv=20 km s-1; angular-resolution=4′), making them currently the most sensitive observations of atomic hydrogen 21-cm line emission at this angular resolution.

Ever since its discovery by the French astronomer Edouard Stephan in 1877, Stephan’s Quintet has continued revealing puzzles related to the complex web of galaxy-galaxy and galaxy-intragroup medium interactions in the group.

The new observations show that large-scale, diffuse, low-density gas (with a column identity less than 1018cm-2) exists far away from the center of the group, and it is likely that the gas has been there for ~1 giga years. The observations challenge the current theory of galaxy-group formation/evolution because it is not clear how the low-density atomic gas can survive ionization by the intergalactic UV background on such a long time scale.

Reference: “A 0.6 Mpc H i structure associated with Stephan’s Quintet” by C. K. Xu, C. Cheng, P. N. Appleton, P.-A. Duc, Y. Gao, N.-Y. Tang, M. Yun, Y. S. Dai, J.-S. Huang, U. Lisenfeld, and F. Renaud, 19 October 2022, Nature.
DOI: 10.1038/s41586-022-05206-x


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  • Let,discuss the following rule and facts about the evolution of galaxies.

    Evry galaxy has a supermassive bĺack hole at the center and a starformation rate.These can be calculated by observation at certain point of time in the time axis,taking origin to big bang consided through dynamics.The supermassive black hole at the center of galaxy is in a typical state of equilibrium with the rate of star formation differently depending upon the phase of evolution.From the common observation of galaxies in the universe,the fact from the experience is noted that a galaxy is in a state of saturation equilibrium 1 billion years after the big bang.At the saturated state star formation rate or mass increase rate of SMBH is a linear fixed process,simple function of rotation with time.So,a rule can be presented on evolution of earĺy galaxies as follows -
    The Octet Rule of evolution of the early gaĺaxies.

    As this rule indicates and dynamic nature of origin of galaxies,this can be concluded that at half of the octet,that is after .5 billion years of big bang,the event horizon around the supermassive black hole come to existance with active function working at higher rate than usual.Though evolution of galaxy is a continuous process,capacity of observation is an accounting factor.Thus,unit of octet,that is 125 + or _ .25 million years the first galaxy of universe is evolved;here lower limit is for most possible smooth observation condition and uppor limit is for fine obsrvation with perfect electronic structures of the atom.

    So,as per the discussion,the evolution of atomic hydrogen gas cloud around Stephan's Quintet can be glanced.Also nonionization of this by intergalactic UV background in long scale can be explained.

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