The fastest rotating disk galaxy – UGC 12591

Last updated on November 10th, 2020 at 03:06 pm

The Moon goes around the Earth at about a kilometer a second; the Earth goes around the Sun at about 30 kilometers a second; and the Sun goes around the Milky Way at about 200 kilometers per second. The stars in UGC 12591 (the fastest spinning galaxy known) go around its center at about 500 kilometers a second (exceeds the previously known fastest rotating galaxy by about 30%); and at that speed, one won’t even require one and a half minute to go around the entire circumference of the Earth!

The History

The speed of the galaxy UGC 12591 was first measured in 1986 by Giovanelli, Haynes, Rubin and Ford and was published in their paper titled “UGC 12591 : The most rapidly rotating disk galaxy” in February, 1986. 

Kent Ford had made an advanced spectrometer, also known as Image Tube Spectrograph, which hiked the then spectrograph resolutions for studying spectroscopy to observe finer difference in wavelengths of light recorded. These new spectrographs then came into use to resolve the finer difference in wavelengths recorded from two ends of a rotating galaxy to calculate the velocity difference on either ends, and hence the rotation speed of the galaxy. Vera Rubin used the spectrograph to observe the rotation curve for galaxies in ‘60s to compare the rotation speeds of the stars in the galaxy around its centre as a function of their distance from the centre. The curve hence obtained showed that the rotation speed of stars near the centre of the target galaxy was smaller as compared to those on the outskirts of the galaxy, in contradiction to the expected fall in rotation speeds as one moves away from the centre because of the observational fact that most of the mass (stars) in the galaxy was concentrated near the centre and fell off rapidly as one moves outwards. This curve served as another verification for the existence of the then hypothesised dark matter. 

The new evidence

The next work on the galaxy UGC 12591 was published by Dai, Anderson, Bregman and Miller in 2012 in their paper titled “XMM-Newton detects a Hot Gaseous Halo in the fastest rotating spiral galaxy UGC 12591”, about two and a half decades after the Giovanelli paper addressing the discovery of UGC 12591 with the fastest rotational speed. This paper described the efforts by Dai and collaborators to discover the halo of hot gas radiating thermal X-ray radiation around the galaxy, which might have been heated by star formation around it or the energy given off by the black hole at the centre of the galaxy. With the data of the optical mass of the galaxy (in stars), mass of the hot unusable gas and the gravitational mass of the galaxy obtained from its rotation curve, Dai and collaborators managed to calculate that only 3-4% of the total gravitational mass (obtained from its rotational curve) in the galaxy was normal matter, 3 times less than the expected normal matter content, and the rest was dark. This was very low as compared to the average normal matter content of a galaxy that is expected from the Tully-Fisher relationship between the normal matter content of a galaxy and its rotational speed.

The graph clearly showed that UGC 12591 lay well below expected behaviour from the known correlation. This led Dai and collaborators to believe that the known correlation is probably not a straight line all the way up, rather it flattens out as we move to higher rotational speed galaxies; and hence they mentioned in their paper that there was a need for observations from more number of fast rotating galaxies to confirm the flattening hypothesis. This led to another hypothesis, that the galaxies might have to lose some of their normal matter to reach that high a rotational speed, and suggested that these fast rotating galaxies might still have the normal matter in the form of cold gas expelled out from the galaxy, and hence is not observable since it is not irradiated by any form of radiation. They then tried to explain the expulsion of this normal matter attributing the cause to supernovae or black hole energy feedback or both mechanisms. Further calculations showed that neither of the two processes could independently generate enough energy to throw out the amount of matter missing from the observable mass, and therefore leading to the belief that both of these processes probably work together for this expulsion to happen.

However, the lack of evidence to explain the phenomenon is still keeping the field open for Astronomers and Astrophysicists to keep a weather eye on possible explanations of the Science behind it.

References:-

• Wikipedia article : https://en.wikipedia.org/wiki/Galaxy_rotation_curve
• Giovanelli et al. 1986 : http://articles.adsabs.harvard.edu/pdf/1986ApJ…301L…7G
• Dai et al. 2012 : https://arxiv.org/pdf/1112.0324.pdf
• YouTube channel Dr. Becky : https://youtu.be/5jjkrugRse4

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Preet Agnihotri

I am a final year undergraduate at the Maharaja Sayajirao University of Baroda. I study the heavens and the beauties it hosts, mostly in Radio spectrum. You may find me talking to dogs, admiring myself in front of a mirror, on a guitar, or on a cricket ground.

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