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Radio image of neutral hydrogen gas in galaxy AGC 114905. Its tilt is estimated by the black ellipse, which best fits the data. Assuming that the galaxy is circular when viewed from the front, this means it has a moderate tilt of 32 degrees. However, the new study suggests that blue ellipses with very low tilts could indeed be true – thus saving the MOND theory – if galaxies were intrinsically somewhat non-circular. The author demonstrates that this is possible by using a custom MOND simulation. Credit: Figure 7 from Mancera-Pina et al. 2022 (MNRAS, 512, 3230)
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An international group of astronomers, led by a physicist at the University of St Andrews, have revived an alternative theory of gravity.
Led by Dr. Indranil Banik of the Faculty of Physics and Astronomy at the University of St Andrews, the study revealed an estimate of the high rotational speed of gases in the dwarf galaxies In accordance with a previously debunked theory known as Milgromian Dynamics (MOND).
An earlier study of the rotational speed of gas in the dwarf galaxy AGC 114905 (Mancera Pena dkk., 2022) discovered that the gas rotated very slowly and thus claimed that the MOND theory was dead.
Such theories are important for understanding our universe because, according to known physics, galaxies are rotating so fast that they must diverge from one another. MOND is a controversial alternative to general relativity, Einstein’s dominant understanding of the phenomenon of gravity. However, general relativity requires dark matter to hold galaxies together, while MOND does not dark matter.
Since dark matter was never found despite decades of highly sensitive search, various theories have been proposed to explain what holds galaxies together. Debate rages over the correct theory. The extremely low spin speeds reported in the Mancera Pina et al study run counter to expectations in a world governed by general relativity with large amounts of dark matter.
Dr. Group. Banek argues that the high spin speed predicted in the MOND theory of gravity is consistent with the observation if the slope galaxy excessive.
The rotation of stars and gas in distant galaxies cannot be directly measured. Only components along the line of sight are known by precise spectroscopic measurements. If galaxies were seen almost directly, most of them would orbit within the celestial plane. This could mislead observers into thinking that galaxies rotate very slowly, which requires them to overestimate the slope between the cylinder and the celestial plane. This tilt is estimated from how elliptical galaxies appear (see image).
The new study explores this important issue using detailed MOND simulations of similar disk galaxies AGC 114905 Created at the University of Bonn by Srikanth Nagesh and at the instigation of Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague. Simulations show that it can appear somewhat elliptical even when viewed directly. This is because the stars and gas in galaxies have gravity and can pull themselves into a somewhat non-circular shape. A similar process causes spiral arms to appear in disk galaxies, a feature so common that they are often called spiral galaxies.
As a result, the galaxy could be closer to confrontation than observers think. This could mean that the galaxy is spinning much faster than has been reported, taking the strain off of the MOD.
Dr Banek, lead author of the new study, said: “Our simulations suggest that the tilt of AGC 114905 may be much lower than what has been reported, meaning that the galaxy is actually rotating much faster than people think, in line. with MOND predictions.”
Dr Hongsheng Zhao, from St Andrews University School of Physics and Astronomy, said: “The reported extremely low spin velocities of this galaxy do not match the MOND and standard approximations with dark matter. But only MOND can overcome this. a real contradiction.”
The new study also argues that a similar “pseudo-tilting” effect is unlikely to occur in the standard dark matter approach because the galaxy is dominated by a fine dark matter halo. The star and gas contribute minimally to gravity, so the disk doesn’t “self-gravitate”.
This means that it is likely to appear very circular when viewed live, as confirmed by simulations by other groups (Selwood and Sanders, 2022). As a result, the observed ellipsoid must be caused by a significant tilt between the disk and the celestial level. The rotational speed will be very small, meaning the galaxy has very little dark matter. In this setting, an isolated dwarf galaxy is unlikely to contain a small amount of dark matter given its mass in stars and gas.
Pavel Krupa, a professor at the University of Bonn and Charles University in Prague, said of the broader context of these findings: “Although MOND performed well in the tests conducted to date, the standard approach causes very serious problems at all scales ranging from dwarf galaxies such as AGC 114905 to the cosmic scale, as found by many independent teams.”
Reference: “Overtendency of the Milgromian Disc Galaxy: The Case of the Ultra-widening Galaxy AGC 114905” By Indranil Banik, Srikanth T Nagesh, Hosein Haghi, Pavel Kroupa, Hongsheng Zhao, 19 Apr 2022, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stac1073
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