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Radio image of neutral hydrogen gas in the galaxy AGC 114905. Its inclination is estimated from the black ellipse, which fits the data better. Assuming the galaxy is circular when viewed from the front, this implies a moderate tilt of 32°. However, the new study suggests that the blue ellipse for very low inclination might actually be correct – saving the MOND theory – if the galaxy is inherently somewhat non-circular. The authors show that this is possible using a dedicated MOND simulation. Credit: Figure 7 from Mancera Pina et al. 2022 (MNRAS, 512, 3230)
An international group of astronomers, led by a physicist from the University of St Andrews, has revived an alternative theory of gravity.
Led by Dr Indranil Banik from the School of Physics and Astronomy at the University of St Andrews, the research has revealed high gas rotational velocity in a dwarf galaxy, in line with the previously debunked theory known as from Milgromian Dynamics (MOND).
A previous study of the gas rotation rate in the dwarf galaxy AGC 114905 (Mancera Pina et al, 2022) discovered that the gas rotated very slowly and therefore claimed that the MOND theory was dead.
Such theories are essential to understanding our universe because, according to known physics, galaxies spin so fast that they should fly away. MOND is a controversial alternative to general relativity, the dominant Einstein-inspired understanding of the phenomenon of gravity. However, general relativity requires dark matter to hold galaxies together, while MOND does not require dark matter.
Because dark matter has never been detected despite decades of highly sensitive searches, various theories have been advanced to alternatively explain what holds the galaxies together. The debate rages over which theory is the correct one. The very low rotational speed reported in the study by Mancera Pina et al is inconsistent with predictions in a universe governed by general relativity with large amounts of dark matter.
Dr. Banik’s group argues that the high rotational velocity predicted in the MOND theory of gravity is consistent with observations if the galaxy’s tilt is overestimated.
The rotation of stars and gases in distant galaxies cannot be measured directly. Only the component along the line of sight is known from precise spectroscopic measurements. If the galaxy is viewed almost face-on, it would rotate mostly in the plane of the sky. This could mislead observers into believing that the galaxy is actually spinning very slowly, causing them to overestimate the tilt between the disk and the celestial planes. This tilt was estimated from the elliptical aspect of the galaxy (see image).
The new study explored this crucial question using detailed MOND simulations of a disk galaxy similar to AGC 114905 carried out 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 from the front. This is because stars and gas in the galaxy have gravity and can form into a somewhat non-circular shape. A similar process causes spiral arms in disk galaxies, features so common that they are often called spiral galaxies.
As a result, the galaxy could be much closer face-on than observers thought. This could mean the galaxy is spinning much faster than expected, removing tension with MOND.
Dr Banik, lead author of the new study, said: ‘Our simulations show that the tilt of AGC 114905 could be significantly less than reported, which would mean that the galaxy is actually spinning much faster than people realize. think so, in line with MOND’s expectations. ”
Dr Hongsheng Zhao, from the School of Physics and Astronomy, University of St Andrews, said: ‘The reported very low rotational velocity of this galaxy is inconsistent with both MOND and the standard approach with dark matter. But only MOND is able to circumvent this apparent contradiction.
The new study also argues that a similar ‘false tilt’ effect is unlikely to occur in the standard dark matter approach, as the galaxy is dominated by the smooth dark matter halo. Stars and gas contribute little to gravity, so the disc is not “self-gravitating”.
This means that it is likely to look very circular when viewed from the front, as confirmed by simulations performed by another group (Sellwood & Sanders, 2022). Therefore, the observed ellipticity must be due to a large tilt between the disc and the planes of the sky. The rotation speed would then be very low, which implies that the galaxy has very little dark matter. It is not possible in this context for an isolated dwarf galaxy to have such a small amount of dark matter given its mass in stars and gas.
Pavel Kroupa, a professor at the University of Bonn and Charles University in Prague, said of the broader context of these results: “While MOND performs well in the tests conducted so far, the standard approach causes very serious problems at all scales, ranging from dwarf galaxies like AGC 114905 to cosmological scales, as many independent teams have found.
Reference: “Overestimated inclinations of Milgromian disc galaxies: the case of the ultradiffuse galaxy AGC 114905” by Indranil Banik, Srikanth T Nagesh, Hosein Haghi, Pavel Kroupa and Hongsheng Zhao, April 19, 2022, Royal Astronomical Society Monthly Notices.
DOI: 10.1093/mnras/stac1073