“In some sense, gravity does not exist,” observed physicist Michio Kaku of the weakest of the four fundamental forces of the Universe,”what moves the planets and the stars is the distortion of space and time.”
An international group of astronomers, led by a physicist at the University of St Andrews, has revealed a high rotation speed of gas in a dwarf galaxy, consistent with the theory known as Milgromian Dynamics or MOdified Newtonian Dynamics (MOND).
Headed by Dr Indranil Banik of the School of Physics and Astronomy at St Andrews, the new study argues that the predicted rotation speed in the MOND gravity theory is consistent with observations if the inclination of the galaxy is overestimated. Banik’s research focuses on testing whether gravity is modified at low accelerations and whether such a modification can remove the need for invisible dark matter halos around galaxies.
Earlier Study Claimed MOND Dead –Fast or Slow Galaxy Rotation?
An earlier study of the rotation speed of gas in the dwarf galaxy AGC 114905 (Mancera Pina et al, 2022) found that the gas rotated very slowly and claimed the MOND theory was dead.
Such theories are essential in understanding our universe because, according to known physics, galaxies rotate so quickly they should fly apart. The most popular explanation is that some sort of dark matter particles, e.g. unknown weakly interacting massive particles not predicted by the Standard Model of Physics, constitute most of the mass and gravity in distant galaxies. As dark matter has never been detected despite decades of very sensitive searches, various theories have been put forward to explain what holds galaxies together, and debate rages over which is right. For example, the controversial MOND theory instead suggests that at very large distances, the gravitational force between two objects falls off as one over the distance, instead of the Newtonian inverse square law empirically measured at smaller distances.
The very low rotation speed reported in the Mancera Pina et al study is inconsistent with predictions in a universe governed by General Relativity with large amounts of dark matter.
The radio image below shows the 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 face-on, this implies a moderate inclination of 32°. However, the new study suggests that the blue ellipse for a very low inclination could actually be correct – thus saving the MOND theory – if the galaxy is intrinsically somewhat non-circular. The authors show that this is possible using a dedicated MOND simulation. Credit: Figure 7 of Mancera Pina et al. 2022 (MNRAS, 512, 3230)
Is Inclination of the Galaxy Overestimated, and Actually Rotating Faster?
The rotation of stars and gas 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, then it would mostly rotate within the plane of the sky. This could mislead observers into thinking that the galaxy is actually rotating very slowly, which would require them to overestimate the inclination between disc and sky planes. This inclination was estimated from how elliptical the galaxy appears (see image).
The new study explored this crucial issue using detailed MOND simulations of a disc galaxy similar to AGC 114905 made at the University of Bonn by Srikanth Nagesh and instigated by Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague.
The simulations show that it can appear somewhat elliptical even when viewed face-on. This is because stars and gas in the galaxy have gravity and can pull themselves into a somewhat non-circular shape. A similar process causes the spiral arms in disc galaxies, features which are so common that these are often called spiral galaxies.
Removing the Contradiction with MOND
As a result, the galaxy could be a lot closer to face-on than the observers thought. This could mean the galaxy is rotating much faster than reported, removing the tension with MOND.
“Our simulations show that the inclination of AGC 114905 might be significantly less than reported, which would mean the galaxy is actually rotating much faster than people think, in line with MOND expectations,” said Dr Banik.
Dr Hongsheng Zhao, of the School of Physics and Astronomy at the University of St Andrews, said: “The very low reported rotation speed of this galaxy is inconsistent with both MOND and the standard approach with dark matter. But only MOND is able to get around this apparent contradiction.”
This one galaxy, AGC 114905, is just that: one galaxy. Even if we believe the claim that it deviates from the established relations for galaxies, that doesn’t really change the fact that the real story is in the relations defined by hundreds and hundreds of other galaxies.”
Fake Inclination’ Effect
The new study also argues that a similar ‘fake inclination’ effect is unlikely to arise in the standard dark matter approach because the galaxy is dominated by the smooth dark matter halo. The stars and gas contribute little to the gravity, so the disc is not ‘self-gravitating’.
This means it is likely to look very circular if viewed face-on, as confirmed by simulations carried out by another group (Sellwood & Sanders, 2022). As a result, the observed ellipticity must be due to a significant inclination between the disc and sky planes. The rotation velocity would then be very small, implying that the galaxy has very little dark matter. It is not possible in this framework that an isolated dwarf galaxy would have such a small amount of dark matter given how much mass it has in stars and gas.
Pavel Kroupa, Professor at the University of Bonn and Charles University in Prague, said of the broader context of these results: “While MOND works well in the tests conducted so far, the standard approach causes very severe problems on all scales ranging from dwarf galaxies like AGC 114905 all the way up to cosmological scales, as found by many independent teams.”
The Last Word
Stacy McGaugh, Chair, Department of Astronomy and Director, Warner & Swasey Observatory at Case Western Reserve University. McGaugh has worked extensively on galaxy dynamics, showing that the Tully-Fisher relation is fundamentally a relation between the rotation speed of a spiral galaxy and its baryonic mass (the sum of stars and gas). He successfully predicted the velocity dispersions of many of the dwarf satellite galaxies of Andromeda as they were discovered.
McGaugh wrote in an email to The Daily Galaxy:
“I’d say that Banik et al are probably correct. This controversy stems in large part from a refusal to see the forest for the trees. This one galaxy, AGC 114905, is just that: one galaxy. Even if we believe the claim that it deviates from the established relations for galaxies, that doesn’t really change the fact that the real story is in the relations defined by hundreds and hundreds of other galaxies. This obsession with one outlying tree is a form of misdirection: many scientists don’t like the implications of the forest, so let’s focus on this one tree over here, shall we? It isn’t a conscious misdirection, but it is an eagerness to see an unpopular theory (MOND) fail without applying the same scrutiny to the most favored theory (dark matter). As Sellwood showed, this galaxy doesn’t make any sense in terms of dark matter either, it being unstable in that context. So all theories are falsified on the basis of this one weird case?
So all theories are falsified on the basis of this one weird case?”
“As Banik and his collaborators have shown, we probably shouldn’t believe the claim that it is an outlier anyway. This is a proverbial tempest in a teapot. Indeed, I suspected myself something along the lines that they have now explicitly demonstrated, that the inclination could be much less well determined than initially thought. I’m glad they did so, because I’ve seen this movie so many times that I didn’t feel the urge to check more deeply myself.”
The paper ‘Overestimated inclinations of Milgromian disc galaxies: the case of the ultradiffuse galaxy AGC 114905’ is published in Publication Monthly Notices of the Royal Astronomical Society and is available online.
Source: DOI [doi.org/10.1093/mnras/stac1073] and Publication Monthly Notices of the Royal Astronomical Society.
The image at the top of the page shows the gas-rich ultra-diffuse, dark-matter free galaxy AGC 114905; the stellar emission is shown in blue; the green clouds show the neutral hydrogen gas. Image credit: Javier Román / Pavel Mancera Piña.
The Galaxy Report newsletter brings you twice-weekly news of space and science that has the capacity to provide clues to the mystery of our existence and add a much needed cosmic perspective in our current Anthropocene Epoch.