The Mystery Of The Missing Dark Matter (Cosmology)

New distance measurements of the diffuse spheroid galaxy Ngc 1052-Df2 place this galaxy at a distance of 72 million light years and confirm that the galaxy is practically devoid of dark matter, a very rare case in the galaxy landscape. This absence of dark matter compared to other galaxies suggests that dark matter exists as a real physical entity and not as a result of a different law of gravitation on a galactic scale.

According to the current paradigm, dark matter makes up about 86% of all matter in our Universe. Its peculiarity is that it does not interact electromagnetically like ordinary matter, but only by gravitational way . For this reason it is difficult to study it, in fact it can only be detected on a large scale by observing the gravitational effects it causes on ordinary matter: unfortunately there is no experimental detection.of dark matter particles. The presence of this matter has been deduced thanks to studies on the velocity curves of spiral galaxies: as we move away from the nucleus of a galaxy, the stars do not decrease their speed as one might expect, but continue to move. quickly. If Newton’s law of gravity holds, this excess of speed indicates that most of the mass of galaxies is made up of invisible matter capable of holding the stars of which they are composed bound together with its own force of gravity: unlike galaxies. they would fall apart. Dark matter in the evolution of the Universe is very important because it is thanks to its intense gravitational effects that, within immense haloes of dark mattergalaxies were formed . Otherwise, after the Big Bang, ordinary matter would never have undergone any process of gravitational collapse and galaxies would not have formed. From this theoretical framework it is expected that each galaxy contains a consistent amount of dark matter: for example the value of the average ratio between dark matter and ordinary , measured for galaxies such as our Milky Way, is of the order of 30 times and increases both for more massive galaxies, and for less massive galaxies.

However, things seem more complex than that, at least as far as the galaxy Ngc 1052-Df2 is concerned . It is an ultra-diffuse galaxy with low surface brightness that is prospectively located in the constellation of the Whale, identified thanks to a large-field survey of the group of galaxies of Ngc 1052. The galaxy contains so little ordinary matter that it is practically transparent, so much is it It is true that in the images that portray it you can see the background galaxies much further away. Morphologically, this galaxy has a spheroidal appearance and does not appear to have a core, spiral arms or a disk of stars. The geometric dimensions are similar to those of the Milky Way.

In a March 2018 article published in Nature, the results of the radial velocity measurements of 10 luminous globular clusters belonging to this evanescent galaxy were published for the estimation of the total mass of the system. The result was that the ratio of dark to bright matter in Ngc 1052-Df2 was about 1, a value about 400 times lower than expected and in stark contrast to what is observed in other galaxies. Put simply, the case of NGC1052-DF2 showed that dark matter is not always coupled with baryon matter , at least on a galactic scale. To confirm this incredible result, the discovery team, led by Pieter van Dokkum of Yale University, focused on precise distance measurementby Ngc 1052-Df2, publishing a new paper in The Astrophysical Journal Letters . In the work of 2018, the distance of the galaxy was assumed to be similar to that of the group of galaxies to which it seemed to belong, namely that of Ngc 1052 at about 65 million light years from us. How does distance fit into estimating the relationship between dark and ordinary matter? To understand this, just think of the fact that the estimation of the mass of a star can be done by measuring its intrinsic brightness and this is obtained by measuring both the apparent brightness and the distance at which the star is located. By scaling this reasoning on a galactic scale we understand that if Df2 were closer to Earth than the 65 million light years adopted, thenits stars would be intrinsically weaker and less massive , so the luminous matter would make a minor contribution to the total mass (which is measured with the radial velocity of globular clusters) and the ratio between dark and luminous matter would increase accordingly. Distance measurement thus becomes a crucial parameter for determining the amount of luminous matter in the galaxy.

To measure the distance of a galaxy you need ” standard candles “, ie stars whose intrinsic brightness is known a priori . The team of astronomers, using the “Hubble” space telescope, focused on measuring the apparent brightness of the red giants located on the periphery of Ngc 1052-Df2 and which, during their evolution, all reach the same brightness peak. In this way, the difference between intrinsic and apparent brightness can be used to measure large intergalactic distances. The new distance estimate tells us that Df2 is 72 million light years awaythat is, the galaxy is further away than the original estimate of 65 million light years. From here it follows that Df2 is really devoid of dark matter, it is not an observational bias .

Moreover, Df2 is not the only galaxy without dark matter, another galaxy, Ngc 1052-Df4 , is also devoid of dark matter. In this case, however, some scientists suggest that dark matter may have been removed from the galaxy due to tidal forces exerted by another passing galaxy.

The discovery of these galaxies devoid of dark matter, paradoxically, confirms that dark matter really exists. In fact, if dark matter were only an effect of a gravitational law different from the Newtonian one, all galaxies should show its presence. The fact that there are galaxies without dark matter means that something is really missing in their structure. Understanding why Df2 is devoid of dark matter will require further observation, the mystery continues.

Featured image: The galaxy poor in dark matter Ngc 1052-Df2 taken with the Hubble Advanced Camera for Surveys between December 2020 and March 2021. The galaxy is so poor in matter that, through it, you can see the background galaxies (Credits: Nasa , Esa, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (Ias), Alyssa Pagan (STScI))

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