Astrophysicists discover that dust in galaxies affects their distance measurements in the Universe

Dust around galactic nuclei significantly affects the measurement of their distances in the Universe. This was discovered by a team of physicists led by Michal Zajaček from the Masaryk University in Brno, who analyzed ultraviolet and X-ray data from a set of 58 active galaxies.

2 Feb 2024 Leoš Verner Michal Zajaček

Using two different methods, astrophysicists have studied the distances of these galaxies, called quasars. The new findings could help clarify some of the "cracks" in the previously known and respected theories of the evolution of the Universe.

Astrophysicists, who have long studied the origin, structure and evolution of the Universe, use exploding stars, or supernovae, as so-called standard candles. In astronomy, a standard candle is a designation for a source of known luminous energy that allows its distance from Earth to be determined. Standard candles are used to measure distances in space. By comparing the measured radiation intensity of a supernova with its energy, the so-called luminosity distances in space can be found. But in the last ten years, some measurements have caused worries among theoretical physicists. This is because of the detection of discrepancies in the measured expansion rate of the Universe, the so-called Hubble constant.

The location of 58 galaxies on the sky along with the distribution of dust in our Milky Way. Source: Zajacek et al.

"It is a known fact that the Universe is expanding and its expansion is accelerating, but the available techniques have yielded different values, especially when measuring the expansion rate," describes Michal Zajaček, a Masaryk University scientist. The discrepancy in the Hubble constant appears when comparing the measurements obtained using near and far sources. Obtaining luminosity distances across the entire visible Universe, which numerous galaxies scattered across the cosmos make possible, could help resolve this problem. But first they need to be calibrated so that they can be used as standard candles to measure distances.

Physicists have identified 58 active galaxies that can be calibrated using two different methods. The first method evaluates the ultraviolet and X-ray radiation emitted by galaxy nuclei. The second method works with ultraviolet radiation luminosities in relation to the radii of the accretion disks of quasars. An accretion disk is a structure made of rapidly rotating gas around a massive black hole in the centres of galaxies. The rapidly circulating gas is heated to high temperatures of around hundreds of thousands of degrees Celsius and glows intensely, particularly in the ultraviolet and X-ray domains of the electromagnetic spectrum.

The image of the galaxy M96 with a prominent dust lane in the centre. Credit: ESA/NASA/Hubble (Leo Schatz)

"The two methods used to find the distances of the galaxies showed a discrepancy, so we looked into what it may be due to. We found that the dust located in the nuclei of galaxies orbiting the central black hole in the form of clouds can absorb and scatter both UV and X-ray radiation from these quasars, thus distorting the measured distances of galaxies from us," explained astrophysicist and lead author of the study Michal Zajaček. "It's like when you imagine a sandstorm in the desert that significantly reduces the sunlight," adds Zajaček.

Michal Zajaček's team, which also includes astronomers from Poland, the USA, Chile and Brazil, showed in their work that extinction caused by dust always contributes to the difference between the two luminosity distances. Based on the data, the physicists found that the dust around galaxies, which is mainly made up of carbon and silicon grains, distorts the distance measurements mainly in the case of the first method, but has little effect on the second method, which deals with the radius-luminosity relation. Based on this finding, the experts propose that the first method should no longer be used to measure the distances of galaxies and subsequently to determine the parameters of the structure and evolution of the Universe.

"This is undoubtedly one of the most important topics that physicists are currently working on. The method could help physicists clarify confusions in current theories of the Universe. And whoever figures it out might get a Nobel Prize for it," smiles Zajaček, summing up the importance of the research.

Michal Zajaček. Photo: Irina Matusevič

For details of the published study, including contacts and other background information, please see Dr. Zajaček's detailed article HERE.

The original article in the Astrophysical Journal is available here. The Astrophysical Journal, Volume 961, Issue 2, Article 229, DOI: 10.3847/1538-4357/ad11dc

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