Is dust responsible for the gaps?

When did the universe begin? When and how were the first stars and galaxies formed? What is the fate of the universe?

The Standard Cosmological Model, also known as the LCDM model, can answer most of these questions. It can also explain the properties of the large-scale spatial structure of the universe, both in its current form and in the past, when the first structures were just beginning to emerge. In addition, thanks to dark energy, it can respond to the accelerated expansion of the universe.

Despite many successes, over the past decade, measurements of nearby Type Ia supernovae and analysis of distant cosmic microwave background data have provided inconsistent values ​​for some cosmological parameters.

In particular, there is a significant difference in the measured value of the current expansion rate, also known as the Hubble constant, between the value determined from distant cosmic microwave background measurements and some values ​​determined at from observations of nearby Type Ia supernovas.

To determine whether this difference is due to systematic problems with one or both data sets or whether it is a problem with the LCDM model, other cosmological probes are sought.

My colleagues and I considered quasars as alternative probes. These are active nuclei located in the centers of galaxies that host supermassive black holes that accumulate matter and emit abundant energy. They can be detected from the local universe to the distant era when the first galaxies were just forming. Therefore, they partially link local measurements of Type Ia supernovae with distant observations of the cosmic microwave background.

Can quasars help resolve current cosmological tensions?

Two methods

It may seem strange that active galactic nuclei (AGNs), which are rather complex objects containing supermassive black holes, whose masses span five orders of magnitude (a factor of 100,000) and accumulate matter in a wide range of rates, can be standardized in a standardized manner. analogous to pulsating Cepheid stars or exploding stars (type Ia supernovae).

Over the past three decades, as better and more multi-wavelength data have been accumulated, AGN measurements have been found to obey two important correlations, both of which involve ionizing electromagnetic radiation originating from the flux of internal accretion around the central black hole in the ultraviolet part of the electromagnetic spectrum.

One of them is based on the correlation between UV and X luminosities (UV/X relationship). In most AGNs, the luminosities of the radiation emitted in the ultraviolet and X-ray parts of the electromagnetic spectrum obey a non-linear relationship. On this basis, the luminosity distance of the quasar can be determined, and for a given redshift, the Hubble diagram of the AGN can be compared to different cosmological models.

The second is based on the discovery that the brightness of ionizing UV radiation emitted near the central black hole correlates with the radius of the more distant region where fast-moving clouds orbit the central black hole. The movement of these clouds is revealed through their characteristic emission in the form of very broad emission lines whose flux is variable.

From the measurement of the delay between the variable UV radiation and the broad line emission, it is possible to deduce the absolute luminosity. From the measured flux we can determine the luminosity distance and then also test cosmological models.

The question remains whether it is possible to find a sample of AGN for which both relationships can be studied. This would make it possible to verify the consistency of the determined luminosity distances and the cosmological models (through their determined cosmological parameter values).

Difference in brightness distances

With my colleague Narayan Khadka of Stony Brook University (formerly at Kansas State University), we identified 58 such AGNs and found that both relationships (UV/X-ray and ray-luminosity ) led to quite different luminosity distances for each of the sources. . This should not happen unless one or both data sets (UV/X-ray and ray-luminosity) do not properly account for some effects. Our study was published in Monthly Notices of the Royal Astronomical Society.

Furthermore, the cosmological parameters obtained from these two relationships were quite different, with the UV/X-ray relationship preferring a higher matter content for the current universe compared to what the ray-luminosity relationship favored. Furthermore, the values ​​of cosmological parameters determined from measurements of the UV/X-ray relationship differ significantly from the values ​​determined using standard cosmological probes. This left us with the puzzle of trying to discover the cause of the discrepancy.

Role of dust in galaxies

By comparing the differences of the two luminosity distances for each of the 58 sources, it appeared to us that the luminosity distance determined from the UV/X-ray relationship is systematically greater than the luminosity distance deduced from the ray-luminosity relationship. . Together with Bozena Czerny (Center for Theoretical Physics PAS), I realized that such an effect could be caused by dust absorbing and scattering UV as well as X-ray photons along the line of sight of the AGN up to us.

Although the 58 observed quasars are located in regions of the sky far from the Milky Way’s dust clouds (see top figure), they are hosted in galaxies that contain many dust clouds through which the emitted photons must travel to reach our telescopes.

In our recent study, published in The Astrophysics Journal, we have shown explicitly that the extinction of the emitted photons due to dust always contributes to a non-zero difference between the two luminosity distances deduced from the AGN correlations, being either positive or negative, depending on whether the X-ray or UV photons are more affected. . Since the distribution peaks are positive for all cosmological models, the quenching of X-ray emission from AGN appears to be more significant for most quasars than the quenching of UV light.

Conclusion

Dust in AGN host galaxies mainly hinders the applicability of the UV/X-ray relationship in cosmology, while the ray-luminosity relationship still seems viable for turning quasars into standard candles. Although the cosmological constraints on the ray-luminosity relationship are still weak due to the limited sample size, the relationship offers a positive side to the use of quasars as cosmological probes, particularly in the era of studies depths of the sky.

This story is part of Science X Dialog, where researchers can report the results of their published research articles. Visit this page for more information about ScienceX Dialog and how to get involved.

Dr Michal Zajaček is a researcher at the Department of Theoretical Physics and Astrophysics at Masaryk University in Brno, Czech Republic. He defended his doctoral thesis in 2017 at the University of Cologne/Max Planck Institute for Radio Astronomy, Germany, on the galactic center, in particular concerning stellar dynamics, star formation and the nature of excess objects in the ‘infrared. From 2017 to 2019 he was a postdoctoral researcher at MPIfR in Bonn, working on jet precession in blazars, and from 2019 to 2021 he was an assistant professor at the Center for Theoretical Physics of the Polish Academy of Sciences in Warsaw , where he studied. the broad region of quasars at intermediate redshift and their application in cosmology.