March 1, 2024

Is dust to blame for the discrepancies?

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Location of 58 active galactic nuclei in the sky along with the distribution of dust throughout the Milky Way. Credit: The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad11dc

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Location of 58 active galactic nuclei in the sky along with the distribution of dust throughout the Milky Way. Credit: The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad11dc

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 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 emerging. Furthermore, through dark energy, it can address 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 measurements of the cosmic microwave background and some values ​​determined from close observations of supernovae of the type Go.

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, alternative cosmological probes are sought.

My colleagues and I consider quasars as alternative probes. These are active nuclei at the center of galaxies that host supermassive black holes that accumulate matter and emit energy profusely. They can be detected from the local universe to distant times, when the first galaxies were just forming. Therefore, they partially connect local measurements of type Ia supernovae with distant cosmic microwave background observations.

Can quasars help resolve current cosmological tensions?

Two methods

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

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

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


The M96 galaxy with the central dust band. Credit: NASA/ESA/Hubble (Leo Shatz)

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The M96 galaxy with the central dust band. Credit: NASA/ESA/Hubble (Leo Shatz)

The second is based on the discovery that the luminosity of ionizing UV radiation emitted near the central black hole is correlated with the radius of the more distant region where fast-moving clouds orbit around 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.

By measuring the time interval between variable UV radiation and broad-line emission, it is possible to infer the absolute luminosity. From the measured flux, we can determine the luminosity distance and subsequently also test cosmological models.

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

Discrepancy in luminosity distances

With my colleague Narayan Khadka at Stony Brook University (formerly at Kansas State University), we identified 58 such AGN and found that the two 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 luminosity-ray) do not adequately 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 favored the ray-luminosity relationship. Furthermore, the values ​​of cosmological parameters determined from UV/X-ray ratio measurements differ significantly from values ​​determined using standard cosmological probes. This left us with the conundrum of trying to figure out 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 became evident to us that the luminosity distance determined from the UV/X-ray ratio is systematically greater than the luminosity distance inferred from the ratio luminosity-ray. With Bozena Czerny (Center for Theoretical Physics PAS), I realized that such an effect could be caused by dust that absorbs and scatters UV as well as X-ray photons along the line of sight from the AGN to us.

Although the 58 observed quasars are located in regions of the sky away from the dust clouds of the Milky Way (see top figure), they are housed in galaxies that contain numerous dust clouds through which the emitted photons must travel on their way to our telescopes. .

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

Conclusion

Dust in AGN host galaxies mainly hampers the applicability of the UV/X-ray relationship in cosmology, while the ray-luminosity relationship still appears viable for turning quasars into standard candles. Although the cosmological constraints on the radius-luminosity relationship are still weak due to limited sample size, the relationship provides a silver lining for the use of quasars as cosmological probes, especially in the era of extensive sky surveys.

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More information:
Narayan Khadka et al, Quasar UV/X-ray ratio luminosity distances are shorter than ray-luminosity ratio luminosity distances measured by reverberation, Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad1040

Michal Zajaček et al, Effect of extinction on quasar luminosity distances determined from UV and X-ray flux measurements, The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad11dc

Diary information:
Astrophysical Journal

Monthly Notices of the Royal Astronomical Society

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