Active galaxies as standard candles: Is dust the culprit behind discrepancies?

When did the universe begin? When and the way did the primary stars and galaxies kind? What’s the destiny of the universe?

The usual cosmological mannequin, often known as the LCDM mannequin, can reply most of those questions. It might probably additionally clarify properties of the large-scale spatial construction of the universe—each in its present kind in addition to prior to now, when the primary buildings have been simply rising. Moreover, by way of darkish power, it could possibly tackle the accelerated enlargement of the universe.

Regardless of many successes, during the last decade, close by sort Ia supernovae measurements and the evaluation of distant cosmic-microwave background knowledge have offered inconsistent values for some cosmological parameters.

Particularly, there’s a vital distinction within the measured worth of the present enlargement fee, often known as the Hubble fixed, between the worth decided from the distant cosmic-microwave background measurements and a few values decided from close by sort Ia supernova observations.

To find out whether or not this distinction is because of systematic issues with one or each of the datasets or whether or not it’s a drawback with the LCDM mannequin, alternate cosmological probes are sought.

My colleagues and I thought-about quasars as such different probes. These are lively nuclei on the heart of galaxies that host supermassive black holes accreting matter and profusely emit power. They are often detected from the native universe to the distant epoch when the primary galaxies have been simply forming. Due to this fact, they partially bridge native measurements of sort Ia supernovae with distant cosmic microwave background observations.

Can quasars assist resolve present cosmological tensions?

Two strategies

It might appear unusual that active galactic nuclei (AGN), that are moderately difficult objects containing supermassive black holes, whose plenty span 5 orders of magnitude (an element of 100,000) and accrete matter over a variety of charges, may be standardized in a similar option to pulsating Cepheid stars or exploding (sort Ia supernovae) stars.

During the last three a long time, as extra and better-quality, multi-wavelength knowledge have been gathered, AGN measurements have been discovered to obey two essential correlations, each of which contain ionizing electromagnetic radiation coming from the inside accretion circulation across the central black hole within the ultraviolet a part of the electromagnetic spectrum.

One among these relies on the correlation between the UV and the X-ray luminosities (UV/X-ray relation). In most AGN, the luminosities of radiation emitted within the ultraviolet and X-ray elements of the electromagnetic spectrum obey a nonlinear relation. Primarily based on this, the luminosity distance of the quasar might be decided, and for a given redshift, the Hubble diagram of AGN might be confronted with totally different cosmological fashions.

The second relies on the invention that the luminosity of the ionizing UV radiation emitted close to the central black hole is correlated with the radius of the extra distant area the place quickly transferring clouds orbit across the central black hole. The movement of those clouds is revealed by way of their attribute emission within the type of very broad emission traces whose flux is variable.

From the measurement of the time delay between the variable UV radiation and the broad-line emission, it’s doable to deduce absolutely the luminosity. From the measured flux, we will decide the luminosity distance and subsequently check cosmological fashions as effectively.

The query stays whether or not it’s doable to discover a pattern of AGN for which each relations might be studied. This is able to enable for a consistency test of the decided luminosity distances and cosmological fashions (by way of their decided cosmological parameter values).

Discrepancy in luminosity distances

With my colleague Narayan Khadka from Stony Brook College (previously at Kansas State College), we recognized 58 such AGN and located that the 2 relations (UV/X-ray and radius-luminosity) led to fairly totally different luminosity distances to every of the sources. This could not occur except one or each of the datasets (UV/X-ray and radius-luminosity) didn’t correctly account for some results. Our examine was published in Month-to-month Notices of the Royal Astronomical Society.

Furthermore, the cosmological parameters obtained from these two relations have been fairly totally different, with the UV/X-ray relation preferring a bigger matter content material for the present-day universe as compared with what the radius-luminosity relation favored. Moreover, the cosmological parameters values decided from the UV/X-ray relation measurements considerably differ from the values decided utilizing normal cosmological probes. This left us with the puzzle of making an attempt to find the reason for the discrepancy.

Function of dust in galaxies

By evaluating variations of the 2 luminosity distances to every of the 58 sources, it grew to become obvious to us that the luminosity distance decided from the UV/X-ray relation is systematically bigger than the luminosity distance inferred from the radius–luminosity relation. With Bozena Czerny (Heart for Theoretical Physics PAS), I spotted that such an impact might be attributable to dust that absorbs and scatters UV in addition to X-ray photons alongside the road of sight from the AGN to us.

Though the 58 noticed quasars are positioned in areas of the sky away from the Milky Way dust clouds (see high determine), they’re hosted in galaxies that comprise quite a few dust clouds by way of which the emitted photons should journey on their path to our telescopes.

In our latest examine, published in The Astrophysical Journal, we confirmed explicitly that the extinction of the emitted photons attributable to dust at all times contributes to a non-zero distinction between the 2 luminosity distances inferred from AGN correlations, being both optimistic or unfavorable, relying on whether or not X-ray or UV photons are extra affected. For the reason that distribution peaks are optimistic for all cosmological fashions, the extinction of X-ray emission from AGN seems to be extra vital for many quasars than the extinction of UV gentle.

Conclusion

Mud in AGN host galaxies hinders primarily the applicability of the UV/X-ray relation in cosmology, whereas the radius–luminosity relation nonetheless seems viable for turning quasars into normal candles. Though the cosmological constraints from the radius–luminosity relation are nonetheless weak attributable to a restricted pattern dimension, the relation offers a silver lining for utilizing quasars as cosmological probes, particularly within the period of intensive sky surveys.

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Dr. Michal Zajaček is a researcher on the Division of Theoretical Physics and Astrophysics, Masaryk College in Brno, Czech Republic. He defended his PhD thesis in 2017 on the College of Cologne/Max Planck Institute for Radioastronomy, Germany, on the galactic heart, particularly regarding stellar dynamics, star formation, and the character of infrared-excess objects. Throughout 2017–2019 he was a postdoctoral researcher on the MPIfR in Bonn, engaged on jet precession in blazars, and through 2019–2021, he was an assistant professor on the Heart for Theoretical Physics, Polish Academy of Sciences in Warsaw, the place he studied the broad-line area of intermediate-redshift quasars and their software in cosmology.