Using spectral data from the Sloan Digital Sky Survey (SDSS), the Chandra Source Catalog 2.0, and the Chandra COSMOS Legacy survey, astronomers have found that X-ray and ultraviolet luminosities of quasars are so tightly correlated that these objects can be used as new ‘standard candles’ to help determine cosmic distances and probe other fundamental cosmological parameters.
This artist’s impression shows how ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun, may have looked. Image credit: M. Kornmesser / ESO.
“In 1929, Edwin Hubble published observations that galaxies’ distances and velocities are correlated, with the distances determined using their Cepheid stars,” said Dr. Susanna Bisogni from the Harvard-Smithsonian Center for Astrophysics and the INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano and her colleagues.
“Harvard astronomer Henrietta Swan Leavitt had discovered that a Cepheid star varies periodically with a period that is related to its intrinsic luminosity.”
“She calibrated the effect, and when Hubble compared those calculated values with his observed luminosities he was able to determine their distances.”
“But even today only Cepheid stars in relatively nearby galaxies can be studied in this way,” they said.
“In order to extend the distance scale back to earlier times in cosmic history, astronomers have used supernovae, which can be seen to much greater distances.”
“By comparing the observed brightness of a supernova with its intrinsic brightness, based on its classification, astronomers are able to determine its distance; comparing that with the host galaxy’s velocity yields the ‘Hubble relation’ relating the galaxy’s velocity to its distance.”
“The most reliable supernovae for this purpose, because of their cosmic uniformity, are so-called Type Ia supernovae, which are thought to be ‘standard candles,’ all having the same intrinsic brightness.”
“However, even supernovae become harder to study in this way as they lie farther away; to date the most distant Type Ia supernova with a reliable velocity determination dates from an epoch about 3 billion years after the Big Bang.”
In their new work. Dr. Bisogni and co-authors propose using quasars as a new standard candle.
“At the heart of a quasar is a supermassive black hole surrounded by a very hot disk of accreting material that emits in the ultraviolet,” they explained.
“The disk in turn is surrounded by hot gas with electrons moving at speeds close to that of light, and when ultraviolet photons encounter these electrons their energy is boosted into the X-rays.”
They analyzed X-ray measurements of 2,332 distant quasars in the new Chandra Source Catalog and compared them to ultraviolet results from the SDSS DR 14 catalog.
They found that the tight correlation already known to exist between the ultraviolet and X-ray luminosity of local quasars continues in distant quasars, back over 85% of the age of the Universe, becoming even tighter at earlier times.
The implication is that these two quantities can determine the distance of each quasar, and those distances can then be used to test cosmological models.
If the results are confirmed, they will provide astronomers with a dramatic new tool with which to measure the properties of the evolving Universe.
“The most distant known quasars have been spotted from an era only about 700 million years after the Big Bang, dramatically extending the range of standard candle redshifts,” the astronomers said.
“Another advantage of quasars is that hundreds of thousands of them have been discovered in the past few years.”
“Not least, the physical processes in quasars are different from those in supernovae, providing completely independent measures of cosmological parameters.”
The team’s work will be published in the journal Astronomy & Astrophysics.
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Susanna Bisogni et al. 2021. The Chandra view of the relation between X-ray and UV emission in quasars. A&A, in press; arXiv: 2109.03252
