**Title:**A New Cosmological Distance Measure Using AGN

**Authors:**D. Watson et al.

Measurement of actual distance to celestial bodies has always been a difficult task from the beginning of astronomy. Every new distance measures have led to fundamental changes in our understanding of the Universe. In particular, the most recent Nobel Prize of physics was awarded to astronomers who took advantage of the powerful distance measure, type Ia supernovae, to prove that the expansion of the universe is actually increasing.

While type Ia supernovae serve as a good distance measure at high redshift, it is still limited within z~2. AGN(active galactic nuclei) reverberation, based on the argument of this paper, provides an alternative way to measure distance at high redshift. The idea of AGN reverberation was proposed by Blandford & McKee in the early 80s : For a standard model AGN, the radiation from the broad line region(BLR) is basically the recombination lines from the gas clumps ionized by the continuum from the central accretion disk. The causal relation between the central continuum and the BLR emissions lines implies that if there's any change in the central continuum, after a time lag, the recombination lines in the broadline region should change accordingly. This time delay, can be measured using the reverberation technique, by continuously observing the AGN spectra. The distance from BLR to the AGN center, equivalent to the ionizing radius of the central continuum, can be measured from the time lag. Since the ionizing radius of the central continuum is a function of the continuum flux, there must be a direct connection between the time delay and the AGN flux measured on Earth. This relation can be used as a measurement of the luminosity distance. This paper shows that the ratio of time lag and the AGN flux is consistent with predictions up to z~1.

The downside of this measure is that reverberation requires a lot of observation times. Even though it is possible to apply this method on AGNs at z>2, it would require more observing time from more powerful telescopes. The uncertainty of the AGN selection effect or the cosmological effects at high z can either hinder the accuracy of this measure, or increase the difficulties in measurement. However, the author did address some of the problems and concludes that the high-z reverberation will soon become possible.

Indeed, if the distance measure can be applied to where no one has been able to measure, it will be extremely exciting to see how it will change our understanding to the Universe.

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