Cosmic dust —— dust in space—— includes intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust, which can cause not only extinction but also reddening for the remote background objects. Cosmic dust extinction plays a very important role in cosmological study. For example, the existence of cosmic dust could cause an apparent dimming for supernovae which is known as the “standard candle” in the Universe. This effect would lead to an overestimate of the distance of the supernovae and further affect our estimation of the induced cosmological parameters. Thus, a better constraint on cosmic dust will provide us a more accurate estimate of cosmological parameters. However, it is quite difficult to observe and measure the average cosmic dust alone the line of sight.

Recently, a PhD graduate student Xiaoyi Xie, under supervision of Prof. Shiyin Shen and Prof. Zhengyi Shao from Shanghai Astronomical Observatory and University of CAS, proposed a new estimation of the average cosmic dust extinction using quasar continuum[1]. In this work. a large and homogeneous quasar sample from the Sloan Digital Sky Survey was selected and further divided into small bins of intrinsic luminosity and cosmic distance (redshift). After that, the possible reddening effects by the cosmic dust is checked. This method is commented as “an excellent idea” by the reviewer of the paper.

“We found that in a fixed luminosity group, the quasar continuum becomes redder towards higher redshift.” Xiaoyi Xie said. “This phenomenon is best explained by the cosmic dust extinction models after the other possibilities have been discussed .” Prof. Shiyin Shen said, “our results are globally consistent with the constraints of cosmic dust extinction from other independent measurements, and has reached to an unprecedented high cosmic distance (redshift z~3.2), which will be very helpful for the future cosmological studies .”

This work will be published in ApJL soon and is currently available on arXiv (http://arxiv.org/abs/1503.02235)..

Fig1. An illustration of the effect of cosmic dust on distant object (extinction and reddening), Credit: Dmitri Pogosyan

[1] Quasars are very bright objects in the sky and can be observed to very large distance (high redshift), The spectrum of the quasars in the Ultraviolet and optical bands is mainly made up of a power-law continuum, which is supposed not to evolve with the distance once its intrinsic properties(e.g. luminosity) have been fixed.