Redshift: Why Does Distance Matter to CTA?
Written by: Paolo Goldoni
Only by knowing the distance of the objects we observe can we begin to understand their physical nature. In 1923, Edwin Hubble demonstrated that some of the so-called “nebulae” he saw were, actually, galaxies located millions of light years away. He did so by observing a particular type of variable star in the “nebulae,” the Cepheids, discovered by Henrietta Leavitt a few years before, whose period of variation is linked to their luminosity. The period of the Cepheids in the nebulae that Hubble observed implied such a luminosity that they were undoubtedly extragalactic. In doing so, he also established a correlation between the distance and the redshift of the optical spectra of the galaxies he observed. Since then, the redshift is the quantity most used to measure the distance of extragalactic objects.
The redshift is an increase of the light’s wavelength (decrease of energy) that occurs when a light source moves away from the observer. It is typically measured in optical and near-infrared by the spectral lines of the source (Figure 1). In extragalactic astronomy, the redshift (also called cosmological redshift) is due to the expansion of the Universe that increases the distance between the galaxies and the Earth.
Figure 1. Absorption spectral lines in the optical spectrum of a supercluster of distant galaxies (upper panel) compared to a close-by object, the Sun (bottom panel). Arrows indicate the redshift, i.e. the increase of the wavelengths (lower energy). Credit: Georg Wiora
Blazars and their Distance in Very High-Energy Astronomy
As for all astronomical instruments, the distance of the observed object is very important to CTA, too. This is particularly true for blazars, the most numerous class of extragalactic sources in the very high-energy (VHE) domain (above tens of GeV). Blazars are a class of Active Galactic Nuclei (AGNs) – compact regions at the centre of galaxies with strong and variable emission across the electromagnetic spectrum. Their emission is caused by accretion on the central supermassive black hole. During the accretion process, a jet of relativistic particles is emitted from the vicinity of the black hole, whose radiation spans from radio to gamma rays. In blazars, the jet is pointed towards the observer.
In the most numerous class of VHE blazars, BL Lacs, the jet emission dominates over the galaxy emission in the optical range. The latter is crucial for redshift measurement, but because of the stronger jet emission, their redshift is very difficult to estimate. Less than half of the known BL Lacs have a known redshift and, with such a low percentage, it is not possible to estimate their properties as a whole: we do not know with precision their density in the Universe and their evolution with time, and their emission cannot be modelled with great precision.
The lack of knowledge of blazars’ redshift (or distance) is particularly problematic in the VHE domain: the blazars’ VHE spectra we observe with our telescopes are distorted by the interaction of the VHE gamma rays with photons and particles encountered along their travel to Earth. This distortion is dependent on the distance and so, its value is needed to correct the spectra and obtain the real emission of the source. Conversely, if a model of the source emission is assumed, the properties of the intergalactic medium can be extracted.
Several physical processes are responsible for the distortion of the VHE spectra, but in this article, we will concentrate on the Extragalactic Background Light (EBL), which is the integration of all the optical and near-infrared light emitted by the stars and galaxies during the history of the Universe, and whose value, usually measured in the optical range, is not very well known yet.
The optical photons from the EBL interact with the VHE gamma rays from blazars, annihilating and giving rise to an electron and a positron (see Figure 2). This effect is strongly dependent on the redshift of the blazar and so, by knowing the redshift and observing VHE gamma rays, we can measure the EBL.
Figure 2: Cosmic journey of gamma rays from a distant galaxy down to the CTA Observatory. Among other physical processes, some gamma rays produce electron-positron pairs when interacting with EBL photons (top left process).
This measurement is one of the main science cases of CTA. Thanks to its unprecedented sensitivity, wide field of view and extended spectral range, in fact, CTA will obtain many more blazar spectra and with a much higher signal-to-noise than previous instruments. These spectra, in principle, will allow to measure the EBL with precision comparable to the optical one, allowing a fundamental cross-check between the two methods. Unfortunately, if the distance of the blazars is not known, the quality of the measurements in general can be affected. For this reason, a group of CTA Consortium members has performed observations from some of the world’s greatest optical observatories (ESO, Keck, SALT) in order to measure the redshift of bright gamma-ray blazars that likely will be detected with CTA. A paper presenting the measurement of 11 blazar redshifts, which will be crucial for future CTA scientific studies, has recently accepted for publication by Astronomy and Astrophysics Journal . This effort is just beginning, and it will increase the number of gamma-ray blazars with known redshift, thus, helping to maximise the scientific return of CTA.
 Goldoni, P. Et al. 2020, ArXiV:201205176