Impact of IGM temperature fluctuations on large-scale clustering
28 July 2015
In 2014, the measurements of the clustering properties from large-scale structures at high redshift by the Baryon Oscillation Spectroscopic Survey (BOSS) solidified our current understanding of the accelerated expansion of the Universe. The wealth and quality of BOSS data further allows for detailed studies of the intergalactic medium (IGM; i.e. diffuse gas between galaxies). By measuring the absorption of light by neutral hydrogen gas in the IGM in the spectra of over 100,000 bright background sources – known as quasars – it is possible to deduce the location and, through detailed modelling, the amount of neutral hydrogen along the line-of-sight. From this, global properties of the IGM, namely its temperature, as well as the intensity of ionising background radiation from star-forming galaxies and quasars, can be calculated. This is an important factor in the evolution of the Universe because helium reionisation at redshift z ~ 3 was driven by radiation from quasars. Due to the sparsity of these sources, along with the energy spectrum of ionising radiation, this process can produce spatial fluctuations in the IGM temperature of more than 10,000 K. These fluctuations impact on the individual absorption properties which could in turn affect the interpretation on these important clustering measurements.
As part of his PhD project at the University of Melbourne, ex-CAASTRO student Dr Bradley Greig (now in Pisa, Italy) and his supervisors used detailed semi-analytical simulations developed to investigate the impact of these temperature fluctuations on the clustering properties. These large-volume simulations employ a simple, physically motivated model for helium reionisation from which the researchers could construct synthetic quasar absorption spectra drawn through the IGM, mimicking observational programmes such as BOSS. This approach had the advantage of efficiently achieving a larger dynamic range than in computationally expensive, fully numerical simulations.
They found that these temperature fluctuations could cause a 20-30% increase in the amplitude of the clustering measurements. While this increase does not affect the precision cosmology inferred from clustering, it has important consequences for recovering information regarding the IGM properties. The team concluded that any attempt to infer information on the IGM temperature or strength of the ionising background from the BOSS data must include detailed forward modelling of the IGM temperature fluctuations from helium reionisation and the quasars responsible.