Paper Review: The Role of Galaxies and AGN in reionising the IGM-II: metal-tracing the faint sources of reionsation at 5≲z≲6

This paper was written by Romain A. Meyer1Sarah E. I. Bosman1, Koki Kakiichi1Richard S. Ellis1 and investigates the influence of galaxies on cosmic reionisation through the use of a common absorbing species, C IV, to measure the escape fraction of faint galaxies directly and probe the ionizing candidates of faint galaxies and gaseous environments. They also investigate the distribution and abundance of C IV absorbers, assessing the impact on the IGM through the cross-correlation with IGM transmission in Lyman-α forest.

Cosmic reionisation is the period in the history of the universe were all neutral hydrogen in the intergalactic medium (IGM) was reionised by photons emitted by galaxies and occurred between redshifts of approximately 6-15. The uncertainty in the fraction of ionizing photons that are able to escape into the IGM makes the investigations into the role of star-forming galaxies and AGN at redshifts higher than approximately 7 very difficult. Due to the increasing absorption of LyC photons, by the IGM, at redshifts greater than about 3 the escape fraction of galaxies beyond this cannot be directly measured through usual means. Thus, this paper attempts to directly measure this escape fraction for higher redshifts.

They used a group of 25 objects, 5 of which were taken from the archives of the Keck Observatory and XShooter, who’s signal to noise ratio was greater than 17 (including, however, one source with a signal to noise ratio of less than 17 due to its broad C IV absorptions). A power law continuum was fitted to the spectra over a range of 1270Å-1450Å and any pixels effected by skylines were excluded. Much of the paper explores the methods and conditions used to determine suitable C IV absorber detections as they developed an algorithm to automate QSO continuum spline fitting for wavelengths above those of Lyman-α. To do this the algorithm determines areas with no broad absorption lines, rejects pixels with S/Nmatched-filter>3  from the continuum and fit BEL features before search for C IV doublets. They often used mocks and randomly generated data to test the algorithm, estimate the completeness of the search and test for spectral anomalies in the data they collected. This resulted in a sample size of 147 C IV absorber detections at  with 37 systems (Figure 4 in the paper, given below, shows the absorbers found). Checking against previous studies they found their automation worked in concordance with previous results.

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From the sample of absorbers they collected they were able to find the comoving cosmic mass density as a function of the redshift, this then shows there is a general decline in the cosmic density with an increase in redshift. This decrease in cosmic density indicates an increase of the total carbon budget, as more metals are ejected into the IGM by galactic outflows from star-forming galaxies, and a change in the ionisation state of carbon, from the evolution of the UV background spectrum.

Lyman-α should be between 1045Å and 1176Å in QSO rest frame, they took only one absorber when there are multiple components (where δv<100km.s-1) and removing C IV absorbers in saturated end regions of Lyman-α forest, they were limited to only 37 suitable absorbers with which to compute the cross-correlation of the data.

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Figure 7 in the paper (shown above) presents the main body of their results from the cross-correlation (ξ) with the IGM transmission (rc). The graph shows there is an excess of absorptionation at rc≲5c Mpc.h-1, potentially from the opaqueness of Lyα, suggesting gas overdensity and indicating outskirts of CGM around  galaxies. For values of rc≳10 c Mpc.h-1 there is an excess of transmission, which is consistent with an enhanced UV background. Large-scale excess transmission also indicates the collective radiation of large scale overdensities of galaxies around C IV absorbers.

Their results also suggest that the halo mass of C IV absorber host galaxies is Mh≳1010Msol, thus a smaller enrichment radius produced by weaker outflows and/or clumpy distributions requires more, lower mass, abundant halos as the C IV absorber hosts. C IV is a highly ionised species; thus, its presence indicates the existence of highly ionising radiation in the area of the C IV hosts.

They do note in the paper however that the linear fit used means that at small scales there are modelling uncertainties and they have a substantial uncertainty which remains in the results. It is also noted that whilst there may be spatial offset and winds which result in an offset between Lyman-α emitters and C IV, this is however unlikely. They also did not however, actually measure the escape fraction of galaxies at redshift greater than 3 within this paper.

I found this paper particularly interesting as it looked into the epoch of reionisation and effect of certain galaxies on the cosmic reionisation. Their research also focused heavily on the spectra of the galaxies under investigation, which is similar to much of the work I have been doing over the course of the internship.

I find it intriguing to see how the reionisation of the IGM may be influenced by star-forming galaxies and how they developed an algorithm, for this paper, to analyse much of their spectra before cross-correlating the data.