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A spectroscopic search for AGN activity in the reionization era

Laporte, N; Nakajima, K; Ellis, R; Zitrin, A; Stark, DP; Mainali, R; Roberts-Borsani, GW; (2017) A spectroscopic search for AGN activity in the reionization era. The Astrophysical Journal , 851 (1) , Article 40. 10.3847/1538-4357/aa96a8. Green open access

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Abstract

The ubiquity of Lyman alpha (Lyα) emission in a sample of four bright [O iii]-strong star-forming galaxies with redshifts above seven has led to the suggestion that such luminous sources represent a distinct population compared with their fainter, more numerous counterparts. The presence of Lyα emission within the reionization era could indicate that these sources created early ionized bubbles due to their unusually strong radiation, possibly because of the presence of active galactic nuclei. To test this hypothesis, we secured long integration spectra with XSHOOTER on the VLT for three $z\simeq 7$ sources selected to have similar luminosities and prominent excess fluxes in the IRAC 3.6 or 4.5 μm band, usually attributed to strong [O iii] emission. We secured additional spectroscopy for one of these galaxies at z = 7.15 using MOSFIRE at the Keck telescope. For the most well-studied source in our sample with the strongest IRAC excess, we detect significant nebular emission from He ii and N v indicative of a non-thermal source. For the other two sources at z = 6.81 and z = 6.85, for which no previous optical/near-infrared spectroscopy was available, Lyα is seen in one and C iii] emission in the other. Although based on a modest sample, our results further support the hypothesis that the phenomenon of intense [O iii] emission is associated preferentially with sources lying in early ionized bubbles. However, even though one of our sources at z = 7.15 suggests the presence of non-thermal radiation, such ionized bubbles may not uniquely arise in this manner. We discuss the unique advantages of extending such challenging diagnostic studies with JWST.

Type: Article
Title: A spectroscopic search for AGN activity in the reionization era
Open access status: An open access version is available from UCL Discovery
DOI: 10.3847/1538-4357/aa96a8
Publisher version: https://doi.org/10.3847/1538-4357/aa96a8
Language: English
Additional information: This version is the version of record. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: early universe; galaxies: distances and redshifts; galaxies: evolution; galaxies: formation; infrared: galaxies; stars: formation
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
URI: https://discovery.ucl.ac.uk/id/eprint/10037674
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