Journal Article

Discovery of a single faint AGN in a large sample of z > 5 Lyman break galaxies

Laura S. Douglas, Malcolm N. Bremer, Elizabeth R. Stanway and Matthew D. Lehnert

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 376, issue 3, pages 1393-1398
Published in print April 2007 | ISSN: 0035-8711
Published online March 2007 | e-ISSN: 1365-2966 | DOI:
Discovery of a single faint AGN in a large sample of z > 5 Lyman break galaxies

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As part of a large spectroscopic survey of z > 5 Lyman break galaxies (LBGs), we have identified a single source which is clearly hosting an active galactic nucleus (AGN). Out of a sample of more than 50 spectroscopically confirmed R-band dropout galaxies at z∼ 5 and above, only J104048.6−115550.2 at z= 5.44 shows evidence for a high ionization potential emission line indicating the presence of a hard ionizing continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Lyα line. Uniquely within this sample of LBGs, emission from N v is also detected, a clear signature of AGN photoionization. The object is spatially resolved in Hubble Space Telescope (HST) imaging. This, and the comparatively high Lyα/N v flux ratio indicates that the majority of the Lyα (and the UV continuum longward of it) originates from stellar photoionization, a product of the ongoing starburst in the LBG. Even without the AGN emission, this object would have been photometrically selected and spectroscopically confirmed as a Lyman break in our survey. The measured optical flux (IAB= 26.1) is therefore an upper limit to that from the AGN and is of order 100 times fainter than the majority of known quasars at these redshifts. The detection of a single object in our survey volume is consistent with the best current models of high redshift AGN luminosity function, providing a substantial fraction of such AGN is found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.

Keywords: galaxies: active; galaxies: high-redshift; galaxies: individual: J104048.6−115550.2

Journal Article.  4846 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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