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

Abstract

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 AGN. Out of a sample of more than fifty 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 ionisation-potential emission-line indicating the presence of a hard ionising continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Ly_alpha line. Uniquely within this sample of LBGs, emission from NV is also detected, a clear signature of AGN photo-ionisation. The object is spatially resolved in HST imaging. This, and the comparatively high Ly_alpha/NV flux ratio indicates that the majority of the Ly_alpha (and the UV continuum longward of it) originates from stellar photo-ionisation, a product of the ongoing starburst in the Lyman break galaxy. 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 (I(AB)=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 are found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.

arXiv comment7 pages, 3 figures, MNRAS in press

References in corpus (30)

• The Sloan Digital Sky Survey: Technical Summary
• Composite Quasar Spectra From the Sloan Digital Sky Survey
• The epochs of early-type galaxy formation as a function of environment
• Constraining the Evolution of the Ionizing Background and the Epoch of Reionization with z ~ 6 Quasars II: A Sample of 19 Quasars
• Cosmological Evolution of the Hard X-ray AGN Luminosity Function and the Origin of the Hard X-ray Background
• A Survey of z>5.8 Quasars in the Sloan Digital Sky Survey I: Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z~6
• The SDSS Quasar Survey: Quasar Luminosity Function from Data Release Three
• Lyman Break Galaxies at Redshift z~3: Survey Description and Full Data Set
• The Complete Star Formation History of the Universe
• The HELLAS2XMM survey. VII. The hard X-ray luminosity function of AGN up to z=4: more absorbed AGN at low luminosities and high redshifts
• Elemental Abundances in QSOs: Star Formation and Galactic Nuclear Evolution at High Redshifts
• Color-selected galaxies at z~6 in the Great Observatories Origins Deep Survey
• The Great Observatories Origins Deep Survey VLT/FORS2 Spectroscopy in the GOODS-South Field: Part II
• Continuum and Emission Line Strength Relations for a large Active Galactic Nuclei Sample
• Lyman Break Galaxies and the Star Formation Rate of the Universe at z~6
• EDisCS -- the ESO Distant Cluster Survey -- Sample Definition and Optical Photometry
• Star Formation at z~6: The UDF-Parallel ACS Fields
• The Evolution in the Faint-End Slope of the Quasar Luminosity Function
• Lyman-break galaxies at z~5 -I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift
• Luminous Lyman Break Galaxies at z>5 and the Source of Reionization
• Q0906+6930: The Highest-Redshift Blazar
• Weak lensing mass reconstructions of the ESO Distant Cluster Survey
• The GLARE Survey II. Faint z=6 Ly-alpha Line Emitters in the HUDF
• X-ray Properties of Lyman Break Galaxies in the Great Observatories Origins Deep Survey
• Discovery of a Color-Selected Quasar at z=5.50
• The X-ray emission of Lyman break galaxies
• The discovery of a significant sample of massive galaxies at redshifts 5 < z < 6 in the UKIDSS Ultra Deep Survey Early Data Release
• Discovery of an Optically-Faint Quasar at z=5.70 and Implications for the Faint End of the Quasar Luminosity Function
• The Properties of Galaxies at z>5
• A Snapshot Survey for Gravitational Lenses Among z>=4.0 Quasars: II. Constraints on the 4.0<z<4.5 Quasar Population

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• The relation between star formation, morphology and local density in high redshift clusters and groups
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• The Role of the Dust in Primeval Galaxies: A Simple Physical Model for Lyman Break Galaxies and Lyman Alpha Emitters
• Spectroscopy of clusters in the ESO distant cluster survey (EDisCS).II. Redshifts, velocity dispersions, and substructure for clusters in the last 15 fields
• Discovery of 16 New z ~ 5.5 Quasars : Filling in the Redshift Gap of Quasar Color Selection
• On Contamination and Completeness in z>5 Lyman Break Galaxy Surveys
• The Near and Mid-infrared photometric properties of known redshift $z\ge 5$ Quasars
• Spectroscopy of z~5 Lyman Break Galaxies in the ESO Remote Galaxy Survey
• Photometric Selection of z~5 Lyman Break Galaxies in the ESO Remote Galaxy Survey
• M-Dwarfs at Large Heliocentric Distances
• Physical Properties of Local Star-Forming Analogues to z~5 Lyman Break Galaxies
• Establishing an analogue population for the most distant galaxies
• Limits on the Molecular Gas Content of z-5 LBGs
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