Heavily Reddened Quasars at z~2 in the UKIDSS Large Area Survey: A Transitional Phase in AGN Evolution

Details Heavily Reddened Quasars at z~2 in the UKIDSS Large Area Survey: A Transitional Phase in AGN Evolution Heavily Reddened Quasars at z~2 in the UKIDSS Large Area Survey: A Transitional Phase in AGN Evolution

2012-03-25

arxiv.org/abs/1203.5530v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

17 pages, 11 figures, Submitted to MNRAS, High-resolution version of all figures available at http://www.ast.cam.ac.uk/~mbaner

Scientific paper

We present a new sample of K_Vega<16.5 extremely red quasar candidates at z~2 from ~900 deg^2 of data in the UKIDSS Large Area Survey Data Release 4. Five of these are spectroscopically confirmed to be heavily reddened Type 1 AGN with broad emission lines. These combined with the 7 reddened quasars with K_Vega<17 from 100 deg^2 of data overlapping the SDSS Stripe82 (Hawthorn et al., 2012) brings our total sample of optically obscured AGN to 12 at z=1.5--2.7. At these redshifts, Halpha (6563A) is in the K-band. However, the mean Halpha equivalent width of the reddened quasars is only 1.1x that of the optically selected population and cannot explain the extreme colours. Instead, dust extinction of the order of A_V~2-6 is required to reproduce the continuum colours of our sources. This is comparable to the dust extinctions seen in submillimeter galaxies at similar redshifts. We argue that our sample are likely being observed in a relatively short-lived phase when they are transitioning from massive starbursts to UV-luminous AGN. Their host-galaxies are therefore likely to be brighter at submillimeter wavelengths than the hosts of the UV-bright quasar population. The mean virial black-hole mass of our quasars is 2x10^9 M_O and we infer typical mass accretion rates of 10-100M_0/yr. Our reddest quasar, ULASJ1234+0907 (z=2.5), has an inferred extinction-corrected absolute magnitude of M_i=-31 making it intrinsically brighter than any SDSS spectroscopically confirmed quasar. Finally, we use the sample to place place new constraints on the fraction of obscured Type 1 AGN likely to be missed in optical surveys and find that this is a strong function of quasar luminosity with optical surveys more incomplete to the brightest, most highly-reddened quasars (abridged).

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