J/ApJ/755/169 3<z<5 quasar luminosity function in the COSMOS (Masters+, 2012)
Evolution of the quasar luminosity function over 3<z<5 in the COSMOS survey
field.
Masters D., Capak P., Salvato M., Civano F., Mobasher B., Siana B.,
Hasinger G., Impey C.D., Nagao T., Trump J.R., Ikeda H., Elvis M.,
Scoville N.
<Astrophys. J., 755, 169 (2012)>
=2012ApJ...755..169M 2012ApJ...755..169M (SIMBAD/NED BibCode)
ADC_Keywords: QSOs ; Photometry ; Redshifts ; X-ray sources
Keywords: cosmology: observations -
galaxies: luminosity function, mass function - Galaxy: evolution -
quasars: general
Abstract:
We investigate the high-redshift quasar luminosity function (QLF) down
to an apparent magnitude of IAB=25 in the Cosmic Evolution Survey
(COSMOS). Careful analysis of the extensive COSMOS photometry and
imaging data allows us to identify and remove stellar and low-redshift
contaminants, enabling a selection that is nearly complete for type-1
quasars at the redshifts of interest. We find 155 likely quasars at
z>3.1, 39 of which have prior spectroscopic confirmation. We present
our sample in detail and use these confirmed and likely quasars to
compute the rest-frame UV QLF in the redshift bins 3.1<z<3.5 and
3.5<z<5. The space density of faint quasars decreases by roughly a
factor of four from z∼3.2 to z∼4, with faint-end slopes of β~-1.7
at both redshifts. The decline in space density of faint optical
quasars at z>3 is similar to what has been found for more luminous
optical and X-ray quasars. We compare the rest-frame UV luminosity
functions found here with the X-ray luminosity function at z>3, and
find that they evolve similarly between z∼3.2 and z∼4; however, the
different normalizations imply that roughly 75% of X-ray bright active
galactic nuclei (AGNs) at z∼3-4 are optically obscured. This fraction
is higher than found at lower redshift and may imply that the
obscured, type-2 fraction continues to increase with redshift at least
to z∼4. Finally, the implications of the results derived here for
the contribution of quasars to cosmic reionization are discussed.
Description:
We utilize the COSMOS broad, intermediate, and narrowband photometric
catalog (Capak et al. 2007ApJS..172...99C 2007ApJS..172...99C, Cat. II/284; Sanders et al.
2007ApJS..172...86S 2007ApJS..172...86S) for the selection of quasars and identification
of contaminants. With 29 bands of well-matched photometry, these data
constitute low-resolution spectra for all objects over the wavelength
range 0.1-8 µm. This is generally sufficient to distinguish stars
from high-redshift quasars. With spectra of sufficient quality for all
sources in the field, the population of quasars can be well
constrained down to the limiting magnitude. Our approach is similar to
that of Wolf et al. (2003A&A...408..499W 2003A&A...408..499W), who used the 17 filters
(5 broad and 12 intermediate band) of the COMBO-17 survey to identify
quasars. COSMOS is of similar resolution to the COMBO-17 survey in the
optical, but significantly deeper and covering a wider wavelength range.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table3.dat 67 155 Summary of the Likely Quasars Found Here and
Used to Compute the Luminosity Function
--------------------------------------------------------------------------------
See also:
II/284 : COSMOS Multi-Wavelength Photometry Catalog (Capak+, 2007)
J/ApJ/690/1236 : COSMOS photometric redshift catalog (Ilbert+, 2009)
J/ApJ/693/8 : High-redshift QSOs in the COSMOS survey (Brusa+, 2009)
Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 I7 --- ID [115356/1970813] Quasar identifier
9- 17 F9.5 deg RAdeg Right Ascension in decimal degrees (J2000)
19- 26 F8.6 deg DEdeg Declination in decimal degrees (J2000)
28- 32 F5.2 mag Imag Auto I band magnitude (AB)
34- 37 F4.2 --- zest Redshift estimated based on visual examination
39- 42 F4.2 --- zqso Photometric redshift from COSMOS
(Ilbert et al. 2009, J/ApJ/690/1236)
44- 49 F6.2 --- zspec ?=-99 Spectroscopic redshift
51- 54 F4.2 --- zused Redshift adopted in computing the luminosity
function
56 I1 --- Flag [2/4] Confidence flag we assigned the source based
on visual examination (1)
58- 60 A3 --- X-ray [yes/no ] Source detected in X-ray ?
62- 67 F6.2 mag M1450 Derived absolute magnitude M1450 (1450Å)
--------------------------------------------------------------------------------
Note (1): Where 4 is highly confident and 2 is somewhat confident.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Prepared by Tiphaine Pouvreau [CDS] 21-Aug-2017