J/AJ/154/269 A new photo-z method for quasars in Stripe 82 (Yang+, 2017)
Quasar photometric redshifts and candidate selection: a new algorithm based on
optical and mid-infrared photometric data.
Yang Q., Wu X.-B., Fan X., Jiang L., McGreer I., Green R., Yang J.,
Schindler J.-T., Wang F., Zuo W., Fu Y.
<Astron. J., 154, 269 (2017)>
=2017AJ....154..269Y 2017AJ....154..269Y (SIMBAD/NED BibCode)
ADC_Keywords: QSOs ; Photometry, ugriz ; Redshifts ; Models
Keywords: catalogs - cosmology: observations -
galaxies: distances and redshifts - methods: statistical -
quasars: general
Abstract:
We present a new algorithm to estimate quasar photometric redshifts
(photo-zs), by considering the asymmetries in the relative flux
distributions of quasars. The relative flux models are built with
multivariate Skew-t distributions in the multidimensional space of
relative fluxes as a function of redshift and magnitude. For 151392
quasars in the SDSS, we achieve a photo-z accuracy, defined as the
fraction of quasars with the difference between the photo-z zp and
the spectroscopic redshift zs, |Δz|=|zs-zp|/(1+zs) within
0.1, of 74%. Combining the WISE W1 and W2 infrared data with the SDSS
data, the photo-z accuracy is enhanced to 87%. Using the Pan-STARRS1
or DECaLS photometry with WISE W1 and W2 data, the photo-z accuracies
are 79% and 72%, respectively. The prior probabilities as a function
of magnitude for quasars, stars, and galaxies are calculated,
respectively, based on (1) the quasar luminosity function, (2) the
Milky Way synthetic simulation with the Besancon model, and (3) the
Bayesian Galaxy Photometric Redshift estimation. The relative fluxes
of stars are obtained with the Padova isochrones, and the relative
fluxes of galaxies are modeled through galaxy templates. We test our
classification method to select quasars using the DECaLS g, r, z, and
WISE W1 and W2 photometry. The quasar selection completeness is higher
than 70% for a wide redshift range 0.5<z<4.5, and a wide magnitude
range 18<r<21.5 mag. Our photo-z regression and classification method
has the potential to extend to future surveys. The photo-z code will
be publicly available.
Description:
We use a sample of spectroscopically identified quasars consisting of
quasars from the SDSS Data Release 7 Quasar catalog (DR7Q; Schneider et al.
2010, Cat. VII/260) and the Data Release 12 Quasar catalog (DR12Q;
Paris et al. 2017, Cat. VII/279). The DESI Legacy imaging survey
(DELS; A. Dey et al. 2018arXiv180408657D 2018arXiv180408657D) will provide images for
target selection, including the DECam Legacy Survey (DECaLS) in the g,
r, and z bands. In this work, we use the three-band (grz) photometry
from the DECaLS DR3. We test the quasar candidate selection algorithm
described in Section 5.3 with the DECaLS g, r, z and WISE W1 and W2
photometry. There is a 15 deg2 region (36°<RA<42° and
-1.25°<DE<1.25) in S82 with spectroscopically identified quasars
as dense as 167 per deg2.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table4.dat 82 45505 Photometric quasar sample in S82
--------------------------------------------------------------------------------
See also:
VII/260 : The SDSS-DR7 quasar catalog (Schneider+, 2010)
VII/279 : SDSS quasar catalog: twelfth data release (Paris+, 2017)
J/AJ/134/973 : SDSS Stripe 82 star catalogs (Ivezic+, 2007)
J/ApJ/768/105 : z∼5 QSO luminosity function from SDSS Stripe 82
(McGreer+, 2013)
J/MNRAS/440/476 : Low-redshift quasars in SDSS Stripe 82 (Falomo+, 2014)
J/MNRAS/441/1802 : Low-redshift QSOs in SDSS Stripe 82 (Karhunen+, 2014)
J/A+A/611/A97 : Photometric quasar candidates in Stripe 82
(Pasquet-Itam+, 2018)
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 12 F12.8 deg RAdeg Right Ascension in decimal degrees (J2000)
14- 24 F11.8 deg DEdeg Declination in decimal degrees (J2000)
26- 30 F5.2 mag gmag [-1/30.75] Magnitude in g filter, galactic
extinction corrected
32- 36 F5.2 mag rmag [16.62/23] Magnitude in r filter, galactic
extinction corrected
38- 41 F4.2 --- zphot [0.1/5.1] Photometric redshift
43- 46 F4.2 --- z1 [0.05/5] The lower limit of zphot (z1 in
Equation 8)
48- 51 F4.2 --- z2 [0.15/5.15] The upper limit of zphot (z2
in Equation 8)
53- 57 F5.3 --- Pprob [0.052/1] The probability that the redshift
locates within (z1, z2) (Pprob in
Equation 8)
59- 64 F6.3 [-] logPqso [-2/1.361] The logarithmic likelihood of an
object to be a quasar over the whole redshift
range (LQSO in Equation 9)
66- 73 F8.3 [-] logPstar [-322.829/-3.792]?=99.0 The logarithmic
likelihood of an object to be a star (LStar
in Equation 15)
75- 82 F8.3 [-] logPgal [-322.829/-5.145]?=99.0 The logarithmic
likelihood of an object being a galaxy
(L'galaxy in Equation 20)
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 10-Sep-2018