UCL, Sept 16th 2008

Photometric redshifts in the SWIRE Survey - the need for infrared bands

Michael Rowan-Robinson

Imperial College London

UCL, Sept 16th 2008

IMPz photometric redshift ‘code’

• designed to exploit Spitzer IRAC (3.6-160 m) data, in addition to broad-band optical (Rowan-Robinson et al 2003,2004, 2008, Babbedge et al 2004)

• template fitting method, with priors: first create colour table as function of z• two passes through data, separate template fits to starlight in optical/near ir

and to dust emission in infrared, extinction AV as optional free parameter• Pass 1: 6 galaxy, 3 QSO templates, fit zph, using 3.6 and 4.5 if lg10{S(3.6)/S(r)}

< 0.5, then fit ir templates to bands with an excess relative to starlight model• Pass 2: 11 galaxy, 3 QSO templates, higher S(3.6)/S(r) limit for galaxies, 3.6

and 4.5 not used if Pass 1 indicated dominant AGN dust torus in the infrared• use priors on absolute magnitude range, AV

• known spectroscopic redshifts used to estimate in-band correction factors (Ilbert et al 2006)

• good performance for galaxies, use of infrared bands significantly reduced catastrophic outlier rate (not so good for QSOs)

• problem is presence of AGN dust torus, which can affect wavelengths down to rest-frame 1 m

UCL, Sept 16th 2008

SWIRE (Spitzer Wide-Area IR Extragalactic Survey)

• SWIRE: 49 sq deg in ELAIS-N1,-N2,-S1, Lockman, XMM-LSS, CDFS areas surveyed at 3.6,4.5,5.8,8.0,24,70 and 160 m• Reliable catalogues released in Dec 05 • Final catalogues being prepared, photometric redshift catalogue released

UCL, Sept 16th 2008

optical templates for photometric redshifts

t

optical templates(Babbedge et al 2004, RR et al 2008)

original templates were empirically modified from Yoshii and Takahari (1988)

these were fitted with stellar synthesis models supplied by Stefano Berta

these are convolved with filter set to create colour table

AGN1 AGN2

UCL, Sept 16th 2008

Photometric redshiftsSWIRE-VVDS sample (with VVDS team, PI LeFevre)

VIRMOS-VLT Deep Survey spectra>1000 sources~3% rms in (1+z)<2% outliers

phot z method of RR 03, Babbedge et al 04, RR et al 05, some refinements

~ IRAC 3.6 and 4.5 m big help in reducing outliers

10 optical bands, 3.6, 4.5 m

UCL, Sept 16th 2008

Photometric redshifts

whole SWIRE catalogue, galaxies, 6 bands, r<23.5, S(3.6)>7.5 Jy

UCL, Sept 16th 2008

Photometric redshifts for quasars

whole SWIRE catalogue,

at least 4 bands

at least 6 bands

UCL, Sept 16th 2008

redshift error as function of r-magnitude

UCL, Sept 16th 2008

Photometric redshifts

Percentage redshift error, (zph-zsp)/(1+zsp) v. zsp, v. I for VVDS

Lockman field red: gals, blue: QSOs

Lockman, 4 optical bands, z from Keck/ Gemini (Smith et al 06)

UCL, Sept 16th 2008

Photometric redshifts

gri ugri

gri 3.6,4.5 ugri 3.6,4.5

UCL, Sept 16th 2008

rms and % outliers, as function of 2 threshold

Histogram of 2

for SWIRE sources with 7 photometric bands

rms as function of 2

threshold (dotted)

% catastrophic outliers (>15% in 1+z) as function of 2

threshold (solid curves, upper: r<24, lower: r<21.5)

UCL, Sept 16th 2008

rms and % outliers, as function of no. of bands

UCL, Sept 16th 2008

Redshift distributions

>=3 bands from U-8m

left: Suburu XDS, R<27.5

below: ELAIS-N1, r<23.5: for optically blank sources use 3.6-8 m for phot-z

UCL, Sept 16th 2008

AV distribution

• filled circles: galaxies, crosses: quasars

• definite improvement in 2 when use AV

• histogram of AV in SWIRE catalogue is consistent with star-formation history

UCL, Sept 16th 2008

Infrared templates

estimation of sfr from 24 mu not very accurate, since galaxies have a range of mid-to-far ir seds (Rowan-Robinson 2001)

UCL, Sept 16th 2008

modelling ir excess with dust emission templates

SEDs of SWIRE galaxies and AGN• use optical and IRAC 3.6, 4.5 to fit

optical SEDs and derive redshifts

Sources with IR excess• where there is a mid-IR excess, fit

infrared templates to the excess(Rowan-Robinson et al, 2005,AJ 129, 1183)

UCL, Sept 16th 2008

SPITZER-IRS spectra of ELAIS sources

• IRS spectra for 70 ELAIS-N1 and -N2 sources with S15> 1mJy validate the template fits

• most are ULIRGs, with z =1-3

• Filled circles: optical, ISO, SWIRE

( and MAMBO) data

• Solid curves: model seds

• Red curve: calibrated IRS data

(Hernan-Caballero et al 2006, Perez-Fournon et al, 2007, in prep)

UCL, Sept 16th 2008

seds of submilllimetre galaxies

SHADES SXDS Clements et al 2007

UCL, Sept 16th 2008

Predicted fluxes at 160, 350 and 850 m• can use ir template fits to predict fluxes at

longer wavelengths, for comparison with expected AKARI, PLANCK and HERSCHEL survey limits

• projections based on N1

Babbedge et al. in prep.

850 m850 m

160 m70 m

UCL, Sept 16th 2008

conclusions

• over one million galaxies in SWIRE photometric redshift catalogue, includes stellar mass, star-formation rate, luminosities in infrared components (cirrus, M82 and A220 starburst, AGN dust torus), predicted far ir and submillimetre fluxes, full 2(z)

• use of infrared bands reduces percentage of catastrophic outliers

• need AV for optimal solution

• SWIRE Photometric Redshift Catalogue is huge resource for high redshift galaxies: 10% have z > 2, 4% have z > 3