Photometric Redshifts of objects in B0218+357 field

Sanhita Joshi

ANGLES Workshop, Crete

07/04/2005

What do we know about 0218+357

• Double image, with Einstein Ring with radius 334 mas (Patnaik et al)

• Time delay of 10.5 ± 0.4 days (Biggs et al)• Redshifts: Quasar – 0.944 (Lawrence et al)

Galaxy – 0.685 (Browne et al)• Galaxy position: (255±6 , 119 ± 4) mas to A.

– LENSCLEAN (Wucknitz et al)– With HST images (York et al)

• Hubble Constant: 73 ± 8 km/s-Mpc (York et al)

Need more work on 0218?

The result can be (im)proved!

• Any other object assisting lensing? – Redshifts of objects in field of view– 3D mapping of these objects

• Time delay

How to find photo-z

• For good enough accuracy, need data in 5 bands

• Wavelength range: – UBVRI from Keck– JK from UKIRT

• Fluxes compared with standard templates, using code HyperZ

Photometric Calibration

• Magnitudes should be corrected for– Zero point of the instrument– Atmospheric extinction (small)– Galactic extinction (small)

Used a standard star for photometric calibration of J and K data

Why ‘Photometric’ Redshift

• Less time consuming

• Accuracy in z up to 0.1, which is sufficient for the purpose

• Objects with z > 1.0 can be neglected

Model spectrum and SED fits

The spectra don’t fit to the model!

Sources of Error

• Sky was not photometric during Keck observations

• Some other systematic errors

What I will find ?

• Redshifts for objects in 0218 field

• (Non) – presence of any cluster assisting lensing