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Laboratoire d’Astrophysique Ecole Polytechnique Fédérale de Lausanne Switzerland. Strong gravitational lensing as a tool for studying galaxy formation and cosmology. Frédéric Courbin For the COSMOGRAIL collaboration Bologna, January 2008.

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slide1

Laboratoire d’Astrophysique

Ecole Polytechnique Fédérale de Lausanne

Switzerland

Strong gravitational lensing as a tool for studying galaxy formation and cosmology

Frédéric Courbin

For the COSMOGRAIL collaboration

Bologna, January 2008

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses

http://www.cosmograil.net

slide2

Image

Source

Observer

Lens

Quasar time delays, H0 and galaxy mass profile

Geometry (plus Ho)

Radial (mass) profile of the lens

slide3

Quasar time delays, H0 and galaxy mass profile

Quasar lensing requires:

  • High-accuracy astrometry (a few milli-arcsec)
  • Detailed mapping of the light distribution in the lens
  • Image of the lensed host galaxy of the quasar
  • Accurate measurement the time delays

Image deconvolution of HST images is extensively used but:

  • Lack of PSF stars in the small HST field of view
  • PSF distortions across the field
  • Colour dependence of the PSF

HST

Ground

slide4

Example of a lensed quasar: RX J1131-123

HST / ACS

1.2 m Euler (La Silla)

3 arcsec

(Sluse et al. 2006, A&A 449, 539, including data from CASTLEs)

slide5

COSMOGRAIL telescopes

Liverpool 2m Robotic Telescope

Euler (Chile)

Mercator (La Palma)

2 x 1.2m telescopes

Uzbek 1.5m Telescope

2m Himalayan Chandra Telescope

Switzerland:

G. Meylan

F. Courbin

C. Vuissoz

A. Eigenbrod

G. Burki

D. Sluse

P. Saha

Belgium:

P. Magain

V. Chantry

E. Eulaers

L. Le Guillou

H. Van Winckel

C. Waelkens

India:

T. Prabhu

D. Sahu

C.S. Stalin

Uzbekistan:

M. Ibrahimov

I. Asfandiyarov

UK:

S. Dye

S. Warren

slide6

WFI 2033-4723

Rather wide angular separation (2-3 arcsec)

Photometry using MCS deconvolution

(Magain, Courbin, Sohy, 1998, ApJ 494, 472)

1.2 m Euler (deconvolved) image

FWHM = 0.35 arcsec

HST ACS image (from CASTLEs)

slide7

WFI 2033-4723

219 epochs !

Sampling ~ 4 days

3 full years

Vuissoz et al. 2008

slide8

WFI 2033-4723

Time delays measured from 3 different methods (Vuissoz et al. 2008) :

B-A = 35.5 ± 1.4 days (3.8%), most of the error is shot noise

B-C = 63.7 ± 3.4 days (5.0%), most of the error comes from systematics

Slow microlensing is negligible, fast microlensing (scales of weeks) is not.

slide9

WFI 2033-4723

Shifted curves

Vuissoz et al. 2008

slide10

WFI 2033-4723

Detailed HST-NICMOS (F160W) imaging (from CASTLE):

  • Light profile of the lens (best fit: de Vaucouleurs)
  • Astrometry to 3 mas accuracy including systematics
  • Faint Einstein ring
slide11

WFI 2033-4723

Non parametric modeling

Twisting of the mass contours

Dynamical perturbation from group ?

slide12

WFI 2033-4723

  • Twisted mass iso-contours, suggesting gravitational perturbation
  • When the time delays are not fitted, all models agree with the

data: isothermal, de Vaucouleurs, NFW

  • When the time delays are used, the best mass model is a central

de Vaucouleur plus NFW dark matter halo

  • The twisting of the mass contours and the steep mass profile of

the lens in WFI 2033-4723 suggest the lens is a satellite rather

than a central galaxy of a group.

slide13

Going beyond HST resolution

An iterative PSF construction algorithm is devised:

  • Start with a TinyTim PSF (Krist & Hook)
  • Carry out a first deconvolution
  • Remove extended sources in the image
  • Estimate a new PSF on the point sources in the data
  • Deconvolve again
  • Do this a few times until the residuals are acceptable

Magain, Courbin, Gillon, et al. 2007, A&A 461, 373

Chantry & Magain 2007, A&A 470, 467

-> For more details

slide14

1 arcsec

Going beyond HST resolution

Example of application to NICMOS images of the « cloverleaf »

Original F160W image

slide15

Going beyond HST resolution

Example of application to NICMOS images of the « cloverleaf »

Modifications applied to the PSF after each deconvolution step

Chantry & Magain 2007, A&A 470, 467

slide16

1 arcsec

Going beyond HST resolution

Example of application to NICMOS images of the « cloverleaf »

New résolution is 0.05 arcsec

Chantry & Magain 2007, A&A 470, 467

(+ see poster)

slide17

Going beyond HST resolution:

the double Einstein ring in SDSS 0924+02

HST (F555W+F814W+F160W)

Deconvolved

(Eigenbrod et al. 2006, A&A 451, 747; data from CASTLEs)

slide18

For the concordance cosmology lenses appear to split in two groups:

  • isothermal lenses
  • lenses with a steeper-than-isothermal inner (15 kpc) mass profile

Most lenses are in groups, often dominated by a brighter member.

Numerical simulations show that galaxies in groups, that have steep mass profiles are satellite galaxies, and that the steep profile is transient (e.g. Dobke, King & Fellhauer 2007).

-> The time delay technique allows to infer the details of the inner

structure of galaxies in a range of environments

-> Estimate Ho independent of standard candles by “stacking” lenses

-> provided there exist sufficiently high resolution images to

interpret the time delays

slide19

COSMOGRAIL papers so far

I: Simulated light curves

Eigenbrod et al. 2005, A&A 436, 25

II: The double Einstein ring in SDSS J0924+02

Eigenbrod et al. 2006, A&A 451, 747

III: Deep VLT spectroscopy of 7 lensed quasars

Eigenbrod et al. 2006, A&A 451, 759

IV: Non parametric modeling of cosmograil objects

Saha et al. 2006, A&A 450, 461

V: First time delay measurement

Vuissoz et al. 2007, A&A 464, 845

IV: Deep VLT spectroscopy of 8 lensed quasars

Eigenbrod et al. 2007, A&A 465, 51

VII: First high-accuracy time delay (~3%)

Vuissoz et al. 2008