SL2S Workshop at the Institut d'Astrophysique de Paris
(June 28-29, 2007)

Registration & logistics

This is a call for registration to the SL2S workshop Thursday 28 June and Friday 29 June 2007, just before the IAP lensing conference.

If you wish to participate to this workshop please contact Rémi Cabanac (rcabanac at before June 25 2007.

Registration fees are ? euros payable by credit card on the first day of the workshop.

It is time to put together a preliminary programme that does not duplicate the presentations that will be given during the conference.

First, for logistic questions you can contact Delphine (Delphine Charbonneau, charbonn at who is in charge of our venue for the conference. For those not knowing where the IAP is:

We can ask a small amount of money to spend on the SL2S workshop, if one or two of you need a complement support for your visit. We'd rather address specific needs (e.g. airfare or accomodations) that are not already covered for the following conference.


For this workshop we will mainly focus on science that can be done with the SL2S data. As you are awared, the SL2S observational programmes have not yet entered an intensive observational follow-up of all our lens candidates (HST ACS failure, very few spectroscopic observations). In addition, the CFHTLS Data Release T004 was delayed to early June 2007. The situation is however not critical because CFHTLS still gives us a unique and promising opportunity for several years. The main objective of the meeting will be to list the scientific topics that we can rapidly address with the SL2S and find the best arguments to get more observing time on ground-based telescopes. We should have an undisputable list of arguments to put forward in our next proposals (next September). We give below a tentative programme with names of possible speakers introducing a topic and stimulating the discussion. The workshop is intended to be very informal.

Registered participants

Christophe Alard (IAP), Kaushala Bandara (UVic), Roger Blandford (Stanford), Rémi Cabanac (OMP), Bernard Fort (IAP), Mireille Dantel-Fort (IAP), Raphaël Gavazzi (UCSB), Stephanie Jouvel (OAMP), Jean-Paul Kneib (OAMP), Alexie Leauthaud (Berkeley), Marceau Limousin (Copenhagen), Veronica Motta (Valparaiso), Phil Marshall (UCSB), Christophe Pichon (IAP), Sherry Suyu (Caltech), Mark Swinbank (Durham), Jean-Francois Sygnet (IAP), David Valls-Gabaud (Meudon), Hong Tu (IAP)

Preliminary agenda

SL2S collaboration meeting
June 28-29 2007

Thursday 28 June

0900Breakfast (Room TBD)
1000Welcome (Fort) Room TBD
1000-1300 Discussion of SL2S immediate scientific goals
Here the aim of the discussion is to compare ideas and to start identifying a list of specific topic that we can really address in the near future. Preliminary list to be completed, please):
- modeling of the first 5 HST snapshot images with lenstool (Hong, Jullo) and modeling with other software (Suyu, Gavazzi, Marshall, Valls-Gabaud, etc..),
An interesting exercise could be for lens modelers to download the HST data between now and June 28 from the www page (I assume everyone knows the username and password by now otherwise contact Remi at rcabanac at and try to model the candidates using F606W+F814W imaging and a common modeling ground (e.g. EIS, PIEMD or equivalent for a single stellar component + DM component + external shear). We compare the resulting best-fit...
Additional comments from David Valls-Gabaud: Regarding the first session on Thursday morning, what I would really like is that we discuss and focus on the estimation of systematic errors in the modelling/fitting procedures. We all have performed reconstructions and assessed errors in the fittings, yet the odds are that when reconstructing the same objects with different methods we'll get different results. How compatible are they? Clearly the ideal case would be a blind test, where none of the modellers know the parameters of a simulation made by a third party, although the biases in the simulation cannot be addressed. We can perhaps set up such an experiment at the meeting. What we can do instead in the meantime is use the five well-observed ACS lenses and compare what we get. Presumably some parameters will be well constrained and we'll all agree on them, and hopefully their uncertainties -as estimated by different methods- will overlap (but by how much?), while other parameters will depend on the priors we take. A second issue is to assess the goodness of fit of the best models inferred by our reconstructions. All in all, a very interesting exercise. Who is willing to participate, so that we can organise that session in some detail ?
Additional comments from Phil Marshall: At least on the galaxy-scale (where the UCSB group have been focussing our efforts in the last 6 months) I think Raphael and I can each say something informal yet useful on this subject, having investigated in some detail the various systematic effects facing us in our individual case studies. Our science emphases may be different from the rest of the group - but then that's what these meetings are all about, eh?
- multi-component models (Phil, Bernard, ?). technical aspect and a first possible scientific objective: Is there DM core radius in massive elliptical as seen in spirals with rotation curves?
- lensing by groups of galaxies (Mamon,...) open questions for (fossil groups). What can we learn from lensing?
- Rings as a robust probe of external shear: An application to A1689 (Hong, Limousin,...). Other possible applications.
13:00 Lunch at the Paris observatory restaurant
14:00-16:00 - Comparison between simulations and models. presentation of the IAP-Horizon lensing simulation (Pichon) Discussion: selecting the immediate applications (modeling of simulated arcs systems, recovering basic lens parameters, probing substructures perturbations on the arcs geometry, etc..)
- Perturbative analysis, new ideas for detection of arcs Talk on the Detection of arcs (Alard, Peirani ?)
- Update on Haggles (Marshall)
- Report on the workshop Searching for Strong Lenses in Large Imaging Surveys Fermilab (Marshall, Gavazzi, Cabanac, Kneib)
- Gravitational telescope topics including the 2D spectroscopy of bright arcs (Swinbank)
- To be completed by any realistic topic you can think of and that you will really follow-up in the near future?
EveningDinner somewhere TBD


Friday 29 June (9:30-13:00)

10:00-10:30 Observational update (Remi/Jean-Paul)
10:30-12:30 Next proposals; discussions of the scientific cases, who is doing what?
Detection of new candidates in T004 (Sygnet, Alard, Gavazzi) Spectroscopic follow-up of arcs ( Cabanac, ...) Spectroscopic follow-up of rings (Gavazzi, .....) Gravitational telescope follow-up ( Soucail, Tanjavur , ...) Adaptative optic with laser stars (Marshall, Gavazzi;..) IR imaging of groups (Soucail,..)
Here the aim of the discussion is both to analyse the possible weakness of previous proposals and to prepare as early as possible indisputable scientific cases for the next ground-based proposals
12:30-13:00 Executive summary (Cabanac)
13:00-14:30 lunch at the Paris Observatory restaurant.
14:30 End of the Friday SL2S session



Science interests

This is a simple copy paste of the same section from last worskhop.1- Study of individual lenses

a - detailed modelling of each case, giving a reference list of all lense parameters (zl, zsi, type: galaxy, multipolar or fossil group/clusters) with all possible structural parameters (tbd) describing the light and DM distribution (all people, publication policy ?). Color and/or spectroscopic redshifts of galaxy (group members) in the field (Suyu et al., Marshall et al., Kneib et al., Fort et al., Thanjavur, Bandara).

b- Source Image reconstructions (Suyu et al., Marshall et al., Gavazzi et al.)

c - 2D dynamical studies of a selected sample of highly magnified and bright distant galaxies (Thanjavur, Crampton, Cabanac, Swinbank)

d- coupling the SL modeling with the K light distribution of galaxies (a la Lin et al 2004)(Soucail)

e- dynamics through spectroscopic follow-ups (Treu galaxies, Thanjavur groups, Gavazzi, Marshall, Cabanac)

f - gravitational telescope detection of faint galaxies (Crampton et al., Pello, Richard, Cabanac, Kneib)

2 - SL2S Statistical ananalysis

a - statistical analysis of structural parameters (splitting angles and sigma, M/L, ellipticity (triaxiallity), PA, occupational number, environmentals, effects, etc. etc....). (all of the above a + Simard: galaxy properties morphology colours vs lensing mass)

b- Here, I would like to look at the correlation between the stellar concentration parameter (C1/3 ~ m SMBH) and the lens velocity dispersion for various lens masses from rings to giant arc in clusters (Fort et al.)

c - strong lensing study of cut-off radius for galaxy members of groups (Kneib, Hong, Limousin, ...) and study of rings in clusters and groups at several Re from the center to put constraints on the main DM profiles and also on cutoff radius on galaxy members (Fort, Sygnet,...)

d - extention of SLACS studies for (ring) lenses at larger redshifts (Gavazzi, Marshall, Treu + SLACS group, Suyu, Cabanac,).

d - Understanding the selection function, comparison of simulated lensing events by groups and distant clusters with observations (Alard et al., Thanjavur, Oguri?, Gavazzi)

e- Group WL (whenever possible) up to Rvir (Soucail, Leauthaud, Marshall, Bradac maybe, Gavazzi, Motta et al.), tests of the universal DM profile. aperture mass detection (after staking similar lenses) to study C200 (M,z). (Soucail,..)

f - investigation of cosmological tests with rings (Gavazzi et al., Marshall , Suyu et al., Fort, Kneib)

g -statistic of arcs and comparison with predictions on large fields (cf Oguri 2006). comparison between SL2S ring density vs field. (Marshall, Gavazzi, Fort, Kneib)

3 - improvement of automated procedure in the SNAP/LSST perspective

a - arcs (Alard, Thanjavur, Cabanac)

b - rings (Gavazzi, Marshall, Simard, Cabanac), possible extension to the search of SL events by massive edge-on spirals (Fort)

c- improve efficiency of selection (visual, automated motors, etc.) (Cabanac, Fort).

4 - Observational follow-up and data reduction

a - HST (Kneib, Marshall, Cabanac, et al.)

b - VLT/GEMINI NS/Chilean telescopes/Keck Spectroscopy of SL2S arcs (Soucail, Cabanac, Thanjavur, Richard, Motta, Swinbank) and rings (Gavazzi et al., Richard, Treu)

c - Gemini/ESO/Keck IR imagery of SL2S lens fields (Fort et al., Thanjavur) will have a possibility to make a test with Hawki during its VLT 2007 commissioning time, Soucail, Cabanac can re-apply at CFHT)

d - X-Ray follow-ups (mass vs effective radius) proposals (Belsole, Soucail, Limousin, Mammon)

e - observation of rings with AO (Treu, Marshall, Bandara, Gavazzi et al.)

f - data reductions (PIs take care of the work)

g - Archive searches (Marshall et al., Simard, Cabanac)

5- lens modeling techniques: cf. Reports