Searching for very high redshift galaxies (z>6) using the gravitational telescopes
I am involved in different projects searching for the earliest galaxies
formed during, or shorlty after, the cosmic reionization epoch.
Characterizing the nature, physical properties, and epoch of formation of these sources, probably responsible for reionizing the intergalactic medium, is one
of the latest challenges of modern cosmology. Extending the searches beyond z~6.5 and back to ages where the Universe re-ionised requires extremely deep observations in the near-IR bands. Until the construction of JWST and 20-40 meter class ground-based telescopes, gravitational lensing surveys offer the only possibility of spectroscopically verifying the presence of an abundant population of low-luminosity sources.
I conducted or largely contributed to three projects targetting the central
regions of strong lensing clusters to search for very high redshift objects:
A search for Optical Dropouts with deep ground-based VLT/ISAAC images of 2 lensing clusters (Richard et al. 2006)
Location of the candidates (left), and corresponding magnification map (right),
with respect to the z~2.5 and z~9 critical lines.
Contours on the right are spaced by 0.5 magnitudes of magnification.
Constraints on the UV Luminosity Function of high redshift galaxies.
A search for Optical Dropouts with deep HST/ACS and NICMOS images of 6 lensing clusters (Richard et al. 2008)
Snapshot images showing the average spectral energy distribution of the candidates in each ACS/NICMOS filter.
Coverage of the central region of one cluster with NICMOS, the high redshift
critical line is shown in blue.
Constraints on the abundance of dropouts from this survey. The luminosities probed,
after correction by the magnification factors, are up to 2 magnitudes fainter than
the Ultra Deep Field (UDF).
A critical lines mapping survey of 10 lensing clusters with Keck/NIRSPEC to search for Lyman-alpha emitters (Stark et al. 2007)
Spectroscopic coverage with NIRSPEC for one of the clusters (Abell 68), showing the
location of one candidate and the high redshift critical line.
Zoom on the 2d spectra of 6 candidate Lyman-alpha emitters found in this survey.
Precise strong-lensing modelling of massive galaxy clusters
I have used the LENSTOOL software to constrain the mass distribution of several lensing clusters, in particular as part of the MACS and the LoCuSS collaborations. I concentrated on measuring the redshifts of the largest number of multiple images identified in each cluster, using mainly the LRIS instrument on the Keck telescope.
Here are a few example of results obtained for individual clusters:
The modelling of lensing clusters constrain the mass distribution
using the positions and redshift of multiple images. Going backward, a well-constrained mass model can predict the redshift for an additional multiple image.
Abell 68 and spectroscopic redshift measurements.
(Richard et al. 2007)
Detailed studies of highly magnified z>2 galaxies
With collaborators at Caltech (Richard Ellis, Tucker Jones), Durham (Mark Swinbank, Ian Smail) and Cambridge (Dan Stark), we have been performing
a survey of the brightest z>2 arcs in clusters, taken from the LoCuSS and MACS surveys (see above), in order to study their physical properties in
Indeed, studying the internal properties, in particular the dynamics of these high redshift galaxies enable to test galaxy formation models.
Dynamics enables to distinguish between chaotic and well-ordered velocity fields, depending on the maturity of the systems. Gravitational magnification provides
a large increase in angular size, enabling to reach source plane resolutions of ~100 pcs.
Our survey strategy is the following:
- Use HST snapshot images to identify bright strongly lensed arcs and other multiple systems
- Perform Optical Spectroscopy using Keck/LRIS or VLT/FORS to measure their redshifts, extinction, line strengths, star-formation rates, and
- Screen the best candidates with near-infrared spectroscopy (using Keck/NIRSPEC): measures the abundances, star-formation rates and extinction independently
- Select the brightest near-infrared emission lines for Integral Field Unit (IFU) follow-up, using the Keck/OSIRIS and VLT/SINFONI near-infrared IFUs: this allows to measure
the resolved dynamics in these objects
- Other follow-up observations include: further HST images to study the resolved stellar populations, measurements of CO and radio continuum, mid-infrared
spectroscopy with Spitzer/IRS