IRAC Lensing Survey


The IRAC Lensing Survey is a Spitzer/IRAC imaging survey of 47 massive clusters. Background sources behind these clusters are significantly magnified due to the gravitational lensing effect, while keeping their surface brightnesses at the same level. It results in an increase of the observed total fluxes, bringing into view faint sources that would otherwise be unobservable. We exploit this potential to achieve JWST depth (~10 nJy) with Spitzer. For each cluster with well-constrained mass-model, we obtained 5 hours of observation in 2 bands, at 3.6 and 4.5 microns, reaching 3-sigma sensitivities of 0.17 and 0.31 micro-Jy, respectively.

With such a large and statistical sample of clusters, we probe the high-redshift Universe with three main scientific objectives:

1. Characterizing z > 6 galaxies. The z>6 universe is the last frontier in observational cosmology. This epoch is thought to define a boundary beyond which the cosmic reionization have taken place and identifying the sources responsible for the reionization remains a challenge. While HST observations with ACS and WFC3 are sensitive to young UV-luminous stellar populations by sampling the rest-frame UV light at z>6, IRAC, on the other hand, provides information on older mature stellar populations by observing the rest-frame optical light longward of the Balmer break. In a parallel effort, we examine the properties of low-luminosity z>6 galaxies identified by our Cycle-17 HST lensing survey (PI: J.-P. Kneib) and by the public Multi-Cycle Treasury program CLASH (PI: M. Postman). Finally, the accurate stellar mass estimates provided by the IRAC observations could also help to infer star formation activities at a higher redshift and bring indirect constraints on the reionization at z>7.

2. Supporting Herschel & IRAM-PdB/ALMA Lensing surveys. By design, the cluster list overlap with that of the Herschel Lensing Survey. The IRAC observations provide essential information for identifying Herschel-detected optically-faint high-redshift infrared-luminous sources and for deriving accurate photometric redshifts.
In the same way, these IRAC observations will also offer the opportunity to identify and study galaxies detected by our observations of 10 lensing clusters with the IRAM/PdB millimeter interferometer (PI: J.-P. Kneib) or detected by future similar cluster surveys conducted with ALMA.

3. Searching for z > 6 supernovae. By observing an early generation of star-forming galaxies, we might also expect to see a number of short-lived massive stars exploding as core-collapse supernovae (SNe). Observations of gamma-ray bursts (GRBs) out to redshift z~8.2 indicate that such explosions do happen at z>6 (under the assumption that this GRB is associated with a SN explosion). In the 3.6 and 4.5 um bands, SNe experience a strong K-correction toward high redshift, they remain visible for a long time because of the (1+z) time dilation factor and contamination by other types of variable sources is thought to be low. In that regard, we have monitored the same fields twice with a time interval of 6 months to a year to identify potential z>6 SNe.


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