[Paper Review] CLASH: Three Strongly Lensed Images of a Candidate z ~ 11 Galaxy

Dan Coe, Adi Zitrin+21 more|Americanae (AECID Library)|Nov 15, 2012

Astronomy and Astrophysical Research4 citations

TL;DR

This paper presents MACS0647-JD, a candidate z ≈ 11 galaxy discovered via strong gravitational lensing by cluster MACSJ0647.7+7015, with three magnified images detected at 12–13σ significance in Hubble WFC3/IR filters. The galaxy's photometric redshift of z = 10.7₊₀.₆⁻⁰.₄ (95% CI) and dropout behavior across UV-to-IR filters, combined with Spitzer non-detections, support a high-redshift origin, making it the most distant known galaxy candidate at the time of publication.

ABSTRACT

We present a candidate for the most distant galaxy known to date with a photometric redshift z = 10.7 +0.6 / -0.4 (95% confidence limits; with z < 9.5 galaxies of known types ruled out at 7.2-sigma). This J-dropout Lyman Break Galaxy, named MACS0647-JD, was discovered as part of the Cluster Lensing and Supernova survey with Hubble (CLASH). We observe three magnified images of this galaxy due to strong gravitational lensing by the galaxy cluster MACSJ0647.7+7015 at z = 0.591. The images are magnified by factors of ~8, 7, and 2, with the brighter two observed at ~26th magnitude AB (~0.15 uJy) in the WFC3/IR F160W filter (~1.4 - 1.7 um) where they are detected at >~ 12-sigma. All three images are also confidently detected at >~ 6-sigma in F140W (~1.2 - 1.6 um), dropping out of detection from 15 lower wavelength HST filters (~0.2 - 1.4 um), and lacking bright detections in Spitzer/IRAC 3.6um and 4.5um imaging (~3.2 - 5.0 um). We rule out a broad range of possible lower redshift interlopers, including some previously published as high redshift candidates. Our high redshift conclusion is more conservative than if we had neglected a Bayesian photometric redshift prior. Given CLASH observations of 17 high mass clusters to date, our discoveries of MACS0647-JD at z ~ 10.8 and MACS1149-JD1 at z ~ 9.6 are consistent with a lensed luminosity function extrapolated from lower redshifts. This would suggest that low luminosity galaxies could have reionized the universe. However given the significant uncertainties based on only two galaxies, we cannot yet rule out the sharp drop off in number counts at z >~ 10 suggested by field searches.

Motivation & Objective

• To identify and characterize the most distant galaxy candidate known, with a redshift approaching z ≈ 11.
• To leverage strong gravitational lensing by massive galaxy clusters to amplify faint, high-redshift galaxies beyond Hubble’s intrinsic sensitivity.
• To test whether the observed photometric dropout and SED shape of MACS0647-JD are consistent with a z ≈ 11 origin, ruling out lower-redshift interlopers.
• To assess the implications of this detection for the luminosity function and reionization history of the early universe.
• To determine the intrinsic properties of the galaxy, including its size, luminosity, and stellar mass, via lens modeling and deprojection.

Proposed method

• Utilized deep Hubble Space Telescope WFC3/IR and ACS imaging from the CLASH survey to detect candidate high-redshift galaxies in 17 massive galaxy clusters.
• Applied strong gravitational lensing models to reconstruct the intrinsic source properties of MACS0647-JD from three observed, magnified images (magnification factors ~8, 7, and 2).
• Conducted photometric redshift analysis using SED fitting with a Bayesian prior, incorporating known galaxy types and luminosity functions from lower redshifts.
• Compared observed HST and Spitzer/IRAC photometry (F140W, F160W, 3.6μm, 4.5μm) to model SEDs, identifying a Lyman break and dropout behavior consistent with z ≈ 11.
• Evaluated a broad range of lower-redshift interlopers, including previously published high-z candidates, to rule out alternative explanations.
• Used lensing magnification to infer intrinsic magnitudes and physical properties (e.g., stellar mass, half-light radius) via deprojection and de-lensing techniques.

Experimental results

Research questions

• RQ1 Can a galaxy at z ≈ 11 be detected using strong gravitational lensing in the Hubble Ultra Deep Field and CLASH survey data?
• RQ2 Is the observed photometric dropout behavior in HST filters consistent with a z ≈ 11 Lyman break galaxy, or could it be explained by lower-redshift interlopers?
• RQ3 What is the intrinsic luminosity, size, and stellar mass of MACS0647-JD after correcting for lensing magnification?
• RQ4 Are the observed number counts of lensed high-redshift galaxies consistent with extrapolations of the luminosity function from lower redshifts?
• RQ5 Can the detection of MACS0647-JD support the hypothesis that low-luminosity galaxies reionized the early universe?

Key findings

• MACS0647-JD is identified as a z ≈ 10.7₊₀.₆⁻⁰.₄ galaxy with 95% confidence limits, and galaxies of known types at z < 9.5 are ruled out at 7.2-σ.
• Three strongly lensed images of the galaxy were detected at >12σ significance in the WFC3/IR F160W filter, with magnifications of ~8, 7, and 2.
• The galaxy exhibits a Lyman break, dropping out of detection in all HST filters shortward of 1.4μm, and shows no significant detection in Spitzer/IRAC 3.6μm and 4.5μm bands.
• The intrinsic (delensed) AB magnitude is ~28.2, corresponding to a luminosity of ~20 nJy (~0.2 μJy in the brightest image), consistent with a z ≈ 11 source.
• The galaxy has a deconvolved, delensed half-light radius of ≤100 pc, smaller than expected from lower-redshift extrapolations, and a stellar mass of ~10⁸–10⁹ M☉.
• The detection of MACS0647-JD at z ≈ 10.8 and MACS1149-JD1 at z ≈ 9.6 in CLASH is consistent with a linearly evolving luminosity function extrapolated from lower redshifts, suggesting low-luminosity galaxies may have contributed to reionization.