Sp1149 II: Spectroscopy of HII Regions Near the Critical Curve of MACS J1149 and Cluster Lens Models

TL;DR

This study compares multiple galaxy cluster lens models with actual measurements of HII regions near the critical curve of MACS J1149, revealing significant discrepancies in predicted magnifications and suggesting that high-redshift HII regions are more luminous than low-redshift relations indicate.

Contribution

It provides a detailed comparison of ten lens models against observed HII region data, highlighting potential biases and the need to revise luminosity relations at high redshift.

Findings

Lens models underestimate magnifications by an average factor of 6.2.

Flux ratios among knots are consistent with model predictions.

High-redshift HII regions are more luminous than low-redshift L-sigma relation predicts.

Abstract

Galaxy-cluster gravitational lenses enable the study of faint galaxies even at large lookback times, and, recently, time-delay constraints on the Hubble constant. There have been few tests, however, of lens model predictions adjacent to the critical curve (<8") where the magnification is greatest. In a companion paper, we use the GLAFIC lens model to constrain the Balmer L-sigma relation for HII regions in a galaxy at redshift z=1.49 strongly lensed by the MACS J1149 galaxy cluster. Here we perform a detailed comparison between the predictions of ten cluster lens models which employ multiple modeling assumptions with our measurements of 11 magnified giant HII regions. We find that that the models predict magnifications an average factor of 6.2 smaller, a 2-sigma tension, than that inferred from the HII regions under the assumption that they follow the low-redshift L-sigma relation. To evaluate the possibility that the lens model magnifications are strongly biased, we next consider the flux ratios among knots in three images of Sp1149, and find that these are consistent with model predictions. Moreover, while the mass-sheet degeneracy could in principle account for a factor of ~6 discrepancy in magnification, the value of H0 inferred from SN Refsdal's time delay would become implausibly small. We conclude that the lens models are not likely to be highly biased, and that instead the HII regions in Sp1149 are substantially more luminous than the low-redshift Balmer L-sigma relation predicts.