Strong-lensing analysis of MACS J0717.5+3745 fromHubbleFrontier Fields observations: How well can the mass distribution be constrained?

Abstract

We present a strong-lensing analysis of MACSJ0717.5+3745 (hereafter MACS J0717), based on the full depth of the Hubble Frontier Field (HFF) observations, which brings the number of multiply imaged systems to 61, ten of which have been spectroscopically confirmed. The total number of images comprised in these systems rises to 165, compared to 48 images in 16 systems before the HFF observations. Our analysis uses a parametric mass reconstruction technique, as implemented in the L software, and the subset of the 132 most secure multiple images to constrain a mass distribution composed of four large-scale mass components (spatially aligned with the four main light concentrations) and a multitude of galaxy-scale perturbers. We find a superposition of cored isothermal mass components to provide a good fit to the observational constraints, resulting in a very shallow mass distribution for the smooth (large-scale) component. Given the implications of such a flat mass profile, we investigate whether a model composed of "peaky" non-cored mass components can also reproduce the observational constraints. We find that such a non-cored mass model reproduces the observational constraints equally well, in the sense that both models give comparable total rms. Although the total (smooth dark matter component plus galaxy-scale perturbers) mass distributions of both models are consistent, as are the integrated two-dimensional mass profiles, we find that the smooth and the galaxy-scale components are very different. We conclude that, even in the HFF era, the generic degeneracy between smooth and galaxy-scale components is not broken, in particular in such a complex galaxy cluster. Consequently, insights into the mass distribution of MACS J0717 remain limited, emphasizing the need for additional probes beyond strong lensing. Our findings also have implications for estimates of the lensing magnification. We show that the amplification difference between the two models is larger than the error associated with either model, and that this additional systematic uncertainty is approximately the difference in magnification obtained by the different groups of modelers using pre-HFF data. This uncertainty decreases the area of the image plane where we can reliably study the high-redshift Universe by 50 to 70%.

Chandra and JVLA Observations of HST Frontier Fields Cluster MACS J0717.5+3745

“…This could imply that the dark matter and galaxies are already past pericenter and in the "return phase" of the merger (Ng et al 2015). Interestingly, the dark matter peak is located even further to the east (as reported by e.g., Johnson et al 2014;Limousin et al 2016), although other lensing models show better agreement between the BGC location and mass surface density (e.g., Ishigaki et al 2015). From a core size of about 0 4 (≈150 kpc) and a temperature of 6.8 keV, we compute a sound crossing time of1 10 8 year for the core.…”

“…This structure seems to be associated with mass componentA, and is also seen in the mass surface density map from Ishigaki et al (2015). However, Johnson et al (2014) and Limousin et al (2016) place the center of the westernmost mass component about 0 5east of the center of the X-ray component. We discuss this "fly-through" bullet-like core in more detail in Section 4.3.…”

“…The cluster consists of at least four separate merging substructures. Regions of the intracluster medium (ICM) are heated to 20 keV (Ma et al 2008(Ma et al , 2009Limousin et al 2012Limousin et al , 2016. A study of the Sunyaev-Zel'dovich (SZ) effect provided further evidence for the presence of shock-heated gas (Mroczkowski et al 2012).…”

Mass distribution in the core of MACS J1206

“…Remarkably, this study has revealed an exceptional number of central multiple images, which, within the inner 20 kpc, is approximately a factor of 4 larger than in any other known cluster to date. As a result, this spectroscopic data set has allowed us to obtain a very robust estimate of the inner slope of the projected total mass density profile and a detailed reconstruction of the inner projected mass distribution, which has been shown to be challenging when using photometrically selected multiple images (Limousin et al 2016). We summarize our results as follows:…”

A systematic search for lensed high-redshift galaxies inHSTimages of MACS clusters

“…those for which such features have been spectroscopically confirmed. Magnification estimates were derived by means of Lenstool (Jullo & Kneib 2009) from the positions of strong-lensing features and a parametric model which includes the contribution from clusterscale and galaxy-scale haloes (following the same approach as, e.g., Richard et al 2010;Limousin et al 2015). The available models allowed us to calculate the magnification due to cluster gravity for the ten high-redshift candidates presented in Table 3.…”