Detailed Mass Map of CL0024+1654 from Strong Lensing

TL;DR

This study presents a high-resolution mass map of the z=$0.39$ cluster CL0024+1654 derived from strong lensing of eight multiple images of a background galaxy observed with HST. The authors combine a parametric source model (58 disks) with a mascon-based mass model (512 free parameters, including 118 galaxy mascons and 25 diffuse mascons) and constrain the mass distribution by pixel-level comparisons to the data, using bootstrap and simulated annealing for error estimation. The results reveal that most of the mass outside galaxies is in a smooth dark-matter component with a soft core of $r_{ m core} = 35\,h^{-1}\, \mathrm{kpc}$ and little substructure, yielding $M_{107} \approx 1.66\times 10^{14}\,h^{-1}\,d^{-1}_{0.57}\,M_\odot$ within $107\,h^{-1}\, \mathrm{kpc}$ and $M/L_V \approx 276\pm40\,h\,(M/L_V)_\odot$ inside that radius. The DM profile is consistent with NFW outside the core but invalidates a strongly cusped inner profile, implying a soft core that is difficult to reconcile with standard CDM simulations and supporting open non-flat cosmologies. The diffuse intra-cluster light accounts for about 15% of the cluster light in the inner region, highlighting a close DM–baryon correspondence on these scales. Overall, the work provides a stringent, high-resolution constraint on cluster mass distributions and their implications for structure formation models.

Abstract

We construct a high resolution mass map of the z=0.39 cluster 0024+1654, based on parametric inversion of the associated gravitational lens. The lens creates eight well-resolved sub-images of a background galaxy, seen in deep imaging with HST. Excluding mass concentrations centered on visible galaxies, more than 98% of the remaining mass is represented by a smooth concentration of dark matter centered near the brightest cluster galaxies, with a 35 h^{-1} kpc soft core. The asymmetry in the mass distribution is <3% inside 107 ~h^{-1} kpc radius. The dark matter distribution we observe in CL0024 is far more smooth, symmetric, and nonsingular than in typical simulated clusters in either Omega=1 or Omega=0.3 CDM cosmologies. Integrated to 107 h^{-1} kpc radius, the rest-frame mass to light ratio is M/L_V = 276\pm 40 h (M/L_V)_solar.

Figures (4)

• Figure 1: [Color Plate] The reconstructed total mass density in CL0024, shown as a color-coded mass image. Dark matter is shown in orange. Mass associated with visible galaxies is shown in blue. Contours are at 0.5, 1, and 1.5 times the critical lensing density (4497 $h ~ d_{0.57}^{-1} ~ M_\odot$ pc$^{-2}$), with the heavier contour at the critical lensing density. This image is 336 $h^{-1}$ kpc across. North is up and east is left.
• Figure 2: The reconstructed mass density not associated with visible galaxies in CL0024 is shown as a contour plot (white contours), superposed on the F450W (blue) HST image for reference. Isomass contours for this dark mass are at $0.1 \Sigma_c$ intervals in projected mass density, with thick contour at 1$\times$$\Sigma_c$, as labeled. The plot is $336~ h^{-1}$ kpc (100 ) across, centered at RA = 00:23:56.6, DEC = 16:53:15 [1950]. On scales larger than 10 kpc, this majority component of the DM is remarkably smooth. Dark matter substructure has already been erased by $z ~=~ 0.39$.
• Figure 3: [Color Plate] Top panel: The cluster mass density not associated with galaxies is shown in orange. The centers of the dark matter (+), total light (x) and diffuse light (o) distributions are marked. The reconstructed optical arcs are shown superposed in blue. Bottom panel: The diffuse light component of the cluster is shown in orange. Galaxies have been fit and subtracted, and their cores are replaced with the fit to the diffuse light. The HST images of the arcs are then superposed in color (one blue and two yellow galaxies projected on two of the arcs are reinserted here, but are not counted in the diffuse light). Note the excellent agreement of the modeled arcs ( top panel) and the data ( bottom panel). The demagnified source image just west of the center strongly constrains the mass profile in the core.
• Figure 4: A radial plot of the mass density and light density. Total ( thick line) and galaxy-only ( thin line) components of the mass are shown. The dotted line is the best NFW fit discussed in the text, and the dashed line is the best-fit single PL model. The 35 $h^{-1}$ kpc soft core in the mass is evident. A singular mass distribution is ruled out. The total rest-frame V light profile ( solid line), and galaxy V light profile ( dashed line), smoothed with a 5 $h^{-1}$ kpc Gaussian, are also shown.