JWST lens model for A370: A very low dark matter fraction for a brightest cluster galaxy and lensing properties for the Dragon arc

TL;DR

The paper develops a JWST-backed hybrid lens model for Abell 370, integrating MAGNIF CANUCS constraints to map the mass distribution near the BCGs and to analyze the Dragon arc’s microlensing regime. It finds a striking dichotomy in dark matter content: the north BCG appears consistent with negligible DM inside $23\,\rm{kpc}$, while the south BCG requires substantial DM, raising questions for MOND/SIDM within a cluster core. The Dragon arc is used to extract detailed lensing properties, including a spatially varying CC strength and pronounced microlensing activity, with time-delay structure that could inform cosmology and exotic DM scenarios. The work also explores the potential to detect long-period Cepheids in the Dragon under microlensing, underlining JWST’s capabilities and the need for further inner-region constraints and follow-up observations.

Abstract

We present a new lens model for the $z=0.375$ galaxy cluster Abell 370 based on previously spectroscopically confirmed lensed galaxies and new lensed systems identified in JWST data, including recent data from the MAGNIF program. Based on the best models able to reproduce two radial arcs near the BCGs, we compare the stellar mass to the total mass from the lens model and find that the fraction of dark matter in the south BCG is consistent with $Λ$CDM while in the north BCG we find a very small amount of dark matter, more consistent with alternative models to $Λ$CDM. We discuss possible causes for this and conclude that additional data is needed to clarify the situation. We study the lensing properties, magnification, time delay and strength of the critical curve, along the Dragon arc, where previous studies have reported tens of alleged microlensing events from supergiant stars at $z=0.7251$. The new lens model is able to reproduce the distribution of microlensing events with great accuracy. Some of the microlensing events may be reinterpreted as long-period Cepheid in future observations. We consider this possibility and study in more detail the challenges for such detection from intervening microlenses.

Figures (15)

• Figure 1: Color image of A370 combining 16 HST and JWST filters (range 0.2--5 micron). The scale and orientation are indicated. Medium-size colored circles mark four new systems (or redefinitions) used in the lens model with redshifts obtained from MAGNIF. Counterimage 70.3 is outside this image $\approx 15$ arcseconds west. Small white circles mark galaxies with MAGNIF photo-z $1.05\pm0.05$ from a lensed group overdensity behind the cluster. Some members of this group are multiply lensed. The two BCGs and the Dragon galaxy are marked. The two big ellipses show the location of two radial arcs (A$_{\rm N}$ and A$_{\rm S}$) from system 7. Each radial arc contains a double image of different portions of the source galaxy in system 7. The yellow circles labeled C$_{\rm E}$, C$_{\rm S}$, C$_{\rm C}$, and C$_{\rm W}$ contain additional counterimages of the source in system 7. The straight features in the NE are diffraction spikes from nearby bright stars.
• Figure 2: Mass profile of the lens models A, B, and C. Red curves correspond to the radial profiles when the center is the north BCG while blue curves are for the profiles centered in the south BCG. The black lines is the same NFW model from Diego2018b with concentration $c=2$ and a virial radius $R_{200}= 3$ Mpc. The profiles around the north and south BCG are remarkably similar, suggesting a mass ratio of 1:1 for the two subgroups.
• Figure 3: Predicted radial arc 7a near the South BCG for models A, B, and C, and for two different counterimages used as templates. The dashed yellow line is identical in all panels and can be used to compare with the position and orientation of the observed arc.
• Figure 4: Predicted radial arc 7b near the North BCG for models A, B, and C, and for two different counterimages used as templates. The dashed yellow line is identical in all panels and can be used to compare with the position and orientation of the observed arc.
• Figure 5: Top. Composite HST+JWST image showing the Dragon arc region. The image is a made form 16 filters with the blue containing HST's F200LP, F435W, F606W and F814W, green with JWST's F090W, F115W, F150W, F182M, F200W, and F210M, and red with F277W, F300M, F335M, F356W, F460M, and F480M. Middle, high-pass filtered color version (scale of 0.3 arcseconds for the high pass Gaussian filter) of the three filters indicated in the top-left corner. The circles mark the constraints in the Dragon used to derive the lens model. Yellow squares mark the position of the critical points also used as constraints. Bottom, high pass filtered (scale 0.1 arcseconds for the high pass Gaussian filter) color version of the three bands indicated in the top-left corner. The portion of the Dragon shown in this image is marked with a rectangle in the middle panel. Circles indicate the position of the transients in Fudamoto25. The white curve is the hybrid WSLAP+ lens model derived with critical points as constraints. The "+" and "-" symbols indicate the parity of the counterimages formed on that side of the CC.