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Register a new userGeometric Support for Dark Matter by an Unaligned Einstein Ring in Abell 3827
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The non-detection of dark matter (DM) particles in increasingly stringent laboratory searches has encouraged alternative gravity theories where gravity is sourced only from visible matter. Here, we consider whether such theories can pass a two-dimensional test posed by gravitational lensing -- to reproduce a particularly detailed Einstein ring in the core of the galaxy cluster Abell 3827. We find that when we require the lensing mass distribution to strictly follow the shape (ellipticity and position angle) of the light distribution of cluster member galaxies, intracluster stars, and the X-ray emitting intracluster medium, we cannot reproduce the Einstein ring, despite allowing the mass-to-light ratios of these visible components to freely vary with radius to mimic alternative gravity theories. Alternatively, we show that the detailed features of the Einstein ring are accurately reproduced by allowing a smooth, freely oriented DM halo in the lens model, with relatively small contributions from the visible components at a level consistent with their observed brightnesses. This dominant DM component is constrained to have the same orientation as the light from the intracluster stars, indicating that the intracluster stars trace the gravitational potential of this component. The Einstein ring of Abell 3827 therefore presents a new challenge for alternative gravity theories: not only must such theories find agreement between the total lensing mass and visible mass, but they must also find agreement between the projected sky distribution of the lensing mass and that of the visible matter, a more stringent test than has hitherto been posed by lensing data.
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We present integral field spectroscopy of galaxy cluster Abell 3827, using ALMA and VLT/MUSE. It reveals an unusual configuration of strong gravitational lensing in the cluster core, with at least seven lensed images of a single background spiral galaxy. Lens modelling based on HST imaging had suggested that the dark matter associated with one of the clusters central galaxies may be offset. The new spectroscopic data enable better subtraction of foreground light, and better identification of multiple background images. The inferred distribution of dark matter is consistent with being centered on the galaxies, as expected by LCDM. Each galaxys dark matter also appears to be symmetric. Whilst we do not find an offset between mass and light (suggestive of self-interacting dark matter) as previously reported, the numerical simulations that have been performed to calibrate Abell 3827 indicate that offsets and asymmetry are still worth looking for in collisions with particular geometries. Meanwhile, ALMA proves exceptionally useful for strong lens image identifications.
The lensing and Einstein ring at the core of the galaxy cluster Abell 3827 are reproduced in the modified gravity theory MOG. The estimated effective lensing mass $M_L=(1+alpha)M_b=5.2times 10^{12} M_odot$ within $R=18.3$~kpc for a baryon mass $M_b=1.0times 10^{12} M_odot$ within the same radius produces the observed Einstein ring angular radius $theta_E=10$. A detailed derivation of the total lensing mass is based on modeling of the cluster configuration of galaxies, intracluster light and X-ray emission. The MOG can fit the lensing and Einstein ring in Abell 3827 without dark matter as well as General Relativity with dark matter.
Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and VLT/MUSE integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62+/-0.48kpc, where the 68% confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section sigma/m=(1.7+/-0.7)x10^{-4}cm^2/g x (t/10^9yrs)^{-2}, where t is the infall duration.
We have discovered strong gravitational lensing features in the core of the nearby cluster Abell 3827 by analyzing Gemini South GMOS images. The most prominent strong lensing feature is a highly-magnified, ring-shaped configuration of four images around the central cD galaxy. GMOS spectroscopic analysis puts this source at z~0.2. Located ~20 away from the central galaxy is a secondary tangential arc feature which has been identified as a background galaxy with z~0.4. We have modeled the gravitational potential of the cluster core, taking into account the mass from the cluster, the BCG and other galaxies. We derive a total mass of (2.7 +- 0.4) x 10^13 Msun within 37 h^-1 kpc. This mass is an order of magnitude larger than that derived from X-ray observations. The total mass derived from lensing data suggests that the BCG in this cluster is perhaps the most massive galaxy in the nearby Universe.
We investigate the mass content of galaxies in the core of the galaxy cluster Abell 611. We perform a strong lensing analysis of the cluster core and use velocity dispersion measurements for individual cluster members as additional constraints. Despite the small number of multiply-imaged systems and cluster members with central velocity dispersions available in the core of A611, the addition of velocity dispersion measurements leads to tighter constraints on the mass associated with the galaxy component, and as a result, on the mass associated with the dark matter halo. Without the spectroscopic velocity dispersions, we would overestimate the mass of the galaxy component by a factor of $sim1.5$, or, equivalently, we would underestimate the mass of the cluster dark halo by $sim5%$. We perform an additional lensing analysis using surface brightness (SB) reconstruction of the tangential giant arc. This approach improves the constraints on the mass parameters of the 5 galaxies close to the arc by up to a factor $sim10$. The galaxy velocity dispersions resulting from the SB analysis are consistent at the 1$sigma$ confidence level with the spectroscopic measurements and with the prediction from the simple pointlike analysis. In contrast the truncation radii for 2-3 galaxies depart significantly from the galaxy scaling relation and suggest differences in the stripping history from galaxy to galaxy.
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