On Dark Peaks and Missing Mass: A Weak-Lensing Mass Reconstruction of the Merging Cluster System A520,

Abstract

Merging clusters of galaxies are unique in their power to directly probe and place limits on the self-interaction cross-section of dark matter. Detailed observations of several merging clusters have shown the intracluster gas to be displaced from the centroids of dark matter and galaxy density by ram pressure, while the latter components are spatially coincident, consistent with collisionless dark matter. This has been used to place upper limits on the dark matter particle self-inteaction crosssection of order 1 cm 2 g −1 . The cluster Abell 520 has been seen as a possible exception. We revisit A520 presenting new HST ACS mosaic images and a Magellan image set. We perform a detailed weak lensing analysis and show that the weak lensing mass measurements and morphologies of the core galaxy-filled structures are mostly in good agreement with previous works. There is however one significant difference -we do not detect the previously claimed "dark core" that contains excess mass with no significant galaxy overdensity at the location of the X-ray plasma. This peak has been suggested to be indicative of a large self-interaction cross-section for dark matter (at least ∼ 5σ larger than the upper limit of 0.7cm 2 g −1 determined by observations of the Bullet Cluster). We find no such indication and instead find that the mass distribution of A520, after subtraction of the X-ray plasma mass, is in good agreement with the luminosity distribution of the cluster galaxies. We conclude that A520 shows no evidence to contradict the collisionless dark matter scenario.

Spectral index image of the radio halo in the cluster Abell 520, which hosts the famous bow shock

“…The NE clump is aligned with the elongated mass distribution (NE to SW): structures 2+3 in Fig. 2 by Clowe et al (2012). The diffuse emission at the shock location in the SW is observed to correspond with the 4-7 structures in the same figure.…”

“…In between, a depression in the radio surface brightness can be observed. The mass distribution (Clowe et al 2012) and the diffuse radio emission are clearly linked. Indeed, we note that the nonthermal emission permeates regions of high mass.…”

“…By analyzing new Hubble Space Telescope data, Clowe et al (2012) did not detect the dark-matter core and, after subtracting the X-ray plasma mass, they found a good agreement between the mass distribution in this cluster and the luminosity distribution of the cluster galaxies, and between weak-lensing mass measurements and the morphology of the core-galaxyfilled structure. Moreover, they found that the mass within a sphere encompassing a mean overdensity of 200 is M 200 = (9.1 ± 1.9) × 10 14 M , in agreement with previous works.…”

Stellar streams as gravitational experiments

“…This paradigm provides a natural explanation to the apparent conspiracy between the observed baryonic distribution and the gravitational force in various types of galaxies (see, e. g. Famaey & McGaugh 2012;McGaugh et al 2016;Lelli et al 2016). Nevertheless, the successes of the paradigm are mostly limited to galactic scales (for problems at other scales, see, e.g., Skordis et al 2006;Clowe et al 2006;Angus 2008;Ibata et al 2011;Dodelson 2011;Clowe et al 2012;Harvey et al 2015). This could mean that it is either an effective way to predict how dark matter (DM) is arranged in galaxies, or that it is an effective modification of gravity resulting from a broader theory of the dark sector (e.g.…”

Is the radio emission in the Bullet cluster due to dark matter annihilation?

“…Markevitch et al 2002) and of the gravitational lensing (Clowe et al 2006, Bradac et al 2006 showed that distributions of the hot gas in the Intra Cluster Medium (ICM) and of the DM are centered at different positions. The other case of A520 where there is some spatial offset between DM and baryons is not a very clean case to study (Mahdavi et al 2007, Clowe et al 2012.…”