Galactic Cluster's Mass Mapped With Unprecedented Precision

ESA/Hubble, NASA, HST Frontier Fields. The enormous galactic cluster MCS J0416.1-2403, which is creating images of more distant galaxies and quasars

Measuring the mass distribution of galaxies is a challenge. However, the Hubble telescope's Frontier Fields program is allowing a level of accuracy never seen before, in turn helping us learn about galaxy formation and dark matter.

The opportunity to map mass distributions arises when a galaxy lies in our line of sight to a bright object behind. The mass acts as a gravitational lens, bending the light from the more distant object. The distribution of the mass affects the way the light is bent, allowing us to calculate the properties required to produce this.

Frontier Fields has chosen six clusters of galaxies to target, using Hubble to examine the refracted light with exceptional detail. Results from the first of these, MCS J0416.1-2403 has been revealed in the image above.

Most galaxies produce “weak lensing”, smearing the more far-flung light source rather than producing a clear image. Large, dense galactic clusters however, can, with the right alignment produce rings of light, huge arcs or multiple images of the same object. Naturally Frontier Fields has chosen six strong lenses.

"The depth of the data lets us see very faint objects and has allowed us to identify more strongly lensed galaxies than ever before," explains Dr Mathilde Jauzac of Durham University. "Even though strong lensing magnifies the background galaxies they are still very far away and very faint. The depth of these data means that we can identify incredibly distant background galaxies." 

In Monthly Notices of the Royal Astronomical Society Jauzac and colleagues identify 51 galaxies where multiple images are visible as a result of MCS J0416.1-2403's lensing. Previous surveys had found 17.

Working back from this the authors claim to have more than doubled the precision with which they have mapped MCS J0416.1-2403's mass, reducing uncertainty to 0.5%. With 1.6 x 1014 solar masses, there is a lot there to map.

“Frontier Fields' observations and gravitational lensing techniques have opened up a way to very precisely characterize distant objects -- in this case a cluster so far away that its light has taken four and a half billion years to reach us," adds author Professor Jean-Paul Kneib Université d'Aix-Marseille . "But, we will not stop here. To get a full picture of the mass we need to include weak lensing measurements too. Whilst it can only give a rough estimate of the inner core mass of a cluster, weak lensing provides valuable information about the mass surrounding the cluster core." 

With recent claims throwing doubt on our understanding of how dark matter is distributed within galaxies, the research could confirm or overthrow existing theories.

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