Tuesday, September 02, 2014
Fig. 1: Image of the galaxy cluster MACS J1149.5+2223, observed with the Hubble Space Telescope. Several background images are multiply imaged (marked by circles). There is one particularly striking example of a face-on spiral galaxy; zoomed images of this galaxy are enlarged as insets. (From Rau et al. 2014)
Fig. 2: Contours of the cluster mass distribution including scaled cluster galaxies for two different models (blue: position based modelling; red: hybrid modelling). The grey background shows the same observation as in figure 1, but in just one colour filter (F555W). (From Rau et al. 2014)
Fig. 3: Results for the surface brightness modelling of the multiply imaged face-on galaxy. Images from left to right are the main images 1.1, 1.2, 1.3.1 and 1.3.2. The top row shows the model reconstruction, the middle row the observed data and the bottom row the residual (i.e. the difference between model and data). (From Rau et al. 2014)
Observational constraints on the central mass distribution of galaxy clusters provide a key test of the nature of dark matter and contain invaluable information on the formation and evolution of the clusters and their brightest cluster galaxies. Scientists at the MPA have used an advanced lens modelling technique to map the mass distribution of the galaxy cluster MACS J1149.5+2223 over a wide range of distances. They found that this cluster has a large core and its mass profile therefore has to be shallower than what is expected from cold dark matter N-body numerical simulations.
At large scales, the cold dark matter model has been very successful in describing the observed properties of the Universe. With the use of N-body numerical simulations, detailed predictions have been made on the formation and evolution of the dark matter distribution at various scales. In particular, cold dark matter haloes are expected to be self-similar over a wide range of masses and to have a well-defined mass density distribution.
Gravitational lensing is a powerful tool to precisely measure the total mass distribution over a wide range of distances from the cluster centre. In combination with other measurements it therefore allows the scientists to study the interaction between the dark matter and the baryons in the cluster. In this work they focused on the galaxy cluster MACS J1149.5+2223 with the aim of providing a robust total mass model over a wide range of distances.
The galaxy cluster MACS J1149.5+2223 (located at redshift 0.544) was initially discovered by the Massive Cluster Survey and acts as a strong gravitational lens for at least 5 galaxies (with redshifts between 1.4 and 3.0), which are lensed in as many as 15 different images. One of the lensed galaxies is an impressive triply-imaged grand-design face-on spiral galaxy, which is also strongly lensed in two distinct Einstein rings by two cluster galaxies (see figure 1).
Previous gravitational lens models of this cluster were based on the reconstructions of a relatively limited number of image positions of the multiply lensed bright clumps and on simple scaling relations for the lensing contribution of the cluster member galaxies. Because of these limitations, many details of this lens system were reproduced only approximately.
In this work, they used a more sophisticated approach instead. They modelled all five cluster galaxies in the cluster centre that are close to multiply lensed images by using individual mass profiles. From the lensed background galaxies, they identified twice as many constraints as in previous models. Their advanced lens modelling technique uses not only the information on the positions of the multiply lensed images, but also their full surface brightness distribution.
The unique configuration of this lens system allowed them to reconstruct in particular the surface brightness distribution of the large central spiral galaxy in great detail. In addition, we were able to measure the mass profiles for several other individual cluster members and to determine the overall mass profile for the galaxy cluster as whole from its centre to a distance of up to 33 arcseconds (from 8 to 80 kpc) from the brightest cluster galaxy.
Our model indicates a large core (about 12 arcseconds) in the dark matter distribution of the cluster and we find that the total mass profile at the very centre is dominated by the brightest cluster galaxy. The inferred slope of the dark matter profile of MACS J1149.5+2223 is therefore shallower than the profile expected from pure dark matter simulations (the Navarro Frenk White (NFW) profile). This suggests that the baryons at the cluster centre have influenced the dark matter distribution.
Stefan Rau, Simona Vegetti and Simon White