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  Current Research Highlight :: March 2009 all highlights

Using Galaxy Clusters as Distant Mirrors of the Universal Acceleration

The gas contained in galaxy clusters acts as a mirror of the Cosmic Microwave Background (the relic radiation from the Big Bang), showing us how this background has evolved in recent cosmological times. Scientists of the Max Planck Institute for Astrophysics (MPA) have studied how distant galaxy clusters reflect the CMB before the Universe underwent a phase of accelerated expansion and what can be learnt from it.

Fig. 1: Sketch depicting the mirroring of CMB radiation from two galaxy clusters: a distant one, for which the CMB has yet no ISW contribution, and a nearby one, which mirrors the total CMB signal containing both the LSS and the ISW contributions to the CMB intensity anisotropy pattern.

Fig. 2: CMB anisotropy pattern versus inverse angular scale (l). Solid and dotted lines correspond to recent epochs, while dashed and dot-dashed lines correspond to a distant past in the history of the Universe. The ISW effect arises at large angular scales (small l-s) only at recent epochs: distant clusters must not reflect it.

Fig. 3: Errors in the cluster's peculiar motion recovery required for performing ISW tomography (at 3-sigma c.l.). Black circles, squares and diamonds depict the errors required on each galaxy cluster for populations more massive than 1, 2 and 3 times 1014 solar masses. The red squares and green circles observe the predictions for two different cluster surveys of the upcoming X-ray mission e-Rosita.

The Cosmic Microwave Background radiation (CMB) was released 370,000 years after the Big Bang, at the so-called Last Scattering Surface (LSS). Since then, the CMB has crossed the whole visible Universe before reaching the observer. It is a very isotropic radiation field, showing tiny angular deviations in intensity and polarization (one part in ~100,000), most of which were generated at the LSS.

At the same time, there is observational evidence that the Universe is currently undergoing a phase of accelerated expansion. If true, this would have a direct impact on the gravitational potential wells: an accelerated growth of the scale of the Universe makes potential wells shallower on the very large scales. The CMB photons crossing those regions leave a potential well that was deeper than when they entered it, i.e., the CMB photons gain some gravitational energy. The amount of energy earned will depend upon the particular potential well, and will not be the same for all CMB photons. Therefore this process introduces secondary anisotropies on the CMB, and it is known as the Integrated Sachs-Wolfe effect (ISW). This process should start occurring when the Universe is around 6-8 Gyrs old (or, equivalently, for redshifts in the range z ~ 0.6 - 1).

MPA researchers Carlos Hernandez-Monteagudo and Rashid Sunyaev have studied how clusters of galaxies placed at different distances can mirror the CMB anisotropies at different epochs, and, in this way, perform tomography of the late ISW effect. Galaxy clusters are the largest collapsed structures in the Universe, and contain electron plasma that interacts with the CMB radiation in different ways. In particular, the electrons blur the CMB anisotropies at the cluster's position by scattering the CMB photons off the line of sight. This blurring is actually sensitive to the CMB anisotropy field at the place of scattering, and effectively mirrors the CMB anisotropy pattern of that cosmological epoch. If this blurring can be measured in galaxy clusters situated at different redshifts (covering the range 0.1 < z < 1.3), then it should describe how the ISW has arosen at recent times (see Fig. 1). The ISW introduces large angle anisotropies, so a typical ISW cold or hot spot should be sampled by a relatively large number of clusters (each of them having independent errors on the estimated "mirrored" CMB).

Fig. 2 shows the theoretical expectations for the anisotropy pattern of the CMB versus inverse angular scale ("l" ~ inverse angle ~ 1 / angle). This plot displays the amount of CMB intensity anisotropy for every inverse angular scale "l". At present (solid lines) there is an excess of power at large angular scales (low l-s) due to the ISW effect. This effect vanishes at earlier epochs (dashed and dot-dashed lines). The top panel displays the anisotropy pattern as seen by nearby (solid, dotted lines) and distant (dashed, dot-dashed lines) clusters, while the bottom panel shows the projection or free-streaming of those signals mirrored by clusters onto a local observer. We see that the CMB mirrored by clusters placed at increasing redshifts should have lower signatures of the ISW.

However, electrons in clusters interact with the CMB radiation via two other channels. There is a thermal channel, that expresses the energy transfer of hot electrons to the CMB radiation, but it becomes zero for a particular choice of the observing frequency. There is also a kinetic channel, that arises due to the peculiar motion of clusters with respect to the CMB radiation. The contribution of this channel can be removed with minimal knowledge of the cluster's peculiar motion. Fig. 3 shows the errors required in the recovery of the individual cluster's peculiar velocity in order to measure the onset of the ISW.

This study provides another reason to accurately measure CMB anisotropies along the direction of galaxy clusters, currently a field of frantic activity.

Carlos Hernandez-Monteagudo and Rashid A. Sunyaev

Further Information:

Carlos Hernandez-Monteagudo & Rashid A. Sunyaev, "Galaxy Clusters as Distant Probes of Gravitational Potential Decay", 2009, submitted to Astronomy & Astrophysics

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