# Research

### Evolution of Diffuse Gas Properties of the Most Distant Clusters

Galaxy clusters form from overdense regions in the large-scale matter distribution, collapsing under the effect of gravitational forces. As the most massive collapsed structures in the Universe, clusters are unique probes of fundamental cosmological parameters (e.g., dark energy equation of state, baryon fraction) and the growth of structure. While properties of the hot (107K) intracluster medium (ICM), which dominates the baryonic mass of clusters, are well understood, such as its metallicity, temperature, entropy, and pressure distributions, how these properties have evolved over the lifetime of a cluster remains poorly constrained. Dr. Bulbul's group investigate the ICM properties and their evolution of the fifteen most massive South Pole Telescope (SPT) selected clusters at z > 1 with XMM-Newton, Chandra, and SPT. This program will place the strongest constraints on the evolution of the ICM over the redshift range 0.1 < z < 1.5 and will provide some of the best constraints on the assumption of self-similar cluster evolution, which is often assumed in current cosmological studies.

Publications

• Ghirardini, V., Bulbul E., et al. 2019, in prep
• Bulbul, E., Chiu, I. N., Mohr, J. J., et al. 2019, ApJ, 871, 50
• McDonald, M., Bulbul, E., de Haan, et al., 2016, ApJ, 826, 12

### Indirect Searches for Dark Matter in the X-ray Band

The puzzle of dark matter has remained one of the greatest unsolved problems of physics for almost a century. Swiss astronomer Fritz Zwicky first conceived dark matter to explain the “missing mass” needed to predict the orbital velocities of galaxies in the galaxy clusters in 1934. The existence of dark matter has only been inferred from its gravitational influence on large scales. Various experiments for the direct detection of the dark matter particle are currently being pursued at a range of facilities including the Large Hadron Collider. Indirect detection through the measurement of the secondary products of dark matter annihilation or decay is also possible and being pursued using X-ray and gamma ray observations of dark-matter-dominated objects. One of the best sites to search for dark matter is in galaxy clusters, as these have some of the highest concentrations of dark matter in the universe. Dr. Bulbul recently developed a method that was used to reduce systematic effects from calibration and background uncertainties in the X-ray instruments, as well as to increase the sensitivity to weak signals from dark matter. This method led to a discovery of an unidentified emission line at 3.5 keV in the stacked observations of 73 galaxy clusters. The signal has independently been detected in other dark matter dominated objects, such as Andromeda galaxy and the Milky Way. The exact origin of the line remains unclear, however, this faint, yet tantalizing detection has the potential to be the discovery of a dark matter particle, which constitutes 85% of the total matter in the Universe. Dr. Bulbul's group combines the 94 Ms Chandra archival observations of the Galactic Halo in bins of angular distance from the Galactic Center to examine the profile of the signal at 3.5 keV. Given that the next calorimeter mission will not launch until 2021, this proposed program has the greatest potential to identify the origin of the 3.5 keV line. Most of the detections in the literature fit within the allowed limits for a sterile neutrino, and if present the detection has major implications for galaxy formation and cosmology.

Publications
• Bulbul, E., Foster, A., Brown, G. V., et al. 2019, ApJ, 870, 21
• Cappelluti, N., Bulbul, E., Foster, A., et al. 2018, ApJ, 854, 179
• Hitomi Collaboration, 2017, ApJ, 837, 1
• Bulbul, E., Markevitch, M., Foster, et al., 2016, ApJ, 831, 1
• Franse, J., Bulbul, E., Foster, et al., 2016, ApJ, 829, 124
• Bulbul, E., Markevitch, M., Foster, A., Smith, R. K., Loewenstein, M., Randall, S. W., 2014, ApJ, 789, 13
• Ruchayskiy, O., Boyarsky, A., Iakubovskyi, D., Bulbul, E., et al., 2015, MNRAS, 460, 1390