Speaker
Description
The isotropy of the late Universe and consequently of the X-ray galaxy cluster scaling relations is an assumption greatly used in astronomy. However, many studies have recently reported deviations from isotropy using various cosmological probes, with a definitive conclusion yet to be made. New, effective and independent methods to robustly test the cosmic isotropy are of crucial importance. We use such a method, namely the directional behavior of the X-ray luminosity-temperature ($L_{\text{X}}-T$) relation of galaxy clusters. While the measured luminosity depends on the underlying cosmology, the temperature can be determined without any cosmological assumptions. Exploiting this property and the homogeneous sky coverage of X-ray galaxy cluster samples, one can effectively test the isotropy of cosmological parameters over the full extragalactic sky. Using >300 X-ray galaxy clusters included in eeHIFLUGCS, we find that the behavior of the $L_{\text{X}}-T$ relation heavily depends ($> 4\sigma$) on the direction of the sky. Several effects that could potentially explain these strong anisotropies were examined, but none was able to explain the obtained results. Interestingly, other cluster samples appear to have a similar behavior throughout the sky. Combining 3 independent samples results in 842 different galaxy clusters with luminosity and temperature measurements. Performing a joint analysis, the final anisotropy is further intensified ($> 5\sigma$), found towards $(l,b)\sim (303^{\circ}, -27^{\circ})$, in very good agreement with other cosmological probes. The maximum variation of $H_0$ seems to be $\sim 16\pm 3\%$ for different regions in the sky ($\sim 35\%$ in terms of the $L_{\text{X}}-T$ normalization). This result robustly demonstrates that X-ray studies that assume perfect isotropy in the properties of galaxy clusters and their scaling relations can produce strongly biased results whether the underlying reason is cosmological or related to X-rays. The identification of the exact nature of these anisotropies is therefore crucial for any statistical cluster physics or cosmology study. Finally, we show how the eROSITA All-Sky Survey, which will provide us with numerous high quality cluster data, will help us to better understand the origin of these anisotropies.
