The unusually high dark matter concentration of the galaxy group NGC 1600

We investigate the properties of the dark matter (DM) halo surrounding the nearby galaxy group NGC 1600. Through the use of deep (252 ks) Chandra observations and 64.3 ks of XMM-Newton observations, we construct surface brightness profiles in multiple energy bands in order to perform hydrostatic equilibrium analysis of the hot plasma within NGC 1600. Regardless of the DM model profile assumed, we measure a halo concentration (c₂₀₀) that is an extreme, positive outlier of the ΛCDM c₂₀₀-M₂₀₀ relation. For a typical NFW DM profile, we measure c₂₀₀ = 26.7 ± 1.4 and M₂₀₀ = (2.0 ± 0.2) × 10¹³ M_⊙; assuming a similar halo mass, the average concentration expected is c₂₀₀ = 6 - 7 for the theoretical ΛCDM c-M relation. Such a high concentration is similar to that of well-known fossil groups MRK 1216 and NGC 6482. While NGC 1600 exhibits some properties of a fossil group, it fails to meet the X-ray luminosity threshold of L_X > 5 × 10⁴¹ erg s^-1. Whether or not it is considered a fossil group, the high concentration value makes it part of a select group of galaxy groups.