The Extremely High Dark Matter Halo Concentration of the Relic Compact Elliptical Galaxy Mrk 1216

Abstract

Spatially compact stellar profiles and old stellar populations have established compact elliptical galaxies (CEGs) as local analogs of the high-redshift “red nuggets” thought to represent the progenitors of today’s early-type galaxies (ETGs). To address whether the structure of the dark matter (DM) halo in a CEG also reflects the extremely quiescent and isolated evolution of its stars, we use a new ≈122 ks Chandra observation together with a shallow ≈13 ks archival observation of the CEG Mrk 1216 to perform a hydrostatic equilibrium analysis of the luminous and relaxed X-ray plasma emission extending out to a radius 0.85 r 2500 . We examine several DM model profiles and in every case obtain a halo concentration ( c 200 ) that is a large positive outlier in the theoretical ΛCDM c 200 – M 200 relation; i.e., ranging from 3.4 σ to 6.3 σ above the median ΛCDM relation in terms of the intrinsic scatter. The high value of c 200 we measure implies an unusually early formation time that firmly establishes the relic nature of the DM halo in Mrk 1216. The highly concentrated DM halo leads to a higher DM fraction and smaller total mass slope at 1 R e compared to nearby normal ETGs. In addition, the highly concentrated total mass profile of Mrk 1216 cannot be described by modified Newtonian dynamics without adding DM, and it deviates substantially from the radial acceleration relation. Our analysis of the hot plasma indicates that the halo of Mrk 1216 contains ≈80% of the cosmic baryon fraction within r 200 . The radial profile of the ratio of cooling time to freefall time varies within a narrow range ( t c / t ff  ≈ 14–19) over a large central region ( r  ≤ 10 kpc), suggesting “precipitation-regulated active galactic nucleus feedback” for a multiphase plasma, although there is little evidence at present for cool gas in Mrk 1216. Finally, other than its compact stellar size, the stellar, gas, and DM properties of Mrk 1216 are remarkably similar to those of the nearby fossil group NGC 6482.