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

Abstract

Compact elliptical galaxies (CEGs) are candidates for 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 $\approx 122$ ks Chandra observation together with a shallow $\approx 13$ 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 $0.85r_{2500}$. 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 $\Lambda$CDM $c_{200}-M_{200}$ relation; i.e., ranging from $3.4\,\sigma - 6.3\, \sigma$ above the median $\Lambda$CDM 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 $1R_e$ compared to nearby normal ETGs. In addition, the highly concentrated total mass profile of Mrk 1216 cannot be described by MOND without adding DM, and it deviates substantially from the Radial Acceleration Relation. Mrk 1216 contains $\approx 80\%$ of the cosmic baryon fraction within $r_{200}$. The radial profile of the ratio of cooling time to free-fall time varies within a narrow range $(t_c/t_{\rm ff}\approx 14-19)$ over a large central region ($r\le 10$ kpc) suggesting "precipitation-regulated AGN feedback" for a multiphase plasma, though presently there is little evidence for cool gas in Mrk 1216. The properties of Mrk 1216 are remarkably similar to those of the nearby fossil group NGC 6482.