2018ApJ...854..143B

There is mounting evidence that compact elliptical galaxies (CEGs) are local analogs of the high-redshift "red nuggets" that are thought to represent progenitors of today's early-type galaxies (ETGs). We report the discovery of extended X-ray emission from a hot interstellar/intragroup medium in two CEGs, Mrk 1216 and PGC 032873, using shallow Chandra observations. We find that PGC 032873 has an average gas temperature of k_BT = 0.67 ± 0.06 keV within a radius of 15 kpc and a luminosity L_x = (1.8 ± 0.2) x 10⁴¹ erg s^–1 within a radius of 100 kpc. For Mrk 1216, which is closer and more luminous (L_x(<100 kpc) = (12.1 ± 1.9) x 10⁴¹ erg s^–1), we used an entropy-based hydrostatic equilibrium (HE) procedure and obtained a good constraint on the H-band stellar mass-to-light ratio, M_stars/L_H = 1.33 ± 0.21 solar, that is in good agreement with stellar dynamical (SD) studies, which supports the HE approximation. We obtain a density slope of 2.22 ± 0.08 within R_e that is consistent with other CEGs and normal local ETGs, while the dark matter fraction within R_e, f_DM = 0.20 ± 0.07 is similar to local ETGs. We constrain the supermasssive black hole mass, M_BH = (5 ± 4) x 10⁹ M_☉, with M_BH > 1.4 x 10¹⁰ M_☉ (90% confidence), which is consistent with a recent SD measurement. We obtain a halo concentration (c₂₀₀ = 17.5 ± 6.7) and mass (M₂₀₀ = (9.6 ± 3.7) x 10¹² M_☉), where c₂₀₀ exceeds the mean ΛCDM value (≃7), which is consistent with a system that formed earlier than the general halo population. We suggest that these galaxies should be classified as fossil groups.