The Luminous X-Ray Halos of Two Compact Elliptical Galaxies

Abstract

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 B T  = 0.67 ± 0.06 keV within a radius of 15 kpc and a luminosity L x  = (1.8 ± 0.2) × 10 41 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) × 10 41 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) × 10 9 M ⊙ , with M BH > 1.4 × 10 10 M ⊙ (90% confidence), which is consistent with a recent SD measurement. We obtain a halo concentration ( c 200  = 17.5 ± 6.7) and mass ( M 200  = (9.6 ± 3.7) × 10 12 M ⊙ ), where c 200 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.