The Luminous X-Ray Halos of Two Compact Elliptical Galaxies

TL;DR

This study discovers extended X-ray emission in two compact elliptical galaxies, revealing their hot gas properties, dark matter content, and central black hole masses, supporting their classification as fossil groups and their similarity to early-type galaxies.

Contribution

First detailed X-ray analysis of two CEGs providing insights into their hot gas, dark matter, and black hole properties, supporting their evolutionary link to early-type galaxies.

Findings

Detected hot gas in both galaxies with specific temperature and luminosity.

Measured dark matter fraction within the effective radius consistent with local ETGs.

Constrained black hole mass and halo concentration, indicating early formation.

Abstract

There is mounting evidence that compact elliptical galaxies (CEGs) are local analogs of the high-redshift "red nuggets" 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 archival Chandra observations. We find that PGC 032873 has an average gas temperature $k_BT=0.67\pm 0.06$ keV within a radius of 15 kpc, and a luminosity $L_{\rm x} = (1.8\pm 0.2)\times 10^{41}$ erg s$^{-1}$ within a radius of 100kpc. For Mrk 1216, which is closer and more luminous $[L_{\rm x}(\rm <100~kpc) = (12.1\pm 1.9)\times 10^{41}$ erg s$^{-1}]$, we performed a spatially resolved spectral analysis in 7 annuli out to a radius of 73 kpc. Using an entropy-based hydrostatic equilibrium (HE) procedure, we obtain a good constraint on the $H$-band stellar mass-to-light ratio, $M_{\rm stars}/L_H=1.33\pm 0.21$ solar, in good agreement with stellar dynamical (SD) studies, which supports the HE approximation. We obtain a density slope $2.22\pm 0.08$ within $R_e$ consistent with other CEGs and normal local ETGs, while the dark matter (DM) fraction within $R_e$, $f_{\rm DM}=0.20\pm 0.07$, is similar to local ETGs. We place a constraint on the SMBH mass, $M_{\rm BH} = (5\pm 4)\times 10^{9}\, M_{\odot}$, with a 90% upper limit of $M_{\rm BH} = 1.4\times 10^{10}\, M_{\odot}$, consistent with a recent SD measurement. We obtain a halo concentration $(c_{200}=17.5\pm 6.7)$ and mass [$M_{200} = (9.6\pm 3.7)\times 10^{12}\, M_{\odot}$], where $c_{200}$ exceeds the mean $\Lambda$CDM value ($\approx 7$), consistent with a system that formed earlier than the general halo population. We suggest that these galaxies, which reside in group-scale halos, should be classified as fossil groups. (Abridged)