X-ray properties of massive compact relic galaxies

Orsolya E. Kovacs, Norbert Werner, Akos Bogdan, Jelle de Plaa

First listed 2025-05-20 | Last updated 2025-11-04

Abstract

We present the X-ray analysis of seven local compact elliptical galaxies (cEGs), selected for their morphological resemblance to high-redshift red nuggets. As likely descendants of the red nugget population, cEGs offer a unique window into the early Universe, enabling the study of early galaxy evolution and the interplay between black holes, stellar bulges, and dark matter halos. Using data from Chandra and XMM-Newton, we investigate the properties of the hot gaseous halos in cEGs. Two galaxies - MRK 1216 and PGC 32873 - host luminous, spatially extended X-ray atmospheres, allowing us to derive radial thermodynamic profiles. For MRK 1216, we performed high-resolution spectral modeling with RGS data, which hints at super-solar $α/\rm{Fe}$ abundance ratios. The remaining galaxies show either faint or undetected X-ray halos, though several display AGN-like (active galactic nucleus) power-law emission. In the context of local scaling relations, cEGs show only mild deviations from the general galaxy population, exhibiting a slightly steeper $M_{\star}-L_{X}$ relation and occupying the lower boundary of the $M_{\star}$-$M_{\rm vir}$ relation. These trends suggest that high-redshift red nuggets may also host a diverse range of X-ray atmospheres. We speculate that the compactness of cEGs may trace back to the population of `little red dots' (LRDs), hinting at a potential link between LRDs, red nuggets, and compact relic galaxies in the local Universe.

Short digest

Chandra and XMM-Newton data are used to map hot atmospheres around seven local compact ellipticals selected as red‑nugget analogs, probing BH–bulge–halo coupling. Only MRK 1216 and PGC 32873 show luminous, spatially extended halos enabling radial thermodynamic profiles, with MRK 1216’s RGS spectrum hinting at super‑solar α/Fe. The others have faint or undetected halos, several instead showing AGN‑like power‑law emission. On galaxy scaling planes, cEGs follow a slightly steeper M⋆–LX and sit on the lower edge of M⋆–Mvir, implying red nuggets—and possibly their LRD progenitors—host a diverse range of halo atmospheres.

Key figures to inspect

• Deep Chandra/XMM images and surface‑brightness profiles of MRK 1216 and PGC 32873: verify the spatially extended halo relative to the PSF and separate central point‑source from diffuse gas.
• Radial thermodynamic profiles (kT, ne, entropy, pressure) for MRK 1216 and PGC 32873: read off cooling times/entropy floors and assess hydrostatic mass trends that place them on the low edge of M⋆–Mvir.
• XMM‑RGS spectrum of MRK 1216: inspect α‑element (O/Ne/Mg/Si) to Fe line ratios supporting super‑solar α/Fe and check for multi‑temperature structure.
• Spectral fits for the fainter cEGs: compare thermal vs power‑law components and photon indices to substantiate AGN‑like emission where halos are undetected.
• Scaling‑relation panels (M⋆–LX and M⋆–Mvir): see the steeper M⋆–LX slope and cEG locus along the lower boundary of M⋆–Mvir relative to the general galaxy population.