X-ray properties of massive compact relic galaxies

TL;DR

This study analyzes the X-ray properties of seven local compact elliptical galaxies, including two confirmed relics, to probe their hot circumgalactic media and inform the connection to high-redshift red nuggets. Using Chandra and XMM-Newton data, the authors derive deprojected thermodynamic profiles for MRK 1216 and PGC 32873, identify or constrain diffuse halos in the other targets, and perform high-resolution spectroscopy with the RGS that suggests super-Solar alpha-element abundances in MRK 1216. They find a broad diversity of X-ray halos, with MRK 1216 and PGC 32873 hosting luminous, extended atmospheres, while others are X-ray faint or AGN-dominated, and they place the cEGs on local scaling relations, noting slightly steeper M_*–L_X and lower M_*–M_vir trends than the general population. The results imply that red nuggets at high z could also exhibit a range of X-ray atmospheres and support a narrative in which local compact relics are linked to little red dots and early-universe progenitors through a path involving limited mergers and AGN feedback, offering valuable constraints on early galaxy evolution and SMBH growth.

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.

Figures (6)

• Figure 1: The $0.3-2$ keV surface brightness profile of MRK 1216 (left panel) and PGC 32873 (right panel), and the best-fitting $\beta$-model with $\beta=0.507\pm0.003$, $r_{\mathrm{c}} = 0.874\pm0.001$ kpc and $\beta=0.526\pm0.011$, $r_{\mathrm{c}}=0.492\pm0.001$ kpc, respectively.
• Figure 2: Deprojected thermodynamic profiles of the CGM surrounding MRK 1216 extracted from Chandra ACIS data, showing temperature ($kT$), total density ($n = n_e + n_i$), entropy ($K$), and cooling time ($t_{\mathrm{cool}}$) profiles extending out to $\gtrsim\!20 \, r_{\mathrm{eff}}$. The effective radius of MRK 1216 is $r_{\mathrm{eff}} = 3.0$ kpc, corresponding to $6.8\arcsec$.
• Figure 3: Deprojected thermodynamic profiles of the CGM surrounding PGC 32873 extracted from Chandra ACIS data, showing temperature ($kT$), total density ($n = n_e + n_i$), entropy ($K$), and cooling time ($t_{\mathrm{cool}}$) profiles extending out to $\sim\!3 \, r_{\mathrm{eff}}$. The effective radius of PGC 32873 is $r_{\mathrm{eff}} = 2.3$ kpc, corresponding to $4.4\arcsec$.
• Figure 4: Left: The $0.5-2$ keV MOS1 CGM luminosity of the cE galaxies with detected X-ray emission within $r_{\mathrm{vir}}$ as a function of their stellar mass. The shaded region shows the best-fit relation and its $1\sigma$ uncertainty from 2024AA...690A.268Z. Right: The virial mass of cEs calculated from the $0.5-2$ keV CGM luminosity as a function of stellar mass, and the best-fit relation at $z=0$ with its $1\sigma$ confidence interval from 2019MNRAS.488.3143B. Overall, based on X-ray measurements, the cEs largely follow both the $M_\star-L_X$ and the $M_\star-M_{\mathrm{vir}}$ relations established for the local galaxy population, however, their virial mass is on the lower side of expectations, possibly hinting at a more compact halo structure.
• Figure 5: The inferred black hole mass of cE galaxies, estimated from $\sigma$, as a function of stellar mass. The cEs in our sample follow the relation observed for inactive elliptical and spiral/S0 galaxies. For comparison, we also include the relation found for local AGNs. Both relations are taken from 2015ApJ...813...82R, with the shaded regions indicating the corresponding $1\sigma$ confidence intervals.