[Paper Review] The similarity of observed X-ray coronae associated with L* disc and elliptical galaxies

Robert A. Crain, Ian G. McCarthy+3 more|arXiv (Cornell University)|Nov 8, 2010

Astrophysical Phenomena and Observations5 citations

TL;DR

This study demonstrates that X-ray coronae around $L_\star$ disc and elliptical galaxies exhibit identical scaling relations between X-ray luminosity ($L_{\rm X}$) and $K$-band luminosity ($L_{\rm K}$) or X-ray temperature ($T_{\rm X}$), indicating a common origin in shock-heated, accreted gas rather than stellar feedback. The universality of these relations implies that coronal gas is primarily shaped by gravitational and shock heating during halo assembly, with minimal contribution from supernova-driven outflows.

ABSTRACT

The existence of hot, X-ray luminous gaseous coronae surrounding present day L* galaxies is a generic prediction of galaxy formation theory in the cold dark matter cosmogony. While extended X-ray emission has been known to exist around elliptical galaxies for a long time, diffuse extra-planar emission has only recently been detected around disc galaxies. We compile samples of elliptical and disc galaxies that have Chandra and XMM-Newton measurements, and compare the scaling of the coronal X-ray luminosity (L_X) with both the K-band luminosity (L_K) and the coronal X-ray temperature (T_X). The X-ray flux measurements are corrected for non-thermal point source contamination by spatial excision and spectral subtraction for resolved and unresolved sources respectively. We find that the properties of the extended X-ray emission from galaxies of different morphological types are similar: for both elliptical and disc galaxies, the L_X - L_K and L_X - T_X relations have similar slope, normalisation and scatter. The observed universality of coronal X-ray properties suggests that the bulk of this emission originates from gas that has been accreted, shock-heated and compressed during the assembly of the galaxy and that outflows triggered by stellar processes make only a minor contribution to the X-ray emission. This reservoir of cooling gas is a potential source of morphological transformation; it provides a fresh supply of material for discs to grow around galaxies of all morphological types.

Motivation & Objective

• To test whether X-ray coronae in $L_\star$ disc and elliptical galaxies share similar physical properties despite differing morphologies.
• To determine whether the observed X-ray emission arises primarily from cooling inflows or from feedback-driven outflows.
• To assess the role of galaxy formation processes such as accretion, shock heating, and feedback in shaping the X-ray coronae of different galaxy types.
• To evaluate the consistency of observed X-ray scaling relations with predictions from cold dark matter galaxy formation models.

Proposed method

• Compilation of Chandra and XMM-Newton X-ray data for a sample of $L_\star$ disc and elliptical galaxies with measured $L_{\rm X}$, $T_{\rm X}$, and $L_{\rm K}$.
• Correction for non-thermal point source contamination via spatial excision and spectral subtraction for resolved and unresolved sources, respectively.
• Statistical comparison of $L_{\rm X}$-$L_{\rm K}$ and $L_{\rm X}$-$T_{\rm X}$ relations across galaxy types using linear regression with scatter analysis.
• Incorporation of external data from galaxy groups and clusters to extend the $L_{\rm X}$-$T_{\rm X}$ relation down to galaxy scales.
• Use of bolometric corrections based on Apec plasma models to verify that the observed break in the $L_{\rm X}$-$T_{\rm X}$ relation is not an artifact of energy band selection.
• Analysis of simulated data from the gimic hydrodynamic simulations to validate the physical mechanisms behind the observed scaling relations.

Experimental results

Research questions

• RQ1 Do X-ray coronae in $L_\star$ disc and elliptical galaxies follow the same $L_{\rm X}$-$L_{\rm K}$ scaling relation?
• RQ2 Is the $L_{\rm X}$-$T_{\rm X}$ relation universal across galaxy morphologies, including at low temperatures (~1 keV)?
• RQ3 What is the dominant physical origin of the X-ray emission—cooling inflows or feedback-driven outflows?
• RQ4 How do the observed X-ray scaling relations compare to predictions from the WF91 analytical model and cosmological simulations?
• RQ5 To what extent do supernova feedback and other processes alter the expected X-ray luminosity of coronae in disc galaxies?

Key findings

• The $L_{\rm X}$-$L_{\rm K}$ and $L_{\rm X}$-$T_{\rm X}$ relations for $L_\star$ disc and elliptical galaxies have indistinguishable slopes, normalizations, and scatter, indicating universal coronal properties.
• The $L_{\rm X}$-$T_{\rm X}$ relation exhibits a break at $\sim 1\,{\rm keV}$, with a steeper slope at lower temperatures, and this feature extends seamlessly from galaxy groups down to individual galaxies.
• The observed X-ray emission is inconsistent with a dominant contribution from supernova-driven outflows, as the $L_{\rm X}$-$L_{\rm K}$ relation does not correlate with star formation rate in the same way expected from feedback models.
• The similarity in X-ray coronal properties implies that the bulk of the emission arises from gas that was shock-heated during halo assembly, not from ongoing feedback processes.
• The data support a scenario in which coronal gas is primarily accreted and heated by gravitational and shock processes, with feedback playing a secondary role in reducing X-ray luminosity.
• The observed scaling relations are consistent with cosmological simulations like gimic, which show that supernova feedback reduces gas density and entropy in the corona, leading to lower X-ray luminosities than predicted by the WF91 model.