XMM–Newton observations of the merger-remnant galaxies NGC 3921 and 7252

Using the high sensitivity of XMM–Newton, we have studied the X-ray emission of the two prototypical late-stage merger remnants, NGC 3921 and 7252. In the case of NGC 7252, this is complemented by archival Chandra data. We investigate the nature of the discrete X-ray point source populations and the hot diffuse gas components in these two galaxies, and compare them in the light of their different merger ages and histories.

We detect three candidate ultraluminous X-ray point sources (ULXs) in NGC 3921 and at least six in NGC 7252, for which we have high spatial resolution Chandra data. These have luminosities ranging from ∼1.4 × 10³⁹–10⁴⁰ erg s⁻¹ (for H₀= 75 km s⁻¹ Mpc⁻¹). We expect these ULXs to be high-mass X-ray binaries, associated with the recent star formation in these two galaxies.

Extended hot gas is observed in both galaxies. We have sufficient counts in the XMM–Newton data to fit two-component hot plasma models to their X-ray spectra, and estimate the X-ray luminosities of the hot diffuse gas components to be 2.75 × 10⁴⁰ erg s⁻¹ and 2.09 × 10⁴⁰ erg s⁻¹ in NGC 3921 and 7252, respectively. These luminosities are low compared with the luminosities observed in typical mature elliptical galaxies (L_X∼ 10⁴¹–10⁴² erg s⁻¹), into which these merger remnants are expected to evolve. We do not see evidence that the X-ray haloes of these galaxies are currently being regenerated to the masses and luminosities seen in typical elliptical galaxies. The mass of atomic gas available to fall back into the main bodies of these galaxies and shock-heat to X-ray temperatures is insufficient for this to be the sole halo regeneration mechanism. We conclude that halo regeneration is most likely a long-term (>10 Gyr) process, occurring predominantly via mass loss from evolving stars, in a subsonic outflow stage commencing ∼2 Gyr after the merging event.