XMM-Newton Observation of an X-Ray Trail between the Spiral Galaxy NGC 6872 and the Central Elliptical Galaxy NGC 6876 in the Pavo Group

Abstract

We present XMM-Newton observations of a trail of enhanced X-ray emission extending along the full 8.7 arcmin ×4^' region between the large spiral galaxy NGC 6872 and the dominant elliptical galaxy NGC 6876 in the Pavo group, the first known X-ray trail associated with a spiral galaxy in a poor galaxy group and, with a projected length of 90 kpc, one of the longest known X-ray trails. The X-ray surface brightness in the trail region is roughly constant beyond ~20 kpc of NGC 6876 in the direction of the spiral galaxy. The trail is hotter (~1 keV) than the undisturbed Pavo IGM (~0.5 keV) and has low metal abundances (0.2 Z_solar). The 0.5-2 keV luminosity of the trail, measured using a 67×90 kpc rectangular region, is 6.6×10⁴⁰ ergs s^-1. We compare the properties of gas in the trail to the spectral properties of gas in the spiral galaxy NGC 6872 and in the elliptical galaxy NGC 6876 to constrain its origin. We suggest that the X-ray trail is either IGM gas gravitationally focused into a Bondi-Hoyle wake, a thermal mixture of ~60% Pavo IGM gas with ~40% galaxy gas that has been removed from the spiral galaxy NGC 6872 by turbulent viscous stripping, or both, due to the spiral galaxy's supersonic motion at angle ξ~40^deg with respect to the plane of the sky, through the densest region of the Pavo IGM. Assuming ξ=40^deg and a filling factor η in a cylindrical volume with radius 33 kpc and projected length 90 kpc, the mean electron density and total hot gas mass in the trail are 1×10^-3η^-1/2 cm^-3 and 1.1×10¹⁰η^1/2 M_solar, respectively.