The diverse hot gas content and dynamics of optically similar low-mass elliptical galaxies

Abstract

The presence of hot X-ray-emitting gas is ubiquitous in massive early-type galaxies. However, much less is known about the content and physical status of the hot X-ray gas in low-mass ellipticals. In the present paper, we study the X-ray gas content of four low-mass elliptical galaxies using archival Chandra X-ray observations. The sample galaxies, NGC821, NGC3379, NGC4278, and NGC4697, have approximately identical K-band luminosities, and hence stellar masses, yet their X-ray appearance is strikingly different. We conclude that the unresolved emission in NGC821 and NGC3379 is built up from a multitude of faint compact objects, such as coronally active binaries and cataclysmic variables. Despite the non-detection of X-ray gas, these galaxies may host low density, and hence low luminosity, X-ray gas components, which undergo an outflow driven by a Type Ia supernova (SN Ia). We detect hot X-ray gas with a temperature of kT ∼ 0.35keV in NGC4278, the component of which has a steeper surface brightness distribution than the stellar light. Within the central 50″ (∼3.9 kpc), the estimated gas mass is ∼3 × 107 M ⊙, implying a gas mass fraction of ∼0.06%. We demonstrate that the X-ray gas exhibits a bipolar morphology in the northeast-southwest direction, indicating that it may be outflowing from the galaxy. The mass and energy budget of the outflow can be maintained by evolved stars and SNe Ia, respectively. The X-ray gas in NGC4697 has an average temperature of kT ∼ 0.3keV and a significantly broader distribution than the stellar light. The total gas mass within 90″ (∼5.1 kpc) is ∼2.1 × 10 8 M⊙, hence the gas mass fraction is 0.4%. Based on the distribution and physical parameters of the X-ray gas, we conclude that it is most likely in hydrostatic equilibrium, although a subsonic outflow may be present. © 2012. The American Astronomical Society. All rights reserved.