Bogdán, Ákos, David, Laurence P., Jones, Christine, Forman, William R., and Kraft, Ralph P. 2012. "The Diverse Hot Gas Content and Dynamics of Optically Similar Low-mass Elliptical Galaxies." The Astrophysical Journal 758:65. https://doi.org/10.1088/0004-637X/758/1/65
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, NGC 821, NGC 3379, NGC 4278, and NGC 4697, 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 NGC 821 and NGC 3379 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 ~ 0.35 keV in NGC 4278, 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 &sun;, 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 NGC 4697 has an average temperature of kT ~ 0.3 keV and a significantly broader distribution than the stellar light. The total gas mass within 90'' (~5.1 kpc) is ~2.1 × 108 M &sun;, 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.