The diverse hot gas content and dynamics of optically similar low-mass
  elliptical galaxies
  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 a Type Ia supernova (SN Ia)
driven outflow. We detect hot X-ray gas with a temperature of kT ~ 0.35 keV in
NGC4278, the component of which has a steeper surface brightness distribution
than the stellar light. Within the central 50 arcsec (~3.9 kpc) the estimated
gas mass is ~3 x 10^7 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 NGC4697 has an average
temperature of kT ~ 0.3 keV, and a significantly broader distribution than the
stellar light. The total gas mass within 90 arcsec (~5.1 kpc) is ~2.1 x 10^8
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.
