We investigate the X-ray vs. optical scaling relations of poor groups to small clusters ( \sigma \approx 100 - 700 km/s ) identified in a cosmological hydrodynamic simulation of a \Lambda CDM universe , with cooling and star formation but no pre-heating .
We find that the scaling relations between X-ray luminosity , X-ray temperature , and velocity dispersion show significant departures from the relations predicted by simple hydrostatic equilibrium models or simulations without cooling , having steeper L _ { X } - \sigma and L _ { X } - T _ { X } slopes and a “ break ” at \approx 200 km/s ( \approx 0.3 keV ) .
These departures arise because the hot ( X-ray emitting ) gas fraction varies substantially with halo mass in this regime .
Our predictions roughly agree with observations .
Thus radiative cooling is a critical physical process in modeling galaxy groups , and may present an alternative to ad hoc models such as pre-heating or entropy floors for explaining X-ray group scaling relations .