### Abstract

We examine the scalings of X-ray luminosity, temperature, and dark matter or galaxy velocity dispersion for galaxy groups in a ΛCDM cosmological simulation, which incorporates gravity, gas dynamics, radiative cooling, and star formation, but no substantial nongravitational heating. In agreement with observations, the simulated L_{X}-σ and L_{X}-T _{X} relations are steeper than those predicted by adiabatic simulations or self-similar models, with L_{X} ∝ σ ^{4.4} and L_{X} ∝ T_{X}^{2.6} for massive groups and significantly steeper relations below a break at σ ≈ 180 km s^{-1} (T_{X} ≈ 0.7 keV). The T_{X}-σ relation is fairly close to the self-similar scaling relation, with T_{X} ∝ σ^{1.75}, provided that the velocity dispersion is estimated from the dark matter or from lsim;10 galaxies. The entropy of hot gas in low-mass groups is higher than predicted by self-similar scaling or adiabatic simulations, and it agrees with observational data that suggest an "entropy floor." The steeper scalings of the luminosity relations are driven by radiative cooling, which reduces the hot (X-ray-emitting) gas fraction from 50% of the total baryons at σ ≈ 500 km s^{-1} to 20% at σ ≈ 100 km s^{-1}. A secondary effect is that hot gas in smaller systems is less clumpy, further driving down L_{X}. A smaller volume simulation with 8 times higher mass resolution predicts nearly identical X-ray luminosities at a given group mass, demonstrating the insensitivity of the predicted scaling relations to numerical resolution. The higher resolution simulation predicts higher hot gas fractions at a given group mass, and these predicted fractions are in excellent agreement with available observations. There remain some quantitative discrepancies: the predicted mass scale of the L_{X}-T_{X} and L_{X}-σ breaks is somewhat too low, and the luminosity-weighted temperatures are too high at a given σ, probably because our simulated temperature profiles are flat or rising toward small radii, while observed profiles decline at r lsim; 0.2.R _{vir}. We conclude that radiative cooling has an important quantitative impact on group X-ray properties and can account for many of the observed trends that have been interpreted as evidence for nongravitational heating. Improved simulations and observations are needed to understand the remaining discrepancies and to decide the relative importance of cooling and nongravitational heating in determining X-ray scalings.

Original language | English (US) |
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Pages (from-to) | 23-41 |

Number of pages | 19 |

Journal | Astrophysical Journal |

Volume | 579 |

Issue number | 1 I |

DOIs | |

State | Published - Nov 1 2002 |

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### Keywords

- Cosmology: observations
- Cosmology: theory
- Galaxies: clusters: general
- Methods: numerical
- X-rays: galaxies: clusters

### ASJC Scopus subject areas

- Astronomy and Astrophysics
- Space and Planetary Science

### Cite this

*Astrophysical Journal*,

*579*(1 I), 23-41. https://doi.org/10.1086/342706