The Physics of Gas in Groups

1. Romeel Davé (Arizona) Neal Katz (UMass) David Weinberg (Ohio State) The Physics of Gas in Groups

2. Galaxy Groups: Tools for Studying Galaxy Formation • Groups (like our Local Group) contain the majority of L* galaxies in the Universe. • M~1013.5-1014.5, s~100-500 km/s, TX~0.1-2 keV. • Groups are hard to see: • Faint in X-rays, large Galactic foreground at lower T. • Hard to identify optically due to chance projections. • ROSAT observations + deep optical imaging have revealed some puzzles, the answers to which may impact our understanding of galaxy formation.

3. Group Scaling Relations: A "Crisis"? • Bound, virialized systems of hot gas are expected to obey self-similar scaling relations: • TX2 (thermal energy = kinetic energy of galaxies) • LX TX2 (assuming free-free emission, M 3) • LX4 • Observed (Mulchaey&Zabludoff 98, Helsdon&Ponman 00): • LX TX3, LX4-5, TX2, for T>1 keV. • LX TX4-5, LX, TXfor T<1 keV.

4. LXTX3 LX4.4 TX2 from Mulchaey (2000)

5. Solutions: Hot and Cold • To reduce luminosity, must do one of three things: • Lower temperature (without raising density) • Lower density • Remove the offending gas • The Hot answer: Add some heat, presumably due to supernovae/AGN/etc, which puffs up gas and reduces density. • The Cool answer: Make galaxy formation more efficient in lower mass systems, removing hot gas.

6. The Pre-Heating Model • Evidence in favor: • The IGrM is enriched, presumably by winds. Those winds must inject energy. • AGN in clusters may be responsible for keeping cooling flow gas at ~1keV. Similar in groups? • Quantitatively, things are not so easy: • Energy needed is ~1-3 keV/baryon over entire IGrM (or entropy ~100-400 keV cm2); a LOT for supernovae. • AGNs emit enough energy, but how to confine? Also, simulations suggest "cooling flows" can be continually disrupted dynamically due to accretion events, so AGN heat not needed.

7. Entropy "Floor" plot: Bryan (2000) data: Ponman, Cannon, Navarro (1999)

8. Pre-Heating Works Borgani et al. 2001

9. Evidence for Cooling Bryan 2000

10. Cooling Works... at least for clusters Bryan 2000

11. What We Know So Far • Pre-heating works... but only at the expense of invoking some fairly mysterious energy source. • Cooling works... but only for cluster-sized systems, and only by assuming a variation in hot gas fraction with temperature, which may or may not be observed. Also has problems with "overcooling". • The real question: What do standard ab initio galaxy formation models predict?

12. Cosmological Hydro Simulation • Tree gravity, Smoothed Particle Hydrodynamics, Massively Parallel. • Radiative cooling (H, He, Compton, No Metals!). • Photoionization (spatially uniform, time-varying). • Star formation, feedback (thermal). • 2x1443 (6 million) particles (NSPH=NDM), L=50 h-1Mpc, =7 h-1kpc. • mgas= 8.5x108 M⊙, mDM= 6.3x109 M⊙. 64-particle galaxy criterion. • m=0.4, =0.6, b=0.02 h-2, h=0.65, 8=0.8. • Groups identified as bound systems with /crit>278; 128 at z=0. • Hot and cold phases explicitly "decoupled" by computing gas density from hot particles (T>105K) only. • X-ray properties calculated using Raymond-Smith code.

13. Scaling relations(Zero metallicity, dark matter ) • Smaller groups are under-luminous relative to self-similar prediction. • Below about 0.7 keV (180 km/s), luminosity relations steepen further. • TX- relation shows not much extra heating (not surprising, since we haven't put any in). • Slopes in reasonable agreement with observations, but other effects (eg metals) are significant.

14. Baryon fraction • T~3 keV groups have 50% hot fraction, T~0.3 keV have 20%. • Second panel shows computing hot fraction out to observable radius (ROSAT surface brightness limit). • Third panel (resolution test): High-res simulation has LESS cold gas! • Fourth panel: Clumping factor. Trend due to WHIM? Mulchaey 2000

15. Profiles • Surface brightness profile fairly self-similar. • Temperature profile ~isothermal, but no cool central region. • Hot gas profile also fairly self-similar, but scaled down due to lower hot gas fraction. • Entropy profile roughly a power-law in radius.

16. Beta Model • Isothermal King model gives: S(r) = S0 (1+r/rc)-3+0.5 , where  = mp2/kBT •  is obtained by fitting SB profile (fit) or finding T from X-ray spectrum (spec). • Our fit shows little variation with group size, but is far from 1, and often is not well-constrained. • Our spec shows our temperatures are high: No cool central region?

17. Entropy-Temperature • We calculate entropy at 0.1Rvir by fitting S(r) with a power law for each group. • Our groups agree with observations, but they do not suggest a "floor", only a sub-self-similar slope. • While entropy is nice in theory, observations of it are noisy and uncertain.

18. Comparison With Observed Scaling Relations • Include metallicity as observed by Davis, Mushotzky, Mulchaey (1999): ZT for T<2 keV. • Include surface brightness effects by computing out to an "observable" radius. • Slopes are in good agreement with observations, but "break" is at slightly too low mass. • LX- amplitude in very good agreement, but amplitude of temperature relations are too high, since T is high by X 1.5-2.

19. Conclusions • Radiative cooling has a significant effect on IGrM properties, despite that fact that current cooling times are longer than a Hubble time over most of the group. • Since cooling is known to occur, any additional physical processes such as pre-heating must be examined as add-ons. • The effect of cooling qualitatively brings simulations into agreement with observations. Simply put: In clusters, most baryons are hot, while in galaxies most baryons are cold; groups around 0.5-1 keV represent the transition objects. • Groups, relative to clusters, spend a larger portion of their assembly history in a state where tcool < tHubble. • Quantitative agreement has yet to be clearly demonstrated, though initial results are encouraging. Better simulations (e.g. two-phase handling) and better observations are in the works.