X-Ray Group Scaling Relations: Insights for Galaxy Formation

1. X-Ray Group Scaling Relations:Insights for Galaxy Formation Romeel Davé (Arizona) Neal Katz (UMass) David Weinberg (Ohio State) (work in progress)

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. • 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: • TXµ s2 (thermal energy = kinetic energy of galaxies) • LXµ TX2 (assuming free-free emission, M µs3) • LXµ s4 • Observed (Mulchaey&Zabludoff 98, Helsdon&Ponman 00): • LXµ TX3, LXµ s4-5, TXµ s2, for T>1 keV. • LXµ TX4-5, LXµ s5-10, TXµ s1-2 for T<1 keV.

4. LXµ TX3 LXµ s4.4 TXµ s2 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. • Alternatively, entropy injection required at level of ~100 keV cm2. • Uh-oh, that's a lotta energy/entropy.

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. • 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, e=7 h-1kpc. • mgas= 8.5x108 MM, mDM= 6.3x109 MM. 64-particle galaxy criterion. • Wm=0.4, L=0.6, Wb=0.02 h-2, h=0.65, s8=0.8. • Groups identified as bound systems with r/rcrit>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 s) • Smaller groups are under-luminous relative to self-similar prediction. • Below about 0.7 keV (180 km/s), luminosity relations steepen further. • TX-s 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). • Our simulation overcools baryons (the usual problem). • But the trend is consistent with observations. 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)-3b+0.5 , where b = mmps2/kBT • b is obtained by fitting SB profile (bfit) or finding T from X-ray spectrum (bspec). • Our bfit shows little variation with group size, but is far from 1, and often is not well-constrained. • Our bspec 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, in observations it is noisy and uncertain.

18. Comparison With Observed Scaling Relations • Include metallicity as observed by Davis, Mushotzky, Mulchaey (1999): ZµT 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-s 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 (e.g. XMM) are in the works.