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

Galaxy Groups: Tools for Studying Galaxy Formation " Groups (like our Local Group) contain the majority of L * galaxies in the Universe.  M~ ,  ~ km/s, T X ~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.

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

from Mulchaey (2000) L X  4.4 L X  T X 3 T X  2

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.

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 cm 2. – Uh-oh, that's a lotta energy/entropy.

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

Pre-Heating Works Borgani et al. 2001

Evidence for Cooling Bryan 2000

Cooling Works... at least for clusters Bryan 2000

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?

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).  2x144 3 (6 million) particles (N SPH =N DM ), L=50 h -1 Mpc,  =7 h -1 kpc.  m gas = 8.5x10 8 M M, m DM = 6.3x10 9 M 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>10 5 K) only. " X-ray properties calculated using Raymond-Smith code.

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.  T X -  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.

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

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.

Beta Model  Isothermal King model gives: S(r) = S 0 (1+r/r c ) -3  +0.5, where  =  m p  2 /k B T   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?

Entropy-Temperature " We calculate entropy at 0.1R vir 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.

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.  L X -  amplitude in very good agreement, but amplitude of temperature relations are too high, since T is high by X

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 keV represent the transition objects. – Groups, relative to clusters, spend a larger portion of their assembly history in a state where t cool < t Hubble. " 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.