190 likes | 293 Views
Romeel Davé (Arizona) Neal Katz (UMass) David Weinberg (Ohio State). The Physics of Gas in Groups. Galaxy Groups: Tools for Studying Galaxy Formation. Groups (like our Local Group) contain the majority of L * galaxies in the Universe. M~10 13.5 -10 14.5 , s~100-500 km/s, T X ~0.1-2 keV.
E N D
Romeel Davé (Arizona) Neal Katz (UMass) David Weinberg (Ohio State) The Physics of Gas in Groups
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.
Group Scaling Relations: A "Crisis"? • Bound, virialized systems of hot gas are expected to obey self-similar scaling relations: • TX2 (thermal energy = kinetic energy of galaxies) • LX TX2 (assuming free-free emission, M 3) • LX4 • Observed (Mulchaey&Zabludoff 98, Helsdon&Ponman 00): • LX TX3, LX4-5, TX2, for T>1 keV. • LX TX4-5, LX, TXfor T<1 keV.
LXTX3 LX4.4 TX2 from Mulchaey (2000)
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 (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.
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. Also has problems with "overcooling". • 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). • 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.
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.
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
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) = S0 (1+r/rc)-3+0.5 , where = mp2/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?
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.
Comparison With Observed Scaling Relations • Include metallicity as observed by Davis, Mushotzky, Mulchaey (1999): ZT 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.
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.