Cosmic Baryons: The IGM Ue-Li Pen 彭威禮

Overview History of Cosmic Baryons: a gas with phase transitions Missing baryons simulations SZ-Power spectrum: direct probe of baryons Prospects for detection

Cosmic Gas Today, 25% of matter is baryons (ordinary matter), the rest is dark matter (interacts through gravity only). C.f. dark energy. Thermal state today very poorly known. Probably in warm/hot/diffuse state, filling most of the universe. Only a small fraction in stars, cold (obervable) gas (Fukogita et al 1999). “Missing Baryons” T range K

Cosmological Context Critical for understanding global cosmology processes: galaxy formation, cluster formation. Impacts precision measurement of cosmology: dark energy, dark matter, lensing. Major efforts underway to map cosmic distribution using SZ (Compton scattering of CMB)

In the beginning Hot big bang: above z>1000, T>3000K. Baryons well understood: linear waves in photon-baryon plasma. Recombination: phase transition to neutral: well understood. Dark Ages(10<z<1000): non-linear passive evolution: well understood. Reionization (6<z<20): phase transition Epoch of Galaxy formation 2<z<6: Lya forest: modestly understood Present: z<2: poorly understood

WMAP 3yr Baryons at recombination: T=3000K, n=10 2 /cm 3

Reionization T:10K K z=6-15 Iliev, Mellema, Pen o FOV

Present day IGM: 3,000,000K (simulation)

Where are the missing baryons?

IGM conundrum Gas falls into gravitational wells. Why haven’t we seen it? =0.3 keV from cosmic virialization: easily visible by ROSAT extragalactic XRB Brightness depends on clumping C=ξ(0)<60 PS prediction: C>200. Need to expel 70% of gas. Simulations: C>>100 (Pen 1999, Dave et al 2001, Kang&Ryu 2003)

Cosmic Fluid Constraint If gas follows dark matter (adiabatic evolution), Press-Schechter theory describes dynamics: all matter in gravitationally bound, hydrostatic halos Missing effects: heating/cooling Cooling: form stars, denser/colder gas, more easily observed. Calculable. Heating: expel gas from dark matter halos, harder to observe. Unpredictable.

Zhang, Pen & Trac 2004

Gas traces DM too well, inconsistent with XRB data XRB limit grid Zhang, Pen and Trac 2004.

Hiding Baryons Heat and eject: ΔE of 1 keV, maybe less if SN are intergalactic (entropy). Cool and hide: cooling catastrophe Problem with simulations? Data interpretation (XRB shadowing)? How can we find them and show that we found them? How does that affect SZ clusters?

IGM balance Heating: hydrostatic equilibrium vs free expansion Gravitational potential determined by dark matter, only weakly affected by baryons Heating scenarios: 1. halo centers 2. uniform

Central Heating Initial halo state: isothermal halo, strong entropy stratification Add heat adiabatically at center, due to winds from SNe, BH outflows, etc. (HII regions are not energetic enough) Raise central entropy adiabatically at convective stability limit Final state: central isentropic “core”, isothermal stratified envelope

Hydrostatic Solution Halo Mass. Given v c (observable) Virial radius Isothermal profile Post heating core profile Core radius From Pen (1999, ApJ 510 L1)

IGM dilemma Cosmic virial temperature is 3,000,000K 1 keV is uncomfortably hot (10 7 K) for SNe and feedback scenarios. What about warm ( K) phase? Difficult to understand in Press-Schechter picture: hydrostatic equilibrium results in high density, rapid cooling. Warm phase may be numerical artifact.

Hunting Baryons Baryons (electrons) interact with light (Thomson scattering): SZ & KSZ against CMB. KSZ is photon Doppler shift from bulk velocity. TSZ is Compton y =τkT/(m c 2 ) ~ SZ is redshift independent! Where are the baryons?

Simulated Universe in tSZ

Power spectrum of baryons: thermal and kinetic Zhang, Pen & Trac 2004

Redshift resolution Equation of state of gas: how hot/dense is the IGM? Hotter means smoother, less correlated than galaxies Cross-correlation with photo-z crucial to quantify results

Zhang & Pen 2001

Prospects New experiments: SPT, SZA, ACT will map SZ and KSZ for large areas of sky, measuring baryon inventory

Conclusions Baryons poorly understood today. Heating/cooling probably important. Adiabatic prediction is E virial = 0.3 keV, inconsistent with observations (groups, XRB) Feedback requires a lot of energy (>E virial ). 1 keV consistent with XRB, LTR (group properties). Debate on temperature (warm?), pressure equilibrium, simulations. SZ a promising physical probe of baryon distribution. This needs to be understood for precision measurement of cosmological parameters, galaxy and cluster formation.