Galaxy Formation

Galaxy Formation James Binney Oxford University TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAA

Outline • Cosmological clustering • Scales introduced by baryons • Timeline • Chemical evolution • Cores of Es • Cooling flows

CDM Background • Power spectrum of fluctuations • ! filaments+voids • ! hierarchy of halos • Analytic model: Extended Press-Schechter theory • characteristic mass(z) • Halo characteristic velocity(M) • Halo mass fn • Halo merger prob

Primary & secondary halos • Secondary halo: one that has fallen in to another halo • Survival time tfric ' tdyn(M/m) • Primary halo: one that hasn’t fallen in • P-S theory applies only to primary halos • Older theory didn’t believe in secondary halos • Primary/Secondary status changes sign of gas accretion/depletion

And baryons? • Have e.m. interactions: • Short-range scattering • adiabatic/shock compressive heating • Exchange E with e.m. waves • emission of bremsstrahlung + line radiation; • photo + Compton heating • Can form stars and BHs, which heat surrounding matter • Mechanically (winds/jets/shocks) • photonically

Characteristic numbers • Photo-heating • T'104K $ cs'10 km/s $ M=108M¯ • SN heating • With Salpeter IMF get 1 SN / 200 M¯ of SF ! ESN=1044J of mechanical E • Tmax=(mp/200M¯)ESN/kB=3£107K

Numbers (cont) • Gravitational heating • Rate of grav heating/unit mass • Hgrav=(GMH/r2)v=G½rv • Rate of radiative cooling/unit mass • Crad=¤(T)n2/(nmp)=¤½B/mp2 • ¤(T) = ¤(T0)(T/T0)1/2 = ¤(T0)v/v0 with T0 ' 106K, v0 = 100 km/s • Crad = ¤(T0)fB½ v/(v0mp2) with fB=0.17 • Hgrav/Crad = Gmp2v0r/fB¤(T0) = r/rcrit where rcrit=160kpc • ! Mcrit' 1012M¯ • Bottom line: smaller systems never get hot • Galaxies don’t form by cooling

Timeline • z'20: small-scale (M~106M¯) structures begin to collapse • Location: where long & short waves at crests, ie what will be centres of rich clusters • Voids shepherd matter into filaments • Larger & larger regions collapse, driving mergers of substructures • Voids merge too • A substructures survives if it falls into sufficiently bigger halo • Action spreads from densest to less dense regions (“downsizing”) • Initially Universe extremely cold (T<1K) • At z'6 photo heated to 104K • Halos less massive than 108 M¯ subsequently can’t retain gas • In low-density regions ! large population dark-dark halos?

Timeline (contd) • At any location scale of halo formation increases, as does Tvir • Until Tvir=106K, M=1012M¯ SN-heated gas escapes • Until Tvir=106K, M=1012M¯ infalling gas cold • Halos with M>1012M¯ acquire hot atmospheres • Heating by AGN counteracts radiative cooling • Hot gas evaporates limited cold infall ! “quenching” of SF

Chemical evolution • Closed-box model • Z=Mh/Mg (Z¯=0.02) • Instantaneous recycling • ±Mh = p±Ms-Z±Ms = (p-Z)±Ms • ±Z = ±(Mh/Mg) = (±Mh-Z±Mg)/Mg • Eliminate ±Mh!± Z = -p±ln(Mg) • ! Z(t)=-p ln[Mg(t)/Mg(0)] • Ok for gas-rich dwarfs but not dSph! • Ms[<Z(t)]=Ms(t)=Mg(0)-Mg(t)=Mg(0)(1-e-Z/p) • Ms(<®Z)/Ms(<Z)=(1-x®)/(1-x) where x=Mg(t)/Mg(0) • G-dwarf problem: with x=0.1 Ms(<Z¯/4)'0.49Ms but only 2% stars <0.25Z¯

In or out? • The box is open! • Outflow or inflow? • Arguments for inflow: • SFR ' const in solar nhd (Hipparcos) • S0 galaxies are spirals that have ceased SF (TF relation & specific GC frequency); they are also in locations where we expect inflow to have been reversed (Bedregal et al 2007) • Arguments for outflow: • in rich clusters ~half of heavy elements are in IGM • in M82 you see ouflow (probably in Galaxy too) • application of leaky box to globular-cluster system

Leaky-box model • dMt/dt=-c dMs/dt • ! • Can also apply to centres of ellipticals with c(¾) by equating E of ejection to ESN (S5.3.2 of Binney & Merrifield)

® enhancement • Most “® elements” (O, Ne, Mg, Si, S, A, Ca) ejected by core-collapse SNe; ¿~10Myr • Majority of Fe injected by type 1a SNe; ¿~1Gyr • Spheroids (metal-poor halo) ® enhanced (relative to Sun) • Implies SF complete inside 1Gyr

Centres of Es • Photometry of Es fitted by Lauer + 07 Conclude: on dry merging cores destroyed by BHs; in gas-rich mergers reformed by SF Nipoti & Binney 07

Cooling flows: mass dropout • In 1980s & 90s X-ray profiles interpreted on assumption that (i) steady-state, (ii) no heating • Imply diminishing flow to centre • ICM multiphase (Nulsen 86) • Field instability analysis implied runaway cooling of overdense regions (tcool/ 1/) • Cooler regions radiate all E while at rÀ 0 • Predicts that there should be (a) cold gas and (b) line radiation from T<106K throughout inner cluster Stewart et al 84

G modes • Malagoli et al (87): overdense regions just crests of gravity waves • In half a Brunt-Vaisala period they’ll be underdensities. • Oscillations weakly overstable (Balbus & Soker 89) but in reality probably damped. • Conclude: over timescale <tcool heating must balance radiative losses • Systems neither cooling nor flowing!

2001 – Chandra & XMM-Newton • XMM doesn’t see lines of <106K gas • XMM shows that deficit of photons at <1keV not due to internal absorption • But associated with “floor” T' Tvir/3 • Chandra shows that radio plasma has displaced thermal plasma (Bohringer et al 02) (Peterson et al 02)

Outward increasing entropy Omma thesis 05 Donahue 04

Summary (cooling flows) • Hot atmospheres not thermally unstable: will cool first @ centre • Clear evidence that weak radio sources associated with BH keep atmospheres hot • Mechanism: probably Bondi accretion at rate controlled by central density • Result: halos M>1012M¯ have little SF • Smaller halos that fall into such big halos gradually sterilized by ablation too • Hence decline in cosmic SF rate at current epoch

Papers to read • Dekel & Silk 1986 • Frenk & White 1991 • Benson et al 2003 • Cattaneo et al 2006

Galaxy Formation