Galaxy formation
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Galaxy Formation. James Binney Oxford University. TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: A A A A A A A. Outline. Cosmological clustering Scales introduced by baryons Timeline Chemical evolution Cores of Es Cooling flows. CDM Background.

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Galaxy Formation

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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


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