Transcript Galaxy Formation - University of Oxford

Galaxy Formation
James Binney
Oxford University
Outline
• Cosmological clustering
• Scales introduced by baryons
• Timeline
• Chemical evolution
• Cores of Es
• Cooling flows
CDM Background
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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
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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
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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,
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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
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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
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M t (0) ³
M s (< Z ) =
1 ¡ e¡
1+ c
( 1+ c) Z =p
´
• 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
1
§ (R) = °
R (a + R) ¯ ¡
°
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
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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):
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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
•(Bohringer
XMM shows
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deficit
of
photons
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et al 02)
<1keV not due to internal absorption
• But associated with “floor” T' Tvir/3
• Chandra shows that radio plasma has
displaced thermal plasma
(Peterson et al 02)
Outward increasing entropy
Omma thesis 05
Donahue 04
Summary (cooling flows)
• Hot atmospheres not thermally unstable: will
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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