Galaxies 626. Lecture 5

Transcription

1 Galaxies 626 Lecture 5

2 Galaxies 626 The epoch of reionization

3 After Reionization After reionization, star formation was never the same: the first massive stars produce dust, which catalyzes H2 formation rapid formation of molecular gas, cooling, and star formation the first massive stars pollute the IGM with heavy elements, which greatly increase the cooling rate will strongly increase the star formation rate As a result, star formation after this epoch on much more similar to

4 Reionization When? Since we cannot (yet) observe the sources of reionization directly, we must rely on indirect methods to determine the epoch of reionization One method is the polarization structure of the cosmic microwave background, where reionization should give a redshift dependent large scale polarization signal (the more electrons there are, the larger the polarization signal)

5 Gnedin 2000 Reionization Simulations redshift evolution of log from mean ionization density log of HI fraction gas density gas temperature z = 11.5

6 Gnedin 2000 Reionization Simulations z = 9.0

7 Gnedin 2000 Reionization Simulations z = 7.7

8 Gnedin 2000 Reionization Simulations z = 7.0

9 Gnedin 2000 Reionization Simulations z = 6.7

10 Gnedin 2000 Reionization Simulations z = 6.1

11 Gnedin 2000 Reionization Simulations z = 5.7

12 Gnedin 2000 Reionization Simulations z = 4.9

13 Polarization From Reionization CMB was emitted at z~1088. Some fraction of CMB was re scattered in a reionized universe. The reionization redshift of ~11 would correspond to 365 million years after the Big Bang. IONIZED First star formation z=1088, τ 1 NEUTRAL REIONIZED z 11, τ 0.1 z=0

14 Meaning of Optical Depth Since polarization is generated by scattering, the amplitude is given by the number of scatterings, or the optical depth of Thomson scattering:

15 Can allow various reionization histories Three reionization histories with same τ=0.16 xe Redshift

16 Galaxies 626 Constraining reionization with quasar absorption lines (lower bound on redshift)

17 Quasars are very luminous objects with very blue colors, which make them relatively easy to detect at high redshifts Because quasars are so bright, any absorbing gas in front of them can accurately be studied through absorption lines Very well developed field of research with excellent statistics Unique probe of the IGM

18 A Forest of Absorption Lines As light from a quasar travels towards Earth it passes through intergalactic Hydrogen clouds and galaxies each cloud leaves absorption lines at a different z on the quasar spectrum this is the only way we can observe protogalactic clouds

19 The Lyman α Forest The IGM at redshifts lower than the quasar redshift is probed by the Lyα forest on the blue side of the quasar Lyα line Lyman α forest (z<2.523) Wavelength (Angstroms)

20 Schematic of Distant Absorbers t = t1 t = t0 t = t2 f f t = t4 t = t2 t = t4 f λ f t = t3 t = t3 f λ t = t1 λ t = t0λ λ

21 Energy Lyman Lines and Continuum Absorption photons that can cause transitions from ground state are absorbed and re emitted (i.e., scattered) (Lyman lines) ground state photons with E > 13.6 ev (Lyman limit) are absorbed (Lyman continuum absorption)

22 Lyα Absorbers Absorbers are detected with a complete range of column densities, 1012 N(HI) cm 2 Three main classifications Lyα forest: 1012 N(HI) 1016 cm 2 Ly limit systems: N(HI) 1018 cm 2 Damped Lyα: N(HI) 1020 cm 2

23 HI cloud in photoionizing radiation Photons E > 13.6 ev

24 When does the cloud remain partly neutral? optical depth τ>1 for E ~13.6 ev photons: N(HI) > 3x1017 cm 2

25 Lyman α forest Metal lines 20 DLA N >2x10 cm 2 HI Wavelength (Angstroms)

26 (all DLAs have metal lines) Damped Lyα SiII CII OI/SiII SiIV SiII FeII AlII CIV Damped Lyman α System (DLA) with its metal lines

27 Lyman α forest Lyman Limit DLA System Metal lines Lyman Limit System (LLS) with its metal lines

28 Several Absorbers LA LA

29 Lyα Absorbers Lyα forest 1012 N(HI) 1016 cm 2 lines are unsaturated metallicity > primordial < solar sizes are > galaxies Ly limit systems N(HI) 1018 cm 2 Lyα lines are saturated NHI sufficient to absorb all ionizing photons Damped Lyα N(HI) 1022 cm 2 Line heavily saturated profile dominated by damped Lorentzian wings

30 Lyα Absorbers Evolve Strongly! Cosmological density evolution of the gas, (1+z)3

31 The Gunn Peterson Effect Observation of quasar absorption systems demonstrate that the universe at high redshifts is ionized Absorption by any neutral hydrogen eats away at quasar continuum However, although we do see individual clouds absorbing some continuum, we do not see an overall drop in the continuum The increase in optical depth (continuum drop) that is expected at redshifts where the gas in the universe is neutral is known as the Gunn Peterson effect Therefore, the Gunn Peterson effect can be used to determine the epoch of reionization

32 The Gunn Peterson Effect observed continuum expected continuum if gas in universe near quasar were neutral

33 The Gunn Peterson Effect

34 Lyman forest very obvious at z=4.5 Songaila 2002; Keck ESI spectrum (11.6 hours)

35 Intergalactic gas is ionized to z=6 (at least) Becker et al Djorgovski et al Keck ESI spectra

36 Summary Reionization occurs somewhere between z=11 and z=6.5 It is a complex process and different parts of the intergalactic gas may reionize at different times. The gas may also ionize, become more neutral and then fully reionize.

37 Galaxies 626 Feedback Feedback mechanisms may halt the formation of early small galaxies (internal feedback) AND may affect their surroundings by injecting energy and metals which will change the character of the subsequent galaxy formation (global feedback)

38 Internal Feedback Small early galaxies are easily disrupted by energy injection from supernovae and stellar winds Releasing metals and energy into their surroundings

39 Conditions for Gas Removal E(n,V ) > ½ Mg V2 esc Limitations esc Only a fraction ε of the available SN energy E is tranferred into gas motions; the rest is radiated. but for these early small galaxies the escape velocity is very small

40 The binding energy of a 10^7 solar mass galaxy is only about 10^53 ergs: about the kinetic energy release of a 100 supernovae

41 So energy input from young stars & supernovae can quench star formation in small galaxies before more than a small gas fraction has been converted into stars.. 1 y

42 α=1 nob(r) ρ α α=2

43 L38 L/1038 erg s 1 3 SNe/Myr

44 Ejection Efficiency 0.1 MECHANICAL LUMINOSITY [1038 erg s 1] E 3 8.4E E E 4 3.4E 4 1.3E E 6 1.9E 5 VISIBLE MASS [MO]

45 Fraction of mass initially inside the virial radius retained after the explosions DISTRIBUTED SF ACTIVITY rvir CENTRALLY CONCENTRATED SF 50% 0.5rvir 15% 0.1rvir Time [Myr] Time [Myr]

46 Global Feedback The effects of galaxies on the surrounding material

47 Mechanical Feedback: Shock stripping S Shock from a neighbour galaxy M=108 Mo z=9 t = Myr NFW density profile

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49 Metal Enrichment Close to Galaxies

50 Metal Enrichment Close to Galaxies (Galaxy Proximity z = 3) QSO Δr

51 Metal Enrichment Close to Galaxies Absorption Signatures HI CIII Cavity/shell interface OVI SiIII CI Infalling gas CIV SiII Infalling gas SiIV

52 Summary Metals and energy escape easily from early small galaxies Ejection occurs preferentially in galaxies less than 1/10th of the Milky Way at all times Small objects pollute the IGM early. Early IGM enrichment up to z 7