Galaxies 626. Lecture 8 The universal metals

Transcription

1 Galaxies 626 Lecture 8 The universal metals

2 The Spectra of Distant Galaxies Distant Galaxy Stellar Continuum Emission Observer Scattering by clouds of HI in the IGM at λline* (1+zcloud) Forest of absorption lines

3 Lyman forest (neutral hydrogen)

4 There are also metals in the Intergalactic Medium CIV forest

5 Metals in the post reionization IGM The presence of heavy elements like C, N and Si in the Lyα forest clouds at z~3 is well established The distribution of metals in the IGM is highly inhomogeneous, with a global cosmic abundance at z=3 of [C/H] = 2.8 ± 0.13 for gas with overdensities 0.3< δ <100

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7 The Metal Forest & Star Formation

8 How do metals get into the IGM? Early very massive stars, Pop III? OR Now we can measure metals to z = 6 Small galaxies at z = 6?? OR Superwinds at z > 2 3?

9 Putting metals into the intergalactic gas Metals are probably transported by powerful outflows from star forming galaxies Superwinds are created when supernova remnants within star forming regions overlap to create highly pressurized bubbles which burst out into intergalactic space At the earliest times when galaxies are small and potential wells are weak metals can be released into the intergalactic gas just by interaction with other galaxies NOAO/AURA/NSF

10 Unknown parameters Redshift and masses of wind hosts Fraction of SNa energy lost to radiation Fraction of galaxy mass entrained by the wind Geometry of the winds (e.g. bipolar vs spherical) Size of the winds and amount of metals produced (both star formation efficiency)

11 Early galactic enrichment Mori, Ferrara & Madau 2002 A number of theoretical arguments suggest that the IGM might have been polluted with metals produced by early star formation when the characteristic mass of galaxy halos was small and gas retainment more difficult.

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13 Suppose metals formed at high redshift z ~ 10 Winds from star forming low mass systems at z~10 are expected to sweep up region of the IGM of comoving size rb< 250 kpc. High density peaks collapse first and, most likely, inhibit star formation (and the production of new metal bubbles) in lower density peaks that collapse at a more recent epoch LBG z=3

14 Difficult to distinguish from lower redshift ejection Host halo of a dwarf galaxy formed at redshift z2 Protohalos dynamics LBG formed at z=3 Halo produced by merging Looks as though later galaxy may have ejected them

15 However subject to the uncertainties in the fraction of metals ejected the observed intergalactic metal density is an integrated history of the galaxy formation So if we can map as a function of redshift we can constrain things

16 How do we measure and interpret the intergalactic metals?

17 Classification of the QSO absorption lines Damped Lyα lines (DLyα). High redshift HI clouds with high column density, N(HI)>1020cm 2 The Lyα forest. Large number of weak, narrow HI absorption lines N(HI)<1017cm 2. Lyman Limit systems intermediate between the two.

18 When Does the Cloud Remain Partly Neutral? optical depth τ>1 for E ~13.6 ev photons: N(HI) > 3x1017 cm 2 Physical distinction between high and low column density systems

19 Lower column density forest clouds are permeated by the metagalactic ionizing flux from all the quasars and star formation optical depth τ<1 for E ~13.6 ev photons: N(HI) < 3x1017 cm 2 Physical distinction between high and low column density systems

20 Absorption Line Profiles Doppler Lorentzian

21 Equivalent Width (EW) EW = (f(λ) fc(λ)) / fc(λ) d λ ~ Flux/ fc(λο) EW Column density physical conditions ionization state abundances FWHM Dynamics. Thermal motion? Turbulence? Opacity and Line Profiles Absorption cross section is σ = f (πe2/mc), where f is the oscillator strength. Opacity τ (ν) = Ν σ φ(ν) = Ν f (πe2/mc) φ(ν) Lorentzian profile: φ(ν) = Fo (γ/4π2) / ((ν νο)2 + (γ/4π)2) Doppler profile: φ(ν) = Fo exp( (ν νο)2 c2 /b2 νο2)(c/(bνο π)) Voigt profile: Convolve the Lorentzian and Doppler profiles

22 Curves of Growth Curve of growth for the line equivalent width is Wν = (1 e τν) dν Square root portion: Wλ /λ ~ (Nfλ) Flat portion: Wλ /λ ~ ln(nfλ) Linear portion: Wλ /λ ~ Nfλ

23 For neutral hydrogen (the Lyman forest) neutral fraction xhi

24 Mean comoving density (for any quantity X): fraction of closure density in these elements or ions 24

25 QSO (GRBs? Galaxies?) Absorption Lines Metals in the IGM Association with galaxies Metallicities and SF

26 High column density clouds are predominantly neutral or singly ionized Neutral hydrogen Metals are mostly neutral or singly ionized. O, Si and C are most abundant elements with strong lines but see an enormous range of elements.

27 Damped Lyman Alpha Absorbers Range of H absorbers Studies of DLAs very important: dominate IGM HI easiest to get precision metal abundances

28 Evolution of Neutral Hydrogen in IGM Dominated by Damped Lyα systems NHI> cm 1

29 Damped Ly α Systems HI : Metals : > Metallicities > Dust content > Kinematics Molecules H2 : > Density/Temperature > UV flux

30 Redshift evolution of metals in the DLAs * Some evolution * No system at Z< 3 Prochaska et al. (2003)

31 Mean Chemical Evolution

32 Lower column density clouds are highly ionized Small fraction of neutral hydrogen (10^( 4)) Metals are mostly triply ionized. C is most abundant element with suitable lines so mostly see CIV (triply ionized C) then Si IV These are doublets so nice and easy to identify

33 e.g., BR R=67000 z = 4.55 R = 18.3

34 Carbon in Lyα Forest Lyα forest log f(n) z 1 log N(CIV) CIV was detected in the Lyα forest in 1995 with N(CIV)/N(HI)~ CIV is now seen in even the weakest Lyα systems (Ellison et al 2000)

35 Column density distributions & omega <z> = 2.2 <z> = 2.8 <z> = 3.9 N(CIV) = <z> = 4.5 N(CIV) = Ω (CIV) from distributions

36 Omega (ION) Average Ω (C IV) Average Ω (Si IV)

37 Status of CIV evolution Essentially the current situation is that, within the variation in the CIV/HI ratio, we see CIV in the IGM to the column density limits that we can detect it to. The distribution functions and total density are remarkably invariant as a function of redshift This is an extremely surprising result. Why aren t there changes in the ionization balance and the metal content?

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