lecture 16 – the intergalactic mediumw.astro.berkeley.edu/~ay216/notes/lecture16.pdf · balance...

Lecture 16 – The Intergalactic Medium

1. Cosmological Scenario2. Results of Observing the Ly-α Forest 3. Photoionization Modeling4. The Gunn-Peterson Effect 5. Reionization and the First Stars

General ReferencesMS Longair Galaxy Formation, Ch. 19M Rauch, ARAA 36 267 1998A Loeb & R Barkana, ARAA 39 19 2001J Miralda-Escude´, Science 300 1904 2003V Bromm & RB Larson, ARAA 42 79 2004

1. Cosmological Scenario

z+1

Big Bang

10 100 10001

log t10 9 7 6

Now

J. Miralda-Escude´, Science, 300, 1904 2003

Cosmological Parameters

04.026.07.07.0

Mpcskm100cmgr1088.183 11

03292

20

=Ω=Ω=Ω=

=×==

++==

Λ

−−−−

Λ

BDM

cr

BDMcr

h

hHhG

H

ρρΩ

πρ

ρρρρ

The look-back (or emission) time is:

Gyr7.13)1( 2

3 ≅+

= nn tz

tt

1. Results of Observing the Ly-α Forest

z = 3.50 quasar withmany features -

Emission linesLyman α forestLyman break

A SongailaAJ 115 2184 1998

breakforest

Ly α

C IV

Analysis & modeling Ly α forest provides basicinformation about the IGM, as follows:

Results Summary for the Ly-α ForestLet Ñ stand for a number distribution function of Ly-αfeatures and N(H) for column density of atomic hydrogen.

1. dÑ(z)/dz is large & increases rapidly with (1+z)γ(γ ~ 3 for large z), referred to as evolution

2. N(H) varies from < 1012 to > 1021 cm-2, with a strongpreference for thin clouds: dÑ)/dN(H) ~ N(H)-1.5.

3. Flat structures for small columns, e.g., 50 kpc thick & Mpc wide, c.f. lensed quasars and quasar pairs

4. x(H) ~ 10-5, T ~ 30,000 K, bD ~ 30-35 km s-1,

c.f. photoionization models (simulations)5. IGM ionizing radiation field & rate:

J ~ 10-21 erg cm-2 s-1 sr-1 Hz-1 G ~ 10-12 s-1

c.f. variation in Ñ near quasars (“proximity effect”)

Comparison with the Diffuse ISMThe Ly-α forest arises from quasi-distinct features, althougha smooth background distribution is not ruled out by theobservations; they are as real as diffuse interstellar clouds.

Compare the IGM at z ~ 3 with the closest ISM phases:WIM n ~ 0.030 cm-3 T ~ 8000 KHIM n ~ 0.003 cm-3 T ~ 106 K

IGM (z ~ 3) n ~ 10-5 cm-3 T ~ 3x104 KThe pressure of the IGM is tiny.

ISM gas is bound to the Milky Way, with an internal distribution determined by several pressures etc.The IGM is governed largely by the gravity of CDM halos plus self-gravity and shocks in a highly clustered medium.

2. Photoionization ModelingWe provide the elementary basis for these models by considering the physical conditions at a typical location in a uniform and flat cosmological model. Of course, current understanding of the Ly α forest is based on non-uniform structures that arose from primordial fluctuations and evolved according to the ΛCDM cosmology (with the parameters given earlier).

a. Post-recombination Density

HHH

HHeH

)1()1(

mY

mYn

Y

crB

B

BB

ρρρρρρρ

Ω−=−=

+=+=

3373

HH )1(cm106.1)1()1( zz

mYn B

cr +×≅+Ω−= −−ρ

e.g., at recombination (z ~1000): nH ~ 200 cm-3

at reionization (z ~ 7): nH ~ 10-4 cm-3

b. Local Photoionization Balance

The basic physics was covered in Lectures 3 & 4 & steady balance equations in Lecture 7. The controlling parameter is the ionization parameter, in this case G/nHα. Although surely variable, typical parameters for the IGM might be:

G ~ 10-12 s-1 T ~ 30,000 K x(H) ~ 10-5

With α ~ 2 x 10-13 cm3 s-1, 37 )1(105.2 −+×= z

nGα

)He()He()H()H()H(

)H()H()H()H(

eH

e+++++

++

++=+=

=

nNnnnnn

nGnnα

This large value means that the photoionization time is much shorter than the recombination time,

αττ

erecph

11nG

=<<=

and implies that the IGM is fully ionized: ne = nH.

c. Atomic Hydrogen Abundance

The chemical balance equations are:

Again using ne = nH, the abundance of H is:

1)1(1041)H( 381

H

1

H

<<+×=⎟⎟⎠

⎞⎜⎜⎝

⎛≈⎟⎟

⎠

⎞⎜⎜⎝

⎛+= −

−−

znG

nGx

αα

d. The Ionization Flux of a Quasar

The IGM radiation field is clearly basic for understandingthe Ly α forest. It depends on cosmic time through the history of star, galaxy, and quasar formation. We give just one estimate, that due to a luminous quasar, analogous tothose made for O & B stars in earlier lectures. For furtherdiscussion, see Loeb & Barkana, ARAA 39 19 2001.

311111 ))(()()(4

1νννσνσνσ

νπν ν

ν

== ∫∞

hJdG

We use an empirical SED for quasars (Madau astro-ph 0005106 2001)

21-1-3021

1 44Hzserg10)( 1

rπL πJLLL ν

ν ≈=≅νν

ν

The result for the ionization rate at Mpc distances is

2

1

1-12 )Mpc(s102rL

LG −×=

This is < or ~ the order of magnitude found in simulations that assume G is everywhere the same.

The problem is more complicated because one has toadd the contributions of all relevant quasars, especiallythose close to the line of sight, and do the radiative transfer.

As for H II regions, one also has to evaluate the heatingrate, or equivalently the mean photo-electron energy

4. Gunn Peterson EffectAlmost immediately after the discovery of quasars, Gunn & Peterson (ApJ 142 1633 1965) speculated on the effect of a continuous distribution of intervening atomic H on quasar spectra via “resonant scattering” of Ly-α photons in the rest frame of the absorber, which must come from the quasar spectrum shortwards of its Ly-α line. Of course, resonant scattering is really the same process that produces the Ly-α forest. It refers to the photon emitted immediately after an absorption.

νν ′+→→+ hHHHh )1()2()1(

O H Q redshift

z = 0 z z = zem

observer absorber/scatterer quasar

λ(Ly α) in frame H has wavelength (1+z) λ(Ly α) in the observer’s frame, which corresponds to the following wavelength in the quasar frame:

All scattered photons following absorption have wavelengths shorter than Ly α in the quasar frame.

)Ly()Ly(11

emem αλαλλ <

++

=zz

GP Optical Depth

dzzz

Hcdl

dlznvzcm

efd

m2

1])1([1/

)(])1[(

30

112e

2

12

−ΛΩ++Ω

+=

+= ϕπτ

Apply the important result of Lecture 3, reviewed in the previous lecture, for the optical depth at line center

and use

The 2nd term is small for finite z. Integrating z from 0 to zem & assuming φ is sharply peaked yields (e.g., Longair or Madau 2001 astro-ph 0005106)

23)1()H(1

012e

2

12−+= z

Hcn

cmefGP νπτ

Estimate of the GP Optical Depth

Substituting the cosmological parameters given at thestart of the lecture (ΩBh2 = 0.019) yields

23)1()H(104 3 zxGP +×≈τ

According to photoionization models, x(H) is sensitive to the ionization rate G (as 1/G), and one expectssmall values at low z and large ones in the early universe after recombination and until the universe is re-ionized as the result of star formation.

Evidence for a finite GP optical depth had been evident in the spectra of high-z quasars, but convincing data has only come from the discovery of z = 6 quasarsby the Sloan Digital Sky Survey (SDSS).

Pre-SDSS Q 1422+23 (zem=3.62)

Rauch 1998 Songaila & Cowie 1996

Si IV

CIVLyα

Lyβ

High red-shift quasar showingrich Ly α forest plus absorptionline systems for Ly β, Si IV, & CIV, but no strong evidence for a GP trough between the lines.

GP Trough for High-z SDSS Quasars

z = 5.80

z = 5.82

z = 5.99

z = 6.28

Becker et al. 2002

Q 1030+0524 hasno detectable flux short of Ly α & Ly β.

Becker et al. (2002) Close Up

20)Ly(5)Ly( GPGP >> βτατ

Conclusion: HI Reionization at z ~ 6 or before

See White et al. AJ 126 1 2003 for a complete report.

He ReionizationBasic facts about He II:

IP = 54.418 eV (227.8 Å)E(Ly α) = 40.81 eV (303.8 Å)

They imply:1. Much harder UV radiation is needed to ionize He IIthan H I (maybe quasars rather than stars).2. Detecting He II in high-z quasars requires UV observations, e.g., for z = 3 HeII Ly α is shifted to 1215Å.

HST had UV capability down to 1150 Å whereas FUSE (Spectroscopic Ultraviolet Spectroscopic Explorer) provides high resolution from 905-1195 Å.He II Ly α observations provides unique information about the spectrum of ionizing radiation in the IGM.

He II in the IGM With FUSEQ 2347-4342 z = 2.885

Green: HST STIS spectrumRed: extrapolated STIS

continuum

Kriss et al. Science, 293 1112 2001 (FUSE + STIS)See: Zheng et al. ApJ 2004 (FUSE + VLT) for a re-analysis of He II Ly clouds in the spectrum of this quasar.

He II re-ionization occurred prior to z ~ 3.

More on Q 2347-4342 z = 2.885

red: FUSE spectrum yellow: common features

(both HI & He II)green: additional HeII features

normalized Keck spectrumof HI Ly α forest

The many common lines have small HI columns (log N < 13)and larger HeII columns.

Still More on Q 2347-4342 (z = 2.885)

z

N(He II)/N(HI)

Ratios < 100 are consistent with quasar radiation.(reanalysis gives ratios > 1000; Zheng et al. 2004).The fluctuations indicate variable radiation fields.

Illustration of the Proximity Effect

He II Ly α forest in Q 0302-003 (Z=3.05) (Heap et al. ApJ 534 69 2000 “marred” by re-ionization of He II by a nearby quasar (Jakobsen et al. A&A 397 891 2003)

5. Reionization and the First StarsObservations of the Ly α forest and especially the GPtroughs for high-z quasars indicate that H I reionization occurred before z = 6 and He II somewhat later.

In accepting this conclusion, it’s important to remember that:(1) the observational data consists of distribution functionsfor cloud properties (e.g., column density & line width) vs. z;(2) physical properties are derived from ΛCDM structure simulations using simplified astrophysical models.

One must now ask: How did reionization occur? Or: How did the first generation of stars (Pop III) form from just H, He, and Li; how did the next generations of stars(Pop II) form from slightly metal-enriched gas; how did thefirst galaxies and first quasars form? Not easy questions!

Reionization Stars with the First Stars

Scenario from Loeb & Barkana, ARAA 39 19 2001that raises the issue of how the first generationsof H II regions merge to re-ionize the entire IGM

Star Formation and ΛCDM Simulationsdensity atomic fraction temperature

Stars are presumed to format the high-density knots in a ΛCDM simulation. Bromm & Larson (2004)

The massive new stars ionize and heat the IGM. Miralda-Escude (2003)

Issues in Early Star Formation• Cooling the gas collected in CDM halos to collapse

bremsstrahlung radiation is weakno heavy elements or dustH I, He I, He II have large energy-level spacings

• Formation of homonuclear molecular hydrogen

• The accretion process, stellar evolution, nucleosynthesis;the first winds, supernovae, & condensed objects

• Mixing of newly synthesized heavy elements into the IGM

•Transition for Population III to II

• Galaxy and quasar formation (massive black holes)