Elements of observational cosmology 16.02.2009 Integalactic medium: in clusters between clusters.

Elements of observational cosmology

16.02.2009 Integalactic medium: in clusters between clusters

Intergalactic medium: clusters

In clusters: intense X-ray

bremsstrahlung at T~108 K

in big clusters: the mass of gas at least as great as the mass of the gas (Coma, Perseus)

Coma


In clusters: strong emission lines of

highly ionized iron Here: the iron

abundance of the gas in clusters from the HEAO-1 A-2 survey derived from their X-ray spectra, plotted versus their X-ray luminosity (Mushotzky 1984). The iron abundance is relative to solar, and the X-ray luminosity is in erg/s.

Intergalactic medium:clusters

In clusters: strong emission lines of

highly ionized iron Here: the iron

abundance of the gas in clusters from the HEAO-1 A-2 survey derived from their X-ray spectra, plotted versus their X-ray luminosity (Mushotzky 1984). The iron abundance is relative to solar, and the X-ray luminosity is in erg/s.

Intergallactic medium:clusters

X-rays emitted from the gas in the AWM7 galaxy cluster. The bright region has a stronger X-ray intensity. The figure below shows the X-ray spectrum. At the marked positions there are emission lines of heavy elements. (provided by: Kosuke Sato, Tokyo University of Science)

Suzaku X-ray satellite


The observations of the lines of the heavy elements show that the intergalactic medium has been enriched relative to its primordial abundance but the heavy elements are underabundant relative to their “mean” cosmic abundance - ~1/2 of the mean galactic abundance

The enrichment of the gas was due to stellar nucleosynthesis inside galaxies expulsion of the enriched galactic gas into the

intergalactic medium

Intergalactic medium: between clusters

Ly-α clouds (systems of the Ly-α absorption lines seen in spectra of distant quasars) the densest were identified with the discs of

protospiral galaxies but the less dense clouds are thought to be

baryonic perturbations of low masses (which never became galaxies)

history of metal aboundances of these systems: history of metal production and feedback processes

during galaxy formation thermal and ionisation history of the Universe during

and after the reionisation epoch (Gunn-Peterson effect)

“missing baryon” problem

Intergalactic medium: the background emission and

absorption Let us assume a uniform distribution of sources

of luminosity L(ν1,z). The background intensity of their light in t

0 is:

I 0[W m−2Hz−1 sr−1]=∫ S 0dN= 1

4 ∫ L 1, zN 0

1zdr

since flux density: number density: and the comoving

number density of sources N

0=const

S 0=L 1

4 D2 1z dN z =N z dz= D 2N 0dr


absorption To adapt this expression to the case of the

diffuse gas, we use the proper number density of objects at redshift z, i.e.:

Them, the background intensity is:

I 0=1

4∫0

∞ 1

1z 4 dr

N z =N 0 1 z 3

Then, let us define emissivity of the intergalactic medium as luminosity per (comoving) volume: 1=L 1, z N z


absorption And in a world model with a cosmological

constant the background intensity becomes:

Then, the corresponding optical depth, assuming the absorption coefficient for ν1 to be α(ν1) (in case of an absorption line it describes a line profile), is:

I 0=c

4 H 0∫0

∞ 1

1 z4 [1 z2 0 z1− z z2 ]1 /2 dz

0=∫1dl=∫1dr

1 z

= cH 0

∫0

z [0 1z ]dz

1 z [1z 2 0 z1− z z2]1/2

The Gunn-Peterson test

Is/was there any diffuse extragalactic neutral hydrogen?

If so, it should cause continuum absorption at the short-wavelength side of the Lyman-α line (Gunn-Peterson effect).

Discovered in the spectra of distant quasars (z~6) -> reionisation and heating of the neutral primordial gas was not completed yet at this epoch

http://map.gsfc.nasa.gov/m_mm.html

Dark Ages = Gunn-Peterson effect


The cross-section for scattering at the wavelength of the Ly-α photons, 121.6 nm, is very large. The radiation in absorbed and re-emitted in random direction many times.

A lot of neutral H -> continuous absorption at the short wavelength side of the Ly-α line.


Let us find the optical depth for the Ly-α scattering.

The photo-excitation cross-section for the Ly-α transition is

where νLy

– the frequency of the Ly-α transition; f – its oscillator strength (0.146 in this case), g(ν-ν

Ly) - the profile of the

absorption line, normalized so that

= e2 f4 me c

g − Ly

∫ g d =1


Then, with α(ν= σ(ν1) NH(z), the optical depth due to the Ly-α scattering is

0=∫0

z 1N H z dr

1z=

e2 f4 0meH 0

∫0

1 NH zg [01z− Ly ]d [01z ]

0 [0 z11z 2− z z2]1/21z


Since for the Ly-α line g is very sharply peaked, it can be approximated by the Dirac delta function, which leads to:

0=e2 f

4 0meH 0 Ly

N H z

[0 z11 z2− z z2]1 /2=

4×104h−1 N H z

[0 z11z 2− z z2]1 /2

NH is measured in atoms per m3

Gunn-Peterson test

This continuous absorption has been looked for in quasars distant enough the Ly-α line (usually in UV) is redshifted into optical waveband, i.e. 1+z > (330nm)/λ

Ly -> z>2.

QSO absorption lines

Various absorption systems - close and distant

Gunn-Peterson effect

Quasars with z up to 5 have shown NO evidence for such a continuous absorption by diffuse intergalactic gas of neutral hydrogen, although with time the continuum becomes more and more affected by individual Ly-α absorption lines (Ly-α forest ).

Typically, for these quasars τ(ν0)<0.1. It means that for a quasar at z~3 NH<10-5m-3 : very small compared to typical baryonic densities ~10 ΩBh

2(1+z)3m-3 = 15 m-3 (for z=1 and ΩBh

2 = 0.0223). If there is a significant amount of H in the intergalactic medium, it must be highly ionized.

Gunn-Peterson trough

First observed in 4 distant quasars (z>5.8) discovered in the SDSS by the i-dropout technique (Becker et al. 2001)

"UV Continuum Spectroscopy of a 6L* z = 5.5 Starburst Galaxy", Dow-Hygelund, C. C. et al. 2005

Gunn-Peterson trough

Becker et al. 2001: optical spectra observed by Keck-2

Gunn Peterson trough

Absorption features increasing with z -> no flux at z~6.28.

“The Universe is approaching the reionisation at z~6”

Gunn Peterson trough

Transmitted flux falls down and optical depths grows after z ~6

dark ages first stars believed to

be formed around z~30

how did they reionize the Universe?

Evolution of the neutral fraction of the IGM

Fan et al. (2005b)

Gunn-Peterson effect


Fan et al. (2005a)

Neutral fraction

Fan et al. (2005a) 　　

Ly


Closer home: Ly-α absorption clouds

Ly-α absorption systems: 3 broad classes: Ly-α forest Lyman-limit systems The damped Ly-α systems


Ly-α forest: the most common: forest of lines dominating spectra of distant quasars


Ly-α forest: forest of lines dominating spectra of distant quasars neutral hydrogen column densities N

HI= 1016-1021

m-2. When N

HI >1019m-2,low abundance of heavy

elements (1/100 solar) usually found Lower N

HI – no detectable CIV, so the

abundance not certain Highly ionised intergalactic clouds, with

unprocessed primordial material Rather condensations in the ubiquitous IGM than

distinct couds


Ly-limit systems (LLS): much rarer 2 ×1021 m-2 ≥ N(HI) ≥ 2 ×1024 m-2

Absoption lines of common elements + continuum break observed at the redshifted wavelength 912Å -> gas dense enough to absorb all the photons below the Lyman limit (912Å).

Associated with the very extended (~50-100 kpc) gaseous haloes of galaxies in which the abundance of cosmic elements is ~10% of their cosmic abundance.


Ly-limit systems (LLS): Observed energy

distribution of QSO 1157+3143 in the optical and UV. The inset shows the model continuum predicted from a QSO power law spectrum and a Lyman limit system at z = 2.77. Vertical bars show the locations of the Lyman limit systems at z = 2.77 and z = 2.94.


The damped Ly-α systems (DLA): even rarer (10% of systems)

• N(HI) ≥ 2 ×1024 cm-2

• Hydrogen layer is optically thick, the profile of the abs. line can be fitted by the Voigt profile

• thought to be a central region of galaxy disk (progenotors of today's spirals).


Various absorption systems

DLAs


The damped Ly-α systems (DLA): N(HI) ≥ 2 ×1024 cm-2

• It has been observed that for local spirals the minimal column density need to start star formation is of the same order (Kennicut 1989)

• Related to the stability criterion for the thin disc:

• (cs – sound speed, related to the chemical content of the disc)

• -> DLAs as the probe for chemical evolution of spirals at high z?

crit=A c s

3.36G


Ionization state

Most of the IGM is ionized at z < 6 (see 8.9). The absorbers are pockets of denser neutral gas.

The QSO absorption lines (QSOALS) provide a useful information to calculate the numbers of ionizing photons.

In the vicinity of a quasar, the extra ionizing flux from the quasar reduces the number of absorbers: the proximity effect.

The proximity effect depends on the ratio of the quasar’s ionizing flux (we can calculate from its spectrum) and the ambient flux at the same z, allowing the latter to be determined.

Ly-α absorption clouds: composition/properties

Low abundances of metals -> must consist of primordial material

• Column densities are determined from the strength of the lines

• Only a few cases with the column density of neutral H possible to be measured -> their material must be highly ionized

• At z~2-4 the flux of ionizing radiation much have been so intense that they were ionized completely.

• masses: 106-109 solar masses

• At low z their content corresponds to ΩIGM~0.1

Ly-α absorption clouds: composition/properties

How do these systems hold together? Not certain. Simulations: they are in a sort of pressure balance with

the lukewarm intergalactic gas with T~3x104 K. More complete studies involve:

stability of clouds role of evaporation photoionisation balance within the clouds feedback effects

and so on...

Evolution of the number density: follows ~power law -> evolution

Solid: no evolution Solid lines: no evolution

Péroux et al. (2005)

Ly-α absorption clouds: evolution

Number counts change with z and characterized by a power law N(z)dz = A (1+z)γdz.

For Ly-α forests: A~10, γ~2-3 For Ly-α systems: A~1, γ~1 If the properties of the absoption clouds did not change,

their comoving density N(z) ~(1+z) to a power slighly larger than 1 (depending on the cosmological model)

the observed slope is steeper -> evolution Ly-α forests: much more at high z than low LLS, LDS – much less variation with z


Fan et al. (2005a)

Red

shif

t

Backgrounds: X-ray background and the cosmic

conspiracy The first measurement of

the X-ray background radiation found that there exist an isotropic X-ray background emission

A spectrum could be beautifully interpreted as a thermal bremsstrahlung of very hot gas: kT ~40 keV, ΩBh

2 > 0.23 (Marshal 1980)


conspiracy However:

number counts – by now it has already been resolved into individual sources

problem: ΩBh2 > 0.23 > 0.0223 resulting from

nucleosynthesis/CMB measurements (this time we would have TOO MANY baryons)

problem: such hot gas should interact with CMB and the signature would be well seen in the CMB spectrum

from CMB: optical depth of hot gas y < 2.5x10-5

while best bremstrahlung fit implies y~5x10-4

with realistic models (heating at large z) the discrepancy even worse, y~(1+z)2.5


conspiracy Then, it was finally concluded that “the perfect

bremsstrahlung shape is just a cosmic conspiracy” (Fabian and Barcons 1992)

Is it enough to explain the shape of the X-ray background?

The lukewarm intergalactic gas

Is rather inferred from the UV background and H, He and metal lines in quasars

The presence of the lukewarm gas with T~3x104 K The missing baryon problem: the baryonic component

of galaxies is only Ωsh~ (2+/-0.6)x10-3 while from the primordial nucleosynthesis and CMB it should be ~0.0223.

At high z, Ly-α systems can contain a large part of it. Where are these baryons now? Most probably in a form of diffuse gas and hot ionized

coulds in clusters and medium between them.

Elements of observational cosmology 16.02.2009 Integalactic medium: in clusters between clusters.

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Transcript of Elements of observational cosmology 16.02.2009 Integalactic medium: in clusters between clusters.