The metallicity of the intergalactic medium and

its evolution

Anthony Aguirre

UCSC

The intergalactic medium

The Ly forest

The intergalactic medium

The Ly forest

The intergalactic medium

Metals in the IGM!

IGM metallicity provides information on:

History of star/galaxy formation.

Formation of unobservably early stars/galaxies.

UV ionizing background.

Feedback in galaxy formation processes.

Ways to get enriched:

1. “Late” enrichment by 2 < z < 6 galaxies. Strong feedback during galaxy-formation epoch.• Observed z ~ 3 galaxies drive

winds that seem likely to escape.• Semi-analytics and simulations:

gas removal seems necessary during galaxy formation.

• Most of cosmic star formation at z < 5.

Ways to get enriched: two straw-man models

Late enrichment

2. “Early” enrichment at z >> 5. Metals just “sprinked in” with no effect on galaxies or IGM at z < 5.• Easier escape from small

potential wells.• Larger filling factor?• Would not disrupt IGM (as not

observed).

Ways to get enriched: two straw-man models

Early enrichment

1. Look for evolution in Z at z < 5.

2. Check temperature of gas (late enrichment should come with/in hot gas).

3. Compare amount of metals with expectations.

4. Look at spatial distribution of metals.

5. Look at abundance ratios for info. on nucleosynthetic sources.

Signatures of early vs. late in observed IGM.

All this and more can be done with:

Pixel method (short version)

HI, CIV, SiIV pixel optical depths

Hydro. simulations

See Aguirre et. al. 2002; 2004Schaye et al. 2003

UVB model19x

Results: Carbon metallicities from CIV

1. The carbon metallicity is inhomogeneous.At fixed and z, p.d.f. for [C/H] is gaussian, i.e. carbon metallicity distribution is lognormal.

Characterize by [C/H] and ([C/H])

Results: Carbon metallicities from CIV

1. The carbon metallicity is inhomogeneous.Primordial enrichment is ruled out.But early vs. late will require detailed modeling.

Results: Carbon metallicities from CIV

2. The median carbon metallicity [C/H] changes with density.

So does scatter ([C/H])

Results: Carbon metallicities from CIV

2. The median carbon metallicity [C/H] changes with density.

Expected and reasonable, but never observed.But again, early vs. late will require detailed modeling.

Results: Carbon metallicities from CIV

3. There is Carbon in underdense gas.2.4 detection in medians3.4 detection in higher percentiles. Most information from z > 3.5.

Results: Carbon metallicities from CIV

3. There is Carbon in underdense gas.The filling factor of metals is high: tens of percent (depending on metallicity threshhold).May be difficult for late enrichment.

Results: Carbon metallicities from CIV

4. The median carbon metallicity [C/H] does not evolve (for our fiducial UVB) from z~4 to z~2.

Neither does ([C/H])

Results: Carbon metallicities from CIV

4. The median carbon metallicity [C/H] does not evolve (for our fiducial UVB) from z~4 to z~2.Clearly favors enrichment at z > 4.But: there is some room for more.

Late enrichment

Results: Carbon metallicities from CIV

5. [C/H] depends on UVB model.

But very different UVBs can be ruled out.

Results: Carbon metallicities from CIV

5. [C/H] depends on UVB model.

Inferences are sensitive to assumed UVB (and its history).

But density-dependence, scatter are robust, and evolution fairly robust.

Gas temperature from CIII, SiIII

6. CIII/CIV, SiIII/SiIV provide thermometer.Bulk of SiIV gas at T<104.9KLittle scatter in gas temp.But some evidence for hotter gas? (< 30%)Similar results using CIII/CIV.

Gas temperature from CIII, SiIII

6. CIII/CIV, SiIII/SiIV provide thermometer.Observed metals are in photoionized, warm gas, not the collisionally ionized warm/hot gas expected from winds.

Late enrichment

Gas temperature from CIII, SiIII

6. CIII/CIV, SiIII/SiIV provide thermometer.Observed metals are in photoionized, warm gas, not the collisionally ionized warm/hot gas expected from winds.But: slight evidence for some missing SiIII, and suggestions of collisionally ionized gas from OVI (in progress).

Silicon metallicities from SiIV, CIV

7. SiIV/CIV vs CIV: ratios depend on , reproduced by simulation.

[Si/C]=0.77+/-0.05 [Si/C] varies w/UVB hardness.No scatter in

inferred [Si/C]

Silicon metallicities from SiIV, CIV

7. SiIV/CIV vs CIV: ratios depend on , reproduced by simulation.Suggests Pop. II enrichment, which can have [Si/C] ~ 0.5. If [Si/C]=0.77 taken seriously, could point to Pop. III contribution as per Heger & Woosley.Lack of scatter -> Si and C from same sources; later C production not important.

Silicon metallicities from SiIV, CIV

8. SiIV/CIV vs CIV: ratios depend little on z, reproduced by simulation.

No jump in UVBhardness at z ~ 3.No evolution in [Si/C] for usual UVB

Silicon metallicities from SiIV, CIV

8. SiIV/CIV vs CIV: ratios depend little on z, reproduced by simulation.Again, more lack of evidence for anything evolving.

Early enrichment

Adding up global C, Si abundances.

9. Median+scatter mean metallicity, and contribution to cosmic C, Si abundance.[C/H] = -2.8, [Si/H] = -2.0

stars hold only < 60-70% of cosmic Si;

rest is in Ly forest.Lots of metals in the forest!

Adding up global C, Si abundances.

9. Lots of metals in the forest.Metal dispersal into IGM is quite efficient before z ~ 3-4. (also note most metals escape cluster galaxies)Could z >> 6 enrichment really provide enough metals?

Late enrichment

The scorecard

Test

Inhomogeneous Z

No evolution in Z observed. X

Warm, photoionized gas X?

[Si/C] ~ 0.75 ? ?

No evolution, scatter in [Si/C] X?

Lots of metal in IGM X?

Late enrichmentEarly enrichment

The real picture: early and late?

Some questions/considerations: Metals sprinked in non-feedback simulation

reproduce all current observations. But…Do the observed winds escape? If so, where do the metals go?

If not winds, how to we fix baryon fraction in galaxies?

Clusters, z ~ 0 observations indicate Z ~ 0.1 Zsol. How do we close the gap?

Metal from late galaxies may be hidden in unobservably hot gas, with low filling factor.

Metal and H absorption does not have to come from same gas.

Data allows some evolution, esp. using freedom in UVB.

To Do:

1. Complete OVI analysis, look for NV: UVB has opposite effect on O inferences than on SiIV. Also, hotter gas can be seen in OVI.

2. Looks at metallicity vs. “distance” from absorber.3. Look at correlations in PODs. See if simulations

reproduce observations.4. Compare observed PODs in detail to hydro

simulations with feedback.5. Try to connect these with simulations of individual

galaxies.

Conclusions

We can learn a lot from the Lya forest and the pollution in it.

Evidence from galaxies suggests that they enrich the IGM.

Evidence from the IGM suggests it was already enriched.

Next step of detailed model/observation comparison holds great promise.