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					  The metallicity of the
intergalactic medium and
       its evolution
     Anthony Aguirre
         UCSC
The intergalactic medium

The Lya forest
The intergalactic medium

The Lya 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:
 Ways to get enriched: two straw-man models

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.
                                           Late enrichment
    • 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

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.                    Early enrichment
    • Larger filling factor?
    • Would not disrupt IGM (as not
       observed).
 Signatures of early vs. late in observed IGM.

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.
All this and more can be done with:
        Pixel method (short version)

                                               UVB model
19x




  HI, CIV, SiIV pixel optical depths




                                       See Aguirre et. al. 2002; 2004
                                       Schaye et al. 2003
 Hydro. simulations
  Results: Carbon metallicities from CIV

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




    Characterize by [C/H] and s([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 s([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.
    2.4s detection in medians
    3.4s 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 s([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<104.9K
    Little 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 d,
    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 d,
    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 UVB
      hardness 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 Lya 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
                              Early enrichment Late enrichment
Inhomogeneous Z                3               3
No evolution in Z observed.    3               X
Warm, photoionized gas         3               X?
[Si/C] ~ 0.75                  3    ?          3     ?
No evolution, scatter in [Si/C] 3              X?
Lots of metal in IGM           X?              3
 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.

				
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posted:6/26/2011
language:English
pages:33