The origin of intergalactic metals around Lyman-break galaxies

1. The origin of intergalactic metals around Lyman-break galaxies Cristiano Porciani (ETH Zurich) in collaboration with Piero Madau (UCSC) 2005, ApJ, 625, L43-L46

2. Metals in the post-reionization IGM • The presence of heavy elements like C, N and Si in the Ly forest clouds at z~3 is well established (e.g. Cowie et al. 1995; Simcoe et al. 2004) • 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<  <100 (Schaye et al. 2003)

3. Galactic superwinds • 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 NOAO/AURA/NSF

4. 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)

5. Mori, Ferrara & Madau 2002 Pre-galactic enrichment 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. (e.g. Tegmark et al. 1993; Madau, Ferrara & Rees 2001; Furlanetto & Loeb 2003)

6. Observational hints • No evidence for evolution in the total mass-weighted abundance of CIV and SiIV between z ~ 1.5 and 5(e.g. Songaila 2001) [However, 1) mass weighted measurements constrain only the metals in most massive systems and tell us little about the total volume of enriched space; 2) Conclusions about the total metallicity based on single ionization states are uncertain] • Dynamical studies of CIV systems point to a rather quiescent gas unlike any known galactic environment (Rauch, Sargent & Barlow 2001)

7. Observed galaxy - IGM connection at z~3 Adelberger et al. (2003, 2005) Method: Survey galaxies and Ly forest in the same cosmic volume using 23 (8) fields with QSOs at z~3.5 Goal: Determine relative distributions of galaxies, HI and metals

8. Kinematics of the IGM around LBGs • ISM absorption lines are always blueshifted with respect to the systemic redshift of the galaxy • Ly in emission (when present) is almost always redshifted Adelberger et al. (2005)

9. Galactic scale outflows? We detect Ly- photons that scattered off the backside of the outflowing material Interstellar medium absorption lines are generated in the outflow coming towards us V+ ~ 300 km/s V- ~ 300 km/s We are here Systemic redshift of the galaxy indicated by (rest-frame optical) nebular lines

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11. Mean Ly transmissivity around LBGs • The Ly absorption produced by the IGM within ~ 1 Mpc/h from an LBG seems to be systematically reduced with respect to random locations Adelberger et al. (2003)

12. Clustering properties LBG auto-correlation CIV-LBG cross-correlation Mhalo > 1011.5 M(Porciani & Giavalisco 2002; Adelberger et al. 2005) Adelberger et al. (2003)

13. Galaxy - CIV association • Bright star-forming galaxies at z=3 and the strongest CIV systems have similar spatial distributions • There could be a causal connection between the 2 (Adelberger et al. 2003, 2005) Adelberger et al. (2005)

14. IGM metal enrichment from LBGs? All these facts led Steidel’s group to argue that metal-rich superwinds from LBGs are responsible for distributing the product of stellar nucleosynthesis on megaparsec scales. I will show you that this interpretation may be an oversimplification of a far more complex physical picture. A number of numerical studies have shown that extreme feedback models (that affect the gas at unrealistically large distances) are required to produce the observed increase in the transmitted flux near galaxies (Croft et al., Kollmeier et al., Desjacques et al., Cen et al., Theuns et al., Bruscoli et al.)

15. Revised Ly transmission around LBGs • A recent re-analysis by Adelberger et al. (2005) finds strong Ly absorption within 1 Mpc/h of LBGs. • For ~1/3 of the galaxies the absorption is weak

16. Can we explain the LBG - CIV cross-correlation function without invoking the help of strong superwinds from LBGs?

17. The environment of LBG formation The ejection of supernova debris from the shallow potential wells of collapsed, rare density fluctuations is expected to lead to an early era of metal pollution and preheating in biased regions of the IGM (Tegmark et al. 1993; Madau et al. 2001).

18. A schematic model for metal enrichment 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 (Mori et al. 2002; Furlanetto & Loeb 2003). 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 (Thacker et al. 2002) z ~ 10 LBG z = 3

19. A model for the cross-correlation function Host halo of a dwarf galaxy formed at redshift z2 Protohalos LBG formed at z=3 dynamics Halo produced by merging Lagrangian description (linear density field) Eulerian description (observed positions)

20. Correlations in the excursion-set formalism Porciani et al. (1998) Rf r Rf See also: Scannapieco & Barkana (2002)

21. C IV - LBG correlation in the pregalactic enrichment scenario • We assume that C IV systems are associated with metal “bubbles” that have been expelled by halos of mass M at redshift z • The 2 free parameters of the model can be tuned to reproduce the cross-correlation function measured by Adelberger et al. 2003

22. Best-fitting solution • The best-fitting solution is not unique. Models such that D(z)1.15 (M) = 2.9 reproduce the data with equal accuracy. • This corresponds to perturbations with =D(z)-0.425 • The observed mean number of CIV systems per unit redshift (Songaila 2001) implies rb~ 100 kpc 200 kpc 150 kpc 100 kpc 50 kpc

23. Conclusions The observed galaxy-C IV spatial association at z=3 DOES NOT need to be generated by superwinds frm LBGs. Enriched outflows of size rb=100 kpc around 109 M halos at z=9 (or of size rb=200 kpc around 1010 M halos at z=6) can quantitatively explain both the mean absorption-line density and the LBG-C IV cross-correlation observed at redshift 3. The reason is twofold. First, both LBGs and high-z dwarfs are biased tracers of the mass distribution and form from high density fluctuations which are strongly clustered. Second, the action of gravity tends to increase the spatial association between metal bubbles and LBGs. Simulations are ongoing

24. Early vs late enrichment • Uncovering the relative contributions of LBGs and early dwarfs to the pollution of the IGM may prove difficult • Absorption against rare quasars and GRBs at z~10 (Furlanetto & Loeb 2003) requires high-res spectroscopy of faint sources in the IR (JWST) • Tests relying on measurements of relative metal abundances (Cen et al. 2004) are made impractical by uncertainties in the ionization corrections and lack of information regarding the IMF