Metals and winds (Reionisation)
From Eric's Wiki
Galactic superwinds (GSW) provide a method to inject metals into the ICM. There is observational evidence for metals at high redshift reported in: Franx et al 1997, Pettini et al 1998, Dawson et al 2002, Pettini et al 2001, Pettini et al 2002, Adelberger 2003, Adleberger et al 2003. Also, at z=2 to 3, low-to-moderate density regions have metals at levels difficult to achieve by sources imbedded in those regions (Tytler et al 1995, Songaila & Cowie 1996, Bergeron et al 2002)
Cosmological hydrodynamical simulations with galactic super winds (GSW): Cen & Ostriker 1992, Cen & Ostriker 1993a, Cen & Ostriker 1993b, Cen et al 1994, Gnedin & Ostriker 1997, Gnedin 1998, Cen & Ostriker 1999b, Sprigel & Hernquist 2003, Kay et al 2002, Theuns et al 2002. The results have been plagued by unexpected effects which may be numerical (CNO04). For example, energy injected by winds is radiated away. The problems are likely due to poor spatial resolution which does not model the multi-phase medium.
Galactic Winds are powered by the cumulative energy injected by supernovae and stellar winds (Ostriker & Cowie 1981), plus radiation pressure on dust (Aguirre 1999).
The ISM has a complicated physical (McKee & Ostriker 1977) and magnetic structure, making calculations of a galactic wind and its evolution difficult. A proper treatment would require multi-phase gas, magnetic fields, and cosmic rays. Magnetic fields of ISM: Koo & McKee 1992a, Koo & McKee 1992b, Smith 1996, Suchkov et al 1996, Nath & Trentham 1997, Hartquist Dyson & Williams 1997, Gnedin & Ostriker 1997, Gneden 1998, Mac Low & Ferrara 1999, Cen & Ostriker 1999b, Ferrara Pettini & Shchekinov 2000, Madau Ferrara & Rees 2001, Aguirre et al 2001, Mori Ferrara & Madau 2002, Scannapieco Thacker & Davis 2001, Scannapieco Ferrara & Madau 2002, Thacker Scannapieco and Davis 2002, White Herquist & Springel 2002, Dyson Arther & Hartquist 2002, Springel & Hernquist 2003.
COW04 inject energy into the surroundings of a galaxy in a cosmological simulation to drive galactic winds. The amount injected is tuned to match the observed galactic winds. Chevalier & Clegg 1985, Strickland & Stevens 2000, Heckman 2001 show that empirical determination of the energy in galactic winds is possible. The energy and mass injected by the galactic winds is taken to be a factor of the star formation rate, dM * / dt:
- dEGSW / dt = eGSWc2dM * / dt
- dMGSW / dt = βGSWdM * / dt
(Pettini et al 2001, Pettini et al 2002, Heckman 2001). The code does not have feedback; it simply models the effects of the galactic winds. The code is called TIGER: (Tvd for Intergalactic Medium and Galaxy Evolution and foRmation). Radiative processes are included. For UV photons, the contributions of all sources are added to a uniform field. For high-energy photons (1ev to 100 kev), "the radiation field is followed in detail with allowance for self-consistently produced radiation sources and skinks in the simulation box and for cosmological effects, i.e., radiation transfer for the mean field Jν is computed with stellar, quasar and bremsstrahlung sources and sinks due to Lyα clouds etc. In addition, a local optical depth approximation is adpoted to crudely mimic the local shielding effects: each cubic cell is flagged with six hydrogen "optical depths" on the six faces, each equal to the product of neutral hydrogen density, hydrogen ionization cross section and scale height, and the appropriate mean from the six values is then calculated; equivalent ones for neutral helium and singly-ionized helium are also computed."
Theuns et al 2002 domeonstrate that galactic winds propagate in the direction of lowest column density and filaments are not significantly affected. Hence, the Lyα forest statistics are not changed significantly.
The simulations of COW04 verify that galactic winds are capable of transporting metals to the low density regions. Metalicities there rise to dex = -2. Metalicity as a function of Lyα column density agrees with observations. But the strength of the wind does not vary the metalicity in the higher-density regions -- the very regions probed by the Lyα forest. "Metalicity in the range NHI = 1014.5 − 1015.5cm − 2 provides an insensitive test of GSW." Local star formation dominates the metalicity in these regions.
Metals and injection via galactic outflows have been included in simulations, but the simulations fail to produce the amount of C IV observed, according to Aguirre04. Also, the simulated distribution of metals is too clumpy. This indicates winds are not the source.
Galactic winds may manifest themselves in the Lyα forest. The winds will sweep away neutral (or partially neutral) gas around galaxies. If these regions are spherical, then gaps will exist in the Lyα forest where the opacity is 0 (Fang05). In any case, there should be some gaps if winds are present.
An interesting connection exists between 6Li and Reionisation: The energy required to create the amount of 6Li observed in low-metalicity stars (which sample the almost-primordial soup) is similar to that required for reionisation (Reeves05). 6Li is observed to have values much (>1000 x) higher than predicted by BBN. This is the Lithium Plateau. Be/Fe and B/Fe ratios are constant in low-metalicity stars. That means Be and 6Li are not corelated, even though they are thought to be primary elements ("primary" means fast CNO particles impinging on protons and α particles in the ISM (from a SN?); "secondary" means fast protons and α particles on CNO nuclei already in the ISM). 6Li can be generated from cosmic cays. So there must have been a lot of cosmic rays during the early universe. Could the mechanism that generated to CR's also have ionised the universe? If so, the mechanism must not have produced many metals.
It is expected that since galaxies enrich their environments with metals, then QSO spectra should have a metal forest that is correlated with galaxies along the line of sight. PSA05 conclude that this correlation exists, but is not as strong as one might expect; the enrichment is more widespread than the immediate surroundings. My thoughts: galaxy formation exists in regions of high overdensity and proceeds hierarchically so enrichment due to the sub-galaxies would occur over a larger area and, due to the violent nature of mergers, would spread over a larger area.
--Etittley 16:19, 24 June 2007 (BST)
Back to Reionisation
