The dramatic impact of background overdensity...

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-0.7 sigma +0.7 sigma

0.0 sigma (MEAN) +1.4 sigma

These simulations cover small (5 Mpc) patches of space at redshift 3 ( when the universe was 1/8th of it's current age ). Shown is HI gas, which is detected in absorption in Quasar spectra. At the vertices of the filamentary network, dwarf galaxies can be seen. The HI is supressed near the larger dwarf galaxies because of high temperatures ( 10^6 K ) associated with shocked gas in the dark matter halos of the dwarfs. These dwarfs would almost certainly form stars. The four panels differ in their deviations from the average density. These deviations are small and thus quite likely, and yet have a dramatic impact. The values compared to the RMS deviation ( on Gaussian scale of 0.5 Mpc ) are -0.7,0.0,0.7 and 1.4. For a more technical description see below.

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Snapshots at redshift $z$=3 of distributions of $n_{HI}$ in the gas in the inner $2.5\mpc$ (comoving) of several SPH calculations illustrate the dramatic role the large scale shear field plays. They evolved from peak-patch initial conditions with varying heights $\nu=-0.7,0.0,0.7$ and $1.4$ on a Gaussian filtering scale of $0.5 \mpc$ with typical shear parameters ($e_v$, $p_v$) for each $\nu$. Self-consistent mean-field constraints were also imposed at $1.0$ and $1.5 \mpc$, all centered at the origin. The high resolution patches are initally $5 \mpc$ spherical regions in diameter, encased in 1/2 resolution boundary particles out to $8\mpc$. The external shearing tides are simulated with additional lower resolution particles out to a diameter of $12.8\mpc$ and a mean tide acting on the entire $12.8\mpc$ region. The cosmology is a standard initially scale-invariant $\Omega_{nr}$=1 CDM model with $\Omega_{B}$=0.05, ${\rm h}$=0.5, normalized to $\sigma_8$=0.67. The ionizing flux is $J_{912}=5.0\times 10^{-22}$. The colours in all panels indicate $HI$ densities (purple $10^{-12}$, blue $10^{-10.5}$, green $10^{-9}$, yellow $10^{-7.5}$ and red $10^{-6}$ $cm^{-3}$).