Images from Hydrodynamic Cosmological Simulations

All of the images below come from smoothed particle hydrodynamics (SPH) simulations by Neal Katz, Lars Hernquist, and David Weinberg. The simulation method is described by Katz, Weinberg, and Hernquist (1996, ApJ Supp 105, 19, referred to below as KWH). We are using these simulations to study galaxy formation, galaxy clustering, and the intergalactic medium in cosmological models based on inflation and cold dark matter.

You are welcome to make overhead transparencies of these images for use in talks; please give appropriate acknowledgment of the source. If you would like to use one of these images in a review article, book, etc., please check first with Neal, Lars, or myself. The images are made with the display program TIPSY, by Katz and Tom Quinn, which is publically available through the University of Washington HPCC web site.

Series 1

All of the images in this series come from a simulation of the "standard" cold dark matter model. The basic tenets of the theory are: (a) structure in the universe forms as a result of gravity acting on quantum fluctuations produced in the first fraction of a second after the Big Bang, and (b) most of the mass of the universe is in the form of some not-yet-discovered elementary particle that interacts with "normal" matter only via gravity. Most of the images in this series show the output of the simulation at "redshift 2" (z=2), a time when the expanding universe is 1/3 of its current size and about 20% of its current age.

For the afficionados: The model parameters are Omega=1, h=0.5, sigma8=0.7. The simulation volume is a periodic cube of comoving size 11.111/h Mpc. The simulation uses 262,144 dark matter particles and 262,144 gas particles, some of which are converted to collisionless "star" particles during the course of the simulation. Details are given in KWH and in various other papers that have made use of this simulation.

(172 Kb GIF)

1. A projection of gas particles in the box at z=2, color coded by temperature (with a logarithmic scale), which ranges from a few thousand degrees (blue) to a million degrees (yellow) to 10 million degrees or above (white).

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2. Same as above, but with particles color-coded by density (again on a logarithmic scale) instead of temperature.

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3. A zoomed-in view of a region 1.4 Mpc/h on a side (in comoving coordinates; the physical size is 0.46 Mpc/h), with particles color-coded by temperature. This region is the most prominent structure in the larger scale view (slightly to the right of center). Note that the gas in this high density region has a two-phase structure: lumps of cold, dense gas embedded in a much hotter medium. The cold gas lumps are comparable in size and overdensity to the luminous regions of observed galaxies.

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4. Same region as above. Blue particles represent dark matter, red particles represent gas, and yellow particles represent gas that has been partially or completely converted into collisionless stars.

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5. A zoomed view of a region in the upper left corner of the large scale view. This region is 0.7 Mpc/h on a side (comoving; physical size is 0.23 Mpc/h). There are three "galaxies" (clumps of yellow "star" particles) embedded in an extended common halo of gas (red particles) and dark matter (blue particles).

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6. Zoomed view of the central object from the above region. Blue particles represent dark matter, red particles represent gas, and yellow particles represent gas that has been partially or completely converted into collisionless stars. Vectors indicate the direction and magnitude of the particle velocities.

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7. Same region as above, showing gas particles color coded by temperature (purple = ten thousand degrees; white = ten million degrees). Note that many of the "star" particles from the previous view are also gas particles (see KWH for a discussion of these dual-identity, star-gas particles). One can see from the velocity vectors that this object is rotating (albeit in a rather floppy fashion); this rotation supports it against further gravitational collapse. However, the whole system is not much larger than the gravitational softening length of the simulation, so the details of its internal structure should not be trusted.

(5 Kb GIFs)

8. Same as above, but the position of the viewer has been changed to show the object "edge on". Note that much of the gas has settled into a relatively thin disk, though the outer gas has a different axis of rotation from the inner gas.

9. "Face on" view of the same object.

(172 Kb GIF)

10. Almost the same as the first image (full box at z=2, gas particles color coded by temperature), except that particles more than 1000 times the mean density and with temperature less than 30,000 degrees are plotted as heavier green dots. The green clumps thus indicate the locations of the high-redshift galaxies.

(172 Kb GIFs)

11-13. Same as above, but at earlier times: redshifts 3, 4, and 5. As time progresses (going from z=5 to z=4, 3, 2), gravity draws matter together into larger structures and more galaxies form.

(224 Kb GIF)

Neutral hydrogen column density through the simulation box at z=2. The color levels correspond to neutral column densities log NHI > 16.5 (white), 15.5 - 16.5 (yellow), 14.5 - 15.5 (red), below 14.5 (black). This is Figure 1 from Katz, Weinberg, Hernquist, and Miralda-Escude (1996, ApJ 457, L57).

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Updated: 1998 June 18[dhw]