Welcome to our journey and quest to understand how galaxies form and
evolve! We will be using an observational technique known as "QSO
Absorption Line" and the theoretical approach of N-body + hydrodynamic
cosmological simulations. We'll get to the QSO absorption lines
shortly, including how we cipher the "codes" of the features in QSO
refers to treating objects as point masses and having them obey the
laws of gravitational attraction. N refers to their being many of
these bodies, sometimes as many an several hundred million. Hydrodynamics,
which involves complex fluid flow physics and energy balancing, is
more difficult to handle. Simulations have now become sophisticated
enough that both N-body treatment of stars (and dark matter particles)
AND hydrodynamics of gas flows can be included in our computations.
The above image is from a hydrodynamic cosmological simulation that
uses an adapted refinement tree (ART) method to employ high resolution
in regions where the gas and stars are changing rapidly, while
allowing the regions that are evolving slowly to be modeled at lower
resolution. The scale of the background image is roughly 5
Megaparsecs* (Mpc) across. The zoom in box shows the gas density (no
stars are shown) centered on a galaxy; the density scale (cm^-3) is
given in the legend. The scale of this image is given by the vertical
bar across the top, which measures 400 kiloparsecs* (kpc) across. The
size of the galaxy (not shown) is on the order of 10 kpc in
diameter. Note the filamentary structure out of which the galaxy
Though not shown, stars form in the highest density regions.
Individual stars cannot be modeled, instead massive star "particles"
represent a population
of stars, including their distribution of masses and lifetimes.
This still allows us to model the evolution of the star, including how
they build up heavy chemical elements and how the supernovae explode
(including the outward force and heating of their energy into the
surrounding medium). The exploding stars pollute the surrounding gas
with heavy elements like carbon, nitrogen, carbon, and oxygen. So,
the complete star-gas cycles taking place in galaxies is modeled.
* A parsec is a unit
of distance equal to 3.26 light years, where a light year is the unit
of distance light travels in a year (~31,500,00 seconds at 300,00
km/s). A kiloparsec (kpc) is a thousand parsecs.A
Megaparsec (Mpc) is a million parsecs.