Abstract
After having described the cosmological model in great detail, as well as the objects that inhabit our Universe at low and high redshifts, we will now try to understand how these objects can be formed and how they evolve in cosmic time.
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Notes
- 1.
Spectroscopy in the NIR is possible in principle, but the high level of night-sky brightness and, in particular, the large number of atmospheric transition lines renders spectroscopic observations in the NIR much more time consuming than optical spectroscopy.
- 2.
To destroy all the H2 in the Universe one needs less than 1 % of the photon flux that is required for the reionization.
- 3.
- 4.
Note that in this case, the baryons are embedded in a dark matter halo, so the consideration of the spin parameter, which applies for the total energy and angular momentum, no longer applies to the baryons only. Therefore, in this case (10.11) does not hold for the baryons alone.
- 5.
In this case of high collision velocity, the time it takes a galaxy from one of the two clusters to cross the gravitational potential of the other cluster is shorter than the time it takes the matter of the second cluster to react to the changing conditions caused by the merger; therefore, the gravitational potential of the second cluster can be considered almost stationary during the collision process. Thus, the galaxy leaves the potential of the second cluster with almost the same velocity it had on entering, i.e., it is not gravitationally bound to the second cluster.
- 6.
The fact that spectacular images of merging galaxies show mainly gas-rich mergers (such as in Fig. 9.25 or 1.16) can be attributed to selection effects. On the one hand, gas-rich mergers lead to massive star formation, yielding a statistically increased luminosity of the systems, whereas dry mergers basically preserve the luminosity. On the other hand, gas-rich mergers can be recognized as such for a longer period of time than dry ones, owing to the clearly visible tidal tails traced by luminous newly formed stars.
- 7.
Calculating the behavior of a binary black, using the equations of General Relativity, turns out to be very difficult endeavor. Only since 2005 it has become possible to find numerical solutions of this problem.
- 8.
The equations of hydrodynamics describe the behavior—or transport—of the mass, momentum and energy in a fluid. Mass conservation is expressed by the continuity equation (7.2). The evolution of the fluid momentum is given, in the simplest case, by the Euler equation (7.3); however, since gas is dissipative, frictional terms need to be included (the resulting equation for the fluid velocity is then called Navier–Stokes equation). Finally, the transport of energy is described by an energy equation, which contains sources and sinks of energy, as they can be caused by absorption and emission of radiation and the local generation of heat by frictional forces.
- 9.
The situation is rather similar in meteorology, where we believe to know all the essential physical processes that affect the Earth atmosphere; nevertheless, we all know that weather predictions can be terribly wrong, even on short time-scales. The reason is that, although the relevant physical laws are known, their consequences cannot be calculated with sufficient accuracy due to the complexity of the underlying equations. Also in this case, small-scale, highly non-linear processes (convection, turbulence) have an impact on the large-scale properties of the atmosphere.
- 10.
In fact, one can obtain a statistical ensemble of such merger trees also analytically from an extension of the Press–Schechter theory (see Sect. 7.5.2), but referring to N-body simulations also yields a prescription of the spatial distribution of the resulting galaxy distribution.
- 11.
Of course, this simple picture ignores all the difficulties in understanding the transport of gas from large distances to the immediate vicinity of the black hole where it can be accreted.
- 12.
This will not be true in every single case; as can be seen from Fig. 10.26, the most massive SMBHs at high redshifts are not necessarily the mass record holder at later epochs.
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Schneider, P. (2015). Galaxy evolution. In: Extragalactic Astronomy and Cosmology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-54083-7_10
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DOI: https://doi.org/10.1007/978-3-642-54083-7_10
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