Abstract
In the previous chapter we explained by what means the cosmological parameters may be determined, and what progress has been achieved in recent years. This might have given the impression that, with the determination of the values for Ω m, Ω Λ etc., cosmology is nearing its conclusion. As a matter of fact, for several decades cosmologists have considered the determination of the density parameter and the expansion rate of the Universe as their prime task, and now this goal has largely been achieved.
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Notes
- 1.
Readers not familiar with the optical/near-IR filter system may find it useful to consult Sect. A.4.2 in the Appendix at this point. We will also follow the usual practice and write R = 22 instead of R = 22 mag in the following.
- 2.
Whereas the symbols for redshift and the z-band magnitudes are identical, we trust that no confusion will arise by that, as the meaning will always be clear by the context.
- 3.
Note that a factor of 5 in magnification corresponds to a factor 25 in the exposure time required for spectroscopy. This factor of 25 makes the difference between an observation that is feasible and one that is not. Whereas the proposal for a spectroscopic observation of 3 h exposure time at an 8-m telescope may be successful, a similar proposal of 75 h would be hopelessly doomed to failure.
- 4.
At the time of writing, ∼ 90 QSOs with z > 5. 7 and known, of which ∼ 40 have z > 6. 0; those were found from several wide-field imaging surveys, including SDSS, the CFHT quasar survey, the UKIRT Infrared Deep Sky Survey (UKIDSS), and Pan-STARRS.
- 5.
The accuracy with which the position of a compact source can be determined is approximately given by the ratio of the FWHM and the signal-to-noise ratio with which this source is observed.
- 6.
The reason for this bias is the very different K-correction in the sub-mm and radio regimes, due to the very different slopes of the spectral energy distribution in these two regimes, as can be seen in Fig. 9.30: Whereas the flux in the sum-mm regime increases as a source is moved to higher redshifts, its radio flux decreases strongly, thus biasing against the detection of high-z SMGs in the radio.
- 7.
There a few TeV blazars at higher redshift, but as we discussed in Sect. 5.2.6, the featureless spectrum of most blazars renders the determination of a secure redshift sometimes uncertain.
- 8.
We recall that the roughly equal energy in the optical and FIR extragalactic background radiation shows that about half of the cosmic star formation occurs in dust-obscured regions.
- 9.
The derivation of the star-formation rate as a function of redshift is largely drawn from galaxy surveys which are based on color selection, such as LBGs, EROs and sub-mm galaxies. The possibility cannot be excluded that additional populations of galaxies which are luminous but do not satisfy any of these photometric selection criteria are present at high redshift. Such galaxies can be searched for by spectroscopic surveys, extending to very faint magnitude limits. This opportunity now arises as several of the 10-m class telescopes are now equipped with high multiplex spectrographs which can thus take spectra of many objects at the same time. One of them is VIMOS at the VLT, another is DEIMOS on Keck. With both instruments, extensive spectroscopic surveys are being carried out on flux-limited samples of galaxies. Among the first results of these surveys is the finding that there are indeed more bright galaxies at redshift z ∼ 3 than previously found, by about a factor of 2, leading to a corresponding correction of the star-formation rate at high redshifts. In a color-color diagram, these galaxies are preferentially located just outside the selection box for LBGs (see Fig. 9.4). Given that this selection box was chosen such as to yield a high reliability of the selected candidates, it is not very surprising that a non-negligible fraction of galaxies lying outside, but near to it are galaxies at high redshift with similar properties.
- 10.
Recall that atmospheric scintillations are due to a space and time dependent refractive index of the air. For propagating radio waves, the same is true, except that the refractive index here is determined by the electron density of the ionized plasma in the ISM.
- 11.
Indeed, it seems that the distribution of GRBs extends further out in redshift than that of the star formation density. This observational fact is most likely related to the finding that GRBs are found in host galaxies with small metallicity. It is possible that the metal enrichment of galaxies suppresses GRBs at later redshifts. The connection to the metallicity may have its origin on a possible metallicity-dependent star formation, i.e., allowing for higher-mass stars from metal-poor gas.
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Schneider, P. (2015). The Universe at high redshift. In: Extragalactic Astronomy and Cosmology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-54083-7_9
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DOI: https://doi.org/10.1007/978-3-642-54083-7_9
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