and Cosmology

6. Clusters and Groups of Galaxies
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Fig. 6.44. The radio galaxy LBDS 53W091 has a redshift of
z = 1.552, and it features a very red color (R − K ≈ 5.8).
Optical spectroscopy of the galaxy provides us with the spectral
light distribution of the UV emission in the galaxy’s
rest-frame. The UV light of a stellar population is almost completely
due to stars on the upper main sequence – see Fig. 3.38.
In the upper left panel, the spectrum of LBDS 53W091 is compared
to those of different F stars; one can see that F6 stars
match the spectral distribution of the galaxy nearly perfectly.
In the bottom panel, synthetic spectra from population synthesis
calculations are compared to the observed spectrum.
A population with an age of about 4 Gyr represents the best
fit to the observed spectrum; this is also comparable to the
lifetime of F6 stars: the most luminous (still existing) stars
dominate the light distribution of a stellar population in the
UV. In combination, this reveals that this galaxy at z = 1.552 is
at least 3 Gyr old. Phrased differently, the age of the Universe
at z = 1.55 must be at least 3 Gyr. In the upper right panel,
the age of the Universe at z = 1.55 is displayed as a function
of H 0 and Ω Λ (for Ω m + Ω Λ = 1). Hence, this single galaxy
provides significant constraints on cosmological parameters