Page 251 - Special Topic Session (STS) - Volume 1

STS426 Didier Fraix-Burnet

Figure 1. Median spectra (black lines) for sixteen groups obtained with a set of 100 000 spectra.
The abscissae are the wavelengths in Ångström (Å), and the ordinates is the flux. All spectra
before the analysis have been normalized using the integrated flux between 4300 and 5000 Å.
The shaded areas show the region between the 10% and 90% quantiles in each group. The
legends give the number N of spectra in each group.

On the two figures, the spectra are ordered according to their global
slopes (continuum). This continuum emission is produced mainly by the
populations of stars: the first groups on the upper left of the two figures are
clearly populated by old (red) stars, while the groups toward the bottom have
young (bluer) stars. The latter also form stars since there are many emission
lines spiking out. These emission lines can also be due to an active galactic
nuclei, a central region where very probably a lot of matter is attracted and
heated by the massive black hole that we now know is present in most of the
galaxies, if not all.
It must be kept in mind that galaxies are made of billions of stars and many
gaseous regions. They are thus mixtures of different populations of stars, of
different ages and chemical compositions. This explains why several types of
spectra are identified among red and blue galaxies. As a consequence, there
seems to be several nearly identical spectra (for instance in the two first lines
in the figures), varying by subtle differences of slopes together with more
significant ones for the emission lines. Another ingredient of galaxies is the

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