and Cosmology

9.5 The Cosmic Star-Formation History
387
Flux
0
(1+z reion.
)
Source
Spectrum
Ly
(1+z source
)
Fig. 9.31. Sketch of a potential observation
of reionization: light from a very
distant QSO propagates through a partially
ionized Universe; at locations where
it passes through HII regions, radiation
will get through – flux will be visible at
the corresponding wavelengths. When the
HII regions start to overlap, the normal
Lyα forest will be produced
Space Telescope, JWST), one hopes to discover the first
light sources in the Universe; this space telescope, with
a diameter of 6.5 m, will be optimized for operation at
wavelengths between 1 and 5 μm.
9.5 The Cosmic Star-Formation History
The scenario for reionization as described above should,
at least for the main part, be close to reality, with its
details still being subject to intense discussion. In particular,
the star-formation rate of the Universe as a function
of redshift can be computed only by making relatively
strong model assumptions, because too many physical
processes are involved; we will elaborate on this
in the next section. However, observations of galaxies
at very high redshifts have also been accomplished
in recent years, through which it has become possible
to empirically trace the star-formation rate up to large
redshifts.
9.5.1 Indicators of Star Formation
We define the star-formation rate (SFR) as the mass of
the stars that are formed per year, typically given in
units of M ⊙ /yr. For our Milky Way, we find a SFR of
∼ 3M ⊙ /yr. Since the signatures for star formation are
obtained only from massive stars, their formation rate
needs to be extrapolated to lower masses to obtain the
full SFR, by assuming an IMF (initial mass function;
see Sect. 3.9.4). Typically, a Salpeter-IMF is chosen between
0.1M ⊙ ≤ M ≤ 100M ⊙ . We will start by listing
the most important indicators of star formation:
• Emission in the far infrared (FIR). This is radiation
emitted by warm dust which is heated by hot young
stars. For the relation of FIR luminosity to the SFR,
observation yields the approximate relation
SFR FIR
M ⊙ /yr ∼
L FIR
5.8 × 10 9 L ⊙
.
• Radio emission by galaxies. A very tight correlation
exists between the radio luminosity of galaxies and
their luminosity in the FIR, over many orders of magnitude
of the corresponding luminosities. Since L FIR
is a good indicator of the star-formation rate, this
should apply for radiation in the radio as well (where
we need to disregard the radio emission from a potential
AGN component). The radio emission of normal
galaxies originates mainly from supernova remnants
(SNRs). Since SNRs appear shortly after the beginning
of star formation, caused by core-collapse
supernovae at the end of the life of massive stars in
a stellar population, radiation from SNRs is a nearly
instantaneous indicator of the SFR. Once again from
observations, one obtains
SFR 1.4 GHz
M ⊙ /yr
∼
L 1.4 GHz
8.4 × 10 27 erg s −1 Hz −1 .
• Hα emission. This line emission comes mainly from
the HII regions that form around young hot stars. As
an estimate of the SFR, one uses
SFR Hα
M ⊙ /yr ∼
L Hα
1.3 × 10 41 erg s −1 .
• UV radiation. This is only emitted by hot young stars,
thus indicating the SFR in the most recent past, with
SFR UV
M ⊙ /yr ∼ L UV
7.2 × 10 27 erg s −1 Hz −1 .