8.5. Pregalactic radiation sources and explosive theories of galaxy formation
One of the most important problems in astronomy concerns the nature of
missing mass. Whilst the dark matter could be in the form of weakly
interacting particles, Occam's Razor invites us to associate its
origin with early star formation and galactic evolution. A crucial
question concerns the masses of the first generation of stars to form
from primordial gas soon after recombination. Whilst there are some
theoretical arguments which indicate a lower mass limit of ~
10M,
theory is not yet refined enough to be convincing (see
Silk, 1983b;
Kashlinsky and Rees,
1983).
There are, however, some observational arguments
(Terlevich, 1982)
which indicate that the mass spectrum of
stars is dependent on metallicity in the sense that for lower metal
abundance the spectrum is weighted towards high mass stars.
However, we cannot at present rule out the possibility that most of
the dark matter is in low-mass stars. Limits on the mass-to-light
ratios in the extended halos of spiral galaxies indicate that low-mass
stars ( 0.1
M
)
of spectral type M8 or later would be required
(Hegyi and Gerber, 1977).
Stars less massive than ~ 0.08
M
would
never have
undergone hydrogen burning and would be extremely difficult to detect.
One novel test has been proposed by
Gott (1981).
Low-mass stars in
galaxy halos could act as gravitational lenses leading to variability
in the intensities of quasar images on time-scales of ~ several years
but so far the effect has not been observed.
Another possibility is that very massive objects (VMO's) form in the
mass range 102 - 105
M
(Carr, Arnett and Bond,
1982;
Arnett et al., 1983).
Arnett, Bond and Carr suggest that the oxygen cores of a
VMO would collapse to a black hole if the precursor hydrogen star has
a mass in excess of Mcrit ~ 200 - 500
M
.
The large uncertainty in Mcrit
arises because it is difficult to calculate how much mass is lost in
the hydrogen and helium burning phases. VMO's offer a way of producing
black hole remnants without producing too many metals, though if most
of the mass is to end up in black hole remnants, in order to explain
the high mass-to-light ratios of clusters of galaxies, one may require
several generations of VMO'S.
Pregalactic stars of mass
< Mcrit may help to solve several
out-standing problems. Pregalactic enrichment may explain the lack of
low metallicity stars in the galaxy
(Truran and Cameron, 1971)
and various abundance anomalies such as the high ratios of O/Fe and N/Fe
observed in metal-poor stars
(Sneden, Lambert and
Whitaker, 1979;
Edmunds and Pagel, 1978).
There are tentative indications of spectral distortions in the
microwave background
(Woody and Richards,
1979).
Rowan-Robinson,
Negroponte and Silk (1979)
suggest that this effect may be explained
if a substantial fraction of the background radiation
( 25%) was
generated by pregalactic stars and the radiation was thermalized by
grains produced by the stars.
It may even be possible to produce the entire microwave background
if pregalactic stars form at a redshift of ~ 103
(Rees, 1978)
in which case one could contemplate a cold big-bang model (s
= photon 1) or a
tepid model (1 << s << 108). A VMO may
return 20-50% of its mass as helium before the oxygen core phase. If most of
the stars have masses
> Mcrit one may be able to explain the
observed helium abundance of
25% in a cold
big bang model without overproducing heavy elements
(Bond and Carr, 1983);
moreover, the deuterium abundance could also be explained
(Audouze and Silk, 1983).
Ostriker and Cowie (1981)
and Ikeuchi (1981)
suggest that explosions
of supermassive stars could provide an amplification mechanism for
generating galaxies and groups of galaxies (see also
Doroshkevich, Zel'dovich
and Novikov, 1967).
In Ostriker and Cowie's model, the explosion of a
preexisting "seed" (a supermassive star or a cluster of supermassive
stars) with an energy release of
~ 10-5 mc2 leads to a blast
wave which
sweeps up a shell of surrounding gas. If the explosion occurs at
z
represents the largest scale on which radiation pressure could be
important in generating density fluctuations, an idea previously
exploited by
McClelland and Silk
(1977).
This is interestingly, close
to the masses of the largest non-linear structures observed
today. Detailed computations of the density fluctuation spectra and
the microwave background anisotropies expected on this model are
presented by
Hogan and Kaiser (1983).
10, the shell can radiatively cool and fragment, leading to new
objects ~ 103 times more massive than the original
seeds. The new
objects may themselves explode, leading to the collapse of still more
massive shells, etc. Thus, if we start with small seeds it may be
possible to generate all structure up to the characteristic mass-scale
that can cool within a Hubble time. Whilst it may be possible to cool
on mass-scales corresponding to galaxies or groups of galaxies
(cf. Eqs. 8.4) it is difficult to see how a cluster of galaxies such
as Coma (~ 1015
M
)
could radiate its binding energy and,
therefore, why rich clusters should be bound at all
(Hogan, 1983).
Ostriker and Cowie argue that explosions which occur between
150
z
10 (when
Compton drag is ineffective and Compton scattering of electrons by the
background radiation provides the dominant cooling mechanism) are
likely to lead to the formation of supermassive stars.