Pre-Phase A Report - Lisa - Nasa

1.2 Low-frequency sources of gravitational radiation 29
galaxies. Confident identifications include, besides the ones mentioned above: NGC 3115
(2×109 M⊙); M104 (5×108 M⊙); NGC 4261 (52×108 M⊙); M106 (3.6×107 M⊙); and M51
(3×106 M⊙). The size of the black hole seems to be roughly proportional to the mass of
the galaxy [44]. For example, limits below 105 M⊙ now exist for MBHs in the Local Group
dwarf galaxies M33 and NGC 205. See the articles by Kormendy & Richstone [44] and by
Rees [45, 46] for reviews and summaries.
What is particularly important for LISA is that massive black holes must be remarkably
abundant, since all the confident black-hole detections are in local galaxies. When the
surveys are continued to larger distances, the evidence remains strong, within the limits
of the available resolution of the observations. Two recent HST surveys bear this out. Of
about 50 sample galaxies outside the Local Group in two studies [47, 48], not a single one
showed a central region that is constant in surface brightness. Instead, 70 % of the galaxies
in the first study showed light profiles similar to the cusps generated by central MBHs,
with inferred MBH masses of roughly 0.1 % to a few % of the galaxy core mass. And the
second study similarly concluded that the cores of many galaxies appear to be similar to
that of M32, with its modest-mass MBH.
Formation of massive black holes. There is still major uncertainty about how MBHs
form, although a great deal has been written on this subject. One approach taken by
Quinlan and Shapiro [49] is to start from an assumed dense cluster of stars in a galactic
nucleus and follow the build-up of 100 M⊙ or larger seed MBHs by collisions. The further
growth to a MBH would then be mainly by accretion of gas made available to the hole
in a number of ways. If the seed MBHs grow to 500 M⊙ before they coalesce with each
other, these coalescences would give signals observable by LISA out to a redshift of z =5.
Provided that roughly 10 or more such coalescences occurred per galaxy now having
a MBH at its centre, the annual number of such events observable by LISA might be
substantial. If instead a number of roughly 100 M⊙ seed MBHs per galaxy are formed
from a very high mass tail on the initial mass function or in some other way, coalescence
signals still might observable out to high redshifts.
However, a very different view concerning the formation of MBHs was presented recently
by Rees [45, 46]. In this view, fragmentation and star formation in a massive and dense
gas cloud at a young galactic center will stop when the opacity becomes high, and the
angular momentum loss will be rapid. The gas will either form a supermassive star, which
evolves rapidly to a MBH, or collapse directly to a MBH. According to such scenarios, the
collapse to a MBH will be nearly Newtonian and quite spherically symmetric until right
at the end, and the amount of gravitational radiation emitted in the LISA frequency band
will be very low.
A similar scenario was presented somewhat earlier by Haehnelt and Rees [50]. They stated
that, if a density concentration of the order of 108 M⊙ occurs in a region 1 pc across, it
will have no nonrelativistic equilibrium state that can be supported for long, and will
collapse to a MBH. Assuming that this scenario is correct for roughly 108 M⊙ and larger
gas clouds, and that it leads to rapid formation of MBHs in the quasars observed at large
redshifts, it still is unclear how massive and how dense the gas cloud has to be for the
collapse to a MBH to occur. In particular, whether it occurs throughout the MBH mass
range of roughly 105 to 107 M⊙ of particular interest for LISA has not been established.
Alternative ways of forming MBHs in this mass range also have been proposed [51, 52].
Corrected version 2.08 3-3-1999 9:33