Pre-Phase A Report - Lisa - Nasa

28 Chapter 1 Scientific Objectives
1.2.2 Massive black holes in distant galaxies
It is clear from the preceding sections that LISA will provide valuable information concerning
the populations of various types of binaries in different parts of our galaxy. However,
the most exciting scientific objectives for LISA involve the search for and detailed study
of signals from sources that contain massive black holes (MBHs).
The most spectacular event involving MBHs will be the coalescence of MBH-MBH binaries.
Because the signal has the unique signature of a “chirp” and can be followed over many
months, and because it is intrinsically very strong, LISA can recognise MBH coalescence
events in its frequency band almost anywhere in the Universe they occur. If LISA sees
even one such event, it would confirm beyond doubt the existence of MBHs. From the
fundamental physics point of view, the waveforms of signals from such objects at times
near coalescence can provide extremely sensitive tests of general relativity for very strong
field conditions [34]. Because the phase of the signals over thousands of cycles or longer
can be tracked accurately for even fairly weak signals, very minor errors in the predictions
of the theory would be detectable [35].
From the astrophysics point of view, sources involving MBHs can provide unique new
information on the numbers, mass distribution, and surroundings of MBHs.
Astronomers invoke MBHs to explain a number of phenomena, particularly quasars and
active galactic nuclei. The most well-known cases involve MBHs of masses roughly 108 –
10 10 M⊙. LISA is sensitive mainly to lower masses, which may be considerably more
abundant.
The key question for LISA is to estimate the likely event rate (see e.g. [36] and [37]).
Identification and abundance of massive black holes. The initial arguments for
the existence of MBHs in quasars and active galactic nuclei were theoretical: there seemed
to be no other way of explaining the extremely high and rapidly varying luminosities that
were observed in the optical and radio bands. Now, however, direct observational evidence
is compelling. For example, Hubble Space Telescope observations of M87 revealed a
central brightness cusp and large asymmetric Doppler shifts, indicating a BH mass of
order 3×109 M⊙ [38, 39]. X-ray observations can see gas much closer to the MBH, and
the ASCA satellite provided remarkable evidence that seems definitive. Observing the
active galaxy MCG-6-30-15, it has detected an iron X-ray line that is Doppler-broadened
by velocities of order 0.3 c and that is strongly redshifted, indicating that the radiation
is coming from within 3 to 10 Schwarzschild radii of the MBH at the galactic centre [40].
The measured radial distances and Doppler shifts for H2O masers in orbit around the
centre of NGC 4258 demonstrate the presence of a mass of 3.6×107 M⊙ in a region less
than 0.13 pc in radius [41].
Evidence for smaller MBHs inthemainLISA mass range is also strong. Recent near-
IR measurements clearly indicate a 2.6×106 M⊙ black hole at the centre of our own
galaxy [42]. Even smaller galaxies have them: HST and ground-based observations
of M32 [43] imply that this, a nearby dwarf elliptical, a satellite of the Andromeda
galaxy M31, contains a 2.8×106 M⊙ black hole at its centre. Indeed, M31 itself contains
a black hole of mass 3×107 M⊙.
Observational evidence for black holes is turning up in every galaxy that has been studied
with enough sensitivity to see them, which restricts the evidence mainly to nearby
3-3-1999 9:33 Corrected version 2.08