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The Question
(Submitted July 01, 1997)
Has anyone calculated the high and low limits of mass in the probable
number of quasars?
The Answer
For your first question, the mass of quasars seems to be included in the
mass of galaxies, so that is not the missing mass.
For nearby galaxies, the mass is often estimated by measuring the rotation
curve
(for spiral galaxies) or the velocity dispersion (for elliptical galaxies).
The
higher the velocities, the greater the mass which must be enclosed. This is
actually a straightforward application of Newtonian gravity (which is still a
reasonable approximation far from the black hole). This method includes the
mass of any single central black hole (or stellar mass black holes) lurking
within.
Relating these masses to the luminosity of the galaxy, we develop a
mass-luminosity relation for galaxies that is applied to estimate masses of
galaxies too distance to obtain a rotation curve or velocity dispersion.
Considering that recent Hubble results suggest black holes may lie in the
centers of many, if not all galaxies, then the mass of these black holes is
already included in our mass estimates of the galaxies and therefore of the
Universe. Our current models of quasars suggest they are simply regular
galaxies being viewed down a jet being ejected near the central black hole.
The
jets are probably formed by complex magneto-hydrodynamical interactions in the
accretion disk with the spin of the black hole.
Anyway, as a result, the quasars and their mass are included in our mass
estimates of the Universe. There have been some studies searching for
possible
populations of small black holes roaming around between galaxies but the
observational constraints do not suggest these could be the source of the
'dark
matter' or 'missing light', whichever you choose to call it.
Tom Bridgman and David Palmer
for Ask a High-Energy Astronomer
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