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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 an Astrophysicist

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