A mass deficit is the amount of mass (in stars) that has been removed from the center of a

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

, presumably by the action of a

binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that t ...

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical obj ...

The density of stars increases toward the center in most galaxies. In small galaxies, this increase continues into the very center. In large galaxies, there is usually a "core", a region near the center where the density is constant or slowly rising. The size of the core – the "core radius" – can be a few hundred

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s in large

elliptical galaxies An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...

. The greatest observed stellar cores reach 3.2 to 5.7 kiloparsecs in radius. It is believed that cores are produced by binary supermassive black holes (SMBHs). Binary SMBHs form during the merger of two galaxies. If a star passes near the massive binary, it will be ejected, by a process called the

gravitational slingshot In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the ...

. This ejection continues until most of the stars near the center of the galaxy have been removed. The result is a low-density core. Such cores are ubiquitous in giant elliptical galaxies. The mass deficit is defined as the amount of mass that was removed in creating the core. Mathematically, the mass deficit is defined as

M_\mathrm = 4 \pi \int_0^ \left rho_i(r) - \rho(r) \right^2 dr,

where ''ρ''_i is the original density, ''ρ'' is the observed density, and ''R''_c is the core radius. In practice, the core-Sersic model can be used to help quantify the deficits. Observed mass deficits are typically in the range of one to a few times the mass of the central SMBH, and observed core radii are comparable to the influence radii of the central SMBH. These properties are consistent with what is predicted in theoretical models of core formation and lend support to the hypothesis that all bright galaxies once contained binary SMBHs at their centers. It is not known whether most galaxies still contain massive binaries, or whether the two black holes have coalesced. Both possibilities are consistent with the presence of mass deficits.

References

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External links

*Ferrarese, L. and Merritt, D. (2002)
Supermassive Black Holes
''Physics World'', June 2002, p. 41. Astrophysics Supermassive black holes