The role of black holes in galaxy formation and evolution (original) (raw)
Supermassive black holes and the evolution of galaxies
Black holesÐan extreme consequence of the mathematics of General RelativityÐhave long been suspected of being the main energy source for quasars, which emit more energy than any other objects in the Universe. Recent evidence indicates that supermassive black holes reside at the centres of most galaxies, where they are probably the remnants of quasars that have been starved of fuel. As our knowledge of the demographics of supermassive black holes improves, we see clues that they participated in the formation of galaxies, and strongly in¯uenced the evolution towards the present-day structure of their surrounding hosts. Black holes are a prediction of Einstein's theory of gravity, fore-shadowed by the work of Michell and later Laplace in the late eighteenth century. K. Schwarzschild discovered the simplest kind of black hole in the ®rst solution of Einstein's equations of General Relativity, and Oppenheimer was among the ®rst to consider the possibility that black holes might actually form in nature. The subject gained life in the 1960s and 1970s, when supermassive black holes were implicated as the powerhouses for quasars, and stellar-mass black holes were touted as the engines for many galactic X-ray sources. In the past decade, we have progressed from seeking supermassive black holes in only the most energetic astrophysical contexts, to suspecting that they might be routinely present at the centres of galaxies 1±3. The de®ning property of a black hole is its event horizon, the boundary of the region surrounding the black hole from which no matter or photons can escape. Because the horizon itself is invisible, we must often settle for evidence of mass without light. All dynamical techniques for ®nding supermassive black holes at the centres of galaxies rely on a determination of mass enclosed within a radius r from the velocity v of test particles. In newtonian physics, this mass is M r av 2 r=G. Determining a requires a detailed dynamical analysis, but it is often of order 1. In cases where there is extra mass above that associated with starlight, we refer to the object as a `massive dark object' (MDO). In most of the cases discussed in this paper, it is likely that the MDO is a supermassive black hole (MBH), but in only a few cases have plausible alternatives to a black hole been ruled out. These are important because they establish the reality of MBHs and justify the interpretation of less compelling objects as MBHs. Black holes as energy sources of quasars Black holes are thought to exist in two mass ranges. Small ones of ,10M (are the evolutionary end points of some massive stars. This paper discusses the much more massive black holes that might power quasars and their weaker kin, active galactic nuclei (AGN). Quasars produce luminosities of L,10 46 erg s-1 (,10 12 L (). Where they power double-lobed radio sources, the minimum energy stored in the lobes is E,10 60 ±10 64 erg. The mass equivalent of this energy is M E=c 2 ,10 6 ±10 10 M (, and the horizon scale associated with that mass is R S GM=c 2 ,10 11 ±10 15 cm. Although most quasars do not vary much at visual wavelengths, a few objects change their luminosity in minutes at high energies 4,5. Because an object cannot causally vary faster than the light-travel time t across it, such objects must be smaller than R,ct,10 13 cm. Although relativistic corrections can alter this limit somewhat in either direction via Doppler boosting or gravitational redshift, there is no escaping the conclusion that many quasars are prodigiously luminous yet tiny, outshining a galaxy in a volume smaller than the Solar System. The small size, together with the enormous energy output of quasars, mandates black-hole accretion as the energy source. Most investigators believe that quasars and AGNs are MBHs accreting mass from their environment, nearly always at the centre of a galaxy 6±8. Black holes of mass .10 7 M (must normally lie at the centre because dynamical friction drags them to the bottom of the potential well. This location is now clearly established for low-redshift (z (0:3) quasars 9. The connection between MBHs and quasars was ®rst made by Zeldovich 10 and Salpeter 11. Lynden-Bell 12 sharpened the argument by computing the ratio of gravitational energy to nuclear energy: E g E n , e g GM 2 =R e n Mc 2 , e g e n R S R ,100e g 1 where R S is the Schwarzschild radius of a black hole of mass M, R is the size of the quasar, and e g and e n are gravitational and nuclear energy conversion ef®ciencies; the last equality follows from the typical astrophysical thermonuclear ef®ciency of ,1% and the size scale from variability noted above. Because quasars were populous in the youthful Universe, but have mostly died out, the Universe should be populated with relic black holes whose average mass density r u matches or exceeds the mass-equivalent of the energy density u emitted by them 13. The integrated co-moving energy density in quasar light (as emitted) is: u # ` 0 # ` 0 L©L j zdL dt dz dz 1:3 3 10 2 15 erg cm 2 3 2 where © is the co-moving density of quasars of luminosity L, and t is cosmic time. The corresponding present-day mass density for a radiative ef®ciency e is r u u=ec 2 2 3 10 5 0:1=eM (Mpc-3. This density can be compared with the luminous density in galaxies, j 1:1 3 10 8 L (Mpc-3 (ref. 14), to obtain the ratio of the mass in relic MBHs to the light of galaxies: ¨ r u j 1:8 3 10 2 3 0:1 e M (L (: 3 Dynamical evidence for massive black holes First steps. The ®rst dynamical evidence for black holes in galactic centres was the measurement 15 of a rising central velocity dispersion , reaching ,400 km s-1 , in the giant elliptical galaxy M87. This object is a prime site to prospect for an MBH by virtue of its AGN featuresÐnon-thermal radio emission, broad nuclear emission lines, and a `jet' of collimated relativistic particles being ejected from the nucleus. Isotropic models of the stellar kinematics, when reviews A14
Science (New York, N.Y.), 2015
Supermassive black holes (SMBHs) and their host galaxies are generally thought to coevolve, so that the SMBH achieves up to about 0.2 to 0.5% of the host galaxy mass in the present day. The radiation emitted from the growing SMBH is expected to affect star formation throughout the host galaxy. The relevance of this scenario at early cosmic epochs is not yet established. We present spectroscopic observations of a galaxy at redshift z = 3.328, which hosts an actively accreting, extremely massive BH, in its final stages of growth. The SMBH mass is roughly one-tenth the mass of the entire host galaxy, suggesting that it has grown much more efficiently than the host, contrary to models of synchronized coevolution. The host galaxy is forming stars at an intense rate, despite the presence of a SMBH-driven gas outflow.
Role of black holes in galaxy formation and regenerated dark matter in a continuous fusion cycle
Physics Essays, 2018
This essay proposes galaxy formation is a direct result of supermassive black holes, which recycle all matter within the galaxy. A continuous cycle results as regenerated matter is expelled from the galaxy core as dark matter. Dark matter forms from relativistic jets of plasma released from polar regions of rotating black holes. Dark matter coalesces from ejected plasma and rains down upon the galaxy as a massive nonluminescent halo, forming the initiation of all new star systems.