Quasars AGN History the first Detection of an Emission Spectrum from a Galaxy Was 1908
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Astronomy 218 Quasars AGN History The first detection of an emission spectrum from a galaxy was 1908. By the 1920s, several more had been added. Seyfert introduced the classification of active galactic nucleus in 1943, showing that ~1% of spirals had AGNs. The development of radio astronomy after World War II led to the discovery of radio galaxies, as these were among the brightest radio sources in the sky. Quasi-Stellar Searches for the optical counterparts to strong radio sources revealed many radio galaxies. Improvements in radio telescopes through the 1950s revealed many more, fainter radio galaxies. In 1960, Matthews & Sandage found a 16th-magnitude point-like optical counterpart to 3C 48. This was joined in 1963 by a similar companion to 3C 273. They were dubbed Quasi-Stellar 1” Radio Sources (QSRs). Peculiar Spectrum The optical spectra of QSRs (pronounced quasars) were perplexing, exhibiting strong emission lines that corresponded to no known elements. Later in 1963, Maarten Schmidt (1929− ) identified the Balmer series, but at a redshift of z = 0.158, indicating a speed 14.6% of the speed of light. It was soon realized that quasar spectra were those of active galactic nuclei, but enormously redshifted. Greenstein & Matthews revealed 3C 48 had z = 0.367. Extremely Luminous Solving the spectral problem introduced a new problem. Applying Hubble’s law to a redshift of z = 0.158 yields a distance of 620 Mpc for 3C273! 3C 48’s redshift of z = 0.367 is equivalent to a Hubble distance of 1.3 Gpc. At the time of this discovery, 3C 48 was among the most distant objects yet discovered. Quasars must be among the most luminous objects (L ~ 1038‒1041 W) in the universe, though they appear quite faint at visible wavelengths because of their distance. Quasar Spectra With the redshift revealed, the optical spectra of nearby quasars look like broad line Active Galactic Nuclei, although higher redshifts move Lyman and other UV lines into the visual wavelength range. Overall, the spectral energy distributions (SEDs) of quasars are similar to those of radio-loud AGN, though brighter and more variable. For example, this SED shows the average and range of fluctuations for 3C273. Finding Galaxies While some early QSR optical counterparts showed “fuzziness” suggesting nebulosity, not until 1982 (by Boronson & Oke for 3C 48) was it firmly established that the nebulosity was stellar and at a similar redshift. This cemented the picture of quasars as distant AGN. The quasars in these images are shown with their host galaxies. Quasi-Stellar Object Like AGN in general, the radio-loud QSRs are joined by a larger class of similar, but radio-quiet objects. For quasars, these historically were termed Quasi-Stellar objects (QSOs), to differentiate them from QSRs, though QSO and quasar are now used almost interchangeably by many astronomers. In modern day, Radio-Quiet Quasars (RQQ), which make up > 90% of quasars, are distinguished from Radio-Loud Quasars (RLQ) to avoid the confusion between QSO and quasar. Superluminal Expansion With the development of Very-Long-Baseline Interferometry (VLBI) in the 1970s, it became possible to resolve the jets of quasars. For 3C273, this showed knots with proper motion of μ ~ 0.7 mas yr−1. At a distance d > 600 Mpc, this implies a tangential velocity, −1 �t ≈ 4.74 km s μ” (d/pc) ≈ 4.74 (6 × 108 pc)(7 × 10-4” yr−1) ≈ 2 × 106 km s−1 ≈ 7 c Such superluminal velocities restarted the simmering debate over the true distance to quasars, but ultimately the solution is relativistic geometry. Trick of the Light A Photons emitted at t1 from B1 arrive at A at time Photons emitted at t2 from B2 arrive at A at time The time interval between the two observations is Δϕ d θ sin t ʋδ For objects moving toward the observer, θ B2 cos θ > 0, thus Δt < δt. θ cos ʋδt t ʋδ For θ ~ 0 and β ~ 1, Δt << δt B1 Relativistic Proper Motion A Over the time interval Δt, the radio source will move through Δϕ = vδt sin θ / d. The resulting proper motion, Δϕ d This corresponds to an apparent velocity of θ sin t ʋδ θ B2 θ cos Explaining 3C273’s βt = 7 requires β ≥ 0.99 and ʋδt t −1 ʋδ θ ≈ cos β ≤ 8° B1 Loud verses Quiet The observation that > 90% of quasars, like 90% of AGNs, are radio-quiet raises the question, Why are only some AGN radio-loud? One answer is that only RL AGN have strong jets, but that merely re-frames the question. Some RQQ have weak but observable jets, while in others the radio core is unresolved. The presence of the jet does not seem to be an issue with accretion rate, as the flux at other wavelengths is similar. Aside from the accretion rate, it is the properties of the black hole that determine behavior of the AGN. Special Black Holes? The only properties of a black hole are mass, charge and angular momentum. Observations indicate that the typical 9 masses of black holes in Radio-Loud Quasars (~ 10 M☉) are not greatly different from their Radio-Quiet brethren (~ 5 × 8 10 M☉). We expect charge to be 0. In General Relativity, rotating masses drag spacetime around with them. Called frame-dragging, this effect alters the shape of the event horizon and introduces an ergosphere where matter must rotate. In 1969, Penrose suggested that energy could be extracted via the ergosphere. In 1977, Blanford & Znajek described a means to achieve this via magnetic fields, converting black hole rotation to EM radiation. Aging Quasars We can observe quasars to very great distances, seeing them as they existed a long time ago. 12 Hasinger et al.: Luminosity-dependent evolution of soft X-ray AGN 35 Analyzing their behavior Lx > 10 W 36 over time, we see that the Lx > 10 W space density of quasars is 37 currently declining, having Lx > 10 W peaked ~ z = 2 (10 Gyr ago). 38 Further, we can see they are Lx > 10 W getting fainter over time. 39 The black holes powering Lx > 10 W the quasars do not shrink (most galaxies have a supermassive black hole at Fig. 5. (a) The space density of AGNs as a function of redshift in different luminosity classes and the sum over all their center) so the accretion mustluminosities slow with over log Lx ≥time42. Densities. from the PLE and LDDE models (Sect. 4.4) are overplotted with solid lines. (b) The same as (a), except that the soft X–ray emissivities are plotted instead of number densities. The uppermost curve (black) shows the sum of emissivities in all luminosityclassesplotted. density shifts from z ∼ 0.7atlogLx ∼ 42.5toz>2 5. An alternate approach using the Vmax method at high luminosities. Based on a similar observation of ahardX–ray–selectedsample,Uedaetal.2003usedan As described in the Introduction, the luminosity function derived from survey data binned in luminosity and red- expression where zc is a simple function of Lx: shift does not necessarily apply to the centers of the (Lx,z) p1 (1 + z) (z ≤ zc) bins. This binning bias tends to be especially a problem ed(z,Lx)= p2 . (9) ! ed(zc)[(1 + z)/(1 + zc)] (z>zc) if data are scarce (often at higher redshifts) and gradi- ents across bins are large. The previous section describes along with aprocedurethatcorrectsthebinnedspacedensitiesto α zc,0(Lx/Lx,c) (Lx ≤ Lx,c) first order. zc(Lx)= . (10) ! zc,0 (Lx >Lx,c) In this section, we avoid deriving densities from binned survey data. Instead, we use the V values of individual The results of the analysis in the previous section max RBS sources to derive the zero redshift luminosity func- shown in Table 4 suggest that considering the dependence tion. We then derive by iteration an analytical density of p1andp2onluminositywouldstillimprovethefit.Thus template at various L values that, together with the zero we have also included the following for our full LDDE ex- x redshift luminosity function, accounts for the observed pression: counts and redshifts of the deeper surveys. The end re- sult of the procedure is a set of observed values of the lu- p1(Lx)=p144 + β1 (log Lx − 44) (11) minosity function that apply to the centers of the (Lx,z) bins, and that is quite insensitive to the precise template p2(Lx)=p244 + β2 (log Lx − 44) (12) employed. A further advantage of employing Vmax of in- The best–fit parameters and the results of the K–S dividual sources is that it can be derived for two or more tests for the PLE and LDDE models are summarized in selection variables. This allows us to account for the effect Table 5. The best–fit PLE and LDDE models are over- of a spectroscopic magnitude limit in some of the deeper plotted on Figs. 4 and 5 with dotted and dashed lines surveys beyond which the redshift is unknown for most of respectively. A detailed discussion of the comparison of the sources. In the first use of Vmax,thisfeaturewasused model and data is given in Sect. 6. to derive the luminosity function of radio quasars from a Quasars as AGNs The picture of quasars as simply ultra-bright AGNs is supported by their relative rarity. Galaxy Density Another useful observation is the Type (Mpc−3) lifetime of a quasar. Field 10−1 The most massive black holes known Bright Spiral 10−2 9 are Mbh ~ 4 × 10 M☉ while the most 14 −4 luminous quasars at L ≈ 3 × 10 L☉ Seyfert 10 ̇ −1 requiring � ~ 200 M☉ yr .