Astronomy 218Q u a s a r s

AGN HistoryThe 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.

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 Radio Sources (QSRs).

Quasi-Stellar

1”

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.

Peculiar Spectrum

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.

Extremely Luminous

Quasar SpectraWith 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.

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.

Finding Galaxies

Quasi-Stellar ObjectLike 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 ExpansionWith 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,

𝑣t ≈ 4.74 km s−1 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 LightPhotons 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

For objects moving toward the observer, cos θ > 0, thus Δt < δt.

For θ ~ 0 and β ~ 1, Δt

Relativistic Proper MotionOver the time interval Δt, the radio source will move through Δϕ = vδt sin θ / d. The resulting proper motion,

This corresponds to an apparent velocity of

Explaining 3C273’s βt = 7 requires β ≥ 0.99 and θ ≈ cos−1 β ≤ 8°

θʋδt

d

ʋδt s

in θ

ʋδt c

os θ

Δϕ

B1

B2

A

Loud verses QuietThe 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 masses of black holes in Radio-Loud Quasars (~ 109 M☉) are not greatly different from their Radio-Quiet brethren (~ 5 × 108 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.

We can observe quasars to very great distances, seeing them as they existed a long time ago.

Analyzing their behavior over time, we see that the space density of quasars is currently declining, having peaked ~ z = 2 (10 Gyr ago). Further, we can see they are getting fainter over time.

The black holes powering the quasars do not shrink (most galaxies have a supermassive black hole at their center) so the accretion must slow over time.

Aging Quasars

12 Hasinger et al.: Luminosity-dependent evolution of soft X-ray AGN

Fig. 5. (a) The space density of AGNs as a function of redshift in different luminosity classes and the sum over allluminosities with log Lx ≥ 42. 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 uppermostcurve (black) shows the sum of emissivities in all luminosity classes plotted.

density shifts from z ∼ 0.7 at log Lx ∼ 42.5 to z > 2at high luminosities. Based on a similar observation ofa hard X–ray–selected sample, Ueda et al. 2003 used anexpression where zc is a simple function of Lx:

ed(z, Lx) =

{

(1 + z)p1 (z ≤ zc)ed(zc)[(1 + z)/(1 + zc)]p2 (z > zc)

. (9)

along with

zc(Lx) =

{

zc,0(Lx/Lx,c)α (Lx ≤ Lx,c)zc,0 (Lx > Lx,c)

. (10)

The results of the analysis in the previous sectionshown in Table 4 suggest that considering the dependenceof p1 and p2 on luminosity would still improve the fit. Thuswe have also included the following for our full LDDE ex-pression:

p1(Lx) = p144 + β1 (log Lx − 44) (11)

p2(Lx) = p244 + β2 (log Lx − 44) (12)

The best–fit parameters and the results of the K–Stests for the PLE and LDDE models are summarized inTable 5. The best–fit PLE and LDDE models are over-plotted on Figs. 4 and 5 with dotted and dashed linesrespectively. A detailed discussion of the comparison ofmodel and data is given in Sect. 6.

5. An alternate approach using the Vmax method

As described in the Introduction, the luminosity functionderived from survey data binned in luminosity and red-shift does not necessarily apply to the centers of the (Lx, z)bins. This binning bias tends to be especially a problemif data are scarce (often at higher redshifts) and gradi-ents across bins are large. The previous section describesa procedure that corrects the binned space densities tofirst order.

In this section, we avoid deriving densities from binnedsurvey data. Instead, we use the Vmax values of individualRBS sources to derive the zero redshift luminosity func-tion. We then derive by iteration an analytical densitytemplate at various Lx values that, together with the zeroredshift luminosity function, accounts for the observedcounts and redshifts of the deeper surveys. The end re-sult of the procedure is a set of observed values of the lu-minosity function that apply to the centers of the (Lx, z)bins, and that is quite insensitive to the precise templateemployed. A further advantage of employing Vmax of in-dividual sources is that it can be derived for two or moreselection variables. This allows us to account for the effectof a spectroscopic magnitude limit in some of the deepersurveys beyond which the redshift is unknown for most ofthe sources. In the first use of Vmax, this feature was usedto derive the luminosity function of radio quasars from a

Lx > 1039 W

Lx > 1038 W

Lx > 1037 W

Lx > 1036 WLx > 1035 W

Quasars as AGNsThe picture of quasars as simply ultra-bright AGNs is supported by their relative rarity.

Another useful observation is the lifetime of a quasar.

The most massive black holes known are Mbh ~ 4 × 109 M☉ while the most luminous quasars at L ≈ 3 × 1014 L☉ requiring �̇� ~ 200 M☉ yr−1.

To estimate quasar lifetime,

≈ 20 Myr

Thus quasars, and by extension AGN, are not so much objects as they are a phase in the life of some galaxies.

Galaxy Type

Density (Mpc−3)

Field 10−1

Bright Spiral 10−2

Seyfert 10−4

Radio 10−6

RQ Quasar 10−7

RL Quasar 10−9

The picture that emerges is that quasars are simply a short-term behavior brought on by high accretion rates, transitioning ultimately to the ellipticals and spirals we see today, with or without an active nucleus. We are probably surrounded by old quasars. The fact that RL AGN occur in giant ellipticals, while RQ AGN occur in spirals, tells us that the conditions to make a rapidly spinning black hole also produce giant ellipticals.

Dormant Quasars

The light from distant quasars travels through a tremendous amount of space to reach us.

For a quasar with z = 6.4, we are seeing it as it was 13 Gyr ago. At the time, it was 1 Gpc away, but the expansion of space means it is now almost 9 Gpc away.

The stretching of space turns a Lyman α photon (λ = 122 nm) photon from ultraviolet into infrared (λ = 900 nm)

Along this path, the intervening space alters the quasar’s light, allowing us to learn about parts of space too dim for us to see.

In the process, we also sample the Universe at every time between the time the light was emitted, when the universe was young, and now.

Exploring with Quasars

Examination of the light from distant quasars reveals a forest of absorption lines, the result of quasar light passing through hundreds of gas clouds, each with a different redshift, on its way to us.

While many of the clouds are optically thin and produce narrow lines, some are denser and optically thick, producing “damped” Lyman lines.

This suggests filaments of neutral gas populated with denser regions, likely disks of young gas-rich galaxies.

Lyman α Forest

The distant quasar light is also subject to the influence of the masses it passes near.

What first appears to be two mirrored quasars on closer inspection turns out to be just one, with an invisible intervening mass.

By identifying identical spectra and luminosity variations, though often lagged in time by several years, at least 8 multiply-imaged quasars have been identified.

Seeing Double

The time difference results from the different lengths of the two paths that the light takes to reach us.

For the quasar, we simultaneously observe two moments in time, a boon to understanding the variability of the quasar.

More importantly, by analyzing the bending of the light we learn about the mass distribution in the lens, another way to learn about the mass to light ratio.

Lensing Galaxy

A gravitational lens is not limited to 2 images.

If the lensing mass and the quasar line up precisely along our line of sight, the quasar appears as a ring of light, called an Einstein Ring. Slight off-center alignments produce partial rings or short arcs of light.

Probably the most notable multiply-imaged quasar is Einstein’s cross. The quasar Q2237+031, at redshift of z = 1.69 is lensed by Huchra’s lens, a nearby spiral at redshift of z = 0.04.

Einstein Cross

Next TimeClusters of Galaxies