Quasars: Characteristics, Observations,

Controversies, and Spatial Distribution

Peter LeimbiglerDavid Rotenberg

Quasi-stellar radio objects• The discovery of quasars in the late 1950s opened a new

window to the cosmos• Quasars first described as ‘radio stars’ or ‘radio galaxies

without galaxies’• are among the farthest and energetic objects in the

Universe (L > 1011L)• Quasars can and have been used to deduce the structure

and conditions of the early Universe• Quasars enable spectrographic mapping of otherwise

undetectable large-scale structure in the intervening space• Spatial distribution of quasars is an active area of research

thanks to recent and extensive quasar surveys (e.g. 2dF, Durham/AAO)

• Lively debate surrounds the nature of quasars

What Quasars are• Thought to be the most luminous form of AGN• Radio-loud quasars were discovered first (hence

‘radio object’); radio-quiet quasars (‘quasi-stellar radio objects’, QSOs) were later found to be 10-30x more common than radio-loud quasars

• Both radio-loud and radio-quiet quasars reside in giant elliptical galaxies (from HST observations)

• in contrast, Seyfert nuclei (another type of AGN) reside mostly in spirals

• “quasars are AGNs” quasars are powered by supermassive black holes (SMBHs)

Redshift• Redshift z is the fractional increase in wavelength

of light• mechanisms:

– Doppler– gravitational– Cosmological– (Arp: non-cosmological?)

• The primary method of distance determination for quasars

• Constitutes the final rung of the cosmic distance ladder

Solar spectrum

spectrum of distant galaxy cluster

The cosmic distance ladderuse for quasars

More about redshift

• Doppler:

• relativistic Doppler: , where

o = observed wavelength,

e = emitted wavelength

More about redshift

• cosmological: , for small

and large z, where R(t) is the scale length (basically the size) of the Universe

• z = 5 quasar emitted its light when the Universe was 1/6 its present size

• Cosmological redshift makes quasars appear to recede; in fact, the whole coordinate system is expanding, carrying them farther away and redshifting their light as it travels through space

•Small yet vocal groups strongly oppose the use of redshifts as distance indicators. The brunt of their arguments are aimed at refuting the validity of the conclusions about the distance and age of quasars based on redshift.

•If redshifts are not related to object distances as maintained by Hubble’s law then the familiar concepts such as the expansion of the universe, and modern cosmological theories would be falsified.

•Therefore the question of quasar distribution becomes elemental in upholding current cosmological theories, including those centered around the age, expansion and creation of our universe.

•One of the most influential contributors to the against the redshift-distance law’s validity is Halton Arp, who claims to have observed through his studies

Impossible Interactions?

Many of Arp’s claims rely on images of what he interprets as galactic objects interacting one another, despite the fact that they possess spectra redshifted by different degrees. If in fact redshifts are valid distance indicators, then the vast distances between the galaxies would make physical interaction impossible.

It is difficult to ascertain, if one ignores the redshift-distance correlation what the relative distances between galaxies are simply by visual interpretation. Arp is scant when it comes to determining the galaxies’ actual positions relative on another.

Without clear data demonstrating that these such galaxies are indeed interacting, of which there is seemingly none, their existence can be explained by other methods fitting with current theory. Such as the projection of more distant galaxies onto those nearby, or seeming interactions due to our particular line of sight. The photographs supplied by Arp are of finite scientific value.

Preferential Distribution – Forbidden Interactions Continued.

Arp also argues that quasars appear preferentially close to bright or active galaxies. He supports this with photographic evidence and imparts that the statistical likelihood of quasars aligned nearby galaxies in this manner is on the order of 10-6.

If this statistically likelihood is supported by a substantial number of observations of such systems then the isotropic distribution of quasars would be violated. However Arp’s method have been strongly challenged.

-Searching for anomalies that depart from what would otherwise be a regular, uniform distribution and taking note of these and these alone seems contrary and counterproductive; using ad hoc explanations to account for anomalies, as each arises-

Not only have claims of galaxies with different red-shift interacting with one another been raised, but images have been produced which suggest that quasars are interacting with nearby galaxies, and that some are even embedded within large host galaxies. If this were the case not only would it invalidate the value of the redshift – distance relation, but would also imply that quasars have completely different origins then what has been stated by current theory.

If quasars are distant and ancient then the light that they radiate would pass through myriad features on its way to earth, some of which would be as ancient as the quasars themselves. The interactions between the quasar’s light and these features are recorded in their spectra, which we can analyze and withdraw vast information from.

According to current big bang theory, at the time when quasars were thought to have formed, the universe was a different environment. Referring to redshift as both a distance and age indicator, the light from highly redshifted quasars should show evidence of the variance between the modern and ancient universe.

Ly α “Forest”

Gunn-Peterson Trough

Of particular interest is the contrast between quasars of different redshift. The oldest quasars, those of z =6 exhibit what is referred to as the Gunn-Peterson trough a depression between the hydrogen lines.

Whereas the Ly α forest is the product of diffuse gas clouds, the Peterson trough is thought to be caused by absorption by neutral hydrogen, before the early universe was completely ionized.

This precise fit into the cosmological model, and big bang theory suggests that indeed quasars are not only as distant, but as old as redshift would indicate.

The question pertaining to the distribution of quasars in space is still in debate by those who argue against upheld theory.

Although the consensus of astronomers is as of 2007 that: quasars are indeed the most distant and ancient objects we have so far observed, and have a nearly isotropic distribution, new discoveries always arise to challenge this consensus.

As for a conclusion: is it possible that there are sources capable of producing high redshift phenomenon without being extremely distant?

Redshift Survey (2dF QZ, 1996-2002)

Distribution• Quasars are observed in all directions of the

sky in a seemingly random distribution

• Clustering is not as strong as that of galaxies

• Quasars are very rare near us: ~ 1 per GPc3, or one quasar among millions of normal galaxies

• Density increases rapidly with increasing z

The quasar epoch

Fit for data collected by Shaver et al. (1988)

Density distribution

Implications of distribution• Quasars preferentially formed in the early

Universe and have died out today

• Stage of galaxy formation

• Galaxy collisions ‘feed’ SMBHs; perhaps galaxy collisions were more common in the early Universe

• Explanations for high-z falloff:– selection effect: only bright high-z quasars are seen– evolution: quasar cores take time to form– both?

Redshift Survey (2dF QZ, 1996-2002)

For comparison: 2dF galaxy redshift survey