Quasars as Standard Candles?

Quasars are extremely luminous active galaxies visible out to high redshifts
and in principle, if well enough calibrated as standard candles, 
could serve as cosmological probes.  Baldwin (1977) showed that the 
equivalent width of the C IV lambda 1549 emission line displays a strong
anti-correlation with the ultraviolet continuum luminosity.  The scatter
in this so-called ``Baldwin Effect'' is too large to provide reliable 
luminosities, however.  In recent years, however, sources of scatter in
the equivalent width of C IV have been identified.  Furthermore, a number
of other parameters have been found to correlate with quasar luminosity
(e.g. the ratio of optical-to-X-ray flux, orientation, etc.).  Either
through the comparison of well-matched samples at two redshifts, or through
an improved multi-parametric predictor of luminosity and the study of
large numbers of SDSS quasars, we can determine the extent to which quasars
can now be used to constrain cosmological parameters.  In the best case,
we may be able to confirm the work with supernova type 1as that were used
to identify the acceleration of the expansion of the universe.  In the 
worst case, we can identify how much scatter persists in predicting 
quasar luminosities and how much improvement is left to go.  Also, it 
may be possible that the large numbers of quasar spectra now available
in the SDSS data releases may overcome the limitations of the intrinsic
scatter.

To do this project:

It will be necessary to read a handful of quasar papers, select samples
of objects, use and/or make measurements of archival data, perform 
statistical tests and fit different cosmological models.  We may need 
to pursue more than one approach.  I envision teams of 2-3 working on 
different parts of the project and sharing the results with the class,
culminating in a final conclusion about the modern utility of quasars
as calibratable candles for cosmology (3C).

Baldwin, J., 1977, ApJ, 214, 679