What makes red quasars red? Observational evidence for dust extinction
  from line ratio analysis
  Red quasars are very red in the optical through near-infrared (NIR)
wavelengths, which is possibly due to dust extinction in their host galaxies as
expected in a scenario in which red quasars are an intermediate population
between merger-driven star-forming galaxies and unobscured type 1 quasars.
However, alternative mechanisms also exist to explain their red colors: (i) an
intrinsically red continuum; (ii) an unusual high covering factor of the hot
dust component, that is, $\rm CF_{HD} = {\it L}_{HD} / {\it L}_{bol}$, where
the ${L}_{\rm HD}$ is the luminosity from the hot dust component and the
${L}_{\rm bol}$ is the bolometric luminosity; and (iii) a moderate viewing
angle. In order to investigate why red quasars are red, we studied optical and
NIR spectra of 20 red quasars at $z\sim$0.3 and 0.7, where the usage of the NIR
spectra allowed us to look into red quasar properties in ways that are little
affected by dust extinction. The Paschen to Balmer line ratios were derived for
13 red quasars and the values were found to be $\sim$10 times higher than
unobscured type 1 quasars, suggesting a heavy dust extinction with $A_V > 2.5$
mag. Furthermore, the Paschen to Balmer line ratios of red quasars are
difficult to explain with plausible physical conditions without adopting the
concept of the dust extinction. The $\rm CF_{HD}$ of red quasars are similar
to, or marginally higher than, those of unobscured type 1 quasars. The
Eddington ratios, computed for 19 out of 20 red quasars, are higher than those
of unobscured type 1 quasars (by factors of $3 \sim 5$), and hence the moderate
viewing angle scenario is disfavored. Consequently, these results strongly
suggest the dust extinction that is connected to an enhanced nuclear activity
as the origin of the red color of red quasars, which is consistent with the
merger-driven quasar evolution scenario.
