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Red quasars are very red in the optical through near-infrared (NIR)wavelengths, which is possibly due to dust extinction in their host galaxies asexpected in a scenario in which red quasars are an intermediate populationbetween merger-driven star-forming galaxies and unobscured type 1 quasars.However, alternative mechanisms also exist to explain their red colors: (i) anintrinsically red continuum; (ii) an unusual high covering factor of the hotdust component, that is, $\rm CF_{HD} = {\it L}_{HD} / {\it L}_{bol}$, wherethe ${L}_{\rm HD}$ is the luminosity from the hot dust component and the${L}_{\rm bol}$ is the bolometric luminosity; and (iii) a moderate viewingangle. In order to investigate why red quasars are red, we studied optical andNIR spectra of 20 red quasars at $z\sim$0.3 and 0.7, where the usage of the NIRspectra allowed us to look into red quasar properties in ways that are littleaffected by dust extinction. The Paschen to Balmer line ratios were derived for13 red quasars and the values were found to be $\sim$10 times higher thanunobscured type 1 quasars, suggesting a heavy dust extinction with $A_V > 2.5$mag. Furthermore, the Paschen to Balmer line ratios of red quasars aredifficult to explain with plausible physical conditions without adopting theconcept of the dust extinction. The $\rm CF_{HD}$ of red quasars are similarto, or marginally higher than, those of unobscured type 1 quasars. TheEddington ratios, computed for 19 out of 20 red quasars, are higher than thoseof unobscured type 1 quasars (by factors of $3 \sim 5$), and hence the moderateviewing angle scenario is disfavored. Consequently, these results stronglysuggest the dust extinction that is connected to an enhanced nuclear activityas the origin of the red color of red quasars, which is consistent with themerger-driven quasar evolution scenario.

What makes red quasars red? Observational evidence for dust extinction from line ratio analysis