Is There an Advection‐dominated Accretion Flow in Radio Galaxies with Double‐peaked Balmer Lines?

In order to examine the prediction that the broad-line radio galaxies (BLRGs)with double-peaked Balmer lines harbor an accretion disk characterized by anadvection-dominated accretion flow (ADAF) in their nuclei, we investigatenarrow emission-line flux ratios of the narrow-line regions which arephotoionized by the nuclear continuum radiation. We compile data from theliterature and confirm the pioneering work of Eracleous & Halpern that theBLRGs with the double-peaked Balmer emission exhibit larger flux ratios of both[O I]6300/[O III]5007 and [O II]3727/[O III]5007 than the BLRGs without thedouble-peaked Balmer emission. To examine whether or not these properties areattributed to the difference in the spectral energy distribution (SED) of theionizing radiation between the BLRGs with and without the double-peaked Balmeremission, we perform photoionization model calculations using two types ofinput continuum radiation; one has the strong big blue bump which is expectedfor standard optically-thick accretion disks and another does not exhibit astrong big blue bump as expected for optically-thin ADAFs. We find that thedata of the BLRGs with the double-peaked Balmer lines are consistent with themodels adopting the SED without a strong big blue bump while the data of theBLRGs without the double-peaked emission lines are well described by the modelsadopting the SED with a strong big blue bump. On the other hand, the observeddifferences in the NLR emission is hard to be explain by the difference in thecontribution of shocks. These results support the idea that the double-peakedBalmer lines arise at an outer region of an accretion disk which is illuminatedby an inner, geometrically-thick ADAF.Comment: 12 pages including figures, to appeare in the Astrophysical Journa