Red Quasars and Quasar Evolution: The Case of BAL QSO FIRST J155633.8+351758

Abstract

We present the first near-infrared spectroscopic observations of the radio-loud broad absorption line quasi-stellar object (QSO), FIRST J155633.8+351758. The spectrum is similar to that of a reddened QSO and shows strong emission lines of H? and H?, as well as strong Fe II emission blends near H?. The redshift of the object, measured from the H? and H? lines, is zBLR = 1.5008 ? 0.0007, slightly larger than the redshift of zmetal = 1.48, estimated from the broad metal absorption features. Thus, the broad metal absorption features are blue shifted with respect to the systemic velocity. The width of the H? emission line (FWHM ? 4100 km s -1) is typical of that observed in QSO broad-line regions, but the Balmer decrement (H?/H? ? 5.8) is larger than that of most optically selected QSOs. Both the Balmer decrement and the slope of the rest-frame UV?optical continuum independently suggest a modest amount of extinction along the line of sight to the broad-line region (EB-V ? 0.5 for SMC-type screen extinction at the redshift of the QSO). The implied gas column density along the line of sight is much less than that implied by the weak X-ray flux of the object, suggesting that either the broad emission and absorption line regions have a low dust-to-gas ratio or that the rest-frame optical light encounters significantly lower mean column density lines of sight than the X-ray emission. From the rest-frame UV-optical spectrum, we are able to constrain the stellar mass content of the system (<3 ? 1011 M?). Comparing this mass limit with the black hole mass estimated from the bolometric luminosity of the QSO, we find it possible that the ratio of the black hole to stellar mass is comparable to the Magorrian value, which would imply that the Magorrian relation is already in place at z = 1.5. However, multiple factors favor a much larger black hole?to?stellar mass ratio. This would imply that if the Magorrian relation characterizes the late history of QSOs and if the situation observed for F1556+3517 is typical of the early evolutionary history of QSOs, central black hole masses develop more rapidly than bulge masses.