THE EMISSION-LINE PROPERTIES OF THE QSO POPULATION AT Z-SIMILAR-TO-2

We compare the ultraviolet emission-line properties of the three subclasses of the QSO population (radio quiet, flat radio spectrum radio loud, steep radio spectrum radio loud) during the "QSO epoch" at z∼2. Our total sample consists of 79 objects that are well matched in luminosity and redshift, which should allow unbiased comparison of their spectral properties. We confirm recent results that radio-loud objects have, on average, narrower and stronger C IV γ1549 and C III] γ1909 emission lines than radio-quiet objects. This is likely the effect of larger average intermediate-line and/or narrow-line regions in radio loud objects, including a relative excess of emission on very extended (∼100 kpc) scales. We additionally find that radio-loud objects show a significantly wider range in the asymmetry of the C IV γ1549 profile than radio-quiet objects, with cases of both redward and blueward asymmetry. By contrast, radio-quiet objects almost exclusively show blueward asymmetry in this line, and cases of strong redward asymmetry appear to be limited to steep-spectrum objects. Statistically, there is a closer correspondence between the emission-line properties of radio-quiet and flat-spectrum objects than between either of these subclasses and steep-spectrum objects, whose mean emission-line properties differ strongly from those of the radio-quiet sample. This result is similar to that found for the properties of the optical emission lines in low-redshift objects and is not easily understood under the proposed unified models of the flat- and steep-spectrum sources. We also find that 20%-30% of radio-loud objects show narrow (∼1000 km s-1) emission in He II γ1640. No cases of such emission are found among radio-quiet objects. There is evidence that this spectral feature arises in extended ionized halos, which may thus be associated with the objects' radio ejecta, cluster cooling flows, or a combination of both.