A Statistical Study of Rest-Frame Optical Emission Properties in Luminous Quasars at 2.0⩽z⩽2.5*

We have obtained H-band spectra of 32 luminous quasars at 2.0≤z≤2.5 with the Multiple Mirror Telescope. The sample contains 15 radio-loud quasars (RLQs) and 17 radio-quiet quasars (RQQs). We have measured emission line properties from the rest-frame wavelength range of approximately λλ4500-5500 by fitting the data with composite model spectra. Our analysis includes comparison of RLQs versus RQQs, as well as comparison between the broad-absorption-line quasar (BALQSO) and non-broad-absorption-line quasar (nonBALQSO) subsets of the RQQ sample. In addition, we calculated the complete correlation matrix of the measured properties. We combined our high-redshift sample with the sample of 87 low-redshift quasars from Boroson & Green to determine the luminosity and redshift dependences of the measured emission properties. Our main results are the following: (1) The RLQ sample has significantly (at more than 97.2% confidence) stronger [O III] λ5007 emission than the RQQ sample, which favors scenarios including two populations of quasars that are intrinsically different. We are not aware of a unified model based upon orientation that can explain enhanced [O III] emission with increased radio power. (2) The RLQ sample has significantly narrower (in full width at half-maximum) Hβ broad component line profiles than the RQQ sample. (3) At the sensitivity of our observations, there are no statistically significant (>95%) differences between the rest-frame optical emission line properties of the BALQSO and nonBALQSO subsamples. This result is consistent with the view that all RQQs have broad-absorption-line clouds with a small (~10%-20%) covering factor and that differences between the two types are merely a function of viewing angle and covering factor. (4) The significant [O III]-Fe II anticorrelation found in lower redshift quasars holds at this higher redshift range; however, it is the [O III] emission in this relationship that appears to be related to the physical distinction between the RLQ and RQQ classes instead of the Fe II emission that distinguishes at low redshifts and luminosities. We also find significant relationships between (i) the [O III] emission strength and the radio power, the broad-emission-line widths, and the X-ray continuum shape; (ii) positive correlations relating the strength of optical Fe II emission to broad-emission-line widths and the shape of the ionizing continuum; and (iii) similar relations for the strength and width of the Hβ emission. Many of these correlations have been found in lower redshift and luminosity studies. (5) We report a previously unknown luminosity and/or redshift dependence of the narrow-line region velocity width over the range 0<z<2.5, such that emission line widths increase with increasing luminosity. We confirm a similar dependence for the Hβ broad-line width. These findings may be evidence for a physical connection between the continuum and line-emitting regions at similar energies. Furthermore, we find a "Baldwin Effect" for the [O III] λ5007 line in the RQQ-only sample over this same range in redshifts.