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 less than or equal to z less than or equal to 2.5 with the Multiple Mirror Telescope. Th e sample contains 15 radio-loud quasars (RLQs) and 17 radio-quiet quasars ( RQQs). We have measured emission line properties from the rest-frame wavele ngth range of approximately lambda lambda 4500-5500 by fitting the data wit h composite model spectra. Our analysis includes comparison of RLQs versus RQQs, as well as comparison between the broad-absorption-line quasar (BALQS O) and non-broad-absorption-line quasar (nonBALQSO) subsets of the RQQ samp le. In addition, we calculated the complete correlation matrix of the measu red properties. We combined our high-redshift sample with the sample of 87 low-redshift quasars from Boroson & Green to determine the luminosity and r edshift dependences of the measured emission properties. Our main results are the following: (1) The RLQ sample has significantly (a t more than 97.2% confidence) stronger [O III] lambda 5007 emission than th e RQQ sample, which favors scenarios including two populations of quasars t hat 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 a t half-maximum) H beta broad component line profiles than the RQQ sample. ( 3) At the sensitivity of our observations, there are no statistically signi ficant (>95%) differences between the rest-frame optical emission line prop erties of the BALQSO and nonBALQSO subsamples. This result is consistent wi th the view that all RQQs have broad-absorption-line clouds with a small(si milar to 10%-20%) covering factor and that differences between the two type s are merely a function of viewing angle and covering factor. (4) The signi ficant [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 a t low redshifts and luminosities. We also find significant relationships be tween (i) the [O III] emission strength and the radio power, the broad-emis sion-line widths, and the X-ray continuum shape; (ii) positive correlations relating the strength of optical Fe II emission to broad-emission-line wid ths and the shape of the ionizing continuum; and (iii) similar relations fo r the strength and width of the H beta 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 O < z < 2.5, such that emission line widths increase with increasing luminosity. We confirm a similar dependence for the H beta broad-line width. These findings may be evidence for a phys ical connection between the continuum and line-emitting regions at similar energies. Furthermore, we find a "Baldwin Effect" for the [O III] lambda 50 07 line in the RQQ-only sample over this same range in redshifts.