)Figure 5.5 shows the histograms of the velocity at which the Pa Beta and Br Gamma line peaks occur.
Figure 5.5: Histogram of the
velocities where the Pa Beta
and Br Gamma
line peaks occur. The Pa Beta
data is
displayed in the upper panel and the Br Gamma
data on the lower panel.
The most important feature is that the distributions are clearly asymmetric relative to zero. The line peaks are, in general, slightly blueshifted and very rarely redshifted. It should be stressed that the blueshift is in most cases very slight, rarely exceeding 60 km/s for Pa Beta and 90 km/s for Br Gamma. The Br Gamma distribution has, like the one for Pa Beta, a strong peak in the bin nearest to the rest velocity, on the blue side, but it also shows a large number of lines with typical peaks near 70 km/s. The Pa Beta lines show velocity displacements as high as these in far less cases.
Since most of the line profiles are blueshifted by less than 50 km/s, the asymmetry observed in the extension of the line wings, mentioned in the previous section, cannot be due to symmetric blueshifted lines.
The slight blueshift of the hydrogen lines is a feature difficult to explain in models for which the line emission arises in outflowing material. On the contrary, models for which emission in the hydrogen lines has its origin in accreting material tend to produce slightly blueshifted lines [Hartmann et al. 1994]. In Section 5.4 this will be further discussed.