Comparison of MgII and CIV emission lines in NGC5548

Abstract

The Seyfert galaxy NGC5548 was monitored in the UV portion of its spectrum every 4 days for an 8 month period by an international collaboration using the IUE satellite. Using these UV spectra, a cross‐correlation analysis shows that the CIV 1550 A line correlates with the underlying continuum with a time delay of approximately 10 days. A similar analysis for the MgII 2798 A line yields a much broader peak and a delay of 50±20 days. Thus the MgII line emitting gas appears to be roughly 5 times more distant from the central source. If the motions of the line‐emitting gases are dominated by the gravity of a central mass, then we should find that the CIV line profile has a width more than twice that of the MgII line. Since the MgII line is blended with a pair of FeII lines at 2726 A and 2838 A, these two lines were removed, and the MgII and CIV profiles in this series of spectra were then compared on the same velocity scale. The two lines are quite similar on the red side of the profile, with the CIV more extended on the blue side. The overall width of the CIV is as much as 40% larger, but it is definitely not larger by a factor of two or more. Thus this evidence indicates that the motion of the broad line emitting gas is not dominated by the gravitational influence of a central massive object.