Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

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Comparison of FOS and Optical Spectra for a Large Sample of AGN

J. W. Sulentic
Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487 USA

P. Marziani
Dipartimento di Astronomia. Università di Padova, I-35122 Padova, Italy

M. Calvani
Osservatorio Astronomico, Vicolo dell'Osservatorio 5, I-35122 Padova, Italy

D. Dultzin-Hacyan
Instituto de Astronomía, UNAM, Apartado 70-264, México D. F. 04510, México

M. Moles
Instituto de Astrofisica de Andalucia, Apartado 3004, E-18080 Granada, Spain

Abstract:

We present first results of a comparison between CIV1549 and H emission lines in a sample of 52 active galactic nuclei. Profile parameters of these two lines provide direct clues about the geometry and kinematics of the high (HIL) and low (LIL) ionization broad line emitting regions. We find significant differences between HIL and LIL properties in radio-loud and radio-quiet sources. The observations virtually rule out the possibility of a single zone HIL/LIL emitting region in the majority (i.e., radio-quiet) of quasars.

Keywords: active galactic nuclei, quasars, emission line spectra

Introduction

The advent of HST has provided the first opportunity to compare the properties of the high and low ionization broad line emission in the same sample of AGN. The principal high excitation lines (CIV1549 and CIII]1909) are only accessible to ground-based observation in objects with redshifts greater than z2. The principal low ionization lines (Balmer series) are shifted out of the visible in such objects. Past comparisons of HIL and LIL have, therefore, involved studies of high and low redshift quasar samples.

Study of the broad emission lines provides direct clues about the kinematics and geometry of gas located 10--20 light days from the central ionizing continuum source. Comparison of the HIL and LIL in the same source provides a way to remove part of the degeneracy between structural and kinematic information contained in the line profile. Our study provides some of the first statistically meaningful clues into the motions and geometry of the broad line region.

Observations

We compare the HIL and LIL properties in the same sample of AGN for the first time. We compare FOS UV spectra of CIV with ground based optical spectra of H. These two lines are representative of the HIL and LIL respectively. They are the cleanest strong spectral features that can be used for such a study. Optical spectra were obtained at the following observatories/telescopes: 1) Kitt Peak National Observatory 2.1m; 2) European Southern Observatory 1.5m; 3) Observatorio Astronomico Nacional, San Pedro Martir, México 2.1m; 4) Calar Alto Observatory 2.2m.

We present here first results of a comparison of three emission line parameters: 1) profile centroid displacement relative to the local rest frame (narrow line [OIII]5007 used as reference); 2) profile asymmetry index; and 3) emission line luminosity. Our sample included 52 AGN (31 radio-loud and 21 radio-quiet) with FOS spectra of CIV available in the HST archive. The sample reflects the heterogeneity and biases present in the archive. It contains Seyfert I, broad line radio galaxies and quasars and is strongly biased towards radio-loud sources. The biases should be secondary as far as line profile properties are concerned because AGN are simply selected because they exhibit broad lines. The details of the broad lines are unlikely to introduce a significant bias in the various AGN selection procedures. The safest approach at this time appears to be a general comparison of all types of AGN. This is especially true if they all arise from the same fundamental mechanism.

It was necessary to make numerous special corrections to the data before the comparisons could be made. Optical corrections include removal of FeII, narrow [OIII] and narrow H emission. UV corrections include removal of UV FeII, HeII1640 and narrow CIV emission contributions. Details of the data reduction and processing can be found in Marziani et al. (1996).

Results

Our comparison focuses on the differences between radio-loud and radio-quiet AGN.

 
Figure: Comparison of CIV and H line profile: 1) displacement, 2) asymmetry and 3) luminosity. Radio-loud emitters are indicated by filled circles.

Figure 1 shows this CIV vs. H comparison in three panels: 1) line centroid displacement relative to the local rest frame in units of 10 ; 2) profile asymmetry; and 3) line luminosity. Radio-loud AGN are distinguished by filled circles in all of the plots and +/- signs correspond to red/blueshifts and asymmetries. Approximate 2 error bars are shown in the first two panels. All of the panels suggest differences between radio-loud and quiet sources.

The first panel shows line centroid displacement measured at half maximum. Results are almost identical at profile peak and 3/4 intensity. At these levels profile asymmetry has no effect on the measured line displacement. 80% of our sample shows measurable displacement in CIV and/or H. Radio-quiet CIV emitters prefer blueshifts with values as large as 4000 . The recent study of the weak-lined radio quiet quasar PG 1407+265 (McDowell et al. 1995) presents data consistent with a CIV blueshift of 10000 (although not recognized as such in that paper because of the use of MgII2798 as a reference frame). Radio-loud CIV emitters show only small displacements with no preference for red or blueshifts.

Radio-loud H shows some preference for redshifts although previous work (Sulentic 1989) has shown that large blueshifts occur in the radio-loud population (e.g., 3C227). It is clear that the radio-loud emitters show more complex line profile properties. Radio-quiet H shows only small (but significant) red and blue displacements.

The second panel of Figure 1 shows that the average asymmetry properties follow the sign of the profile displacement in the sense that radio-quiet and radio-loud sources prefer blue and red asymmetries, respectively. It is important to note that the asymmetry index as defined here is independent from the profile displacement above half maximum. The third panel shows that radio loud sources tend to have higher luminosity in both LIL and HIL lines. This may be due in part to a higher mean redshift in the loud sample. K-S tests indicate probabilities of 10, 10 and 10 that the radio-loud and quiet samples are drawn from the same population.

The first panel also suggests that the HIL and LIL in the majority (i.e., radio-quiet) of quasars show strikingly different kinematics. The situation for radio-loud AGN is much less clear.

Conclusions

We identify three components of motion in the broad line emitting region. 1) Random velocities of the individual emitting clouds. The envelope of these velocities is measured by the line profile width (usually FWHM). 2) Small (10 ) bulk displacements (both red and blueshifts) of the ensemble of broad line emitting clouds. 3) Large (up to 10000 ) bulk displacements (especially blueshifted CIV in radio-quiet and redshifted H in radio-loud sources) of the ensemble of broad line emitting clouds.

Radio-loud and quiet AGN can be distinguished on the basis of their broad emission line properties. We find strong evidence for fundamentally different geometry/kinematics in radio-loud and quiet samples. The kinematic difference between HIL and LIL in radio quiet AGN rule out single source models for the majority of AGN. We cannot rule out models for a single broad line region (giving rise to both HIL and LIL) in radio-loud sources.

Ironically, the shift/asymmetry statistics for radio-quiet AGN fit better the predictions of radiating accretion disk models. It is the radio-loud sources that have been proposed as the most likely AGN where accretion disk line emission is observed (Eracleous & Halpern 1994). The shift/asymmetry statistics for radio-quiet sources suggest that CIV arises in a wind or outflow with rather large opening angle that can only be seen on the near side of the central structure. The onset of radio activity appears to disrupt the well defined kinematic relationship between the HIL and LIL emitting clouds. The alternative interpretation would be that radio-loud AGN have fundamentally different BLR structure than the radio-quiet majority of quasars.

References:

Eracleous M. & Halpern, J. 1994, ApJS, 90, 1.

Marziani, P., Sulentic, J. W., Dultzin-Hacyan, D., Calvani, M & Moles, M. 1996, ApJS, in press.

McDowell, J. et al. 1995, ApJ, 450, 585.

Sulentic, J. W. 1989, ApJ, 343, 54.