High-resolution spectral scans of several QSOs and Seyfert galaxies show broad Balmer lines and permitted Fe II lines in emission. It is suggested that a careful comparison of the Fe II and Balmer profiles may distinguish between the different proposed mechanisms for broadening of the lines. In particular, if the Fe II profile is much narrower that the hydrogen profile, the suprathermal particle mechanism is most likely presented

Ptak, R.

Stoner, R.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19760015037 2020-03-22T15:15:21+00:00Z
by
Roger Ptak
-and Ronald Stoner
Department of Physics
Bowling Green State University
Bowling Green, Ohio 43403
Received
APR 1976,
RECEIVED
NASA STI F ITY
INPUT g^NCy
jv ny ^;^.h-a
(NASA-CF.-146816) ON THE INTEEPRETATICN OF
	 N76-22125
FE 2 EMISSION PROFILES IN SEYFERT GALAXIES
AND QSOs (Bowling Green State Univ.) 9 p HC
$3.50	 CSCL 03B	 Onclas
G3/90 25428
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ABSTRACT
Recent high-resolution spectral scans of several ()SOs and
Seyfert galaxies show broad Balmer lines and permitted FeII lines
in emission. We suggest that a careful comparison of the FeII and
Balmer profiles may distinguish between the different proposed
mechanisms for broadening of the lines. In particular, if the FeII
profile is much narrower than the hydrogen profile, the suprathermal
particle mechanism is most likely present.
Permitted FeII emission lines have recently been identified
in two QSQs by Baldwin (1975), they seem to be definitely present
in spectra published by Shlectman and MacAlpine (1975) for the
Seyfert galaxy NGC 7469, and in two other Seyfert spectra reported
by Shields and oke (1975). osterbrock (1975) reports that most
Seyfert galaxies of type I show these permitted FeII lines in
-emission. The purpose of this letter is to suggest that a careful
comparison of the profiles of the observed FeII emission lines
with the profiles of Balmer emission lines could provide a critical
test of theoretical models for the origin of the broad emission
lines in these objects.
Three mechanisms for the production of broad permitted lines
have been proposed, with varying degrees of success in fitting
observed profiles: 1) electron scattering, 2) mass motions,
3) emission from suprathermal particles. If the permitted iron
emission arises in the same physical location as the broad hydrogen
emission, then any combinations of the first two mechanisms should
produce nearly identical profiles for FeII and Balmer lines. But
4f the Felt emission comes from a gas excited by suprathermal
hydrogen which is responsible for the broad Balmer lines, then the
profiles should be different.
It is important to determine whether the FeII lines are as
broad as the Balmer lines or substantially narrower. Rough visual
estimates from the published spectra, especially those obtained by
Baldwin (1975), indicate to us that the permitted FeII lines are
substantially narrower than the Balmer lines. If this visual
estimate is substantiate) by detailed analysis of the spectra,
REPRODUCIBILITY OF THE
3
_	 1\
_2
then, as we discuss ' below, the suprathermal particle mechanism is
the most viable, of the three mentioned.
The suprathermal particle model is described elsewhere (Stoner,
Ptak and Ellis 1974 and Ptak and Stoner 1975); in this model, the
broad permitted lines observed in type I Seyfert galaxies and QSOs
represent emission from suprathermal atoms and ions interacting
with a dense ambient gas of low ionization. The Balmer emission
from the atoms of the ambient gas is largely self-absorbed, and
the narrow component of the hydrogen lines as well as the narrow
forbidden lines arise from a separate, lower density region. Narrow
permitted lines of other species could be produced in either of
these regions. However, since forbidden lines of FeII are not
seen, the FeII emission must originate from a gas of fairly high
density, with Ne > 10 7cm-3 ._ If the FeII lines were produced by
suprathermal Fe ions, they would be slightly broader than the
Balmer lines. Since this does not appear to be the case, we
conclude that the FeII emission is produced by the ambient gas
with which the suprathermal particles interact.
There is a range of physical conditions of the ambient gas
which is compatible with the required flux of suprathermals. Such
conditions have been calculated by Ptak and Stoner (1975), by
Kimmer (1976) and by Stoner and Ptak (1975). Typical values for
the electron density and temperature are Ne = 10 cm -3 and T = 10 4o K.
If we assume these values for the ambient gas, we can easily cal-
culate the abundance ratios for the various ionic species of iron.
In equilibrium, there must be a balance between the ionization rate
3_	 for a given ionic species and the recombination rate to form that
ion. Applying this balance to neutral iron, we have:
tV Lr C ; Pte 
_T12 - N ^r C J Eve ul ,	 % L;
where Weo) and N(Fe+) are the number densities of the two ionic
stages, Ne is the electron number density, Y12 is the ionization
rate, and al is the recombination rate. Simple approximate for-
mulas for the required rates are given by Allen (1973), and using
there, we obtain from equation (1):
N (F 
o
— ) ~ 4.7T5/4exp - 91290	 (2)
N (Fe )
where T is the electron temperature. So for T = 104oK,
N (Fe+) 
= 50
N (Feo)
Similarly, for the Fe+ balance, we have
N (Fe+ ) Ne Y23 r N (Fe++ ) Ne a 2	 (3)
Again, using the formulas given by Allen, we find that
N(Fe
++ 
= 2T-5/4exo 1.87 T
x 105
	 (4)
We )
For T = 104oK, this yields
rz
4
N (Fe±)
	 2600
N (Fe ++
Thus the self-consistent physical conditions obtained from the
suprathermal particle model imply that essentially all of the iron
in the ambient gas should be singly ionized.
The presence of FeII lines significantly narrower than Balmer
lines, then, is compatible with a picture in which the permitted
FeII lines arise from singly ionized iron in the dense, fairly
neutral ambient gas. These lines may be produced via the fluorescence
mechanism suggested by trampler and oke (1967) and discussed by
several others. Since the upper levels for these lines are about
5 eV above the ground level, the collisional excitation rate at
T 104oK may be more than 104 times greater than the ionization
rate for Fe+ , which is also the recombination rate. At high enough
electron density, collisional excitation of the FeII multiplets
may be sufficient to replace the fluorescence mechanism as the
origin of the iron emission.
If the FeII lines are significantly narrower than the Balmer
widths, then this can be explained by mass motions in photo-
ionization models only by postulating two high-density regions:
a low-velocity, Fe± rich region and a high-velocity, Fe± poor
region. According to the calculations of MacAlpine (1972), the
FeII emitting region is located outside the H+ zone where the
free electrons come primarily from ionization of helium. If this
is the case, it is difficult to see how high-velocity Balmer-line-
emitting clouds with a small filling factor can shield the low-
velocity clouds which emit the FeII lines and keep them from being
T' T"
6
U1.
5highly ionized. If these objects are characterized by narrow per-
mitted FeII lines and broad Balmer lines, then the photo-ionization
models must be modified in some presently unforeseen way to satisfy
the constraints imposed by the line profiles and by the relative
line intensities.
All of this depends on the widths of the FeII lines; it appears
to us that the observed FeII lines are significantly narrower than
the Balmer lines in these objects. Shields and Oke (1975),
however, call the FeII emission "broad". Estimates of the observed
widths are hampered by blending of lines, both from the same
multiplet and from different multiplets. The spectrum of NGC 7469
published by Shectman and Mac Alpine (1975) does not seem well
enough resolved for a definitive statement regarding the FeII line
widths. Those spectra which Osterbrock and his group have obtained
for type I Seyfert galaxies are not yet published. For the QSOs
3C273 and ?KS 0736+01, Baldwin (1975) obtained spectra which
apparently show many FeII lines, most of them blended into.broad
bands. However, these spectra contain some FeII lines, notably two
from multiplets (27) and (28), which Baldwin says are unblended.
Since thes ,.y lines are about 60 R apart, their half widths are less
than about 20 R, which is substantially less than the Balmer line
widths.
Since the determination of the profiles of individual FeII
emission lines seems to be a crucial observational matter, it is
very important to have high-resolution scans of the spectra of
these objects, especially in those regions in which there is little
f	 1
6blending. Perhaps the spectra already available, if carefully
analyzed, are sufficient. if not, suitable spectra should soon be
available.
In subsequent work, we hope to make a detailed comparison of
Balmer and FeII line profiles calculated in the context of the
suprathermal particle model and the observed profiles. Since in
the picture described here, the motion of the ambient gas, combined
with any electron scattering contributions, determines the FeII
profiles, the broad wings of the Balmer lines should be obtainable
by folding the theoretical suprathermal profiles from a gas at
rest (Ptak and Stoner, 1973) with the observed FeII profiles.
The :authors gratefully acknowledge useful communicat-ions with
Dr. cordon MacAlpine. This work has been supported by NASA through
grant number NSG 7022.
REFERENCES
Allen, C. W. 1973, Astrophysical Quantities (London: Athlone Press).
Baldwin, J. A. 1975, Ap. J. (Letters), 196, L91.
Kim*mer, E. 1976, to appear in Ap. J. .
MacAlpine, G. M. 1975, Private Communication.
Osterbrock, D. E. 1975, Bull. A. A. S., 7, 516.
Ptak, R. and Stoner, R. 1973, Av. J., 185, 121.
Ptak, R. and Stoner, R. 1975, Ap. J., 200, 558.
Shectman, S. A. and MacAlpine, G. M. 1975, Ap. J. (Letters), 199, L85.
Shields, G. A. and Oke, J. B. 1975, preprint.
Stoner, R. E., Ptak, R. and Ellis, D. 1974, Ap. J., 191, 291.
Wampler, E. J. and Oke, J. B. 1967, Ap. J., 148, 695.


On the interpretation of FE 2 emission profiles in Seyfert galaxies and QSOs

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