Repeated observations of the Seyfert 1 galaxies I Zw 1 and II Zw 136, which have very strong Fe II emission lines in the optical region, were made at low resolution with the IUE Satellite. The ultraviolet spectra are very similar: both are variable and show broad emission features of Fe II (especially the UV multiplets 1, 33, 60, 62, and 63) as well as the emission lines usually strong in Seyferts and quasars. The data strongly support the hypothesis that the optical Fe II emission lines are primarily due to collisional excitation and that resonance fluorescence makes only a minor contribution to the excitation of these lines

Boksenberg, A.

Fosbury, R. A. E.

Haskell, J. D. J.

Penston, M. V.

Snijders, M. A. J.

English

NASA Technical Reports Server

IUE OBSERVATIONS OF Fe II GALAXIES
M.V. Penston
Astronomy Division,ESTEC,
_illafranca Satellite Tracking Station,
European Space Agency,
Apartado 54065, Madrid, Spain
M.A.J. Snijders . A. Boksenberg , J.D.J. Haskell
Department of Physics & Astronomy,
University College London ,
Gower Street, London WCIE 6BT
R.A.E. Fosbury
Royal Greenwich Observatory,
Herstmonceux Castle,
Hailsham, Sussex
ABSTRACT
Repeated observations of the Seyfert i galaxies I Zw I and II Zw 136,
which have very strong Fe II emission lines in the optical region, were made
at low resolution with the IUE Satellite. The ultraviolet spectra are very
similar: both are variable and show broad emission features of Fe II (espe-
cially the UV multiplets I, 33, 60, 62 and 63) as well as the emission lines
usually strong in Seyferts and quasars e.g: Lye, Mg II, C 1111, C IV and N V.
The data strongly support the hypothesis that the optical Fe II emission lines
are primarily due to collisional excitation and that resonance fluorescence
makes only a minor contribution to the excitation of these lines.
INTRODUCTION
Fe II emission lines in the optical spectra of quasars and Seyfert gal-
axies were first discovered in 3C273 (ref. i) and I Zw I (ref. 2) and are
actually present in many objects (refs. 3, 4). Usually these lines are
thought to be excited by either resonance fluorescence or collisional excita-
tion (e.g. refs. I, 4, 9). Both mechanisms involve excitation of Fe+ atoms
from 4 even-parity levels, the ground state and 3 low-lying metastable levels,
to 6 odd levels at 5 to 6 eV, and subsequent cascade to even metastable levels
around 3 eV. In Fig. I we show an Fe+ energy level diagram with the strongest
observed optical and ultraviolet (this paper) multiplets indicated.
The excitation can be through either collisional excitation by thermal
electrons or absorption of UV continuum photons in the UV resonance lines,
but in both cases large optical depths in the resonance lines are required
for an efficient conversion of ultraviolet to visual photons. For the case
of collisional excitation we would expect both the optical and the ultraviolet
lines to be in emission. For resonance fluorescence the optical lines are in
757
https://ntrs.nasa.gov/search.jsp?R=19810017442 2020-03-21T12:06:17+00:00Z
emission but the ultraviolet lines could be either in absorption or emission
depending on the geometrical situation. In principle, there are amply suf-
ficient ultraviolet continuum photons available for the fluorescence mechan-
ism; however the Fe II lines are formed in the broad line region which may
cover only a small part of the continuum source and the absorption lines th_
must be very broad in order to intercept sufficient ultraviolet photons, re-
quiring uncomfortably large turbulent velocities (ref. 5). Using better
atomic data and more extensive model atoms it has been recently shown (refs.
5, 8) that the collisional theory can explain the observed optical Fe II lines.
However attempts to observe the predicted Fe II ultraviolet emission lines
in Seyfert nuclei have so far only resulted in upper limits to either absorp-.
tion or emission lines (refs. i0, 12). In quasars, older work also resulted
only in upper limits (e.g. ref. 13) but recently better data for several
intermediate redshift objects show evidence for Fe II emission in the ultra-
violet in the form of broad, low-contrast emission features (refs. 14, 15).
In this paper we present ultraviolet observations for two Seyfert gal-
axies, with very strong optical Fe II emission lines, which have been exten-
sively studied in the optical. Optical studies of I Zw 1 show two redshift
systems at Z = 0.0608 for low ionization lines, (ref. 16). The broad line
component has relatively narrow features which greatly facilitate the study
of the Fe II emission lines (refs. 2, 3-6, 16, 17). II Zw 136 has broader
lines, FWHM about 2000 km sec -I for I Zw I, and a shallower spectral slope,
e = 0.49 compared with 1.33 for 1Zw 1 (ref. 3), but in general the optical
spectra of these two objects are similar (ref. 3, 4, 6). By studying these
three objects simultaneously we might be able to obtain insight into the
vexing problem of why some extragalactic objects show strong Fe II emission
and others virtually none (refs. 4, 18). It has been suggested that Fe II
emission line objects have a higher density and that the C III] 1908/C IV
1549 ratio might be lower than is normal (ref. 5). However, the available
material on differences between line ratios for objects with and without
Fe II optical emission is very uncertain and the separation for example of
optical depth and density effects is problematical (ref. 5).
In section 2 we discuss the observations and data reduction, in section
3 we present the results for I Zw 1 and II Zw 136 and in section 4 we discuss
and summarize the conclusions.
OBSERVATIONS AND DATA REDUCTION
All spectra were obtained using the IUE satellite in the low resolution
mode and with the large apertures. A summary of the observations is given
in Table I. For both I Zw 1 and II Zw 136 the exposure times were determined
by the strength of the strongest emission lines, Lye and Mg II, and in the
SWP images the continuum is rather faint. The last 2 SWP spectra of I Zw i
were obtained near the end of a shift and these exposures had to be curtailed,
so both are underexposed.
758
The SWP images were originally processed with the faulty ITF, but after
subsequent correction of the ITF errors in the GPHOT image (ref. 19) we re-
extracted the spectra directly from GPHOT image using improved software (ref.
19). For the LWR spectra we only reprocessed the background in order to
remove particle events and image defects. An improved net spectrum was then
obtained from the standard gross spectrum and the new background signal.
Only a relative wavelength scale is given, as the position of the source
in the large aperture is unknown. Zero points for the absolute wavelength
scales were obtained from the Lye and Mg II emission lines in the ease of
I Zw i and II Zw 136.
Emission line strengths given in this preliminary report on these data
are based on estimated continua and simply integrating the observed flux
between suitable wavelength limits. The final results (ref. 20) will be based
on profile fitting techniques where possible. For I Zw I part of the profile
fitting has been done and we can accurately separate complex features like
the Si II, Si III, Lye, N V blend over 1180 < _ < 1250, and estimate the
strength of the individual components.
THE UV SPECTRA OF I ZW 1 AND II ZW 136
In Fig. 2 the mean long wavelength spectra of I Zw i and II Zw 136 are
shown. For I Zw I rest wavelengths were calculated assuming %o = %/(i+0.0608),
appropriate for the low ionization lines like Fe II and Mg II (ref. 7).
Comparison of the near UV spectra of Fe II Seyfert galaxies as shown in
Fig. 2 with those of other Seyferts and quasars (ref. 10-12) shows two
remarkable differences.
i) The Si 1113 _o 1892 intercombination line is greatly increased in
strength with respect to C III] 1o 1908. Normally Si III]/C III] is 0.i to
0.2, in I Zw i $i III]/CIIIJ = 0.5. Clavel (private communication) has
noticed the same effect in another Seyfert galaxy with strong optical and
ultraviolet Fe II emission lines.
ii) The strong "blue wing" of the Mg II line previously noticed in some
medium redshift quasars (ref. 21) is also present in I Zw I and II Zw 136 but
here clearly is not due to Mg II alone. In particular, for I Zw i, profile
fitting shows that most of the "Mg II line wing" cannot be due to Mg II at
all and that Fe II UV.multiplets 62 and 63 provide a good identification.
Comparison between I Zw 1 and II Zw 136 shows that the Fe II multiplets
are clearly stronger in I Zw I, as in the optical region. Only 3 multiplets
UV 33, UV 62 and UV 63 can be measured easily in both objects. Multiplets
_N 2, 3, 34, 35 and 36 are rather weak and blended with C II] _ _ 2326,
e IV] %o _ 2422 and [Ooll] _o 2470 (the Ne IV lines _fall virtually on top
of the fiducial at 2580 A but inspection of the original data shows that a
substantial emission line is present in both objects at %o _ 2422 . The red
759
wing of multiplet UV 1 coincides with possible galactic Mg II k _ 2798,
absorption, and in II Zw 136 the blue half of the UV 1 emission feature is
absent. The latter could be a result of absorption in the object in the two
resonance lines which arise from the ground state, %% 2585 and 2600. From
theoretical calculations including both fluorescence and collisional processes
it appears that even in situations where collisional excitation is the domi-
nant process, in forming the Fe II lines, some resonance fluorescence should
take place (ref. 5). Some absorption could also be present in UV 2 and UV 3
and contribute to the apparent weakness of these multiplets,
In table 2 we list the observed emission line strength for both objects.
In general, the emission lines do not show evidence for variability and only
mean values are given; a possible exception are the weak, far UV lines of
II Zw 136 and for these we give values at the two epochs. In December 1978
the weak lines in the SWP region appear to be substantially stronger than in
May 1979. However the strong lines Lye, N V and C IV do not show this effect
and the "variability" could be due to errors in the estimated continuum level,
which is difficult to determine in all these spectra. Even in the long wave-
length spectra the results for the weak broad Fe II emission blends are
sensitive to small errors in the adopted continuum. In particular, the
results for UV 60 and UV 61, which in the observer's frame fall longward of
3000 X, where the IUE sensitivity decreases rapidly, are of very low accuracy.
At wavelengths lo < 2100 A there exist additional as yet unidentified broad
features. The C III], Si III] blend has very extended wings which could be
due to Fe III emission, and the 0 IV], Si IV feature, % _ 1400, has extra
emission in both the red and blue wings which appears present on all spectra.
Finally, there are extensive weak emission blends for 1560 _ < %a _< 1800 A
which cannot be explained by the presence of the He II %1640, O II_ %1666
and N lid _ %1750 lines. These could be due to Fe II emission from levels
above 7 eV, which are strong in solar spectra, RR Tel and Nova Cygni 1978
(ref. 22-24).
We include in table 2 the total optical Fe II emission line flux and the
strength of H_. For the strongest optical Fe II lines there is good agreement:
between the measurements of different observers (refs. 3, 4, 16, 17) but for
the total emission line strength the weaker features have to be included and
here substantial differences do exist. For I Zw I we used only the most
recent results (ref. 17); comparison between various H_ measurements (refs. 6,
17) shows that the older data (ref. 6) substantially overestimated the H8
line strength. For II Zw 136 the H8 flux (ref. 6) could also be up to 30% too
high. Comparison between recent measurements of Fe II lines in both objects
indicates that the most extensive set of data (ref. 6) probably still under-
estimates the total Fe II emission. After comparing all available data we
estimate
F (Fe II, optical)_ 10.3 × 10"13 erg cm-2 s-I
for II Zw 136. The optical multlplets I, 6 and 7, which arise from the meta-
stable a_P level, have been excluded from the total optical Fe II emission
strength. Their role is probably comparable to the Fe II ultraviolet multi-
760
plets and they have been added to the total Fe II UV flux. Combining all
Fe II UV data and applying approximate corrections for blends, e.g. [O11]
1o 2470, or multiplets not measured (UV 60 and UV 61 for II Zw 136), we obtain
for the ratio of optical to ultraviolet Fe II photons: 2.8 for I Zw 1 and
2.1 for II Zw 136. The line strengths for II Zw 136 are in rough agreement
with recent collisional excitation models (ref. 5), which predict comparable
strength for HS, C III], Mg II and the ultraviolet Fe II total line strength,
but with at least a factor 2 uncertainty. The highest conversion efficiency
of ultraviolet to optical photons is obtained in I Zw i and in this object
the Fe II lines appear to be considerably stronger than so far predicted;
especially the optical lines. These conversion efficiencies are only rough
estimates and for individual levels th_ conversion efficiency can differ
substantially; for example for level z po the optical to UV photon ratio is
3.2 ±_'_ where the errors are based on the uncertainties in the UV flux alone.
Very high column densities are indicated by these ultravioletto optical
photon ratios (ref. 9).
Towards shorter wavelengths the continuum variability increases. The
December 1978 spectrum of II Zw 136 is shown in Fig. 3 and the 2 mean spectra
of I Zw 1 in Fig. 4. Just as in the optical, I Zw I has a steeper spectral
slope than II Zw 136. The C IV lines in I Zw 1 are very weak but as C III],
0 IV] + Si IV and N V are normal this cannot be explained by simply a lack
of ionizing UV photons. There is no evidence for an intrinsic 12200 extinction
feature in either of the two Seyfert galaxies, but a weak galactic extinction
feature could be present in I Zw 1 (E(B-V) < 0.03).
SUMMARY
We have observed 2 Seyfert i galaxies which emit strong Fe II emission
lines in the optical region. They have similar UV spectra, and only differ
from normal Seyferts and QSOs (refs. 10-12, 25) in the presence ofeasily
noticable Fe II UV emission lines. In both objects the conversion of UV to
optical Fe II photons seems to be comparable: for every detected UV photon
there are i to 3 optical photons, which implies very high optical depth in the
UV lines (refs. 5, 8, 9). The present data appear in good agreement with
theoretical predictions based on the collisional excitation theory(refs. 5,
8, 9). The apparent absence of UV Fe II emission in earlier work is possibly
due to the nature of the UV emission lines, consisting of broad blends of low
contrast against the continuum which can cover the whole region between
12300 and _2650 (Fig. i and ref. 14) and the region _ > 2700 _. Some expected
UV Fe II multiplets, e.g. UV 64 a _D to z_PO, are not discovered so far; these
have rest wavelengths around _3000 and the typical observed wavelengths at
_3200 in the minimum efficiency zone between the IUE and optical domains.
Again this points to the need for better observations and highlights the
inherent problems in measuring these often weak, broad features• So far only
one obvious difference between Fe II and normal Seyfert galaxies has been
discovered: the Si III] % 1892 line appears much stronger than is normal. I£
has been stressed that theoretical predictions for Fe II lines should give
761
self-consistent results for other strong lines, and usually Lye, H_, C III_,
C IV and Mg II are mentioned in this respect (refs. 548). The present
results suggest that Si IIl] should be included and t_et a check on Fe III,
which has many strong lines in this wavelength region (ref. 26) in early typ_
stars, could be useful.
ACKNOWLEDGEMENTS :
Two of us M.A.J.S. and A.B. wish to acknowledge financial support from
the S.R.C.. It is a pleasure to thank the resident astronomers at VILSPA
ground station for help in acquiring the data,
REFERENCES
i. Wampler, E.J. and Oke, J.B., 1967, _., 148, 695.
2. Sargent, W.L.W., 1968, Ap. J. Left, 152, L 31.
3. Philips, M.M., 1977, AP" J" Suppl, 38, 187.
4. Osterbrock, D.E., 1977, Ap. J., 215, 733.
5. Netzer, H., 1980, Ap. J., 236, 406.
6. Oke, J.B. and Shields, G.A., 1976, A__ J., 207, 713.
7. Philips, M.M. 1978, A____., 226, 736.
8. Collin-Souffrin, S., Joly, M., Heidmann, N. and Dumont, S., 1979, Astr. &
Ap., 72, 293.
9. C. Jordan, 1979, Progress in Atomic Spectroscopy, part B, 1453.
i0. Boksenberg, et al., 1978, Nature, 275, 404.
II. Oke, J.B. and Zimmerman, B, 1979, Ap.. J. Left., 231, LI3.
12. Ulrich et al., 1980, M.N.R.A.S., in press.
13. Baldwin, J.A. and Netzer, H., 1978, _., 226, I.
14. Wills, B.J., Netzer, H., Uomoto, A.K., and Wills, D., 1980, preprint.
15. Philips, M.bl., 1980, preprint.
16. Philips, M.M., 1976, Ap. J., 208, 37.
17. Oke, J.B. and Lauer, T.R., 1979, A.__u__ , 230, 360.
762
18. Dovidson, K. and Netzer, H., 1979, Re____v.Modern, Phys. 51, 715.
19. Snijders, M.A.J., 1980, SRC IUE Newsletter N° 5
20. Snijders, M.A.J., Boksenberg, A., Haskell, J.D.J., Penston, M.V. and
Fosbury, R.A.E., 1980, to be submitted to M.N.R.A.S.
21. Grandi, S.A. and Philips, M.M., 1979, Ap. J., 232, 659.
22. Doschek, G.A., Feldman, U., Van Hoosier, M.E. and Bartoe, J.-D.F., 1976,
Ap. J. Suppl., 31, 417.
23. Penston, M.V., et al., 1980, in preparation.
24. Stiekland, D.J., Penn, C.J., Seaton, M.J., Snijders, M.A.J., Storey, P.J.,
and Kitchin, C.R., 1979, "The First Year of IUE", ed. A.J. Willis, p. 63.
25. Green, R.F., Pier, J.R., Schmidt, M., Estabrook, F.B., Lane, A.L., and
Wahlquist, H.D., 1980, Ap. J., in press.
26. Viotti, R., 1976, M.N.R.A.S., 177, 617.
TABLE 1
OBJECT DATE IUE EXPOSURE REMARKS
IMAGE NR. TIME (SEC)
I ZW I 2.08.78 LWR 1955 210
I ZW 1 5.08.78 SWP 2216 200
I ZW I 18.08.78 SWP 2333 155
II ZW 136 18.12.78 SWP 3637 120
II ZW 136 27.05.79 LWR 4610 180 -
II ZW 136 29.05.79 LWR 4628 180
II ZW 136 29.05.79 SWP 5389 i00
I ZW I 31.05.79 SWP 5411 60 underexposed
I ZW 1 2.06.79 LWR 4673 180
I ZW i 2.06.79 SWP 5427 60 underexposed
763
TABLE 2 Emission Line Intensities
llne _o F(lO -13 erg cm-2s-l_ -I)
Z ZW 1 IZ Z;; 136
HB 4861 3.9 ± 0.3 (I) 5.6 + 0.5 (2)
Fe IT optical 26.3 (I) 10? (3)
8.7 (2)
Dec 78 May 79
Fe I1 UV 60 2.4 + 1.2
T 290 7.6 (4) I.I + 0.5 present
Fe Ii UV 61 1.4 ± 0.8
0 lII 2837 0.8 + 0.4
Mg 11 2798 6.0 + 1.5 4.2 ± 0.6
¥e II UV 62 I 3.2 t 1.6 1.o _ 0.4
UV 63 J
Fe I7 UV I 1.9 + t,O 0,9 _t 0,6 (5)
Fe I1 UV 33 1,6 ± 0,9 1,1 .t 0,3
_e II- UV 34 ] 1.2 ± 0.5 (6) 1.5 + 0.6 (6)
II] 2470 I
[Ne IV] present present
Fe TI UV 2
UV _ <3.1 ± 1.7 2.4 _ 1.0
Uv 35
UV 36
C I1] 2326
Fe II UV 6 1.0 ± 0.5 (7) (7)
C ITI 3 1908 (8)l 6.0 + 1.2 6.6 ± 0.7
Sl Ill] 1892 J
OZV 1549 ] 4.5 t 1.5 12.1 ± 2.0 13.5 + 2.5
Si _II 1531 J
OIV] 1402 ] 4,0 ± 1,0 4,8 + 1,0 2,9"_ 0,8
Si IV 1393 J
C 11 1335 0.3 ± 0.2 1.1 _ 0.4 0.30 ± O.15
0 _ 1304 0.7" :t 0.3 3.7 :t 1.2 1.2 :t 0.4
SJ. 1I 1263 1,5 • 1,0 1,1 _ 0,4 1,5 :t 0,3
NV 1240 6.0 ± 1.5 (9) 4.9 ± 2.0 3.4 * 1.5
Lya 1216 16.O -+ 4.0 (9) ] 32.1 ± 3.0 31.4 _ 3.0
Si II1 1206 3.1 ± 1.0 (9) /
Sl II 1192 1.5 ± 0.5 (9) J
notes to table 2:
(I) re±. 22, (2) re±. 7, (3) see text, (4) calculated rest wavelcngtb:co_parislon
between I ZN i and II ZW 136 suggests that this is a Io_ ionization feature (5) see text,
(6) [011] _o 2470 could contribute up to 307.of the total strength, (7) the reality of
multiplet 6 of re II is questionable, (8) could include Fe II_ emission, (9) this
four features have been separated u#ng i_c£11e fitting t_ethods.
764
_-I I I I f I I i [ [ ! I I -
• -Z
/ I Z Z/! b2 N
'.' .._'_'a-_ .,',/ " j ; t |o _ _ i_ _ :,b_b observedmult{pZetsin the optical
_2_ _._ '_ :/ _ , (--) and_v<--)resions,For.
I , _/ . ,,, L_ _ _ few nultipletsm_an rest wavelength
L _t_ii._ _La (_) are sho%_nand the multipletnumbersI /? /'h _'_ for mst-transitions are g ven.
" Fe', a0 I i i'_a! I l I I l I I I I_
+S +P'60 60"_F"_P 9e _O _0" '? +'F'"G 2H
Fig. 2 Mean long wavelength spectra for l ZW1
and ll ZW136.
765
! Zwl
i2 Si-I] N-V 0-[ Si-IV C-IV He-11 N-_ 10
I I I il I I I
Ly_- Si-lI C-I] O-IVl Si-II O_
10 I I I I I
._8 6
'
, k ,4 :I 31-_11-_-,9
0-
, ! I I t I _ I _ I _ I f I I f I
1300 1500 _l,_l 1700 1900
rl$* $ _au Ihort nvele_th 81_cc_ri £c_r| Z_; I*
12 ll-Zw-136-
CzK O_I Six Hel
10 I I II l"
_.--. Si_ C'_ O_J Sin
_c III I I II
0 I t I I I ,, t I _ I
1200 1400 1600 1800 2000
1|$, A 0:_ Of tl_ t_ *holt _Jvelenlc h Ipectt*
for I! Z_ 136,
766


IUE observations of Fe 2 galaxies

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server