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OCR for page 91
Optical Observations of Active Galactic Nuclei
ALEXE! V. FTLIPPENKO
University of California, Berkeley
ABSTRACT
I describe the most important observed characteristics of active galac-
tic nuclei, concentrating on their optical spectra. Connections with the
"standard model" of such objects are emphasized. Some of the new ob-
se~vational developments that are leading to modifications of the standard
model are outlined; these include spatial variations of the ionization pa-
rameter in the broad-line region, the possible existence of a very dense
component in the broad-line region, and the presence of an intermediate-
density zone between the broad-line and narrow-line regions. Evidence
for "hidden" Seyfert 1 nuclei and anisotropic ionizing radiation is also
reviewed. The search for, and properties of, intrinsically weak Seyfert 1
nuclei are subsequently summarized, with special attention given to the very
low-lummosibr object in the late-type dwarf galaxy NGC 4395. I conclude
with a discussion of the possibility that low-level activity in some galactic
nuclei might actually be produced by bursts of star foundation and their
associated supernovae.
BASIC OBSERVATIONS AND THE STANDARD MODEL
The optical and ultraviolet (UV) spectra of QSOs and Seyfert 1 nuclei
are dominated by strong, broad-permitted emission lines superposed on
a featureless continuum (Osterbrock and Mathews 1986, and references
therein). These lines typically have full-widths at half maximum (FWHM) of
3,000~,000 km s~i, and full-widths near zero intensity (E;WZI) of 10,00
20,000 km sol. In addition, one often sees narrower cores (FWHM ~ 500
91
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92
AMERICAN AND SOVIET PERSPECTIVES
km sol) to the permitted lines, as well as forbidden lines of comparable
(relatively narrow) width. The forbidden lines include a wide range of
ionization states, from [0 I] to [Fe VII] or higher. In Seyfert 2 nuclei, the
broad permitted lines seem to be absent, although new data indicate that
in at least some cases they are actually present at a very low leveL
The featureless continuum of classical AGNs has a roughly power-law
shape with index ~ ~ 1 - 2 (f`, or I'm) at red through near-infrared wave-
lengths, but it generally becomes flatter (a ~ O-1) at higher frequencies
(Neugebauer et at 1979~. In the UV region, the slope is often quite flat
(a ~ 0~. There appears, in essence, to be a broad excess, or "big blue
bump," over an extrapolation of the near-infrared power laws Starlight is
an important contaminant in the spectra of low-luminosity Seyfert 1 nuclei
(MaLkan and Filippenko 1983), but their nonstellar spectra are similar to
those of QSOs.
It has long been recognized that the flux of H,B and other permitted
lines is, to first order, directly proportional to the flux density of the
optical nonstellar continuum in large samples of AGNs (Tee 1980; Shuder
1981~. Thus, the lines are probably produced by gas photoionized by
the UV extension of the featureless continuum emerging from the active
nucleus. Photoionization calculations have reinforced our belief in this
simple picture, since most of the emission-line intensity ratios can be
reproduced (Netzer and Ferland 1984~. The models depend primarily
on the ionization parameter, which is simply the ionizing photon number
density divided by the nucleon (or electron) number density at the exposed
face of a slab of gas.
The broad permitted lines are formed by gas having ne ~ 109 - 10~°
cm~3. This is deduced from the weakness or absence of broad forbidden
[0 IlI] AJ4959, 5007 emission, and by the presence of broad semi-forbidden
(intercombination) C III] A1909. Other, similar diagnostics also exist.
Photoioni7~tion models imply that Te ~ 10, 000-20, 000 K Comparable
temperatures are also found from various intensity ratios among the narrow
lines, although the derived densities are much lower (ne ~ 103 - 105 cm~3~.
with such low temperatures, the thermal widths of emission lines are
only 1~20 lan sol. Bulk motions of gas are undoubtedly responsible for
broadening the lines to the observed widths; other proposed mechanisms
(e.g., electron scattering) have serious difficulties (Davidson and Netzer
1979~.
The gas emitting the line radiation is distributed in clouds or filaments
occupying only a small fraction (10-4 - 10-7) of the volume they encompass.
This is deduced directly from the observed luminosities of the lines and their
average distances from the central source, as determined by photoionization
models. In low-luminosity AGNs, these clouds often have a covering factor
of nearly unity and absorb soft X-rays traveling along our line of sight
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HIGH-ENERGY ASTROPHYSICS
93
pelvis and Lawrence 1985), but in luminous QSOs the clouds only cover
1~10% of the sly. Supporting evidence includes the fact that a sharp
drop at 912 &, the hydrogen Lyman limit, is rarely seen in the continua of
high-redshift QSOs.
T;he calculated radial distances for clouds in the broad-line region
(BLR) are usually 0.01-1 pc, and in the narrow-line region (NLR) 1
1,000 pc. It is therefore not surprising that the BLR has never been
spatial resolved in optical images or spectra, while the NLR is often
resolved in nearby AGNs. This is also consistent with the absence of
narrow-line variability in most AGNs (Peterson 1988), whereas vanabili~
of Me broad lines over time scales of weeks, months, or years is common.
Photoioni~tion models and obsenations at X-ray energies (Halpern 1982)
show that the column density of neutral hydrogen, 1VH, is ~ 1023 ~-2
and ~ 1022 cm~2 in the BER and NLR, respectively, almost independent
of the intrinsic luminosity of the AGN.
Accretion of matter onto a central, massive black hole has long been
thought of as the ultimate source of energy emitted by QSOs and Seyfert 1
nuclei (Zel'dovich and Novikov 1964; Salpeter 1964), largely because of the
absence of other viable mechanisms. Although spherical accretion has been
considered, it is now generally believed that the gas forms an accretion disk
around the black hole. Infalling matter almost certainly has specific angular
momentum greater than that of the least stable orbit around a black hole,
and accretion disks provide efficient release of energy (up to 0.37 mc2 for a
Kerr black hole). Also, the well collimated jets spewing out of some AGNs
suggest the presence of disks (Rees 19843. Direct observational evidence
for disks is difficult to find, but many (not all!) researchers believe that
the "big blue bump" represents thermal emission from an accretion disk
(Shields 1978; Sun and MaLkan 1989, and references therein).
COMPLICATIONS TO THE STANDARI) MODEL
Despite its success, the simple picture described above is far from
complete; many problems have been pointed out over the past decade.
In particular, there remain serious difficulties in reproducing the observed
emission-line spectrum of active galaxies (Ferland and Shields 1985; Netzer
1989~. Here ~ briefly discuss several complications that must be addressed
in current and future calculations.
Spatially Variable Ionization Parameter
A single set of clouds, characterized by one value for the ionization
parameter, probably cannot be invoked to explain all the intensity ratios
of broad emission lines in AGNs (Collin-Souffrin et at 1988~. Specifically,
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94
AMERICAN AND SOVIET PERSPECTIVES
a large ionization parameter is needed lo generate highly ionized species
such as C IV and O VI, whereas great quantities of X-rays are necessary to
penetrate deep into thick clouds to produce the observed Fe II emission.
As one possible solution, Netzer (1987) suggests that the ionizing UV
radiation is emitted by a thin accretion disk whereas the X-ray continuum
comes from a hot, spherical bulge at the center of the dish Clouds near
the disk ems would have a much higher ionization parameter than those
at large angles from the ems. An additional requirement is that the cloud
covering factor must increase with increasing angle away from the disk ens.
This is reasonable if the clouds have considerable angular momentum. It
may also be the case that extra heating sources, other than the normal
photoionizing continuum, are necessary for a good fit to the data (e.g., Joly
198 7~.
Ultra-High and Intermediate Densities
Another, equally serious problem is that the broad emission lines are
observed to vary rapidly in response to changes in the luminosity of the
ionizing continuum; see Peterson (1988) for a thorough review. Based on
the sizes of the BLR calculated from the ionization parameter (inferred
from spectra), time delays of a month to a year were expected in many
Seyfert 1 gal~es. The measured delays, on the other hand, can be as
short as a week to a month. This means that the BLR is typically a
factor of three to ten smaller than had previously been thought. In the
simplest models, the gas density must be 1~100 times higher to prevent
the calculated values of the ionization parameter to deviate significantly
from those deduced with spectra. Such high densities, however, produce
over problems. The C IIT] A1909 line, for example, should be collisionally
suppressed (relative to permitted lines) at the newb derived densities of
no ~ TO]}-10~2 cnn~3e Thus, it probably arises from gas of lower densitr
(~ 109 cm-3), yet it often has nearly the same profile as the permitted
lines.
There is also evidence for multiple zones in the gas primarily responsi-
ble for the forbidden lines in AGNs. Many studies conducted over the past
few years have shown that the concept of distinct, well-separated NLRs and
BERs Is often an oversimplification. Rather, it appears that there exists
an "intermediate zone," in which ne ~ 106 - 108 cm~3 and velocities of
clouds are between those in the NLR and BLR. The evidence is the strong
correlation, found in many objects, between line width and critical density
for collisional de-excitation (Filippenko 1985; Whittle 1985; De Rober-
tis and Osterbrock 19863; see Figure 1. Forbidden lines having ne~cnt)
~ 106 - 107 cm~3 are quite broad in some objects. Moreover, detailed
analysis of the HE spectral region in Seyfert 1 galaxies indicates that broad
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HIGH-ENERGY ASTROPHYSICS
(b)
-
, 0,
E
A
~ .
I
ID
o
95
Pictor A
Widths of forbidden lines
o
-
Slope ~ 0.103 ~ 0.015
r ~ 0.91
~ 257.
.
I I
3 ~s 6
7 8
Log ne(crit) (cm~3)
FIGURE 1 The Spyfert 1 galaxy Pictor A shows a strong Correlation between the F\VHM
and the critical density of emission lines ~lippenko 19853. A very high correlation
coefficient of 0.91 is found. The data were obtained with the 2.5 m du Pant reflector at
Las Campanas Obsenratorye
[O III1 emission may exist at a low intensity level, and is probably produced
by clouds having ne ~ 107 - 108 cm~3 (Crenshaw and Peterson 1986, and
references therein). The simple two-zone models are clearly inadequate,
except for first-order approximations to the emission-line intensity ratios.
Anisotropic Ionizing Radiation
There is now considerable evidence that in some AGNs, ionizing
radiation might be preferentially beamed along certain directions. The
radio galaxy PKS 2152~9, for example, exhibits a high-excitation knot
of emission at a projected distance of 8 kpc from its nucleus (headhunter
et aL 1987~. without the presence of beamed radiation, it is difficult to
reproduce this knot's isolated nature and its very high ionization parameter.
The polarized blue continuum detected in this lmot is probably caused by
scattering of the beamed radiation (di Serego Alighieri et at 19~.
Powerful evidence for a disk-l~e (rather than spherically symmetnc)
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96
AMERICAN AND SOVIET PERSPECTIVES
geometry is found in the study of NGC 1068 done by Antonucm and Miller
(1985~. Optical spectra of NGC 1068 show it to be a bright type 2 Seyfert
galaxy, with permitted and forbidden lines having similar widths (FWHM
~ 1100 En sob. A spectrum of the poled flux alone, however, is virtually
indistinguishable from normal (total flop spectra of type 1 Seyferts and
QSOs! Broad permitted lines of hydrogen and Fe II are visible, and there
is a very blue, featureless continuum. Furthermore, Miller (1989) has
observed broad Balmer lines scattered from an ofI-nuclear H II region in
NGC 1068. The obvious interpretation of these data is that NGC 1068
actually harbors a type 1 Seyfert nucleus that is located inside an optically
and geometrically thick torus. Photons from the continuum source and the
BLR are scattered into our line of sight, and therefore polarized, by free
electrons above and below the disk. The NLR is sufficiently extended Hat
it is not obscured by the disk. It is quite possible that the thick torus feeds
into an accretion disk close to the black hole.
Pogge's (1988) discovery of a high-ioniz~tion emission-line "cone" in
NGC 1068 supports the hypothesis advanced by Antonucci and Miller
(1985~. Moreover, the luminosity of the ionizing continuum from the
nucleus of NGC 106S, deduced from the total flux spectrum and the
assumption of spherical symmetry, cannot account for the luminosity of
forbidden lines in the NLR. No problem exists, on the other hand, if the
NLR clouds have a much more direct view of the compact nucleus. Similar
conclusions regarding hidden BLRs (Miller and Goodrich 1990), high-
ionization emission-line cones (Pogge 1989), and the energetics of the NLR
Abelson et at 1988) have been made for many other Seyfert 2 galaxies; thus,
NGC 1068 is not an isolated example of this phenomenon. In addition,
radio observations (Antonucci 1984) show that the optical polarization
position angle of Seyfert 2 galaxies is almost always perpendicular to the
radio axis, as expected if they contain an opaque torus which we view i rom
the side.
LOW-LUMINOSITY ShYFERT 1 NUCLEI
Searching for Broad Ha Emission
The nonstellar continuum of QSOs Epically has an absolute blue
magnitude (MB; ~ ~ 4500 A' between-30 and-23 (i.e., LB ~ 1011-
10~4 Log, while that of Markarian Seyfert 1 nuclei lies in the range-23 to
-18 (LB ~ 109 - 1011 Log. The well-known continuity in many observed
characteristics of these objects "Ives strong evidence for the idea that their
physical properties are also similar. Observations of low-redshift QSOs
suggest that all QSOs occur in galaxies; hence, we certainly expect there
to be at least a few local Seyfert 1 nuclei with very low luminosity, if
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HIGH-ENERGY ASTROPHYSICS
97
some of these objects result from the fading of old QSOs as their fuel
suppler steadier dwindles. Continuity in the observed properties of QSOs
and Seyfert 1 nuclei further suggests the existence of such objects, but there
might be a lower limit to the activity. If so, it could be of fundamental
importance, like the lower mass limit (~ 0.08 Me,) of main-sequence stars.
The nucleus of the nearby, frequently studied spiral galaxy NGC 4051 was
for many years the least luminous known Seyfert 1 (MB ~-16; V6ron
1979~.
Owing to contamination by starlight, it is quite difficult to detect the
nonstellar blue continuum in very low-luminosity Seyfert 1 nuclei. Broad
Ha emission, on the other hand, is easier to discern. For convenience, we
will adopt the relation MB ~-2.5 log L(Hc~) ~ 84.7 (where log ~ _ logic z),
as empirically derived by Weedman (1985; Ho = 75 lan s~i Mpc-~) for
the nuclei of Markarian galaxies having broad Ho emission with luminosity
L(Ha) ergs s-l. Low-luminosibr Seyfert 1 nuclei, arbitrarily defined to be
those win MB ~ - 18 may, have a corresponding broad He luminosity
of L(Hc~) ~ 104~ ergs sol. The standard interpretation for such objects
(e.g., Filippenko 1988) is that they have rather small central black holes
(M < 106 M(S,Y, or that they are Secreting material at very sub-Eddington
rates.
Many low-luminosibr Seyfert 1 nuclei have now been found in nearby
galaxies (Stauffer 1982; Keel 1983; Filippenko and Sargent 1985~. An
excellent example is M81 (Figure 2), whose distance is about 3.3 Mpc;
weak, broad Ha emission was discovered by Peimbert and ~rres-Peimbert
(1981) and confirmed by Shuder and Osterbrock (1981~. An accurate
measurement of the broad Ha luminosity, L(Hcr) ~ 1.2 x 1039 ergs s~t
(Filippenko and Sargent 1988), together with Weedman's (1985) relation
given above, yields MB ~ -13.0 mag for the blue nonstellar continuum,
which has not yet been detected. The nucleus of this galaxy is also a
strong, variable X-ray source, as well as a bright, compact, variable, flat-
spectrum radio source. There is almost no doubt that its activity is similar
to that in QSOs; it could reasonably be called a "microquasar" (Elvis 1984~.
Of relevance to the discussion above, its spectrum (Figure 2) shows that
forbidden lines associated with high critical density (e.g., [O I] AA6300,
6364; ne~crit) = 1.4 x 106 cm~3) are considerably broader than those with
low critical density (e.g., IS II] AN6716, 6731; ne~crit) ~ 2 x 103 cm~3~.
In an attempt to quantifier the luminosity function of intrinsically faint
Seyfert 1 nuclei W. L. W. Sargent (Caltech) and ~ are searching for
broad Ho emission in optical spectra of the nuclei of the 500 brightest
galaxies in the northern sky; see Filippenko and Sargent (1985) for initial
results. NGC 4639 is an excellent example of a bright, nearby Shapley-
Ames galaxy whose Sequent 1 characteristics had previously gone unnoticed.
The spectrum at blue wavelengths (Figure 3) is comparable to those of
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98
AMERICAN AND SOVIET PERSPECTIVES
, ~ r-l T-~-,
I 1 1 1, I I 1 1 ' ' ' ' 1 ' ' 1 ~ ! '
00
so
o
6200 6300
1
M8 1 minus NGC 4339
1986 Feb. 25 UT
1`' x 4, P.\ - 180
(a)
~ A
1 ~ _\, -I ~ 1 ~ 1
; - \1
[o I]
Am Jig (c)=(a)-(b) 1 Ha
1 ~
[N II]
[N II] [S II]
~ -~ _ _ = wow ~
,,,,1,,,,1,,,,1,,,,1,,,,1,,,,1
6400 6s00 6600
Rest wavelength (A)
6700 6800
FIGURE 2 Red spectrum of M81 (Filippenko and Sargent 1988), obtained with the 5 m
Hale reflector at Palomar Observatory. A spectrum of the absorption-line template galaxy
NGC 4339 is subtracted Tom M81 in (c). M81 has a low-luminosi~ Spyfert 1 nucleus;
broad Hcz is clearly visible. Note that the widths of the forbidden lines span a wide range.
normal, inactive galaxies devoid of strong emission lines. The red spectrum
(Figure 43, by contrast, exhibits prominent, broad He emission with FW~
~ 3, 000 - 4, 000 km s~: and FWZI ~ 8, 000 - 9, 000 lan sol. A Malone
of old redshift surveys of galaxies did not include the red spectral region,
so it is possible that quite a few objects of this type have been overlooked.
Furthermore, in most galaxies the broad Hc' emission line is very weak,
requiring careful deconvolution of the He + IN II] AA6548, 6583 blend for
detection and measurement.
A Seyfert 1 Nucleus in a Dwarf Sd Galaxy
It seems reasonable that most low-luminosity Seyfert 1 nuclei have
been found in early-Wpe spiral galaxies, given the drstn~ution of classical
Seyferts among different Hubble types. Moreover, early-lope spirals with
well-developed bulges are thought to have much deeper potential wells
than late-type spirals, perhaps promoting the formation of supermassive
black holes and the retention of gas. Indeed, with the possible exception
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HIGH-ENERGY ASTROPHYSICS
4
99
1 ' ' ' ' 1 ' '
' ' 1 ' ' 1 1 1 i,, 1
NGC 4639
1985 June 19 UT
[0 II]
[O III]
o , , , , , , , 1 , , , , 1 , , , , 1 , ,
3500 4000 4500 5000
Wavelength (it)
FIGURE 3 Blue spectrum of NGC 4639, obtained with the 3 m Shane reflector et Lick
Observatory. The galaxy redshift has been removed. An absence of strong emission lines
suggests that this is not a Se~rfert galaxy.
Of G1200 2038 (Kunth et al 1987), until recently not a single Seyfert 1
nucleus had been found in Sd, irregular, or dwarf galaxies.
We were very surprised, therefore, to discover that NGC 4395, a nearby
(d ~ 2.6 Mpc) low-luminositr (MB ~-16.4 may) Sd galaxy, harbors a very
faint Seyfert 1 nucleus (Filippenko and Sargent 1989~. A superb photograph
of this object is shown in panels 10 and 56 of the Atlas of Galaxies compiled
by Sandage and Bedke (1988~. The faint star-like nucleus emits a narrow-
line spectrum similar to that of a type 2 Seyfert, with emission lines of
very high ionization (up to [Ne V1 and [Fe X1) superposed on a featureless
condouum. Photoion~tion by a reasonably hard continuum is almost
certainly responsible for Heir relative strengths, but tO I] A6300 and IS lI]
A)6716, 6731 are unusually intense with respect to IN Il] A)6548, 6583,
as illustrated in Figure 5. The narrow lines have FWHM < 60 km s~i
smaller than in any other Mown Seyfert nucleus.
Weak, broad components are clearly visible in the per~itted-line pro-
files, but not in the forbidden lines. The FWZI of Ha is 6,000 7,000 km
OCR for page 100
100
.
=- -
- ~
- - ~
-
~ -
oD
c -·
~ -
~>
- ~
c,
+. -
NGC 4639
1985 February 24 UT
[0 1]
AMERICAN AND SOY7ET PERSPECTIVES
H`x
[N II]
J
fit
[N lI]
1
[S 11]
Ah
. . . .
6300 6400 6500 6600 6700
Wavelength (A)
FIGURE 4 Red spectrum of NGC 4639 (Filippenko and Sargent 1986), obtained with
the 5 m Hale redector at Palomar Observatory. The galaxy redshift has been removed.
Atmospheric O2 produces the absorption line near 6260 ~ The obvious presence of broad
Hat emission indicates that the nucleus is a type 1 Se~rfert.
s-1 in the very high-quality spectrum (Figure 5), although the FWHM
is only about 60~700 km sol. The luminosity of the broad Ha line is
~ 1.2 x 1038 ergs s~i, about one tenth that in Met, and MB ~-9.8 mag
is measured from the spectrum. Hence, NGC 4395 has the intrinsically
weakest known Seyfert 1 nucleus, with a blue luminosity no greater than
that of the most luminous supergiant stars! If NGC 4395 were much farther
away, so that the spectroscopic entrance aperture included a large amount
of extranuclear light, its spectrum would have been dominated by emission
lines from H II regions and continuum from OB associations, making the
Seyfert activity difficult to detect.
At optical wavelengths the featureless continuum of NGC 4395 can be
described by a power law with spectral index 1.5, comparable to that of
other Seyfert 1 nuclei. Moreover, the equivalent width of the broad Ho
emission, 270 A, is also Apical of type 1 Seyferts. If we assume that the
bolometnc luminosity of the nucleus is roughly 1000 times that of the broad
H,0 emission, as for other Seyfert 1 nuclei (Weedman 1976), we find that
LAO! ~ 1.5 x 104° ergs sol. This could be emitted by a black hole having
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HIGH-ENERGY ASTROPHYSICS
50
40
30
20
83 10
lo_
2.0
1.0
[o I]
(b)
[o I]
[/m] [Fe X]
_ ~ Hi/ [ArV]
0.0 I I I I I I I I t I I
6200 6300
101
J I I I I I I I I I I I I I I I I l I I I I I I i I I I I I I Ill
- (a) NGC 4395: Nucleus
Ha
[N II]
1 1 ~ 1,,,, 1, ., .
[N lo]
at_
[S B]
I ~ ~ I
6400 6500 6600 6700 6800
Observed Wavelength (~)
FIGURE 5 Red spectrum of the nucleus of NGC 4395 ~lippenko and Sargent 19&9),
obtained with the 5 m Hale reflector at Palomar Observatory. The gala~s heliocentric
radial velocity is 320 An s-1. The luminosity of the broad Ha line is the lowest measured
in any Spyfert 1 nucleus
a mass of only ~ 100 Me if it were accreting at the Eddington limit! Such
a black hole could be produced in only 200 million years if it started out
with one solar mass and always Secreted at the Eddington limit Adopting
the formalism of Wandel and Yahil (1985), on the other hand, we find
that the current dynamical mass is about 104 M ,, if gravity accelerates the
emission-line clouds to typical observed speeds of ~ 500 lan sol. Thus, it
is possible that L ~ 0.01 LE&~.
Given its low luminosity, it is interesting to consider whether the
nucleus of NGC 4395 is actually a single very massive star (M ~ 200 Mom.
Its spectrum, of course, is unlike that of known objects such as main-
sequence O-type stars or even Wolf-Rayet stars, but this is nonetheless
an interesting idea; we might never have observed such a star, because
they form rarely and have very short lives. If the broad emission lines are
caused by an outflowing wind, then the mass loss rate can be calculated
under the assumption of steady-state conditions. Photoioni~ntion models
give a distance of ~ 10~5 an between the star and the emission-line clouds,
and the absence of broad forbidden lines suggests that ne ~ 109 am~3.
Adopting an outflow velocity of SOO km s~i, the equation of cont~nui~r
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102
AMERICAN AND SOVIET PERSPECTIVES
yields M ~ 0.02 M<3 yr~l. At this rate, the 200 Me star would only last
about 104 years!
It Is very important to obtain additional observations, at all wave-
lengths, of low-luminosi~ Seyfert 1 nuclei such as that in NGC 4395. They
offer counterexamples to the conventional wisdom that activity is restricted
to early-type galaxies with large bulges. If a massive black hole is ultimately
responsible for the observed phenomena in the nucleus of NGC 4395, we
must explain how it was able to form in this particular object, but not
in a majority of other extended, low-mass galaxies. On the other hand,
perhaps most dwarf and very late-type galaxies can, in principle, be active,
but are usually quiescent because the fueling mechanism does not operate
efficiently. Basically, we do not yet know what conditions are necessary for
a galaxy to have an active nucleus.
Warmers: A Different Point of View
The narrow emission lines in classical AGNs, even those without
broad emission lines, are almost certainly the result of photoionization of
gas by a relatively flat UV and X-ray continuum. The intensities of the
lines generally satistr, very roughly, [O III] A5007/H,B ~ 10 and rN III
A6583/Hc~ ~ 1, and high-ionization lines such as [He V1 A3426 and He II
A4686 are present (e.g., Osterbrock 1977; Koski 1978). [O I] )6300 is
considerably stronger than in H II regions, whose O and B stars produce
Stromgren spheres with thin, well-defined boundaries between the zones of
neutral and ionized hydrogen.
Most researchers believe that the continuum arises from the vicinity of
a massive black hole, and is of nonstellar origin, as described above. There
is, however, a very different possibility, at least for AGNs of relatively low
luminosity. In a thought-provoking paper, Terlevich and Melnick (1985)
showed that ionizing radiation from evolved, very massive (M > 60 M<~)
stars in metal-rich H II regions can produce emission-line intensity ratios
typical of Seyfert 2 nuclei. These Wolf-Rayet stars, which they call "Warm-
ers" (extreme WC or WO stars), have effective surface temperatures of
(1 - 2) x 105 K; plenty of high-energy ionizing photons are therefore emit-
ted. A key point is that stellar mass loss increases with increasing metal
abundance in massive stars, so that very massive stars formed in high-
metallici~ environments can end their lives as bare He-C-D cores with
surface temperatures comparable to those of the hottest lmown nuclei of
planetary nebulae but with considerably higher luminosities.
Calculations show that after 3 million years, the ionizing continuum of
a cluster created during a large burst of star formation (initial mass function
slope = 3.0; LRqueux 1979) is nearly a power law, fit or u-i 5, with a cutoff
at ~ 20 Ryd. Thus, the resulting emission-line spectrum from surrounding
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HIGH-ENERGY ASTROPHYSICS
103
clouds of gas having roughly solar composition closely resembles that of
Seyfert 2 galaxies. Moreover, the weak featureless continuum observed
in some type 2 Seyferts may be the reddened spectrum of the ionizing
cluster. As the cluster ages, the spectrum evolves into one characterized
by low-ioni~tion emission lines, similar to those seen in Heckman's (1980)
LINERs (low-ionmation nuclear emission-line regions). Motivated by the
success of their earlier work, Terlevich et al (1987) went on to postulate that
typical Seyfert 1 nuclei may also ultimately be produced by starburst activity.
In their scenano, the broad emission lines are produced by supernovae
(SNe), and by the interaction of SNe with a dense interstellar medium.
Although there are a number of potential difficulties with this hypoth-
esis, here I speculate that NGC 4395 may represent an example of a Seyfert
nucleus powered by a burst of star formation; in this case, the narrow-line
spectrum is produced by a cluster of Warmers. Detailed photoionization
models must be computed in order to see whether the observed emission-
line spectrum can be reproduced under conditions appropriate for the
Warmers scenario. It is not clear, for example, whether the low ~ Ill
strength reflects peculiar abundances or unusual excitation.
The observed velocity of the broad-line gas Is unlikely to be produced
by gravitational effects; a sufficiently compact and massive star cluster would
also be very short lived, due to stellar collisions. If, on the other hand, the
broad permitted lines are produced by SNe, as suggested by Terlevich et
aL (1987), only one very old SN is needed to explain their low luminosity.
Indeed, if the total broad-line luminosity is 1039 ergs s~: (a reasonable
value, given the observed strengths of HE and H,B), the entire visible
energy emitted by a typical SN (~ 1049 ergs) could power the nucleus for
300 years at a constant rate! Adding even a small fraction of the kinetic
energy (105: ergs) greasy increases this estimate. Near maximum, a SN is
far more luminous than the nucleus of NGC 4395; the SN in NGC 4395
must therefore be very old. Goodrich et at (1989) detected weak, broad
Ha emission nearer three decades after the discovery of the more distant
(d ~ 8 Mpc) SN 1961V, so a late-tune detection of a SN in NGC 4395 is
not unreasonable. Note, however, that Goodrich et aL (1989) believe that
SN 1961V was actually not a SN, but rather an exaggerated y-Carinae-like
outburst This may be consistent with the single-star hypothesis of NGC
4395 briefly discussed in the preceding section.
It might be argued that the spectral characteristics of Seyfert 1 nuclei
cannot possibly be produced by SNe, since the spectra of SNe are not
generally thought to resemble those of Seyfert 1 nuclei. Figure 6, however,
shows Mat there are some exceptions; the spectrum of SN 1987F was quite
similar to that of 3C 48 at optical wavelengths (Filippenko 1989~. There
are, of course, some important differences if the spectra are examined
carefully. The broad line near 5900 3, for example, is attributed to Na I
OCR for page 104
104
14
15
16
._
17
18
AMERICAN AND SOVIET PERSPECTIVES
1 ' ' I I ~I ' 1 ~ ~ 1 1 I ~1 ' ' ' ~ 1 1 ~i, I, 1 _
(a) 3C 48, 10 - 3 - 88 Ha
to m]
H.'
Her
Fe II
_ Ha
it'
Fe II He ~
l~/iI
~f - ~ n . it _ ~ s I
~f -' Phi '-"--WOW
~ AIL
_ ~
1 1,,, 1
4000 4500
19 1 1 I I I 1 1 1 1 ~1 1 I ', .
(b) SN 1987F, 12 - 26 - 87
I,,,, 1,,,, 1 1
5000 5500 6000 6500
Rest Wavelength (at)
FIGURE 6 Spectra of (a) the quasar 3C 48, and (by SN 1987F in NGC 4615 roughly
9 months after discovery. The data were obtained with the Shane 3 m reflector at lick
Observatory (F-llippenko 1989~. AB magnitude = - 2.5 log fit -48.6, where the unites of
A, are ergs s~ 1 cm-2 Hz- ~
D in SN 1987F and to He I in 3C 48. Moreover, the centroid of the broad
Ha line in SN 1987F and in other, similar SNe is blueshifted relative to
the narrow component, whereas the broad Ha in Seyfert 1 nuclei tends to
be redshifted at least as often as blueshifted. Finally, the continuum of SN
1987F begins to drop rapidly at near-UV wavelengths, whereas that of 3C
48 remains quite strong.
Nevertheless, had SN 1987F occurred in the nucleus of a normal
galaxy, a low-resolution spectrum undoubtedly would have led to a Seyfert
1 classification for the galaxy, especially if some Warmers were present to
produce high-ioni~tion narrow lines and a blue continuum. If we further
assume that physical conditions (e.g., a dense interstellar medium, as may
have been He case for SN 1987F) in certain galactic nuclei somehow
enhance the production of SNe Lee SN 1987F, then the spectrum could
remain Seyfert-l~e for long periods of time, and exhibit variability as well.
OCR for page 105
HIGH-ENER~ ~TROP~ICS
105
Thus, it is possible that some catalogued type 1 Seyfert galaxies, especially
those of low luminosity, owe their observed characteristics to objects such
as SN 1987F; we simply do not know enough about the evolution of SNe
and their remnants in dusty, dense environments.
I do not want to use this spectral resemblance to argue for the above
scenario as a general explanation for Seyfert 1 galaxies, and certainly not for
luminous radio-loud QSOs. However, the spectral similarity of SN l987F
and 3C 48 at optical wavelengths can be used to teach us a great deal about
the physical processes in both types of objects. Many more comparisons
must be made, over as large a range of wavelengths as possible, in order
to obtain a more thorough physical understanding of AGNs. I am certain
we will encounter numerous surprises along the way.
ACKNOWLEDGMENTS
Some of the observations reported here were taken at Palomar Obser-
vatory, where I was a Guest Investigator collaborating with W. L. W. Sargent.
Data were also obtained at Lick Observatory, which receives partial fund-
ing from NSF Core Block grant AST 8614510, and from Ins Campanas
Observatory, which is owned and operated by the Carnegie Institution of
Washington. My research on AGNs and SNe has been supported by the
California Space Institute, most recently through grant CS 41 - , and by
NSF grants AST 8957063 (Presidential Young Investigator Award) and
AST-9003~9. I thank the Once of International Affairs of the National
Research Council, especially Kathleen [livers, as well as George Clark,
Walter Lewin, and Rashid Sllnyaev, for organizing the successful NAS-
ASUSSR Workshop on High Energy Astrophysics. I am also very grateful
for the warm hospitality of our hosts in Moscow and Soviet Georgia.
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Representative terms from entire chapter:
galactic nuclei
