Active galactic nuclei (Seyfert nuclei and their relatives) and intense star formation can both deliver substantial amounts of energy to the vicinity of a galactic nucleus. Many luminous nuclei have energetics dominated by one of these mechanisms, but detailed observations show that some have a mixture. Seeing both phenomena at once raises several interesting questions: (1) Is this a general property of some kinds of nuclei? How many AGNs have surround starbursts, and vice versa? (2) As in 1, how many undiscovered AGNs or starbursts are hidden by a more luminous instance of the other? (3) Does one cause the other, and by what means, or do both reflect common influences such as potential well shape or level of gas flow? (4) Can surrounding star formation tell us anything about the central active nuclei, such as lifetimes, kinetic energy output, or mechanical disturbance of the ISM? These are important points in the understanding of activity and star formation in galactic nuclei. Unfortunately, the observational ways of addressing them are as yet not well formulated. Some preliminary studies are reported, aimed at clarifying the issues involved in study of the relationships between stellar and nonstellar excitement in galactic nuclei

Keel, William C.

English

NASA Technical Reports Server

N87- 24363
Star Formation Around Active Galactic Nuclei
William C. Keel
Sterrewacht Leiden
Leiden, The Netherlands
Emission-line images and high-dispersion optical spectra have been used
to investigate star-forming regions in the vicinity of active galactic nuclei,
including objects covering a wide range in luminosity of the central source.
Rings of HII regions around the nucleus on 100-500 pc scales occur
preferentially in galaxies with active nuclei, perhaps indicating a common
response to gas flow in certain potential shapes. Multiaperture spectra of
nearby HIl-region nuclei shows many to have centrally condensed LINER-like
components, suggesting that such configuration may be quite common.
The stellar population, and its history, may be probed in some of the
most favorable cases. Observed stellar absorption features and crude
starburst models imply ages _ 108 years and < 3xi07 years for the starbursts
around the nuclei of NGC 1068 and 7469, respectively. These are in rough
concordance with lifetimes of the nuclear activity based on radio structures;
if a direct link between nuclear activity and surrounding starbursts can be
demonstrated, this could become an important way to study the history of
individual objects. A somewhat different situation is found in Mkn 231, where
the whole galaxy (merger?) exhibits optical colors and spectra suggesting
either an IMF deficient in OB stars or a sudden turnoff of star formation, on
scales so large that it is unlikely the nucleus is directly responsible.
There thus appears to be evidence of at least two kinds of links between AGN
and star formation.
Active galactic nuclei (Seyfert nuclei and their relatives, in which
nonstellar sources of energy are important) and intense star formation can
both deliver substantial amounts of energy to the vicinity of a galactic
nucleus. Many luminous nuclei have energetics dominated by one of these
mechanisms, but detailed observations show that some have a mixture. Seeing
both phenomena at once raises several interesting questions:
1) Is this a general property of some kinds of nuclei? How many AGNs have
surrounding starbursts, and vice versa?
2) As in 1), how many undiscovered AGNs or starbursts are hidden by a more
luminous instance of the other?
3) Does one cause the other, and by what means, or do both reflect common
influences such as potential-well shape or level of gas flow?
4) Can surrounding star formation tell us anything about the central active
nuclei, such as lifetimes, kinetic energy output, or mechanical disturbance of
the ISM?
These are important, and perhaps crucial, points in our understanding of
activity and star formation in galactic nuclei. Unfortunately, the
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 661
https://ntrs.nasa.gov/search.jsp?R=19870014930 2020-03-20T11:07:04+00:00Z
W.C.KEEL
observational ways of addressing them are as yet not well formulated. I
report here some preliminary studies aimed at clarifying at least the issues
involved in study of the relationships between stellar and nonstellar
excitement in galactic nuclei.
HII RinKs and Active Nuclei
Many spirals, particularly of intermediate Hubble type, show
substructures in the nuclear region often termed "hot spots" (e.g. Sersic and
Pastoriza 1967). A large fraction of these show a very regular morphology
upon more detailed study: a nucleus with late-type (old) stellar population
and Seyfert or LINER emission spectrum, surrounded by an elliptical ring of
luminous giant HII regions. Such rings may be associated with inner
resonances in the stellar orbits (see, for example, Sanders and Huntley 1976,
Sanders and Tubbs 1980). Relatively few such objects are known; at the
redshifts of even most Markarian Seyferts, they would be blended with the
nucleus in ground-based observations. Galaxies in which this morphology is
well-established are listed in Table i (after Hummel, van der Hulst, and Keel
1986). Note that only one of these (NGC 5427) does not appear in the lists of
Sersic and Pastoriza. Typical appearances of these are illustrated in Figure
I, by an Ha image of the nucleus of NGC 5248 (from the KPNO 2.1-m with TI
CCD).
TABLE 1
Galactic Nuclei with HII Rings
NGC Ring Diameter (kpc) Nucleus Hubble Type
1097 1.1 LINER/Sy2 SBb
2297 0.4 LINER? SABc
3351 0.4 LINER SBb
4303 0.3 LINER/Sy2 SABbc
4321 1.4 LINER/Sy2 SABbc
5236 0.3 HII SABc
5248 0.7 LINER SABbc
5427 1.9 Sy2 Sc
From the data summarized in Table 1, it appears that galaxies with
nuclear rings of HII regions are more likely to have indicators of nuclear
activity (Seyfert or LINER spectra) than the norm for the Hubble types
involved. I Particularly noteworthy is the presence of such activity even in
1The LINERS here are not the "blue LINERS" that are often associated with
starbursts (Rieke, these proceedings); the nuclei here (except the HII
nucleus of M83) show no real evidence of current star formation inside the HII
rings.
662
STAR FORMATION AROUND ACTIVE NUCLEI 
ORIGINAL 
OF POOR 
Figure  1. Continuum-subtracted H a  + [NII ]  image of t h e  n u c l e a r  r e g i o n  of NGC 
5248. The r i n g  of H I 1  r e g i o n s  around t h e  a c t u a l  nuc leus  i s  e v i d e n t .  The 
f i e l d  i s  160" square ,  w i t h  n o r t h  a t  t h e  top. 
Sc g a l a x i e s ,  i n  which i t  is rare among magnitude-limited samples (e.g. Keel 
1983). The a v a i l a b l e  sample c l e a r l y  poin ts  t o  some kind of connec t ion  between 
H I 1  r i n g s  and n u c l e a r  a c t i v i t y ,  both perhaps formed by c e r t a i n  k i n d s  of 
p o t e n t i a l  shapes  t h a t  induce  r a d i a l  gas  motion. Complete surveys  of t h e  
i n c i d e n c e  of such r i n g s  and p r o p e r t i e s  of t h e  a s s o c i a t e d  n u c l e i  w i l l  be 
i m p o r t a n t ,  bu t  can be complicated by s e v e r a l  e f f e c t s .  The r e s o l u t i o n  e f f e c t  
mentioned above means t h a t  p r e s e n t  d a t a  would have d i f f i c u l t y  f i n d i n g  such 
c o n f i g u r a t i o n s  at  t h e  d i s t a n c e s  c h a r a c t e r i s t i c  of very luminous S e y f e r t s ;  i n  
such cases, s p e c t r o s c o p i c  i d e n t i f i c a t i o n  of an HII-region component o r  use  of 
I R  c o l o r s  t o  i n f e r  a star forming r e g i o n  might be p o s s i b l e ,  but  d i f f i c u l t  due 
t o  t o  t h e  g r e a t  l u m i n o s i t y  of t h e  nucleus.  Second, i n  such very  a c t i v e  
o b j e c t s ,  some of t h e  s i g n a t u r e s  of H I 1  r ings  can be l o s t .  I n  t h e  t y p e  2 
S e y f e r t  NGC 5728 (M. M. P h i l l i p s  e t  a1 p r i v a t e  communication) t h e r e  is  --* ' 
dynamical evidence of a gas  r i n g ,  but  i t s  i o n i z a t i o n  i s  dominated by r a d i a t i o n  
from t h e  nuc leus .  Very d e t a i l e d  s tudy  i s  needed f o r  such o b j e c t s ;  more H I 1  
r i n g s  w i l l  l i k e l y  emerge from d e t a i l e d  s t u d i e s  of i n d i v i d u a l  a c t i v e  n u c l e i .  
PAGE IS 
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663 
W.C.KEEL
Histories of Star-Formin$ Regions around .Seyfert Nuclei
The presence of "starburst disks" - regions of intense star formation on
scales of a few kiloparsecs is well established for a few nearby Seyferts.
They may be recognized from emission-line properties (Morris et al. 1985,
Alloin et al. 1983), infrared mapping (Cutri et al. 1984), and radio-source
morphology (A. S. Wilson, these proceedings). The properties of these regions
are of interest as possible probes of the history of the AGN itself, if it can
be established that the onset of extensive star formation and the turn-on of
the nucleus are roughly contemporaneous (admittedly a tall order, and a large
question for the future). To explore the possibilities of learning the age
and history of these "starburst disks", spectra of _1% resolution have been
obtained for the H_, HB, [0III] X5007 regions in nuclei showing a range of
activity and some evidence for surrounding star formation. The observations
used the IPCS at the 2.5-m Isaac Newton Telescope on La Palma and CCDs at the
INT and the Kitt Peak 4-m.
Figure 2 shows the HB-X5007 regions approximately 16" SW of the nucleus
of NGC 1068. This region has been studied in the UV by Weedman and
Huenemoerder (1985), and shows HIl-region emission in the images by Balick and
Heckman (1985); a discrete near-IR source here also suggests a local clump of
star formation (Telesco et al. 1984). The present data shows, in addition to
narrow HB emission from the HII region and broad [OIII] associated with the
nucleus (see the kinematic analysis by Alloin et al. 1983), stellar absorption
features of H_, MgI, Hy, and the G-band. These features, and the Balmer
emission diagnostic of the stellar Lyman continuum, have very different
weightings with stellar age in an aging burst, and for a given burst model can
serve as age or history diagnostics.
To illustrate this approach, consider a simple model of star formation
with a Salpeter IMF turning on at some time and retaining a constant star-
formation rate thereafter. INT blue spectra of the circumnuclear regions of
NGC 1068 and 7469 may be roughly compared with such a model. In NGC 1068, HB
and MgI _ X5175 are seen with equivalent widths of 7 and 3 A, while neither is
detected in NGC 7469 at a level _2 A. These data imply ages of _i08 and < 107
years, respectively. These are at least consistent with timescales needed to
produce the extended ratio excess in each case (following de Bruyn and Wilson
1978). More detailed work, including more objects, emission-line results, and
varying star forming histories, is in progress.
Star Formation in Markarian 231
The nucleus of Mkn 231 shows a number of unusual properties, including
broad, blueshifted absorption, strong continuum reddening, and very strong IR
emission (Adams 1972, Boksenberg et al. 1977, Rieke and Low
1972). The surrounding galaxy is also peculiar in morphology and
spectroscopic properties and may be a nearby example of the conditions seen
in, for example, 3C 48 (Boroson and Oke 1982). This section describes a study
of the host galaxy of Mkn 231, conducted in collaboration with D. Hamilton
(CTIO).
664
STARFORMATIONAROUNDACTIVENUCLEI
N1068 S 40-89
0
0
0
0
0
0
£b
0
I I I
, , , , I , _ _ , I , , L , I , _ , , I
500 1000 1500 2000
Figure 2. INT + IPCS spectrum in the 4300-5200 % range, for the "starburst
disk" of NGC 1068 16" SW of the nucleus. Stellar absorption of H6 (a broad
feature underlying the narrow emission line) is strong; other identifiable
absorption lines include MgI, Hy, and the G-band. [OIII] emission redward (to
the right) of HB, _mos_y due to the effects of the Seyfert nucleus.
Multi-color imaging shows the galaxy to be very asymmetric, with smooth
amorphous condensations and, at very faint levels, a pair of probable tidal
tails. A large region of nearly constant surface brightness west of the
nucleus offers a good opportunity to study the stellar population. A low-
resolution spectrum of this region was obtained with the Cryogenic Camera at
the KPNO 4-m, and shows relatively strong HB as well as MgI and MgH
absorption, indicating a large contribution from stars of types A and F. In
itself, this indicates active star formation within the last _5 x 108 years.
However, the current star formation rate must be much smaller, as deduced from
the lack of Ha attributable to HII regions in the disk and lack of direct
observation of such regions in H= images. Using the precepts of Kennicutt
(1983), the SFR has dropped more than fourfold in this period.
It would be remarkable for a whole galaxy (of extent > 30 kpc) to turn
off star formation everywhere in this time. The alternative is equally
665
W.C.KEEL
remarkable, an IMF deficient in high mass stars. This is just the opposite of
the case usually inferred for starburst galaxies. The stellar population here
does appear similar to those in a few other active galaxies such as 3C 48 and
3C 459 (Miller 1981). The physics of star formation during galaxy mergers
could provide important clues to what is seen here; Mkn 231 shows a pair of
tails, and most giant radio galaxies, including 3C 459, show morphological or
kinematic evidence of being merger products (Heckman et al. 1986). Mkn 231,
at least, raises the possibility that conditions in some merging galaxies may
favor formation of low-mass stars. If so, some interpretation of observations
of many interacting and post-interaction systems would be much more
complicated than previously thought.
Future Directions
The most pressing needs in understanding star formation around active
nuclei are solid indications of how often, and at what levels, it occurs, and
a detailed study of representative objects that could provide clues to the
physical operation there. Schemes exist that could have a link between the
phenomena going in either direction, or with both as a result of host galaxy
properties. Weedman (1983) has considered the fate of condensed stellar
remnants from a starburst; if they can reach a compact enough configuration,
relativistic effects (Shapiro and Teukolsky 1985) could rapidly produce a
single massive central object. Weedman notes that the process might be so
rapid that both would be seen simultaneously.
In the other direction, active nuclei could induce surrounding star
formations through shocks, perhaps via jets. There is clear evidence of jet-
induced star formation on larger scales, in Cen A and Minkowski's Object (van
Breugel et al. 1985, Brodie, Bowyer and McCarthy 1985, Graham and Price
1981). Finally as discussed for HII regions above, some kinds of potential
might efficiently funnel gas both into kiloparsec and parsec scales, fuelling
star formation and nuclear activity at the same time.
Surveys for extranuclear star formations might take several forms. IR
colors (Rodriguez-Espinosa et al., these proceedings) are relatively unbiased
and reddening-free, but from the IRAS database it is not clear that the star
formation is on a small enough scale to be connected with the nuclei. Ha
studies (imaging or slit spectroscopy) complement IR studies in spatial
resolution, but can be of limited use when the nucleus is so luminous as to
dominate the gas ionization at relevent distances within the disks. Finally,
the work of Wilson et al. (these proceedings) suggests that radio morphology
may be an additional useful diagnostic. All these kinds of surveys should be
pursued.
More detailed studies of individual objects are in part available, and
will continue as byproducts of studies of active nuclei.
Observations described here have been obtained at the Isaac Newton
Telescope, now happily ensconced on the island of La Palma, and at Kitt Peak
National Observatory and Cerro Tololo InterAmerican Observatory, National
666
STAR FORMATION AROUND ACTIVE NUCLEI
Optical Astronomy Observatories, operated by A.U.R.A. under contract with the
National Science Foundation. I am especially grateful to Susan Davidson for
helping produce this manuscript on time.
REFERENCES
Adams, T.F. 1972, Ap. J. (Lett.) 176, LI.
Alloin, D., Pelat, D., Boksenberg, A., and Sargent, W. L. W. 1983, A_. J. 275,
493.
Balick, B. and Heckman, T. 1985, Astr. J. 90, 197.
Boksenberg, A., Carswell, R. F., Allen, D. A., Fosbury, R. A. E., Penston, M.
V. and Sargent, W. L. W. 1977, M.N.R.A.S., 178, 451.
Boroson, T. A. and Oke, J. B. 1982, Nature, 296, 397.
Brodie, J. P., Bowyer, S. and McCarthy, P. 1985, Ap. J. (Lett.) 293, L59.
Cutri, R. M., Rudy, R. J., Rieke, G. H., Tokanuga, A. T. and Willner, S. P.
1984, Ap. J., 280, 521.
de Bruyn, A. G. and Wilson, A. J. 1978, Astr. Ap., 64, 433.
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K., lllingworth, G. D., Bothun, G. D., and Balick, B. 1986, Ap. J. (in
press).
Hummel, G., van der Hulst, J. M., and Keel, W. C. 1986, Astr. Ap. (submitted).
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Shapiro, S. L. and Teukolsky, S. A. 1985, Ap. J., 298, 58.
Telesco, C. M., Beeklin, E. E., Wynn-Williams, C. G. and Harper, D. A. 1984,
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van Breugel, W., Filippenko, A. V., Heckman, T. and Miley, G. 1985, A_. J.,
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DISCUSSION
WILSON:
In our study of the circum-nuclear starburst in NGC 7469 (to be published in Ap. J. Nov. 1986), we
do find evidence of Hfl absorption in some locations. Presumably, this would make the starburst somewhat
older than you estimated.
KEEL:
This emphasizes an important issue in the study of these systems - spatial resolution and averaging. My
data so far pertain only to limited slices through the circum-nuclear regions; since present star formation is
obviously patchy, stars too young to have completely diffused will also have more or less 'lumpy' distributions.
A more proper observational approach would involve mapping absorption features at many positions around
the nuclei.
667


Star formation around active galactic nuclei

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