New observations of the 10 and 20 micron size of the emission region in Arp 220 are presented. Also given are ground based photometry from 1 to 32 micron including measurements of the strength of the silicate feature at 10 micron. The results show that the 20 micron size of Arp is smaller than 1.5 arcsec (500 pc); comparison of IRAS and ground based observations show that IRAS 12 micron flux measured with a large arcmin beam is the same as that seen from the ground with a 3 arcsec aperture. At 10 micron a deep silicate absorption feature is seen that corresponds to a visual extinction of about 50 mag. These results suggest that a very significant portion of the 10 to the 12th power L sub 0 infrared luminosity from Arp 220 comes from a region less than or of the order of 500 pc in diameter. When these results are combined with recent measurement of a broad Brackett alpha line by DePoy and an unresolved 2.2 micron source by Neugebauer, Matthews and Scoville, a very attractive possibility for the primary luminosity source Arp 220 is a dust embedded compact Seyfert type nucleus

Becklin, E. E.

Wynn-Williams, C. G.

English

NASA Technical Reports Server

N87- 2436 1
GROUND-BASED I- TO 32-_m 0BSERVATIONS OF ARP 220:
EVIDENCE FOR A DUST-EMBEDDED "AGN"?
E. E. Becklln and C. G. Wynn-Willlams
University of Hawaii, Institute for Astronomy
Honolulu, Hawaii 96822 USA
ABSTRACT. New observations of the I0- and 20-_m size of the emission region in
Arp 220 are presented. We also give ground-based photometry from 1-32 _m
including measurements of the strength of the silicate feature at I0 _m. The
results show that the 20-_m size of Arp 220 is smaller than 1.5 arcsec (500
pc); comparison of IRAS and ground-based observations show that IRAS 12-_m flux
measured with a large arcmln beam is the same as that seen from the ground with
a 3-arcsec aperture. At i0 _m a deep silicate absorption feature is seen that
corresponds to a visual extinction of about 50 mag.
These results suggest that a very significant portion of the 1012 Le
infrared luminosity from Arp 220 comes from a region less than or of the order
of 500 pc in diameter. When these results are combined wlth recent measurement
of a broad Brackett-_ llne by DePoy and an unresolved 2.2-_m source by
Neugebauer, Matthews and Scovllle, a very attractive possibility for the pri-
mary luminosity source Arp 220 is a dust-embedded compact Seyfert-type nucleus.
INTRODUCTION
hrp 220 (IC 4553) Is a relatively nearby galaxy that IRAS found to be
extremely luminous at infrared wavelengths (Solfer et al. 1984). It has a
total luminosity of 1012 Le at a distance of 70 Mpc (Ho = 75 km/sec/Mpc). It
emits 50 to i00 times more luminosity at infrared than visible wavelengths.
Optically the galaxy appears highly disturbed and has been classified as a
merger (Joseph and Wright 1985). It contains several compact radio sources
(Norris 1985) and at visual wavelengths shows a thick dust lane across its
center (Schild 1985).
Arp 220 appears to be one of a number of galaxies discovered by IRAS that
have a total luminosity > 1012 Le. The space density of these galaxies is as
high or higher than optical, radio, or x-ray selected galaxies or QSOs (Solfer
et al. 1986). A critical question to be answered is the dominant source of
energy in Arp 220. A number of suggestions have been made such as (i) a burst
of star formation (Rieke et al. 1985; Joseph and Wright 1985), (2) a dust-
embedded active galactic nucleus "AGN" (Solfer et al. 1984; Norris 1985), and
(3) mechanical energy from a collision of two galaxies (Harwlt et al. 1986).
In this paper we present new observations that suggest that a dust-embedded
"AGN" is the most likely explanation for the luminosity in Arp 220.
OBSERVATIONAL RESULTS
We present previously unpublished results on the size of the emission
region at i0 _m (N) and 20 _m (Q). The data were obtained on the IRTF in March
1985 and 1986 using the facility photometer with a Ga:Ge bolometer and standard
interference filters.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies
F._ECEDING PAGE BLANK NOT FII.MED
643
https://ntrs.nasa.gov/search.jsp?R=19870014928 2020-03-20T11:06:53+00:00Z
E. E. BECKLIN AND C. G. WYNN-WILLIAMS
Figure I shows a 20-_m profile of Arp 220 along a north-south llne made
wlth a 3-aresec aperture. The data have been folded about the central point.
Also shown Is the profile of a star. Similar data also exist in the east-west
direction. These data show that the galaxy and stellar profiles are similar;
in other words Arp 220 is unresolved. Assuming that the two profiles are
Gausslan in shape, the Arp 220 source has a characteristic slze (FWHM) which is
less than 1.5 arcsec. This corresponds to <500 pc at the distance of Arp 220.
I I I I
' 20 Fm
ST_,R
0
0 1.0 2.0 5.0 4.0 5.0
DEC Offset Arcsec
Figure l: North-south profile at
20 _m taken with a 3-arcsec beam.
X
I,
._> 0.5
tr
200
E
A
E
=L I00
0
l,l.
_
I I I
0 Rieke eta/
• Becklin end
Wynn-Williams
I 1 I 1 I
0 6 12
Displocement north-west (orcsec)
Figure 2: North-west profile at
I0 _m taken with a 5.5 arcsec beam.
A 10-Pa profile with 5.5-arcsec resolution along a north-west direction is
shown in Figure 2 along wlth the profile of a star; again Arp 220 is unre-
solved. These data disagree with the results of Rieke et al. (1985) whose data
are also shown in Figure 2. Both observations were made wlth the same system
on the IRTF; we do not understand the discrepancy. The 20-_m profile and the
12.5-pa aperture growth curves discussed below would suggest that the present
results, which show that the source is unresolved at i0 _m, are correct.
Figure 3 is an aperture growth curve at 12.5 _m, l.e., flux versus
aperture size. It includes ground-based observations wlth beam diameters of
3.0, 5.5, and 7.0 arcsec and the IRAS data with a beam >I arcmln in diameter.
The IRAS point has been corrected for the different filter responses using the
energy distribution for Arp 220 given below. The result is that >90% of the
12.5-pa flux measured by IRAS is observed in a 3-arcsec-dlameter aperture.
In Figure 4 we present the energy distribution of Arp 220 from 7.8- to
12.5-P_n observed with a 5.5-arcsec aperture and standard IRTF filters with
Ah/k _ 0. I. There is an obvious deep absorption feature at I0 _m. The shape
of the feature agrees wlth that seen in other galactic and extragalactic
sources and is ascribed to silicate dust (Roche et al. 1986). The shape of the
spectrum indicates an absorption optical depth of z ~ 3, implying a visual dust
extinction of Av ~ 50 mag.
"_ 644
1-32_m OBSERVATIONS OF ARP 220
A
X
-n
J
it
-'I
tart
N
l
ARP 220
S" IRAS
I , I
1 2
tog Aperture (arcsec)
Figure 4:
Energy distribution
from 7.8 to 12.5 _m.
--%
t--
QJ
X
=3
IJ_
Figure 3:
Aperture growth curve at 12.5 Pan.
i
I i I I I
ARP 220
I I I I
8 9 10 11 12
Wavelength (microns)
Figure 5 shows the complete spectrum of Arp 220 from 1 _m to 350 _m plot-
ted as log vSv vs log v; both IRAS and ground-based data are shown. The 1.65-,
2.2-, and 3.8-_m points are unpublished University of Hawaii measurements in a
4-arcsec aperture that have had an estimated stellar contribution subtracted.
All of the points from 7.8 to 32 _m were made wlth a 5.5-arcsec aperture on the
IRTF. The 350-_m point is from Emerson et al. (1984). The energy distribution
shows that:
1) The energy peaks at about 50 Pa; the distribution corresponds to
a blackbody at a temperature of 62 K (Solfer et al. 1984).
2)
3)
Silicate absorption is a dominant feature.
The IRAS fluxes at 25 and 60 Pa are consistent with the observed
ground-based fluxes at 20 and 32 pa measured with a 5.5-arcsec
aperture.
645
E.E.BECKLINANDC.G.WYNN-WILLIAMS
-12
'E
-13
0=.,
-- -lZ,
I ' I ' I
/ 1= \ Arp 220
• ""\ Non-stettar
/
/
/ \
/ o,\
/
0 • / \
I / \
• IRAS \
00round \ ._.
Y
J I L I , I
12 13 1/+
Log v (Hz)
i
i
i Figure 5: Energy dis tribution
from 1 to 350 P4n.
A 6-cm VLA map with 0.4-arcsec resolution is shown in Figure 6. It shows
two slightly resolved sources (e ~ 0.2 arcsec) separated by about 1 arcsec in
right ascension. The structure we see at 6 cm differs significantly from the
triplet morphology found by Norris (1985) using the MERLIN array at 18 cm. We
have not yet established whether this difference is attributable to time varia-
bility, spectral index variations, or to differences in the data reduction pro-
cedure. The compact I0- and 20-_m source lies within 1 arcsec of the midpoint
of the radio peaks; this has been determined by offsetting the IRTF from nearby
AGK 3 stars.
Figure 6: VLA 6-cm map.
23"z,0'10"
23%0'0B'
23%0'06"
t t I t t (3
Arp 220 ,
6 cm
c?
0
o
o
i ,L I o I 0 I I
15h32'=¢7'.1 15h32=¢6'-.9 15h32=_6=.7
Right ascension
NEW OBSERVATIONS BY OTHERS
A student of ours at the Institute for Astronomy (D. DePoy) has measured
the Brackett-= line at 4.1 _m on the UKIRT using CGS2 with a 5.5-arcsec beam.
From his results, presented at this conference (DePoy 1986), he finds that the
line is broad (~1300 km/sec FWHM). The line strength implies an infrared
excess [Lbol/LLy= -i] ~ 300. Correction for the reddening based on the sill-
646
1-32/_m OBSERVATIONS OF ARP 220
cate absorptlon depth would reduce the excess to about i00. Because the 32-_m
flux originates from a region (Figure 5) with the same angular diameter as the
beam used for the B= measurements, no correction for aperture size is
necessary.
The Palomar infrared group has made sllt scans of Arp 220 at 2.2 _m using
the 200-inch telescope (Neugebauer, Matthews, and Scoville 1986). These data
are shown in Figure 7 along with a scan of a star; the sllt width was _I.0 arc-
see. Model fits with Gausslan profiles indicate that there is an unresolved
source at the center with a limit to the full width at half maximum of 0.2 arc-
see and a 2.2-_m flux of 8 mJy. Correcting for the reddening discussed above
implies an intrinsic 2.2 P_n-flux of 500-1000 mJy.
Figure 7: Sllt scan at 2.2 Itm
from the 200-inch (Neugebauer,
Matthews and Scoville 1980).
OJ
= 05
m
L-I
E
p
0
I
Arp 220
I
100
I
200
' 1
Sample number
DISCUSSION
A. Thermal Infrared Component
The energy distribution shown in Figure 5 suggests that the emission mech-
anism in Arp 220 is thermal emission from dust (Solfer et al. 1984). If half
of the 20-Pan flux observed in the 3-arcsec beam is coming from a region whose
diameter is equal to or less than 1.5 arcsec, then the brightness temperature
at 20 Pa is greater than or equal to 60 K. From the spectrum the color temper-
ature is about 62 K (Soifer et al. 1984). Thus the optical depth in emission
at 20 P4n is close to one. This agrees well with the depth of the silicate
absorption feature at I0 Pa if x9.7 ~ 2 xlS; such a relationship is observed in
galactic sources (Forrest et al. 1979).
Because the source is thick at 20 Pa and emits approximately like a black-
body at its 20-_m brightness temperature, it is very probable that much of the
longer wavelength emission near the peak of the energy distribution is coming
from a region 0 N 1.5 arcsec (NS00 pc in diameter). Although radiative trans-
fer effects could make the emission region at 50 _m somewhat larger than at 20
Pa, it should be noted that this size is consistent with a 50-Pan upper limit to
the size of 8 arcsec measured by Joy et al. (1986) from the KAO and our 32-_m
measurement with a 5.5-arcsec beam (Figure 4). The infrared radlat]on from
647
E.E.BECKLINANDC.G.WYNN-WILLIAMS
Arp 220 appears highly concentrated toward the center.
B. I- to 3-P_n Infrared Component
Both the 2.2-_m scan on the 200-1nch telescope and our unpublished aper-
ture studies from the IRTF, show that there is a component in the 1.65- to
3.8-P_n range which may not be stellar; it is observed to be about i0 mJy at
2.2 _m or 500 mJy when corrected for reddening. If this component has a spec-
trum similar to the QSO 3C273, then its total luminosity would be of the order
of 1012 Le.
C. Collisional Ener_
The present observations can be used to test the various luminosity
sources in Arp 220 that have been suggested to date.
The model of Harwit et al. (1986) which predicts the amount of mechanical
energy converted into infrared luminosity when two galaxies merge does not give
a size for the emission region. Such a mechanism would not appear to naturally
explain the emission size that we observe.
D. Star Formation
The hypotheses that a burst of star formation is the primary energy
source in Arp 220 has a number of problems in light of the present set of
observations.
One of the strongest arguments used to support star formation has been
extended 10-_m emission (Rieke et al. 1985; Joseph and Wright 1985). The
observation of extended 10-_m emission in Arp 220 appears to be in error. The
surface density of star formation that is required in Arp 220 is 25 times
larger than in M82; prior to IRAS, M82 had one of the highest densities of
nuclear star formation. Is it reasonable to have such a rate of star formation
in such a limited volume? Also a region of star formation in the central 500
pc of Arp 220 should have a corresponding 500-pc diameter radio source, but
only two sources <50 pc diameter are seen at 6 cm.
Several parameters that are used to determine the amount of star formation
have also been measured in Arp 220. These include the Brackett-_ line strength
which gives the number of ionizing photons (DePoy 1986), the CO llne strength
which gives an estimate of the interstellar material (Sanders and Mirabel
1985), and the 2.2-_m continuum which gives a limit on the number of M super-
giants. In each ease, if one scales from M82 using the total luminosity, one
finds that the star formation indicators are a factor of I0 less than expected.
There appears to be very little evidence for star formation producing more than
10% of the observed luminosity in Arp 220.
E. An Active Nucleus
There does appear to be a number of observations that suggest that a
compact active nucleus is the primary luminosity source in Arp 220. It is the
easiest and most natural way to explain the compact infrared morphology. This
includes both the direct size measurement and the deep silicate absorption
feature. It is also the easiest way to explain the compact radio sources, the
648
1-32#mOBSERVATIONSOFARP220
broad Brackett-= line, and the presence a point-like 2-_m source. The latter
source, in fact, could contain enough energy to produce the observed infrared
luminosity, if it is a power law source in the uv and x-ray region.
In summary, the present observations are consistent with the idea that
more than 90% of the luminosity in Arp 220 originates from a radio quiet active
nucleus.
SPECULATION
Based on Arp 220, MKN 231, NGC 6240, and NGC 1068 it appears that both a
burst of star formation and an active nucleus are present in luminous IRAS
galaxies. A burst of star formation produces L ~ few x i0 II Le. If L ~ 1012
Le, the dominant source of luminosity is a radio quiet active nucleus. Since
interstellar material certainly feeds the star formation, it is natural to
speculate that it also feeds the active nucleus.
ACKNOWLEDGMENTS
We thank D. DePoy, G. Neugebauer, K. Matthews, and N. Scoville for pro-
viding us with their unpublished data and N. Devereux and G. Hill for useful
discussions. This work was supported under NSF grant AST 84-18197.
REFERENCES
DePoy, D. 1986, this conference.
Emerson, J. P., Clegg, P. E., Gee, G., Cunningham, C. T., Griffin, M. J.,
Brown, L. M. J., Robson, E. I., and Longmore, A. J. 1984, Nature, 311,
237.
Forrest, W. J., McCarthy, J. F., and Houck, J. R. 1979, Ap. J., 233, 611.
Harwlt, M. O., Houck, J. R., Soifer, B. T., Palumbo, G. G. C. 1986, preprint
and this conference.
Joseph, R. D. and Wright, G. S. 1985, M.N.R.A.S, 214, 87.
Joy, M., Lester, D. F., Harvey, P. M., Frueh, M. 1986, Ap. J., 307, Ii0.
Neugebauer, G., Matthews, K., and Scoville, N. 1986, private communication.
Norris, R. P. 1985, M.N.R.A.S., 216, 701.
Rieke, G. H., Cutrl, R. H., Black, J. H., Kailey, W. F., McAlary, C. W.,
Lebofsky, M. J., and Elston, R. 1985, Ap. J., 290, 116.
Roche, P. F., Aitken, D. K., Smith, C. H., James, S. D. 1986, M.N.R.A.S.,
218, 19P.
Schild, R. E. 1985, Sky and Telescope, 69, 24.
Sanders, D. B., and Mirabel, I. F. 1985, Ap. J. (Letters), 298, L31.
Seller, B. T., et al. 1984, Ap. J., (Letters), 283, LI.
Solfer, B. T., Sanders, D. B., Neugebauer, G., Danielson, G. E., Lonsdale,
C. J., Madore, B. F., and Persson, S.E. 1986, Ap. J. (Letters), 303,
L41.
DISCUSSION
HARWIT:
The small size of Arp 220 is impressive. In the model we presented yesterday, we took 2 kpc diameter x
0.1 kpc thick disks colliding with each other in order to accommodate 10 t° M® of H2 at a normal molecular
density of n ,,_ 10Scm -3. We know that Arp 220 has that much gas from CO observations. Its compactness
simply says densities are higher;, but that does not affect the way such a model works. Higher densities tend
to lead to higher luminosities for a given size source.
649
E. E. BECKLIN AND C. G. WYNN-WILLIAMS
BECKLIN:
That is the nice thing about a theoretical model, it can always be fixed up to agree with observations.
UNGER:
The radio continuum emission at 18cm has an extent of about 1.5 arcsec, rather larger than at 6cm. You
give an upper limit to the angular extent at 20#m of 1.5 arcsec - can you push this upper limit any further to
say that the radio and 20#m emission aren't coincident?
BECKLIN:
That is unlikely, since if the source is thermal, it cannot be made smaller than 1.5 arcsec.
ELIAS:
Since you know the approximate size of the 20 micron source, and also that it has an optical depth of at
least 1, what is the minimum amount of dust required?
BECKLIN:
About 107 M e in dust. If the dust to gas ratio is 0.01 by mass, this corresponds to 109 M e in gas, not
too different from the amount estimated from the CO.
YOUNG:
Could you please elaborate on the point that the CO is too low by a factor of 5. Given the M(H2) and
the dust temperature in Arp 220, the LtR/M(H2) ratio is precisely what you expect for a galaxy with Arp
220's $6o/$100 ratio.
BECKLIN:
For a 'typical' star forming region there is a 'standard' LIR/M(H2) ratio. If LIR/M(H2) is larger than
this ratio, physics demands that the dust temperature increase (unless Mdust/M(H2) were also larger). As
pointed out by Phil Solomon, your relationship has physical significance, but not astrophysical significance.
Astrophysically, Arp 220 has a larger Lm/M(H2) ratio than 'standard' star forming galaxies.
TELESCO:
As you know, in NGC 1068 the 10#m emission comes from a very compact region, whereas the far-
infrared luminosity originates from a much larger region. So you have to be careful in concluding that all the
luminosity of Arp 220 comes from a region 500 pc in size.
BECKLIN:
I know very well that we have not proven a thing. However, unlike NGC 1068, Arp 220, from its
spectrum, appears to have only one component, which is thick at 20#m.
650


Ground-based 1- to 32-microns observations of ARP 220: Evidence for a dust-embedded AGN?

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