High resolution CO observations of the IRAS galaxies Arp 220, IC 694/NGC 3690, NGC 6240 and NGC 7469 were made with the Millimeter Wave Interferometer of the Owen Valley Radio Observatory. These yield spatial information on scales of 1 to 5 kpc and allow the separation of compact condensations from the more extended emission in the galaxies. In the case of the obviously interacting system IC 694/NGC 3690 the contributions of each component can be discerned. For that galaxy, and also for Arp 220, the unusually high lumonisities may be produced by nonthermal processes rather than by intense bursts of star formation

Sanders, D. B.

Sargent, A. I.

Scoville, N. Z.

Soifer, B. T.

English

NASA Technical Reports Server

N87- 2433 1
INTERFEROMETRIC CO OBSERVATIONS OF THE ULTRALUMINOUS IRAS
GALAXIES ARP 220, IC 694/NGC 3690, NGC 6240, and NGC 74691
A. I. Sargent, D. B. Sanders, N. Z. Scoville, B. T. Soifer
Division of Physics, Mathematics and Astronomy
Califomia Institute of Technology, Pasadena, CA 91125
ABSTRACT. High resolution CO observations of the IRAS galaxies Arp 220, IC 694/NGC 3690, NGC
6240 and NGC 7469 have been made with the Millimeter Wave Interferometer of the Owens Valley Radio
Observatory. These yield spatial information on scales of 1 to 5 kpc and allow the separation of compact
condensations from the more extended emission in the galaxies. In the case of the obviously interacting
system IC 694/NGC 3690 the contributions of each component can be discemed. For that galaxy, and
also for Arp 220, the unusually high luminosities may be produced by non-thermal processes rather than
by intense bursts of star formation.
1. INTRODUCTION
IRAS observations have shown that the galaxies Arp 220, IC 694/NGC 3690, NGC 6240 and NGC
7469 are all extremely luminous with LFm > 1011 L® (Sanders and Mirabel 1985). The molecular gas
masses, M(H2), derived from single dish measurements (Sanders and Mirabel 1985) are considerably in
excess of 109 M®. In addition, the ratio LFIR/M(HE), which can be interpreted as an indicator of star
formation efficiency (Sanders et al. 1986a), is significantly enhanced. A comparison of the properties of
these four galaxies with those of the starburst galaxy M82 is presented in Table I.
TABLEI
Galaxy D a M(H2) b LFIRb LFm/M(H2) b
(Mpc) (101°M®) (1011L®) (Lo/M®)
Arp220 77 1.3 13.4 100
NGC6240 101 1.99 5.3 26
NGC7469 66 0.92 2.6 28
NGC3690 I 48 0.73 5.3 72
IC694 I
M82 3 0.17 0.2 12
Notes:
a) Assumes Ho = 75 km s-1 Mpc-1.
b) From Sanders and Mirabel (1985) and Sanders et al. (1986a).
Although the presence of a significant quantity of molecular gas in these galaxies was discernible
from the single dish CO observations, their 45" resolution was not sufficiently high to determine the
distribution. Here we report 5" resolution, aperture synthesis CO maps of the four galaxies. These
1 These results are presented in full by Scoville et al. (1987) and Sargent et al. (1986).
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 471 ;";_-CF..DING PAGE BLANK NOT FILMED
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A. I. SARGENT ET AL. 
observations permit a study of the detailed structure of the molecular gas and the origins of the unusually 
high luminosities. 
2. OBSERVATIONS AND RESULTS 
Observations were made in the 2.6 mm CO line, using the Owens Valley Millimeter Wave Inter- 
ferometer in May, 1986. For each galaxy, two configurations of the three 10.4 meter telescopes were 
employed, with spacings out to 80 m north-south and 65 m east-west. The shortest projected baseline was 
10m, so that the interferometer is insensitive to structures greater than 30” in size. A recently-completed, 
broad-band filterbank consisting of 32 channels, each 5 MHz wide, provided velocity coverage of 416 km 
s-l, with 13 km s-l resolution. System temperatures were typically 300 K (SSB). 
The resulting contour maps of integrated CO intensity for Arp 220, IC694/NGC3690, and NGC 7469 
are shown in Figures 1, 2, and 3, respectively, overlayed on optical images of the galaxies. No emission 
was detected from NGC 6240 up to a level of 15% of the single dish flux, suggesting that the molecular 
gas in this galaxy is distributed on scales > 30”, rather than being confined to compact structures. In Arp 
220 and NGC 7469, the CO emission is unresolved. This is also the case for each of the two compact 
regions detected in IC 694/NGC 3690. Upper limits to the diameters of the compact sources range from 
1 to 2 kpc. Precise values, as well as the fraction of the single dish flux detected by the interferometer, 
Fi/F,, are given in Table 11. Average densities were calculated assuming a spherical distribution of gas 
and are also presented in Table 11. 
Figure 1: Contours of the integrated CO emission in 
Arp 220 superimposed on an optical photograph of 
the galaxy. Contour levels are spaced by 12 Jybeam 
km s-’, the lowest being 12 Jybeam km s-’. 
Figure 2: Contours of the integrated CO emission in 
Arp 299 (IC694-NGC3690) overlayed on an optical 
photograph of the galaxy. Contour levels begin at, 
and are spaced by, 10 Jybeam km s-I. 
TABLE I1 
Galaxy Phase Center Beam Scale Fi/Fs M(H2) NH 2 
~((1950) S(1950) &PC) (10~h.1~) ( ~ m - ~ )  
Arp220 15 32 46.92 23 40 07.9 6”.4 x 4”.0 1.4 0.7 9.1 130 
NGC6240 16 50 28.00 02 09 00.0 7”.6 x 6”.4 2.2 < 0.15 < 3  < 10 
NGC7469 23 00 44.00 08 36 18.0 13”.7 x 6”.4 2.0 0.3 2.8 10 
NGC3690 IC694 1 11 25 42.60 58 50 14.0 8”.3 x 4”.8 1.0 0.5 3.2 70 
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ORIGINAL PAGE 1s 
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INTERFEROMETRIC CO OBSERVATIONS 
PAGE IS 
Q U A L m  
Figure 3: Contours of the integrated CO emission in 
NGC 7469 superimposed on an optical photograph of 
the galaxy. Contour levels are spaced by 8 Jyheam 
km s-l ,  the lowest being 8 Jyheam km s-l. 
3. DISCUSSION 
3.1. ,Arp 220 
A substantial mass of molecular gas, N 9 x lo9 Ma, is concentrated in a region of radius < 700 pc at 
the nucleus of Arp 220 (Figure 1). The minimum mean density is 130 cm-3 and the ratio LFIR/M(Hz) is 
100 L,/M,. The star formation rate could be considerably enhanced as a result of an increased frequency 
of cloud-cloud collisions (cf. Scoville, Sanders and Clemens 1986), and the unusually high luminosity 
could be attributable to an intense burst of star formation (cf. Reike et al. 1985). However, it has recently 
been discovered that the spatial extents of the 20 pm and radio continuum emission are comparable, and 
less than 1.5” (Becklin 1986). This extreme concentration of the infrared and radio emission, the fact 
that the Br c( emission is both unusually broad (de Poy 1986) and much weaker than expected if the 
luminosity is the result of high mass star formation (Beck, Turner and Ho 1986), the compact, dense 
molecular core, and the uncommonly high value of L ~ R / M ( H ~ ) ,  provide compelling evidence that the 
energy in Arp 220 derives from non-thermal processes. 
3.2. IC 694/NGC 3690 
The interacting system Arp 299 comprises the galaxies IC 694 and NGC 3690. In the interferometer 
maps, two compact CO components were detected (see Figure 2), each of mass - 1.4 x 109M0. The 
eastern component coincides with the nucleus of IC 694 (Telesco, Decher and Gatley 1985) where there is 
an unresolved, flat spectrum radio source, termed A by Gehrz, Sramck, and Weedman (1983); the western 
component spans their compact radio components C and C’, which span the region where IC 694 and 
NGC 3690 overlap. Although CO is known to be prcsent at the nucleus of NGC 3690 (Sanders et al. 
1986b), no emission was detected by the interferomcter, indicating that there the molecular gas must be 
distributed on scales greater than 30”. 
Obsewations at 60 and 100 pm, at 20” and 30” resolution respectively (Harper 1984), show that about 
75% of the total luminosity from Arp 299 arises from the nucleus of IC 694. If the luminosity source is 
as concentrated as the CO, L F ~ / M ( H z )  is N 280 La/Ma. The H2 mass in a 16” region centered on this 
473 
A. I. SARGENT ET AL.
nucleus is _ 2.4 x 109 M® (Sanders et al. 1986b), leading to LFIR/M(H2) = 150 L®/M®. These ratios
are considerably greater than the values for the Milky Way, ,,_ 3, and for starburst galaxies, _ 20 L®/M®
(Sanders et al. 1986a, Scoville and Good 1986, Young et al. 1986). Taken together'with the presence
of the fiat-spectrum radio source at the nucleus, these results strongly suggest that the source of energy
in IC 694 may well be nonthermal. By contrast the ratio LFm/M(H2) for the remainder of the Arp 299
complex is between 40 and 70 L®/M®, indicating that the energy source here is probably massive star
formation.
The effect of the interaction between IC694 and NGC3690 appears to have been to channel a
substantial mass into the nucleus of IC 694, thereby producing an unusual energy source, probably fuelled
by non-thermal processes. It has also induced increased density and, by implication, enhanced star
forming activity in the region of overlap between the two galaxies.
3.3. NGC 7469
NGC 7469 is a type 1 Seyfert galaxy. From Figure 3, it is evident that the one CO source detected is
offset from the nucleus by about 8". Its mass, 2.8 x 109 M®, is approximately 30% of the total molecular
gas mass measured by Sanders and Mirabel (1985). On the basis of 3.3 #m emission measures, Cutri et
al. (1984) have suggested that star formation complexes may reside in a 2-8" annulus around the nucleus
of the galaxy. Since the interferometer observations were somewhat limited, the resolution here is not
optimal and the precise location and extent of the CO emission region is not well-determined. However,
it is tempting to speculate that it represents the densest part of such an annulus, perhaps similar to that
found around another Seyfert nucleus, NGC 1068 (Myers and Scoville 1986).
Acknowledgements
This work was supported in part by NASA through the IRAS extended mission program and by NSF
grant AST 84-12473.
REFERENCES
Beck, S. C., Tumer, J. L, Ho, P. T. P. 1986, Ap. J., in press.
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INTERFEROMETRICCOOBSERVATIONS
Sargent,A. I., Sanders,D. B., Scoville,N. Z., andSoifer,B. T. 1986, Ap. J. (Letters),
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DISCUSSION
A. S. Wilson. I'm a little puzzled by the offset of the CO emission from the nucleus of NGC 7469.
The centimetric radio continuum and circumnuclear optical line emission center very well on the nucleus.
Could atmospheric effects be responsible? It would be valuable if you could directly measure the relative
positions of the CO and the radio continuum in your data.
Sargent. Our observations of NGC 7469 were somewhat limited so that the exact size and location of
the compact CO region cannot be determined. It is possible that we have detected only the densest part
of an mad.us of gas centered on the nucleus. Higher sensitivity measures are needed to confirm this.
J. Carlstrom. Do you see any 3 mm continuum emission from these galaxies?
Sargent. We have not yet searched for continuum emission from the galaxies, although such a measure-
ment might permit a more accurate determination of the CO offset from the nucleus.
475


Interferometric CO observations of the ultraluminous IRAS galaxies ARP 220, IC 694/NGC 3690, NGC 6420 and NGC 7469

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