Galaxy-galaxy collisions are known to produce drastic changes in morphology and, in many cases, enhance the level of star formation activity in galaxies. In order to better quantify the effects that interactions have on the star formation characteristics of galaxies the authors undertook a multiwavelength survey of a large sample of interacting disk-type galaxies. The sample is optically-selected, the inclusion of systems having been based upon the presence of unusual morphological features--such as tidal tails, plumes, rings, warped disks--suggestive of tidal interaction. The sample is composed of about 115 systems, most of which are spiral-spiral pairs, with a few spiral-elliptical pairs and a few merging systems (see Bushouse 1986 for more details of the sample selection). This sample has now been studied in the optical, infrared, and radio regimes, including optical spectra and H alpha images, near-infrared photometry and imaging, far-infrared photometry, H I 21 cm emission-line measurements, Very Large Array (VLA) 20 cm maps, and CO emission-line measurements. This paper presents an overview and comparison of the results of the optical, infrared and CO surveys. With these data the authors can compare the far-infrared and CO properties of the galaxies with the classic optical and radio indicators of star formation activity and thereby determine what, if any, relationships exist between star formation activity and the far-infrared and CO properties of the galaxies

Bushouse, Howard A.

Lamb, Susan A.

Lo, K.-Y.

Lord, S.

Werner, M.

English

NASA Technical Reports Server

A MULTIWAVELENGTH SURVEY OF INTERACTING GALAXIES 
H. Bushousel, S. Lamb2, K.-Y. Lo2, S. Lord3, and M. Werner3 
IDept. of Physics and Astronomy, Northwestern Univ., Evanston, IL 
2Dept. of Astronomy, University of Illinois, Urbana, IL 
3Space Sciences Division, NASA-Ames Res. Ctr., Moffett Field, CA 
I. Introduction 
Galaxy-galaxy collisions are known to produce drastic changes in morphology and, in 
many cases, enhance the level of star formation activity in galaxies. In order to better quan- 
tify the effects that interactions have on the star formation characteristics of galaxies we have 
undertaken a multiwavelength survey of a large sample of interacting disk-type galaxies. The 
sample is optically-selected, the inclusion of systems having been based upon the presence of 
unusual morphological features-such as tidal tails, plumes, rings, warped disks-suggestive 
of tidal interaction. The sample is composed of about 115 systems, most of which are spiral- 
spiral pairs, with a few spiral-elliptical pairs and a few merging systems (see Bushouse 1986 
for more details of the sample selection). This sample has now been studied in the optical, 
infrared, and radio regimes, including optical spectra and Ha images, near-infrared photom- 
etry and imaging, far-infrared photometry,, H I 21cm emission-line measurements,, VLA 2Ocm 
maps, and CO emission-line measurements. This paper presents ?n overview and comparison 
of the results of the optical, infrared and CO surveys. With these data we can compare the 
far-infrared and CO properties of the galaxies with the classic optical and radio indicators of 
star formation activity and thereby determine what, if any,, relationships exist between star 
formation activity and the far-infrared and CO properties of the galaxies. 
11. Optical Properties 
Global star-formation rates (SFRs) derived from Ha, emission-line luminosities span a 
wide range with a large fraction of the interacting galaxies having SFRs indistinguishable 
from that of isolated spirals (Bushouse 1987). On average, however, the global SFRs of the 
interacting galaxies are a factor of 2.5 higher than that of isolated spiral galaxies of similar 
luminosity. The level of enhanced activity does not depend strongly on either galaxy size or 
the proximity of cmpanions, although high levels of star formation activity are seen more 
often in close pairs. All possible combinations of activity, i.e. pairs in which both galaxies 
are active or inactive and pairs in which only one galaxy is active, appear to occur in roughly 
equal numbers. 
Quantitative analysis of the spatial distribution of Ha emission in the interacting galax- 
ies indicates that the majority of interaction-induced star formation activity occurs preferen- 
tially in the near-nuclear regions. Modest enhancements in the level of disk star formation are 
also seen to occur in a few system@. Given the fact that the radial distribution of star-forming 
regions in isolated spirals approximately follows the integrated light of the stellar disk (Hodge 
and Kennicutt 1983), we conclude that interaction-induced star formation usually does not 
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follow the same pattern as preinteraction star-formation activity. 
Emission-line ratios of most nuclear region spectra are consistent with photoionization 
by H 11 regions, with only a few showing LINER and Seyfert characteristics. Thus it appears 
that in the vast majority of systems in this sample interactions are triggering “normal” 
,modes of star formation activity, while the creation or activation of nuclear “monsters” occurs 
infrequently. 
111. Far-Infrared Properties 
Far-infrared (42-122pm) luminosities derived from IRAS data for the interacting sys- 
tems again cover a wide range, showink considerable overlap with the luminosities of isolated 
spirals (Bushouse, Lamb and Werner 1988). However, the median far-infrared luminosity of 
the interacting galaxies is about a factor of four greater than that of a sample of isolated 
spiral galaxies. Sonie of this increase is most certainly due to the fact that the sample of 
interacting galaxies is somewhat biased, towards intrinsically more luminous galaxies, there- 
fore the ratio of infrared-to-optical luminosity is a more appropriate property for comparison. 
We find that the median €or the interacting galaxies is about a factor of two 
greater than that of isolated spirals. Thus, by this measure, interactions are producing an 
approximate doubling of infrared luminbsity. Assuming that this increase is tied to increased 
heating of interstellar dust due to enhm‘ced star formation, this result is entirely consistent 
with the factor of two or so increase in optically-derived star formation rates which was found 
previously. 
The median 12pm/25pm and 60pm/100pm colors of the interacting galaxies are also dif- 
ferent from those of isolated spirals, with the interacting galaxies having larger 60pm/100pm 
and smaller 12pm/25pm flux ratios. The higher 60pm/100pm flux ratios, which imply higher 
dust temperatures, are consistent with the assumption that the increase in far-infrared emis- 
sion is due to an increase in the population of young stars. 
Two curious phenomena are revealed when comparing the relationship between LIR /LB 
and LB for the interacting and isolated galaxy samples. First, there are no isolated galaxies, 
of any optical luminosity, with LIR/LB much greater than 10, whereas fully one-third of 
the interacting systems have LIR/LB ratios above this level and several have LIR/LB > 30 
(see figure 6 in Bushouse, Lamb and Werner 1988). Therefore it is likely that galaxy-galaxy 
interactions are a necessary requirement for significant enhancements in infrared luminosity. 
Second, there appears to be a cutoff at LB N lo9 La, below which no systems-neither 
interacting nor isolated-show a significant enhancement in infrared luminosity. This result 
is consistent with Smith et al. (1987) who find that galaxies of low blue luminosity are not 
strong infrared emitters. 
We have searched for correlations between classic optical indicators of star-formation 
activity and far-infrared properties in order to determine to what extent the far-IR emission is 
driven by star formation. We have found that on an individual galaxy-by-galaxy basis there is 
only a modest correlation between the star-formation rate per unit area of a galaxy (as derived 
from Ha emission-line luminosities) and the far-infrared “excess” ( LIR/LB)  and infrared 
colors. There is a much stronger correlation, however, between star-formation activity and 
both LIR/LB and far-infrared colors when the sample of galaxies is broken into groups based 
on their optical spectral characteristics. Those galaxies having spectra characteristic of an 
val 
286 
old “dormant” stellar population have an average LIR/&B-~ .~  and very low 60pm/100pm 
and 25pm/12pm flux ratios, while galaxies showing strong H 11 region-type emission features 
have an average l & / L ~ - 1 4  and 60pm/100pm and 25pm/12pm flux ratios a factor of 2-3 
higher than galaxies with low SFRs. Systems that appear to be in the process of merging 
into a single body have the highest values of all, with &IR/&&% and colors a factor of 3-4 
higher than low SFR systems. 
While the majority of interacting pairs are too closely spaced to be resolved by IRAS, 
detailed analysis of the IRAS data has allowed us to measure the levels of 25pm and 60pm 
emission from the individual galaxies in approximately 20 pairs. Contrary to the results of 
Telesco, Wolstencroft, and Done (1988), we find that in most spiral-spiral pairs both galaxies 
are far-infrared emitters. It is usually the case, however, that one of the galaxies is brighter 
at 60pm than its companion by a factor of 2 4 ,  although in a few pairs both galaxies are 
equally bright. 
IV. Molecular Gas Properties 
CO (J=l -O)  emission-line observations have been obtained for 24 of the interacting 
systems with the NRAO 12-m telescope (Bushouse et al. 1990). CO observations for another 
11 systems in the present sample are available in the literature. In agreement with previous 
studies, we find a strong correlation between far-infrared luminosity and molecular gas content 
for the interacting systems that have been observed. We also find strong correlations between 
the FIR luminosity-to-molecular gas ratio and both the 60pm/100pm flux ratio and LIR/LB. 
The molecular gas contents of the interacting galaxies span a large range but are, on average, 
a factor of 3 4  higher than that of isolated spiral galaxies. These systems also have an 
average LIR/M(H~) ratio about a factor of 2 higher than isolated spirals, which others have 
interpreted as a higher SFR per unit gas mass for interacting galaxies (e. g. Young e t  al. 1986; 
Solomon and Sage 1988). Systems suspected to be in the process of merging-e. g. Arp 193, 
Arp 243, and Arp 299-have the largest LIR/M(H~) values in the sample. The molecular-to- 
atomic gas ratio, M(B2)/M(H I), has a relatively constant value of -1.5 among the systems 
that have been observed and does not depend strongly on LIR/LB,  which has been found to 
correlate with star-formation activity (5111). Only those systems with very low LIR/LB also 
have unusudly low M(H2)/M(H I) ratios 5 0.4. Therefore systems with higher SFRs are not 
necessarily overabundant in terms of molecular gas relative to atomic gas. 
A comparison of optically-determined SFRs with the molecular gas content of the inter- 
acting galaxies shows that there is a modest correlation between the ratio of SFRs for the two 
galaxies in an individual pair and the ratio of molecular gas content for that pair of galaxies. 
In other words, it appears that the relative molecular gas content of the two galaxies in a pair 
is at least partidly responsible for the relative level of interaction-induced star-formation 
activity. There are exceptions to this, however, as there are some pairs composed of two 
galaxies with nearly equal molecular gas content and yet one galaxy has a SFR as much as 
9 times higher than its companion. Therefore, orbital and other internal properties of the 
galaxies must also play a role in determining the level of interaction-induced star-formation 
activity. 
287 
V. Unusual Interacting Systems 
In spite of the correlations between various observable properties as discussed in the 
previous sections there are, of course, some systems that do not fit the general trends. As is 
often the case, these exceptions can reveal many interesting properties. First, while many of 
the interacting galaxies have very high SFRs, there are also many systems that have very low 
optically-determined SFRs and low far-infrared luminosities. Furthermore, many of these 
low SFR galaxies occur in pairs with a companion that has a very high SFR. These low 
SFR galaxies have optical luminosities that are typical for normal isolated spirals and also 
have normal atomic H I gas contents, while their molecular gas contents span a large range. 
Therefore these are not intrinsically small or “anemic?’ galaxies and so the low levels of star 
formation activity are not simply due to a lack of fuel. Orbital and other internal properties 
must again play a role here. 
Second, while we found that galaxies with optical indications of high SFRs usually have 
average LIR/LB and “warm” far-infrared colors, there are several , 
region-type spectra that have very low far-infrared luminosities and 
analysis has shown that these systems have low optical luminosities (LB 
N 5 x lo8 Le), but relatively normal atomic gas content (M(H I) - 4 x lo9 M e ) .  Optical 
emission-line ratios indicate that these galaxies are metal-poor, having O/H ratios 3 4  times 
lower than the high SFR systems that have high far-infrared luminosities. CO observations 
of these systems have yielded null detections, indicating very low molecular gas contents of 
M(W2) < IO9 Ad,. Therefore, the lack of far-infrared emission from these galaxies may be 
due to the absence of ufficiently massive ISM necessary to convert UV/optical radiation 
into the far-infrared. hermore, if these galaxies maintain their current SFRs, they will 
exhaust their molecular gas supplies in a few x lo8 years. 
VI. Near-Infrared Imaging 
We have obtained J (1.25pm) and K (2.2pm) images of about 20 of the interacting 
galaxies using the Kitt Peak National Observatory 58 x 62 element infrared camera (Bushouse 
and Werner 1990). In most systems the large-scale infrared morphology is similar to that seen 
in optical continuum images. However, in many systems there are small-scale features that 
are strikingly different from their optical counterparts, which we believe to be due mainly to 
the obscuring effect of dust at optical wavelengths. In some galaxies the infrared images have 
revealed the locations of previously invisible nuclei. In NGC 520, for example, the infrared 
images show the primary nucleus to be located at the very center of the massive dust lane 
that dominates optical images of the system. They also suggest that the fainter concentration 
to the northwest may be the nuclear remnant of a second galaxy. In general there is little 
correlation between the near-infrared and Ha morphologies of the galaxies. Only the very 
1.1 complexes stand out from the background stellar population in the infrared 
images. 
An analysis of the spatial distribution of infrared colors has shown that virtually all 
of the galaxies become redder towards their nuclei. On average, the nuclei are 0.4 and 0.9 
mag redder in J-K and E-K, respectively, than the disk regions. The majority of this effect 
appears to be due to an increase in the amount of dust in the nuclei of the galaxies. The ratio 
of color changes in and J-K is, however, somewhat less than the value for reddening 
by dust alone, suggesting that metallicity and population gradients are also present. The 
“excess” reddening in J-K relative to R-K might also arise if the dust is well mixed with 
the stars, in which case the infrared colors would have a larger contribution from stars with 
large amounts of extinction. It appears that little, if any, of the red nuclear colors is due to 
thermal emission from dust since of the two galaxies in-the sample that are experiencing the 
largest nuclear starbursts-NGC 1614 and NGC 7714-neither show unusually red nuclear 
colors relative to lower SFR systems. 
Bushouse, H. A. 1986, Ph.D. thesis, University of Illinois. 
Bushouse, H. A. and Werner, M. W. 1990, Ap.J., in press. 
Bushouse, H. A., Lamb, S. A. and Werner, M. W. 1988, Ap.J., 335, 74. 
Bushouse, H. A., Lamb, S. A., Lo, K.-Y., Lord, S. D. and Werner, M. W. 1990, in preparation. 
Hodge, P. W. and Kennicutt, R. C. 1983, Ap.J.,  267,563. 
Smith, B. J., Kleinmann, S. G., Huchra, J. P. and Low, F. J. 1987, Ap. J., 318, 161. 
Solomon, P. M. and Sage, L. J. 1988, Ap. J., 334, 613. 
Telesco, C. M, Wolstencroft, R. D., and Done, C. 1988, Ap.J . ,  329, 174. 
Young, J. S. et  al. 1986, Ap. J.(Letters), 311, L17. 
. 1987, Ap.J., 320, 49. 
289 
DISCUSSION 
Osterbroek: I believe the relatively small number of AGNs or 
Seyfert galaxies which you mentioned can be understood as 
resulting from the fact that your sample is selected as strongly 
interacting, violently disturbed systems. Dahari and others have 
shown observationally that mild interactions tend to enhance 
Seyfert activity, but violent.$nteractions do not. Presumably 
the explanation is connected with the situations in which gas can 
be delivered near the nucleus with nearly zero angular momentum. 
Bushouse: Yes, this result may be consistent’ with the study by 
Dahari, although it’s interesting to note that the few liner’s in 
my sample seem to occur’preferentially in the on-going merger 
systems which are obviously the most strongly interacting 
systems. I think the sample of ultraluminous I R A S  galaxies 
studied by Sanders et al. also shows evidence for larger than 
normal contributions from AGN-type activity. 
Lasov: Can you compare briefly the behavior of late- and early- 
type interacting galaxies? For example, is there any noticable 
difference between fractions of galaxies which have experienced 
enhanced SF among different morphological types? 
Bushouse: Unfortunately the disturbed morphologies of many of 
these galaxies do not allow us to accurately classify them by 
Hubble sub-types. However, I believe that in their sample, Keel 
and Kennicutt did determine Hubble sub-types and essentially 
found no correlation between star formation activity and 
morphological type across the spiral sequence . 
Kennicutt: (Comment in reply to a question from Zasov to 
Bushouse - dependence of star formation on morphological type. 1 
In our study the distributions of H-alpha emission are nearly 
independent of Hubble type. However there may be some 
differences in the spatial distributions of the emission. In 
those early-type spirals (Sa-Sb) which have high levels of 
emission the activity is often confined to the nuclear regions. 
2 90 


A multiwavelength survey of interacting galaxies

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