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New Observations of Extra-Disk Molecular Gas in Interacting Galaxy Systems, Including a Two-Component System in Stephan's Quintet

We present new CO (1 - 0) observations of eleven extragalactic tails and bridges in nine interacting galaxy systems, almost doubling the number of such features with sensitive CO measurements. Eight of these eleven features were undetected in CO to very low CO/HI limits, with the most extreme case being the NGC 7714/5 bridge. This bridge contains luminous H II regions and has a very high HI column density (1.6 X 10^21 cm^-2 in the 55"CO beam), yet was undetected in CO to rms T(R)* = 2.4 mK. The HI column density is higher than standard H2 and CO self-shielding limits for solar-metallicity gas, suggesting that the gas in this bridge is metal-poor and has an enhanced N(H2)/I(CO) ratio compared to the Galactic value. Only one of the eleven features in our sample was unambiguously detected in CO, a luminous HI-rich star formation region near an optical tail in the compact group Stephan's Quintet. We detect CO at two widely separated velocities in this feature, at ~6000 km/s and ~6700 km/s. Both of these components have HI and H-alpha counterparts. These velocities correspond to those of galaxies in the group, suggesting that this gas is material that has been removed from two galaxies in the group. The CO/HI/H-alpha ratios for both components are similar to global values for spiral galaxies.


Introduction
Large amounts of interstellar gas can be removed from the main disks of spiral galaxies by four main processes: tides due to the gravitational force of a companion, ram pressure stripping during a near-head-on collision between gas-rich galaxies, ram pressure stripping by intracluster gas, and galactic winds driven by supernovae. By ejecting processed gas into intergalactic space, these mechanisms contribute to the metal-enrichment of the intergalactic medium. In many cases, which of these four processes is active in a given galaxy system can be determined from the optical, radio, and/or X-ray morphology. The signature of a tidal encounter between two galaxies is the presence of long stellar and/or HI tails and bridges (e.g., Toomre & Toomre 1972), while head-on collisions between gas-rich galaxies can produce ring galaxies (e.g., Lynds & Toomre 1976;Theys & Spiegel 1977) as well as gaseous bridges between the galaxies (Struck 1997). Ram pressure stripping by intracluster gas leads to an HI deficiency (e.g., Giovanelli & Haynes 1983), truncation of the outer HI disk of a galaxy (Cayette et al. 1990), and in some cases, bending of the HI disk (Kenney & Koopmann 1999), but not usually removal of large quantities of molecular gas (Kenney & Young 1989). Galactic winds are identified by extended extra-disk ionized gas without stellar counterparts (e.g., Rand, Kulkarni, & Hester 1990).
Star formation sometimes occurs in gas clouds far removed from the main disks of galaxies. Of the four processes that can remove gas from disks, the best-known to trigger the formation of young stars in the stripped gas is tidal forces: numerous examples of luminous H II regions in tidal features have been found (e.g., Schweizer 1978;Mirabel et al. 1991Mirabel et al. , 1992. Young stars have also been found in extra-disk gas clouds thought to have been stripped during head-on collisions (Smith et al. 1999) and interstellar-intracluster encounters (Xu et al. 1999).
In order to better understand the processes that lead to the removal of gas from galaxies and the triggering of star formation in this gas, it is important to make a complete inventory of the gas in these features. This means not just the atomic gas, which has been surveyed in a large number of extragalactic tails and bridges (e.g., Haynes et al. 1984;Smith 1991;Hibbard & van Gorkom 1996), but also the molecular gas. Molecular gas has proved elusive in classical tidal tails; in our earlier CO survey of six tidal tails (Smith & Higdon 1994), no CO was found to very low levels, while only very low mass concentrations of molecular gas (≤10 7 M ⊙ ) were found in the tidal features of the nearby interacting system M81/M82/NGC 3077 (Brouillet, Henkel, & Baudry 1992;Walter & Heithausen 1999).
In contrast to tidal features, gas removed from galaxy disks by ram pressure during head-on collisions between two gas-rich galaxies may be richer in CO. The proto-type of this class of object is the CO-rich gas concentration found outside the disk of the Virgo Cluster galaxy NGC 4438 by Combes et al. (1988). If the standard Galactic N H 2 /I CO ratio holds in this feature, it contains M H 2 ∼ 10 9 M ⊙ . The ring-like morphology of NGC 4438 and scan was 6 minutes long. The beamsize FWHM is 55 ′′ at 115 GHz. The pointing was checked periodically with bright continuum sources and was consistent to 10 ′′ . The system temperatures ranged from 200 to 500 K. Calibration was accomplished using an ambient chopper wheel.
A total of 45 positions in nine interacting systems were observed. These nine systems are listed in Table 1, along with a brief description of their optical morphologies. In Figure   1, we display optical images of these systems, obtained from the Digitized Sky Survey 2 (DSS). Table 2 lists the observed positions in the galaxies and tails/bridges; these positions and the CO FWHM beamwidth are marked in Figure 1. Fourteen of the observed positions were in tails or bridges; the rest were in the main disks of the galaxies. A total of eleven tails and bridges were observed. In three systems (Arp 144, NGC 2814/20, and NGC 3628), we observed multiple positions in a single tail or bridge. The results of the CO observations are given in Table 3: the line fluxes, velocities, widths, and rms noise levels. The summed spectra for each observed position are displayed NGC 7252, and Arp 143 (Smith & Higdon 1994), Arp 245 (Braine et al. 2000), NGC 4410 (Smith 2000), NGC 3561 (Braine et al. 2000), M81 (Brouillet et al. 1992) and NGC 3077 (Walter & Heithausen 1999), as well as the tentative (4σ) detection of CO in the southern tail of NGC 4038/9 (Gao et al. 2000). Note that in this paper we present CO data for the northern tail in the NGC 3561 system, while data for the southern feature are available from Braine et al. (2000). In Tables 5 and 6, we also give the Hα luminosities for these sources, when available, as well as the L Hα /M H 2 ratio.
For the sources in Tables 5 and 6 which have HI data available, in Figure 3 we plot the HI column density in the CO beam N HI against M H 2 /M HI . For those with Hα fluxes also available, in Figure 4 we compare L Hα /M H 2 with M H 2 /M HI . Figures 3 and 4 show a wide range in L Hα /M H 2 and M H 2 /M HI ratios for the features in our sample. The most extreme cases are 1) the NGC 7714/5 bridge, with a very high HI column density but very low CO flux, 2) the eastern feature of NGC 2782, with a low L Hα /M H 2 ratio, 3) the molecular clump near NGC 4438, with a very high implied M H 2 /M HI ratio and a low L Hα /M H 2 value, and in spite of its high column density of atomic hydrogen (1.6 × 10 21 cm −2 in the CO beam; from the data in Smith et al. 1997). At this bridge, our upper limit implies N H 2 ≤ 5.0 × 10 19 cm −2 and M H 2 /M HI ≤ 0.063, more than three times smaller than our upper limit for the system with the second lowest M H 2 /M HI ratio in a tail or bridge, the NGC 3561 irregular. The HI column density in this bridge is above the H 2 and CO self-shielding limits for solar metallicity gas (∼5 × 10 20 cm −2 and 10 21 cm −2 , respectively; Federman et al. 1979;van Dishoeck & Black 1988). However, the nucleus of NGC 7714 is known to be low metallicity (French 1980;García-Vargas et al. 1997), so it is likely that the gas in the bridge would also have low abundances, leading to an enhanced N H 2 /I CO ratio. Thus there may be more molecular gas in this system than implied by the CO measurements. We note that the mid-infrared camera on the Infrared Space Observatory (ISOCAM) did not detect this bridge (O'Halloran et al. 2000), but it did detect the more distant extra-disk source in Stephan's Quintet (Xu et al. 1999). This suggests a smaller warm dust component in the NGC 7714/5 bridge.
The smaller galaxy NGC 7715, the eastern tail of NGC 7715, and the HI loop north of NGC 7714 are also weak in CO, with inferred M H 2 /M HI ratios ≤0.16, ≤0.16, and ≤0.24, respectively. These may also have enhanced N H 2 /I CO ratios. Note that NGC 7715, its tail, and the NGC 7714 loop have not been detected in Hα (Smith et al. 1997). Also note that NGC 7714 and NGC 7715 have absolute blue magnitudes of −19.8 and −18.1, respectively (de Vaucouleurs et al. 1991), compared to −17.7 for the Large Magellanic Cloud (de Vaucouleurs et al. 1991;Sandage, Bell, & Tripicco 1999). At absolute magnitudes fainter than about −19, both irregular and spiral galaxies typically have less than solar metallicities (Skillman et al. 1989;Vila-Costas & Edmunds 1992;Storchi-Bergmann et al. 1994), thus NGC 7715 may also be metal-poor.

NGC 2782 and NGC 4438
At the other extreme from the NGC 7714/5 bridge in terms of CO brightness is the eastern tail of NGC 2782 and the extra-disk gas cloud near NGC 4438, with low L Hα /M H 2 ratios compared to the other features (see Figure 4). These features have strong CO emission, but little on-going star formation (Combes et al. 1988;Kenney et al. 1995;Smith et al. 1999). None of the features in our new sample are as rich in CO as the NGC 2782 and NGC 4438 features. As discussed at length in Smith et al. (1999), the gas in these features may be metal-rich material removed from the interiors of their disks by near-head-on collisions, leading to high CO fluxes. Star formation may be inhibited in these features because of the collision (Smith et al. 1999). Gas clouds pushed out of a galactic disk by a high velocity collision may compress and then adiabatically expand, reducing their self-gravity. This may decrease the rate of star formation and therefore the L Hα /M H 2 ratio. 4.3. Arp 245,NGC 3561,and NGC 4038/9 In Figure 3 and Tables 5 and 6, the structure with the highest HI column density in the CO beam is the northern tail of the relatively nearby galaxy Arp 245, which has N HI = 2.3 × 10 21 cm −2 averaged over the 3.5 kpc beam . As discussed in Duc et al. (2000), this feature is relatively gas-rich and CO-rich (M H 2 ∼ 1.5 × 10 8 M ⊙ , M HI ∼ 4.8 × 10 8 M ⊙ , and M H 2 /M HI ∼ 0.31 in the CO beam) and has many properties in common with spiral galaxies. It has an L Hα /M H 2 ratio similar to spirals, and a stellar population dominated by an underlying old population . Furthermore, it has a relatively high extinction (A B = 2.6, from the Balmer decrement), a high blue luminosity (M B = −17.2 and L B = 1.2 × 10 9 L ⊙ , uncorrected for internal extinction), and a metallicity of 12 + log(O/H) ∼ 8.6 . With a modest correction for internal extinction, these values are consistent with the metallicity-luminosity relationship for late-type spirals (Zaritsky, Kennicutt, & Huchra 1994;Garnett et al. 1997). Thus it is possible that this structure may not be a tidal tail, but rather a pre-existing edge-on disk galaxy that is interacting with the other two galaxies in the system. High resolution kinematic data may be useful in testing this hypothesis. In any case, this feature appears to be richer in CO relative to HI and Hα than most dwarf galaxies.
The fact that the Arp 245 feature was detected in CO and many of the other features were not may be due to its high HI column density and relatively high metallicity. There is some evidence that N H 2 /I CO is correlated with metallicity (Wilson 1995;Verter & Hodge 1995;Arimoto et al. 1996). The trends implied in these papers suggest that at the metallicity of the Arp 245 feature, the N H 2 /I CO ratio is enhanced only slightly, between 1.5 − 3 times bigger than the Galactic value. Thus, at the high HI column density of the Arp 245 feature, the CO self-shielding limit is exceeded. We note that IC 10, which has a lower oxygen abundance that the Arp 245 feature, shows strong CO at positions where N HI exceeds 10 21 cm −2 (Ohta, Sasaki, & Saito 1988). At present, the metal abundances of most of the features listed in Tables 5 and 6 are unknown. The NGC 3561 system (Arp 105; Figure 1d) contains a spiral (NGC 3561A), an S0 or E galaxy (NGC 3561B), and two tidal features with on-going star formation, presumably pulled out from the spiral galaxy. The concentration of gas and star formation in the northern tail is classified as a Magellanic irregular by Duc & Mirabel (1994). The southern feature, which crosses the companion NGC 3561B, is visible as an optical knot in Figure 1d, and is classified as a compact dwarf by Duc & Mirabel (1994). This southern feature was detected in CO by Braine et al. (2000), yielding a high M H 2 /M HI ratio in the CO beam of ∼0.8. This ratio is higher than that of the Arp 245 feature, in spite of its more modest HI column density in the CO beam, ∼ 2 × 10 20 cm −2 (Duc et al. 1997) and its lower oxygen abundance (12 + log(O/H) ∼ 8.4; Duc & Mirabel 1994). This feature also has a lower blue luminosity (M B = −16.9; L B = 9.0 × 10 8 L ⊙ , uncorrected for internal extinction) and a lower extinction (A B = 1.0) (Duc et al. 1994) than the Arp 245 feature.
The northern tail of NGC 3561 (the Magellanic irregular) has a peak HI column density four times higher and a blue luminosity five times larger than the southern tail (Duc et al. 1997), and has a higher oxygen abundance 12 + log(O/H) ∼ 8.6 (Duc & Mirabel 1994), but was undetected in CO, with a lower M H 2 /M HI ratio of ≤0.2. This difference may be due in part to the difference in beam sizes of our CO observations and the higher resolution observations of Braine et al. (2000), which are less affected by beam dilution. In the interaction scenario for this system presented by Duc et al. (1997), the spiral galaxy NGC 3561A interacts with the elliptical NGC 3561B, drawing out a long tail to the north, and a shorter countertail to the south. Since both features may have originated in NGC 3561A, it is reasonable to expect they would have similar gas properties. The southern feature may have a relatively small angular size, high column density clump in which the self-shielding limits are exceeded.
If the tentative detection of CO in the NGC 4038/9 tail (Gao et al. 2000) is confirmed by more sensitive observations, it places this tail in the same category as the NGC 3561 and Arp 245 features: it is richer in CO than dwarf galaxies, relative to its Hα flux. The metallicity in this feature has been estimated to be somewhat lower than that in the Arp 245 feature, 12 + log(O/H) ∼ 8.4 (Mirabel et al. 1992).

M81 and NGC 3077
The tidal features of M81 and NGC 3077 have also been detected in CO (Brouillet et al. 1992;Walter & Heithausen 1999). Like those in Arp 245 and NGC 3561, they also have high inferred M H 2 /M HI ratios (Table 6). However, the inferred H 2 masses in the CO beam are small (∼10 6 M ⊙ , using the standard Galactic N H 2 /I CO ratio; 10 7 M ⊙ , using the virial theorem) and there is no evidence for on-going star formation in these locations (Brouillet et al. 1992;Henkel et al. 1993;Walter & Heithausen 1999). The M81/M82/NGC 3077 group is very nearby, so the area subtended by the CO beam (∼360 pc) is much smaller than in the other systems listed. The other features plotted in Figure 3 have physical beam diameters ranging from 1.8 kpc (NGC 4438) to 43 kpc (I Zw 192). As with the Arp 245 and NGC 3561 features, the CO self-shielding threshold may be exceeded locally in these features. We note that the HI column density in the CO beam is quite high for the M81 and NGC 3077 positions, 10 21 cm −2 .
A recent larger area CO survey of the NGC 3077 tail (Walters & Heithausen 2000) shows CO in two clumps covering an area of about 3 ′ (2.8 kpc). In this region, the standard Galactic conversion factor gives M H 2 ∼ 6 × 10 6 M ⊙ and an average H 2 column density of ∼8 × 10 19 cm −2 . As in IC 10, CO is only detected at HI column densities above ∼ 10 21 cm −2 . In this larger area, the inferred M H 2 /M HI ratio is smaller, ∼ 0.1, consistent with the upper limits on the undetected tails and bridges in the sample.

Stephan's Quintet
Of the nine extra-disk locations in our new sample, only one was detected in CO: the position in Stephan's Quintet (HCG 92; Arp 319). Stephan's Quintet is a compact group of four galaxies with similiar velocities (∼6000 km s −1 ; NGC 7317, NGC 7318A/B, and NGC 7319), and a probable foreground galaxy (NGC 7320) with a much lower velocity (∼800 km s −1 ). The outlying galaxy NGC 7320C also has a velocity of ∼6000 km s −1 , and so probably also belongs to this group. Strong gravitational interactions are clearly present in this group (Figure 1g). A long optical tail stretches more than 1 ′ (26 kpc) to the southeast of NGC 7319; this tail is also detected in HI (Shostak et al. 1984;Verdes-Montenegro et al. 2000). Two more optical tails are seen extending to the north of the close pair NGC 7318A/B (see Figure 1g). Along the easternmost of these features, X-rays (Pietsch et al. 1997) and radio continuum emission (van der Hulst & Rots 1981) are visible, suggesting a shock front. At the location where the two optical tails of NGC 7318A/B intersect, a large concentration of ionized gas has been detected (Moles, Sulentic, & Márquez 1997;Ohyama et al. 1998;Xu et al. 1999). This source is also present in the optical DSS image (Figure 1g), in the mid-infrared map of Xu et al. (1999), and in HI (Shostak, Sullivan, & Allen 1984;Verdes-Montenegro et al. 2000). Optical spectroscopy shows that the ionization mechanism for this source is young stars, while spectral signatures of shocks are present to the south, along the radio continuum and X-ray ridge (Moles, Márquez, & Sulentic 1998).
At the location of the extra-disk star formation region in Stephan's Quintet, we have detected two CO components, at 6000 km s −1 and 6700 km s −1 , at the 5σ and 6σ significance level, respectively. These two components have similar CO fluxes (Table 3).
The large velocity separation of these two features argues against the idea that they are caused by the double-horned signature of a rotating disk, but are instead two distinct gas clouds. These two components each contain 1.0 × 10 9 M ⊙ of molecular gas, if the standard Galactic N H 2 /I CO ratio holds. One of these components (6000 km s −1 ) was detected at a 4σ level in the CO interferometer data of Gao & Xu (2000). We have also detected CO in NGC 7319 and NGC 7318B. NGC 7319 had previously been seen in CO (Verdes-Montenegro et al. 1998;Yun et al. 1997), however, the weaker CO flux from 7318B was not seen in those less sensitive studies. The CO detection in NGC 7318B is at 6000 km s −1 , a somewhat higher velocity than the optical velocity of this galaxy (5765 km s −1 ; Moles et al. 1998).
Both of the CO components in the extra-disk star formation region have HI counterparts (Shostak et al. 1984). They also both appear to have on-going star formation. Moles et al. (1998) obtained optical spectra of three H II regions in this vicinity; two have velocities of ∼6020 km s −1 and one has a velocity of 6680 km s −1 . Two Hα velocity components are also indicated by the narrowband imaging of Xu et al. (1999).
The Hα luminosity, molecular gas mass, and HI mass for the extra-disk source in Stephan's Quintet are similar to those of many normal spiral galaxies (Young et al. 1996), but unlike those of typical dwarf galaxies (Sage et al. 1992). The implied M H 2 /M HI ratios for both velocity components in this region are very high, ∼0.65, higher than values typically found for dwarf galaxies (Sage et al. 1992). The L Hα /M H 2 ratio for the Stephan's Quintet source, 0.016 L ⊙ /M ⊙ , is consistent with global values for spirals (Young et al. 1996), as well as the values found for the NGC 4676 tail, the Arp 245 feature, and the southern tail of NGC 3561, but is higher than the values seen for the eastern tail of NGC 2782 and the NGC 4438 clump. In Smith et al. (1999), we suggested that star formation was inhibited in the eastern tail of NGC 2782 and the NGC 4438 CO clump because of strong shocks sustained during a head-on collision between two gas-rich galaxies. If the extra-disk cloud in Stephan's Quintet was formed in this way, star formation is clearly not inhibited.
To explain the peculiar morphology of Stephan's Quintet, a number of different interaction scenarios have been suggested. Some of these scenarios involve ram pressure stripping during a head-on collision between two galaxies. For example, Peterson & Shostak (1980) suggest that a head-on collision between NGC 7318B and NGC 7319 removed the gas and caused the radio continuum ridge between them. A different scenario was suggested by Shostak et al. (1984): a prograde tidal encounter of NGC 7320C and NGC 7319 occurred, pulling gas out of these galaxies, creating the long tail south of NGC 7319. This was followed by a high-speed collision between NGC 7318A and NGC 7318B. Other scenarios involve a collision between an 'intruder galaxy' (usually NGC 7318B) and intragalactic gas that had previously been tidally-stripped or ram pressure-stripped in an earlier encounter -17between two different galaxies in the group (van der Hulst & Rots 1981; Moles et al. 1997). Moles et al. (1997) suggest that a direct collision between the outlying galaxy NGC 7320C and NGC 7319 occurred in the recent past (10 8 years ago), removing a large quantity of gas from these galaxies. After this event, NGC 7318B entered the group at high velocity, colliding with this stripped gas and triggering star formation.
Our new data shed some light on the question of the origin of the extra-disk gas in Stephan's Quintet. Our detection of two CO-rich components in the extra-disk star formation region shows that the star formation in this feature was triggered by a collision involving relatively high metallicity, molecule-rich gas concentrations, rather than metal-poor intracluster medium. This does not rule out ram pressure stripping of NGC 7318B by an intergalactic medium, but it does require that this intergalactic medium be relatively metal-rich gas: gas previously removed from a galaxy. How this gas was originally removed from the disk is unclear: as shown in Arp 245 and NGC 3561, structures apparently created by tidal forces can be relatively CO-rich. Thus this intergalactic gas may have been removed either by tidal forces, or via ram pressure stripping.
The 6700 km s −1 extra-disk component may have originally come from NGC 7319 or NGC 7318A, which have similar optical velocities (6650 km s −1 and 6620 km s −1 , respectively; Moles et al. 1998). The most likely candidate of these two is the barred spiral NGC 7319, which shows a surprising lack of H II regions (Arp 1973), a long optical tail, and a very offset CO distribution (Yun et al. 1997). NGC 7318A also has an optical tail, however, this may have been created by another interaction.
The 6000 km s −1 extra-disk gas may have originated in either the outlying galaxy NGC 7320C or in NGC 7318B. NGC 7320C has an optical radial velocity of 6000 ± 150 km s −1 (Lynds 1972), consistent with this velocity. Previous arguments for the involvement of NGC 7320C (Moles et al. 1997) cited the long tail south of NGC 7319 that points towards NGC 7320C. Alternatively, this component of the extra-disk gas may have come from NGC 7318B. As noted above, our CO observations show a faint CO line in NGC 7318B at this velocity, redshifted 300 km s −1 from the optical velocity. This detection corresponds to a molecular gas mass of 1.7 × 10 9 M ⊙ , assuming the standard N H 2 /I CO conversion factor.
We therefore suggest that the extra-disk gas undergoing star formation in Stephan's Quintet originated in NGC 7318B and NGC 7319. At the present time it is unclear whether a single collision between NGC 7318B and NGC 7319 occurred, ram pressure stripping the gas and creating the shock front between the two galaxies, or whether gas had been removed from NGC 7319 earlier, by an earlier encounter with another galaxy, and then this gas was impacted by NGC 7318B.

Conclusions
We have obtained new CO (1 − 0) observations of eleven extragalactic tails and bridges, and compared these measurements with previously-published data for another thirteen features. Of these 24 structures, four have inferred M H 2 /M HI upper limits (assuming the Galactic N H 2 /I CO conversion factor) less than global values for spiral galaxies, consistent with those found for irregular galaxies. Three of these four CO-poor features are in the NGC 7714/5 system; the most extreme case is the star forming bridge, which has a very high HI column density (1.6 × 10 21 cm −2 in the 55 ′′ CO beam), yet was undetected in CO, giving an inferred M H 2 /M HI upper limit of ≤0.063. These features may have enhanced N H 2 /I CO ratios compared to the Galactic value due to low metallicities.
Of the 24 structures in our combined sample, eight have been detected in CO. Three of these eight (the Stephan's Quintet extra-disk source, the NGC 4038/9 tail, and the Arp 245 tail) have inferred efficiencies of star formation (L Hα /M H 2 ) consistent with global values for spiral galaxies. In contrast, the L Hα /M H 2 ratios for the eastern tail of NGC 2782 and the NGC 4438 clump are lower than typical for disk galaxies, suggesting inhibited star formation. In none of the features were our CO limits low enough to conclusively show L Hα /M H 2 ratios as high as those found in dwarf galaxies.
Of the eight detected features, two (NGC 2782 and NGC 4438) have optical morphologies indicating head-on collisions. The Stephan's Quintet feature may also have been formed in this manner. In these systems, we suggest that metal-rich gas with an approximately Galactic N H 2 /I CO ratio was pulled from the inner parts of the galaxies.
A few recent studies have reported the detection of CO in a number of classical tidal features (e.g., NGC 4038/9, Arp 245,and NGC 3561). In these structures, the HI column density may be locally high enough that the CO self-shielding limit is exceeded, in spite of modestly enhanced N H 2 /I CO ratios. There is a slight tendency for these tidal features to have lower M H 2 /M HI ratios and higher L Hα /M H 2 ratios than ram pressure stripped features discussed above, but there is a large amount of scatter and the sample size is small. More observations are needed to determine whether there is truly a statistical difference in the gaseous and star formation properties of tidal and ram pressure stripped features.
We are grateful for the help of the NRAO 12m telescope operators and staff in obtaining these data. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory under contract with NASA. We are pleased to acknowledge partial funding for this project from a NASA grant administrated by the American Astronomical Society and from NSF grant INT-9908542. Figure 1. Optical images of the nine interacting galaxy systems in our sample, with the locations and FWHM beamsizes of the CO beams marked. All images are from the Digitized Sky Survey (DSS), except when noted. a) Arp 144 (NGC 7828/9). There is an HI tail without a known optical counterpart to the southeast of the optical pair   (Rots 1978;Haynes, Giovanelli, & Roberts 1979); this tail has a faint optical counterpart not visible in the DSS image shown here (Kormendy & Bahcall 1974). The two marked locations are in this tail. We also observed the center of NGC 3628. f) I Zw 192. g)

Captions
Stephan's Quintet. h) A narrowband red continuum image of NGC 7714/5 (Arp 284), from Smith et al. (1997). i) UGC 12914/5 (Taffy Galaxies).   Tables 5   and 6 with both HI and CO data. For comparison purposes, the standard Galactic N H 2 /I CO ratio is used to calculate the molecular gas masses. As discussed in the text, this ratio may not hold in all cases. The unlabeled points near M H 2 /M HI ∼ 0.8 − 0.9 and N HI ∼ 10 20 cm −2 are the data for the Arp 143 tail, the two positions in the Arp 144 tail, the NGC 2782 western tail, and the NGC 4410 tail (see Tables 5 and 6).        S V dV, where D is the distance in Mpc; Bloemen et al. 1986), H o = 75 km s −1 Mpc −1 , and assuming 34 Jy/K for the 12m telescope and the source fills the beam (η c = 0.82). For the two sources with more than one position detected in the disk, the data were fit to a Mpc −1 , and assuming 34 Jy/K for the 12m telescope and the source fills the beam (η c = 0.82). As noted in the text, these assumptions may not hold for all these sources; they are used for comparison purposes here. c Not corrected for extinction. d Because this position is -37separated from the main disk of the galaxies by less than half of the FWHM beamwidth, the molecular gas mass is given as an upper limit here. e Hα luminosity from Duc & Mirabel 1994. This was obtained via spectroscopy with a 1.5 ′′ slit and so is a lower limit. f HI values from Shostak et al. 1984. Both velocity components are included in the L Hα and L Hα /M H 2 values given here. g Derived from the data presented in Smith et al. 1997. Stephan's Quintet Region A