Astron. Astrophys. 346, 45-57 (1999)

4. Discussion

4.1. Gas surface densities

In the central beam, the surface density of UGC 2855 (corrected for an inclination of ) is pc^-2. In the inner part of the bar, this value drops to pc^-2. UGC 2866 has a central mass surface density of pc^-2 (for ). These numbers (remember, however, that they are based on the SCF) are high enough to place UGC 2855 and especially UGC 2866 in the range of IR-bright starburst galaxies (e.g. Scoville 1991), but only UGC 2866 clearly is a starburst. A significantly lower gas mass (as discussed in Sect. 3.1.2) would reduce ; however, a similar correction is likely to be also necessary for the sample of IR bright starbursts, so that UGC 2855 and UGC 2866 would remain in this group. Starburst properties depend on morphology and dynamics (e.g. Kenney 1997), but the central concentration found in UGC 2855 should be a fairly favorable environment. We will, however, speculate below that UGC 2855 may be in a preburst phase.

4.2. Kinematics of the gas: UGC 2855

4.2.1. Rigid rotation and a high velocity feature

A position-velocity diagram of UGC 2855 along the major axis of the bar (Fig. 6) shows solid body rotation in the bar out to its end at a radius of kpc. Close to the center, extending to only a radius of 3" or 290 pc, there is another feature with a total velocity width of almost 300 km s ^-1. Fig. 7 illustrates the clumpiness of this structure at higher resolution.

Fig. 6. Position-velocity diagram of UGC 2855 along the major axis, from the naturally weighted data set. The cut is centered on the CO integrated emission peak at . Contours start at the level (0.22 K or 68 mJy beam-1) and range to 2.33 K in steps of .

Fig. 7a and b. Position-velocity diagrams of the central part of the bar at higher resolution. Center and orientation of the cut are the same as in Fig. a. The robustly weighted data. The contours range from (0.86 K or 104 mJy beam-1) to 4.38 K in steps of . b  The high resolution track only. The lowest contour is again at (2.68 K or 124 mJy beam-1), and the contour step is . While the `main bar' is almost resolved out, the high velocity structure breaks up into a number of clumps, labels 1-6.

An important question to be asked of a galactic bar is whether an Inner Lindblad Resonance (ILR) exists and where it is located. The presence of an ILR affects the ease with which gas can reach the galactic nucleus: At an ILR, material is often trapped in a circumnuclear structure and prevented from further infall.

The general shape of the position-velocity diagram of UGC 2855 could be modelled in terms of a bar with an ILR. Models like those applied by García-Burillo & Guélin (1995) to NGC 891 match the pattern we see almost perfectly (see especially their Fig. 8). The best fit occurs if the bar of UGC 2855 is seen almost side-on, i.e. along its minor axis, the angle also implied by the orientation of the bar parallel to the major axis of the galaxy. In this model, the `high velocity feature' represents gas on (anti-bar) orbits, while gas on bar-enforcing orbits causes the `main' feature.

Fig. 8. Position-velocity diagram along the major axis for UGC 2866. As is Fig. , the center of the cut coincides with the peak of the integrated CO emission, at . The lowest contour () is at 0.28 K or 54 mJy beam-1. The distribution is dominated by one feature only.

Despite the close match, caution is necessary in applying this model directly to UGC 2855. The modelling of NGC 891 assumed a weak bar, while the bar of UGC 2855 is strong. Strong bars, however, are not expected to have an ILR (Friedli 1998), and therefore orbits may not occur. A possible way out is an ILR that is located very close to the nucleus, so that the -to- coverage ratio remains very small. In this case, long, gas-rich bars with an ILR may still be possible (Ishizuki 1997). From the structure we see, we can thus place the ILR at an outermost radius of (290 pc). Still, even an ILR at radius might betray itself by the presence of gas on -orbits elongated along the minor bar axis. Often, galaxies with an ILR show circumnuclear rings with emission maxima (`twin peaks') where the and orbits intersect and gas accumulates (see Kenney et al. 1992, Downes et al. 1996, Sempere et al. 1995a for examples). No such structure along the minor bar axis is visible in UGC 2855.

Thus, the bar of UGC 2855 may not have an ILR at all . Since the high velocity pattern shows no forbidden velocities (which are, however, not expected for a viewing angle along the minor axis), it may also be a circularly rotating structure, i.e. a clumpy disk. This structure may be fed efficiently by gas falling in along the bar, especially if no ILR is present.

The end of the bar at kpc gives a firm lower limit to the location of the corotation radius (e.g. Elmegreen 1996). If corotation occurs close to the bar end, as is often assumed and indicated by numerical simulations (e.g. Athanassoula 1992), a pattern speed of the bar of 5.2 km s ^-1 arcsec^-1 or 53 km s ^-1 kpc^-1 is derived. However, Combes & Elmegreen (1993) argue that, especially in late-type galaxies, the length of the bar is determined by the scale length of the disk and corotation may be far outside the radius of the bar ends.

The rotation curve along the major axis, which coincides with the bar axis, does not represent the circular rotation curve of the galaxy and cannot be used to determine . Thus, a stringent determination of the location of the resonances has to await an evaluation of the potential of the galaxy, e.g. using a K-band image to obtain the mass distribution.

4.2.2. Velocity gradients

The isovelocity contours along the bar of UGC 2855 are almost perpendicular to the bar major axis. However, some deviations are found. Velocity differences, indicating streaming motions across the bar measured parallel to the minor bar axis, are typically 20 km s ^-1 in the plane of the galaxy (assuming ). There are variations, especially between the two sides of the galaxy (see Table 3): The velocity gradient is generally higher in the southeastern part of the bar, reaching a maximum of 46 km s ^-1 kpc^-1 at an offset of 14" from the center.

Table 3. Streaming motions across the bar of UGC 2855. gives the difference in velocity in the plane of the galaxy, measured perpendicular to the bar axis.

These velocity gradients perpendicular to the bar axis of UGC 2855 are comparatively small and smooth. One could suspect that, if there is a very sharp discontinuity, even the high resolution of our interferometric map might be insufficient to show it. However, the low dispersion we find in the region of the bar of UGC 2855 where the streaming motions reach a maximum implies that there is indeed no smoothed out, unresolved shock feature hidden.

A largely perpendicular orientation of the velocity contours with respect to the bar major axis is expected from simulations of gas streamlines in bar potentials if the bar is seen side-on. For this case, van Albada & Roberts (1981) predict, however, a very sharp, spikelike discontinuity in velocity across the bar, indicating the presence of a shock. In a large number of model runs Athanassoula (1992) almost always finds sharp velocity jumps, connected to dust lanes, shocks and gas density enhancements, usually on the leading side of the bar, no matter whether the bar has an ILR or not. Interestingly, the only model with no shock in her sequence is the one having the lowest central mass concentration (and no ILR) . Since infall along a bar should increase the central mass with time, it seems likely that a situation like this prevails early in the evolution of a bar. This agrees with the expectations of a linear theory of swing amplification for the initialisation of a bar, which requires the bar to set out with no ILR (Toomre 1981).

The general shape of the velocity contours we observe agrees with the predictions of the "no-shock"-model, especially on the more undisturbed northwestern side of the bar, where there are even hints of the expected `bulges' in the contours at offsets of about along the bar.

The only velocity discontinuity found along the bar occurs very locally, in a curved region at a major axis offset of about , close to the bar end. Here, a jump of close to 80 km s ^-1 over little more than is seen, i.e. the discontinuity is unresolved and corresponds to a gradient of at least 600 km s ^-1 kpc^-1. It is noteworthy that this region is located on the more disturbed side of the bar.

4.2.3. Properties of the inner structure

If the inner high velocity structure is a circularly rotating disk, presumed to be aligned with the plane of the galaxy, i.e. seen at an inclination of 60^o, its dynamical mass will be (assuming a radius of 290 pc and a circular velocity of 160 km s ^-1 in the plane of the galaxy, estimated from Fig. 7). The total mass in the central region of UGC 2855 may thus be smaller than the mass in the inner 300 pc radius of the Milky Way ( , e.g. McGinn et al. 1989).

The H₂ mass from the high resolution data in this area is . This rises to if the naturally weighted map of the central region is evaluated. After the helium correction, or more than half of the dynamical mass in the central 3" radius along the bar may thus be gaseous (for a lower gas mass, this percentage drops by a factor of up to 8).

The clumpiness of the high velocity feature (Fig. 7b) allows the identification of 6 molecular cloud complexes above the level, labeled 1 to 6 in Fig. 7b. The complexes 5 and 6 show a clear drop in velocity and are likely to belong to the main bar structure.

The diameters of these complexes, which can only be resolved as distinct entities in a pv-diagram, are of order 300 pc. With a velocity width of km s ^-1, they have virial masses of roughly . This matches the definition of Giant Molecular Associations (GMAs) coined by Vogel et al. (1988). We find an H₂ (SCF) mass of typically for these structures, which may suggest that they are bound objects.

It is interesting to note that no intensity peak at the center corresponds to the systemic velocity of 1200 km s ^-1. This explains the slight blueshift observed in the velocity field (Fig. 1b) at the center. It might also suggest an explanation for the moderate star formation even in the high surface density central region. The inner structure may be better described as a torus with a central hole of radius than as a centrally peaked disk. It it even conceivable that this structure is an inner bar, possibly with its own pattern speed (and an ILR at 0.5" radius). There is, in fact, a hint of a slight misalignment of the major axis of the inner structure with respect to the main bar: both the high resolution ¹²CO and the ¹³CO data are matched better by PA of than by the determined for the large scale bar.

4.3. UGC 2866

The position-velocity diagram along the major axis of UGC 2866 is displayed in Fig. 8. It confirms that all the molecular gas in this galaxy is part of one kinematic structure. It is, however, not possible to decide whether this structure is a bar or a circularly rotating disk. In the case of a rotating disk, inclined by 30^o, the dynamical mass is . However, if a circular disk was assumed, the measured axis ratio of the CO structure of would imply an inclination of . Thus, the value of deduced from the catalogued axis ratios seems too small. Alternatively, the observed discrepancy can be interpreted as pointing toward a bar morphology for the molecular structure.

4.4. Not all gaseous bars are equal: Evolutionary effects?

From the rarity of continuous gas-rich bars it can already be inferred that these phenomena have to be transient. This is true even though some caution is necessary: Only few galaxies have been fully mapped in molecular gas, and thus bars rich in molecular gas may be more frequent than is apparent today (Turner 1996). We found one gas-rich system with a long bar (UGC 2855) and another system the morphology of which is compatible with all gas being in bar-like structure (UGC 2866) among only three galaxies we have inspected so far. The ready success of our search for such systems suggests that such objects might not be extremely uncommon.

Evolutionary scenarios suggest that infall of matter along bars drives a change from a later to an earlier galaxy type, since the flow concentrates mass in the center, leading to a more pronounced bulge or nuclear region. UGC 2855, classified as SBc, does not seem to have a large central mass concentration (though we do not yet have K-band data to derive the mass distribution of the galaxy), so it may be at the beginning of the concentration process. In this case, the bar may be very young, in line with the notion that interaction with UGC 2866 has recently triggered its formation.

4.4.1. Gas and star formation properties: a comparison between UGC 2855 and NGC 7479

Two of the longest gas-rich bars known, the one of NGC 7479 and the one of UGC 2855 reported here, are very dissimilar objects. This is especially obvious when one inspects the H images: The bar of NGC 7479 is clearly outlined in H emission, indicating vigorous star formation all along it (Martin & Friedli 1997, Quillen et al. 1995). Dust lanes at the leading edge of the bar are widely regarded as indicators of a shock front (e.g. Athanassoula 1992). In contrast, the bar in UGC 2855 is only barely visible in H (Fig. 4a). It is interesting to note that the part of the bar southeast of the nucleus, which is weaker in (interferometrically detected) CO, but more disturbed in velocity , is the region of the bar where faint H can be most clearly detected.

May the gas in the bar of UGC 2855 be in a more quiescent state than in the bar of NGC 7479? Clearly, the degree of star formation as indicated by H emission varies over a wide range in barred galaxies (Martin & Friedli 1997).

¹²CO/¹³CO line ratios may give a hint here, since they allow insight into gas properties: In the cool ISM of the Galatic disk, where the ¹²CO line has a high optical depth, this ratio is (Polk et al. 1988). Centers of normal galaxies (including, at least on scales averaged over a large part of the Central Molecular Zone, our own Galactic center region) typically have ¹²CO/¹³CO ratios ranging from 10-20. A high ratio is () can indicate higher gas kinetic temperatures, the presence of diffuse molecular gas (e.g. Aalto et al. 1995), or, alternatively, higher gas densities, though, from non-LTE models, this effect alone cannot account for ratios exceeding . These relations can only serve as a rough guide, since they do not include abundance effects, and a multi-transition study including more than two lines is needed for a proper excitation analysis that allows us to distinguish between the possibilities listed above. Still, a varying ¹²CO/¹³CO line ratio is a clear indicator of changes in the gas properties.

In NGC 7479, large variations are found, with the ratio exceeding 20 along the bar, where the OVRO interferometer does not detect ¹³CO, and a very variable (15-35) ratio in the center (Aalto et al. 1998). Our interferometric and complimentary single dish measurements, which have detected ¹³CO in several positions along the bar, indicate a more constant line ratio of just below 10 along the bar in UGC 2855, with only a slight trend toward an increase (to 10) in the center. This might mean that the amount of diffuse or hot gas in the bar of UGC 2855 is lower than in NGC 7479 (see Table 4). The narrow line widths found in the bar of UGC 2855 support this notion. Of course, caution is required when interpreting these line ratios, since the 33" beam of the OSO single dish telescope picks up emission from the spiral arms. Still, the excellent agreement of the ratios between the interferometer and the single dish telescope in the center implies that not much diffuse emission missed by the interferometer is present there. This is confirmed by the flux comparison between interferometer and single dish data (Sect. 3.1.2).

Table 4. ¹²CO/¹³CO total integrated line ratios for UGC 2855 and, for comparison purposes, NGC 7479 and typical galactic ratios. errors are given in parantheses.
Notes:
a) Polk et al. 1988
b) Aalto et al. 1995

It is suggestive (though only marginally significant) that the OSO ¹²CO/¹³CO line ratios in the northwestern part of the bar, which shows a more regular velocity field than the southeastern part, are slightly lower, possibly indicating even more quiescent, `disk-like' gas properties. Along the same line of argument, more flux seems to be missing from the interferometer map in the southwestern bar, and more H emision is detected, indicating a larger amount of diffuse gas and less quiescent conditions in this region.

We would expect shocks along the bar to trap the diffuse gas and prevent it from moving into the center more effectively than the dense gas. Thus, the observation of lower ¹²CO/¹³CO line ratios along the bar of UGC 2855 than in the bar of NGC 7479 ties in very well with the indication of less or no shocks along its bar derived from its velocity field (Sect. 4.1.2).

The velocity field of NGC 7479 is distinctly different from the regular field of UGC 2855, where the contours are almost perpendicular to the bar major axis. In contrast, the velocity field in NGC 7479 appears far more disturbed. The velocity contours close to the center of NGC 7479 at high resolution are almost parallel to the bar axis (Aalto et al. 1999 in prep. the field shown by Sempere et al. (1995b) based on single dish observations does not show this clearly). Part of the difference between the two galaxies can be attributed to different viewing geometries, since the angle between the bar axis and the line of nodes in NGC 7479 () suggests that the bar is not seen entirely side on. Indeed, early optical work on the velocity fields of barred galaxies (Pence & Blackman 1984) as well as the numerical simulations (van Albada & Roberts 1981) already clearly indicate that `S shaped' velocity contours, which are partially parallel to the bar axis, are best seen in galaxies where the orientation of the bar is close to 45^o with respect to the line-of-sight.

Still, all evidence, both dynamical and excitational, points toward there being no large-scale shock present along the bar of UGC 2855, while the discontinuities in the velocity structure of NGC 7479 are consistent with a shock. This is also indicated by a shock-like feature discovered at the leading edge of the bar in NIR color maps by Quillen et al. (1995).

In the future, an optical image of UGC 2855 that allows the detection (or exclusion) of the presence of dust lanes will be helpful to finally decide the question of whether the gas in the bar of this galaxy indeed manages to escape being shocked.

The FIR color index is routinely used to indicate dust (color) temperatures. The value of 2.6 found for UGC 2855 suggests K, assuming a spectral emissivity index of 1.5 and 1.0 for the lower and upper temperature limits, respectively, and applying color corrections. for NGC 7479 may be slightly higher, with corresponding values K. Estimating dust temperatures using the 60 µm flux is problematic since the 60 µm flux can have a large contribution from stochastically-heated dust grains, thereby over-estimating the temperature of the thermal-equilibrium grains that dominate the mass of the dust (see e.g. Désert et al. 1990, Wall et al. 1996). This effect is likely to be most pronounced in large spirals like UGC 2855 and NGC 7479 where the IRAS flux encompasses more than the nucleus and the bar. Still, these color temperatures hint at more star formation activity in NGC 7479. Interestingly, UGC 2866 has, again under the conventional assumptions, a high of K. This appears to be consistent with the high degree of star formation activity indicated by the H emission.

4.4.2. The evolutionary state of the bar of UGC 2855

Is it possible to place the bar of UGC 2855 in an evolutionary scheme? Martinet & Friedli (1997) use the IRAS color index as an indicator of star formation activity, relate it to bar strength and length and calculate 3-dimensional self-consistent models of bar evolution to predict the variation of these parameters with time. In their scheme, UGC 2855, having , (barely) belongs to a the group of galaxies with less pronounced star formation. Since the bar is both long and strong, this places it in either an early, preburst stage ( Myr) or in a late, postburst state ( Myr). The central-to-bar H emission ratio suggests an early evolutionary stage in the models of Martin & Friedli (1997). However, these models are still unable to explain all observed bar properties and differ significantly depending on the initial parameters, e.g. the star formation efficiency and the mechanical energy released into the ISM.

On the side of the observations, it has also to be kept in mind that the large IRAS beam may pick up significant emission from the gas-rich spiral arms of UGC 2855, and that the H activity is generally low in the bar, making the ratio uncertain. Still, these indicators together with the high gas content of the bar and the moderate mass concentration in the center let us speculate that the bar in UGC 2855 may be young. Then, this presently inconspicious galaxy has the potential to become a spectacular starburst in the future.

© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999
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