9.7. CO, IR, and Radio Continuum Observations of Ringed Galaxies

Most molecular, IR, and radio continuum studies of ringed galaxies have focussed on those objects with the most intense rings of star formation. Of the three types of rings, nuclear rings are most commonly found in such studies because the gas density in these regions can be very high and the star formation rate may be much higher than elsewhere in the galaxies.

A particularly well-studied case is NGC 1097. Telesco & Gatley (1981) showed that 10µm emission from the central region is ring-like and coincides with the optical nuclear ring. The total IR luminosity in the central 30" was estimated to be 10¹¹ L , most of which originates in the nuclear ring. The IR emission was suggested to be due to a burst of star formation in the ring. Hummel, van der Hulst, and Keel (1987 = HHK; see also Wolstencroft et al. 1984 and Hummel et al. 1984) obtained VLA radio continuum (1.5 and 4.9 GHz) and optical BV and H observations of the nuclear ring. The radio continuum observations revealed a closed ring pattern nearly coincident with a ring of HII regions. The HII regions in the ring have a low excitation much like that in NGC 3351 (Rubin et al. 1975). Detection of IR hydrogen recombination lines is the strongest indicator of a starburst in the NGC 1097 ring according to HHK. HHK compare the ring morphology in the different wavebands and separate the thermal and nonthermal radio emission. They find that one-third of the flux density from the ring at 4.9 GHz is thermal. The average extinction found is a factor of 3 at H. They find a smooth transition in the radio continuum emission from the dust lanes to the nuclear ring. From the ring H flux, they found a star formation rate of 5 M per year, after a factor of ten correction for internal extinction. Geometrically, they deduced that the nuclear ring is elongated perpendicular to the bar in the galaxy plane.

The nuclear ring of NGC 1097 has also been observed in the CO(1-0) emission line by Gerin et al. (1988). These authors determined that most of the CO emission comes from the nuclear ring and estimated a large molecular hydrogen mass of 1.3 x 10⁹ M. They suggest that the high gas pileup there is due to the bar and possibly also to a small interacting companion, NGC 1097A. Most importantly, they estimated the time-scale for consumption of all of this gas as a few hundred million years, similar to the time-scale for infall.

Sofue (1991) has reviewed CO observations of nuclear-ringed galaxies. Many CO-bright galaxies have a nuclear CO disk structure in the form of a ring, outside of which one normally finds dense molecular arms and sometimes bars. Sofue identifies two types of molecular nuclear rings: the first type has a radius of about 200 pc and a central cavity within the ring; the second type has a radius of 0.7-1 kpc. Examples of the first, compact type are M82, IC 342, Maffei 2, and the Galaxy; these do not necessarily have optical counterparts. Examples of the second, broad type of ring are NGC 1068 and NGC 1097; in these cases, the nuclear rings are prominent in optical images.

Not surprisingly, one of the first star-forming rings to be studied for its molecular gas content was NGC 4736. From a cut along the major axis, Garman & Young (1987) showed that the inner ring of this galaxy is a concentration of molecular gas. The central region of the galaxy was mapped more completely by Gerin et al. (1991), who detected CO(1-0) and CO(2-1) emission from the whole region of the ring and its interior. The molecular gas ring in NGC 4736 coincides closely with the HI inner ring (Mulder & van Driel 1993). Gerin et al. determine that only 10% of the total hydrogen in the ring is in atomic form.

Radio continuum emission at 20 cm has been detected from the central regions of NGC 4736 by Duric & Dittmar (1988). In this case the inner ring was detected as a weak secondary peak outside a bright central source of emission. Several compact sources, connected with bright HII regions or supernova remnants, were found along the ring.

NGC 4736 is near enough that far-infrared observations can resolve the location of emitting dust. Smith et al. (1991) observed the galaxy at 100µm in a series of low-resolution scans across the nucleus and inner ring region. They determined that the emission from this region is definitely resolved and non-gaussian, with peaks at the nucleus and secondary peaks where the scans cross the inner ring. The flux density of 100µm emission from the ring was found to be 1.5 times that from the nuclear source. The 100µm ring emission was also found to be coincident with the well-known bright H emission from the ring as well as the molecular gas ring. Smith et al. conclude that the far-IR emission from the ring comes from dust connected with the molecular gas ring. The total far-IR luminosity is about 100 times the H luminosity. To explain this, Smith et al. concluded that the ring dust is being heated by massive young OB stars, while the nuclear emission is from dust heated by non-OB stars.

Dahlem et al. (1991) discuss the detection of a molecular gas nuclear ring in NGC 1808, and conclude the gas is partly optically thin. This complex region has been studied in more detail by Phillips (1993b), who detected an H nuclear ring and analyzed large-scale outflow connected with the nucleus and striking optical filaments.

Garcia-Barreto et al. (1991) obtained H, CO, radio continuum, and near IR observations of NGC 1326. Virtually all of the CO and radio continuum emission was determined to come from the region of the bright nuclear ring of this galaxy. High resolution radio continuum emission maps revealed ``hotspots'' imbedded in diffuse emission in the nuclear ring. A large fraction of the radio continuum emission from the ``hotspot'' sources was determined to be thermal in nature. The CO mass in the region was determined to be 2.2 x 10⁸ M.

Combes et al. (1992) observed CO (1-0) emission from the central region of NGC 4314. They detected molecular gas on the inner side of the radio continuum nuclear ring, and concluded that the optical mini-spiral in this region is due to dust obscuration. Since the radio continuum ring coincides with recent star formation, the smaller H₂ nuclear ring implies that the ring is decreasing in diameter with time. Combes et al. suggest that dynamical friction of GMC's with bulge stars could account for this angular momentum loss. It would be a transient situation, seen after gas accretion in the ring has ceased. A similar explanation has been suggested for the nuclear dust ring in NGC 7217, which lies on the inner edge of the stellar nuclear ring (Buta et al. 1995b; Figure 37).

Kenney et al. (1992) brought attention to the phenomenon of ``twin peaks'' in barred spirals. In this structure, the CO emission in the bar has two bright peaks at the inner ends of the leading bar dust lanes. The prototype example is NGC 3351, where the two bright peaks are symmetrically placed with respect to the nucleus and oriented perpendicular to the primary bar. The spots are connected with a bright nuclear ring, and Kenney et al. state that the peaks are where inward flowing gas from the bar region intersects the nuclear ring of gas. The phenomenon may be connected with orbit crowding near the ILR regions of the bar. Kenney (1996) illustrates some of the variation of CO morphology of conventional nuclear rings. In addition to twin peaks, the nuclear ring of NGC 6951 appears as a CO spiral, while that in NGC 4314 appears as a partial ring. He suggests that some of the variation is due to variations in central mass concentration and bar strength. Some barred spirals with particularly young central starbursts do not have ring-like CO distributions in the center of the bar. The CO distribution in the nuclear ring of NGC 4314 has been further studied by Benedict et al. (1996), who found the ring to be incomplete, clumpy (five distinct clumps), with an off-center minimum.

Garcia-Burillo et al. (1993a, b) have obtained the highest resolution CO(2-1) and CO(1-0) observations of the quintessential grand design spiral galaxy M51. Their Figure 1 shows a clear ring of molecular gas in the central regions, a feature which would be classified as a nuclear ring. They state that the highest density of molecular gas is found in the ring, which corresponds to the beginnings of two spiral arms.

Wilson et al. (1991) discovered a nuclear ring in the Seyfert galaxy NGC 7469 via radio continuum observations. Near-IR observations at 3.3µm by Cutri et al. (1984) had revealed a very concentrated source of emission and dust with a high temperature. These authors concluded that the dust must be heated by hot stars, not the Seyfert nucleus or clouds in the surrounding regions. The galaxy exhibits unidentified emission lines in the 8-13 µm region typical of starbursts. Gas associated with hot stars in the central region is of low excitation compared to gas ionized by the Seyfert nucleus. IRAS properties of the system support the existence of nuclear HII regions. Strong ¹²CO emission and an IR H₂ emission line suggest that there is definite molecular gas within a few arcseconds of the nucleus. Cutri et al.'s observations give definitive evidence of a circumnuclear starburst in NGC 7469, but little information on its structure. Wilson et al.'s high resolution 6-cm observations detected a partial nuclear ring 1."5 from a strong compact source coincident with the Seyfert nucleus. The extended radio emission morphology seen is unlike most Seyfert nuclei. They use the ratio log[S₆₀ / S_{1.4 GHz}] = 2.11-2.27 to connect the emission mainly to a starburst. The emission-line morphologies are not like aligned narrow-line regions in Seyfert galaxies. Their radio map shows that much of the emission is from a ring of diameter 1 kpc (H₀ = 75). The size is typical of nuclear rings. The supernova rate was estimated to be 0.97 yr^-1.

Forbes et al. (1994a, b) reported the discovery of a radio continuum nuclear ring in the nearly face-on barred spiral NGC 7552. The ring is not distinguishable in single color broad-band optical images because of dust, but its presence is detectable in 3cm and 6cm radio continuum images. The ring has dimensions 9" x 7" and shows no obvious central source. In B - I, a partial dust nuclear ring is evident with a blue break. H emission is strongest in the blue break. In a J - K color index map, the nuclear ring is seen with similar dimensions and shape to the radio continuum image. The ring is 1 kpc in diameter and is nearly circular in the galaxy plane. U - B and V - K colors of the ring region support a burst of star formation < 5 x 10⁷ years old. The K-band flux is dominated by red supergiants, and the SN rate was estimated to be 0.4 yr^-1. They connect the ring to an ILR and suggest that the nucleus is in a dormant (non-AGN) phase. They also suggest that the best wavelength regime to search for small-scale nuclear rings is in the radio regime. They state the nuclear rings may form a depository for inflowing material which is driven to the nucleus in cycles, and may act as as ``nozzle'' for an outflowing superwind.

The spiral galaxy M100 (NGC 4321) is an example of a nuclear pseudoring that has been well-studied recently. As we have noted, this region shows a ring-shaped morphology in H images (Arsenault et al. 1988). In broadband ( V) HST images distributed for publicity, this region is a grand-design spiral that mimics the large-scale structure of the galaxy. Knapen et al. (1995a) obtained K-band images of this region at good resolution and detected a bar within the pseudoring (see also Pierce 1986) and two short leading spiral arms. The latter were determined to be not due to dust, and were not distinguishable at shorter wavelengths. Two symmetrically-placed knots were also identified at the ends of the inner bar. Knapen et al. (1995b) attribute much of the structure seen in this region to a density wave driven by the primary bar of this galaxy. The nuclear pseudoring was connected to a double inner Lindblad resonance, which is the only way to explain the leading arms near the ends of the inner stellar bar. An important conclusion made by these authors is that the inner bar is not necessarily a ``nuclear bar'' decoupled from the primary bar (see section 16); the leading armlets cannot be explained in this context. The double ILR interpretation of the central structure of M100 has been further supported by Sakamoto et al. (1995), who obtained high resolution CO data in this region. The CO distribution consists of two molecular spiral arms and a nuclear gas concentration. Sakamoto et al. interpret the molecular arms as due to orbit crowding of molecular clouds near the outer ILR, and the nuclear gas concentration as due to self-gravitating gas which fell into the center from the region of the inner ILR. Sakamoto et al. also determined that the nuclear bar is the dominant contribution to the potential, and that the molecular arms connected to the nuclear pseudoring are driven mainly by this small bar.

Knapen (1996a, b) has also discussed some of the general characteristics of nuclear rings in the near IR. In K-band images of the nuclear ring of NGC 6951, star forming sites line the nuclear ring in much the same manner as in a B - I color index map (see also Friedli et al. 1996). Knapen notes that this turns out to be the rule, while the structure in M100 is the exception. He suggests the differences might be due to differences in the mass distribution in the central kiloparsec, such that the inner ILR might be closer to the center in some galaxies as opposed to M100.

The recent availability of larger format near-IR arrays has led to JHK imaging of the global light distribution in some ringed galaxies. We show in Figure 49 H-band (1.6µm) images of the inner regions of NGC 3081 and the whole disk of IC 4214. It is remarkable how prominent the inner ring of NGC 3081 is in the near IR. If H-band surface photometry, as stated by Benedict et al. (1992), ``best represents the distribution of red giants, supergiants, and low mass dwarfs'' and essentially ``traces stars only'', then the inner ring has a clear old component and represents a true surface density enhancement in the old disk population. The structure of the ring is smooth but still shows traces of some of the smaller scale structure seen in blue light images. In IC 4214, the near IR image shows all three of the prominent rings of the galaxy.

Figure 49. B-band (left) and H-band (right) images of the ringed, weakly-barred galaxies NGC 3081 and IC 4214. Only the inner ring region of NGC 3081 is shown in H.

Isobe & Feigelson (1992) have derived far-IR luminosity functions of normal galaxies using a volume-limited sample and the technique of ``survival analysis'' as a means of allowing for the effects of censored data. The study used IRAS fluxes in the 12, 25, 60, and 100 µm passbands, and Gaussians were fitted to the integral luminosity function. In addition to examining for a possible dependence of this luminosity function on Hubble type (E-S0, early S, late S), they also considered the influence of family (SA, SAB, SB) and variety (r, rs, s). Among spirals, no difference was found between early and late types. However, differences were found among family and variety. The authors concluded that, contrary to previous studies, barred spirals are the least IR-luminous as a class. Galaxies of the inner ring variety were found to have significantly less IR emission than those of the (rs) variety, and the authors concluded that SAB(rs) galaxies have the highest IR luminosity among the class of normal galaxies. It was concluded that variety is more important than Hubble type or family in determining the IR emission of a galaxy, an unexpected result. They suggested that bars and rings may affect the dust distribution in a galaxy, by either reducing it or confining it, so that the efficiency of conversion of optical and UV photons to the IR is reduced.

These are interesting results, but we note that selecting galaxies on the basis of having inner rings is not necessarily the same as selecting them on the basis of having nuclear rings. Hawarden et al. (1986) found that barred spirals have higher far-IR luminosities than nonbarred or weakly barred galaxies, based on a different sample selection, and suggested that the enhancement was connected to the ability of a bar to transport large amounts of gas to the central regions where a circumnuclear starburst could take place. It is likely that if galaxies could be selected on the basis of the presence of a nuclear ring, then such galaxies would be IR-bright compared to non-nuclear-ringed galaxies. The IR emission from barred galaxies is further reviewed by Hawarden et al. (1996).