9.5. HII Regions in Ringed Galaxies
The previous section showed that colors of nuclear, inner, and outer rings indicate that such features are usually sites of active star formation, particularly the inner and nuclear types. It is therefore not surprising that rings are also often concentrations of HII regions (see Figure 41). Van der Kruit (1976b) showed that the inner ring of NGC 4736 is an intense bounded zone of both discrete HII regions and diffuse H emission. In NGC 3351, Rubin et al. (1975) and Peterson et al. (1976) found that HII regions are concentrated within the nuclear ring, the bright inner ring, and the outer arms, but that there was little emission from the region between the nuclear and inner rings. The same was found for the inner and nuclear rings of NGC 5728 by Rubin (1980).
Buta (1984) obtained HII region distributions in 8 ringed galaxies: NGC 1433, 1512, 3351, 4725, 4736, 5364, 6300, and 7531, based on both spectroscopy and Fabry-Perot interferometry. In the five barred spirals (NGC 1433, 1512, 3351, 4725, and 6300) of the sample, the bar regions were largely devoid of discrete HII regions, although NGC 6300 was found to have diffuse H emission in its bar region. In each case, emission is concentrated in the inner ring regions or beyond, and in a nuclear ring in three cases. In the two weakly-barred sample objects (NGC 4736 and 7531), the inner rings are very intense sources of H emission and HII regions, and are the brightest regions in H in their respective galaxies. Diffuse emission fills the entire region interior to the inner ring of NGC 7531. In the one true nonbarred case, NGC 5364, the inner ring includes HII regions, but is not the strongest source of emission in the galaxy.
Detector technology has improved since these early studies, and many more ringed galaxies have been imaged in H or H+[NII], usually as parts of other studies. For example, imaging Fabry-Perot interferometry revealed a small nuclear ring of H emission in the grand design spiral NGC 4321 (Arsenault et al. 1988). This small feature is actually a spiral in broad-band images. Pogge (1989) imaged the ionized gas in 91 nearby non-Seyfert galaxies, and discovered a wide variety of nuclear emission morphologies. Inner rings or pseudorings were prominent in NGC 4736 and 5921, while bright nuclear rings of emission were detected in 13 galaxies ranging from distinct rings to partial rings with ``hotspots''. Pogge found that the relative contribution of such rings to the total H+[NII] luminosity ranged from 1% in NGC 4254 to 94% in NGC 4314, and that the rings may or may not surround a nuclear emission source. This study also underscored once again the tendency for the interior regions of some inner rings (e.g., in NGC 4736 and 5921) to be relatively devoid of emission except for the nucleus.
Ryder & Dopita (1993) carried out an H imaging survey of bright southern galaxies which included several inner-ringed galaxies: NGC 1187, 1398, 6300, 5643, and 6744. They suggested that there may be a reciprocal relationship between the number of HII regions in a bar and the number in the inner ring, such that when the inner ring is well-populated with HII regions, the bar region is devoid of HII regions. The one outer-ringed galaxy in their sample, NGC 2217, showed HII regions only in parts of the outer ring.
Pogge & Eskridge (1993) found from another imaging survey that the most common HII region distribution in S0 galaxies is an HII ring. Though they refer to the observed rings only as ``inner'' or ``outer'' types, it is clear that conventional nuclear, inner, and outer rings are represented. The most interesting H map in this paper is for NGC 7742, a face-on example of a ringed SA galaxy (see Figure 42). Just as for barred galaxies, the inner ring is a strong concentration of HII regions. In their sample, the inner and nuclear rings tend to be fully populated in azimuth by HII regions, but they note that the outer rings are more sparsely populated and patchy. It is possible that some of these ``gas-rich'' S0's are probably early-type spirals (i.e., misclassified S0/a types).
Phillips (1993a; see also Kennicutt 1994 and Phillips 1996) surveyed the distribution of HII regions in a sample of SBb and SBc galaxies. The tendency for the bar regions of SBb galaxies to be devoid of HII regions was again noted, as well as the frequent presence of circumnuclear rings of ionized gas. He also compared the luminosity function of HII regions in inner pseudorings with that in the outer disk of a few galaxies. The outer disk HII regions in NGC 1300 have a standard Type I luminosity function with no break (see Kennicutt et al. 1989), while the inner pseudoring (arms enveloping the bar in this case) appears to show a Type II luminosity function with a break, perhaps implying an upper limit to the masses of giant molecular clouds allowed in that region. Phillips also found circumnuclear star formation to be common in SBb galaxies: eleven of twelve SBb galaxies in his sample of RSA spirals include such emission.
From an H survey of 52 RSA barred spirals, Garcia-Baretto et al. (1996) found nuclear ionized gas rings in 10 galaxies, three of type SBa, six of type SBb, and one of type SBbc. Their sample was chosen to have IRAS colors indicative of star formation and high dust temperatures. Most of the nuclear emission rings they identified were found to be misaligned with the primary bar.
The most detailed study of the HII region distribution in ringed galaxies has been made by Crocker, Baugus, and Buta (1996, hereafter CBB). This study included H+[NII] images of 32 galaxies from the CSRG. Besides verifying the results from previous studies, CBB were able to investigate connections between HII region distributions and dynamics. The main results from the paper are as follows:
Finally, we note the existence of HII regions connected with the subtle ``dimpling'' aspect of R'1 outer pseudorings in some galaxies. This has been noted in a study of the (R'1)SAB(rs)a spiral IC 4214 by Buta et al. (1996). Their H distribution (based on Fabry-Perot interferometry) is shown in Figure 46, and the arrows point to HII regions connected with weak dimples seen in blue light. These dimples are regions where the gas would be slowing down in the bar reference frame (Schwarz 1981), and perhaps bright HII regions might be expected in such regions depending on the gas available. This is yet another aspect of the distribution of HII regions in ringed galaxies which can be connected to internal dynamics. IC 4214 is also discussed by Buta & Crocker (1991) and Saraiva (1996).
A special topic in HII regions in ringed galaxies concerns the nature of nuclear ``hotspot'' HII regions compared to HII regions away from the nucleus. These hotspots are commonly found in nuclear rings, as we have noted. Kennicutt, Keel, and Blaha (1989) have made a spectrophotometric and radio continuum comparison between nuclei, hotspot, and disk HII regions to determine the mechanisms responsible for the ionization and the validity of assumptions concerning abundance determinations. These authors first of all determined that many of the hotspots seen in the nuclear rings are not HII regions but are continuum knots, i.e., star clusters or associations with no surrounding ionized cloud. The luminosities of disk HII regions and hotspot HII regions were found to be similar, but the hotspot HII regions were found to be more compact and had H equivalent widths 7 times lower than disk HII regions of comparable luminosity (see also McCall et al. 1985 and Mayya 1994). The stellar continua in the hotspots were also found to be more significant than in disk HII regions. Kennicutt et al. suggested that the optical and radio continuum properties of these regions are not easily explained in a simple picture whereby the hotspots are normal, photoionized HII regions located in an unusual environment.
Korchagin et al. (1995) have examined the star formation mechanism in hotspots. The most favored idea is that the high continuum emission reflects an accumulation of stars over many generations, so that star formation has to take place in hotspots over a period of time longer than the normal lifetime of a disk HII region. They conclude that hotspots are regions of self-regulated star formation where ultraviolet radiation from young, massive stars both triggers star formation and regulates it. The unique conditions at the centers of galaxies help to explain the behavior of the mechanism as compared to ordinary HII regions in the outer disk regions. Korchagin et al. conclude that the low equivalent widths and red optical colors of hotspots rule out an instantaneous burst interpretation but favor self-regulated sequential star formation lasting for 10-70 million years.