Annu. Rev. Astron. Astrophys. 2004. 42: 603-683 Copyright © 2004 by Annual Reviews. All rights reserved

5.2. General Properties of Circumnuclear Regions

The circumnuclear star-forming rings discussed above are not extreme cases; even higher SFRs are observed at the centers of NGC 1097 and some other nearby galaxies. Nevertheless, before we attempt to characterize the global rates of star formation in these objects, it is important to review the properties of circumnuclear star-forming rings and disks in general. This subject was reviewed by Kennicutt (1998a), with emphasis on the most luminous starburst galaxies. These are nearly always associated with major mergers of gas-rich galaxies that are forming high-mass bulges and elliptical galaxies (Sanders & Mirabel 1996; Kennicutt, Schweizer, & Barnes 1998, and references therein). Here, we focus exclusively on the central regions of normal spiral galaxies, where the circumnuclear activity is fed by the kinds of secular processes discussed in this paper.

The frequency of occurrence of dense central gas disks and vigorous star formation can be estimated from two independent lines of evidence, surveys of central star formation in the ultraviolet, visible, or mid-infrared, and CO surveys of central molecular gas. Prominent circumnuclear rings like those in Figure 8 are easily identified. Maoz et al. (1996) estimate that approximately 10 % of Sc and earlier-type spirals contain such strong circumnuclear star-forming regions, based on an ultraviolet imaging survey of 110 nearby spirals. This is roughly consistent with the frequency of circumnuclear "hotspot" galaxies in the survey of Sérsic (1973), which was based on blue photographic plates. Most of these galaxies are barred. This includes all of galaxies that have ultraviolet-bright rings identified by Maoz et al. (1996), 88 % of the hotspot galaxies in the Sérsic (1973) compilation, and 81 % of all galaxies with peculiar nuclei identified by Sérsic. Galaxies with strong circumnuclear star formation tend to have Hubble types between Sa and Sbc (Devereux 1987; Pogge 1989; Ho et al. 1997), although there are earlier- and later-type exceptions. Putting all this together, the frequency of circumnuclear rings among the core population of massive, intermediate-type barred galaxies is of order 20 %.

Quantifying the star formation statistics in less spectacular star-forming galaxy centers is more difficult. In early-type galaxies, the typical levels of extended disk star formation are relatively low (Kennicutt 1998a and references therein). Any nuclear star formation stands out. However, in the gas-rich, late-type spirals that dominate the total star formation in the local universe, it can be difficult to distinguish central star formation that might be associated with pseudobulge growth from the central extrapolation of the general disk star formation.

CO interferometer surveys give a clearer picture. Some of the more comprehensive aperture synthesis CO surveys include a study of 20 spirals by Sakamoto et al. (1999), the BIMA Survey of Nearby Galaxies (SONG), which observed 44 nearby spirals (Regan et al. 2001; Sheth et al. 2002; Helfer et al. 2003; Jogee 1998; Jogee et al. 2004). Notable studies of individual galaxies include Kenney et al. (1992); Garcia-Burillo et al. (1998, 1999, 2000); Sofue et al. (1999); Sakamoto et al. (2000); Jogee et al. (2002); and Schinnerer et al. (2002, 2003). Larger samples of galaxies have been observed in the ¹²CO (1-0) and (2-1) rotational lines with single-dish telescopes, with typical beam diameters of 11 - 50". Such large surveys include Young & Devereux (1991); Braine et al. (1993); and Böker et al. (2003a). A survey in HCN including a large subsample of normal galaxies is given in Gao & Solomon (2004).

These surveys show that central molecular disks are common but not universal. For example, 45 % of the spiral galaxies in the BIMA SONG show central gas concentrations that exceed the highest peak column densities anywhere else in the disks. This fraction was even higher (75 %) in the Sakamoto et al. (1999) CO survey of 20 spiral galaxies, but in this case the sample was partly selected on the basis of strong CO emission. The presence and masses of these disks are strongly enhanced in barred galaxies (Sakamoto et al. 1999, Helfer et al. 2003), as is the case for central star formation. Although a few objects show centrally peaked distributions that one might be tempted to associate with the exponential profiles of stellar disks or pseudobulges, the predominant structures are barlike distributions, bipolar "twin-peak" distributions (Kenney et al. 1992), circumnuclear rings, spiral arms, or combinations of these structures. In many systems, the gas is unlikely to be in steady-state equilibrium, and the interpretation is complicated by the likely presence of spatial variations in gas temperature that will modulate the distribution of CO emission. We can conclude only that the gas disks have radii that are characteristic of central bars, bulges, and pseudobulges.

The first CO observations that spatially resolved galaxies showed that the distribution of molecular gas often follows the starlight (e.g., Young & Scoville 1991). Recent observations confirm this result (Regan et al. 2001; Böker et al. 2003a). Even when the stellar brightness rises steeply toward the center above the inward extrapolation of the outer exponential - this is what we would conventionally call a "bulge" - the CO emission often does so also. Five examples are shown in Figure 20. All are excellent examples of objects in which a bar (top row), oval (middle row), or global spiral structure that reaches the center (NGC 4321 in the bottom row) provides an engine for inward gas transport. Consistent with this, the molecular gas is very centrally concentrated. Since star formation rates increase faster than linearly with gas density, the observation that the molecular gas density follows the starlight guarantees that star formation will further enhance the density contrast between the (pseudo)bulge and the outer disk. We discussed several of these objects as typical pseudobulges. The exception in Figure 20 is NGC 7331, a galaxy that contains a probable classical bulge. Other galaxies in Regan et al. (2001) behave similarly.

Figure 20. Radial profiles of CO and stellar K-band surface brightness from the BIMA SONG (adapted from Regan et al. 2001). CO surface brightness is in magnitudes of Jy km s-1 arcsec-2 with zeropoint at 1000 Jy km s-1 arcsec-2. The stellar surface brightness profiles have been shifted vertically to the CO profiles. Morphological types are from the RC3. NGC 2903 and 3627 are clearly barred in the K-band images shown in Regan et al. (2001). NGC 2903 and 4736 are oval galaxies (Section 3.2). NGC 4736 contains a prototypical pseudobulge; it is also illustrated here in Figures 2, 8, and 17. NGC 4321 is an unbarred galaxy with no ILR; it was discussed in Section 3.4. All galaxies in this figure except NGC 7331 have structures that are expected to cause gas to flow toward the center. NGC 7331 is included to show the very different CO profile in a galaxy with a probable classical bulge.

When one combines the data from the above aperture synthesis surveys with small-beam, single dish measurements (Braine et al. 1993, Böker et al. 2003), the resulting gas masses show a large range, from ~ 10⁶ M to 2 × 10⁹ M. These values assume a "standard" CO-to-H₂ conversion factor, X_CO = 2.8 × 10²⁰ cm^-2 (K km s^-1)^-1 (Bloemen et al. 1986). If instead we use a variable, metallicity-dependent conversion factor (e.g., Wilson 1995; Paglione et al. 2001; Boselli et al. 2002), then this range narrows to ~ 10⁷ -10⁹ M (Böker et al. 2003). Pseudobulges are expected to grow to at least these masses. More massive pseudobulges would be result if gas continues to be added to the observed nuclear disks.

Studies of the SFRs in individual systems are too numerous to be listed here, but some of the most extensive and notable studies include Kennicutt et al. (1989b), Pogge (1989), Phillips (1993), Maoz et al. (1996, 2001), Elmegreen et al. (1997, 1998, 1999, 2002), Usui et al. (1998, 2001), Buta et al. (2000), Colina & Wada (2000), Inoue et al. (2000), Alonso-Herrero & Knapen (2001), Ryder et al. (2001), Benedict et al. (2002), and Knapen et al. (2002). A variety of star formation tracers have been used, including measurements of ultraviolet and infrared continua, and H, P, Br, and other hydrogen recombination lines (see Kennicutt 1998a). The Spitzer Space Observatory will have a major impact on this subject by providing spatially-resolved maps of the far-infrared dust emission in these regions.

SFRs measured by different authors are generally consistent at the factor-of-two level; this is comparable to the uncertainties that are typically quoted for these highly dust-attenuated regions. Although this limits the reliability of SFRs for any individual object, good measurements are available for about 40 galaxies, and this is sufficient to characterize the range of star formation properties. The absolute SFRs within circumnuclear rings and disks range over a factor of about a thousand, from 0.01 to 10 M yr^-1. This brackets the range of SFRs observed in our four case studies and is comparable to the range observed in the integrated SFRs of normal spiral galaxies (Kennicutt 1998a and references therein). The central star formation accounts for 10 - 100% of the total SFR of spirals galaxies. The highest fractions occur in early-type galaxies, which typically have low SFRs in their outer disks (Kennicutt 1983, 1998a).