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

5. CENTRAL STAR FORMATION & PSEUDOBULGE GROWTH

In this section, we estimate the current rate at which star formation is building stellar mass density in pseudobulges. We wish to see whether the picture of secular evolution in Sections 2 and 3 is consistent via plausible formation timescales with the properties of pseudobulges in Section 4.

Figure 8 showed central gas disks in barred and oval galaxies that have radii and masses comparable to those of pseudobulges and that are intensely forming stars. They are a window on pseudobulge formation. We begin this section by discussing well-studied systems in which star formation rates (SFRs), gas masses, stellar mass deposition rates, and hence evolution timescales can be constrained accurately. We then review the broader body of observations of circumnuclear gas disks and SFRs. Star formation is ubiquitous in late-type galaxies. This means that it cannot be driven by episodic events such as mergers. It must be secular.

5.1. Case Studies: NGC 1326, NGC 1512, NGC 4314, NGC 5248

These galaxies are excellent prototypes for studying circumnuclear disks, because each has been studied in depth using a combination of HST and ground-based imaging (Buta et al. 2000, Maoz et al. 2001, Benedict et al. 2002, Jogee et al. 2002). Also, the four objects span a representative range of host galaxy properties. They are all barred, and they all have similar luminosities (-19.0 M_B⁰ - 20.3), but they cover a wide range of morphological types (SB0/a - SBbc) and environments. NGC 1326 is in the Fornax cluster; NGC 1512 is in an interacting pair with NGC 1510; and NGC 4314 and NGC 5248 are relatively isolated field galaxies located in loose groups. Nearly all (> 80 %) of the star formation in NGC 1326 and NGC 4314 is contained in the circumnuclear rings, while NGC 1512 and NGC 5248 have actively star-forming outer disks, with less than 40 % of the total SFR near the center. The diversity in galaxy properties and environments already suggests that internal structure (e.g., bars) is more important than external influences in feeding the central star formation.

The central star-forming rings of NGC 1512, NGC 1326, and NGC 4314 are illustrated in Figure 8. At high spatial resolution, the rings of HII regions and young star clusters often are revealed to be pairs of tightly-wound spiral arms. This is shown for NGC 1512 in Figure 3. The spiral structure is seen most clearly in red continuum images, where networks of dust features spiraling toward the center from within the star-forming rings can be seen. The continuum images also reveal large numbers of bright stellar knots (> 70 in NGC 4314; 500 - 1000 in the others). The luminosities and dereddened colors of these knots indicate that they are not single stars but instead are luminous associations or star clusters. The brightest of these have stellar masses of order 10⁵ M, placing them in the class of populous blue clusters observed in the Magellanic Clouds and other gas-rich galaxies. Some may be young progenitors of globular clusters. Many of the clusters are coincident with HII regions, but most are free of surrounding nebulosity, and these probably are older than the 5 - 10 Myr lifetimes of typical HII regions.

Current SFRs in these regions can be estimated from H or Pa measurements converted using the SFR calibrations of Kennicutt (1998a). The largest uncertainties come from heavy and patchy dust obscuration. When both H and Pa data are available, the flux ratio of the two lines can be used to infer the extinction A. Typically, A_H = 1 to 3 mag across these regions, larger than normal values of ~ 1 mag in spiral disks (Kennicutt 1983; Kewley et al. 2002), but low enough so that the extinction-corrected emission-line fluxes should provide reasonable measures of the SFRs. Star formation rates estimated independently from extinction-corrected ultraviolet or far-infrared photometry of the regions (when available) are in general agreement with the above results. This increases our confidence in the SFR measurements. The resulting SFRs range from ~ 0.13 M yr^-1 in NGC 4314 (Benedict et al. 2002) to 1 M yr^-1 in NGC 1326 and NGC 1512 (Buta et al. 1999, Maoz et al. 2001) and ~ 2 M yr^-1 in NGC 5248 (Maoz et al. 2001 corrected to a distance of 15 Mpc). These values are probably accurate to within ± 50 %, given uncertainties in the amounts and patchiness of the extinction and in the assumed distances to the galaxies. This is sufficient to characterize the evolutionary properties and physical conditions in these regions.

SFRs of 0.1 - 2 M yr^-1 are modest compared to the total SFRs in giant spiral galaxies, which typically range from 0.1 - 1 M yr^-1 in normal Sa galaxies to 1 - 10 M yr^-1 in Sb-Sc galaxies (Kennicutt 1983, 1998a). However, they are quite exceptional considering the physical compactness of the star-forming regions. The star-forming rings have radii of 500 - 700 pc, so the surface densities of star formation are 0.1 - 1 M yr^-1 kpc^-2. This is 1 - 3 orders of magnitude larger than the typical disk-averaged SFR densities in normal galaxies. It approaches the SFR densities seen in some infrared starburst galaxies (Kennicutt 1998a, b). If these SFRs were to persist over a Hubble time, they would produce "bulges" with stellar masses of 10⁹ -10¹⁰ M. Thus, while the total amounts of star formation in these regions are not unusual by galactic standards, the character of the star formation is quite distinct.

The distinctive character of the star formation is underscored by large populations of luminous young star clusters. Their extinction-corrected absolute magnitudes range from M_V⁰ = - 13 to M_V⁰ ~ - 8. Fainter than this, HST photometry becomes very incomplete. The corresponding masses, corrected as discussed below for the ages of the clusters, are ~ 10³ to 10⁵ M. These are similar to the masses of giant OB associations such as those in supergiant HII regions like 30 Doradus in the LMC and to the masses of the populous blue star clusters found in the LMC and other gas-rich galaxies (e.g., Kennicutt & Chu 1988). The luminosity functions of the knots are well fitted by a power law with slope dN / dm ~ - 2. They are consistent with the luminosity functions of HII regions and their embedded OB associations (e.g., Kennicutt et al. 1989a; Bresolin & Kennicutt 1997). They are also similar to the young star cluster populations in merger remnants such as the Antennae (e.g., Zhang & Fall 1999). However, no examples are found in these galaxies of the so-called "super star clusters" (SSCs) with M_V < - 15 that are often seen in merger remnants and luminous starburst galaxies. This may be a reflection of the lower total amounts of star formation in these rings rather than any sign of a different cluster mass spectrum. Even if the power-law cluster mass spectra extend to the realm of the SSCs in these objects, the number of SSCs that we expect to observe at any one moment is less than one, based on the total size of the populations observed. We need to observe more central star-forming rings to determine whether they can form SSCs.

The star clusters can be age-dated using multi-color photometry and synthesis models (e.g., Leitherer et al. 1999). They provide a powerful probe of the star formation histories in these circumnuclear regions. In all four of these galaxies, multiband HST imaging in different combinations of U, B, V, I, H, and H have been used to derive reddening-corrected colors, luminosities, and hence age distributions. The galaxies all show a spread in cluster ages from zero to 200 - 300 Myr. The age distributions are heavily weighted toward younger clusters, but this is readily accounted for by dimming with age and by dynamical disruption effects. When corrections are applied for these processes, the age distributions are generally consistent with a roughly constant cluster formation rate over the past 200 - 300 Myr (Maoz et al. 2001). However, more sporadic histories cannot be ruled out.

If the rings have been forming stars at the current rate for 0.2 - 0.3 Gyr, then the total mass of stars formed is (2 - 6) × 10⁸ M in NGC 1326, NGC 1512, and NGC 5248 and about 2 × 10⁷ M in NGC 4314. We can check the consistency of these results by comparing the SFRs with the masses of the circumnuclear gas disks. Millimeter measurements of CO emission in the centers of NGC 1326, NGC 4314, and NGC 5248 have been reported by Garcia-Barreto et al. (1991), Combes et al. (1992), Benedict et al. (1996), Sakamoto et al. (1999), Jogee et al. (2002), and Helfer et al. (2003). The corresponding molecular gas masses range from 0.7 × 10⁸ M in NGC 4314 to (5 - 12) × 10⁸ M in NGC 1326 and NGC 5248. These values assume a "standard" Galactic conversion factor between CO intensity and H₂ column density. Several authors (e.g., Wilson 1995; Paglione et al. 2001; Regan 2000) have advocated using a lower conversion factor for these metal-rich environments; doing so would reduce the above masses by factors of up to 2 - 3. The gas masses are comparable to the masses of stars already formed in the central disks during the current star formation burst, as one would expect if one typically observes these systems at random times during the burst. Combining the gas masses with the SFRs also shows that there is sufficient fuel to feed the current circumnuclear SFRs for another 0.2 - 1 Gyr. By the time the gas is exhausted, central stellar disks with masses of 10⁸ to 10⁹ M will have formed. Of course, the masses will be even larger if gas from the galaxies' bars continues to feed the star formation. In the cases of NGC 1326, NGC 1512, and NGC 5248, the above masses are factors of several higher than the mass in stars, ~ 1 × 10⁸ M (see below), formed in the main exponential disks if the parameters of these disks are extrapolated to the center. In fact, the stellar disks being formed by the star formation rings have characteristic masses and sizes that are comparable to those of pseudobulges. In these four galaxies, we almost certainly are observing the formation of pseudobulges, or the continued growth of pre-existing pseudobulges.