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

8. CAVEATS

8.1. Stellar Populations in Classical Bulges and Pseudobulges

Published studies of stellar populations and metallicities promote quite a different view of bulges than the one discussed in this paper. They emphasize that stellar populations are generally old, metal-rich, and similar to those of elliptical galaxies. On the basis of such evidence, many papers suggest that observations of stellar population are inconsistent with secular evolution and instead point to early and rapid formation. Do these results reveal a problem with the present picture?

The themes of this section are: (1) Most stellar population studies concentrate on early-type galaxies; their results are consistent with our conclusion that these generally contain classical bulges. (2) In contrast, many Sbc and later-type bulges, plus a few early-type objects with independent evidence for pseudobulges, do contain young stars. However, (3) some galaxies with clearcut evidence for pseudobulges certainly have old stellar populations. Point (2) provides some "wiggle room"; tentatively, we see no compelling collision between stellar population and secular evolution studies. But the situation is not clearcut, and a collision is possible. More than any other subject discussed in this review, the stellar populations of late-type (pseudo)bulges need further work to make sure that there is no fundamental problem or else to uncover it.

Stellar populations in galaxies are a giant subject; we do not have space to review it in detail. Excellent recent reviews are available, including Sandage (1986); Wyse, Gilmore, & Franx (1997), papers in Carollo, Ferguson, & Wyse (1999), particularly Renzini (1999), and may others. We concentrate on a few seminal papers that illustrate the above points.

Peletier et al. (1999) studied the B - I versus I - H color-color diagram of bulges using HST NICMOS. High spatial resolution is important: they show convincingly that dust absorption dominates the central colors in many galaxies and that colors at the effective radius r_e of the bulge are much less affected by dust. Their conclusions are as follows.

For 13 bulges of S0 - Sab galaxies, colors at r_e mostly show little scatter and are consistent with the colors of Coma cluster ellipticals. The authors conclude that the age spread of these bulges is small, at most 2 Gyr, and that the stars formed 12 Gyr ago. Two early-type galaxies have blue colors indicative of younger ages. NGC 5854 is classified Sa in Sandage & Bedke (1994), who note, "The inner spiral pattern [of two] is in the bulge (it forms the bulge) and has the form and the sense of the opening of a stubby S." This observation is more consistent with a pseudobulge than with a classical bulge. Spiral structure requires a dynamically cold system, and indeed, the weighted mean of the best velocity dispersion measurements is 102 ± 4 km s^-1 (Simien & Prugneil 1997; Vega Beltrán et al. 2001; Falcón-Barroso, Peletier, & Balcells 2002). The second "young" bulge is in the S0 galaxy NGC 7457; Kormendy (1993) showed that it has an unusually small velocity dispersion indicative of a pseudobulge.

Of four Sb bulges, one (NGC 5443) is young; it is also barred. The other three are made of old stars. It is important to note that these include the boxy pseudobulge NGC 5746 (Figure 15).

Peletier et al. (1999) note that their three Sbc bulges are "considerably bluer, have lower surface brightness, show patchy dust and star formation together, and are rather different from the rest of the galaxies." So the sample is small, but the results in Peletier et al. (1999) are consistent with the present picture that secular evolution dominates late-type bulges but that early-type bulges, on the whole, are like ellipticals. One important new result is that at least one pseudobulge is made of old stars. Generally similar results are found in Bica & Alloin (1987).

One stellar population result that is consistent with and even suggestive of secular evolution is an observed correlation between bulge color and the color of the adjacent part of the disk (Peletier & Balcells 1996; Gadotti & dos Anjos 2001). Bulges and disks both show large ranges in colors, but "bulges are more like their disk than they are like each other" (Wyse et al. 1997). Also, some bulge colors found in the above studies are indicative of young ages, especially for Sc - Sm galaxies (de Jong 1996c). Bulge and disk scale lengths and surface brightnesses also correlate (see the above papers; Courteau, de Jong, & Broeils 1996; Courteau 1996b). Courteau and collaborators interpret these correlations as products of "secular dynamical evolution ... via angular momentum transfer and viscous [gas] transport."

Finally, Trager (2004) reviews recent work which shows that bulges of S0/a - Sbc spirals span a large range of ages and [-element/Fe] abundance enhancements. The latter are a particularly important indicator of star formation history because -elements are ejected by massive stars when they explode as supernovae of type II; their abundances are diluted with Fe when type I supernovae become important ~ 1 Gyr after a starburst. After that, [-element/Fe] can never again be much greater than solar. So overabundances of the -elements indicate that almost all of the star formation occurred quickly (e.g., Terndrup 1993; Bender & Paquet 1995; Thomas, Greggio, & Bender 1999; Thomas, Maraston, & Bender 2002; cf. Worthey, Faber, & Gonzalez 1992). Trager (2004) reviews evidence (e.g., Jablonka, Martin, & Arimoto 1996; Proctor & Sansom 2002; Thomas et al. 2002; Mehlert et al. 2003) that bulges with high luminosities or velocity dispersions show -element enhancements but those with low luminosities or velocity dispersions do not. For example, many S0 bulges, which tend to be high in luminosity, tend to have -element enhancements indicative of rapid formation (e.g., Bender & Paquet 1995; Fisher, Franx, & Illingworth 1996), although even they show a large range in -element enhancements and ages (Kuntschner & Davies 1998; Kuntschner 2000; Kuntschner et al. 2002; Mehlert et al. 2003). These results do not ring alarm bells, but they would be a more decisive test of secular evolution if they included more late-type galaxies.

It is well known that a few S0 bulges have post-starburst spectra; these are more likely to be the result of galaxy accretions than processes that form either classical or pseudo bulges (e.g., NGC 4150: Emsellem et al. 2002; NGC 5102: van den Bergh 1976c; Pritchet 1979; Rocca-Volmerange & Guideroni 1987; Burstein et al. 1988; Deharveng et al. 1997).

In summary, stellar population data appear reasonably consistent with the conclusion of previous sections that S0 - Sb galaxies tend to have classical bulges and that Sbc - Sm galaxies usually have pseudobulges. However, (1) the galaxy samples studied are too heavily weighed toward early-type galaxies to be a decisive test, and (2) there certainly exist galaxies that are difficult to understand. For example, while the boxy bulge of the S0 galaxy NGC 7332 is likely to be younger than its disk (Bender & Paquet 1995), the even more boxy bulge of NGC 5746 appears to be old (Peletier et al. 1999).

Finally, the best-studied boxy bulge is the one in our Galaxy. The low-absorption field that has received the most attention is Baade's window. At a Galactic latitude of -4°, it is well up into the boxy part of the bulge revealed by COBE (see Figure 1 in Wyse, Gilmore, & Franx 1997). Still, it is almost along the minor axis, so there is a small chance that the stars that define the boxiness are not completely the same as the ones in Baade's window. In any case, the evidence is overwhelmingly that bulge stars far from the Galactic plane are old (Terndrup 1993; Ortolani et al. 1995; Feltzing & Gilmore 2000; Kuijken & Rich 2002; Zoccali et al. 2003, all of which review earlier work; see Sandage 1986; Wyse, Gilmore, & Franx 1997; Renzini 1999; and Rich 1999 for further review). The absolute age is uncertain but is approximately 11 - 13 Gyr. Moreover, moderate -element overabundances with respect to iron (McWilliam & Rich 1994; Barbuy et al. 1999) again imply rapid star formation over a period of 1 Gyr. Finally, the observed correlation that more metal-poor bulge/halo stars have more eccentric, plunging orbits continues to point to an early collapse with accompanying self-enrichment (Eggen, Lynden-Bell, & Sandage 1962; Sandage 1986, 1990). The Galactic bulge is clearly older than a secular evolution picture can easily accommodate. If we try to solve this problem by postulating that the boxy structure was made by heating a pre-existing disk of old stars, then the fact that the bulge and the metal-poor halo are similar in age becomes a coincidence. Also, the Galactic center is currently forming stars at a rate that, if sustained for an appreciable fraction of a Hubble time, adds up to much of the stellar density observed there (Rich 1999). We have argued in this paper for the importance of secular evolution, but we would be the last to suggest that the above results are easily understood. On the other hand, the Galactic bulge is clearly boxy. At present, the only model that we have for its origin is via secular processes.