6 The age of the bulge

Taking advantage of the sharp turnoff region of the decontaminated near-IR CMD derived in Sect. 3 we proceed to estimate the age of the bulge stellar population. As in paper I, we adopt a differential procedure, comparing the luminosity difference between the HB clump and the MS turnoff of the bulge to that of a globular cluster of similar metallicity.

Figure 23: The bulge and the NGC 6528 CMDs are shown side by side. The magnitude difference between the HB and the turnoff of in the bulge CMD (left) is compared with the same quantity for NGC 6528 (right). The CMD for NGC 6528, originally in the NICMOS instrumental (m110, m110-m160) plane, has been shifted both in magnitude and in color in order to match the location of the bulge HB.

Figure 23 shows the comparison between the bulge CMD and that of the cluster NGC 6528, whose metallicity is close to the average of the bulge. The near-IR CMD of NGC 6528 is based on the NICMOS photometry obtained by Ortolani et al. (2001). The magnitude difference between the HB clump and the turnoff is virtually identical in the two diagrams, as emphasized by the two horizontal lines.

This ensures that the difference between the age of the cluster and the mean age of the bulge cannot exceed $\sim$$20\%$ (thanks to the rule of thumb according to which $\delta$age/age  $\simeq \delta(\Delta M^{\rm HB}_{\rm TO})$ (Renzini 1991).

This confirms and reinforces the conclusion in Paper I that the bulk of the bulge population and the clusters NGC 6528 and NGC 6553 are coeval. The absence of any appreciable extension of the bulge main sequence beyond the obvious turnoff makes it clear that no trace of an intermediate-age population is detectable in the bulge CMD.

Figure 24: 10 Gyr isochrones (Cassisi & Salaris 1997) for the two extremes of the bulge MD are overplotted on the CMD.

The proper motion decontaminated and differential reddening corrected CMD of NGC 6553 (Zoccali et al. 2001a) and NGC 6528 (Feltzing & Johnson 2002) confirm the results of Paper I that the HB to TO magnitude difference in these two bulge clusters is virtually identical to that of the inner halo clusters NGC 104 (47 Tuc). Figure 23 now shows that this magnitude difference is essentially identical also for the bulge field population, strengthening the case for the bulk of the whole population of the Galactic spheroid (i.e. bulge and halo) being essentially coeval, though an age difference of $\sim$$20\%$ ($\sim$2-3 Gyr) either way cannot be excluded.

One aspect of the cluster to bulge CMD comparison still deserves some attention. Indeed, the bulge population is affected by dispersion in both distance and metallicity, while the cluster stars are chemically homogeneous and all at the same distance (although affected by some differential reddening). In Fig. 24 two 10 Gyr isochrones spanning the full metallicity range of the bulge are overplotted to the bulge CMD, assuming the same distance and reddening for both of them. This illustrates that the wider dispersion affecting the bulge CMD (compared to the HST/NICMOS CMD of NGC 6528) can be well accounted by the bulge metallicity dispersion, also taking into account the $\sim$0.13 mag 1-$\sigma$ dispersion due to the distance distribution along the line of sight.

According to recent attempts to determine the relative ages of Galactic globular clusters the bulk of clusters are coeval within a $\pm$1.5 Gyr uncertainty, with only the most metal rich ones in the sample appearing to be slightly younger than the others (Rosenberg et al. 1999; Salaris & Weiss 2002). However, these studies do not extend to the high-metallicity clusters of the bulge. For example, Rosenberg et al. assign to 47 Tuc an age $1.2\pm1.2$ Gyr "younger'' than that the bulk of the halo globular clusters. Salaris & Weiss (2002) assign to the same cluster an age of $10.7\pm 1.0$ Gyr, compared to $11.7\pm 0.8$ Gyr for the prototypical metal poor cluster NGC 7078 (M 15). On the other hand, Ortolani et al. (2001) date NGC 6528 at 13 $\pm$ 3 Gyr from the value of $\Delta J^{\rm HB}_{\rm TO}$.

Figure 25: Comparison of the bulge CMD with two younger isochrones of 3 (left panel) and 5 Gyr (right panel). Two models are plotted in each panel, both referring to the same age. The reddest curve in each panel is for solar metallicity, while the one on the blue side is for [M/H] =-1.3.

It is clear that, within the uncertainties of the currently available data and dating methods, no appreciable age difference has been unambiguously detected between the bulk of bulge field stars and the globular clusters of either the bulge or the halo. On the other hand, the absolute age of the clusters remain more uncertain than the formal error bars sometime quoted by individual authors. Just to mention one example, the age of the globular cluster 47 Tuc has been recently estimated to be 12.5 $\pm$ 2 (Carretta et al. 2000), 13 $\pm$ 2.5 Gyr (Zoccali et al. 2001b), and 10.7 $\pm$ 1.0 Gyr (Salaris & Weiss 2002), the difference being partly due to a difference in the cluster distance and partly to the use of different sets of models.

Significantly younger ages can be excluded, as shown in Fig. 25, where 3 and 5 Gyr isochrones of both solar and [M/H] =-1.3 metallicity are overplotted on the bulge CMD. After the submission of this paper, during the refereing process, we became aware of the paper by Cole & Weinberg (2002), in which the Authors argue that the bulk of the stellar population of the Galactic "bar'' formed less than 6 Gyr ago, with an age of $\sim$3 Gyr being favored. As they state "the main sequence turnoff of a 3 Gyr old population should be readily traceable along the Galactic bar from $V\approx17$ at the near end to $V\approx19$ at the far end''. Note that the Galactic component called "bar'' in Cole & Weinberg (2002) has a mass of $2\times 10^{10}~M_\odot$ and therefore is not a minor component, but rather the whole population of the system called here "the bulge''. As evident from Fig. 26, no such intermediate age population is actually detected in the present data.

Figure 26: The disk decontaminated optical CMD, together with the 3 Gyr isochrone for solar metallicity. The box shows the region where the stars of a hypotetical 3 Gyr old "bar'' population, spread along the line of sight, would be expected.

The region in the CMD just above the main sequence turnoff is so devoid of stars that very few, if any, blue stragglers stars (BSS) may be present in the field (see, e.g., Fig. 24). Among Galactic globular clusters, Ferraro et al. (1995) estimate an average frequency of $\sim$1 BSS every $10^3~L_\odot$ of bolometric light of the parent cluster, but with very large cluster to cluster variations that are not merely statistical fluctuations. Scaling from the SOFI-LARGE field, the SOFI-SMALL field samples $\sim$ $177~000/4.6\simeq 38~000~L_\odot$, and one would expect to recover $\sim$38 BSSs, if the bulge has the same BSS frequency as the average globular cluster. Clearly it has not. The bulge is far less productive of BSSs than a typical globular cluster, indicative that the cluster environment favors the formation of binaries with the right separation for producing BSSs. Most likely this is due to the dynamical processes that are germaine to the clusters.