Most stellar population studies in bulges have been done using the integrated properties inside a certain aperture. In case of spectroscopic studies, this aperture commonly encloses just the very central parts. However, if we want to have a full understanding of bulge formation, it is necessary to gain knowledge of the variations of the stellar populations with radius. These variations are intimately connected with the dynamical processes that led to the formation of these structures, the degree of dissipation, and the possible re-arrangement of material.
Mergers with gas dissipation or monolithic collapse scenarios predict steep metallicity gradients (Eggen et al. 1962, Larson 1974, Arimoto & Yoshii 1987) and strong gradients in [α / Fe] Ferreras & Silk 2002). The predictions for secularly formed bulges are more complicated. As they formed from redistribution of disk stars, the final metallicity gradient will depend on the original gradient in the disk and the scale-length of the final bulge and also on the disk heating (Moorthy & Holtzman 2006). However, lower metallicity gradients are expected compared to those of the first scenario. Observationally, the Galactic bulge, which manifests many characteristics of a peanut-shaped bulge, has a clear vertical metallicity gradient, such that the more metal-rich part of the metallicity distribution thins out towards high latitudes (Minniti et al. 1995, Zoccali et al. 2008, Gonzalez et al. 2011). This result has long been taken as a signature for a classical bulge in the MW. However, recent results have shown that the stars that have been scattered furthest from the disk are the oldest stars and, consequently formed from the least metal-enriched fuel (Freeman 2008, Martinez-Valpuesta & Gerhard 2013). The buckling process may hence establish a negative minor-axis metallicity gradient (which is observed in the MW and NGC 4565 (Proctor et al. 2000).
Several articles have studied the variation of the spectral features with radius in bulges (Moorthy & Holtzman 2006, Jablonka et al. 2007, Morelli et al. 2008, Pérez & Sánchez-Blázquez 2011, MacArthur et al. 2009, Sánchez-Blázquez et al. 2011, Ganda et al. 2007) and compared them with stellar population models to obtain either SSP-equivalent parameters or mean values, based on the recovery of the star formation history. These studies find that most bulges have SSP-equivalent or luminosity-weighted negative gradients in the metallicity, almost no gradients in age, and slightly positive or null [α / Fe] gradients. Metallicity gradients in the bulge regions are generally steeper than those in the disk region (Moorthy & Holtzman 2006, Sánchez-Blázquez et al. 2011). Figure 10 shows the distribution of the SSP-equivalent gradients for a sample of bulges taken from the work of (Morelli et al. 2008).
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Figure 10. Distribution of the gradients of age (left), metallicity (central), and [α / Fe] enhancement (right) for a sample of galaxies. The dashed line represents the median of the distribution and its values is also reported. The solid line represents a Gaussian centered on the median value of the distribution. Their σ is approximated by the value containing the 68% of objects of the distribution and is noted in the inset label. The green and blue arrows show the average gradient found for early-type galaxies and bulges by Mehlert et al. (2003) and Jablonka et al. (2007), respectively (figure from Morelli et al. 2008). |
These values are similar to the ones found in elliptical galaxies, although the quantitative comparison is not that clear. Jablonka et al. (2007) and Morelli et al. (2008) do not find any difference in the magnitude of the bulge gradients and those of elliptical galaxies, while Williams et al. (2012), on the other hand, found that the gradients in boxy bulges are shallower than those in elliptical galaxies at a given σ.
Several studies have also looked for correlations between the gradients and other properties of the galaxies, such as the central σ, the luminosity or the mass, to check if they are related with the potential well of the galaxy, as the central values are. Goudfrooij et al. (1999) and Proctor et al. (2000) found that gradients were correlated with luminosity and central σ, respectively, although from very small samples, while Jablonka et al. (2007) found no such correlation. However, there seems to be a trend for which small bulges have lower gradients (see also Moorthy & Holtzman 2006, González Delgado & et al. 2014). This, in principle, could be attributed to the fact that secularly formed bulges are more common in low mass galaxies.
The possible differences between the gradients of bulges with and without bars have also been explored in a few works, and none of them find any significant one (Moorthy & Holtzman 2006, Jablonka et al. 2007, Pérez & Sánchez-Blázquez 2011). However, Moorthy & Holtzman (2006) reported that when a positive age gradient was present, it was always in barred galaxies, which could indicate that these objects have more extended star formation in their centers due to bar-driven inflow of gas. This result agrees with that of Gadotti & dos Anjos (2001), who found a greater prevalence of null or positive color gradients in barred galaxies than in non-barred galaxies, which they interpret as an evidence for gradients being erased by bar-driven mixing. Nonetheless, this has not been confirmed in other studies (Jablonka et al. 2007). The reason for the discrepancies could be, once again, the orientation of the samples. The majority of authors agree that positive age gradients are normally the consequence of the the presence of central disks or nuclear rings with recent star formation (e.g. Morelli et al. 2008). As central disks and rings are likely more common in barred galaxies, this can explain the differences between barred and non-barred galaxies found by Moorthy & Holtzman (2006) and also the lack of differences reported in the edge-on sample of Jablonka et al. (2007), as these flattened central structures will not contribute to the observed light of the bulge in these orientations.
This interpretation of the age gradients as the consequence of the presence of central younger structure is supported by the fact that the mass-weighted age gradients are, in the majority of cases, much flatter than the luminosity-weighted or the SSP-equivalent ones, indicating the the majority of the stars in the bulge are old and share a common age, while a small fraction of stars concentrated in central structures are causing the observed radial trends (MacArthur et al. 2009, Sánchez-Blázquez et al. 2011, Sánchez-Blázquez & et al. 2014, González Delgado & et al. 2014).
Interestingly, Jablonka et al. (2007) find that the line-strength indices at 1 reff 12 were very similar for all the galaxies, independent of their mass or morphological type, and the different gradients come from the differences in their central indices.
An issue in measuring the gradients of age, metallicity and [α / Fe] in bulges could be the contamination of their stellar population by the light coming from the underlying disk stellar component. This effect is not important in the galaxy center but it can have an enormous impact in the stellar population estimates of the external parts where the contribution from the disk to the total light is more important. Different authors have tried to quantify one way or the other this contamination from the disk to the bulge light (see Jablonka et al. 1996, Moorthy & Holtzman 2006, Morelli et al. 2008) and it does not seem to be very important, but the number of tests is small and disk contamination is still an issue in the measurement of gradients. Studies of edge-on galaxies (Jablonka et al. 2007) do not have this problem although they have the drawback of being blind to flatter components in the center of the galaxy.
12 The radius that contains half of the total luminosity of the bulge. Back.