4.1. The metallicity distribution
The characterisation of the chemical abundance of Bulge stars is perhaps the field that has evolved most quickly thanks to the advent of multi-object spectrographs in large telescopes. Different surveys have − and still are − pointing towards different regions of the bulge and collecting samples of thousands of stars. The argument for such surveys is clear: it became evident that a few low extinction regions were no longer representative of the global chemical abundance patterns of the Bulge. First attempts to derive the metallicity distribution of the Bulge based on low resolution spectra (e.g. Sadler et al 1996, Ramírez et al 2000), together with a small number of available spectra obtained with high resolution (McWilliam and Rich 1994, Fulbright et al 2006), had shown from the start that the mean population of the bulge was overall metal-rich. The shape of the metallicity distribution was, on the other hand, less clear. Zoccali et al (2003) used a set of observations using WFI to obtain a photometric metallicity distribution based on the colour of red giant stars. They found a rather broad metallicity distribution, in good agreement with that derived from spectroscopy in the same field but with a larger statistical sample, with [Fe/H] values ranging from -1.0 to 0.4 and which peak at solar metallicity.
With a well-characterised metallicity distribution in Baade's window, it was time to answer the following question: how spatially uniform were these properties? Minniti et al (1995) had already discussed the possibility for a metallicity gradient in the Bulge, impressively enough based on low resolution spectra of less than a hundred giant stars. It then became the era of multi-object spectroscopy where hundreds of stars could be observed in one single shot. Using FLAMES on the Very Large Telescope, Zoccali et al (2008) derived the metallicity distribution for different fields along the Bulge minor axis at different latitudes (b = −4, −6, and −12). They found a clear metallicity gradient of ∼ 0.6 dex/kpc, with mean metallicities varying from -0.4 at the largest latitudes and up to solar metallicity at b = -4. This gradient has since then been confirmed thanks to several subsequent observations across different regions and the variation in the metallicity distributions have been further characterised (Johnson et al 2011, Uttenthaler et al 2012, Johnson et al 2013, Ness et al 2013a, Rojas-Arriagada et al 2014).
Figure 3. Left panel: Metallicity distributions for a compilation of studies taken from Johnson et al (2013). The (l, b) = (+1, −4), (0, −6) and (0, −12) fields are from Zoccali et al 2008, the (0, −8) field is from Johnson et al (2011), the (+5, −3) field is from Gonzalez et al (2011), and the (−5.5, −7), (−4, −9), and (+8.5, +9) fields are from Johnson et al (2013). Right panel: Map of the mean photometric metallicities of the bulge constructed with the VVV survey data from Gonzalez et al (2013). [Left panel adapted from Fig. 8 in Johnson et al 2013, ©AAS reproduced with permission. Right panel adapted from Fig. 2 in Gonzalez et al 2013, ©ESO reproduced with permission.]
Gonzalez et al (2013) recently complemented these results by presenting a photometric metallicity map, constructed with the same technique used by Zoccali et al (2008) but based on the Vista Variables in the Via Lactea (VVV) ESO public survey, for almost the entire Bulge region providing the global picture of the Bulge metallicity gradient. The metallicity gradient is therefore strongly established by an increasing number of spectroscopic studies obtained with different techniques and stellar samples (see Fig. 3 for a compilation of the latest results). However, the metallicity distributions obtained in the innermost regions of the Bulge (|b| < 4) based solely on high-resolution, near-infrared spectroscopy have provided evidence for the flattening-out of the gradient in the inner 700 pc (Ramírez et al 2000, Rich et al 2012).
What is the implication of finding such a metallicity gradient in the Bulge? At first, similarly to the domination of old ages found in Bulge stars, the metallicity gradient was interpreted as direct evidence for a bulge formed as a classical bulge via mergers in the early stages of the galaxy, similarly to elliptical galaxies. It was also interpreted as evidence against the secular evolution scenario, since it was thought that bars would mix the stellar orbits well enough to erase any existing vertical gradient. Models of bar formation in disc galaxies, however, proved otherwise, showing that a bar might produce a gradient similar to the one seen in the Milky Way depending on the original disc radial gradients (Martinez-Valpuesta and Gerhard 2013), vertical gradients (Bekki and Tsujimoto 2011), or both (Di Matteo et al 2014).
However, the existence of metallicity gradients has also been interpreted as a consequence of having two or more underlying components each one with a characteristic metallicity distribution. This mixing of components would naturally produce a variation on the mean metallicity according to the bulge region which is being studied. Evidence for such a multiple component scenario has been suggested based on a bimodal metallicity distribution of red clump stars in Baade's window by Hill et al (2011) and has been also suggested from a similar bi-modality seen in the metallicity distribution of microlensed dwarfs. Both of this distributions show a metal-poor and a metal-rich peaks located approximately at [Fe/H] ∼ −0.3 and [Fe/H] ∼ +0.3, respectively. Recently, the same bi-modality has also been found in the metallicity distributions based on the Gaia-ESO survey observations of the Bulge (Rojas-Arriagada et al 2014). By producing a Gaussian decomposition of the metallicity distribution functions, Rojas-Arriagada et al (2014) showed a clear bi-modality in all the analysed fields with relative sizes of components depending of the specific position on the sky. This change in the relative sizes of each component can be clearly seen in Fig. 4.
Figure 4. Upper panels: Metallicity distributions for the five bulge fields from the ESO Gaia survey, presented in Rojas-Arriagada et al (2014). Black dashed and solid lines show the components identified in each field as dashed Gaussian functions, with the sum of them shown as a solid line. The three minor-axis fields located at (+1,-4), (0,-6), and (-1,-10) are shown in the central panels, and the lateral fields (+7,-9) and (-10,-8) at the left and right. Lower panels: Metallicity distributions from the ARGOS survey from Ness et al (2013b). From left to right at b = −5, −7.5 and −10, for l = ±15. The different contribution of the adopted Gaussian components are marked in each field, with the three main components being A, B and C. [Upper panel adapted from Fig. 6 in Rojas-Arriagada et al, 2014, ©ESO reproduced with permission. Bottom panel adapted from Fig. 1 in Ness et al 2013b]
On the other hand, the ARGOS Galactic bulge survey (Freeman et al 2013), which consists of the largest sample of homogeneously analysed RC stars, constructed large-number statistics metallicity distributions at different latitude stripes, using a total of more than 10,500 stars located within a galacto-centric radius of 3.5 kpc. The overall metallicity distribution of the ARGOS survey was interpreted in Ness et al (2013a), as being composed of five Gaussian components. Each of these components would be sampling a different stellar population and thus any changes in their relative contribution fraction as a function of latitude could be the origin of the observed mean metallicity gradient seen in the Bulge. The metallicity distributions for stars within Galactic longitudes l = ±15 and latitudes b = −5, −7.5, and −10 from the ARGOS survey (Ness et al 2013a) are also shown in Fig. 4. This effect has lead the ARGOS survey to suggest the three main components of the metallicity distribution to be associated with the metal rich B/P bulge (mean [Fe/H] ∼ +0.15), the thick B/P bulge (mean [Fe/H] ∼ −0.25) and the inner thick disc (mean [Fe/H] ∼ −0.70).
Although the results from Ness et al (2013a) are based on the identification of several components, it can be safely understood that most studies converge into a similar conclusion, the bulge metallicity distribution is the result of not a single but of a mixture of populations with at least two main components, one metal-poor and the other metal-rich. Certainly, this is everything that can be concluded from the metallicities alone and one has to be extremely cautious when attempting to link these different components with a given bulge formation scenario. All evidence needs to be considered when interpreting results in terms of such scenarios, i.e. such an assessment must also include the kinematics, spatial distribution and ages of the stars.
4.2. The Bulge alpha-element abundances
The α-element abundances can provide us with further constraints for the origin of the Bulge stellar populations, specifically with respect to its formation time-scale. Tinsley (1979) suggested that the ratio of [α/Fe] compared to [Fe/H] is a function of the time delay between SNe II, which produce both α- and iron-peak elements (e.g. Woosley and Weaver 1995), and SNe Ia, which yield mostly iron-peak with little α-element production (e.g. Nomoto et al 1984). Therefore, only after sufficient time has passed for the SNe Ia events occur, the [α/Fe] ratio will decline from the SNe II value. Clearly, the critical ingredient on this relation is the SNe Ia delay time, for which different production channels might be present.
In the Bulge, the α-element abundances of Bulge stars with [Fe/H] < -0.3 have been found to be enhanced over iron by [α/Fe] ∼ +0.3 dex (McWilliam and Rich 1994, Rich and Origlia 2005, Cunha and Smith 2006, Fulbright et al 2007, Lecureur et al 2007, Rich et al 2007) calling for a fast formation scenario, while metal-rich stars [Fe/H]>-0.3 showed a decrease in [α/Fe] reaching solar values for metallicities larger than Solar. However, is important to note that not all elements were found to follow the same yield trends.
A relative approach has been commonly adopted instead of an absolute interpretation of the Bulge α-element ratio. The direct comparison of [α/Fe] values in Bulge stars against those of other galactic components then provides a relative time constraint on the bulge formation. Fulbright et al (2007), Zoccali et al (2006), and Lecureur et al (2007) all came to the conclusion that the [α/Fe] ratio was enhanced by nearly +0.1 dex with respect to the trends of both the thin and the thick disc, thus implying a shorter formation time scale for the bulge than from both discs. However, as first pointed out by Meléndez et al (2008), the bulge α-element over-enhancement with respect to the thick disc was a result of systematic offsets between abundance measurements in dwarf stars of the disc and giant stars from the Bulge. Later on, Alves-Brito et al (2010) and Gonzalez et al (2011) confirmed that when giants from the both bulge and the disc are homogeneously analysed the Bulge followed the same over-abundance in α-elements as the thick disc, both being enhanced with respect to the thin disc at metallicities [Fe/H]<-0.2. At solar metallicities, the Bulge stars are found to be α-poor, as poor as those of the thin disc. The way these trends are interpreted is that the metal-poor population of the bulge underwent a similarly fast formation scenario to that of the thick disc, while the metal-rich population of the bulge must have had a longer formation time scale, in similar time-scale to that of the thin disc stars. Similar conclusions have been reached in several other studies carried in different regions of the Bulge (Bensby et al 2010, Bensby et al 2011, 1Ryde et al 2010, Hill et al 2011, Johnson et al 2011, Johnson et al 2013, Johnson et al 2014).
However, open questions remain regarding the chemical similarities of Bulge stars with those of the thick disc stars, particularly in light of a few recent findings. Bensby et al (2013a) suggested that the position in the [α/Fe] – [Fe/H] plot where [α/Fe] starts to decrease (referred to as the knee in the literature) is located at higher metallicities in the Bulge than in the thick disc. The position of the knee in the bulge may be 0.1–0.2 dex higher in metallicity in the Bulge than in the thick disc thus suggesting that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. As the sample of Bulge micro-lensed dwarfs increases, it would be of great interest to further confirm the findings of Bensby et al (2013a). As a matter of fact, a similar result was proposed by Johnson et al (2014) who also added the analysis of Fe-peak elements finding in particular that Co, Ni, and Cu appear enhanced compared to the disc. It is important to recall that the results presented in Johnson et al (2014) have been obtained by comparing Bulge giants to dwarf stars from the local disc. This technique has been shown to suffer from systematic offsets (Meléndez et al 2008). However, the detailed analysis by Johnson et al (2014) has been carefully calibrated internally so it would be of great interest to confirm if these results are also found when bulge giant stars are compared to (inner) disk giant stars. These findings certainly highlight the importance of the future multi-object spectroscopic surveys on different galactic components to obtain a definitive answer.