Several studies (e.g. Babusiaux et al. 2010; Hill et al. 2011; Gonzalez et al. 2011; Freeman 2012; Rojas-Arriagada et al. 2014) have interpreted the MDF of the bulge across a broad region in (l,b) as being made up of multiple populations. Ness et al. (2013a) interpret the MDF as having 5 populations which are shown in Figure 3: 3 dominant populations (A–C) with [Fe/H] > −1 and two minor metal-poor populations (D & E). From the ARGOS bulge survey described in Ness et al. (2013a), the three populations A-C with [Fe/H] > −1 have peaks at metallicities of about +0.15, -0.25, -0.7 dex, respectively and provide about 95% of stars in the bulge. They associate these populations with the stars of the boxy/peanut bulge (A and B), the thick disk (C), the metal weak thick disk (D) and stellar halo (E). They find that these populations are present in different proportions throughout the inner region of the Milky Way.
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Figure 3. The MDF for stars within RGC < 3.5 kpc from the ARGOS survey: (a) for stars at b = −5∘, (b) stars at b = −7.5∘, and (c) stars at b = −10∘, for all stars across longitudes |l| < 15∘ showing the changing contribution of metallicity fractions with latitude. The Gaussian components A-E are indicated. |
Using red clump stars, Ness et al. (2012) and Uttenthaler et al. (2012) showed that only the more metal rich stars in populations A and B (i.e. stars with [Fe/H] > −0.5) are part of the split density distribution of stars in the innermost region that reflects the X-shaped morphology of the bulge. This implies that stars with [Fe/H] < −0.5 that are present in the inner region are not part of the boxy/peanut bulge morphology and are not on X-profile-supporting x1 orbit families. The most metal-rich stars show the largest minima between peaks in the K-magnitude distribution of stars and are the most strongly involved in the X-shape (Ness et al. 2012). Population A was found by Ness et al. (2013a) to be concentrated toward the plane and to be the thinner part of the boxy/peanut bulge. Population B corresponds to the vertically thicker stars in the bulge, with a similar contribution fraction across b = 5 to 10∘. Population C is not significantly involved in the split density distribution or boxy/peanut structure but transitions smoothly from the bulge to disk with longitude; it is identified with the inner thick disk. The two populations of stars (A and B) associated with the boxy/peanut structure have similar peaked velocity dispersion profiles with longitude. The velocity dispersion profiles for the more metal-poor stars have a different shape (e.g. Shen et al. 2010; Ness et al. 2013b; Portail et al. 2015) (see below). The 5% of stars in the bulge with metallicities [Fe/H] < −1.0 are chemically similar to stars of the metal-weak thick disk and halo near the Solar neighbourhood (e.g. Alves-Brito et al. 2010; Bensby et al. 2013) and were associated by Ness et al. (2013a) with these populations.
The rotation and dispersion profiles reported by Ness et al. (2013b) and shown in Figure 4 as a function of [Fe/H] support the differentiation of the populations. The kinematics of stars in populations A & B are related and are different from the other components. The stars in populations A and B, which are part of the split clump and the boxy/peanut bulge, show the same characteristic peaked pattern of velocity dispersion in the two left hand panels of Figure 4, with population A being a colder replica of population B. Population C, associated with the thick disk in the inner Galaxy, is rotating as rapidly as the more metal-rich populations (all show the latitude-independent rotation profiles that are commonly seen in boxy bulges) but its velocity dispersion profile has a different shape. The most metal-poor stars with [Fe/H] < −1 have the slow rotation profile and high dispersion that might be expected of a population that had no ancestral association with a disk, such as a spheroidal population in the bulge or the stars of the inner halo or an underlying merger component.
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Figure 4. The rotation curves (top) and dispersion profiles (bottom) for the 17,500 bulge stars from the ARGOS survey within distances 5 – 11 kpc. The populations from most metal rich to most metal poor indicated in Figure 3 correspond to populations A,B,C and D/E. Ness et al. (2013b). |
Other studies (Babusiaux et al. 2010; Hill et al. 2011; Gonzalez et al. 2011; Rojas-Arriagada et al. 2014) have proposed that the MDF of the bulge comprises two populations, a metal-rich population that is part of the boxy/peanut bulge and a metal poor population that is an old spheroid (i.e. with a possible formation history that is distinct from the disk and halo components of the Milky Way). Although note that Gonzalez et al. (2015) concludes that whether their two MDF components have different formation histories is yet to be determined. The decomposition of stars into two roughly equal components from Gonzalez et al. (2015) is shown in Figure 5, from the GIBS survey (Zoccali et al. 2014), where these two populations peak at [Fe/H] of about +0.25 dex and -0.3 dex.
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Figure 5. From Gonzalez et al. (2015): The MDF obtained from the combination of four GIBS (high-resolution) fields and the red clump stars from Hill et al. (2011). The probability density distribution is shown in the upper panel as a dashed line, with the variance on the probability densities in blue. The lower panel shows the best two Gaussian fit to the upper panel distribution. |
The 2-component decomposition places fewer stars within the boxy/peanut or X-shaped structure, attributing approximately 50% to an old spheroid (Hill et al. 2011). The 5-population model of Ness et al. (2013a) attributes 95% of stars to be disk stars, with the population C (at [Fe/H] = –0.7) being disk stars but simply not part of the X-shaped morphology (possibly because they were originally part of a hotter thick disk which was dynamically less responsive to the instability). These two interpretations therefore have different implications for the stars in the inner region in terms of their origin from the disk and the contribution of any additional population that is distinct from any other Milky Way population and specifically from the bulge, such as an old spheroid formed via mergers at high redshift. Whereas a large fraction, up to 50% of stars in the two-component decomposition, are associated with an old-spheroid component, only ∼ 5% of the population in the 5 population decomposition is associated with any old spheroid component, and this component may simply be metal-poor halo stars in the inner region and not a unique population of the bulge.