The formation of the galactic bulge of the milky way

Abstract

Infrared imaging with the COBE satellite shows that the Milky Way has a small boxy / peanut-shaped bulge. Disk galaxies with small bulges pose a problem for the ACDM model of galaxy formation, which predicts large central bulges formed in galaxies via merger activity. Dynamical N-body simulations of stellar disks show that small boxy / peanut bulges can form via dynamical instabilities from the buckling of an inner bar which evolves from the disk at early times. These bulges have cylindrical rotation and they contain potentially identifiable chemical signatures of the early disk trapped within the bulge. The aim of the Abundances and Radial velocities Galactic Origins Survey (ARGOS) is to understand the formation of the Milky Way bulge and to determine if it has a merger or instability origin. The observations were completed at the Anglo-Australian telescope over 42 nights from 2008 to 2011. A total of 28,000 potential giants in the bulge and inner disk were selected for observations across a grid of two degree fields spanning longitudes of -31 to +26{u00B0} at latitudes of b = -5{u00B0}, b = -7.5{u00B0}, b = -10{u00B0}, plus some fields at b = +8{u00B0}. From the spectra of the ARGOS stars, stellar parameters, (Teff, log g, [Fe/H] and [a/Fe]) were determined using a minimum x{u00B2} method and radial velocities were measured using a cross correlation of the Ca-triplet region with stellar templates. To test for the signatures of formation for the bulge, the inner stellar population was isolated using the distance estimates calculated from the stellar parameters. The key signatures of formation examined in this work are (i) the stellar density distribution of the red clump stars in the inner region, (ii) the metallicity distribution functions (MDFs) and stellar populations of the bulge and (iii) kinematics of the inner Galaxy as a function of metallicity. Recent photometric surveys have revealed a split in the density distribution of the red clump stars in the inner region at latitudes of [b] >5{u00B0} . This split appears as a bimodal distribution of apparent magnitude for the clump stars, and is related to the boxy / peanut structure of the bulge. The ARGOS data show that only the more metal-rich stars with [Fe/H] > -0.5 are involved in the split. Comparisons with N-body models of instability bulges formed via internal evolution demonstrates that the split clump structure is a generic feature of the instability process. For the stars with [Fe/H] > -0.5 that are involved in the split structure, the stars with [Fe/H] > 0 are more centrally concentrated and have a lower dispersion than stars with -0.5 < [Fe/H] < 0, indicating the presence of multiple stellar populations within the boxy / peanut bulge. The MDFs obtained with ARGOS across the bulge have been decomposed into a number of components which represent the multiple population of the inner Galaxy. Some of these populations are part of the boxy / peanut bulge and some are not. The stars with [Fe/H] > -0.5 belong to the boxy / peanut structure that is observed in the near infrared with the COBE satellite. We associate the more metal-poor stars with [Fe/H] < -0.5 with the thick disk, metal weak thick disk and inner halo populations that are present in the inner Galaxy. The metal-rich fraction of stars decreases sharply with height from the plane. The changing fractions with latitude of the individual populations identified in the ARGOS survey is responsible for a large overall vertical metallicity gradient for the stars in the inner Galaxy. In the context of the multiple MDF populations, however, the vertical gradient of each individual population measured in the bulge is small ({u2248} -0.1 dex/kpc). The kinematics analysis confirms that the mean rotation of the boxy / peanut bulge is cylindrical and transitions smoothly out into the disk, as predicted by boxy / peanut bulge N-body models. The stars in the inner Galaxy with -1.0 < [Fe/H] < -0.5, that are not part of the boxy / peanut bulge and which are likely members of the thick disk in the central regions, show similar rotation to the boxy / peanut bulge. The metal-poor population of the bulge with [Fe/H] < -1.0 is rotating slowly, consistent with an inner halo population. N-body simulations of boxy bulge formation via dynamical instability of the inner disk are examined. They show that stars initially in the disk in the inner region, and also at galactic radii larger than the present extent of the boxy bulge, become trapped in the bulge. The phase space locations of the stars in the initial disk determine whether they migrate into orbit families which produce the peanut structure and split clump in the inner region, and also the vertical height to which they are redistributed via the instability. Stars with low vertical velocity dispersions are preferentially mapped into the boxy bulge at low latitudes. This may account for the large fraction of metal-rich bulge stars with [Fe/H] > 0 observed at low latitudes. The fraction of these metal-rich stars decreases rapidly with height above the plane. In contrast, the fraction of the kinematically hotter and more metal-poor bulge stars with -0.5 > [Fe/H] > 0 is approximately constant across the ARGOS latitudes. The ARGOS results are consistent with bulge formation from the early disk via an internal dynamical instability process. For the Milky Way, we conclude that the stars from the early disk which were mapped by the instability into the boxy / peanut bulge had a metallicity > -0.5. Although more metal-poor stars with [Fe/H] < -0.5 are present in the inner region, they do not appear to have been part of the initial phase space regions mapped into the boxy / peanut bulge structure. An underlying merger-generated component is not excluded by the ARGOS data but is not needed to explain any of our observations. Identifying an underlying classical bulge is complicated by the possibility that it may have been spun up over time. Our work clearly identifies several components in the inner Galaxy, and they can be readily interpreted via the instability scenario and within the context of the same Galactic populations (thin disk, thick disk and halo) that are observed near the Sun. Show more