Abstract
We aim to determine if the bulge formed via mergers as predicted by Cold Dark Matter (CDM) theory, or from disk instabilities, as suggested by its boxy shape, or both processes. We are observing about 28,000 bulge stars in fields that span longitudes of − 31 to + 26° and latitudes of − 5° to − 10°, targeting mostly red clump giants and we are measuring stellar velocities and chemical abundances. We have almost concluded our observations and have analysed data of 23,000 stars. We find a cylindrical rotation profile for the bulge which blends smoothly out into the disk and from the [Fe/H] results we find the bulge to be comprised of separate components, with an underlying slowly rotating metal poor subsample which we believe to be the inner halo stars and metal weak thick disk. We find only a small [Fe/H] gradient with latitude in the bulge, of − 0.07dex/kpc. This weak gradient does not necessarily support a merger origin for our bulge and the composite nature of the bulge is consistent with formation out of the thin disk as per instability formation models.
Highlights
Galactic bulges are relevant to galaxy formation as they are signatures of formation and we see two primary classes of bulges in our universe, large classical bulges and smaller boxy bulges
We find a cylindrical rotation profile for the bulge which blends smoothly out into the disk and from the [Fe/H] results we find the bulge to be comprised of separate components, with an underlying slowly rotating metal poor subsample which we believe to be the inner halo stars and metal weak thick disk
The aims of our study are to determine if the Milky Way bulge was formed via mergers, or disk instability and to achieve our aims we are undertaking a spectroscopic survey at medium resolution in order to characterise the metallicity and kinematics of the bulge
Summary
Galactic bulges are relevant to galaxy formation as they are signatures of formation and we see two primary classes of bulges in our universe, large classical bulges and smaller boxy bulges. The aims of our study are to determine if the Milky Way bulge was formed via mergers (classical bulges), or disk instability (boxy bulges) and to achieve our aims we are undertaking a spectroscopic survey at medium resolution in order to characterise the metallicity and kinematics of the bulge. The signatures of formation between classical merger generated bulges and disk instability boxy bulges are quite distinct. Boxy bulges show a cylindrical rotation profile, with the bulge rotating as rapidly at high latitudes as it does in the plane and this has been seen both observationally [1] and in simulations [2]. Classical bulges should have a larger metallicity gradient, due to the rapid star formation and CDM predicts that the first stars will be found in the central regions of the galaxy, ie. Classical bulges should have a larger metallicity gradient, due to the rapid star formation and CDM predicts that the first stars will be found in the central regions of the galaxy, ie. the bulge [3]
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