The Pristine Inner Galaxy Survey (PIGS) I: tracing the kinematics of metal-poor stars in the Galactic bulge

Abstract

Our Galaxy is known to contain a central boxy/peanut-shaped bulge, yet the importance of a classical, pressure-supported component within the central part of the Milky Way is still being debated. It should be most visible at low metallicity, a regime that has not yet been studied in detail. Using metallicity-sensitive narrow-band photometry, the Pristine Inner Galaxy Survey (PIGS) has collected a large sample of metal-poor ([Fe/H] < −1.0) stars in the inner Galaxy to address this open question. We use PIGS to trace the metal-poor inner Galaxy kinematics as function of metallicity for the first time. We find that the rotational signal decreases with decreasing [Fe/H], until it becomes negligible for the most metal-poor stars. Additionally, the velocity dispersion increases with decreasing metallicity for −3.0 < [Fe/H] < −0.5, with a gradient of −44 ± 4 km s −1 dex −1 . These observations may signal a transition between Galactic components of different metallicities and kinematics, a different mapping onto the boxy/peanut-shaped bulge for former disk stars of different metallicities and/or the secular dynamical and gravitational influence of the bar on the pressure-supported component. Our results provide strong constraints on models that attempt to explain the properties of the inner Galaxy.

The COMBS Survey - II. Distinguishing the metal-poor bulge from the halo interlopers

“…We show the position of the Sun as a black star at (8.3,0) kpc (Reid et al 2014). We also show the edge of what we consider the bulge as a black line, which corresponds to a distance of 3.5 kpc from the Galactic center and is consistent with what is typically used in the literature (e.g., Ness et al 2013b;Arentsen et al 2020a;Kunder et al 2020). Our sample clearly has some contamination from disk stars that are along the lineof-sight towards the bulge.…”

“…For example, Kunder et al (2020) found that only 25% of their sample of RR Lyrae stars had apocenters> 3.5 kpc. However, we note that the kinematic results, specifically the Galactocentric line-of-sight velocity distributions as a function of Galactic longitude, for studies which did not target RR Lyrae stars (e.g., Ness et al 2013b;Arentsen et al 2020a) show results similar to ours when we do not remove the contamination. This may indicate similar rates of contamination with halo interlopers in these studies.…”

“…Furthermore, the confined stars appear to be rotating slower than expectations from the simulation (see right panel of Figure 10). This has previously been observed among metal-poor bulge stars in Arentsen et al (2020a). However, since they cannot distinguish between the halo interlopers and confined stars, it is difficult to determine if the slower rotation observed in Arentsen et al (2020a) is a result of halo contamination or the confined bulge stars.…”

“…Studies of the kinematics of metal-poor bulge stars indicate that they are distinct from the metal-rich population and do not participate in the B/P bulge. Using the lineof-sight velocities, it has been shown that the metal-poor component of the bulge rotates slower than the metal-rich component and has higher velocity dispersion (Ness et al 2013b;Kunder et al 2016;Arentsen et al 2020a). Furthermore, the vertex deviation, which measures the orientation of the covariance between the radial and tangential motion, approaches zero for metal-poor bulge stars while it is large for metal-rich stars (Soto et al 2007;Babusiaux et al 2010).…”

Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way’s quiescent merger history and in-situ bulge

“…Therefore, in order to detect the ex-situ population of stars in the MW b/p bulge we will likely have to probe the extremely metal-poor tail of the inner regions (see e.g. Starkenburg et al 2017;Arentsen et al 2019 and Figure A4 where we show the kinematics of the [Fe/H]<-1 population in Auriga).…”