Stellar halo substructure generated by bar resonances

Abstract

Using data from the Gaia satellite’s Radial Velocity Spectrometer Data Release 3 (RVS, DR3), we find a new and robust feature in the phase space distribution of halo stars. It is a prominent ridge at constant energy and with angular momentum Lz > 0. We run test particle simulations of a stellar halo-like distribution of particles in a realistic Milky Way potential with a rotating bar. We observe similar structures generated in the simulations from the trapping of particles in resonances with the bar, particularly at the corotation resonance. Many of the orbits trapped at the resonances are halo-like, with large vertical excursions from the disc. The location of the observed structure in energy space is consistent with a bar pattern speed in the range Ωb ≈ 35 − 40 km s−1 kpc−1. Overall, the effect of the resonances is to give the inner stellar halo a mild, net spin in the direction of the bar’s rotation. As the distribution of the angular momentum becomes asymmetric, a population of stars with positive mean Lz and low vertical action is created. The variation of the average rotational velocity of the simulated stellar halo with radius is similar to the behaviour of metal-poor stars in data from the APOGEE survey. Though the effects of bar resonances have long been known in the Galactic disc, this is strong evidence that the bar can drive changes even in the diffuse and extended stellar halo through its resonances.

Shiva and Shakti: Presumed Proto-Galactic Fragments in the Inner Milky Way

“…In the remaining study, we characterize the stellar populations of Shakti and Shiva and aim to understand their nature and origin (i.e., in situ or accreted). At this juncture, it is important to note that the (L z , E) location of Shakti is similar to two previously identified substructures: the ridge-like component pointed out in Dillamore et al (2023), who attribute them to halo stars that got spun up as they got trapped in the resonances of the Galactic bar (see our Figure 1); and also a sparse population of metal-poor disk stars (e.g., Carollo et al 2019;Naidu et al 2020). To what extent these substructures are identical, overlapping, or connected with Shakti is a question we address in Section 6.…”

“…However, there are important aspects in which Shakti and Shiva match expectations for ancient stars that gradually picked up angular momentum by resonant orbit trapping caused by the Galactic bar, a scenario proposed recently by Dillamore et al (2023). This scenario predicts ridges in the E − L z plane toward the prograde direction.…”

“…Second, while its (L z , E) location is similar to that of Dillamore et al's (2023) ridge-like component, its compact "blob"-like appearance in (L z , E) space argues against them being a ridge of former halo stars that became trapped in resonances with the Galactic bar. The feature that Dillamore et al (2023) identified had a lower contrast than Shakti. If Shakti were part of a resonancedriven ridge in orbit space, we would expect the chemical properties to be similar to that of the halo stars, in particular to GSE (because the majority of halo stars presumably originated from GSE alone; Helmi 2020).…”

“…The "spread" is the interquartile difference between the 16th and 84th percentiles and the corresponding uncertainties represent their 16th and 84th percentiles. Dillamore et al 2023), or (e) accreted origin (i.e., representing a population that was accreted inside a foreign galaxy, much like the GSE). The following discussion is summarized in Table 2.…”

“…Likewise, the "R" feature indicated in Figure 1, which represents the ridge-like overdensity of stars, may lend itself to the interpretation as an accreted population. But Dillamore et al (2023) showed that this feature comprises both metal-rich and metal-poor stars and their orbits lie somewhere between disk-and halo-like. Given that these stars' azimuthal orbit frequency coincides with that of the Galactic bar, this orbit-space substructure can be very plausibly attributed to the trapping of the (inner) halo stars in bar resonances.…”