A Diffuse Metal-poor Component of the Sagittarius Stream Revealed by the H3 Survey

Abstract

The tidal disruption of the Sagittarius dwarf galaxy has generated a spectacular stream of stars wrapping around the entire Galaxy. We use data from Gaia and the H3 Stellar Spectroscopic Survey to identify 823 high-quality Sagittarius members based on their angular momenta. The H3 Survey is largely unbiased in metallicity, and so our sample of Sagittarius members is similarly unbiased. Stream stars span a wide range inWe identify a strong metallicity dependence to the kinematics of the stream members. At [Fe/H] >−0.8 nearly all members belong to the well-known cold (s < -20 km s v 1 ) leading and trailing arms. At intermediate metallicities (−1.9<[Fe/H]<−0.8) a significant population (24%) emerges of stars that are kinematically offset from the cold arms. These stars also appear to have hotter kinematics. At the lowest metallicities ([Fe/H]−2), the majority of stars (69%) belong to this kinematically offset diffuse population. Comparison to simulations suggests that the diffuse component was stripped from the Sagittarius progenitor at earlier epochs, and therefore resided at larger radius on average than the colder metal-rich component. We speculate that this kinematically diffuse, low-metallicity population is the stellar halo of the Sagittarius progenitor system.

Dwarf stellar haloes: a powerful probe of small-scale galaxy formation and the nature of dark matter

“…The potential existence of satellite stellar haloes in the MW halo is an exciting prospect, and in the current and upcoming era of Gaia and wide-field spectroscopic surveys, such as DESI, WEAVE and 4MOST, this is a testable prediction. Indeed, the stellar halo of the (disrupting) Sagittarius has potentially already been uncovered by the H3 survey (Johnson et al 2020, but also see Peñarrubia & Petersen 2021). Interestingly, Johnson et al (2020) identify the diffuse halo component by selecting Sagittarius stars in angular momentum space, in excellent agreement with the outputs of our toy models.…”

“…Indeed, the stellar halo of the (disrupting) Sagittarius has potentially already been uncovered by the H3 survey (Johnson et al 2020, but also see Peñarrubia & Petersen 2021). Interestingly, Johnson et al (2020) identify the diffuse halo component by selecting Sagittarius stars in angular momentum space, in excellent agreement with the outputs of our toy models. It is worth bearing in mind that there is a significant amount of parameter space to explore in our models (we only discuss a few examples here).…”

“…Indeed, Tarumi et al (2021) showed that early major mergers can form an extended stellar halo similar to what is seen in Tucana II. On slightly more massive scales, Johnson et al (2020) use the H3 survey (Conroy et al 2019) to find a kinematically diffuse, low metallicity population related to the disrupting Sagittarius (Sgr) dwarf galaxy. These authors suggest that this could be the stellar halo of the Sgr dwarf progenitor.…”

Orbital Clustering Identifies the Origins of Galactic Stellar Streams

“…This recently dissolved population might shed light on the origin of scatter in the relation between the globular cluster system and the galaxy mass for lowmass galaxies (e.g., Harris et al 2013). On the other hand, assuming that streams were formed only recently would suggest larger pre-infall halo masses of their host galaxies, many of which already have a sizeable population of globular clusters (e.g., 5 − 7 in Sagittarius, Johnson et al 2020;up to 6 in Sequoia, Myeong et al 2019). This is especially pertinent for the accretion of Sagittarius as its impact throughout the Milky Way strongly depends on its mass (Laporte et al 2019).…”

“…Stars from both the Cetus stream and the Sagittarius dwarf galaxy have been observed at these E − L z (Figure 2). We associate these streams with Sagittarius based on their low L y angular momenta (Johnson et al 2020). All of these streams are thin and no metallicity spreads have been detected, which suggests they were originally globular clusters associated with Sagittarius that were disrupted by the Milky Way tidal force.…”