The Global Dynamical Atlas of the Milky Way Mergers: Constraints from Gaia EDR3–based Orbits of Globular Clusters, Stellar Streams, and Satellite Galaxies

Abstract

The Milky Way halo was predominantly formed by the merging of numerous progenitor galaxies. However, our knowledge of this process is still incomplete, especially in regard to the total number of mergers, their global dynamical properties and their contribution to the stellar population of the Galactic halo. Here, we uncover the Milky Way mergers by detecting groupings of globular clusters, stellar streams, and satellite galaxies in action ( J ) space. While actions fully characterize the orbits, we additionally use the redundant information on their energy (E) to enhance the contrast between the groupings. For this endeavor, we use Gaia EDR3‒based measurements of 170 globular clusters, 41 streams, and 46 satellites to derive their J and E. To detect groups, we use the ENLINK software, coupled with a statistical procedure that accounts for the observed phase-space uncertainties of these objects. We detect a total of N = 6 groups, including the previously known mergers Sagittarius, Cetus, Gaia‒Sausage/Enceladus, LMS-1/Wukong, Arjuna/Sequoia/I’itoi, and one new merger that we call Pontus. All of these mergers, together, comprise 62 objects (≈25% of our sample). We discuss their members, orbital properties, and metallicity distributions. We find that the three most-metal-poor streams of our galaxy—“C-19” ([Fe/H] = −3.4 dex), “Sylgr” ([Fe/H] = −2.9 dex), and “Phoenix” ([Fe/H] = −2.7 dex)—are associated with LMS-1/Wukong, showing it to be the most-metal-poor merger. The global dynamical atlas of Milky Way mergers that we present here provides a present-day reference for galaxy formation models.

Probability of forming gaps in the GD-1 stream by close encounters of globular clusters

“…We note that the GD-1 stream and NGC3201 share similar orbital properties. Indeed, Malhan et al (2022) recently claimed that these systems are part of the same merging event dubbed 'The Arjuna/Sequoia/I'itoi merger' (see also Bonaca et al 2021). Although their orbital similarity is intriguing, the fact that most of the high-P gap-forming GCs have very different orbital properties means that even if a GC has an orbital property similar to that of the GD-1 stream, such a GC is not necessarily a good candidate for forming a gap.…”

“…In reality, the stripping process may be continuous and time-dependent. Also, we do not take into account the host system of the progenitor system that could affect the morphology of the GD-1 stream (Malhan et al 2022;Qian et al 2022). If we were to reproduce the density profile of the GD-1 stream as a function of φ 1 , the effects mentioned above are important and the only way to faithfully take these effects into account is to run N -body simulations.…”

A New Member of the Milky Way’s Family Tree: Characterizing the Pontus Merger of Our Galaxy

“…Pontus was recently discovered by Malhan et al (2022) as a grouping of seven globular clusters that are tightly clumped in the action-energy (J, E) space of the Milky Way. This group possesses low energy (i.e., the orbits of its components are more tightly bound to the Milky Way compared to other merger groups) and slightly retrograde motion (this is also shown in Figure 1, which we explain below).…”

“…While Malhan et al (2022) could only analyze the globular cluster population of Pontus, our central objective here is to analyze its general, unbound stellar population. This is important to understand, for instance: How are the Pontus stars distributed in the phase space of the Milky Way?…”

Revisit NGC 5466 Tidal Stream with $Gaia$, SDSS/SEGUE and LAMOST

“…The discoveries of stellar streams offer an opportunity to probe the shape of the Galactic gravitational field (e.g., Küpper et al 2015;Malhan & Ibata 2019) and the formation history of the stellar halo (e.g., Bullock & Johnston 2005;Bell et al 2008;Koposov et al 2010;Law & Majewski 2010;Bowden et al 2015;Grillmair & Carlin 2016;Bernard et al 2016;. In addition, the scenario of hierarchical formation for galaxies in the standard ΛCDM cosmological model is supported by the existence of stellar streams (e.g., Springel et al 2008;Malhan et al 2022).…”