Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy (2007.10374v2)

Abstract: We report evidence from APOGEE for the presence of a new metal-poor stellar structure located within $\sim$4~kpc of the Galactic centre. Characterised by a chemical composition resembling those of low mass satellites of the Milky Way, this new inner Galaxy structure (IGS) seems to be chemically and dynamically detached from more metal-rich populations in the inner Galaxy. We conjecture that this structure is associated with an accretion event that likely occurred in the early life of the Milky Way. Comparing the mean elemental abundances of this structure with predictions from cosmological numerical simulations, we estimate that the progenitor system had a stellar mass of $\sim5\times10^8M_\odot$, or approximately twice the mass of the recently discovered Gaia-Enceladus/Sausage system. We find that the accreted:{\it in situ} ratio within our metal-poor ([Fe/H]$<$--0.8) bulge sample is somewhere between 1:3 and 1:2, confirming predictions of cosmological numerical simulations by various groups.

Analysis of Stellar Accretion Within the Inner Galaxy from APOGEE Data

The paper by Danny Horta et al. offers intriguing evidence, derived from APOGEE data, indicating the existence of a distinct, metal-poor stellar structure within approximately 4 kpc of the Galactic center. This finding contributes to the nuanced understanding of the Milky Way's accretion history, especially considering the potential association between this structure and early accreted components of the Galaxy. The paper, underpinned by a combination of spectroscopic and kinematic analyses, provides insights into the chemical and dynamical detachment of this structure from more metal-rich counterparts within the inner Galaxy.

Key Findings and Methodology

1. Detection of a Unique Metal-Poor Structure: The researchers identify a new Inner Galaxy Structure (IGS), distinct in both chemical composition and dynamical properties from surrounding stellar populations. This discovery is initiated via chemical signatures reminiscent of low-mass satellites of the Milky Way, specifically focusing on elemental abundances such as Fe, Mg, Al, and Mn derived from APOGEE spectroscopy.
2. Chemical and Dynamical Detachment: The IGS is characterized by low [Fe/H] values and elemental compositions aligning more closely with accreted systems rather than in situ Galactic formation. Using integrals of motion, it appears dynamically detached from other inner Galaxy components.
3. Accretion Event: A conjecture is proposed that the IGS is the result of an early massive accretion event, potentially involving a progenitor system twice the mass of the Gaia-Enceladus/Sausage (5 x 10^8 M_\odot), informed by comparisons to cosmological simulations.
4. Accreted vs. In Situ Formation: The paper extends its analysis to estimate accreted to in situ ratios in sample populations, supporting simulation predictions that imply substantial early accreted contributions to the mass of the Galaxy.

Implications

These findings have significant implications for both the theoretical understanding of Galactic evolution and the accretion history of the Milky Way. This work suggests higher contribution from accreted structures to the Halo than previously acknowledged, presenting necessary amendments in models to account for these chemically and dynamically detached populations.

Future Research Directions

1. Model Improvements: Further refinement in cosmological simulations to better model the implications of multiple accreted events, especially in how these events shape inner Galaxy populations.
2. Detailed Age Analysis: Further research into the age distribution of stars contributing to the Halo versus those formed in situ could provide more precise constraints on accretion timelines and overlap with in situ formation periods.
3. Extended Observations: Future surveys such as the Pristine Inner Galaxy Survey (PIGS) and developments from the MOONS spectrograph could substantially augment the data available for these analyses, offering deeper insights into the metal-poor, dynamically distinct populations within the inner Galaxy.

The analysis provided in this paper enriches our comprehension of the complex accretion processes shaping our Galaxy's Halo and emphasizes the pivotal role of detailed spectroscopic surveys in uncovering the intricate layers of Galactic history.