APOGEE Chemical Abundances of Stars in the MW Satellites Fornax, Sextans, Draco and Carina

TL;DR

This study leverages APOGEE near-infrared spectra to derive LTE abundances for Fe, C, N, O, Mg, Al, Si, Ca, Ti, Cr, Mn, Ni, and Ce in 74 member stars from four Milky Way satellites (Fornax, Sextans, Draco, Carina). The results show a strong mass dependence of alpha-element patterns, with more massive dwarfs reaching higher metallicities at fixed $[ m Si/Fe]$ and exhibiting a characteristic alpha knee, while individual elements display varying levels of scatter and NLTE-related offsets. Al remains sub-solar across the dwarfs (around $[ m Al/Fe]\, ightarrow\, -0.5$), and a subset of Fornax stars reveals N-rich field stars likely tied to dissolved globular clusters; Ce highlights limited AGB-like enrichment in a few cases. The analysis uncovers a metallicity gradient in Fornax but no strong radial gradients in most alpha-elements, reinforcing the significance of galaxy mass and merger histories in shaping chemical evolution and offering evidence for GC disruption in dwarf environments.

Abstract

During its evolution, the Milky Way (MW) incorporated numerous dwarf galaxies, particularly low-mass systems. The surviving dwarf galaxies orbiting the MW serve as exceptional laboratories for studying the unique properties of these systems. Their metal-poor environments and shallow gravitational potentials likely drive significant differences in star formation and star cluster properties compared to those in the MW. Using high-quality near-infrared spectra from the APOGEE survey, we determined abundances of Fe, C, N, O, Mg, Al, Si, Ca, Ti, Cr, Mn, Ni, and Ce for 74 stars in four MW satellite dwarf galaxies: Fornax, Sextans, Draco, and Carina. Our analysis reveals that the distribution of $α$ elements (e.g., [Si/Fe]) strongly correlates with galaxy luminosity (and hence mass), underscoring the critical role of galaxy mass in shaping chemical evolution. These dwarf galaxies exhibit [Al/Fe$]\sim -0.5$, which is comparable to those of the metal-poor stars in the MW. Additionally, we identified nitrogen-rich field stars in the Fornax dwarf galaxy, which display distinct metallicities compared to its known globular clusters (GCs). If these stars originated in GCs and subsequently escaped, their presence suggests we are observing relics of destroyed GCs, offering possible evidence of cluster disruption.

Figures (9)

• Figure 1: Target information. (a): Spatial distribution of target stars in Fnx dwarf galaxy. The black ellipse indicates the tidal radius of Fnx and its center is labelled with black cross. Target stars from 2020AA...641A.127R and this work are marked as blue triangles and red dots, respectively. The N-rich stars are further marked with red circles. The black stars represent GC Fnx 1-5 while the purple star represents GC Fnx 6. (b): Color--magnitude diagram of Fnx stars based on $Gaia\ DR3$ (grey dots).
• Figure 2: [$\alpha$/Fe] vs. [Fe/H]. Fnx, Dra, Car and Sex stars from this work are labeled as red, cyan, blue and green stars respectively. The error bar in each panel indicates the median uncertainty of available measurements. Black dots correspond to MW stars from the halo 2000AJ....120.1841F2004AA...416.1117C2005AA...439..129B2013ApJ...762...27Y2014AJ....147..136R, and MW stars from the disc 2003MNRAS.340..304R2006MNRAS.367.1329R2014AA...562A..71B. Orange dots represent Sgr stars from 2021ApJ...923..172H. Purple squares represent GCs in Fnx from 2022AA...660A..88L
• Figure 3: [Al/Fe] vs. [Fe/H] relations. Symbols are the same as in Fig. \ref{['fig: alpha']}.
• Figure 4: Derived LTE abundances of [Si/Fe] over metallicities for individual dwarf galaxies. MW stars (black dots) are described in Fig. \ref{['fig: alpha']}. The best polynomial fits to the data of Sgr, Fnx, Dra, Car, Sex and Scl member stars are labeled as orange, red, cyan, blue, green, and yellow lines.
• Figure 5: Abundances of iron peak elements vs. [Fe/H]. Symbols are the same as in Fig. \ref{['fig: alpha']}.