Unveiling the Sagittarius Dwarf Spheroidal Galaxy Core with Gaia DR3: A Red Clump Distance Precise to 2%

Abstract

The Sagittarius dwarf spheroidal galaxy provides us with the unique opportunity to study an ongoing Galactic cannibalistic event between our Milky Way Galaxy and a satellite dwarf galaxy. Understanding this event crucially requires memberships and high-precision metallicities. Here, we present the first major membership star catalog of the Sagittarius dwarf core ($\approx$140,000 sources) and Messier 54 ($\approx$2000 sources) with positions, proper motions, and parallaxes from $Gaia$ DR3, supplemented with metallicities from the Apache Point Observatory Galactic Evolution Experiment. We initially isolate the Sagittarius dwarf core and Messier 54 spatially from prior literature positions. Using evolutionary sub-samples separated within a color-magnitude diagram, we analyze the substructures of the Sagittarius core and infer its positional relationship with Messier 54 within 5D phase space. A sample of Milky Way stars from a similar galactic latitude were used to identify contaminants and separate member stars from the core of the Sgr dSph and Messier 54 using a Gaussian Mixture Model. We present the derived proper motion, parallaxes, and metallicities for these evolutionary sub-samples while demonstrating the precision of our sample using red clump standard candles. We find a distance modulus for the Sagittarius core and Messier 54 of $(m-M)_{0}=16.958^{+0.044}_{-0.044}$ mag and $(m-M)_{0}=16.94^{+0.047}_{-0.056}$ mag, corresponding to a heliocentric distance of $d=24.635^{+0.49}_{-0.49}$ kpc and $d=24.452^{+0.537}_{-0.602}$ kpc respectively. With red clump distance analysis, our results imply there is no separation between the Sagittarius core and Messier 54. Finally, we describe the metallicity distributions of the evolved stars within these two systems, finding evidence for the infall scenario.

Figures (6)

• Figure 1: Sky density distribution of stars spatially selected as members of the (left) Sgr dSph core and (right) Messier 54 from $Gaia$ DR3. The black density contours present visualize the location of 80% of stars within our sample. The bottom panels show Sgr dSph core and Messier 54 after removing Milky Way contaminants from Section \ref{['sec:mw_contam']}.
• Figure 2: A bivariate contour plot of proper motion space ($\mu_\alpha*$ vs $\mu_{\delta}$) for the Sgr core (left) and M54 (right). Pink contours and histograms represent our high probability (95%) identified members through a GMM. The purple contours and histograms represent the MW field stars. We identify the Sgr/M54 population using literature values from 2018GaiaZhaozhou respectively.
• Figure 3: Color-magnitude diagrams (CMDs) for the Sgr core (left panels) and M54 (right panels), shown before (top) and after (bottom) minimizing MW contamination through a GMM for identified evolutionary areas and an spatial over density cut in proper motion. The background colors in all four plots indicate the blue, green, and red zones within $G_{BP}-G_{RP}$ which informed our evolutionary subsample selection detailed in \ref{['sec: evolution']}. The top panel points are colored by stellar density, while the multi-colored, larger, star shaped points show the APOGEE DR17 [Fe/H] values of identified member stars in the bottom panel. Over-plotted in all panels are identified evolutionary subsamples (defined as polygons within Table \ref{['table:sgrPoly']} and Table \ref{['table:m54Poly']}). The black points in the bottom two panels are the high-probability identified member stars without APOGEE DR17 [Fe/H] value using a GMM. No correction of reddening has been applied.
• Figure 4: Distance moduli of the Sgr Core+M54 from previous works of varying distance determination methods Mateo1995AlardMonacoDistanceSiegelKunderSollima2010 compared to the Sgr Core and M54 distance moduli from this work.
• Figure 5: Measured heliocentric velocities (top panel), metallicities ([Fe/H]; middle panel) from APOGEE DR17 for available member stars identified within this paper. Top panel is measured total proper motion ($\mu$) from $Gaia$ DR3. Identified Sgr members are in blue; identified M54 members are in red.