Recovering the infall mass for Milky Way satellite galaxy Sextans

Abstract

Understanding the formation and evolution of the Milky Way (MW) requires detailed knowledge of its satellite galaxies. In this study, we focus on the Sextans dwarf spheroidal (dSph) galaxy, a faint, dark matter (DM)-dominated satellite, to investigate the role of tidal and baryonic effects in shaping its observed properties. Using tailored $N$-body simulations, we explore possible orbits of Sextans in different MW models to reconstruct its progenitor's properties. Our simulations demonstrate the stars in Sextans are only mildly affected by galactic tides and the stellar kinematics provide robust constraints on its dynamical mass within the half-light radius, while the tidal mass loss of its DM component depends primarily on MW mass. The recovered infall mass of Sextans ranges from $1.22$ to $3.14\times10^9\rm\,M_\odot$ for MW masses from $0.8$ to $2\times10^{12}\rm\,M_\odot$. If the DM density remained as cuspy as NFW profile, the infall mass would be smaller by a factor of 2. Although with large ranges, the possible infall masses of Sextans recovered by our simulations are consistent with the stellar mass-halo mass relation in TNG50 and abundance matching results. We find some cases for the cuspy DM density profile where the infall mass is smaller than $10^9\rm\,M_\odot$, possibly indicating that star formation in Sextans is more efficient than in other satellites. The recovered DM halo structural parameters from our simulations provide valuable constraints for future studies on the DM content and formation history of Sextans.

Figures (9)

• Figure 1: The observed half-light radius and LOS velocity dispersion of Sextans from literatures. In the left panel, we convert the elliptical radius to circular one based on the provided ellipticity in each work and the blue squares are used to calculate the average $R_{1/2}$ and its uncertainty, which is marked by the blue star. For LOS velocity dispersion, we adopt the value in battaglia_study_2011, which is shown by the blue star in the right panel.
• Figure 2: In the top-left panel, we show the circular velocity curves of our three MW models at $z=0$. The solid, dotted and dashed black lines represent the Fiducial, Light and Heavy MW model, respectively. The best-fitting model in mcmillan_mass_2017 are also shown by the gray line for comparison. Studies that constrain the MW mass at larger radii are shown by the gray dots with error bars (1:zhang2025; 2:ou2024; 3:bird_milky_2022; 4:liConstrainingMilkyWay2020; 5:fritzMassOurGalaxy2020; 6:eilersCircularVelocityCurve2019; 7:vasiliev2019; 8:eadie2019; 9:watkinsEvidenceIntermediatemassMilky2019; 10:fardal2019; 11:sohnAbsoluteHubbleSpace2018; 12:mcmillan_mass_2017; 13:bovy_galpy_2015; 14:kafle2014; 15:gibbons2014; 16:boylan-kolchin2013; 17:deason2012; 18:xue2008). In the rest of panels, we show the evolution of the MW parameters as a function of time. The top-right panel shows the virial mass evolution. The thick gray line shows the averaged halo mass evolution of the MW-like haloes in TNG50 and the shaded region represents the $1\sigma$ deviation. The averaged halo mass evolution is well approximated by a quadratic function shown by the blue dashed line. The lower panels show evolution of parameters of stellar disk and stellar bulge.
• Figure 3: Top: the distribution of $r_{\rm p,rec}$ of the 10,000 Monte Carlo realizations of orbits for Sextans in three MW models. The downward arrows denote the $r_{\rm p,rec}$ of three representative orbits chosen for simulations. Blue represents orbits with medium pericenter distance, while red and green represent orbits with smaller and larger values, which are $2\sigma$ away from the medium value. As is denoted by the text, we name these orbits as L81, L86, L91, M74, M81, M88, H70, H78 and H85 according to the MW model and $r_{\rm p,rec}$. Middle: the specifically chosen orbits that have different pericenter distance. The black lines mark the evolution of virial radius $r_{\rm MW,200}$ of the host MW. Squares mark the point of infall and upper triangles mark the most recent pericenter. The solid circles denote the start point of simulations. Bottom: the bound DM mass evolution over time of the 9 simulations. Colors are marked according to the upper conventions.
• Figure 4: The simulated properties of the satellite galaxies at infall (solid lines) and present time (dashed lines) across different MW models, with red, blue and green denoting simulations respectively with close, medium, and far pericenter orbits, compared with those of observation (black squares). Top: LOS velocity dispersion as a function of projected radius $R$ of stellar component in different simulations compared with observed one battaglia_study_2011. See the text for details. Middle: the projected surface density profile of stellar component in simulations compared with observation. The black squares are 2D Plummer profile with scale radius of half-light radius $R_{1/2}$ and total stellar mass of $m_*$ in Table \ref{['tab:SextansObs']}. The insets show the zoom-in profiles within $1^\circ$. For comparison, the surface density profiles measured by a few observation work are also shown using color dots. Bottom: the circular velocity of DM component as a function of radial distance $r$ in simulations. The black squares with error bars are derived circular velocity at $r_{1/2}$ as in Table \ref{['tab:SextansObs']}.
• Figure 5: The dependence on the MW model and pericenter distance of the tidal history of Sextans with dotted lines for Light MW model, solid lines for Fiducial MW model and dashed lines for Heavy MW model. Symbols with different colors represent different simulations, with the simulation name labeled in the right panel. The left panel shows the bound mass loss fraction from infall to present time, defined by $f_{\rm stripping}\equiv1-m_{\rm bound}(t=0)/m_{\rm bound, inf}$, versus the the most recent pericenter distance $r_{\rm p,rec}$. The points with solid box are bound mass loss fraction for DM, while those with hollowed box are for stellar component. The middle panel shows the DM mass loss fraction within a radius of $r_{1/2}$. The right panel shows the infall virial mass of Sextans recovered by our simulations.