Epicyclic Density Variations in the Indus Stellar Stream

Abstract

Longitudinal density fluctuations observed in stellar streams can result from gravitational interactions with massive perturbers in the Milky Way, such as dark matter subhalos. Analysing these density variations provides a powerful probe of properties (motion, mass, size, etc.) of the perturbing objects. However, caution is needed because density variations may arise naturally from internal dynamics of streams, namely epicycles. In this work, we focus on the Indus stellar stream, a remnant of an ancient dwarf satellite of the Galaxy. An Indus stream spanning $\sim 90^\circ$ is revealed in the southern Galactic sky using a comprehensive matched-filter analysis utilizing data from the Gaia mission. A spatial density model is fitted to the filtered map to quantitatively characterize the morphology, which demonstrates episodic density peaks and gaps in the stream. Through N-body simulations, we show that there are strong epicyclic motions of stars happening during tidal disruptions. The present-day longitudinal densities from simulations are comparable to the measurement from data, with similar numbers and locations of peaks and gaps, suggesting that the observed density should mainly be caused by epicycles. We also find that a cuspy dark matter halo for the Indus dwarf is likely to produce milder stellar epicyclic peaks compared to a cored halo which results in steeper peaks. This arises from different instantaneous mass loss due to distinct central mass distributions of halos, where a cored halo usually leads to severer tidal stripping. The observed density exhibits moderate peak sharpness, implying that Indus may have originally possessed a cuspy halo.

Figures (16)

• Figure 1: Comparisons between the trial model and real data of the Indus stream in phase spaces. Five panels display $\phi_2$, $d_\odot$, $\mu^*_{\alpha}$, $\mu_{\delta}$, and $V_r$, respectively, as a function of $\phi_1$. Grey dots show the model, while cyan and magenta points show Indus members from $S^5$2022ApJ...928...30L and STREAMFINDER2024ApJ...967...89I.
• Figure 2: Top six panels: kinematic filters created based on the trial model of the Indus stream. The three pairs of panels show the mean track and width in $\phi_2$, $\mu^*_{\alpha}$, and $\mu_{\delta}$, respectively. Blue points are computed from the model stream, while red lines are tracks and widths designed for filtering. The green line is the model width $s_{\phi_2}$ fitted using splines. Bottom two panels: the CMD filter including colour $BP-RP$ and its median error (calculated from Gaia data and used as the filter width) as a function of apparent $G$.
• Figure 3: The first row presents the filtered map on the sky. Stars are shown in black dots in the left panel. The right panel is a binned two dimensional histogram of the left one. The $\phi_2$ filter is overplotted in the red curve as a comparison. The second and third rows are similar but for $\mu^*_{\alpha}$ and $\mu_{\delta}$. The last row presents the CMD of Indus in three segments. Yellow and red lines show the isochrone filters. Cyan dashed lines indicate $G_0=17$ mag, above which Indus dominates.
• Figure 4: Distance modulus and metallicity gradients of the Indus stream. Red and blue points are crossmatched RRL and BHB stars in the CMD of Figure \ref{['fig:indus_kinematic_cmd']}. The black line represents the distance of the trial model. Magenta and cyan lines show a linear fit and the mean and dispersion of metallicity of Indus reported in 2022ApJ...928...30L.
• Figure 5: The top panel is a histogram of all stars (stream $+$ background) located in the stream region between the red lines in the upper-right panel of Figure \ref{['fig:indus_kinematic_cmd']}. The rest three panels are measurements of stream density N (plotted as $\mathrm{exp}(I(\phi_1))/0.5$), stream trajectory $\phi_2$ (i.e. $\Phi_2(\phi_1)$) and stream width $\sigma_{\phi_2}$ ($\mathrm{exp(S(\phi_1))}$) in Equation \ref{['eq:rho_s']}. Dividing $\mathrm{exp}(I(\phi_1))$ by $\phi_2$ bin width $=0.5\degr$ is used to derive total star counts within each $\phi_1$ bin. The Solid lines and bands show results at 16th, 50th, and 84th percentiles. Additionally, black dashed lines are third-order polynomial fits to respective profiles, used to normalize densities to compute power spectra later.