The shape-velocity alignment of satellites forged by tidal locking and dynamical friction

TL;DR

This study uses the high-resolution TNG50 simulation to quantify two intrinsic alignment signals for satellites and subhalos: radial alignment of their major axes with galactocentric directions and orbital alignment with their velocity vectors. It finds strong radial alignment, especially for subhalos, largely insensitive to host halo mass, and a weaker but significant orbital alignment that varies with orbital phase and halo mass due to dynamical friction and tidal stripping. The authors show that observed MW-like alignment signals in projection can largely arise from the coplanarity of major axes with orbital planes, amplified for highly elliptical systems. Together, the results link tidal torque, orbit decay, and tidal streams to orientation statistics, with implications for interpreting weak-lensing systematics and halo assembly histories.

Abstract

Utilizing the TNG50 simulation, we study two types of alignments for satellites/subhalos: 1) the alignment of their major axes with the galactocentric radial directions (radial alignment), and 2) with the motion directions (orbital alignment). We find that radial alignment is substantially stronger than orbital alignment, with both signals being consistently stronger for subhalos than for satellites. Interestingly, inward- and outward-moving satellites/subhalos show contrasting orbital alignment behaviors, which can be understood in terms of their radial alignment, orbit decay due to dynamical friction and the effect of tidal stripping. The orbital alignment is stronger in more massive halos. In the end, we explore the orbital alignment measured by a mock observer, and find that the alignment reported by Pace et al. (2022) for MW satellites is due to projection effects, as the major axes of satellites lie within their orbital planes, approximately coplanar with the observer.

Figures (7)

• Figure 1: The cumulative distribution function (CDF) of the cosine of the radial alignment angle $\theta_\mathrm{sr}$ between the major axes of the systems and their galactocentric radial directions. Each row corresponds to a given mass bin in $\log_{10}(M_\mathrm{host}/M_\odot)$ of their host halos. The left column shows the results of satellites, and the right column shows the results of their subhalos. The blue (red) curves refer to systems moving inwards (outwards). The solid (dashed) curves refer to systems at the galactocentric radii $r$ smaller (larger) than the half of the virial radii $R_\mathrm{host}$ of their host halos. The black dotted line in each panel denotes the CDF of a uniform distribution, and curves above (below) it correspond to misalignment (alignment) signals. The error bars represent the 1$\sigma$ scatters of 50 bootstrapped subsamples of the systems in the same bin.
• Figure 2: Similar to Figure \ref{['fig:thetasr']}, but of the cosine of the orbital alignment angle $\theta_\mathrm{sv}$ between the major axes of the systems and their motion directions.
• Figure 3: The evolution of different properties along the orbits of subhalos in a host halo mass bin. In all panels, the $x$-axis is the orbit phase angle, which starts from subhalos' first pericentric passages (i.e. orbit phase angle $= 0$) and lasts for the following three periods (i.e. orbit phase angle $= 6\pi$). From top to bottom, these panels show the evolution of the galactocentric distance normalized by its largest value in the orbit, the radial alignment angle $\theta_\mathrm{sr}$ (or $\alpha_\mathrm{sr}$), the orbital alignment angle $\theta_\mathrm{sv}$ (or $\alpha_\mathrm{sr}$), the angle between the galactocentric radial direction and the motion direction $\theta_\mathrm{vr}$, the ellipticity of the subhalos and a direct comparison between $\alpha_\mathrm{sv}$ and $\alpha_\mathrm{vr}$. Note that our original definition of alignment angle (Equation \ref{['eqn:theta']}), $\theta$, is between 0 and 90$^\circ$, which is shown as black curves in the third and fourth panels from the top. The definition of $\alpha$ in Equation \ref{['eqn:alpha']} ranges from 0 to 180$^\circ$, which is shown as other shallow colored curves (yellow and green, see the legend). The solid curves represent the median values, with shaded regions denoting the 16% to 84% percentiles. The blue and red vertical dashed lines represent pericentric (P) and apocentric (A) passages, respectively. The black horizontal dotted lines in the second, third, and fourth panels denote the median value of the angle $\theta$ with random orientations in 3-dimensional space (i.e., $\cos \theta =1/2$).
• Figure 4: Top: The illustration of a representative subhalo orbit around its host in an orbital period. The purple star represents the center of the host. The long black arrows denote the direction of the orbit. Each point represents the location of the subhalo at different time, with the blue and red points denoting the pericenter and the apocenter, respectively. The gray, green and yellow arrows represent the galactocentric radial direction, the motion direction and the major axis of the subhalo at each point, respectively. The points with dashed lines extending arrows backwards denote locations where the angles $\theta$ and $\alpha$ of two vectors differ. Bottom: The illustration of the angles $\theta$ (Equation \ref{['eqn:theta']}) and $\alpha$ (Equation \ref{['eqn:alpha']}) of two vectors. In the left panel, when the dot product of two vectors is positive, $\theta=\alpha$. While in the right panel, when the dot product of two vectors is negative, $\theta=180^\circ-\alpha$.
• Figure 5: The cumulative distribution function (CDF) of the observed orbital alignment angle $\theta_\mathrm{sv,observed}$ between the projected major axes of the systems and their projected motion directions, for satellites/subhalos surrounding MW/M31-like central galaxies. The left column shows the results of satellites, and the right column shows the results of subhalos. The blue (red) curves refer to systems moving inwards (outwards). The solid (dashed) curves refer to systems at the galactocentric radii $r$ smaller (larger) than 150pc. The black dotted line in each panel denotes the CDF of a uniform distribution, and curves above (below) it correspond to alignment (misalignment) signals. The error bars represent the 1$\sigma$ scatters of 50 bootstrapped subsamples of the systems in the same bin.