Detectability of Satellite Planes in Mock Observations of Isolated L* Galaxies

Abstract

The existence and prevalence of planar, co-rotating distributions of satellite galaxies around L* host galaxies in the local universe remains a subject of ongoing debate. Despite numerous observational efforts over the past decade, a statistically robust sample of "satellite planes" across the diversity of host galaxy environments is lacking. To guide future observing strategies, we construct a controlled suite of mock observations of on-sky positions and line-of-sight (LOS) velocities of isolated L* host galaxies and their satellite systems, based on samples drawn from the Illustris TNG100-1 cosmological simulation to build a statistical sample. In these mock systems, satellite planes are defined by three key parameters: the number of satellites ($N_{\mathrm{sat}}$), the fraction residing in a thin co-rotating plane ($f_{p}$), and the orientation angle relative to the observer ($θ_{\mathrm{rot}}$). We evaluate the sensitivity of three observational metrics, $N_{\mathrm{cor}}$ (number of co-rotating satellites), $b/a$ (projected flattening of the satellite distribution), and $v_\mathrm{los}$ (mean absolute LOS velocity), to the presence of such planes. Our results show that detection rates are strongly dependent on $θ_{\mathrm{rot}}$ and $N_{\mathrm{sat}}$. Satellite planes that are viewed nearly edge-on or face-on, are the most readily detected. In contrast, intermediate orientations and systems with fewer satellites yield low detection success rates. Generally, only satellite planes with $N_{\mathrm{sat}}>20$ have high chances of being detected. These findings provide a practical framework for prioritising observational targets and designing future surveys aimed at detecting and characterising satellite planes.

Figures (12)

• Figure 1: Histograms of the mass distribution of our sample of 920 systems from TNG100-1. In the left histogram, we show the distribution of $M_{200}$, or the virial mass of the FOF groups. In the right histogram, we show $M_*$, or the stellar mass of the host halos that are included in our sample.
• Figure 2: A mock observation of a satellite system containing a satellite plane with $N_{\mathrm{sat}}=20$ and $f_{p}=75\%$. In both graphs, downward triangles represent the positions of in-plane satellites that are blueshifted relative to the host, and upward triangles are redshifted. Crosses represent satellites not in the plane. The large dot indicates the location of the host galaxy. The colour of the symbols indicates the velocity magnitude relative to the virial velocity, and is indicated on the plot within the colour bar. The top panel is an edge-on ($\theta_{\mathrm{rot}}=0^\circ$) view of the satellite plane, bottom panel is a face-on view ($\theta_{\mathrm{rot}}=90^\circ$).
• Figure 3: A mock observation of a simulated satellite system with $N_{\mathrm{sat}}=20$, $f_{p}=0.75$ and $\theta_{\mathrm{rot}}=20^\circ$, where the satellites that are determined by the iterative weighting scheme to be outliers (weight is zero) are shown with crosses, and inliers (weight is $>0$) with dots. Whether a satellite was originally generated as in-plane or out-plane is shown by the symbol shape, a circle is an in-plane satellite, and a cross an out-plane satellite. Green symbols have been correctly identified as outliers or inliers, and black symbols incorrectly identified. The size of the green dots is scaled with the weight given to to them. We overlay the figure with an ellipse of the $b/a$ metric from all the satellites (dotted line), and with outliers rejected (dashed line.)
• Figure 4: The distributions of the metrics $N_{\mathrm{cor}}$, $b/a$ and $v_{\mathrm{los}}$ across simulations of 20 satellites whose distribution is defined by the parameters $f_p$ and $\theta_{\mathrm{rot}}$. $b/a$ and $v_{\mathrm{los}}$ have been smoothed by a Kernel Density function to approximate the distribution. The value $f_p$ is set by the y-axis, and $\theta_{\mathrm{rot}}$ by the shading of the lines, with a legend in the figure. The dashed line is the reference isotropic case ($f_p=0$) for each metric.
• Figure 5: A diagram detailing how co-rotating in-plane satellites can appear to be counter-rotating to distant observers, if one of the satellites is on a highly eccentric orbit. The left panel demonstrates this observing the plane 'face-on', with coloured arrows at points along the orbital paths indicating the magnitude of the LOS velocity of the observer. In the right panel, the full path of satellite 1 and satellite 2 is plotted in the LOS velocity - x spatial coordinate space. The green path indicates the part of satellite 2's orbit where it appears to be co-rotating with satellite 1, while the orange part where its counter-rotating. Co-rotation can occur when $v_{\mathrm{los}}>0$ or $v_{\mathrm{los}}<0$ for both satellites, as long as both are on the same side of the host. Both satellites have $v_{\mathrm{los}}<0$, but reside on opposite sides of the host, and thus appear to be counter-rotating.