Dark Matter — Question 6

Question 6

A cluster collision separates gas drag from the lensing mass pattern

In a hypothetical merger, a lensing mass peak is 150 kpc ahead of the X-ray gas peak along the collision axis. Assume a constant relative speed 1500 km/s and 1 kpc=$3.086\times10^{16}$ km.

1. Estimate the time needed to produce the offset and express it in Myr using 1 year=$3.156\times10^7$ s.

2. Explain how mass following collisionless galaxies rather than the dominant baryonic gas tests a gas-only mass model.

Question 6 - Solution

A cluster collision separates gas drag from the lensing mass pattern

(a) Method and calculation

$$t=d/v=\frac{150(3.086\times10^{16})}{1500}=3.086\times10^{15}\ {\mathrm{s}}.$$$$\boxed{t\simeq97.8\ {\mathrm{Myr}}}.$$

(b) Interpretation and further calculation

$$\Sigma_{\mathrm{lens}}=\Sigma_{\mathrm{gas}}+\Sigma_\star+\Sigma_{\mathrm{other}}.$$$$\boxed{\text{offset mass and gas peaks require more than a gas-traces-all-mass model}}.$$

Physical check

The time is a projected, constant-speed estimate. Combined lensing, gas and galaxy observations constrain mass distribution without determining the dark particle identity.