Dry galaxy merger

A dry galaxy merger joins galaxies with little cold gas, so dissipation and merger-triggered star formation are weak. Conservation of energy followed by re-virialization can make repeated dry minor mergers increase a galaxy's radius faster than its mass.

In a dry galaxy merger, zero orbital energy at infinite separation and retention of all mass and energy give $E_F=E_I+E_A$. If the progenitors and remnant satisfy the virial theorem, their gravitational radii obey $R_F/R_I=(1+\eta {)}^2/(1+ϵ\eta )$, where $\eta =M_A/M_I$ and $ϵ=⟨v_A^2⟩/⟨v_I^2⟩$. The latter average is weighted by satellite mass. Nonzero orbital energy, escaping stars, or dissipative losses change the energy accounting.

A minor galaxy merger has a small satellite-to-host mass ratio. Its stars are commonly deposited at large host radii after tidal stripping and Chandrasekhar dynamical friction.

For repeated minor galaxy mergers with negligible internal satellite binding energy and zero orbital energy at infinity, parabolic dry-merger energy accounting leaves total energy nearly constant. The virial theorem then gives gravitational radius $R_g\propto M^2$ and mean-square speed proportional to $M^{-1}$. Small satellite mass does not by itself imply negligible internal binding energy.