Resonant Stripping


I have  recently proposed that interactions between dwarfs can pre-process dwarf galaxies, e.g transforming dwarf irregular galaxies into dwarf spheroidals , before they are accreted into larger galaxies like the Milky Way. This scenario, published as a Letter to Nature,  illustrates  the formation of dwarf spheroidal galaxies by  encounters between dwarf disk galaxies in groups of dwarfs. In particular, it suggests that these encounters of dwarf galaxies excite a process, which I termed “resonant stripping” that can transform a dwarf rotating disk into a dwarf spheroidal.

The evolution of  the dwarf occurs through a resonance between the spin angular frequency of a small dwarf with its orbital angular frequency about a larger system. The gravitational resonance strips stars from a small  dwarf  galaxy, producing very extended tails and bridges when the orbits are prograde (see the top row in Figure 1) and drives the  morphological evolution of the small disk galaxy into  a dwarf spheroidal  in a very short time. 

The most interesting implication of this process is that resonant stripping  can alter the mass to light ratio of  dwarfs by changing their baryonic mass relative to their dark matter content. 

Note that this  process is distinct from merging, galaxy-galaxy harassment or more general heating processes or tidal stripping but it can be much more efficient at removing the stars at each pericentric passage of the little galaxy within the larger dwarf system. 

Figure 1. Top Row: Interaction between a dwarf galaxy orbiting around a larger dwarf. Only the stellar components are plotted. The upper left panel illustrates the initial set up where the two dwarfs approach one another on a somewhat prograde orbit (the disks are seen face on). The upper middle panel gives the state of the system after 2 billion years, following the first pericentric passage, and the upper right panel shows the appearance of the galaxies after 7 billion years. Bottom Row: Shown is the orbit of the same small galaxy (in white) around the Milky Way today (in magenta), which has 10,000 times its mass. Although the encounter is mostly prograde, the spin and orbital frequencies are no longer well-matched and the resonant response is suppressed.

“Resonant stripping as the origin of dwarf spheroidal galaxies”.

D’Onghia, E. Besla, G., Cox, T.J., Hernquist, L. 2009, Nature, 460, 605.