2012ApJ...748..149N

Dwarf galaxies in the Local Group appear to be stripped of their gas within 270 kpc of the host galaxy. Color-magnitude diagrams of these dwarfs, however, show clear evidence of episodic star formation (Δt ∼a few Gyr) over cosmic time. We present a simple model to account for this behavior. Residual gas within the weak gravity field of the dwarf experiences dramatic variations in the gas cooling time around the eccentric orbit. This variation is due to two main effects. The azimuthal compression along the orbit leads to an increase in the gas cooling rate of ~([1 + ε]/[1 - ε])². The Galaxy's ionizing field declines as 1/R ² for R > R_diskalthough this reaches a floor at R ∼ 150 kpc due to the extragalactic UV field ionizing intensity. We predict that episodic star formation is mostly characteristic of dwarfs on moderately eccentric orbits (ε > 0.2) that do not come too close to the center (R > R_disk) and do not spend their entire orbit far away from the center (R ≳ 200 kpc). Up to 40% of early infall dwarf spheroidals can be expected to have already had at least one burst since the initial epoch of star formation, and 10% of these dwarf spheroidals experiencing a second burst. Such a model can explain the timing of bursts in the Carina dwarf spheroidal and restrict the orbit of the Fornax dwarf spheroidal. However, this model fails to explain why some dwarfs, such as Ursa Minor, experience no burst post-infall.