Spectacular Hubble Space Telescope Observations of the Coma Galaxy D100 and Star Formation in Its Ram Pressure-stripped Tail

We present new HST F275W, F475W, and F814W imaging of the region of the Coma cluster around D100, a spiral galaxy with a remarkably long and narrow (60 × 1.5 kpc) ram pressure–stripped gas tail. We find blue sources coincident with the Hα tail, which we identify as young stars formed in the tail. We also determine they are likely to be unbound stellar complexes with sizes of ~50–100 pc, likely to disperse as they age. From a comparison of the colors and magnitudes of the young stellar complexes with simple stellar population models, we find ages ranging from ~1 to 50 Myr and masses ranging from 103 to ~105 M ⊙. We find that the overall rate and efficiency of star formation are low, ~6.0 × 10−3 M ⊙ yr−1 and ~6 × 10−12 yr−1, respectively. The total Hα flux of the tail would correspond to a star formation rate 7 times higher, indicating that some other mechanism for Hα excitation is dominant. From an analysis of the colors, we track the progression of outside-in star formation quenching in the main body of D100, as well as its apparent companion, S0 D99. Finally, we observe that the dust extinction in the base of the tail has an outer envelope with remarkably smooth and straight edges and linear filamentary substructure strongly suggestive of magnetic fields. These features and the small amount of tail broadening strongly suggest gas cooling restricting broadening and the influence of magnetic fields inhibiting turbulence.