BLUEPRINT: Blue-dominant Lyman-α emission as evidence of gas inflow in ultra-low-mass galaxies at z=3

We report the detection of a clumpy, blue-dominated Lyman-α (łya) emission at z = 3.066 located in the heart of a cosmic web filament in the MUSE eXtremely Deep Field (MXDF). It is spatially associated with the formation of two compact star-forming regions revealed by deep JWST/NIRCam imaging. Gas accretion in these regions is indicated by the blue-dominated łya profiles, spectral signatures that are rarely observed. Radiative transfer simulations of the łya profile using a clumpy multiphase model suggest a radial inflow of gas clumps with a velocity of ∼ 100 km/s. Embedded in this łya structure, the associated main galaxy dominates the stellar mass budget, while the two compact ultra-low-mass systems (łog (M_⋆/M_⊙)= 6.3 - 6.9) have formed the bulk of their stellar mass in less than 7 Myr. These two components also have high specific star-formation rates and elevated ionisation parameters, consistent with recent bursty star formation. This system provides compelling observational evidence of how gas accretion, most likely from the cosmic web, can induce starbursts in ultra-low-mass galaxies.