BLUEPRINT: Blue-dominant Lyman-alpha (Ly$α$) emission as an evidence of gas inflow in ultra-low-mass galaxies at z = 3

We report the detection of a clumpy, blue-dominated Ly$α$ emission at z = 3.066 located in the heart of a cosmic web filament in the MUSE eXtremely Deep Field (MXDF), 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 Ly$α$ profiles, spectral signatures that are rarely observed. Radiative transfer simulation of the Ly$α$ profile using a clumpy multiphase model suggests a radial inflow of gas clumps with a velocity of 100 km/s. Embedded in this Ly$α$ structure, we find that the associated main galaxy dominates the stellar mass budget, while the two compact ultra-low-mass systems ($\log(M_\star/M_\odot) = 6.3\text{-}6.9$) 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 direct observational evidence that how gas accretion, most likely from cosmic web, can induce starburst in ultra-low-mass galaxies.

💡 Research Summary

The authors present a multi‑wavelength study of a rare blue‑dominant Ly α emitter at redshift z = 3.066, discovered in the MUSE eXtremely Deep Field (MXDF) and examined with deep JWST NIRCam imaging, JWST NIRSpec spectroscopy, and ancillary HST data. The Ly α line shows a double‑peaked profile in which the blue peak contains roughly 65 % of the total flux, a signature that theoretical models associate with inflowing neutral hydrogen rather than the more common outflow‑driven red‑peak dominance. By applying Voronoi binning to the MUSE datacube, the authors isolate two spatial regions within the Ly α halo and extract high‑signal‑to‑noise spectra for each.

Radiative‑transfer modeling is performed with a clumpy, multiphase framework in which cool (∼10⁴ K) H I clumps are embedded in a hot (∼10⁶ K) ionised inter‑clump medium. Five free parameters—clump filling factor, residual H I density in the inter‑clump medium, clump column density, clump velocity dispersion, and a constant radial velocity—are constrained via Markov Chain Monte Carlo fitting. The best‑fit solution yields a radial inflow speed of v_cl ≈ ‑98 km s⁻¹ (± ≈ 50 km s⁻¹), a modest clump covering factor (F_v ≈ 0.11), and a low residual H I density (log n_HI,ICM ≈ ‑7.5 cm⁻³). These results provide quantitative evidence that neutral gas is streaming toward the central galaxy at roughly 100 km s⁻¹.

The Ly α structure is embedded in a 4 cMpc cosmic‑web filament identified in previous MXDF analyses. JWST NIRCam imaging in 14 filters reveals three distinct stellar components: a massive primary galaxy (log M★/M⊙ ≈ 9.2) and two ultra‑low‑mass companions (designated primary‑1 and companion) with stellar masses log M★/M⊙ = 6.3–6.9. Both low‑mass systems are detected in all NIRCam bands, display very blue colours, and are spatially offset by ∼0.2–0.85 arcsec from the Ly α surface‑brightness peak.

Spectral energy distribution (SED) fitting is carried out with BAGPIPES, employing a double‑power‑law star‑formation history, a Kroupa IMF, BC03 stellar population models, Calzetti dust attenuation, and self‑consistent nebular emission. The fits indicate that the two low‑mass components formed the bulk of their stellar mass in less than 7 Myr, have extremely high specific star‑formation rates (sSFR ≈ 10–15 Gyr⁻¹), elevated ionisation parameters (log U ≈ ‑2.2 to ‑2.4), and modest dust extinction (A_V ≈ 0.2–0.3). The primary galaxy’s NIRSpec spectrum shows strong nebular lines (