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Quiescent filaments appear as absorption features on the solar disk whenobserved in chromospheric lines and at continuum wavelengths in the millimeter(mm) range. Active region (AR) filaments are their small-scale, low-altitudeanalogues, but they could not be resolved in previous mm observations. Thisspectral diagnostic can provide insight into the details of the formation andphysical properties of their fine threads, which are still not fullyunderstood. Here, we shed light on the thermal structure of an AR filamentusing high-resolution brightness temperature ($T_{\rm b}$) maps taken with ALMABand 6 complemented by simultaneous IRIS near-UV spectra, Hinode/SOTphotospheric magnetograms, and SDO/AIA extreme-UV images. Some of the darkthreads visible in the AIA 304 {\AA} passband and in the core of Mg IIresonance lines have dark ($T_{\rm b}<5000$K) counterparts in the 1.25 mm maps,but their visibility significantly varies across the filament spine and intime. These opacity changes are possibly related to variations in temperatureand electron density in filament fine structures. The coolest $T_{\rm b}$values ($<$5000 K) coincide with regions of low integrated intensity in the MgII h and k lines. ALMA Band 3 maps taken after the Band 6 ones do not clearlyshow the filament structure, contrary to the expectation that the contrastshould increase at longer wavelengths based on previous observations ofquiescent filaments. The ALMA maps are not consistent with isothermalconditions, but the temporal evolution of the filament may partly account forthis.

Subarcsecond imaging of a solar active region filament with ALMA and IRIS