2017ApJ...837..182O

We present ultrahigh spatial resolution (∼20 mas or 150 pc) ALMA observations of the dust continuum at 920 µm and 1.2 mm in two submillimeter sources at z = 3.442, ALMACAL-1 (A-1: S_870µm_=6.5±0.2 mJy) and ALMACAL-2 (A-2: S_870µm_=4.4±0.2 mJy). About half of the star formation in each of these sources is dominated by a single compact clump (FWHM size of ∼350 pc). In A-1, two additional fainter clumps are found. The star formation rate (SFR) surface densities of all these clumps are extremely high, Σ_SFR∼1200 to ∼3000 M_☉ yr^–1 kpc^–2, the highest rates found in high-redshift galaxies. Given their geometry and identical redshifts, there is a possibility that A-1 and A-2 are the lensed images of a single background source that are gravitationally amplified by the blazar host. If this were the case, the effective radius of the dusty galaxy in the source plane would be R_eff∼40 pc and the demagnified SFR surface density would be Σ_SFR ∼ 10,000  M_☉ yr^–1 kpc^–2, comparable with the eastern nucleus of Arp 220. Although we cannot rule out an AGN contribution, our results suggest that a significant percentage of the enormous far-IR luminosity in some dusty starbursts is extremely compact. The high Σ_SFR in these sources could only be measured thanks to the ultrahigh-resolution ALMA observations used in this work, demonstrating that long-baseline observations are essential to study and interpret the properties of dusty starbursts in the early Universe.