ALMACAL II: Extreme Star Formation Rate Densities in Dusty Starbursts Revealed by ALMA 20 mas Resolution Imaging

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}870μ {{m}}=6.5+/- 0.2 {mJy}) and ALMACAL-2 (A-2: {S}870μ {{m}}=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.