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

Abstract

We present ultrahigh spatial resolution (∼20 mas or 150 pc) ALMA observations of the dust continuum at 920 μ m and 1.2 mm in two submillimeter sources at z  = 3.442, ALMACAL–1 (A–1: ) and ALMACAL–2 (A–2: ). 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, to , 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 and the demagnified SFR surface density would be ∼ 10,000 , 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 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.