ALMACAL II: Extreme star-formation-rate densities in a pair of dusty starbursts at $z=3.442$ revealed by ALMA 20-milliarcsec resolution imaging

Abstract

We present ALMA ultra-high-spatial resolution ($\sim 20\mathrm{mas}$) observations of dust continuum at $920\mu \mathrm{m}$ and $1.2\mathrm{mm}$ in a pair of submm galaxies (SMGs) at $z=3.442$, ALMACAL-1 (A-1: $S_{870\mu \mathrm{m}}=6.5\pm 0.2\mathrm{mJy}$) and ALMACAL-2 (A-2: $S_{870\mu \mathrm{m}}=4.4\pm 0.2\mathrm{mJy}$). The spectroscopic redshifts of A-1 and A-2 have been confirmed via serendipitous detection of up to nine emission lines. Our ultra-high-spatial resolution data reveal that about half of the star formation in each of these starbursts is dominated by a single compact clump (FWHM size of $\sim 350\mathrm{pc}$). This structure is confirmed by independent datasets at $920\mu \mathrm{m}$ and $1.2\mathrm{mm}$. The star-formation rate (SFR) surface densities of all these clumps are extremely high, ${\Sigma }_{\mathrm{SFR}}\sim 1200$ to $\sim 3000M_{\odot }{\mathrm{yr}}^{-1}{\mathrm{kpc}}^{-2}$, the highest found in high-redshift galaxies. There is a small probability that A-1 and A-2 are the lensed components of a background source gravitationally amplified by the blazar host. If this was the case, the effective radius of the source would be $R_{\mathrm{eff}}\sim 40\mathrm{pc}$, and the de-magnified SFR surface density would be ${\Sigma }_{\mathrm{SFR}}\sim 10000M_{\odot }{\mathrm{yr}}^{-1}{\mathrm{kpc}}^{-2}$, comparable with the eastern nucleus of Arp 220. Despite being unable to rule out an AGN contribution, our results suggest that a significant percentage of the enormous far-IR luminosity in some dusty starbursts is concentrated in very small star-forming regions. The high ${\Sigma }_{\mathrm{SFR}}$ in our pair of SMGs could only be measured thanks to the ultra-high-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.

Submitted to ApJ. Comments are welcome

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