# Discovery of four apparently cold dusty galaxies at z=3.62-5.85 in the COSMOS field: direct evidence of CMB impact on high-redshift galaxy observables

## TL;DR

This study uses ALMA observations to identify four high-redshift dusty galaxies with unexpectedly cold dust temperatures, providing evidence of CMB influence on their observed properties and revealing a previously underrepresented population of cold star-forming galaxies.

## Contribution

It presents the first direct evidence of CMB impact on high-redshift galaxy observables and characterizes a population of colder, compact dusty galaxies at z=3.62-5.85.

## Key findings

- Detection of four high-redshift dusty galaxies with z=3.62-5.85.
- Cold dust temperatures suggest optically thick FIR dust rather than low star formation efficiency.
- CMB significantly biases observed spectral slopes and reduces CO fluxes, which can be corrected for.

## Abstract

We report Atacama Large Millimetre Array (ALMA) observations of four high-redshift dusty star-forming galaxy candidates selected from far-Infrared (FIR)/submm observations in the COSMOS field. We securely detect all galaxies in the continuum and spectroscopically confirm them at z=3.62--5.85 using ALMA 3mm line scans, detecting multiple CO and/or [CI] transitions. This includes the most distant dusty galaxy currently known in the COSMOS field, ID85001929 at z=5.847. These redshifts are lower than we had expected as these galaxies have substantially colder dust temperatures (i.e., their SEDs peak at longer rest frame wavelengths) than most literature sources at z>4. The observed cold dust temperatures are best understood as evidence for optically thick dust continuum in the FIR, rather than the result of low star formation efficiency with rapid metal enrichment. We provide direct evidence that, given their cold spectral energy distributions, CMB plays a significant role biasing their observed Rayleigh-Jeans (RJ) slopes to unlikely steep values and, possibly, reducing their CO fluxes by a factor of two. We recover standard RJ slopes when the CMB contribution is taken into account. High resolution ALMA imaging shows compact morphology and evidence for mergers. This work reveals a population of cold dusty star-forming galaxies that were under-represented in current surveys, and are even colder than typical Main Sequence galaxies at the same redshift. High FIR dust optical depth might be a widespread feature of compact starbursts at any redshift.