# The ALMA Spectroscopic Survey in the HUDF: CO emission lines and 3 mm continuum sources

## TL;DR

This paper presents the results of the ALMA spectroscopic survey in the HUDF, detecting CO emission lines and continuum sources, and analyzing their properties and number counts to improve understanding of distant galaxy molecular gas and dust emission.

## Contribution

It introduces a new search strategy for emission lines and continuum sources, compares line search algorithms, and provides the most comprehensive constraints on faint 3 mm number counts to date.

## Key findings

- Identified 16 high-fidelity emission lines with optical/infrared counterparts.
- Detected 6 continuum sources, all with counterparts in shorter wavelengths.
- Found 3 mm number counts higher than expected for synchrotron sources, consistent with dust emission models.

## Abstract

The ALMA SPECtroscopic Survey in the {\it Hubble} Ultra Deep Field is an ALMA large program that obtained a frequency scan in the 3\,mm band to detect emission lines from the molecular gas in distant galaxies. We here present our search strategy for emission lines and continuum sources in the HUDF. We compare several line search algorithms used in the literature, and critically account for the line-widths of the emission line candidates when assessing significance. We identify sixteen emission lines at high fidelity in our search. Comparing these sources to multi-wavelength data we find that all sources have optical/infrared counterparts. Our search also recovers candidates that have lower significance that can be used statistically to derive, e.g. the CO luminosity function. We apply the same detection algorithm to obtain a sample of six 3 mm continuum sources. All of these are also detected in the 1.2 mm continuum with optical/near-infrared counterparts. We use the continuum sources to compute 3 mm number counts in the sub-mJy regime, and find them to be higher by an order of magnitude than expected for synchrotron-dominated sources. However, the number counts are consistent with those derived at shorter wavelengths (0.85--1.3\,mm) once extrapolating to 3\,mm with a dust emissivity index of $\beta=1.5$, dust temperature of 35\,K and an average redshift of $z=2.5$. These results represent the best constraints to date on the faint end of the 3 mm number counts.