ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Survey Description

Abstract: We present the rationale for and the observational description of ASPECS: The ALMA SPECtroscopic Survey in the Hubble Ultra-Deep Field (UDF), the cosmological deep field that has the deepest multi-wavelength data available. Our overarching goal is to obtain an unbiased census of molecular gas and dust continuum emission in high-redshift (z$>$0.5) galaxies. The $\sim$1$'$ region covered within the UDF was chosen to overlap with the deepest available imaging from HST. Our ALMA observations consist of full frequency scans in band 3 (84-115 GHz) and band 6 (212-272 GHz) at approximately uniform line sensitivity ($L'_{\rm CO}\sim$2$\times$10$^{9}$ K km/s pc$^2$), and continuum noise levels of 3.8 $\mu$Jy beam$^{-1}$ and 12.7 $\mu$Jy beam$^{-1}$, respectively. The molecular surveys cover the different rotational transitions of the CO molecule, leading to essentially full redshift coverage. The [CII] emission line is also covered at redshifts $6.0<z<8.0$. We present a customized algorithm to identify line candidates in the molecular line scans, and quantify our ability to recover artificial sources from our data. Based on whether multiple CO lines are detected, and whether optical spectroscopic redshifts as well as optical counterparts exist, we constrain the most likely line identification. We report 10 (11) CO line candidates in the 3mm (1mm) band, and our statistical analysis shows that $<$4 of these (in each band) are likely spurious. Less than 1/3 of the total CO flux in the low-J CO line candidates are from sources that are not associated with an optical/NIR counterpart. We also present continuum maps of both the band 3 and band 6 observations. The data presented here form the basis of a number of dedicated studies that are presented in subsequent papers.

Overview of the ASPECS in the Hubble Ultra Deep Field

The paper presents the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS), detailing the design, methodology, and preliminary results of a significant observational program. The central objective is to secure an unbiased census of molecular gas and dust continuum emissions in high-redshift galaxies (z > 0.5), with a particular emphasis on the characterisation of CO line emissions. This effort is a critical component in advancing our understanding of the evolutionary processes governing cosmic star formation rate density and stellar mass build-up over time.

The survey exploits the unparalleled sensitivity and resolution of ALMA, conducting full frequency scans in the 84-115 GHz (band 3) and 212-272 GHz (band 6) ranges. These bands were chosen for their capacity to cover the various rotational transitions of the CO molecule, facilitating a near-complete redshift range analysis. The observational setup incorporates uniform line sensitivity metrics and continuity in noise levels, ensuring robust datasets across both bands.

The novel algorithm developed to identify line candidates in the molecular scans is a noteworthy feature of the study, enhancing the precision of line detection. The study reports several CO line candidates, with a meticulous assessment of their likelihood to be genuine, anchored in calculations of fidelity and completeness. Furthermore, the survey reports less than one-third of the CO flux in the detected candidates originates from sources lacking optical/NIR counterparts.

Numerical Highlights and Methodological Insights

In band 3, 10 CO line candidates were identified, with <4 expected to be spurious. Similarly, in band 6, 11 candidates were found, with a similar proportion possibly being false positives. The methodology makes use of a blind line search, complemented by targeted searches based on optical counterparts, increasing the chance of capturing true signals.

The paper meticulously outlines the challenges in redshift determination associated with the equidistant spacing of CO rotational transitions. Multiple CO line detection serves as a critical check against false identifications, enabling more confident constraints on redshift for certain candidates. The authors also delineate cases where optical/NIR spectroscopic redshifts were matched to CO line candidates to solidify identifications.

Implications and Future Directions

ASPECS represents an indispensable effort in cosmic molecular gas census, yielding high-value datasets that hold implications for models of galaxy evolution, CO luminosity functions, and cosmic molecular gas density profiles. The comprehensive frequency coverage achieved by simultaneous use of multiple ALMA bands establishes a new benchmark for future surveys.

Moreover, ASPECS opens new avenues for exploring the interplay between gas content and star formation in galaxies across redshifts, particularly those not pre-selected by optical/NIR criteria. The observed alignment of findings with expectations based on previous empirical data reinforces the reliability of the observations.

However, it is acknowledged that the survey’s current scope, constrained by the operational limitations of early ALMA cycles, only provides a partial view. Once ALMA attains full operational capacity, more extensive surveys could map the complete UDF, potentially re-defining our interpretation of the molecular universe.

In summary, ASPECS offers a detailed framework and preliminary insight into the multi-faceted architecture of high-redshift galaxies. The dataset and methods presented promise substantial contributions to the fields of cosmology and galaxy evolution, particularly as subsequent analyses unfold.