ACES Survey Design

Survey Area: ACES fundamental requirement is to observe all gas in the inner 100 pc of the Galaxy expected to form stars. To meet this requirement ACES survey area is defined using a Herschel column density threshold of 1E22 cm-2 (see figure below). This threshold is chosen because it defines the column density of gas above which stars are observed to form in the Galactic disc. This allows a direct comparison between the disc and CMZ to quantify the impact that Galactic environment has on the star formation process.

Angular resolution and sensitivity to different spatial scales: ACES angular resolution of 1.5" (0.05 pc) matches the observed size of dense (pre)star-forming cores in CMZ clouds. Complementary observations from ALMA's 7m and total power dishes are essential to recover large spatial scales for the line data. Existing 3mm continuum single-dish data from Mustang Galactic Plane Survey will be used to recover the zero-spacing flux.

Spectral setup: ACES uses ALMA's Band 3 (~100 GHz) receiver to ensure that the observations have a sufficiently large primary beam (field of view) to be able to map the full survey area. ACES uses a spectral setup containing molecular line transitions with critical density and excitation energy ranges to trace the gas kinematic, physical and chemical properties from global (tens of pc), to cloud (few pc), to core (0.05 pc) scales. Two high spectral resolution (0.2 km/s) windows centred on reliable gas kinematic tracers HCO+ and HNCO resolve the thermal linewidth (0.4 km/s in the 60K CMZ gas) in global-to-cloud and cloud-to-core scale gas, respectively. Intermediate spectral resolution (1.7 km/s) windows centred on dense gas and shock tracers and their isotopologues trace gas kinematics in intermediate density gas and solve degeneracies in opacity and excitation. The 13C, 18O, and 15N isotopic substitutions of HCO+, HCN, and HNC trace stellar nucleosynthesis gas enrichment and ever smaller scale gas properties/kinematics when the main isotope transitions become optically thick. Other lines trace specific physical processes/regimes such as shocks (SiO), cosmic ray irradiation and star formation activity (HC5N/HC3N), ionised gas (H40, He40) and chemical complexity (complex organic molecules). Two broad bands with 2.9km/s resolution are sufficient to trace the bulk kinematics and chemical complexity while allowing removal of lines to obtain continuum measurements.

Sensitivity: ACES RMS line sensitivity is 1.0, 0.6 and 0.25 K in 0.2, 1.7 and 3 km/s channels, respectively. Based on (i) the observed brightness temperatures in existing single-dish data, (ii) previous ALMA observations of CMZ clouds at similar resolution and sensitivity, (iii) numerical simulations with chemistry and radiative transfer modelling ACES expects to detect all key lines at high significance in essentially all pointings. The corresponding continuum sensitivity is 0.07 mJy/beam, sufficient to detect 10 Msun, 30 K cores at 5 sigma significance, making ACES complete to precursors of high-mass stars. The corresponding column density sensitivity of 1E22 cm-2 matches the (Herschel) column density used to define the survey area.