ALMA Central Molecular Zone Exploration Survey (ACES) I: Overview

Abstract

The mass flows and energy cycles within the inner regions of galaxies exert a powerful influence on the evolution of the galaxy population. The centre of the Milky Way is the only galactic nucleus for which it is possible to resolve the physical mechanisms that drive these cycles, namely star formation and feedback, while also tracing global (>100 pc) processes which determine where and when star formation and feedback occur. We present an overview of ACES, the 'Atacama Large Millimeter/submillimeter Array (ALMA) CMZ Exploration Survey', a ~1.5" angular resolution, 0.2-3 km/s spectral resolution ALMA Band 3 (85-102 GHz), survey of the 'Central Molecular Zone' (CMZ) -- the inner-100 pc of the Galaxy (l = 359.4 deg to 0.8 deg). ACES spectral setup is tuned to observe optimal tracers of the physical, chemical, and kinematic conditions in over 70 spectral features (e.g. HCO+, HNCO, SiO, H40alpha, complex molecules) of the gas in the CMZ, to derive the properties of all potentially star-forming Galactic Centre gas, from global scales (100 pc) to dense ~0.05 pc structures that are expected to host individual star-forming cores, down to sub-sonic (<0.4 km/s) velocity resolution. In this overview paper, we provide the scientific justification for the ACES survey, explain the choice of observational setup, and describe the data legacy products. Finally, we show some of the initial ACES data which highlight the power of ACES' combination of high angular resolution, unprecedented spatial dynamic range, sensitivity, spectral resolution and spectral bandwidth as an illustration of how ACES aims to understand how global processes set the location, intensity, and timescales for star formation and feedback in the CMZ.

Figures (11)

• Figure 1: Finding chart for the Galactic Center region. A colour composite of the 4.5 $\mu$m (white) and 8 $\mu$m (green) emission from the Spitzer GLIMPSE survey Churchwell2009, 24 $\mu$m (yellow) emission from the Spitzer MIPSGAL survey Carey2009, and 20 cm (red) emission observed by MeerKAT Heywood2019Heywood2022 and the Green Bank Telescope (GBT; Law2008). Overlaid are labels highlighting several features of interest across the Galactic Centre, including the central few 100 pc known as the Central Molecular Zone (CMZ). Overlaid as a white contour is the coverage of the ACES survey (see Figure \ref{['fig:ACES_coverage']}). The inset zoom-in shows the ACES HNCO(1-0) peak intensity map. Figure \ref{['fig:ACES_coverage']} shows the coordinates for the inset image. The background image for this Figure is adapted from Henshaw2023. An interactive version of the figure is available on the project homepage, https://sites.google.com/view/aces-cmz/home.
• Figure 2: Coverage of the ACES survey. The upper panel displays the spatial coverage of the ACES survey, indicated by the red contour (see also Figure \ref{['fig:ACES_rgb']}). Overlaid orange contours denote the pre-ACES archival observations available in ALMA Band 3. The outline of the SMA $1\,\mathrm{mm}$ CMZoom survey Battersby2020Hatchfield2020_cmzoomCallanan2023 is shown as the cyan contour, while the magenta rectangle represents the coverage of the CARMA survey pound2018. The lower panel presents the individual mosaics comprising the ACES survey, each labelled with an alphabetical identifier. Both panels are superimposed on the Spitzer$8\,\mu\mathrm{m}$ image from the GLIMPSE survey Churchwell2009.
• Figure 3: Three-colour images of ACES observations. The upper panel shows an image of the ACES maximum intensity maps for CS $2-1$ (blue), HNCO $4-3$ (red), and HC$_{3}$N $11-10$ (green). The lower panel shows an image of a coarse rendering of the velocity structure in the ACES HNCO data, made by integrating the HNCO emission across three velocity ranges: 46 -- 145 km s$^{-1}$ (red), 1 -- 45 km s$^{-1}$ (green), and -145 -- 0 km s$^{-1}$ (blue). Both panels are superimposed on the Spitzer$8\,\mu\mathrm{m}$ image from the GLIMPSE survey Churchwell2009, and the red contour outlines the ACES coverage shown in Fig. \ref{['fig:ACES_coverage']} (see upper panel).
• Figure 4: Example of simulations in development that will help with interpretation of ACES data. White boxes illustrate zoom-in/-out regions in other panels. Panels A-B: a top-down and plane-of-the-sky H$_2$ projection of a large-scale simulation of the gas flow in the Galactic bar with star formation, stellar feedback (supernovae, ionising radiation) and magnetic fields (ideal MHD). Simulations like this one can follow the formation of the CMZ and of individual molecular clouds self-consistently from kpc scales down to the cloud scale, but cannot resolve star formation. Panel C: simulation of an individual molecular cloud on a CMZ orbit through the Galactic potential, that can resolve star formation (Adapted from Dale2019Kruijssen2019). Panel D: a synthetic line emission image created by post-processing the simulation snapshot from Panel C (Adapted from Petkova2023a).
• Figure 5: Multi-wavelength view of the central region of the Galaxy. The top panel shows a composite using data from the Spitzer Space Telescope at 3.6, 4.5, 5.8, and 8.0 $\mu$m Churchwell2009, Hubble Space Telescope (HST) Paschen-$\alpha$ emission (F187N) and (F190N) continuum Wang2010Dong2011, and Chandra X-ray Observatory data in three energy bands: 1--3 keV (soft), 3--5 keV (medium), and 5--8 keV (hard; see Wang2021Wang2022). The bottom panel shows the same region overlaid with the ACES HNCO data from Figure \ref{['fig:ACES_rgb']}.