ALMA Central Molecular Zone Exploration Survey (ACES) II: 3mm continuum images

Abstract

The ALMA Central Molecular Zone Exploration Survey, ACES, has mapped $\gtrsim1000$ square arcminutes at 3 mm toward the center of our Galaxy. ACES provides the first large-scale, high-resolution ($\sim2.5$") view of the central $\sim200$ parsecs of the Milky Way. In this work, we describe the continuum data processing and present the continuum data products. In the combined mosaic of 45 individual ALMA mosaics, the typical RMS noise achieved is $\sim0.1$ mJy per $\sim2.5$" beam, though there is a tail of substantially higher noise toward regions with bright continuum structure, especially around Sgr A* and Sgr B2. In-band spectral indices are measurable for a small fraction of the brightest and most compact sources, enabling distinction between dust-dominated and free-free- or synchrotron-dominated sources. To recover emission on large angular scales, we present the GBT MUSTANG-2 Three millimeter Extended Nucleus Survey (TENS), a new 10"resolution survey of the CMZ, which we combine with the ACES image by feathering. To demonstrate the quality and reliability of the ACES data, we compare to previously-published ALMA data obtained with higher resolution and sensitivity, finding overall good agreement with past results, but some disagreement toward the brightest sources.

Figures (35)

• Figure 1: The GBT MUSTANG-2 TENS image combined with Planck all-sky data that is used to fill in the short spacings missing in the ALMA interferometric data.
• Figure 2: Overview of the ACES fields. The survey footprint is comprised of 45 mosaics, labeled a through as (1 through 45). The mapping from letters to Galactic coordinates can be found in Table \ref{['tab:observation_metadata_12m']}.
• Figure 3: Visualization of the (u,v) coverage per field. (left) The box-and-whisker plots in each row show the 5th and 95th percentile as whiskers and the 25th and 75th percentile as box ends. The field identifiers listed on the left side of the plot refer to the labels shown in Figure \ref{['fig:fieldidlabels']}. (right) The beam geometric average sizes for each image product, in arcseconds, for each imaged field. The points show the high-frequency spectral windows, the aggregate bandwidth image including all spectral windows, and the low-frequency spectral windows in blue, red, and orange, respectively from left to right.
• Figure 4: Map of the aggregate-resolution beam relative sizes and orientations by field. Each ellipse has major and minor diameter proportional to the full-width half-max (FWHM) of the corresponding beam. The concentric blue circles at the bottom-center show the size scale to associate with the beams; they have diameters of 1 and 3 (the beams are not shown to scale with respect to the axis labels).
• Figure 5: Noise vs beam size for each field. The plotted beam size is the geometric average between the major and minor axes (top) and the major axis size (bottom). The noise is the MAD-estimated standard deviation.