SDSS-ALMA Legacy Value Archival Gas Exploration (SALVAGE) -- I: global star formation is governed by central (not global) molecular gas

TL;DR

This study introduces SALVAGE, a large archival dataset combining SDSS optical data with ALMA CO(1--0) maps to yield semi-resolved measurements of stellar mass, SFR, and molecular gas in the inner and outer regions of 277 galaxies at $0.02 \lesssim z \lesssim 0.25$. The authors demonstrate that global star formation on the SFMS is primarily governed by central, not global, molecular gas properties, with the inner gas fraction and especially the inner star formation efficiency driving a galaxy's position, while the global molecular gas reservoir plays a secondary role. Across the three classical scaling relations—SFMS, MGMS, and KS—the MGMS and KS are largely insensitive to gas distribution, whereas the SFMS shifts with central SFE and gas availability; this semi-resolved view clarifies how quenching and starbursts relate to the location and state of central gas. By testing an alternative, metallicity- and density-dependent CO-to-H2 conversion and by comparing to resolved studies, SALVAGE reinforces the central region as a critical driver of galaxy evolution at low redshift, while highlighting the value and limitations of archival, semi-resolved data for understanding quenching mechanisms.

Abstract

Star-forming galaxies form tight relations between their stellar mass, star-formation rate, and molecular gas reservoir on global and resolved scales. On the path to quiescence, the exchange between gas and stars must inevitably be broken. Understanding the mechanisms governing star formation and quenching therefore requires observations of both the stellar and molecular gas components. To this end, we have assembled a sample of 277 galaxies ($0.02 \lesssim z \lesssim 0.25$) with semi-resolved optical and millimetre $^{12}$CO(1-0) data, wherein the properties of the inner $\thicksim$2 kpc can be distinguished from the outer regions. This effort was made possible by the Sloan Digital Sky Survey (SDSS) catalogues and the maturing archive of the Atacama Large (sub-)Millimetre Array (ALMA). We call this dataset the SDSS-ALMA Legacy Value Archival Gas Exploration (SALVAGE). In this work, we leverage SALVAGE to provide a semi-resolved perspective on global scaling relations and why some galaxies deviate from them. In agreement with previous work, we find that the offset of a galaxy from the global star-forming main sequence (SFMS) is driven by its inner star formation rate. With the relative inner and outer distributions of molecular gas fraction and star formation efficiency, we investigate whether the central star formation driving global changes is due to fuel availability or efficiency. We find that the position of a galaxy within the SFMS is largely due to the inner star-formation efficiency, while departure from the SFMS is driven by availability of central gas. The central few kpc are thus the most consequential region for galaxy evolution at low redshift.

Figures (11)

• Figure 1: Example data products for one target (SDSS object ID: 588015508213989555) in SALVAGE. Left panel: The DECaLS gri colour image with the SDSS fibre aperture overlaid in red and the approximate extent of the SDSS photometric model in blue. Centre-left panel: the ALMA CO moment 0 map with the colour corresponding to the CO line strength according to the colorbar to the right. The size of the circularized beam and a scale bar representing 10" are overlaid in black along the bottom. The ALMA project code and target name (as per that project) are along the top. Centre-right panel: the spatially-integrated inner (red) and outer (blue) spectra extracted from the cleaned ALMA cube within the SDSS fibre aperture (red circle in centre-left panel) and the annulus between the fibre and photometric limit (blue dashed circle in centre-left panel), respectively. Channels for which the area under the line is shaded represent those identified as contributing flux. The signal-to-noise ratio of the lines is shown in the top right. Right panel: the inner (red), outer (blue), and total (green) locations on the SFR-M$_\star$ plane with all SDSS galaxies in shaded black for reference. To demonstrate the combination of semi-resolved optical and mm data, the ratio between the inner and outer $f_\text{gas}$ (see Eq. \ref{['fgas']}) and SFE (see Eq. \ref{['SFE']}) are written in text along the bottom of the panel.
• Figure 2: Left: The stellar mass and redshift of SALVAGE targets (blue points) compared to all of SDSS (gray shading). SALVAGE is biased towards high stellar mass and low-redshift targets. Centre: SFR and stellar mass of SALVAGE targets (blue points) compared to all of SDSS (gray shading). SALVAGE consists of a diverse sampling of galaxies above, on, and below the SFMS across a broad range of stellar masses. Right: BPT diagram of SALVAGE targets (blue points) compared with all of SDSS. The orange dot-dash line represents the K03 AGN/SF demarcation and the dashed green line represent the Kewley01 AGN criteria. SALVAGE includes galaxies with a variety of emission line characteristics including pure star-forming galaxies, AGN, and composites.
• Figure 3: Images for 25 example galaxies with strong CO detections. On the left are the 50" $\times$ 50" cutouts from DECaLS (centred on the SDSS fibre) and on the right are the CO moment 0 maps (centred on the ALMA observation RA and Dec. and with a scale matched to the 50" optical cutouts). The galaxies are ordered according to their $\Delta$SFR, which is listed in the bottom left corner of the DECaLS cutouts. The circularized ALMA beam is shown as a circle in the bottom left corner of the CO moment 0 maps. However, in practice, the molecular gas mass measurements are computed from the cube with the original synthesized beam, without circularization.
• Figure 4: Key global scaling relations for the CO-detected targets in SALVAGE (coloured points) compared with detections from xCOLDGASS (gray contours) demonstrating their positions relative to the SFMS (top panel), MGMS (centre panel), and the KS relation (bottom panel). SALVAGE targets are coloured by the log ratio of the inner to the outer region of their sSFR (top panel), $f_\text{gas}$ (centre panel), and SFE (bottom panel). All points must have a 5$\sigma$ global CO detection as well as an individual inner or outer detection (also at 5$\sigma$) such that the ratio in the colour is constrained by at least an upper or lower limit; hexagon symbols are used to distinguish cases where the colour grading represents an upper or lower limit. The semi-resolved perspective provided by SALVAGE allows us to unpack these scaling relations in more detail.
• Figure 5: The median sSFR (left panel), $f_\text{gas}$ (centre panel), and SFE (right panel) of the inner (red circles) and outer (black squares) regions of SALVAGE targets as a function of $\Delta$SFR. Each $\Delta$SFR bin spans 0.5 dex except for the region $-0.5 < \Delta$SFR$< 0.5$ where there are enough targets to reduce bins to 0.25 dex. The shaded area around the lines represent the standard error in the median ($\sigma / \sqrt{N}$) and the blue and green shading represents $\Delta$SFRs consistent with star-forming and green valley galaxies, respectively.