Molecular Gas and Star Formation in Dwarf Galaxies Observed by the Atacama Large Millimeter/submillimeter Array

TL;DR

This work investigates whether dwarf galaxies with low mass and metallicity follow the same molecular star formation law as massive spirals. Using high-resolution ALMA CO(1→0) maps for six dwarfs and multiwavelength tracers (NUV and 12 μm) to estimate SFR, the authors derive radial relations between molecular gas and star formation, finding SFL slopes broadly consistent with spiral galaxies (≈0.8–0.9 on robust data, rising toward ≈1.1 when including marginal detections). The molecular gas depletion times are short (≈0.2–1.1 Gyr), influenced by CO-to-H2 conversion uncertainties rather than intrinsic efficiency, and the disks remain near marginal gravitational stability (Qtot ≈ 0.9–1.5) across radii, consistent with self-regulated star formation. Together, these results suggest that the same fundamental processes govern star formation in dwarfs and spirals when observed at comparable resolution and sensitivity, emphasizing the relevance of high-quality CO observations for understanding galaxy evolution across mass scales.

Abstract

We present a spatially resolved analysis of the molecular star formation law (SFL) and gravitational instability in a sample of nearby dwarf galaxies (NGC 1035, NGC 4310, NGC 4451, NGC 4701, NGC 5692, and NGC 6106), using high-resolution $^{12}$CO ($J=1\rightarrow0$) data from the Atacama Large Millimeter/submillimeter Array. We estimate the star formation rate (SFR) by combining the Galaxy Evolution Explorer near-ultraviolet and the Wide-field Infrared Survey Explorer 12 $μ$m imaging data to examine the relationship between molecular gas and SFR densities on scales of several hundred parsecs. We find that the power-law slope of the molecular SFL ranges from 0.62 to 1.08, with an average value of N$=0.81\pm0.18$, increasing to N$=0.87\pm0.05$ when excluding galaxies with poorly constrained CO data. These results are roughly consistent with values observed in massive spiral galaxies, suggesting a universal molecular SFL when analyzed with sufficient resolution and sensitivity. Radial profiles of the Toomre $Q$ parameter remain close to unity across the disks, with minimal radial variation, consistent with a self-regulated star formation model. Our results suggest that, despite their lower mass and metallicity, star formation in dwarf galaxies is governed by the same fundamental physical processes as in larger systems. This highlights the significance of high-resolution molecular gas observations in low-mass galaxies.

Figures (6)

• Figure 1: CO integrated intensity maps for the dwarf galaxies in our sample. Contour levels begin at approximately 5$\sigma$ to clearly delineate reliable molecular gas detections. The synthesized beam size is indicated by the ellipse in the lower-right corner of each panel.
• Figure 2: NIR images used to trace stellar mass distributions. For NGC 5692, the WISE 3.4 $\micron$ image is shown, while $Spitzer$ IRAC 3.6 $\micron$ images are displayed for all other galaxies. The contours represent regions of significant stellar emission, beginning at approximately 15$\sigma$. The point spread function of the images is marked by the ellipse in the lower-right corner of each panel, illustrating the differences in spatial resolution between the WISE (7$\farcs$5) and $Spitzer$ (1$\farcs$6) images.
• Figure 3: WISE 12 $\micron$ images and contours (black) for our dwarf galaxy sample, overlaid with contours from the GALEX NUV emission (blue). The lowest contour level for both 12 $\micron$ and NUV emissions is approximately 10$\sigma$. The PSF, with a Gaussian FWHM of $7\farcs5$, is makred by the ellipse in the lower-right corner of each panel.
• Figure 4: Radial surface density profiles of H$_2$ (blue diamonds), stars (brown stars), SFR (red circles), along with the individual contributions to the total SFR from NUV (orange triangles) and 12 $\micron$ (magenta squares). All profiles are measured in concentric elliptical annuli, each with a fixed radial width of 7.5$\arcsec$. The vertical error bars represent the standard deviation of data points in each annulus.
• Figure 5: (Left) Relationship between $\Sigma_{\rm SFR}$ and $\Sigma_{\rm H_2}$ for the six dwarf galaxies. The dashed diagonal lines represent constant molecular gas depletion times ($\tau_{H_2}$) of 0.1 and 1 Gyr for reference. The best-fit power-law index (N) is shown in the lower-right corner. (Right) SFE of the molecular gas (SFE$_{\rm H_2}$) as a function of radius normalized by the optical radius (R$_{25}$). The molecular gas depletion time ($\tau_{H_2}$), measured in Gyr, is indicated for each galaxy in the upper-right corner. The horizontal dotted line marks $\tau_{H_2}$ = 1 Gyr as a reference point.