Revisiting the Origin of the Star-Forming Main Sequence Based on a Volume-Limited Sample of ~25,000 Galaxies

Abstract

We revisit the extensively debated star-forming main sequence (SFMS)-a tight correlation between the star formation rate and stellar mass in both kiloparsec-resolved and integrated galaxies. We statistically explore the fundamental drivers of star formation at global scales, using a large volume-limited sample of 24,954 local star-forming galaxies to overcome the limitations of previous works. Based on the mid-infrared 12 micron luminosity, stellar mass, and g-r color, we estimate the molecular gas mass for the considered sample. At galaxy-wide scales, we establish global relations between the surface densities of the star formation rate, stellar mass, and molecular gas mass . These global density relations are connected with and follow similar trends as the resolved SFMS, the Kennicutt-Schmidt (KS) relation, and the molecular gas main sequence (MGMS). Taking advantage of this large catalog, we show that the scatters in the global KS and MGMS relations are smaller than that of the global relation between the star formation rate surface density and stellar mass surface density, and their Pearson correlation coefficients are higher. More importantly, multivariate regression and partial correlation analyses demonstrate that the apparent correlation between the star formation rate surface density and stellar mass surface density is entirely mediated by the molecular gas surface density, with its best-fit parameters directly derivable from those of the KS and MGMS relations. Overall, our findings suggest that the correlation between stellar mass and molecular gas, as well as that between molecular gas and star formation, are more direct and fundamental. The star-forming main sequence thus appears to be a natural by-product of these two tighter relations.

Figures (3)

• Figure 1: The molecular gas mass fraction ($f_{\rm mol} \equiv M_{\rm mol}/M_\ast$) is plotted against (from left to right): stellar mass ($M_\ast$), stellar mass surface density ($\Sigma_{\ast}$), NUV--r color, and the specific star formation rate (sSFR). The green circles represent xCOLD GASS main sequence galaxies with CO detections, while the contour shows the distribution of our estimated volume-limited main sequence sample. The black and red points with error bars respectively indicate the median and 1 $\sigma$ scatters of the observed and estimated MS galaxies in each bin.
• Figure 2: Similar to Figure \ref{['fig1']}, but for molecular gas depletion time scaling $t_{\rm dep}$(mol) relations. In the rightmost panel, we plot the adjusted best--fitting bisector linear relation provided by Saintonge2011b based on the COLD GASS sample, assuming a constant Galactic conversion factor.
• Figure 3: The global scaling relations between the star formation rate surface density ($\Sigma_{\rm SFR}$), the stellar mass surface density ($\Sigma_{\ast}$), and the molecular gas mass surface density ($\Sigma_{\rm mol}$) measured at galaxy-wide scales for 24,954 main-sequence galaxies. The top-left panel shows the three-parameter distribution in logarithmic space. The remaining panels present the regression fits to the three projected two-dimensional relations: $\Sigma_{\rm SFR}$ vs. $\Sigma_{\ast}$, $\Sigma_{\rm SFR}$ vs.$\Sigma_{\rm mol}$ ( KS relation), and $\Sigma_{\rm mol}$ vs. $\Sigma_{\ast}$ (MGMS relation). Contours show the distribution of our sample. The best-fit equations, the standard deviation of the residuals ($\sigma$), and the Pearson correlation coefficient (r) are indicated in each panel. The green circles are 198 xCOLD GASS MS galaxies for demonstration and were not included in the fitting.