Metal Abundances across Cosmic Time ($\mathcal{MACT}$) Survey. III. The Relationship between Stellar Mass and Star Formation Rate in Extremely Low-Mass Galaxies

Kaitlyn Shin, Chun Ly, Matthew A. Malkan, Sangeeta Malhotra, Mithi de los Reyes, James E. Rhoads

Published 2019-10-23Version 1

Extragalactic studies have demonstrated that there is a moderately tight ($\approx$ 0.3 dex) relationship between galaxy stellar mass ($M_{\star}$) and star formation rate (SFR) that holds for star-forming galaxies at $M_{\star} \sim 3 \times 10^8$-10$^{11}~M_{\odot}$. This relationship has often been referred to as the "star formation main sequence." However, it has yet to be determined whether such a relationship extends to even lower mass galaxies. Here, we present new results using observations for 777 narrowband H$\alpha$-selected galaxies with stellar masses between $10^6$ and $10^{10}~M_{\odot}$ (average of $10^{8.2}~M_{\odot}$) at $z \approx$ 0.07--0.5. These galaxies have sensitive UV to near-infrared photometric measurements and optical spectroscopy. The latter allows us to correct our H$\alpha$\ SFRs for dust attenuation using Balmer decrements. Our study reveals: (1) for low-SFR galaxies, our H$\alpha$ SFRs systematically underpredicts compared to FUV measurements; (2) on average, sSFR increases with decreasing stellar mass, but at a slower rate compared to more massive galaxies; (3) the SFR--$M_{\star}$ relation holds for galaxies down to $\sim$10$^6~M_{\odot}$ ($\sim$1.5 dex below previous studies), and follows a redshift-dependent main-sequence relation of $\log{({\rm SFR})} \propto \alpha \log{M_{\star}} + \beta z$ with $\alpha=0.58 \pm 0.01$ and $\beta=2.08 \pm 0.07$, over lookback times of up to 5 Gyr; (4) the intrinsic dispersion in the SFR--$M_{\star}$ relation at low stellar masses is $\approx0.3$ dex with no strong evidence that lower mass galaxies display a larger dispersion, consistent with results obtained at similar or higher stellar masses.