Σ<sub>SFR</sub>–M <sub>∗</sub> Diagram: A Valuable Galaxy Evolution Diagnostic to Complement (s)SFR–M <sub>∗</sub> Diagrams
Samir Salim, Sandro Tacchella, Chandler Osborne, S. M. Faber, Janice Lee, Sara L. Ellison
Abstract
Abstract The specific star formation rate (sSFR) is commonly used to describe the level of galaxy star formation (SF) and to select quenched galaxies. However, since it is a relative measure of the young-to-old population, an ambiguity in its interpretation may arise because a low sSFR can be due to either a substantial previous mass buildup or SF activity that is low. We show, using large samples spanning 0 < z < 2, that the normalization of the star formation rate (SFR) by the physical extent over which SF is taking place (i.e., the SFR surface density, Σ SFR ) overcomes this ambiguity. Σ SFR has a strong physical basis, being tied to the molecular gas density and the effectiveness of stellar feedback, so we propose Σ SFR – M * as an important galaxy evolution diagram to complement (s)SFR– M * diagrams. Using the Σ SFR – M * diagram we confirm the Schiminovich et al. result that the level of SF along the main sequence today is only weakly mass-dependent—high-mass galaxies, despite their redder colors, are as active as blue, low-mass ones. At higher redshift, the slope of the “Σ SFR main sequence” steepens, signaling the epoch of bulge buildup in massive galaxies. We also find that Σ SFR based on the optical isophotal radius more cleanly selects both starbursting and spheroid-dominated (early-type) galaxies than the sSFR. One implication of our analysis is that the assessment of the inside-out versus outside-in quenching scenarios should consider both sSFR and Σ SFR radial profiles, because ample SF may be present in bulges with low sSFRs (red color).