THE QUENCHED MASS PORTION OF STAR-FORMING GALAXIES AND THE ORIGIN OF THE STAR FORMATION SEQUENCE SLOPE

Abstract

ABSTRACT Observationally, a massive disk galaxy can harbor a bulge component that is comparably inactive as a quiescent galaxy. It has been speculated that the quenched component contained in star-forming galaxies (SFGs) is the reason why the star formation main sequence (MS) has a shallow slope at high masses. In this paper, we present a toy model to quantify the quenched mass portion of SFGs (f Q) at fixed stellar mass (M *) and to reconcile the MS slopes in both the low- and the high-mass regimes. In this model, each SFG is composed of a star-forming plus a quenched component. The mass of the star-forming component (M SF) correlates with the star formation rate (SFR) following a relation SFR , where α SF ∼ 1.0. The quenched component contributes to the stellar mass but not to the SFR. It is thus possible to quantify f Q based on the departure of the observed MS slope α from α SF. Adopting the redshift-dependent MS slope reported by Whitaker et al., we explore the evolution of the relations over z = [0.5, 2.5]. We find that Milky Way-like SFGs (with ) typically have an f Q = 30%–40% at z ∼ 2.25, whereas this value rapidly rises up to 70%–80% at z ∼ 0.75. The origin of an α ∼ 1.0 MS slope seen in the low-mass regime is also discussed. We argue for a scenario in which the majority of low-mass SFGs stay in a “steady-stage” star formation phase. In this phase, the SFR is mainly regulated by stellar feedback and not significantly influenced by the quenching mechanisms, thus remaining roughly constant over cosmic time. This scenario successfully produces an α ∼ 1.0 MS slope, as well as the observed MS evolution from z = 2.5 to z = 0 at low masses.