The quenched mass portion of star-forming galaxies and the origin of the
  star formation sequence slope
  Observationally, a massive disk galaxy can harbor a bulge component that is
comparably inactive as a quiescent galaxy (QG). 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 ($f_{\rm Q}$) at fixed stellar mass ($M_{\ast}$) and to reconcile the MS
slopes both in the low and the high mass regimes. In this model, each SFG is
composed by a star-forming plus a quenched component. The mass of the
star-forming component ($M_{\rm SF}$) correlates with the star formation rate
(SFR) following a relation SFR $\propto M_{\rm SF}^{\alpha_{\rm SF}}$, where
$\alpha_{\rm SF}\sim 1.0$ . The quenched component contributes to the stellar
mass but does not to the SFR. It is thus possible to quantify $f_{\rm Q}$ based
on the departure of the observed MS slope $\alpha$ from $\alpha_{\rm SF}$.
Adopting the redshift-dependent MS slope reported by \citet{Whitaker 2014}, we
explore the evolution of the $f_{\rm Q}-M_{\ast}$ relations over $z=[0.5,2.5]$.
We find that Milky-Way mass SFGs (with $M_{\ast}\approx 10^{10.7}M_{\odot}$)
typically have a $f_{\rm Q}=30\%-40\%$ at $z\sim 2.25$, whereas this value
rapidly rises up to $70\%-80\%$ at $z\sim 0.75$. The origin of an $\alpha\sim
1.0$ 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 keeping
roughly constant over cosmic time. This scenario successfully produces an
$\alpha \sim 1.0$ MS slope, as well as the observed MS evolution from $z=2.5$
to $z=0$ at low masses.
