Intermittent Self-Sustaining Star Formation in Low-Redshift Galaxies
  Exhibiting a Peak Metallicity Plateau
  The decline of star formation in massive low-redshift galaxies, often
referred to as quenching, has been attributed to a variety of factors. Some
proposals suggest that erupting active galactic nuclei may strip galaxies of
their interstellar medium, and thus the ability to form stars. Here, we note
that, whereas star formation is universal in small, low-redshift galaxies,
fractional duty cycles of star formation steadily decline in galaxies of
increasing mass, although star formation may not cease entirely. We show that,
when infall of gas from extragalactic space ceases, galaxies of high stellar
mass appear to sustain star formation on gas liberated in mass loss from
evolved low- and intermediate-mass stars admixed with occasional Type II
supernova ejecta. This model quantitatively accounts for the universal limiting
metallicity plateau at a ratio of oxygen to hydrogen atoms, Z(O) = n(O)/n(H) =
0.0013, characterizing high-mass intermittently star-forming galaxies. We show
that, when fractional duty cycles are specifically taken into account, the star
formation rates of galaxies on this plateau correspond to mass loss rates from
evolving stars in rough agreement with observed estimates. Far-infrared
continuum and fine-structure line observations, as well as molecular data, may
soon be able to resolve whether or not low levels of sporadic star formation
can be sustained indefinitely in massive galaxies.
