Suppressing star formation in quiescent galaxies with supermassive black hole winds

Abstract

Quiescent galaxies with little or no ongoing star formation dominate the galaxy population above M * ∼ 2 × 10 10 M⊙, where their numbers have increased by a factor of ∼ 25 since z ∼ 2 1-4 . Once star formation is initially shut down, perhaps during the quasar phase of rapid accretion onto a supermassive black hole [5][6][7] , an unknown mechanism must remove or heat subsequently accreted gas from stellar mass loss 8 or mergers that would otherwise cool to form stars 9, 10 . Energy output from a black hole accreting at a low rate has been proposed 11-13 , but observational evidence for this in the form of expanding hot gas shells is indirect and limited to radio galaxies at the centers of clusters 14,15 , which are too rare to explain the vast majority of the quiescent population 16 . Here we report bisymmetric emission features co-aligned with strong ionized gas velocity gradients from which we infer the presence of centrallydriven winds in typical quiescent galaxies that host low-luminosity active nuclei. These galaxies are surprisingly common, accounting for as much as 10% of the population at M * ∼ 2 × 10 10 M⊙. In a prototypical example, we calculate that the energy input from the galaxy's low-level active nucleus is capable of driving the observed wind, which contains sufficient mechanical energy to heat ambient, cooler gas (also detected) and thereby suppress star formation.Using optical imaging spectroscopy from the Sloan Digital Sky Survey-IV Mapping Nearby Galaxies at Apache Point Observatory 17 (SDSS-IV MaNGA) program, we define a new class of quiescent galaxies (required to have red rest-frame colors, NUV − r > 5) that is characterized by the presence of narrow bisymmetric patterns in equivalent width (EW) maps of strong emission lines, such as Hα and [O III]. Our selection employs multiband imaging to exclude galaxies with dust lanes and other disk signatures. The observed enhanced emission features are oriented randomly with respect to the optical surface brightness morphology, but roughly align with strong, systematic velocity gradients as traced by the ionized gas emission lines. The gas velocity fields in these galaxies are decoupled from their stellar motions. These galaxies are surprisingly common among the quiescent population, accounting for ∼10% of quiescent galaxies with log M * /M⊙ ∼ 10.3. To illuminate the salient features of this class, we focus on a prototypical example, nicknamed "Akira" (Fig. 1). The SDSS imaging shows Akira to be an unremarkable spheroidal galaxy of moderate stellar mass (log M * /M⊙ = 10.78) that is interacting with a low-mass companion (nicknamed "Tetsuo") at a projected separation of ≈ 32 kpc (67 ′′ ); they are not classified as members of a larger galaxy group 18 and the properties of both galaxies are listed in Table 1. Spectral energy distribution (SED) fitting indicates that Akira is nearly dormant, with almost no detection of ongoing star formation 19 . Resolved spectroscopy, however, reveals intriguing and complex patterns among spectral tracers of gas in Ak...

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