Galaxy Bulge Formation: Interplay with Dark Matter Halo and Central Supermassive Black Hole

Abstract

We present a simple scenario where the formation of galactic bulges was regulated by the dark halo gravity and regulated the growth of the central supermassive black hole. Assuming the angular momentum is low, we suggest that bulges form in a runaway collapse due to the "gravothermal" instability once the central gas density or pressure exceeds certain threshold (Xu & Zhao 2007). We emphasize that the threshold is nearly universal, set by the background NFW dark matter gravity $g_{DM} \sim 1.2 \times 10^{-8}{\rm cm} {\rm sec}^{-2}$ in the central cusps of halos. Unlike known thresholds for gradual formation of galaxy disks, we show that the universal "halo-regulated" star formation threshold for spheroids matches the very high star formation rate and star formation efficiency shown in high-redshift observations of central starburst regions. The starburst feedback also builds up a pressure shortly after the collapse. This large pressure could both act outward to halt further infall of gas from larger scale, and act inward to counter the Compton-thick wind launched from the central black hole in an Eddington accretion. Assuming the feedback balancing inward and outward forces, our scenario naturally gives rise to the black hole-bulge relationships observed in the local universe.