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

We present a simple scenario where the formation of galactic bulges wasregulated by the dark halo gravity and regulated the growth of the centralsupermassive black hole. Assuming the angular momentum is low, we suggest thatbulges form in a runaway collapse due to the "gravothermal" instability oncethe central gas density or pressure exceeds certain threshold (Xu & Zhao 2007).We emphasize that the threshold is nearly universal, set by the background NFWdark matter gravity $g_{DM} \sim 1.2 \times 10^{-8}{\rm cm} {\rm sec}^{-2}$ inthe central cusps of halos. Unlike known thresholds for gradual formation ofgalaxy disks, we show that the universal "halo-regulated" star formationthreshold for spheroids matches the very high star formation rate and starformation efficiency shown in high-redshift observations of central starburstregions. The starburst feedback also builds up a pressure shortly after thecollapse. This large pressure could both act outward to halt further infall ofgas from larger scale, and act inward to counter the Compton-thick windlaunched from the central black hole in an Eddington accretion. Assuming thefeedback balancing inward and outward forces, our scenario naturally gives riseto the black hole-bulge relationships observed in the local universe.Comment: 7 pages, 2 figures, accepted for publication in Ap