Abstract
We use hydrodynamical simulations to study the color transformations induced by star formation and active galactic nuclei (AGNs) during major mergers of spiral galaxies. Our modeling accounts for radiative cooling, star formation, and supernova feedback. Moreover, we include a treatment of accretion onto supermassive black holes embedded in the nuclei of the merging galaxies. We assume that a small fraction of the bolometric luminosity of an accreting black hole couples thermally to surrounding gas, providing a feedback mechanism that regulates its growth. The encounter and coalescence of the galaxies triggers nuclear gas inflow, which fuels both a powerful starburst and strong black hole accretion. Comparing simulations with and without black holes, we show that AGN feedback can quench star formation and accretion on a short timescale, particularly in large galaxies where the black holes can drive powerful winds once they become sufficiently massive. The color evolution of the remnant differs markedly between mergers with and without central black holes. Without AGNs, gas-rich mergers lead to elliptical galaxies that remain blue owing to residual star formation, even after more than 7 Gyr have elapsed. In contrast, mergers with black holes produce elliptical galaxies that redden much faster, an effect that is more pronounced in massive remnants where a nearly complete termination of star formation occurs, allowing them to redden to u - r 2.3 in less than 1 Gyr. AGN feedback may thus be required to explain the population of extremely red massive early-type galaxies, and it appears to be an important driver in generating the observed bimodal color distribution of galaxies in the local universe.
Highlights
In hierarchical theories of galaxy formation, large systems are built up from mergers of smaller progenitors
We have demonstrated that gas-rich galaxies do not necessarily consume all their gas in the starbursts that accompany major mergers
If the merging galaxies host supermassive black holes at their centers, active galactic nuclei (AGN) feedback provides a mechanism to quench star formation on a short timescale. This introduces a marked difference in the color evolution of galaxies: mergers of massive galaxies can produce remnants that redden to u − r ≃ 2.2 − 2.3 in about 1 − 2 Gyrs
Summary
In hierarchical theories of galaxy formation, large systems are built up from mergers of smaller progenitors. Theoretical models conjecture that the correlation arises because black hole growth stalls once the energy deposition associated with the accretion can expel the remaining gas from the halo or bulge (Ciotti & Ostriker 1997, 2001; Silk & Rees 1998; Wyithe & Loeb 2003). This would have an immediate bearing on star formation in the galaxy.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
