Abstract

1. Quasars, Mergers, and the Formation of Elliptical Galaxies Philip F. Hopkins (Harvard-CfA) phopkins@cfa.harvard.edu Lars Hernquist (CfA), T. J. Cox (CfA), Brant Robertson (Chicago), Volker Springel (MPA), Adam Lidz (CfA), Tiziana Di Matteo (Carnegie), Rachel Somerville (MPIA), Kevin Bundy (Caltech), Gordon Richards (JHU), Yuexing Li (CfA), Sukanya Chakrabarti (CfA), Ramesh Narayanan (CfA), Adam Myers (Illinois), Alison Coil (Steward) Abstract Observed Populations We develop a model of the co-formation of quasars, supermassive black holes (BHs), starbursts, and spheroids in major galaxy mergers. Utilizing high-resolution merger simulations, including the effects of radiative feedback, a multi-phase dynamically star-forming ISM, BH accretion based on the nearby gas properties, and feedback from BH growth and star formation (Springel et al.). These simulations allow us to self-consistently track simultaneous BH and galaxy evolution, testing models against a wide variety of observations. Taking advantage of the detail we can extract from the simulations, we can model various (potentially) merger-driven populations and their relations to one another. - = How Do We Test the Models? Morphological probes provide the most obvious and potentially powerful test of merger-driven unification schemes. Studying merger remnants and their structural or kinematic properties (Cox et al.; Robertson et al.) we can determine not only whether mergers are required for certain galaxy populations, but specifically what kinds of mergers. Unfortunately, detailed morphological studies or identification of faint tidal features are extremely difficult at high redshift, and by the final starburst or quasar stages the central regions of the merger remnant have already relaxed to resemble a “normal” elliptical. We therefore turn to indirect, but robust tests of the possible links between these populations. Hopkins, Bundy, et al. Comparing the early-type galaxy mass function at two times (upper; Borch et al), we obtain the rate of “formation”/”buildup” of spheroids (colored points in lower figure). We compare this to observed merger mass functions (black points; Hopkins et al.), using our simulations to measure in detail how long mergers will be identified in a given sample (Lotz et al.) -- this detail allows us to convert merger fractions to rates. Given a merger mass function, or the rate at which spheroids are forming at a given cosmic time, we assume each such even must occur through a major, gas-rich merger. Using a large suite of ~500 such merger simulations, each of which has a self-consistent quasar lightcurve, we can predict the resulting, merger-driven quasar population. Inverting these tests or applying them to other merger-driven populations demonstrates that mergers can self-consistently explain these populations, and enables a diversity of new observational tests. Right: Stellar morphology of a typical merger with a quasar 100x brighter than its host (top), equal in brightness to its host (middle), and 0.1x as bright as the host (bottom). Chronology of a typical gas-rich merger (Cox et al.). Lower panels show surface density at each labeled time. Ingredients of the Models Star Formation and Accretion: Systems experience periods of activity on the first passage of the galaxies, where tidal tails and morphological disturbances are obvious. The final coalescence drives gas to the central regions of the galaxies, fueling a powerful starburst and “buried’’ BH growth (hidden by the large surrounding column densities), typical of local ULIRGs. Feedback: After a couple ~108 years, as the gas supply declines and the BH continues to grow rapidly, feedback from accretion heats the surrounding gas and drives a powerful outflow, terminating BH growth and residual star formation. This leaves rapidly reddening, elliptical remnant, which lies on the appropriate correlations between BH and host galaxies properties. = From Schweizer (1982): Surface brightness dimming or limited integration times can make even “obvious” mergers appear as normal spheroids. + For example, by integrating forward the observed accretion (quasar) history of the universe, we obtain the average “age” (formation redshift/lookback time) of BHs as a function of mass. Comparing this to the stellar ages of the bulges hosting them reveals that the major quasar phase of the BH and the termination/final star formation of the host occurred at the same epoch. This and other empirical constraints demonstrate that there is some association between quasars, the termination of star formation, and the formation of ellipticals. Merger LF => QLF QLF => Merger LF Left: M-sigma relation in simulated merger remnants (colored; Di Matteo et al.) and observed spheroids (Tremaine et al.) Left: Predicted (dark shaded) quasar luminosity function, given the observed merger mass/luminosity function at each z and our suite of simulations of similar mergers. Points are the observed Ueda et al. LF. Right: Same, but inverting the comparison, with data from Xu et al. & Wolf et al. Hopkins, Hernquist, et al. Right: Evolution of quasar luminosity & obscuring column in a typical simulation. Left: Formation times of BHs (lines), compared to observed stellar ages of their host bulges (points). Hopkins, Somerville, et al. Hopkins, Bundy, et al. Merger fraction observed (black points), with that required to account for the observed buildup of the red sequence (colored points), and that required to account for the quasar LF at each redshift. Right: Predicted clustering of QSOs vs. z, if they are triggered at the same time star formation in their host ceases (black), vs. observed QSO clustering (colored points). Hopkins, Lidz, et al.