A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies: I. Galaxy Mergers & Quasar Activity

TLDR: In this article, the authors developed a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies.

Abstract: (Abridged) We develop a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies. Combining halo mass functions (MFs) with empirical halo occupation models, we calculate where major galaxy-galaxy mergers occur and what kinds of galaxies merge, at all redshifts. We compare with observed merger MFs, clustering, fractions, and small-scale environments, and show that this yields robust estimates in good agreement with observations. Making the simple ansatz that major, gas-rich mergers cause quasar activity, we demonstrate that this naturally reproduces the observed rise and fall of the quasar luminosity density from z=0-6, as well as quasar LFs, fractions, host galaxy colors, and clustering as a function of redshift and luminosity. The observed excess of quasar clustering on small scales is a natural prediction of the model, as mergers preferentially occur in regions with excess small-scale galaxy overdensities. We show that quasar environments at all observed redshifts correspond closely to the empirically determined small group scale, where mergers of gas-rich galaxies are most efficient. We contrast with a secular model in which quasar activity is driven by bars/disk instabilities, and show that while these modes probably dominate at Seyfert luminosities, the constraints from clustering (large and small-scale), pseudobulge populations, disk MFs, luminosity density evolution, and host galaxy colors argue that they must be a small contributor to the z>1 quasar luminosity density.

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FIG. 13.— Predicted quasar luminosity density, if quasars are triggered in mergers, as a function of redshift.Left: Prediction from a simplified toy model in which all halos hosting∼ L∗ galaxies undergo major mergers near their characteristic small group mass scale, and build a BH which obeys the appropriateMBH − Mhalo relation for that redshift (estimatedMBH − Mhalo as a function of redshift from Hopkins et al. 2007d; Fine et al. 2006; Hopkins et al. 2007c, corresponding to solid, long dashed, and dot-dashed lines, respectively). Points show observational estimates from the measured QLFs of Ueda et al. (2003, red circles), Hasinger et al. (2005, blue triangles), Richards et al. (2005, green diamonds), and the large compilation of multiwavelength QLF data in Hopkins et al. (2007e, black stars). The observations from specific bands are converted to a bolometric luminosity density using the bolometric corrections calibrated in Hopkins et al. (2007e). Right: Same, but the predicted luminosity density is calculated properly accounting for all galaxy and halo masses from the merger rate functions determined in § 2, and adopting the observed ratio of BH to host galaxy spheroid mass as a function of redshift (e.g. Peng etal. 2006). Linestyles correspond to different means of estimating the exact merger rates, as in Figure 9. Red lines assume all mergers will trigger quasars, black (lower) lines assume only gas-rich (“wet”) mergers can trigger bright quasar activity (adopting the observed fraction of gas-rich/star-forming/blue galaxies as a function ofMgal andMhalo as in Figure 5). A merger-driven model naturally predicts both the rise and fall of the global quasar luminosity density to high precision.

FIG. 11.— Comparing our predicted clustering of∼ M∗ major mergers (lines; style as in Figure 9) as a function of redshift to thatvarious populations usually associated with galaxy mergers (points): post-starburst (E+A/K+A) galaxies (Blake et al. 2004, star), close galaxy pairs (Infante et al. 2002, diamond), and sub-millimeter galaxies (Blain et al. 2004, square).

FIG. 10.— Predicted merger fraction as a function of redshift (lines, same style as Figure 9), above two approximate mass limit . Observations (points) are shown from Patton et al. (2002, filled inverted triangles), Conselice et al. (2003, filled circles), Bundy et al. (2004, filled triangles), Lin et al. (2004, open diamonds), Xu et al. (2004, open stars), De Propris et al. (2005, open circles), Cassata et al. (2005, filled diamonds), Wolf et al. (2005, filled stars), Bundy et al. (2005, open triangles), Lotz et al. (2006a, open inverted triangles), Lotz et al. (2006b, open squares), Bell et al. (2006, filled squares), and Bridge et al. (2007, ×’s). Note that the mass limit is only approximate in several of these cases, as they are selected by optical luminosity. The predicted merger fractions agree well, especially for the deeper case which resolvesM∗ galaxies.

FIG. 4.— Merger efficiency (arbitrary units; defined in the same manner as the lower panel of Figure 2, with different linestyles in the same style for various mass galaxies) for different classes of mergers. Using the subhalo mass functions and halo occupation models, we can separate major ergers onto the central galaxy in a halo (top), minor (mass ratio> 3 : 1 but< 10 : 1) mergers onto the central galaxy (middle), and satellite-satellite mergers (bottom). Major mergers occur efficiently in central galaxies near the small group scale for eachMgal. When galaxies live in very massive halos, they experience a large number of minor mergers from the satellite population. Satellite-satellite mergers are a relatively small effectat all galaxy and halo masses.

FIG. 5.— Merger efficiency (arbitrary units; calculated as in Figure 2) as a function of halo mass (adopting the meanMgal(Mhalo) from Figure 3). Using the type-separated galaxy mass functions from Bell etal. (2003b); Borch et al. (2006); Fontana et al. (2004) atz= 0, 1, 2, respectively, we show the fraction of galaxies at each mass expected to be gas-richand gas-poor, at each of three redshifts. At high redshifts, all but the most massive merging galaxies will be gas-rich, whereas at low masses the gas-poor population dominates at most masses where mergers are efficient.

FIG. 20.— Left: Predicted bias as a function of quasar luminosity from our merger-driven model (black lines, style as in Figure 9). To contrast, the expected biasb(L) from the semi-analytic models of Wyithe & Loeb (2002, cyan)d Kauffmann & Haehnelt (2000, orange with diamonds) are plotted (dot-dashed lines); these adopt simplified (constant or exponential “on/off”) quasar lightcurves. Points are measurements from Croom et al. (2005, red squares), Adelberger & Steidel (2005, orange crosses), Porciani & Norberg (2006, purple diamonds), Myers et al. (2006a, blue circles), da Angela et al.(2006, magenta stars), and Coil et al. (2007, black open circles). For ease of comparison, all luminosities are converted to bolometric luminosities using the corrections from Hopkins et al. (2007e). Vertical blue dotted lines showL∗ in the QLF at each redshift, from Hopkins et al. (2007e).Right: The best-fit slope of the dependence of bias on luminosity at the QLFL∗, i.e. d(b/b∗)/d log (L/L∗), whereb∗ ≡ b(L∗). Points are determined from the observations at left, withthe observations from Myers et al. (2006a, cyan circles) and Grazian et al. (2004); Wake et al. (2004, black open diamond) added. Lines are in the style of the left panel, with the red dashed line showing no dependence of bias on luminosity. Adopting an a priori model for merger-triggered quasar activity reproduces the empirical prediction from Lidz et al. (2006), that quasar bias should depend weakly on quasar luminosity.