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

TLDR: In this paper, a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies is proposed.

Abstract: We develop a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies. By combining theoretically well-constrained halo and subhalo mass functions as a function of redshift and environment with empirical halo occupation models, we can estimate where galaxies of given properties live at a particular epoch. This allows us to calculate, in an a priori cosmological manner, where major galaxy-galaxy mergers occur and what kinds of galaxies merge, at all redshifts. We compare this with the observed mass functions, clustering, fractions as a function of halo and galaxy mass, and small-scale environments of mergers, and we show that this approach yields robust estimates in good agreement with observations and can be extended to predict detailed properties of mergers. Making the simple Ansatz that major, gas-rich mergers cause quasar activity (but not strictly assuming they are the only triggering mechanism), we demonstrate that this model naturally reproduces the observed rise and fall of the quasar luminosity density at -->z = 0–6, as well as quasar luminosity functions, fractions, host galaxy colors, and clustering as a function of redshift and luminosity. The recent observed excess of quasar clustering on small scales at -->z ~ 0.2–2.5 is a natural prediction of our model, as mergers will preferentially occur in regions with excess small-scale galaxy overdensities. In fact, we demonstrate that quasar environments at all observed redshifts correspond closely to the empirically determined small group scale, where major mergers of ~L* gas-rich galaxies will be most efficient. We contrast this with a secular model in which quasar activity is driven by bars or other disk instabilities, and we show that, while these modes of fueling probably dominate the high Eddington ratio population at Seyfert luminosities (significant at -->z = 0), the constraints from quasar clustering, observed pseudobulge populations, and disk mass functions suggest that they are a small contributor to the -->z 1 quasar luminosity density, which is dominated by massive BHs in predominantly classical spheroids formed in mergers. Similarly, low-luminosity Seyferts do not show a clustering excess on small scales, in agreement with the natural prediction of secular models, but bright quasars at all redshifts do so. We also compare recent observations of the colors of quasar host galaxies and show that these correspond to the colors of recent merger remnants, in the transition region between the blue cloud and the red sequence, and are distinct from the colors of systems with observed bars or strong disk instabilities. Even the most extreme secular models, in which all bulge (and therefore BH) formation proceeds via disk instability, are forced to assume that this instability acts before the (dynamically inevitable) mergers, and therefore predict a history for the quasar luminosity density that is shifted to earlier times, in disagreement with observations. Our model provides a powerful means to predict the abundance and nature of mergers and to contrast cosmologically motivated predictions of merger products such as starbursts and active galactic nuclei.

Figures

FIG. 21.—Left: Predicted total IR (8−1000µm) luminosity functions at different redshifts (as labeled). Green, blue, and red lines shows the estimated contribution from non-merging systems, star formation in mergers, and obscured AGN in mergers, respectively. Linestyles are asin Figure 9, for the variants of the merger calculations. Points show observational estimates from Saunders et al. (1990, magenta stars), Soifer & Neugebauer (1991, blue triangles), Yun et al. (2001, black circles), Le Floc’h et al. (2005, black diamonds), Chapman et al. (2005, black inverted triangles), Babbedge et al. (2006, black squares), and Caputi et al. (2007, black×’s). Right: Integrated IR luminosity density. Solid lines show the contributions from non-merging systems (green), star formation in mergers (blue), and obscured quasars in mergers (red). Blue dotted shows the total (star formation+AGN) merger contribution, black dashedshows the total from all sources. Orange points show observational estimates ofρIR from the compilation of Hopkins (2004, circles; only the direct IR observations therein are plotted here), as well as Le Floc’h et al. (2005, diamonds), Pérez-González et al. (2005), and Caputi et al. (2007,×’s). Red stars show the bolometric quasar luminosity density from Figure 13, rescaled by a constant (mean) obscured-to-unobscured ratio of∼ 2 : 1. The agreement in all cases is good – our model reproducesthe star formation history of the Universe and distribution of star formation rates and bolometric luminosities.

FIG. 22.— Left: Total IR luminosity, as a function of redshift, above which mergers (star formation+AGN) dominate the total IR luminosity functions (solid line, from Figure 21; dashed lines show the range above which 25/75% of systems on the luminosity function are mergers). Point shows the corresponding transition point (and range) observed inlow-redshift systems (Sanders et al. 1988b).Right: Same, but for the transition between star formation (in non-merging+merging systems) and (obscured) AGN dominating the IR luminosity functions (generally a factor∼ a few larger luminosity than the quiescent system-merger transition). Points showt e observed estimates from comparison of PAH feature strengths in Lutz et al.(1998, low redshift) and Sajina et al. (2007, high redshift). A similar estimate is obtained (at low redshift) from comparison of emission line strengths (Sanders & Mirabel 1996; Kewley et al. 2007), full SED template fitting (Farrah et al. 2003), or indirect comparison with Type 2 AGN luminosity functions (Chary & Elbaz 2001). The model predicts the local transitions, and that byz& 1, the LIRG population is dominated by quiescent star formation in gas-rich systems (even as the total and fractional luminosity density in mergers increases rapidly).

FIG. 23.— As Figure 17 (upper center panel), but comparing the clustering (quasar-galaxy cross-correlation) as a function ofscale measured by Serber et al. (2006) for bright optical quasars and dimmer Seyfert galaxies. Quasar clustering is consistent with our predicted excess on small scales, indicating a merger-driven origin, but low-luminosity systems show no such dependence, suggesting that processes independent of the lcal, small-scale density (e.g. secular processes) may dominate at these luminosities.

FIG. 8.— Excess galaxy overdensity on small scales predicted for mergers from our model. Because mergers are more likely when there isa galaxy overdensity on small scales (Figure 7), mergers will, on averag , occur in regions with slightly enhanced small-scale densities. We show the real-space correlation function (bottom; technically the merger-galaxy cross correlation function) and corresponding number of companions within a given radius (top) of all field galaxies (Goto 2005), and then this multiplied by the predicted excess on small scales for mergers (essentially integrating over the probability bias to large overdensity on small scales in Figure 7). Dashed blue lines indicate the errors in our estimate from the combination of uncertainties in the field galaxy correlation function, the rangeof galaxy masses considered (which slightly shifts the physical scale on which the effect is important), and the inclusion/exclusion of Poisson noise in the distribution of overdensities for a given halo mass. The observed number of companions and clustering of post-starburst (likely merger remnant) galaxies is shown for comparison, from Goto (2005, red circles) and Hogg et al.(2006, purple diamonds).

FIG. 17.— Excess small-scale clustering of quasars expected ifthey are triggered in mergers, as in Figure 8.Left: Observed correlation functions from Myers et al. (2006b, blue circles) and Hennawi et al. (2006, green diamonds), measured for∼ L∗ quasars over the redshift rangesz∼ 1 − 2. Dashed black line shows the expected correlation function (nonlinear dark matter clustering from Smith et al. (2003), multiplied bythe appropriate constant large-scale bias factor) without a small-scale excess. Red lines multiply this by the predicted additional bias as a function of scale from § 2.3, namely the fact that small-scale overdensities increase the probability of mergers. Solid line shows our mean prediction, dashed the approximate∼ 1σ range, as in Figure 8.Center: Same, but dividing out the best-fit large-scale correlation function(i.e. bias as a function of scale). Black squares in upper panel show the measurement for true optical quasars (−23.3 > Mi > −24.2) from Serber et al. (2006) atz∼ 0.1− 0.5. Right: Ratio of the mean bias at small radii (r < 100h−1 kpc) to that at large radii (the asymptotic values in the center panel), at all redshifts where this has been observed. Lines show the predicted excess from the previous panels (lower line averages down to a minimum radiusr = 50h−1 kpc, upper line to a – potentially unphysical – minimumr = 10h−1 kpc).

FIG. 26.— Fraction of the integrated quasar luminosity densityowing to non-merger driven secular mechanisms.Top: Upper limit to the contribution from BHs in disk galaxy hosts at eachz (see text). Limits are derived from the observed type-separated mass functions in Figure 16 (same style) and Franceschini et al. (2006, cyan stars). Solid line assume the disk mass function does not evolve withz. Second from Top:Fractional contribution from systems in pseudobulges atz = 0. Local distribution of pseudobulge masses is estimated from the observed pseudobulge fractionversus galaxy type (Noordermeer & van der Hulst 2007, red dashed line, with∼ 1σ shaded range), or assuming all bulges with Sersic indexn< 2 are pseudobulges (with the distribution ofn versus bulge mass from Balcells et al. 2004, black solid line and shading), or from directly measured pseudobulge mass functions (Driver et al. 2007, blue long-dashed line and shading).Second from Bottom: Probability (fromχ2) that observed clustering of quasars (data in Figure 15) and star-forming galaxies reflect the same hosts. Solid line is derived from the best-fit to the compilation of Hopkins et al. (2007d)points from the individual measurements included (see Figure 15).Bottom: Predicted fraction of the luminosity density from the the model for secularfueling from Hopkins & Hernquist (2006), when combined with the merger-driven model herein.