MERGERS AND BULGE FORMATION IN ΛCDM: WHICH MERGERS MATTER?

By P. Hopkins, K. Bundy, Darren Croton, L. Hernquist, D. Keres̆, S. Khochfar, Kyle R. Stewart, Andrew Wetzel, J. Younger. 2009

302 citations recorded by the source corpus.

We use a suite of semi-empirical models to predict the galaxy–galaxy merger rate and relative contributions to bulge growth as a function of mass (both halo and stellar), redshift, and mass ratio. The models use empirical constraints on the halo occupation distribution, evolved forward in time, to robustly identify where and when galaxy mergers occur. Together with the results of high-resolution merger simulations, this allows us to quantify the relative contributions of mergers with different properties (e.g., mass ratios, gas fractions, redshifts) to the bulge population. We compare with observational constraints, and find good agreement. We also provide useful fitting functions and make public a code to reproduce the predicted merger rates and contributions to bulge mass growth. We identify several robust conclusions. (1) Major mergers dominate the formation and assembly of ∼L* bulges and the total spheroid mass density, but minor mergers contribute a non-negligible ∼30%. (2) This is mass dependent: bulge formation and assembly is dominated by more minor mergers in lower-mass systems. In higher-mass systems, most bulges originally form in major mergers near ∼L*, but assemble in increasingly minor mergers. (3) The minor/major contribution is also morphology dependent: higher B/T systems preferentially form in more major mergers, with B/T roughly tracing the mass ratio of the largest recent merger; lower B/T systems preferentially form in situ from minor mergers. (4) Low-mass galaxies, being gas-rich, require more mergers to reach the same B/T as high-mass systems. Gas-richness dramatically suppresses the absolute efficiency of bulge formation, but does not strongly influence the relative contribution of major versus minor mergers. (5) Absolute merger rates at fixed mass ratio increase with galaxy mass. (6) Predicted merger rates agree well with those observed in pair and morphology-selected samples, but there is evidence that some morphology-selected samples include contamination from minor mergers. (7) Predicted rates also agree with the integrated growth in bulge mass density with cosmic time, but with a factor ∼2 uncertainty in both—up to half the bulge mass density could come from non-merger processes. We systematically vary the model assumptions, totaling ∼103 model permutations, and quantify the resulting uncertainties. Our conclusions regarding the importance of different mergers for bulge formation are very robust to these changes. The absolute predicted merger rates are systematically uncertain at the factor ∼2 level; uncertainties grow at the lowest masses and high redshifts.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/052105ccb19d7ac2c7c29f128544f898b08b52f7

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Citations in the corpus

THE MAJOR AND MINOR GALAXY MERGER RATES AT z < 1.5 (2011, background): “…fixed total mass) does not automatically translate into a (1+z)3 evolution in the galaxy merger rate (per galaxy above a fixed stellar mass) because there is not a simple connection between observed galaxies and dark matter halos (e.g. Berrier et al. 2006; Hopkins et al. 2010a, Moster et al. 2010).”
The SLUGGS Survey: kinematics for over 2500 globular clusters in 12 early-type galaxies (2012, background): “In contrast, beyond this radius (Reff > 1), the halo of the galaxy contains precious dynamical signatures of early merging or early collapse events that might have eventually built up the galaxy (Hopkins et al. 2009, 2010; Hoffman et al. 2010; Oser et al. 2010).”
HYDRODYNAMICS OF HIGH-REDSHIFT GALAXY COLLISIONS: FROM GAS-RICH DISKS TO DISPERSION-DOMINATED MERGERS AND COMPACT SPHEROIDS (2010, background): “In particular, mergers of dark halos do not necessarily lead to the merging of their central galaxies, and the baryonic mass ratio of galaxy mergers can significantly differ from the dark matter mass ratio of their host haloes (Hopkins et al. 2010).”
WHAT TURNS GALAXIES OFF? THE DIFFERENT MORPHOLOGIES OF STAR-FORMING AND QUIESCENT GALAXIES SINCE z ∼ 2 FROM CANDELS (2011, result): “…cases quantitatively) consistent with this picture: the approximate equality of the merger rate and the quiescent galaxy formation rate (e.g., Hopkins et al. 2010; Robaina et al. 2010); the detailed kinematic structure of early-type galaxies (Naab et al. 2006; Hoffman et al. 2010); and, the…”
The merger rate of galaxies in the Illustris simulation: a comparison with observations and semi-empirical models (2015, methodology): “As before, the three resolutions of Illustris are indicated with blue, red and green solid lines, and predictions from the semi-empirical models of Stewart et al. (2009) and Hopkins et al. (2010a) are shown with dotdashed and solid black lines, respectively.”
The origin of discs and spheroids in simulated galaxies (2011, background): “…(Efstathiou, Lake & Negroponte 1982; Christodoulou, Shlosman & Tohline 1995; Mo et al. 1998), as well as repeated minor encounters, are now claimed to be the main formation path of spheroids (e.g. Parry, Eke & Frenk 2009; Hopkins et al. 2010; Bournaud et al. 2011; De Lucia et al. 2011).”
THE DEPENDENCE OF QUENCHING UPON THE INNER STRUCTURE OF GALAXIES AT 0.5 ⩽ z < 0.8 IN THE DEEP2/AEGIS SURVEY (2012, background): “Specifically, they agreed with the merger rates of Hopkins et al. (2010a), who, using a combination of semi-empirical models and high-resolution merger simulations, concluded that there are enough major mergers, to within a factor of∼ 2, to account for the observed growth of the bulge population.”
Hierarchical formation of bulgeless galaxies: why outflows have low angular momentum (2010, background): “Bulges are commonly asserted to be formed in mergers, with galaxies which have more mergers, or mergers with larger mass ratios, assumed to have larger bulges (e.g. Barnes & Hernquist 1996; Hopkins et al. 2010).”
ON SIZES, KINEMATICS, M/L GRADIENTS, AND LIGHT PROFILES OF MASSIVE COMPACT GALAXIES AT z ∼ 2 (2010, methodology): “…mass ratio, redshift and (redshift-dependent) gas fraction is required to properly model spheroid evolution (Khochfar & Silk 2006b; Naab et al. 2009; Hopkins et al. 2010a), but for our present purpose it is sufficient to highlight two characteristic features notable in our re-merger simulations.”
The effects of gas on morphological transformation in mergers: implications for bulge and disc demographics (2009, methodology): “In practice, many of these bulges are made by minor mergers, but the relative suppression is similar (Hopkins et al. 2009e; Weinzirl et al. 2009).”
Simulations of binary galaxy mergers and the link with fast rotators, slow rotators, and kinematically distinct cores (2011, background): “From Hopkins et al. (2010) for a galaxy of mass 1011 M (i.e. with the mass of our main progenitor), if P is the probability of having a 1:1 merger, the probability to have a 2:1 merger (respectively 3:1 and 6:1) is 4P (respectively 6P and 9P).”
CAN MINOR MERGING ACCOUNT FOR THE SIZE GROWTH OF QUIESCENT GALAXIES? NEW RESULTS FROM THE CANDELS SURVEY (2011, methodology): “They are therefore expected theoretically to experience less dissipation in their formation, possibly leading to less compact remnants (Robertson et al. 2006; Khochfar & Silk 2006; Hopkins et al. 2010b; Shankar et al. 2011).”
MERGERS IN ΛCDM: UNCERTAINTIES IN THEORETICAL PREDICTIONS AND INTERPRETATIONS OF THE MERGER RATE (2010, methodology): “Figure 1 summarizes the comparisons from Hopkins et al. (2010).12 The different model choices are discussed therein, and we will break each down in detail in what follows.”
The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted stars (2015, background): “This is consistent with expectations fromN - body and hydrodynamic simulations of galaxy mergers, in which the binding energy ranks of the interacting particles tend to be conserved during a galaxy merger (e.g. Amorisco 2015; Barnes 1988; Hopkins et al. 2009).”