HOW DO DISKS SURVIVE MERGERS?

By P. Hopkins, Thomas J. Cox, J. Younger, L. Hernquist. 2008

390 citations recorded by the source corpus.

We develop a general physical model for how galactic disks survive and/or are destroyed in mergers and interactions. Based on simple dynamical arguments, we show that gas primarily loses angular momentum to internal torques in a merger, induced by the gravity of the secondary. Gas within some characteristic radius, determined by the efficiency of this angular momentum loss (itself a function of the orbital parameters, mass ratio, and gas fraction of the merging galaxies), will quickly lose angular momentum to the stars sharing the perturbed host disk, fall to the center, and be consumed in a starburst. We use a similar analysis to determine where violent relaxation of the premerger stellar disks is efficient on final coalescence. Our model describes both the dissipational and dissipationless components of the merger, and allows us to predict, for a given arbitrary encounter, the stellar and gas content of the material that will survive (without significant angular momentum loss or violent relaxation) to re-form a disk in the merger remnant, versus being dissipationlessly and violently relaxed or dissipationally losing angular momentum and forming a compact central starburst. We test these predictions with a large library of hydrodynamic merger simulations, and show that they agree well (with small scatter) with the properties of simulated merger remnants as a function of merger mass ratio, orbital parameters, and gas distributions in simulations which span a wide range of parameter space in these properties as well as prescriptions for gas physics, stellar and active galactic nucleus (AGN) feedback, halo and initial disk structural properties, redshift, and galaxy masses. We show that, in an immediate (short-term) sense, the amount of stellar or gaseous disk that survives or re-forms following a given interaction can be understood purely in terms of simple, well understood gravitational physics, independent of the details of the interstellar medium gas physics or stellar and AGN feedback. This allows us to demonstrate and quantify how these physics are in fact important, in an indirect sense, to enable disks to survive mergers by lowering star formation efficiencies in low-mass systems (allowing them to retain large gas fractions) and distributing the gas to large radii. The efficiency of disk destruction in mergers is a strong function of gas content—our model allows us to explicitly predict and demonstrate how, in sufficiently gas-rich mergers (with quite general orbital parameters), even 1:1 mass-ratio mergers can yield disk-dominated remnants, and more realistic 1:3–1:4 mass-ratio major mergers can yield systems with <20% of their mass in bulges. We discuss a number of implications of this modeling for the abundance and morphology of bulges as a function of mass and redshift, and provide simple prescriptions for the implementation of our results in analytic or semianalytic models of galaxy formation.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/0ee489d06460f95274b95477336fc32e99aeb3b6

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Citations in the corpus

A unified multiwavelength model of galaxy formation (2015, background): “Numerical simulations of galaxy mergers imply fellip ∼ 0.3 andfburst ∼ 0.1 (e.g. Mihos & Hernquist 1994; Barnes 1998; Hopkins et al. 2009).”
Galaxy Zoo: the fraction of merging galaxies in the SDSS and their morphologies (2009, result): “On the contrary, recent studies have argued convincingly that, in some cases at least, disc galaxies are able to survive (multiple) major mergers (Hopkins et al. 2009).”
Galaxy Zoo: the properties of merging galaxies in the nearby Universe – local environments, colours, masses, star formation rates and AGN activity (2009, background): “The simulations studied in Hopkins et al. (2009a) emphasise the role of the progenitor gas-to-stellar-mass ratio as well as feedback mechanisms that serve to retain gas at large radii during the merger process.”
Discriminating Between the Physical Processes that Drive Spheroid Size Evolution (2009, methodology): “…high-redshift observations do not resolve the small radii needed to directly measure the central stellar mass density c; we adopt the estimates from Hopkins et al. (2009a) based on the best-fitting profiles presented in van Dokkum et al. (2008) and van der Wel et al. (2008), extrapolating the fits…”
There's no place like home? Statistics of Milky Way-mass dark matter haloes (2009, background): “…galaxies can have a substantial impact on remnant properties (Springel & Hernquist 2005; Kazantzidis et al. 2008, 2009; Scannapieco et al. 2009; Hopkins et al. 2009; Purcell et al. 2009; Stewart et al. 2009; Moster et al. 2010; Governato et al. 2009; D’Onghia et al. 2010) Nevertheless, we can…”
Evolution of the atomic and molecular gas content of galaxies (2013, methodology): “The efficiency of disc destruction and build up of spheroids is a function of progenitor merger mass ratio and gas fractions, parameterised based on the simulations of binary galaxy systems (Hopkins et al. 2009a).”
Forming a large disc galaxy from a z < 1 major merger (2008, background): “This result supports the suggestion that major mergers do not always completely destroy the pre-existing discs (Hopkins et al. 2009), although they heat them quite substantially, making them thicker.”
GALAXY ZOO: THE FUNDAMENTALLY DIFFERENT CO-EVOLUTION OF SUPERMASSIVE BLACK HOLES AND THEIR EARLY- AND LATE-TYPE HOST GALAXIES (2010, background): “Simulations show that mergers involving particularly gas-rich progenitors can result in a disk-dominated remnant (Springel & Hernquist 2005; Robertson et al. 2006; Hopkins et al. 2009) , but only for particular orientation of the angular momentum vectors.”
CAN MINOR MERGING ACCOUNT FOR THE SIZE GROWTH OF QUIESCENT GALAXIES? NEW RESULTS FROM THE CANDELS SURVEY (2011, background): “Simple virial arguments based on energy conservation show that the growth efficiency can be higher for more minor mergers (Hopkins et al. 2009; Bezanson et al. 2009; Naab et al. 2009).”
Dark halo response and the stellar initial mass function in early-type and late-type galaxies (2010, background): “This basic trend can be understood as a consequence of the mass dependence of the frequency of major mergers and the mass dependence of gas fractions of the progenitor galaxies (Maller 2008; Hopkins et al. 2009a,b).”
The Morphologies of Massive Galaxies at 1<z<3 in the CANDELS-UDS Field: Compact Bulges, and the Rise and Fall of Massive Disks (2012, background): “Various physical mechanisms have been suggested as the primary drivers of this process, including major or minor mergers (Khochfar & Silk 2006; Naab et al. 2007; Hopkins et al. 2009; Shankar et al. 2011) or AGN feedback (Fan et al. 2008, 2010).”
How do massive black holes get their gas (2009, methodology): “This process has been described in detail in e.g. Hopkins et al. (2009c) and references therein, but for completeness we briefly summarize the important physics.”
MERGERS AND BULGE FORMATION IN ΛCDM: WHICH MERGERS MATTER? (2009, methodology): “A much more detailed investigation of e.g. differences in predicted merger rates between semi-empirical models, semi-analytic models, and simulations will be the subject of a companion paper (Hopkins et al. 2009f).”
Simulations of binary galaxy mergers and the link with fast rotators, slow rotators, and kinematically distinct cores (2011, background): “…distinct cores (KDC) (e.g. Hernquist & Barnes 1991; Jesseit et al. 2009; Bois et al. 2010; Hoffman et al. 2010), the presence of gaseous discs (e.g. Hopkins et al. 2009; Martig et al. 2009), the relation between nuclear black holes and the dynamics of the stars (e.g. Springel, Di Matteo &…”
Dusty Star Forming Galaxies at High Redshift (2014, background): “For example, numerical simulations by Springel and Hernquist (2005) and Hopkins et al. (2009) have shown that very gas-rich mergers at high-redshift can quickly re-form a gaseous disk soon after the merger, due to the dissipational nature of gas.”
The role of dwarf galaxy interactions in shaping the Magellanic System and implications for Magellanic Irregulars (2012, background): “Indeed, Hopkins et al. (2008, 2009) showed that discs can survive even a 1:1 mass ratio major merger.”
BULGE n AND B/T IN HIGH-MASS GALAXIES: CONSTRAINTS ON THE ORIGIN OF BULGES IN HIERARCHICAL MODELS (2008, methodology): “Due to the reduced stellar mass that ends up in the bulge after a major merger, the Hopkins et al. (2009) models tend to yield lower B/T after such a merger.”
Star formation in semi-analytic galaxy formation models with multiphase gas (2015, methodology): “The treatment of mergerdriven bursts is based on the results of hydrodynamic simulations of binary galaxy mergers (Robertson et al. 2006; Hopkins et al. 2009a).”