The effects of gas on morphological transformation in mergers: implications for bulge and disc demographics

By P. Hopkins, R. Somerville, Thomas J. Cox, L. Hernquist, S. Jogee, D. Keres̆, Chung-Pei Ma, Brant Robertson, Kyle R. Stewart. 2009

176 citations recorded by the source corpus.

Transformation of discs into spheroids via mergers is a well-accepted element of galaxy formation models. However, recent simulations have shown that the bulge formation is suppressed in increasingly gas-rich mergers. We investigate the global implications of these results in a cosmological framework, using independent approaches: empirical halo-occupation models (where galaxies are populated in haloes according to observations) and semi-analytic models. In both, ignoring the effects of gas in mergers leads to the overproduction of spheroids:low- and intermediate-mass galaxies are predicted to be bulge-dominated (B/T ∼ 0.5 at < 10 10 M ⊙ , with almost no 'bulgeless' systems), even if they have avoided major mergers. Including the different physical behaviour of gas in mergers immediately leads to a dramatic change: bulge formation is suppressed in low-mass galaxies, observed to be gas-rich (giving B/T ∼ 0.1 at < 10 10 M ⊙ , with a number of bulgeless galaxies in good agreement with observations). Simulations and analytic models which neglect the similarity-breaking behaviour of gas have difficulty reproducing the strong observed morphology-mass relation. However, the observed dependence of gas fractions on mass, combined with suppression of bulge formation in gas-rich mergers, naturally leads to the observed trends. Discrepancies between observations and models that ignore the role of gas increase with redshift; in models that treat gas properly, galaxies are predicted to be less bulge-dominated at high redshifts, in agreement with the observations. We discuss implications for the global bulge mass density and future observational tests.

Subfields: Astronomy and Astrophysics, Computer Vision and Pattern Recognition, Instrumentation

View Paper: https://www.semanticscholar.org/paper/2abd7322179f912900e74c24a98cb0bb1ab83fdd

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Citations in the corpus

MERGERS IN ΛCDM: UNCERTAINTIES IN THEORETICAL PREDICTIONS AND INTERPRETATIONS OF THE MERGER RATE (2010, methodology): “In Hopkins et al. (2009a), these simulations are used to show that, to lowest order, the efficiency of gas angular momentum loss scales with a factor∝ (1− fgas) – i.e. in a merger with 50% gas, the ultimate gas angular momentum loss per unit mass is (on average) half as efficient as in a merger…”
Galaxy Zoo: Passive Red Spirals . (2009, background): “Most major mergers destroy spiral structure (Toomre & Toomre 1972) unless they involve very gas rich progenitors (Hopkins et al. 2009), but some interactions can be quite gentle (e.g. Walker, Mihos & Hernquist 1996), for example minor-mergers, tidal interactions etc. (2) Interaction with the…”
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).”
A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, background): “…properties of metallicities in mergers (Cox et al. 2006a; Torrey et al. 2011), the broad-band colours of post-starburst galaxies (Snyder et al. 2011), and the structural properties of merging and elliptical galaxies (Hopkins et al. 2008c,d; Hopkins, Cox & Hernquist 2008a; 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.”
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.”
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).”
WHAT DOES A SUBMILLIMETER GALAXY SELECTION ACTUALLY SELECT? THE DEPENDENCE OF SUBMILLIMETER FLUX DENSITY ON STAR FORMATION RATE AND DUST MASS (2010, methodology): “…equal to the sum of the isolated values for the two progenitor disks, because the disks are too gas-rich at first passage (t ∼ 0.1 Gyr) for tidal torques to cause a strong starburst, as a significant stellar bar is required for the gas to efficiently loose angular momentum (Hopkins et al. 2009b).”
VERY HIGH GAS FRACTIONS AND EXTENDED GAS RESERVOIRS IN z = 1.5 DISK GALAXIES (2009, background): “Numerical models predict that, under some physical assumptions, merger remnants can contain extended gas disks (e.g. Hopkins et al. 2009).”
Compact high-redshift galaxies are the cores of the most massive present-day spheroids (2009, background): “This is a relatively efficient channel for size–mass evolution, yielding a factor of several size evolution with only a factor of 1.5–2 increase in stellar mass (Hopkins et al. 2009d).”
THE RISE AND FALL OF PASSIVE DISK GALAXIES: MORPHOLOGICAL EVOLUTION ALONG THE RED SEQUENCE REVEALED BY COSMOS (2009, methodology): “Given observations of fgas as a function of M∗, mergers with M∗ ≈ 1010M are much more likely to result in a remnant with a disk-like morphology than similar mergers at M∗ ≈ 1011M (see Hopkins et al. 2009a).”
Hierarchical formation of bulgeless galaxies: why outflows have low angular momentum (2010, background): “Secondly, in our simulation as in Hopkins et al. (2009), a bar does not form in gas dominated mergers, limiting the amount of gas driven toward the centre.”
Physical Models of Galaxy Formation in a Cosmological Framework (2014, background): “Similarly, the observed tilt of the Fundamental Plane can be explained by the expected trends in galaxy gas content with mass and redshift, and the physics of gas dissipation in mergers (Covington et al. 2011, Hopkins et al. 2009b, Porter et al. 2014).”
HYDRODYNAMICS OF HIGH-REDSHIFT GALAXY COLLISIONS: FROM GAS-RICH DISKS TO DISPERSION-DOMINATED MERGERS AND COMPACT SPHEROIDS (2010, background): “Some orbital parameters not modeled here might also be more favorable to disk survival (e.g. Hopkins et al. 2009).”
BULGE GROWTH AND QUENCHING SINCE z = 2.5 IN CANDELS/3D-HST (2014, methodology): “The starburst, black hole accretion and morphological transformation induced by mergers depends on the mass ratio and gas fraction, as calibrated using a large suite of binary merger simulations (Somerville et al. 2008; Hopkins et al. 2009; Somerville et al. 2012).”
Star formation in galaxy mergers with realistic models of stellar feedback and the interstellar medium (2012, methodology): “Hopkins et al. (2009b) showed that in those models, this is governed by global gravitational torques being inefficient at removing angular momentum from gas at large radii in discs, and so is independent of the details of the EOS implementation or introduction of an ad hoc stellar wind model.”
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).”
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.”
An analytic model of angular momentum transport by gravitational torques: from galaxies to massive black holes (2010, background): “We find reasonable consistency with our simulation results and the profiles of observed cusp ellipticals by taking ηb ≈ ηd ≈ 1/2 over this radial range (see e.g. Lauer et al. 2007; Hopkins et al. 2009a).”