MERGERS IN ΛCDM: UNCERTAINTIES IN THEORETICAL PREDICTIONS AND INTERPRETATIONS OF THE MERGER RATE

By P. Hopkins, Darren Croton, K. Bundy, S. Khochfar, Frank C. van den Bosch, R. Somerville, Andrew Wetzel, D. Keres̆, L. Hernquist, Kyle R. Stewart and 3 more. 2010

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Different theoretical methodologies lead to order-of-magnitude variations in predicted galaxy–galaxy merger rates. We examine how this arises and quantify the dominant uncertainties. Modeling of dark matter and galaxy inspiral/merger times contribute factor of ∼2 uncertainties. Different estimates of the halo–halo merger rate, the subhalo “destruction” rate, and the halo merger rate with some dynamical friction time delay for galaxy–galaxy mergers, agree to within this factor of ∼2, provided proper care is taken to define mergers consistently. There are some caveats: if halo/subhalo masses are not appropriately defined the major-merger rate can be dramatically suppressed, and in models with “orphan” galaxies and under-resolved subhalos the merger timescale can be severely over-estimated. The dominant differences in galaxy–galaxy merger rates between models owe to the treatment of the baryonic physics. Cosmological hydrodynamic simulations without strong feedback and some older semi-analytic models (SAMs), with known discrepancies in mass functions, can be biased by large factors (∼5) in predicted merger rates. However, provided that models yield a reasonable match to the total galaxy mass function, the differences in properties of central galaxies are sufficiently small to alone contribute small (factor of ∼1.5) additional systematics to merger rate predictions. But variations in the baryonic physics of satellite galaxies in models can also have a dramatic effect on merger rates. The well-known problem of satellite “over-quenching” in most current SAMs—whereby SAM satellite populations are too efficiently stripped of their gas—could lead to order-of-magnitude under-estimates of merger rates for low-mass, gas-rich galaxies. Models in which the masses of satellites are fixed by observations (or SAMs adjusted to resolve this “over-quenching”) tend to predict higher merger rates, but with factor of ∼2 uncertainties stemming from the uncertainty in those observations. The choice of mass used to define “major” and “minor” mergers also matters: stellar–stellar major mergers can be more or less abundant than halo–halo major mergers by an order of magnitude. At low masses, most true major mergers (mass ratio defined in terms of their baryonic or dynamical mass) will appear to be minor mergers in their stellar mass ratio—observations and models using just stellar criteria could underestimate major-merger rates by factors of ∼3–5. We discuss the uncertainties in relating any merger rate to spheroid formation (in observations or theory): in order to achieve better than factor of ∼3 accuracy, it is necessary to account for the distribution of merger orbital parameters, gas fractions, and the full efficiency of merger-induced effects as a function of mass ratio.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/918920c378d50104180681155a4e47522d924858

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Citations in the corpus

Galaxy and mass assembly (GAMA): End of survey report and data release 2 (2015, methodology): “Although galaxy merger rate predictions are less accurate (e.g. Hopkins et al. 2010), limiting the testability of the CDM structure formation model by merger rate observations, GAMA merger rate measurements will also be used to constrain the extent to which mergers are driving various aspects of…”
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 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.”
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).”
The clustering of intermediate-redshift quasars as measured by the Baryon Oscillation Spectroscopic Survey (2012, background): “…interval is only weakly dependent on the mass of the halo (Lacey & Cole 1993; Percival et al. 2003; Cohn & White 2005; Wetzel, Cohn & White 2009; Fakhouri & Ma 2009; Hopkins et al. 2010), i.e. the mass function of such halos is almost proportional to the mass function of the parent population.”
The role of dwarf galaxy interactions in shaping the Magellanic System and implications for Magellanic Irregulars (2012, background): “From Hopkins et al. (2010), the expected galaxy major merger (mass ratio > 1/3) rate is relatively flat as a function of host galaxy mass at z = 0, so it is not expected that such encounters would be more likely for low-mass systems.”
The baryonic assembly of dark matter haloes (2011, background): “…al. 2006; Somerville et al. 2008; Dekel C© 2011 The Authors Monthly Notices of the Royal Astronomical Society C© 2011 RASDownloaded from https://academic.oup.com/mnras/article-abstract/417/4/2982/1100091 by guest on 29 July 2018 et al. 2009a; Bouché et al. 2010; Benson 2010; Hopkins et al. 2010).”
SIZES AND SURFACE BRIGHTNESS PROFILES OF QUIESCENT GALAXIES AT z ∼ 2 (2011, background): “Most studies seem to point towards gas-poor galaxy merging as the dominant growth process (e.g., Bezanson et al. 2009; Naab et al. 2009; Hopkins et al. 2010); however, it is unclear whether this can account for all the observed size growth.”
Constraining the galaxy–halo connection over the last 13.3 Gyr: star formation histories, galaxy mergers and structural properties (2017, result): “This question has been studied in a number of previous works (Zheng, Coil & Zehavi 2007; Stewart et al. 2009; Hopkins et al. 2010a,b; Zavala et al. 2012; AvilaReese, Zavala & Lacerna 2014; Rodriguez-Gomez et al. 2015, 2016).”
MAJOR GALAXY MERGERS ONLY TRIGGER THE MOST LUMINOUS ACTIVE GALACTIC NUCLEI (2012, background): “Specifically, it was observed that the merger rate increases not only with redshift as reported above but also with galaxy mass (e.g., Hopkins et al. 2010).”
The Evolution of Galaxy Structure Over Cosmic Time (2014, background): “While the merger fractions from Hopkins et al. (2010) are higher than those from the CDM models, they are still lower than the observations (see also Jogee et al. 2009 and Lotz et al. 2011 for further discussions).”
Massive Black Hole Binary Mergers in Dynamical Galactic Environments (2016, background): “Higher-order, less observationally constrained parameters can have systemic biases however, for example galaxy merger rates (see, e.g. Hopkins et al. 2010)—which are obviously critical to understanding MBHB evolution.”
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): “…case, size growth driven primarily by major mergers would require many more major mergers since z 2 than appears plausible from N-body simulations (which suggest <2 per massive galaxy by the present day; e.g. Hopkins et al. 2010b), or indeed from observed merger rates (e.g. Robaina et al. 2010).”
The sizes, masses and specific star formation rates of massive galaxies at 1.3 < z < 1.5: strong evidence in favour of evolution via minor mergers (2012, background): “This level of major merging is 1102 R. J. McLure et al. completely inconsistent with N-body simulations, which suggest that the typical number of major mergers experienced in the redshift interval 0 < z < 1.5 by galaxies with stellar masses M ≥ 1011 M is 1 (e.g. Hopkins et al. 2010).”
Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution, and implications for the IMF (2012, methodology): “Furthermore, in our model, merger-induced starbursts account for 5 per cent of the SFR density of the universe at all redshifts (Hopkins et al. 2010c); this is consistent with the starburst-mode contribution inferred from the stellar densities, surface brightness profiles, kinematics and stellar…”
Galaxy formation theory (2010, methodology): “…utilizing N-body simulations have lead to the development of an empirical understanding of halo formation histories (Wechsler et al., 2002; van den Bosch, 2002) and halo–halo merger rates (Fakhouri and Ma, 2008a, 2009; Stewart et al., 2009; Fakhouri et al., 2010; Hopkins et al., 2010).”