Discriminating Between the Physical Processes that Drive Spheroid Size Evolution

By P. Hopkins, K. Bundy, L. Hernquist, S. Wuyts, Thomas J. Cox Berkeley, Cfa. 2009

188 citations recorded by the source corpus.

Observations have shown that massive galaxies at high redshift have much smaller effective radii than galaxies of similar mass today; however, recent work has shown that they have similar central densities. The primary growth of size, therefore, relates to the apparent relative abundance of low-density material at low redshifts. But various models have been proposed to accomplish this, and the exact contribution of these mechanisms, relative to others that would, for example, lower the density of the system uniformly, or relate to possible observational misestimates of the stellar mass distribution, remain uncertain, as does the degree to which this evolution is driven by processes of initial spheroid formation versus subsequent ‘dry’ assembly of spheroids. These different possibilities also yield dramatically different constraints on any possible evolution in the MBH–σ relation. Here, we compile observations of spheroid properties as a function of redshift and use them to test the different proposed models, each of which we have calibrated and studied in a suite of high-resolution hydrodynamic simulations. We show that the evolution in progenitor disc gas fractions with redshift gives rise to the initial formation of smaller spheroids at high redshift. We then consider how these early-forming systems must evolve to be consistent with the larger sizes of old spheroids today. We consider (1) equal-density ‘dry’ mergers, (2) later major or minor ‘dry’ mergers with less dense galaxies, (3) adiabatic expansion, after significant gas mass loss, (4) gradients in stellar mass-to-light ratios from young nuclear stellar populations (yielding smaller Re at early times, which vanish as the system fades), (5) biases in the stellar mass estimation of high-redshift (young) systems (from e.g. uncertain asymptotic giant branch starlight contributions) and (6) observational effects (possible biases in fitting or missed light from surface brightness dimming, or the effects of different definitions of effective radii). In principle, any of these models could be tuned to explain any observed effective radius evolution. However, the predicted evolution in velocity dispersions, central stellar mass surface densities and profile shape are very distinct. Comparing with observations, only model (2), later or minor ‘dry’ mergers with less dense systems, is consistent with the constraints as an explanation of the entire effect. Moreover, it is the only model which allows for any evolution in MBH–σ towards more massive black holes (BHs) at high redshift. Still, the amount of merging needed for this to explain the observed factor of ∼6 size evolution is larger than that predicted by hierarchical growth and clustering constraints. We, therefore, consider a cosmologically motivated model with high-resolution simulations, in which the initial galaxy forms in a gas rich merger and is observed at an appropriate age under representative conditions, then evolves undergoing a ‘typical’ level of dry merging and mass loss. We show that this case is consistent with all the observational constraints without tension with cosmological expectations. Effect (2), which builds up an extended, low-density envelope, dominates the evolution (giving factors ∼2–3 size evolution), but effects (1), (3), (4) and (possibly) (6) each contribute an additional ∼20 per cent size evolution (net factor of ∼2), together bringing the natural cosmological predictions into good agreement with the combination of observational constraints. We discuss implications for the evolution in correlations between BH and host bulge properties and show that this naturally predicts some evolution similar to that observed; better observations of BH masses could also constrain host galaxy merger histories.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/c90c8b087eecbd9aead39d7b3dbbdda2af6a8c0b

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Citations in the corpus

The ATLAS3D project – XX. Mass–size and mass–σ distributions of early-type galaxies: bulge fraction drives kinematics, mass-to-light ratio, molecular gas fraction and stellar initial mass function (2012, background): “A way to reconcile this very mild (or lack of) σ evolution is by assuming that the compact primordial ETGs grow mostly by accretion of small satellites in their outer haloes, while preserving the central structure (Naab et al. 2009; Hopkins et al. 2010; Oser et al. 2010, 2012).”
NEWLY QUENCHED GALAXIES AS THE CAUSE FOR THE APPARENT EVOLUTION IN AVERAGE SIZE OF THE POPULATION (2013, background): “…Q-ETGs maintain a nearly constant mass within their innermost few kpcs, and gradually grow inside-out, building up extended stellar envelopes/halos around such compact, dense cores (e.g., Cimatti et al. 2008; Hopkins et al. 2009, 2010; Taylor et al. 2010; Feldmann et al. 2010; Szomoru et al. 2011).”
A REVISED PARALLEL-SEQUENCE MORPHOLOGICAL CLASSIFICATION OF GALAXIES: STRUCTURE AND FORMATION OF S0 AND SPHEROIDAL GALAXIES (2011, background): “The extra light in these galaxies that converts n ≃ 3 ± 1 into n ≫ 4 may be the debris accumulated in many minor mergers (e. g., Naab et al. 2009; Hopkins et al. 2010; van Dokkum et al. 2010; Oser et al. 2010, 2011).”
The DEEP3 Galaxy Redshift Survey: the impact of environment on the size evolution of massive early-type galaxies at intermediate redshift (2011, background): “Furthermore, the mass loss needed to produce a factor of 2 increase in size is of order 30–50 per cent (Zhao 2002; Hopkins et al. 2010b), beyond the expected impact of feedback from lower luminosity AGN or evolved stars (e.g. Damjanov et al. 2009).”
The size evolution of star-forming and quenched galaxies in the IllustrisTNG simulation (2017, background): “…it has been argued that individual quenched galaxies must undergo significant size growth (Faisst et al. 2017), and various physical mechanisms have been proposed as the drivers of this evolution, with a particular emphasis on the role of dry minor mergers (Naab et al. 2009; Hopkins et al. 2010).”
The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted stars (2015, methodology): “…since many of them are believed to eventually become the ‘cores’ of large, massive ellipticals in the local Universe (e.g. Naab, Johansson & Ostriker 2009; Hopkins et al. 2010b; Feldmann et al. 2010; van Dokkum et al. 2010; Cimatti, Nipoti & Cassata 2012; Oser et al. 2012; Wellons et al. 2016).”
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 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 ATLAS3D project XV: benchmark for early-type galaxies scaling relations from 260 dynamical models: mass-to-light ratio, dark matter, fundamental plane and mass plane (2012, background): “Hopkins et al. (2010) pointed out the usefulness of adopting as size parameter the major axis Rmaje of the half-light isophote instead of the circularized radius Re, when analysing results of simulations.”
THE TWO PHASES OF GALAXY FORMATION (2010, background): “This distinction is useful, e.g. for understanding the size evolution of massive galaxies (Khochfar & Silk 2006a; Naab et al. 2007, 2009; Bezanson et al. 2009; Nipoti et al. 2009a; Hopkins et al. 2010; Feldmann et al. 2010).”
How do minor mergers promote inside-out growth of ellipticals, transforming the size, density profile and dark matter fraction? (2012, background): “…major merg- ers alone cannot explain the observed size growth but minor mergers with mass-ratios of 1:5 or 1:10 show a significantly stronger size growth per added stellar mass (Khochfar & Silk 2006; Naab et al. 2009; Nipoti et al. 009a; Hopkins et al. 2010; Hilz et al. 2012; Oser et al. 2012).”
The ATLAS3D Project – XXX. Star formation histories and stellar population scaling relations of early-type galaxies (2015, methodology): “We use the robust size parameter Rmaje , corresponding to the major axis of the half-light isophote (Hopkins et al. 2010) as derived in Cappellari et al. (2013a), and taken from table 1 there.”
The formation of massive, compact galaxies at z = 2 in the Illustris simulation (2014, background): “…of galaxies and puff up the existing system are one way to achieve this physical size growth (Naab et al. 2007; Naab, Johansson & Ostriker 2009; Hopkins et al. 2009b, 2010; Bezanson et al. 2009; Oser et al. 2010, 2012; Johansson, Naab & Ostriker 2012; Hilz, Naab & Ostriker 2013; Feldmann &…”
FORMING COMPACT MASSIVE GALAXIES (2015, background): “…dominated by minor mergers: such mergers are expected, and other mechanisms cannot easily produce the observedṙe/Ṁstars≈ 2 scaling between size growth and mass growth (Bezanson et al. 2009; Naab, Johansson, & Ostriker 2009; Hopkins et al. 2010; Trujillo et al. 2011; Hilz, Naab, & Ostriker 2013).”
THE COSMOLOGICAL SIZE AND VELOCITY DISPERSION EVOLUTION OF MASSIVE EARLY-TYPE GALAXIES (2011, background): “This process seems to be disfavored by observations (e.g. Hopkins et al. 2010b; Trujillo et al. 2011) and the absence of a significant young stellar population would indicate that such hypothesized secular processes would need to occur without significant star formation.”
Physical Models of Galaxy Formation in a Cosmological Framework (2014, background): “…works have pointed out that accounting for the effects of dissipation in gas-rich mergers, can lead to important changes in the scaling relations (Covington et al. 2008, Covington et al. 2011, Hopkins et al. 2010, Hopkins et al. 2009b, Porter et al. 2014, Shankar et al. 2010, Shankar et al. 2013).”
Size evolution of normal and compact galaxies in the EAGLE simulation (2015, background): “…in particular, may result in significant size growth even if relatively little mass is added to the system (e.g. Naab, Johansson & Ostriker 2009; Hopkins et al. 2010; Oser et al. 2012; Hilz, Naab & C© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical…”
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).”