How do minor mergers promote inside-out growth of ellipticals, transforming the size, density profile and dark matter fraction?

By M. Hilz, T. Naab, Jeremiah P. Ostriker. 2012

208 citations recorded by the source corpus.

There is observational evidence for inside-out growth of giant elliptical galaxies since z & 2 3, which is - in contrast to disk galaxies - not driven by in-situ star formation. Many of the � 10 11 M⊙ systems at high redshift have small sizes � 1kpc and surface brightness profiles with low Sersic indices n. The most likely descendants at z = 0 have, on average, grown by a factor of two in mass and a factor of four in size, indicating r / Mwith α & 2. They also have surface brightness profiles with n & 5. This evolution can be qualitatively explained on the basis of two assumptions: compact ellipticals predominantly grow by collisionless minor (mass-ratio 1:10) or intermediate (mass-ratio 1:5) 'dry' mergers, and they are embedded in massive dark matter halos which support the stripping of merging satellite stars at large radii. We draw these conclusions from idealized collisionless mergers spheroidal galaxies - with and without dark matter - with mass ratios of 1:1, 1:5, and 1:10. The sizes evolve as r / Mwith α 8 (n � 5.5 without dark matter), reaching a final value of n = 9.5 after doubling the stellar mass. This is accompanied by a significant increase of the dark matter fraction (from � 40 per cent to & 70 per cent) within the stellar half- mass radius, driven by the strong size increase probing larger, dark matter dominated regions. For equal-mass mergers the effect is much weaker. We conclude that only a few intermediate mass-ratio mergers (� 3 5 with initial mass-ratios of 1:5) of galaxies embedded in massive dark matter halos can result in the observed concurrent inside-out growth and the rapid evolution in profile shapes. This process might explain the existence of present day giant ellipticals with sizes, r > 4kpc, high Sersic indices, n > 5, and a significant amount of dark matter within the half-light radius. Apart from negative stellar metallicity gradients such a 'minor' merger scenario also predicts significantly lower dark matter fractions for z � 2 compact quiescent galaxies and their rare present day analogues.

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

View Paper: https://www.semanticscholar.org/paper/61fc8ce49eeaa4bf91dd604440f597715989b66e

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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, result): “This seems consistent with the shape of the photometric profiles of the early ETGs (Bezanson et al. 2009; Hopkins et al. 2009a; van Dokkum et al. 2010; Hilz et al. 2013).”
Selection bias in dynamically measured supermassive black hole samples: its consequences and the quest for the most fundamental relation (2016, background): “…depleted core, along with their possible correlations with the mass of the central black hole, are also believed to contain information about the types of mergers responsible for shaping the host spheroids (e.g., Aguerri et al. 2001; Merritt 2006; Hilz et al. 2013; Graham & Scott 2013, 2015).”
EMERGE – an empirical model for the formation of galaxies since z ∼ 10 (2017, background): “It is well established that the mass from accreted satellites is mainly located at the outskirts of massive galaxies (Oser et al. 2012; Hilz et al. 2013).”
The first and second data releases of the Kilo-Degree Survey (2015, background): “The relationship between Re or Sérsic index and mass, luminosity and its evolution with redshift has been demonstrated to be a fundamental probe of galaxy evolution and the role of mass accretion in galaxy merging (Trujillo et al. 2007; Hilz et al. 2013; Tortora et al. 2014).”
THE ASSEMBLY OF MILKY-WAY-LIKE GALAXIES SINCE z ∼ 2.5 (2013, background): “0±0.1 (van Dokkum et al. 2010; Hilz et al. 2013; Patel et al. 2013), whereas those of galaxies with the mass of the Milky Way grow asre ∝ M0.”
The size evolution of star-forming and quenched galaxies in the IllustrisTNG simulation (2017, unknown): “This is roughly consistent with r ∝M2 as expected from growth by minor dry mergers (Naab et al. 2009; van Dokkum et al. 2010; Hilz et al. 2013).”
FORMING COMPACT MASSIVE GALAXIES (2015, background): “The likely explanation is that the compact galaxies accrete extended envelopes through merging fromz∼ 1.5 to the present day (e.g., Bezanson et al. 2009; Naab et al. 2009; van Dokkum et al. 2010, 2014; Newman et al. 2012; Hilz et al. 2013).”