arXiv subjects

Thorsten Naab

Publications and source records attributed to Thorsten Naab.

At least 181 records · Page 10Linked to original sources

arXiv2013

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

We study the volume-limited and nearly mass selected (stellar mass M>6*10^9 Msun) Atlas3D sample of 260 early-type galaxies. We construct detailed axisymmetric dynamical models (JAM), which allow for orbital anisotropy, include a dark matter halo, and reproduce in detail both the galaxy images and the high-quality integral-field stellar kinematics. We derive accurate total M/L and dark matter fractions f_DM, within a sphere of radius r=Re. We also measure the stellar M/L and derive a median dark matter fraction f_DM=13%. We find that the thin two-dimensional subset spanned by galaxies in the (M_JAM,sigma_e,R_e) coordinates system, which we call the Mass Plane (MP) has an observed rms scatter of 19% and an intrinsic one of 11%. The MP satisfies the scalar virial relation M_JAM sigma_e^2 R_e within our tight errors. However, the details of how both Re and sigma_e are determined are critical in defining the precise deviation from the virial exponents. We revisit the (M/L)-sigma_e relation, which describes most of the deviations between the MP and the FP. The best-fitting relation is (M/L) sigma_e^0.72 (r-band). It provides an upper limit to any systematic increase of the IMF mass normalization with sigma_e. We study of the link between sigma_e and the galaxies circular velocity V_circ within 1Re (where stars dominate) and find the relation max(V_circ)~1.76*sigma_e, which has an observed scatter of 7%. The accurate parameters described in this paper are used in the companion Paper XX of this series to explore the variation of global galaxy properties, including the IMF, on the projections of the MP. [Abridged]

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arXiv2013

The Dark Halo - Spheroid Conspiracy and the Origin of Elliptical Galaxies

Dynamical modeling and strong lensing data indicate that the total density profiles of early-type galaxies are close to isothermal, i.e., rho_tot ~ r^gamma with gamma approx -2. To understand the origin of this universal slope we study a set of simulated spheroids formed in isolated binary mergers as well as the formation within the cosmological framework. The total stellar plus dark matter density profiles can always be described by a power law with an index of gamma approx -2.1 with a tendency toward steeper slopes for more compact, lower-mass ellipticals. In the binary mergers the amount of gas involved in the merger determines the precise steepness of the slope. This agrees with results from the cosmological simulations where ellipticals with steeper slopes have a higher fraction of stars formed in situ. Each gas-poor merger event evolves the slope toward gamma ~ -2, once this slope is reached further merger events do not change it anymore. All our ellipticals have flat intrinsic combined stellar and dark matter velocity dispersion profiles. We conclude that flat velocity dispersion profiles and total density distributions with a slope of gamma ~ -2 for the combined system of stars and dark matter act as a natural attractor. The variety of complex formation histories as present in cosmological simulations, including major as well as minor merger events, is essential to generate the full range of observed density slopes seen for present-day elliptical galaxies.

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arXiv2013

The Growth in Size and Mass of Cluster Galaxies since z=2

We study the formation and evolution of Brightest Cluster Galaxies starting from a $z=2$ population of quiescent ellipticals and following them to $z=0$. To this end, we use a suite of nine high-resolution dark matter-only simulations of galaxy clusters in a $Λ$CDM universe. We develop a scheme in which simulation particles are weighted to generate realistic and dynamically stable stellar density profiles at $z=2$. Our initial conditions assign a stellar mass to every identified dark halo as expected from abundance matching; assuming there exists a one-to-one relation between the visible properties of galaxies and their host haloes. We set the sizes of the luminous components according to the observed relations for $z\sim2$ massive quiescent galaxies. We study the evolution of the mass-size relation, the fate of satellite galaxies and the mass aggregation of the cluster central. From $z=2$, these galaxies grow on average in size by a factor 5 to 10 of and in mass by 2 to 3. The stellar mass growth rate of the simulated BCGs in our sample is of 1.9 in the range $0.3<z<1.0$ consistent with observations, and of 1.5 in the range $0.0<z<0.3$. Furthermore the satellite galaxies evolve to the present day mass-size relation by $z=0$. Assuming passively evolving stellar populations, we present surface brightness profiles for our cluster centrals which resemble those observed for the cDs in similar mass clusters both at $z=0$ and at $z=1$. This demonstrates that the $Λ$CDM cosmology does indeed predict minor and major mergers to occur in galaxy clusters with the frequency and mass ratio distribution required to explain the observed growth in size of passive galaxies since $z=2$. Our experiment shows that Brightest Cluster Galaxies can form through dissipationless mergers of quiescent massive $z=2$ galaxies, without substantial additional star formation.

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arXiv2013

The Atlas3D project - XIX. The hot-gas content of early-type galaxies: fast versus slow rotators

For early-type galaxies, the ability to sustain a corona of hot, X-ray emitting gas could have played a key role in quenching their star-formation history. Yet, it is still unclear what drives the precise amount of hot gas around these galaxies. By combining photometric and spectroscopic measurements for the early-type galaxies observed during the Atlas3D integral-field survey with measurements of their X-ray luminosity based on X-ray data of both low and high spatial resolution we conclude that the hot-gas content of early-type galaxies can depend on their dynamical structure. Specifically, whereas slow rotators generally have X-ray halos with luminosity L_X,gas and temperature T values that are in line with what is expected if the hot-gas emission is sustained by the thermalisaton of the kinetic energy carried by the stellar-mass loss material, fast rotators tend to display L_X,gas values that fall consistently below the prediction of this model, with similar T values that do not scale with the stellar kinetic energy as observed in the case of slow rotators. Considering that fast rotators are likely to be intrinsically flatter than slow rotators, and that the few L_X,gas-deficient slow rotators also happen to be relatively flat, the observed L_X,gas deficiency in these objects would support the hypothesis whereby flatter galaxies have a harder time in retaining their hot gas. We discuss the implications that a different hot-gas content could have on the fate of both acquired and internally-produced gaseous material, considering in particular how the L_X,gas deficiency of fast rotators would make them more capable to recycle the stellar-mass loss material into new stars than slow rotators. This is consistent with the finding that molecular gas and young stars are detected only in fast rotators in the Atlas3D sample, and that fast rotators tend to dustier than slow rotators. [Abridged]

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arXiv2012

The ATLAS3D project - XXII. Low-efficiency star formation in early-type galaxies: hydrodynamic models and observations

We study the global efficiency of star formation in high resolution hydrodynamical simulations of gas discs embedded in isolated early-type and spiral galaxies. Despite using a universal local law to form stars in the simulations, we find that the early-type galaxies are offset from the spirals on the large-scale Kennicutt relation, and form stars 2 to 5 times less efficiently. This offset is in agreement with previous results on morphological quenching: gas discs are more stable against star formation when embedded in early-type galaxies due to the lower disc self-gravity and increased shear. As a result, these gas discs do not fragment into dense clumps and do not reach as high densities as in the spiral galaxies. Even if some molecular gas is present, the fraction of very dense gas (above 10^4 cm-3) is significantly reduced, which explains the overall lower star formation efficiency. We also analyse a sample of local early-type and spiral galaxies, measuring their CO and HI surface densities and their star formation rates as determined by their non-stellar 8um emission. As predicted by the simulations, we find that the early-type galaxies are offset from the Kennicutt relation compared to the spirals, with a twice lower efficiency. Finally, we validate our approach by performing a direct comparison between models and observations. We run a simulation designed to mimic the stellar and gaseous properties of NGC524, a lenticular galaxy, and find a gas disc structure and global star formation rate in good agreement with the observations. Morphological quenching thus seems to be a robust mechanism, and is also consistent with other observations of a reduced star formation efficiency in early-type galaxies in the COLD GASS survey. This lower efficiency of star formation is not enough to explain the formation of the whole Red Sequence, but can contribute to the reddening of some galaxies.

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arXiv2012

The ATLAS 3D project - XVI. Physical parameters and spectral line energy distributions of the molecular gas in gas-rich early-type galaxies

[Abridged] We present a detailed study of the physical properties of the molecular gas in a sample of 18 molecular gas-rich early-type galaxies (ETGs) from the ATLAS$ 3D sample. Our goal is to better understand the star formation processes occurring in those galaxies, starting here with the dense star-forming gas. We use existing integrated $^{12}$CO(1-0, 2-1), $^{13}$CO(1-0, 2-1), HCN(1-0) and HCO$^{+}$(1-0) observations and present new $^{12}$CO(3-2) single-dish data. From these, we derive for the first time the average kinetic temperature, H$_{2}$ volume density and column density of the emitting gas, this using a non-LTE theoretical model. Since the CO lines trace different physical conditions than of those the HCN and HCO$^{+}$ lines, the two sets of lines are treated separately. We also compare for the first time the predicted CO spectral line energy distributions (SLEDs) and gas properties of our molecular gas-rich ETGs with those of a sample of nearby well-studied disc galaxies. The gas excitation conditions in 13 of our 18 ETGs appear analogous to those in the centre of the Milky Way. Such results have never been obtained before for ETGs and open a new window to explore further star-formation processes in the Universe. The conclusions drawn should nevertheless be considered carefully, as they are based on a limited number of observations and on a simple model. In the near future, with higher CO transition observations, it should be possible to better identify the various gas components present in ETGs, as well as more precisely determine their associated physical conditions. To achieve these goals, we show here from our theoretical study, that mid-J CO lines (such as the $^{12}$CO(6-5) line) are particularly useful.

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arXiv2012

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

There is observational evidence for inside-out growth of elliptical galaxies since $z \gtrsim 2-3$, which is not driven by in-situ star formation. Many systems at high redshift have small sizes $\sim 1kpc$ and surface brightness profiles with low Sersic indices n. The most likely descendants have, on average, grown by a factor of two in mass and a factor of four in size, indicating $r \propto M^α$ with $α\gtrsim 2$. They also have surface brightness profiles with $n \gtrsim 5$. This evolution can be qualitatively explained on the basis of two assumptions: compact ellipticals predominantly grow by collisionless minor or intermediate 'dry' mergers, and they are embedded in massive dark matter halos. 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 $r \propto M^α$ with $α< 2$ for mass-ratios of 1:1. For minor mergers of galaxies embedded in dark matter halos, the sizes grow significantly faster and the profile shapes change more rapidly. Mergers with moderate mass-ratios of 1:5 give $α\sim 2.3$ and a final Sersic index of $n = 9.5$ after doubling the stellar mass. This is accompanied by a significant increase of the dark matter fraction within the stellar half-mass radius, driven by the strong size increase probing larger, dark matter dominated regions. Only a few intermediate mass-ratio mergers of galaxies embedded in massive dark matter halos can result in the observed concurrent inside-out growth and the rapid evolution in profile shapes. Apart from negative stellar metallicity gradients such a 'minor' merger scenario also predicts significantly lower dark matter fractions for $z \sim 2$ compact quiescent galaxies and their rare present day analogues (abbreviated).

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arXiv2012

Modelling the formation of today's massive ellipticals

The discovery of a population of massive, compact and quiescent early-type galaxies has changed the view on plausible formation scenarios for the present day population of elliptical galaxies. Traditionally assumed formation histories dominated by 'single events' like early collapse or major mergers appear to be incomplete and have to be embedded in the context of hierarchical cosmological models with continuous gas accretion and the merging of small stellar systems (minor mergers). Once these processes are consistently taken into account the hierarchical models favor a two-phase assembly process and are in much better shape to capture the observed trends. We review some aspects of recent progress in the field.

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arXiv2012

The SINS/zC-SINF survey of z~2 galaxy kinematics: Outflow properties

Based on SINFONI Ha, [NII] and [SII] AO data of 30 z \sim 2 star-forming galaxies (SFGs) from the SINS and zcSINF surveys, we find a strong correlation of the Ha broad flux fraction with the star formation surface density of the galaxy, with an apparent threshold for strong outflows occurring at 1 Msun yr^-1 kpc^-2. Above this threshold, we find that SFGs with logm_\ast>10 have similar or perhaps greater wind mass loading factors (eta = Mdotout/SFR) and faster outflow velocities than lower mass SFGs. This trend suggests that the majority of outflowing gas at z \sim 2 may derive from high-mass SFGs, and that the z \sim 2 mass-metallicity relation is driven more by dilution of enriched gas in the galaxy gas reservoir than by the efficiency of outflows. The mass loading factor is also correlated with the SFR and inclination, such that more star-forming and face-on galaxies launch more powerful outflows. For galaxies that have evidence for strong outflows, we find that the broad emission is spatially extended to at least the half-light radius (\sim a few kpc). We propose that the observed threshold for strong outflows and the observed mass loading of these winds can be explained by a simple model wherein break-out of winds is governed by pressure balance in the disk. Using the ratio of the [SII] doublet in a broad and narrow component, we find that outflowing gas has a density of \sim10-100 cm^-3, significantly less than that of the star forming gas (600 cm^-3).

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arXiv2012

The ATLAS3D Project - XXI. Correlations between gradients of local escape velocity and stellar populations in early-type galaxies

We explore the connection between the local escape velocity, V_esc, and the stellar population properties in the ATLAS3D survey, a complete, volume-limited sample of nearby early-type galaxies. We make use of ugriz photometry to construct Multi-Gaussian Expansion models of the surface brightnesses of our galaxies. We are able to fit the full range of surface brightness profiles found in our sample, and in addition we reproduce the results of state-of-the-art photometry in the literature with residuals of 0.04 mags. We utilise these photometric models and SAURON integral-field spectroscopy, combined with Jeans dynamical modelling, to determine the local V_esc derived from the surface brightness. We find that the local V_esc is tightly correlated with the Mgb and Fe5015 linestrengths and optical colours, and anti-correlated with the Hbeta linestrength. In the case of the Mgb and Colour - V_esc relations we find that the relation within individual galaxies follows the global relation between different galaxies. We intentionally ignored any uncertain contribution due to dark matter since we are seeking an empirical description of stellar population gradients in early-type galaxies that is ideal for quantitative comparison with model predictions. We identify a population of galaxies that occur only at low V_esc that exhibit negative gradients in the Mgb - and Colour -V_esc relations. These galaxies typically have young central stellar populations and contain significant amounts of molecular gas and dust. Combining these results with N-body simulations of binary mergers we use the Mgb-V_esc relation to constrain the possible number of dry mergers experienced by the local early-type galaxy population - a typical massive ETG can have experienced only ~1.5 major mergers before becoming a significant outlier in the Mgb-V_esc relation. [Abridged]

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arXiv2012

The ATLAS3D project - XVII. Linking photometric and kinematic signatures of stellar discs in early-type galaxies

[Abridged] We analyse the morphological structures in galaxies of the ATLAS3D sample by fitting a single Sersic profile and decomposing all non-barred objects (180 of 260 objects) in two components parameterised by an exponential and a general Sersic function. The aim of this analysis is to look for signatures of discs in light distributions of nearby early-type galaxies and compare them to kinematic properties. Using Sersic index from single component fits for a distinction between slow and fast rotators, or even late- and early-type galaxies, is not recommended. Assuming that objects with n>3 are slow rotators (or ellipticals), there is only a 22 per cent probability to correctly classify objects as slow rotators (or 37 per cent of previously classified as ellipticals). We show that exponential sub-components, as well as light profiles fitted with only a single component of a low Sersic index, can be linked with the kinematic evidence for discs in early-type galaxies. The median disk-to-total light ratio for fast and slow rotators is 0.41 and 0.0, respectively. Similarly, the median Sersic indices of the bulge (general Sersic component) are 1.7 and 4.8 for fast and slow rotators, respectively. Overall, discs or disc-like structures, are present in 83 per cent of early-type galaxies which do not have bars, and they show a full range of disk-to-total light ratios. Discs in early-type galaxies contribute with about 40 per cent to the total mass of the analysed (non-barred) objects. The decomposition into discs and bulges can be used as a rough approximation for the separation between fast and slow rotators, but it is not a substitute, as there is only a 59 per cent probability to correctly recognise slow rotators. Kinematics (i.e. projected angular momentum) remains the best approach to mitigate the influence of the inclination effects.

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arXiv2012

The Atlas3D project - XIV. The extent and kinematics of molecular gas in early-type galaxies

We use interferometric CO observations to compare the extent, surface brightness profiles and kinematics of the molecular gas in CO-rich Atlas3D early-type galaxies (ETGs) and spiral galaxies. We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scale-lengths. Virgo cluster ETGs have less extended molecular gas reservoirs than field counterparts. Approximately half of ETGs have molecular gas surface brightness profiles that follow the stellar light profile. These systems often have relaxed gas out to large radii, suggesting they are unlikely to have had recent merger/accretion events. A third of the sample galaxies show molecular gas surface brightness profiles that fall off slower than the light, and sometimes show a truncation. We suggest that ram pressure stripping and/or the presence of hot gas has compressed/truncated the gas in these systems. The remaining galaxies have rings, or composite profiles, that we argue can be caused by the effects of bars. We investigated the kinematics of the molecular gas using position-velocity diagrams, and compared the observed kinematics with dynamical model predictions, and the observed stellar and ionised gas velocities. We confirm that the molecular gas reaches beyond the turnover of the circular velocity curve in ~70% of our CO-rich Atlas3D ETGs. In general we find that in most galaxies the molecular gas is relaxed and dynamically cold. Molecular gas is a better direct tracer of the circular velocity than the ionised gas, justifying its use as a kinematic tracer for Tully-Fisher and similar analyses. (abridged)

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arXiv2012

The ATLAS3D project - XVIII. CARMA CO imaging survey of early-type galaxies

We present the Combined Array for Research in Millimeter Astronomy (CARMA) ATLAS3D molecular gas imaging survey, a systematic study of the distribution and kinematics of molecular gas in CO-rich early-type galaxies. Our full sample of 40 galaxies (30 newly mapped and 10 taken from the literature) is complete to a 12CO(1-0) integrated flux of 18.5 Jy km/s, and it represents the largest, best-studied sample of its type to date. A comparison of the CO distribution of each galaxy to the g-r color image (representing dust) shows that the molecular gas and dust distributions are in good agreement and trace the same underlying interstellar medium. The galaxies exhibit a variety of CO morphologies, including discs (50%), rings (15%), bars+rings (10%), spiral arms (5%), and mildly (12.5%) and strongly (7.5%) disrupted morphologies. There appear to be weak trends between galaxy mass and CO morphology, whereby the most massive galaxies in the sample tend to have molecular gas in a disc morphology. We derive a lower limit to the total accreted molecular gas mass across the sample of 2.48x10^10 Msuns, or approximately 8.3x10^8 Msuns per minor merger within the sample, consistent with minor merger stellar mass ratios.

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arXiv2012

Origin of the anti-hierarchical growth of black holes

Observational studies have revealed a "downsizing" trend in black hole (BH) growth: the number densities of luminous AGN peak at higher redshifts than those of faint AGN. This would seem to imply that massive black holes formed before low mass black holes, in apparent contradiction to hierarchical clustering scenarios. We investigate whether this observed "downsizing" in BH growth is reproduced in a semi-analytic model for the formation and evolution of galaxies and black holes, set within the hierarchical paradigm for structure formation (Somerville et al. 2008; S08). In this model, black holes evolve from light seeds (\sim100M\odot) and their growth is merger-driven. The original S08 model (baseline model) reproduces the number density of AGN at intermediate redshifts and luminosities, but underproduces luminous AGN at very high redshift (z > 3) and overproduces them at low redshift (z < 1). In addition, the baseline model underproduces low-luminosity AGN at low redshift (z < 1). To solve these problems we consider several modifications to the physical processes in the model: (1) a 'heavy' black hole seeding scenario (2) a sub-Eddington accretion rate ceiling that depends on the cold gas fraction, and (3) an additional black hole accretion mode due to disk instabilities. With these three modifications, the models can explain the observed downsizing, successfully reproduce the bolometric AGN luminosity function and simultaneously reproduce galaxy and black hole properties in the local Universe. We also perform a comparison with the observed soft and hard X-ray luminosity functions of AGN, including an empirical correction for torus-level obscuration, and reach similar conclusions. Our best-fit model suggests a scenario in which disk instabilities are the main driver for moderately luminous Seyfert galaxies at low redshift, while major mergers are the main trigger for luminous AGN.

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arXiv2012

Discovery of a giant HI tail in the galaxy group HCG 44

We report the discovery of a giant HI tail in the intra-group medium of HCG 44 as part of the Atlas3D survey. The tail is ~300 kpc long in projection and contains ~5x10^8 M_sun of HI. We detect no diffuse stellar light at the location of the tail down to ~28.5 mag/arcsec^2 in g band. We speculate that the tail might have formed as gas was stripped from the outer regions of NGC 3187 (a member of HCG 44) by the group tidal field. In this case, a simple model indicates that about 1/3 of the galaxy's HI was stripped during a time interval of <1 Gyr. Alternatively, the tail may be the remnant of an interaction between HCG 44 and NGC 3162, a spiral galaxy now ~650 kpc away from the group. Regardless of the precise formation mechanism, the detected HI tail shows for the first time direct evidence of gas stripping in HCG 44. It also highlights that deep HI observations over a large field are needed to gather a complete census of this kind of events in the local Universe.

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arXiv2012

Gemini GMOS and WHT SAURON integral-field spectrograph observations of the AGN driven outflow in NGC 1266

We use the SAURON and GMOS integral field spectrographs to observe the active galactic nucleus (AGN) powered outflow in NGC 1266. This unusual galaxy is relatively nearby (D=30 Mpc), allowing us to investigate the process of AGN feedback in action. We present maps of the kinematics and line strengths of the ionised gas emission lines Halpha, Hbeta, [OIII], [OI], [NII] and [SII], and report on the detection of Sodium D absorption. We use these tracers to explore the structure of the source, derive the ionised and atomic gas kinematics and investigate the gas excitation and physical conditions. NGC 1266 contains two ionised gas components along most lines of sight, tracing the ongoing outflow and a component closer to the galaxy systemic, the origin of which is unclear. This gas appears to be disturbed by a nascent AGN jet. We confirm that the outflow in NGC 1266 is truly multiphase, containing radio plasma, atomic, molecular and ionised gas and X-ray emitting plasma. The outflow has velocities up to \pm900 km/s away from the systemic velocity, and is very likely to be removing significant amounts of cold gas from the galaxy. The LINER-like line-emission in NGC 1266 is extended, and likely arises from fast shocks caused by the interaction of the radio jet with the ISM. These shocks have velocities of up to 800 km/s, which match well with the observed velocity of the outflow. Sodium D equivalent width profiles are used to set constraints on the size and orientation of the outflow. The ionised gas morphology correlates with the nascent radio jets observed in 1.4 GHz and 5 GHz continuum emission, supporting the suggestion that an AGN jet is providing the energy required to drive the outflow.

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arXiv2012

Radiative and Momentum Based Mechanical AGN Feedback in a 3-Dimensional Galaxy Evolution Code

We study the growth of black holes (BHs) in galaxies using three-dimensional smoothed particle hydrodynamic simulations with new implementations of the momentum mechanical feedback, and restriction of accreted elements to those that are gravitationally bound to the BH. We also include the feedback from the X-ray radiation emitted by the BH, which heats the surrounding gas in the host galaxies, and adds radial momentum to the fluid. We perform simulations of isolated galaxies and merging galaxies and test various feedback models with the new treatment of the Bondi radius criterion. We find that overall the BH growth is similar to what has been obtained by earlier workers using the Springel, Di Matteo, & Hernquist algorithms. However, the outflowing wind velocities and mechanical energy emitted by winds are considerably higher (v_w ~ 1000-3000 km/s) compared to the standard thermal feedback model (v_w ~ 50-100 km/s). While the thermal feedback model emits only 0.1 % of BH released energy in winds, the momentum feedback model emits more than 30 % of the total energy released by the BH in winds. In the momentum feedback model, the degree of fluctuation in both radiant and wind output is considerably larger than in the standard treatments. We check that the new model of the BH mass accretion agrees with analytic results for the standard Bondi problem.

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arXiv2012

Relaxation and Stripping: The Evolution of Sizes, Dispersions and Dark Matter Fractions in Major and Minor Mergers of Elliptical Galaxies

We revisit collisionless major and minor mergers of spheroidal galaxies in the context of the size evolution of elliptical galaxies. The simulations are performed as a series of mergers with mass-ratios of 1:1 and 1:10 for models representing pure bulges as well as bulges embedded in dark matter halos. For major and minor mergers, respectively, we identify and analyze two different processes, violent relaxation and stripping, leading to size evolution and a change of the dark matter fraction within the observable effective radius. Violent relaxation - which is the dominant mixing process for major mergers but less important for minor mergers - scatters relatively more dark matter particles than bulge particles to small. Stripping in minor mergers assembles stellar satellite particles at large radii in halo dominated regions of the massive host. This strongly increases the size of the bulge into regions with higher dark matter fractions leaving the inner host structure almost unchanged. A factor of two mass increase by minor mergers increases the dark matter fraction by 20 per cent. We present analytic corrections to simple one-component virial estimates for the evolution of the gravitational radii. If such a two-component system grows by minor mergers alone its size growth, $r_{\mathrm{e}} \propto M^α$, reaches values of $α\approx 2.4$, significantly exceeding the simple theoretical limit of $α= 2$. For major mergers the sizes grow with $α\lesssim 1$. Our results indicate that minor mergers of galaxies embedded in massive dark matter halos provide a potential mechanism for explaining the rapid size growth and the build-up of massive elliptical systems predicting significant dark matter fractions and radially biased velocity dispersions at large radii (abbreviated)

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