Ludwig Oser

Constraints on the relationship between stellar mass and halo mass at low and high redshift

We use a statistical approach to determine the relationship between the stellar masses of galaxies and the masses of the dark matter halos in which they reside. We obtain a parameterized stellar-to-halo mass (SHM) relation by populating halos and subhalos in an N-body simulation with galaxies and requiring that the observed stellar mass function be reproduced. We find good agreement w ith constraints from galaxy-galaxy lensing and predictions of semi-analytic models. Using this mapping, and the positions of the halos and subhalos obtained from the simulation, we find that our model predictions for th e galaxy two-point correlation function (CF) as a function of stellar mass are in excellent agreement with the observed clustering properties in the SDSS at z = 0. We show that the clustering data do not provide additional strong constraints on the SHM function and conclude that our model can therefore predict clustering as a functio n of stellar mass. We compute the conditional mass

The cosmological size and velocity dispersion evolution of massive early-type galaxies

We analyze 40 cosmological re-simulations of individual massive galaxies with present-day stellar masses of M∗ > 6.3 × 10 10 M⊙ in order to investigate the physical origin of the observed strong increase in galaxy sizes and the decrease of the stellar velocity dispersions since redshift z ≈ 2. At present 25 out of 40 galaxies are quiescent with structural parameters (sizes and velocity dispersions) in agreement with local early type galaxies. At z=2 all simulated galaxies with M∗ & 10 11 M⊙ (11 out of 40) at z=2 are compact with projected half-mass radii of ≈ 0.77 (±0.24) kpc and line-of-sight velocity dispersions within the projected half-mass radius of ≈ 262 (±28) kms −1 (3 out of 11 are already quiescent). Similar to observed compact early-type galaxies at high redshift the simulated galaxies are clearly offset from the local mass-size and mass-velocity dispersion relations. Towards redshift zero the sizes increase by a factor of ∼ 5 − 6, following R1/2 ∝ (1 + z) α with α = −1.44 for quiescent galaxies (α = −1.12 for all galaxies). The velocity dispersions drop by about one-third since z ≈ 2 , following σ1/2 ∝ (1 + z) β with β = 0.44 for the quiescent galaxies (β = 0.37 for all galaxies). The simulated size and dispersion evolution is in good agreement with observations and results from the subsequent accretion and merging of stellar systems at z . 2 which is a natural consequence of the hierarchical structure formation. A significant number of the simulated massive galaxies (7 out of 40) experience no merger more massive than 1:4 (usually considered as major mergers). On average, the dominant accretion mode is stellar minor mergers with a mass-weighted mass-ratio of 1:5. We therefore conclude that the evolution of massive early-type galaxies since z ≈ 2 and their present-day properties are predominantly determined by frequent ’minor’ mergers of moderate mass-ratios and not by major mergers alone.

The ATLAS3D project - XXV : Two-dimensional kinematic analysis of simulated galaxies and the cosmological origin of fast and slow rotators

We present a detailed two-dimensional stellar dynamical analysis of a sample of 44 cosmological hydrodynamical simulations of individual central galaxies with stellar masses of 2 × 1010 M⊙ ≲ M* ≲ 6 × 1011 M⊙. Kinematic maps of the stellar line-of-sight velocity, velocity dispersion and higher order Gauss-Hermite moments h3 and h4 are constructed for each central galaxy and for the most massive satellites. The amount of rotation is quantified using the λR-parameter. The velocity, velocity dispersion, h3 and h4 fields of the simulated galaxies show a diversity similar to observed kinematic maps of early-type galaxies in the ATLAS3D survey. This includes fast (regular), slow and misaligned rotation, hot spheroids with embedded cold disc components as well as galaxies with counter-rotating cores or central depressions in the velocity dispersion. We link the present-day kinematic properties to the individual cosmological formation histories of the galaxies. In general, major galaxy mergers have a significant influence on the rotation properties resulting in both a spin-down as well as a spin-up of the merger remnant. Lower mass galaxies with significant (≳18 per cent) in situ formation of stars since z ≈ 2, or with additional gas-rich major mergers - resulting in a spin-up - in their formation history, form elongated (ɛ ˜ 0.45) fast rotators (λR ˜ 0.46) with a clear anticorrelation of h3 and v/σ. An additional formation path for fast rotators includes gas-poor major mergers leading to a spin-up of the remnants (λR ˜ 0.43). This formation path does not result in anticorrelated h3 and v/σ. The formation histories of slow rotators can include late major mergers. If the merger is gas rich, the remnant typically is a less flattened slow rotator with a central dip in the velocity dispersion. If the merger is gas poor, the remnant is very elongated (ɛ ˜ 0.43) and slowly rotating (λR ˜ 0.11). The galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. In general, more massive galaxies have less in situ star formation since z ˜ 2, rotate slower and have older stellar populations. We discuss general implications for the formation of fast and slowly rotating galaxies as well as the weaknesses and strengths of the underlying models.

Short-lived star-forming giant clumps in cosmological simulations of z~2 disks

Many observed massive star-forming z 2 galaxies are large disks that exhibit irregular morphologies, with 1 kpc, 108-1010M⊙ clumps. We present the largest sample to date of high-resolution cosmological smoothed particle hydrodynamics simulations that zoom-in on the formation of individual M * 1010.5M⊙ galaxies in 1012M⊙ halos at z 2. Our code includes strong stellar feedback parameterized as momentum-driven galactic winds. This model reproduces many characteristic features of this observed class of galaxies, such as their clumpy morphologies, smooth and monotonic velocity gradients, high gas fractions (f g 50%), and high specific star formation rates (1 Gyr–1). In accord with recent models, giant clumps (M clump (5 × 108-109)M⊙) form in situ via gravitational instabilities. However, the galactic winds are critical for their subsequent evolution. The giant clumps we obtain are short-lived and are disrupted by wind-driven mass loss. They do not virialize or migrate to the galaxy centers as suggested in recent work neglecting strong winds. By phenomenologically implementing the winds that are observed from high-redshift galaxies and in particular from individual clumps, our simulations reproduce well new observational constraints on clump kinematics and clump ages. In particular, the observation that older clumps appear closer to their galaxy centers is reproduced in our simulations, as a result of inside-out formation of the disks rather than inward clump migration.

The effect of metal enrichment and galactic winds on galaxy formation in cosmological zoom simulations

We investigate the differential effects of metal cooling and galactic stellar winds on the cosmological formation of individual galaxies with three sets of cosmological, hydrodynamical zoom simulations of 45 halos in the mass range 10^11 1 and predict reasonable star formation histories, (ii) produce galaxies with high cold gas fractions (30-60 per cent) at high redshift, (iii) significantly reduce the galaxy formation efficiencies for halos (M_halo<10^12M_sun) at all redshifts in agreement with observational and abundance matching constraints, (iv) result in high-redshift galaxies with reduced circular velocities matching the observed Tully-Fisher relation at z~2, and (v) significantly increase the sizes of low-mass galaxies (M_stellar<3x10^10M_sun) at high redshift resulting in a weak size evolution - a trend in agreement with observations. However, the low redshift (z<0.5) star formation rates of massive galaxies are higher than observed (up to ten times). No tested model predicts the observed size evolution for low-mass and high-mass galaxies simultaneously. Due to the delayed onset of star formation in the wind models, the metal enrichment of gas and stars is delayed and agrees well with observational constraints. Metal cooling and stellar winds are both found to increase the ratio of in situ formed to accreted stars - the relative importance of dissipative vs. dissipationless assembly. For halo masses below ~10^12M_sun, this is mainly caused by less stellar accretion and compares well to predictions from semi-analytical models but still differs from abundance matching models. For higher masses, the fraction of in situ stars is over-predicted due to the unrealistically high star formation rates at low redshifts.

The mass and angular momentum distribution of simulated massive early-type galaxies to large radii

We study the dark and luminous mass distributions, circular velocity curves (CVC), line-of-sight kinematics, and angular momenta for a sample of 42 cosmo- logical zoom simulations of galaxies with stellar masses from 2.0 × 10 10 M⊙ h −1 to 3.4× 10 11 M⊙ h −1 . Using a temporal smoothing technique, we are able to reach large radii. We find that: (i) The dark matter halo density profiles outside a few kpc follow simple power-law models, with flat dark matter CVCs for lower-mass systems, and rising CVCs for high-mass haloes. The projected stellar density distributions at large radii can be fitted by Sersic functions with n � 10, larger than for typical early-type galaxies (ETGs). (ii) The massive systems have nearly flat total (luminous plus dark matter) CVCs at large radii, while the less massive systems have mildly decreasing CVCs. The slope of the circular velocity at large radii correlates with circular velocity itself. (iii) The dark matter fractions within the projected stellar half mass radius Re are in the range 15-30% and increase to 40-65% at 5Re. Larger and more massive galaxies have higher dark matter fractions. The fractions and trends with mass and size are in agreement with observational estimates, even though the stellar-to-total mass ratio is �2-3 times higher than estimated for ETGs. (iv) The short axes of simulated galaxies and their host dark matter haloes are well aligned and their short-to-long axis ratios are correlated. (v) The stellar root mean square velocity vrms(R) profiles are slowly declining, in agreement with planetary nebulae observations in the outer haloes of most ETGs. (vi) The line-of-sight velocity fieldsshow that rotation properties at small and large radii are correlated. Most radial profiles for the cumulative specific angular momentum parameter λ(R) are nearly flat or slightly rising, with values in (0.06, 0.75) from 2Re to 5Re. A few cases show local maxima in |¯|/σ(R). These prop- erties agree with observations of ETGs at large radii. (vii) Stellar mass, ellipticity at large radii ǫ(5Re), and λ(5Re) are correlated: the more massive systems have less an- gular momentum and are rounder, as for observed ETGs. (viii) More massive galaxies with a large fraction of accreted stars have radially anisotropic velocity distributions outside Re. Tangential anisotropy is seen only for galaxies with high fraction of in-situ stars.

Why stellar feedback promotes disc formation in simulated galaxies

We study how feedback influences baryon infall onto galaxies using cosmological, zoom-in simulations of haloes with present mass

SPHGal: smoothed particle hydrodynamics with improved accuracy for galaxy simulations

M_{vir}=6.9\times10^{11} M_{\odot}

The stellar accretion origin of stellar population gradients in massive galaxies at large radii

to

Galaxy formation in semi-analytic models and cosmological hydrodynamic zoom simulations

1.7\times10^{12} M_{\odot}