The distribution of stellar orbits in eagle galaxies – the effect of mergers, gas accretion, and secular evolution

The merger history of a galaxy is thought to be one of the major factors determining its internal dynamics, with galaxies having undergone different types or mergers (e.g. dry, minor, or major mergers) predicted to show different dynamical properties. We study the instantaneous orbital distribution of galaxies in the EAGLE simulation, colouring the orbits of the stellar particles by their stellar age, in order to understand whether stars form in particular orbits (e.g. in a thin or thick disc). We first show that EAGLE reproduces well the observed stellar mass fractions in different stellar orbital families as a function of stellar mass and spin parameter at z = 0. We find that the youngest stars reside in a thin disc component that can extend to the very inner regions of galaxies, and that older stars have warmer orbits, with the oldest ones showing orbits consistent with both hot and counter-rotating classifications, which is consistent with the trend found in the Milky Way and other disc galaxies. We also show that counter-rotating orbits trace galaxy mergers – in particular dry mergers, and that in the absence of mergers, counter-rotating orbits can also be born from highly misaligned gas accretion that leads to star formation

Disentangling the formation history of galaxies via population-orbit superposition: method validation

We present population-orbit superposition models for external galaxies based on Schwarzschild's orbit-superposition method, by tagging the orbits with age and metallicity. The models fit the density distributions, as well as kinematic, age and metallicity maps from Integral Field Unit (IFU) spectroscopy observations. We validate the method and demonstrate its power by applying it to mock data, similar to those obtained by the Multi-Unit Spectroscopic Explorer (MUSE) IFU on the Very Large Telescope (VLT). These mock data are created from Auriga galaxy simulations, viewed at three different inclination angles ($\vartheta=40^o, 60^o, 80^o$). Constrained by MUSE-like mock data, our model can recover the galaxy's stellar orbit distribution projected in orbital circularity $\lambda_z$ vs. radius $r$, the intrinsic stellar population distribution in age $t$ vs. metallicity $Z$, and the correlation between orbits' circularity $\lambda_z$ and stellar age $t$. A physically motivated age-metallicity relation improves recovering the intrinsic stellar population distributions. We decompose galaxies into cold, warm and hot + counter-rotating components based on their orbit circularity distribution, and find that the surface density, mean velocity, velocity dispersion, age and metallicity maps of each component from our models well reproduce those from simulation, especially for projections close to edge-on. These galaxies exhibit strong global age vs. $\sigma_z$ relation, which is well recovered by our model. The method has the power to reveal the detailed build-up of stellar structures in galaxies, and offers a complement to local resolved, and high-redshift studies of galaxy evolution.Comment: 20 pages, submitted to MNRA

The MAGPI Survey: impact of environment on the total internal mass distribution of galaxies in the last 5 Gyr

We investigate the impact of environment on the internal mass distribution of galaxies using the Middle Ages Galaxy Properties with Integral field spectroscopy (MAGPI) survey. We use 2D resolved stellar kinematics to construct Jeans dynamical models for galaxies at mean redshift z ∼ 0.3, corresponding to a lookback time of 3–4 Gyr. The internal mass distribution for each galaxy is parametrized by the combined mass density slope γ (baryons + dark matter), which is the logarithmic change of density with radius. We use a MAGPI sample of 28 galaxies from low-to-mid density environments and compare to density slopes derived from galaxies in the high density Frontier Fields clusters in the redshift range 0.29 < z < 0.55, corresponding to a lookback time of ∼5 Gyr. We find a median density slope of γ = −2.22 ± 0.05 for the MAGPI sample, which is significantly steeper than the Frontier Fields median slope (γ = −2.00 ± 0.04), implying the cluster galaxies are less centrally concentrated in their mass distribution than MAGPI galaxies. We also compare to the distribution of density slopes from galaxies in ATLAS3D at z ∼ 0, because the sample probes a similar environmental range as MAGPI. The ATLAS3D median total slope is γ = −2.25 ± 0.02, consistent with the MAGPI median. Our results indicate environment plays a role in the internal mass distribution of galaxies, with no evolution of the slope in the last 3–4 Gyr. These results are in agreement with the predictions of cosmological simulations

The SAMI Galaxy Survey: a statistical approach to an optimal classification of stellar kinematics in galaxy surveys

Large galaxy samples from multi-object IFS surveys now allow for a statistical analysis of the z~0 galaxy population using resolved kinematics. However, the improvement in number statistics comes at a cost, with multi-object IFS survey more severely impacted by the effect of seeing and lower S/N. We present an analysis of ~1800 galaxies from the SAMI Galaxy Survey and investigate the spread and overlap in the kinematic distributions of the spin parameter proxy $\lambda_{Re}$ as a function of stellar mass and ellipticity. For SAMI data, the distributions of galaxies identified as regular and non-regular rotators with \textsc{kinemetry} show considerable overlap in the $\lambda_{Re}$-$\varepsilon_e$ diagram. In contrast, visually classified galaxies (obvious and non-obvious rotators) are better separated in $\lambda_{Re}$ space, with less overlap of both distributions. Then, we use a Bayesian mixture model to analyse the observed $\lambda_{Re}$-$\log(M_*/M_{\odot})$ distribution. Below $\log(M_{\star}/M_{\odot})\sim10.5$, a single beta distribution is sufficient to fit the complete $\lambda_{Re}$ distribution, whereas a second beta distribution is required above $\log(M_{\star}/M_{\odot})\sim10.5$ to account for a population of low-$\lambda_{Re}$ galaxies. While the Bayesian mixture model presents the cleanest separation of the two kinematic populations, we find the unique information provided by visual classification of kinematic maps should not be disregarded in future studies. Applied to mock-observations from different cosmological simulations, the mixture model also predicts bimodal $\lambda_{Re}$ distributions, albeit with different positions of the $\lambda_{Re}$ peaks. Our analysis validates the conclusions from previous smaller IFS surveys, but also demonstrates the importance of using kinematic selection criteria that are dictated by the quality of the observed or simulated data.Comment: 30 pages and 17 figures, accepted for publication in MNRAS. Abstract abridged for Arxiv. The key figures of the paper are: 3, 7, 8, and 1

Evolution in the orbital structure of quiescent galaxies from MAGPI, LEGA-C, and SAMI surveys: direct evidence for merger-driven growth over the last 7 Gyr

We present the first study of spatially integrated higher-order stellar kinematics over cosmic time. We use deep rest-frame optical spectroscopy of quiescent galaxies at redshifts z = 0.05, 0.3, and 0.8 from the SAMI, MAGPI, and LEGA-C surveys to measure the excess kurtosis h4 of the stellar velocity distribution, the latter parametrized as a Gauss-Hermite series. Conservatively using a redshift-independent cut in stellar mass (⁠⁠) and matching the stellar-mass distributions of our samples, we find 7σ evidence of h4 increasing with cosmic time, from a median value of 0.019 ± 0.002 at z = 0.8 to 0.059 ± 0.004 at z = 0.06. Alternatively, we use a physically motivated sample selection based on the mass distribution of the progenitors of local quiescent galaxies as inferred from numerical simulations; in this case, we find 10σ evidence. This evolution suggests that, over the last 7 Gyr, there has been a gradual decrease in the rotation-to-dispersion ratio and an increase in the radial anisotropy of the stellar velocity distribution, qualitatively consistent with accretion of gas-poor satellites. These findings demonstrate that massive galaxies continue to accrete mass and increase their dispersion support after becoming quiescent

Different higher order kinematics between star-forming and quiescent galaxies based on the SAMI, MAGPI, and LEGA-C surveys

We present the first statistical study of spatially integrated non-Gaussian stellar kinematics spanning 7 Gyr in cosmic time. We use deep, rest-frame optical spectroscopy of massive galaxies (stellar mass ⁠) at redshifts z = 0.05, 0.3, and 0.8 from the SAMI, MAGPI, and LEGA-C surveys, to measure the excess kurtosis h4 of the stellar velocity distribution, the latter parametrized as a Gauss–Hermite series. We find that at all redshifts where we have large enough samples, h4 anticorrelates with the ratio between rotation and dispersion, highlighting the physical connection between these two kinematic observables. In addition, and independently from the anticorrelation with rotation-to-dispersion ratio, we also find a correlation between h4 and M⋆, potentially connected to the assembly history of galaxies. In contrast, after controlling for mass, we find no evidence of independent correlation between h4 and aperture velocity dispersion or galaxy size. These results hold for both star-forming and quiescent galaxies. For quiescent galaxies, h4 also correlates with projected shape, even after controlling for the rotation-to-dispersion ratio. At any given redshift, star-forming galaxies have lower h4 compared to quiescent galaxies, highlighting the link between kinematic structure and star-forming activity

Stellar IMF and mass profiles of ETGs

Empirical thesis.Spine title: The stellar IMF and mass profiles of ETGs.Bibliography: pages [49]-52.1. Introduction -- 2. Common methods -- 3. Scaled potential -- 4. General power law -- 5. Discussion & conclusions.The universality of the stellar Initial Mass Function (IMF) remains an open question to this day. Studies where individual stars can be counted provide strong evidence for universality in and around the Milky Way, while recent extragalactic studies find evidence for a dependence on galactic properties. In this work, we constrain the systematic variations of the IMF that have been observed in early-type galaxies, by constructing detailed dynamical models from kinematic observations. We account for the effects of spatially-varying stellar populations by computing star-formation histories from the observed spectra, and incorporating the resulting stellar mass-to-light ratio variations into the dynamical models. In addition, we determine total mass distributions, and separate the contributions from the stellar and dark matter components. This allows for inferences on the central dark matter fraction, as well as constraints on scaling relations of the total mass profile slope with galactic observables.Mode of access: World wide web1 online resource (x, 62 pages) colour illustration

Comparing lensing and stellar orbital models of a nearby massive strong-lens galaxy

Exploiting the relative proximity of the nearby strong-lens galaxy SNL-1, we present a critical comparison of the mass estimates derived from independent modelling techniques. We fit triaxial orbit-superposition dynamical models to spatially resolved stellar kinematics, and compare to the constraints derived from lens modelling of high-resolution photometry. From the dynamical model, we measure the total (dynamical) mass enclosed within a projected aperture of radius the Einstein radius to be log10(MEin/M) = 11.00 ± 0.02, which agrees with previous measurements from lens modelling to within 5 per cent. We then explore the intrinsic (de-projected) properties of the best-fitting dynamical model. We find that SNL-1 has approximately constant, intermediate triaxiality at all radii. It is oblate like in the inner regions (around the Einstein radius) and tends towards spherical at larger radii. The stellar velocity ellipsoid gradually transforms from isotropic in the very central regions to radially biased in the outskirts. We find that SNL-1 is dynamically consistent with the broader galaxy population, as measured by the relative fraction of orbit ‘temperatures’ compared to the CALIFA survey. On the mass–size plane, SNL-1 occupies the most-compact edge given its mass, compared to both the MaNGA and SAMI surveys. Finally, we explore how the observed lensing configuration is affected by the orientation of the lens galaxy. We discuss the implications of such detailed models on future combined lensing and dynamical analyses