Stellar Half-mass Radii Research Articles

Shape Asymmetries and the Relation between Lopsidedness and Radial Alignment in Simulated Galaxies

Galaxies are observed to be lopsided, meaning that they are more massive and more extended along one side than the opposite side. In this work, we provide a statistical analysis of the lopsided morphology of 1780 isolated satellite galaxies generated by the TNG50-1 simulation, incorporating the effect of tidal fields from halo centers. The isolated satellites are galaxies without nearby substructures whose mass is over 1% of the satellites within their virial radii. We study the radial alignment (RA) between the major axes of satellites and the radial direction of their halo centers in radial ranges of 0–2, 2–5, and 5–10 R h , with R h being the stellar half-mass radius. According to our results, the RA is virtually undetectable in inner and intermediate regions, yet it is significantly evident in outer regions. We also calculate the far-to-near-side semiaxial ratios of the major axes, denoted by a −/a +, which measure the semiaxial ratios of the major axes in the hemispheres between those facing away from (far side) and facing toward (nearside) halo centers. In all the radial bins of the satellites, the numbers of satellites with longer semiaxes on the far side are found to be almost equal to those with longer semiaxes on the near side. Therefore, the tidal fields from halo centers play a minor role in the generation of lopsided satellites. The long semimajor-axes radial alignment (LRA), i.e., an alignment between the long semimajor axes of satellite galaxies and the radial directions to their halo centers, is further studied. No clear evidence of LRA is found in our sample within the framework of ΛCDM Newtonian dynamics. Finally, we briefly discuss the possible origins of the asymmetry of galaxies in TNG50-1.

Stellar Half-mass Radii of 0.5 z < 2.3 Galaxies: Comparison with JWST/NIRCam Half-light Radii

We use CEERS JWST/NIRCam imaging to measure rest-frame near-IR light profiles of 435 M ⋆ > 1010 M ⊙ galaxies in the redshift range of 0.5 < z < 2.3. We compare the resulting rest-frame 1.5–2 μm half-light radii (R NIR) with stellar half-mass radii ( RM⋆ ) derived with multicolor light profiles from CANDELS Hubble Space Telescope imaging. In general agreement with previous work, we find that R NIR and RM⋆ are up to 40% smaller than the rest-frame optical half-light radius R opt. The agreement between R NIR and RM⋆ is excellent, with a negligible systematic offset (<0.03 dex) up to z = 2 for quiescent galaxies and up to z = 1.5 for star-forming galaxies. We also deproject the profiles to estimate RM⋆,3D , the radius of a sphere containing 50% of the stellar mass. We present the R−M ⋆ distribution of galaxies at 0.5 < z < 1.5, comparing R opt, RM⋆ , and RM⋆,3D . The slope is significantly flatter for RM⋆ and RM⋆,3D compared to R opt, mostly due to downward shifts in size for massive star-forming galaxies, while RM⋆ and RM⋆,3D do not show markedly different trends. Finally, we show rapid evolution of the size (R ∝ (1 + z)−1.7±0.1) of massive (M ⋆ > 1011 M ⊙) quiescent galaxies between z = 0.5 and z = 2.3, again comparing R opt, RM⋆ , and RM⋆,3D . We conclude that the main tenets of the evolution of the size narrative established over the past 20 yr, based on rest-frame optical light profile analysis, still hold in the era of JWST/NIRCam observations in the rest-frame near-IR.

The isolated dark matter-poor galaxy that ran away

Since the discovery of dark matter-deficient galaxies, numerous studies have shown that these exotic galaxies naturally occur in the ΛCDM model due to stronger tidal interactions. They are typically satellites, with stellar masses in the 108 − 109 M⊙ range, of more massive galaxies. The recent discovery of a massive galaxy lacking dark matter and also lacking a more massive neighbor is puzzling. Two possible scenarios have been suggested in the literature: either the galaxy lost its dark matter early or it had been lacking ab initio. As a proof of concept for the former assumption, we present an example from IllustrisTNG300. At present, the galaxy has a stellar mass of M⋆ ≃ 6.8 × 109 M⊙, with no gas, MDM/MB ≃ 1.31, and a stellar half-mass radius of R0.5, ⋆ = 2.45 kpc. It lost the majority of its dark matter early, between z = 2.32 and z = 1.53. Since then, it has continued to dwell in the cluster environment, interacting with the cluster members without merging, while accelerating on its orbit. Eventually, it left the cluster and it has spent the last ∼2 Gyr in isolation, residing just outside the most massive cluster in the simulation. Thus, the galaxy represents the first example found in simulations of both an isolated dark matter-poor galaxy that lost its extended envelope early and a fairly compact stellar system that has managed to escape.

Gas-phase metallicity break radii of star-forming galaxies in IllustrisTNG

ABSTRACT We present radial gas-phase metallicity profiles, gradients, and break radii at redshift z = 0–3 from the TNG50-1 star-forming galaxy population. These metallicity profiles are characterized by an emphasis on identifying the steep inner gradient and flat outer gradient. From this, the break radius, Rbreak, is defined as the region where the transition occurs. We observe the break radius having a positive trend with mass that weakens with redshift. When normalized by the stellar half-mass radius, the break radius has a weaker relation with both mass and redshift. To test if our results are dependent on the resolution or adopted physics of TNG50-1, the same analysis is performed in TNG50-2 and Illustris-1. We find general agreement between each of the simulations in their qualitative trends; however, the adopted physics between TNG and Illustris differ and therefore the breaks, normalized by galaxy size, deviate by a factor of ∼2. In order to understand where the break comes from, we define two relevant time-scales: an enrichment time-scale and a radial gas mixing time-scale. We find that Rbreak occurs where the gas mixing time-scale is ∼10 times as long as the enrichment time-scale in all three simulation runs, with some weak mass and redshift dependence. This implies that galactic discs can be thought of in two-parts: a star-forming inner disc with a steep gradient and a mixing-dominated outer disc with a flat gradient, with the break radius marking the region of transition between them.

Inferring galaxy dark halo properties from visible matter with machine learning

ABSTRACT Next-generation surveys will provide photometric and spectroscopic data of millions to billions of galaxies with unprecedented precision. This offers a unique chance to improve our understanding of the galaxy evolution and the unresolved nature of dark matter (DM). At galaxy scales, the density distribution of DM is strongly affected by feedback processes, which are difficult to fully account for in classical techniques to derive galaxy masses. We explore the capability of supervised machine learning (ML) algorithms to predict the DM content of galaxies from ‘luminous’ observational-like parameters, using the TNG100 simulation. In particular, we use photometric (magnitudes in different bands), structural (the stellar half-mass radius and three different baryonic masses), and kinematic (1D velocity dispersion and the maximum rotation velocity) parameters to predict the total DM mass, DM half-mass radius, and DM mass inside one and two stellar half-mass radii. We adopt the coefficient of determination, R2, as a metric to evaluate the accuracy of these predictions. We find that using all observational quantities together (photometry, structural, and kinematics), we reach high accuracy for all DM quantities (up to R2 ∼ 0.98). This first test shows that ML tools are promising to predict the DM in real galaxies. The next steps will be to implement the observational realism of the training sets, by closely selecting samples that accurately reproduce the typical observed ‘luminous’ scaling relations. The so-trained pipelines will be suitable for real galaxy data collected from Rubin/Large Synoptic Survey Telescope (LSST), Euclid, Chinese Survey Space Telescope (CSST), 4-metre Multi-Object Spectrograph Telescope (4MOST), Dark Energy Spectroscopic Instrument (DESI), to derive e.g. the properties of their central DM fractions.

The scatter in the galaxy–halo connection: a machine learning analysis

ABSTRACT We apply machine learning (ML), a powerful method for uncovering complex correlations in high-dimensional data, to the galaxy–halo connection of cosmological hydrodynamical simulations. The mapping between galaxy and halo variables is stochastic in the absence of perfect information, but conventional ML models are deterministic and hence cannot capture its intrinsic scatter. To overcome this limitation, we design an ensemble of neural networks with a Gaussian loss function that predict probability distributions, allowing us to model statistical uncertainties in the galaxy–halo connection as well as its best-fitting trends. We extract a number of galaxy and halo variables from the Horizon-AGN and IllustrisTNG100-1 simulations and quantify the extent to which knowledge of some subset of one enables prediction of the other. This allows us to identify the key features of the galaxy–halo connection and investigate the origin of its scatter in various projections. We find that while halo properties beyond mass account for up to 50 per cent of the scatter in the halo-to-stellar mass relation, the prediction of stellar half-mass radius or total gas mass is not substantially improved by adding further halo properties. We also use these results to investigate semi-analytic models for galaxy size in the two simulations, finding that assumptions relating galaxy size to halo size or spin are not successful.

Lopsided galactic disks in IllustrisTNG

A significant fraction of nearby late-type galaxies are lopsided. We study the asymmetry of the stellar component in a sample of well-resolved disky galaxies selected from the last snapshot of the Illustris TNG100 simulation based on their flatness and rotational support. Among 1912 disks, we identify 161 objects with significant asymmetry in terms of them = 1 Fourier mode of the stellar component within (1−2) stellar half-mass radii and describe their properties using three representative examples. The profiles of them = 1 mode typically increase with radius, and the corresponding phase is constant in the asymmetric region, signifying a global distortion. Following the evolution of the lopsided disks over time, we find that their history is rather uneventful and the occurrence of the asymmetry is fairly recent. Only about 1/3 of the lopsided disks experienced any strong interaction recently that could have led to the distortion of their shape: 24% were affected by a more massive object and 9% underwent a gas-rich merger. Still, a majority of lopsided disks show a significant increase in their recent star formation rate. The most frequent mechanism for the formation of lopsided disks thus seems to be asymmetric star formation probably related to gas accretion, although the distortions in the gas and stars are not strongly correlated. This picture is supported by the finding that the lopsided population on average contains more gas, has higher star formation rate, lower metallicity and bluer color than the remaining disks. These correlations are similar to those seen in real galaxies, even though the fraction of simulated lopsided disks (8%) is much lower than in observations (30%). The observed correlation between the presence of the asymmetry and a bar is not reproduced either. These discrepancies may be due to overquenching or insufficient resolution of IllustrisTNG simulations.

The impact of dust on the sizes of galaxies in the Epoch of Reionization

ABSTRACT We study the sizes of galaxies in the Epoch of Reionization using a sample of ${\sim 100\, 000}$ galaxies from the BlueTides cosmological hydrodynamical simulation from z = 7 to 11. We measure the galaxy sizes from stellar mass and luminosity maps, defining the effective radius as the minimum radius that could enclose the pixels containing 50 per cent of the total mass/light in the image. We find an inverse relationship between stellar mass and effective half-mass radius, suggesting that the most massive galaxies are more compact and dense than lower mass galaxies, which have flatter mass distributions. We find a mildly negative relation between intrinsic far-ultraviolet luminosity and size, while we find a positive size–luminosity relation when measured from dust-attenuated images. This suggests that dust is the predominant cause of the observed positive size–luminosity relation, with dust preferentially attenuating bright sightlines resulting in a flatter emission profile and thus larger measured effective radii. We study the size–luminosity relation across the rest-frame ultraviolet and optical, and find that the slope decreases at longer wavelengths; this is a consequence of the relation being caused by dust, which produces less attenuation at longer wavelengths. We find that the far-ultraviolet size–luminosity relation shows mild evolution from z = 7 to 11, and galaxy size evolves with redshift as R ∝ (1 + z)−m, where m = 0.662 ± 0.009. Finally, we investigate the sizes of z = 7 quasar host galaxies, and find that while the intrinsic sizes of quasar hosts are small relative to the overall galaxy sample, they have comparable sizes when measured from dust-attenuated images.

Observed structural parameters of EAGLE galaxies: reconciling the mass–size relation in simulations with local observations

ABSTRACT We use mock images of z = 0.1 galaxies in the 100 Mpc EAGLE simulation to establish the differences between the sizes and morphologies inferred from the stellar mass distributions and the optical light distributions. The optical, r-band images used were constructed with a radiative transfer method to account for the effects of dust, and we measure galaxy structural parameters by fitting Sérsic models to the images with galfit. We find that the derived half-light radii differ systematically from the stellar half-mass radii, as the r-band sizes are typically 0.1 dex larger, and can deviate by as much as ${\approx}0.5\,$ dex, depending on the dust attenuation and star formation activity, as well as the measurement method used. Consequently, we demonstrate that the r-band sizes significantly improve the agreement between the simulated and observed stellar mass–size relation: star-forming and quiescent galaxies in EAGLE are typically only slightly larger than observed (by 0.1 dex), and the slope and scatter of the local relation are reproduced well for both populations. Finally, we compare the obtained morphologies with measurements from the GAMA survey, finding that too few EAGLE galaxies have bulge-like light profiles (Sérsic indices of n ∼ 4). Despite the presence of a significant population of triaxial systems among the simulated galaxies, the surface brightness and stellar mass density profiles tend to be closer to exponential discs (n ∼ 1–2). Our results highlight the need to measure the sizes and morphologies of simulated galaxies using common observational methods in order to perform a meaningful comparison with observations.

The dust-continuum size of TNG50 galaxies at z = 1–5: a comparison with the distribution of stellar light, stars, dust, and H2

ABSTRACT We present predictions for the extent of the dust-continuum emission of main-sequence galaxies drawn from the TNG50 simulation in the range z = 1–5. We couple the radiative transfer code SKIRT to the output of the TNG50 simulation and measure the dust-continuum half-light radius of the modelled galaxies, assuming a Milky Way dust type and a metallicity-dependent dust-to-metal ratio. The dust-continuum half-light radius at observed-frame 850 $\mu$m is up to ∼75 per cent larger than the stellar half-mass radius, but significantly more compact than the observed-frame 1.6 $\mu$m (roughly corresponding to H band) half-light radius, particularly towards high redshifts: the compactness compared to the 1.6 $\mu$m emission increases with redshift. This is driven by obscuration of stellar light from the galaxy centres, which increases the apparent extent of 1.6 $\mu$m disc sizes relative to that at 850 $\mu$m. The difference in relative extents increases with redshift because the observed-frame 1.6 $\mu$m emission stems from ever shorter wavelength stellar emission. These results suggest that the compact dust-continuum emission observed in z &amp;gt; 1 galaxies is not (necessarily) evidence of the build-up of a dense central stellar component. We find that the dust-continuum half-light radius closely follows the radius containing half the star formation and half the dust mass in galaxies and is ∼80 per cent of the radius containing half the H2 mass. The presented results are a common feature of main-sequence galaxies.

The tidal evolution of the Fornax dwarf spheroidal and its globular clusters

ABSTRACT The dark matter content of the Fornax dwarf spheroidal galaxy inferred from its kinematics is substantially lower than expected from LCDM cosmological simulations. We use N-body simulations to examine whether this may be the result of Galactic tides. We find that, despite improved proper motions from the Gaia mission, the pericentric distance of Fornax remains poorly constrained, mainly because its largest velocity component is roughly antiparallel to the solar motion. Translating Fornax’s proper motion into a Galactocentric velocity is, thus, sensitively dependent on Fornax’s assumed distance: the observed distance uncertainty, $\pm 8\,{{\rm per\,cent}}$, implies pericentric distances that vary between rperi ∼ 50 and ∼150 kpc. Our simulations show that for rperi in the lower range of that estimate, an LCDM subhalo with maximum circular velocity Vmax = 40 km s−1 (or virial mass $M_{200}\approx 10^{10}\, \rm M_\odot$, as expected from LCDM) would be tidally stripped to Vmax ∼ 23 km s−1 over 10 Gyr. This would reduce the dark mass within the Fornax stellar half-mass radius to about half its initial value, bringing it into agreement with observations. Tidal stripping affects mainly Fornax’s dark matter halo; its stellar component is affected little, losing less than $5\,{{\rm per\,cent}}$ of its initial mass in the process. We also explore the effect of Galactic tides on the dynamical friction decay times of Fornax’s population of globular clusters (GCs) and find little evidence for substantial changes, compared with models run in isolation. A population of GCs with initial orbital radii between 1 and 2 kpc is consistent with the present-day spatial distribution of Fornax GCs, despite assuming a cuspy halo. Neither the dark matter content nor the spatial distribution of GCs of Fornax seems inconsistent with a simple model where Fornax inhabits a tidally stripped cuspy cold dark matter halo.

Spurious heating of stellar motions in simulated galactic discs by dark matter halo particles

ABSTRACT We use idealized N-body simulations of equilibrium stellar discs embedded within course-grained dark matter (DM) haloes to study the effects of spurious collisional heating on disc structure and kinematics. Collisional heating artificially increases the vertical and radial velocity dispersions of disc stars, as well as the thickness and size of discs; the effects are felt at all galacto-centric radii. The integrated effects of collisional heating are determined by the mass of DM halo particles (or equivalently, by the number of particles at fixed halo mass), their local density and characteristic velocity dispersion, but are largely insensitive to the stellar particle mass. The effects can therefore be reduced by increasing the mass resolution of DM in cosmological simulations, with limited benefits from increasing the baryonic (or stellar) mass resolution. We provide a simple empirical model that accurately captures the effects of spurious collisional heating on the structure and kinematics of simulated discs, and use it to assess the importance of disc heating for simulations of galaxy formation. We find that the majority of state-of-the-art zoom simulations, and a few of the highest-resolution, smallest-volume cosmological runs, are in principle able to resolve thin stellar discs in Milky Way-mass haloes, but most large-volume cosmological simulations cannot. For example, DM haloes resolved with fewer than ≈106 particles will collisionally heat stars near the stellar half-mass radius such that their vertical velocity dispersion increases by ≳ 10 per cent of the halo’s virial velocity in approximately one Hubble time.

Neutral CGM as damped Ly α absorbers at high redshift

ABSTRACT Recent searches for the hosts of z ∼ 4 damped Ly α absorbers (DLAs) have detected bright galaxies at distances of tens of kpc from the DLA. Using the FIRE-2 cosmological zoom simulations, we argue that these relatively large distances are due to a predominantly cool and neutral inner circumgalactic medium (CGM) surrounding high-redshift galaxies. The inner CGM is cool because of the short cooling time of hot gas in ${\lesssim}10^{12}\, {\rm M_{\odot }}$ haloes, which implies that accretion and feedback energy are radiated quickly, while it is neutral due to high volume densities and column densities at high redshift that shield cool gas from photoionization. Our analysis predicts large DLA covering factors (${\gtrsim}50{{\ \rm per\ cent}}$) out to impact parameters ∼0.3[(1 + z)/5]3/2Rvir from the central galaxies at z ≳ 1, equivalent to a proper distance of ${\sim}21\, M_{12}^{1/3} \left(\left(1+z\right)/5\right)^{1/2}\, {\rm kpc}$ (Rvir and M12 are the halo virial radius and mass in units of $10^{12}\, {\rm M_{\odot }}$, respectively). This implies that DLA covering factors at z ∼ 4 may be comparable to unity out to a distance ∼10 times larger than stellar half-mass radii. A predominantly neutral inner CGM in the early universe suggests that its mass and metallicity can be directly constrained by absorption surveys, without resorting to the large ionization corrections as required for ionized CGM.

Bar-like galaxies in IllustrisTNG

We study a sample of bar-like galaxies in the Illustris TNG100 simulation, in which almost the whole stellar component is in the form of a prolate spheroid. The sample is different from the late-type barred galaxies studied before. In addition to the requirement of a high enough stellar mass and resolution, the 277 galaxies were selected based on the single condition of a low enough ratio of the intermediate to long axis of the stellar component. We followed the mass and shape evolution of the galaxies as well as their interactions with other objects and divided them into three classes based on the origin of the bar and the subsequent history. In galaxies of class A (comprising 28% of the sample), the bar was induced by an interaction with a larger object, most often a cluster or group central galaxy, and the galaxies were heavily stripped of dark matter and gas. In classes B and C (27% and 45% of the sample, respectively) the bars were induced by a merger or a passing satellite, or they were formed by disk instability. Class B galaxies were then partially stripped of mass, while those of class C evolved without strong interactions, thus retaining their dark matter and gas in the outskirts. We illustrate the properties of the different classes with three representative examples of individual galaxies. In spite of the different evolutionary histories, the bars are remarkably similar in strength, length, and formation times. The gas fraction in the baryonic component within two stellar half-mass radii at the time of bar formation is always below 0.4 and usually very low, which confirms in the cosmological context the validity of this threshold, which has previously been identified in controlled simulations. Observational counterparts of these objects can be found among early-type fast rotators, S0 galaxies, or red spirals with bars.

Early-type galaxy density profiles from IllustrisTNG – I. Galaxy correlations and the impact of baryons

ABSTRACT We explore the isothermal total density profiles of early-type galaxies (ETGs) in the IllustrisTNG simulation. For the selected 559 ETGs at z = 0 with stellar masses $10^{10.7}\, \mathrm{M}_{\odot } \leqslant M_{\ast } \leqslant 10^{11.9}\, \mathrm{M}_{\odot }$, the total power-law slope has a mean of 〈γ′〉 = 2.011 ± 0.007 and a scatter of $\sigma _{\gamma ^{\prime }} = 0.171$ over the radial range 0.4–4 times the stellar half-mass radius. Several correlations between γ′ and galactic properties including stellar mass, effective radius, stellar surface density, central velocity dispersion, central dark matter fraction, and in situ-formed stellar mass ratio are compared to observations and other simulations, revealing that IllustrisTNG reproduces many correlation trends, and in particular, γ′ is almost constant with redshift below z = 2. Through analysing IllustrisTNG model variations, we show that black hole kinetic winds are crucial to lowering γ′ and matching observed galaxy correlations. The effects of stellar winds on γ′ are subdominant compared to active galactic nucleus (AGN) feedback, and differ due to the presence of AGN feedback from previous works. The density profiles of the ETG dark matter haloes are well described by steeper than NFW profiles, and they are steeper in the full physics (FP) run than their counterparts in the dark matter-only (DMO) run. Their inner density slopes anticorrelate (remain constant) with the halo mass in the FP (DMO) run, and anticorrelate with the halo concentration parameter c200 in both the types of runs. The dark matter haloes of low-mass ETGs are contracted whereas high-mass ETGs are expanded, suggesting that variations in the total density profile occur through the different halo responses to baryons.

The fraction of dark matter within galaxies from the IllustrisTNG simulations

We use the IllustrisTNG (TNG) cosmological simulations to provide theoretical expectations for the dark matter mass fractions (DMFs) and circular velocity profiles of galaxies. TNG predicts flat circular velocity curves for $z = 0$ Milky Way (MW)-like galaxies beyond a few kpc from the galaxy centre, in better agreement with observational constraints than its predecessor, Illustris. TNG also predicts an enhancement of the dark matter mass within the 3D stellar half-mass radius ($r_\mathrm{half}$; $M_\mathrm{200c} = 10^{10}-10^{13}\mathrm{M}_{\odot}$, $z \le2$) compared to its dark matter only and Illustris counterparts. This enhancement leads TNG present-day galaxies to be dominated by dark matter within their inner regions, with $f_\mathrm{DM}(<r_\mathrm{half})\gtrsim0.5$ at all masses and with a minimum for MW-mass galaxies. The 1$\sigma$ scatter is $\lesssim$ 10~per~cent at all apertures, which is smaller than that inferred by some observational datasets, e.g. 40 per cent from the SLUGGS survey. TNG agrees with the majority of the observationally inferred values for elliptical galaxies once a consistent IMF is adopted (Chabrier) and the DMFs are measured within the same apertures. The DMFs measured within $r_\mathrm{half}$ increase towards lower redshifts: this evolution is dominated by the increase in galaxy size with time. At $z\sim2$, the DMF in disc-like TNG galaxies decreases with increasing galaxy mass, with $f_\mathrm{DM}(<r_\mathrm{half}) \sim 0.10-0.65$ for $10^{10} \lesssim M_{\rm stars}/\mathrm{M}_{\odot} \lesssim 10^{12}$, and are two times higher than if TNG galaxies resided in Navarro-Frenk-White dark matter haloes unaffected by baryonic physics. It remains to be properly assessed whether recent observational estimates of the DMFs at $z\sim2$ rule out the contraction of the dark matter haloes predicted by the TNG model.

How to break the density-anisotropy degeneracy in spherical stellar systems

We present a new non-parametric Jeans code, GravSphere, that recovers the density $\rho(r)$ and velocity anisotropy $\beta(r)$ of spherical stellar systems, assuming only that they are in a steady-state. Using a large suite of mock data, we confirm that with only line-of-sight velocity data, GravSphere provides a good estimate of the density at the projected stellar half mass radius, $\rho(R_{1/2})$, but is not able to measure $\rho(r)$ or $\beta(r)$, even with 10,000 tracer stars. We then test three popular methods for breaking this $\rho-\beta$ degeneracy: using multiple populations with different $R_{1/2}$; using higher order `Virial Shape Parameters' (VSPs); and including proper motion data. We find that two populations provide an excellent recovery of $\rho(r)$ in-between their respective $R_{1/2}$. However, even with a total of $\sim 7,000$ tracers, we are not able to well-constrain $\beta(r)$ for either population. By contrast, using 1000 tracers with higher order VSPs we are able to measure $\rho(r)$ over the range $0.5 < r/R_{1/2} < 2$ and broadly constrain $\beta(r)$. Including proper motion data for all stars gives an even better performance, with $\rho$ and $\beta$ well-measured over the range $0.25 < r/R_{1/2} < 4$. Finally, we test GravSphere on a triaxial mock galaxy that has axis ratios typical of a merger remnant, $[1:0.8:0.6]$. In this case, GravSphere can become slightly biased. However, we find that when this occurs the data are poorly fit, allowing us to detect when such departures from spherical symmetry become problematic.

Dark matter cores all the way down

We use high resolution simulations of isolated dwarf galaxies to study the physics of dark matter cusp-core transformations at the edge of galaxy formation: M200 = 10^7 - 10^9 Msun. We work at a resolution (~4 pc minimum cell size; ~250 Msun per particle) at which the impact from individual supernovae explosions can be resolved, becoming insensitive to even large changes in our numerical 'sub-grid' parameters. We find that our dwarf galaxies give a remarkable match to the stellar light profile; star formation history; metallicity distribution function; and star/gas kinematics of isolated dwarf irregular galaxies. Our key result is that dark matter cores of size comparable to the stellar half mass radius (r_1/2) always form if star formation proceeds for long enough. Cores fully form in less than 4 Gyrs for the M200 = 10^8 Msun and 14 Gyrs for the 10^9 Msun dwarf. We provide a convenient two parameter 'coreNFW' fitting function that captures this dark matter core growth as a function of star formation time and the projected stellar half mass radius. Our results have several implications: (i) we make a strong prediction that if LCDM is correct, then 'pristine' dark matter cusps will be found either in systems that have truncated star formation and/or at radii r > r_1/2; (ii) complete core formation lowers the projected velocity dispersion at r_1/2 by a factor ~2, which is sufficient to fully explain the 'too big to fail problem'; and (iii) cored dwarfs will be much more susceptible to tides, leading to a dramatic scouring of the subhalo mass function inside galaxies and groups.

Evolution of galaxy shapes from prolate to oblate through compaction events

We study the evolution of global shapes of galaxies using cosmological simulations. The shapes refer to the components of dark matter (DM), stars and gas at the stellar half-mass radius. Most galaxies undergo a characteristic compaction event into a blue nugget at $z\sim2-4$, which marks the transition from a DM-dominated central body to a self-gravitating baryonic core. We find that in the high-$z$, DM-dominated phase, the stellar and DM systems tend to be triaxial, preferentially prolate and mutually aligned. The elongation is supported by an anisotropic velocity dispersion that originates from the assembly of the galaxy along a dominant large-scale filament. We estimate that torques by the dominant halo are capable of inducing the elongation of the stellar system and its alignment with the halo. Then, in association with the transition to self-gravity, small-pericenter orbits puff up and the DM and stellar systems evolve into a more spherical and oblate configuration, aligned with the gas disc and associated with rotation. This transition typically occurs when the stellar mass is $\sim 10^9$ M$_\odot$ and the escape velocity in the core is $\sim 100$ km s$^{-1}$, indicating that supernova feedback may be effective in keeping the core DM-dominated and the system prolate. The early elongated phase itself may be responsible for the compaction event, and the transition to the oblate phase may be associated with the subsequent quenching in the core.

The formation of massive, compact galaxies at z = 2 in the Illustris simulation

Massive, quiescent galaxies at high redshift have been found to be considerably more compact than galaxies of similar mass in the local universe. How these compact galaxies formed has yet to be determined, though several progenitor populations have been proposed. Here we investigate the formation processes and quantify the assembly histories of such galaxies in Illustris, a suite of hydrodynamical cosmological simulations encompassing a sufficiently large volume to include rare objects, while simultaneously resolving the internal structure of galaxies. We select massive (∼1011 M⊙) and compact (stellar half-mass radius <2 kpc) galaxies from the simulation at z = 2. Within the Illustris suite, we find that these quantities are not perfectly converged, but are reasonably reliable for our purposes. The resulting population is composed primarily of quiescent galaxies, but we also find several star-forming compact galaxies. The simulated compact galaxies are similar to observed galaxies in star formation activity and appearance. We follow their evolution at high redshift in the simulation and find that there are multiple pathways to form these compact galaxies, dominated by two mechanisms: (i) intense, centrally concentrated starbursts generally triggered by gas-rich major mergers between z ∼ 2–4, reducing the galaxies’ half-mass radii by a factor of a few to below 2 kpc, and (ii) assembly at very early times when the universe was much denser; the galaxies formed compact and remained so until z ∼ 2.