Simulating Globular Clusters in Dark Matter Subhalos

A cosmological zoom-in simulation that develops into a Milky Way-like halo begins at redshift 7. The initial dark matter distribution is seeded with dense star clusters of median mass 5 × 105 M ⊙, placed in the largest subhalos present, which have a median peak circular velocity of 25 km s−1. Three simulations are initialized using the same dark matter distribution with the star clusters starting on approximately circular orbits having initial median radii 6.8, 0.14 kpc, and, at the exact center of the subhalos. The simulations are evolved to the current epoch at which time the median galactic orbital radii of the three sets of clusters are 30, 5, and 16 kpc, with the clusters losing about 2%, 50%, and 15% of their mass, respectively. Clusters beginning at small orbital radii have so much tidal forcing that they are often not in equilibrium. Clusters that start at larger subhalo radii have a velocity dispersion that declines smoothly to ≃20% of the central value at ≃20 half-mass radii. The clusters that begin in the subhalo centers can show a rise in velocity dispersion beyond 3–5 half-mass radii. That is, the clusters that form without local dark matter always have stellar-mass-dominated kinematics at all radii, whereas about 25% of the clusters that begin in subhalo centers have remnant local dark matter.

Crucial Factors for Lyα Transmission in the Reionizing Intergalactic Medium: Infall Motion, H ii Bubble Size, and Self-shielded Systems

Using the CoDa II simulation, we study the Lyα transmissivity of the intergalactic medium (IGM) during reionization. At z > 6, a typical galaxy without an active galactic nucleus fails to form a proximity zone around itself due to the overdensity of the surrounding IGM. The gravitational infall motion in the IGM makes the resonance absorption extend to the red side of Lyα, suppressing the transmission up to roughly the circular velocity of the galaxy. In some sight lines, an optically thin blob generated by a supernova in a neighboring galaxy results in a peak feature, which can be mistaken for a blue peak. Redward of the resonance absorption, the damping-wing opacity correlates with the global IGM neutral fraction and the UV magnitude of the source galaxy. Brighter galaxies tend to suffer lower opacity because they tend to reside in larger H ii regions, and the surrounding IGM transmits redder photons, which are less susceptible to attenuation, owing to stronger infall velocity. The H ii regions are highly nonspherical, causing both sight-line-to-sight-line and galaxy-to-galaxy variation in opacity. Also, self-shielded systems within H ii regions strongly attenuate the emission for certain sight lines. All these factors add to the transmissivity variation, requiring a large sample size to constrain the average transmission. The variation is largest for fainter galaxies at higher redshift. The 68% range of the transmissivity is similar to or greater than the median for galaxies with M UV ≥ −21 at z ≥ 7, implying that more than a hundred galaxies would be needed to measure the transmission to 10% accuracy.

Orbital pericentres and the inferred dark matter halo structure of satellite galaxies

ABSTRACT Using the phat-ELVIS suite of Milky Way-sized halo simulations, we show that subhalo orbital pericentres, rperi, correlate with their dark matter halo structural properties. Specifically, at fixed maximum circular velocity, Vmax, subhaloes with smaller rperi are more concentrated (have smaller rmax values) and have lost more mass, with larger peak circular velocities, Vpeak, prior to infall. These trends provide information that can tighten constraints on the inferred Vmax and Vpeak values for known Milky Way satellites. We illustrate this using published pericentre estimates enabled by Gaia for the nine classical Milky Way dwarf spheroidal satellites. The two densest dSph satellites (Draco and Ursa Minor) have relatively small pericentres, and this pushes their inferred rmax and Vmax values lower than they would have been without pericentre information. For Draco, we infer $V_{\rm max} = 23.5 \, \pm 3.3$ km s−1 (compared to $27.3 \, \pm 7.1$ km s−1 without pericentre information). Such a shift exacerbates the traditional Too Big to Fail problem. Draco’s peak circular velocity range prior to infall narrows from Vpeak = 21–51 km s−1 without pericentre information to Vpeak = 25–37 km s−1 with the constraint. Over the full population of classical dwarf spheroidals, we find no correlation between Vpeak and stellar mass today, indicative of a high level of stochasticity in galaxy formation at stellar masses below ∼107 M⊙. As proper motion measurements for dwarf satellites become more precise, they should enable useful priors on the expected structure and evolution of their host dark matter subhaloes.

3 per cent-accurate predictions for the clustering of dark matter, haloes, and subhaloes, over a wide range of cosmologies and scales

ABSTRACT Predicting the spatial distribution of objects as a function of cosmology is an essential ingredient for the exploitation of future galaxy surveys. In this paper, we show that a specially designed suite of gravity-only simulations together with cosmology-rescaling algorithms can provide the clustering of dark matter, haloes, and subhaloes with high precision. Specifically, with only three N-body simulations, we obtain the power spectrum of dark matter at z = 0 and 1 to better than 3 per cent precision for essentially all currently viable values of eight cosmological parameters, including massive neutrinos and dynamical dark energy, and over the whole range of scales explored, 0.03 < $k/{h}^{-1}\, {\rm Mpc}^{-1}$ < 5. This precision holds at the same level for mass-selected haloes and for subhaloes selected according to their peak maximum circular velocity. As an initial application of these predictions, we successfully constrain Ωm, σ8, and the scatter in subhalo-abundance-matching employing the projected correlation function of mock SDSS galaxies.

Measuring dynamical masses from gas kinematics in simulated high-redshift galaxies

ABSTRACT Advances in instrumentation have recently extended detailed measurements of gas kinematics to large samples of high-redshift galaxies. Relative to most nearby, thin disc galaxies, in which gas rotation accurately traces the gravitational potential, the interstellar medium (ISM) of $z$ ≳ 1 galaxies is typically more dynamic and exhibits elevated turbulence. If not properly modelled, these effects can strongly bias dynamical mass measurements. We use high-resolution FIRE-2 cosmological zoom-in simulations to analyse the physical effects that must be considered to correctly infer dynamical masses from gas kinematics. Our analysis covers a range of galaxy properties from low-redshift Milky-Way-mass galaxies to massive high-redshift galaxies (M⋆ > 1011 M⊙ at $z$ = 1). Selecting only snapshots where a disc is present, we calculate the rotational profile $\bar{v}_\phi (r)$ of the cool ($10^{3.5}\,\lt {\it T}\lt 10^{4.5}~\rm {K}$) gas and compare it to the circular velocity $v_{\rm c}=\sqrt{GM_{\rm enc}/r}$. In the simulated galaxies, the gas rotation traces the circular velocity at intermediate radii, but the two quantities diverge significantly in the centre and in the outer disc. Our simulations appear to over-predict observed rotational velocities in the centres of massive galaxies (likely from a lack of black hole feedback), so we focus on larger radii. Gradients in the turbulent pressure at these radii can provide additional radial support and bias dynamical mass measurements low by up to 40 per cent. In both the interior and exterior, the gas’ motion can be significantly non-circular due to e.g. bars, satellites, and inflows/outflows. We discuss the accuracy of commonly used analytic models for pressure gradients (or ‘asymmetric drift’) in the ISM of high-redshift galaxies.

Modelling the tightest relation between galaxy properties and dark matter halo properties from hydrodynamical simulations of galaxy formation

ABSTRACT We investigate how a property of a galaxy correlates most tightly with a property of its host dark matter halo, using state-of-the-art hydrodynamical simulations of galaxy formation: EAGLE, Illustris, and IllustrisTNG. Unlike most of the previous work, our analyses focus on all types of galaxies, including both central and satellite galaxies. We find that the stellar mass of a galaxy at the epoch of the peak circular velocity with an evolution correction gives the tightest such correlation to the peak circular velocity Vpeak of the galaxy’s underling dark matter halo. The evolution of galaxy stellar mass reduces rather than increases scatter in such a relation. We also find that one major source of scatter comes from star stripping due to the strong interactions between galaxies. Even though, we show that the size of scatter predicted by hydrodynamical simulations has a negligible impact on the clustering of dense Vpeak-selected subhalo from simulations, which suggests that even the simplest subhalo abundance matching (SHAM), without scatter and any additional free parameter, can provide a robust prediction of galaxy clustering that can agree impressively well with the observations from the Sloan Digital Sky Survey (SDSS) main galaxy survey.

The H i velocity function: a test of cosmology or baryon physics?

Accurately predicting the shape of the H i velocity function (VF) of galaxies is regarded widely as a fundamental test of any viable dark matter model. Straightforward analyses of cosmological N-body simulations imply that the Λ cold dark matter (ΛCDM) model predicts an overabundance of low circular velocity galaxies when compared to observed H i VFs. More nuanced analyses that account for the relationship between galaxies and their host haloes suggest that how we model the influence of baryonic processes has a significant impact on H i VF predictions. We explore this in detail by modelling H i emission lines of galaxies in the shark semi-analytic galaxy formation model, built on the surfs suite of ΛCDM N-body simulations. We create a simulated ALFALFA survey, in which we apply the survey selection function and account for effects such as beam confusion, and compare simulated and observed H i velocity width distributions, finding differences of ≲ 50 per cent, orders of magnitude smaller than the discrepancies reported in the past. This is a direct consequence of our careful treatment of survey selection effects and, importantly, how we model the relationship between galaxy and halo circular velocity – the H i mass–maximum circular velocity relation of galaxies is characterized by a large scatter. These biases are complex enough that building a VF from the observed H i linewidths cannot be done reliably.

Peculiar motions of the gas at the centre of the barred galaxy UGC 4056

We derive the circular velocity curves of the gaseous and stellar discs of UGC 4056, a giant barred galaxy with an active galactic nucleus (AGN). We analyse UGC 4056 using the 2D spectroscopy obtained within the framework of the Mapping Nearby Galaxies at APO (MaNGA) survey. Using images and the colour index g − r from the Sloan Digital Sky Survey (SDSS), we determined the tilt of the galaxy, which allows us to conclude that the galaxy rotates clockwise with trailing spiral arms. We found that the gas motion at the central part of the UGC 4056 shows peculiar features. The rotation velocity of the gaseous disc shows a bump within around three kiloparsecs while the rotation velocity of the stellar disc falls smoothly to zero with decreasing galactocentric distance. We demonstrate that the peculiar radial velocities in the central part of the galaxy may be caused by the inflow of the gas towards the nucleus of the galaxy. The unusual motion of the gas takes place at the region with the AGN-like radiation and can be explained by the gas response to the bar potential.