X-Ray Ionization of Planet-Opened Gaps in Protostellar Disks

Young planets with masses approaching Jupiter's have tides strong enough to clear gaps around their orbits in the protostellar disk. Gas flow through the gaps regulates the planets' further growth and governs the disks' evolution. Magnetic forces may drive that flow if the gas is sufficiently ionized to couple to the fields. We compute the ionizing effects of the X-rays from the central young star, using Monte Carlo radiative transfer calculations to find the spectrum of Compton-scattered photons reaching the planet's vicinity. The scattered X-rays ionize the gas at rates similar to or greater than the interstellar cosmic ray rate near planets the mass of Saturn and of Jupiter, located at 5 au and at 10 au, in disks with the interstellar mass fraction of sub-micron dust and with the dust depleted a factor 100. Solving a gas-grain recombination reaction network yields charged particle populations whose ability to carry currents is sufficient to partly couple the magnetic fields to the gas around the planet. Most cases can undergo Hall shear instability, and some can launch magnetocentrifugal winds. However the material on the planet's orbit has diffusivities so large in all the cases we examine, that magneto-rotational turbulence is prevented and the non-ideal terms govern the magnetic field's evolution. Thus the flow of gas in the gaps opened by the young giant planets depends crucially on the finite conductivity.Comment: 24 pages, 6 figures. Gap depths are now chosen to match recent hydrodynamical results. Accepted for publication in Ap

The Milky Way satellite velocity function is a sharp probe of small-scale structure problems

Twenty years ago, the mismatch between the observed number of Milky Way satellite galaxies and the predicted number of cold dark matter subhalos was dubbed the "missing satellites problem". Although mostly framed since in terms of satellite counts in luminosity space, the missing satellites problem was originally posed in velocity space. The stellar velocity dispersion function encodes information about the density profile of satellites as well as their abundance. We compare the completeness-corrected MW satellite velocity function down to its ultrafaint dwarfs (L > 340 L$_\odot$) against well-motivated, semi-empirical predictions based on galaxy-halo scaling relations. For our most conservative completeness correction, we find good agreement with a simple CDM model in which massive, classical satellites (M$_{\rm vir} \gtrsim 10^9~$M$_\odot$) have baryon-driven cores, while low-mass, ultrafaint satellites (M$_{\rm vir} \lesssim 10^9~$M$_\odot$) inhabit cuspy halos that are not strongly tidally stripped. This bifurication is required to explain a non-power-law feature in the velocity function at $\sigma_{\rm los}^* \approx 10$ km/s. Intriguingly, this feature could point to a flattening of the stellar-mass--halo-mass relation. Tidal destruction of satellites by the Milky Way's disk must be minimal, or the corrected velocity function exceeds any plausible prediction -- a "too many satellites" problem. We rule out non-core-collapsing self-interacting dark matter models with a constant cross section $\gtrsim$ 0.3 cm$^2$/g. Constraints on warm dark matter are stronger than those based on the luminosity function on account of the velocity function's additional sensitivity to the central densities of subhalos. Reducing uncertainties on stellar kinematics and the amount of tidal stripping experienced by the faintest dwarfs is key to determining the severity of the too many satellites problem.Comment: 19 pages, 13 figures. Key results are summarized in Figure 6. To be submitted to MNRAS. Comments welcome

EDGE: The shape of dark matter haloes in the faintest galaxies

Collisionless Dark Matter Only (DMO) structure formation simulations predict that Dark Matter (DM) haloes are prolate in their centres and triaxial towards their outskirts. The addition of gas condensation transforms the central DM shape to be rounder and more oblate. It is not clear, however, whether such shape transformations occur in `ultra-faint' dwarfs, which have extremely low baryon fractions. We present the first study of the shape and velocity anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of $f_{\rm gas}(r<R_{\rm half}) < 0.06$. These dwarfs are drawn from the Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high resolution simulations that allow us to resolve DM halo shapes within the half light radius ($\sim 100\,$pc). We show that gas-poor ultra-faints ($M_{\rm 200c} \leqslant 1.5\times10^9\,$M$_\odot$; $f_{\rm gas} < 10^{-5}$) retain their pristine prolate DM halo shape even when gas, star formation and feedback are included. This could provide a new and robust test of DM models. By contrast, gas-rich ultra-faints ($M_{\rm 200c} > 3\times10^9\,$M$_\odot$; $f_{\rm gas} > 10^{-4}$) become rounder and more oblate within $\sim 10$ half light radii. Finally, we find that most of our simulated dwarfs have significant radial velocity anisotropy that rises to $\tilde{\beta} > 0.5$ at $R \gtrsim 3 R_{\rm half}$. The one exception is a dwarf that forms a rotating gas/stellar disc because of a planar, major merger. Such strong anisotropy should be taken into account when building mass models of gas-poor ultra-faints.Comment: 16 pages and 11 figures (excluding appendices), accepted by MNRA

EDGE: the puzzling ellipticity of Eridanus II's star cluster and its implications for dark matter at the heart of an ultra-faint dwarf

The Eridanus II (EriII) 'ultra-faint' dwarf has a large ($15\,\text{pc}$) and low mass ($4.3\times10^3\,\text{M}_\odot$) star cluster (SC) offset from its centre by $23\pm3\,\text{pc}$ in projection. Its size and offset are naturally explained if EriII has a central dark matter core, but such a core may be challenging to explain in a $\Lambda$CDM cosmology. In this paper, we revisit the survival and evolution of EriII's SC, focussing for the first time on its puzzlingly large ellipticity ($0.31^{+0.05}_{-0.06}$). We perform a suite of 960 direct $N$-body simulations of SCs, orbiting within a range of spherical background potentials fit to ultra-faint dwarf (UFD) galaxy simulations. We find only two scenarios that come close to explaining EriII's SC. In the first, EriII has a low density dark matter core (of size $\sim70\,\text{pc}$ and density $\lesssim2\times10^8\,\text{M}_{\odot}\,\text{kpc}^{-3}$). In this model, the high ellipticity of EriII's SC is set at birth, with the lack of tidal forces in the core allowing its ellipticity to remain frozen in for long times. In the second, EriII's SC orbits in a partial core, with its high ellipticity owing to its imminent tidal destruction. However, this latter model struggles to reproduce the large size of EriII's SC, and it predicts substantial tidal tails around EriII's SC that should have already been seen in the data. This leads us to favour the cored model. We discuss potential caveats to these findings, and the implications of the cored model for galaxy formation and the nature of dark matter.Comment: 16 pages, 12 figures + appendices. Published with MNRAS. Comments welcom

EDGE -- Dark matter or astrophysics? Clear prospects to break dark matter heating degeneracies with HI rotation in faint dwarf galaxies

Low-mass dwarf galaxies are expected to showcase pristine `cuspy' inner dark matter density profiles compared to their stellar sizes, as they form too few stars to significantly drive dark matter heating through supernovae-driven outflows. Here, we study such simulated faint systems ($10^4 \leq M_{\star} \leq 2\times 10^6 \, M_\mathrm{\odot}$) drawn from high-resolution (3 pc) cosmological simulations from the `Engineering Dwarf Galaxies at the Edge of galaxy formation' (EDGE) project. We confirm that these objects have steep and rising inner dark matter density profiles at $z=0$, little affected by galaxy formation effects. But five dwarf galaxies from the suite showcase a detectable HI reservoir ($M_{\mathrm{HI}}\approx 10^{5}-10^{6} \, M_\mathrm{\odot}$), analogous to the observed population of faint, HI-bearing dwarf galaxies. These reservoirs exhibit episodes of ordered rotation, opening windows for rotation curve analysis. Within actively star-forming dwarfs, stellar feedback easily disrupts the tenuous HI discs ($v_{\phi} \approx 10\, \mathrm{km} \, \mathrm{s}^{-1}$), making rotation short-lived ($\ll 150 \, \mathrm{Myr}$) and more challenging to interpret for dark matter inferences. Contrastingly, we highlight a long-lived ($\geq 500 \, \mathrm{Myr}$) and easy-to-interpret HI rotation curve extending to $\approx 2\, r_{1/2, \text{3D}}$ in a quiescent dwarf, that has not formed new stars since $z=4$. This stable gas disc is supported by an oblate dark matter halo shape that drives high angular momentum gas flows. Our results strongly motivate further searches for HI rotation curves in the observed population of HI-bearing low-mass dwarfs, that provide a key regime to disentangle the respective roles of dark matter microphysics and galaxy formation effects in driving dark matter heating.Comment: Main text 10 pages, submitted to MNRAS. Comments welcome

Andromeda XXV -- a dwarf galaxy with a low central dark matter density

Andromeda (And) XXV has previously been reported as a dwarf spheroidal galaxy (dSph) with little-to-no dark matter. However, the uncertainties on this result were significant. In this study, we double the number of member stars and re-derive the kinematics and mass of And XXV. We find that And XXV has a systemic velocity of $\nu_\mathrm{r}=-107.7\pm1.0 \mathrm{~km s}^{-1}$ and a velocity dispersion of $\sigma_\nu=4.5\pm1.0\mathrm{~km s}^{-1}$. With this better constrained velocity dispersion, we derive a mass contained within the half-light radius of $M(r< r_\mathrm{h})=6.9^{+3.2}_{-2.8}\times10^6\mathrm{~M}_\odot$. This mass corresponds to a mass-to-light ratio of $\mathrm{[M/L]}_\mathrm{r_\mathrm{h}}=37^{+17}_{-15}\mathrm{~M}_\odot/\mathrm{L}_\odot$, demonstrating, for the first time, that And XXV has an unambiguous dark matter component. We also measure the metallicity of And XXV to be $\mathrm{[Fe/H]}=-1.9\pm0.1$$\mathrm{~}$dex, which is in agreement with previous results. Finally, we extend the analysis of And XXV to include mass modelling using GravSphere. We find that And XXV has a low central dark matter density, $\rho_\mathrm{DM}(150\mathrm{pc})= 2.7^{+1.8}_{-1.6}\times10^7\mathrm{~M}_\odot\mathrm{kpc}^{-3}$, making And XXV a clear outlier when compared to other Local Group (LG) dSphs of the similar stellar mass. In a companion paper, we will explore whether some combination of dark matter cusp-core transformations and/or tides can explain And XXV's low density.Comment: 13 pages, 8 figures (7 main, 1 appendix). Submitted to MNRA

The PAndAS View of the Andromeda Satellite System. IV Global properties

We build a statistical framework to infer the global properties of the satellite system of the Andromeda galaxy (M31) from the properties of individual dwarf galaxies located in the Pan-Andromeda Archaelogical Survey (PAndAS) and the previously determined completeness of the survey. Using forward modeling, we infer the slope of the luminosity function of the satellite system, the slope of its spatial density distribution, and the size-luminosity relation followed by the dwarf galaxies. We find that the slope of the luminosity function is $\beta=-1.5\pm0.1$. Combined with the spatial density profile, it implies that, when accounting for survey incompleteness, M31 hosts $92_{-26}^{+19}$ dwarf galaxies with $M_\textrm{V}<-5.5$ and a sky-projected distance from M31 between 30 and 300kpc. We conclude that many faint or distant dwarf galaxies remain to be discovered around Andromeda, especially outside the PAndAS footprint. Finally, we use our model to test if the higher number of satellites situated in the hemisphere facing the Milky Way could be explained simply by the detection limits of dwarf galaxy searches. We rule this out at $>99.9\%$ confidence and conclude that this anisotropy is an intrinsic feature of the M31 satellite system. The statistical framework we present here is a powerful tool to robustly constrain the properties of a satellite system and compare those across hosts, especially considering the upcoming start of the Euclid or Rubin large photometric surveys that are expected to uncover a large number of dwarf galaxies in the Local Volume.Comment: Submitted to ApJ - 12 pages, 6 figures, 2 table

EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies

Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. In this paper, we study the projected stellar ellipticities and faint stellar outskirts of tidally isolated ultra-faints drawn from the 'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) cosmological simulation suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range of projected ellipticities ($0.03 < \varepsilon < 0.85$), with many possessing anisotropic extended stellar haloes that mimic tidal tails, but owe instead to late-time accretion of lower mass companions. Furthermore, we find a strong causal relationship between ellipticity and formation time of an UFD, which is robust to a wide variation in the feedback model. We show that the distribution of projected ellipticities in our suite of simulated EDGE dwarfs matches well with that of 21 Local Group dwarf galaxies. Given the ellipticity in EDGE arises from an ex-situ accretion origin, the agreement in shape indicates the ellipticities of some observed dwarfs may also originate from a similar non-tidal scenario. The orbital parameters of these observed dwarfs further support that they are not currently tidally disrupting. If the baryonic content in these galaxies is still tidally intact, then the same may be true for their dark matter content, making these galaxies in our Local Group pristine laboratories for testing dark matter and galaxy formation models.Comment: 10 pages, 4 figures; submitted to MNRA