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"Halos"
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The Stellar Halo of the Galaxy is Tilted and Doubly Broken
Modern Galactic surveys have revealed an ancient merger that dominates the stellar halo of our galaxy (Gaia–Sausage–Enceladus, GSE). Using chemical abundances and kinematics from the H3 Survey, we identify 5559 halo stars from this merger in the radial range r Gal = 6–60kpc. We forward model the full selection function of H3 to infer the density profile of this accreted component of the stellar halo. We consider a general ellipsoid with principal axes allowed to rotate with respect to the galactocentric axes, coupled with a multiply broken power law. The best-fit model is a triaxial ellipsoid (axes ratios 10:8:7) tilted 25° above the Galactic plane toward the Sun and a doubly broken power law with breaking radii at 12 kpc and 28 kpc. The doubly broken power law resolves a long-standing dichotomy in literature values of the halo breaking radius, being at either ∼15 kpc or ∼30 kpc assuming a singly broken power law. N-body simulations suggest that the breaking radii are connected to apocenter pile-ups of stellar orbits, and so the observed double-break provides new insight into the initial conditions and evolution of the GSE merger. Furthermore, the tilt and triaxiality of the stellar halo could imply that a fraction of the underlying dark matter halo is also tilted and triaxial. This has important implications for dynamical mass modeling of the galaxy as well as direct dark matter detection experiments.
Journal Article
How Do the Velocity Anisotropies of Halo Stars, Dark Matter, and Satellite Galaxies Depend on Host Halo Properties?
We investigate the mass (M 200) and concentration (c 200) dependencies of the velocity anisotropy (β) profiles for different components in the dark matter halo—including halo stars, dark matter, and subhalos—using systems from the IllustrisTNG simulations. Beyond a critical radius, β becomes more radial with the increase of M 200, reflecting more prominent radial accretion around massive halos. The critical radius is r ∼ r s , 0.3 r s , and r s for halo stars, dark matter, and subhalos, with r s being the scale radius of the host halos. This dependence on M 200 is the strongest for subhalos and the weakest for halo stars. In central regions, the β of halo stars and dark matter particles get more isotropic with the increase of M 200 in TNG300 due to baryons. By contrast, the β of dark matter from the dark-matter-only TNG300-Dark run shows much weaker dependence on M 200 within r s . Dark matter in TNG300 is slightly more isotropic than in TNG300-Dark at 0.2 r s < r < 10 r s and log10M200/M⊙<13.8 . Halo stars and dark matter also become more radial with the increase in c 200, at fixed M 200. Halo stars are more radial than the β profile of dark matter by approximately a constant beyond r s . Dark matter particles are more radial than subhalos. The differences can be understood, as subhalos on more radial orbits are more easily stripped, contributing more stars and dark matter to the diffuse components. We provide the fitting formula for the differences between the β of halo stars and dark matter at r s < r < 3 r s as βstar−βDM=(−0.034±0.012)log10M200/M⊙+(0.772±0.163) for log10M200/M⊙≥13 and as β star − β DM = 0.328 for log10M200/M⊙<13 .
Journal Article
Cosmological Predictions for Minor Axis Stellar Density Profiles in the Inner Regions of Milky Way–mass Galaxies
ΛCDM cosmology predicts the hierarchical formation of galaxies, which build up mass by merger events and accreting smaller systems. The stellar halo of the Milky Way (MW) has proven to be useful a tool for tracing this accretion history. However, most of this work has focused on the outer halo where dynamical times are large and the dynamical properties of accreted systems are preserved. In this work, we investigate the inner galaxy regime, where dynamical times are relatively small and systems are generally completely phase mixed. Using the FIRE-2 and Auriga cosmological zoom-in simulation suites of MW-mass galaxies, we find the stellar density profiles along the minor axis (perpendicular to the galactic disk) within the Navarro–Frenk–White scale radii (R ≈ 15 kpc) are best described as an exponential disk with scale height < 0.3 kpc and a power-law component with slope α ≈ −4. The stellar density amplitude and slope for the power-law component are not significantly correlated with metrics of the galaxy’s accretion history. Instead, we find the stellar profiles strongly correlate with the dark matter profile. Across simulation suites, the galaxies studied in this work have a stellar-to-dark-matter mass ratio that decreases as 1/r2 along the minor axis.
Journal Article
Cool outflows in galaxies and their implications
Neutral-atomic and molecular outflows are a common occurrence in galaxies, near and far. They operate over the full extent of their galaxy hosts, from the innermost regions of galactic nuclei to the outermost reaches of galaxy halos. They carry a substantial amount of material that would otherwise have been used to form new stars. These cool outflows may have a profound impact on the evolution of their host galaxies and environments. This article provides an overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe. The remaining outstanding issues that have not yet been resolved are summarized at the end of the review to inspire new research directions.
Journal Article
Mapping Dark Matter with Extragalactic Stellar Streams: The Case of Centaurus A
In the coming decade, thousands of stellar streams will be observed in the halos of external galaxies. What fundamental discoveries will we make about dark matter from these streams? As a first attempt to look at these questions, we model Magellan/Megacam imaging of the Centaurus A (Cen A) disrupting dwarf companion Dwarf 3 (Dw3) and its associated stellar stream, to find out what can be learned about the Cen A dark matter halo. We develop a novel external galaxy stream-fitting technique and generate model stellar streams that reproduce the stream morphology visible in the imaging. We find that there are many viable stream models that fit the data well, with reasonable parameters, provided that Cen A has a halo mass larger than M 200 > 4.70 × 1012 M ⊙. There is a second stream in Cen A’s halo that is also reproduced within the context of this same dynamical model. However, stream morphology in the imaging alone does not uniquely determine the mass or mass distribution for the Cen A halo. In particular, the stream models with high likelihood show covariances between the inferred Cen A mass distribution, the inferred Dw3 progenitor mass, the Dw3 velocity, and the Dw3 line-of-sight position. We show that these degeneracies can be broken with radial-velocity measurements along the stream, and that a single radial velocity measurement puts a substantial lower limit on the halo mass. These results suggest that targeted radial-velocity measurements will be critical if we want to learn about dark matter from extragalactic stellar streams.
Journal Article
Probing the Dark Matter Halos of External Galaxies with Stellar Streams
Stellar streams have proven to be powerful tools for measuring the Milky Way’s gravitational potential and hence its dark matter (DM) halo. In the coming years, the Vera Rubin Observatory, Euclid, ARRAKIHS, and the Nancy Grace Roman Space Telescope will uncover a plethora of streams around external galaxies. Although great in number, observations of these distant streams will often be limited to only the on-sky position of the stream. In this work, we explore how well we will be able to measure the DM halos of these galaxies by fitting simplified mock streams with a variety of intrinsic and orbital properties in a range of data availability scenarios. We find that results vary based on the interplay between the amount of information provided by a stream’s intrinsic properties versus that of the observational uncertainties, as well as on the form of potential assumed. In general, we find streams with multiple wraps around their host galaxy can constrain the overall radial profile and scale radius of the potential without radial velocities. In many other cases, a single radial velocity measurement often provides a significant boost to constraining power for the radial profile, scale radius, and enclosed mass of the DM halo. Given the wealth of data expected soon, this suggests that we will be able to measure the DM halos of a statistically significant sample of galaxies with stellar streams in the coming years.
Journal Article
Solutions of the Einstein Equations for a Black Hole Surrounded by a Galactic Halo
Various profiles of matter distribution in galactic halos (such as the Navarro–Frenk–White, Burkert, Hernquist, Moore, Taylor–Silk models, and others) are considered here as the source term for the Einstein equations. We solve these equations and find exact solutions that represent the metric of a central black hole immersed in a galactic halo. Even though in the general case the solution is numerical, very accurate general analytical metrics, which include all the particular models, are found in the astrophysically relevant regime, when the mass of the galaxy is much smaller than the characteristic scale in the halo.
Journal Article
A massive galaxy that formed its stars at z ≈ 11
The formation of galaxies by gradual hierarchical co-assembly of baryons and cold dark matter halos is a fundamental paradigm underpinning modern astrophysics
1
,
2
and predicts a strong decline in the number of massive galaxies at early cosmic times
3
–
5
. Extremely massive quiescent galaxies (stellar masses of more than 10
11
M
⊙
) have now been observed as early as 1–2 billion years after the Big Bang
6
–
13
. These galaxies are extremely constraining on theoretical models, as they had formed 300–500 Myr earlier, and only some models can form massive galaxies this early
12
,
14
. Here we report on the spectroscopic observations with the JWST of a massive quiescent galaxy ZF-UDS-7329 at redshift 3.205 ± 0.005. It has eluded deep ground-based spectroscopy
8
, it is significantly redder than is typical and its spectrum reveals features typical of much older stellar populations. Detailed modelling shows that its stellar population formed around 1.5 billion years earlier in time (
z
≈ 11) at an epoch when dark matter halos of sufficient hosting mass had not yet assembled in the standard scenario
4
,
5
. This observation may indicate the presence of undetected populations of early galaxies and the possibility of significant gaps in our understanding of early stellar populations, galaxy formation and the nature of dark matter.
A massive galaxy observed with the JWST indicates that the bulk of its stars formed within the first 500 million years of the Universe.
Journal Article
Strong Dark Matter Self-interactions Diversify Halo Populations within and surrounding the Milky Way
We perform a high-resolution cosmological zoom-in simulation of a Milky Way (MW)–like system, which includes a realistic Large Magellanic Cloud analog, using a large differential elastic dark matter self-interaction cross section that reaches ≈100 cm2 g−1 at relative velocities of ≈10 km s−1, motivated by the diverse and orbitally dependent central densities of dwarf galaxies within and surrounding the MW. We explore the effects of dark matter self-interactions on satellite, splashback, and isolated halos through their abundance, central densities, maximum circular velocities, orbital parameters, and correlations between these variables. We use an effective constant cross section model to analytically predict the stages of our simulated halos’ gravothermal evolution, demonstrating that deviations from the collisionless Rmax – Vmax relation can be used to select deeply core-collapsed halos, where Vmax is a halo’s maximum circular velocity, and Rmax is the radius at which it occurs. We predict that a sizable fraction (≈20%) of subhalos with masses down to ≈108 M ⊙ is deeply core collapsed in our SIDM model. Core-collapsed systems form ≈10% of the isolated halo population down to the same mass; these isolated, core-collapsed halos would host faint dwarf field galaxies with extremely steep central density profiles. Finally, most halos with masses above ≈109 M ⊙ are core-forming in our simulation. Our study thus demonstrates how self-interactions diversify halo populations in an environmentally dependent fashion within and surrounding MW-mass hosts, providing a compelling avenue to address the diverse dark matter distributions of observed dwarf galaxies.
Journal Article