Small-Scale Challenges to the ΛCDM Paradigm Bullock, James S; Boylan-Kolchin, Michael
Annual review of astronomy and astrophysics,
08/2017, Letnik:
55, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The dark energy plus cold dark matter (ΛCDM) cosmological model has been a demonstrably successful framework for predicting and explaining the large-scale structure of the Universe and its evolution ...with time. Yet on length scales smaller than ∼1 Mpc and mass scales smaller than ∼10
11
M
, the theory faces a number of challenges. For example, the observed cores of many dark matter-dominated galaxies are both less dense and less cuspy than naïvely predicted in ΛCDM. The number of small galaxies and dwarf satellites in the Local Group is also far below the predicted count of low-mass dark matter halos and subhalos within similar volumes. These issues underlie the most well-documented problems with ΛCDM: cusp/core, missing satellites, and too-big-to-fail. The key question is whether a better understanding of baryon physics, dark matter physics, or both is required to meet these challenges. Other anomalies, including the observed planar and orbital configurations of Local Group satellites and the tight baryonic/dark matter scaling relations obeyed by the galaxy population, have been less thoroughly explored in the context of ΛCDM theory. Future surveys to discover faint, distant dwarf galaxies and to precisely measure their masses and density structure hold promising avenues for testing possible solutions to the small-scale challenges going forward. Observational programs to constrain or discover and characterize the number of truly dark low-mass halos are among the most important, and achievable, goals in this field over the next decade. These efforts will either further verify the ΛCDM paradigm or demand a substantial revision in our understanding of the nature of dark matter.
ELVIS: Exploring the Local Volume in Simulations Garrison-Kimmel, Shea; Boylan-Kolchin, Michael; Bullock, James S. ...
Monthly Notices of the Royal Astronomical Society,
03/2014, Letnik:
438, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We introduce a set of high-resolution dissipationless simulations that model the Local Group (LG) in a cosmological context: Exploring the Local Volume in Simulations (ELVIS). The suite contains 48 ...Galaxy-size haloes, each within high-resolution volumes that span 2-5 Mpc in size, and each resolving thousands of systems with masses below the atomic cooling limit. Half of the ELVIS galaxy haloes are in paired configurations similar to the Milky Way (MW) and M31; the other half are isolated, mass-matched analogues. We find no difference in the abundance or kinematics of substructure within the virial radii of isolated versus paired hosts. On Mpc scales, however, LG-like pairs average almost twice as many companions and the velocity field is kinematically hotter and more complex. We present a refined abundance matching relation between stellar mass and halo mass that reproduces the observed satellite stellar mass functions of the MW and M31 down to the regime where incompleteness is an issue, M
* ∼ 5 × 105 M. Within a larger region spanning approximately 3 Mpc, the same relation predicts that there should be ∼1000 galaxies with M
* > 103 M awaiting discovery. We show that up to 50 per cent of haloes within 1 Mpc of the MW or M31 could be systems that have previously been within the virial radius of either giant. By associating never accreted haloes with gas-rich dwarfs, we show that there are plausibly 50 undiscovered dwarf galaxies with H i masses >105 M within the local volume. The radial velocity distribution of these predicted gas-rich dwarfs can be used to inform follow-up searches based on ultracompact high-velocity clouds found in the ALFALFA survey.
ABSTRACT
We show that dissipationless Λ cold dark matter simulations predict that the majority of the most massive subhaloes of the Milky Way are too dense to host any of its bright satellites (LV > ...105 L⊙). These dark subhaloes have peak circular velocities at infall of Vinfall= 30–70 km s−1 and infall masses of (0.2–4) × 1010 M⊙. Unless the Milky Way is a statistical anomaly, this implies that galaxy formation becomes effectively stochastic at these masses. This is in marked contrast to the well‐established monotonic relation between galaxy luminosity and halo circular velocity (or halo mass) for more massive haloes. We show that at least two (and typically four) of these massive dark subhaloes are expected to produce a larger dark matter annihilation flux than Draco. It may be possible to circumvent these conclusions if baryonic feedback in dwarf satellites or different dark matter physics can reduce the central densities of massive subhaloes by order unity on a scale of 0.3–1 kpc.
We use Local Group galaxy counts together with the ELVIS N-body simulations to explore the relationship between the scatter and slope in the stellar mass versus halo mass relation at low masses, M... ...10 super( 5)-10 super( 8) M... Assuming models with lognormal scatter about a median relation of the form M... M..., the preferred log-slope steepens from a ... 1.8 in the limit of zero scatter to a ... 2.6 in the case of 2 dex of scatter in M... at fixed halo mass. We provide fitting functions for the best-fitting relations as a function of scatter, including cases where the relation becomes increasingly stochastic with decreasing mass. We show that if the scatter at fixed halo mass is large enough (... 1 dex) and if the median relation is steep enough (a ... 2), then the 'too-big-to-fail' problem seen in the Local Group can be self-consistently eliminated in about ~5-10 per cent of realizations. This scenario requires that the most massive subhaloes host unobservable ultra-faint dwarfs fairly often; we discuss potentially observable signatures of these systems. Finally, we compare our derived constraints to recent high-resolution simulations of dwarf galaxy formation in the literature. Though simulation-to-simulation scatter in M... at fixed Mhalo is large among different authors (~2 dex), individual codes produce relations with much less scatter and usually give relations that would overproduce local galaxy counts. (ProQuest: ... denotes formulae/symbols omitted.)
We present spectroscopic metallicities of individual stars in seven gas-rich dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same mass-metallicity relation as the dwarf spheroidal ...(dSph) satellites of both the Milky Way and M31: Z sub(*) is proportional to M super(0.30+ or -0.02) sub(*). The uniformity of the relation is in contradiction to previous estimates of metallicity based on photometry. This relationship is roughly continuous with the stellar mass-stellar metallicity relation for galaxies as massive as M sub(*) = 10 super(12) M sub(middot in circle). Although the average metallicities of dwarf galaxies depend only on stellar mass, the shapes of their metallicity distributions depend on galaxy type. The metallicity distributions of dIrrs resemble simple, leaky box chemical evolution models, whereas dSphs require an additional parameter, such as gas accretion, to explain the shapes of their metallicity distributions. Furthermore, the metallicity distributions of the more luminous dSphs have sharp, metal-rich cut-offs that are consistent with the sudden truncation of star formation due to ram pressure stripping.
We use the Aquarius simulations to show that the most massive subhaloes in galaxy-mass dark matter (DM) haloes in Λ cold dark matter (ΛCDM) are grossly inconsistent with the dynamics of the brightest ...Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf spheroidals all have
, ΛCDM simulations predict at least 10 subhaloes with V
max > 25 km s−1. These subhaloes are also among the most massive at earlier times, and significantly exceed the reionization suppression mass back to z∼ 10. No ΛCDM-based model of the satellite population of the Milky Way explains this result. The problem lies in the satellites' densities: it is straightforward to match the observed Milky Way luminosity function, but doing so requires the dwarf spheroidals to have DM haloes that are a factor of ∼5 more massive than is observed. Independent of the difficulty in explaining the absence of these dense, massive subhaloes, there is a basic tension between the derived properties of the bright Milky Way dwarf spheroidals and ΛCDM expectations. The inferred infall masses of these galaxies are all approximately equal and are much lower than standard ΛCDM predictions for systems with their luminosities. Consequently, their implied star formation efficiencies span over two orders of magnitude, from 0.2 to 20 per cent of baryons converted into stars, in stark contrast with expectations gleaned from more massive galaxies. We explore possible solutions to these problems within the context of ΛCDM and find them to be unconvincing. In particular, we use controlled simulations to demonstrate that the small stellar masses of the bright dwarf spheroidals make supernova feedback an unlikely explanation for their low inferred densities.
ABSTRACT
We perform high-resolution simulations of an MW-like galaxy in a self-interacting cold dark matter model with elastic cross-section over mass of $1~\rm cm^2\, g^{-1}$ (SIDM) and compare to a ...model without self-interactions (CDM). We run our simulations with and without a time-dependent embedded potential to capture effects of the baryonic disc and bulge contributions. The CDM and SIDM simulations with the embedded baryonic potential exhibit remarkably similar host halo profiles, subhalo abundances, and radial distributions within the virial radius. The SIDM host halo is denser in the centre than the CDM host and has no discernible core, in sharp contrast to the case without the baryonic potential (core size ${\sim}7 \, \rm kpc$). The most massive subhaloes (with $V_{\mathrm{peak}}\gt 20 \, \rm km\, s^{-1}$) in our SIDM simulations, expected to host the classical satellite galaxies, have density profiles that are less dense than their CDM analogues at radii less than 500 pc but the deviation diminishes for less massive subhaloes. With the baryonic potential included in the CDM and SIDM simulations, the most massive subhaloes do not display the too-big-to-fail problem. However, the least dense among the massive subhaloes in both these simulations tend to have the smallest pericenter values, a trend that is not apparent among the bright MW satellite galaxies.
We present a series of high-resolution cosmological simulations1 of galaxy formation to z = 0, spanning halo masses ∼108–1013 M⊙, and stellar masses ∼104–1011 M⊙. Our simulations include fully ...explicit treatment of the multiphase interstellar medium and stellar feedback. The stellar feedback inputs (energy, momentum, mass, and metal fluxes) are taken directly from stellar population models. These sources of feedback, with zero adjusted parameters, reproduce the observed relation between stellar and halo mass up to M
halo ∼ 1012 M⊙. We predict weak redshift evolution in the M
*–M
halo relation, consistent with current constraints to z > 6. We find that the M
*–M
halo relation is insensitive to numerical details, but is sensitive to feedback physics. Simulations with only supernova feedback fail to reproduce observed stellar masses, particularly in dwarf and high-redshift galaxies: radiative feedback (photoheating and radiation pressure) is necessary to destroy giant molecular clouds and enable efficient coupling of later supernovae to the gas. Star formation rates (SFRs) agree well with the observed Kennicutt relation at all redshifts. The galaxy-averaged Kennicutt relation is very different from the numerically imposed law for converting gas into stars, and is determined by self-regulation via stellar feedback. Feedback reduces SFRs and produces reservoirs of gas that lead to rising late-time star formation histories, significantly different from halo accretion histories. Feedback also produces large short-time-scale variability in galactic SFRs, especially in dwarfs. These properties are not captured by common ‘sub-grid’ wind models.
Cold dark matter: Controversies on small scales Weinberg, David H; James S. Bullock; Fabio Governato ...
Proceedings of the National Academy of Sciences - PNAS,
10/2015, Letnik:
112, Številka:
40
Journal Article
Recenzirano
Odprti dostop
The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations ...that probe the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. We review the current observational and theoretical status of these “small-scale controversies.” Cosmological simulations that incorporate only gravity and collisionless CDM predict halos with abundant substructure and central densities that are too high to match constraints from galaxy dynamics. The solution could lie in baryonic physics: Recent numerical simulations and analytical models suggest that gravitational potential fluctuations tied to efficient supernova feedback can flatten the central cusps of halos in massive galaxies, and a combination of feedback and low star formation efficiency could explain why most of the dark matter subhalos orbiting the Milky Way do not host visible galaxies. However, it is not clear that this solution can work in the lowest mass galaxies, where discrepancies are observed. Alternatively, the small-scale conflicts could be evidence of more complex physics in the dark sector itself. For example, elastic scattering from strong dark matter self-interactions can alter predicted halo mass profiles, leading to good agreement with observations across a wide range of galaxy mass. Gravitational lensing and dynamical perturbations of tidal streams in the stellar halo provide evidence for an abundant population of low-mass subhalos in accord with CDM predictions. These observational approaches will get more powerful over the next few years.
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