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Register a new userStreams and caustics: the fine-grained structure of LCDM haloes
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Mark Vogelsberger MPA
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We present the first and so far the only simulations to follow the fine-grained phase-space structure of galaxy haloes formed from generic LCDM initial conditions. We integrate the geodesic deviation equation in tandem with the N-body equations of motion, demonstrating that this can produce numerically converged results for the properties of fine-grained phase-space streams and their associated caustics, even in the inner regions of haloes. Our effective resolution for such structures is many orders of magnitude better than achieved by conventional techniques on even the largest simulations. We apply these methods to the six Milky Way-mass haloes of the Aquarius Project. At 8 kpc from halo centre a typical point intersects about 10^14 streams with a very broad range of individual densities; the ~10^6 most massive streams contribute about half of the local dark matter density. As a result, the velocity distribution of dark matter particles should be very smooth with the most massive fine-grained stream contributing about 0.1% of the total signal. Dark matter particles at this radius have typically passed 200 caustics since the Big Bang. The peak densities on present-day caustics in the inner halo almost all lie well below the mean local dark matter density. As a result caustics provide a negligible boost (<0.1%) to the predicted local dark matter annihilation rate. The effective boost is larger in the outer halo but never exceeds about 10%. Thus fine-grained streams and their associated caustics have no effect on the detectability of dark matter, either directly in Earth-bound laboratories, or indirectly through annihilation radiation, with the exception that resonant cavity experiments searching for axions may see the most massive local fine-grained streams because of their extreme localisation in energy/momentum space. (abridged)
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We simulate the growth of isolated dark matter haloes from self-similar and spherically symmetric initial conditions. Our N-body code integrates the geodesic deviation equation in order to track the streams and caustics associated with individual simulation particles. The radial orbit instability causes our haloes to develop major-to-minor axis ratios approaching 10 to 1 in their inner regions. They grow similarly in time and have similar density profiles to the spherical similarity solution, but their detailed structure is very different. The higher dimensionality of the orbits causes their stream and caustic densities to drop much more rapidly than in the similarity solution. This results in a corresponding increase in the number of streams at each point. At 1% of the turnaround radius (corresponding roughly to the Suns position in the Milky Way) we find of order 10^6 streams in our simulations, as compared to 10^2 in the similarity solution. The number of caustics in the inner halo increases by a factor of several, because a typical orbit has six turning points rather than one, but caustic densities drop by a much larger factor. This reduces the caustic contribution to the annihilation radiation. For the region between 1% and 50% of the turnaround radius, this is 4% of the total in our simulated haloes, as compared to 6.5% in the similarity solution. Caustics contribute much less at smaller radii. These numbers assume a 100 GeV c^-2 neutralino with present-day velocity dispersion 0.03 cm s^-1, but reducing the dispersion by ten orders of magnitude only doubles the caustic luminosity. We conclude that caustics will be unobservable in the inner parts of haloes. Only the outermost caustic might potentially be detectable.
We study how tidal streams from globular clusters may be used to constrain the mass of ultra-light dark matter particles, called `fuzzy dark matter (FDM). A general feature of FDM models is the presence of ubiquitous density fluctuations in bound, virialized dark matter structures, on the scale of the de Broglie wavelength, arising from wave interference in the evolving dark matter distribution. These time-varying fluctuations can disturb the motions of stars, leading to potentially observable signatures in cold thin tidal streams in our own Galaxy. The study of this effect has been hindered by the difficulty in simulating the FDM wavefunction in Milky Way-sized systems. We present a simple method to evolve realistic wavefunctions in nearly static potentials, that should provide an accurate estimate of this granulation effect. We quantify the impact of FDM perturbations on tidal streams, and show that initially, while stream perturbations are small in amplitude, their power spectra exhibit a sharp cutoff corresponding to the de Broglie wavelength of the FDM potential fluctuations. Eventually, when stream perturbations become nonlinear, fold caustics generically arise that lead to density fluctuations with universal behavior. This erases the signature of the de Broglie wavelength in the stream density power spectrum, but we show that it will still be possible to determine the FDM mass in this regime, by considering the fluctuations in quantities like angular momenta or actions.
The mass aggregation and merger histories of present-day distinct haloes selected from the cosmological Millennium Simulations I and II are mapped into stellar mass aggregation and galaxy merger histories of central galaxies by using empirical stellar-to-halo and stellar-to-gas mass relations. The growth of bulges driven by the galaxy mergers/interactions is calculated using dynamical prescriptions. The predicted bulge demographics at redshift z~0 is consistent with observations (Zavala+2012). Here we present the evolution of the morphological mix (traced by the bulge-to-total mass ratio, B/T) as a function of mass up to z=3. This mix remains qualitatively the same up to z~1: B/T<0.1 galaxies dominate at low masses, 0.1<B/T<0.45 at intermediate masses, and B/T>0.45 at large masses. At z>1, the fractions of disc-dominated and bulgeless galaxies increase strongly, and by z~2 the era of pure disc galaxies is reached. Bulge-dominated galaxies acquire such a morphology, and most of their mass, following a downsizing trend. Since our results are consistent with most of the recent observational studies of the morphological mix at different redshifts, a LCDM-based scenario of merger-driven bulge assembly does not seem to face critical issues. However, if the stellar-to-halo mass relation changes too little with redshift, then some tensions with observations appear.
We use the Millennium I and II cosmological simulations to revisit the impact of mergers in the growth of bulges in central galaxies in the LCDM scenario. We seed galaxies within the growing CDM haloes using semi-empirical relations to assign stellar and gaseous masses, and an analytic treatment to estimate the transfer of stellar mass to the bulge of the remnant after a galaxy merger. We find that this model roughly reproduces the observed correlation between the bulge-to-total (B/T) mass ratio and stellar mass in present-day central galaxies as well as their observed demographics, although low-mass B/T<0.1 (bulgeless) galaxies might be scarce relative to the observed abundance. In our merger-driven scenario, bulges have a composite population made of (i) stars acquired from infalling satellites, (ii) stars transferred from the primary disc due to merger-induced perturbations, and (iii) newly formed stars in starbursts triggered by mergers. We find that (i) and (ii) are the main channels of mass assembly, with the first being dominant for massive galaxies, creating large bulges with different stellar populations than those of the inner discs, while the second is dominant for intermediate/low-mass galaxies creating small bulges with similar stellar populations to the inner discs. We associate the dominion of the first (second) channel to classical (pseudo) bulges, and compare the predicted fractions to observations. We remark that our treatment does not include other mechanisms of bulge growth such as intrinsic secular disc instabilities or misaligned gas accretion. We find that the evolution of the stellar and gaseous contents of the satellite as it moves towards the central galaxy is a key ingredient in setting the morphology of the remnant, and that a good match to the observed bulge demographics occurs when this evolution proceeds closely to that of the central galaxy.
We investigate the internal structure and density profiles of halos of mass $10^{10}-10^{14}~M_odot$ in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These follow the formation of galaxies in a $Lambda$CDM Universe and include a treatment of the baryon physics thought to be relevant. The EAGLE simulations reproduce the observed present-day galaxy stellar mass function, as well as many other properties of the galaxy population as a function of time. We find significant differences between the masses of halos in the EAGLE simulations and in simulations that follow only the dark matter component. Nevertheless, halos are well described by the Navarro-Frenk-White (NFW) density profile at radii larger than ~5% of the virial radius but, closer to the centre, the presence of stars can produce cuspier profiles. Central enhancements in the total mass profile are most important in halos of mass $10^{12}-10^{13}M_odot$, where the stellar fraction peaks. Over the radial range where they are well resolved, the resulting galaxy rotation curves are in very good agreement with observational data for galaxies with stellar mass $M_*<5times10^{10}M_odot$. We present an empirical fitting function that describes the total mass profiles and show that its parameters are strongly correlated with halo mass.
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