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Register a new userDynamical Instability of Collapsed Dark Matter Halos
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A self-interacting dark matter halo can experience gravothermal collapse, resulting in a central core with an ultrahigh density. It can further contract and collapse into a black hole, a mechanism proposed to explain the origin of supermassive black holes. We study dynamical instability of the core in general relativity. We use a truncated Maxwell-Boltzmann distribution to model the dark matter distribution and solve the Tolman-Oppenheimer-Volkoff equation. For given model parameters, we obtain a series of equilibrium configurations and examine their dynamical instability based on considerations of total energy, binding energy, fractional binding energy, and adiabatic index. The numerical results from our semi-analytical method are in good agreement with those from fully relativistic N-body simulations. We further show for the instability to occur in the classical regime, the boundary temperature of the core should be at least $10%$ of the mass of dark matter particles; for a $10^9~{rm M_odot}$ seed black hole, the particle mass needs to be larger than a few keV. These results can be used to constrain different collapse models, in particular, those with dissipative dark matter interactions.
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Dissipative dark matter self-interactions can affect halo evolution and change its structure. We perform a series of controlled N-body simulations to study impacts of the dissipative interactions on halo properties. The interplay between gravitational contraction and collisional dissipation can significantly speed up the onset of gravothermal collapse, resulting in a steep inner density profile. For reasonable choices of model parameters controlling the dissipation, the collapse timescale can be a factor of 10-100 shorter than that predicted in purely elastic self-interacting dark matter. The effect is maximized when energy loss per collision is comparable to characteristic kinetic energy of dark matter particles in the halo. Our simulations provide guidance for testing the dissipative nature of dark matter with astrophysical observations.
We consider a dark matter halo (DMH) of a spherical galaxy as a Bose-Einstein condensate of the ultra-light axions interacting with the baryonic matter. In the mean-field limit, we have derived the integro-differential equation of the Hartree-Fock type for the spherically symmetrical wave function of the DMH component. This equation includes two independent dimensionless parameters: (i) b{eta}- the ratio of baryon and axion total mases and (ii) {xi}- the ratio of characteristic baryon and axion spatial parameters. We extended our dissipation algorithm for studying numerically the ground state of the axion halo in the gravitational field produced by the baryonic component. We calculated the characteristic size, Xc, of DMH as a function of b{eta} and {xi} and obtained an analytical approximation for Xc.
Cosmological simulations indicate that cold dark matter (CDM) halos should be triaxial. Verifying observationally this theoretical prediction is, however, less than straightforward because the assembly of galaxies is expected to modify the halo shapes and to render them more axisymmetric. We use a suite of N-body simulations to investigate quantitatively the effect of the growth of a central disk galaxy on the shape of triaxial dark matter halos. As expected, the halo responds to the presence of the disk by becoming more spherical. The net effect depends only weakly on the orientation of the disk relative to the halo principal axes or the timescale of disk assembly, but strongly on the overall gravitational importance of the disk. Our results show that exponential disks whose contribution peaks at less than ~50% of their circular velocity are unable to modify noticeably the shape of the gravitational potential of their surrounding halos. Many dwarf and low surface brightness galaxies are expected to be in this regime, and therefore their detailed kinematics could be used to probe halo triaxiality, one of the basic predictions of the CDM paradigm. We argue that the complex disk kinematics of the dwarf galaxy NGC 2976 might be the reflection of a triaxial halo. Such signatures of halo triaxiality should be common in galaxies where the luminous component is subdominant.
Gamma rays and microwave observations of the Galactic Center and surrounding areas indicate the presence of anomalous emission, whose origin remains ambiguous. The possibility of dark matter (DM) annihilation explaining both signals through prompt emission at gamma-rays and secondary emission at microwave frequencies from interactions of high-energy electrons produced in annihilation with the Galactic magnetic fields has attracted much interest in recent years. We investigate the DM interpretation of the Galactic Center gamma-ray excess by searching for the associated synchrotron in the WMAP-Planck data. Considering various magnetic field and cosmic-ray propagation models, we predict the synchrotron emission due to DM annihilation in our Galaxy, and compare it with the WMAP-Planck data at 23-70GHz. In addition to standard microwave foregrounds, we separately model the microwave counterpart to the Fermi Bubbles and the signal due to DM, and use component separation techniques to extract the signal associated with each template from the total emission. We confirm the presence of the Haze at the level of 7% of the total sky intensity at 23GHz in our chosen region of interest, with a harder spectrum $I sim u^{-0.8}$ than the synchrotron from regular cosmic-ray electrons. The data do not show a strong preference towards fitting the Haze by either the Bubbles or DM emission only. Inclusion of both components provides a better fit with a DM contribution to the Haze emission of 20% at 23GHz, however, due to significant uncertainties in foreground modeling, we do not consider this a clear detection of a DM signal. We set robust upper limits on the annihilation cross section by ignoring foregrounds, and also report best-fit DM annihilation parameters obtained from a complete template analysis. We conclude that the WMAP-Planck data are consistent with a DM interpretation of the gamma-ray excess.
We use a 200 $h^{-1}Mpc$ a side N-body simulation to study the mass accretion history (MAH) of dark matter halos to be accreted by larger halos, which we call infall halos. We define a quantity $a_{rm nf}equiv (1+z_{rm f})/(1+z_{rm peak})$ to characterize the MAH of infall halos, where $z_{rm peak}$ and $z_{rm f}$ are the accretion and formation redshifts, respectively. We find that, at given $z_{rm peak}$, their MAH is bimodal. Infall halos are dominated by a young population at high redshift and by an old population at low redshift. For the young population, the $a_{rm nf}$ distribution is narrow and peaks at about $1.2$, independent of $z_{rm peak}$, while for the old population, the peak position and width of the $a_{rm nf}$ distribution both increases with decreasing $z_{rm peak}$ and are both larger than those of the young population. This bimodal distribution is found to be closely connected to the two phases in the MAHs of halos. While members of the young population are still in the fast accretion phase at $z_{rm peak}$, those of the old population have already entered the slow accretion phase at $z_{rm peak}$. This bimodal distribution is not found for the whole halo population, nor is it seen in halo merger trees generated with the extended Press-Schechter formalism. The infall halo population at $z_{rm peak}$ are, on average, younger than the whole halo population of similar masses identified at the same redshift. We discuss the implications of our findings in connection to the bimodal color distribution of observed galaxies and to the link between central and satellite galaxies.
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