Double-Disk Dark Matter

TL;DR

The paper argues that a subdominant, dissipative dark matter component—DDDM—can cool and collapse into a thin galactic disk, offering a rich phenomenology beyond standard cold dark matter. It presents a minimal PIDM framework with a dark U(1) gauge interaction, analyzes the dark sector's early thermal history, and shows how a thin, dense dark disk can amplify indirect-detection signals and produce detectable dark radiation, while remaining consistent with BBN, CMB, and local mass constraints. A central result is that the DDDM disk could contain up to ~5% of the Milky Way's dark matter, achieving high boost factors for annihilation signals and potentially explaining features like the Fermi 130 GeV line, especially when combined with Sommerfeld enhancement. The work lays out concrete observational signatures, including enhanced gamma-ray flux in the plane of the galaxy, dark radiation signatures for Planck/ Gaia-era data, and distinctive gravitational effects, and maps out a broad program of future simulations and observational tests to validate or falsify the DDDM scenario.

Abstract

Based on observational constraints on large scale structure and halo structure, dark matter is generally taken to be cold and essentially collisionless. On the other hand, given the large number of particles and forces in the visible world, a more complex dark sector could be a reasonable or even likely possibility. This hypothesis leads to testable consequences, perhaps portending the discovery of a rich hidden world neighboring our own. We consider a scenario that readily satisfies current bounds that we call Partially Interacting Dark Matter (PIDM). This scenario contains self-interacting dark matter, but it is not the dominant component. Even if PIDM contains only a fraction of the net dark matter density, comparable to the baryonic fraction, the subdominant component's interactions can lead to interesting and potentially observable consequences. Our primary focus will be the special case of Double-Disk Dark Matter (DDDM), in which self-interactions allow the dark matter to lose enough energy to lead to dynamics similar to those in the baryonic sector. We explore a simple model in which DDDM can cool efficiently and form a disk within galaxies, and we evaluate some of the possible observational signatures. The most prominent signal of such a scenario could be an enhanced indirect detection signature with a distinctive spatial distribution. Even though subdominant, the enhanced density at the center of the galaxy and possibly throughout the plane of the galaxy can lead to large boost factors, and could even explain a signature as large as the 130 GeV Fermi line. Such scenarios also predict additional dark radiation degrees of freedom that could soon be detectable and would influence the interpretation of future data, such as that from Planck and from the Gaia satellite. We consider this to be the first step toward exploring a rich array of new possibilities for dark matter dynamics.

Figures (13)

• Figure 1: $\alpha_D$ that yields a thermal relic abundance of $X, {\bar{X}}$ that is a 5% fraction of the total DM density for different $m_X$.
• Figure 2: Above the curves, the recombination rates are larger than the Hubble rate, leading to $X{\bar{X}}$ annihilation that depletes the abundance of the symmetric relic component.
• Figure 3: Fixing $(\sigma v)( \phi \phi \to \gamma \gamma) = 10^{-27}$ cm$^{3}$s$^{-1}$, $\lambda_{\phi S}=1$ and $m_\phi = 130$ GeV, the boost factor $B$ needed to generate a signal that current Fermi line search is sensitive to.
• Figure 4: Comparison of the rates of bremsstrahlung and Compton cooling. At left: the value of $m_C$ for which the rates are equal, as a function of redshift. To the right of the curves, i.e. at early times, Compton cooling dominates. At right: the contour in the $(m_C, \alpha_D)$ plane along which the bremsstrahlung cooling rate equals the Compton cooling rate (black dashed line) and the contour along which the cooling rate equals the age of the universe (solid purple line). This shows that Compton cooling is the dominant effect at small $m_C$ and $\alpha_D$, while bremsstrahlung dominates for larger values. In both plots, we have taken an NFW virial cluster of radius 20 kpc.
• Figure 5: Cooling in the $(m_C, \alpha_D)$ plane. The purple shaded region is the allowed region that cools adiabatically within the age of the universe. The light blue region cools, but with heavy and light particles out of equilibrium. We take redshift $z = 2$ and $T_D = T_{\rm CMB}/2$. The two plots on the left are for $m_X = 100$ GeV; on the right, $m_X = 1$ GeV. The upper plots are for a 110 kpc radius virial cluster; the lower plots, a 20 kpc NFW virial cluster. The solid purple curves show where the cooling time equals the age of the universe; they have a kink where Compton-dominated cooling (lower left) transitions to bremsstrahlung-dominated cooling (upper right). The dashed blue curve delineates fast equipartition of heavy and light particles. Below the dashed black curve, small $\alpha_D$ leads to a thermal relic $X,{\bar{X}}$ density in excess of the Oort limit. To the upper right of the dashed green curve, $B_{XC}$ is high enough that dark atoms are not ionized and bremsstrahlung and Compton cooling do not apply (but atomic processes might lead to cooling).