Project A2: "Cosmic-ray signatures in dwarf galaxies: astrophysical foreground, dark-matter background"

Cosmic rays carry a significant part of the energy budget of the interstellar medium (ISM) and are important to model the development of the different phases of the ISM. Observations of radio continuum emission allow observational access to the magnetic field's strength, its direction, and the relative contributions of ordered and turbulent magnetic field components in disks and halos of galaxies. Cosmic-ray transport through a magnetized ISM is often modeled as an ensemble via the solution of the transport equation. This concept is well-developed and tested in particular for the Milky Way (e.g. Strong et al., 2000). The solutions typically use simplified descriptions of the diffusion tensor, either as a constant or a space-dependent scalar. In the past few years, the importance of the diffusion tensor has started to become an aspect of improved modeling (Gaggero et al., 2014; Kissmann, 2014).

Here, we plan to apply our recently developed transport code to dwarf galaxies, the smallest and structurally simplest galactic systems, combining our modeling (JT) with astrophysical data (DB) on the structure of the magnetic field and gas densities that will be taken in the context of this project. Our results will be compared to high-energy multimessenger data, in particular concerning gamma-ray maps in order to investigate the role of cosmic rays in these small-scale galactic systems, and compare transport effects to larger and more energetic systems investigated in other projects of this CRC. Specifically, in this first phase of the CRC, we plan to model multimessenger signatures of the nearest dwarf starbursts and the nearest elliptical and dwarf spheroidal galaxies. With our new code, it is the first time this can be done using concrete magnetic fields as input provided by the observational part of this project. In doing so, we will start to develop a common understanding of the sequence of dwarf galaxies, from luminous to dark ones. Together with the other projects in CIM, we will start to develop a unified understanding of outflow signatures in galaxies for the lowest-mass galaxies. Finally, we will use our results to provide background estimates of astrophysical signatures for dark-matter searches in order to improve current limits on indirect dark-matter detections.