Calorimetric Dark Matter Detection with Galactic Center Gas Clouds

We demonstrate that dark matter heating of gas clouds, hundreds of parsecs from the Milky Way Galactic Center, provides a powerful new test of dark matter interactions. To illustrate, we set a new bound on nucleon scattering for 10–100 MeV mass dark matter. We also constrain millicharged dark matter models, including those proposed to match the recent EDGES 21 cm absorption anomaly. For Galactic Center gas clouds, the Galactic fields’ magnetic deflection of electromagnetically charged dark matter is mitigated, because the magnetic fields around the Galactic Center are poloidal, as opposed to being aligned parallel to the Milky Way disk. We discuss prospects for detecting dark matter using a population of Galactic Center gas clouds warmed by dark matter.

Figure 1

Sensitivity is shown for spin-independent dark matter scattering with nucleons in cold Galactic Center gas clouds. Specifically, gas cloud $\mathrm{G}1.4-1.8+87$ measured in Ref. [35] sets the tightest constraint. We have shown the effect of increasing the gas cloud metallicity to the highest metallicity observed in a star, $[\mathrm{Fe}/\mathrm{H}]=0.5$, although it is most likely the gas cloud will have solar or sub-solar metal content, resulting in a stronger bound, as discussed in the text. Alongside this bound, we plot representative bounds from dark matter detectors and cosmology. The dashed line labelled “CMB scattering” indicates bounds from spectral distortions of the cosmic microwave background [36, 37, 38, 39]; we plot the results of Ref. [37]. Terrestrial detection constraints from the above-ground run of CRESST-SAPPHIRE [22, 25, 40] and the XQC Rocket [20, 24] are also indicated. For XQC, we plot the 2% XQC efficiency curve recently discussed in [41]. For the sake of clarity, we omitted some overlapping bounds for $m_x\gtrsim \mathrm{GeV}$, including DAMIC [42] and the RRS balloon [13].

Figure 2

Constraints are shown on millicharged dark matter scattering with cold Galactic Center gas clouds, for dark matter particles with charge $\epsilon \equiv Q/e$, which compose all ($f_{\mathrm{DM}}=1$) or one percent ($f_{\mathrm{DM}}=0.01$) of the dark matter. The metallicity of the gas cloud does not substantially alter the result, as explained in the main text. Galactic Center gas cloud bounds on millicharged particles are likely unaffected by galactic magnetic fields [43, 44], because magnetic fields in the Galactic Center are oriented towards the edge of the halo [45, 46], as opposed to being oriented parallel to the disk of the Milky Way. Parameter space that would explain the EDGES 21 cm anomaly for $f_{\mathrm{DM}}=0.01$ is shown with a dotted grey line, taken from [47] (see also [47, 48, 49, 50, 51, 52, 53]). Bounds from supernova 1987a production of millicharged particles [54], from the SLAC millicharged particle search [55], and from the millicharged particle contribution to the number of relativistic degrees of freedom in the early Universe ($N_{\text{eff}}$) [56] are indicated. For some recently reported experimental results on millicharged dark matter, see [41, 57, 58]. For additional cosmological bounds, which can be sensitive to the millicharged fraction of dark matter ($f_{\mathrm{DM}}$), see [36, 44, 59, 60].