Searching for Dark Matter Annihilation in Recently Discovered Milky Way Satellites with Fermi-LAT (1611.03184v1)

Abstract: We search for excess gamma-ray emission coincident with the positions of confirmed and candidate Milky Way satellite galaxies using 6 years of data from the Fermi Large Area Telescope (LAT). Our sample of 45 stellar systems includes 28 kinematically confirmed dark-matter-dominated dwarf spheroidal galaxies (dSphs) and 17 recently discovered systems that have photometric characteristics consistent with the population of known dSphs. For each of these targets, the relative predicted gamma-ray flux due to dark matter annihilation is taken from kinematic analysis if available, and estimated from a distance-based scaling relation otherwise, assuming that the stellar systems are dark-matter-dominated dSphs. LAT data coincident with four of the newly discovered targets show a slight preference (each ~$2 \sigma$ local) for gamma-ray emission in excess of the background. However, the ensemble of derived gamma-ray flux upper limits for individual targets is consistent with the expectation from analyzing random blank-sky regions, and a combined analysis of the population of stellar systems yields no globally significant excess (global significance $<1 \sigma$). Our analysis has increased sensitivity compared to the analysis of 15 confirmed dSphs by Ackermann et al. 2015. The observed constraints on the dark matter annihilation cross section are statistically consistent with the background expectation, improving by a factor of ~2 for large dark matter masses ($m_{{\rm DM},b \bar b} \gtrsim 1$ TeV and $m_{{\rm DM},\tau^{+}\tau^{-}} \gtrsim 70$ GeV) and weakening by a factor of ~1.5 at lower masses relative to previously observed limits.

• The paper presents a refined search for dark matter annihilation using six years of Fermi-LAT gamma-ray data in 45 Milky Way satellite galaxies.
• The analysis employs binned likelihood methods across various dark matter mass ranges, comparing individual satellite signals with ensemble background expectations.
• The study improves DM annihilation cross-section limits by up to a factor of 2 and outlines necessary future refinements in spectral and J-factor analyses.

Dark Matter Annihilation Probes in Milky Way Satellites with Fermi-LAT Observations

The paper under review presents an in-depth analysis of dark matter (DM) annihilation searches using data from the Fermi Large Area Telescope (LAT), specifically targeting recently discovered Milky Way satellite galaxies. This research is a part of the collaborative effort by the Fermi-LAT and the Dark Energy Survey (DES) Collaborations. The paper utilizes six years of accumulated LAT data, probing gamma-ray emissions which are potential indicators of DM annihilation in the Universe.

Study Overview

The investigation focuses on a sample of 45 stellar systems, comprising both kinematically confirmed and candidate dwarf spheroidal galaxies (dSphs). These objects were chosen based on their photometric properties, consistent with known dSphs, which are considered prime venues for DM searches due to their proximity, high DM density, and relative simplicity in astroparticle backgrounds.

Analytical Approach

The authors employed a comprehensive gamma-ray data analysis aligned with the procedures outlined in prior works, notably those by Ackermann et al. (2015b). This included binned likelihood analyses and comparisons across various DM annihilation spectra and mass distributions. Notably, the paper contrasts individual limits from suspected targets with ensemble analyses of all 45 systems, providing a nuanced understanding of potential DM-induced gamma emissions.

Results

Although the paper revealed slight gamma excess in a few cases—specifically towards newly identified targets like Indus II and Tucana III—none of these demonstrated global significance post-statistical corrections. Individual targets with local significance peaks remained consistent with background expectations upon broader analyses across masses and annihilation channels. Importantly, the constraints on DM annihilation cross-section obtained are consistent with previous estimates, albeit improved (in some cases by a factor of 2 for specific DM mass ranges compared to Ackermann et al.).

Implications and Future Directions

Despite the lack of statistically robust detections for DM, this research advances our experimental sensitivity and statistical methods which are crucial in cosmic DM surveys. The researchers underscore that further spectroscopic analysis and more refined J-factor calculations would enhance our understanding, facilitating better identification of DM characteristics in satellite systems. Future exploration with both the current LAT dataset and upcoming high-sensitivity instruments could yield further insights into the nature of DM.

Conclusion

In summary, this paper reflects meticulous research that elevates our search methodologies and provides integral insights into dark matter annihilation phenomena in the cosmos. Although definitive claims of discovery are not asserted, the work systematically refines our observational capabilities and theoretical expectations, laying a strong foundation for subsequent studies. As advancements in detection and spectral analysis proceed, endeavors like these will be pivotal to uncovering the elusive components of our Universe.