Dark matter search in dwarf irregular galaxies with the Fermi Large Area Telescope

We analyze 11 years of Fermi-Large Area Telescope (LAT) data corresponding to the sky regions of seven dwarf irregular (dIrr) galaxies. DIrrs are dark matter (DM)-dominated systems, proposed as interesting targets for the indirect search of DM with gamma rays. The galaxies represent interesting cases with a strong disagreement between the density profiles (core versus cusp) inferred from observations and numerical simulations. In this work, we addressed the problem by considering two different DM profiles, based on both the fit to the rotation curve (in this case, a Burkert cored profile) and results from $N$-body cosmological simulations (i.e., Navarro-Frenk-White cuspy profile). We also include halo substructure in our analysis, which is expected to boost the DM signal by a factor of 10 in halos such as those of dIrrs. For each DM model and dIrr, we create a spatial template of the expected DM-induced gamma-ray signal to be used in the analysis of Fermi-LAT data. No significant emission is detected from any of the targets in our sample. Thus, we compute upper limits on the DM annihilation cross section versus mass parameter space. Among the seven dIrrs, we find IC10 and NGC6822 to yield the most stringent individual constraints, independently of the adopted DM profile. We also produce combined DM limits for all objects in the sample, which turn out to be dominated by IC10 for all DM models and annihilation channels, i.e., $b\overline{b}$, ${\tau }^{+}{\tau }^{-}$, and $W^{+}{W}^{-}$. The strongest constraints are obtained for $b\overline{b}$ and are at the level of $⟨\sigma v⟩\sim 7\times {10}^{-26}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ at $m_{\chi }\sim 6\mathrm{GeV}$. Though these limits are a factor of $\sim 3$ higher than the thermal relic cross section at low weakly interacting massive particles masses, they are independent from and complementary to those obtained by means of other targets.

Figure 1

Two-dimensional templates of the expected spatial morphology of the DM annihilation fluxes from the IC10 dIrr, as obtained with clumpy for each of the four benchmark DM substructure models in Table 4; see text for details. The $z$-axis represents the values of the differential $J$-factors ($\frac{dJ}{d\mathrm{\Omega }}$). Because the values of $\frac{dJ}{d\mathrm{\Omega }}$ vary by many orders of magnitude in these plots, we chose the colour scale in such a way that we still could appreciate details in the MED and MAX models. However, for the MIN case, this color scale implies that the outer regions of the object are not visible, as their expected fluxes already lie below the minimum $J$-factor value shown by the color scale.

Figure 2

Likelihood profiles as a function of the WIMP mass for each of the dIrrs and the combined targets, assuming the MED model. The $⟨\sigma v⟩$ is left free for each mass bin. Top, middle, and bottom panels are for the $b\overline{b}$, ${\tau }^{+}{\tau }^{-}$, and $W^{+}{W}^{-}$ annihilation channels, respectively.

Figure 3

Upper limits for $⟨\sigma v⟩$ for each individual source, for the $b\overline{b}$ annihilation channel. The different DM models considered are, from top to bottom, MIN, MED, MAX-Bur, and MAX-NFW; see Table 4 for details on each of them.

Figure 4

Limits for the combined signal in the four considered scenarios, for $b\overline{b}$ (top panel), ${\tau }^{+}{\tau }^{-}$ (middle panel), and $W^{+}{W}^{-}$ (bottom panel) annihilation channels.

Figure 5

Comparison between different limits for the $b\overline{b}$ annihilation channel. We include the ones derived in this work, assuming the spatial template (solid blue), the theoretical predictions from [13] (dashed orange), the null extended simulations 95% containment band (see Appendix pp2), and the LAT dSphs [74] in dot-dashed green.

Figure 6

Rotation curve data plotted along with the fit results taking only the DM component. All the observed data are based on the HI measurements with the exception of Phoenix galaxy for which optical observations are used. The blue solid line represents the Burkert profile, while the blue dashed the NFW predicted profile (see Sec. 2 for further details). For each galaxy, the main plots show the RC till $R_{200}$, indeed an indirect representation of consistency of the $M_{200}^{\text{Burk}}=M_{200}^{\mathrm{NFW}}$ approximation; in the zoomed-in panel, we show the results of the total fit ($\mathrm{DM}+\mathrm{baryons}$) for the Burkert profile, indeed the discrepancy with the DM-only profile at small radius.

Figure 7

Control simulations for the $b\overline{b}$ annihilation channel in the MED model. 100 simulations are run to compute the expected DM limits, shown here as the 95% containment bands for both the null simulation using the spatial template (cyan band) and the pointlike analysis in random sky positions (orange band). The actual data constraints are given by the dot-dashed red and dotted blue lines, which were obtained, respectively, for the pointlike and extended (spatial template) cases.

Figure 8

95% C.L. upper limits to the flux and likelihood values found for the dIrrs in the gamma-ray analysis performed in Sec. 4 with the Fermi-LAT. The color code traces the change in log-likelihood. The arrows indicate upper limits, as no signal is detected in any of the bins. Panels show, from left to right and top to bottom, DDO210, DD0216, IC10, IC1613, NGC6822, Phoenix, and WLM.

Figure 9

Left column: Upper limits for $⟨\sigma v⟩$ to each individual source, for the ${\tau }^{+}{\tau }^{-}$ annihilation channel. The different models considered are, from top to bottom, MIN, MED, MAX-Bur, and MAX-NFW. Right column: Same as left column but for the $W^{+}{W}^{-}$ annihilation channel.