A Targeted Gamma-Ray Search of Five Prominent Galaxy Merger Systems with 17 years of Fermi-LAT Data

TL;DR

Five prominent galaxy mergers were targeted with 16.9 years of Fermi-LAT data to search for gamma-ray emission in the 1–300 GeV band. A uniform binned likelihood framework, background modeling with 4FGL-DR4 sources, and iterative residual-source searches were used to derive spectral energy distributions for each system. Marginal gamma-ray detections were found for NGC 3256 and NGC 660, consistent with star-formation–driven hadronic emission, while the other mergers yielded only upper limits, already above simple hadronic predictions. The results establish baseline gamma-ray fluxes for these mergers and constrain cosmic-ray calorimetry in complex merger environments, highlighting the need for deeper LAT observations and future gamma-ray facilities to probe fainter merger-driven emission.

Abstract

Galaxy mergers are among the most energetic astrophysical phenomena, driving intense star formation and potentially fueling cosmic ray acceleration, which can produce high energy $γ$-ray emission through hadronic processes. We present a targeted search for $γ$-ray emission from five prominent galaxy merger systems, NGC~3256, NGC~660, UGC~813/816, UGC~12914/12915, and VV~114 using 16.9 years of Fermi-LAT data in the 1--300~GeV energy range. Employing a binned maximum likelihood analysis, we model the emission with power-law spectra and derive spectral energy distributions (SEDs) to constrain $γ$-ray fluxes and spectral indices. Marginal detections are found for NGC~3256 (TS = 15.4, $\sim$3.51$σ$) and NGC~660 (TS = 8.16, $\sim$2.39$σ$), with photon fluxes of $(7.21 \pm 3.17) \times 10^{-11}$ and $(8.28 \pm 3.56) \times 10^{-11}$ ph cm$^{-2}$ s$^{-1}$, respectively, suggesting merger driven star formation contributes to $γ$-ray emission. The remaining systems yield non-detections (TS $< 5$). This is the first targeted study of $γ$-ray emission from these aforementioned galaxy merger systems.

Figures (16)

• Figure 1: TS map of the NGC 3256 in the 1--300 GeV range. The color scale indicates the TS intensity revealing a localized excess near the target position with TS of 15.40. Contours represent $\sqrt{\mathrm{TS}} = 3$ (orange), 4 (deepskyblue), and 5 (lime). The cyan star marks the optical position of NGC 3256, while the white cross indicates the location of the maximum TS pixel. Nearby 4FGL-DR4 catalog sources are shown as green circles with source names annotated.
• Figure 2: SED of NGC 3256 in the 1--300 GeV range, showing marginal gamma-ray emission. The first four energy bins represent detections with TS > 9, while the last bin shows an upper limit with TS < 9. TS values obtained in the first four bins are 9.39 (2.61$\sigma$), 13.12 (3.19$\sigma$) , 11.73 (2.98$\sigma$) and 13.48 (3.24$\sigma$) respectively.
• Figure 3: Residual map of the NGC 3256 field in the 1--300 GeV range. The color bar indicates the TS values ranging from $-16$ to $+16$. The yellow star marks the optical position of NGC 3256. No significant residual excesses are detected, confirming that the background and source models adequately describe the observed emission.
• Figure 4: TS map of NGC 660 in the 1--300 GeV range. A mild excess with TS of 8.16 is observed near the target position. The plotting conventions are the same as in Figure \ref{['fig:Figure1']}.
• Figure 5: SED of NGC 660 in the 1--300 GeV range, showing upper limits in all five energy bins.