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Galaxy Mergers Collectively Illuminate the $γ$-Ray Sky

Jaya Doliya, Deep Jyoti Das, Subhadip Bouri, Pooja Bhattacharjee, Mousumi Das, Ranjan Laha

TL;DR

This paper tests the hypothesis that galaxy mergers are sites of high-energy γ-ray emission and cosmic-ray acceleration. By leveraging 16.7 years of Fermi-LAT data across 31,464 mergers from eight catalogs, the authors perform both individual ROI analyses and a population-wide stacking study. They detect eight mergers with TS>25 and uncover a highly significant stacked signal with spectral index Γ≈2.07 and an energy flux around 2×10^{-14} erg cm^{-2} s^{-1}, while also finding spatial coincidences with 18 unassociated γ-ray sources. These results establish galaxy mergers as a new class of HE γ-ray emitters and imply that such systems contribute to the extragalactic γ-ray sky, with CTA and neutrino observatories poised to further illuminate the underlying CR acceleration mechanisms.

Abstract

The origin and acceleration mechanism of cosmic rays (CRs) remain fundamental open questions. Galaxy mergers are proposed as very high-energy CR accelerators, which are expected to produce high-energy (HE) $γ$ rays and neutrinos through interactions with the ambient gas and low-energy background radiation fields. For the first time, we systematically study the HE $γ$-ray emission from galaxy mergers utilising 16.7 years of Fermi Large Area Telescope (Fermi-LAT) data with the sample list compiled from eight survey catalogs. Our analysis finds 8 galaxy mergers that exhibit $γ$-ray emission with significance $\gtrsim5σ$ in the 1-500 GeV energy range. A stacking analysis of the remaining faint galaxy mergers yields a combined $γ$-ray emission detected at $\sim 35σ$ significance, a best-fit spectral index of $Γ\approx 2.07$, and an energy flux of $\sim \rm 2\times10^{-14}~erg~cm^{-2}~s^{-1}$. We compare the stacked spectral energy distributions of the galaxy mergers with the projected sensitivity of the upcoming $γ$-ray telescope Cherenkov Telescope Array (CTA). Furthermore, we find that 18 previously unassociated Fermi-LAT sources are spatially coincident with galaxy mergers. Our findings establish galaxy mergers as a new class of HE $γ$-ray sources. Future neutrino and $γ$-ray observatories will be crucial to discover the particle acceleration mechanism in these newly identified CR sources.

Galaxy Mergers Collectively Illuminate the $γ$-Ray Sky

TL;DR

This paper tests the hypothesis that galaxy mergers are sites of high-energy γ-ray emission and cosmic-ray acceleration. By leveraging 16.7 years of Fermi-LAT data across 31,464 mergers from eight catalogs, the authors perform both individual ROI analyses and a population-wide stacking study. They detect eight mergers with TS>25 and uncover a highly significant stacked signal with spectral index Γ≈2.07 and an energy flux around 2×10^{-14} erg cm^{-2} s^{-1}, while also finding spatial coincidences with 18 unassociated γ-ray sources. These results establish galaxy mergers as a new class of HE γ-ray emitters and imply that such systems contribute to the extragalactic γ-ray sky, with CTA and neutrino observatories poised to further illuminate the underlying CR acceleration mechanisms.

Abstract

The origin and acceleration mechanism of cosmic rays (CRs) remain fundamental open questions. Galaxy mergers are proposed as very high-energy CR accelerators, which are expected to produce high-energy (HE) rays and neutrinos through interactions with the ambient gas and low-energy background radiation fields. For the first time, we systematically study the HE -ray emission from galaxy mergers utilising 16.7 years of Fermi Large Area Telescope (Fermi-LAT) data with the sample list compiled from eight survey catalogs. Our analysis finds 8 galaxy mergers that exhibit -ray emission with significance in the 1-500 GeV energy range. A stacking analysis of the remaining faint galaxy mergers yields a combined -ray emission detected at significance, a best-fit spectral index of , and an energy flux of . We compare the stacked spectral energy distributions of the galaxy mergers with the projected sensitivity of the upcoming -ray telescope Cherenkov Telescope Array (CTA). Furthermore, we find that 18 previously unassociated Fermi-LAT sources are spatially coincident with galaxy mergers. Our findings establish galaxy mergers as a new class of HE -ray sources. Future neutrino and -ray observatories will be crucial to discover the particle acceleration mechanism in these newly identified CR sources.

Paper Structure

This paper contains 15 sections, 2 equations, 14 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Galaxy merger catalogs
  3. Fermi-LAT data analysis
  4. Pre-processing: Single Merger Analysis
  5. Stacking Analysis
  6. Background Stacking
  7. Results
  8. Individual Galaxy Merger Analysis
  9. Stacked Galaxy Merger Analysis
  10. Galaxy Merger Association with 4FGL-DR4 sources
  11. Discussion and Conclusion
  12. TS maps of significant Galaxy Mergers
  13. SED of significant Galaxy Mergers
  14. List of Photon Events above 200 GeV from Galaxy Mergers
  15. List Galaxy Mergers Associated with 4FGL-DR4 Unassociated Sources

Figures (14)

  • Figure 1: Locations of 31,464 galaxy mergers considered in the analysis from eight different catalogs are shown in the equatorial coordinate system, where the horizontal axis and the vertical axis represent the Right Ascension (RA) and Declination (Dec), respectively. The sage, blue, cyan, purple, black, orange, red and green colored points represent the galaxy mergers from UNIONS DR5-SDSS DR7 2024MNRAS.533.2547F, https://cdsarc.cds.unistra.fr/viz-bin/cat/J/ApJS/181/2332009yCat..21810233H, https://cdsarc.cds.unistra.fr/viz-bin/cat/J/AJ/130/2043Yee:2000qj, https://cdsarc.u-strasbg.fr/viz-bin/Cat?J/AJ/128/622004AJ....128...62G, Dual AGN 2023MNRAS.524.4482B, https://cdsarc.cds.unistra.fr/viz-bin/cat/J/AJ/127/1883Allam:2003me, https://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/479/4152018MNRAS.479..415A and https://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/388/1537McIntosh:2007hh catalogs, respectively.
  • Figure 2: TS distribution for 1,214 galaxy mergers (green solid), with the same number of randomly selected blank-sky regions (red dashed), and the chi-squared distribution with 2 degrees of freedom representing the null hypothesis (navy dashed). The vertical black dotted line marks $\rm TS=25$.
  • Figure 3: TS map centred on RCS2338 14991 for $\gamma$ rays in the energy range 1–500 GeV. The left panel shows the TS map without including the test source at the centre with a single PowerLaw spectrum. The right panel shows the TS map after inserting a test source at the centre. Both maps include existing 4FGL-DR4 catalog sources.
  • Figure 4: Light curve of RCS2338 14991 spanning MJD 52697 to 60727. The blue points show the photon flux (1–500 GeV) in eight time bins, with error bars. Time bins with TS values very close to zero are shown with upper limits (95% confidence level). The grey histogram displays the TS values for each time bin when TS is positive. The green curve represents the cumulative TS. The black dotted vertical line indicates the release date of 4FGL-DR4.
  • Figure 5: Left: Stacked TS profile of 1,206 faint galaxy mergers (TS $<$ 25) with best-fit photon index and flux represented by a black cross. Blue, green, and red contours represent $1\sigma$, $2\sigma$, and $3\sigma$ confidence levels, respectively. The color bar represents the stacked TS values. Right: Stacked TS profile of 1,206 randomly selected blank-sky regions.
  • ...and 9 more figures