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

Galaxy mergers are among the most energetic astrophysical phenomena, driving intense star formation and potentially fueling cosmic ray acceleration, which can produce high energy <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>γ</mml:mi> </mml:math> -ray emission through hadronic processes. We present a targeted search for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>γ</mml:mi> </mml:math> -ray emission from five prominent galaxy merger systems, NGC~3256, NGC~660, UGC~813/816, UGC~12914/12915, and VV~114 using 16.8 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>γ</mml:mi> </mml:math> -ray fluxes and spectral indices. Marginal detections are found for NGC~3256 (TS = 15.4, $ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>3.51</mml:mn> </mml:math> $) and NGC~660 (TS = 8.16, $ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>2.39</mml:mn> </mml:math> $), with photon fluxes of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mn>7.21</mml:mn> <mml:mo>±</mml:mo> <mml:mn>3.17</mml:mn> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mn>8.28</mml:mn> <mml:mo>±</mml:mo> <mml:mn>3.56</mml:mn> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ph cm s, respectively, suggesting merger driven star formation contributes to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>γ</mml:mi> </mml:math> -ray emission. The remaining systems yield non-detections (TS <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>&lt;</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:math> ). This is the first targeted study of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>γ</mml:mi> </mml:math> -ray emission from these aforementioned galaxy merger systems.