Are X-Ray Detected Active Galactic Nuclei in Dwarf Galaxies Gamma-Ray Bright?

Abstract

The ${\hat{I}}^3$-ray emission from active galactic nuclei (AGN), including both beamed blazars and misaligned-AGN, dominates the extragalactic ${\hat{I}}^3$-ray point-source population count and flux. While multi-wavelength studies have detected an increasing number of AGN within dwarf galaxies in the local Universe, ${\hat{I}}^3$-ray emission has so far only been associated with systems hosting supermassive black holes (SMBHs). Dwarf-galaxy AGN are of particular interest because their central black holes fall in the intermediate-mass black hole (IMBH) regime, offering insight into the early evolution of SMBHs. Using 15~years of \textit{Fermi}-LAT data, we present the first search for ${\hat{I}}^3$-ray emission from dwarf-galaxy AGN. In the sample of 74 X-ray-selected dwarf-galaxy AGN, we find no sources that exceed the \textit{Fermi}-LAT detection threshold. However, a joint-likelihood analysis reveals a modest, trials-corrected population-level excess ($\sim 2\ddot{I}ƒ$) above blank-field expectations at very soft photon indices $\hat{I}“\gtrsim 3.8$ above 500~MeV. This hint is most pronounced when source contributions are weighed by $M^{\hat{I}}{\pm }_{\mathrm{IMBH},i}/d_i^2$, with $\hat{I}\pm \simeq 1$--$1.5$, suggesting -- but not confirming -- that ${\hat{I}}^3$-ray emission could scale with the central black hole mass or a property correlated with it (e.g., accretion rate), but with a markedly softer spectrum than in SMBH-hosted AGN.

11 pages, 5 figures. Appendices add 3 pages and 3 figures. Comments welcome! v2: Fixed typo in Eq. 6 (now written in fluxes to match Ref. 65); results unchanged