We present a search for spatial extension in high-latitude ( |b| > 5 \mbox { $~ { } ^ { \circ } $ } ) sources in recent Fermi point source catalogs .
The result is the Fermi High-Latitude Extended Sources Catalog , which provides source extensions ( or upper limits thereof ) and likelihood profiles for a suite of tested source morphologies .
We find 24 extended sources , 19 of which were not previously characterized as extended .
These include sources that are potentially associated with supernova remnants and star forming regions .
We also found extended \gamma -ray emission in the vicinity of the Cen A radio lobes and—at GeV energies for the first time—spatially coincident with the radio emission of the SNR CTA 1 , as well as from the Crab Nebula .
We also searched for halos around active galactic nuclei , which are predicted from electromagnetic cascades that are induced by the e ^ { + } e ^ { - } pairs that are deflected in intergalactic magnetic fields .
These are produced when \gamma -rays interact with background radiation fields .
We do not find evidence for extension in individual sources or in stacked source samples .
This enables us to place limits on the flux of the extended source components , which are then used to constrain the intergalactic magnetic field a coherence length \lambda \gtrsim 10 kpc , even when conservative assumptions on the source duty cycle are made .
This improves previous limits by several orders of magnitude .