The Displaced Halo of NGC 3311: Hydra I's Diffuse Light as One Unfinished Event

The Hydra I cluster's central galaxy, NGC 3311, wears its diffuse light displaced: deep imaging shows an extended halo whose centre is offset from the galaxy's nucleus, the signature of a system still sloshing after the accretion events that built it. Adopting a total cluster light of 6.5e11 solar luminosities within 500 kpc and the published diffuse fraction of 0.22, Hydra's intracluster component carries 1.43e+11 solar luminosities. The offset halo around NGC 3311 accounts for 6.0e+10 solar luminosities of that total, a fraction 0.42 of the cluster's diffuse light and 0.092 of all its light. Spread within 120 kpc, the halo's mean surface brightness is 26.10 magnitudes per square arcsecond. Its 15 kpc displacement, at a relative velocity scale of 250 kilometres per second, corresponds to a crossing time of only 59 million years, far shorter than the time phase mixing needs to erase such an asymmetry, which is why the offset can persist as a visible fossil of the cluster's recent assembly. Hydra's diffuse light is not a smooth equilibrium component; nearly half of it sits in one displaced structure around one galaxy.