Galaxy Zoo : 'Hanny's Voorwerp', a quasar light echo?

TL;DR

Hanny's Voorwerp is a peculiar emission-line object near galaxy IC 2497, potentially representing a quasar light echo that reveals recent quasar activity history through ionized gas observations.

Contribution

This study presents the first multi-wavelength observations of Hanny's Voorwerp, proposing it as the first detected quasar light echo and offering insights into quasar variability over 10^5 years.

Findings

Hanny's Voorwerp exhibits strong OIII emission lines indicating highly ionized gas.

The source of ionization is unclear, with evidence suggesting a past or obscured quasar activity.

The object may be the first direct observation of a quasar light echo, revealing quasar activity decline or obscuration changes.

Abstract

We report the discovery of an unusual object near the spiral galaxy IC 2497, discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part of the Galaxy Zoo project. The object, known as Hanny's Voorwerp, is bright in the SDSS g band due to unusually strong OIII 4959-5007 emission lines. We present the results of the first targeted observations of the object in the optical, UV and X-ray, which show that the object contains highly ionized gas. Although the line ratios are similar to extended emission-line regions near luminous AGN, the source of this ionization is not apparent. The emission-line properties, and lack of x-ray emission from IC 2497, suggest either a highly obscured AGN with a novel geometry arranged to allow photoionization of the object but not the galaxy's own circumnuclear gas, or, as we argue, the first detection of a quasar light echo. In this case, either the luminosity of the central source has decreased dramatically or else the obscuration in the system has increased within 10^5 years. This object may thus represent the first direct probe of quasar history on these timescales.