Hanny's Voorwerp

Lore & Background

Hanny's Voorwerp (HsV) is about the size of a small galaxy and has a central hole over 16,000 light years across. In an image taken with the Hubble Space Telescope, HsV is colored green, a standard false color that is used to represent the presence of several luminous emission lines of glowing oxygen. HsV has been shown to be at the same distance from Earth as the adjacent galaxy IC 2497, which is about 650 million light-years away. Star birth is occurring in the region of HsV that faces IC 2497. Radio observations indicate that this is due to an outflow of gas from IC 2497's core which is interacting with a small region of HsV to collapse and form stars. The youngest of these stars are several million years old.

One hypothesis suggests that HsV consists of remnants of a small galaxy showing the impact of radiation from a bright quasar event that occurred in the center of IC 2497 about 100,000 years before how it is observed today. The quasar event is thought to have stimulated the bright emission that characterizes HsV. The quasar might have switched off in the last 200,000 years and is not visible in the available images. This might well be due to a process known as AGN feedback. One possible explanation for the missing light-source is that illumination from the assumed quasar was a transient phenomenon. In this case, its effects on HsV would be still visible because of the distance of several tens of thousands of light years between HsV and the quasar in the nearby galaxy: HsV would show a 'light echo' or 'ghost image' of events that are older than those currently seen in the galaxy. On 17 June 2010, a group of researchers at the European VLBI Network (EVN) and the UK's Multi-Element Radio Linked Interferometer Network (MERLIN), proposed another related explanation. They hypothesized that the light comes from two sources: (1) a supermassive black hole at the center of IC 2497, and (2) light produced by an interaction of an energetic jet from the black hole and the gas surrounding IC 2497.

Reader's Guide

Hanny's Voorwerp is significant as a notable example of a quasar ionization echo, discovered through citizen science. Its study has spurred further research into similar ionized clouds, called 'Voorwerpjes' (Dutch for 'small objects'). In February 2012, W. C. Keel and others published a paper in the Monthly Notices of the Royal Astronomical Society, reporting a search by Galaxy Zoo participants using Sloan Digital Sky Survey data that yielded a list of 19 galaxies with AGN-photoionized clouds detected beyond 10 kiloparsecs. Subsequent studies, including those in August 2013 (F. Schweizer et al.), May 2015 (W.C. Keel et al.), February 2018 (Treister et al.), March 2018 (Lansbury et al.), and March 2019 (Keel et al.), have examined these Voorwerpjes in various galaxies, using telescopes such as the BTA-6, VLT/MUSE, ALMA, Swift, Chandra, and the Very Large Array. In April 2016, a study using Chandra X-ray Observatory data found extended soft X-ray emission in IC 2497, suggesting a bubble or cavity surrounding the AGN, possibly inflated by the AGN or a past luminous quasar. The legacy of Hanny's Voorwerp lies in its role as a catalyst for systematic searches and detailed studies of AGN-photoionized clouds, advancing understanding of AGN history and obscuration.

Did You Know?

• Hanny's Voorwerp was discovered in 2007 by Dutch schoolteacher Hanny van Arkel while volunteering for the Galaxy Zoo project.
• The object is colored green in Hubble Space Telescope images, a false color representing glowing oxygen emission lines.
• A 40-page comic about Hanny's Voorwerp was presented at the 24th Dragon Con in Atlanta on 3 September 2010.