The strange shape of the dwarf galaxy NGC 1156 observed by the Hubble Space Telescope

The dwarf galaxy NGC 1156 seen by Hubble (Image ESA/Hubble & NASA, R. B. Tully, R. Jansen, R. Windhorst)
The dwarf galaxy NGC 1156 seen by Hubble (Image ESA/Hubble & NASA, R. B. Tully, R. Jansen, R. Windhorst)

An image captured by the Hubble Space Telescope portrays the dwarf galaxy NGC 1156, a truly unique object because it’s an irregular dwarf galaxy that has various characteristics that belong to different classes of galaxies. Typically, galaxies that are out of the ordinary are the result of interactions or mergers, which are sometimes still in progress and offer the possibility to see the shapes of the original galaxies. NGC 1156 is among other things an isolated galaxy, and that means that there are no other galaxies close enough to affect its shape with a much higher force of gravity.

About 25 million light-years from Earth, the dwarf galaxy NGC 1156 is a curiosity due to its strange shape. In some ways, it looks like a spiral galaxy whose arms have been broken and emits a diffuse glow, typical of elliptical galaxies instead. In short, a strange irregular structure that glows thanks to young bright stars whose emissions generate a reddish glow in the ionized hydrogen gas. An older generation of stars occupies the center of this dwarf galaxy. A further oddity is given by the clouds of gas that rotate in the opposite direction to the rest of NGC 1156.

When a dwarf galaxy is part of a galaxy cluster, its shape can be distorted by the proximity of a much larger and more massive galaxy. Interactions with other galaxies can also lead to mergers and in the case of dwarf galaxies even an absorption if the other galaxy is much more massive. NGC 1156 is isolated and consequently, there are no nearby galaxies that could have caused distortion like the one seen in the images. The event that distorted its shape might go back to a remote era.

The interest in the dwarf galaxy NGC 1156 is confirmed also by the fact that it’s not the first time that ESA chose it for its photo of the week. A little over three years ago, ESA released the bottom image of NGC 1156, generated using two optical frequency filters applied to observations conducted with the Hubble Space Telescope’s Advanced Camera for Surveys (ACS) instrument. This time, the top image was generated using a combination of five optical frequency filters applied to observations conducted with the ACS instrument and an ultraviolet frequency filter applied to observations conducted with the Wide Field and Planetary Camera 2 (WFPC2) instrument.

The new image was generated using data collected during the “Every Known Nearby Galaxy” program, which aims to conduct observations that have a better quality than the ones already available of many galaxies less than 30 million light-years from Earth. Perhaps, the analyzes of the new observations of the dwarf galaxy NGC 1156 will enable to understand the origin of its strange shape.

The dwarf galaxy NGC 1156 seen by Hubble (Image ESA/Hubble, NASA, R. Jansen)
The dwarf galaxy NGC 1156 seen by Hubble (Image ESA/Hubble, NASA, R. Jansen)

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