The focus of this week’s Hubble Picture of the Week is the blue compact dwarf galaxy NGC 5253, located in the constellation Centaurus around 11 million light-years from Earth. This new image combines data taken with Hubble’s Advanced Camera for Surveys (ACS), using its Wide Field Channel, and with the older Wide Field and Planetary Camera 2 (WFPC2). As a bonus for this Picture of the Week, there is also a second new image made using data from the High Resolution Channel (HRC) of ACS, a sub-instrument only operational for a few years that was optimised for detailed studies of environments dense with stars.
What has interested astronomers so much about this galaxy that three of Hubble’s instruments were used to study it in depth over ten years? It turns out to lie at the focus of a few areas of research where Hubble’s capabilities are essential. Dwarf galaxies are considered important for understanding the evolution of both stars and galaxies through time, since they resemble ancient, distant galaxies. NGC 5253 is called both a “starburst galaxy” and a “blue compact dwarf”: these names mean it is forming clusters of bright, massive stars at an exceptional rate. This Hubble image clearly shows the dense nebula which is being consumed to birth these stars, and which makes NGC 5253 a laboratory to investigate stellar composition, star formation and star clusters, all at once.
A tremendously high rate of star formation is a recipe for star clusters, but NGC 5253 goes beyond that: in a small region of the core, the star formation is so intense that the galaxy contains no less than three “super star clusters” (SSCs). SSCs are very bright, populous and massive open clusters which are believed to evolve into globular clusters. Globular clusters themselves offer unique insights into how stars form and evolve, but their origins are poorly understood. Astronomers were therefore eager to make use of the HRC sub-instrument, with its superb resolution, to hone in on these small, very dense clusters of stars.
[Image Description: An oval-shaped galaxy, made up of many point-like stars. It is softly lit from the centre, brightest and slightly blue at the very centre and fading to darkness at the edges. Surrounding the galaxy’s core are reddish clouds of gas and dust, most around or behind the core, but a few wisps are in front of it and block some light. Some faraway galaxies and two foreground stars can be seen around the galaxy.]