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Left panel: map of the surface brightness of the "Antennae" in
emission from ionized hydrogen. The brightest zones are the zones
occupied by clusters of massive stars. Right panel: map of the expansion
velocity of the bubbles detected in the Antennae, pushed outwards by
the stellar winds and supernova explosions of the stars in the cluster.
The colours give the values of the expansion velocity at each point on
the map. The largest bubble is 1500 light years in size. Credit: IAC.
A team of scientists led by members of the Instituto de Astrofísica de Canarias (IAC), in collaboration with the Universidad Nacional Autónoma de México (UNAM), have detected and measured a complete "carpet" of expanding bubbles in the interstellar medium of the "Antennae", a pair of galaxies in interaction which will eventually merge. The work, published in Monthy Notices of the Royal Astronomical Society is based on observations with GHaFaS on the William Herschel Telescope (WHT). This instrument is capable of making a map of the velocities of a complete galaxy using the emission from the ionized hydrogen in its interstellar medium.
To detect the huge bubbles in the combined galaxy disc, the team has used BUBBLY, a method developed by some of the current authors which has already been published in the same journal in 2015. These huge bubbles in galaxy discs are caused by the stellar winds and supernova explosions in clusters of very hot very massive stars. They can range in size from a couple of light years to a thousand light years, depending on the number of stars in the cluster and how massive they are. The larger ones are often called "superbubbles".
In the study, the BUBBLY method has been applied to the "Antennae", in which the interaction between the galaxies is causing a major burst of star formation, leading to many star clusters, each surrounded by a bubble of expanding gas. The researchers have been able to calculate how much energy is being fed into the interstellar medium from each individual bubble, and from the sum total of all the bubbles, including a decent estimate for those which are too small to be fully detected.
"The importance of the bubbles", says Artemi Camps-Fariña, the lead author on both of the articles, "is that they let us measure the effects of feedback caused by massive star clusters on the rest of the galaxy in which they lie. This is being recognised as very important. Theorists who aim to model how galaxies are formed and evolve had a major problem when they made models without feedback. In those models star formation was far too rapid, so that all the available gas would have been used up when the universe had reached no more than one tenth of its present age. All galaxies would now be passive without any current star formation. But the feedback process, in which massive star clusters blow large bubbles, slows down the star formation rate, as it stops new gas from condensing so quickly into stars. It reduces the overall star formation rate by a big factor and has let galaxies such as the Milky Way produce their stellar populations in a much more extended time frame".
"Although this general idea is not new", says John Beckman, one of the authors of both articles, "our ability to measure the properties of the bubbles is giving us a way to quantify the effect and to match basic theory with the observed properties of galaxies".
Artemi Camps-Fariña, et al., 2017, "Physical properties of superbubbles in the Antennae galaxies", MNRAS, 468, 4134 [ADS].
Artemi Camps-Fariña, et al., 2015, "BUBBLY: a method for detecting and characterizing interstellar bubbles using Fabry-Perot spectroscopy", MNRAS, 447, 3840 [ ADS].