Monster galactic outflow powered by exploding stars

Enlarge / All galaxies have large amounts of gas that influence their star-formation rates.

Galaxies pass gas—in the case of galaxy NGC 4383, so much so that its gas outflow is 20,000 light-years across and more massive than 50 million Suns.

Yet even an outflow of this immensity was difficult to detect until now. Observing what these outflows are made of and how they are structured demands high-resolution instruments that can only see gas from galaxies that are relatively close, so information on them has been limited. Which is unfortunate, since gaseous outflows ejected from galaxies can tell us more about their star formation cycles.

The MAUVE (MUSE and ALMA Unveiling the Virgo Environment) program is now changing things. MAUVE’s mission is to understand how the outflows of galaxies in the Virgo cluster affect star formation. NGC 4383 stood out to astronomer Adam Watts, of the University of Australia and the International Centre for Radio Astronomy Research (ICRAR), and his team because its outflow is so enormous.

The elements it releases into space can reveal the galaxy’s potential to form (or stop forming) stars. “Understanding the physics of stellar feedback-driven outflows… is essential to completing our picture of galaxy evolution,” the researchers said in a study recently published in Monthly Notices of the Royal Astronomical Society.

Star potential

Stellar feedback, which is all the radiation, particle winds, and other materials that stars blast into the interstellar medium, is what forms outflows as huge as that in NGC 4383. Much of this material comes from either bursts of star formation or the insides of massive stars when they die and go supernova. It includes heavier elements that escape into space with the outflow and float there for an indefinite amount of time, sometimes ending up in other galaxies.

Star formation in a galaxy depends on several processes. There has to be the right balance of gas accretion (growth from added gas), consumption (the burning of hydrogen and helium by stars), and ejection (when interstellar gas is blown out of the galaxy) between the intergalactic medium and circumgalactic medium, the gas surrounding galaxies. Some of the gas and other materials, such as iron and other heavy elements, that form stars can be recycled from supernova explosions.

The supply of gas is key because large amounts of gas eventually collapse in on themselves because of their immense gravity, eventually forming stars. A deficit of gas can squelch the formation of potential stars.

Watts and his team think that one source of the stellar feedback pushing star-forming gas out of NGC 4383 is multiple supernovae that occurred relatively close together. Supernovae can form gargantuan bubbles of scorching gas that eventually break out of a galactic disk vertically, extending from the top and bottom of the galaxy.

Hot gas continues into cooler regions of the interstellar medium, with its gravity pulling in more gas on the way out of the galaxy and increasing the total mass of the outflow (known as mass loading). The loss of so much gas decreases the chances of star formation even further.

Lost in space

Outflows can be observed at many different wavelengths. Emissions of X-rays from elements such as hydrogen and compounds such as carbon monoxide can be detected. It is also possible to observe outflows using UV, optical, and infrared. Some of the region’s emissions had already been observed with other telescopes, which was combined with MAUVE imaging of the Virgo Cluster and NGC 4383 at different wavelengths.

The problem with observing outflows accurately is that the scattered materials are notoriously difficult to spatially resolve, which means figuring out the distance of the entire outflow based on pixels. MAUVE, NGC 4383, and the Virgo Cluster were observed at a spatial resolution of about 261 light-years, so each pixel represented a square in space that measured 261 light-years on every side. Clumps of ionized gas that showed up in these pixels told the research team there was a bipolar outflow leaving the galaxy from the top and bottom.

So, does NGC 4383 have reduced star formation because of its massive outflow of star stuff? It turns out that stars are actually forming at the galaxy’s edge. While no stars form in the stream escaping the galaxy, there are still areas where there is enough accreted gas to give birth to them.

These starbursts, or areas of rapid star formation, are also providing stellar feedback—it’s not just supernovae. “There is an extension of blue knots that are much brighter in the near-UV and are clear evidence of star formation occurring outside the main body of the galaxy,” the researchers said in the same study.

Something that remains unclear about NGC 4383 is whether the gas outflow was set off by stellar feedback alone or whether a gravitational interaction with another galaxy intensified existing outflows. There is possibly evidence for this on the eastern side, where a disturbance in the gas suggests that a nearby dwarf galaxy might have interacted with it. For now, the research team is confident that the outflow is primarily driven by starbursts and supernovae.

There is still more that the researchers want to find out about NGC 4383 and its outflow. As telescopes become more advanced and spatial resolution improves, maybe something else will be revealed inside those clouds of gas.

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