Cosmic dust and dying galaxies

Gas in galaxies can exist in two forms: atomic and molecular. Atomic gas is composed of hydrogen atoms separated from each other, whereas molecular gas is composed of hydrogen atoms forming a molecule (two atoms bound together).

Michał Jerzy Michałowski

Gamma-ray bursts (GRBs) are the explosions of the most massive stars, 20-30 times more massive than the Sun. There were previous claims that galaxies hosting such explosions have very little molecular gas, which would be surprising, because this gas is the fuel for making new stars (including those exploding as GRBs). However, those claims were based on a small number of galaxies. I studied a dozen of such galaxies and concluded that on average their molecular gas properties are as expected, given their star formation activity. This has important consequences on our understanding of the explosion mechanism and on using GRBs as a tool in galaxy evolution.

I discovered large amounts of atomic gas in the environment of an explosion of a relativistic supernova 2009bb type Ic. These supernovae explode when the most massive stars in the Universe die (several dozen times more massive than the Sun). This is the first time when atomic gas in the environment of a supernova explosion has been studied. This discovery brings us closer to the full understanding of stellar explosions because the concentration of gas in this case suggests that stars exploding as this kind of supernovae are formed when gas flows into a galaxy from the space between galaxies. This will also allow us to study gas inflows into galaxies using supernovae.

I characterised the atomic gas in the host galaxy of an unusual and poorly-understood explosion called AT 2018cow. Its nature is still being discussed by scientists. I have found typical gas distribution unlike those of GRB and supernova host galaxies. Therefore, the environment of AT 2018cow suggests that its progenitor may not have been a massive star. This brings us closer to the understanding of this unusual explosion.

I reported the discovery of the second-closest GRB (number 111005A). I found that this GRB was surprisingly not accompanied by a supernova so it likely represents a rare class of GRBs that is different from typical explosions of massive stars. I have measured for the first time how quickly galaxies which are ‘dying’ (shutting down their star formation activity) are getting rid of dust. This is a key aspect of galaxy evolution.

Dust affects the optical light of a galaxy in a complex way, so that dusty galaxies appear redder than in reality (in the same way as the setting Sun appears red). It is important to measure the details of this effect in order to understand the properties of distant galaxies. I am a member of an international team, which measured this effect for very distant galaxies using the best telescopes in the world: James Clerk Maxwell Telescope (JCMT) in Hawaii and the Atacama Large Millimeter Array (ALMA) in Chile. We found that the standard parametrisation of this effect derived for nearby galaxies also applies for distant ones.

I led the Poznań team, a part of a larger collaboration, in observations of a gamma-ray burst number 171205A. This allowed us for the first time to confirm the existence of a hot gas cocoon ripped from the centre of an exploding star. Our observations were executed using the Roman Baranowski Telescope. This is a robotic telescope owned by the Adam Mickiewicz University, located in Arizona (USA) and operated from Poznań via the internet.

Gravitational waves, travelling disturbances of space-time predicted by the theory of general relativity, were discovered 4 years ago by LIGO (Laser Interferometer Gravitational Wave Observatory). On the 17 August 2017 an international team of astronomers, of which I was a member, for the first time detected radiation from colliding neutron stars – the sources of gravitational waves. Neutron stars are extremely dense objects with sizes of around ten kilometres and a few times more massive than the Sun. The discovery of this radiation provided the confirmation that gravitational waves can also be emitted by colliding neutron stars. Moreover, the observation of gamma-rays provided the proof that Understanding the Universe so-called short gamma-ray bursts result from the collisions of such stars. This discovery opens new ways of using gravitational waves to study very massive stars.

The results of your research were published as 42 papers in top astronomical journals, including one in Nature and one in Nature Astronomy. You also found time for public lectures, press releases and TV interviews. Why are such outreach activities important?

For me outreach activities are a chance to connect with the wider public to talk about the exciting results we obtain. This is needed in order to inform non-specialists about the importance of our work and to inspire people to study physics and astronomy. Finally, the interaction with the audience attending outreach events can simply be fun!


Dr Michał J. Michałowski is an astronomer at the Astronomical Observatory Institute at the Adam Mickiewicz University in Poznań. He did his PhD in the Dark Cosmology Centre at the University of Copenhagen under the supervision of Jens Hjorth and Darach Watson. He was a postdoc at the Institute for Astronomy, University of Edinburgh and at the University of Gent. His research concentrates on exploding stars and the so-called interstellar medium in distant galaxies, i.e. gas and dust between stars.