The Square Kilometer Array (SKA), construction of which has begun this year, will soon be the largest radio telescope in the world. Thanks to its sensitivity and survey speed, this observatory will shed light on important topics in astronomy and astrophysics, such as the formation of structures in the early universe, the formation of first stars and galaxies, and galaxy evolution. In most cases these will be studied through multi-frequency surveys covering different sky areas in different deployment stages. An international team of researchers (this work is conducted within the framework of the SKA Science Working Group ‘Extragalactic Continuum’) have investigated the capability of potential legacy surveys to detect the interstellar medium in normal spiral galaxies such as M51 and M33 back to when the universe had only a small fraction of its current age. They show that the SKA surveys proposed for array deployment phase 1 can already detect the radio continuum emission from the interstellar medium in galaxies like M51 back to Cosmic Noon and even earlier.  

Galaxies are quenched after a golden epoch of star formation about 10 billion years ago called “Cosmic Noon” following which star-formation activity declined for reasons that are still not fully understood. A drop in the amount of cool gas within galaxies, which serves as the fuel for star formation, is often considered as the main reason.  However, observations show that many galaxies still had sufficiently large gas reservoirs for star formation to take place. “Another possibility is that the cool gas in galaxies was increasingly stabilized by magnetic field pressure, high-energy particles, and turbulence.” says Fatemeh Tabatabaei, a faculty member at the Institute for Research in Fundamental Sciences-IPM and a researcher at the Max-Planck-Institutes for Astronomy (MPIA), Max-Planck-Institutes for Radio Astronomy (MPIfR), and Institute of Astrophysics in Canary Islands (IAC) who is leading this research. “Understanding the importance of these factors requires energy balance studies as a function of redshift.” she continues.

Observations of the radio continuum emission are a powerful way to trace energetic processes in galaxies. This emission emerges mainly from the interaction of high-energy particles with magnetic fields, an energetic component of the interstellar medium. Deep and spatially resolved observations at different radio frequencies with SKA allow us to map these processes in galaxies near and far. This is the key step toward understanding the energy balance and structure formation in galaxies over cosmic time, and for shedding light on the processes governing galaxy evolution and quenching. “Assessing for which galaxies, and out to which cosmic distances, it will be possible to study these processes is an essential part of preparing for the actual SKA data.” says Mark Sargent from the International Space Science Institute in Bern, Switzerland, and co-author on the study.

“As the first step, we were interested in simulating the radio continuum emission from the ISM of typical high-redshift galaxies, using normal present-day spiral galaxies such as M51, NGC6946, and M33 as templates. This simulation takes into account two different radiation mechanisms, the thermal free-free and the non-thermal synchrotron radiation.” says Masoumeh Ghasemi-Nodehi, a postdoc at IPM, and collaborator of this project.  “We showed that the SKA phase 1 MID radio frequency (SKA1-MID) surveys can map the synchrotron radiation in M51–like galaxies up to a redshift of 3, when the universe had only 1/7 of its current age.” she continues.

“Both the relativistic particles and magnetic fields are expected to insert higher pressures into the interstellar medium at earlier times, due to the higher level of star formation activity in these early galaxies. This expectation shown by our studies needs to be further confirmed by the SKA observations.” says Fatemeh Tabatabaei.

The first part of this research is published in the Journal of Monthly notices of the Royal Astronomical Society.

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