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X-Ray Emission from Star-Forming Galaxies

X-Ray Emission from Star-Forming Galaxies

The star forming galaxy NGC 694 as seen in the X-ray (blue) and optical. The X-ray emission is due in large part to processes associated with star formation, and astronomers have figured out how to determine the star formation rate from the measured X-ray flux.

X-ray: NASA/CXC/CfA/R. Tuellmann et al.; Optical: NASA/AURA/STScI

Star formation lights up a galaxy because many newly formed stars are massive, hot and bright. These young stars are made in dusty clouds of material that obscure their visible light, and so luminous galaxies in our universe are often optically dim. But the dust absorbs the light and re-radiates it at infrared wavelengths, and astronomers can use the infrared from galaxies to infer the rate of star formation activity underway, even without seeing those stars. The method does not always work well, however: other processes can heat up dust and lead to an overestimate of the star formation rate, for example an active black hole at the nucleus; on the other hand sometimes the dust does not effectively absorb all of the light, leading to an underestimate of the rate.

There are three dominant sources of X-ray emission in galaxies: very hot interstellar gas, massive compact binary stars that emit X-rays (both of these a result of star formation), and accretion that heats material around a black hole nucleus. Astronomers would like to use X-ray emission as an alternate measure of star birth activity because of the issues arising with infrared dust emission, but the problem is addressing contamination from nuclear emission.

CfA astronomer Stefano Mineo and four colleagues have been able to calibrate the strength of X-ray emission against star formation rate and find a very good correlation. To solve the nuclear contamination problem, they limited their study to a sample of sixty-six nearby galaxies with no signature of activity in the nucleus, as confirmed by their low X-ray fluxes. In these objects they report finding a linear relation between the amount of X-ray emission and the star formation rate. Moreover, they determine that about 66% of the X-ray brightness comes from the X-ray binary phenomenon, and the rest mostly from the hot ISM. The result provides astronomers with an alternate method to infrared for estimating star formation activity. The astronomers also report that their findings appear not to depend on the distance (cosmic epoch) of a galaxy, at least in the not-too-remote universe, making these fainter and harder-to- analyze galaxies amenable to this new diagnostic.

"X-ray Emission from Star-Forming Galaxies – III. Calibration of the LX−SFR Relation Up to Redshift z ≈ 1.3," S. Mineo, M. Gilfanov, B. D. Lehmer,G. E. Morrison and R. Sunyaev, MNRAS 437, 1698, 2014.