Galactic superwinds may help galaxies leak ionizing radiation, Haro 11 study finds

Our new study of the nearby starburst galaxy Haro 11 has shown that strong X-ray-emitting superwinds may be a key mechanism enabling galaxies to leak ionizing radiation, which is believed to have played a major role in the cosmic reionization in the early universe.

In our study published in Astronomy & Astrophysics, we looked at detailed X-ray data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton to examine the X-ray activity of Haro 11, which is a unique Lyman continuum emitter in our nearby universe.

Galaxies like Haro 11 give us a unique opportunity to study the physics of ionizing radiation escape up close. These mechanisms were likely central to reionizing the early universe, but they are extremely difficult to study at high redshift due to high distances.

Two X-ray sources, two stories

Haro 11 is home to two prominent X-ray sources—X1 and X2—which are spatially associated with massive star-forming regions known as Knots B and C. Both are thought to harbor ultraluminous X-ray sources, and they both emit in the ionizing Lyman continuum (LyC).

But curiously, the X-ray source X2, the fainter X-ray source, leaks more LyC radiation than the more X-ray luminous source X1. In addition, the X-ray source X1 with a higher luminosity may host an accreting compact object, like an intermediate-mass black hole.

To characterize the X-ray properties of these two X-ray sources, we conducted detailed spectral modeling of X-ray spectra. Our analysis revealed that the X-ray source X2, which leaks more LyC radiation, exhibits a significantly lower line-of-sight absorbing material—about 10 times less than that of the more X-ray luminous source X1.

This evidence indicates that X2 is less covered by gas and dust, potentially due to the action of strong superwinds generated by intense star formation and clustered supernovae. These outflows can potentially form cavities in the interstellar medium, opening channels for escaping LyC photons.

A superwind-powered escape route?

To investigate the spectral variability in the X-ray sources X1 and X2, we performed a principal component analysis (PCA), a linear dimensionality reduction technique that decomposes different modes of spectral and temporal change.

PCA is often used in data preprocessing for data mining and machine learning. Our analysis showed different emission patterns with peaks at energy levels matching H-like and He-like ion lines, like those of magnesium and silicon, indicating hot, ionized gaseous outflows.

These signatures mirror what has been observed in other starburst-driven superwinds, such as in the Cigar galaxy Messier 82 and Sculptor galaxy (NGC 253).

We believe that the observed X-ray variations in the X-ray sources X1 and X2 are likely linked to these starburst-driven superwinds, making it easier for ionizing radiation to leak out of the galaxy. The present study is the first time we have statistically linked X-ray variability to the tarburst-driven outflows and LyC leakage in Haro 11.

It supports the idea that feedback from stellar winds and supernovae may play an important role in forming galactic channels, facilitating the escape of ionizing photons.

Implications for cosmic reionization

Our findings support a growing body of evidence that superwinds and feedback-driven outflows are instrumental in allowing young, compact galaxies to leak Lyman continuum radiation. This process is thought to be what contributed to the cosmic reionization of the intergalactic medium a billion years after the Big Bang.

While direct X-ray observations of early high-redshift galaxies remain challenging due to their distance and faintness, local analogs like Haro 11 offer a powerful proxy.

Our study also highlights the need for more detailed and clearer X-ray observations to identify weak emission lines and better understand how hot plasma, ionizing radiation, and the surrounding interstellar medium interact.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.

Ashkbiz Danehkar is an astrophysicist funded by NASA grants at Eureka Scientific, Inc. (Oakland, CA). Prior to this position, Danehkar worked as a postdoctoral researcher at the University of Michigan (Ann Arbor, MI) and the Smithsonian Astrophysical Observatory (Cambridge, MA) after completing his PhD in astrophysics at Macquarie University (Sydney, Australia). Danehkar has conducted research on galactic outflows in star-forming galaxies and active galactic nuclei, in addition to stellar feedback in ionized nebulae and symbiotic stars.