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Starts With A Bang

One Cosmic Mystery Illuminates Another, As Fast Radio Burst Intercepts A Galactic Halo

There’s so much we don’t know about fast radio bursts and galactic halos. Combined, we get a unique window on the Universe.

Deep in space, mysterious signals known as Fast Radio Bursts (FRBs) stream towards Earth.

The host galaxies of fast radio bursts remain mysterious for most of the FRBs we’ve seen, but a few of them have had their host galaxy’s detected. For FRB 121102, whose repeating bursts were extremely polarized, the host was identified as a dwarf galaxy with an active galactic nucleus. Perhaps interestingly, the stars within it, on average, have far fewer heavy elements (and hence, rocky, potentially habitable planets) than the ones in our Milky Way. (GEMINI OBSERVATORY/AURA/NSF/NRC)

These FRBs last milliseconds or less, originate in ultra-distant galaxies, and sometimes repeat.

Waterfall plot of the fast radio burst FRB 110220 discovered by Dan Thornton (University of Manchester). The image shows the power as a function of time (x axis) for more than 800 radio frequency channels (y axis) and shows the characteristic sweep one expects for sources of galactic and extragalactic origin. FRBs come as either single or multiple discrete bursts lasting from tens of microseconds to a few milliseconds, but no longer. (MATTHEW BAILES / SWINBURNE UNIVERSITY OF TECHNOLOGY / THE CONVERSATION)

Although scientists have studied them intensely since their discovery, their origins remain mysterious.

The Universe is full of two trillion galaxies, each containing hundreds of billions of stars on average, with countless more to come in the future. Yet we need two galaxies to be very well aligned, serendipitously so, for a FRB originating in one to pass through the halo of another in the foreground. (NASA, ESA, J. JEE (UNIVERSITY OF CALIFORNIA, DAVIS), J. HUGHES (RUTGERS UNIVERSITY), F. MENANTEAU (RUTGERS UNIVERSITY AND UNIVERSITY OF ILLINOIS, URBANA-CHAMPAIGN), C. SIFON (LEIDEN OBSERVATORY), R. MANDELBUM (CARNEGIE MELLON UNIVERSITY), L. BARRIENTOS (UNIVERSIDAD CATOLICA DE CHILE), AND K. NG (UNIVERSITY OF CALIFORNIA, DAVIS))

Meanwhile, an estimated 2 trillion galaxies populate our observable Universe.

For a FRB originating from a galaxy with the magnitude that the observed host galaxy of FRB 181112 possesses, the probabilities of having a random association within one arc-second (1/3600th of a degree) of another galaxy can be computed. Typical odds of such an association range between 0.25% and 0.40%, with the median value of 0.31%: about 1-in-300 odds. We’ve clearly gotten lucky, as humanity has not yet detected anywhere near 300 FRBs in total. (ESO/X. PROCHASKA ET AL.)

With incredibly large distances for FRBs to traverse, each one risks passing through an intervening galaxy.

In November of 2018, the fast radio burst FRB 181112 arrived here at Earth, but not before passing through the halo of the brighter foreground galaxy at the upper left. The burst passed through the galactic halo at a distance of approximately 95,000 light-years away from the galaxy’s center. (ESO/X. PROCHASKA ET AL.)

Giving off multiple pulses of under 40 microseconds apiece, FRB 181112 became the first burst to intercept a galactic halo.

This diagram shows how scientists determined the size of the halo of the Andromeda galaxy: by looking at absorption features from distant quasars, whose light either did or did not pass through the halo surrounding Andromeda. Where the halo is present, its gas absorbs some of the quasar light and darkens it across a very small wavelength range. By measuring the tiny dip in brightness at that specific range, scientists could tell how much gas is between us and each quasar. Doing this for more distant galaxies requires not only alternative techniques, but also serendipitous alignments. (NASA, ESA, AND A. FEILD (STSCI))

Halos are their own enigmas, populated with cool, enriched gas extending for hundreds of thousands of light-years.

The galaxy Centaurus A has a dusty disk component in it, but is dominated by an elliptical shape and a halo of satellites: evidence of a highly evolved galaxy that has experienced many mergers in its past. It is the closest active galaxy to us, but accelerates away from our Local Group. Each galaxy ought to be unique in terms of the properties of the normal matter in its halo, but broad categorizations by galaxy type, age, mass. morphology, metallicity, and star formation history should be possible. (CHRISTIAN WOLF & SKYMAPPER TEAM/AUSTRALIAN NATIONAL UNIVERSITY)

This gas is necessary for fueling future star-formation, but its physical properties remain largely unexplored.

A distant quasar will have a big bump (at right) coming from the Lyman-series transition in its hydrogen atoms. To the left, a series of lines known as a forest appears. These dips are due to the absorption of intervening gas clouds, and the fact that the dips have the strengths they do place constraints on many properties, such as the temperature of dark matter, which must be cold. However, this can also be used to constrain and/or measure the properties of any intervening galactic halos, including the gas within them. (M. RAUCH, ARAA V. 36, 1, 267 (1998))

Absorption features previously revealed abundant, cool (~10,000 K), low-density gas in these halos.

FRB 181112 comes to us from a distance of nearly 6 billion light-years away. However, it passed through the halo of an intervening foreground galaxy perhaps a billion light-years closer: a rare event with just a 0.3% probability of occurring for an FRB this distant. The vertical line at about 1.5 Gpc (~5 billion light-years) represents where the FRB signal passed through the foreground galaxy’s dark matter (and normal matter) halos. (ESO/X. PROCHASKA ET AL.)

But properties like total halo mass and hot (~1,000,000+ K) gas density are still undetermined.

The positions of the known fast radio bursts as of 2013, including four who had identifiable host galaxies, helped prove the extragalactic origins of these objects. The remaining radio emissions show the locations of galactic sources like gas and dust. The absorption features, polarizations, and pulse-lengthening of the FRBs we receive can tell us information about our own galaxy’s halo, but a serendipitous close pass to a foreground extragalactic object is an even greater probe of the outer galactic halos present in our nearby Universe. (MPIFR/C. NG; SCIENCE/D. THORNTON ET AL.)

When FRB 181112’s pulses traversed this galaxy’s halo, they were surprisingly unaffected.

This burst revealed a tranquil halo for this Milky Way-like galaxy, with:

  • very low-density gas,
  • no turbulence,
  • no clumps,
  • and negligible magnetization.
In searching for the free electron density (x-axis) and the magnetic field parallel to the FRB’s propagation direction (y-axis), scientists measured numerous properties of the arriving radiation. Only constraints could be placed: the magnetic field can be no stronger than about one-millionth the field strength generated by planet Earth at its surface, or about one-millionth the strength of a typical refrigerator magnet. (ESO/X. PROCHASKA ET AL.)

Are these properties universal to all Milky Way-like galaxies?

Within a dark matter halo, which could extend for millions of light-years, the normal matter collects towards the center. When the densities reach large enough amounts, owing either to gravitational collapse or the funneling of the gas into the disk/core, the gas will trigger the formation of new stars within. Having a foreground signal pass close to another galaxy is a rare, 1-in-300 odds event. (J. TURNER)

More observations, with additional FRBs, hold the answers.

Fast Radio Bursts (FRBs) have opened up an entirely new realm of astronomy for the 21st century. This discovery marks the first time a burst has passed through a foreground galaxy, giving us indicators of the properties of the halo gas within it. (DANIELLE FUTSELAAR)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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