Our telescopes have never detected black hole more massive than twenty times the mass of the Sun. Nonetheless, we now know of their existence, as dozens of these black holes have recently been “heard” to merge via gravitational wave radiation. A team of astronomers led by Peter Jonker (SRON / Radboud University) has now discovered that these seemingly disparate results can be explained by biases against massive black holes in conventional telescope observations.
In 2015, the LIGO installations detected gravitational waves for the first time. They were emitted by two massive black holes several dozen the mass of the Sun in the process of merging. This discovery shook the Universe, as well as the astronomical community, because few astronomers had predicted the existence of such massive black holes, let alone their fusion. Before gravitational wave detections, our conventional telescopes had found proof of the existence of stellar mass black holes in about twenty cases. However, none had ever been found that was as massive as those now seen by gravitational wave radiation emitted during fusion. To date, around 50 of these merging black hole pairs have been detected, including by the European detector Virgo, still in most cases involving massive black holes. Telescopes still haven’t found such black holes.
This disparity can be explained in part by the greater volume of the Universe which is probed by gravitational wave detectors. LIGO-Virgo can more easily find such more massive black holes because their waves are stronger than those of lighter black holes, implying that they could be rare but noisy events. But zero detection of such black holes using telescopes? Black holes, or at least their surroundings, light up when they slowly devour a companion star. Through measurements of the unfortunate star’s orbital motion, the mass of the black hole can be determined.
A team of astronomers led by Peter Jonker (Radboud University / SRON) have found that telescope observations are biased compared to the detection of massive black holes. Such massive black holes can, in principle, be observed if they eat the mass of a companion star. However, the circumstances of these observations were too difficult in practice, explaining the lack of detections of massive black holes through telescope observations. The largest black holes are formed by the implosion of massive stars, instead of explosions of massive stars (“supernova”). Formed by an implosion, these massive black holes remain in the same place where their predecessor (the massive star) was born, the plane of the Milky Way galaxy. However, this does mean that they remain shrouded in dust and gas. Their lighter black hole siblings, born from massive stars in supernova explosions, are kicked out of the plane of the Milky Way, making them more easily observable for our telescopes measuring their mass.
To compound this bias, as Jonker and colleagues realized, any companion star in a massive black hole must orbit a relatively large distance, making it more rare for a companion star to be devoured in an observable frenzy. Such episodes are what reveal the existence and location of black holes. Thus, the most massive black holes will more rarely reveal their location.
The imminent launch of the James Webb Space Telescope (JWST) on December 18 will allow astronomers to test these ideas. JWST will make it possible for the first time to measure the mass of several candidate black hole systems in the plane of the Milky Way. JWST will be sensitive to infrared light, and this light is much less affected by dust and gas than optical light typically used by ground-based telescopes. In addition, JWST’s large size and its advantageous position in space allow JWST to choose the lucky star to study from among the millions of stars in the Milky Way plane. Finally, being above the Earth’s atmosphere, JWST will not be bothered by infrared light emitted by the atmosphere.
Reference: “The observed mass distribution of LMXBs in galactic black holes is biased against massive black holes” by Peter G. Jonker, Karamveer Kaur, Nicholas Stone and Manuel AP Torres, November 9, 2021, The Journal of Astrophysics.
DOI: 10.3847 / 1538-4357 / ac2839