The White Dots in This Image Are Not Stars or Galaxies. They're Black Holes - ScienceAlert
Totalling 25,000 such dots, astronomers have created the most detailed map to date of black holes at low radio frequencies, an achievement that took years and a Europe-sized radio telescope to compile.When they're just hanging out not doing much, black holes don't give off any detectable radiation, making them much harder to find. When a black hole is actively accreting material - spooling it in from a disc of dust and gas that circles it much as water circles a drain - the intense forces involved generate radiation across multiple wavelengths that we can detect across the vastness of space.What makes the above image so special is that it covers the ultra-low radio wavelengths, as detected by the LOw Frequency ARray (LOFAR) in Europe.Currently, LOFAR is the only radio telescope network capable of deep, high-resolution imaging at frequencies below 100 megahertz, offering a view of the sky like no other.At frequencies below 5 megahertz, the ionosphere is opaque for this reason.