Black holes are one of the most exotic phenomena in astrophysics and represent a breakdown in fundamental physics between gravity and quantum mechanics. We would like to understand their origins, demographics, evolution, and nature. But, how can we find something that doesn’t emit light?
The only way to detect black holes is through their influence on their surroundings. Black holes can be detected when their extreme gravity effects nearby luminous gas and stars. They can also be detected from the gravitational waves generated when two black holes merge. Both of these methods require that the black hole is in close proximity to other objects - gas, stars, or another black hole in a binary system. None of these methods can be used to find free-floating, isolated black holes. Luckily, general relativity gives us another way that these dark objects can be detected.
We are searching for black holes in the Milky Way using gravitational lensing. Our Galaxy likely contains 100 million stellar-mass black holes. The number and mass statistics of black holes can provide important constraints on the star formation history, the stellar mass function, supernova physics and how BHs form, the equation of state of nuclear matter, and the existence of primordial black holes. To date, isolated stellar-mass black holes have never been definitively detected and only two dozen black holes in our Galaxy have measured masses – all in binaries.
Microlensing events, where a black hole’s gravity lenses the light of a background star as observed from Earth, provide a way to detect isolated black holes and measure their masses. Black hole lensing events produce a photometric magnification that has a long-duration (>3 months) and an astrometric signature that can be >1 mas. However, the astrometric signature has only recently become detectable with technological advancements in high-resolution imaging, including adaptive optics. Only the combination of photometry and astrometry can be used to precisely measure the mass of the lensing object and determine if it is indeed a black hole or a chance slow-motion lensing event between two normal stars.
We aim to find the first isolated stellar−mass black holes by detecting the astrometric signature of microlensing. Finding just a few black holes would already reduce the orders of magnitude uncertainty on the total number of black holes in the Galaxy and constrain theories of black hole formation and evolution. The proposed development of astrometry techniques, needed for astrometric microlensing, will also lay a foundation for new explorations in many areas of astrophysics.
Principles of Gravitational Microlensing
General relativity tells us that mass can bend light just like a physical lens. Massive objects behave like converging lenses, magnifying the image of more distant objects. Because the brightness of the image is conserved, the total flux overall increases as the size of the image increases. This leads to a characteristic transient brightening effect as the lens and the source move across each other along our line of sight. While the images cannot be resolved over the course of the event the centroid of the combined image will shift. With very precise astrometry this shift can be measured.
The animation below shows a black hole passing in front of background source star (yellow). The black hole acts as a lens and instead of seeing a single image of the star, two temporary images are produced instead (orange). Even our largest telescopes usually aren’t able to resolve (i.e. separate) these two images from each other. Instead, we still see a single object. When this occurs, we call this “microlensing”. The single object will get temporarily brighter (bottom), which we call the photometric microlensing signal. And the position of the object on the sky doesn’t stay still… instead it moves in an elliptical fashion (top red), which we call the astrometric microlensing signal.
Source: The background object that has its light manipulated by the lens.
Lens: The foreground object which is affecting the light of the source by virtue of its mass.
DOL: Distance between the observer and the lensing object.
DOS: Distance between the observer and the source.
Einstein Radius (θE): Angular radius of the circular image created when the source and lens are aligned along the line of sight of the observer.
Where G is the gravitational constant, M is mass of the lens, and c is the speed of light.
µrel: Relative source-lens proper motion on the sky plane.
tE: Einstein crossing time. The time it takes for the lens to traverse the Einstein radius
Microlensing with ZTF
Members: Michael Medford, Jessica Lu, Will Dawson (LLNL)