Where Are All the 'Inbetweener' Black Holes?
The magenta spots in this image show two black holes in the Circinus galaxy: the supermassive one at its heart, and a smaller black hole closer to the edge that belongs to a class called ultraluminous X-ray sources, or ULXs. The magenta X-ray data come from NASA's Nuclear Spectroscopic Telescopic Array, and are overlaid on a visible/infrared image from the Digitized Sky Survey.
NASA/JPL-Caltech/K. Su (Univ. of Arizona)
NASA's Spitzer Space Telescope was launched 10 years ago and has since peeled back an infrared veil on the Cosmos. The mission has worked in parallel with NASA's other "Great Observatories" (Hubble and Chandra) to provide coverage of the emissions from galaxies, interstellar dust, comet tails and the solar system's planets. But some of the most striking imagery to come from the orbiting telescope has been that of nebulae. Supernova remnants, star-forming regions and planetary nebulae are some of the most iconic objects to be spotted by Spitzer. So, to celebrate a decade in space, here are Discovery News' favorite Spitzer nebulae.
First up, the Helix Nebula -- a so-called planetary nebula -- located around 700 light-years from Earth. A planetary nebula is the remnants of the death throes of a red giant star -- all that remains is a white dwarf star in the core, clouded by cometary dust.
NASA/JPL-Caltech/B. Williams (NCSU)
Spitzer will often work in tandem with other space telescopes to image a broad spectrum of light from celestial objects. Here, the supernova remnant RCW 86 is imaged by NASA's Spitzer, WISE and Chandra, and ESA's XMM-Newton.
Staring deep into the Messier 78 star-forming nebula, Spitzer sees the infrared glow of baby stars blasting cavities into the cool nebulous gas and dust.
The green-glowing infrared ring of the nebula RCW 120 is caused by tiny dust grains called polycyclic aromatic hydrocarbons -- the bubble is being shaped by the powerful stellar winds emanating from the central massive O-type star.
NASA/JPL-Caltech/J. Stauffer (SSC/Caltech)
Spitzer stares deep into the Orion nebula, imaging the infrared light generated by a star factory.
X-Ray: NASA/CXC/J.Hester (ASU); Optical: NASA/ESA/J.Hester & A.Loll (ASU); Infrared: NASA/JPL-Caltech/R.Gehrz (Univ. Minn.)
In the year 1054 A.D. a star exploded as a supernova. Today, Spitzer was helped by NASA's other "Great Observatories" (Hubble and Chandra) to image the nebula that remains. The Crab Nebula is the result; a vast cloud of gas and dust with a spinning pulsar in the center.
The Tycho supernova remnant as imaged by Spitzer (in infrared wavelengths) and Chandra (X-rays). The supernova's powerful shockwave is visible as the outer blue shell, emitting X-rays.
NASA/JPL-Caltech/E. Churchwell (University of Wisconsin - Madison)
Over 2,200 baby stars can be seen inside the bustling star-forming region RCW 49.
X-ray: NASA/CXC/Univ.Potsdam/L.Oskinova et al; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech
The "Wing" of the Small Magellanic Cloud (SMC) glitters with stars and warm clouds of dust and gas. By combining observations by Spitzer, Chandra and Hubble, the complex nature of this nebulous region can be realized.
There's small black holes and supermassive black holes, but where are all the "inbetweener" black holes?
This question has been foxing astrophysicists for years; the apparent dearth of medium-sized or "intimediate" black holes -- between 100 to 1 million solar masses -- doesn't make logical sense. But when it comes to black holes, you can often check logic at the door.
One would assume that to make a supermassive black hole, there must be some growth mechanism that causes small black holes, say around 100 solar masses, to pack on the pounds and grow to the gravitational behemoths that occupy the centers of most known galaxies.
Black holes at the lower end of the mass spectrum are stellar-mass black holes and, as their name suggests, they were formed by the collapse of massive stars and the result of supernoavae. The most massive black holes that are found in the cores of galaxies -- often reaching tens of millions to billions of solar masses -- are less well understood and astronomers are currently trying to understand how they grew to be so massive.
But the scarcity of intermediate-mass black holes poses a quandary: Is there some black hole growth mechanism that is stranger than we can possibly imagine? Or are current observatories simply not sensitive to the emissions from these middleweight objects?
"Exactly how intermediate-sized black holes would form remains an open issue," said Dominic Walton of the California Institute of Technology (Caltech), Pasadena. "Some theories suggest they could form in rich, dense clusters of stars through repeated mergers, but there are a lot of questions left to be answered."
In an effort to get to know the nature of intermediate mass black holes, a collaboration of international observatories "went to town" on two ultraluminous X-ray sources (or ULXs) that were thought to contain black holes in the 100 to 10,000 solar mass range.
ULXs are likely composed of a star and a nearby black hole. The black hole does what it does best, sucking material from the unfortunate binary partner, generating radiation in the process. These compact sources of X-rays have led astronomers to believe that the feeding black holes in ULXs fall into the intermediate-mass category.
NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) has joined Europe's XMM-Newton satellite in an effort to study a recently-identified ULX in the Circinus spiral galaxy some 13 million light-years distant (pictured top). Combining these X-ray observations with archival data from NASA's Chandra, Swift and Spitzer space telescopes plus the Japanese Suzaku satellite, this has become one of the most intensely-scrutinized ULXs ever.
In a paper published in the Astrophysical Journal, this collaboration deduced that the Circinus ULX is around 100 solar masses -- but it may not be an intermediate-mass black hole at all. It could actually just be a large stellar-mass black hole that has an exotic "feeding" mechanism that generates intense X-ray emissions.
In another study also accepted for publication in the Astrophysical Journal, two ULXs in NGC 1313, a spiral galaxy 13 million light-years away, were examined. Those too, after being studied by NuSTAR, appear to also be large stellar-mass black holes and not the much sought-after intermediate-mass black holes. So what's going on?
"It's possible that these objects are ultraluminous because they are accreting material at a high rate and not because of their size," said Matteo Bachetti of the Institut de Recherche en Astrophysique et Planétologie. "If intermediate-mass black holes are out there, they are doing a good job of hiding from us."
Despite the hope that ULXs may provide some answers to the apparent lack of medium black holes in the Universe, so far the evidence is suggesting otherwise.