Black Hole Week is in full swing right now, and to celebrate, NASA has released stunning observations of the heart of a distant spiral galaxy - as well as the gigantic supermassive black hole that resides at that heart.
The observations were conducted by the X-ray Imaging and Spectroscopy Mission (XRISM), led by the Japan Aerospace Exploration Agency (JAXA) with contributions from NASA; they show the center of spiral galaxy NGC 4151.
Located some 43 million light-years away (as well as its incumbent supermassive black hole, which is estimated to have as many as 20 million suns), this galaxy is seen in vibrant reds and bright blues thanks to the addition of radio waves. That addition comes via data from the Very Large Array (VLA) and the Isaac Newton Group of Telescopes.
Yet there is more to the XRISM (pronounced 'crism') observations than just their aesthetic value. The X-ray space telescope was able to pick out fingerprints of iron in this galaxy's Active Galactic Core (AGN), and that could help determine the fate of the matter swirling around its monstrous black hole.
Related: An image of the central region of the spiral galaxy NGC 4151, home to a supermassive black hole
"XRISM's Resolve instrument captured a detailed spectrum of the region around the black hole," Brian Williams, NASA's XRISM project scientist at the Goddard Space Flight Center, said in a statement. "The peaks and valleys are like chemical fingerprints that can tell us what elements are present and provide clues about the fate of matter as it approaches the black hole."
A supermassive black hole engine
Like all AGNs, NGC 4151's central engine shines brightly because it's powered by a supermassive black hole that's actively feeding on surrounding matter.
However, not all supermassive black holes are so greedy. For example, Sagittarius A* (Sgr A*) at the heart of our galaxy, the Milky Way, has such a sparse diet that if it were human, it would be sustained by about one grain of rice every 1 million years.
On the other hand, for an AGN, gas and dust feeding the monstrous black hole form an oblate cloud, called an accretion disk, around the black hole itself. The black hole's enormous gravity also generates intense tidal forces in this accretion disk, heating the disk and causing it to glow brightly.
Furthermore, matter that does not fall into the maw of the monstrous black hole can be channeled toward the object's poles by powerful magnetic fields that accelerate these particles to near-light speeds, ejecting them as twin jets, one from each pole. This situation is accompanied by a burst of electromagnetic radiation that, when combined with emissions from the accretion disk, often makes an AGN brighter than the combined light of every star in the galaxy around it.
The AGN of NGC 4151 is exceptionally bright, even for such an active region of a galaxy, and shows a high degree of variability. This, combined with the fact that it is located at the heart of one of the nearest known active galaxies, makes NGC 4151's black hole an ideal subject for research.
The AGN has previously been examined by the Hubble Space Telescope and NASA's Chandra X-ray Observatory, in an effort to learn more about the interactions between its supermassive black hole and its environment. Understanding the dynamics of these two things can reveal how the growth of these cosmic titans affects the growth of galaxies around them.
XRISM counts calories
XRISM has a particular advantage over Hubble when it comes to studying the AGN of NGC 4151. This galaxy is unusually bright in the specific X-rays that XRISM is adept at.
Using its Resolve instrument, which studies the universe with just 36 pixels, XRISM was able to reconstruct the spectrum of light coming from the AGN. Because chemical elements and compounds absorb and emit light at characteristic wavelengths, they leave their 'fingerprints' in such spectra. Sure enough, in the Resolve spectrum from the heart of NGC 4151, scientists were able to determine the energies associated with this AGN peak, just below the characteristic emission line associated with the element iron.
Scientists have theorized that most of the power of AGNs manifests through X-rays emanating from regions of hot and blazing matter embedded in the accretion disk near their central black hole. When these X-rays are reflected from the cooler and denser regions of the same swirling cloud of matter, they are thought to flare up iron in those regions - which indeed explains this discovery.
This means that the detection of iron and an X-ray peak around this supermassive black hole paints a clearer picture of the phenomena found in such eruptive disks and spots.
The spectrum of the AGN at the heart of NGC 4151 also shows absorption lines characteristic of iron in its environment. It appears that iron in these regions absorbs rather than radiates because they are cooler than the blazing spots of the accretion disk near the black hole.
All this radiation is about 2,500 times more energetic than light in the visible region of the electromagnetic spectrum, the only kind of light our eyes have evolved to see.
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Although this specific result from XRISM focuses on iron, that is certainly not the only element that the X-ray telescope can distinguish. Depending on the source, the satellite can also detect the elements sulfur, calcium, argon and others in AGNs (and other celestial bodies).
Each of these elements can tell scientists a different aspect of the stories of the celestial bodies they surround or even compose. That makes XRISM an essential tool in the future of astronomy and efforts to decode the X-ray sky.
