Quasars & Active Galaxies

Shifu Zhu

Shifu Zhu, a 5th-year graduate student of Astronomy & Astrophysics at Pennsylvania State University, is our guest blogger for this post. He received his B.S. in Astronomy from the University of Science and Technology of China (USTC) in 2013. He received his M.S. in Astrophysics from USTC in 2016.

“So, the answer to the nature of the X-ray emission from radio-loud quasars is simpler than we previously had thought,” I said to myself after staring for a while at our new correlations between how bright radio-loud quasars are in X-ray and ultraviolet light.

The term “quasar” was originally coined for bright radio sources that look like stars in visible-light images, i.e., quasi-stellar radio sources. Shortly after their discovery, researchers realized that quasars are supermassive black holes (with masses of millions to billions of times that of the Sun) feeding on material that is gravitationally attracted to them. Notably, despite this strong gravitational attraction, some material can also be ejected in powerful jets, narrow streams of material shooting away from the supermassive black hole in opposite directions. These jets are fueled by material in an “accretion disk” falling towards the black hole.

Erini Lambrides

We are delighted to welcome Erini Lambrides from Johns Hopkins University (JHU) in Baltimore Maryland as a guest blogger. She is the first author of a paper in the Astrophysical Journal that is the subject of our latest press release. Erini is a PhD candidate at JHU in the Department of Physics and Astronomy and will be defending her thesis next year. Prior to starting her PhD, she spent a year as a research assistant at Gemini Observatory North, and a year as a research assistant at the American Museum of Natural History in NYC. She got her BS with Honors in Physics at the University of Rochester. She was awarded a NASA Maryland Space Grant Fellowship to fund her PhD and outreach efforts. Her research is focused on quantifying the extent and impact of AGN-host galaxy co-evolution.

When one normally thinks of the scientific method, the following mantra rooted in elementary school days comes to mind: The Question, The Research, The Hypothesis, The Data, The Conclusion. Neatly lined, linear steps that seemingly underpin the entirety of humanity’s quest for knowledge about our physical world. I learned this when I was ten years old, but nearly twenty years later, as I trundle along my own scientific journey, I’ve learned that the steps of the scientific method more resemble an M.C. Escher painting.

My group and I have recently completed work on a peculiar class of astrophysical objects: growing supermassive black holes that are embedded in a thick cocoon of gas and dust. This work is exciting because we discovered that this particularly difficult-to-observe class of objects can be detected in greater numbers than previously thought. However, when I started this work, my goal was not to find these objects, nor was it even about this class of objects!

This article appeared on NASA.gov, written by NASA public information officer Elizabeth Landeau, based on a press release from Keck Observatory.

Examples of Einstein ring gravitational lenses taken with the Hubble Space Telescope.
Credit: NASA/ESA/SLACS Survey Team: A. Bolton (Harvard/Smithsonian), S. Burles (MIT), L. Koopmans (Kapteyn), T.Treu (UCSB), L. Moustakas (JPL/Caltech)

Determined to find a needle in a cosmic haystack, a pair of astronomers time traveled through archives of old data from W. M. Keck Observatory on Mauankea in Hawaii and old X-ray data from NASA’s Chandra X-ray Observatory to unlock a mystery surrounding a bright, lensed, heavily obscured quasar.

This celestial object, which is an active galaxy emitting brilliant amounts of energy due to a black hole devouring material, is an exciting object in itself. Finding one that is gravitationally lensed, making it appear brighter and larger, is exceptionally exciting. While slightly over 200 lensed unobscured quasars are currently known, the number of lensed obscured quasars discovered is in the single digits. This is because the feeding black hole stirs up gas and dust, cloaking the quasar and making it difficult to detect in visible light surveys.

Andrew King

We are pleased to welcome Andrew King from the University of Leicester in the United Kingdom as a guest blogger. Andrew is the author of a paper that is the subject of our latest press release. He graduated in Mathematics from the University of Cambridge (UK), and then researched there for his PhD in General Relativity. After postdoctoral positions in London and Hamburg, he moved to the University of Leicester, where he is now Professor of Theoretical Astrophysics. He is a long-term visitor at the Anton Pannekoek Astronomical Institute in the University of Amsterdam, and Visiting Professor at Leiden Observatory. His interests include ultraluminous X-ray sources, accretion and feedback involving supermassive black holes, and how this affects their host galaxies.

A few months ago, Giovanni Miniutti from ESA's Center for Astrobiology in Spain, and collaborators observed that X-ray emission from the low-mass nucleus (that is, a relatively small black hole at its center) of the galaxy GSN 069 brightened by factors of about 100 roughly every 9 hours, staying bright for about an hour each time before returning to the faint state. From the X-ray spectrum — the intensity of X-rays at different wavelengths — they deduced that the X-rays came from an accretion disk around the central black hole of the galaxy, which has the rather low mass of about 400,000 times that of the Sun. I was intrigued by these observations: the eruptions implied that a lot of mass was being fed into the accretion disc every 9 hours, and the reasonably stable period suggested there was something in a very close orbit around the black hole.

Ophiuchus Galaxy Cluster
Credit: X-ray: Chandra: NASA/CXC/NRL/S. Giacintucci, et al., XMM-Newton: ESA/XMM-Newton;
Radio: NCRA/TIFR/GMRT; Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF

We are pleased to welcome two guest bloggers, Maxim Markevitch and Simona Giacintucci, who led the study described in our latest press release. Markevitch, an expert on galaxy clusters X-ray studies, got his PhD at the Space Research Institute of the Russian Academy of Sciences. He worked on ASCA X-ray data in Japan, then at the Chandra X-ray Center for the first 10 years of Chandra operations, and is now at the NASA Goddard Space Flight Center. He received the AAS Rossi Prize. Giacintucci, the lead author of the study, is an expert in radio phenomena in galaxy clusters. She got her PhD at Bologna University. She was a postdoc at the CfA and an Einstein fellow at the University of Maryland, and is now at the Naval Research Lab.

Galaxy clusters are colossal concentrations of dark matter, galaxies, and tenuous, 100-million-degree plasma. This plasma — gas where the electrons have been stripped from their atoms — slowly loses heat by emitting radiation in the form of X-rays. Around the central peaks of many clusters, where matter concentrates, the plasma gets dense enough* to cool quite fast, on a timescale shorter than the cluster's lifetime (a few billion years). The higher the plasma's density, the more X-rays it emits and the faster it cools. As it cools down, it contracts and becomes denser still, and so on, entering a runaway cooling process. Left unchecked, this process should deposit vast quantities of cold gas in the cluster centers.

We know for a fact that the plasma cools down because we do observe those X-rays — but we don't find nearly as much cold gas in the cluster centers as such runaway cooling must deposit. This has been a puzzle for a long while, and the solution the astronomers converged upon is that there must be some source of additional heat in the central regions of clusters — their “cores” — that doesn't let the plasma cool below 10 million degrees or so.

Early Chandra X-ray images of galaxy clusters pointed to the likely source: the supermassive black holes (SMBH) that sit in the centers of the cluster central galaxies, pull in the surrounding matter, and eject a tiny part of it (just before it sinks irretrievably into the black hole) at nearly the speed of light back into the surrounding gas. Where those jets hit the gas, they blow huge bubbles in it, stir it, generate shocks like sonic booms, etc. (all of these features have been seen in the Chandra images of the cluster cores). The current wisdom holds that these processes together supply the needed heat to prevent runaway cooling from occurring, but at the same time are not so powerful that they blow up the whole plasma cloud, implying some kind of a gentle, self-regulated feedback loop may be occurring.

Supermassive Black Hole Jet in M87
Credit: NASA/CXC/SAO/B.Snios et al.

These images show evidence from NASA's Chandra X-ray Observatory that the black hole in the galaxy Messier 87 (M87) is blasting particles out at over 99% the speed of light, as described in our latest press release. While astronomers have observed features in the M87 jet blasting away from its black hole this quickly at radio and optical wavelengths for many years, this provides the strongest evidence yet that actual particles are travelling this fast. Astronomers required the sharp X-ray vision from Chandra in order to make these precise measurements.

The main panel of the graphic shows the entire length of M87's jet seen by Chandra, stretching for about 18,000 light years. "Knots" of X-ray emission seen here are created when material falls onto the M87 sporadically, creating bursts of X-ray light that travel along the jet and away from the black hole. The insets show Chandra observations taken in 2012 and 2017 of a small region near the base of the jet. The source in the lower left is X-ray emission from material around the black hole, and the other source is a knot in the jet about 900 light years from the black hole. This knot moves away from the black hole between 2012 and 2017 and also fades by 70%.

Phoenix Cluster
Credit: X-ray: NASA/CXC/MIT/M.McDonald et al; Radio: NRAO/VLA; Optical: NASA/STScI

The Phoenix galaxy cluster contains the first confirmed supermassive black hole that is unable to prevent large numbers of stars from forming in the core of the galaxy cluster where it resides. This result, reported in our latest press release, was made by combining data from NASA's Chandra X-ray Observatory and Hubble Space Telescope, and the NSF's Karl Jansky Very Large Array (VLA). A new composite image shows data from each telescope. X-rays from Chandra depict hot gas in purple and radio emission from the VLA features jets in red. Optical light data from Hubble show galaxies (in yellow), and filaments of cooler gas where stars are forming (in light blue).

SDSS J084905.51+111447.2
Credit: X-ray: NASA/CXC/George Mason Univ./R. Pfeifle et al.; Optical: SDSS & NASA/STScI

A new study using data from NASA's Chandra X-ray Observatory and other telescopes provides the strongest evidence yet for a system of three supermassive black holes, as described in our latest press release. Astronomers think these triplet collisions, while extremely rare, play a critical role in how the biggest black holes grow over time.

The system is known as SDSS J084905.51+111447.2 (SDSS J0849+1114 for short) and is located a billion light years from Earth. In this graphic, X-rays from Chandra (purple) are shown in the pull-out in comparison with optical light from the Hubble Space Telescope and the Sloan Digital Sky Survey (red, green, and blue) in the main panel.

GSN 069
Credit: X-ray: NASA/CXO/CSIC-INTA/G.Miniutti et al.; Optical: DSS

A supermassive black hole is blasting out X-rays about every nine hours, according to data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton, as described in our latest press release. This indicates that this black hole, containing about 400,000 times the mass of our Sun, is consuming significant amounts of material about three times per day.

The main panel of this graphic is a visible light image taken by the Digitized Sky Survey (DSS) around the galaxy known as GSN 069, located in the center of the image. The inset gives a time-lapse of Chandra data taken over a period of about 20 hours on February 14 and 15, 2019, centered on the X-ray source in the middle of GSN 069. The sequence runs in a loop to show that the X-ray brightness of the source changes regularly and dramatically over the Chandra observation. Three X-ray eruptions are observed. (Note that to clearly show the Chandra source is located in GSN 069, the size of the box in the center of the DSS image is about ten times larger than the Chandra field in the inset.)

Fabio Vito

We are very pleased to welcome Fabio Vito as our guest blogger. Vito is the first author of a paper that is the subject of our latest press release, on the discovery of a distant, cloaked black hole. He obtained his PhD in 2014 at the University of Bologna, Italy, before moving to Penn State as a postdoctoral researcher. He is now a postdoctoral fellow at the Pontificia Universidad Católica de Chile. He mainly works on the properties and evolution of high-redshift AGN, with the final goal of understanding how they formed and grew in the first billion years of the Universe.

Imagine you are a teacher in a kindergarten starting the school year. You enter the classroom, but instead of finding little children, you see fully grown people — men and women — staring at you. Puzzled, you check with the principal, and they confirm that those people are supposed to be the new kindergarteners, just a handful of years old. Two things come to your mind immediately: 1) this is definitely going to be a very long school year, 2) what happened? Why are adults sitting in your kindergarten classroom?

Astrophysicists find themselves in a similar situation today. According to our theoretical knowledge, supermassive black holes (SMBHs) should grow from "seeds" with masses not larger than hundreds of thousands of solar masses. We then use the most powerful telescopes to find the most distant — both in space and in time — growing SMBHs, shining as “quasars,” about 13 billion years ago, when the Universe was just a few hundred million years old. We look for them because astronomers want to study how they grew to become the monsters that populate the older Universe, with masses of billions of solar masses. However, the SMBHs powering the quasars that we find in the kindergarten of the Universe are already fully grown! They are indeed already as massive as the most massive SMBHs in the local Universe.