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.

Terzan 5
Credit: X-ray: NASA/CXC/Univ. of Amsterdam/N.Degenaar, et al.; Optical: NASA, ESO/F.Ferraro

A double star system has been flipping between two alter egos, according to observations with NASA's Chandra X-ray Observatory and the National Science Foundation's Karl F. Jansky Very Large Array (VLA). Using nearly a decade and a half worth of Chandra data, researchers noticed that a stellar duo behaved like one type of object before switching its identity, and then returning to its original state after a few years. This is a rare example of a star system changing its behavior in this way.

Astronomers found this volatile double, or binary, system in a dense collection of stars, the globular cluster Terzan 5, which is located about 19,000 light years from Earth in the Milky Way galaxy. This stellar duo, known as Terzan 5 CX1, has a neutron star (the extremely dense remnant left behind by a supernova explosion) in close orbit around a star similar to the Sun, but with less mass.

Since ancient times, the study of astronomy has largely been limited to the flat, two-dimensional projection of what appears on the sky. However, just like a botanist puts a plant under a microscope or a paleontologist digs for fossils, astronomers want more "hands on" ways to visualize objects in space.

A new set of computer simulations represents an exciting step in that direction. Each is a three-dimensional (3D) visualization of an astronomical object based on data from NASA's Chandra X-ray Observatory and other X-ray observatories. While unable to fly to these distant objects and travel around them, astronomers have used data from these observatories to learn about the geometry, velocity, and other physical properties of each of these cosmic sources.

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%.

In the year 1054 AD, Chinese sky watchers witnessed the sudden appearance of a "new star" in the heavens, which they recorded as six times brighter than Venus, making it the brightest observed stellar event in recorded history. This "guest star," as they described it, was so bright that people saw it in the sky during the day for almost a month. Native Americans also recorded its mysterious appearance in petroglyphs.

Observing the nebula with the largest telescope of the time, Lord Rosse in 1844 named the object the "Crab" because of its tentacle-like structure. But it wasn't until the 1900s that astronomers realized the nebula was the surviving relic of the 1054 supernova, the explosion of a massive star.

Now, astronomers and visualization specialists from the NASA's Universe of Learning program have combined the visible, infrared, and X-ray vision of NASA's Great Observatories to create a three-dimensional representation of the dynamic Crab Nebula. Certain structures and processes, driven by the pulsar engine at the heart of the nebula, are best seen at particular wavelengths.

NGC 6338
Credit: X-ray: Chandra: NASA/CXC/SAO/E. O'Sullivan; XMM: ESA/XMM/E. O'Sullivan; Optical: SDSS

As the holiday season approaches, people in the northern hemisphere will gather indoors to stay warm. In keeping with the season, astronomers have studied two groups of galaxies that are rushing together and producing their own warmth.

The majority of galaxies do not exist in isolation. Rather, they are bound to other galaxies through gravity either in relatively small numbers known as "galaxy groups," or much larger concentrations called "galaxy clusters" consisting of hundreds or thousands of galaxies. Sometimes, these collections of galaxies are drawn toward one another by gravity and eventually merge.

Using NASA's Chandra X-ray Observatory, ESA's XMM-Newton, the Giant Metrewave Radio Telescope (GMRT), and optical observations with the Apache Point Observatory in New Mexico, a team of astronomers has found that two galaxy groups are smashing into each other at a remarkable speed of about 4 million miles per hour. This could be the most violent collision yet seen between two galaxy groups.

Roberto Gilli

We are very happy to welcome Roberto Gilli as our guest blogger. Dr. Gilli is the first author of a paper that is the subject of our latest Chandra press release. He received his Ph.D. in astronomy from Firenze University in Italy in 2001. Afterward, he did a post-doctoral fellowship at The Johns Hopkins Observatory before returning to Firenze at the Arcetri Astrophysical Observatory. Today, he is an astronomer at the National Institute of Astrophysics (INAF) in Bologna, Italy, a position he has held since 2005. His research interests include active galactic nuclei, quasars, and deep X-ray surveys.

Black holes are usually perceived as dangerous, disruptive systems. On the one hand they swallow copious amount of matter. On the other, they release a large amount of energy in the form of both radiation and matter when enormous quantities of material fall onto them.

The most extreme manifestation of such phenomenon is known as a "quasar" or an "active galactic nucleus" (AGN) that are powered by growing supermassive black holes (SMBHs) at galaxy centers. During these growth phases, part of the gravitational energy of the infalling gas is converted into strong electromagnetic radiation. Meanwhile, some of the gas, rather than being swallowed by the black hole, is instead accelerated and pushed very far away in the form of fast winds or even faster jets that can approach the speed of light.

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).

2019 Chandra Archive Collection
Credit: Enhanced Image by Judy Schmidt (CC BY-NC-SA) based on
images provided courtesy of NASA/CXC/SAO & NASA/STScI.

It is both an art and a science to make images of objects from space. Most astronomical images are composed of light that humans cannot detect with their eyes. Instead, the data from telescopes like NASA’s Chandra X-ray Observatory are “translated,” so to speak, into a form that we can understand. This process is done following strict guidelines to ensure scientific accuracy while trying to achieve the highest levels of aesthetics possible.

Over the two decades of the Chandra mission, we have had many talented people who have been involved with making our publicly-released images. We interviewed our current team and share some of their answers to questions posed to all of them below. Kim Arcand is Chandra’s visualization lead and has been with the mission since before launch; Nancy Wolk has been involved with Chandra’s data analysis, software, and spacecraft science operations before joining the image processing team; Lisa Frattare spent years making images from the Hubble Space Telescope before switching career gears but continues to lend her expertise part-time to Chandra’s efforts; Judy Schmidt is a citizen scientist who spends some of her free time using public data to make gorgeous images of space, including those featured in our latest release.

Gerrit Schellenberger

We welcome Gerrit Schellenberger as our guest blogger. He received his PhD in Bonn, Germany in 2016, and has been a post-doctoral researcher at the Center for Astrophysics | Harvard & Smithsonian since March 2016. His research includes working on galaxy clusters and groups in large samples for cosmology, but also on individual objects in the X-rays and in the radio regime.

From the beginning of my astronomical career, I was fascinated by studying galaxy clusters, consisting of hundreds, sometimes even thousands of galaxies held together by gravity only. Yet, the galaxies alone do not — by far — sum up to the mass necessary to keep the cluster bound together. Beginning in the 1970s after the birth of X-ray astronomy and the first imaging satellites such as Einstein and ROSAT, scientists discovered that a very hot gas exists between the galaxies of the cluster. The mass of this gas exceeds the mass of all the stars in the galaxies together.

Although this gas is the most dominant, visible structure in galaxy clusters, it is only about 10% of the total mass (while the stars in the galaxies make only about 1%). The rest, roughly 90%, is dark matter, which cannot be observed directly. However, we can see its effect on the hot gas and galaxies in galaxy clusters: the gravity not only keeps the galaxies within the cluster, but also compresses the gas, heating it to the point where it emits X-rays. So we can study dark matter in clusters by measuring the properties (like temperature) of the hot gas from the X-ray emission.

Intrigued by this, I started to analyze a sample comprising 64 clusters during my PhD in Bonn, Germany, with the goal of obtaining total masses (including the dark matter component) for all of them. It turns out that smaller and lighter galaxy clusters, also called galaxy groups, do not follow the expected scaling between X-ray emission and temperature at a given cluster mass, meaning that the X-ray properties of gas in these systems cannot be used to give reliable mass estimates. Therefore, galaxy groups can only be of limited use for cosmological studies, where it is crucial to estimate the amount of matter in objects and how it changes with cosmic time.