Cosmology/Deep Field

X-Rays Help Prove Some Galaxies are True Relics of the Ancient Universe

Professor David Buote
Professor David Buote

We welcome Professor David Buote as our guest blogger. Buote was one of the first Chandra Postdoctoral Fellows and is now a Professor at the University of California at Irvine. He has studied X-rays from massive elliptical galaxies and galaxy clusters since the time he was a graduate student. His new work with Aaron Barth on the dark matter in a relic elliptical galaxy is the subject of our latest press release.

This year marks the 20th anniversary of the Chandra X-ray Observatory and a chance to celebrate its many and diverse accomplishments. A critical aspect of Chandra's impact on astrophysics is its synergies with observations of phenomena throughout the electromagnetic (EM) spectrum and through other channels like gravity waves and neutrinos. Our study highlights how studies of the X-ray emission of a rare type of galaxy complement and augment what has been learned from observations of the stellar light at longer wavelengths.

Galaxies are broadly divided into two types — disks and spheroids — with substantial overlap in their properties. The spheroids — or elliptical galaxies — are approximately round but range in shape as observed on the sky from nearly circular to elongated somewhat like an American football viewed from the side. Most of what we know about the stars in galaxies comes from observations of visible light photons with lots of help from observations in the nearby ultraviolet and infrared (IR) parts of the EM spectrum.

Heart of Lonesome Galaxy is Brimming with Dark Matter

Image of Mrk 1216
Markarian 1216
Credit: X-ray: NASA/CXC/Univ. of CA Irvine/D. Buote; Optical: NASA/STScI

Data from NASA's Chandra X-ray Observatory (left) have helped astronomers reveal that a galaxy has more dark matter packed into its core than expected after being isolated for billions of years, as reported in our press release. The image on the right shows the galaxy called Markarian 1216 (abbreviated as Mrk 1216) in visible light from NASA's Hubble Space Telescope over the same field of view.

Mrk 1216 belongs to a family of elliptically shaped galaxies that are more densely packed with stars in their centers than most other galaxies. Astronomers think they have descended from red, compact galaxies called "red nuggets" that formed about a billion years after the Big Bang, but then stalled in their growth about 10 billion years ago.

Hide and Seek: Tracking Down the Invisible Filaments

Orsolya Kovács
Orsolya Kovács

We welcome Orsolya Kovács, a third-year PhD student at the Eötvös Loránd University, Hungary where she obtained her MSc degree in astronomy, as our guest blogger. Currently, she is a pre-doctoral fellow at the Smithsonian Astrophysical Observatory, and is the first author on a recent paper on the WHIM featured in our latest press release.

I was working on a totally different subject before I started the missing baryon project with a small group of scientists at the Smithsonian Astrophysical Observatory (SAO) about two years ago. Before I came to the United States as a Ph.D. student, I was involved in analyzing optical data of variable stars observed at the beautiful Piszkéstető Station in the Mátra Mountains, Hungary. In my master’s thesis, I focused on the variable stars of an extremely old open cluster in the Milky Way, and at that time, I also got the chance to gain some observing skills from my Hungarian supervisor.

So the very beginning of my astronomy career was all about optical astronomy. But before getting really into optical astronomy and mountain life, I decided to interrupt this idyllic period, and find some new challenges: I wanted to spend part of my Ph.D. years learning X-ray astrophysics. With this in my mind, I applied to the SAO’s pre-doctoral program, and a few months later I arrived in Massachusetts.

Shortly after introducing me to the basics of X-ray astronomy, Ákos Bogdán at SAO proposed a crazy idea about how to observe the ‘invisible’, i.e. the missing part of the ordinary (baryonic) matter that could possibly solve the long-standing missing baryon problem. The missing baryon problem is related to the mismatch between the observed and theoretically predicted amount of matter.

Where is the Universe Hiding its Missing Mass?

Plot and Simulation
WHIM Simulation
Credit: Illustration: Springel et al. (2005); Spectrum: NASA/CXC/CfA/Kovács et al.

New results from NASA's Chandra X-ray Observatory may have helped solve the Universe's "missing mass" problem, as reported in our latest press release. Astronomers cannot account for about a third of the normal matter — that is, hydrogen, helium, and other elements — that were created in the first billion years or so after the Big Bang.

Scientists have proposed that the missing mass could be hidden in gigantic strands or filaments of warm (temperature less than 100,000 Kelvin) and hot (temperature greater than 100,000 K) gas in intergalactic space. These filaments are known by astronomers as the "warm-hot intergalactic medium" or WHIM. They are invisible to optical light telescopes, but some of the warm gas in filaments has been detected in ultraviolet light. The main part of this graphic is from the Millenium simulation, which uses supercomputers to formulate how the key components of the Universe, including the WHIM, would have evolved over cosmic time.

Cosmology with Quasars

Guido Risaliti
Guido Risaliti

We are pleased to welcome Guido Risaliti as our guest blogger. Guido is the first author of a paper that is the subject of our latest press release. He is an astrophysicist whose main research field is the study of giant black holes in the center of galaxies. He got a Ph.D. from the University of Florence, Italy, in 2002. He then worked as a researcher at INAF - Arcetri Observatory from 2002 until 2015, and was a Research Associate at the Center for Astrophysics / Harvard and Smithsonian from 2002 to 2014. Since 2015, he has been an Associate Professor at the University of Florence.

For about 20 years I have studied the emission of quasars, the most luminous persistent sources in the Universe, powered by an “accretion disk” made of gas spiraling into a giant black hole. Quasars emit most of their radiation in the optical/ultraviolet (UV) band, through their accretion disk, and a small fraction in the X-rays, produced by a cloud of hot electrons called a “corona”. This corona needs a continuous flow of energy from the disk in order not to cool down and stop producing X-rays.

We do not know much about this energy exchange: a self-consistent model linking these two components has not been found yet. However, we have observed an interesting relation: the X-ray fraction of the total emission of radiation by quasars decreases with its luminosity. For example, if observing two quasars, we find that the first one is ten times more UV luminous than the second one, it will be only four times more luminous in X-rays.

Finding the Happy Medium of Black Holes

COSMOS Survey
Credit: X-ray: NASA/CXC/ICE/M.Mezcua et al.;
Infrared: NASA/JPL-Caltech; Illustration: NASA/CXC/A.Hobart

This image shows data from a massive observing campaign that includes NASA's Chandra X-ray Observatory. These Chandra data have provided strong evidence for the existence of so-called intermediate-mass black holes (IMBHs). Combined with a separate study also using Chandra data, these results may allow astronomers to better understand how the very largest black holes in the early Universe formed, as described in our latest press release.

The COSMOS ("cosmic evolution survey") Legacy Survey has assembled data from some of the world's most powerful telescopes spanning the electromagnetic spectrum. This image contains Chandra data from this survey, equivalent to about 4.6 million seconds of observing time. The colors in this image represent different levels of X-ray energy detected by Chandra. Here the lowest-energy X-rays are red, the medium band is green, and the highest-energy X-rays observed by Chandra are blue. Most of the colored dots in this image are black holes. Data from the Spitzer Space Telescope are shown in grey. The inset shows an artist's impression of a growing black hole in the center of a galaxy. A disk of material surrounding the black hole and a jet of outflowing material are also depicted.

A Serendipitous Discovery May Provide Our First View of a New Population of X-ray Transients

Franz E. Bauer
Franz E. Bauer

It is a pleasure to welcome Franz E. Bauer as a guest blogger. Franz led the study that is the subject of our latest press release. He is an associate professor at Pontificia Universidad Catolica de Chile in Santiago, Chile, where his group studies the cosmic evolution of star-forming galaxies and supermassive black holes, as well as a variety of transient phenomena. He completed his PhD at the University of Virginia in 2001, then worked at Pennsylvania State University, University of Cambridge (UK), and Columbia University before finally moving to Chile.

Like many discoveries in astrophysics, the subject of our recent study was an act of serendipity. Our large international collaboration had been allocated a series of long observations with Chandra to push the exposure from 45 days to 75 days for the deepest X-ray image on the sky to date, the Chandra Deep Field-South (CDF-S). The primary goal of this project was to explore the poorly understood realm of the ultra-faint X-ray universe, to learn how supermassive black holes form in the early Universe and by what mechanisms they grow to become the present day "monsters" that we see today (for details, see a January 2017 press release led by Bin Luo from Nanjing University and Fabio Vito from Penn State University and an associated blog post by Fabio Vito). However, the leaders of this project, colleagues Drs. Niel Brandt (Penn State University) and Bin Luo, had studied variability from known X-ray objects in the previous data containing 45 days of exposure, and were thus monitoring the individual observations as they arrived to check for large deviations.

To our surprise, during one 13-hour observation on October 1st, 2014, a bright, new source emerged (see Figure 1), at a location where no source had been detected, even when summing up all of the previous exposures together. Two days later, in the next Chandra observation, it was gone! We had never anticipated that our observations would capture such a rare, fast transient. After convincing ourselves that it was not some weird instrumental effect, we reported it to the astronomy community as Luo, Brandt & Bauer (2014) in ATEL 6541, to encourage follow-up observations at other wavelengths and gain more clues as to the origin of this unique event.

Hunting For the Most Distant Galaxy Cluster

Tao Wang
Tao Wang

We are pleased to welcome Dr. Tao Wang as a guest blogger. Tao is the first author of a paper that is the subject of our latest press release, about an extremely distant galaxy cluster. Tao is now a postdoc in CEA/Saclay, France, working with Dr. David Elbaz on high-redshift galaxies and galaxy clusters, and received a PhD in astrophysics from Nanjing University, China in 2012. During his PhD, he worked for two years in the Harvard-Smithsonian Center for Astrophysics and then worked as an associate researcher back in Nanjing University for one year before starting his postdoctoral work at CEA/Saclay in 2013.

Galaxy clusters are the largest known gravitationally bound structures in the universe and usually consist of hundreds of galaxies distributed in a relatively small area a few million light-years across. One of the most prominent features of clusters is the presence of a predominant population of massive, elliptical galaxies in the cluster core. These galaxies are among the most massive galaxies in the universe and are believed to have rapidly formed their stars a long time ago. However, how these galaxies formed and why have they stopped forming new stars remain mysteries. Solving these mysteries is essential to our understanding of both galaxy and cluster formation. To answer these questions, the key is to search for and study galaxy clusters (or their progenitors) in the early universe, right when they form.

Record-breaking Galaxy Cluster Discovered

CL J1001
This image contains the most distant galaxy cluster, a discovery made using data from NASA's Chandra X-ray Observatory and several other telescopes. The galaxy cluster, known as CL J1001+0220, is located about 11.1 billion light years from Earth and may have been caught right after birth, a brief, but important stage of cluster evolution never seen before.

Cosmic Genealogy: the Ancestors of Supermassive Black Holes

Fabio Pacucci
Fabio Pacucci

It is a pleasure to welcome Fabio Pacucci as a guest blogger. Fabio led the study that is the subject of our latest press release. He is going to defend his Ph.D. Thesis at the Scuola Normale Superiore in Pisa (Italy), under the supervision of Andrea Ferrara. During his Ph.D. he spent several months at the Institute d’Astrophysique de Paris (IAP) in France, Yale University and Harvard University in the USA. In September he is starting his first postdoctoral position at Yale University. Fabio has mainly been working on understanding the properties of the first black hole seeds, formed when the Universe was less than one billion years old.

It was a sunny and hot afternoon in Pisa when Andrea Ferrara, my Ph.D. supervisor, suggested that I study the first black holes formed in the Universe. This topic is among the most interesting in cosmology. We know that almost every galaxy hosts a supermassive black hole (SMBH) at its center. In the Milky Way there is a black hole about 4 million times more massive than the Sun, but objects up to 10 billion times the mass of the Sun have also been observed.

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