Category: Featured Science

An IFU View of the Active Galactic Nuclei in MaNGA Galaxy Pairs

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The role of active galactic nuclei (AGNs) during galaxy interactions and how they influence the star formation in the system are still under debate. Recently, a research team led by the Chinese Academy of Sciences South America Center for Astronomy (CASSACA) used the integral field unit (IFU) data to study the star-formation distributions of a paired active galactic nuclei (AGN) sample. They found that AGNs are likely follow an inside-out quenching and the merger impact on the star formation in AGNs is less prominent than in star-forming galaxies (SFGs). 


Upper panel: An example of AGN identification using IFU data.
Lower left: Similar distributions of paired AGNs and isolated AGNs in the SFR-Mass diagram.
Lower right: Interaction show different impact on the resolved properties of AGNs and SFGs.

This pair sample of 1156 IFU-covered galaxies were selected by the velocity offset, projected separation, and morphology from SDSS IV-MaNGA survey, and is further classified into four cases along the merger sequence based on morphological signatures. A total of 61 (5.5%) AGNs in pairs were identified based on the emission-line diagnostics. No evolution of the AGN fraction was found, either along the merger sequence or compared to isolated galaxies (5.0%). There was a higher fraction of passive galaxies in galaxy pairs, especially in the pre-merging cases, which was related to a denser environment. The isolated AGN and AGNs in pairs showed similar distributions in their global stellar mass, star-formation rate (SFR), and central [O iii] surface brightness. AGNs in pairs showed radial profiles of increasing specific SFR and declining Dn4000 from center to outskirts, and no significant difference from the isolated AGNs. This was clearly different from SFGs in this pair sample, which showed enhanced central star formation, as reported before. AGNs in pairs had lower Balmer decrements at outer regions, possibly indicating less dust attenuation.

The research paper is recently published in The Astrophysical Journal (https://doi.org/10.3847/1538-4357/ac2901). CASSACA student Gaoxiang Jin and Prof. Y. Sophia Dai are the first author and the corresponding author, respectively.

The First FAST detection of neutral hydrogen emission from extragalactic galaxies

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The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) is the largest telescope with the highest sensitivity in the world. Extragalactic neutral hydrogen detection is one of important scientific goals of FAST.

Recently, an international research team led by Dr. Cheng Cheng from Chinese Academy of Sciences South America Center for Astronomy (CASSACA), successfully detected the neutral hydrogen line emission from three local galaxies using the FAST 19-beam receiver with only five minutes of exposure. The research paper is recently published in Astronomy & Astrophysics Letter. This is the first publication on FAST observation of extragalactic neutral hydrogen.

Fig. 1. The optical color images of the four galaxies for FAST observation. The red contours are the previous CO observation by ALMA. The white spectra in each panel are the results from FAST.

Neutral hydrogen gas is the most extended baryons in galaxies, while cold gas traced by CO is more concentrated to a galaxy center (red contour in Figure 1). With dynamical measurements of neutral hydrogen and CO, we can estimate the mass distribution of galaxies at different radii. Dynamical masses of these four galaxies estimated from the newly observed the neutral hydrogen line are 10 times higher than the observed baryon masses, indicating contribution of dark matter. On the other hand, dynamical masses estimated using previous CO observations were equivalent to their observed baryon masses. Therefore, the new FAST observation illustrates its ability of studying dark matter in galaxies using the neutral hydrogen 21cm emission line.

The FAST observation of these galaxies was an important part of an international research project, the Valparaíso ALMA Line Emission Survey (VALES), led by Prof. Edo Ibar from Valparaiso University in Chile. The VALES is a project of observing star forming galaxies using first-class international facilities such as SDSS, Herschel space observatory, ALMA, APEX and VLT. This international team has been publishing a series of scientific results.

FAST with the unpreceded sensitivity provides a very unique chance to observe the extra-galactic neutral hydrogen, and therefore has been adding to the list modern astronomical facilities used by this international collaboration. The FAST observation time was awarded in the Share-risk Observing call during the FAST Commissioning Phase. The team will apply for FAST time to further study the neutral hydrogen properties of VALES galaxies.

The paper can be accessed at https://www.aanda.org/articles/aa/full_html/2020/06/aa38483-20/aa38483-20.html

[Reprinted] Cloaked Black Hole Discovered in Early Universe using NASA’s Chandra

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NASA/CXC

Astronomers have discovered evidence for the farthest “cloaked” black hole found to date, using NASA’s Chandra X-ray Observatory. At only about 6% of the current age of the universe, this is the first indication of a black hole hidden by gas at such an early time in the history of the cosmos.

Supermassive black holes, which are millions to billions of times more massive than our Sun, typically grow by pulling in material from a disk of surrounding matter. Rapid growth generates large amounts of radiation in a very small region around the black hole. Scientists call this extremely bright, compact source a “quasar.”

According to current theories, a dense cloud of gas feeds material into the disk surrounding a supermassive black hole during its period of early growth, which “cloaks” or hides much of the quasar’s bright light from our view. As the black hole consumes material and becomes more massive, the gas in the cloud is depleted, until the black hole and its bright disk are uncovered.

“It’s extraordinarily challenging to find quasars in this cloaked phase because so much of their radiation is absorbed and cannot be detected by current instruments,” said Fabio Vito of the Pontificia Universidad Católica de Chile, in Santiago, Chile, who led the study. “Thanks to Chandra and the ability of X-rays to pierce through the obscuring cloud, we think we’ve finally succeeded.”

The new finding came from observations of a quasar called PSO167-13, which was first discovered by Pan-STARRS, an optical-light telescope in Hawaii. Optical observations from these and other surveys have detected about 180 quasars already shining brightly when the universe was less than a billion years old, or about 8 percent of its present age. These surveys were only considered effective at finding unobscured black holes, because the radiation they detect is suppressed by even thin clouds of gas and dust. Since PSO167-13 was part of those observations, this quasar was expected to be unobscured, too.

Vito’s team were able to test this idea by using Chandra to observe PSO167-13 and nine other quasars discovered with optical surveys. After 16 hours of observation, only three X-ray light particles were detected from PSO167-13, all with relatively high energies. Since low-energy X-rays are more easily absorbed than higher energy ones, the likely explanation is that the quasar is highly obscured by gas, allowing only high-energy X-rays to be detected.

“This was a complete surprise”, said co-author Niel Brandt of Penn State University in University Park, Pennsylvania. “It was like we were expecting a moth but saw a cocoon instead. None of the other nine quasars we observed were cloaked, which is what we anticipated.”

An interesting twist for PSO167-13 is that the galaxy hosting the quasar has a close companion galaxy, visible in data previously obtained with the Atacama Large Millimeter Array (ALMA) of radio dishes in Chile and NASA’s Hubble Space Telescope. Because of their close separation and the faintness of the X-ray source, the team was unable to determine whether the newly-discovered X-ray emission is associated with the quasar PSO167-13 or with the companion galaxy.

If the X-rays come from the known quasar, then astronomers need to develop an explanation for why the quasar appeared highly obscured in X-rays but not in optical light. One possibility is that there has been a large and rapid increase in cloaking of the quasar during the three years between when the optical and the X-ray observations were made.

On the other hand, if instead the X-rays arise from the companion galaxy, then it represents the detection of a new quasar in close proximity to PSO167-13. This quasar pair would be the most distant yet detected.

In either of these two cases, the quasar detected by Chandra would be the most distant cloaked one yet seen, at 850 million years after the Big Bang. The previous record holder has been observed 1.3 billion years after the Big Bang.

The authors plan to follow up with more observations to learn more.

“With a longer Chandra observation we’ll be able to get a better estimate of how obscured this black hole is,” said co-author Franz Bauer, also from the Pontificia Universidad Católica de Chile, “and make a confident identification of the X-ray source with either the known quasar or the companion galaxy.”

The authors also plan to search for more examples of highly obscured black holes.

“We suspect that the majority of supermassive black holes in the early universe are cloaked: it’s then crucial to detect and study them to understand how they could grow to masses of a billion suns so quickly,” said co-author Roberto Gilli of INAF in Bologna, Italy.

A paper describing these results is accepted for publication in Astronomy and Astrophysics and is available online. NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science and flight operations from Cambridge, Massachusetts.
Other materials about the findings are available at:
http://chandra.si.edu

CASSACA scientist reveals the connection between radiation and shape of circum-nuclear materials around super-massive black holes

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[Dr. Claudio Ricci, a CAS-CONICYT Postdoctoral Fellow in astronomy,  led an important paper in《Nature》on September 27th 2017, in which he reported recent major progress regarding how radiation feedback controls the shape of close environment around super-massive black holes, drawn from a multi-band survey of a sample of black holes selected in the hard X-ray band.]

A black hole is a place in space-time where gravity pulls are so strong that even light cannot get out, it is therefore “black” in any bands. The theory of general relativity predicts that a sufficiently large and dense mass would deform space -time and give birth to a black hole. Despite its invisible interior, a black hole can be indirectly inferred and investigated at various wavelengths through its interactions with the surrounding and in-falling materials.

It is known for decades that very heavy black holes inhabit the centers of galaxies (including our own galaxy, the Milky Way), but are hidden by gas and dust. Some of these black holes can “eat” materials from their environment, and emit a lot of light during this process. Most of these “luminous” black holes are surrounded by large amount of gas and dust, distributed in a doughnut-like structure. Such a structure could resemble a pantry, which guarantees that the black hole can keep eating, radiating and growing. However, it is not known where exactly this material is located, and what the relationship is between light produced by the black hole and the dusty gas.

In order to address this long-standing issue, Dr. Claudio Ricci, a postdoc fellow supported by Chinese Academy of Sciences South America Center for Astronomy (CASSACA), and his collaborators made use of observations carried out in the X-ray band, similar to what is typically used for radiographies in hospitals. With each observation performing these “space radiographies”, they could measure the amount of material around the black hole, and then study its evolution.

This project started in 2013, and it took the authors many years to create the large database used for their research, using data from space telescopes as well as ground-based observatories, such as those in Chile. The Chilean telescopes were extremely important for measuring the properties of the black holes, and in particular, for “weighing” their masses. The main X-ray instrument used was the NASA satellite Swift, but also data from the satellites XMM-Newton of ESA, Suzaku of JAXA, and another NASA telescope, Chandra, were used. In the optical band the facilities used include the Sloan Digital Sky Survey, the UK Schmidt telescope, Gemini, CTIO, DuPont and SAAO.

With this work, Ricci and collaborators discovered the process that controls the interaction between light produced by the black hole and the gas that surrounds it, and showed that most of the material around black holes is located close to it. The authors found that, when the black hole emits a lot of light, this light pushes away the material from its vicinity; in other words, the gas can “evaporate” because of the large amount of energy released by the material falling rapidly onto the black hole. This could also mean that, if the black hole “eats” too rapidly, the energy produced could destroy the “food” available for the future.

It is a major step forward to reveal a clear picture of the connection between radiation feedback and the surrounding material’s shape. “The next step will be to further understand the details of this behavior, and what happens to the material that is pushed away from the black hole”, said Dr. Ricci, the leading author of this work.

Figure 1. Artistic impression of the gas and dust surrounding an accreting supermassive black hole. Taken from NASA/JPL/Caltech.

Figure 2. Schematic representation of the material surrounding supermassive black holes for different ranges of Eddington ratio. The Eddington ratio is the ratio between the bolometric and the Eddington luminosity, where the latter is defined as the luminosity at which the radiation pressure from a source, in this case the accreting SMBH, balances the gravitational attraction. Taken from Ricci et al. (2017, Nature Letter).

Young scientist from CASSACA pens review on accreting supermassive black holes in Nature Astronomy

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Dr. Claudio Ricci, a CAS-CONICYT Postdoctoral Fellow in astronomy, recently authored a review paper in 《Nature Astronomy》 with Cristina Ramos Almeida of IAC Tenerife, in which they summarized the most important developments of the past ten years in the fields of accreting black holes and circum-nuclear materials, as revealed by observations in the X-ray and infrared bands.

All massive galaxies host supermassive black holes (SMBH) at their centers, and these objects are often found to be hidden behind large amounts of gas and dust. This circum-nuclear material is what eventually accretes onto the black hole, allowing it to grow, and its structure and evolution have been the subject of intense study in the past decade. Chinese Academy of Sciences South America Center for Astronomy (CASSACA)’s postdoctoral fellow Claudio Ricci and Dr. Cristina Ramos Almeida (IAC Tenerife) were recently invited to write a review for《Nature Astronomy》 on this subject, with the idea of combining results obtained from X-ray and infrared studies of the close environments of supermassive black holes. These two energy bands are highly complementary: while X-rays are produced very close to the supermassive black hole and allow the study of radiation absorbed and reflected, infrared radiation is directly produced by the dust around the black hole.

A black hole is a place in space where gravity pulls so much that even light cannot get out, and therefore itself is “black” in any bands. The theory of general relativity predicts that a sufficiently large and compact mass can deform space time and give birth to a black hole. Black holes of stellar masses are expected to form when very massive stars collapse at the end of their life cycle. After a black hole is formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. The first widely-accepted black hole is Cygnus X-1 discovered in 1964, and it weighs about 15 solar masses (Figure 1). The well-known radio source named Sagittarius A*, sitting at the core of our own Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.

Despite its invisible interior, the presence of a black hole can be inferred through its interactions with matter. Circum-nuclear materials falls onto a black hole, and forms an AGN (active galactic nucleus), one of the brightest objects in the universe, including an external accretion disk, an X-ray emitting corona, a broad-line region (BLR), a torus and a narrow-line region (NLR) (Figure 2), which have various physical characteristics, and their radiation are widely used to study the black hole and its surroundings.

The review summarizes the most important developments of the past ten years in this field, and particularly in light of the most advanced X-ray and IR facilities. The authors discuss why the circum-nuclear material is anisotropic, clumpy, and connects with the host galaxy via gas inflow/outflows. They also highlight the importance of dust emission from the polar region of the AGN, possibly related to outflows caused by radiation pressure from the accreting supermassive black hole. Future observing facilities will allow a better understanding of the nuclear environment of AGN, and a fuller description on how it links the black hole to its host galaxy. In particular, the upcoming James Webb Space Telescope (JWST) will allow us to probe the structure and evolution of AGN’s polar dust, while high-resolution spectrometers, such as the ones on board XARM and Athena satellites, will shed light on the physical characteristics of the gas and dust.

Dr. Ricci was invited to write this review because of his significant contribution to this field of research, and in particular in the light of his recent studies on the properties of the most heavily obscured accretion events in the Universe, and of the discovery that the merger of two or more galaxies can strongly affect the surroundings of the supermassive black hole, enriching it in gas and dust. Being able to publish review papers in such prestigious scholarly journals is usually an indication of wide acceptance and recognition of the author’s research work.

For young scientists such as Ricci and Almeida, this is an even more remarkable testimonial to their creative contribution to the relevant subject matter.

Figure 1. An artist’s drawing of a black hole named Cygnus X-1. It formed when a large star caved in. This black hole pulls matter from the blue star beside it. Taken from NASA/CXC/M. Weiss.

Figure 2. Sketch of the main AGN structures, seen along the equatorial and polar directions. From the center to host-galaxy scales: SMBH (Super Massive Black Hole), accretion disk and X-ray emitting corona, BLR (Broad-Line Region), torus and NLR (Narrow-Line Region). Different colors indicate different compositions or densities. Taken from Ramos Almeida & Ricci (2017, Nature Astronomy).

For more information about the Nature Astronomy Review, please visit https://www.nature.com/articles/s41550-017-0232-z.

CAS-CONICYT Postdoc Claudio Ricci Finds that Merging Galaxies Have Enshrouded Black Holes

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This illustration compares growing supermassive black holes in two different kinds of galaxies. A growing supermassive black hole in a normal galaxy would have a donut-shaped structure of gas and dust around it (left). In a merging galaxy, a sphere of material obscures the black hole (right).

Credits: National Astronomical Observatory of Japan

Black holes get a bad rap in popular culture for swallowing everything in their environments. In reality, stars, gas and dust can orbit black holes for long periods of time, until a major disruption pushes the material in.

A merger of two galaxies is one such disruption. As the galaxies combine and their central black holes approach each other, gas and dust in the vicinity are pushed onto their respective black holes. An enormous amount of high-energy radiation is released as material spirals rapidly toward the hungry black hole, which becomes what astronomers call an active galactic nucleus (AGN).

A study using NASA’s NuSTAR telescope shows that in the late stages of galaxy mergers, so much gas and dust falls toward a black hole that the extremely bright AGN is enshrouded. The combined effect of the gravity of the two galaxies slows the rotational speeds of gas and dust that would otherwise be orbiting freely. This loss of energy makes the material fall onto the black hole.

“The further along the merger is, the more enshrouded the AGN will be,” said Claudio Ricci, lead author of the study published in the Monthly Notices Royal Astronomical Society. “Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust.”

Ricci and colleagues observed the penetrating high-energy X-ray emission from 52 galaxies. About half of them were in the later stages of merging. Because NuSTAR is very sensitive to detecting the highest-energy X-rays, it was critical in establishing how much light escapes the sphere of gas and dust covering an AGN.

The study was published in the Monthly Notices of the Royal Astronomical Society. Researchers compared NuSTAR observations of the galaxies with data from NASA’s Swift and Chandra and ESA’s XMM-Newton observatories, which look at lower energy components of the X-ray spectrum. If high-energy X-rays are detected from a galaxy, but low-energy X-rays are not, that is a sign that an AGN is heavily obscured.

The study helps confirm the longstanding idea that an AGN’s black hole does most of its eating while enshrouded during the late stages of a merger.

“A supermassive black hole grows rapidly during these mergers,” Ricci said. “The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy.”

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory for NASA’s Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR’s mission operations center is at UC Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror archive. JPL is managed by Caltech for NASA.

For more information on NuSTAR, visit:

http://www.nasa.gov/nustar

http://www.nustar.caltech.edu

Reproduced from https://www.nasa.gov/feature/jpl/merging-galaxies-have-enshrouded-black-holes