Research & Science Highlights

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Galaxy Cluster Mergers - the most energetic events in the Universe since the Big Bang

Discovered "double" clusters with the Einstein Observatory and analyzed other merging clusters with Chandra and XMM-Newton. With energies up to 1064 ergs, galaxy cluster mergers are the most energetic events in the Universe since the Big Bang. These major cluster mergers, along with smaller accretion events, dramatically stir and heat the cluster atmosphere, produce shocks, and accelerate particles to relativistic energies. The relativistic electrons emit synchrotron radiation in spectacular Mpc-scale radio rellics.

Cluster mergers provide unique insights on dark matter and particle acceleration

Outbursts from Supermassive Black Holes

Discovered filamentary structures in the buoyantly uplifted arms, multiple cavities, and shocks in M87 arising from multiple supermassive black hole (SMBH) outbursts. Proposed a deep, 500 ks Chandra observation of M87 and measured the shock parameters (age and energy) as well as details of the filaments. The multiple outbursts are likened to the SMBH singing a tune 56 octaves below middle C! Modeled the SMBH outburst to understand the energy partition between shocks and bubbles. M87 hosts the first supermassive black hole imaged with the Event Horizon Telescope. M87 is a prototype for SMBH outbursts which mitigate the radiative cooling of hot X-ray atmospheres. SMBH outbursts "solve" the enigmatic cooling flow problem - the gas in galaxy clusters (and groups and galaxies) radiates its energy in the X-ray band, but the expected amount of cold/cold matter was never detected. The SMBH outbursts are "re-energizing" the cooling atmospheres in individual galaxies, groups, and clusters.

The solution to the "cooling flow" problem began with the study of M87 in the Virgo cluster and NGC1275 in the Perseus cluster.

Discovered "double" clusters with the Einstein Observatory and analyzed other merging clusters with Chandra and XMM-Newton. With energies up to 1064 ergs, galaxy cluster mergers are the most energetic events in the Universe since the Big Bang. These major cluster mergers, along with smaller accretion events, dramatically stir and heat the cluster atmosphere, produce shocks, and accelerate particles to relativistic energies. The relativistic electrons emit synchrotron radiation in spectacular Mpc-scale radio rellics.

Cluster mergers provide unique insights on dark matter and particle acceleration

The solution to the "cooling flow" problem began with the study of M87 in the Virgo cluster and NGC1275 in the Perseus cluster.

Although Chandra observations provided a detailed view of feedback (from M87 as well as a gallery of feedback on scales from clusters to individual galaxies), the first clues of SMBH feedback came from ROSAT observations of Perseus and M87.

The detection of a pair of symetrically placed cavities around the active nucleus of NGC1275, the central galaxy in the Perseus cluster provided a first clue. As Boehringer et al. noted, the inner core of Perseus is "far from being a quiescent cluster". Earlier, with UHURU's discovery of galaxy clusters as bright X-ray sources with extensive gaseous atmospheres, Gull & Northover (1973) argued that paired radio sources are "bubbles of relativistic plasma rising through the hot gas which produces the X-ray emission from clusters of galaxies." and "energy in the nucleus of a galaxy will lead to the formation of two such bubbles, moving in opposite directions along the axis of rotation."

(click to expand / collapse section)

Building on these early ideas, Churazov and collaborators used ROSAT HRI observations to model the properties of the buoyant bubbles around NGC1275. Using the inflation time of the bubbles by the central SMBH, the bubble buoyancy times, and the bubble sizes (volumes), Churazov et al. showed that the nuclear power output was of order 1045 ergs/s, comparable to the radiated power in the cooling flow.

In a second ROSAT HRI analysis, Churazov and colleagues modeled the evolution of the outbursts from M87's SMBH (the first supermassive black hole imaged by the Event Horizon Telescope). This work showed the close connection between the radio and X-ray images, the transformation of nearly spherical bubbles into torii and flattened "pancakes", and the bubble uplift of cooler X-ray gas generating tails that trailed behind the rising bubbles. The paper argued that in M87, as in Perseus/NGC1275, there was sufficient energy produced by the SMBH to compensate for the energy radiated by the hot X-ray atmosphere and that the buoyantly rising bubbles would deposit the bulk of their energy in the "cooling flow" region.

Churazov and collaborators investigated feedback from supermassive black holes:

Fabian and collaborators used Chandra observations to study, in great detail, feedback in NGC1275/Perseus:

Talks on AGN Feedback

Eugene Churazov - Murray Lecture: Why there are so many successful feedback models
Andrew Fabian - Kavli Prize Lecture: Luminous Accreting Black Holes and AGN Feedback

A detailed view of AGN feedback and M87 - from clusters to groups and galaxies

External link - Chandra Observations of M87

The Interstellar Medium of Early Type Galaxies

Discovered the interstellar medium of early-type galaxies. The launch of the Einstein Observatory, the first fully imaging focusing X-ray telescope with an angular resolution of a few arc seconds, provided more than a 100-fold increase in sensitivity for X-ray astronomy. Exploiting the increase in sensitivity, Forman, Jones & Tucker (1985) discovered the hot interstellar medium (ISM) surrounding early type (E and S0) galaxies (see also Nulsen, Stewart & Fabian 1984). Instead of being expelled in a galactic wind, the mass loss from evolving stars in these galaxies, formed a hot corona. As in more massive halos of groups and galaxy clusters, these hot coronae provide a unique tracer of the gravitational potential and show that these galaxies are surrounded by dark matter halos. While the bulk of the ISM of spiral galaxies resides in cool components, in more massive E and S0 galaxies, the bulk of the ISM is hot (kT ~ 10 million degrees K) and radiates predominantly in the soft X-ray band. For their studies of hot coronae, Forman and Jones were awarded the first Bruno Rossi Prize (1985) for "pioneering work in the study of X-ray emission from early type galaxies."

UHURU - Galaxy Clusters, an X-ray Catalog, and Compact Sources

Galaxy Clusters - Observations of the Extended X-Ray Sources in the Perseus and Coma Clusters from UHURU Studied the UHURU X-ray observations of the Perseus and Coma galaxy clusters and measured their extents and spectra (as part of my PhD thesis). As described below, clusters of galaxies were the single largest class of extragalactic objects identified in the final UHURU catalog of sources.

The Fourth Uhuru Catalog of X-ray Sources Forman et al. 1978 ApJ - Led the analysis and activities preparing the Fourth UHURU Catalog (649 citations as of December 2022; 4U has become a "household" name in astronomy with over 3000 listings in astronomy journal abstracts, from ADS). The catalog was the second most cited paper in all of astronomy/astrophysics in the first year following its publication (The Third UHURU Catalog was the most cited paper in all of science in its year (1973) of publication, covered by the Science Citation Index). The paper/catalog includes positions and intensities for 339 X-ray sources observed by the Uhuru (SAS A) X-ray satellite observatory. Optical and radio counterparts are suggested based on positional coincidence. The major classes of identified objects include binary stellar systems, supernova remnants, Seyfert galaxies, and clusters of galaxies.

(at left) The full catalog of sources in Galactic coordinates showing the source concentration to the Galactic plane and Galactic center. Source sizes are logarithmically proportional to peak brightness. Well-known sources are labeled.

The observations were accumulated from 429 days of operation between 12 December 1970 to March 18, 1973 (failure of the battery and transmitter). To prepare the Fourth UHURU Catalog, a new aspect solution was developed using 1) a triad of magnetometers, 2) the Sun sensor, and 3) locations of well-located X-ray sources (see Forman, Jones, and Tananbaum 1976 for details). The largest single class of identified extragalactic objects in the catalog are clusters of galaxies.

Top - section (from Centaurus to Circinus) of a single scan of the Galactic plane (1/2 degree per second)

Bottom - superposition of multiple scans (while the spin axis was held fixed; typically one day) to increase sensitivity. The source response (triangle with base 1 degree) is fit to locate and measure the flux of each source. Each fit yields a line of position (10 degrees long) on the sky.

UHURU engineering model on display at the Smithsonian Air and Space Museum (1976-1997)