Found: Three Black Holes on Collision Course

A team of astronomers, led by Graduate Student Ryan Pfeifle of George Mason University (GMU) in Fairfax Virginia, used observations spanning the electro-magnetic spectrum to provide the strongest evidence to-date of the existence of three Active Galactic Nuclei (AGN) residing in a merging galaxy.  

Beginning with a sample of hundreds of likely merging galaxies identified by the Galaxy Zoo Citizen Science Program and using thermal infrared observations from NASA's Wide-field Infrared Survey Explorer (WISE), Pfeifle and his team identified a few dozen candidate systems which were thought to host two Active Galactic Nuclei (AGN).   AGN are supermassive black holes at least 1 million times more massive than our Sun which are energized by the presence of vast quantities of gas drawn into them by gravitational forces. 

Space-based X-ray observations obtained with the Chandra Observatory Nuclear Spectroscopic Telescope Array (NuStar) found evidence to support the presence of two AGN in most of the merging galaxies.  However, one system appeared to be significantly more unusual than the rest, SDSS J084905.51+111447.2 (shortened to SDSS J0849+1114). Here, the team found X-ray evidence to suggest three AGN resided in this merging galaxy.

In order to provide more definitive support, and rule out competing astrophysical explanations, the team turned to the Large Binocular Telescope Observatory (LBT), currently the world's largest optical/infrared telescope.  Led by Dr. Barry Rothberg, who is a staff astronomer at LBT and holds a concurrent affiliate faculty position at GMU, optical and infrared spectroscopic observations of SDSS J0849+1114 were obtained using the Multi-Object Double Spectrographs (MODS), and the LBT Utility Camera in the Infrared (LUCIs).

The spectroscopic observations allow optical and near-infrared light to be separated so that atomic and molecular species can be identified.  The MODS and LUCI LBT observations conclusively demonstrated the presence of atomic species which can only exist in conditions found in AGN, as well as the presence of fast flowing gas that could only be powered by these monsters.  Moreover, the spectroscopic data allowed the team to determine the ages of the stars in the host galaxy, further ruling out competing astrophysical explanations for the presence of the three X-ray detections.  

Recent work by the team has expanded their study of this object to radio wavelengths.  "The use of multi-wavelength space- and ground-based observatories shows the synergies needed to discover such unusual, but important astrophysical phenomenon.  There is no way we could have confirmed the existence of a Triple AGN without contributions from all of these observatories," says Dr Rothberg.

However, the team's work is far from finished.  Pfiefile notes that the team's new multi-wavelength technique will allow them to extend their work to finding more triples.  "We already have more candidates that we are actively pursuing. I look forward to LBT once again demonstrating its importance in finding the optical and near-infrared signatures that either confirm or reject the presence of AGN in merging galaxies."

More details on the findings here with pictures and videos here.

Chemical element potassium detected in an exoplanet atmosphere

A team of astronomers led by AIP PhD student Engin Keles detected the chemical element potassium in the atmosphere of an exoplanet, for the first time with overwhelming significance and applying high-resolution spectroscopy. The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) in Arizona was used to study the atmosphere on the Jupiter-like exoplanet HD189733b. 

Ever since the earliest theoretical predictions 20 years ago, the chemical elements potassium and sodium were expected to be detectable in atmospheres of “hot jupiters”, gaseous planets with temperatures of a few thousand Kelvin that orbit closely around far-away stars. While sodium was detected with high resolution observations already early on, potassium was not, which created a puzzle for atmospheric chemistry and physics.

The elements can be discovered by analyzing the home star’s spectrum of light when the planet passes in front of it as seen from Earth. Different elements cause specific absorption signals in the spectrum, dark lines, that hint at the chemical composition of the atmosphere. However, the presence of clouds in hot Jupiter atmospheres strongly weakens any spectral absorption features and thus makes them very hard to detect. Even for HD189733b, the best studied hot Jupiter, so far scientists only possessed a very vague and imprecise knowledge of the potassium absorption. The exoplanet, 64 light years away and about the size of Jupiter, orbits its home star – an active K dwarf – in 53 hours and is 30 times closer to it than the Earth to the Sun. It needed the light gathering capability of the 2x8,4m LBT and the high spectral resolution of PEPSI to definitely measure potassium for the first time at high resolution in atmospheric layers above the clouds. With these new measurements, researchers can now compare the absorption signals of potassium and sodium and thus learn more about processes such as condensation or photo-ionization in these exoplanet atmospheres.

Potassium detection in HD189733b. The figure depitcts the excess absorption in the potassium line in the expoplanet’s atmosphere during transit. The horizontal axis shows the time in minutes, 0 means the exoplanet is at the central meridian near the middle of the stellar disk.  Vertical dashed lines indicate the transit duration. The blue line shows the modelled planetary absorption. Credit: AIP/Engin Keles, Kristin Riebe

The technique that was applied for this study at LBT is called transmission spectroscopy. It requires that the exoplanet transits in front of the host star. “We took a time series of light spectra during the transit and compared the absorption depth,” says the lead author of the study, Engin Keles, PhD student at AIP in the group Stellar Physics and Exoplanets. “During transit, we then detected the potassium signature, which disappeared before and after transit as expected, which indicates that the absorption is induced by the planetary atmosphere.” Investigations by other teams already attempted to detect potassium on the same exoplanet, however, either nothing was found or what was found was too weak to be statistically significant. Until now there has been no significant detection of potassium in high resolution observations for any exoplanet. 

“Our observations clearly made the breakthrough” emphasizes project co-leader Dr. Matthias Mallonn, who is seconded by PEPSI’s principal investigator Prof. Klaus Strassmeier:  “PEPSI is well suited for this task because of its high spectral resolution that allows collecting more photons per pixel from very narrow spectral lines than any other telescope-spectrograph combination.” “Both as a spectrograph and as a spectropolarimeter, PEPSI has already made significant contributions to stellar physics,” adds Christian Veillet, LBT Observatory's Director. “This strong detection of potassium in the atmosphere of an exoplanet establishes PEPSI as an amazing tool for exoplanet characterization as well as a unique asset for the members of the LBT community.” The team included colleagues from Denmark, The Netherlands, Switzerland, Italy and the United States and has now presented the results in the journal Monthly Notices of the Royal Astronomical Society.

Original Publication Engin Keles, Matthias Mallonn, Carolina von Essen, Thorsten A. Carroll, Xanthippi Alexoudi, Lorenzo Pino, Ilya Ilyin, Katja Poppenhager, Daniel Kitzmann, Valerio Nascimbeni, Jake D. Turner, Klaus G. Strassmeier (2019), MNRAS, “The potassium absorption on HD189733b and HD209458b” https://doi.org/10.1093/mnrasl/slz123

Science contacts

At AIP (Potsdam, Germany)    Engin Keles, 0331-7499-538, ekeles@aip.de

                                                 Prof. Dr. Klaus G. Strassmeier, 0331-7499-223, kstrassmeier@aip.de

Aty LBTO (Tucson, USA)       Dr. Christian Veillet, 1 (520) 349-4576, cveillet@lbto.org

Mapping Stars with PEPSI

The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) in Arizona released its first image of the surface magnetic field of another star. In a paper in the European journal Astronomy & Astrophysics, the PEPSI team presents a so-called Zeeman-Doppler-Image (ZDI) of the surface of the magnetically active star II Pegasi. 

A special technique allows astronomers to resolve the surfaces of faraway stars. Those are otherwise only seen as point sources, even in the largest telescopes and interferometers. This technique, referred to as Doppler imaging (DI) or Doppler tomography, requires a high-resolution spectrograph, usually a large telescope, lots of observing time, and nifty analysis software.

But PEPSI can go a major step further. Because its two polarimeters also feed polarized light to the spectrograph, PEPSI captures the otherwise hidden profile deformation due to the Zeeman-effect. (splitting and polarization of spectral lines due to an external magnetic field). Combined with the rotational Doppler-effect it allows the reconstruction of the star’s surface magnetic field geometry. 

Stellar environment of the star II Pegasi. Shown is the magnetic-field extrapolation out to 2.2 stellar radii. Open field lines are depicted in colour (magenta: negative polarity, green: positive polarity, closed loops are in white.) Credit: AIP

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