Saturday, December 12, 2020

That young but already evolved entirely self-made galaxy

So young and already so evolved: thanks to observations obtained at the Large Binocular Telescope, an international team of researchers coordinated by Paolo Saracco of the Istituto Nazionale di Astrofisica (INAF, Italy) was able to reconstruct the wild evolutionary history of an extremely massive galaxy that existed 12 billions years ago, when the Universe was only 1,8 billions years old, less than 13% of the present age. This galaxy, dubbed C1-23152, formed in "just" 500 million years, an incredibly short time to give rise to a mass of about 200 billion suns. To do so, it produced as many as 450 stars per year, more than one per day, a star formation rate almost 300 times higher than the current rate in our galaxy, the Milky Way.  The information obtained from this study will be fundamental for galaxy formation models for which the nature of objects such as C1-23152 is still difficult to account for.

Color image of the galaxy C1-23152 at redshift z=3.352, when the Universe was 1.8 billion years old. The image is the sum of 3 images at different wavelengths taken with the Hubble Space Telescope.  C1-23152 appears a regular spheroidal galaxy, its light profile matches exactly those of typical elliptical galaxies in the local Universe. Its stellar mass is about 200 billions of stars like sun and it is formed in less than 500 million years.


The most massive galaxies that we observe in the Universe reach masses several hundred billion times that of the Sun and although they are numerically just one third of all galaxies, they contain more than 70% of the stars in the Universe. For this reason, how and how rapidly these galaxies formed are among the most debated questions of modern astrophysics. The current model of galaxy formation - the so-called hierarchical model - predicts that smaller galaxies formed earlier, while more massive systems formed later, through subsequent mergers of the pre-existing smaller galaxies. On the other hand, some of the properties of the most massive galaxies observed in the local Universe, such as the age of their stellar populations, suggest instead that they were formed at early epochs. Unfortunately, the variety of evolutionary phenomena that galaxies can undergo during their lives does not allow us to uniquely define, through studies conducted in the nearby Universe, the way in which they formed, leaving large margins of uncertainty. However, an answer to these questions can come from the study of the properties of massive galaxies in the early Universe, as close as possible to the time when they formed most of their mass.

Seventeen hours of spectroscopic observations with the Large Binocular Telescope (LBT) of the elliptical galaxy C1-23152, previously identified at a distance at which the Universe age was less than 13% of its current age, allowed Saracco’s team to reconstruct its evolutionary history. “The data show that the formation time of C1-23152, that is the time elapsed between the formation of the first stars from the pre-existing gas to the moment when the star formation has almost completely ceased, is less than 500 million of years” says Paolo Saracco, researcher at INAF in Milan and first author of the article published in The Astrophysical Journal. “Also, from the data collected with LBT we were able to establish that in this short time, corresponding to less than 4 hundredths of the age of the Universe, the galaxy formed a mass equal to about 200 billion stars like the Sun, that is about 450 suns per year. Our galaxy, the Milky Way, now forms no more than two a year", adds Danilo Marchesini, full professor at Tufts University and second author of the article. But that is not all. The large amount of information collected allowed the team to quantify for the first time in a galaxy so distant the abundance of chemical elements heavier than helium (the so-called metallicity): the stars of this galaxy have, surprisingly, a higher metallicity than that of the Sun, similar to that observed in the most massive galaxies in the Universe today.

Spectrum of galaxy C1-23152. The top panel shows the atmospheric transmission in the wavelength range of observations. In the middle panel the one-dimensional spectrum of galaxy C1-23152 is shown in the original form (dark-gray curve) and smoothed by a boxcar filter over three pixels (black curve) corresponding to the instrumental resolution. The main absorption and emission lines are marked by solid and dashed lines, respectively. The red curve is the best-fitting composite model obtained with STARLIGHT. The shaded gray regions are those masked in the fitting because of bad sky transmission or the presence of emission lines. For comparison, the bottom panel shows the observed spectrum of a typical post-starburst galaxy in the local Universe selected from the Sloan Digital Sky Survey (SDSS).

“These observations showed that the formation of the most massive galaxies in the Universe can occur extremely quickly, through an extremely intense star formation process in the early Universe, as for C1-23152", underlines Francesco La Barbera, researcher at INAF in Naples, in the team that conducted the study. "Understanding whether the scenario that describes the formation of C1-23152 is a particular case or whether, on the contrary, it is what happens for most of the most massive galaxies in the Universe is of fundamental importance since this would require a profound revision of the galaxy formation models”, adds Adriana Gargiulo, also a researcher at INAF in Milan and co-author of the study.


Likely formation scenario of massive elliptical galaxies like C1-23152. Massive primordial gas clouds, falling in the same region under the effect of gravitational force, collide triggering violent and massive star formation processes. The starburst phase is expected to last few hundreds of million years during which hundreds to thousands stars per year are formed, as for C1-23152. The resulting massive elliptical galaxy will then evolve with time, possibly experiencing different evolutionary phenomena. 

The formation of stellar masses as high as for C1-23152 requires both high masses of gas to convert into stars and particular physical conditions. A possible scenario hypothesized by the researchers is that massive primordial gas clouds, falling under the effect of gravitational force in the same region, collide, triggering violent and massive star formation processes. From the observational point of view, the precursors of the most massive galaxies could therefore be remote galaxies with a very high rate of star formation.

This image shows an example of starburst galaxies forming about a thousand of stars per year at the time of observation. This phase is most likely the formation phase of massive galaxies in the early Universe, like C1-23152.

"To test our hypotheses, the observations that the next generation of instrumentations will allow us to carry out will be decisive, in particular the James Webb Space Telescope (JWST) which will be launched in orbit at the end of 2021, and the Extremely Large Telescope (ELT) the largest ground-based telescope ever built, with a main mirror of 39 meters in diameter, which will be operational in 2026”, concludes Saracco.

Science Contacts:

INAF Press Release     https://www.media.inaf.it/2020/12/10/galassia-vega/

Publication: “The Rapid Build-up of Massive Early-type Galaxies. Supersolar Metallicity, High Velocity Dispersion and Young Age for an ETG at z=3.35”, di Paolo Saracco, Danilo Marchesini, Francesco La Barbera, Adriana Gargiulo, Marianna Annunziatella, Ben Forrest, Daniel J. Lange Vagle, Z. Cemile Marsan, Adam Muzzin, Mauro Stefanon, Gillian Wilson

Thursday, October 1, 2020

The web of the Giant: spectroscopic confirmation of a Large Scale Structure around the z=6.31 quasar SDSS J1030+0524

Using three of the largest telescopes around the world - namely, the Large Binocular Telescope (LBT), the ESO Very Large Telescope (VLT), and the W.M. Keck Observatory Telescope - astronomers found a Large Scale Structure made of six galaxies lying around a massive galaxy harboring a supermassive black hole (SMBH), the first time such a close grouping has been seen within the first billion years of the Universe. The finding helps us better understand how supermassive black holes, one of which exists at the center of our Milky Way, formed and grew to their enormous sizes. It supports the theory that black holes can grow quickly within large web-like structures, which contain plenty of gas to fuel them.

The Large Scale Structure with six galaxies around the Quasar/SMBH (Artist view - Credit: ESO)

“This research was mainly driven by the desire to understand some of the most challenging astronomical objects — supermassive black holes in the early Universe. These are extreme systems and, to date, we have no good explanation for their existence,” says Marco Mignoli, an astronomer at the National Institute for Astrophysics (INAF) in Bologna, Italy and lead-author of the new research published today in Astronomy & Astrophysics Letters.

The new observations revealed multiple galaxies surrounding a supermassive black hole, lying in a cosmic ‘spider web’ of gas and galaxies extending over 300 times the size of the Milky Way. “The cosmic web filaments are like spider-web threads,” explains Mignoli. “The galaxies stand and grow where the filaments cross, and streams of gas — available to fuel the galaxies and the central supermassive black hole — can flow along the filaments”.

Mignoli and his team found this large web-like structure with a black hole of one billion solar masses at a time when the Universe was only 0.9 billion years old. “Our work has placed an important piece in the largely incomplete puzzle that is the formation and growth of such extreme, yet relatively abundant, objects so quickly after the Big Bang,” says co-author Roberto Gilli, also an astronomer at INAF in Bologna.

The very first black holes, thought to have formed from the collapse of the first stars, must have grown very fast to reach masses of a billion Suns within the first 0.9 billion years of the Universe’s life. But astronomers have struggled to explain how sufficiently large amounts of ‘black hole fuel’ could be available to help these objects grow to such enormous sizes in a short time. The newfound structure offers a likely explanation: the large amounts of gas in it provide the fuel the central black hole needs to quickly become a supermassive giant.

But how did such large web-like structures form in the first place? Astronomers think giant halos of mysterious dark matter are key. These large areas of invisible matter are thought to attract huge amounts of gas in the early Universe; together, the gas and invisible matter form the web-like structures where galaxies and black holes can evolve.

“Our finding lends support to the idea that the most distant and massive black holes form and grow within massive dark matter halos in large-scale structures, and that the absence of earlier detections of such structures was likely due to observational limitations,” says Colin Norman of Johns Hopkins University in Baltimore, US, also a co-author on the study.

The galaxies now detected are some of the faintest that current astronomical instruments can observe. This discovery required observations from the largest optical telescopes available. The hunt started eight years ago, when the team - using the superb imaging quality of the Large Binocular Camera (LBC) mounted on the LBT - selected about twenty galaxies with peculiar photometric colors that could be possible neighbors of the central black hole. In the following years, using spectrographs mounted on the LBT Observatory (MODS), the ESO VLT (FORS2), and the Keck Observatory (DEIMOS), the team actually confirmed that six galaxies are located around the black hole.

“We expect to discover further, fainter galaxies around this supermassive black hole. We may have just seen the tip of the iceberg" says co-author Felice Cusano, an astronomer at INAF in Bologna and observer for the LBT-Italy team.

These results contribute to our understanding of how supermassive black holes and large cosmic structures formed and evolved. The future ESO’s Extremely Large Telescope will be able to build on this research by observing higher numbers of fainter galaxies around massive black holes in the early Universe using its powerful spectroscopic instruments.


More Information

This research was presented in the paper “Web of the Giant: Spectroscopic confirmation of a Large Scale Structure around the z=6.31 quasar SDSS J1030+0524” to appear in Astronomy & Astrophysics.

The team is composed of M. Mignoli (INAF, Bologna, Italy), R. Gilli (INAF, Bologna, Italy), R. Decarli (INAF, Bologna, Italy), E. Vanzella (INAF, Bologna, Italy), B. Balmaverde (INAF, Pino Torinese, Italy), N. Cappelluti (Department of physics, University of Miami, Florida, USA), L. Cassarà (INAF, Milano, Italy), A. Comastri (INAF, Bologna, Italy), F. Cusano (INAF, Bologna, Italy), K. Iwasawa (ICCUB, Universitat de Barcelona & ICREA, Barcelona, Spain), S. Marchesi (INAF, Bologna, Italy), I. Prandoni (INAF, Istituto di Radioastronomia, Bologna, Italy), C. Vignali (Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Bologna, Italy & INAF, Bologna, Italy), F. Vito (Scuola Normale Superiore, Pisa, Italy), G. Zamorani (INAF, Bologna, Italy), M. Chiaberge (Space Telescope Science Institute, Maryland, USA), C. Norman (Space Telescope Science Institute & Johns Hopkins University, Maryland, USA).

INAF Press Release

Contacts

Marco Mignoli
INAF Bologna
Bologna, Italy
Tel: +39 051 6357 382
Email: marco.mignoli@inaf.it

Roberto Gilli
INAF Bologna
Bologna, Italy
Tel: +39 051 6357 383
Email: roberto.gilli@inaf.it

Barbara Balmaverde
INAF Torino
Pino Torinese, Italy
Email: barbara.balmaverde@inaf.it

Colin Norman
Johns Hopkins University
Baltimore, USA
Email: norman@stsci.edu

Friday, March 6, 2020

Where there's one, there's one hundred more

Although it may have a difficult designation to remember, PSO J030947.49+271757.31, its importance is unique.  It is the most distant Blazar observed to date.  The light we see from it began its journey when the Universe was less than 1 billion years old, almost 13 billion years ago

PSO J0309 + 27 – in short - was discovered by a team of researchers led by Silvia Belladitta, a PhD student at the University of Insubria, working for the Italian National Institute for Astrophysics (INAF) in Milan, under the supervision of Alberto Moretti and Alessandro Caccianiga. While it was suspected that the object was distant, and observations from the Swift Space Telescope (of which INAF is a major contributor) showed its X-ray power matched that of other Blazars, it was the observations obtained with the optical Multi-Double Object Spectrographs (MODS) at the Large Binocular Telescope (LBT) that confirmed it indeed broke the record as the most distant Blazar in the known Universe. 

Blazars are one of the brightest of a class of objects called AGN - or Active Galactic Nuclei - which are supermassive black holes (SMBHs) in the centers of galaxies.  They are active due to the presence of a disk or sphere of ionized gas around them which "fuels" the emission seen at many wavelengths.  Blazars emit powerful relativistic jets bright enough to be seen across the known Universe.  The beam of a Blazar is only visible along a narrow line of sight.  If the Earth is not within that line of sight, it would not be easily recognizable. Thus detecting objects can be extremely difficult (and fortuitous).  But more importantly, this Blazar is one of the earliest, most distant SMBHs seen that is not obscured by dust (unlike most AGN). This allows astronomers to study this object across the entire electromagnetic spectrum and build a complete picture of its properties.

"The spectrum that appeared before our eyes confirmed first that PSO J0309 + 27 is actually an AGN, or a galaxy whose central nucleus is extremely bright due to the presence, in its center, of a supermassive black hole fed by the gas and the stars it engulfs, ”says Belladitta. "In addition, the data obtained by LBT also confirmed that PSO J0309 + 27 is really far away from us using the shift of the color of its light towards red or redshift with a record value of 6.1, never measured before for a similar object," adds Belladitta, first author of the paper describing the discovery, published today in the journal Astronomy & Astrophysics Letters.

PSO J0309 + 27 has therefore proved to be, at the moment, the most powerful persistent radio source in the primordial universe, within the first billion years since its formation. Observations taken by the XRT telescope on board the Swift satellite - a mission with a fundamental contribution by INAF and the Italian Space Agency - have also made it possible to establish that, even in X-rays, PSO J0309 + 27 is the brightest cosmic source ever observed at these distances.

MODS/LBT discovery spectrum of PSO J0309+27 at z=6.10±0.03. The O[VI]λ1033Å, the Ly-αλ1216Å the OIλ1304Å and the CIIλ1336Å lines are marked. The red-dashed line is the quasar template from Vanden Berk et al. (2001) at the redshift of the object for comparison.
Belladitta notes further,  “Observing a blazar is extremely important, for every discovered source of this type, we know that there must be a hundred similar, but oriented differently and therefore too weak to be seen directly". Therefore, the discovery of PSO J0309 + 27 allows astronomers to quantify, for the first time the number of AGN with powerful relativistic jets present in the primordial universe. The Blazars at these early epochs represent the "seeds" for all SMBHs that exist in the Universe today.

“From these new LBT observations, still under development, we also estimate that the central engine that powers PSO J0309 + 27 is a black hole with a mass equal to about a billion times the mass of our Sun. Thanks to our discovery, we are able to say that already in the first billion years of life of the universe, there existed a large number of very massive black holes emitting powerful relativistic jets. This result places tight constraints on the theoretical models that try to explain the origin of these huge black holes in our universe" concludes Belladitta.

Science contact     Silvia Belladitta    silvia.belladitta@inaf.it
Media contact at INAF (Italy)        ufficiostampa@inaf.it
Media contact at LBT (Arizona)    pr_officer@lbto.org

INAF press release: https://www.media.inaf.it/2020/03/06/blazar-da-record/

Publication:
S. Belladitta, A. Moretti, A. Caccianiga, C. Spingola, P. Severgnini, R. Della Ceca, G. Ghisellini, D. Dallacasa, T. Sbarrato, C. Cicone, L. P. Cassarà,  and M. Pedani.    link to the pdf version
A&A 635, L7 (2020) 

Monday, March 2, 2020

Total lunar eclipse: observing the Earth as a transiting planet

Astronomers succeeded in recording sunlight shining through the Earth’s atmosphere in a manner similar to the study of distant exoplanets. During the extraordinary occasion of a lunar eclipse, the Large Binocular Telescope observed the light that was filtered by the Earth’s atmosphere and reflected by the Moon in unique detail. In addition to oxygen and water, atomic spectral lines of sodium, calcium and potassium were detected in our atmosphere in this way first time.

Total lunar eclipse: observing the Earth as a transiting planet
The Sun as seen from the Tycho crater on the Moon during a total lunar eclipse on Earth. When the Sun sets behind the northern Pacific, its disk completely disappears behind Earth. Credit: AIP/Strassmeier/Fohlmeister
When an exoplanet transits in front of its host star, astronomers may be able to record both the dimming of the starlight that the planet blocks and also the starlight that shines through the planet’s atmosphere. While it is only a tiny signal, it contains the imprint of the planet’s chemical and physical signature and provides the principal possibility to measure the planet’s atmospheric constituents. In astrophysics, this technique is called transmission spectroscopy, and is a relatively young technique booming since many exoplanet transits were detected from space. “While, so far, only applicable to super-sized Jupiters, that is oversized Jupiter-like planets orbiting close to their host star, we are most interested in Earth-like planets and whether we could detect more complex molecular signatures in an exo-Earth transmission spectrum possibly even hinting for life”, explains Klaus Strassmeier from the Leibniz Institute for Astrophysics in Potsdam (AIP), the leading author of the now published study. „While not yet doable for any Earth-like exoplanet transit, a total lunar eclipse, which is a total solar eclipse when seen from our own Moon, is nothing else than a transit of our own Earth, and indirectly observable.”

The sunlight that passes through the Earth’s atmosphere before it reaches the Moon and back reflects to Earth is called the Earthshine. The Earth’s atmosphere contains many by-products of biological activity, such as oxygen and ozone in association with water vapor, methane and carbon dioxide. These biogenic molecules present attractive narrow molecular bands at optical and near infrared wavelengths for detection in atmospheres of other planets. Taking the Earth as the prototype of a habitable planet, Earthshine observations provide the possibility to verify biogenic and related chemical elemental presence with the same techniques that otherwise are being used for observing stars with super Jupiter planets. Earthshine is thus an ideal test case for future exo-Earth detections with the new generation of extremely large telescopes.

January 2019 featured a total lunar eclipse. The Moon dimmed by a factor of 20,000 during totality which is the reason why the light gathering capability of the 11.8 m Large Binocular Telescope (LBT) in Arizona was needed for the observations. Additionally, the high spectral resolution of the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) was necessary to separate the expected tiny spectral-line absorptions of the Earth’s atmosphere from the normal solar spectrum at unprecedented spectral resolution and in polarized light.

“PEPSI has already made significant contributions to the study of exoplanets through the observation of their transit in front of their sun.” adds Christian Veillet, LBT Observatory's Director. “Looking at the Earth as an exoplanet thanks to a total lunar eclipse well-suited to LBT's location in Arizona, and adding polarimetry to the exquisite resolution of the PEPSI spectrograph, resulted in the detection of sodium, calcium, and potassium in Earth's atmosphere."
Snapshot spectra of terrestrial molecular oxygen and water vapor absorption. Intensity is plotted versus wavelength in Angstroem. Time increases from bottom up as indicated in UT hh:mm:ss. Immediately noticeable is the dramatic increase of O2 and H2O absorption during eclipse (central four spectra) with respect to outside eclipse (other spectra). Oxygen molecules create the so-called A-band at 7600 Å, H2O is seen as myriads of individual absorption lines in the range 7850–9100 Å. Credit: AIP/Strassmeier. 
Detailed look at the wavelengths around the potassium line at 7699 Å. Time increases bottom up and is again indicated as UT. The bottom spectrum is a comparison spectrum of the full moon outside of eclipse. Red color denotes times of totality, black times of partiality, and blue out of eclipse. Note that the spectral lines flanking the potassium line are from two terrestrial water vapor absorptions. Credit: AIP/Strassmeier.



More information on PEPSI and the LBT: https://pepsi.aip.de | http://www.lbto.org


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

Media contacts: 
        Dr. Janine Fohlmeister (Potsdam, Germany)  0331-7499-803, presse@aip.de
        Dr. Christian Veillet (Tucson, USA)   1-520-349-4576, pr_officer@lbto.org



Publication:

Klaus G. Strassmeier, Ilya Ilyin, Engin Keles, Matthias Mallonn, Arto Järvinen, Michael Weber, Felix Mackebrandt, and John M. Hill, 2020, Astronomy & Astrophysics, in press


Friday, September 27, 2019

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.




Wednesday, September 4, 2019

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


Monday, March 18, 2019

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