This section has obviously not been updated for a long, long time. Sorry about that...
Associate Professor (Docent) in Astronomy
The first stars and galaxies
Department of Physics and Astronomy
751 20 Uppsala
Phone: +46 (0)18 471 5975
Email: erik.zackrisson [@] physics.uu.se
Click on the image captions for project descriptions, with links to talks and articles.
The very first stars likely formed when the Universe was about 100 million years old, prior to the formation of the first galaxies. As the elements that make up most of planet Earth had not yet formed, these primordial objects – known as population III stars – were made almost entirely of hydrogen and helium. As they exploded as supernovae, they ejected the heavy elements produced in their interiors into the interstellar medium. This started the cosmic chemical enrichment that led to the formation of the stars that we see in the Milky Way today, to rocky planets and eventually humans. Simulations and theoretical arguments predict population III stars to be substantially more massive than the chemically enriched stars that formed at later epochs. However, none of this has been observationally confirmed, since no population III objects have so far been found.
As most population III stars are predicted to be short-lived and to have formed only for a brief period of cosmic time, very few – if any – have survived to the present epoch. One of the most promising techniques to hunt for such objects is therefore to exploit the time machine provided by the deepest astronomical images. Since the light from the most distant regions of the Universe has taken billions of years to reach us, we are seeing these objects as they were early on, when population III stars still roamed the cosmos. The problem is simply to catch the population III signatures among the myriad of more mundane objects that also turn up in these ultradeep fields.
In a sequence of papers, we have used cosmological simulations, gravitational lensing models and spectral synthesis codes to study the detectability of various types of population III objects (Zackrisson et al. 2010ab, 2011ab, 2012; Rydberg et al. 2013, 2015; Zackrisson et al. 2015). While individual population III stars in the early Universe are beyond the reach of existing and upcoming telescopes (unless we can catch their supernova explosions), small galaxies that for a brief period come to be dominated by the light from many population III stars may well be within reach. Using imaging data from the Hubble Space Telescope, we have uncovered the first population III galaxy candidates at redshifts beyond z ≈ 7 (Zackrisson et al. 2011b, Rydberg et al. 2015) and are using the largest groundbased telescopes to do follow-up spectroscopy of these objects.
Searching for Population III galaxies at high redshifts [pdf]
South by High Redshift, Austin, April 1-3, 2015
Searching for gravitationally lensed population III galaxies with HST and JWST [pdf]
CosmoLens 2013 meeting, Marseille, May 28, 2013
Finding high-redshift dark stars with the James Webb Space Telescope [pdf]
Zackrisson et al. 2010a, ApJ, 717, 257
Observational constraints on supermassive dark stars [pdf]
Zackrisson et al. 2010b, MNRAS, 407, L74
The spectral evolution of the first galaxies. I. JWST detection limits and color criteria for population III galaxies [pdf]
Zackrisson et al. 2011a, ApJ 740, 13
The HST colours of high-redshift population III galaxies with strong Lyman-alpha emission [pdf]
Zackrisson et al.. 2011b, MNRAS, 418, L104
Detecting gravitationally lensed population III galaxies with HST and JWST [pdf]
Zackrisson et al. 2012, MNRAS, 427, 2212
Detection of isolated population III stars with JWST [pdf]
Rydberg et al. 2015, ApJ, in press (arXiv1411.5691)
Primordial star clusters at extreme magnification [pdf]
Zackrisson et al. 2015, MNRAS, in press (arXiv1411.6628)
After the Big Bang, the Universe expanded, cooled off and eventually allowed protons and electrons to form neutral hydrogen atoms. However, at some point during the first billion years of cosmic history, the Universe was flooded with highly energetic photons which brought the hydrogen in the intergalactic medium back into a highly ionized state. The origin of these ionizing photons remains unclear, which makes cosmic reionization one of the great unsolved puzzles of the early Universe.
While a host of astronomical objects could in principle have contributed to this process, star-forming galaxies are usually seen as the top candidates, since they are already known to exist in great numbers at the relevant epoch. However, this scenario hinges on the assumption that ionizing radiation can escape from these objects and into the intergalactic medium. Can we somehow find out whether this really happened?
At low to intermediate redshifts, hydrogen-ionizing (Lyman continuum) photons that escape into the intergalactic medium can be measured directly, and our team has made the two first direct detections of Lyman continuum leakage in the local Universe (Bergvall et al. 2006, Leitet et al. 2013). In the reionization epoch, indirect methods must instead be used – even if Lyman continuum photons make it out of these galaxies, the increasingly neutral intergalactic medium will inevitably absorb these photons before they can reach our telescopes. We have pioneered the first method capable of constraining the Lyman continuum escape fractions of individual galaxies in the reionization epoch (Zackrisson et al. 2013), and are eagerly awaiting the upcoming James Webb Space Telescope to make the required observations.
The LYCAN project:
Constraining the escape of ionizing photons from z>6 galaxies with JWST [pdf]
Exploring the Universe with JWST, ESTEC, Netherlands, Oct 12, 2015
First detection of Lyman continuum photon escape from a local starburst galaxy. Observations of the luminous blue compact galaxy Haro 11 with FUSE [pdf]
Bergvall et al. 2006, A&A 448, 513
Escape of Lyman continuum radiation from local galaxies. Detection of leakage from the young starburst Tol 1247-232 [pdf]
Leitet et al. 2013, A&A, 553, 106
The Spectral Evolution of the First Galaxies. II. Spectral Signatures of Lyman Continuum Leakage from Galaxies in the Reionization Epoch [pdf]
Zackrisson et al. 2013, ApJ 777, 39
Planet hunters are finding more and more Earth-like planets in the Milky Way, but how are such objects distributed on cosmological scales? When did most of the planets form, and in what type of galaxies? In a recent simulation project, we use semi-analytical models of galaxy formation to trace the formation of Earth-like planets across space and time. We find that there may be a billion billion (ten to the eighteenth) such planets in the observable Universe, and that the typical Earth-like planet in our local Universe may be about 3 billion years older than Earth!
If Earth-like planets are so common, why is the Milky Way not teeming with aliens? The Search for Extraterrestrial Intelligence (SETI) has been on for over 50 years, yet no intelligent signals have ever been discovered. And if the typical Earth-like planet is much older than Earth, why is not the entire Milky Way already colonized by some advanced super-civilization? One potential solution to this conundrum (known as the Fermi paradox) is that even though planets are dime a dozen, intelligent life is exceedingly rare. If this is the case, the only chance of detecting intelligent life beyond Earth may be to extend the search radius beyond the Milky Way.
As part of the first Swedish SETI project, we use galaxy scaling relations to set upper limits on so-called Kardashev type III civilizations – extraterrestrials with the means to harvest a substantial fraction of the radiative energy from the galaxies in which they reside. Our results indicate that, if such star-harvesting civilizations exist at all, less than 0.3% of the disk galaxies in the local Universe seem to be hosting them.
Project Hephaistos:Selected papers:
Extragalactic SETI: The Tully-Fisher relation as probe of Dysonian astroengineering in disk galaxies [pdf]
Zackrisson, E., Calissendorff, P., Asadi, S., Nyholm, A. 2015, Astrophysical Journal, 810, 23
Searching for Extraterrestrial Intelligence Beyond the Milky Way – the First Swedish SETI project [pdf]
Astronomdagarna, October 11, 2013, Lund, Sweden
Works of fiction that I've authored (in Swedish). If you can't find them in online book stores, you can always order them directly from Artelligens. Snyltos bok is also available for free as an e-book in Ibooks.
Bilder av det vrängda köttet och andra berättelser (2005)
Träd in i en värld av mörkrumsteknik, parallella universa och medicinska experiment… Droger, sjukdomar och högre väsen… Se flickan som fick chans att stänga av världen och mannen som ville ta livet av sig inför publik. Möt döda som tjuvlyssnar på de levandes tankar och uppfinnaren som tvingat ned Gud på knä. Beskåda köttorgeln, cancerlampan och keloidsamlaren… Fakirer, mordbrännare och talande dockor… Stig fram, gott folk. Välkomna till vårt kuriosakabinett. Visningen börjar om några få minuter. Låt er förfäras och förundras.
Snyltos bok (2011)
En dag dimper ett vykort ned i brevlådan. Det kommer från pappas kompis Snylto, som meddelar att han är tillbaka i stan. Redan nästa morgon dyker Snylto upp på familjens tröskel och förväntar sig att få stanna några dagar. Snart har han vänt upp och ned på familjens mellanmjölkstillvaro och lärt den femåriga dottern hur man skolkar från dagis och snattar i affären. Snylto verkar komma undan med allt – han spelar hög musik, stökar ned och äter hur mycket sötsaker som helst. Men pappa förklarar att det faktiskt är ganska synd om Snylto. Att vara som Snylto är kanske inte så roligt som det först verkar. Eller är det?
This section has obviously not been updated for a long, long time. Sorry about that...
As part of a grant for pedagogical renewal awarded by TUFF at Uppsala University, we have developed a number of exercises that will teach the students about the night sky, using augmented-reality observations with free astronomy apps for Android and Iphone. In these exercises, the students will learn how to find the North star, identify stars and planets in the sky, learn more about solar eclipses, spherical coordinates and the difference between astronomy and astrology. These exercises have now been used in this year's edition of the course Introduction to Astronomy and are just about to be introduced in the third-year course Astrophysics I. So far, the feedback has been great! The students seem to enjoy these exercises and the technical problems have been remarkably few.
The first paper describing the simulations used within the LYCAN project was recently submitted and is publicly available as a preprint. In this paper, we explain how observations with the upcoming James Webb Space Telescope (launch in late 2018) will allow the identification of galaxies that leak large amounts of ionizing photons into the intergalactic medium at redshifts z>6. This is important for scenarios where galaxies provide the ionizing photons that caused the reionization of the Universe - the cosmic phase transition that took place at slightly less than one billion years after the Big Bang.
The first Gaia data release is out, and astronomers across the globe are busy studying the catalogs to learn more about topics as diverse as stars, exoplanets and dark matter. I myself have gotten interested in the potential of using Gaia to search for Dyson spheres. A super-brief outline of the idea is available here. The proposed method can be applied to some 200,000 objects from the Gaia data release, and MSc student Ansgar Wehrhahn has agreed to work on this throughout the fall.
Martin Sahlén has received research funding for two years from Stiftelsen Olle Engkvist Byggmästare to join our group. Congratulations, Martin!
In a paper led by Akio Inoue (also featuring Erik Zackrisson) and published in the journal Science, we present some game-changing ALMA observations of an early-Universe galaxy at a redshift of z=7.2. This constitutes the most distant detection of the element oxygen so far, and confirms previous simulation predictions that the [OIII] emission line at a rest-frame wavelength of 88 micron may be the best way to hunt down galaxies at very high redshifts with ALMA. An analysis of all the spectral information available for the target galaxy moreover indicates that it may be leaking copious amounts of ionizing radiation into the intergalactic medium, with important implications for the reionization of the Universe.
Our first paper on machine learning in the study of first galaxy spectra has been submitted to the Astrophysical Journal and is now available as a preprint.
A couple of weeks ago, we posted our paper on the cosmological distribution of terrestrial planets on the arXiv preprint server. It got some fair coverage on Astrobites and by Scientific American, but then something curious happended... As more bloggers and journalists got interested, the claimed content of the paper got increasingly distorted, until headlines like "Earth is different from 700 quintillion planets in the Universe" and "Latest Research Says that the Earth Shouldn't Exist" started appearing. And based on the emails I've been getting lately, I strongly suspect that someone out there is trying to put a religious spin on all of this. Internet - you gotta love it...
Today, Saghar successfully defended her Licentiate thesis "Gravitational lensing and radio interferometry as a probe of the small–scale structure of dark matter" at Stockholm University. Congratulations, Saghar!
As an exercise in making concrete representations of abstract scientific concepts, we asked the students in the cosmology course to explain the four hierarchies of parallel universes defined by Max Tegmark, using lego. This turned out to be both fun and educational. Here's an example of how an encounter with an armed stormtrooper branches off into three different Everett worlds (level III parallel universes) with different, colour-coded probabilities.
Most galaxies in the local Universe are old - several billion years at least. This image, which has been constructed from Very Large Telescope data that I obtained a few years ago, captures a number of genuinely young galaxies that have formed in the tidal debris left over from the collision of two larger galaxies. This image now features in a fancy image/video release by ESO, in connection to a paper in which my old FORS data is combined with spectroscopy from the new Very Large Telescope instrument MUSE to reveal a number of puzzling features in the spectra of these objects.
We now have a new PhD student in our group - Christian Binggeli. Christian will be working on high-redshift galaxies and their role in the reionization of the Universe. Welcome, Christian!
This week, I attended a James Webb Space Telescope (JWST) meeting at the European Space Agency centre ESTEC in the Netherlands and gave a talk on the role of JWST in studying the role of galaxies in the reionization of the Universe (here's a link to the talk). After a couple of days, I rushed off to the Square Kilometer Array (SKA) epoch-of-reionization key science project meeting in Groningen to discuss how to coordinate SKA with other telescopes to make the most of the surveys planned for the SKA low-frequency array in Australia. Busy week, but pretty exciting!
This semester, me and Hannes Jensen (new postdoc in our group) are involved in a cross-disciplinary project together with Kristiaan Pelckmans, Rubén Cubo, Ulrika Lundholm and Kristiina Ausmees from the Department of Information Technology. The idea is to develop machine learning techniques to study galaxies in the early Universe, and more specifically to extract information about the escape of ionizing radiation from high-redshift (z>6) galaxies observed with the upcoming James Webb Space Telescope (JWST). In a paper of ours from a couple of years ago (Zackrisson et al. 2013), we proposed that this information should be retrievable from JWST spectra, but did not go to great lengths to figure out just how far this method could be pushed. With the help of this crew, we should be able to find out!
Over the last three days, astronomers from across the globe have gathered at the Wennergren centre in Stockholm to discuss key science projects for the upcoming Square Kilometer Array (SKA), a huge radio telescope to be constructed in 2020-2023 in South Africa and Australia. Since the SKA will be a game changer in the study of the reionization of the Universe and the emergence of the first stars and galaxies, I'm of course very eager to get onboard. Several astronomers (including yours truly) were interviewed by national television during this meeting, but the only snippets that actually aired were about - you guessed it - aliens. Yes, with the SKA, you could indeed eavesdrop on radio signals from nearby civilizations, but this is not the reason why the SKA is built - it's more or less just a side effect of constructing a radio telescope of this scale. Even though I'm all in favour of SETI (Searching for Extraterrestrial Intelligence), I have to confess that I feel pretty ambivalent about SETI getting all the limelight on this particular occasion. The SKA is primarily built to shed new light on the formation of planets and complex molecules in space, on cosmic magnetism and the early Universe, on putting theories of gravity to the test and to learn more about the large-scale structure of the Universe. Given the monumental cost of a telescope like this, perhaps it's a good idea to let the tax payers know that we're not just building a giant ear to gauge how deep the eerie silence goes? The problem, of course, is that it's so much easier for journalists to spin a tale about leakage from alien radar stations than to make the state of the intergalactic medium sound sexy. What would the best media strategy at a meeting like this be? To simply refuse to answer questions on SETI? Not sure, but this deserves some pondering, since this is a problem that is going to resurface again and again.
The first paper from our SETI project is now in print in Astrophysical Journal. We've also launched the Project hephaistos homepage. Be sure to check out Saghar Asadi's galactic colonization simulations - they're supercool.
Lots of changes in the gang lately... Claes-Erik Rydberg has taken up a postdoc in Heidelberg, where he will continue his work on the first stars and galaxies in the group of Ralf Klessen. We wish Claes-Erik the best of luck (but secretly hope that he will return to us one day). Per Calissendorff recently presented his MSc thesis Planets throughout space and time (see picture below) and is now off to the Canary Islands to work at the Swedish Solar Telescope. On top of that, Hannes Jensen, a PhD student that I've been co-supervising at Stockholm University, defended his PhD thesis Simulating observational probes of reionization. However, if all goes well, both Per and Hannes will be joining us in Uppsala in the fall - Per to work on SETI and Hannes to work on machine learning algorithms for the analysis of James Webb Space Telescope spectra.
We now have an opening for a PhD student eager to explore the mysteries of the high-redshift Universe. The application deadline is May 31, 2015. Please use this link if you're interested in applying!
This week, I attended the South by High Redshift conference in Austin and gave a talk on our paper about Population III galaxies at extreme magnifications. Lots of cool stuff was presented at this meeting, but the talk that really blew my mind was the one by David Sobral on a new Population III galaxy candidate at redshift z = 6.6. This could be a game-changer! The slides from the talks can be downloaded from the conference homepage.
Our paper on extreme forms of gravitational lensing has just been accepted by the Monthly Notices of the Royal Astronomical Society. Gravitational lensing allows you to observe objects in the distant Universe that are otherwise too faint to be detected with telescopes. These days, lensed galaxies are routinely detected with magnification factors of ten to a hundred, but not with magnifications much higher than this. Why not? Basically because the probability for this is too low and because galaxies are too big. Magnification factors of thousands are by no means impossible, but this requires light sources that are fairly compact (just a few parsecs across). You also need a source population with a high number density (many sources per square degree in the sky), and a survey with a very large areal coverage to find them. In this paper, we use gravitational lensing simulations to quantify the probabilities for extreme magnifications in the distant Universe. We also investigate what sort of light sources and surveys this may be relevant for. As it turns out, extreme magnification factors may allow the Ultra Deep Survey considered for the Japanese WISH telescope to detect primordial star clusters (with zero-metallicity stars, so-called Population III stars) at redshifts z = 7 to 13. Pretty cool! A preprint of the paper is available here.
This semester, the Galaxies and Cosmology group is hosting three MSc students: Per Calissendorff (left), Kostas Liolios (middle) and Christian Binggeli (right). Per is modelling exoplanet formation in a cosmological perspective, Kostas is analyzing Keck observations of galaxies at redshift z=3 and Christian is simulating the spectra of galaxies in the reionization epoch. We're also hosting one BSc student - Marika Ljungberg (not in picture), working on black hole signatures in high-redshift galaxies.
The local radio station P4 Uppland did a short piece on our search for extraterrestrial intelligence. The live interview is availble here. Also, Florent Duval, a PhD student that I've been co-supervising at Stockholm University, successfully defended his thesis. Congratulations Dr. Duval!
Beatriz Villarroel gave a talk at the Transient Universe for All meeting in Lund. You can find the meeting webpage here.
Today, my student Claes-Erik (Utte) Rydberg successfully defended his PhD thesis "Gravitational lensing as a probe of the first stars and galaxies" at Stockholm University. A link to his thesis is available here.
Nils Bergvall's monumental paper on starburst galaxies is the Sloan Digital Sky Survey has been brewing for a long time, but has now finally been submitted. Conrats! Local co-authors include Thomas Marquart, Michael Way, Anna Blomqvist, Emma Holst and myself. A preprint of the paper is available here.
We've received a small research grant for our SETI (Searching for Extraterrestrial Intelligence) projects. As far as I know, this is the first SETI grant ever awarded in Sweden. Major kudos to the Magnus Bergvall Foundation! The funding will likely be used to pay the salary of a summer student to work on SETI. If this had happended when I was an undergrad, I would've killed for that job!
My student Claes-Erik Rydberg has just gotten his PhD thesis "Gravitational lensing as a probe of the first stars and galaxies" back from the printer. The thesis defence will take place in Stockholm on January 30.
Some of the courses I've taught, with links to course homepages when available
Orienteringskurs i astronomi (5 ECTS), Uppsala University, Fall 2016
Översiktskurs i astronomi (7.5 ECTS), Stockholm University, Spring 2009
Orienteringskurs i astronomi (7.5 ECTS), Uppsala University, Fall 2004
Courses for BSc students
Courses for MSc students
Galaxies (7.5 ECTS), Stockholm University, 2012
Galaxies (7.5 ECTS), Uppsala University, 2004
Courses for PhD students
Gravitational Lensing (7.5 ECTS), Stockholm University, 2011
Modern Cosmology (7.5 ECTS), Uppsala University, 2005