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A daily view of all the goings-on at ASTRON and JIVE.

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    © R. F. Pizzo

    The Third LOFAR Data Processing School was organized and hosted by ASTRON between 17-21 November 2014. The event was sponsored by the host institute, the International LOFAR Telescope (ILT), and RadioNet.

    The School has provided a week of lectures and tutorials to introduce the LOFAR system to users and new members of the collaboration who will analyse Cycle data. The topics ranged from the capabilities of the basic station hardware to the software pipelines and science products they produce. 48 regular participants attended the event, together with lecturers, tutors, and helpers. The social events included a School dinner at ASTRON and a visit to the LOFAR core.

    The School will deliver very important documentation to future radio astronomers. The proceedings of the lectures will contribute to the first LOFAR Data Book, which will be published by Springer within 2015. We believe that this book will be an important reference for the scientists who will use LOFAR and will work on low frequency radio astronomy in general.

    Here you see the school picture, taken on the third day of the school at the LOFAR Superterp. For the participants, the LOC, and SOC, it has been an intense, fruitful, and successful week. The School LOC thanks all the people who contributed to the school!


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  • 11/23/14--16:00: The Periphery of Disks
  • © CASS/ASTRON

    One of the key science themes for SKA and its pathfinders is the role of atomic gas in the evolution of galaxies. So far, studies on this topic have been limited, for technical reasons, to either small samples of nearby galaxies, or to studying the global, overall gas properties of larger samples. With upcoming or upgraded instruments such as Apertif, ASKAP, MeerKat, JVLA and, slightly later, SKA1, it will be possible, for the first time, to image in detail the HI distribution and kinematics of large samples of galaxies out to cosmologically interesting distances. Combining these HI data with those from large surveys done in other wave bands is likely to create a breakthrough in our understanding of how galaxies evolve and the role gas plays in this.

    A central factor in determining how galaxies evolve is in what kind of environment galaxies live; whether there are many other galaxies nearby, or whether a galaxy is isolated. One of the powers of HI work is that it is the best diagnostic of how galaxies interact with their environment. HI is often the most extended component of a galaxy and is therefore most sensitive to influences of nearby galaxies. Inspired by this fact, a meeting was organised in the first week of November by our colleagues in Sydney on the theme 'The Periphery of Disks', i.e. what can we learn, now and in the future, from HI observations of the outer regions of galaxies on the important theme of galaxy evolution.

    One can argue that HI radio astronomy is a Dutch invention and this fact still has a strong impact on Dutch astronomy. The picture is the usual group photo of the participants, here in front of the Powerhouse Museum in central Sydney where the meeting was held. If you look carefully, you will notice that about 25% of the participants are from Dutch institutes, and if you include the participants that have a Dutch past, the fraction is even higher. This underlines the role of the Dutch astronomical community in getting ready for the new radio telescopes.


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    © ASTRON

    The AARTFAAC(*) correlator that is currently under development will correlate all 288 dual polarization antennas of the six LOFAR superterp stations. The signals arrive with an aggregated bandwidth of 30Gbps. At each station, UniBoards collect subband data from the LOFAR ring interface and feed the formatted data to the GPU correlator. The UniBoards format and re-order the data such that the correlator can perform optimally.

    To verify that each antenna signal arrives at the GPU machine correctly, a simulation was made to predict what the phase differences and amplitudes of dipole cross covariances should look like when the dipole inputs are complex sinusoids with a fixed phase and amplitude relationship between themselves. The resulting plots are in the left column and show the amplitude and phase gradients across antennas as fed in.

    The goal of the first field test was to produce the same plots as the simulation, but with 288 waveforms generated by the RSP boards in the field. This resulted in the pieces of modern art in the second column. It is caused by setting the amplitude of the waveform generators too high, leading to clipping. The correct levels were applied in the second field test, which proved that all antenna signals were arriving at the GPU machine correctly.

    Until now, the correlation was done offline, and for one time-sample only. The next step was to let the GPU correlator produce the same plots, at full speed. Some artifacts can be seen in the right column, which were traced to dropped packets in the correlator code. This was fixed by tweaking the network and interrupt subsystems of the GPU machine.

    So, the AARTFAAC correlator is ready for action. Stay tuned!

    (*) This elegant acronym stands for All-sky Transient Detector for LOFAR


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    © Image Credit: R.Kelly

    Radio emission from low-mass X-ray binaries (LMXBs) is dominated by synchrotron emission from relativistic jets that are powered by accretion onto the black hole. Polarization measurements allow us to study the orientation of unresolved LMXB jets, the structure of the associated magnetic fields, as well as the jets' interaction with the surrounding medium. In the presence of ordered magnetic fields, linear polarizations of up to ~70% are expected.

    However, polarization has been observed in very few LMXBs and at relatively low levels, though it is unclear if this is due to a lack of intrinsic polarization, depolarization, or a lack of observations -- a question complicated by the tendency for null results to remain unpublished.

    I will discuss a number of recent (and not so recent) LMXB outbursts where we obtained, among other broadband data, radio polarization measurements. These will include Swift J1745-26, which had linear polarizations of up to 50%, and the spatially resolved polarimetry of XTE J1748-288.

    I will also present our recent VLA and VLBA observations of XTE J1908+094, showing expanding polarized ejecta.


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  • 11/27/14--16:00: Meet the Radio Life team!
  • © Astron

    The ERC Advanced Grant "Exploiting new radio telescopes to understand the role of AGN in galaxy evolution", better known as "RadioLife", is up-and-running since 1.5 years. The proposal has obtained 2.5 million euro to set up a science team dedicated to exploring the full impact of radio AGN on galaxy evolution using LOFAR and Apertif. This team is now finally complete!

    The picture shows the great team of students, postDocs and software engineer which the PI, Raffaella Morganti, has put together.

    From left to right top row: Francesco Santoro(PhD), Filippo Maccagni (PhD), Kristina Nyland (PostDoc), Jeremy Harwood (PostDoc), Leith Godfrey (PostDoc), Bradley Frank (PostDoc, part-time RadioLife), Nicolas Vilchez (software engineer), Raymond Oonk (PostDoc, part-time RadioLife). Bottom row, from left to right: Marisa Brienza (PhD), Raffaella Morganti (PI), Elizabeth Mahony (PostDoc, part-time RadioLife).

    The goal is to achieve a better understanding of the dynamic life of radio sources, their duty-cycle and whether HI has a role in all this. Several exciting results have already been obtained in this initial period and you can find them in some of the past Daily Images and in published papers. To list just some of the results: the self-calibration pipeline for LOFAR data (Daily Image 27-Jul-2014 from Nicolas as part of the CITT), the identification and characterization of HI absorption and HI absorption stacking (part of Filippo's thesis Daily Images 20-Aug-2014 and 7-Jul-2014) and the tracing of the interaction of the radio jet with the surrounding quiescent medium in Centaurus A using integral field spectroscopy with VIMOS and MUSE@VLT (part of Francesco's thesis).

    We expect many more successes to come and many more Daily Images posted!


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  • 12/01/14--16:00: The HI absorption Zoo
  • © Katinka Gereb

    Radio nuclear activity is thought to be connected with the properties of the gas in the circumnuclear region of galaxies. Neutral hydrogen 21-cm (HI) absorption is a sensitive measure of the properties of even small amounts of gas, and it provides one of the key tracers of the interplay between gas and AGN.

    Recently, we published a selection of 32 HI absorption lines detected in a survey using the Westerbork Synthesis Radio Telescope, aimed to probe the gas content of radio galaxies through HI absorption stacking (Daily Image of August 20, 2014 ).

    The HI absorption spectra show a broad variety of widths and shapes, really an "HI Zoo". We identify three different groups of absorption lines, representing physically different HI structures. Narrow lines are likely to trace regularly rotating HI disks or single gas clouds (see #13, #30). Broader profiles appear to be more asymmetric, possibly tracing unsettled components of HI gas. The broadest lines are associated with gas-rich merging systems (#7, # 22) and are profiles composed by several blended HI components. Furthermore, three profiles (#15, #17, #29) show highly blue-shifted wings, which are the typical signs of outflows driven by interactions between the jets and the ambient medium.

    We find a correlation between the power of the radio source and the velocity shift of the HI with respect to the optical systemic velocity. This suggests that more powerful AGN are more efficient in affecting the kinematics of the surrounding gas. This relation appears to be tighter in compact, young sources (shown in red in Fig. 1), where the radio jets are starting their growth. This is also supported by the fact that both stacking and the individual analysis of the HI lines show that in compact sources the profiles are overall wider and more asymmetric.

    These results have been presented by Gereb, K; Maccagni, F, M; Morganti, R; Oosterloo, T; in a paper now accepted for publication in A&A: ArXiv:1411.036


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    © Astron

    Tom Oosterloo, one of the senior astronomers at ASTRON and professor at the Kapteyn Institute Groningen, has been awarded a prestigious TOP grant from NWO.

    The grant will allow Tom to expand his work on dwarf galaxies, for which he is one of the leading experts. The project aims at searching for the smallest and dimmest galaxies in the nearby Universe, and to understand their properties in terms of models for galaxy formation. With this grant, Tom will be able to do this search by exploiting the new and unique capabilities of Apertif, the upgraded Westerbork Synthesis Radio Telescope.

    The importance of these extremely small galaxies Tom will search for is that they carry unique information about the young Universe, when the very first stars and galaxies have formed. They also represent buildings blocks for larger galaxies: they play, therefore, a key role in the Universe. These small galaxies are delicate systems. Because of their small mass and, consequently, weak gravity, they are very sensitive to external conditions: small changes in these conditions may result in large changes in the properties of the resulting galaxies.

    Until a few years ago, theoretical models tended to predict far too many dwarf galaxies. The predictions have now improved, assuming that the young galaxies actually manage to loose their gas. This seems to be the case for the dwarf galaxies located close to the Milky Way. But there are major exceptions! The presence of a number of very gas-rich dwarf galaxies (some discovered and studied by Tom) provides still a headache for astronomers modelling galaxy formation!

    A proper census of how many of these dwarf galaxies exist outside the Local Universe requires observations with a large field of view, good sensitivity and good spatial resolution. In other words: Apertif!!

    One lesson learned from Tom's proposal: even a terrible acronym can give you a grant!

    Congratulations Tom and good luck with the HuDaGa project!


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  • 12/02/14--16:00: Heel Nederland Kijkt Sterren
  • © ASTRON

    Tonight at 21:25 (local time) the famous Westerbork Radio Telescope (WSRT) will shine in the Dutch TV show Heel Nederland Kijkt Sterren (All of the Netherlands watches the Stars) on NPO1. The show will be presented by Govert Schilling and Jeroen Latijnhouwers, who will switch between different locations.

    Our very own Joeri van Leeuwen (top left image) will also strut his stuff. He will host a part of the show and will explain to people at home what we are doing at ASTRON, and what we can see in the sky.

    On 21 November, Joeri also glittered in the show MAX TV wijzer. Don't forget to check out that episode too; you can watch it here: http://www.omroepmax.nl/televisie/alle-programmas/max-tv-wijzer/uitzending/tv/max-tv-wijzer-vrijdag-21-november-2014/

    So, everybody tune in on NPO1 tonight at 21:25.


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    © Alberto Sanna

    I will review the current state of the Bar and Spiral Structure Legacy Survey (BeSSeL). The BeSSeL Survey is a Very Long Baseline Array (VLBA) key science project to measure trigonometric parallaxes and proper motions of massive star-forming regions throughout the 1st, 2nd, and in the 3rd Galactic quadrant of the Milky Way. Sites of star formation which give birth to massive O-B stars trace rich gas condensations along spiral arm segments, and are therefore the most suitable targets to pinpoint the spiral structure of our Galaxy.

    Very Long Baseline Interferometry (VLBI) observations of strong maser emission, associated with young massive stars, currently yield parallax accuracy as good as 10 micro-arcsecond. That allows us to explore our Galaxy reaching objects with distances more than 10 kpc away from the Sun, and still locating them with an accuracy better than 10%. Currently, over 100 trigonometric parallaxes and proper motions have been measured with the VLBA, the European VLBI Network, and the Japanese VLBI Exploration of Radio Astrometry facility. I will summarize the major achievements obtained so far and put them in the context of a global view of our own Galaxy.


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    © ASTRON

    After the successful completion and measurement of the Medium Size Tile (MST) in May of this year, the MST, with a 5x6 array of new LNT-elements in two polarizations, has been measured at the Parkes telescope in Australia. This was done as part of a campaign of the SKA Dish Consortium to compare the noise performance of the available Focal Plane Arrays: The Dutch MST, the Australian checkerboard array and the Canadian AFAD thick Vivaldi array. The measurements were done using the BETA digital backend, used for ASKAP.

    Preliminary results show similar noise performance for the three FPAs. The final results will be the subject of a future AJDI, as soon as all the data have been processed in detail.

    In the meantime, it is interesting to compare the Parkes results for the MST with the previous measurements in Dwingeloo. Both are presented in the accompanying noise plot and show that the Dwingeloo measurements are quite accurately reproduced at Parkes. The noise temperatures are between 35 K and 40 K in the 1000-1500 MHz frequency range, gradually increasing up to 50 K near 1800 MHz.

    The picture shows the MST with the Parkes telescope in the background.


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    © Astron

    Today, the project proposal EXTRIMA was submitted to H2020 call FET HPC 2014 together with University of Cambridge, University of Oxford, Fraunhofer MEVIS, mediri GmbH and NL eScience Center.

    EXTRIMA focuses on Reciprocal Space Imaging (RSI): the data acquisition is performed in the reciprocal space of the domain where the information extraction is needed. Instruments rely mostly on Fourier algorithms but alternative algorithms such as wavelets are increasingly being researched and applied.

    The following challenges in RSI can be discerned: higher dynamic ranges and higher sensitivity, larger image sizes, faster imaging, lower energy consumption, multimodal imaging, efficient data acquisition, higher information extraction, and calibration for object motion. The challenges described above will be met by the EXTRIMA project by delivering a radically new approach on the entire Reciprocal Imaging chain with respect to the combination of algorithms and the HPC platforms involved. EXTRIMA focuses on Magnetic Resonance Imaging (MRI) and Radio Astronomy.

    The Astron team is composed of Monique Sluiman, Rob van der Meer, Emmy Boerma and Gert Kruithof.


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    © JvL

    PSR J1906+0746 is a young pulsar in the relativistic binary with the second-shortest known orbital period, of 3.98 hours. Our timing study based on five years of observations, conducted with the 5 largest radio telescopes in the world, including Westerbork, aimed at determining the companion nature. The paper was recently accepted for ApJ and is now in arxiv (van Leeuwen et al. 2014).

    The complete set of 28,000 pulsar times of arrival (TOAs) is one the largest, most detailed datasets ever fit. From these we derived parameters describing the state and evolution of both the individual pulsar and the binary system. These include three post-Keplerian orbital parameters -- changes in the orbit that only occur in the General Relativistic regime. These are the rate of periastron advance, the gravitational redshift, and the orbital period derivative. These all depend, in different ways, on the two masses of the system. Thus by plotting these three relations on a plane spanned by the masses of the two stars, the highest-confidence value is determined. That plot (Fig. 7 in the paper) is shown above. The colors range from yellow (low probability) to red (highest probability). Above, the figure is further stretched by an impression of the time-space distortions produced by the two components of the binary system.

    Through the measurement of these three post-Keplerian orbital parameters we find the pulsar mass to be 1.29 solar mass, and the companion mass 1.32 solar mass respectively. These masses fit well in the observed collection of double neutron stars, but are also compatible with other white dwarfs around pulsars that are young like PSR J1906+0746. We conclude that young pulsar J1906+0746 is likely part of a double neutron star -- one of only ten known!


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    © ASTRON

    The first small prototype of a Dense Dipole Array (DDA) was manufactured and S-parameter measurements were done. The design consists of a 4x4 array of densely packed printed dipoles above a ground plane. Array elements are connected in parallel-pairs and fed differentially through a common-mode suppressing feed-board. Preliminary results of the measurements conducted so far have indicated a frequency range of 670-1643 MHz.

    The work forms part of the PhD work of Jacki Gilmore from Stellenbosch University in collaboration with Astron through MIDPREP. The PCB manufacturing was done in-house by Albert van Duin and the mechanical design and manufacturing was organised by Raymond van den Brink. The measurements were done with the assistance of Michel Arts. Thank you all for your support!


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    © AARTFAAC group

    The AARTFAAC project piggybacks on the LOFAR superterp stations in order to implement a commensal, near real-time All-sky monitor for transient sources. This is achieved by additional hardware in the stations which tap the station RSP ring network and reformat the data, before routing them to Groningen for correlation with a GPU based system.

    Needless to say, the 288 dual-pol inputs passing through multiple hardware, and arriving at ~30 Gbps on the GPU machine provides ample scope for generating formatting and other issues.

    Recently, we were able to verify the correctness of the data chain from the stations, through the AARTFAAC station hardware, and upto the output of the correlator.

    Now, the calibration and imaging component of the pipeline has also been verified. This block carries out per-dipole calibration, including compensation for ionospheric refraction, and subtracts the A-team to generate images with a dynamic range of ~2000:1. The above gif shows such a sequence all-sky images in the XX polarization, over a timespan of a minute during the night of 08Nov14. Each image has an integration time of 1 second, and a bandwidth of 1 subband (~200 kHz) around 57 MHz. The LBA_OUTER array configuration was used. In the image, sources from the 3C catalog with a flux above 50 Jy are indicated by their 3C numbers, while the flux colorbar is in arbit. units.

    The correlator latency is about 0.45 sec, with the calibration and imaging taking about 0.25 second per frequency channel. The next step for AARTFAAC is interfacing to the transients pipeline (TraP), which will do the actual job of transient detection via image plane lightcurve extraction and analysis.

    Stay tuned for more exciting news!


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    © ASTRON

    In the Digital and Embedded Signal Processing group we are constantly seeking ways to reduce the development time of digital systems and simplify the workflow in order to reduce the time to science. In this ongoing quest we are not the only ones. Recently we had a visit of two firmware engineers from SKA-SA (Jason Manley and Wesley New) to team up towards a common design flow for our firmware designs. They have been sitting together with Daniel, Eric and Hajee in one room for 1.5 week to agree on a way forward. Early next year we will plan another visit to keep the momentum at a high level. In the meanwhile monthly video conferences are organised to make sure we keep ourselves aligned.

    The digital systems we develop are targetted to large scale systems with a relative low power budget. The main focus area is processing on massive amounts of streaming data.

    Our vision is to have hardware and firmware modules ready on the shelf, such that these items can quickly be integrated in radio astronomical instruments when the need arises. With UniBoard and UniBoard^2 we are exactly working towards that vision.


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    © ASTRON

    In our daily image of October 31, we started an investigation of the number of sources that will be visible anywhere in the field-of-view of a 15-m filled aperture at 1 GHz (shown in the right panel for convenience). We argued that, as telescope sensitivity increases, the number of sources may grow so large that their cumulative sidelobes may increase the image noise to a level that nullifies the (very expensive) nominal sensitivity of the telescope, in particular the SKA. Since ASTRON has a particular interest in the Low-Frequency Aperture Array (LFAA) for the SKA, we now present the results for a 35-m filled aperture at 100 MHz (left panel).

    The curves in these plots indicate the total number of sources brighter than a given apparent flux. This is shown for various lobes of the primary beam pattern, i.e. the LFAA station beam on the left, and a typical SKA dish beam on the right. Although the shape of the curves, as well as the distribution of sources over various parts of the primary beam, are very similar for both cases, the 35-m aperture at 100 MHz detects about a hundred times more sources than the 15-m aperture at 1 GHz.

    This is caused by two effects. Firstly, the primary beam size of the 35-m aperture at 100 MHz is about 20 times larger than that of the 15-m aperture at 1 GHz. The second effect is, that the synchrotron spectrum of most continuum sources is about 7 times brighter at 100 MHz than at 1 GHz. Although the second effect is damped a little bit by the decrease in telescope sensitivity due to increasing sky noise towards lower frequencies, the net result, as shown in these plots, is that the LFAA system is likely to detect about a hundred times more sources than the SKA-dish system, in an observation with the same integration time and bandwidth.

    It is interesting to note that the ability to detect about 100 million sources in a single observation, implies that the SKA-low system will be capable of detecting virtually every galaxy in the universe(*). Of course that would require a resolution of 1", i.e. baselines up to 600km.

    In a future daily image, we will show that, to achieve this lofty goal, we have to think carefully about the design of our instrument to keep the PSF(**) sidelobe level as low as possible. Almost certainly this will require a station configuration that is different from the one currently proposed for the LFAA.

    (*) Based on an estimate of a total of 100 billion galaxies.

    (**) Point Spread Function


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  • 12/15/14--16:00: ERIC decision for JIVE
  • © JIVE

    On the 12th of December 2014, the president of the European Commission Jean-Claude Juncker signed the decision that will allow the Joint Institute for VLBI in Europe (JIVE) to operate as a European Research Infrastructure Consortium (ERIC). ERICs are truly international organizations in Europe that enjoy several administrative advantages and tax exemptions. The new status will allow JIVE to function (even) more efficiently.

    JIVE has supported the EVN already for 20 years, and we foresee that it will continue this excellent job for many years to come. On 20-21 April 2015 there will be a science symposium followed by the inaugural ceremony.


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    © Art, Education & Outreachtprojects

    Wednesday November 26, the project Kijk eens wat verder dan de sterren (Look beyond the stars) has started. Primary schools are invited to register for the project, a boundless adventure to interest children in science and technology. The project is a virtual journey, featuring a logbook, a suitcase full of tasks, excursions to unique places, engaging fellow travellers, and an exciting game in which everyone can participate.

    The journey is based on a story, a sequel to Het Logboek, the book that Anke den Duyn wrote and which was distributed to all primary schools in Drenthe. Ralf, one of the characters in the book gets involved in a new adventure. He travels through the universe in a special vessel, passing different time zones, and discovers the secrets of the universe. In each time zone an astronomical object or physical phenomenon will be visited.

    Schools that sign up for the trip will receive a travel budget from the Bank of Astronomia, which allows them to subscribe to interesting lectures and excursions. These have an astronomical topic as a starting point, but are always presented from an unexpected angle.

    Throughout the project runs a game: "What is the final destination of the trip?" Clues will be offered everywhere, on the website, in the episodes of the story, in magazines, etc.. Parents, grandparents, friends, .. everyone can join.

    Please visit the website www.kijkeenswatverderdandesterren.nl, follow the adventures of Ralf, and keep your eyes and ears open.


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    © Pictures: Dieter Engels, Ronald Halfwerk

    The latest International LOFAR station, located at Norderstedt, close to the city of Hamburg, was handed-over successfully on December 16th by AstroTec Holding BV to its new owners: The universities of Hamburg and Bielefeld. After careful site-selection by the universities, and receiving the building permits, the entire station was built and validated in a mere month and a half. Good team work proves its value again.

    Although the station is still in the process of validation and calibration, it has already received two stars: the precious LOFAR hardware ("made by ASTRON") in the EMC-container is now kept safe by the strongest available 2-star graded German-made padlock, including two custom-made "Norderstedt key-labels".

  • On the middle left picture: Nico Ebbendorf (ASTRON) hands over the mutually agreed list of hardware deliverables to Prof Marcus Brueggen (University of Hamburg), including the CE declaration of the station.

  • On the bottom right picture, from left to right: Nils Boehmer (Uni Hamburg), Joern Kuensemoeller (Uni Bielefeld), Nico Ebbendorf (ASTRON), Dieter Engels (Uni Hamburg), Dominik Schwarz (Uni Bielefeld), Ronald Halfwerk ( AstroTec Holding)

    The new LOFAR station will be used by researchers at the University of Hamburg, led by Prof. Marcus Brueggen, who specializes in studying the formation and evolution of clusters of galaxies from the early Universe to the present era. A group of researchers at Bielefeld University, led by Prof Dominik Schwarz, studies the distribution of galaxies on the largest observable distances in the Universe, which carry imprints from the era of cosmological inflation.


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    © Betsey Adams

    Our model of cosmology, including dark matter and dark energy, does a good job at explaining large scale structure in the Universe but predicts many more small structures than we observe dwarf galaxies. These small structures may exist but as dark matter only (no stars or gas to observe) as they don't have enough mass to keep their gas and form stars. We want to study the smallest galaxies that have formed to understand what processes are important in these low-mass systems.

    In order to find some of these smallest galaxies, we identified isolated neutral hydrogen (HI) clouds with no optical counterpart from ALFALFA, a survey of neutral hydrogen in the nearby Universe using the Arecibo radio telescope. These clouds are good candidates to represent dark matter halos containing gas but (nearly) no stars within our own Local Group of galaxies.

    We recently published follow-up observations with the Westerbork Synthesis Radio Telescope (WSRT) of one very intriguing cloud, known as AGC198606. This cloud is of particular note as it is located nearby another very small, gas-rich galaxy, Leo T, and has similar HI properties as measured in ALFALFA. The WSRT observations allow the neutral hydrogen to be imaged in more detail and show that that the gas is distributed smoothly and has an ordered motion of ~25 km/s aligned with the major axis of the system. Deep optical images with the WIYN telescope at Kitt Peak reveal no stars associated with this cloud.

    Without any stars, we don't know the distance to AGC198606. Is it physically associated with Leo T at a distance of 420 kpc? Is it at a closer distance of 150 kpc that theoretical models might suggest? Or is it very nearby and a cloud of gas that is part of our own Galaxy? We can't say for sure, but the distribution of HI and ordered velocity motion suggests AGC198606 might be a dark matter halo containg gas and (nearly) no stars. If AGC198606 is at the distance of Leo T it has a neutral hydrogen mass of 620,000 solar masses, a radius of 1.4 kpc, and a total mass (including dark matter) within the HI extent of 150,000,000 solar masses.

    These results have been presented by E. A. K. Adams, Y. Faerman, W. F. Janesh, S. Janowiecki, T. A. Oosterloo, K. L. Rhode, R. Giovanelli, M. P. Haynes, J. J. Salzer, A. Sternberg, J. M. Cannon, and R. R. Munoz in a paper now accepted for publication in A&A and available at http://arxiv.org/abs/1412.0286.


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