Monday Seminar for the year 2015

The presentations are shown in order of increasing time.









Monthly Discussion






Andrew Cameron (1) 

Innovative Pulsar Searching Techniques

chair: Dhanya G. Nair 

Abstract: Pulsars, rapidly-rotating highly-magnetised neutron stars, are fantastic natural laboratories and allow for a wide range of fundamental scientific applications. To date, over 2400 pulsars have been found, and the search to find more is constantly ongoing, with the development of new surveys and search techniques pushing the boundaries of what we are able to observe and discover. In this talk, I will present both an overview of and the latest progress from the High Time Resolution Universe South Low-Latitude (HTRU-S LowLat) survey, a highly sensitive survey taken using the 64m Parkes Radio Telescope in Australia, which promises to provide the deepest large-scale search ever for the Galactic plane region. I will also discuss some of the limitations of this survey's data-reduction and searching strategy when it comes to longer period pulsars, and present the revival of the Fast Folding Algorithm (FFA), an alternative searching technique which may be able to overcome some of these limitations and the study of which will provide a substantial portion of my ongoing thesis work.






Axel Buddendiek (3)

Finding Massive Distant Galaxy Clusters in RASS and SDSS

chair: A. Cameron

Finding a sample of the most massive clusters with redshifts z > 0.6 can provide an interesting  consistency check of the Lambda-CDM model. Here we present results from our search for clusters with 0.6 < z < 1.0 where the initial candidates were selected by cross-correlating the RASS faint and bright source catalogs with red galaxies from the Sloan Digital Sky Survey DR8. Our survey thus covers roughly 10,000 deg^2, much larger than previous studies. Deeper follow-up observations in three bands using the William Herrschel Telescope and the Large Binocular Telescope were performed to confirm the candidates, resulting in a sample of 44 clusters for which we present richnesses and red sequence redshifts, as well as spectroscopic redshifts for a subset. At least two of  the clusters in our sample are comparable in richness to RCS2-J232727.7-020437, one of the richest systems discovered to date. We also obtained new observations with the Combined Array for Research in Millimeter Astronomy for a subsample of 21 clusters. For eleven of those we detect the Sunyaev-Zel'dovich effect signature. The Sunyaev-Zel'dovich signal allows us to estimate M200  and check for tension with the cosmological standard model. We find no tension between our cluster masses and the Lambda-CDM model.

Kostas Markakis (3) 

Galactic Nuclei in the NIR: From SgrA* to extragalactic sources

chair: A. Buddendiek

NGC3718 is a LINER L1.9 galaxy, lying at a distance of about ∼ 17.4 Mpc away from earth and its similarities with NGC5128 often award it the name “northern Centaurus A”. The presence of a compact radio source with a candidate jet structure, a prominent dust lane and a strongly warped molecular and atomic gas disk are indicative that, NGC3718 has undergone some sort of a large scale gravitational interaction sometime in the recent past, which channeled gas towards the center, feeding the black hole and igniting the central engine. One proposed scenario involves an encounter with the close neighboring galaxy NGC3729, while other authors favor a merging event with mass ratio ≥ (3 − 4) : 1, as the origin of NGC3718. We use high angular resolution (∼ 100 mas) e-Merlin radio and SUBARU NIR (∼ 170 mas) data, that allow us to take a detailed view of the processes that take place in its central region. In order to preserve some objectivity in our interpretation, we combine our results with literature values and findings from previous studies. Our NIR maps suggest, on the one hand, that towards the stellar bulge there are no large scale absorption phenomena caused by the apparent dust lane and, on the other, that there is a significant (local) contribution from hot (∼ 1000 K) dust to the nuclear NIR emission. The position where this is happening, appears to be closer to the offset compact radio emission from our e-Merlin 6 cm map which, altogether, lie offset by ∼ 4 pc from the center of the underlying stellar bulge. The shape of the radio map suggests the presence of one (or possibly two, forming an X-shape) bipolar structure(s) ∼ 1 (∼ 0.6) arcsec across, which combined with the reported balance between the gas and the stellar velocity dispersions and the presence of hard X-ray emission, points towards effects expected by AGN feedback. We also argue that NGC3718 is the case of a core object despite the fact that it is a gas-rich galaxy and we discuss its mixed photometric and spectroscopic characteristics. The latter combined with the observed spatial offset, the reported relative redshift between the broad and the narrow Hα, the limited star formation activity and AGN feedback, strongly imply the existence of a possible SMBH recoil. Finally, we discuss a possible interpretation, that could naturally incorporate all these findings into one physically consistent picture.






Monthly Discussion






 Michal Zajacek (1)

Physics in the vicinity of the Galactic centre: conditions for star-formation and properties of young stellar objects

chair: K. Markakis

The Galactic centre (GC) of the Milky Way offers us a unique laboratory for studying the physics close to the supermassive black hole (SMBH), which is associated with the compact radio source Sgr A*. I will briefly introduce the complex environment of GC, which contains both early- and late-type stars, compact remnants, molecular, neutral and ionized gas and dust. All of these components interact with each other to a certain extent, bringing plethora of unique phenomena. The focus of the presentation will be the analysis of the conditions for star-formation in the sphere of influence of the SMBH (< 1.7 pc). Although it has been argued that the star-formation is inefficient in the potential of the SMBH, it will be shown that the infalling gas clumps can reach densities leading to their collapse. The properties of young stellar objects in the Galactic centre will be discussed from both the theoretical and the observational point of view (see also Zajacek, Karas, Eckart, A&A, 2014). A special attention will be paid to a puzzling Dusty S-cluster object G2 (DSO/G2) observed mainly in NIR L-band, which passed the peribothron in April 2014 at about 150 AU from the SMBH (see also Valencia-S, Eckart, Zajacek et al. 2015, ApJ in print). Its plausible association with an embedded pre-main sequence star will be discussed, although further observations are needed to confirm this hypothesis.












Alessandro Ridolfi (2)

Timing of pulsars in globular clusters

chair: M. Zajacek

Globular clusters (GCs) are spherical, gravitationally bound groups of stars which are very often home of a number of pulsars. Their very high densities, especially near their cores, make GCs also ideal environments for the formation (and disruption) of binary systems and for the spin-up of pulsars through accretion processes.
47 Tucanae and M15 are among the richest GCs in terms of the number of pulsars, hosting 23 and 8 known pulsars, respectively. In this talk I will outline the procedures needed to detect and study some of the faintest pulsars of these clusters.
I will introduce some search techniques that have been applied to first detect and then get the timing solution of some pulsars in 47 Tucanae, and I will show some preliminary results that we have obtained.
In the second part of the talk I will review the main characteristics of the pulsars in M15. One of them, called PSR B2127+11C, is a binary millisecond pulsar in a double neutron star system and its peculiarity is that it is showing evidence of geodetic precession occurring, an effect predicted by Einstein's General Relativity. I will give a update on the ongoing work, showing some results from our recent observation campaign made with the Arecibo radio telescope.






Monthly Discussion






Bia Boccardi (3)

The stratified two-sided jet of Cygnus A

chair: A. Ridolfi

At a distance of 249 Mpc (z=0.056), Cygnus A is the only powerful FR II radio galaxy for which a detailed sub-parsec scale imaging of the base of both jet and counter-jet can be obtained. Observing with VLBI at milimeter wavelengths is fundamental for this object, as it uncovers those regions which appear self-absorbed or free-free absorbed by a circumnuclear torus at longer wavelengths. We performed 7 mm Global VLBI observations, achieving ultra-high resolution imaging on scales down to 90 microarcsec. This resolution corresponds to a linear scale of only ∼400 Schwarzschild radii. We studied the transverse structure of the jets through a pixel-based analysis, and kinematic properties of the main emission features by modelling the interferometric visibilities with two-dimensional Gaussian components. Both jets appear limb-brightened, and their opening angles are relatively large. The flow is observed to accelerate within the inner-jet up to scales of ∼ 1 pc, while lower speeds and uniform motions are measured further downstream. A single component seen in the counter-jet appears to be stationary. These observational properties are explained assuming the existence of transverse gradient of the bulk Lorentz factor across the jet, consisting of a fast central spine surrounded by a slower boundary layer.






Alice Pasetto (3)

Probing the environment of high Rotation Measure AGNs through multifrequency radio observations

chair: B. Boccardi 

This work aims at characterising and studying, in the radio band, a sample of candidates of high Rotation Measure (RM). The point-like objects (at all JVLA scales) suffer for a strong depolarisation at 21cm (our main selection criteria from the NVSS catalogue). This feature suggests the presence of a very dense medium surrounding them in a combination of a strong magnetic field.
Single-dish observations were performed with the 100-m Effelsberg telescope to characterise the SEDs of the sample and to well determine their RM in the 11 to 2 cm wavelength range.
Besides, a wideband (L, C and X band ) full polarisation observational campaign is running at the JVLA facility. It will allow us to analyse the in-band RM for the most extreme objects. VLBI observations have been scheduled as well, to study the polarisation and morphological properties at high angular resolution.
Effelsberg analysis and preliminary JVLA results will be presented. First considerations about the presence of several Faraday screens, intervening the radio wavelengths, will be discussed.


Michael Mattern (1)

Structure development in filamentary, molecular clouds

chair: A. Pasetto 

Molecular clouds provide the conditions of star formation. Recent observations show that filamentary structures are very common in both star-forming and quiescent clouds . These filaments are very likely to play an important role in star formation. Turbulence leads to density fluctuations. When the density reaches a critical value, gravity becomes the dominating force and the filament collapses into dense clumps. These are the seeds of star formation. But the evolution and fragmentation of filaments into dense clumps and prestellar cores is still not well understood. We studied the Nessie filamentary cloud in more detail and analyzed its fragmentation on different size scales. The resulting fragmentation curve follows for larger scales (> 1pc) the predictions of  a fragmentation model of a non-turbulent, infinite long cylinder. On smaller scales it is closer to the model of spherical Jeans' fragmentation.






Visit to Jülich 






Katharina Borm  (3) 

How to Constrain Cosmology? Using the eROSITA instrument as an example

chair: Michael Mattern 

One of the currently most driving questions in cosmology is the study of the nature of dark energy, whose characteristics are especially imprinted in the redshift and mass distribution of galaxy clusters. The up-coming X-ray instrument eROSITA, which is scheduled for launch in late 2016, is expected to observe around 100,00 clusters of galaxies up to redshifts of z 1.5. With this sample of clusters, the eROSITA instrument allows for very tight constraints on the characteristics of dark energy. In my talk, I give a brief summary of how cosmology and thus also the nature of dark energy can be studied with the help of galaxy clusters and I present forecasts for the cosmological constraints from the eROSITA-instrument. To do so, the halo mass function, which describes the distribution of galaxy clusters with mass and redshift, is introduced and I explain how we need to alter this function for eROSITA forecasts and for the anaylsis of future eROSITA data. These forecasts are based on Markov Chain Monte Carlo simulations and the routine Cosmo MC. Both of these utilities are illustrated in this talk and I present first preliminary results of my forecasts. 

Jason Wu (2)

Fermi-LAT guided pulsar survey

chair: K. Borm 

Pulsars are one of the main contributor of astronomical point sources in gamma-ray. A few years after the discovery of pulsars by radio astronomers, the Crab and Vela pulsars were detected at gamma-ray energies. Pulsars accelerate particles to tremendous energies in their magnetospheres. These particles are ultimately responsible for the gamma-ray emission seen from pulsars. Seven years after the launch of Fermi-LAT, over 150 gamma-ray pulsars has been found, unfortunately the number of the gamma-ray pulsars is still very small compared to the 2400 known pulsars in our Milky way. In my talk, I will present the latest gamma-ray blind search survey using the volunteer computing project Einstein at Home. Pulsar candidates and followup radio observation results will be presented. 












Fateme Kamali (1) & Monthly Discussion

Radio Continuum Study of water-Disk-Megamaser Galaxies

chair: Jason Wu

Detailed studies of the jet-accretion disk connection are essential for a better understanding of outflow formation and the launch of jets.
Water-disk-masers in the very central region of active galaxies provide a unique sample of targets which their Observations provide information on accretion disks surrounding super massive black holes. In this work we investigate the accretion disk – jet paradigm via combining the water maser observations at 22 GHz and radio continuum observations.
In today's seminar I will give a brief introduction to masers, then try to explain my project in a qualitative way that is easy for everyone to follow.






Maria Strandet (3) 

ALMA observations of Strongly Lensed Dusty Star Forming Galaxies selected from the South Pole Telescope survey

chair: F. Kamali

A population of rare and extremely bright mm-selected galaxies has been found with the South Pole Telescope (SPT) from a 2500 deg^2 survey at 1.4 mm and 2.0 mm. Follow up observations with HST, Herschel, ALMA and other ground based facilities have shown that these sources are strongly lensed high redshift dusty star forming galaxies (Hezaveh et al. 2013, Vieira et al. 2013).
In ALMA Cycle 0 a CO redshift search of a subsample of 28 sources showed a mean redshift of z=3.5 and a flat distribution between z=2-4 (Weiss et al. 1013). This mean redshift is significantly larger than those of radio identified submm galaxies (z=2.3, Chapman et al. 2005) and it seems that the SPT survey has uncovered the long missing high redshift tail of the submm galaxy population.
In ALMA Cycle 1 we searched for CO in a subsample of 15 sources and in this talk I will present the updated redshift distribution of our sample.
I will also discuss how our sample might be biased compared to other samples and show you that even when correcting for these biases we still see a higher mean redshift than other samples.






Denise Keller (3)

Tracing the clumpy shells around IRC+10216 with the VLA

chair: Maria Strandet

Asymptotic giant branch (AGB) stars are evolved red giants with strong mass loss. They provide dust and metal-enriched gas to the interstellar medium. The circumstellar outflow builds up a circumstellar envelope (CSE) which we trace through the emission of molecules, so AGB stars are fundamentally important to constrain the formation channels of molecules
in the Universe.

Due to its proximity (~130 pc), the carbon-rich and high-mass loss AGB star IRC+10216, also known as CW Leonis, is an exemplary object. About 80 molecules have been identified in its CSE, so it is is one of the most important chemical laboratories in astronomy. However, there is no global view on the morphology of its CSE on different scales. Carbon-bearing molecules in the CSE of IRC+10216 are distributed in clumpy shells similar to the dust which indicates a gas-dust coupling. It is not well understood to what extent these density-enhanced shells influence the chemistry in CSEs in general. Chemical models and observations are inconsistent.

I will talk about our new key dataset using the Karl G. Jansky Very Large Array (VLA). It consists of a spectral and imaging survey of IRC+10216 which covers 18–50 GHz with a sensitivity on a mJy level and a resolution of 1 arcsec. I perform the calibration and imaging synthesis with the data reduction program CASA. This survey traces the molecular distribution in unprecedented detail and strongly constrains the (mainly UV-induced) chemistry in these clumpy shells. I will present first results of identified spectral lines and example images of interesting molecules and will briefly discuss future work.

Elvijs Matrozis (2)

Post-mass-transfer evolution of Carbon-Enhanced Metal-Poor (CEMP) stars

chair: Denise Keller

About 20% of all metal-poor stars in the Galaxy are enriched in carbon, most of which also show enhanced s-process element abundances. Radial velocity measurements are consistent with all of these so-called CEMP-s stars being in binaries, suggesting that binarity factors in their abundance anomalies. Thus, the currently preferred model for explaining these anomalies is mass accretion from an asymptotic giant branch (AGB) companion. CEMP-s stars therefore bear the chemical imprint of nucleosynthesis in low-metallicity AGB stars, which play an important role in the chemical evolution of galaxies and stellar clusters. To infer the processes in the AGB star from observations of the secondary, the extent and nature of mixing of the accreted material in the secondary has to be understood. In this talk I will discuss some of the mixing mechanisms that may be operating in these CEMP-s stars and their consequences on the evolution of the stars' surface abundances.






Anna Mikler (1)

SZ and Galaxy Clusters: Contamination of Radio Sources on SZ detections

chair: Elvijs Matrozis

The first discovery of a galaxy cluster through SZ observation was made only on 2009. However since then hundreds of galaxy clusters have been found trough recent SZ surveys. These surveys allows us to identify in an unbiased way galaxy clusters, which are powerful tools for cosmological and dark energy studies. In this work we used data from the South Pole Telescope SZ Survey to study the effect of radio sources on the SZ detections at the center of the galaxy clusters from redshift 0.1 – 0.6. We model the radio sources on the galaxy clusters using the radio luminosity function and the isothermal β-model. The contamination, or effect of the radio sources on the SZ detections, is determine by comparing the flux of the radio sources and the flux of the SZ emission from the galaxy cluster. We present some preliminary results were ~15% of the galaxy clusters studied do not show any contamination and ~37% show a contamination of < 10%. Furthermore, we did not find any correlation between the contamination percentage and the redshift or the radius (R200) of the galaxy cluster.

Afterwards: Monthly Discussion 






Luca Grassitelli (3)

On the observational effects of turbulent pressure during the evolution of massive stars

chair: Ana Mikler

The major mass fraction of the envelope of hot luminous stars is radiatively stable. However, the partial ionization of hydrogen, helium and iron gives rise to extended sub-surface convection zones in all of them. We investigate the effect of the pressure induced by the turbulent motion in these zones and search for observable consequences. We find that the turbulent pressure fraction can amount up to 5% in OB supergiants, and to 30% in cooler supergiants. Although the evolutionary tracks are not significantly affected, a comparison between an extra-broadening mechanism influencing the spectral line shapes in OB stars  with  the  turbulent  pressure  fraction  obtained in corresponding stellar models reveals a strong correlation of both quantities. We discuss a possible physical connection, and conclude that turbulent pressure fluctuations may drive oscillations in the photosphere of hot luminous stars.

Gerrit  Schellenberger (3)

New results on X-ray cross calibration and cosmology with HIFLUGCS

chair: Luca Grassitelli

Cosmological constraints from clusters rely on accurate gravitational mass estimates, which strongly depend on cluster gas temperature measurements. Therefore, systematic calibration differences may result in biased, instrument-dependent cosmological constraints. I quantify in detail the systematic uncertainties between Chandra, XMM-Netwon and Suzaku, and show methods to identify the best calibrated instrument. Finally I will summarize the cosmology project using HIFLUGCS galaxy cluster, highlight some crucial aspects and point out the connection to the cross calibration work.






Ka Tat Wong (2)

Ammonia Emission from Oxygen-rich Circumstellar Envelopes

chair: Gerrit Schellenberger

One of the long-standing mystery in the circumstellar chemistry around oxygen-rich evolved stars is the abundance of ammonia (NH3) molecule. NH3 has been found to be dramatically overabundant in O-rich circumstellar envelopes (CSEs) as compared to thermo-equilibrium predictions by several orders of magnitude. In order to probe the physical conditions of NH3-emitting regions, multiple rotational and inversion transitions were observed towards CSEs of various O-rich evolved stars.

My work focuses on the observation and modelling of NH3 emission from four oxygen-rich evolved stars (IK Tau, OH 231.8+4.2, VY CMa and IRC+10420) which are among the best studied and most representative samples in their respective evolution stages. In my talk, I will present the spectra and maps of ammonia emission observed with the Herschel Space Observatory and the Very Large Array. I will also discuss the preliminary results from radiative transfer modelling with the code RATRAN. The modelling suggests that vibrationally excited energy levels have to be included, in addition to the vibrationally ground states, in order to derive consistent NH3 abundance
in the CSEs.












Monthly Discussion






Marina Berezina (3)

The High Time Resolution Universe survey

chair: Ka Tat Wong

I will report on the progress of the  High Time Resolution Universe survey for pulsars and fast transients in the northern hemisphere, being conducted with the 100-m Effelsberg telescope (Barr et al,  2013). Having unprecedentedly high time and frequency resolution, the survey is potentially sensitive to the most interesting pulsars in binary systems.  Up to now, we have observed about 30 % of the mid-latitude region ( i.e., galactic latitudes |b|< 15 deg) .  With our new  pipeline using GPUs,  it is possible to finish processing these data (thousands of TB!) within a year.  The survey so far resulted in the total number of 17 new discoveries.  Here I  present timing solutions for two newly discovered millisecond pulsars which might be a valuable addition to the pulsar timing arrays now being used for the detection of gravitational waves.






 no Seminar






 no Seminar






Behnam Javanmardi  (3)

Investigations towards a better cosmological model

chair: Marina Berezina

In my third IMPRS talk I will report on the two main projects of my PhD thesis. In the first project we developed a method to probe the isotropy of cosmic acceleration with type Ia Supernovae. By applying this method to the recent Union2.1 compilation, we found that though the anisotropy is small, the most discrepant direction is alined with the dipole of the cosmic microwave background at more than 95 percent confidence level. If this result is not due to a statistical fluke, it can either point towards some systematic issues with the data in that direction, or towards new physics. In the second (ongoing) project of my PhD, I am using long exposure optical images from small telescopes to search for dwarf satellite galaxies in the Local Volume. After calibrating our flux measurements, we search for low surface brightness objects, remove the foreground stars, and fit Sersic model to dwarf candidates to measure their properties. The final aim of this project is to test the correlation between the bulge-mass and the satellite-number of galaxies already observed in the Local Group.

Dominik Klaes (2)

Weak lensing analysis of galaxy clusters in the ATLAS and Kilo Degree Survey

chair: Behnam Javanmardi






Monthly Discussion






no seminar






no seminar






Cosmos Yeh (3)

Large-scale CO Observations on Star Forming regions in the Far Carina Arm

chair: Dominik Klaes

We present the first result of our recent CO survey observations targeted on the star forming regions/complexes in the Far Carina Arm, which covered more than 10.75 square degree of the sky in between l=280-300, b=-2-2. The CO(2-1) on-the-fly mapping data were taken form the Swedish Heterodyne Facility Instrument (SHeFI)/APEX-1 receiver on the APEX telescope, with an angular resolution of 27.27 arcsec at 230.538 GHz, and rms of 0.67K in 2 sec of integration for each spectrum. Star forming regions/complexes on the Far Carina Arm were identified by combining ATLASCAL compact source catalog with 13CO(2-1)/C18O(2-1) velocity measurements, and high-resolution infrared surveys from 8 µm to 870µm. The Star forming regions/complexes are further mapped in CO(2-1) to understand the link between nearby ATLASCAL compact sources, an to disentangle components in the same line-of-sight. When finished, the survey will covered 12.25 square degree of the sky, and become a complementary dataset of existing surveys in Carina area, such as Vela-Car, WISE, MSX, Hi-GAL, and ATLASGAL etc. From the first result by comparing Herschel image with CO(2-1) data cube, we found that the Herschel fluxes in star forming regions/complexes close to the tangent point reveal multiple component in the complementary CO(2-1) cubes, but behave relatively simple for star forming regions/complexes far away from the tangent point.






Naftali Kimani (3) 

chair: Cosmos Yeh

The Kinematics Study of M81-M82 Galaxies

The M81 Galaxy Group at a distance of 3.63Mpc is one of the nearest to our Milky Way. It has a fascinating M81-M82-NGC3077 trio-galaxy interaction that has a well defined interaction scenario. Multiwavelength observations and numerical simulations has been used in this group to obtain a comprehensive understanding of the dynamics and the consequence of tidal interactions in a group environment. However, Spatial Motion, a key parameter that has to been known for an accurate modelling of the history and evolution of this group has been a guessed estimate. As a contribution to the understanding of the history and evolution of this interacting system, I will briefly discuss the observationally derived Proper Motion of M81 and M82 galaxies.

Jens Erler (1)

chair: Naftali Kimani

Planck's view on the spectrum of the Sunyaev Zel'dovich Effect

The Sunyaev Zel'dovich Effect (SZE) is a secondary anisotropy of the cosmic microwave background caused by the interaction of CMB photons and the hot intra-cluster medium. Its unique properties enable studies of the pressure profile of galaxy clusters and its mass dependent signal allows to constrain cosmological parameters. With this years release of the full data products of the Planck mission one of the most largest SZE data sets became available and the wide frequency coverage of Planck allows to probe the spectrum the SZE for small deviations of its standard non-relativistic formulation. In my first IMPRS seminar talk, I will present common data reduction strategies for Planck SZE data as well as their extension to handle all-sky data. Furthermore, I will investigate consequences of using those data reduction techniques and ignoring relativistic corrections to the SZE and present two strategies to directly observe the relativistic SZE spectrum for the first time.






 Joey Martinez (1)

chair: Jens Erler

Title: Pulsar Searching and Timing with the Effelsberg and Arecibo Radio Telescopes

Pulsars have been used to study our galaxy, uncovering secrets that have helped scientists investigate theories of gravity, the interstellar medium, neutron star population, and binary system
dynamics. Even after discovering over ~2300 radio pulsars, many large scale surveys are still on the hunt for these exotic stars to help us further understand the laws that govern our universe. My project is to work on Effelsberg and Arecibo pulsar surveys to discover pulsars and exploit their scientific stories. I will talk about my findings and their current status as well as future work on the surveys.

Monthly Discussion



                       --- SUMMER BREAK ---









17-08-2015, 24-08-2015, 31-08-2015







Monthly Discussion






Marzieh Parsa (1)

chair: J. Martínez

Investigating stellar orbits near SMB

Sgr A* is surrounded by a small cluster of stars called S-stars. These stars and specially one important and bright star called S2, are prefect probes to search for effects of General Relativity caused by high mass of black hole and its effect on spacetime in its vicinity. Deviation from Keplerian orbit can occur as a result of relativistic correction to the orbital solution. This correction can show an advance in periapse after each orbit. Modeling the relativistic orbit of stars using astronomical data and radial velocity and therefore measuring the relativistic effects is of great interest.

Seungkyung Oh (3)

chair: M. Parsa

The dynamical ejections of massive stars from young star clusters

Most, if not all, stars form in a clustered environment. But a small fraction of massive stars are found in apparent isolation. Some of them are moving with a high velocity and are called runaways. One scenario for the origin of such stars is dynamical ejection: a star can obtain a high velocity via an energetic close encounter with other cluster members and then leaves its birth cluster. I have performed a large set of direct N-body calculations for young star clusters with diverse initial conditions, building a theoretical young star cluster library and have studied the dynamical ejection of massive stars from their parent cluster. I will present the works I have done with the library: (1) the influence of dynamical ejections of the initially most massive stars on the maximum-stellar-mass and cluster-mass relation, (2) the dependency of the O star ejection fraction on cluster mass, and (3) the effects of initial conditions on the dynamical ejection of massive stars. In particular, work (2) has led to the discovery of a peak in the ejection fraction of O stars at a young cluster mass of about 3200 Msun. In addition, it will be shown that very massive (> 100 Msun) single stars and binaries can be dynamically ejected from a young massive star cluster such as R136 in the Large Magellanic cloud, as case studies of VFTS 682 and R144.






no seminar (retreat)






no seminar






Monthly Discussion






Maja Kierdorf (1) 

chair: S. Oh

Probing the Magnetized Medium of AGNs and Galaxies using Wideband Polarimetry

In this talk, I will briefy summarize the plan for my PhD thesis. Wideband radio polarimetry is a powerful tool to reveal magnetic elds on dierent scales in dierent environments: in galaxies and in Active Galactic Nuclei (AGNs). I will introduce RM synthesis and QU tting which are excellent tools to interpret wideband polarization data. The aim of my thesis is to use new polarization data with a wide frequency coverage and these tools to map magnetic elds near and far in order to shed new light on the origin of cosmic magnetism. A perfect laboratory for the magnetic eld study provide the grand design spiral galaxy M51. To complete the study at several frequency ranges I observed this galaxy with the VLA at 2-4 GHz where currently no polarization data of M51 exists. Because of Faraday depolarization, data in this frequency range allow us to probe the disk-halo interaction region that has never been done so far to see how the magnetic eld structure behave in the layer between disk and halo of M51. The goal is to understand what is the origin of large scale magnetic elds in halos of galaxies and how do they are connected to the disk eld. Besides directly mapping the diuse polarized synchrotron emission of galaxies, I also observed polarized emission from unresolved point sources, selected from the NVSS catalog which seems to have very high fractional polarizations (> 30%). Except for the high degree of polarization, those sources appear to have normal AGN properties. Therefore, they could represent a class of sources with very well ordered magnetic elds. For this, I am conducting a deep, high resolution, broad band polarization survey of around 100 of those sources to make one more step towards understanding the origin of the unusual high degree of polarization. Understanding the intrinsic polarization properties of these objects are crucial since these unresolved point sources are used as background probe of intervening systems in these so-call RM grid experiments.

Tobias Roehser (3)

chair: M. Kierdorf

Searching For Molecular Intermediate-Velocity Clouds

Intermediate-Velocity Clouds (IVCs) are HI clouds located within the disk/halo interface of the Milky Way galaxy. They are thought to be related to a Galactic fountain process which is itself an important mechanism in the evolution of star-forming galaxies. IVCs are predominantly atomic with a small fraction of molecular hydrogen (H_2). So far only a few molecular IVCs (MIVCs) are known with a significant amount of H_2. MIVCs are identified by their bright far-infrared (FIR) emission which is in excess with respect to their environment. Based on correlation studies of the HI-FIR data from the Effelsberg-Bonn HI Survey, the Parkes-Galactic Allsky Survey and the Planck Satellite, we deduce the H_2 distribution towards large Galactic latitudes and search for MIVCs on Galactic scales. A total number of 206 MIVC candidates is inferred with a pronounced different abundance for Galactic hemispheres and for negative and positive radial velocities. The kinematics of the IVC gas shows the signature of the Galactic rotation in combination to a vertical in-fall. The distribution and properties of the MIVCs are compared to models of Galactic fountains and to ballistic orbits within the fountain cycle constraining the possible origins of the IVC sky.






Eleni Graikou (2)

chair: T. Roehser

High precision pulsar timing

Pulsars - a special kind of neutron starts, emits periodic -- or repeating -- bursts of radio waves, X-rays or gamma rays. Millisecond pulsars, a subset of the pulsar population, having a rotational stability compared to atomic clocks, are ideal for studying a wide range of physical and astrophysical phenomena, which allow us to investigate the interstellar medium and test theories of gravity. In this talk I will present how by using timing (the precise measurement of the exact time that the signal of pulsars arrives to Earth) we can increase the timing precision in order to direct detect gravitational waves, test general relativity by monitoring one of its predictions: geodetic precession, in the double neutron star system PSR B1913+16 and, finally, understand the evolution of the pulsar - white dwarf system PSR J1933-6211.





Abhijeet Borkar (3)

chair: E. Graikou

Monitoring the Galactic Center at 3mm

The supermassive black hole, Sagittarius A* (Sgr A*), at the centre of the Milky Way undergoes regular flaring activity which is thought to arise from the innermost region of the accretion flow.\@ We performed  the monitoring observations of the Galactic Centre to study the  flux-density variations at 3mm using the Australia Telescope Compact Array (ATCA) between 2010 and 2014. We obtain the light curves of Sgr  A* by subtracting the contributions from the extended emission around  it and the elevation and time dependent gains of the telescope. The observations detect five instances of significant variability in the flux density of Sgr A*, with variations between 0.5 to 1.0 Jy, which last for $1.5-3$ hours. We use the adiabatically expanding plasmon model to explain the short time-scale variations in the flux density. We derive the physical quantities of the modelled flare emission, which give a source expansion speed of $v_{\mathrm{exp}}$ ${\sim}$  $0.014-0.035$ $c$, source sizes of $\sim 1$ Schwarzschild radii,  spectral indices of $\alpha_{\mathrm{synch}} = 0.35 - 0.80$, with the  peak of the synchrotron radiation occurring at frequencies of few  
hundred GHz. These parameters imply that the expanding source components are either confined to the immediate vicinity of Sgr A* by contributing to the corona or the disc, or have a bulk motion greater than $v_{\mathrm{exp}}$. No exceptional flux density variation on short flare time-scales was observed during the approach and the flyby of the dusty S-cluster object (DSO alias G2). This is consistent with its compactness and the absence of a large bow shock.

I also present the observations of SiO maser sources observed in the central parsec of the GC. SgrA* was observed with two intermediate frequencies (IFs) centered at 86.243 GHz and 85.640 GHz corresponding to the two rotational transition lines of the SiO molecule with 2 GHz bandwidth each, and 1 MHz frequency resolution, which corresponds to 3.477 $kms^{-1}$ velocity resolution. Our spatial resolution is limited by the available baselines with best resolution of $0.2 \mathrm{arcsec}$. These are the most comprehensive observations of the central parsec of the GC at 3mm, with wide band that allow us to investigate high velocity stars. I present the method to detect the maser sources. In total, 11 sources were detected, of which 8 are previously known sources, like: IRS 1W, IRS 2L, IRS 7, IRS 9, IRS 10EE, IRS 12N, IRS 28 and IRS 34. Three new sources were detected. I present the method to calculate the accurate positions and proper motions of the maser sources. The proper motions of strong sources IRS 7 \& IRS 12N are calculated precisely and are in agreement with previous results. Comparative study of the relative strength of the  
SiO transition lines indicates that the 86.243 GHz line is stronger than the 85.640 GHz line. Among the detected stars, 3 are cool stars, 2 HE I stars, 1 AGB star and 1 red giant. I also present an upper limit on the detection of several maser sources which have been detected in previous studies but were not detected in my dataset. This is most likely due to the strong variability of the maser emission.











 Monthly Discussion












Won Ju Kim (2)

chair: tbd

Hydrogen Recombination Line Surveys toward ATLASGAL Sources

In today talk, I would like to show millimetre hydrogen recombination line surveys.
We have carried out first large millimetre hydrogen recombination lines (mm HRLs) surveys toward 991 ATLASGAL sources using IRAM 30 m and MOPRA 21m telescopes. These 991 ATLASGAL sources were only selected by 870 micro m dust continuum emission. It means that we do not know what kind of evolutionary stages of massive star formation they belong to. The recombination line surveys are therefore useful to find embedded HII region stage in the molecular cloud traced by 870 micro m dust continuum emission. From these surveys, we detected 177 mm HRLs source which they were considered to trace different condition HII regions.
In addition, we also found that the ATLASGAL sources associated with mm HRLs showed to be turbulent when compare to H13CO+(1-0) line widths. Also, that the mm HRLs flux show to be correlated with 6cm radio continuum flux seems the mm HRLs origin sources are associated with 6cm radio continuum emission sources.
The results of the surveys will be useful for statistic study and to give us basic understanding recombination lines and HII regions in massive star formation.






Dhanya G. Nair (2)

chair: W. Kim

86 GHz VLBI survey of Ultra compact radio emission in Active Galactic Nuclei

Very Long Baseline Interferometry (VLBI) Observations at 86 GHz reach a resolution of about 50 microarcseconds and sample the scales as small as 10^3 - 10^4 Schwartzchild radii of the central black hole in Active Galactic Nuclei (AGN), and uncover the jet regions where accelaration and collimation of the relativistic flow takes place. Synchrotron radiation becomes optically thin at millimetre wavelengths ; making it possible to look deeper into the core and inner jets of AGN which is invisible at cm and longer wavelengths due to self absorption or free-free absorption by the torus. In today's talk,I will present results from a large global VLBI survey of 168 ultracompact radio sources at 86 GHz conducted in 2010 - 2011. All the sources were detected and imaged ; increasing by a factor of ~ 2 the total number of AGN imaged with VLBI at 86 GHz. We use gaussian modelfitting to represent the structure of the observed sources. The modelfitting yields estimates of the brightness temperature (Tb) of the VLBI bright core (base) of the jet and inner jet components of AGN, taking into account the resolution limits of the data at 3mm. I will present the brightness temperature of the VLBI cores from this 86 GHz survey with similar estimates from the MOJAVE VLBI surveys at 15 GHz and compare the brightness temperatures obtained from the model fits with estimates of the brightness temperature limits made directly from the visibility data. For objects with sufficient structural detail detected, we investigate the effect of adiabatic energy losses on the evolution of brightness temperature along the jet.

Sandra Unruh (1)

chair: D. Nair

Generalized shear ratio tests in weak gravitational lensing

In this talk I will give a brief overview about shear ratio tests and their potential in determining cosmological parameters with weak gravitational lensing.

Nowadays, exploring the nature dark energy has become a major research field in physics. By analyzing the large-scale structure of our Universe it is possible to infer whether the dark energy density is constant or a function of time. The potentially most powerful method to probe cosmological parameters uses weak gravitational lensing and its distortion of background images of distant galaxies. Future surveys, like ESA’s space-based mission Euclid, will cover about one-third of sky and detect an unparalleled number of 2 billion galaxies at high redshift. However, the full statistical potential that these large data sets offer is tainted by possible systematic uncertainties far higher than the expected level of statistical uncertainty. The trade-off between redshift uncertainties and the poorly understood multiplicative bias poses the main challenge in shear measurements. Schneider et al. (in prep.) proposed a generalized shear ratio test to tackle these two major problems separately. This test does not depend on any assumptions but a metric theory in our Universe and a statistically isotropic Universe. First results from a maximum-likelihood analysis indicate relative changes in the multiplicative shear bias can be detected at the 1% level (1σ confidence).






Sac Medina (1) and Monthly Discussion






Natalya Poroyka (1)

chair: S. Medina

Investigation of interstellar medium (ISM) with LOFAR

LOFAR (or low frequency array) is group of phase antennas spread over the Europe, six
of which are located in Germany. The LOFAR sensitivity band covers the frequency range from almost ionosphere cut-off frequency up to 240 MHz.On this particularly low frequencies pulsars' radiation is strongly affected by ionized and magnified interstellar medium through dispersive effect, scattering and Faraday rotation. That means that with LOFAR data we will be able to determine RM (rotation measure) and DM (dispersion measure) with higher accuracy than using high frequency data.
During weekends German LOFAR stations are exploited locally to conduct the observations of ~100 pulsars. The longest timespan of observations is 3 years with approximately 1-week cadence. Using the data of highly polarized pulsars from German stations we will investigate the RM variation as the function of time due to proper motion of the pulsar along the sky. Found variations are believed to show red noise features in frequency spectrum due to red colored energetic scale spectrum of the medium inhomogeneities predicted by different regimes of turbulent medium. Some results on automation of data processing and ionospheric corrections have been made.
















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