August-September 2015: summer break

July 2015

July 24: GMC Populations of Nearby Galaxies - Annie Hughes, Toulouse

The observed ~kpc scaling relations between molecular gas and star formation in galaxies must ultimately be due to highly localised star formation activity that is occurring within individual giant molecular clouds (GMCs). In this talk, I will summarise what we have learnt about the dynamical properties and evolution of GMCs from recent high resolution surveys of CO emission in nearby galaxies (M51, NGC628, LMC), which provide a unique view of the relationship between star-forming clouds and their host galaxy. I will also present some of our ongoing work to characterise the connection between molecular gas and star formation on cloud- to galactic scales in these systems.

July 17: CAS bbq - no talk

July 10: Establishing super- and sub-Chandrasekhar limiting mass white dwarfs to explain peculiar type Ia supernovae - Upasana Das, Indian Institute of Science

Type Ia supernovae (SNeIa), a key to unravel the evolutionary history of the universe, are extremely bright thermonuclear explosions. They are believed to be triggered in carbon-oxygen white dwarfs having mass close to their maximum possible value of about 1.44 solar mass, which is known as the famous Chandrasekhar limit. However, observations of several peculiar, highly over- and under-luminous SNeIa do not conform to this conventional picture and argue for exploding masses widely different from the Chandrasekhar limit. The over-luminous SNeIa seem to invoke super-Chandrasekhar white dwarf progenitors, having mass 2.1-2.8 solar mass. While, the under-luminous SNeIa seem to favor sub-Chandrasekhar explosion scenarios. In our venture to obtain a fundamental basis behind the formation of such super-Chandrasekhar white dwarfs, we have exploited the enormous potential of magnetic fields, which can affect the structure and properties of the underlying white dwarf in a variety of ways. We have progressed from a simplistic to more rigorous and self-consistent models. In this talk I will try to give a brief overview of our results, with emphasis on the latest results obtained from an extensive GRMHD numerical formulation, whereby we have constructed stable equilibrium models of strongly magnetized, static, non-spherical white dwarfs. Very interestingly, our study establishes that strongly magnetized white dwarfs can be significantly super-Chandrasekhar, having mass 1.7-3.4 solar mass, irrespective of the nature of origin of the underlying magnetic effect. On a different note, we have also explored the effect of modification to Einstein’s gravity in white dwarfs, for the first time in the literature to the best of our knowledge. I will try to briefly motivate how this can lead to significantly super- as well as sub-Chandrasekhar limiting mass white dwarfs, determined by a single model parameter. Explosions of these white dwarfs can explain both the peculiar, over- and under-luminous SNeIa respectively, thus unifying these two apparently disjoint sub-classes of SNeIa.

July 3: The HIX galaxy survey - How spirals accrete gas and form stars - Katharina Lutz, Swinburne

When comparing the gas content of galaxies with their current star formation rate, it has been found that the gas consumption time scale is much smaller than the age of galaxies. This discrepancy leads to the conclusion that galaxies need to replenish their gas reservoirs to sustain star formation. In order to investigate this process of gas replenishment in more detail we target galaxies that contain at least 2.5 times more atomic hydrogen (HI) than expected from their optical properties using scaling relations. For this set of galaxies, we are building a rich data set consisting of deep HI interferometry (Australia Telescope Compact Array), optical integral field spectroscopy (WiFeS spectrograph on the SSO 2.3m telescope), deep imaging (DECam) and publicly available photometry from GALEX (ultraviolet), WISE (infrared) and DSS-II (optical). This data set will enable us to distinguish between multiple scenarios that might lead to an excess in HI content, among them a phase of elevated gas accretion, minor mergers or an inefficient conversion of gas into stars. In a next step it allows us to investigate the respective scenario in more detail. In my talk I will first introduce the survey, then compare the HI excess galaxies to the general galaxy population with respect to star formation and stellar mass and finally present first results of the more detailed analysis of the ATCA HI data combined with the optical IFU spectroscopy.

June 2015

June 26: Where is Sgr A* (precisely) - Philipp Plewa

Near-infrared observations of stellar orbits at the Galactic Center provide conclusive evidence for a massive black hole associated with the compact radio source Sgr A*. In this talk I will explain how to construct a precise and stable (infrared) astrometric reference frame for these observations, in which (radio-)Sgr A* is localized to within a factor five better than previously. This improvement is mainly the result of modeling and correcting optical distortion in the NACO imager, but also other methods that unlock a new level of high-precision astrometry using our existing, decade-spanning data set. A further improvement will follow future observations and facilitate the detection of relativistic orbital effects. More immediately, we will be able to measure the orbits of even more stars and make refined estimates of the black hole’s mass and distance.

June 19: Obscuring gas and dust structures in nearby galactic nuclei - Marc Schartmann, Swinburne

A summary of our recent and ongoing work on the distribution of gas and dust in the nuclei of nearby galaxies will be given. 
I will focus on our latest work and present time-resolved images and spectral energy distributions (SEDs) of geometrically thick obscuring structures which are thought to enshroud the central engines of nearby Seyfert nuclei. The 3D dust continuum radiative transfer simulations are based on the radiation-driven dynamical model for the central obscurer by Wada (2012). When comparing the results to high spatial resolution SEDs as well as observed relations for nearby Seyfert galaxies, we find that a model which comprises of a clear three-component structure gives the best comparison with the data: a thin disc with spiral and filamentary high density features, a surrounding fluffy component (the obscurer) and a low density outflow along the rotation axis. Strong differences are found depending on wavelength: whereas the mid-infrared images are dominated by the elongated appearance of the outflow cone (reminiscent of recent interferometric observations), the long wavelength emission is mainly given by the cold and dense disc component. I will end my talk by giving a short outlook on our preliminary work on understanding the distribution and dynamics of gas and dust in a sample of nearby radio galaxies.

June 12: no talk

June 5: no talk

May 2015

May 29: ([CII]) intensity mapping - Dieter Lutz

Given a recent surge of papers discussing the potential of [CII] and CO intensity mapping of the high-z universe up to reionisation, I plan to give a journal-club like overview of those works.

May 22: no talk

May 15: A new view of the torus: a story of optical obscuration and x-ray absorption - Ric Davies

May 8: no talk

May 1: holiday

April 2015

April 24: The Nature of [CII] emission in Lensed Dusty Star-forming Galaxies from the SPT survey - Bitten Gullberg, ESO

ALMA spectroscopy (cycle 0 and 1) of point sources from the South Pole Telescope survey has uncovered a population of high-redshift (z = 2-5.7), strongly lensed dusty star-forming galaxies (DSFGs). This has resulted in an unbiased redshift distribution for DSFGs peaking for z~3.5, i.e. higher than previously believed of z~2.5, and doubled the number of sources at z > 4. In this talk I will present the latest result from our fine-structure line survey of 20 DSFGs. Comparing [CII] velocity profiles (APEX and Herschel) with CO velocity profiles from ALMA reveals consistent velocity profiles, suggesting little differential lensing between these species. 
Combining the [CII] detections with low-J CO detections (ATCA), we find [CII]/CO(1--0) luminosity ratios of 5200+/-1800, and argue that this line ratio is best described by [CII] and CO emitting gas with higher [CII] than CO excitation temperature,  high CO optical depth (tau>>1), and low to moderate [CII] optical depth (tau<1). The geometric structure of photodissociation regions (PDRs) allows for such conditions.

April 17: no talk

April 10: The role of disk instabilities and galaxy interactions in triggering AGN activity - Marco Gatti, Rome

At present, it has become widely accepted that Active Galactic Nuclei (AGN) are powered by short and repetitive accretion episodes onto Super Massive Black Holes (SMBHs) and that their evolution is tightly correlated with that of their host galaxies. However, understanding what triggers AGN activity producing at the same time the observed co-evolution remains one of the long-standing questions in astrophysics. A detailed statistical study about the role of different AGN triggering mechanisms can be performed using a state-of-the-art semi analytic model (SAM) for galaxy formation. In this talk I will discuss the effects of assuming different mechanisms for triggering AGN activity on several AGN and host galaxy properties (e.g. AGN luminosity function, Eddington ratio distribution, AGN 2PCF, host galaxy SSFRs). Two accretion modes will be considered: a first mode where AGN activity is triggered by disk instabilities in isolated galaxies, and a second mode where the mass inflow onto the central SMBH is induced by galaxy mergers and fly-by events (interacti

April 1: Graph theory and Molecular Gas Clusters - Dario Colombo, University of Alberta

In the present Universe, all stars born in cold clouds of molecular gas which inner and outer physical phenomena play a key role to set the star formation capabilities of the galaxies. The study of a molecular-dominated spiral galaxy as M51 has underlined the importance of the ISM clump characterization to provide fundamental insight within the physics involved into the process of star formation. In the same way, however, it challenged the performance of the most advanced cloud identification method to date, indicating the need for new, more powerful tools. 

Some of the limitations of commonly used algorithms can be overcome by considering the cloud segmentation problem in the broad framework of the graph theory.  Additionally, the clustering analysis provides a natural and robust mathematical description of the molecular ISM discrete features that might be viewed as “Molecular Gas Clusters”.

In particular, the algorithm we designed (SCIMES - Spectral Clustering for Molecular Emission Segmentation) applies the spectral clustering approach to look for relevant objects within topological graphs of emission (dendrograms) from star-forming clouds. SCIMES appears especially useful for the cloud identification within complex molecular emission data cubes since, in contrast to other algorithms, it does not over-divide structures, faithfully reproducing the work of the human eyes.

Moreover, SCIMES introduces a new philosophy in the identification of the molecular clouds, where virtually every property of the molecular emission might be used for the ISM segmentation. This may be helpful for distinguishing between the dominant physical mechanisms responsible for the formation of those molecular clusters.

March 2015

March 27: updates on LUCI - Peter Buschkamp

March 20: Astrophysical Levi Flights - Re'em Sari, the Hebrew University of Jerusalem

Levy flights are a form of random walk, where the step size is drawn from a distribution with infinite second moment. They appear quite ubiquitously in nature, like in the motion of Spider Monkeys or Albatrosses in their search for food, as well as in the stock market. We discuss their implication to astrophysical situations including eccentricity of planetesimals, eccentricities of binaries, formation of the Oort Cloud, collisional spins of asteroids, and perhaps astrometry and photometry of IR sources in the galactic center.

March 13: The incidence of kpc-scale outflows and conditions for star formation in luminous unobscured QSOs - Bernd Husemann, ESO

I will highlight the advantages to study the QSO-host galaxy connection with optical IFU
spectroscopy based on more than 50 luminous radio-quiet unobscured QSO (z<0.3). In particular I
will highlight the difficulty and our solution to deal with the beam smearing of the bright nucleus.
We find a much lower incidence of large-scale outflows which is in contradiction with
recent studies of luminous unobscured and obscured QSOs. Part of this can be explained by
ignoring seeing. We also find that our QSO host galaxies are consistent with conditions
for star formation in normal galaxies on the star forming main sequence.
Interestingly, we recover a correlation between bolometric AGN luminosity and molecular gas
mass for disc-dominated host, while elliptical hosts have a higher bolometric AGN luminosity
at a given gas mass. At the end I am going to briefly introduce the Close AGN Reference Survey (CARS)
which is a spatially-resolved multi-wavelength survey of ~40 luminous broad-line AGN at
0.01<z<0.06 with VLT-MUSE combined with other facilities.

March 6: proposal writing workshop

February 2015

February 27: What drives the intense star formation of high-z, massive, star-forming galaxies? - Matthieu Béthermin, ESO

Deep Spitzer and Herschel surveys revealed the important contribution of ULIRGs (SFR>100 Msun/yr)
to the star formation history at z>2. Standard theoretical models and numerical simulations cannot
reproduce their high number density easily. This intense star formation could be explained by
large gas reservoirs fed by a strong accretion of cold gas or by a higher star formation efficiency
caused by major mergers? I will present results from statistical advocating for that the first hypothesis.

Using a stacking analysis of mid-IR-to-mm data, we measured the evolution of the dust and gas content
of massive galaxies up to z=4. In average, they lie on the sequence of local spirals in the integrated
Schmidt-Kennicutt diagram, and their high SFR is explained by their large gas fraction (~60% at z=4).
The extreme starbursts (defined as being 10 times above the main-sequence) have similar gas fractions,
but much higher star-formation efficiencies.

The clustering of these objects provides interesting insights about the nature of their host dark matter
structures and the origin of these gas reservoirs. Both ~3x1010 Msun starbursts and main-sequence galaxies
are hosted in dark matter halos of few 1012 Msun. The accretion of baryons on these halos is sufficient
to refill the gas consumed by the star formation in main-sequence galaxies, but not in starbursts that
can maintain their SFR only during <100 Myr. The most massive (>1011 Msun) main-sequence galaxies are
hosted by group mass halos (>1013 Msun), progenitors of today’s clusters.

I will finally discuss the selection biases induced by the galaxy surveys using the Béthermin et al. (2012)
model of galaxy evolution. I will especially focus on the South Pole Telescope (SPT) sample of lensed
high-redshift galaxies. This sample is expected to contain mainly z~3.5 gas-rich main-sequence galaxies,
and opens interesting opportunity to study in detail the physics of these objects.

February 20: The HI and H2 content and sub-mm emission of galaxies over cosmic time: a semi-analytic and semi-empirical approach - Gergely Popping, ESO

The star-formation activity of our Universe increased from early epochs (z~6), peaked around z=2, and then decreased by an order of magnitude until present age. To fully appreciate the physical origin of the star-formation activity of our Universe we need to focus on the gas content of galaxies over cosmic time. The most recent versions of cosmological models of galaxy formation explicitly include the detailed tracking of the atomic and molecular hydrogen content of galaxies and make predictions for the sub-mm line emission from galaxies. New semi-empirical approaches provide data-driven predictions for the atomic and molecular gas content of galaxies. I will discuss the predictions made by these different types of models for the HI and H2 content and sub-mm line emission of galaxies. These predictions include a weak evolution in the HI content and HI mass function of galaxies, strong evolution in the H2 content of galaxies, the weak evolution in the cosmic density of HI,  CO SLEDs of galaxies over cosmic time, and predictions for CO luminosity functions. I will compare these predictions to current observational samples, discuss future observing strategies, and will also demonstrate how the combination of cosmological and semi- empirical models can help to reveal caveats in our understanding of galaxy formation.

February 13: no talk

February 6: The Galactic Center cloud G2 and its gas streamer - Oliver Pfuhl

I will discuss the latest observations of of the gas cloud G2 in the Galactic Center, from late 2013 and 2014. The cloud has reached its minimum distance to the MBH at 1950 Schwarzschild radii in July 2014. At this point roughly half of the gas is found at the redshifted, pre-pericenter side of the orbit, while the other half is at the post-pericenter, blueshifted side. Last years deep observations revealed a long stream of gas, which is following the path of G2. Furthermore we (re-)discovered a precursor cloud, named  G1. This cloud was first described a decade ago based on L′-band images when it was spatially almost coincident with Sgr A∗. The orientation of the G1 orbit in the three angles is almost identical to that of G2, although at somewhat lower eccentricity and smaller semi-major axis. We could show that the observed astrometric positions and radial velocities of G1 are compatible with the G2 orbit, assuming that (1) G1 was originally on the G2 orbit preceding G2 by 13 yr, and (2) a simple drag force acted on it during pericenter passage. Taken together with the previously described tail of G2, which we detect in recombination line emission and thermal broadband emission, we propose that G2 may be a bright knot in a much more extensive gas streamer. This matches purely gaseous models for G2, such as a stellar wind clump or the tidal debris from a partial disruption of a star.

January 2015

January 30: Does the Dense Gas Mass set the Star Formation Rate of a Galaxy? - Andreas Schruba

Stars form in the dense interstellar medium. Observations of HCN, a tracer of dense gas, in Milky Way cloud cores and entire (U)LIRG galaxies suggest a constant ratio of current star formation rate to HCN intensity which is interpreted by a density threshold for star formation with fixed star formation efficiency. I summarize results of a recent survey of HCN emission utilizing the IRAM 30m that targets the little-explored regime of normal star-forming disk galaxies. While our observations confirm the common picture that the dense gas fraction increases towards the galaxy centers, they reveal systematic variations in the SFR-to-HCN ratio. Under the assumption of a constant HCN-to-dense gas conversion factor these observation are not conform with the "density threshold" model. However, our knowledge on this conversion factors is still poor. Therefore, we analyze the range of (variable) conversion factors which are required for the density threshold model to hold. We also analyze another popular model of star formation in which the properties of entire molecular clouds regulate the star formation efficiency which can match our observations more naturally.

January 23: Test and Characterization of the GRAVITY Laser Metrology Injection - Johannes Weber (Master's thesis defense)

We investigated the laser metrology injection of GRAVITY, a second generation four-way beam combiner instrument for the ESO Very Large Telescope Interferometer (VLTI). This has been characterized in previous studies, but interference effects within the detector have limited the accuracy of these measurements to several ten nm. The goal of my master thesis, to develop a new technique to overcome these fringing effects and to characterize the metrology injection was achieved with a Fizeau-type interferometer, which allows separating the interference from the metrology injection and the detector fringing in Fourier domain. We will demonstrate that we can overcome residual errors from spectral leakage and thermal induced camera motion with an optimized data analysis, resulting in a closure phase accuracy of better than 1 nm. Subsequently we will present the results of the stability and sensitivity tests of the two different metrology injection designs and present our new model for the laser power induced path length variations. Finally we will draw conclusions concerning GRAVITY's metrology injection.

January 16: Does the NGC 1068 CO line SED constrain the amount/position of the X-ray obscuring gas? - Annemieke Janssen

NGC 1068 is  a nearby, compton thick, Seyfert 2  galaxy, so the AGN is obscured from our view by a gas column of 10^24 cm ^-2 or more. This obscuring gas can lie anywhere between a fraction of a parsec and tens of parsecs away from the AGN. Depending on its position and its total column density, the gas may be molecular and could possibly be observed in high J CO lines.

We observed NGC 1068 with PACS and retrieved the CO line SED up to J=30, while a deeper observation of CO(40-39) resulted in an upper limit of 2e-17 Wm^-2. We use the CO SED and an XDR code developed in our group by Simon Bruderer, to find the answer to 2 questions:
1) Does the upper limit on CO(40-39) constrain the position, density and amount of the X-ray obscuring gas?
2) Could the 'Highly Excited component' (which peaks around J=25) be attributed to the X-ray obscuring gas?

January 9: no talk

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