Recent Talks

With the advent of GPU accelerators the landscape of High Performance Computing has started to change rapidly. While this is in principle good news, the increased compute power comes with a steep price tag in that new languages (CUDA, OpenCL) must be used. Recently Intel has announced their own coprocessor Many Integrated Cores (MIC) technology which will deliver competitive performance but will be programmed through familiar languages (Fortran, C/C++ and OpenMP). In my talk I will introduce Intel's MIC architecture and will discuss the ongoing efforts at the Texas Advanced Computing Center to build a 10 PetaFlop cluster with MIC coprocessors in early 2013. Coprocessors (MIC) and accelerators (GPU) are here to stay and the changing hardware will spur considerable changes in general software design. Astrophysics codes of all varieties (for example highly parallel simulations, data-intensive software pipelines for large surveys, and even data reduction software on desktops) will have to adapt to the new environment. I will discuss software design, performance considerations, and optimizations in general and specifically with respect to the MIC technology. In the second part of my talk I will introduce the software package ASSET (Advanced Spectral Synthesis 3D Tool) that allows for the fast and efficient calculation of spectra from 3D hydrodynamical models and will highlight recent projects that have employed high-resolution (> 1,000,000), wide-range (1000's of Angstroem) synthetic spectra derived from 3D radiation transfer.

Solar magnetism may look deceptively boring (a rather common star with relatively low activity). As it turns out, even the most quiet areas of the Sun (away from the sunspots) harbour a rich and interesting magnetic activity which is extremely complex and dynamic at spatial scales as small as ~100 km. And more importantly, this magnetism permeates most of the Sun, all the time. Therefore, it is not surprising that it might play an important role for solving some longstanding questions of stellar magnetism as: how is the million degree corona maintained when all sunspots have disappeared during the minimum of magnetic activity? And this is of interest not only for solar physics but for stellar astrophysics too, since it is expected that every star with a convective envelope harbours small-scale magnetic activity that we cannot hope to observe with the great detail we observe it in the Sun. From the first evidence of the presence of magnetic fields in the quiet areas of the Sun to the discovery of the smallest organised magnetic structures ever observed in a stellar surface just 30 years have passed. In this seminar, I will give an overview of our present knowledge about the small-scale quiet Sun magnetism. In particular, I will show how small loops of sizes of several hundreds of kilometers appear in the surface and travel across the solar atmosphere, reaching upper layers and having direct implications on chromospheric (coronal) magnetism. I will also show some of the properties of these newly discovered magnetic structures such as their spatial distribution, a key ingredient for understanding their origin.

The mechanism by which AGN activity is triggered has long been debated. One, often suggested, method of doing so is major, gas-rich mergers and galaxy interactions. I will present deep Gemini GMOS-S images of a sample of type II quasar host galaxies, demonstrating that 75% show clear signs that they are undergoing some kind of interaction. We compare these results with a control sample of quiescent early-type galaxies and find a similar rate of interaction (68%). However, we also find that the surface brightness of the features of the type II quasars are up to 2 mags brighter than those for the control sample, suggesting a difference in the types of mergers that the two groups are undergoing. We also compare our results with those for a sample of powerful radio galaxies and find very similar values for the surface brightness of the detected features.

The status of instrumentation at GTC and other ORM/OT telescopes will be reviewed. A short introduction regarding the different ways to access telescope time (normal call for proposals, service nights and DDT) will also be given. The aim of this talk is to help preparing observing proposals for the coming semester 12B.

Global warming has often been portrayed as being connected only to greenhouse gasses in widespread media. However, these are just one of many factors influencing Earth’s climate. Over long timescales the Sun has been the major force driving climate changes. So-called global warming skeptics often use arguments of natural (solar driven) climate changes to argue that anthropogenic influences on the climate over last century have been largely overestimated. These arguments frequently involve hypothesized solar – climate linkages, for which there is a low level of scientific understanding, making the arguments problematic to easily prove or refute. There are three solar parameters proposed which may influence the Earth’s climate: total solar irradiance (TSI), ultraviolet (UV) spectral irradiance, or the galactic cosmic ray (GCR) flux. In recent years there has been a vigorous debate in scientific community regarding the notion of a cosmic rays influence on clouds cover. If true, such a link could have serious implications for our understanding of climate change: consequently, this has become one of the most frequent arguments of global warming skeptics. This talk will give a short overview of different forcing factors in the climate system, give a description of some hypothesized mechanisms linking solar activity to Earth’s climate, and present our current work aiming to resolve the hypothesized link between cosmic rays and clouds.

We present the completion of a program to cross-correlate the SDSS Data Release 1 and 2MASS Point Source Catalog in search for extremely red L and T dwarfs. The program was initiated by Metchev and collaborators, who presented the findings on all newly identified T dwarfs in SDSS DR1, and estimated the space density of isolated T0--T8 dwarfs in the solar neighbourhood. In the current work we present most of the L dwarf discoveries. Our red-sensitive (z-J > 2.75 mag) cross-match proves to be efficient in detecting peculiarly red L dwarfs, adding two new ones, including one of the reddest known L dwarfs. Our search also nets a new peculiarly blue L7 dwarf and, surprisingly, two M8 dwarfs. We further broaden our analysis to detect unresolved binary L or T dwarfs through spectral template fitting to all L and T dwarfs presented here and in the earlier work by Metchev and collaborators. We identify nine probable binaries, six of which are new and eight harbour likely T dwarf secondaries. We combine this result with current knowledge of the mass ratio distribution and frequency of substellar companions to estimate an overall space density of 0.005--0.05 pc^{-3} for individual T0--T8 dwarfs.

Galaxy groups have an important role in the hierarchical assembly of structures in the Universe. Since galaxy groups are much more massive than galaxy-scale halos and are concentrated enough, they can act as lenses. The study of dark matter profiles can be very successfully using group-scale lenses, being that galaxy groups are quite abundant compared to galaxy clusters, and are easy to model. However, these have the disadvantage of the lack of constraints. In this talk, I will show how is possible to constrain the scale radius of the NFW profile using the velocity dispersion of the galaxy group. In particular I will present the results obtained with SL2SJ 02140-0535, a group which belongs to the Strong Lensing Legacy Survey - ARCS (SARCS) sample compiled from the final T0006 data release of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS).

Luminous Infrared Galaxies (LIR=10^11-10^12Lsun) have star formation rates in the range of ~20-200Msun/yr. In the local Universe ~50% LIRGs show AGN or AGN/SB composite nuclear activity from optical spectroscopy. We decompose Spitzer/IRS 5-35micron spectra of a complete sample of 50 local (d<75Mpc) LIRGs using SB and AGN clumpy torus model templates. We derive a mid-IR AGN detection rate in our sample of local LIRGs of 50%. We also compare the continuum mid-IR AGN detection with other indicators in the mid-IR, optical and X-rays. We estimate for the first time the AGN bolometric contribution to the IR luminosity of the galaxies in local LIRGs. We find that one-third of local LIRGs have LAGN(bol)/LIR>0.05, with only ~10% having a significant contribution LAGN(bol)/LIR>0.25. This is in line with results of Nardini et al. (2010) that only at LIR>3x10^12Lsun the AGN starts dominating bolometrically the IR luminosity in the majority of the systems.

I will present new deep and wide narrow-band surveys undertaken with UKIRT, Subaru and the VLT; a unique combined effort to select large, robust samples of H-alpha (Ha) emitters at z=0.40, 0.84, 1.47 and 2.23 (corresponding to look-back times of 4.2, 7.0, 9.2 and 10.6 Gyrs) in a uniform manner over ~2 sqdeg in the COSMOS and UDS fields. The deep multi-epoch Ha surveys are sensitive to Milky-Way SFRs out to z=2.2 for the first time, while the wide area and the coverage over two independent fields allows to greatly overcome cosmic variance. A total of over 600 sources per epoch are homogeneously selected. Overall, the evolution seen in Ha is in good agreement with the evolution seen using inhomogeneous compilations of other tracers of star formation, such as FIR and UV, jointly pointing towards the bulk of the evolution in the last 11 Gyrs being driven by a strong luminosity/SFR increase from z~0 to z~2.2. Our uniform analysis allows to derive the Ha star formation history of the Universe, for which a simple time-parametrisation is a good approximation for the last 11Gyrs. Both the shape and normalisation of the Ha star formation history are consistent with the measurements of the stellar mass density growth, confirming that our Ha analysis traces the bulk of the formation of stars in the Universe up to z~2.2. We are also exploring the large, multi-epoch and homogeneously selected samples of Ha emitters to conduct detailed morphology, dust, clustering, environment and mass studies which are providing us with a unique view on the evolution of star-forming galaxies and what has been driving it for the past 11 Gyrs.