Projects completed in Spring 2019 Semester
Proposed by: Cameron Lemon (Postdoc) and Frédéric Courbin (Faculty)
Type of Project: Master/CSE project
Project flavour: Observation/Data Science/Cosmology
Abstract: Over 40 years after the discovery of the first gravitationally lensed quasar, only ~50 systems useful for time-delay cosmography have been found, and only two lensed supernovae have been discovered. Recent all-sky, multi-epoch, optical surveys now allow variability to be a selection tool for quasars and supernovae. This project will use difference imaging of these datasets (e.g. Pan-STARRS and DES) to identify extended variable sources as candidates for lensed quasars and lensed supernovae. Candidate selection will be aided with machine learning (convolutional neural nets) with the possibility of obtaining and analysing spectroscopy of the most promising candidates.
URL/References:
– https://ui.adsabs.harvard.edu/abs/2006ApJ…637L..73K/abstract
– https://ui.adsabs.harvard.edu/abs/2009ApJ…698..428L/abstract
Proposed by: C. Lardo (Postdoc) & P. Jablonka (Faculty))
Type of Project: Master/CSE project
Project flavour: Observation/Data Science
The next generation of large spectroscopic surveys of the Milky Way (WEAVE, 4MOST, DESI to quote a few) which will deliver millions of spectra that need to be analyzed with methods developed in the framework of big data tools. This project aims at developing and testing machine learning codes capable of automatically delivering the stellar atmospheric parameters (effective temperature, gravity, chemical abundances) out of high resolution spectra.
Proposed by: Martin Millon (PhD) and James Chan (Postdoc) and Frédéric Courbin (Faculty)
Type of Project: TP4b or Master/CSE project
Project flavour: Observation/Data Science
Abstract: Quasars accretion disks are known to be the most luminous objects in the Universe. They are powered by matter falling on a central black hole, releasing the gravitational energy in the form of radiations. As the high-energy photons emitted at the center travel across the disk, they trigger delayed emission at longer wavelength. Reverberation mapping consists in measuring the time-delays between different spectral bands, which correlate to the physical size of the accretion disks.
This project will imply to reduce the high-cadence data currently taken at the Euler 1.2m Swiss Telescope, and measure the time-delays between the different filters. This will involve to apply the existing techniques and to develop new methods to measure time-delays between distorted light-curves.
URL/References:
– https://arxiv.org/abs/1711.11588
Proposed by: P. Jablonka. (Faculty) and F. Courbin. (Faculty)
Type of Project: TP4b and/or Master/CSE project
Project flavour: Image analysis technique – Computing techniques
Abstract: We enter an era of extragalactic research with benefit from an unprecedented wealth of multi-wavelength sky surveys. Even of good quality, the images that are gathered will never have a spatial resolution of comparable quality as the Hubble Space Telescope ones. However, these are mandatory to address some of the most crucial questions related to galaxy evolution.
Hence, one solution is to apply deconvolution techniques to these ground-based images. This allows to reach an HST-like spatial resolution. The Laboratory of Astrophysics has worked on an efficient code, FireDec, which has been tested already on ground based images of distant galaxy clusters. The image quality is improved by a factor 10. The goal of this project is to improve further the treatment of the noise in the images, make it run efficient of sets of large images, and to make the code “user-friendly »
URL/References:
https://arxiv.org/abs/1601.05192
https://arxiv.org/abs/1602.02167
Proposed by: V. Bonvin (Postdoc) and F. Courbin (Faculty)
Type of Project: Master/CSE project
Project flavour: Observation/Modelisation/Simulation
Abstract: Strong gravitational lensing occurs when a large and massive object, such a galaxy or a cluster of galaxies, acts like a lens by distorting the path of the light rays emitted by a background source, and makes it appear multiply imaged. Stars and other compact objects in the lens create a second-order lensing effect, called microlensing, that creates small difference of magnitude between the light curves of the various lensed images. The study of these variations shed a light on the nature of these microlenses. The project revolves around the study of a large database of existing light curves of lensed quasars by single elliptical galaxies. The goal is to determine if one can confidently reproduce the observed microlensing using a standard stellar population, or if more exotic sources are required, such a clumpy dark matter or free-floating black holes.
URL/References:
Proposed by: C. Zhao (Postdoc) and J.-P. Kneib (Faculty)
Type of Project: TP4b and Master/CSE project
Project flavour: Modelisation/Data Science/Simulation
Abstract:
Baryon Acoustic Oscillation (BAO) is known as a standard ruler for measuring distances in the Universe, and is thus a probe for the cosmic evolution history, including the structure formation process dominated by dark matter, and the expansion of the Universe due to dark energy. As a complementary type of tracers to galaxies, under-densities, represented by cosmic voids, also encode the BAO signature. In particular, the BAO scale constraint from a combined sample of LRGs and voids is tighter than that from LRGs alone. Therefore, we expect a better understanding of dark matter density and dark energy equation of state, with a complete 3D map of structures consist of galaxies and voids. To this end, we shall resolve voids from the eBOSS LRG/ELG/QSO samples, and study the auto- and cross-correlations. We aim at a joint constraint on BAO and cosmological parameters with multi-tracers.
Project-1: apply the combined void+galaxy BAO constraint to eBOSS data
Project-2: investigate the response of void BAO to systematic effects using mocks
Project-3: improve the method by including a void radius dependent weighting scheme
URL/References:
http://www.sdss.org/surveys/eboss/
https://arxiv.org/abs/1802.03990
Proposed by: J. Schober (Postdoc)
Type of Project: Master project
Project flavour: Theory / Simulations
Abstract:
The Universe is permeated by magnetic fields, yet their origin is not fully understood. In particular the strongest magnetic fields in nature, which are found in neutron stars, cannot be explained satisfactorily.
Recently, a novel mechanism of magnetic field amplification related to the chirality of fermions as been identified, which is relevant for neutron stars. First numerical simulations have shown that an asymmetry of the handedness of fermions can be converted efficiently into magnetic energy. However, when the mass of the fermions becomes comparable or less than the temperature of the system, chirality flipping reactions need to be included in the theory.
The goal of this project is to study the role of these reactions in the amplification and the final state of the magnetic field with numerical simulations with the Pencil Code.
URL/References:
https://arxiv.org/abs/1711.09733
http://pencil-code.nordita.org/
Proposed by: A. Raichoor (Postdoc) and J.-P. Kneib (Faculty)
Type of Project: TP4b / Master/CSE project
Project flavour: Instrumentation/Observation/Data Science
Abstract: One of the main cosmological probes is galaxy clustering (where both the Baryon Acoustic Oscillation (BAO) and the Redshift Space Distorsion (RSD) can be measured). The on-going SDSS/eBOSS program – currently the largest redshift survey program for cosmology – uses several tracers to measure the galaxy cluster, one of which are the Emission Line Galaxies (ELGs): ~250k star-forming galaxies at redshift ~0.85.
The goal of this project is to improve the sky subtraction in the pipeline estimating the redshift from the galaxy spectra. This step is critical for the ELG in SDSS/eBOSS project – and future projects as DESI and 4MOST: ELGs are star-forming galaxies whose redshift is mainly estimated thanks to the OII emission line. The sky emission lines are numerous and close to the OII line for the ELGs at redshift ~1. An incorrect subtraction of the sky introduces spurious features which confuses the redshift measurement. Furthermore, a clean sky subtraction would enable many science projects, as the search for supernovae in the galaxy spectra. The goal is to use wavelet filtering or other advanced signal processing techniques.
URL/References:
eBOSS: http://adsabs.harvard.edu/abs/2016AJ….151…44D ; eBOSS/ELG: http://adsabs.harvard.edu/abs/2017MNRAS.471.3955R
Proposed by: A. Raichoor (Postdoc) and J.-P. Kneib (Faculty)
Type of Project: TP4b / Master/CSE project
Project flavour: Instrumentation/Observation/Data Science
Abstract: One of the main cosmological probes is galaxy clustering (where both the Baryon Acoustic Oscillation (BAO) and the Redshift Space Distorsion (RSD) can be measured). The SDSS is the leader on this type of experiment, with the BOSS (2008-2014, 1.5M spectra) and eBOSS (2014-2019, 1M spectra) programs.
The goal of this project is to look for high-redshift, faint objects, serendipitously observed with sky fibers. SDSS observations are done per plate, where 1,000 spectra are observed at the same time. On each observation, 80 fibers are allocated to sky measurement. Those sky regions are chosen where there is no object detection in the (rather shallow) SDSS imaging. Current BOSS+eBOSS observations gather ~290k sky spectra. We thus expect that a non-negligible number of interesting faint objects being observed by those sky fibers. Such a detection algorithm will promise fruitful results on the DESI experiment (2021-2026, 35M spectra), which will be one order of magnitude larger.
URL/References:
BOSS: http://adsabs.harvard.edu/abs/2013AJ….145…10D
eBOSS: http://adsabs.harvard.edu/abs/2016AJ….151…44D
BOSS/eBOSS spectral pipeline: http://adsabs.harvard.edu/abs/2012AJ….144..144B
Proposed by: A. Raichoor (Postdoc) and J.-P. Kneib (Faculty)
Type of Project: TP4b / Master/CSE project
Project flavour: Instrumentation/Observation/Data Science
Abstract: One of the main cosmological probes is galaxy clustering (where both the Baryon Acoustic Oscillation (BAO) and the Redshift Space Distorsion (RSD) can be measured). The SDSS is the leader on this type of experiment, with the BOSS (2008-2014, 1.5M spectra) and eBOSS (2014-2019, 1M spectra) programs.
The goal of this project is to look for superposed spectra observed with the BOSS/eBOSS programs. When an object is observed with a fiber, it can happen that a second object is in the same line of sight: the observed spectrum will thus be the addition of the two individual spectra. If the second object is a star-forming galaxy, we can identify its redshift thanks to its emission lines, once the first object spectrum has been subtracted. We will conduct a systematic search for such emission lines in both galaxy and quasar samples and will use machine learning to obtain an automated classification of the lens candidates. That approach has various applications, as for instance looking for strong lensing events.
URL/References:
eBOSS: http://adsabs.harvard.edu/abs/2016AJ….151…44D ; BOSS/eBOSS spectral pipeline: http://adsabs.harvard.edu/abs/2012AJ….144..144B ; strong lenses detection with double-spectra: http://adsabs.harvard.edu/abs/2012ApJ…744…41B
Proposed by: J. Blazek (Postdoc) and J.-P. Kneib (Faculty)
Type of Project: Master/CSE project
Project flavour: Observation and connections to modeling
Abstract: Understanding the properties of galaxies in their broader environment of large-scale structure is critical for the success of future cosmological analyses. Combining the imaging data from the Dark Energy Survey (DES) and the spectroscopic information from the extended Baryon Oscillation Spectroscopic Survey (eBOSS) provides a unique opportunity to measure these properties. There are several possible measurements that can be made with this combined data set. For example, we can examine correlations between the intrinsic shapes, luminosities, or colors of galaxies with the large-scale environment. This project will involve assembling a combined galaxy catalog with information on distance (redshift) and other galaxy properties, adapting existing code to estimate the relevant correlations, and comparing the results to different theoretical predictions.
URL/References: Here is one example of an earlier analysis with similar goals: https://arxiv.org/abs/0911.5347