Master thesis

The goal of the master thesis is to provide students with a first flavor of a genuine research work in astrophysics. The master thesis topics can be the natural continuation of the spring semester series of TP4 (but this is not a pre-requisite) and are all closely connected to the current research of the members of the laboratory.

Many master theses will bring up new original results. Consequently, students are encouraged to participate as one of the authors of the publication of these results. For some research project, extension in the framework of a summer work may be possible.

Projects proposed for Spring 2019 Semester

TITLE The chemical evolution and structure of the Milky Way revealed  by machine learning techniques.

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.

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TITLE: Measuring the size of black holes’ accretion disks from Reverberation Mapping

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://cosmograil.epfl.ch/

https://arxiv.org/abs/1711.11588

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TITLE: Deconvolution of images for the next generation of cosmological imaging surveys

Proposed by:  PJablonka. (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

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TITLE: Quantifying the clumpiness of dark matter with microlensing in lensed quasars.

Proposed by:  Vivien Bonvin (Postdoc) and Frédéric 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:

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TITLE: BAO measurement and cosmological parameter constraints using eBOSS galaxies and voids

Proposed by:  Cheng Zhao (Postdoc) and Jean-Paul 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

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TITLE: The role of chirality flipping in the generation of astrophysical magnetic fields

Proposed by:  Jennifer 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/

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TITLE: Advanced data analysis of massive spectroscopic surveys of galaxies with eBOSS and DESI

Proposed by:  A. Raichoor (Postdoc) and Jean-Paul 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

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TITLE: Data mining of massive spectroscopic surveys of galaxies with SDSS BOSS and eBOSS: sky fibers

Proposed by:  A. Raichoor (Postdoc) and Jean-Paul 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

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TITLE: Data mining of massive spectroscopic surveys of galaxies with SDSS BOSS and eBOSS: double-spectra

Proposed by:  A. Raichoor (Postdoc) and Jean-Paul 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

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TITLE: Galaxy physics with combined eBOSS and DES data

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

Projects completed in Spring 2018 Semester

  • The chemical evolution and structure of the Milky Way revealed  by machine learning techniques. (C. Lardo & P. Jablonka)

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 developped in the framework of big data tools.  This project aims at developping and testing a machine learning code capable to deliver the stellar atmospheric parameters (effective temperature, gravity, metallicity) out of high resolution spectra.

  • Cosmic Web (E. Ricciardelli & P. Jablonka)

The upcoming large sky surveys will deliver the deepest views on the cosmic web via the analysis of photometric and spectroscopic redshifts. This project aims at testing and improving different algorithms that should reconstruct the large structures of the Universe, in particular its filaments, from incomplete datasets as robustly as possible. The tests will be conducted on existing observational sample as well as on simulations of the ESA mission Euclid.

http://www2.iap.fr/users/sousbie/web/html/indexd41d.html

https://arxiv.org/pdf/1512.03057.pdf

https://arxiv.org/pdf/1407.7860.pdf

  • Deconvolution of images for the next generation of cosmological imaging surveys (Jablonka, Courbin, Sperone)

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 master 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 »

  • A detailed study of the chemical multiple populations in the globular cluster NGC 2808 (with C. Lardo and P. Jablonka)

During the last two decades, our understanding of the physical mechanisms driving globular clusters (GCs) formation and early evolution has been seriously challenged by the discovery of multiple populations (MPs) with spreads in He and other light elements(e.g. Na, O, Al). This phenomenon appears to be ubiquitous, as it is observed in all GCs in the Galaxy, in nearby galaxies, and in massive early type galaxies.

In the most popular MP formation models, second population (SP) stars are formed from a combination of the ejecta of evolved massive stars from a first population (FP) and from unprocessed material.  Crucial constraints on the nature of the stellar polluters can be obtained from a detailed investigation of the MP chemistry. For this project we propose to study the “template” GCs NGC 2808 to characterise its MP chemical pattern. State-of-the-art techniques will be employed to derive accurate abundance measurements from high-resolution spectra for all the elements relevant for MPs, like light-, alpha-, and heavier elements. Different NLTE corrections will be applied to  to assess the impact of NLTE on the measured abundance spreads.

  • Fast mass modeling of galaxy clusters and galaxies using GPUs  (with Schafer, Kneib)

Strong gravitational lensing offers the most powerful way to constrain the mass distribution of massive structure in the universe (galaxies and cluster of galaxies). By combining information collected with the Hubble Space Telescope and spectroscopy using for example the MUSE instrument on the ESO-VLT, we will explore fast reconstruction mass models(based on GPU infrastructure) that can probe the mass reconstruction of galaxies and/or galaxy clusters. We will apply these techniques to data challenges and real observations.

  • Sloan data mining: searching for strong lensing systems in BOSS and eBOSS (with Raichoor & Kneib)

Strong lensing happens when the path of the light coming from a background galaxy or quasar is distorted by a foreground massive and compact object. The project is to search for strong lensing systems in the ~2 millions galaxy and quasar spectra of the BOSS and eBOSS catalogs. Spectra are provided with PCA templates of galaxy or quasar automatically fitted by the BOSS pipeline. Strong lensing candidates are identified by looking for emission lines in the spectra not associated with the template, indicating the presence of an additional galaxy along the line of sight of the object. The goal is to 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.
http://www.sdss.org/surveys/eboss/
http://www.sdss.org/surveys/boss/

  • Cosmology with galaxy clustering (with Gorgoni, Raichoor, Kneib)

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): star-forming galaxies at redshift ~0.85. The proposed master project is to work on the clustering analysis for the ELGs. This requires understanding from the ELG spectra observations down to the cosmological requirements. The developed tools will be tested against the complete, well-understood BOSS data and with the current ELG data, the observations of which will be completed in June 2018.

  • An automated inteligent detection of atypical quasar spectra for DESI (with Schafer & Kneib)

Quasars are supermassive black holes living at the centre of galaxies. The Dark Energy Spectroscopic Instrument (DESI) will obtain 700’000 quasar spectra to map the Universe at redshift z>2.1 using a recent technique called the Lyman-alpha forest. The detection of atypical spectra including broad absorption line and damped lyman-alpha systems will be essential for cosmological analyses. The project is to exploit the large quasar spectra database from BOSS and eBOSS (~100’000 object using machine learning algorithm to setup an automatic finder for atypical quasars to be used for DESI.
http://desi.lbl.gov/
http://www.sdss.org/surveys/eboss/
http://www.sdss.org/surveys/boss/

  • Advanced data analysis of massive spectroscopic surveys of galaxies with eBOSS and DESI (with Raichoor & Kneib)

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): 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.

http://desi.lbl.gov/
http://www.sdss.org/surveys/eboss/
http://www.sdss.org/surveys/boss/

  • Measuring the expansion rate of the Universe with lensed supernovae (with Bonvin & Courbin)

The expansion rate of the Universe, also called the Hubble constant – or H0 – is one of the most sought-after prizes in modern cosmology. The currently best model to explain our cosmological observations is called LambdaCDM, and postulates notably the existence of dark matter and dark energy as the main components of the Universe. A change in the nature of dark energy would produce a change in the predicted value of H0, that is to be compared with direct measurement.

Time-delay cosmography, or measuring the time delays between the images of a background source strongly lensed by a foreground galaxy is one of the two most precise techniques currently used to directly measure H0. LASTRO is a leading force in this field, with more than a decade of monitoring and time-delay measurements of strongly lensed quasars. In the scope of future large-scale surveys of the sky that will discover an unprecedented amount of strongly lensed Supernovae, the proposed project aims at implementing the physical and numerical tools into the current time-delay measurement codes in order to analyse precisely and accurately these new data.

  • Quantifying the relation between a galaxy’s star formation rate and its radio emission
     (with Schober & Kneib)

Measuring the physical properties of galaxies accurately is a key for understanding their evolution in cosmic history. One particularly interesting galaxy property is its star formation rate (SFR) which depends on various parameters like the gas density, the activity of the central supermassive black hole, and the interaction rate with neighboring galaxies.

In this project, we aim to determine the SFR from radio observations of synchrotron emission which originates from cosmic rays, i.e. highly energetic charged particles that spiral around magnetic field lines in the interstellar medium. As cosmic rays are produced in supernova remnants, the remains of massive stars, there is a direct link to the SFR. The goal of this project is to model the cosmic ray spectrum in detail with GALPROP ( https://galprop.stanford.edu/ code.php ) and use it as an input for a semi-analytical galaxy model. In particular, we are interested in applying such models to nearby low metallicity galaxies, like Haro 11, which seem to be local counterparts to the first galaxies in the Universe.

Related papers:

http://adsabs.harvard.edu/abs/ 2017MNRAS.468..946S

http://adsabs.harvard.edu/abs/ 2016ApJ…827..109S

  • The impact of the Milky Way heating on the dwarf galaxy gas content (with Revaz, Hausammann & Jablonka)

Heating of the hydrogen and helium gas by the diffuse UV background revealed to be a crucial element for the evolution of galaxies, responsible in particular of the star formation quenching of the faintest galaxies, namely, the dwarf galaxies.In this master project, we want to go one step further by estimating the UV flux generated by a Milky Way like galaxy and its possible impact on a nearby dwarf galaxies. Among different questions, we would like to understand in which conditions the Milky Way UV flux dominates over the diffuse UV background. In a first step, relying on the pyrates and pNbody library, the student will have to build a simple Milky Way model including a realistic stellar population to estimate the UV flux, taking into account absorption processes. In a second step, he/she will build a self-consistent dynamical model based on GEAR, where the UV fluxes are computed on the fly, during the the simulation. In addition to the predictions of the hydrogen and helium heating, this work will also shed light on the molecular hydrogen content and its evolution with time.

Related papers:

http://adsabs.harvard.edu/abs/2016A%26A…588A..21R
http://adsabs.harvard.edu/abs/2012A%26A…538A..82R
http://adsabs.harvard.edu/abs/2017MNRAS.466.2217S

  • The first stars in the first galaxies (with Revaz, Hausammann & Jablonka)

In this work, we would like to explore a new star formation receipt allowing the realisation of ultra-high resolution simulations of galaxies, overpassing current numerical limitations. This new scheme  which is already implemented in our code GEAR relies on the grouping of particles tracing gas to ensure a complete initial stellar mass function. During his/her master, the student will have to test it through different kind of simulations that will help to constrain some free parameters. The final goal will be to apply this new method in a cosmological context to compute the formation of the first stars giving birth to the first galaxies. An important piece of the master will be to explore the impact of those first stars on the chemical evolution of the galaxies, owing to their metals ejection.

Related papers:

http://adsabs.harvard.edu/abs/2016A%26A…588A..21R
http://adsabs.harvard.edu/abs/2012A%26A…538A..82R

  • Tidal stripping of dwarf galaxies born in a cosmological context (with Revaz, Hausammann & Jablonka)

Dwarf galaxies are the least luminous galactic systems with the largest dark to stellar mass ratio in the Universe. Due to they small mass, they are very sensitive to any perturbation. As such they represent a unique laboratories for testing different key ingredients to understand the formation and evolution of galaxies. Based on new high resolution numerical simulations of dwarf galaxies formed in a cosmological context, we would like to review our understanding of they interaction with a Milky Way-like galaxy, in particular the so-called tidal stripping effect. During the master thesis, the student will have to run a bunch of simulations where the dwarf galaxy orbit around a disky potential representing the host galaxy. He/her will have to understand for which mass and which kind of orbits the dwarf sees its star formation being quenched and when it is completely stripped. Understanding this process is a key ingredient to reconcile the formation of dwarf galaxies with the standard cosmological model.

Related papers:

http://adsabs.harvard.edu/abs/2016A%26A…588A..21R
http://adsabs.harvard.edu/abs/2014A%26A…564A.112N
http://adsabs.harvard.edu/abs/2012A%26A…538A..82R

  • A search for gravitationally lensed quasars with the ESA Gaia satellite and the 8m Subaru telescope (with Chan & Courbin)

Gravitationally lensed quasars have wide applications ranging from 1) measuring the expansion rate of the Universe, 2) measuring the stellar content of the galaxies, 3) probing dark matter structures/sub-structures in galaxies, 4) constraining spatial structure of line emitting region of quasars, and 5) diagnosis of the inner structure of quasars via micro-lensing. The detection of lensed quasar also provides us a chance to study distant objects, which cannot be achieved without gravitationally lensing effect. The proposed project aims at detecting more lensed quasars with public database from Gaia (space) and HSC (ground-based) surveys.

Projects completed in Spring 2017 Semester

  •  PSF Interpolation (with Kuntzer, Courbin)

The point spread function (PSF) is among the most critical signal to reconstruct for Weak Gravitational Lensing (WL) analysis. The PSF can only be sampled at the position of the stars, but should be known that the position where the WL is measured, i.e. the position of the galaxies. We want to apply state-of-the art machine learning tools (such as Auto-Encoders and Artificial Neural Networks) to the problem of PSF reconstruction and interpolation. We will work with the CosmoStat Laboratory at CEA (France) to make benchmark tests the methods we will develop.

  • The Colour-Magnitude Diagram of NGC 6397 (with Kuntzer, Courbin)

Colour-Magnitude Diagrams (CMDs) are useful tools to study the stellar formation and evolution. They are constructed from the measurement of the magnitude of stars in two filter bands. We developed a tool based on detailed light profile of the stars that are members of the globular cluster. This profile is used to predict a magnitude in a band based on the image taken in the other band. With that scheme, we can build a CMD with only one image. The scheme can also be applied to future data to recover the spectral type of the object. This procedure was showed to be working in simple circumstances using Artificial Neural Networks. To goal of this work is apply the method to data without any simplification to test the limit of the scheme.

  • using a Machine Learning tool for denoising galaxy images while preserving their shapes (with Gentile)

Future space cosmological surveys will measure the weak gravitational lensing effect of large scale structures on the image of distant galaxies, providing new clues on the nature of dark matter, dark energy, and on the content of the Universe. In practice, this requires to measure precisely the shape of billions of faint and small galaxies and will requires “image denoising”.  The work will consist in applying a denoising autoencoder (DA) to galaxy images with the objective of removing noise without altering the shapes of galaxies e.g. size  and ellipticity. An autoencoder is a specific type of Artificial Neural Network where the input and output layers are identical. The objective is to have the autoencoder learn how to build a new representation of the input data (the galaxy image) that has less noise while preserving the galaxy shape attributes. The DA will be applied to simulated galaxy images with noise of different levels and types (Gaussian, Poisson). The alteration of shapes before and after applying the DA will be assessed using a galaxy shape measurement tool.

  • Premières étoiles : optimisation de la détermination des paramètres stellaires (with  Jablonka & North)

Il s’agit de mettre au point une méthode rapide (C + python) pour la détermination automatique des paramètres atmosphériques d’étoiles: température effective, gravité, vitesse de micro-turbulence, et métallicité. Cette méthode s’appuyera sur des méthodes de minimisation robustes, inclura des prises de décision automatiques et des accès web à des bases de données extérieures.

Le code sera testé sur des données réelles et servira au plus grand survey actuellement en cours d’étoiles extrêmement déficientes en métaux dans le halo de notre galaxie pour comprendre les premières étapes dans la formation des galaxies.

Exemple de code existant:  http://www.aanda.org/articles/aa/pdf/2014/04/aa22430-13.pdf

  • The influence of collisions and merger on the stellar properties of dwarf galaxies(Revaz/Jablonka)

Accurately reproducing the observed stellar properties of dwarf spheroidal galaxies (dSphs),

the smallest and faintest galaxies in our universe, represents a challenging task for the standard cosmological model. The work proposed here will consist in understanding the impact on the stellar properties of dSphs (star formation history, metallicity distribution, metallicity gradient, stellar populations, rotation) of the interactions and collisions that dSphs may suffer during a Hubble time.

The study will consist in extracting dwarf galaxies formed in cosmological numerical simulations and make them collide, running idealised N-body simulations based on the chemo-dynamical-C/MPI-code GEAR (Revaz & Jablonka 2012, Revaz et al. 2016) which includes

a complete treatment of the baryonic physics (gas cooling, star formation, chemical enrichment, supernova feedback). The aim will be to explore the effect of parameters like the mass of the progenitors, the impact parameter and the relative velocity on the final properties of the dwarf galaxies. In this purpose, a set of python routines will be developed to allow for a comparison between the model predictions and the observations. The final goal will be to try to reproduce peculiar features observed in local group dwarf galaxies, like distinct stellar populations and metallicity gradients.

  • The influence of Self-Interacting Dark Matter (SIDM) on dwarf spheroidal galaxies (with Revaz/Harvey/Jablonka)

The aim of this work is to study the influence of the Self-Interacting Dark Matter (SIDM) on the evolution of dwarf spheroidal galaxies (dSphs) and compare their properties with models run with normal Dark Matter. This is a unique occasion to constrain the cross section of SIDM  on a mass and density regime, the one of dSphs, poorly studied up to now.

The study will consist in running isolated models of dSphs formed out of a LCDM universe and compare their evolution with and without SIDM, focusing particularly on the inner density profile of the dark matter (Cusp/Core) but also on the evolution of merger galaxies. The work will rely on a modified version of the the chemo-dynamical-C/MPI-code GEAR (Revaz & Jablonka 2012, Revaz et al. 2016) which includes a complete treatment of the baryonic physics (gas cooling, star formation, chemical enrichment, supernova feedback) but also a recent implementation of (SIDM). Python routines will be developed to perform the reduction of the simulations.

  • Observing dark matter around massive galaxies with gravitational lensing (with Blazek, Kneib)

 Because the path of light is bent by gravity, massive galaxies act as “gravitational lenses” which distort the observed images of more distant galaxies. This gravitational lensing effect is a powerful tool to trace the location of dark matter, which cannot be directly seen by telescopes even though it constitutes most of the matter in the universe. The goal of this project is to combine two new data sets, eBOSS (a spectroscopic survey) and DECaLS (an imaging survey), to measure the gravitational lensing effect around distant, massive galaxies. In addition to learning about the dark matter surrounding these galaxies, these measurements will aid in validating the DECaLS imaging data. In the future, this type of measurement can also be combined with other observations to constrain the underlying cosmological model.

  • Supersonic baryon velocities, galaxy formation, and future measurements (with Blazek, Kneib)

The clustering signal of galaxies is a primary cosmological probe for current and future surveys, including the Dark Energy Spectroscopic Instrument (DESI) and Euclid. In particular, interactions between photons and baryons in the early universe imprinted a preferred length scale, known as the BAO scale, that can be used to trace the geometry of the universe. These same interactions caused an initially supersonic flow of baryons, relative to the dominant dark matter, which may have impacted the formation of galaxies. The goal of this project is to construct a consistent model of this effect and include the model in a new software package designed to do cosmological calculations. We will use these results to forecast how sensitive DESI and other future surveys will be to these baryonic effects.

  • The most precise mass models of galaxy clusters (with Schafer, Kneib)

 Strong gravitational lensing offers the most powerful way to constrain the mass distribution of massive cluster of galaxies. By combining information collected with the Hubble Space Telescope and the MUSE instrument on the ESO-VLT, we will explore what are the detailed mass models that can reproduce the most precisely the observational constraints of the Hubble Frontier Field clusters. Potentially this can also leads to new important measurement of the cosmological parameters or the properties of the dark matter particles.

  • Sloan data mining: searching for strong lensing systems in BOSS and eBOSS (with Delubac & Kneib)

Strong lensing happens when the path of the light coming from a background galaxy or quasar is distorted by a foreground massive and compact object. The project is to search for strong lensing systems in the ~2 millions galaxy and quasar spectra of the BOSS and eBOSS catalogs. Spectra are provided with PCA templates of galaxy or quasar automatically fitted by the BOSS pipeline. Strong lensing candidates are identified by looking for emission lines in the spectra not associated with the template, indicating the presence of an additional galaxy along the line of sight of the object. The goal is to 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.
http://www.sdss.org/surveys/eboss/
http://www.sdss.org/surveys/boss/

  • Cosmology with galaxy clustering (with Raichoor, Delubac, Kneib)

One of the main cosmological probes is the Baryon Acoustic Oscillation (BAO), which is measured through the clustering of a large number of galaxies. The on-going SDSS/eBOSS program – currently the largest BAO program – uses several tracers to measure BAOs, one of which are the Emission Line Galaxies (ELGs): star-forming galaxies at redshift ~0.9. The proposed master project is to work on the clustering analysis for the ELGs. This requires understanding from the ELG spectra observations down to the cosmological requirements. The developed tools will be tested against the complete, well-understood BOSS data and with preliminary ELG data, the observations of which has started in September 2016.

  • An automated inteligent detection of atypical quasar spectra for DESI (with Delubac & Kneib)

Quasars are supermassive black holes living at the centre of galaxies. The Dark Energy Spectroscopic Instrument (DESI) will obtain 700’000 quasar spectra to map the Universe at redshift z>2.1 using a recent technique called the Lyman-alpha forest. The detection of atypical spectra including broad absorption line and damped lyman-alpha systems will be essential for cosmological analyses. The project is to exploit the large quasar spectra database from BOSS and eBOSS (~100’000 object using machine learning algorithm to setup an automatic finder for atypical quasars to be used for DESI.
http://desi.lbl.gov/
http://www.sdss.org/surveys/eboss/
http://www.sdss.org/surveys/boss/

  • Toile cosmique: Identification des filaments de galaxies (Ricciardelli, Jablonka & Revaz)

Il s’agit de tester et d’améliorer un algorithme d’identification de filaments, c’est à dire de galaxies corrélées spatialement en 3 dimensions, dans des champs dans lesquels nous avons les redshifts des systèmes. La méthode proposée s’appuie sur une technique de tesselation. Elle sera testée sur des simulations numériques et des données observées.

Code: http://www2.iap.fr/users/sousbie/web/html/indexd41d.html

Références:

http://adsabs.harvard.edu/abs/2011MNRAS.414..350S

http://adsabs.harvard.edu/abs/2011MNRAS.414..384S

https://arxiv.org/abs/1611.07045

  • Morphologie et populations stellaires de galaxies d’amas (with Joseph, Jablonka & Courbin)

Il s’agit d’analyser des images de galaxies autour d’amas à z~0.5 et de séparer les différentes  composantes stellaires: étoiles vieilles ou nouvellement formées. La méthode utilise les notions de sparsité des données astrophysique et décompose les distributions spectrales d’énergie des galaxies sur des bibliothèques choisies. Pour la première fois, la PSF des images sera prise en compte dans les calculs. Les résultats serviront à comprendre l’influence des environnements denses sur l’évolution des galaxies et en particulier comment la formation s’éteint.

Référence: https://arxiv.org/abs/1603.00473