Detecting the first clusters of galaxies in the Universe in the maps of the cosmic microwave background
Galaxy clusters, located at the node of the cosmic web, are the largest gravitationally bound structures in the Universe. Their abundance and spatial distribution are very sensitive to cosmological parameters, such the matter density in the Universe. Galaxy clusters thus constitute a powerful cosmological probe. They have proven to be an efficient probe in the last years (Planck, South Pole Telescope, XXL, etc.) and they are expected to make great progress in the coming years (Euclid, Vera Rubin Observatory, Simons Observatory, CMB- S4, etc.).
The cosmological power of galaxy clusters increases with the size of the redshift range covered by the catalogue. The attached figure shows the redshift ranges covered by the catalogues of galaxy clusters extracted from experiments observing the cosmic microwave background (first light emitted in the Universe 380,000 years after the Big Bag). One can see that Planck detected the most massive clusters in the Universe in the redshift range 0<z<1. SPT and ACT are more sensitive but covered less sky: they detected tens of clusters between z=1 and z=1.5, and a few clusters between z=1.5 and z=2. The next generation of instruments (Simons Observatory starting in 2024 and CMB- S4 starting in 2032) will routinely detect clusters in 1<z<2 and will observe the first clusters formed in the Universe in 2<z<3.
Only the experiments studying the cosmic microwave background will be able to observe the hot gas in these first clusters at 2<z<3, thanks to the SZ effect, named after its discoverers Sunyaev and Zel’dovich. This effect is due to high energetic electrons of the gas, which distorts the frequency spectrum of the cosmic microwave background, and is detectable in current experiments. But the gas is not the only component emitting in galaxy clusters: galaxies inside the clusters can also emit in radio or in infrared, contaminating the SZ signal. This contamination is weak at z<1 but increases drastically with redshift. One expects that the emission from radio and infrared galaxies in clusters are of the same order of magnitude as the SZ signal in 2<z<3.
One thus needs to understand and model the emission of the gas as a function of redshift, but also the emission of radio and infrared galaxies inside the clusters to be ready to detect the first clusters in the Universe. Irfu/DPhP developed the first tools for detecting clusters of galaxies in cosmic microwave background data in the 2000s. These tools have been used successfully on Planck data and on ground-based data, such as the data from the SPT experiment. They are efficient at detecting clusters of galaxies whose emission is dominated by the gas, but their performance is unknown when the emission from radio and infrared galaxies is significant.
This thesis will first study and model the radio and infrared emission from galaxies in the clusters detected in the cosmic microwave background data (Planck, SPT and ACT) as a function of redshift.
Secondly, one will quantify the impact of these emissions on existing cluster detection tools, in the redshift range currently being probed (0<z<2) and then in the future redshift range (2<z<3).
Finally, based on our knowledge of these radio and infrared emissions from galaxies in clusters, we will develop a new cluster extraction tool for high redshift clusters (2<z<3) to maximize the detection efficiency and control selection effects, that is the number of detected clusters compared to the total number of clusters.
STUDY OF THE GALACTIC CENTER AND DIFFUSE EMISSION SEARCHES IN VERY-HIGH-ENERGY GAMMA RAYS WITH H.E.S.S. AND PROSPECTS WITH CTA
Very-high-energy (E>100 GeV) gamma-ray observations of astrophysical objects are a crucial tool for the understanding of the most violent non-thermal acceleration processes taking place in the Universe. The gamma rays allow to attack fundamental questions across a broad range of topics, including supermassive black holes, the origin of cosmic rays, and searches for new
physics beyond the Standard Model. Multi-wavelength observations of the center of the Milky Way unveil a complex and active region with the acceleration of cosmic rays to TeV energies
and beyond in astrophysical objects such as the supermassive black hole Sagittarius A* lying at the center of the Galaxy, supernova remnants or star-forming regions. The Galactic Centre (GC) stands out as one of the most studied regions of the sky in nearly every wavelength, and has been the target of some of the deepest exposures with high-energy observatories. Beyond the diversity of astrophysical accelerators, the GC should be the brightest source of dark matter particle annihilations in gamma rays.
The GC region harbors a cosmic Pevatron, i.e., a cosmic-ray particle accelerator to PeV energies, diffuse emissions from GeV to TeV such as the Galactic Centre Excess (GCE) whose origin is still unknown, potential variable TeV sources as well as likely unresolved source population. The interaction of electrons accelerated in these objects produces very-high-energy gamma rays
via the inverse Compton process of electrons scattering off ambient radiation fields. These gamma rays can also be efficiently produced through decays of neutral pions from inelastic
collisions protons and nuclei with the ambient gas. Among possible unresolved source populations in the GC region are millisecond pulsars in the Galactic bulge or an intermediate-mass (~20-10^5 Msun) black holes following the dark matter distribution of the Galactic halo. About 10^3 sources would be needed to explain the GCE emission. Such source population would leave characteristic imprints in the background fluctuations for which surveys of the GC region in TeV gamma rays with the H.E.S.S. observatory and the forthcoming CTA are unique to scrutinize them.
The H.E.S.S. observatory composed of five atmospheric Cherenkov telescopes detects gamma rays from a few ten GeV up to several ten TeV. H.E.S.S. has carried out extensive observations
of the GC with recently an observational campaign of the inner several degrees around the GC. The dataset accumulated so far provides an unprecedented sensitivity to study the acceleration and propagation of TeV cosmic rays and search for dark matter signals in the most promising region of the sky. These observations are unique to shape the observation programs of the future observatory CTA, optimize their implementation, and prepare future analyses.
The PhD thesis project will be focused on the analysis and interpretation of the observations carried out in the GC region by the H.E.S.S. over about 20 years. The first part of the work will be devoted to the low-level analysis of the GC data, the study of the systematic uncertainties in this massive GC dataset and the development of dedicated background models. In the second part, the PhD student will combine all the GC observations in order to search for TeV diffuse emissions, unresolved population of sources, and dark matter signals using multi-template analysis techniques including background modelling approaches. The third part will be dedicated to the implementation of the new analysis framework to CTA forthcoming data to prepare future GC analyses using the most up-to-date signal and background templates. In addition, the PhD student will be involved in the data taking and data quality selection of H.E.S.S. observations.
Multi-messenger analysis of core-collapse supernovae
Core-collapse supernovae play a crucial role in the stellar evolution of massive stars, the birth of neutron stars and black holes, and the chemical enrichment of galaxies. How do they explode? The explosion mechanism can be revealed by the analysis of multi-messenger signals: the production of neutrinos and gravitational waves is modulated by hydrodynamic instabilities during the second following the formation of a proto-neutron star.
This thesis proposes to use the complementarity of multi-messenger signals, using numerical simulations of the stellar core- collapse and perturbative analysis, in order to extract physical information on the explosion mechanism.
The project will particularly focus on the multi-messenger properties of the stationary shock instability ("SASI") and the corotational instability ("low T/W") for a rotating progenitor. For each of these instabilities, the signal from different species of neutrinos and the gravitational waves with different polarization will be exploited, as well as the correlation between them.
Measuring the assembly of massive primordial galaxies with the James Webb Space Telescope (JWST)
The James Webb Space Telescope (JWST) is revolutionizing our view of the first billion years after the big bang, by enabling us to detect the primordial galaxies formed by the collapse of the Universe's first overdensities. Initial studies of the properties of these galaxies, partly carried out by our team, have revealed that their formation is still largely misunderstood and potentially in tension with the Lambda Cold Dark Matter (LCDM) model. Indeed, these studies have uncovered a potential excess of massive primordial galaxies, implying accelerated growth of these galaxies at star formation efficiencies well beyond the predictions of theoretical models. Before invoking radically different cosmological and galaxy evolution models, however, it is necessary to confirm these tensions, which are currently based only on highly uncertain measurements of the stellar mass of a few galaxies.
The aim of this thesis is to confirm or refute these tensions by accurately constraining, for the first time, the stellar mass of a large statistical sample of primordial galaxies. To do this, we will combine data from four JWST extragalactic surveys with an original statistical approach of image stacking, enabling us to obtain the average stellar mass of primordial galaxies that are otherwise too faint to be detected individually by the JWST in the critical mid-infrared window. This information, together with that obtained on their star-forming activity, will be decisive in understanding the growth of the Universe's first galaxies.
Data analysis and fundamental physics with LISA and Pulsar Timing Array
There are two types of instruments to observe gravitational waves (GW) at low frequency: space-based interferometer in the milliHertz (mHz) band, and Pulsar Timing Array (PTA) in the nanoHertz (nHz) band. They are complementary either by observing two parts of the same sources as for stochastic backgrounds or two parts of the same population of sources as for massive black hole binaries.
LISA is space-based GWs observatory which is planned for launch in 2035. It consists of three satellites in the free fall in the heliocentric orbit forming an equilateral triangle. Satellites exchange laser light forming multiple interferometers allowing to observe a plethora of astrophysical and cosmological sources of GWs. These sources include galactic white dwarf binaries, extreme mass-ratio inspirals, massive black hole binaries, stochastic backgrounds.
PTA is using the timing of millisecond pulsars to observe GWs. Millisecond pulsars emit about hundreds of radio pulses per second with very high regularity. GWs passing between pulsar and Earth, modifies the time of arrival of the pulses. The timing an array of pulsars, enable to make a galactic scale GW detector. Multiple radio-telescopes contribute to PTA, in particular the Nançay Radio-Telescope. In June 2023, 4 PTA collaborations announced the results of 20 years of pulsar timing: strong evidence for a GWs signal. The signal still needs to be characterized and its origin established. It could have been emitted by an ensemble of super-massive black holes or by processes in the primordial Universe. While the two observing systems are different, the data analysis methods are similar. A large parameter space needs to be sampled to extract overlapping sources and disentangle them from the non-stationary noises.
GWs are a new way to learn about fundamental physics. For example, we can test general relativity with the merger of super-massive black holes binary and Extreme Mass ratio Inspiral and test particle physics beyond the standard model, thanks to the detection of stochastic background (SGWB) from phase transitions in the early Universe. The candidate will work at the CEA-IRFU (Institut de Recherche sur les Lois Fondamentales de l'Univers) as part of a cross-disciplinary team conducting research into GWs. This activity ranges from instrumental involvement in the LISA mission to the astrophysical or cosmological consequences of exploiting the signals, via the development of algorithms, simulations and data analysis. IRFU is also involved in PTA-France and International PTA. Developing methods for detecting gravitational wave sources and deducing the associated physical consequences is at the heart of the proposed thesis topic. The candidate will have the opportunity to take an interest in all aspects of the host team's activity and to interact with each of its members. The main objectives of the proposed work are to develop data analysis methods for LISA, taking advantage of developments in PTA and LISA, and to study the synergy between LISA and PTA observations for fundamental physics, in particular with SGWBs and Massive Black Holes (MBHs). The methods developed can also be adapted and applied to real PTA data. The candidate will be a member of the collaborations LISA, PTA-France, EPTA and IPTA. He/she will interact with members of the Groupement de Recherche Ondes Gravitationnelles and collaborate with physicists from the Astroparticles et Cosmologie (APC) laboratory. He will present his results within the LISA and PTA consortiums and at international conferences.
Understanding the formation of bulges based on morphology and kinematics information from JWST
Present-day bulges of spiral galaxies and elliptical galaxies contain very old stars and are thought to be formed in the early Universe. How this actually happened in practice is not well understood, and the most relevant physical processes at play are still unclear. In the last decade, evidence has been growing of the existence of compact star bursting galaxies that might be signposts of bulges caught at the event of formation. More recently, also thanks to new findings from our group based on JWST, a number of further puzzling results has accumulated, currently difficult to explain: A) these star-bursting galaxies are always embedded in larger disk-like systems that are less active but contain most of the existing stellar mass, as if there was no ’naked’ bulge formation; B) in some cases, the outer disks have actually stopped forming stars, thus representing cases of quenching progressing from the outside-in, reversing the standard more familiar pattern (as observed in local spirals and the MilkyWay, where the center is quenched and the outskirts are forming stars); C) the disks are often strongly lopsided in their stellar mass distribution, a feature becoming more and more dominant when looking at earlier times. This phenomenology is currently unexplained. It could be related to merging activity, gas accretion or also feedback effects. If these are forming bulges, how they would evolve in present-day bulges and elliptical galaxies is unclear. Still, these new challenging observations promise breakthrough in the understanding of bulge formation if more progress can be made and further insight gathered. We propose a PhD project where the student will be using imaging and spectroscopy data from JWST to illuminate these issues. Imaging from deep and ultra deep public surveys that is accumulating will be used to increase the statistics and put on more solid grounds the early results gathered so far. The spectroscopy from JWST holds the key to detailed understanding of specific systems, providing information on kinematics of the compact star bursting cores as well as of the outer disks: if these subsystems are co-rotating without major disturbances would support non violent, gas accretion related evolution. On the contrary, counter-rotating subsystems or kinematics disturbances would betray merging events. This kind of test has not yet been carried out. We will use targeted spectroscopy in part already available from the Early-Release project CEERS of which we are members, from the large archive that is accumulating, and from dedicated proposals (pending, and to be submitted in future cycles).
A window on interstellar nanoparticle evolution: the NIKA2 images of nearby galaxies
Interstellar nanoparticles are a major physical component of galaxies, reprocessing starlight, controlling the heating and cooling of the gas, catalyzing chemical reactions and regulating star formation. The abundance, composition, structure and size distribution of these small solid particles, mixed with the interstellar gas, are however poorly-known. They indeed evolve within the interstellar medium and present systematic differences among galaxies. It is thus crucial to obtain detailed, carefully analyzed, empirical constraints of these properties, in a wide diversity of environments. Progress in this field are absolutely necessary to properly interpret observations of nearby star-forming regions and distant galaxies, as well as for precisely modeling interstellar physics.
Of particular interest are the long-wavelength optical properties of the nanoparticle mixture in the millimeter range. This spectral window is currently the least known. Yet, the millimeter opacity of the grain mixture has a central importance, since mass estimates based on spectral energy distribution fitting primarily rely on this quantity. A bias or systematic evolution of the millimeter opacity will directly translate in an inaccuracy in the nanoparticle mass, which is often used as a proxy to infer the gas mass of a region or galaxy.
Our guaranteed time program, IMEGIN (Interpreting the Millimeter Emission of Galaxies at IRAM with NIKA2; PI Madden; 200 hours), with the NIKA2 camera at the 30-m IRAM radiotelescope, has fully mapped 20 nearby galaxies at 1.2 mm and 2 mm. In addition, our open time program, SEINFELD (Submillimeter Excess In Nearby Fairly-Extended Low-metallicity Dwarfs; PI Galliano; 36 hours), is completing the survey down to low-metallicities (the metallicity is the relative abundance of elements heavier than Helium). These new and exceptional data are the first good quality maps of resolved galaxies at millimeter wavelengths, allowing us to study how the grain properties vary with the physical conditions.
The goal of the present PhD project is to combine these observations with other, already existing, multi-wavelength data (in particular, WISE, Spitzer and Herschel), in order to demonstrate how the millimeter opacity depends on the local physical conditions. The first step will consist in processing and homogenizing the data. The student will also have the opportunity to participate in our observing campaigns at Pico Veleta. In a second time, the student will model the spatially-resolved emission, using our in-house, state-of-the-art hierarchical Bayesian code, HerBIE. This will allow the student to produce maps of the nanoparticle properties and compare them to maps of the physical conditions. Finally, these results will be used to model the evolution timescales of the grain properties under the effects of radiation field and gas accretion. The laboratory measurements recently produced by the Toulouse group will be put to profit. This work will be performed within the IMEGIN international collaboration.
Numerical study of core collapse supernovae
Context : Numerical study of core collapse supernovae. A core collapse supernova begins with the collapse of the core. Once the density goes beyond nuclear matter density, it becomes extremely hard. The collapsing matter bounces on it and creates a shock. The shock propagates and then stalls. The situation is the following : the shock is stationary. Neutrino coming from deep in the core heat the matter close to the shock and tend to push the shock forward, onto stellar explosion. On the other hand, the rest of the star is still collapsing, and this pushes the shock inwards, which in turn tends to black hole formation. The knowledge of which progenitor (which massive star) explodes and which creates a black hole is an active research topic : there is no clear and reliable way, without doing a detailed numerical simulation, to know whether a given progenitor explodes or whether it forms a black hole.
Objectives : Acquire a good knowledge of the supernovae physics, stellar physics, and also the neutron star physics and the black hole physics. Acquire a good knowledge on the development of a numerical physics code. Acquire a good knowledge on the link between numerical physics and laser physics.
Process : The student will acquire knowledge on radiative hydrodynamics with neutrinos, in a relativistic context. The student will also acquire knowledge on general relativity. The possibility of reproducing some aspects of the supernova explosion in laboratory with laser experiments will be studied. The possible link between the progenitor (the massive star about to collapse) and the explosion (whether it explodes or it forms a black hole) will be studied in details numerically. The student will produce simplified progenitors. In these, the student will be able to vary some well chosen parameters. Finally, many possibilities exist to improve this study : implementation of other numerical methods, 3d, implementation of nucleosynthesis, etc. The student can also suggest its own way.
How large are dust particles at the onset of disk formation ? A multi-wavelength investigation
The formation and properties of exoplanetary systems is a fascinating question, which has been at the heart of our quest to define mankind and the conditions for life to develop in a broader context. Observations suggest there should be billions of planets in our Galaxy alone. What are the physical process that make planet formation so likely ? Which local conditions are required to transform the stardust of the interstellar medium into pebbles around young stars, and grow these further into planets ? Investigating the dust evolution along the star formation sequence is key to provide a complete picture of the planet formation scenario.
Moreover, the dust grains are crucial because they regulate some key physical processes: for example, the amount of small grains is a key parameter to set the coupling of magnetic fields, hence regulating the sizes and masses of the protostellar disks when they are assembled.
Recently, the star formation group at CEA has obtained some of the first observational clues that the dust particles contained in the pristine disk-forming reservoirs that are the embedded protostars may already have significantly evolved from the submicronic dust populating the interstellar medium.
The proposed PhD aims at exploring new dataset, observations of young protostars from the infrared to the millimeter wavelengths and investigate wether dust particles are indeed growing significantly already during the first 0.5 Myrs of the star formation process.
To improve our understanding of early dust evolution during the disk-building phase, this analysis of multi-scale observational data will be compared to the predictions of evolved dust models implemented in MHD numerical simulations of disk formation.
The student will analyze data obtained with the NIRSpec/NIRCam, then MIRI, instruments aboard the James webb Space Telescope, towards nearby embedded protostars. This data should probe the presence of micronic dust grains in the close vicinity of the young forming disks. The analysis of complementary dust emission maps from the ALMA and NOEMA interferometers, probing the colder dust, will complete the picture, allowing a multi-wavelength approach to constrain the models.
Inverse Problems in Astrophysics and Machine Learning
AI (artificial intelligence) is significantly changing the way we solve inverse problems in astrophysics.
In radio interferometry, the detection of radio sources and their classification require taking into account numerous effects such as non-Gaussian noise, incomplete sampling of Fourier space, and the need to construct a sufficient data set for the training. The difficulty increases when the source to be reconstructed evolves over time. Such examples of temporal variations are found in various inverse problems in astrophysics such as transient objects (supernovae, fast radio burst, etc.). ARGOS is a pilot study for a radio interferometer that will perform real-time continuous wide-field observations in centimetre wavelengths. The combination of a wide field of view with high sensitivity will allow ARGOS to detect transient sources that vary on timescales shorter than one second. ARGOS will be able to detect thousands of fast radio bursts per year. These events will need to be accurately differentiated from other transient sources detected by ARGOS, such as supernovae, gamma-ray bursts, white dwarfs, neutron stars, blazars, etc. Given the short time scales of some of these transient events and the need for quick follow up, ARGOS will require state-of-the-art classification solutions employing cutting-edge machine learning architectures. This thesis consists of developing innovative tools from machine learning to solve image reconstruction and source classification problems.