Boris Sindhu Kalita

Plongée dans les facteurs environnementaux de la formation d'étoiles dans les halos denses de l'Univers primitif: L'interaction avec l'environnement est un facteur majeur de l'évolution des galaxies à travers le temps cosmique. Ce facteur est particulièrement important dans les premières phases de formation des galaxies et des amas (z ~ 2 - 4), où les galaxies les plus massives (> 1011 M_sun) de notre Univers se formaient, évoluaient et mouraient rapidement en l'espace de quelques milliards d'années. Cette thèse vise à caractériser par l'observation le rôle que l'environnement pourrait jouer dans l’évolution des propriétés de ces galaxies, avec un accent particulier sur l'accrétion filamentaire de gaz froid. Ce phénomène essentiel est censé fournir le carburant nécessaire à la formation d'étoiles dans les halos de matière noire traçant des surdensités de matière, qui sont autrement trop chauds (avec des températures de Viriel ~ 106 K) pour conserver un réservoir de gaz froid (< 104 K) qui peut être facilement converti en étoiles dans les galaxies membres du halo. La théorie prédit que de tels flux continus de gaz froid sont capables de pénétrer profondément dans les halos dans une gamme de paramètres caractérisée par la masse du halo et le décalage vers le rouge. Ainsi, les halos dont la masse de matière noire est supérieure au seuil de 1012 M_sun, au-delà duquel tout gaz disponible est chauffé par choc, sont encore capables de fournir du gaz froid à leurs galaxies. Étant donné l'importance de ce phénomène, qui a récemment été soutenu soulignée par l'évolution de la luminosité Lyman-alpha des halos à haut décalage vers le rouge (à 1,8 < z < 3,3), nous avons cherché à trouver des preuves dans des observations très sensibles et bien résolues de galaxies à forte formation d'étoiles dans une telle structure. RO-1001, un groupe de galaxies à z = 2,91 est l'un des rares cas où il existe des preuves convaincantes d'une accrétion filamentaire de gaz froid, tracée par son halo Lyman-alpha. En utilisant une combinaison d'images profondes HST et ALMA, ainsi que d'autres images et spectres en optique et dans l'infrarouge proche, nous trouvons des preuves prometteuses des effets de l'accrétion sur trois galaxies membres de RO-1001 qui forment des étoiles massives. Nous trouvons une interaction entre les disques stellaires ne formant pas d’étoile et les noyaux fortement formateurs d'étoiles mais obscurcis par la poussière qui est en accord avec les prédictions des simulations de l'évolution des galaxies par accrétion. Cependant, une quatrième galaxie aussi massive mais ne formant aucune étoile est également trouvée dans la même structure riche en gaz, confirmée photométriquement. Ceci peut donc être interprété comme un cas limite de l'accrétion de gaz de rôle, fournissant ainsi une image complète de la formation d'étoiles dans les galaxies au sein de telles structures. De plus, la galaxie ne formant aucune étoile ouvre la voie à la partie suivante de la thèse qui étudie comment la formation d'étoiles est inhibée dans les environnements denses, qui, lorsque nous progressons vers des amas plus évolués, est caractérisée par l'injection d'entropie dans les environnements intra-amas

Context. Clumps in the rest-frame UV emission of galaxies at z ≲ 3 have been observed for decades. Since the launch of the James Webb Space Telescope ( JWST ), a large population is detected in the rest-frame near-infrared (NIR), raising questions about their formation mechanism. Aims. We investigate the presence and properties of NIR overdensities (hereafter substructures, including clumps) in star-forming and quiescent galaxies at 1 < z < 4 to understand their link to the evolution of their host galaxy. Methods. We identified substructures in JWST /NIRCam F277W and F444W residual images at a rest-frame wavelength of 1 μm. Results. The fraction of galaxies with substructures with M _★ > 10 ⁹ M _⊙ has steadily decreased with cosmic time from 40% at z = 4 to 10% at z = 1. NIR clumps, the most common type of small substructure, are much fainter (2% of the total galaxy flux) than similar UV clumps in the literature. Nearly all galaxies at the high-mass end of the main sequence (MS), starburst, and green valley regions have substructures. However, we do not find substructures in low-mass galaxies in the green valley and red sequence. Although massive galaxies on the MS and in the green valley have a 40% probability of hosting multiple clumps, the majority of clumpy galaxies host only a single clump. Conclusions. The fraction of clumpy galaxies in the rest-frame NIR is determined by the stellar mass and star formation rate (SFR) of the host galaxies. Its evolution with redshift is due to galaxies moving toward lower SFRs at z ≲ 2 and the buildup of low-mass galaxies in the green valley and red sequence. Based on their spatial distribution in edge-on galaxies, we infer that most substructures are produced in situ via disk fragmentation. Galaxy mergers may still play a role at high stellar masses, especially at a low SFR.

In this work, we explore the nature of z > 1 galactic bars. Once thought to be highly transient, our results demonstrate otherwise. Our sample consists of nine massive (>10 ^10.5 M _⊙ ) star-forming barred-spiral galaxies at z _spec ∼ 1.5. Using rest-frame near-IR (F444W) JWST/NIRCam imaging, we apply ellipse fitting along with 1D and 2D morphological modeling to directly measure bar properties. We find that five galaxies host flat surface brightness profiles (bar Sérsic index < 0.4), indicative of highly evolved, “mature” bars. By contrast, only two galaxies show exponential profiles, characteristic of young bars, and these are also shorter in absolute length than the flat bars. We therefore conclude that a large fraction of bars at this epoch have already matured, thereby indicating the presence of well-settled disks required to facilitate bar formation and sustained evolution well before z ∼ 1.5. To assess the gravitational impact of the bars, we calculate the maximum transverse-to-radial force ratio ( Q _b ). We find that the Q _b values are comparable to, or weaker than, those of bars in the local Universe; seven of the nine bars show only a marginal increase in strength with maturity (from exponential to flat bars). Contrarily however, the remaining two bars are flat, but have the lowest Q _b values in our sample. We hence propose that the mature bars at z ∼ 1.5 may experience phases of weakening due to rapid gas inflows and/or minor mergers. In conclusion, our work sheds light on the rapidly evolving nature of high- z bars and paves the way for larger statistical studies.

We present the discovery and characterisation of two Ly α nebulae (LANs), RO-1001-Sat and RO-0959-Sat, as satellite structures of two giant LANs at z = 2.920 and 3.092. These two satellite LANs are found neighbouring two out of four known giant LANs at z ∼ 3 in our Multi-Unit Spectroscopic Explorer (MUSE) follow-up observations, reinforcing the idea that Ly α emission can be used as a tracer of massive dark matter halos at high z . This high occurrence of massive satellite halos is in agreement with simulations. With sizes of ≃80 × 160 and 80 × 100 pkpc ² , the two nebulae are both ∼300 pkpc from the main LANs. With Ly α emission only shifted by ≃100 − 300 km s ⁻¹ between each of the two pairs, these two satellite structures are likely connected to their main halos by the large-scale structure. RO-1001-Sat and RO-0959-Sat are estimated to have log( M _h / M _⊙ )≃13.2 ± 0.3 and 12.8 ± 0.3, putting them potentially close to the regime of cold-mode accretion according to several models, which suggests that cold streams should be able to penetrate the virial radii to reach the halo centres without being shock-heated. The central brightest galaxies in the two halos are morphologically distinct despite having a similar stellar mass of ∼10 ¹¹ M _⊙ , one being an elliptical quiescent galaxy in RO-1001-Sat and the other being a dusty star-forming spiral in RO-0959-Sat. Intriguingly, the quiescent galaxy aligns well with the peak of the LAN as well as the potential well of the host halo, making it the first clear-cut case in which the cold gas ought to be accreting onto the galaxy but with no observable star formation, either due to morphological quenching or, more likely, radio-mode feedback from an active galactic nucleus, as supported by excess yet weak radio emission. Finally, we show a tentative detection of a Ly α filament connecting RO-1001 and RO-1001-Sat. This work shows how panoramic MUSE (and in the future, BlueMUSE) observations of massive halo seeds can be used to efficiently search for additional halos, unveiling their large-scale structure and enabling the study of Ly α -selected galaxy groups.

JWST has enabled detecting and spatially resolving the heavily dust-attenuated stellar populations of submillimeter galaxies, revealing detail that was previously inaccessible. In this work, we construct a sample of 289 submillimeter galaxies with joint Atacama Large Millimeter/submillimeter Array (ALMA) and JWST constraints in the COSMOS field. Sources are originally selected using the SCUBA-2 instrument and have archival ALMA observations from various programs. Their JWST NIRCam imaging is from COSMOS-Web and PRIMER. We extract multiwavelength photometry in a manner that leverages the unprecedented near-infrared (NIR) spatial resolution of JWST, and we fit the data with spectral energy distribution models to derive photometric redshifts, stellar masses, star formation rates, and optical attenuation. The sample has an average 〈 z 〉 = 2 . 6 − 0.8 + 1.0 , 〈 A V 〉 = 2 . 5 − 1.0 + 1.5 , 〈 SFR 〉 = 30 0 − 200 + 400 M ⊙ yr − 1 , and 〈 log ( M * / M ⊙ ) 〉 = 11 . 1 − 0.5 + 0.3 . There are 81 (30%) galaxies that have no previous optical/NIR detections, including 75% of the z > 4 subsample (n = 28). The faintest observed NIR sources have the highest redshifts and largest A _V = 4 ± 1. In a preliminary morphology analysis we find that ∼10% of the members of our sample exhibit spiral arms and 5% host stellar bars, with one candidate bar found at z > 3. Finally, we find that the clustering of JWST sources within 10″ of a submillimeter galaxy is a factor of 2 greater than what is expected based on either random clustering or the distribution of sources around any red galaxy irrespective of a submillimeter detection.

Recent JWST observations have revealed the prevalence of spiral structures at z > 1. Unlike in the local Universe, the origin and the consequence of spirals at this epoch remain unexplored. We use public JWST/NIRCam data from the COSMOS-Web survey to map spiral structures in eight massive (>10^10.5 M _⊙) star-forming galaxies at z _spec ∼ 1.5. We present a method for systematically quantifying spiral arms at z > 1, enabling direct measurements of flux distributions. Using rest-frame near-IR images, we construct morphological models accurately tracing spiral arms. We detect offsets (∼0.2–0.8 kpc) between the rest-frame optical and near-IR flux distributions across most arms. Drawing parallels to the local Universe, we conclude that these offsets reflect the presence of density waves. For 9 out of 18 arms, the offsets indicate spiral shocks triggered by density waves. In all, 5 arms have offsets in the opposite direction and are likely associated with tidal interactions. For the remaining cases with no detected offsets, we suggest that stochastic “clumpy” star formation is the primary driver of their formation. In conclusion, we find a multifaceted nature of spiral arms at z > 1, similar to that in the local Universe.

We investigate the deconvolved color profiles of 223 disk galaxies at redshifts of z = 1–3 observed by the James Webb Space Telescope (JWST) as part of the Cosmic Evolution Early Release Science survey (CEERS). The filters were selected to approximate the rest-frame B − Y color, which is used to identify U-shaped color profiles –those becoming progressively bluer with increasing radius, then turning redder beyond a specific point. We find that 36% of Type II (down-bending) disks exhibit U-shaped color profiles with a minimum at or near the disk break. In contrast, no Type I (single-exponential) disks and only 9% of Type III (up-bending) disks show such a profile. The presence of U-shaped color profiles in Type II disks likely arises from the interplay between a star-formation threshold and spiral- or bar-driven secular radial migration of older stars outward. The fraction of Type II disks exhibiting a U-shaped color profile remains almost consistent across two redshift bins, z = 1–2 and z = 2–3, but is significantly lower than that observed in the local Universe, likely because the secular process of radial migration at high redshift may not have had sufficient time to significantly influence the disk structure. The absence of U-shaped color profiles in Type II disks could point to rapid rather than secular radial star migration potentially caused by violent clump instabilities, transporting both younger and older stars to the outer disk. Our results provide useful constraints on the formation and evolution models of disk galaxies in the early Universe.