Recent observations have suggested that the C III] lambda 1907/1909 emission lines could be alternative diagnostic lines for galaxies in the reionization epoch. We use the F128N narrowband filter on the Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3) to search for C III] emission in a sample of five galaxies at z = 5.7 in the Subaru Deep Field and the Subaru/XMM-Newton Deep Field. Using the F128N narrowband imaging, together with the broadband imaging, we do not detect C III] emission for the five galaxies with JAB ranging from 24.10 to 27.00 in our sample. For the brightest galaxy J132416.13+274411.6 in our sample (z = 5.70, J(AB) = 24.10), which has a significantly higher signal to noise, we report a C III] flux of 3.34 +/- 1.81 x 10(-18) erg s(-1)cm(-2), which places a stringent 3 sigma upper limit of 5.43 x 10(-18) erg s(-1)cm(-2) on C III] flux and 6.57 angstrom on the C III] equivalent width. Using the stacked image, we put a 3 sigma upper limit on the mean C III] flux of 2.55 x 10(-18) erg s(-1) cm(-2) and a 3 sigma upper limit on the mean C III] equivalent width of 4.20 angstrom for this sample of galaxies at z = 5.70. Combined with strong C III] detection reported among high-z galaxies in the literature, our observations suggest that the equivalent widths of C III] from galaxies at z > 5.70 exhibit a wide range of distribution. Our strong limits on C III] emission could be used as a guide for future observations in the reionization epoch.

Luminous high-redshift quasars can be used to probe of the intergalactic medium in the early universe because their UV light is absorbed by the neutral hydrogen along the line of sight. They help us to measure the neutral hydrogen fraction of the high-z universe, shedding light on the end of reionization epoch. In this paper, we present a discovery of a new quasar (PSO J006.1240+39.2219) at redshift z = 6.61 +/- 0.02 from Panoramic Survey Telescope & Rapid Response System 1.Including this quasar, there are nine quasars above z > 6.5 up to date. The estimated continuum brightness is M-1450 = -25.96 +/- 0.08. PSO J006.1240+39.2219 has a strong Ly alpha emission compared with typical low-redshift quasars, but the measured near-zone region size is R-NZ = 3.2 +/- 1.1 proper megaparsecs, which is consistent with other quasars at z similar to 6.

Theoretical models predict that some of the first stars ended their lives as extremely energetic pair-instability supernovae (PISNe). With energies approaching 10⁵³ ergs, these supernovae are expected to be within the detection limits of the upcoming James Webb Space Telescope (JWST), allowing observational constraints to be placed on the properties of the first stars. We estimate the source density of PISNe using a semi-analytic halo mass function based approach, accounting for the effects of feedback from star formation on the PISN rate using cosmological simulations. We estimate an upper limit of ~0.2 PISNe per JWST field of view at any given time. Feedback can reduce this rate significantly, e.g., lowering it to as little as one PISN per 4000 JWST fields of view for the most pessimistic explosion models. We also find that the main obstacle to observing PISNe from the first stars is their scarcity, not their faintness; exposures longer than a few times 10⁴ s will do little to increase the number of PISNe found. Given this we suggest a mosaic style search strategy for detecting PISNe from the first stars. Even rather high redshift PISNe are unlikely to be missed by moderate exposures, and a large number of pointings will be required to ensure a detection. / text

The formation of the first stars, also known as Population III (Pop III), marked a pivotal point in the universe's evolution from relative smoothness and homogeneity to its current highly structured state. In this dissertation we study key aspects of Pop III star formation. We utilize three-dimensional cosmological simulations to follow the evolution of gas and DM from z ~100 until the first minihalo forms. Once the gas infalls toward the center of the minihalo and condenses, we implement the 'sink particle' method to represent regions that will form a star, and we follow the evolution of the metal-free, star-forming gas for many free-fall times. A disk forms around the initial Pop III star and fragments to form secondary stars with a range of masses (1 - 50 [solar mass]). This is markedly different from the previous paradigm of one single, massive star forming per minihalo. Using a ray-tracing technique, we also examine the effect of radiative feedback on protostellar growth and disk fragmentation. This feedback will not prevent the formation of secondary stars within the disk, but will reduce the final mass reached by the largest Pop III star. Measuring the angular momentum of the gas that falls onto the sink regions, we also find that the more massive Pop III stars accrete sufficient angular momentum to rotate at nearly break-up speeds, and can potentially end their lives as collapsar gamma-ray bursts or hypernovae. We furthermore numerically examine the recently discovered relative streaming motions between dark matter and baryons, originating from the era of recombination. Relative streaming will slightly delay the redshift at which Pop III stars first form, but will otherwise have little impact on Pop III star formation and the history of reionization. We finally evaluate the possible effect of a cosmic ray (CR) background generated by the supernova deaths of massive Pop III stars. A sufficiently large CR background could indirectly enhance the H₂ cooling within the affected minihalos. The resulting lower temperatures would lead to a reduced characteristic stellar mass (~ 10 [solar mass]), providing another possible pathway to form low-mass Pop III stars. / text

The first stars are something many scientists are curious about. How did they formand how did the universe look like at that time? These stars however probably died along time ago, or are at a distance too far away from us to be observable. If these starsexploded and formed supernovae there might be stars observable today that formed fromthe ejecta of these supernovae. Models of nucleosynthesis in the first stars may potentiallybe used to infer parameters of progenitor supernovae from chemical abundances in old,metal-poor stars. This thesis aim is to find out how precise these abundances need to beto achieve a certain precision in the supernova parameters. This is done by perturbingthe abundances for one element at a time in four different stars and see how the recoveredsupernova parameters change. The first conclusion is that it isn’t necessarily the sameelements that are important for determining the supernova parameters in all stars, andif there is one thing that decides which elements are important it would be the star’smetallicity. The stars HE1327-2326 and HE0107-5240 metallicites are close to each other,with [Fe/H] < -5.0 and [Fe/H] = -5.3 respectively, and behaved similarly, the elementmost important in both stars was nitrogen. The star SMSS0313-6708, [Fe/H] < -7.3, hadmore elements that changed the parameters, the most important beeing C, O and Ca.The star CS 31028-001, [Fe/H] = -2.9, also had many elements that were important, andsome of them are Na, K and Ni. / De första stjärnorna är någonting många forskare är nyfikna på. Hur formades deoch hur såg universum ut vid den tiden? Dessa stjärnor dog antagligen länge sedan,eller så är de på ett avstånd för långt bort från oss för att kunna observeras. Om dessastjärnor exploderade och bildade supernovor är det möjligt att det finns stjärnor som gåratt observera som bildades ur dessa supernovor. Modeller av nukleosyntes i dessa förstastjärnor kan potentiellt användas för att anta parametrar för de föregående supernovorur den kemiska sammansättningen för gamla, metalfattiga stjärnor. Målet för dennauppsats är att ta reda på hur exakta värden som behövs på dessa kemiska sammansättningarför att nå en viss säkerhet på supernovaparametrarna. Det görs genom att ändraämneshalten för ett ämne i taget i fyra olika stjärnor och se hur de beräknade supernovamodellsparametrarnaändras. Den första slutsatsen är att det inte nödvändigtvis ärsamma ämnen som är viktiga för att bestämma supernovaparametrarna i varje stjärna,och om det är en sak som bestämmer vilket ämne som är viktigt skulle det vara stjärnansmetallicitet. Stjärnorna HE1327-2326 och HE0107-5240 har metalliciteter som är näravarandra, med respektive [Fe/H] < -5.0 och [Fe/H] = -5.3, och betedde sig snarlikt, så vardet kväve som var det viktigaste ämnet i dessa två stjärnor. Stjärnan SMSS0313-6708,[Fe/H] < -7.3, hade fler ämnen som ändrade parametrarna och de viktigaste var C, O ochCa. Stjärnan CS 31028-001, [Fe/H] = -2.9, hade också många ämnen som var viktiga ochnågra av dem var Na, K och Ni.

The fraction of ionizing photons escaping from high-redshift star-forming galaxies is a key obstacle in evaluating whether galaxies were the primary agents of cosmic reionization. We previously proposed using the covering fraction of low-ionization gas, measured via deep absorption-line spectroscopy, as a proxy. We now present a significant update, sampling seven gravitationally lensed sources at 4 < z < 5. We show that the absorbing gas in our sources is spatially inhomogeneous, with a median covering fraction of 66%. Correcting for reddening according to a dust-in-cloud model, this implies an estimated absolute escape fraction of similar or equal to 19% +/- 6%. With possible biases and uncertainties, collectively we find that the average escape fraction could be reduced to no less than 11%, excluding the effect of spatial variations. For one of our lensed sources, we have sufficient signal-tonoise ratio to demonstrate the presence of such spatial variations and scatter in its dependence on the Ly alpha equivalent width, consistent with recent simulations. If this source is typical, our lower limit to the escape fraction could be reduced by a further factor similar or equal to 2. Across our sample, we find a modest anticorrelation between the inferred escape fraction and the local star formation rate, consistent with a time delay between a burst and leaking Lyman continuum photons. Our analysis demonstrates considerable variations in the escape fraction, consistent with being governed by the small-scale behavior of star-forming regions, whose activities fluctuate over short timescales. This supports the suggestion that the escape fraction may increase toward the reionization era when star formation becomes more energetic and burst-like.

We present the HST WFC3/F275W UV imaging observations of A2218-Flanking, a lensed compact dwarf galaxy at redshift z approximate to 2.5. The stellar mass of A2218-Flanking is log(M-*/M-circle dot) = 9.14(-0.04)(+0.07) and SFR is 12.5(-7.4)(+3.8) M-circle dot yr(-1) after correcting the magnification. This galaxy has a young galaxy age of 127. Myr and a compact galaxy size of r(1/2) = 2.4 kpc. The HST UV imaging observations cover the rest-frame Lyman continuum (LyC) emission (similar to 800 angstrom) from A2218-Flanking. We firmly detect (14s) the LyC emission in A2218-Flanking in the F275W image. Together with the HST F606W images, we find that the absolute escape fraction of LyC is f(abs,esc) > 28%-57% based on the flux density ratio between 1700 and 800 angstrom (f(1700)/f(800)). The morphology of the LyC emission in the F275W images is extended and follows the morphology of the UV continuum morphology in the F606W images, suggesting that the f(800) is not from foreground contaminants. We find that the region with a high star formation rate surface density has a lower f(1700)/f(800) (higher f(800)/f(1700)) ratio than the diffused regions, suggesting that LyC photons are more likely to escape from the region with the intensive star-forming process. We compare the properties of galaxies with and without LyC detections and find that LyC photons are easier to escape in low-mass galaxies.

This thesis investigates the potential for detection and identification of primordial stars, galaxies, and supernovae at high redshift. Simulations indicate that the first Population III stars should appear in minihalos of mass M = 105-106 Msol at z ≈ 10-30. To assess the detectability of these objects, theoretical models of these stars and their surrounding HII regions are used. We assess the plausibility of detection with the upcoming James Webb Space Telescope (JWST), using the gravitational lensing provided by the galaxy cluster MACSJ0717.5+3745. The conclusion is that the detection of these objects is highly improbable but not impossible. To investigate the prospects of detecting and identifying the first galaxies, the spectral synthesis code Yggdrasil is introduced. According to this code, JWST may be able to detect Population III galaxies with stellar masses as low as 105 Msol at z ≈ 10 in unlensed fields. We find that, over limited redshift intervals, it could be possible to use Hubble Space Telescope (HST) and/or JWST broadband color criteria to single out Population III galaxy candidates. The prospects of detecting gravitationally lensed Population III galaxies with JWST and HST is investigated. A lower limit to detect ≈1 Population III galaxy of ε ≈ 10-2 (HST/CLASH) and ε ≈ 10-3 (JWST using MACS J0717.5+3745 as lens) is derived, where ε is the baryon fraction converted to Population III stars in a host halo. By fitting HST/CLASH data to Yggdrasil and comparison grids, two Population III galaxy candidates are discovered. These two candidates are the first Population III galaxy candidates discovered at z &gt; 6.5. A highly-magnified and doubly lensed extremely high-redshift (z ≈ 7.8) object is also identified. Finally the prospects of detecting core-collapse (CC) supernovae (SN) from the first galaxies at z ≈ 5-12 are investigated. The prediction is that no primordial SN is detectable, but 2-3 CC SN should be discovered by the HST/CLASH. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Submitted. Paper 6: Submitted. Paper 7: Manuscript.</p>

We investigate the evolution of dust content in galaxies from redshifts z = 0 to z = 9.5. Using empirically motivated prescriptions, we model galactic-scale properties-including halo mass, stellar mass, star formation rate, gas mass, and metallicity-to make predictions for the galactic evolution of dust mass and dust temperature in main-sequence galaxies. Our simple analytic model, which predicts that galaxies in the early universe had greater quantities of dust than their low-redshift counterparts, does a good job of reproducing observed trends between galaxy dust and stellar mass out to z approximate to 6. We find that for fixed galaxy stellar mass, the dust temperature increases from z = 0 to z = 6. Our model forecasts a population of low-mass, high-redshift galaxies with interstellar dust as hot as, or hotter than, their more massive counterparts; but this prediction needs to be constrained by observations. Finally, we make predictions for observing 1.1 mm flux density arising from interstellar dust emission with the Atacama Large Millimeter Array.

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