>Magister Scientiae - MSc / The radio emission of radio-quiet active galactic nuclei (AGN) is thought to be from star formation and AGN related emission. I investigate these sources using 1.4 GHz radio data from FIRST and three optical quasars samples from the SDSS: (i) a volume-limited sample in the redshift range 0:2 < z < 0:4 defined by Mi < -23 (ii) magnitude-limited sample in the redshift range 1:8 < z < 2:5 defined by mr ≤ 18:5 and (iii) a uniform sample in the redshift range 0:2 < z < 3:5 (divided into 12 redshift bins). I constructed radio source counts and radio luminosity functions (RLFs) using the optical quasars detected in FIRST, which are consistent with literature results obtained using SDSS and NVSS quasars. There are differences at the low uxs end because of the different resolutions of FIRST and NVSS. I applied a median stack method to the 12 redshift bins of the uniform sample and found that the median ux decreases from 182 µJy in the lowest redshift bin to 39 µJy and the highest redshift bin. This is because the high redshift quasars although more luminous than their low redshift counterparts, they are much further away so they have lower uxes. I probed the quasar radio source counts to lower levels using reconstructed source counts obtained by applying the Bayesian stack technique. The reconstructed radio source counts were then used to constructed the quasar RLF to lower levels, where I found: (i) for z < 1 the constructed quasar RLF has the same slope as the detected quasars, suggesting that like the detect quasars their radio emission is dominated by AGN related emission (ii) above z = 1 the constructed RLF steepens with redshift, which suggests the strong link between accretion rate and radio jet power is gradually breaking down towards faint optical luminosities at high redshift. / National Astrophysics and Space Science Program (NASSP) and SKA Africa

I carried out systematic studies on the assembly history of dark matter halos, using numerical simulations and semi-analytical methods. First, I look into dark halo mass assembly history. I confirmed that the halo mass assembly is divided into a fast accretion phase and a slow accretion phase. These two phases are found to be separated by the epoch when the dark halo potential reaches its maximum. The fast accretion phase is dominated by mergers, especially major mergers; the slow accretion phase is dominated by slow mass accretion. Each halo experiences about 3±2 major mergers since its main progenitor had a mass equal to 1 percent of halo mass. However, the average redshift at which these major mergers occur is strongly mass dependent. Secondly, I investigate the formation times and the assembly bias of dark halos. I use eight different definitions of halo formation times to characterize the different aspects of the halo assembly history. I find that these formation times have different dependence on halo mass. While some formation times characterize well the hierarchical nature of halo formation, the trend is reversed for other definitions of the formation time. In addition, the formation-time dependence of halo bias is quite strong for some definitions of formation time but weak or absent for others. Thirdly, I study sub-halo mass function in the halo assembly history, with the generally known unevolved sub-halo mass functions (USMFs). I find that for subhalos that merge into the main progenitor of a present-day halo, their USMF can be well described by a universal functional form; the same conclusion can also be reached for the USMF of all sub-halos that have merged during the entire halo merging history. In these two cases, the USMFs do not seem to depend on the redshift of the host halo either. However, due to the mass loss caused by dynamical effects, only small part of the accreted halos survived and became sub-structures in the present-day dark halos. In the cluster-sized halos, 30% survived sub-halos are sub-subhalos. The sub-halo mass function at given accretion time (redshift) is also investigated to find the origin of the statistics mentioned above.

The stellar populations harboured by some of the Universe’s earliest galaxies are within observational reach. Determining the details of these stellar populations and their formation histories within the first billion years after the Big Bang is crucial for both understanding the earliest stages of galaxy evolution and for assessing the contribution of early star-forming galaxies to cosmic reionization. This thesis presents observational measurements of the rest-frame UV and optical colours of star-forming Lyman Break galaxies (LBGs) at redshifts 4 < z < 9, and their inferred stellar population parameters. By combining ground-based ~1 deg² surveys with deeper, narrower space-based deep-field surveys, we have constrained the rest-frame UV spectral slope of galaxies over a wide-range of cosmic time (4 < z < 9) and luminosity (−23 < MUV < −17) in a self-consistent way. To do so, we developed simulations to allow the inference of intrinsic colours from noisy, potentially biased observations. With these simulations, a robust UV colour measurement method was devised in preparation for the Hubble Ultra Deep Field 2012 (UDF12) survey. Then, after delivery of the UDF12 data, our technique and simulations were applied to yield the first bias-free measurements of the UV spectral slope of galaxies at z ≈ 7 and 8. We found no support for the previously claimed dominant sub-population of exotically blue, faint galaxies at z ≈ 7. In fact with careful consideration of their errors and selection biases, even the most extreme galaxies we observed can have their colours explained by stellar population synthesis models of unremarkable parameters. Expanding this study to brighter, rarer, galaxies required the inclusion of wide-area ground-based survey data, and consequently a more focused examination of galaxies at z ≈ 5. We selected high signal-to-noise galaxies from four fields, with absolute magnitudes spanning MUV = −22.5 to −17.5, and measured their rest-frame UV spectral slopes. Coupling these measurements with our simulated observations, we were able to determine the width of the intrinsic colour distribution of galaxies at z ≈ 5. We found that brighter galaxies are not only on average redder than their fainter counterparts, but they are also less self-similar in their colours. The redder average UV colours of brighter galaxies can be attributed to those galaxies being either older, or more dust reddened. By pairing these measurements, which are primarily a probe only of the presently forming portion of the stellar population, with those of LBG’s Balmer Breaks, which are more sensitive to bygone star formation, we were able to break this age–dust degeneracy and conclude that, at z ≈ 5, brighter galaxies are more heavily reddened than fainter galaxies even though their stars are no older.

The ignition of the first sources of light marked the end of the cosmic dark ages, an era when the Universe transitioned from the relatively simple conditions following the Big Bang to the complex tapestry of dark matter, baryons, and pervasive cosmic radiation fields we see today. To better understand this uncharted cosmic epoch, we primarily utilize hydrodynamical, N-body simulations to model the assembly of the first galaxies at redshifts greater than ten and the stars that form within them. These simulations begin from cosmological initial conditions, employ a robust, non-equilibrium chemo-thermodynamic model, and take advantage of adaptive-grid-refinement to probe the multi-scale, complex process of star formation from ab initio principles. We explore the consequences that metal enrichment has on the process of star formation, confirming the presence of a critical metallicity for low-mass star formation. To assess the observational prospects of these primeval stellar populations with next-generation telescopes, like the James Webb Space Telescope, we constrain the star formation efficiency of both metal-enriched and metal-free star formation in a typical first galaxy. We also resolve the formation of individual metal-enriched stars in simulations that ultimately began from cosmological scales, allowing meaningful comparisons between our simulations and the recently discovered ultra-faint dwarf satellite galaxies, the suspected analogs of the first galaxies in the local Universe. / text

This thesis presents results from two simulation studies of galaxy formation. In the first project, a dark-matter-only HORIZON simulation is used to investigate the environment and redshift dependence of mass accretion onto haloes and subhaloes. It is found that the halo accretion rate varies less strongly with redshift than predicted by the Extended Press--Schechter formalism, and that low accretion events may drive the radio-mode feedback hypothesized for recent galaxy formation models. The subhaloes at $z<0.5$ in the simulation accrete at higher rates than haloes, on average, and it is argued that this is due to their enhanced clustering at small scales. There is no dependence of accretion rate on environment at $zsim2$, but a weak correlation emerges at $zleq0.5$. The results further support previous suggestions that at $z>1$, dark matter haloes and their associated black holes grew coevally, but imply that haloes could be accreting at fractional rates that are up to a factor of 3--4 higher than their associated black holes by the present day. In the second project, outputs from one of the Adaptive Mesh Refinement NUT simulations are analyzed in order to test whether filamentary flows of cold gas are responsible for the build-up of angular momentum within a Milky Way type disk at $zgeq3$. A set of algorithms are presented that use the resolved physical scale of $12,mathrm{pc}$ to identify: (i) the central gas disk and its plane of orientation; (ii) the complex individual filament trajectories that connect to the disk, and; (iii) the infalling satellites. The results suggest that two filaments at $zgtrsim 5.5$, which later merge to form a single filament at $zlesssim 4$, drive the angular momentum and mass budget of the disk between $3lesssim zlesssim 8$, whereas luminous satellite mergers make negligible fractional contributions. These findings hence provide strong quantitative evidence that the growth of thin disks in low mass haloes at high redshift is supported via inflowing streams of cold gas.

The processes leading to the widespread presence of crystalline silicates throughout the galaxy and the origin of silicon nanoparticles thought to be responsible for the observed extended red emission in diffuse galactic background are still far from being understood. One of the most abundant oxygen bearing species in molecular astronomical regions is SiO. It has been conjectured that silicate formation probably proceeds via the agglomeration of these molecular species; however there are no studies to reveal the microscopic mechanism. We have used a synergistic approach combining experiments in molecular beams and first principles theoretical calculation to demonstrate that the passage from SiO to Si02 proceeds via gradual oxygen enrichment of SinOm clusters and that the smallest cascade involves Si203, S304, Si405, Si506 as the intermediate products. We also demonstrate that as the SiO molecules cluster together, the chemistry drives the agglomerates towards configurations such that the central core are pure Sin clusters while the outer shell are SiO2 molecules. The gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital range from 0.84 to 3.84 eV and hence can contribute to the observed extended red emission and blue luminescence. The findings are of general interest in Astrophysics but are also critical to a fundamental understanding of the interstellar extinction.

The observable properties of galaxy groups, and especially the thermal and chemical properties of the intragroup medium (IGrM), provide important constraints on the different feedback processes associated with massive galaxy formation and evolution. In this {work}, we present a detailed analysis of the global properties of simulated galaxy groups with X-ray temperatures in the range $0.5 - 2$ keV over the redshift range $0 \leq z \leq 3$. The groups are drawn from a cosmological smoothed particle hydrodynamics simulation that includes a well-constrained prescription for momentum-driven, galactic outflows powered by stars and supernovae but no explicit treatment of AGN feedback. Our aims are (a) to establish a baseline against which we will compare future models; (b) to identify model successes that are genuinely due to stellar/supernovae-powered outflows; and (c) to pinpoint mismatches that not only signal the need for AGN feedback but also constrain the nature of this feedback. We find that even without AGN feedback, our simulation successfully reproduces the observed present-day group properties such as the IGrM mass fraction, the various X-ray luminosity-temperature-entropy scaling relations, as well as both the mass-weighted and the emission-weighted IGrM iron and silicon abundance versus IGrM temperature relationships, for all but the most massive groups. We also show that these trends evolve self-similarly for $z < 1$, in agreement with the observations. In contrast to the usual expectations, we do not see any evidence of the IGrM undergoing catastrophic cooling. And yet, the $z=0$ group stellar mass is a factor of $\sim 2$ too high. Probing further, we find that the latter is due to the build-up of cold gas in the massive galaxies {\it before} they are incorporated inside groups. This not only indicates that another feedback mechanism must activate as soon as the galaxies achieve $M_*\approx$ a few $\times 10^{10}\;\rm{M_{\odot}}$ but that this feedback mechanism must be powerful enough to expel a significant fraction of the halo gas component from the galactic halos. ``Maintenance-mode" AGN feedback of the kind observed in galaxy clusters will not do. At the same time, we find that stellar/supernovae-powered winds are essential for understanding the metal abundances in the IGrM and these results are expected to be relatively insensitive to the addition of AGN feedback. We further examine the detailed distribution of the metals within the groups and their origin. We find that our simulated abundance profiles fit the observational data pretty well except that in the innermost regions, there appears to have an excess of metals in the IGrM, which is attributed to the overproduction of stars in the central galaxies. The fractional contribution of the different types of galaxies varies with radial distances from the group center. While the enrichment in the core regions of the groups is dominated by the central and satellite galaxies, the external galaxies become more important contributors to the metals at $r\simgreat R_{500}$. The IGrM at the groups' outskirts is enriched at comparatively higher redshifts, and by relatively less massive galaxies. / Graduate

In my thesis I investigate the luminous z ~ 3 Lyman break galaxies in deep wide field surveys. In the first part of the thesis, I use the LBT/LUCIFER to observe a lensed high-redshift star-forming galaxy (J0900+2234) at z = 2.03. With the high S/N near-IR spectroscopic observations, I reveal the detailed physical properties of this high-redshift galaxy, including SFR, metallicity, dust extinction, dynamical mass, and electron number density. In the second part of the thesis, I select a large sample of LBGs at z ~ 3 from our new LBT Bootes field survey, and study the bright end luminosity function (LF), stellar mass function (SMF) and clustering properties of bright LBGs (1L* < L < 2.5L*). Together with other LF and SMF measurements, the evolution of LF and SMF can be well described by continuously rising star formation history model. Using the clustering measurements in this work and other works, a tight relation between the average host galaxy halo mass and the galaxy star formation rate is found, which can be interpreted as arising from cold flow accretion. The relation also suggests that the cosmic star formation efficiency is about 5%-20% of the total cold flow mass. This cosmic star formation efficiency does not evolve with redshift (from z ~ 5 to z ~ 3), hosting dark matter halo mass (10¹¹-10¹³ M⊙), or galaxy luminosity (from 0.3L* to 3L*).In the third and fourth parts, with the spectroscopic follow-up observations of the bright LBGs, I establish a sample of spectroscopically-confirmed ultra-luminous LBGs (ULBGs) in NOAO Bootes field. With this new ULBG sample, the rest-frame UV LF of LBG at M(1700Å) = -23.0 was measured for the first time. I find that the ULBGs have larger outflow velocity, broader Lyα emission and ISM absorption line profiles, and more prominent CIV P-Cygni profile. This profile may imply a top-heavy IMF in these ULBGs. The ULBGs have larger stellar mass and SFR, but smaller dust extinction than the typical L* LBGs at z ~ 2 - 3. We proposed two evolutionary scenarios, pre-burst and post-burst. The properties of the ULBGs, especially the morphologies, prefer the pre-starburst scenario. Further high spatial resolution HST imaging and IFU spectroscopic observations will allow us to distinguish these two scenarios.

This thesis considers various topics and open questions in galaxy formation during the epoch of reionization and presents multiple new computational techniques developed specifically to study this era. This work naturally divides into two main sections: 1) The formation of the first massive black holes and 2) Interpreting ALMA observations of galaxy formation during the epoch of reionization. The first topic addresses the existence of super massive black holes (SMBHs) with $M_{\rm BH} > 10^9$M$_{\odot}$ at $z > 6$. It is well established that stellar mass black holes are very unlikely to be able to accrete matter efficiently enough to grow to this mass at this redshift. For this reason, many alternative channels have been proposed for black hole formation that produce objects with significantly larger initial masses. In this thesis, I consider a mechanism whereby runaway stellar collisions in dense primordial star clusters form a very massive star that is likely to collapse to an intermediate mass black hole (IMBH) with $M_{\rm BH} > 10^3$M$_{\odot}$. In order to test this scenario, I added 12 species non-equilibrium chemistry to the massively parallel adaptive mesh refinement code RAMSES, and simulated, at sub-pc resolution, the collapse of the first metal-enriched halo which is likely to host a Population II star cluster. The properties of the central gas cloud in the collapsing halo were then extracted from the simulation and used to create initial conditions for the direct N-body integrator, NBODY6. These star clusters were simulated for 3.5Myr (until the first supernova is expected to occur) and it was determined that the properties of the gas clouds that form in cosmological simulations were indeed suitable to form a very massive star by collisional runaway. This suggests that this mechanism is a promising channel for forming the seeds of SMBHs at high redshift. The second topic of this thesis aims to help interpret the plethora of recent and upcoming ALMA observations of star forming galaxies during the epoch of reionization. These observations target far-infrared lines such as [CII] and [OIII] which directly probe the interstellar medium (ISM) of these $z > 6$ galaxies. In order to study this epoch, I employ the RAMSES-RT code, which allows for the computation of multifrequency radiative transfer on-the-fly. I modified this code in a number of ways so that it can handle radiation-coupled H$_2$ non-equilibrium chemistry (including Lyman-Werner band radiation) and I developed the variable speed of light approximation which changes the speed of light in the simulation depending on the density of gas so that ionisation fronts propagate at the correct speed in all gas phases. Cosmological boxes were initialised to include galaxies with masses comparable to the observations of Maiolino et al. (2015) and run at various resolutions to test convergence properties. One of the major goals of this study was to identify the physical mechanism responsible for the spatial offset observed between [CII] and UV/Lyα in many high-redshift galaxies.

abstract: New measurements of the Hα luminosity function (LF) and star formation rate (SFR) volume density are presented for galaxies at z∼0.62 in the COSMOS field. These results are part of the Deep And Wide Narrowband Survey (DAWN), a unique infrared imaging program with large areal coverage (∼1.1 deg 2 over 5 fields) and sensitivity (9.9 × 10 −18 erg/cm 2 /s at 5σ). The present sample, based on a single DAWN field, contains 116 Hα emission- line candidates at z∼0.62, 25% of which have spectroscopic confirmations. These candidates have been selected through comparison of narrow and broad-band images in the infrared and through matching with existing catalogs in the COSMOS field. The dust-corrected LF is well described by a Schechter function with L* = 10 42.64±0.92 erg s −1 , Φ* = 10 −3.32±0.93 Mpc −3 (L* Φ* = 10 39.40±0.15 ), and α = −1.75 ± 0.09. From this LF, a SFR density of ρ SF R =10 −1.37±0.08 M○ yr −1 Mpc −3 was calculated. An additional cosmic variance uncertainty of ∼ 20% is also expected. Both the faint end slope and luminosity density that are derived are consistent with prior results at similar redshifts, with reduced uncertainties. An analysis of these Hα emitters’ sizes is also presented, showing a direct corre- lation between the galaxies’ sizes and their Hα emission. / Dissertation/Thesis / Masters Thesis Astrophysics 2017