We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer and the Ultraviolet-Visual Echelle Spectrograph on the Very Large Telescope. Following up on our earlier study, we focus here on two major topics: (1) small-scale variability (Δz ≈ 10-3) in the ratio η = N(He II)/N(H I) and (2) an observed correlation of high-η absorbers (soft radiation fields) with voids in the (H I) Lyα distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Given the photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 < τHe II < 2.3 and 0.02 < τH I < 3.9 and reliably determine values of η ≈ 4τHe II/τH I over the range 0.1-460. Values η = 20-200 are consistent with models of photoionization by quasars with observed spectral indices α s = 0-3. Values η > 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with η < 30 may indicate the presence of local hard sources. We find that η is higher in "void" regions, where H I is weak or undetected and ∼80% of the path length has η > 100. These voids may be ionized by local soft sources (dwarf starbursts) or by QSO radiation softened by escape from the active galactic nucleus cores or transfer through the "cosmic web." The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs of H I (zH I ∼ 6.2 ± 0.2) and He II (zHe II ∼ 2.8 ± 0.2).

The slope of the ultraviolet continuum emissions from a galaxy between 1250 and 2600 Å provides insights about several facets of the galaxy. Mainly, it is well-correlated with the amount of dust. This work presents a search for objects whose UV-continuum slopes are excessively steep, as well as suggestions for follow-up. The method used is looking through existing data sets, and proposing follow-up of the outliers in the distribution of slopes. Close to fifteen objects with slopes beyond what is easily explained by theory are presented. Since these lie beyond the realm of current theories, confirmations of these may hint at more extreme stellar populations than those currently known. This may include excessively metal-poor stars such as population III stars, or stellar populations where the initial mass function (IMF) for some reason may be biased towards massive stars. Steeper slopes are in general indicative of a lack of dust and an abundance of hot, blue stars; this is due to the reddening caused by dust, and emissions from cooler stars being peaked at longer wavelengths.

Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z(circle dot)/2. Such tests are particularly important for interpreting the surprising high-ionization ultraviolet (UV) line emission detected at z > 6 in recent years. We present HST/COS UV spectra of 10 nearby metal-poor star-forming galaxies selected to show He II emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + logO/H < 8.5) and present uniformly large specific star formation rates (sSFR similar to 10(2) Gyr(-1)). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching CIII] equivalent widths comparable to those seen in systems at z similar to 6-7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + logO/H less than or similar to 8.0 (Z/Z(circle dot) less than or similar to 1/5) presenting minimal stellar wind features and prominent nebular emission in He II and C IV. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He+-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often-neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C IV and He II powered by these stars may provide useful metallicity constraints in the reionization era.

We compare the UV-optical colors of a well-defined set of optically selected pre-merger interacting galaxy pairs with those of normal spirals. The shorter wavelength colors show a larger dispersion for the interacting galaxies than for the spirals. This result can best be explained by higher star formation rates on average in the interacting galaxies, combined with higher extinctions on average. This is consistent with earlier studies which found that the star formation in interacting galaxies tends to be more centrally concentrated than in normal spirals, perhaps due to gas being driven into the center by the interaction. As noted in earlier studies, there is a large variation from galaxy to galaxy in the implied star formation rates of the interacting galaxies, with some galaxies having enhanced rates but others being fairly quiescent.

We have used the Galaxy Evolution Explorer (GALEX) ultraviolet telescope to study stellar populations and star formation morphology in a well-defined sample of 42 nearby optically selected pre-merger interacting galaxy pairs. Galaxy interactions were likely far more common in the early universe than in the present; thus our study provides a nearby well-resolved comparison sample for high-redshift studies. We have combined the GALEX near-ultraviolet (NUV) and far-ultraviolet images with broadband optical maps from the Sloan Digitized Sky Survey to investigate the ages and extinctions of the tidal features and the disks. The distributions of the UV/optical colors of the tidal features and the main disks of the galaxies are similar; however, the tidal features are bluer on average in NUV - g when compared with their own parent disks; thus tails and bridges are often more prominent relative to the disks in UV images compared to optical maps. This effect is likely due to enhanced star formation in the tidal features compared to the disks rather than reduced extinction; however, lower metallicities may also play a role. We have identified a few new candidate tidal dwarf galaxies in this sample. Other interesting morphologies such as accretion tails and "beads on a string" are also seen in these images. We also identify a possible "Taffy" galaxy in our sample, which may have been produced by a head-on collision between two galaxies. In only a few cases are strong tidal features seen in H I maps but not in GALEX.

Depuis le milieu des années 1990, la taille des échantillons de galaxies situées à très grande distance (“redshift”) de nous, au tout début de la vie de l'univers a augmenté grace à la sensibilité croissante des télescopes optiques / infrarouge proche. Cependant, les propriétés des poussières continues dans ces galaxies sont mal connues en raison de l'absence d'observations profondes en infrarouge lointain et en sous-mm. Cette thèse explore les propriétés multi-longueur d'onde d'une population de galaxies observées ~ 2Gyr après le Big Bang. Notre échantillon comprend 22000 galaxies, et il a été sélectionné à partir de la discontinuité de Lyman. Nous utilisons une technique statistique, appelée analyse d'empilement, qui combine le signal provenant d'un grand nombre de sources individuellement non détectées dans le but d’ameliorer les limites de détection par rapport aux observations actuelles, à ces longueurs d'onde. Elle nous permet d'obtenir une distribution spectrale d’énergie complete de l’ultraviolet lointain à l’infrarouge lointain, et d'étudier la formation des étoiles et l'atténuation par la poussière sur ces galaxies. / Since the mid-1990s, the sample of galaxies in the early universe has been growing thanks to the increasing sensitivities in the Optical/NIR telescopes. However, their dust properties are poorly known due to the lack of deep far-infrared or sub-mm observations. This thesis explores the multi-wavelength properties of a population of galaxies observed ~2Gyr after the Big Bang. Our sample includes 22000 galaxies, and it has been selected by the classical U-dropout or Lyman Break technique. We use a statistical technique, called stacking analysis, that combines the signal from a large number of sources to lower the detection limits on the current long wavelengths observations. It allows us to obtain data over the full FUV-to-FIR spectral domain, and study the star formation and dust attenuation of these galaxies.

We present a survey of 72 Seyfert galaxies and quasars observed by the Far Ultraviolet Spectroscopic Explorer (FUSE). Our survey is the largest to date searching for intrinsic UV absorption with high spectral resolution, and is the first step toward a more comprehensive study of intrinsic absorption in low-redshift AGN. We have determined that 72 of 253 available active galactic nuclei (AGN) are viable targets for detection of intrinsic absorption lines. We examined these spectra for signs of intrinsic absorption in the O VI doublet (lambda lamdba 1031.9, 1037.6) and Lyman beta (lambda 1025.7). The fraction of Seyfert 1 galaxies and low-redshift quasars at z<0.15 that show evidence of intrinsic UV absorption is ~50, which is slightly lower than Crenshaw et al. found (60%) based on a smaller sample of C IV absorption in Seyfert 1 galaxies observed with the Hubble Space Telescope (HST). The intrinsic absorption lines are mostly blueshifted with respect to the rest frame of the galaxy, indicating outflow of ionized gas from the AGN. With this new fraction we find a global covering factor of the absorbing gas with respect to the central nucleus of ~0.4. We also provide a deeper analysis of the intrinsic absorption features we found in 35 objects. We have characterized the relation between luminosity and velocity, and examined the relationships between equivalent width, full width at half maximum, velocity and continuum flux. The luminosity/velocity correlation has been explored previously by Laor & Brandt (2002), but at a significantly higher redshift and heavily weighted by Broad Absorption Line quasars. Our survey is for lower redshift and lower luminosity objects, mostly Seyfert galaxies. We have also explored each object with multiple observations for variablity in each of the aforementioned quantities and characterized the variation of equivalent width with continuum flux. Variability for low-z AGN has been seen in the past. In our survey, we find that variability of O VI (lambda lambda 1032, 1038) is less common than for the UV doublets of C IV and N V seen at longer wavelengths, because the O VI absorption is usually saturated. Lyman beta absorption variability is more frequent. In the target-by-target examination we find that Broad Absorption Line (BAL) features and Narrow Absorption Line (NAL) features are related, in that they follow a single relationship between the maximum outflow velocity and the AGN luminosity, and both can be exhibited in similar luminosity objects. IRAS F22456-5125 is one particular Seyfert galaxy that we have selected for modelling due to its interesting assortment of intrinsic absorption lines. It shows a system of five individual kinematic components of absorption features in both O VI lines and in several of the Lyman series lines. We find that each of the components are relatively simple to model and appear to be weak in the X-ray.

We present new spatially resolved surface photometry in the far-ultraviolet (FUV) and near-ultraviolet (NUV) from images obtained by the Galaxy Evolution Explorer (GALEX) and IRAC1 (3.6 mu m) photometry from the Spitzer Survey of Stellar Structure in Galaxies (S(4)G). We analyze the radial surface brightness profiles mu(FUV), mu(NUV), and mu[3.6], as well as the radial profiles of (FUV - NUV), (NUV -[3.6]), and (FUV -[3.6]) colors in 1931 nearby galaxies (z < 0.01). The analysis of the 3.6 mu m surface brightness profiles also allows us to separate the bulge and disk components in a quasi-automatic way and to compare their light and color distribution with those predicted by the chemo-spectrophotometric models for the evolution of galaxy disks of Boissier & Prantzos. The exponential disk component is best isolated by setting an inner radial cutoff and an upper surface brightness limit in stellar mass surface density. The best-fitting models to the measured scale length and central surface brightness values yield distributions of spin and circular velocity within a factor of two of those obtained via direct kinematic measurements. We find that at a surface brightness fainter than mu([3.6]) = 20.89 mag arcsec(-2), or below 3 x 10(8) M-circle dot kpc(-2) in stellar mass surface density, the average specific star formation rate (sSFR) for star-forming and quiescent galaxies remains relatively flat with radius. However, a large fraction of GALEX Green Valley galaxies show a radial decrease in sSFR. This behavior suggests that an outside-in damping mechanism, possibly related to environmental effects, could be testimony of an early evolution of galaxies from the blue sequence of star-forming galaxies toward the red sequence of quiescent galaxies.

We combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium (ISM) to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. We start by assessing the reliability of our stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large Magellanic Cloud clusters. In doing so, we find that neglecting stochastic sampling of the stellar initial mass function in these young (similar to 10-100 Myr), low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass. Then, we proceed and develop a simple approach, based on an idealized description of the main features of the ISM, to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies. Our model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds, as well as for radiative transfer through a diffuse intercloud medium. We use this approach to explore the entangled signatures of stars, the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find that, aside from a few notable exceptions, most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM, which increases with metallicity. We also identify several nebular-emission and interstellar-absorption features, which stand out as particularly clean tracers of the different phases of the ISM.

We have conducted a statistical analysis of the ultra-luminous X-ray point sources (ULXs; L X ≥ 10 39 ergs -1) in a sample of galaxies selected from the Arp Atlas of Peculiar Galaxies. We find a possible enhancement of a factor of 2-4 in the number of ULXs per blue luminosity for the strongly interacting subset. Such an enhancement would be expected if ULX production is related to star formation, as interacting galaxies tend to have enhanced star formation rates on average. For most of the Arp galaxies in our sample, the total number of ULXs compared to the far-infrared luminosity is consistent with values found earlier for spiral galaxies. This suggests that for these galaxies, ULXs trace recent star formation. However, for the most infrared-luminous galaxies, we find a deficiency of ULXs compared to the infrared luminosity. For these very infrared-luminous galaxies, active galactic nuclei may contribute to powering the far-infrared; alternatively, ULXs may be highly obscured in the X-ray in these galaxies and therefore not detected by these Chandra observations. We determined local UV/optical colors within the galaxies in the vicinity of the candidate ULXs using Galaxy Evolution Explorer UV and Sloan Digitized Sky Survey optical images. In most cases, the distributions of colors are similar to the global colors of interacting galaxies. However, the u - g and r - i colors at the ULX locations tend to be bluer on average than these global colors, suggesting that ULXs are preferentially found in regions with young stellar populations. In the Arp sample there is a possible enhancement of a factor of 2-5 in the fraction of galactic nuclei that are X-ray-bright compared to more normal spirals.