To search for Tidal Dwarf Galaxies (TDGs) and to study star formation (SF) in tidal features, we are conducting a large UV imaging survey of interacting galaxies selected from the Arp (1996) Atlas using the Galaxy Evolution Explorer (GALEX) telescope. As part of that study, we present a GALEX UV and Sloan Digital Sky Survey and SARA optical study of the gas-rich interacting galaxy pair Arp 305 (NGC 4016/7). The GALEX UV data reveal much extended diffuse UV emission and SF outside the disks. This includes a luminous star-forming region between the two galaxies, and a number of such regions in tidal tails. We have identified 45 young star-forming clumps in Arp 305, including several TDG candidates. By comparing the UV and optical colors to population synthesis models, we determined that the clumps are very young, with several having ages ∼6 Myr. We do not find many intermediate age clumps in spite of the fact that the last closest encounter was about 300 Myr ago. We have used a smooth particle hydrodynamics code to model the interaction and determine the fate of the star clusters and candidate TDGs.

We present Spitzer infrared, Galaxy Evolution Explorer UV, and Sloan Digitized Sky Survey and Southeastern Association for Research in Astronomy optical images of the peculiar interacting galaxy pair Arp 285 (NGC 2856/4), and compare with a new numerical model of the interaction. We estimate the ages of clumps of star formation in these galaxies using population synthesis models, carefully considering the uncertainties on these ages. This system contains a striking example of "beads on a string": a series of star-formation complexes 1 kpc apart. These "beads" are found in a tail-like feature that is perpendicular to the disk of NGC 2856, which implies that it was formed from material accreted from the companion NGC 2854. The extreme blueness of the optical/UV colors and redness of the mid-infrared colors implies very young stellar ages (4-20 Myr) for these star-forming regions. Spectral decomposition of these "beads" shows excess emission above the modeled stellar continuum in the 3.6 μm and 4.5 μm bands, indicating either contributions from interstellar matter to these fluxes or a second older stellar population. These clumps have -12.0 < M B< -10.6, thus they are less luminous than most dwarf galaxies. Our model suggests that bridge material falling into the potential of the companion overshoots the companion. The gas then piles up at apogalacticon before falling back onto the companion, and star formation occurs in the pile-up. There was a time delay of 500 Myr between the point of closest approach between the two galaxies and the initiation of star formation in this feature. A luminous (M B -13.6) extended (FWHM 1.3 kpc) "bright spot" is visible at the northwestern edge of the NGC 2856 disk, with an intermediate stellar population (400-1500 Myr). Our model suggests that this feature is part of a expanding ripple-like "arc" created by an off-center ring-galaxy-like collision between the two disks.

We present Spitzer mid-infrared imaging of a sample of 35 tidally distorted premerger interacting galaxy pairs selected from the Arp Atlas. We compare their global mid-infrared properties with those of normal galaxies from the SINGS Spitzer Legacy survey, and separate the disk emission from that of the tidal features. The [8.0 μm] - [24 μm], [3.6 μm] - [24 μm], and [5.8 μm] - [8.0 μm] colors of these optically selected interacting galaxies are redder on average than those of spirals, implying enhancements to the mass-normalized star formation rates (SFRs) of a factor of ∼2. Furthermore, the 24 μm emission in the Arp galaxies is more centrally concentrated than that in the spirals, suggesting that gas is being concentrated into the inner regions and fueling central star formation. No significant differences can be discerned in the shorter wavelength Spitzer colors of the Arp galaxies compared to the spirals, and thus these quantities are less sensitive to star formation enhancements. No strong trend of Spitzer color with pair separation is visible in our sample; this may be because our sample was selected to be tidally disturbed. The tidal features contribute ≤ 10% of the total Spitzer fluxes on average. The SFRs implied for the Arp galaxies by the Spitzer 24 μm luminosities are relatively modest, ∼1 M⊙ yr-1 on average.

We present Spitzer infrared imaging of the peculiar galaxy pair Arp 107 and compare with an optical Hα map and a numerical model of the interaction. The [3.6] - [4.5] colors of clumps in the galaxy do not vary around the ringlike primary spiral arm and are consistent with those of stars; thus, these bands are dominated by starlight. In contrast, the [5.8 μm] - [8.0 μm] colors are consistent with those of interstellar dust and vary by about 0.2 mag around the ring/spiral, with redder colors associated with regions with stronger star formation as indicated by Ha and mid-infrared luminosity. The [4.5 μm] - [5.8 μm] colors for clumps in this arm are bluer than dust and redder than stars and vary by 1.3 mag around the arm. This color is therefore a measure of the relative number of young stars to old stars, with a redder color indicating a higher proportion of young stars. There is a clear azimuthal sequence in the [4.5] - [5.8] color around the arm, indicating a sequence in average stellar age. The L HQ/L 8.0 μm ratio varies around the arm by a factor of ≈7; this variation may be due to extinction or to polycyclic aromatic hydrocarbon excitation by nonionizing photons. Our model of Arp 107 accounts for the general morphology of the system and explains the age variation along the arm as the result of differences in the time of maximum compression in the arm. Using Spitzer colors, we are able to distinguish background quasars and foreground stars from star-forming regions associated with Arp 107.

Using archival data from the Chandra X-ray telescope, we have measured the spatial extent of the hot interstellar gas in a sample of 49 nearby interacting galaxy pairs, mergers, and merger remnants. For systems with SFR > 1 M yr-1, the volume and mass of hot gas are strongly and linearly correlated with the star formation rate (SFR). This supports the idea that stellar/supernovae feedback dominates the production of hot gas in these galaxies. We compared the mass of X-ray-emitting hot gas M X(gas) with the molecular and atomic hydrogen interstellar gas masses in these galaxies (MH2 and MH i, respectively), using published carbon monoxide and 21 cm H i measurements. Systems with higher SFRs have larger M X(gas)/(MH2 + M H i) ratios on average, in agreement with recent numerical simulations of star formation and feedback in merging galaxies. The M X(gas)/( MH2 + M H i) ratio also increases with dust temperature on average. The ratio M X(gas)/SFR is anticorrelated with the Infrared Astronomical Satellite 60-100 μm flux ratio and with the Spitzer 3.6-24 μm color. These trends may be due to variations in the spatial density of young stars, the stellar age, the ratio of young to old stars, the initial mass function, and/or the efficiency of stellar feedback. Galaxies with low SFR (<1 M ⊙ yr-1) and high K band luminosities may have an excess of hot gas relative to the relation for higher SFR galaxies, while galaxies with low K band luminosities (and therefore low stellar masses) may have a deficiency in hot gas, but our sample is not large enough for strong statistical significance.

In this paper, we introduce the Local Volume TiNy Titans sample (LV-TNT), which is a part of a larger body of work on interacting dwarf galaxies: TNT . This LV-TNT sample consists of 10 dwarf galaxy pairs in the Local Universe (< 30 Mpc from Milky Way), which span mass ratios of M-*,M- 1/M-*,M- 2 < 20, projected separations < 100 kpc, and pair member masses of log(M-*/M-aS (TM)) < 9.9. All 10 LV-TNT pairs have resolved synthesis maps of their neutral hydrogen, are located in a range of environments and captured at various interaction stages. This enables us to do a comparative study of the diffuse gas in dwarf-dwarf interactions and disentangle the gas lost due to interactions with haloes of massive galaxies, from the gas lost due to mutual interaction between the dwarfs. We find that the neutral gas is extended in the interacting pairs when compared to non-paired analogues, indicating that gas is tidally pre-processed. Additionally, we find that the environment can shape the H i distributions in the form of trailing tails and that the gas is not unbound and lost to the surroundings unless the dwarf pair is residing near a massive galaxy. We conclude that a nearby, massive host galaxy is what ultimately prevents the gas from being re-accreted. Dwarf-dwarf interactions thus represent an important part of the baryon cycle of low-mass galaxies, enabling the 'parking' of gas at large distances to serve as a continual gas supply channel until accretion by a more massive host.

It has become clear in recent years that the study of interacting/merging galaxies plays an important role in understanding important astrophysical phenomena. This thesis presents an observational study of interacting/merging galaxies at radio frequencies. The observations have been carried out at extremely high resolution using very long baseline interferometry. The observations described here include: (1) A study of the high velocity Hi absorption associated with the peculiar galaxy NGC 1275; (2) A study of the radio continuum and Hi absorption of the ULIRG IRAS 172080014; (3) A study of the radio continuum and Hi absorption of the LIRG NGC 7674. Some of the most prominent results of these observations include: Detection of several narrow Hi absorption features in the high velocity system associated with NGC 1275. These Hi absorption lines were observed toward the strong radio nucleus 3C 84. The results indicate the existence of several Hi clouds with velocity differences and widths similar to those seen in Galactic neutral hydrogen absorption and similar to some of the Hi absorption seen in the Large Magellanic Cloud. The discovery of an extreme nuclear starburst region in the advanced merger system IRAS 172080014. Our results suggest a star formation rate of 84 M yr-1, and a supernova rate of 4 yr-1. Hi absorption is detected in multiple components with optical depths ranging between 0.3 and 2.5, and velocity widths between 58 and 232 km s-1. The detection of complex jet structures in the inner 1 kpc region of the galaxy NGC 7674. At full resolution, several compact sources are observed with brightness temperatures on the order of 107 K. While it is possible that one of these compact structures could host an AGN, they could also be shock-like features formed by the interaction of the jet with compact interstellar clouds in the nuclear region of this galaxy. At least eight Hi absorption lines are detected toward some of the continuum emission regions in NGC 7674. If the widest Hi feature in our observations is rotationally broadened by a central supermassive black hole, the implied dynamical mass is about 107 M.

We present a combined morphological and X-ray analysis of Was. 49, an isolated, dual-AGN system notable for the presence of a dominant AGN, Was 49b, in the disk of the primary galaxy, Was 49a, at a projected radial distance of 8. kpc from the nucleus. Using X-ray data from Chandra, the Nuclear Spectroscopic Telescope Array, and Swift, we find that this AGN has a bolometric luminosity of L-bol similar to 10(45) erg s(-1), with a black hole mass of M-BH = 1.3(-0.9)(+10)M(circle dot) . Despite the large mass, our analysis of optical data from the Discovery Channel Telescope shows that the supermassive black hole (SMBH) is hosted by a stellar counterpart with a mass of only 5.6(-2.6)(+4.9)M(circle dot), which makes the SMBH potentially larger than expected from SMBH-galaxy scaling relations, and the stellar counterpart exhibits a morphology that is consistent with dwarf elliptical galaxies. Our analysis of the system in the r and K bands indicates that Was. 49 is a minor merger, with the mass ratio of Was 49b to Was 49a between similar to 1:7 and similar to 1:15. This is in contrast with findings that the most luminous merger-triggered AGNs are found in major mergers and that minor mergers predominantly enhance AGN activity in the primary galaxy.

There is strong observational evidence indicating a time lag of order of some 100 Myr between the onset of starburst and AGN activity in galaxies. Dynamical time lags have been invoked to explain this. We extend this approach by introducing a viscous time lag the gas additionally needs to flow through the AGN's accretion disc before it reaches the central black hole. Our calculations reproduce the observed time lags and are in accordance with the observed correlation between black hole mass and stellar velocity dispersion.

Un essai épistémologique sur l'évolution des concepts cosmologiques depuis l'antiquité occupe la première partie de ce travail. Il porte une attention particulière aux domaines frontaliers qui ont pu favoriser les concepts dominants. Une étude des tests observationnels des modèles d'univers au fil du temps replace dans son contexte la signification cosmologique présente de la relation entre diamètre apparent $\theta$ et décalage spectral cosmologique $z_c$.\\ Dans les modèles d'univers de Friedmann-Lemaître, issus de la relativité générale et du principe cosmologique, la relation $\theta \longleftrightarrow z_c$ dépend de la géométrie de l'espace-temps que parcourt la lumière entre la source et l'observateur et donc de ses paramètres fondamentaux, notamment de son taux d'expansion, de la densité des différents <> qu'il contient et de la <>.\\ En inversant la relation $\theta \longleftrightarrow z_c$ observée on doit donc pouvoir contraindre ces paramètres. L'interaction gravitationnelle des galaxies et la première {\it activation} mutuelle ({\it <>}) qu'elle suscite est ici envisagée comme génératrice d'étalons de longueur relativement indépendants du temps cosmique.\\ Cette idée directrice a accompagné, durant plus de vingt ans, un ensemble de démarches observationnelles pointées (imagerie, spectrophotométrie 1D, 2D, 3D) et à grand champ (astrométrie, photométrie, mouvements propres) dont les retombées sont multiples.\\ L'étude se termine par une qualification de la méthode, basée sur les grands relevés de galaxies disponibles en 2006. À partir des observations locales, des échantillons synthétiques sont générés et la résolution du problème inverse fournit les précisions avec lesquelles peuvent être retrouvés les paramètres de densité en fonction de la dimension des échantillons réels à venir.