We have identified 1027 star forming complexes in a sample of 46 galaxiesfrom the Spirals, Bridges, and Tails (SB&T) sample of interacting galaxies, and693 star forming complexes in a sample of 38 non-interacting spiral (NIS)galaxies in $8\rm{\mu m}$ observations from the Spitzer Infrared Array Camera.We have used archival multi-wavelength UV-to IR observations to fit theobserved spectral energy distribution (SED) of our clumps with the CodeInvestigating GALaxy Emission (CIGALE) using a double exponentially declinedstar formation history (SFH). We derive SFRs, stellar masses, ages andfractions of the most recent burst, dust attenuation, and fractional emissiondue to an AGN for these clumps. The resolved star formation main sequence holdson 2.5kpc scales, although it does not hold on 1kpc scales. We analyzed therelation between SFR, stellar mass, and age of the recent burst in the SB&T andNIS samples, and we found that the SFR per stellar mass is higher in the SB&Tgalaxies, and the clumps are younger in the galaxy pairs. We analyzed the SFRradial profile and found that SFR is enhanced through the disk and in the tidalfeatures relative to normal spirals.

The cosmological model is at present not tested between the redshift of thefarthest observed supernovae (z ~ 1.4) and that of the Cosmic MicrowaveBackground (z ~ 1,100). Here we introduce a new method to measure thecosmological parameters: we show that quasars can be used as "standard candles"by employing the non-linear relation between their intrinsic UV and X-rayemission as an absolute distance indicator. We built a sample of ~ 1,900quasars with available UV and X-ray observations, and produced a Hubble Diagramup to z ~ 5. The analysis of the quasar Hubble Diagram, when used incombination with supernovae, provides robust constraints on the matter andenergy content in the cosmos. The application of this method to forthcoming,larger quasar samples, will also provide tight constraints on the dark energyequation of state and its possible evolution with time.

In the hierarchical two-phase formation scenario, the extended halos of earlytype galaxies (ETGs) are expected to have different physical properties fromthose of the galaxies' central regions. This work aims at characterizing thekinematic properties of ETG halos using planetary nebulae (PNe) as tracers,which allow us to overcome the limitations of absorption line spectroscopy ofcontinuum at low surface brightness. We present two-dimensional velocity andvelocity dispersion fields for 33 ETGs, including both fast and slow rotators,making this the largest kinematic survey to-date of extragalactic PNe. Thevelocity fields typically extend out to 6 effective radii (Re), with a range[3Re-13Re] for the PN.S ETGs. We complemented the PN kinematics with absorptionline data from the literature. We find that ETGs typically show a kinematictransition between inner regions and halo. Estimated transition radii in unitsof Re anticorrelate with stellar mass. Slow rotators have increased but stillmodest rotational support at large radii, while most of the fast rotators showa decrease in rotation, due to the fading of the stellar disk in the outer,more slowly rotating spheroid. 30% of the fast rotators are dominated byrotation also at large radii. Most ETGs have flat or slightly falling halovelocity dispersion profiles, but 15% of the sample have steeply fallingprofiles. One third of the fast rotators show kinematic twists, misalignments,or rotation along two axes, indicating that they turn from oblate near thecenter to triaxial in the halo. ETGs have more diverse kinematic properties intheir halos than in the central regions, and a significant fraction showssignatures of triaxial halos in the PNe data. The observed kinematic transitionto the halo and its dependence on stellar mass is consistent with LambdaCDMsimulations and supports a two-phase formation scenario. [abridged]

The rate of major galaxy-galaxy merging is theoretically predicted tosteadily increase with redshift during the peak epoch of massive galaxydevelopment ($1{\leq}z{\leq}3$). We use close-pair statistics to objectivelystudy the incidence of massive galaxies (stellar$M_{1}{\geq}2{\times}10^{10}M_{\odot}$) hosting major companions($1{\leq}M_{1}/M_{2}{\leq}4$; i.e., $<$4:1) at six epochs spanning $0{<}z{<}3$.We select companions from a nearly complete, mass-limited($\geq5{\times}10^{9}M_{\odot}$) sample of 23,696 galaxies in the five CANDELSfields and the SDSS. Using $5-50$ kpc projected separation and close redshiftproximity criteria, we find that the major companion fraction$f_{\mathrm{mc}}(z)$ based on stellar mass-ratio (MR) selection increases from6% ($z{\sim}0$) to 16% ($z{\sim}0.8$), then turns over at $z{\sim}1$ anddecreases to 7% ($z{\sim}3$). Instead, if we use a major F160W flux ratio (FR)selection, we find that $f_{\mathrm{mc}}(z)$ increases steadily until $z=3$owing to increasing contamination from minor (MR$>$4:1) companions at $z>1$. Weshow that these evolutionary trends are statistically robust to changes incompanion proximity. We find disagreements between published results areresolved when selection criteria are closely matched. If we compute mergerrates using constant fraction-to-rate conversion factors($C_{\mathrm{merg,pair}}{=}0.6$ and$T_{\mathrm{obs,pair}}{=}0.65\mathrm{Gyr}$), we find that MR rates disagreewith theoretical predictions at $z{>}1.5$. Instead, if we use an evolving$T_{\mathrm{obs,pair}}(z){\propto}(1+z)^{-2}$ from Snyder et al., our MR-basedrates agree with theory at $0{<}z{<}3$. Our analysis underscores the need fordetailed calibration of $C_{\mathrm{merg,pair}}$ and $T_{\mathrm{obs,pair}}$ asa function of redshift, mass and companion selection criteria to betterconstrain the empirical major merger history.

Leo T is the lowest mass gas-rich galaxy currently known and studies of itsgas content help us understand how such marginal galaxies survive and formstars. We present deep neutral hydrogen (HI) observations from the WesterborkSynthesis Radio Telescope in order to understand its HI distribution andpotential for star formation. We find a larger HI line flux than the previouslyaccepted value, resulting in a 50% larger HI mass of 4.1 x 10^5 Msun. Theadditional HI flux is from low surface brightness emission that was previouslymissed; with careful masking this emission can be recovered even in shallowerdata. We perform a Gaussian spectral decomposition to find a cool neutralmedium component (CNM) with a mass of 3.7 x 10^4 Msun, or almost 10% of thetotal HI mass. Leo T has no HI emission extending from the main HI body, butthere is evidence of interaction with the Milky Way circumgalactic medium inboth a potential truncation of the HI body and the offset of the peak HIdistribution from the optical center. The CNM component of Leo T is large whencompared to other dwarf galaxies, even though Leo T is not currently formingstars and has a lower star formation efficiency than other gas-rich dwarfgalaxies. However, the HI column density associated with the CNM component inLeo T is low. One possible explanation is the large CNM component is notrelated to star formation potential but rather a recent, transient phenomenonrelated to the interaction of Leo T with the Milky Way circumgalactic medium.

Stephan's Quintet, a compact group of galaxies, is often used as a laboratoryto study a number of phenomena, including physical processes in theinterstellar medium, star formation, galaxy evolution, and the formation offossil groups. As such, it has been subject to intensive multi-wavelengthobservation campaigns. Yet, models lack constrains to pin down the role of eachgalaxy in the assembly of the group. We revisit here this system withmulti-band deep optical images obtained with MegaCam on theCanada-France-Hawaii Telescope (CFHT), focusing on the detection of low surfacebrightness (LSB) structures. They reveal a number of extended LSB features,some new, and some already visible in published images but not discussedbefore. An extended diffuse, reddish, lopsided, halo is detected towards theearly-type galaxy NGC 7317, the role of which had so far been ignored inmodels. The presence of this halo made of old stars may indicate that the groupformed earlier than previously thought. Finally, a number of additional diffusefilaments are visible, some close to the foreground galaxy NGC 7331 located inthe same field. Their structure and association with mid--IR emission suggestcontamination by emission from Galactic cirrus.