Using a mid-infrared calibration of the Cepheid distance scale based on recent observations at 3.6 µm with the Spitzer Space Telescope, we have obtained a new, high-accuracy calibration of the Hubble constant. We have established the mid-IR zero point of the Leavitt Law (the Cepheid Period-Luminosity relation) using time-averaged 3.6 µm data for ten high-metallicity, Milky Way Cepheids having independently-measured trigonometric parallaxes. We have adopted the slope of the PL relation using time-averaged 3.6 µm data for 80 long-period Large Magellanic Cloud (LMC) Cepheids falling in the period range 0.8 < log(P) < 1.8. We find a new reddening-corrected distance to the LMC of 18.477 ± 0.033 (systematic) mag. We re-examine the systematic uncertainties in H 0 , also taking into account new data over the past decade. In combination with the new Spitzer calibration, the systematic uncertainty in H 0 over that obtained by the Hubble Space Telescope (HST) Key Project has decreased by over a factor of three. Applying the Spitzer calibration to the Key Project sample, we find a value of H 0 = 74.3 with a systematic uncertainty of ± 2.1 (systematic) km s −1 Mpc −1 , corresponding to a 2.8% systematic uncertainty in the Hubble constant. This result, in combination with WMAP7 measurements of the cosmic microwave background anisotropies and assuming a flat universe, yields a value of the equation of state for dark energy, w 0 = -1.09 ± 0.10. Alternatively, relaxing the constraints on flatness and the numbers of relativistic species, and combining our results with those of WMAP7, Type Ia supernovae and baryon acoustic oscillations yields w 0 = -1.08 ± 0.10 and a value of N ef f = 4.13 ± 0.67, mildly consistent with the existence of a fourth neutrino species.

It is well established that (1) star-forming galaxies follow a relation between their star formation rate (SFR) and stellar mass (M  ), the "star formation sequence," and (2) the SFRs of galaxies correlate with their structure, where star-forming galaxies are less concentrated than quiescent galaxies at fixed mass. Here, we consider whether the scatter and slope of the star formation sequence is correlated with systematic variations in the Sérsic indices, n, of galaxies across the SFR-M  plane. We use a mass-complete sample of 23,848 galaxies at 0.5 < z < 2.5 selected from the 3D-HST photometric catalogs. Galaxy light profiles parameterized by n are based on Hubble Space Telescope Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey near-infrared imaging. We use a single SFR indicator empirically calibrated from stacks of Spitzer/MIPS 24 μm imaging, adding the unobscured and obscured star formation. We find that the scatter of the star formation sequence is related in part to galaxy structure; the scatter due to variations in n at fixed mass for star-forming galaxies ranges from 0.14 ± 0.02 dex at z ∼ 2 to 0.30 ± 0.04 dex at z < 1. While the slope of the M log SFR log - relation is of order unity for disk-like galaxies, galaxies with n > 2 (implying more dominant bulges) have significantly lower M SFR  than the main ridgeline of the star formation sequence. These results suggest that bulges in massive z ∼ 2 galaxies are actively building up, where the stars in the central concentration are relatively young. At z < 1, the presence of older bulges within star-forming galaxies lowers global M SFR  , decreasing the slope and contributing significantly to the scatter of the star formation sequence.

We present a detailed analysis of multi-wavelength HST/WFC3 imaging and Keck/OSIRIS near-IR AO-assisted integral field spectroscopy for a highly magnified lensed galaxy at z = 1.70. This young starburst is representative of UV-selected star-forming galaxies (SFG) at z ∼ 2 and contains multiple individual star-forming regions. Due to the lensing magnification, we can resolve spatial scales down to 100 pc in the source plane of the galaxy. The velocity field shows disturbed kinematics suggestive of an ongoing interaction, and there is a clear signature of a tidal tail. We constrain the age, reddening, SFR and stellar mass of the star-forming clumps from SED modelling of the WFC3 photometry and measure their Hα luminosity, metallicity and outflow properties from the OSIRIS data. With strong star formation driven outflows in four clumps, RCSGA0327 is the first high redshift SFG at stellar mass < 10 10 M ⊙ with spatially resolved stellar winds. We compare the Hα luminosities, sizes and dispersions of the star-forming regions to other high-z clumps as well as local giant H II regions and find no evidence for increased clump star formation surface densities in interacting systems, unlike in the local Universe. Spatially resolved SED modelling unveils an established stellar population at the location of the largest clump and a second mass concentration near the edge of the system which is not detected in Hα emission. This suggests a picture of an equal-mass mixed major merger, which has not triggered a new burst of star formation or caused a tidal tail in the gas-poor component.

We present high-energy (3-30 keV) NuSTAR observations of the nearest quasar, the ultraluminous infrared galaxy (ULIRG) Markarian 231 (Mrk 231), supplemented with new and simultaneous low-energy (0.5-8 keV) data from Chandra. The source was detected, though at much fainter levels than previously reported, likely due to contamination in the large apertures of previous non-focusing hard X-ray telescopes. The full band (0.5-30 keV) X-ray spectrum suggests the active galactic nucleus (AGN) in Mrk 231 is absorbed by a patchy and Compton-thin (N H ∼ 1.2 +0.3 −0.3 × 10 23 cm −2 ) column. The intrinsic X-ray luminosity (L 0.5−30 keV ∼ 1.0 × 10 43 erg s −1 ) is extremely weak relative to the bolometric luminosity where the 2-10 keV to bolometric luminosity ratio is ∼0.03% compared to the typical values of 2%-15%. Additionally, Mrk 231 has a low X-ray-to-optical power law slope (α OX ∼ −1.7). It is a local example of a low-ionization broad absorption line quasar that is intrinsically X-ray weak. The weak ionizing continuum may explain the lack of mid-infrared [O iv], [Ne v], and [Ne vi] fine-structure emission lines which are present in sources with otherwise similar AGN properties. We argue that the intrinsic X-ray weakness may be a result of the super-Eddington accretion occurring in the nucleus of this ULIRG, and may also be naturally related to the powerful wind event seen in Mrk 231, a merger remnant escaping from its dusty cocoon.