We measure the star formation efficiency (SFE), the star formation rate per unit gas, in 23 nearby galaxies and compare it to expectations from proposed star formation laws and thresholds. We use H I maps from THINGS and derive H 2 maps from CO measured by HERACLES and BIMA SONG. We estimate the star formation rate by combining GALEX FUV maps and SINGS 24µm maps, infer stellar surface density profiles from SINGS 3.6µm data, and use kinematics from THINGS. We measure the SFE as a function of: the free-fall and orbital timescales; midplane gas pressure; stability of the gas disk to collapse (including the effects of stars); the ability of perturbations to grow despite shear; and the ability of a cold phase to form. In spirals, the SFE of H 2 alone is nearly constant at 5.25 ± 2.5 × 10 −10 yr −1 (equivalent to an H 2 depletion time of 1.9 × 10 9 yr) as a function of all of these variables at our 800 pc resolution. Where the ISM is mostly H I, on the other hand, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2-0.25 r 25 . We interpret this decline as a strong dependence of GMC formation on environment. The ratio of molecular to atomic gas appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H 2 -dominated to H I-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe -phase balance in the H I, H 2 formation and destruction, and stellar feedback -governs the formation of GMCs from H I.

We present a comprehensive analysis of the relationship between star formation rate surface density, Σ SFR , and gas surface density, Σ gas , at sub-kpc resolution in a sample of 18 nearby galaxies. We use high resolution H i data from THINGS, CO data from HERACLES and BIMA SONG, 24 µm data from the Spitzer Space Telescope, and UV data from GALEX. We target 7 spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H 2 -dominated centers of spiral galaxies, their H i-dominated outskirts and the H i-rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N = 1.0 ± 0.2 relates Σ SFR and Σ H2 across our sample of spiral galaxies, i.e., that H 2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ∼ 2 · 10 9 years. The range of Σ H2 over which we measure this relation is ∼ 3 − 50 M ⊙ pc −2 , significantly lower than in starburst environments. We find the same results when performing a pixel-by-pixel analysis, averaging in radial bins, or when varying the star formation tracer used. We interpret the linear relation and constant depletion time as evidence that stars are forming in GMCs with approximately uniform properties and that Σ H2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas surface density (Σ gas ) and Σ SFR varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between Σ HI and Σ SFR . As a result, the star formation efficiency (SFE), Σ SFR /Σ gas , varies strongly across our sample and within individual galaxies. We show that this variation is systematic and consistent with the SFE being set by local environmental factors: in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. We attribute the similarity to common environments (low-density, low-metallicity, H i-dominated) and argue that shear (which is typically absent in dwarfs) cannot drive the SFE. In addition to a molecular Schmidt law, the other general feature of our sample is a sharp saturation of H i surface densities at Σ HI ≈ 9 M ⊙ pc −2 in both the spiral and dwarf galaxies. In the case of the spirals, we observe gas in excess of this limit to be molecular.

We present here the Ðnal results of the Hubble Space T elescope (HST ) Key Project to measure the Hubble constant. We summarize our method, the results, and the uncertainties, tabulate our revised distances, and give the implications of these results for cosmology. Our results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60È400 Mpc. The analysis presented here beneÐts from a number of recent improvements and reÐnements, including (1) a larger LMC Cepheid sample to deÐne the Ðducial period-luminosity (PL) relations, (2) a more recent HST Wide Field and Planetary Camera 2 (WFPC2) photometric calibration, (3) a correction for Cepheid metallicity, and (4) a correction for incompleteness bias in the observed Cepheid PL samples. We adopt a distance modulus to the LMC (relative to which the more distant galaxies are measured) of mag, or 50 kpc. New, revised distances are given for the 18 spiral galaxies for k 0 (LMC) \ 18.50^0.10 which Cepheids have been discovered as part of the Key Project, as well as for 13 additional galaxies with published Cepheid data. The new calibration results in a Cepheid distance to NGC 4258 in better agreement with the maser distance to this galaxy. Based on these revised Cepheid distances, we Ðnd values (in km s~1 Mpc~1) of (random)^6 (systematic) (Type Ia supernovae),

An evolutionary connection between ultraluminous infrared galaxies and quasars is deduced from the observations of all 10 infrared galaxies with luminosities L(8-1000 µm) ~ 10 12 L 0 , taken from a flux-limited sample of infrared bright galaxies. Images of the infrared galaxies show that nearly all are strongly interar•ing merger systems with exceptionally luminous nuclei. Millimeter-wave CO observations show that these objects typically contain 0.5-2 x 10 10 M 0 of H2. Optical spectra indicate a mixture of starburst and active galactic nucleus (AGN) energy sources, both of which are apparently fueled by the tremendous reservoir of molecular gas. It is proposed that these ultraluminous infrared galaxies represent the initial, dust-enshrouded stages of quasars. Once these nuclei shed their obscuring dust, allowing the AGN to visually dominate the decaying starburst, they become optically selected quasars. The origin of quasars through the merger of molecular gasrich spiral galaxies can account for both the increased number of high-luminosity quasars at large redshift, when the universe was smaller and gas supplies less depleted, and the observed "redshift-cutoff" of quasars which represents the epoch after galaxy formation when the first collisions occur.

We measure star formation rates of ~50,000 optically-selected galaxies in the local universe (z~0.1), spanning a range from gas-rich dwarfs to massive ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (UV) and SDSS (optical) photometry to a library of population synthesis models that include dust attenuation. For star-forming galaxies, our UV-based SFRs compare remarkably well with those derived from SDSS H alpha. Deviations from perfect agreement between these two methods are due to differences in the dust attenuation estimates. In contrast to H alpha, UV provides reliable SFRs for galaxies with weak or no H alpha emission, and where H alpha is contaminated with an emission from an AGN. We use full-SED SFRs to calibrate a simple prescription that uses GALEX UV magnitudes to produce good SFRs for normal star-forming galaxies. The specific SFR is considered as a function of stellar mass for (1) star-forming galaxies with no AGN, (2) those hosting an AGN, and for (3) galaxies without H alpha emission. We find that the three have distinct star formation histories, with AGN lying intermediate between the star-forming and the quiescent galaxies. Normal star forming galaxies (without an AGN) lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. Weak AGN, while also massive, have lower SFR, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent (red sequence) galaxies via strong and weak AGN. We confirm that some galaxies with no H alpha emission show signs of SF in the UV. We derive a UV-based cosmic SFR density at z=0.1 with smaller total error than previous measurements (abridged).Comment: Accepted for publication in ApJ (Special GALEX Supplement issue - Dec 2007). v2: Typo in Eq. 2 correcte

We give an overview of the Galaxy Evolution Explorer (GALEX), a NASA Explorer Mission launched on 2003 April 28. GALEX is performing the first space UV sky survey, including imaging and grism surveys in two bands (1350-1750 and 1750-2750 galaxy survey. Spectroscopic (slitless) grism surveys ( ) are underway with various depths and sky R p 100-200 coverage. Many targets overlap existing or planned surveys in other bands. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the relationship of the UV and global star formation rate in local galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the redshift range 0 ! z ! and probe the physical drivers of star formation in galaxies. The GALEX mission includes a guest investigator 2 program, supporting the wide variety of programs made possible by the first UV sky survey.

We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX ). These releases have identical pipeline calibrations that are significantly improved over the GR1 data release. GALEX continues to survey the sky in the far-ultraviolet (FUV, $154 nm) and near-ultraviolet (NUV, $232 nm) bands, providing simultaneous imaging with a pair of photon-counting, microchannel plate, delay line readout detectors. These 1.25 field of view detectors are well suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations, and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 m AB in the FUV and NUV, respectively. We have detected a long-term drift of order 1% FUV and 6% NUVover the mission. Astrometric precision is of order 0.5 00 rms in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.

On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer detected gravitational waves (GWs) emanating from a binary neutron star merger, GW170817. Nearly simultaneously, the Fermi and INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) telescopes detected a gamma-ray transient, GRB 170817A. At 10.9 hours after the GW trigger, we discovered a transient and fading optical source, Swope Supernova Survey 2017a (SSS17a), coincident with GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40 megaparsecs. The precise location of GW170817 provides an opportunity to probe the nature of these cataclysmic events by combining electromagnetic and GW observations.