The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg 2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

Type Ia supernovae (SNe Ia) have been used as excellent standardizable candles for measuring cosmic expansion, but their progenitors are still elusive.Here we report that the spectral diversity of SNe Ia is tied to their birthplace environments. We find that those with high-velocity ejecta are substantially more concentrated in the inner and brighter regions of their host galaxies than are normal-velocity SNe Ia. Furthermore, the former tend to inhabit larger and more-luminous hosts. These results suggest that high-velocity SNe Ia likely originate from relatively younger and more metal-rich progenitors than normal-velocity SNe Ia, and are restricted to galaxies with substantial chemical evolution.Type Ia supernovae (SNe Ia) are among the most energetic and relatively uniform stellar explosions in the Universe, and were used to discover its accelerating expansion (1, 2). They 1 are thought to originate from a thermonuclear explosion of an accreting carbon-oxygen white dwarf (WD) near the Chandrasekhar mass limit (M Ch ≈ 1.4 M ⊙ ) in a close binary system (3,4). Two competing scenarios have been proposed for the progenitor systems: single-degenerate (SD) (5, 6) and double-degenerate (DD) models (4, 7). In the former, the mass-donating star could be a main-sequence (MS)/subgiant star (8), a red-giant star (RG; 9), or even a helium star (10, 11), while it is another WD in the latter scenario (4, 7). Recent results suggest that both scenarios are possible (12-18).There is increasing evidence for spectral diversity among SNe Ia. Of particular interest are those showing higher expansion velocities as inferred from the blueshifted Si II 615 nm feature in optical spectra (19). These fast-expanding SNe Ia also generally exhibit a steep Si II temporal velocity gradient (20). This spectral difference in velocity or velocity evolution of the ejecta has been proposed to be a geometric effect of an asymmetric explosion (21,22). Given a common origin for SNe Ia having different ejecta velocities, they should be found in similar stellar environments. This can be tested by examining SN positions in their hosts, the surface brightness at these locations, and the properties of their hosts.We conducted such an analysis with a well-defined SN sample having 188 SNe Ia [see supporting online material (SOM) text S1] from the Lick Observatory Supernova Search (LOSS; 23). The SN Ia sample consists of 123 "Branch-normal" (spectroscopically normal) objects (24), 30 peculiar ones of the SN 1991bg variety (25), 13 peculiar ones like SN 1991T (26, 27), and 7 peculiar ones like SN 2002cx (28), with respective fractions of 65.4%, 16.0%, 6.9%, and 3.7% (Table S1). There are 15 SNe Ia (8.0% of all) that cannot be subclassified due to an absence of early-time spectra. We concentrate on the Branch-normal SNe Ia which are thought to be relatively uniform. We obtained the main parameters of the host galaxies from two large online astronomical databases: the NASA/IPAC Extragalactic Database (NED; 29) and HyperLeda (30). 2The location of a SN in its hos...

Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.

We present extensive optical (U BV RI), near-infrared (JK) light curves and optical spectroscopy of the Type Ia supernova (SN) 2006X in the nearby galaxy NGC 4321 (M100). Our observations suggest that either SN 2006X has an intrinsically peculiar color evolution, or it is highly reddened [E(B − V ) host = 1.42 ± 0.04 mag] with R V = 1.48 ± 0.06, much lower than the canonical value of 3.1 for the average Galactic dust. SN 2006X also has one of the highest expansion velocities ever published for a SN Ia. Compared with the other SNe Ia we analyzed, SN 2006X has a broader light curve in the U band, a more prominent bump/shoulder feature in the V and R bands, a more pronounced secondary maximum in the I and near-infrared bands, and a remarkably smaller late-time decline rate in the B band. The B − V color evolution shows an obvious deviation from the Lira-Phillips relation at 1 to 3 months after maximum brightness. At early times, optical spectra of SN 2006X displayed strong, high-velocity features of both intermediate-mass elements (Si, Ca, and S) and iron-peak elements, while at late times they showed a relatively blue continuum, consistent with the blue U − B and B − V colors at similar epochs. A light echo and/or the interaction of the SN ejecta and its circumstellar material may provide a plausible explanation for its late-time photometric and spectroscopic behavior. Using the Cepheid distance of M100, we derive a Hubble constant of 72.8 ± 8.2 km s −1 Mpc −1 (statistical) from the normalized dereddened luminosity of SN 2006X. We briefly discuss whether abnormal dust is a universal signature for all SNe Ia, and whether the most rapidly expanding objects form a subclass with distinct photometric and spectroscopic properties.

All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M ⊙ 1-4. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries 5, 6. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying Hα emission line require the presence of a dark companion with a mass of

We present the [Oii] (λλ3729, 3726) luminosity function measured in the redshift range 0.1 < z < 1.65 with unprecedented depth and accuracy. Our measurements are based on medium resolution flux-calibrated spectra of emission line galaxies with the FORS2 instrument at VLT and with the SDSS-III/BOSS spectrograph. The FORS2 spectra and the corresponding catalog containing redshifts and line fluxes are released along with this paper. In this work we use a novel method to combine the aforementioned surveys with GAMA, zCOSMOS and VVDS, which have different target selection, producing a consistent weighting scheme to derive the [Oii] luminosity function.The measured luminosity function is in good agreement with previous independent estimates. The comparison with two state-of-theart semi-analytical models is good, which is encouraging for the production of mock catalogs of [Oii] flux limited surveys. We observe the bright end evolution over 8.5 Gyr: we measure the decrease of log L * from 42.4 erg/s at redshift 1.44 to 41.2 at redshift 0.165 and we find that the faint end slope flattens when redshift decreases. This measurement confirms the feasibility of the target selection of future baryonic acoustic oscillation surveys aiming at observing [Oii] flux limited samples.

We present extensive ultraviolet, optical, and near-infrared observations of the type IIP supernova (SN IIP) 2013ej in the nearby spiral galaxy M74. The multicolor light curves, spanning from ∼ 8-185 days after explosion, show that it has a higher peak luminosity (i.e., M V ∼−17.83 mag at maximum light), a faster post-peak decline, and a shorter plateau phase (i.e., ∼ 50 days) compared to the normal type IIP SN 1999em. The mass of 56 Ni is estimated as 0.02±0.01 M ⊙ from the radioactive tail of the bolometric light curve. The spectral evolution of SN 2013ej is similar to that of SN 2004et and SN 2007od, but shows a larger expansion velocity (i.e., v F eII ∼ 4600 km s −1 at t ∼ 50 days) and broader line profiles. In the nebular phase, the emission of Hα line displays a double-peak structure, perhaps due to the asymmetric distribution of 56 Ni produced in the explosion. With the constraints from the main observables such as bolometric light curve, expansion velocity and photospheric temperature of SN 2013ej, we performed hydrodynamical simulations of the explosion parameters, yielding the total explosion energy as ∼0.7× 10 51 erg, the radius of the progenitor as ∼600 R ⊙ , and the ejected mass as ∼10.6 M ⊙ . These results suggest that SN 2013ej likely arose from a red supergiant with a mass of 12-13 M ⊙ immediately before the explosion.

We present well-sampled optical observations of the bright Type Ia supernova (SN Ia) SN 2011fe in M101. Our data, starting from ∼16 days before maximum light and extending to ∼463 days after maximum, provide an unprecedented time series of spectra and photometry for a normal SNIa. Fitting the early-time rising light curve, we find that the luminosity evolution of SN 2011fe follows a t n law, with the index n being close to 2.0 in the VRI bands but slightly larger in the U and B bands. Combining the published ultraviolet (UV) and near-infrared (NIR) photometry, we derive the contribution of UV/NIR emission relative to the optical. SN 2011fe is found to have stronger UV emission and reaches its UV peak a few days earlier than other SNeIa with similar Δm 15 (B), suggestive of less trapping of high-energy photons in the ejecta. Moreover, the U-band light curve shows a notably faster decline at late phases (t≈100-300 days), which also suggests that the ejecta may be relatively transparent to UV photons. These results favor the notion that SN 2011fe might have a progenitor system with relatively lower metallicity. On the other hand, the early-phase spectra exhibit prominent high-velocity features (HVFs) of O I λ7773 and the Ca IINIR triplet, but only barely detectable in Si II6355. This difference can be caused byeitheran ionization/temperature effect or an abundance enhancement scenario for the formation of HVFs; it suggests that the photospheric temperature of SN 2011fe is intrinsically low, perhaps owing to incomplete burning during the explosion of the white dwarf.