None of the approximately 750,000 known asteroids and comets in the Solar System is thought to have originated outside it, despite models of the formation of planetary systems suggesting that orbital migration of giant planets ejects a large fraction of the original planetesimals into interstellar space. The high predicted number density of icy interstellar objects (2.4 × 10 per cubic astronomical unit) suggests that some should have been detected, yet hitherto none has been seen. Many decades of asteroid and comet characterization have yielded formation models that explain the mass distribution, chemical abundances and planetary configuration of the Solar System today, but there has been no way of telling whether the Solar System is typical of planetary systems. Here we report observations and analysis of the object 1I/2017 U1 ('Oumuamua) that demonstrate its extrasolar trajectory, and that thus enable comparisons to be made between material from another planetary system and from our own. Our observations during the brief visit by the object to the inner Solar System reveal it to be asteroidal, with no hint of cometary activity despite an approach within 0.25 astronomical units of the Sun. Spectroscopic measurements show that the surface of the object is spectrally red, consistent with comets or organic-rich asteroids that reside within the Solar System. Light-curve observations indicate that the object has an extremely oblong shape, with a length about ten times its width, and a mean radius of about 102 metres assuming an albedo of 0.04. No known objects in the Solar System have such extreme dimensions. The presence of 'Oumuamua in the Solar System suggests that previous estimates of the number density of interstellar objects, based on the assumption that all such objects were cometary, were pessimistically low. Planned upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to result in the detection of more interstellar objects in the coming years.

The Great Observatories All-Sky LIRG Survey (GOALS) is a comprehensive, multiwavelength study of luminous infrared galaxies (LIRGs) in the local universe. Here we present low resolution Spitzer Infrared Spectrograph spectra covering 5-38 μm and provide a basic analysis of the mid-IR spectral properties observed for nearby LIRGs. In a companion paper, we discuss detailed fits to the spectra and compare the LIRGs to other classes of galaxies. The GOALS sample of 244 nuclei in 180 luminous (10 11 L IR /L < 10 12 ) and 22 ultraluminous (L IR /L 10 12 ) IR galaxies represents a complete subset of the IRAS Revised Bright Galaxy Sample and covers a range of merger stages, morphologies, and spectral types. The majority (>60%) of the GOALS LIRGs have high 6.2 μm polycyclic aromatic hydrocarbon (PAH) equivalent widths (EQW 6.2 μm > 0.4 μm) and low levels of silicate absorption (s 9.7 μm > −1.0). There is a general trend among the U/LIRGs for both silicate depth and mid-infrared (MIR) slope to increase with increasing L IR . U/LIRGs in the late to final stages of a merger also have, on average, steeper MIR slopes and higher levels of dust obscuration. Together, these trends suggest that as gas and dust is funneled toward the center of a coalescing merger, the nuclei become more compact and more obscured. As a result, the dust temperature increases also leading to a steeper MIR slope. The sources that depart from these correlations have very low PAH equivalent width (EQW 6.2 μm < 0.1 μm) consistent with their emission being dominated by an active galactic nucleus (AGN) in the MIR. These extremely low PAH EQW sources separate into two distinct types: relatively unobscured sources with a very hot dust component (and thus very shallow MIR slopes) and heavily dust obscured nuclei with a steep temperature gradient. The most heavily dust obscured sources are also the most compact in their MIR emission, suggesting that the obscuring (cool) dust is associated with the outer regions of the starburst and not simply a measure of the dust along the line of sight through a large, dusty disk. A marked decline is seen for the fraction of high EQW (star formation dominated) sources as the merger progresses. The decline is accompanied by an increase in the fraction of composite sources while the fraction of sources where an AGN dominates the MIR emission remains low. When compared to the MIR spectra of submillimeter galaxies (SMGs) at z ∼ 2, both the average GOALS LIRG and ULIRG spectra are more absorbed at 9.7 μm and the average GOALS LIRG has more PAH emission. However, when the AGN contributions to both the local GOALS LIRGs and the high-z SMGs are removed, the average local starbursting LIRG closely resembles the starburst-dominated SMGs.

We present optical spectroscopy of a sample of 77 luminous AGN and quasars selected on the basis of their mid-infrared colors. Our objects are selected from the Spitzer Extragalactic First Look Survey and SWIRE XMM-LSS fields, with a typical 24µm flux density of 5mJy. The median redshift is 0.6, with a range of ∼ 0.05 − 4. Only 33% (25/77) of these objects are normal type-1 quasars, with no obscuration. 44% (34/77) are type-2 objects, with highionization, narrow emission lines, 14% (11/77) are dust-reddened type-1 quasars, showing broad lines but a dust-reddened or unusually weak quasar continuum. 9% (7/77) show no sign of an AGN in the optical spectrum, having either starburst spectra or spectra which could be of either starburst or LINER types. These latter objects are analogous to the X-ray detected population of AGN with weak or non-existent optical AGN emission (the "XBONGs"). 21 of our objects from the SWIRE field fall within moderately-deep XMM exposures. All the unobscured quasars, and about half the obscured quasars are detected in these exposures. This sample, when taken together with other samples of Spitzer selected AGN and quasars, and results from X-ray studies, confirms that obscured AGN dominate the AGN and quasar number counts of all rapidly-accreting supermassive black hole systems, at least for z < ∼ 4. This implies a high radiative efficiency for the black hole accretion process.

We present a statistical analysis of the mid-infrared (MIR) spectra of 248 luminous infrared (IR) galaxies (LIRGs) which comprise the Great Observatories All-sky LIRG Survey (GOALS) observed with the Infrared Spectrograph (IRS) on-board the Spitzer Space Telescope. The GOALS sample enables a direct measurement of the relative contributions of star-formation and active galactic nuclei (AGN) to the total IR emission from a large sample of local LIRGs.The AGN contribution to the MIR emission (f AGN ) is estimated by employing several diagnostics based on the properties of the [NeV], [OIV] and [NeII] fine structure gas emission lines, the 6.2 µm PAH and the shape of the MIR continuum. We find that 18% of all LIRGs contain an AGN and that in 10% of all -2sources the AGN contributes more than 50% of the total IR luminosity. Summing up the total IR luminosity contributed by AGN in all our sources suggests that AGN supply ∼ 12% of the total energy emitted by LIRGs.The average spectrum of sources with an AGN looks similar to the average spectrum of sources without an AGN, but it has lower PAH emission and a flatter MIR continuum. AGN dominated LIRGs have higher IR luminosities, warmer MIR colors and are found in interacting systems more often than pure starbursts LIRGs. However we find no linear correlations between these properties and (f AGN ). We used the IRAC colors of LIRGs to confirm that finding AGN on the basis of their MIR colors may miss ∼ 40% of AGN dominated (U)LIRGs. * Notes: All fluxes are given in W m −2 . The fluxes presented here agree with previously published values within the errors, in all cases with the exception of fluxes for Mrk 266, NGC5135.

The Great Observatories All-sky LIRG Survey (GOALS) combines data from NASA's Spitzer, Chandra, Hubble and GALEX observatories, together with ground-based data into a comprehensive imaging and spectroscopic survey of over 200 low redshift Luminous Infrared Galaxies (LIRGs). The LIRGs are a complete subset of the IRAS Revised Bright Galaxy Sample (RBGS). The LIRGs targeted in GOALS span the full range of nuclear spectral types defined via traditional optical line-ratio diagrams as well as interaction stages. They provide an unbiased picture of the processes responsible for enhanced infrared emission in galaxies in the local Universe. As an example of the analytic power of the multi-wavelength GOALS dataset, we present data for the interacting system VV 340 (IRAS F14547+2449). Between 80-95% of the total far-infrared emission (or about 5E11 solar luminosities) originates in VV 340 North. While the IRAC colors of VV 340 North and South are consistent with star-forming galaxies, both the Spitzer IRS and Chandra ACIS data indicate the presence of a buried AGN in VV 340 North. The GALEX far and near-UV fluxes imply a extremely large infrared "excess" (IRX) for the system (IR/FUV = 81) which is well above the correlation seen in starburst galaxies. Most of this excess is driven by VV 340 N, which alone has an IR excess of nearly 400. The VV 340 system seems to be comprised of two very different galaxies - an infrared luminous edge-on galaxy (VV 340 North) that dominates the long-wavelength emission from the system and which hosts a buried AGN, and a face-on starburst (VV 340 South) that dominates the short-wavelength emission.Comment: 17 pages, 2 tables, 7 postscript figures. Accepted for publication in PASP. Updated manuscript includes complete source table (Table 1