This paper describes the Fourth Data Release of the Sloan Digital Sky Survey (SDSS), including all survey-quality data taken through 2004 June. The data release includes five-band photometric data for 180 million objects selected over 6670 deg(2) and 673,280 spectra of galaxies, quasars, and stars selected from 4783 deg(2) of those imaging data using the standard SDSS target selection algorithms. These numbers represent a roughly 27% increment over those of the Third Data Release; all the data from previous data releases are included in the present release. The Fourth Data Release also includes an additional 131,840 spectra of objects selected using a variety of alternative algorithms, to address scientific issues ranging from the kinematics of stars in the Milky Way thick disk to populations of faint galaxies and quasars.

We report the discovery of a giant stellar tidal stream in the halo of NGC 4631, a nearby edge-on spiral galaxy interacting with the spiral NGC 4656, in deep images taken with a 40 cm aperture robotic telescope. The stream has two components: a bridge-like feature extending between NGC 4631 and NGC 4656 (stream(SE)) and an overdensity with extended features on the opposite side of the NGC 4631 disk (stream(NW)). Together, these features extend more than 85 kpc in projection. The orientation of stream(SE) relative to the orientations of NGC 4631 and NGC 4656 is not consistent with an origin from an interaction between these two spirals, and is more likely debris from a satellite encounter. The stellar tidal features can be qualitatively reproduced in an N-body model of the tidal disruption of a single, massive dwarf satellite on a moderately eccentric orbit (e = 0.6) around NGC 4631 over similar to 3.5 Gyr. Both modeling and inferences from the morphology of the streams indicate these are not associated with the complex HI tidal features observed between both spirals, which likely originate from a more recent, gas-rich accretion event. The structure of stream(NW) suggests that it may contain the progenitor of the stream, in agreement with the N-body model. However, we cannot exclude other possibilities such as the satellite dwarf galaxy NGC 4627 being the progenitor based on these data. In addition, streamNW is roughly aligned with two very faint dwarf spheroidal candidates. The system of dwarf galaxies and the tidal stream around NGC 4631 can provide an additional interesting case for exploring the anisotropy distribution of satellite galaxies recently reported around Local Group spiral galaxies by means of future follow-up observations.

In order to place the highly substructured stellar halos of the Milky Way and M31 in a larger context of hierarchical galaxy formation, it is necessary to understand the prevalence and properties of tidal substructure around external galaxies. This chapter details the current state of our observational knowledge of streams in galaxies in and beyond the Local Group, which are studied both in resolved stellar populations and in integrated light. Modeling of individual streams in extragalactic systems is hampered by our inability to obtain resolved stellar kinematics in the streams, though many streams contain alternate luminous kinematic tracers, such as globular clusters or planetary nebulae. We compare the observed structures to the predictions of models of galactic halo formation, which provide insight into the number and properties of streams expected around Milky Way like galaxies. More specifically, we discuss the inferences that can be made about stream progenitors based only on observed morphologies. We expand our discussion to consider hierarchical accretion at lower mass scales, in particular the observational evidence that substructure exists on smallermass scales and the effects accretion events may have on the evolution of dwarf galaxies (satellite or isolated). Lastly, we discuss potential correlations between the presence of substructure in the halo and the structural properties of the disk. While many exciting discoveries have been made of tidal substructures around external galaxies, the "global" questions of galaxy formation and evolution via hierarchical accretion await a more complete census of the low surface brightness outskirts of galaxies in and beyond the Local Group.

We report the discovery of DGSAT. I, an ultra-diffuse, quenched galaxy located 10 degrees 4 in projection from the Andromeda galaxy (M31). This low-surface brightness galaxy (mu(V) = 24.8 mag arcsec(-2)), found with a small amateur telescope, appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by optical surveys relying on resolved star counts, in spite of its relatively large effective radius (R-e(V) = 12") and proximity (15') to the well-known dwarf spheroidal galaxy And II. Its red color (V - I = 1.0), shallow Sersic index (n(V) = 0.68), and the absence of detectable H alpha emission are typical properties of dwarf spheroidal galaxies and suggest that it is mainly composed of old stars. Initially interpreted as an interesting case of an isolated dwarf spheroidal galaxy in the local universe, our radial velocity measurement obtained with the BTA 6 m telescope (V-h = 5450 +/- 40 km s(-1)) shows that this system is an M31-background galaxy associated with the filament of the Pisces-Perseus supercluster. At the distance of this cluster (similar to 78 Mpc), DGSAT. I would have an R-e similar to 4.7 kpc and M-V similar to -16.3. Its properties resemble those of the ultra-diffuse galaxies (UDGs) recently discovered in the Coma cluster. DGSAT. I is the first case of these rare UDGs found in this galaxy cluster. Unlike the UDGs associated with the Coma and Virgo clusters, DGSAT I is found in a much lower density environment, which provides a fresh constraint on the formation mechanisms for this intriguing class of galaxy.

Plant respiration constitutes a massive carbon flux to the atmosphere, and a major control on the evolution of the global carbon cycle. It therefore has the potential to modulate levels of climate change due to the human burning of fossil fuels. Neither current physiological nor terrestrial biosphere models adequately describe its short-term temperature response, and even minor differences in the shape of the response curve can significantly impact estimates of ecosystem carbon release and/or storage. Given this, it is critical to establish whether there are predictable patterns in the shape of the respiration-temperature response curve, and thus in the intrinsic temperature sensitivity of respiration across the globe. Analyzing measurements in a comprehensive database for 231 species spanning 7 biomes, we demonstrate that temperature-dependent increases in leaf respiration do not follow a commonly used exponential function. Instead, we find a decelerating function as leaves warm, reflecting a declining sensitivity to higher temperatures that is remarkably uniform across all biomes and plant functional types. Such convergence in the temperature sensitivity of leaf respiration suggests that there are universally applicable controls on the temperature response of plant energy metabolism, such that a single new function can predict the temperature dependence of leaf respiration for global vegetation. This simple function enables straightforward description of plant respiration in the land-surface components of coupled earth system models. Our cross-biome analyses shows significant implications for such fluxes in cold climates, generally projecting lower values compared with previous estimates.

We present deep optical images of the Large and Small Magellanic Clouds (LMC and SMC) using a low cost telephoto lens with a wide field of view to explore stellar substructure in the outskirts of the stellar disk of the LMC (< 10 degrees from the LMC center). These data have higher resolution than existing star count maps, and highlight the existence of stellar arcs and multiple spiral arms in the northern periphery, with no comparable counterparts in the south. We compare these data to detailed simulations of the LMC disk outskirts, following interactions with its low mass companion, the SMC. We consider interaction in isolation and with the inclusion of the Milky Way tidal field. The simulations are used to assess the origin of the northern structures, including also the low density stellar arc recently identified in the Dark Energy Survey data by Mackey et al. at similar to 15 degrees. We conclude that repeated close interactions with the SMC are primarily responsible for the asymmetric stellar structures seen in the periphery of the LMC. The orientation and density of these arcs can be used to constrain the LMC's interaction history with and impact parameter of the SMC. More generally, we find that such asymmetric structures should be ubiquitous about pairs of dwarfs and can persist for 1-2 Gyr even after the secondary merges entirely with the primary. As such, the lack of a companion around a Magellanic Irregular does not disprove the hypothesis that their asymmetric structures are driven by dwarf-dwarf interactions.

We present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the Carina. II ( Car II) and Carina. III ( Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey ( MagLiteS). We identify 18 member stars in Car. II, including two binaries with variable radial velocities and two RR Lyrae stars. The other 14 members have a mean heliocentric velocity v(hel) = 477.2 +/- 1.2 km s(-1) and a velocity dispersion of sigma(v) 3.4(-0.8)(vertical bar 1.2) km s(-1). Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of 1.0(-0.4)(+0.8) x 10(6) M circle dot, indicating a mass-to-light ratio of 369(-161)(+309) M circle dot/L circle dot. From equivalent width measurements of the calcium triplet lines of nine red giant branch (RGB) stars, we derive a mean metallicity of [Fe/H] = -2.44 +/- 0.09 with dispersion sigma([Fe/H]) = 0.22(-0.07)(+0.10). Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify four member stars, from which we calculate a systemic velocity of vhel = 284.6(-3.1)(+3.4) km S-1. The brightest RGB member of Car. III has a metallicity of [Fe/H] = -1.97 +/- 0.12. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation (Delta d similar to 10 kpc) among Local Group satellites, the large difference in their systemic velocities, similar to 200 km s(-1), indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of gamma-ray emission is found at the locations of Car II and Car III in eight years of Fermi-LAT data.

We report the discovery of two ultra-faint satellites in the vicinity of the Large Magellanic Cloud (LMC) in data from the Magellanic Satellites Survey (MagLiteS). Situated 18 deg (similar to 20 kpc) from the LMC and separated from each other by only 18 arcmin, Carina II and III form an intriguing pair. By simultaneously modelling the spatial and the colour-magnitude stellar distributions, we find that both Carina II and Carina III are likely dwarf galaxies, although this is less clear for Carina III. There are in fact several obvious differences between the two satellites. While both are well described by an old and metal poor population, Carina II is located at similar to 36 kpc from the Sun, with M-V similar to -4.5 and r(h) similar to 90 pc, and it is further confirmed by the discovery of 3 RR Lyrae at the right distance. In contrast, Carina III is much more elongated, measured to be fainter (M-V similar to -2.4), significantly more compact (r(h) similar to 30 pc), and closer to the Sun, at similar to 28 kpc, placing it only 8 kpc away from Car II. Together with several other systems detected by the Dark Energy Camera, Carina II and III form a strongly anisotropic cloud of satellites in the vicinity of the Magellanic Clouds.

Context. Understanding the evolutionary history of the Magellanic Clouds requires an in-depth exploration and characterization of the stellar content in their outer regions, which ultimately are key to tracing the epochs and nature of past interactions.