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Satellites orbiting disk galaxies can induce phase space features such as spirality, vertical heating and phase-mixing in their disks. Such features have also been observed in our own Galaxy, but the complexity of the Milky Way disk has only recently been fully mapped by Gaia Data Release 2 (DR2) data. This complex behaviour is mainly ascribed to repeated perturbations induced by the Sagittarius dwarf galaxy (Sgr) along its orbit, pointing to this satellite as the main dynamical architect of the Milky Way disk. Here, we model Gaia DR2-observed colour–magnitude diagrams to obtain a detailed star formation history of the ~2 kpc bubble around the Sun. It reveals three conspicuous and narrow episodes of enhanced star formation that we can precisely date as having occurred 5.7, 1.9 and 1.0 Gyr ago. The timing of these episodes coincides with proposed Sgr pericentre passages according to (1) orbit simulations, (2) phase space features in the Galactic disk and (3) Sgr stellar content. These findings most probably suggest that Sgr has also been an important actor in the build-up of the stellar mass of the Milky Way disk, with the perturbations from Sgr repeatedly triggering major episodes of star formation. Galactic close encounters can induce gravitational effects in the participants. Here Ruiz-Lara et al. have reconstructed the star formation history of the region of our Galaxy close to the Sun, finding that three recent visits of the neighbouring Sagittarius dwarf galaxy have resulted in well-defined episodes of star formation.

The new cardinalfish Gymnapogon sagittarius sp. nov. is described on the basis of eight specimens from Vietnam, Malaysia and the Andaman Sea. The new species is most similar to the Red Sea species G. melanogaster Gon and Golani 2002 (known only from two specimens) in having 9 soft dorsal-fin rays, 8 soft anal-fin rays, and 13 total gill rakers, but differs in having 16 pectoral-fin rays (vs. 14 or 15 rays in G. melanogaster), body depth 12.0–15.1% of SL (mean 14.2%) (vs. 21.7–24.0%), eye diameter 5.2–6.1% (5.6%) (vs. 10.0–11.6%), interorbital width 3.7–4.4% (4.0%) (vs. 5.7–8.2%), and longest pelvic-fin soft ray length 12.0–15.6% (14.1%) (vs. 25.4–34.3%).

A method is introduced for constructing two-color maps for the in-plane component of the magnetic field of our galaxy in (R, l) and (DM, l) coordinates. It is shown that, in agreement with the standard models of the galactic magnetic field, the magnetic field in neighboring spiral arms reverses direction. However, the magnetic field in the spiral arm of Sagittarius differs significantly from the standard magnetic field model, with the major difference being that the magnetic fields in the southern and northern hemispheres are oppositely directed in the spiral arm of Sagittarius. It is proposed that this distribution of the magnetic field can be explained best by assuming that the spiral arm of Sagittarius, or, at least, a magnetic spiral arm in that region, is not symmetric with respect to the galactic plane and lies mainly in the northern hemisphere.

Faraday rotation data on 40 pulsars are used in a detailed study of the magnetic field and its fluctuations in the direction of the spiral arm of Sagittarius. These results mostly agree with standard models for the galactic magnetic field. A magnetic field on the order of 3.2 μG is directed from galactic longitude l 0=55° (toward the sun). However, an asymmetry has been found in the degrees of rotation relative to a plane lying in the southern hemisphere parallel to the galactic plane and at a distance of 50-60 pc from it. All the pulsars with measures of dispersion greater than 30 pc·cm-3 and lying to the north of this plane have positive measures of rotation which increase linearly with distance, while the pulsars lying to the south of this plane have unusually absolutely low negative measures of rotation. We propose that the spiral arm of Sagittarius lies entirely to the north of this plane, while the negative measures of rotation of the pulsars below this plane are caused by the magnetic field of the halo of the southern hemisphere of the galaxy. The magnetic field in the arm of Sagittarius is regular to a great extent and its fluctuating component is roughly half the regular component.

A new zoarcid fish, Lycodes mcallisteri, is described on the basis of nine specimens (230–375 mm SL) from eastern Arctic Canada, Nunavut (Hudson Strait, 298 m; Cumberland Sound and Baffin Bay, 579 m). It resembles about 15 other Lycodes spp. in having a long tail and a "ventromediolateral" type of lateral line. It is distinguished from Arctic members of this group (Lycodes marisalbi, L. palearis, and L. sagittarius) by the high number of pectoral fin rays (22–23), short pelvic fins, low submental crests, and coloration (6–10 light patches dorsally). It resembles two Pacific species (L. macrochir and L. soldatovi) by the high number of pectoral fin rays, but differs by coloration, meristics, and shape of the pectoral fin.

We present preliminary results of a long-term project to investigate the process of accretion and tidal disruption of dSph satellites in the Galactic halo and, in particular, to search for new tidal tails in a sample of nearby dSph satellites of the Milky Way. Here we present our finding of extra-tidal debris in the Ursa Minor dSph and discuss the detection by the Sloan Digitized Sky Survey team of what could be a tidal stream associated to the Sagittarius dSph.

In this paper we present the first results of the analysis of intermediate resolution (Δλ ∼ 3.5 A) spectra of giants of the Sagittarius dwarf galaxy acquired using the ESO NTT telescope. From the deep CCD photometry of Marconi et al. (1998a) we have selected a sample of giants representative of the metallicity spread suggested by the comparison of the colour-magnitude diagram of Sagittarius with those of galactic globular clusters. The spectra have been used to measure radial velocities, to confirm the membership to Sagittarius, and to provide a metallicity estimate by using spectral synthesis techniques. The analyzed stars show a spread in metallicities in the range −1.0≤ [Fe/H]≤ +0.7, some 0.5 dex more metal-rich than the photometric estimates.

We show the results of numerical simulations of dwarf spheroidal galaxies (DSphs), which allow to understand their evolution in the framework of no internal virial equilibrium. Under this hypothesis, we reproduce with a low dark matter (DM) model the high values of the virial M/L ratio measured observationally for these objects. In this work, we present a model of the Sagittarius (Sgr) DSph, with all its observational features.

Radio luminosities of pulsars depend on their periods and derivatives of periods. The parameters of these dependences and the independent distances for 288 pulsars are determined. Known dispersion measures are used for determination of the mean electron densities in the direction of pulsars. The results obtained are used for investigation of the large-scale distribution of electron concentration in the galaxy. The maximum value of that distribution is found at a distance of 9 kpc from the galactic center in the Sagittarius arm. In the interarm regions electron density decreases roughly exponentially.

The radio luminosities o f pulsars are given as functions of their period and the time variation of the period. The parameters of that dependence are calculated and independent distances are determined for 288 pulsars. The average electron densities toward the pulsars are determined from the known dispersion measures. The results obtained are used to study the large-scale electron density distribution in the Galaxy at a distance of 4-5 kpc from the sun. The distribution maximum lies at a distance of 9 kpc from the center o f the Galaxy in the vicinity of the Sagittarius spiral arm. The electron density falls o f f exponentially in the regions between arms (toward the Perseus and Centaurus spiral arms).

The distribution of C/O ratio in carbon stars in the solar neighbourhood and Cygnus, Aquila, Sagittarius regions is obtained. The group of carbon-enriched C stars is selected.

The discovery of the Sagittarius dwarf galaxy, which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars, and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is omega Centauri, and here we report that omega Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that omega Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that omega Centauri was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and omega Centauri appears to be a relict of this era., 7 pages, 3 figures, Latex+nature.sty (included), To appear in November 4th issue of Nature

A gas cloud has been spotted approaching the Milky Way's central black hole. Observations of its closest approach, expected to occur in mid-2013, may offer insight into the black hole's immediate surroundings. See Letter p.51 The radio source Sgr A* in Sagittarius is thought to be the site of a supermassive black hole lying at the centre of the Milky Way. A study of stellar orbits has identified an object moving towards Sgr A* at a speed of 1,700 kilometres per second. Its low temperature and spectral properties suggest that it is a dusty cloud of ionized gas, three times the mass of Earth, in the process of falling into the black hole. Models predict that as the cloud gets closer to the black hole, X-ray emissions will become much brighter, and a giant radiation flare may be emitted in a few years if the cloud breaks up and feeds gas into the black hole.

Knowledge of how the Milky Way formed and evolved is deficient. Simulations show that a past encounter with another galaxy may account for the Galaxy's intricate morphology. See Letter p.301 Since its discovery more than a decade ago, the Sagittarius dwarf galaxy (Sgr), a satellite galaxy of our own Milky Way, has been recognized as a local analogue to the numerous mergers thought to be common in galaxies throughout the Universe. Traditionally, Sgr has been treated as a negligible perturber to the Galactic disk. New simulations of the response of the Milky Way to the infall of the Sgr reveal that, on the contrary, Sgr has played an important part in shaping the disk morphology. Past impacts have triggered the formation of spiral structure and influenced bar evolution.

Giant flashes from soft γ-ray repeaters are spectacular but rare events — only three have ever been observed in our Galaxy. The suspicion is that we have been missing some from farther afield. On 27 December last year, SGR1806–20, a soft γ-ray repeater in Sagittarius, released a giant flare that has been called the brightest explosion ever recorded. SGRs are X-ray stars that sporadically emit low-energy γ-ray bursts. They are thought to be magnetars: neutron stars with observable emissions powered by magnetic dissipation. Five papers in this issue report initial and follow-up observations of this event. The data are remarkable: for instance in a fifth of a second, the flare released as much energy as the Sun radiates in a quarter of a million years. Such power can be explained by catastrophic global crust failure and magnetic reconnection on a magnetar. Releasing a hundred times the energy of the only two previous SGR giant flares, this may have been a once-in-a-lifetime event for astronomers, and for the star itself.

The article discusses the planned route of the "New Horizons" spacecraft past Pluto as of April 2013, with information on the U.S. National Aeronautics and Space Administration's (NASA) search for celestial bodies and planets for it to observe. Topics include the proximity of three known objects to the spacecraft in 2018, NASA's hope to discover a closer planet for it to explore in detail, and the difficulty of identifying possible targets near the constellation Sagittarius.

The discovery last year of Sagittarius, a new dwarf galaxy, prompts a train of speculation about the degree to which the Galaxy has scavenged some of its bulk from the surroundings.

A large body of newly determined rotation measures (RM) for extragalactic radio sources gives a new, much clearer picture of the magnetic structure of our Galaxy. The RM sky is dominated by a very strong zone which extends to large (down to b≈ −40°) negative galactic latitudes. Both the strength and extent of this feature are greater than appreciated from earlier data. It coincides in angular position with loop II of the non-thermal radio background radiation of the Galaxy. This, and two other strong RM features suggest that our Galaxy has very large-scale magnetic field zones, probably between the spiral arms, and that the prevailing sense of the field is opposite in the interarm regions on each side of the Sagittarius arm.

Low energy X ray spectra of Sco X-1 and Sagittarius sources measured by Be and Al window proportional counters during rocket flight

THE first experimental evidence for the presence of 18-cm absorption lines of the hydroxyl (OH) radical in the radio absorption spectrum of Cassiopeia A, as suggested by Lilley1, was obtained by Weinreb et al.2 based on observations made during the period October 15–29, 1963. The Radio Physics Laboratory, Commonwealth Scientific and Industrial Research Organization, in Sydney, Australia, has reported the successful measurement of OH absorption in the spectrum of Sagittarius (see preceding communication).

Chromosome studies of 1 species of Cariamidae and Sagittarius show great karyological differences and tend to dispel the suggestion of a possible relationship between the families.

WE have observed two separate and intense sources of cosmic X-rays in the region of the constellations Scorpius and Sagittarius during a rocket experiment launched on August 28, 1964. This is the same general region of the sky from which non-solar cosmic X-rays were first detected in June 1962 by Giacconi et al.1. In that earlier work a Geiger tube detector of large area and wide angular response was mounted, looking out of the side of a spinning rocket. A strong maximum in the counting rate was observed when the detector pointed in a southerly direction. A detailed discussion of the results led to the conclusion that the X-rays were of non-terrestrial origin, and that they emanated from a source close to, but apparently not coincident with, the galactic centre. These conclusions were confirmed in two later rocket flights, also employing wide-angle detectors2. In April 1963 Bowyer et al. conducted a rocket experiment using a detector with a field of view 10° wide at half maximum intensity3. At the time of that flight the galactic centre was below the horizon. They gave a location for the source in Scorpius at about 16 h 15 min right ascension and − 15° declination and stated that its angular diameter is less than 5°.

WE have measured the 18-cm OH absorption spectrum of Sagittarius A over a range of 6.4 Mc/s using the 60-ft. radio telescope of Harvard College Observatory. This frequency range covers both the 1,667 Mc/s F(2→2) and the 1,665 Mc/s F (1→1) transitions. The spectrum is dominated by two features at +40 km/sec and −120 km/sec with respect to the local standard of rest. Parts of this spectrum have been previously reported1–3, notably the feature at +40 km/sec4. We have also obtained an absorption spectrum for 1II = 4°, bII = 0°. The characteristics of the background continuum radiation near 18 cm and this absorption spectrum allow us to establish an upper limit on the state temperature of interstellar OH.

THERE have been several claims for variability in the 2–10 keV emission from X-ray sources in regions of high source density1,2. In all such early surveys the separations of the sources were typically much smaller than the collimator resolution. More recent high resolution surveys of the Sagittarius and Cygnus regions3,4 have usually resulted in source locations differing markedly from those used in the analyses of the early surveys and have cast some doubt on the variable nature of many of the sources.

Angular diameters of X-ray source regions in Scorpio and Sagittarius constellations by rocket measurement with collimator