EDP Sciences

Vol. 601
In section 1. Letters

The HI content of isolated ultra-diffuse galaxies: A sign of multiple formation mechanisms?

by E Papastergis, E.A.K. Adams, and A.J. Romanowsky A&A 601, L10


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Ultra-diffuse galaxies (UDG) are distinctive because of their extremely low surface density of stars, with respect to the mass and size of their dark matter halo. The latter are derived from their relatively large rotational velocity, and large size. UDG are very abundant in galaxy clusters, and one of the best scenarios is that they have been stripped of their gas by the cluster environment, which stopped their star formation. In isolation, they could be quenched by starburst-driven outflows. The present article reports of 21cm observations of the HI component in four isolated UDG, leading to three upper limits and one detection. Combining with the literature, this means six field UDG have been observed, with three upper limits. The observed dichotomy in the gaseous properties of field UDGs is interpreted as a sign of the existence of multiple mechanisms for their formation, with the formation of the quiescent gas-poor UDGs remaining a mystery.

Vol. 601
In section 5. Galactic structure, stellar clusters and populations

The Gaia-ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations

byA. Rojas-Arriagada, A. Recio-Blanco, P. de Laverny, et al. A&A 601, A140


This paper uses the fourth internal data release of the Gaia-ESO spectroscopic survey to characterize the properties of 2500 individual red clump stars in the Galactic bulge, sampling the range of longitude from -10deg to +8deg and latitude from -10deg to -4deg. This is the first time that such a large and homogeneous sample, over such a wide area of the bulge, has been analyzed looking at [Fe/H] and [Mg/Fe] with such high resolution and high S/N spectra. The authors present the spatial distribution of these measurements, and the correlation of these properties with kinematics. The metalicity distribution function of the bulge is confirmed to be bimodal across the whole sampled area, with metal-poor stars dominating at higher latitudes. The metal-poor stars have isotropic hot kinematics, not consistent with an X-shaped bulge, that could be the product of an early prompt dissipative collapse dominated by massive stars. The population shows a knee in the [Mg/Fe] measurements at higher [Fe/H] than in the thick disk, which suggests a higher star-formation rate. In contrast, the metal-rich stars exhibit bar-like kinematics and appear tightly associated with an X-shape bulge. These metal-rich stars also overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. The bar boxy-peanut bulge thus seems likely to have formed from the secular evolution of the early thin disk. The authors present a chemical evolution model for the entire bulge that requires a fast (< 1 Gyr) intense burst of stellar formation at early epochs.

Vol. 601
In section 9. The Sun

Simulating Ellerman bomb-like events

by S. Danilovic A&A 601, A122


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Discovered exactly a century ago, Ellerman bombs are prominent brightenings in seen in H-alpha observations of newly emerging active regions (sunspots) on the Sun. These events play a crucial role in our understanding of the magnetic reconnection on the Sun, and cartoon scenarios have been drawn for decades. This this paper shows in a 3D MHD simulation how these events form in a self-consistent, way and highlights that solar convection pushes together magnetic field of opposite magnetic polarity and through this initiates reconnection in the photosphere of the Sun. The H-alpha emission derived from the simulations shows the same characteristic flame-like appearance as that found in observations.

Vol. 601
In section 10. Planets and planetary systems

Evidence for depletion of heavy silicon isotopes at comet 67P/Churyumov-Gerasimenko

by M. Rubin, K. Altwegg, H. Balsiger, et al. A&A 601, A123


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The heterogeneity of comets as carriers of primordial abundances is highlighted by the identification of the silicon isotopic ratio variations between comets - especially from this detailed, in situ study. Most importantly, the isotopic ratios point to different origins, espcially among the AGB stars and supernovae. While this has been known for some time from meteorites, this study is a first for the in situ sampling of a comet (thought to be the most primitive material in the solar system). It is important that the silicon is a far less chemistry-dependent tracer of the source(s) than the CNO isotopes.

Vol. 601
In section 5. Galactic structure, stellar clusters and populations.

The compactness of the isolated neutron star RX J0720.4-3125

by V. Hambaryan, V. Suleimanov, F.Haberl, et al., A&A 601, A108


RX J0720.4-3125 (here referred to as J0720) is one of the "magnificent seven", the few isolated neutron stars to have been discovered by ROSAT. J0720 is one of the brightest and best studied (partly because it has been used to calibrate different X-ray telescopes). Pulsations at 8.39 s have been readily observed, establishing a connection with magnetars (i.e. highly magnetic neutron star). In this paper, Hambaryan and co-authors convincingly show that the true spin period is actually twice this reported value (i.e. 16.78 s), highlighting subtle variations in the light curve profile and energy spectrum from one (old) period to the following one. This configuration is achieved with a nearly orthogonal rotator with an angle between the magnetic and the rotation axis of 90 degrees.

Vol. 601
In section 1. Letters to the Editor

First detection of the 448 GHz H2O transition in space

by M. Pereira-Santaella, E. Gonzalez-Alfonso, A. Usero, et al. A&A 601, L3


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Water is a key molecule to probe dense and warm regions in the interstellar medium, because of its chemistry as well as its coupling to the far-infrared. This work presents the first detection of the ortho-H2O 4_23 - 3_30 transition at 448 GHz in space. Due to a strong opacity in the terrestrial atmosphere, this line had not till now been observed in nearby molecular clouds. The line is found in emission with ALMA in a redshifted luminous infrared (IR) galaxy ESO 320-G030. Water excitation models have long predicted the maser nature of this transition, pumped by collisions when the kinetic temperature is around 1000K and the hydrogen density 10^5 cm3. On the contrary, and surprisingly, the line is primarily excited by the intense far-IR radiation field that is present in the nucleus of this galaxy. This field extends over a region of 65 pc in diameter and shows a regular rotation pattern compatible with the global molecular and ionized gas kinematics. This H2O transition is therefore a probe of deeply buried galactic nuclei thanks to the high dust optical depths (NH > 4 10^24 cm2) required to efficiently excite it.