We present the discovery of a new dwarf galaxy, Hydra II, found serendipitously within the data from the ongoing Survey of the Magellanic Stellar History (SMASH) conducted with the Dark Energy Camera on the Blanco 4m Telescope. The new satellite is compact (r h = 68 ± 11 pc) and faint (M V = −4.8 ± 0.3), but well within the realm of dwarf galaxies. The stellar distribution of Hydra II in the color-magnitude diagram is well-described by a metal-poor ([Fe/H] = −2.2) and old (13 Gyr) isochrone and shows a distinct blue horizontal branch, some possible red clump stars, and faint stars that are suggestive of blue stragglers. At a heliocentric distance of 134 ± 10 kpc, Hydra II is located in a region of the Galactic halo that models have suggested may host material from the leading arm of the Magellanic Stream. A comparison with N-body simulations hints that the new dwarf galaxy could be or could have been a satellite of the Magellanic Clouds.
We present a detailed study of the viewing angles of the LMC disk plane. We Ðnd that our viewing direction di †ers considerably from the commonly accepted values, which has important implications for the structure of the LMC. The discussion is based on an analysis of spatial variations in the apparent magnitude of features in the near-IR color-magnitude diagrams extracted from the Deep Near-Infrared Southern Sky Survey (DENIS) and Two Micron All-Sky Survey (2MASS). Sinusoidal brightness variations with a peak-to-peak amplitude of D0.25 mag are detected as a function of position angle. The same variations are detected for asymptotic giant branch stars (using the mode of their luminosity function) and for red giant branch stars (using the tip of their luminosity function), and these variations are seen consistently in all of the near-IR photometric bands in both DENIS and 2MASS data. The observed spatial brightness variations are naturally interpreted as the result of distance variations because of one side of the LMC plane being closer to us than the opposite side. There is no evidence that any complicating e †ects, such as possible spatial variations in dust absorption or the age/metallicity of the stellar population, cause large-scale brightness variations in the near-IR at a level that exceeds the formal errors (D0.03 mag). The best-Ðtting geometric model of an inclined plane yields an inclination angle and line-of-nodes position angle The quoted errors are conservai \ 34¡ .7^6¡ .2 # \ 122¡ .5^8¡ .3. tive estimates that take into account the possible inÑuence of systematic errors ; the formal errors are much smaller, and respectively. There is tentative evidence for variations of D10¡ in the viewing 0¡ .7 1¡ .6, angles with distance from the LMC center, suggesting that the LMC disk plane may be warped. Traditional methods to estimate the position angle of the line of nodes have used either the major-axis position angle of the spatial distribution of tracers on the sky or the position angle of the line of # maj # max maximum gradient in the velocity Ðeld, given that for a circular diskThe present # maj \ # max \ #. study does not rely on the assumption of circular symmetry and is considerably more accurate than previous studies of its kind. We Ðnd that the actual position angle of the line of nodes di †ers considerably from both and for which measurements have fallen in the range 140¡È190¡. This indi-# maj # max , cates that the intrinsic shape of the LMC disk is not circular but elliptical. Paper II of this series explores the implications of this result through a detailed study of the shape and structure of the LMC. The inclination angle inferred here is consistent with previous estimates, but this is to some extent a coincidence, given that also for the inclination angle most previous estimates were based on the incorrect assumption of circular symmetry.
The thermally-pulsing asymptotic giant branch (TP-AGB) experienced by low-and intermediate-mass stars is one of the most uncertain phases of stellar evolution and the models need to be calibrated with the aid of observations. To this purpose, we couple high-quality observations of resolved stars in the Small Magellanic Cloud (SMC) with detailed stellar population synthesis simulations computed with the TRILEGAL code. The strength of our approach relies on the detailed spatially-resolved star formation history of the SMC, derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds, as well as on the capability to quickly and accurately explore a wide variety of parameters and effects with the COLIBRI code for the TP-AGB evolution. Adopting a well-characterized set of observations -star counts and luminosity functions -we set up a calibration cycle along which we iteratively change a few key parameters of the TP-AGB models until we eventually reach a good fit to the observations. Our work leads to identify two best-fitting models that mainly differ in the efficiencies of the third dredge-up and mass loss in TP-AGB stars with initial masses larger than about 3 M . On the basis of these calibrated models we provide a full characterization of the TP-AGB stellar population in the SMC in terms of stellar parameters (initial masses, C/O ratios, carbon excess, mass-loss rates). Extensive tables of isochrones including these improved models are publicly available.
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