The Proper Motion Field Along the Magellanic Bridge: a New Probe of the LMC-SMC Interaction

We present the first detailed kinematic analysis of the proper motions (PMs) of stars in the Magellanic Bridge, from both the \textit{Gaia} Data Release 2 catalog and from \textit{Hubble Space Telescope} Advanced Camera for Surveys data. For the \textit{Gaia} data, we identify and select two populations of stars in the Bridge region, young main sequence (MS) and red giant stars. The spatial locations of the stars are compared against the known H {\small I} gas structure, finding a correlation between the MS stars and the H {\small I} gas. In the \textit{Hubble Space Telescope} fields our signal comes mainly from an older MS and turn-off population, and the proper motion baselines range between $\sim 4$ and 13 years. The PMs of these different populations are found to be consistent with each other, as well as across the two telescopes. When the absolute motion of the Small Magellanic Cloud is subtracted out, the residual Bridge motions display a general pattern of pointing away from the Small Magellanic Cloud towards the Large Magellanic Cloud. We compare in detail the kinematics of the stellar samples against numerical simulations of the interactions between the Small and Large Magellanic Clouds, and find general agreement between the kinematics of the observed populations and a simulation in which the Clouds have undergone a recent direct collision.Comment: 13 pages, 10 figures, 2 tables, submitted to ApJ, accepted February 8th, 201

Star Formation Histories of Ultra-faint Dwarf Galaxies: Environmental Differences between Magellanic and Non-Magellanic Satellites?*

We present the color–magnitude diagrams and star formation histories (SFHs) of seven ultra-faint dwarf galaxies: Horologium 1, Hydra 2, Phoenix 2, Reticulum 2, Sagittarius 2, Triangulum 2, and Tucana 2, derived from highprecision Hubble Space Telescope photometry. We find that the SFH of each galaxy is consistent with them having created at least 80% of the stellar mass by z ∼ 6. For all galaxies, we find quenching times older than 11.5 Gyr ago, compatible with the scenario in which reionization suppresses the star formation of small dark matter halos. However, our analysis also reveals some differences in the SFHs of candidate Magellanic Cloud satellites, i.e., galaxies that are likely satellites of the Large Magellanic Cloud and that entered the Milky Way potential only recently. Indeed, Magellanic satellites show quenching times about 600 Myr more recent with respect to those of other Milky Way satellites, on average, even though the respective timings are still compatible within the errors. This finding is consistent with theoretical models that suggest that satellites’ SFHs may depend on their host environment at early times, although we caution that within the error bars all galaxies in our sample are consistent with being quenched at a single epoch

Extraction of Thermal Spectra of the Four Large Satellites of Uranus

International audienceThe present interest in a Uranus orbiter has refocused attention on the distant planet's mysterious moons. The latter form a regular satellite system whose evolution - during and after the event that tilted Uranus' pole - is poorly understood. Near-infrared observations of the moons have been analyzed, but the mid-infrared (MIR) range remains relatively unexplored and could provide additional insight into the composition of the surfaces and the chemistry occurring thereon. Such MIR spectra cannot be collected from the ground and require spacecraft or high altitude aircraft. We present here data of Ariel, Umbriel, Titania, and Oberon, obtained by the Infrared Spectrograph (IRS) aboard the Spitzer Space Telescope during its cold mission. We developed a state-of-the-art technique to extract the mid-infrared spectra of the moons, making use of the Spectroscopic Modeling Analysis and Reduction Tool (SMART). The observations of the moons were affected by stray light from the planet reaching the detector, making this dataset obtained in 2005 too difficult to tackle until now. To disentangle the light contributions from the moon and the planet's stray light, we generated theoretical point spread functions (PSF) using Spitzer Tiny Tim (STINY_TIM). We then calculated the effective, expected, normalized contributions from the moon's and planet's PSFs inside the slit. Finally, we used a multiple linear regression to extract the moon and planet fluxes simultaneously, for each wavelength element. These first MIR spectra of Uranus' moons are now being examined for molecular features and blackbody temperatures. Preliminary results show the lack any molecular absorption features, in particular CO2. The inferred temperatures are in complete agreement with the theoretical models. Implications for future observations with JWST will be discussed

Extraction of Thermal Spectra of the Four Large Satellites of Uranus

International audienceThe present interest in a Uranus orbiter has refocused attention on the distant planet's mysterious moons. The latter form a regular satellite system whose evolution - during and after the event that tilted Uranus' pole - is poorly understood. Near-infrared observations of the moons have been analyzed, but the mid-infrared (MIR) range remains relatively unexplored and could provide additional insight into the composition of the surfaces and the chemistry occurring thereon. Such MIR spectra cannot be collected from the ground and require spacecraft or high altitude aircraft. We present here data of Ariel, Umbriel, Titania, and Oberon, obtained by the Infrared Spectrograph (IRS) aboard the Spitzer Space Telescope during its cold mission. We developed a state-of-the-art technique to extract the mid-infrared spectra of the moons, making use of the Spectroscopic Modeling Analysis and Reduction Tool (SMART). The observations of the moons were affected by stray light from the planet reaching the detector, making this dataset obtained in 2005 too difficult to tackle until now. To disentangle the light contributions from the moon and the planet's stray light, we generated theoretical point spread functions (PSF) using Spitzer Tiny Tim (STINY_TIM). We then calculated the effective, expected, normalized contributions from the moon's and planet's PSFs inside the slit. Finally, we used a multiple linear regression to extract the moon and planet fluxes simultaneously, for each wavelength element. These first MIR spectra of Uranus' moons are now being examined for molecular features and blackbody temperatures. Preliminary results show the lack any molecular absorption features, in particular CO2. The inferred temperatures are in complete agreement with the theoretical models. Implications for future observations with JWST will be discussed

Astrometry with MCAO at Gemini and at ELTs

We present in this study a first analysis of the astrometric error budget of absolute astrometry relative to back- ground galaxies using adaptive optics. We use for this analysis multi-conjugated adaptive optics (MCAO) images obtained with GeMS/GSAOI at Gemini South. We find that it is possible to obtain 0.3 mas reference precision in a random field with 1 hour on source using faint background galaxies. Systematic errors are correctable below that level, such that the overall error is approximately 0.4 mas. Because the reference sources are extended, we find it necessary to correct for the dependency of the PSF centroid on the used aperture size, which would oth- erwise cause an important bias. This effect needs also to be considered for Extremely Large Telescopes (ELTs). When this effect is corrected, ELTs have the potential to measure proper motions of dwarfs galaxies around M31 with 10 km/s accuracy over a baseline of 5 years