The Pristine Dwarf-Galaxy survey -- V. The edges of the dwarf galaxy Hercules

We present a new spectroscopic study of the dwarf galaxy Hercules (d ~ 132 kpc) with data from the Anglo-Australian Telescope and its AAOmega spectrograph together with the Two Degree Field multi-object system to solve the conundrum that whether Hercules is tidally disrupting. We combine broadband photometry, proper motions from Gaia, and our Pristine narrow-band and metallicity-sensitive photometry to efficiently weed out the Milky Way contamination. Such cleaning is particularly critical in this kinematic regime, as both the transverse and heliocentric velocities of Milky Way populations overlap with Hercules. Thanks to this method, three new member stars are identified, including one at almost 10rh of the satellite. All three have velocities and metallicities consistent with that of the main body. Combining this new dataset with the entire literature cleaned out from contamination shows that Hercules does not exhibit a velocity gradient (d/dX = 0.1+0.4/-0.2 km s-1 arcmin-1) and, as such, does not show evidence to undergo tidal disruption

The Pristine Dwarf-Galaxy survey - II. In-depth observational study of the faint Milky Way satellite Sagittarius II

We present an extensive study of the Sagittarius II (Sgr II) stellar system using MegaCam g and i photometry, narrow-band, metallicity-sensitive calcium H&K doublet photometry and Keck II/DEIMOS multiobject spectroscopy. We derive and refine the Sgr II structural and stellar properties inferred at the time of its discovery. The colour–magnitude diagram implies Sgr II is old (12.0 ± 0.5 Gyr) and metal poor. The CaHK photometry confirms the metal-poor nature of the satellite ([Fe/H] CaHK = −2.32 ± 0.04 dex) and suggests that Sgr II hosts more than one single stellar population (σ CaHK [FeH] = 0.11+0.05 −0.03 dex). Using the Ca infrared triplet measured from our highest signal-to-noise spectra, we confirm the metallicity and dispersion inferred from the Pristine photometric metallicities ([Fe/H]spectro = −2.23 ± 0.05 dex, σspectro [Fe/H] = 0.10+0.06 −0.04 dex). The velocity dispersion of the system is found to be σv = 2.7+1.3 −1.0 km s−1 after excluding two potential binary stars. Sgr II’s metallicity and absolute magnitude (MV = −5.7 ± 0.1 mag) place the system on the luminosity–metallicity relation of the Milky Way dwarf galaxies despite its small size. The low but resolved metallicity and velocity dispersions paint the picture of a slightly dark-matter-dominated satellite (M/L = 23.0+32.8 −23.0 M L−1 ). Furthermore, using the Gaia Data Release 2, we constrain the orbit of the satellite and find an apocentre of 118.4+28.4 −23.7 kpc and a pericentre of 54.8+3.3 −6.1 kpc. The orbit of Sgr II is consistent with the trailing arm of the Sgr stream and indicates that it is possibly a satellite of the Sgr dSph that was tidally stripped from the dwarf’s influence.ES, KY, and AA gratefully acknowledge funding by the Emmy Noether programme from the Deutsche Forschungsgemeinschaft (DFG). This work has been published under the framework of the IdEx Unistra and benefits from a funding from the state managed by theFrench National Research Agency as part of the investments for the future program. NFM, RI, and NL gratefully acknowledge support from the French National Research Agency (ANR) funded project ‘Pristine’ (ANR-18-CE31-0017) along with funding from CNRS/INSU through the Programme National Galaxies et Cosmologie and through the CNRS grant PICS07708. The authors thank the International Space Science Institute (ISSI), Berne, Switzerland for providing financial support and meeting facilities to the international team ‘Pristine’. JIGH acknowledges financial support from the Spanish Ministry project MINECO AYA2017- 86389-P, and from the Spanish MINECO under the 2013 Ramon y ´ Cajal program MINECO RYC-2013-14875. BPML gratefully acknowledges support from FONDECYT postdoctoral fellowship No. 316051

The Pristine Survey – VI. The first three years of medium-resolution follow-up spectroscopy of Pristine EMP star candidates★

We present the results of a 3-year long, medium-resolution spectroscopic campaign aimed at identifying very metal-poor stars from candidates selected with the CaHK, metallicity-sensitive Pristine survey. The catalogue consists of a total of 1007 stars, and includes 146 rediscoveries of metal-poor stars already presented in previous surveys, 707 new very metal-poor stars with [Fe/H]<−2.0⁠, and 95 new extremely metal-poor stars with [Fe/H]<−3.0⁠. We provide a spectroscopic [Fe/H] for every star in the catalogue, and [C/Fe] measurements for a subset of the stars (10% with [Fe/H]<−3 and 24% with −3<[Fe/H]<−2⁠) for which a carbon determination is possible, contingent mainly on the carbon abundance, effective temperature and S/N of the stellar spectra. We find an average carbon enhancement fraction ([C/Fe] ≥ +0.7) of 41 ± 4% for stars with −3<[Fe/H]<−2 and 58 ± 14% for stars with [Fe/H]<−3⁠, and report updated success rates for the Pristine survey of 56 % and 23 % to recover stars with [Fe/H]<−2.5 and [Fe/H]<−3⁠, respectively. Finally, we discuss the current status of the survey and its preparation for providing targets to upcoming multi-object spectroscopic surveys such as WEAVE

The First Post-Kepler Brightness Dips of KIC 8462852

We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in October 2015, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1-2.5% dips, named "Elsie," "Celeste," "Skara Brae," and "Angkor", which persist on timescales from several days to weeks. Our main results so far are: (i) there are no apparent changes of the stellar spectrum or polarization during the dips; (ii) the multiband photometry of the dips shows differential reddening favoring non-grey extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale <<1um, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term "secular" dimming, which may be caused by independent processes, or probe different regimes of a single process

Étude approfondie des galaxies les moins lumineuses avec spectroscopie dédiée, basée sur le relevé Pristine

Dwarf galaxies are metal-poor, faint stellar systems orbiting in general around massive host galaxies. They can be used as cosmological probes as well as laboratories to constrain galaxy formation and evolution. However, most of the recently discovered faint satellites have an ambiguous nature. This ambiguity can be lifted with spectroscopy. But using spectroscopy to that end is only effective if enough stars members of these systems can be identified, which is not always the case given their faintness. My thesis presents the detailed studies of three Milky Way satellites, Draco II, Sagittarius II and Laevens 3, that aim to unveil their nature using deep broadband MegaCam photometry and DEIMOS multi-object spectroscopy. These data are supplemented with the new narrow-band, metallicity-sensitive photometry provided by the Pristine survey that complements spectroscopy by identifying the typical metal-poor population of dwarf galaxies. The size of Draco II places the satellite in the realm of dwarf galaxies. If the spectroscopy is not informative in this case, the Pristine survey allows to estimate the metallicity of the system that also suggests a galactic nature. The spectroscopic and Pristine observations of Sagittarius II also marginally hint that it is a dwarf galaxy, although the satellite is slightly more compact than expected for a dwarf at that luminosity. Its orbit suggests that it is in fact a former satellite of the Sagittarius dwarf galaxy that was stripped from its influence by the Milky Way. Finally, the properties of Laevens 3 unambiguously show that it is anouter halo globular cluster.Les galaxies naines sont des systèmes stellaires pauvres en métaux et peu lumineux, orbitant en général autour de galaxies plus massives. Elles peuvent être utiles pour contraindre la cosmologie autant que les processus de formation et d’évolution des galaxies. Cependant, les satellites peu brillants découverts récemment sont de nature ambiguë. Cette ambiguïté peut être levée par des observations spectroscopiques. Utiliser la spectroscopie à cette fin n’est efficace que si suffisamment d’étoiles membres de ces systèmes peuvent être identifiées, ce qui n’est pas toujours le cas pour ces satellites peu brillants. Ma thèse présente les études détaillées de trois satellites de la Voie Lactée, Draco II, Sagittaire II et Laevens 3, visant à lever le voile sur leur nature par l’utilisation de photométrie profonde MegaCam à bande large et de spectroscopie multi-objet DEIMOS. À ces données s’ajoute de la photométrie bande étroite, sensible à la métallicité des étoiles, provenant du relevé Pristine. Celle-ci est complémentaire à la spectroscopie car elle permet d’identifier les populations pauvres en métaux typiques des galaxies naines. La taille de Draco II place le satellite dans le domaine des galaxies naines. Si la spectroscopie n’est pas informative dans ce cas, le relevé Pristine permet d’estimer la métallicité du système, celle-ci suggérant également une nature galactique. Les observations spectroscopiques et Pristine de Sagittaire II laissent également penser qu’il s’agit d’une galaxie naine, même si le satellite est légèrement plus compact qu’attendu pour une naine de cette luminosité. Son orbite, elle, suggère que c’est en fait un ancien satellite de la galaxie naine du Sagittaire, qui a été arraché de son influence par la Voie Lactée. Enfin, les propriétés de Laevens 3 montrent, sans ambiguïté, que le système est un amas globulaire du halo externe

Post-adaptive optics bispectral speckle imaging

International audienc

Current Velocity Data on Dwarf Galaxy NGC 1052-DF2 do not Constrain it to Lack Dark Matter

International audienceIt was recently proposed that the globular cluster system of the very low surface brightness galaxy NGC 1052-DF2 is dynamically very cold, leading to the conclusion that this dwarf galaxy has little or no dark matter. Here, we show that a robust statistical measure of the velocity dispersion of the tracer globular clusters implies a mundane velocity dispersion and a poorly constrained mass-to-light ratio. Models that include the possibility that some of the tracers are field contaminants do not yield a more constraining inference. We derive only a weak constraint on the mass-to-light ratio of the system within the half-light radius ( at the 90% confidence level) or within the radius of the furthest tracer ( at the 90% confidence level). This limit may imply a mass-to-light ratio on the low end for a dwarf galaxy, but many Local Group dwarf galaxies fall well within this contraint. With this study, we emphasize the need to reliably account for measurement uncertainties and to stay as close as possible to the data when determining dynamical masses from very small data sets of tracers