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

Is Draco II one of the faintest dwarf galaxies? First study from Keck/DEIMOS spectroscopy

We present the first spectroscopic analysis of the faint and compact stellar system Draco II (Dra II, M_V=-2.9+/-0.8, r_h=19^{+8}_{-6} pc), recently discovered in the Pan-STARRS1 3{\pi} survey. The observations, conducted with DEIMOS on the Keck II telescope, establish some of its basic characteristics: the velocity data reveal a narrow peak with 9 member stars at a systemic heliocentric velocity =-347.6^{+1.7}_{-1.8} km/s, thereby confirming Dra II is a satellite of the Milky Way; we infer a velocity dispersion with \sigma_{vr}=2.9+/-2.1 km/s (<8.4 km/s at the 95% confidence level), which implies log_{10}(M_{1/2})=5.5^{+0.4}_{-0.6} and log_{10}((M/L)_{1/2})=2.7^{+0.5}_{-0.8}, in Solar units; furthermore, very weak Calcium triplet lines in the spectra of the high signal-to-noise member stars imply [Fe/H]<-2.1, whilst variations in the line strengths of two stars with similar colours and magnitudes suggest a metallicity spread in Dra II. These new data cannot clearly discriminate whether Draco II is a star cluster or amongst the faintest, most compact, and closest dwarf galaxies. However, the sum of the three --- individually inconclusive --- pieces of evidence presented here, seems to favour the dwarf galaxy interpretation.Comment: 5 pages, 4 figures, 1 table. Excepted for publication in MNRAS. Full table 1 available upon request. v2: moderate revisions of the text, conclusion unchange

Triangulum II: A Very Metal-poor and Dynamically Hot Stellar System

We present a study of the recently discovered compact stellar system Triangulum II. From observations conducted with the DEIMOS spectrograph on Keck II, we obtained spectra for 13 member stars that follow the CMD features of this very faint stellar system and include two bright red giant branch stars. Tri II has a very negative radial velocity (=-383.7^{+3.0}_{-3.3} km/s) that translates to ~ -264 km/s and confirms it is a Milky Way satellite. We show that, despite the small data set, there is evidence that Tri II has complex internal kinematics. Its radial velocity dispersion increases from 4.4^{+2.8}_{-2.0} km/s in the central 2' to 14.1^{+5.8}_{-4.2} km/s outwards. The velocity dispersion of the full sample is inferred to be \sigma_{vr}=9.9^{+3.2}_{-2.2} km/s. From the two bright RGB member stars we measure an average metallicity =-2.6+/-0.2, placing Tri II among the most metal-poor Milky Way dwarf galaxies. In addition, the spectra of the fainter member stars exhibit differences in their line widths that could be the indication of a metallicity dispersion in the system. All these properties paint a complex picture for Tri II, whose nature and current state are largely speculative. The inferred metallicity properties of the system however lead us to favor a scenario in which Tri II is a dwarf galaxy that is either disrupting or embedded in a stellar stream.Comment: 8 pages, 6 figures, 2 tables. ApJ, in press. v2: only minor changes to the tex

Lacerta I and Cassiopeia III. Two Luminous and Distant Andromeda Satellite Dwarf Galaxies Found in the 3π Pan-STARRS1 Survey

We report the discovery of two new dwarf galaxies, Lacerta I/Andromeda XXXI (Lac I/And XXXI) and Cassiopeia III/Andromeda XXXII (Cas III/And XXXII), in stacked Pan-STARRS1 r P1- and i P1-band imaging data. Both are luminous systems (MV ~ –12) located at projected distances of 20fdg3 and 10fdg5 from M31. Lac I and Cas III are likely satellites of the Andromeda galaxy with heliocentric distances of $756^{+44}_{-28}{\rm \,kpc}$ and $772^{+61}_{-56}{\rm \,kpc}$, respectively, and corresponding M31-centric distances of 275 ± 7 kpc and $144^{+6}_{-4}{\rm \,kpc}$. The brightest of recent Local Group member discoveries, these two new dwarf galaxies owe their late discovery to their large sizes ($r_h = 4.2^{+0.4}_{-0.5}$ arcmin or $912^{+124}_{-93}{\rm \,pc}$ for Lac I; $r_h = 6.5^{+1.2}_{-1.0}$ arcmin or 1456 ± 267 pc for Cas III) and consequently low surface brightness (μ0 ~ 26.0 mag arcsec–2), as well as to the lack of a systematic survey of regions at large radii from M31, close to the Galactic plane. This latter limitation is now alleviated by the 3π Pan-STARRS1 survey, which could lead to the discovery of other distant Andromeda satellite dwarf galaxies.Astronom

A Rogues’ Gallery of Andromeda's Dwarf Galaxies. I. A Predominance of Red Horizontal Branches

We present homogeneous, sub-horizontal branch photometry of twenty dwarf spheroidal satellite galaxies of M31 observed with the Hubble Space Telescope. Combining our new data for sixteen systems with archival data in the same filters for another four, we show that Andromeda dwarf spheroidal galaxies favor strikingly red horizontal branches or red clumps down to ~10^{4.2} Lsun (M_V ~ -5.8). The age-sensitivity of horizontal branch stars implies that a large fraction of the M31 dwarf galaxies have extended star formation histories (SFHs), and appear inconsistent with early star formation episodes that were rapidly shutdown. Systems fainter than ~10^{5.5} Lsun show the widest range in the ratios and morphologies of red and blue horizontal branches, indicative of both complex SFHs and a diversity in quenching timescales and/or mechanisms, which is qualitatively different from what is currently known for faint Milky Way (MW) satellites of comparable luminosities. Our findings bolster similar conclusions from recent deeper data for a handful of M31 dwarf galaxies. We discuss several sources for diversity of our data such as varying halo masses, patchy reionization, mergers/accretion, and the environmental influence of M31 and the Milky Way on the early evolution of their satellite populations. A detailed comparison between the histories of M31 and MW satellites would shed signifiant insight into the processes that drive the evolution of low-mass galaxies. Such a study will require imaging that reaches the oldest main sequence turnoffs for a significant number of M31 companions.Comment: 11 pages, 5 figures, 2 tables. ApJ in press. v2: small tweaks to the results and discussion sectio

Lacerta i and cassiopeia III. Two luminous and distant andromeda satellite dwarf galaxies found in the 3π pan-starrs1 survey

We report the discovery of two new dwarf galaxies, Lacerta I/Andromeda XXXI (Lac I/And XXXI) and Cassiopeia III/Andromeda XXXII (Cas III/And XXXII), in stacked Pan-STARRS1 r_P1- and i_P1-band imaging data. Both are luminous systems (M_V ~ -12) located at projected distances of 20.3{\deg} and 10.5{\deg} from M31. Lac I and Cas III are likely satellites of the Andromeda galaxy with heliocentric distances of 756^{+44}_{-28} kpc and 772^{+61}_{-56} kpc, respectively, and corresponding M31-centric distances of 275+/-7 kpc and 144^{+6}_{-4} kpc . The brightest of recent Local Group member discoveries, these two new dwarf galaxies owe their late discovery to their large sizes (r_h = 4.2^{+0.4}_{-0.5} arcmin or 912^{+124}_{-93} pc for Lac I; r_h = 6.5^{+1.2}_{-1.0} arcmin or 1456+/-267 pc for Cas III), and consequently low surface brightness (\mu_0 ~ 26.0 mag/arcsec^2), as well as to the lack of a systematic survey of regions at large radii from M31, close to the Galactic plane. This latter limitation is now alleviated by the 3{\pi} Pan-STARRS1 survey, which could lead to the discovery of other distant Andromeda satellite dwarf galaxies.Comment: 7 pages, 5 figures. Accepted for publication in Ap

A la recherche et la caractérisation des galaxies plus sombres dans le groupe local avec le relevé Pan-STARRS 1

This thesis uses the Panoramic Survey Telescope and Rapid Response System 1 Survey to find new Local Group satellites such as dwarf galaxies and globular clusters. This survey is instrumental in helping resolve tensions that have become apparent between observation and theories. In a first phase, a search algorithm is developed, discovering five new satellites. Though yielding five discoveries, this number is lower than one would expect, assuming isotropy of the dwarf galaxies. This leads to the second aim of this thesis, namely quantifying the detection limits of the PS1 Survey. The detection efficiency maps over the entire PS1 sky can be used as a stepping–stone towards the quantification of the (an)isotropy of the Milky Way satellites’ distribution. Using this information, the luminosity function of these satellites can be re–derived.Cette thèse utilise le relevé de donné du Panoramic Survey Telescope and Rapid Response System 1 Survey pour trouver de nouveaux satellites du Groupe Local: les galaxies naines et les amas globulaires. Le relevé est important pour résoudre les tensions entre les observations et les modèles. Premièrement, un algorithme de détection est développé, découvrant cinq nouveaux satellites. Bien que cinq découvertes soient faites, le nombre de découvertes est inférieur à ce qu’on s’attendrait, en présumant une distribution isotrope de galaxies naines. Ce résultat mène au deuxième objectif de la these: quantifier les limites de détections du relevé PS1. Les cartes d’efficacité de détection du ciel complet peuvent être utilisées pour quantifier la distribution (an)isotrope des galaxies satellites de la Voie Lactée. En outre, ces informations peuvent mener a redériver la fonction de luminosité des satellites

Iodide-Mediated Synthesis of Spirooxindolo Dihydrofurans from Iodonium Ylides and 3‑Alkylidene-2-oxindoles

An iodide-mediated reaction between cyclic iodonium ylides of 1,3-dicarbonyls and 3-alkylidene-2-oxindoles results in 3<i>H</i>-spiro­[furan-2,3′-indolin]-2′-ones. The reaction was tolerant to substitutions on both the alkylidene and ylide substrates and provided access to 19 new, densely functionalized polycyclic spirocycles in typically high yield