We present the deep and wide $V$ and $I_c$ photometry of the Sextans dwarf spheroidal galaxy (dSph) taken by Suprime-Cam imager on the Subaru Telescope, which extends out to the tidal radius. The colour-magnitude diagram (CMD) reaches two magnitudes below the main sequence (MS) turn-off, showing a steep red giant branch, blue and red horizontal branch (HB), sub-giant branch (SGB), MS, and blue stragglers (BS). We construct the radial profile of each evolutionary phase and demonstrate that blue HB stars are more spatially extended, while red HB stars are more centrally concentrated than the other components. The colour distribution of SGB stars also varies with the galactocentric distance; the inner SGB stars shift bluer than those in the outskirt. The radial differences in the CMD morphology indicate the existence of the age gradient. The relatively younger stars ($\sim10$ Gyr) are more centrally concentrated than the older ones ($\sim13$ Gyr). The spatial contour maps of stars in different age bins also show that the younger population has higher concentration and higher ellipticity than the older one. We also detect the centrally concentrated bright BS stars, the number of which is consistent with the idea that a part of these stars belongs to the remnant of a disrupted star cluster discovered in the previous spectroscopic studies.
We have measured the absolute proper motion of the Sextans dwarf spheroidal galaxy using Subaru Suprime-Cam images taken at three epochs, with a time baseline of ~ 10 years. We astrometrically calibrate each epoch by constructing distortion-correction maps from the best available Subaru Suprime-Cam dithered data sets and from Gaia DR1 positions. The magnitude limit of the proper-motion study is V ~ 24. The area covered is 26.7 x 23.3 arcmin, which is still within the core radius of Sextans. The derived proper motion is (mu_a, mu_d) = (-0.409 +/- 0.050, -0.047 +/- 0.058) mas/yr. The direction o…
We report the discovery of clear observational signs of past accretion/merger events in one of the Milky Way satellite galaxies, the Sextans dwarf spheroidal (dSph). These were uncovered in the spatial distribution, internal kinematics and metallicity properties of Sextans stars using literature CTIO/DECam photometric and Magellan/MMFS spectroscopic catalogues. We find the spatial distribution of stars to vary as a function of the colour/metallicity, being rather regular and round for the blue (metal-poor) red giant branch and main-sequence turn-off stars but much more elliptical and irregular…
We report the discovery of two Mira variable stars (Miras) toward the Sextans dwarf spheroidal (dSph) galaxy. We performed optical long-term monitoring observations for two red stars in the Sextans dSph. The light curves of both stars in the $I_{\rm c}$ band show large-amplitude (3.7 and 0.9 mag) and long-period ($326\pm 15$ and $122\pm 5$ days) variations, suggesting that they are Miras. We combine our own infrared data with previously published data to estimate the mean infrared magnitudes. The distances obtained from the period-luminosity relation of the Miras ($75.3^{+12.8}_{-10.9}$ and $7…
We model the large kinematic data sets for the four Milky Way dwarf spheroidal (dSph) satellites: Carina, Fornax, Sculptor and Sextans, recently published by Walker et al. The member stars are selected using a reliable dynamical interloper removal scheme tested on simulated data. Our member selection is more restrictive than the one based on metallicity indicators as it removes not only contamination due to Milky Way stars but also the unbound stars from the tidal tails. We model the cleaned data sets by adjusting the solutions of the Jeans equations to the profiles of the projected velocity d…
Spatially resolved star formation history of Sextans dSph
Instituto Interdisciplinario de Ciencias Básicas (ICB), CONICET-UNCuyo（基础科学跨研究所 (ICB)，CONICET-UNCuyo）；Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)（国家科学研究委员会 (CONICET)）
We introduce a likelihood analysis of multi-epoch stellar line-of-sight velocities to constrain the binary fractions and binary period distributions of dwarf spheroidal galaxies. This method is applied to multi-epoch data from the Magellan/MMFS survey of the Carina, Fornax, Sculptor and Sextans dSph galaxies, after applying a model for the measurement errors that accounts for binary orbital motion. We find that the Fornax, Sculptor, and Sextans dSphs are consistent with having binary populations similar to that of Milky Way field binaries to within 68% confidence limits, whereas the Carina dSp…
We present the analysis of the FLAMES dataset targeting the central 25 arcmin region of the Sextans dSph. This dataset is the third major part of the high resolution spectroscopic section of the ESO large program 171.B-0588(A) obtained by the Dwarf galaxy Abundances and Radial-velocities Team (DART). Our sample is composed of red giant branch stars down to the level of the horizontal branch in Sextans. It allows to address questions related to both stellar nucleosynthesis and galaxy evolution. We provide metallicities for 81 stars, which cover the wide [Fe/H]=$-$3.2 to $-$1.5 dex range. The ab…
Dwarf spheroidal galaxies (dSph) orbiting the Milky Way are complex objects often with complicated star formation histories and internal dynamics. In this work, we search for stellar substructures in four of the classical dSph satellites of the Milky Way: Sextans, Carina, Leo I, and Leo II. We apply two methods to search for stellar substructure: the minimum spanning tree method, which helps us to find and quantify spatially connected structures, and the "brute-force" method, which is able to find elongated stellar substructures. We detected the previously known substructure in Sextans, and al…
We use Gaia EDR3 data to identify stars associated with six classical dwarf spheroidals (Draco, Ursa Minor, Sextans, Sculptor, Fornax, Carina) at their outermost radii, beyond their nominal King stellar limiting radius. For all of the dSphs examined, we find radial velocity matches with stars residing beyond the King limiting radius and with $> 50\%$ astrometric probability (four in Draco, two in Ursa Minor, eight in Sextans, two in Sculptor, twelve in Fornax, and five in Carina), indicating that these stars are associated with their respective dwarf spheroidals (dSphs) at high probability. We…
We use VLT/FLAMES intermediate resolution (R~6500) spectra of individual red giant branch stars in the near-infrared CaII triplet (CaT) region to investigate the wide-area metallicity properties and internal kinematics of the Sextans dwarf spheroidal galaxy (dSph). Our final sample consists of 174 probable members of Sextans with accurate line-of-sight velocities (+- 2 km/s) and CaT [Fe/H] measurements (+- 0.2 dex). We use the MgI line at 8806.8 Å\, as an empirical discriminator for distinguishing between probable members of the dSph (giant stars) and probable Galactic contaminants (dwarf star…
We present the star formation history (SFH) of the Sextans dwarf spheroidal galaxy based on deep archive $B$,$I$ photometry taken with Suprime-Cam at Subaru telescope focusing our analysis on the inner region of the galaxy, fully located within the core radius. Within the errors of our SFH we have not detected any metallicity gradient along the considered radial distance interval. As a main result of this work we can state that the Sextans dwarf spheroidal stopped forming stars less than $\sim1.3$ Gyr after Big Bang in correspondance to the end of the reionization epoch. We have been able to c…
Recovering the infall mass for Milky Way satellite galaxy Sextans
Institute of Astronomy, School of Physics, Zhejiang University（浙江大学物理学院天体物理研究所）；Purple Mountain Observatory（紫金山天文台）；Center for Cosmology and Computational Astrophysics, Zhejiang University（浙江大学宇宙学与计算天体物理中心）
We present a new chemical evolution model for dwarf spheroidal galaxies (dSphs) in the Local Universe. Our main aim is to explain both their observed star formation histories and metallicity distribution functions simultaneously. Applying our new model for the four local dSphs, that is, Fornax, Sculptor, Leo II, and Sextans, we find that our new model reproduces the observed chemical properties of the dSphs consistently. Our results show that the dSphs have evolved with both a low star formation efficiency and a large gas outflow efficiency compared with the Milky Way, as suggested by previous…
We present a homogeneous set of accurate atmospheric parameters for a sample of eleven very metal-poor (-3.32 <= [Fe/H] <= -2.61) stars in the Sextans dwarf spheroidal galaxy (dSph) and the non-local thermodynamic equilibrium (NLTE) abundances of, at least, seven chemical elements based on high-resolution UVES/VLT and HDS/Subaru spectra. For each star, its effective temperature and surface gravity were derived from the spectral energy distribution and the known distance, and the Fe abundance was obtained as the average from lines of Fe I and Fe II. Based on abundances of Mg, Ca, and Ti, we sho…
We present results from deep and very spatially extended CTIO/DECam $g$ and $r$ photometry (reaching out to $\sim$ 2 mag below the oldest MSTO and covering $\sim$ 20 deg$^2$) around the Sextans dSph. We use this data-set to study the structural properties of Sextans overall stellar population and its different stellar evolutionary phases, as well as to search for signs of tidal disturbance from the MW, which would indicate departure from dynamical equilibrium. We perform the most accurate structural analysis to-date of Sextans' stellar components by applying Bayesian MCMC methods to the indivi…
A few dozen dwarf satellite galaxies of the Milky Way have been discovered, which are often viewed as the remaining building blocks of our Galaxy. The follow-up spectroscopy showed that dwarf galaxies have a sizeable spread in their metallicities. Several scenarios were suggested to explain the metallicity spread, which can be tested by the structural patterns of stellar subpopulations with distinct metallicities. However, such chemical plus structural examination, to which we refer to as "chemostructural study" is hindered by the lack of stars with spectroscopic metallicity. Here we propose t…
We take deep images of four ultra faint dwarf (UFD) galaxies, Canes Venatici I (CVn I), Bootes I (Boo I), Canes Venatici II (CVn II), and Leo IV using the Suprime-Cam on the Subaru Telescope. The colour-magitude diagrams (CMDs) extend below the main sequence turn-offs (MSTOs) and yield measurements of the ages of stellar populations. The stellar populations of faint three galaxies, Boo I, CVn II and Leo IV dwarf spheroidal galaxies (dSphs) are estimated as old as a Galactic globular cluster M92. We confirm that Boo I dSph has no intrinsic colour spread in the MSTO, and no spatial difference in…
We present a large homogeneous set of stellar parameters and abundances across a broad range of metallicities, involving $13$ classical dwarf spheroidal (dSph) and ultra-faint dSph (UFD) galaxies. In total this study includes $380$ stars in Fornax, Sagittarius, Sculptor, Sextans, Carina, Ursa Minor, Draco, Reticulum II, Bootes I, Ursa Major II, Leo I, Segue I, and Triangulum II. This sample represents the largest, homogeneous, high-resolution study of dSph galaxies to date. With our homogeneously derived catalog, we are able to search for similar and deviating trends across different galaxies.…
We present an analysis of the kinematics of 14 satellites of the Milky Way (MW). We use proper motions (PMs) from the $Gaia$ Early Data Release 3 (EDR3) and line-of-sight velocities ($v_{\mathrm{los}}$) available in the literature to derive the systemic 3D motion of these systems. For six of them, namely the Carina, Draco, Fornax, Sculptor, Sextans, and Ursa Minor dwarf spheroidal galaxies (dSph), we study the internal kinematics projecting the stellar PMs into radial, $V_R$ (expansion/contraction), and tangential, $V_T$ (rotation), velocity components with respect to the centre of mass. We fi…
Measuring the frequency of binary stars in dwarf spheroidal galaxies (dSphs) requires data taken over long time intervals. We combine radial velocity measurements from five literature sources taken over the course of ~30 years to yield the largest multi-epoch kinematic sample for stars in the dSphs Draco and Ursa Minor. With this data set, we are able to implement an improved version of the Bayesian technique described in Spencer et al. (2017b) to evaluate the binary fraction of red giant stars in these dwarf galaxies. Assuming Duquennoy & Mayor (1991) period and mass ratio distributions, the …
We report the discovery of NGC 253-dw2, a dwarf spheroidal (dSph) galaxy candidate undergoing tidal disruption around a nearby spiral galaxy, NGC 253 in the Sculptor group: the first such event identified beyond the Local Group. The dwarf was found using small-aperture amateur telescopes, and followed up with Suprime-Cam on the 8 m Subaru Telescope in order to resolve its brightest stars. Using g- and R_c-band photometry, we detect a red giant branch consistent with an old, metal-poor stellar population at a distance of ~ 3.5 Mpc. From the distribution of likely member stars, we infer a highly…
We present a large sample of fully self-consistent hydrodynamical Nbody/Tree-SPH simulations of isolated dwarf spheroidal galaxies (dSphs). It has enabled us to identify the key physical parameters and mechanisms at the origin of the observed variety in the Local Group dSph properties. The initial total mass (gas + dark matter) of these galaxies is the main driver of their evolution. Star formation (SF) occurs in series of short bursts. In massive systems, the very short intervals between the SF peaks mimic a continuous star formation rate, while less massive systems exhibit well separated SF …
We present a kinematic study of six dwarf spheroidal galaxies (dSph) satellites of the Milky Way (MW), namely Carina, Draco, Fornax, Sculptor, Sextans, and Ursa Minor. We combine proper motions (PMs) from the $Gaia$ Data Release 3 (DR3) and line-of-sight velocities ($v_{\mathrm{los}}$) from the literature to derive their 3D internal kinematics and to study the presence of internal velocity gradients. We find velocity gradients along the line-of-sight for Carina, Draco, Fornax, and Ursa Minor, at $\geq 1σ$ level of significance . The value of such gradients appears to be related to the orbital …
We present the non-local thermodynamic equilibrium (NLTE) abundances of up to 10 chemical species in a sample of 59 very metal-poor (VMP, -4 < [Fe/H] < -2) stars in seven dwarf spheroidal galaxies (dSphs) and in the Milky Way (MW) halo. Our results are based on high-resolution spectroscopic datasets and homogeneous and accurate atmospheric parameters determined in PaperI. We show that once the NLTE effects are properly taken into account, all massive galaxies in our sample, that is, the MW halo and the classical dSphs Sculptor, Ursa Minor, Sextans, and Fornax, reveal a similar plateau at [alph…
The spatial segregation between dSphs and dIrrs in the Local Group has long been regarded as evidence of an interaction with their host galaxies. In this paper, we assume that ram-pressure stripping is the dominant mechanism that removed gas from the dSphs and we use this to derive a lower bound on the density of the corona of the Milky Way at large distances (50-90 kpc) from the Galactic centre. At the same time, we derive an upper bound by demanding that the interstellar medium of the dSphs is in pressure equilibrium with the hot corona. We consider two dwarfs (Sextans and Carina) with well-…
The Apache Point Observatory extra-Galactic Evolution Experiment (APOeGEE): Chemical Abundance Trends for Seven Dwarf Spheroidal Galaxies in the APOGEE Survey
Leo P is a low-luminosity dwarf galaxy discovered through the blind HI Arecibo Legacy Fast ALFA (ALFALFA) survey. The HI and follow-up optical observations have shown that Leo P is a gas-rich dwarf galaxy with active star formation, an underlying older population, and an extremely low oxygen abundance. We have obtained optical imaging with the Hubble Space Telescope to two magnitudes below the red clump in order to study the evolution of Leo P. We refine the distance measurement to Leo P to be 1.62+/-0.15 Mpc, based on the luminosity of the horizontal branch stars and 10 newly identified RR Ly…
We provide manganese abundances (corrected for the effect of the hyperfine structure) for a large number of stars in the dwarf spheroidal galaxies Sculptor and Fornax, and for a smaller number in the Carina and Sextans dSph galaxies. Abundances had already been determined for a number of other elements in these galaxies, including alpha and iron-peak ones, which allowed us to build [Mn/Fe] and [Mn/alpha] versus [Fe/H] diagrams. The Mn abundances imply sub-solar [Mn/Fe] ratios for the stars in all four galaxies examined. In Sculptor, [Mn/Fe] stays roughly constant between [Fe/H]\sim -1.8 and -1…
In order to minimize environmental effects and gain an insight into the internal mechanisms that shape the properties of the early-type dwarf systems, we study one of the few isolated dwarf spheroidal galaxies (dSphs) of the Local Group (LG): Cetus. We obtained VLT/FORS2 spectra ($R\sim2600$) in the region of the nIR CaII triplet lines for 80 candidate red giant branch stars. The analysis yielded line-of-sight velocities and metallicities ([Fe/H]) for 54 bona fide member stars. The kinematic analysis shows that Cetus is a mainly pressure-supported ($σ_v = 11.0_{-1.3}^{+1.6}$ km/s), dark-matter…
We present a homogeneous set of accurate atmospheric parameters for a complete sample of very and extremely metal-poor stars in the dwarf spheroidal galaxies (dSphs) Sculptor, Ursa Minor, Sextans, Fornax, Boötes I, Ursa Major II, and Leo IV. We also deliver a Milky Way (MW) comparison sample of giant stars covering the -4 < [Fe/H] < -1.7 metallicity range. We show that, in the [Fe/H] > -3.5 regime, the non-local thermodynamic equilibrium (NLTE) calculations with non-spectroscopic effective temperature (Teff) and surface gravity (log~g) based on the photometric methods and known distance provid…
Dwarf galaxies provide insights on the processes of star formation and chemical enrichment at the low end of the galaxy mass function, as well as on the clustering of dark matter on small scales. In studies of Local Group dwarf galaxies, spectroscopic samples of individual stars are used to derive the internal kinematics and abundance properties of these galaxies. It is therefore important to clean these samples from Milky Way stars, not related to the dwarf galaxy, since they can contaminate the analysis of the properties of these objects. Here we introduce a new diagnostic for separating Mil…
