The effect of the local environment on the evolution of dwarf spheroidal galaxies is poorly understood. We have undertaken a suite of simulations to investigate the tidal impact of the Milky Way on the chemodynamical evolution of dwarf spheroidals that resemble present day classical dwarfs using the SPH code GEAR. After simulating the models through a large parameter space of potential orbits the resulting properties are compared with observations from both a dynamical point of view, but also from the, often neglected, chemical point of view. In general, we find that tidal effects quench the star formation even inside gas-endowed dwarfs. Such quenching, may produce the radial distribution of dwarf spheroidals from the orbits seen within large cosmological simulations. We also find that the metallicity gradient within a dwarf is gradually erased through tidal interactions as stellar orbits move to higher radii. The model dwarfs also shift to higher $\langle$[Fe/H]$\rangle$/L ratios, but only when losing $>$$20\%$ of stellar mass.
Estimates of the mass distribution and dark-matter (DM) content of dwarf spheroidal galaxies (dSphs) are usually derived under the assumption that the effect of the tidal field of the host galaxy is negligible over the radial extent probed by kinematic data-sets. We assess the implications of this assumption in the specific case of the Fornax dSph by means of N-body simulations of a satellite orbiting around the Milky Way. We consider observationally-motivated orbits and we tailor the initial distributions of the satellite's stars and DM to match, at the end of the simulations, the observed st…
Dwarf spheroidal galaxies (dSphs) appear to be some of the most dark matter dominated objects in the Universe. Their dynamical masses are commonly derived using the kinematics of stars under the assumption of equilibrium. However, these objects are satellites of massive galaxies (e.g.\ the Milky Way) and thus can be influenced by their tidal fields. We investigate the implication of the assumption of equilibrium focusing on the Sculptor dSph by means of ad-hoc $N$-body simulations tuned to reproduce the observed properties of Sculptor following the evolution along some observationally motivate…
The unusually low velocity dispersion and large size of Crater II pose a challenge to our understanding of dwarf galaxies in the Lambda Cold Dark Matter (LCDM) cosmogony. The low velocity dispersion suggests either a dark halo mass much lower than the minimum expected from hydrogen cooling limit arguments, or one that is in the late stages of extreme tidal stripping. The tidal interpretation has been favoured in recent work and is supported by the small pericentric distances consistent with available kinematic estimates. We use N-body simulations to examine this interpretation in detail, assum…
Dwarf spheroidal galaxies are regarded as the basic building blocks in the formation of larger galaxies and are believed to be the most dark matter dominated systems known in the Universe. There are several models that attempt to explain their formation and evolution, but they have problems to model the formation of isolated dwarf spheroidal galaxies. Here, we will explain a possible formation scenario in which star clusters form inside the dark matter halo of a dwarf spheroidal galaxy. Those star clusters suffer from low star formation efficiency and dissolve while orbiting inside the dark ma…
The mass-metallicity relation shows that the galaxies with the lowest mass have the lowest metallicities. As most dwarf galaxies are in group environments, interaction effects such as tides could contribute to this trend. We perform a series of smoothed particle hydrodynamics (SPH) simulations of dwarf galaxies in external tidal fields to examine the effects of tides on their metallicities and metallicity gradients. In our simulated galaxies, gravitational instabilities drive gas inwards and produce centralized star formation and a significant metallicity gradient. Strong tides can contribute …
I review the current status of dynamical modelling of dwarf spheroidal galaxies focusing on estimates of their dark matter content. Starting with the simplest methods using the velocity dispersion profiles I discuss the inherent issues of mass-anisotropy degeneracy and contamination by unbound stars. I then move on to methods of increasing complexity, aiming to break the degeneracy, up to recent applications of the Schwarzschild orbit superposition method. The dynamical modelling is placed in the context of possible scenarios for the formation of dwarf spheroidal galaxies, including the tidal …
We present deep stellar photometry of the Boötes I dwarf spheroidal galaxy in g and i band filters, taken with the Dark Energy Camera at Cerro Tololo in Chile. Our analysis reveals a large, extended region of stellar substructure surrounding the dwarf, as well as a distinct over-density encroaching on its tidal radius. A radial profile of the Boötes I stellar distribution shows a break radius indicating the presence of extra-tidal stars. These observations strongly suggest that Boötes I is experiencing tidal disruption, although not as extreme as that exhibited by the Hercules dwarf spheroidal…
We use cosmological N-body simulations from the Aquarius Project to study the tidal effects of a dark matter halo on the shape and orientation of its substructure. Although tides are often assumed to enhance asphericity and to stretch subhaloes tangentially, these effects are short lived: as in earlier work, we find that subhaloes affected by tides become substantially more spherical and show a strong radial alignment toward the centre of the host halo. These results, combined with a semi-analytic model of galaxy formation, may be used to assess the effect of Galactic tides on the observed pop…
We present a series of chemodynamical simulations of Magellanic-like systems consisting of two interacting, equal-mass dwarf galaxies orbiting a massive host galaxy, including feedback and star formation, tides, and ram pressure. We study the star formation and chemical enrichment history of the dwarfs, and the production of a Magellanic Stream analogue. The dwarfs interact with each other through tidal forces, distorting their morphologies and triggering star formation. A stream is naturally produced as outflows, induced by feedback and interactions, are stretched by tidal forces. Counter to …
We present the results of the analysis of three XMM-Newton observations of the Willman 1 dwarf spheroidal galaxy (Wil 1). X-ray sources are classified on the basis of spectral analysis, hardness ratios, X-ray-to-optical flux ratio, X-ray variability, plus cross-correlation with available catalogues in optical and infrared wavelengths. We catalogued 97 sources in the field of Wil 1. Our classification shows the presence of a $β$-type symbiotic star in Wil 1. We classified one M dwarf foreground star in the field of Wil 1. Moreover, fifty-four sources are classified as background AGNs and galaxi…
We use a cosmological simulation of the formation of the Local Group to explore the origin of age and metallicity gradients in dwarf spheroidal galaxies. We find that a number of simulated dwarfs form "outside-in", with an old, metal-poor population that surrounds a younger, more concentrated metal-rich component, reminiscent of dwarf spheroidals like Sculptor or Sextans. We focus on a few examples where stars form in two populations distinct in age in order to elucidate the origin of these gradients. The spatial distributions of the two components reflect their diverse origin; the old stellar…
Precise measurements of mass in dark matter dominated dwarf spheroidal galaxies are of great importance for testing the theories of structure formation. We use $N$-body simulations of the tidal evolution of a dwarf galaxy orbiting the Milky Way to generate mock kinematical data sets and use them to test the reliability of a simple mass estimator proposed by Wolf et al. The evolution of the initially disky dwarf galaxy embedded in a dark matter halo was traced for 10 Gyr on a rather tight orbit. After about half of the time a dwarf spheroidal galaxy is formed that retains some remnant rotation …
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…
Chemical evolution of Local Group dwarf galaxies in a cosmological context -- I. A new modelling approach and its application to the Sculptor dwarf spheroidal galaxy
Low mass dwarf spheroidal galaxies are key objects for our understanding of the chemical evolution of the pristine Universe and the Local Group of galaxies. Abundance ratios in stars of these objects can be used to better understand their star formation and chemical evolution. We report on the analysis of a sample of 11 stars belonging to 5 different ultra faint dwarf spheroidal galaxies (UfDSph) based on X-Shooter spectra obtained at the VLT. Medium resolution spectra have been used to determine the detailed chemical composition of their atmosphere. We performed a standard 1D LTE analysis to …
Dwarf spheroidal galaxies (dSphs) are mostly investigated in the Local Group. DSphs are difficult targets for observations because of their small size and very low surface brightness. Here we measure spectroscopic and photometric parameters of three candidates for isolated dSphs, KKH65=BTS23, KK180, and KK227, outside the Local Group. The galaxies are found to be of low metallicity and low velocity dispersion. They are among the lowest surface brightness objects in the Local Universe. According to the measured radial velocities, metallicities, and structural and photometric parameters, KKH65 a…
We present the metallicities and carbon abundances of four newly discovered metal-poor stars with $ -2.2 <$ [Fe/H] $< -1.6$ in the Sagittarius dwarf spheroidal galaxy. These stars were selected as metal-poor member candidates using a combination of public photometry from the SkyMapper Southern Sky Survey and proper motion data from the second data release from the Gaia mission. The SkyMapper filters include a metallicity-sensitive narrow-band $v$ filter centered on the Ca II K line, which we use to identify metal-poor candidates. In tandem, we use proper motion data to remove metal-poor stars …
We use images from the Hubble Space Telescope to determine the distances to a dozen objects with low surface brightness recently observed around the bright nearby spiral M101. Only two dwarf galaxies, M101-DwA and M101-Dw9, turn out to be actual satellites of M101 at distances of about 7 Mpc. The other objects are probably members of a distant group around the S0-galaxies NGC 5485/5473. Based on the radial velocities and projected separations of the 9 satellites, we obtain an estimate of $(8.5\pm3.0)\times 10^{11} M_{\odot}$ for the total mass of M101, which is consistent with a ratio $16\pm6$…
We report on the discovery and chemical abundance analysis of the first CEMP-r/s star detected in the Sagittarius dwarf Spheroidal Galaxy, by means of UVES high resolution spectra. The star, found in the outskirts of Sgr dSph, along the main body major axis, is a moderately metal poor giant (T$_{eff}$=4753 K, log g=1.75, [Fe/H]=-1.55), with [C/Fe]=1.13 placing it in the so-called "high-carbon band", and strong s-process and r-process enrichment ([Ba/Fe]=1.4, [Eu/Fe]=1.01). Abundances of 29 elements from C to Dy were obtained. The chemical pattern appears to be best fitted by a scenario where a…
We constrain the parameters of a self-interacting massive dark matter scalar particle in a condensate using the kinematics of the eight brightest dwarf spheroidal satellites of the Milky Way. For the case of a repulsive self-interaction the condensate develops a mass density profile with a characteristic scale radius that is closely related to the fundamental parameters of the theory. We find that the velocity dispersion of dwarf spheroidal galaxies suggests a scale radius of the order of 1 kpc, in tension with previous results found using the rotational curve of low-surface-brightness and dwa…
We study the density structures of dark matter subhalos for both cold dark matter and self-interacting dark matter models using high-resolution cosmological $N$-body simulations. We quantify subhalo's central density at 150 pc from the center of each subhalo at the classical dwarf spheroidal and ultrafaint dwarf scales found in Milky-Way sized halos. By comparing them with observations, we find that the self-interacting scattering cross-section of $σ/m<3\ \rm{cm^{2}g^{-1}}$ is favored. Due to the combination of hosts' tide and self-interactions, the central density of subhalos with small peric…
We apply a flexible parametric model, a combination of generalized Plummer profiles, to infer the shapes of the stellar density profiles of the Milky Way's satellite dwarf spheroidal galaxies (dSphs). We apply this model to 40 dSphs using star counts from the Sloan Digital Sky Survey, PanStarrs-1 Survey, Dark Energy Survey, and Dark Energy Camera Legacy Survey. Using mock data, we examine systematic errors associated with modelling assumptions and identify conditions under which our model can identify `non-standard' stellar density profiles that have central cusps and/or steepened outer slopes…
We study the impact of the dark matter velocity distribution modelling on signals from velocity-dependent dark matter annihilation in Milky Way dwarf spheroidal galaxies. Using the high resolution APOSTLE simulations, we identify analogues corresponding to Milky Way dwarf spheroidal galaxies, and from these directly determine the dark matter pair-wise relative velocity distribution, and compare to best-fitting Maxwell-Boltzmann distribution models. For three velocity-dependent annihilation models, p-wave, d-wave, and the Sommerfeld model, we quantify the errors introduced when using the Maxwel…
We present the spectral analysis of an 87~ks \emph{XMM-Newton} observation of Draco, a nearby dwarf spheroidal galaxy. Of the approximately 35 robust X-ray source detections, we focus our attention on the brightest of these sources, for which we report X-ray and multiwavelength parameters. While most of the sources exhibit properties consistent with AGN, few of them possess characteristics of LMXBs and CVs. Our analysis puts constraints on population of X-ray sources with $L_X>3\times10^{33}$~erg~s$^{-1}$ in Draco suggesting that there are no actively accreting BH and NS binaries. However, we …
Recent surveys have discovered a population of faint supernovae, known as Ca-rich gap transients, inferred to originate from explosive ignitions of white dwarfs. In addition to their unique spectra and luminosities, these supernovae have an unusual spatial distribution and are predominantly found at large distances from their presumed host galaxies. We show that the locations of Ca-rich gap transients are well matched to the distribution of dwarf spheroidal galaxies surrounding large galaxies, in accordance with a scenario where dark matter interactions induce thermonuclear explosions among lo…
Dwarf spheroidal galaxies provide well-known challenges to the standard cold and collisionless dark matter scenario: The too-big-to-fail problem, namely the mismatch between the observed mass enclosed within the half-light radius of dwarf spheroidals and cold dark matter N-body predictions; The hints for inner constant-density cores. While these controversies may be alleviated by baryonic physics and environmental effects, revisiting the standard lore of cold and collisionless dark matter remains an intriguing possibility. Self-interacting dark matter may be the successful proposal to such a s…
Dwarf spheroidal galaxies are among the best environments that can be studied with Cherenkov telescopes for indirect searches of $γ$-ray signals coming from dark matter self-interaction (annihilation or decay), due to their proximity and negligible background emission. We present new determinations of the dark-matter amount - i.e. the astrophysical factors $J$ and $D$ - in dwarf-galaxy halos obtained through the MCMC Jeans analysis of their brightness and kinematic data. Such factors are of great importance to test the performances of the next-generation $γ$-ray instruments such as the Cherenk…
We report a new star formation history for the Tucana dwarf spheroidal galaxy, obtained from a new look at a deep HST/ACS colour-magnitude diagram. We combined information from the main sequence turn-off and the horizontal branch to resolve the ancient star formation rates on a finer temporal scale than previously possible. We show that Tucana experienced three major phases of star formation, two very close together at ancient times and the last one ending between 6 and 8 Gyr ago. We show that the three discrete clumps of stars on the horizontal branch are linked to the distinct episodes of st…
We present Iron, Magnesium, Calcium, and Titanium abundances for 235 stars in the central region of the Sagittarius dwarf spheroidal galaxy (within 9.0 arcmin ~70 pc from the center) from medium-resolution Keck/DEIMOS spectra. All the considered stars belong to the massive globular cluster M54 or to the central nucleus of the galaxy (Sgr,N). In particular we provide abundances for 109 stars with [Fe/H] > -1.0, more than doubling the available sample of spectroscopic metallicity and alpha-elements abundance estimates for Sgr dSph stars in this metallicity regime. Also, we find the first confirm…
Influence of tides and self-gravity on Ultra-Light Dark Matter Bounds from Dwarf Galaxies
CERN（欧洲核子研究组织）；Dipartimento di Fisica, “Sapienza” Università di Roma & Sezione INFN Roma1（罗马第一大学物理系与INFN罗马一分部）；Weizmann Institute of Science（魏茨曼科学研究所）；Instituto de Astrofísica de Canarias（加那利天体物理研究所）；Universidad de La Laguna, Departamento de Astrofísica（拉各纳大学天体物理学系）
Fermi-LAT observations have strongly constrained dark matter annihilation through the joint-likelihood analysis of dwarf spheroidal galaxies (dSphs). These constraints are expected to be robust because dSphs have measurable dark matter content and produce negligible astrophysical emission. However, each dSph is dim, with a predicted flux that typically falls below the accuracy of the background model. We show that this significantly diminishes the reliability of previous joint-likelihood algorithms, and develop an improved analysis that directly accounts for the effect of background mismodelin…
We have found that the high velocity dispersions of dwarf spheroidal galaxies (dSphs) can be well explained by Milky Way (MW) tidal shocks, which reproduce precisely the gravitational acceleration previously attributed to dark matter (DM). Here we summarize the main results of Hammer et al. (2019) who studied the main scaling relations of dSphs and show how dark-matter free galaxies in departure from equilibrium reproduce them well, while they appear to be challenging for the DM model. These results are consistent with our most recent knowledge about dSph past histories, including their orbits…
Dwarf spheroidal galaxies (dSphs) are promising targets of indirect detection experiments searching for dark matter (DM) at present universe. Toward robust prediction for the amount of signal flux originating in DM annihilation inside dSphs, a precise determination of DM distributions as well as $J$-factors of the dSphs is particularly important. In this work, we estimate those of Draco, Sculptor, and Ursa Minor dSphs by an improved statistical method in which both foreground stars and dSph member stars are simultaneously taken into account. We define the likelihood function of the method as t…
