Sgr Dwarf Research Articles

THE NUCLEUS OF THE SAGITTARIUS DSPH GALAXY AND M54: A WINDOW ON THE PROCESS OF GALAXY NUCLEATION

We present the results of a thorough study of the nucleus of the Sgr dwarf spheroidal galaxy (Sgr dSph) and of the bright globular cluster M54 (NGC 6715) that resides within the same nucleus (Sgr,N). We have obtained accurate radial velocities and metallicity estimates for 1152 candidate red giant branch stars of Sgr and M54 lying within ∼9' from the center of the galaxy, from Keck/DEIMOS and VLT/FLAMES spectra of the infrared Ca ii triplet. Using both velocity and metallicity information we selected two samples of 425 and 321 very likely members of M54 and of Sgr,N, respectively. The two considered systems display significantly different velocity dispersion profiles. M54 has a steeply decreasing profile from r = 0', where σ ≃ 14.2 km s−1, to r ≃ 35 where it reaches σ ≃ 5.3 km s−1, then it appears to rise again to σ ≃ 10 km s−1 at r ∼ 7'. In contrast Sgr,N has a uniformly flat profile at σ ≃ 9.6 km s−1 over the whole 0' ⩽ r ⩽ 9' range. Using data from the literature we show that the velocity dispersion of Sgr remains constant at least out to r ∼ 100' and there is no sign of the transition between the outer flat-luminosity-profile core and the inner nucleus in the velocity profile. These results, together with a re-analysis of the surface brightness profile of Sgr,N and a suite of dedicated N-body simulations, provide very strong support for the hypothesis that the nucleus of Sgr formed independently of M54, which probably plunged to its present position, coincident with Sgr,N, because of significant decay of the original orbit due to dynamical friction.

Is the sky falling? Searching for stellar streams in the local Milky Way disc in the CORAVEL and RAVE surveys

We have searched for in-falling stellar streams on to the local Milky Way disc in the CORrelation RAdial VELocities (CORAVEL) and RAdial Velocity Experiment (RAVE) surveys. The CORAVEL survey consists of local dwarf stars (Nördstrom et al. Geneva–Copenhagen survey) and local Famaey et al. giant stars. We select RAVE stars with radial velocities that are sensitive to the Galactic vertical space velocity (Galactic latitude b < −45°). Kuiper statistics have been employed to test the symmetry of the Galactic vertical velocity distribution functions in these samples for evidence of a net vertical flow that could be associated with a (tidal?) stream of stars with vertically coherent kinematics. In contrast to the ‘Field of Streams’ found in the outer halo, we find that the local volumes of the solar neighbourhood sampled by the CORAVEL dwarfs (complete within ∼3 × 10−4kpc3), CORAVEL giants (complete within ∼5 × 10−2kpc3) and RAVE (5–15 per cent complete within ∼8 kpc3) are devoid of any vertically coherent streams containing hundreds of stars. This is sufficiently sensitive to allow our RAVE sample to rule out the passing of the tidal stream of the disrupting Sagittarius (Sgr) dwarf galaxy through the solar neighbourhood. This agrees with the most-recent determinations of its orbit and dissociates it from the Helmi et al. halo stream. Our constraints on the absence of the Sgr stream near the Sun could prove a useful tool for discriminating between Galactic potential models. The lack of a net vertical flow through the solar neighbourhood in the CORAVEL giants and RAVE samples argues against the Virgo overdensity crossing the disc near the Sun. There are no vertical streams in the CORAVEL giants and RAVE samples with stellar densities ≳1.6 × 104 and 1.5 × 103 stars kpc−3, respectively, and therefore no evidence for locally enhanced dark matter.

A 2MASS All‐Sky View of the Sagittarius Dwarf Galaxy. V. Variation of the Metallicity Distribution Function along the Sagittarius Stream

We present reliable measurements of the metallicity distribution function (MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy, based on high-resolution, echelle spectroscopy of candidate M giant members of the Sgr system. The Sgr MDF is found to evolve significantly from a median [Fe/H] ~ -0.4 in the core to ~-1.1 dex over a Sgr leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e., tidal stripping) age. This is direct evidence that there can be significant chemical differences between current dSph satellites and the bulk of the stars they have contributed to the halo. Our results suggest that Sgr experienced a significant change in binding energy over the past several gigayears, which has substantially decreased its tidal boundary across a radial range over which there must have been a significant metallicity gradient in the progenitor galaxy. By accounting for MDF variation along the debris arms, we approximate the MDF Sgr would have had several gigayears ago. We also analyze the MDF of a moving group of M giants we previously discovered toward the north Galactic cap. These objects have the opposite radial velocities to the infalling Sgr leading arm stars there, and we propose that most of them represent Sgr trailing arm stars overlapping the Sgr leading arm in this part of the sky. If so, these trailing arm stars further demonstrate the strong MDF evolution within the Sgr stream.

High-resolution spectroscopy of RGB stars in the Sagittarius streams

Context. The Sagittarius (Sgr) dwarf spheroidal galaxy is currently being disrupted under the strain of the Milky Way. A reliable reconstruction of Sgr star formation history can only be obtained by combining core and stream information. Aims. We present radial velocities for 67 stars belonging to the Sgr Stream. For 12 stars in the sample we also present iron (Fe) and $\alpha$-element (Mg, Ca) abundances. Methods. Spectra were secured using different high resolution facilities: UVES@VLT, HARPS@3.6m, and SARG@TNG. Radial velocities are obtained through cross correlation with a template spectra. Concerning chemical analysis, for the various elements, selected line equivalent widths were measured and abundances computed using the WIDTH code and ATLAS model atmospheres. Results. The velocity dispersion of the trailing tail is found to be $\sigma$=8.3$\pm$0.9 km s$^{-1}$, i.e., significantly lower than in the core of the Sgr galaxy and marginally lower than previous estimates in the same portion of the stream. Stream stars follow the same trend as Sgr main body stars in the [$\alpha$/Fe] vs [Fe/H] plane. However, stars are, on average, more metal poor in the stream than in the main body. This effect is slightly stronger in stars belonging to more ancient wraps of the stream, according to currently accepted models of Sgr disruption.

Tidal tails test the equivalence principle in the dark-matter sector

Satellite galaxies currently undergoing tidal disruption offer a unique opportunity to constrain an effective violation of the equivalence principle in the dark sector. While dark matter in the standard scenario interacts solely through gravity on large scales, a new long-range force between dark-matter particles may naturally arise in theories in which the dark matter couples to a light scalar field. An inverse-square-law force of this kind would manifest itself as a violation of the equivalence principle in the dynamics of dark matter compared to baryons in the form of gas or stars. In a previous paper, we showed that an attractive force would displace stars outwards from the bottom of the satellite's gravitational potential well, leading to a higher fraction of stars being disrupted from the tidal bulge further from the Galactic center. Since stars disrupted from the far (near) side of the satellite go on to form the trailing (leading) tidal stream, an attractive dark-matter force will produce a relative enhancement of the trailing stream compared to the leading stream. This distinctive signature of a dark-matter force might be detected through detailed observations of the tidal tails of a disrupting satellite, such as those recently performed by the Two-Micron All-Sky Survey (2MASS) and Sloan Digital Sky Survey (SDSS) on the Sagittarius (Sgr) dwarf galaxy. Here we show that this signature is robust to changes in our models for both the satellite and Milky Way, suggesting that we might hope to search for a dark-matter force in the tidal features of other recently discovered satellite galaxies in addition to the Sgr dwarf.

Galilean Equivalence for Galactic Dark Matter

Satellite galaxies are tidally disrupted as they orbit the Milky Way. If dark matter (DM) experiences a stronger self-attraction than baryons, stars will preferentially gain rather than lose energy during tidal disruption, leading to an enhancement in the trailing compared to the leading tidal stream. The Sgr dwarf galaxy is seen to have roughly equal streams, challenging models in which DM and baryons accelerate differently by more than 10%. Future observations and a better understanding of DM distribution should allow detection of equivalence violation at the percent level.

Variable Faint Optical Sources Discovered by Comparing the POSS and SDSS Catalogs

We present a study of variable faint optical sources discovered by comparing the Sloan Digital Sky Survey (SDSS) and the Palomar Observatory Sky Survey (POSS) catalogs. We use SDSS measurements to photometrically recalibrate several publicly available POSS catalogs (USNO-A2.0, USNO-B1.0, DPOSS [the Digitized Second Palomar Observatory Sky Survey], and GSC2.2 [the Guide Star Catalog 2.2]). A piecewise recalibration of the POSS data in 100 arcmin2 patches (one SDSS field) generally results in an improvement of photometric accuracy (rms) by nearly a factor of 2, compared to the original data. In addition to the smaller core width of the error distribution, the tails of the distribution become much steeper after the recalibration. These improvements are mostly due to the very dense grid of calibration stars provided by SDSS, which rectifies the intrinsic inhomogeneities of Schmidt plates. We find that the POSS I magnitudes can be improved to ∼0.15 mag accuracy, and POSS II magnitudes to ∼0.10 mag accuracy. The smallest final errors are obtained with the GSC2.2 catalog, for which they approach 0.07 mag at the bright end. We use the recalibrated catalogs for the ∼3300 deg2 of sky in the SDSS Data Release 2 to construct a catalog of ∼60,000 sources that are variable on timescales of 10-50 yr, and make it publicly available. Using this catalog, we find that at least 1% of faint optical sources appear variable at the >0.25 mag level, and that about 20% of the variable population are quasars, although they represent only 0.6% of all point sources in the adopted flux-limited sample (g < 19). A series of statistical tests based on the morphology of SDSS color-magnitude and color-color diagrams, as well as visual comparison of images and comparison with repeated SDSS observations, demonstrate the robustness of the selection methods: three out of four candidate variable sources are correctly recognized to vary. We also demonstrate that candidate RR Lyrae stars trace the same halo structures, such as the Sgr dwarf tidal stream, that were discovered using repeated SDSS observations. We utilize the POSS-SDSS selected candidates to constrain the halo structure in the parts of sky for which repeated SDSS observations do not exist. We quantify the distribution of variable sources in the SDSS color-color diagrams and the variability characteristics of quasars. The observed long-term quasar variability (structure function) is smaller than predicted by the extrapolation of the power law measured for short timescales using repeated SDSS imaging (0.35 vs. 0.60 mag for SDSS-POSS I and 0.24 vs. 0.35 mag for SDSS-POSS II, rms). This turnover in structure function suggests that the characteristic timescale for quasar variability is on the order of 1 yr in the rest frame.

A search for interstellar absorptions towards Sgr dSph

We searched for Na I D1,D2 interstellar absorption lines towards 12 giant stars of the Sgr dwarf spheroidal and 3 stars of the associated globular cluster Terzan 7, observed at high resolution with the UVES spectrograph at the 8.2 m Kueyen VLT telescope. These stars are not an ideal background source for the analysis of interstellar (IS) absorption lines since they have been observed in the framework of a stellar abundances study. However they are sufficiently luminous to allow a decent S/N ratio at a resolution of about 43000, i.e. ~7 km/s, which allows an exploratory study. We detected two distinct groups of IS absorptions, a ``local'' group, with radial velocities ranging from -43 km/s to 50 km/s, and a high velocity group with radial velocities ranging from 150 km/s to 165 km/s. Likely the high velocity components are due to gas falling on the Sgr dSph since have been observed only in 2 stars which are separated by only 1.5' in the sky. We argue that the observations suggest a stripping of gas due to the passage of Sgr through the Galactic disc.

Detectability of weakly interacting massive particles in the Sagittarius dwarf tidal stream

Tidal streams of the Sagittarius dwarf spheroidal galaxy (Sgr) may be showering dark matter onto the solar system and contributing $\ensuremath{\sim}(0.3--23)%$ of the local density of our galactic halo. If the Sagittarius galaxy contains dark matter in the form of weakly interacting massive particles (WIMPs), the extra contribution from the stream gives rise to a steplike feature in the energy recoil spectrum in direct dark matter detection. For our best estimate of stream velocity (300 km/s) and direction (the plane containing the Sgr dwarf and its debris), the count rate is maximum on June 28 and minimum on December 27 (for most recoil energies), and the location of the step oscillates yearly with a phase opposite to that of the count rate. In the CDMS experiment, for 60 GeV WIMPs, the location of the step oscillates between 35 and 42 keV, and for the most favorable stream density, the stream should be detectable at the $11\ensuremath{\sigma}$ level in four years of data with 10 keV energy bins. Planned large detectors like XENON, CryoArray, and the directional detector DRIFT may also be able to identify the Sgr stream.

A Two Micron All‐Sky Survey View of the Sagittarius Dwarf Galaxy. IV. Modeling the Sagittarius Tidal Tails

M giants recovered from the Two Micron All-Sky Survey have recently been used to map the position and velocity distributions of tidal debris from the Sagittarius (Sgr) dwarf spheroidal galaxy around the entire Galaxy. We compare this data set to both test-particle orbits and N-body simulations of satellite destruction run within a variety of rigid Milky Way potentials, and we find that the mass of the Milky Way within 50 kpc of its center should be × 1011 M☉ in order for any Sgr orbit to simultaneously fit the velocity gradient in the Sgr trailing debris and the apocenter of the Sgr leading debris. Orbital pole precession of young debris and leading debris velocities in regions corresponding to older debris provide contradictory evidence in favor of oblate/prolate Galactic halo potentials, respectively, leading us to conclude that the orbit of Sgr has evolved over the past few gigayears. In light of this discrepancy, we consider constraints from only the younger portions of the debris within three models of the flattening of the Galactic potential [q = 0.90 (O), 1.0 (S), and 1.25 (P), i.e., oblate, spherical, and prolate] in our further N-body simulations. On the basis of the velocity dispersion and width along the trailing tidal stream, we estimate the current bound mass of Sgr to be MSgr = × 108 M☉ independent of the form of the Galactic potential; this corresponds to a range of mass-to-light ratios (M/L)Sgr = 14-36(M/L)☉ for the Sgr core. Models with masses in this range best fit the apocenter of leading Sgr tidal debris when they orbit with a radial period of roughly 0.85 Gyr and have perigalactica and apogalactica of about 15 and 60 kpc, respectively. These distances scale with the assumed distance to the Sgr dwarf and the assumed depth of the Galactic potential. The density distribution of debris along the orbit in these models is consistent with the M giant observations, and debris at all orbital phases in which M giants are obviously present is younger (i.e., was lost more recently from the satellite) than the typical age of a Sgr M giant star.

The Selection of RR Lyrae Stars Using Single-Epoch Data

We use a complete sample of RR Lyrae stars discovered by the Quasar Equatorial Survey Team survey using light curves to design selection criteria based on Sloan Digital Sky Survey (SDSS) colors. Thanks to the sensitivity of the u - g color to surface gravity and of the g - r color to effective temperature and to the small photometric errors (~0.02 mag) delivered by SDSS, RR Lyrae stars can be efficiently and robustly recognized even with single-epoch data. In a 100% complete color-selected sample, the selection efficiency (the fraction of RR Lyrae stars in the candidate sample) is 6%, and, by adjusting color cuts, it can be increased to 10% with a completeness (the fraction of selected RR Lyrae stars) of 80% and to 60% with 28% completeness. Such color selection produces samples that are sufficiently clean for statistical studies of the Milky Way's halo substructure, and we use it to select 3,643 candidate RR Lyrae stars from SDSS Data Release 1. We demonstrate that this sample recovers known clumps of RR Lyrae stars associated with the Sgr dwarf tidal tail and the Palomar 5 globular cluster and use it to constrain the halo substructure away from the Sgr dwarf tidal tail. These results suggest that it will be possible to study the halo substructure out to ~70 kpc from the Galactic center in the entire area imaged by the SDSS, and not only in the multiply observed regions.

Stellar Chemical Signatures and Hierarchical Galaxy Formation

To compare the chemistries of stars in the Milky Way dwarf spheroidal (dSph) satellite galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the literature. When kinematic information is available, we have assigned the stars to standard Galactic components through Bayesian classification based on Gaussian velocity ellipsoids. As found in previous studies, the [α/Fe] ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm this for the Galactic halo, thin-disk, and thick-disk components. There is marginal overlap in the low [α/Fe] ratios between dSph stars and Galactic halo stars on extreme retrograde orbits (V < -420 km s-1), but this is not supported by other element ratios. Other element ratios compared in this paper include r- and s-process abundances, where we find a significant offset in the [Y/Fe] ratios, which results in a large overabundance in [Ba/Y] in most dSph stars compared with Galactic stars. Thus, the chemical signatures of most of the dSph stars are distinct from the stars in each of the kinematic components of the Galaxy. This result rules out continuous merging of low-mass galaxies similar to these dSph satellites during the formation of the Galaxy. However, we do not rule out very early merging of low-mass dwarf galaxies, since up to one-half of the most metal-poor stars ([Fe/H] ≤ -1.8) have chemistries that are in fair agreement with Galactic halo stars. We also do not rule out merging with higher mass galaxies, although we note that the LMC and the remnants of the Sgr dwarf galaxy are also chemically distinct from the majority of the Galactic halo stars. Formation of the Galaxy's thick disk by heating of an old thin disk during a merger is also not ruled out; however, the Galaxy's thick disk itself cannot be comprised of the remnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarf galaxy like the LMC or Sgr, because of differences in chemistry.

The distance to the Sagittarius dwarf spheroidal galaxy from the red giant branch tip

We derived the distance to the central region of the Sagittarius (Sgr) dwarf spheroidal galaxy from the red giant branch tip. The obtained distance modulus is (m - M) 0 = 17.10 ± 0.15, corresponding to a heliocentric distance D = 26.30 ± 1.8 Kpc. This estimate is in good agreement with the distance obtained from RR Lyrae (RR Ly) stars of the globular cluster M 54, located in the core of the Sgr galaxy, once the most accurate estimate of the cluster metallicity and the most recent calibration of the M V (RR Ly) versus [Fe/H] relation are adopted.

Detectability of the Sgr dwarf leading tidal stream with Auger, EUSO, or OWL

We point out that if heavy metastable particles composing the dark matter of our galaxy are responsible for the ultrahigh energy cosmic rays (UHECR) then the leading tidal stream of the Sagittarius dwarf galaxy could be detected through UHECR. The signal would be an anisotropy in the UHECR flux smaller than the telltale anisotropy towards the galactic center that would first establish unstable dark matter as the origin of the UHECR.

Velocity Trends in the Debris of Sagittarius and the Shape of the Dark Matter Halo of Our Galaxy

Recently, radial velocities have been measured for a large sample of M giants from the Two Micron All Sky Survey catalog, selected to be part of the Sgr dwarf leading and trailing streams. Here we present a comparison of their kinematics to models of the Sgr dwarf debris orbiting Galactic potentials, with halo components of varying degrees of flattening and elongation. This comparison shows that the portion of the trailing stream mapped so far is dynamically young and hence does not provide very stringent constraints on the shape of the Galactic dark matter halo. The leading stream, however, contains slightly older debris, and its kinematics provide for the first time direct evidence that the dark matter halo of our Galaxy has a prolate shape with an average density axis ratio within the orbit of Sgr close to 5/3.

A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream

We have obtained moderate resolution (~6 km s-1) spectroscopy of several hundred M giant candidates selected from Two Micron All Sky Survey photometry. Radial velocities are presented for stars mainly in the southern Galactic hemisphere, and the primary targets have Galactic positions consistent with association to the tidal tail system of the Sagittarius (Sgr) dwarf galaxy. M giant stars selected from the apparent trailing debris arm of Sgr have velocities showing a clear trend with orbital longitude, as expected from models of the orbit and destruction of Sgr. A minimum 8 kpc width of the trailing stream about the Sgr orbital midplane is implied by verified radial velocity members. The coldness of this stream (σv ~ 10 km s-1) provides upper limits on the combined contributions of stream heating by a lumpy Galactic halo and the intrinsic dispersion of released stars, which is a function of the Sgr core mass. We find that the Sgr trailing arm is consistent with a Galactic halo that contains one dominant, LMC-like lump; however, some lumpier halos are not ruled out. An upper limit to the total mass-to-light ratio of the Sgr core is 21 in solar units. Evidence for other velocity structures is found among the more distant (>13 kpc) M giants. A second structure that roughly mimics expectations for wrapped, leading Sgr arm debris crosses the trailing arm in the southern hemisphere; however, this may also be an unrelated tidal feature. Among the bright, nearby (<13 kpc) M giants toward the south Galactic pole are a number with large velocities that identify them as halo stars; these too may trace halo substructure, perhaps part of the Sgr leading arm near the Sun. The positions and velocities of southern hemisphere M giants are compared with those of southern hemisphere globular clusters potentially stripped from the Sgr system. Support for association of the globular clusters Pal 2 and Pal 12 with Sgr debris is found, based on positional and radial velocity matches. Our discussion includes description of a masked-filtered cross-correlation methodology that achieves better than 1/20 of a resolution element velocities in moderate-resolution spectra. The improved velocity resolution achieved allows tighter constraints to be placed on the coldness of the Sgr stream than previously established.

The Gaseous Trail of the Sagittarius Dwarf Galaxy

A possible gaseous component to the stream of debris from the Sagittarius dwarf galaxy is presented. We identify (4-10) × 106 M☉ of neutral hydrogen along the orbit of the Sgr dwarf in the direction of the Galactic anticenter (at 36 kpc, the distance to the stellar debris in this region). This is 1%-2% of the estimated total mass of the Sgr dwarf. Both the stellar and gaseous components have negative velocities, but the gaseous component extends to higher negative velocities. If associated, this gaseous stream was most likely stripped from the main body of the dwarf 0.2-0.3 Gyr ago during its current orbit after a passage through a diffuse edge of the Galactic disk with a density greater than 10-4 cm-3. This gas represents the dwarf's last source of star formation fuel and explains how the galaxy was forming stars 0.5-2 Gyr ago.

The Sgr dSph hosts a metal-rich population

We report on abundances of O, Mg, Si, Ca and Fe for 10 giants in the Sgr dwarf spheroidal derived from high resolution spectra obtained with UVES at the 8.2 m Kueyen-VLT telescope. The iron abundance spans the range -0.8 ≤ [Fe/H] ≤ 0.0 and the dominant population is relatively metal-rich with [Fe/H] ∼ -0.25. The a/Fe ratios are slightly subsolar, even at the lowest observed metallicities suggesting a slow or bursting star formation rate. From our sample of 12 giants (including the two observed by Bonifacio et al. 2000) we conclude that a substantial metal rich population exists in Sgr, which dominates the sample. The spectroscopic metallicities allow one to break the age-metallicity degeneracy in the interpretation of the colour-magnitude diagram (CMD). Comparison of isochrones of appropriate metallicity with the observed CMD suggests an age of 1 Gyr or younger, for the dominant Sgr population sampled by us. We argue that the observations support a star formation that is triggered by the passage of Sgr through the Galactic disc, both in Sgr and in the disc. This scenario has also the virtue of explaining the mysterious bulge C stars as disc stars formed in this event. The interaction of Sgr with the Milky Way is likely to have played a major role in its evolution.

Dark Matter Constraints from the Sagittarius Dwarf and Tail System

2MASS has provided a three-dimensional map of the &gt; 360°, wrapped tidal tails of the Sagittarius (Sgr) dwarf spheroidal galaxy, as traced by M giant stars. With the inclusion of radial velocity data for stars along these tails, strong constraints exist for dynamical models of the Milky Way-Sgr interaction. N-body simulations of Sgr disruption with model parameters spanning a range of initial conditions (e.g., Sgr mass and orbit, Galactic rotation curve, halo flattening) are used to find parameterizations that match almost every extant observational constraint of the Sgr system. We discuss the implications of the Sgr data and models for the orbit, mass and M/L of the Sgr bound core as well as the strength, flattening, and lumpiness of the Milky Way potential.

The Galaxy's Eating Habits

The possibility of a gaseous halo stream which was stripped from the Sagittarius dwarf galaxy is presented. The total mass of the neutral hydrogen along the orbit of the Sgr dwarf in the direction of the Galactic Anti-Center is 4 − 10 × 106 M⊙ (at 36 kpc, the distance to the stellar debris in this region). Both the stellar and gaseous components have negative velocities in this part of the sky, but the gaseous component extends to higher negative velocities. We suggest this gaseous stream was stripped from the main body of the dwarf 0.2 – 0.3 Gyr ago during its current orbit after a passage through a diffuse edge of the Galactic disk with a density &gt; 10−4 cm−3. The gas would then represent the dwarf's last source of star formation fuel and explains how the galaxy was forming stars 0.5-2 Gyr ago.