Local Group Dwarf Spheroidal Galaxies Research Articles

A probabilistic deep learning model to distinguish cusps and cores in dwarf galaxies

ABSTRACT Numerical simulations within a cold dark matter (DM) cosmology form haloes whose density profiles have a steep inner slope (‘cusp’), yet observations of galaxies often point towards a flat central ‘core’. We develop a convolutional mixture density neural network model to derive a probability density function (PDF) of the inner density slopes of DM haloes. We train the network on simulated dwarf galaxies from the NIHAO and AURIGA projects, which include both DM cusps and cores: line-of-sight velocities and 2D spatial distributions of their stars are used as inputs to obtain a PDF representing the probability of predicting a specific inner slope. The model recovers accurately the expected DM profiles: $\sim 82{{\ \rm per\ cent}}$ of the galaxies have a derived inner slope within ±0.1 of their true value, while $\sim 98{{\ \rm per\ cent}}$ within ±0.3. We apply our model to four Local Group dwarf spheroidal galaxies and find results consistent with those obtained with the Jeans modelling based code GravSphere: the Fornax dSph has a strong indication of possessing a central DM core, Carina and Sextans have cusps (although the latter with large uncertainties), while Sculptor shows a double peaked PDF indicating that a cusp is preferred, but a core cannot be ruled out. Our results show that simulation-based inference with neural networks provide a innovative and complementary method for the determination of the inner matter density profiles in galaxies, which in turn can help constrain the properties of the elusive DM.

Evolution of neutron capture elements in dwarf galaxies

ABSTRACT We study the evolution of europium (Eu) and barium (Ba) abundances in Local Group dwarf spheroidal and ultrafaint dwarf galaxies by means of detailed chemical evolution models and compare our results with new sets of homogeneous abundances. The adopted models include gas infall and outflow and have been previously tested. We investigate several production scenarios for r-process elements: merging neutron stars and magnetorotational-driven supernovae. Production of Ba through the main s-process acting in low- and intermediate-mass stars is considered as well. We also test different sets of nucleosynthesis yields. For merging neutron stars we adopt either a constant and short delay time for merging or a delay time distribution function. Our simulations show that (i) if r-process elements are produced only by a quick source, it is possible to reproduce the [Eu/Fe] versus [Fe/H], but those models fail in reproducing the [Ba/Fe] versus [Fe/H]. (ii) If r-process elements are produced only with longer delays the opposite happens. (iii) If both a quick source and a delayed one are adopted, such as magnetorotational-driven supernovae and merging neutron stars with a delay time distribution, the [Eu/Fe] abundance pattern is successfully reproduced, but models still fail in reproducing the [Ba/Fe]. (iv) On the other hand, the characteristic abundances of Reticulum II can be reproduced only if both the Eu and the r-process fraction of Ba are produced on short and constant time delays during a single merging event. We discuss also other possible interpretations, including an inhomogeneous mixing of gas that might characterize this galaxy.

The infall of dwarf satellite galaxies are influenced by their host’s massive accretions

ABSTRACT Recent progress in constraining the massive accretions (>1:10) experienced by the Milky Way (MW) and the Andromeda galaxy (M31) offers an opportunity to understand the dwarf galaxy population of the Local Group. Using zoom-in dark matter-only simulations of MW-mass haloes and concentrating on subhaloes that are thought to be capable of hosting dwarf galaxies, we demonstrate that the infall of a massive progenitor is accompanied with the accretion and destruction of a large number of subhaloes. Massive accreted progenitors do not increase the total number of infalling subhaloes on to a MW-mass host, but instead focus surrounding subhaloes on to the host causing a clustering in the infall time of subhaloes. This leads to a temporary elevation in the number of subhaloes as well as changes in their cumulative radial profile within the virial radius of the host. Surviving subhaloes associated with a massive progenitor have a large diversity in their orbits. We find that the star formation quenching times of Local Group dwarf spheroidal galaxies ($10^{5} \mathrm{\, M_{\odot }} \lesssim \mathrm{\mathit{ M}}_{*} \lesssim 10^{7} \mathrm{\, M_{\odot }}$) are clustered around the times of the most massive accretions suffered by the MW and M31. Our results imply that (a) the quenching time of dwarf spheroidals is a good proxy of their infall time and b) the absence of recently quenched satellites around M31 suggests that M33 is not on its first infall and was accreted much earlier.

The Araucaria Project: Deep Near-infrared Photometric Maps of Local and Sculptor Group Galaxies. I. Carina, Fornax, and Sculptor

Abstract Deep near-infrared (NIR) J- and K-band photometry of three Local Group dwarf spheroidal galaxies—Fornax, Carina, and Sculptor—is made available for the community. Until now, these data have only been used by the Araucaria Project to determine distances using the tip of the red giant branch and RR Lyrae stars. Now, we present the entire data collection in the form of a database, consisting of accurate J- and K-band magnitudes, sky coordinates, ellipticity measurements, and timestamps of observations, complemented by stars’ loci in their reference images. The depth of our photometry reaches about 22 mag at the 5σ level, and is comparable to NIR surveys, such as the UKIRT Infrared Deep Sky Survey (UKIDSS) or the VISTA Hemisphere Survey (VHS). Small overlap with VHS and no overlap with UKIDSS makes our database a unique source of quality photometry.

Stellar profile independent determination of the dark matter distribution of the Fornax Local Group dwarf spheroidal galaxy

ABSTRACT We detail a method to measure the correspondence between dark matter (DM) models and observations of stellar populations within Local Group dwarf spheroidal galaxies (LG dSphs) that assumes no parametric stellar distribution. Solving the spherical or cylindrical Jeans equations, we calculate the consistency of DM and stellar kinematic models with stellar positions and line-of-sight velocities. Our method can be used to search for signals of standard and exotic DM distributions. Applying our methodology to the Fornax LG dSph and using statistical bootstrapping, we find: (i) that oblate or prolate cored DM haloes match the stellar data, respectively, ≃60 or ≃370 times better than oblate or prolate cusped DM haloes for isotropic and isothermal stellar velocity dispersions, (ii) that cusped spherical DM haloes and cored spherical DM haloes match the Fornax data similarly well for isotropic stellar velocity dispersions, (iii) that the semiminor to semimajor axial ratio of spheroidal DM haloes are more extreme than 80 per cent of those predicted by Lambda cold dark matter with baryon simulations, (iv) that oblate cored or cusped DM haloes are, respectively, ≃5 or ≃30 times better matches to Fornax than prolate cored or cusped DM haloes, and (v) that Fornax shows no evidence of a disc-like structure with more than two per cent of the total DM mass. We further note that the best-fitting cusped haloes universally favour the largest mass and size fit parameters. If these extreme limits are decreased, the cusped halo likelihoods decrease relative to those of cored haloes.

Kinematics of the Tucana Dwarf Galaxy: an unusually dense dwarf in the Local Group

We present new FLAMES$+$GIRAFFE spectroscopy of 36 member stars in the isolated Local Group dwarf spheroidal galaxy Tucana. We measure a systemic velocity for the system of $v_{\mathrm{Tuc}}=216.7_{-2.8}^{+2.9}$kms$^{-1}$, and a velocity dispersion of $\sigma_{\mathrm{v,Tuc}}=14.4_{-2.3}^{+2.8}$kms$^{-1}$. We also detect a rotation gradient of $\frac{dv_{r}}{d\chi}=7.6^{+4.2}_{-4.3}$ kms$^{-1}$ kpc$^{-1}$, which reduces the systemic velocity to $v_{\mathrm{Tuc}}=215.2_{-2.7}^{+2.8}$ kms$^{-1}$ and the velocity dispersion to $\sigma_{v,\mathrm{Tuc}}=13.3_{-2.3}^{+2.7}$ kms$^{-1}$. We perform Jeans modelling of the density profile of Tucana using the line--of--sight velocities of the member stars. We find that it favours a high central density consistent with `pristine' subhalos in $\Lambda$CDM, and a massive dark matter halo ($\sim$$10^{10}$\,M$_\odot$) consistent with expectations from abundance matching. Tucana appears to be significantly more centrally dense than other isolated Local Group dwarfs, making it an ideal laboratory for testing dark matter models.

SMHASH: a new mid-infrared RR Lyrae distance determination for the Local Group dwarf spheroidal galaxy Sculptor

We present a new distance estimation for the Milky Way dwarf spheroidal satellite Sculptor obtained from multi-epoch mid-infrared observations of RR Lyrae stars. The 3.6 {\mu}m observations have been acquired with the Infrared Array Camera on board the Spitzer Space Telescope as part of the SMHASH Program. Mid-infrared light curves for 42 RRL were obtained, from which we measured Sculptor's distance modulus to be {\mu} = 19.60 $\pm$ 0.02 (statistical) $\pm$ 0.04 (photometric) mag (with $\sigma_{sys}=$ = 0.09 mag), using the 3.6 {\mu}m empirical period-luminosity relations derived from the Galactic globular cluster M4, or {\mu} = 19.57 $\pm$ 0.02 (statistical) $\pm$ 0.04 (photometric) mag (with $\sigma_{sys}=$ = 0.11 mag) using empirical relations in the same passband recently derived from the Large Magellanic Cloud globular cluster Reticulum. Both these measurements are in good agreement with values presented in previous works with Sculptor RR Lyrae stars in optical bands, and are also consistent with recent near-infrared RR Lyrae results. Best agreement with the literature is found for the latter modulus which is equivalent to a distance of d = 82 $\pm$ 1 (statistical) $\pm$ 2 (photometric) kpc (with $\sigma_{sys}=$ = 4 kpc). Finally, using a subsample of RR Lyrae stars with spectroscopic metallicities, we demonstrate that these distance estimates are not affected by metallicity effects.

Pushing back the limits: detailed properties of dwarf galaxies in a ΛCDM universe

We present the results of a set of high-resolution chemo-dynamical simulations of dwarf galaxies in a ΛCDM cosmology. Out of an original (3.4 Mpc/h)3 cosmological box, a sample of 27 systems are re-simulated from z = 70 to z = 0 using a zoom-in technique. Gas and stellar properties are confronted to the observations in the greatest details: in addition to the galaxy global properties, we investigated the model galaxy velocity dispersion profiles, half-light radii, star formation histories, stellar metallicity distributions, and [Mg/Fe] abundance ratios. The formation and sustainability of the metallicity gradients and kinematically distinct stellar populations are also tackled. We show how the properties of six Local Group dwarf galaxies, NGC 6622, Andromeda II, Sculptor, Sextans, Ursa Minor and Draco are reproduced, and how they pertain to three main galaxy build-up modes. Our results indicate that the interaction with a massive central galaxy could be needed for a handful of Local Group dwarf spheroidal galaxies only, the vast majority of the systems and their variety of star formation histories arising naturally from a ΛCDM framework. We find that models fitting well the local Group dwarf galaxies are embedded in dark haloes of mass between 5 × 108 to a few 109 M⊙, without any missing satellite problem. We confirm the failure of the abundance matching approach at the mass scale of dwarf galaxies. Some of the observed faint however gas-rich galaxies with residual star formation, such as Leo T and Leo P, remain challenging. They point out the need of a better understanding of the UV-background heating.

Subaru Hyper Suprime-Cam Survey for the Local Group Dwarf Galaxies: Ursa Minor

AbstractWe have carried out a wide and deep imaging survey for the Local Group dwarf spheroidal galaxy Ursa Minor (UMi) using Hyper Suprime-Cam (HSC). The data cover out beyond the nominal tidal radius down to ~25 mag in i band, which is ~2 mag below the main sequence turn-off point. The structural parameters of UMi are derived using red giant branch (RGB) stars and sub-giant branch (SGB) stars, and the tidal radius is suggested to be larger than those estimated by the previous studies. It is also found that the distribution of bluer RGB/SGB stars is more extended than that of redder RGB/SGB stars. The fraction of binary systems is estimated to be ~0.4 from the morphology of the main sequences.

The chemical connection between damped Lyman-α systems and Local Group dwarf galaxies

Abundances of the volatile elements S and Zn have now been measured in around 80 individual stars in the Sculptor dwarf spheroidal galaxy, covering the metallicity range − 2.4 ≤ [Fe/H] ≤−0.9. These two elements are of particular interest as they are not depleted onto dust in gas, and their ratio, [S/Zn], has thus commonly been used as a proxy for [α/Fe] in Damped Lyman-α systems (DLAs). The S abundances in Sculptor are similar to other α-elements in this galaxy, consistent with S being mainly created in core-collapse supernovae, but also having some contribution from type Ia supernovae. However, our results show that Zn and Fe do not trace all the same nucleosynthetic production channels. In particular, (contrary to Fe) Zn is not significantly produced by type Ia supernovae. Thus, [S/Zn] cannot be reliably used as a proxy for [α/Fe]. We propose [O/S] as a function of [S/H] as a possible alternative. At higher metallicities, the values of [S/Zn] measured in DLAs are inconsistent with those in local dwarf galaxies, and are more compatible with the Milky Way disk. Low-metallicity DLAs are, however, consistent with the most metal-poor stars in Local Group dwarf spheroidal galaxies. Assuming that the dust depletions of S and Zn are negligible, our comparison indicates that the star formation histories of DLAs are on average different from both the Milky Way and the Sculptor dwarf spheroidal galaxy.

HSC Wide and Deep Imaging Survey for the Milky Way Satellite Galaxies

AbstractWe have carried out an imaging survey for Local Group dwarf spheroidal galaxy Ursa Minor (UMi) using Hyper Suprime-Cam (HSC) on the 8.2m Subaru Telescope. Wide and deep data obtained by HSC enable us to investigate the extent of UMi which is revealed to extend out to twice the nominal tidal radius. The fraction of binary systems is also investigated from the morphology of the main sequence and estimated to be ~0.4.

Stellar systems in the direction of the Hickson Compact Group 44

Context. In spite of the numerous studies of low-luminosity galaxies in different environments, there is still no consensus about their formation scenario. In particular, a large number of galaxies displaying extremely low-surface brightnesses have been detected in the last year, and the nature of these objects is under discussion. Aims. In this paper we report the detection of two extended low-surface brightness (LSB) objects (mueff_g'~27 mag) found, in projection, next to NGC 3193 and in the zone of the Hickson Compact Group (HCG) 44, respectively. Methods. We analyzed deep, high-quality, GEMINI-GMOS images with ELLIPSE within IRAF in order to obtain their brightness profiles and structural parameters. We also search for the presence of globular clusters (GC) in these fields. Results. We have found that, if these LSB galaxies were at the distances of NGC 3193 and HCG 44, they would show sizes and luminosities similar to those of the ultra-diffuse galaxies (UDGs) found in the Coma cluster and other associations. In that case, their sizes would be rather larger than those displayed by the Local Group dwarf spheroidal (dSph) galaxies. We have detected a few unresolved sources in the sky zone occupied by these galaxies showing colors and brightnesses typical of blue globular clusters. Conclusions. From the comparison of the properties of the galaxies presented in this work, with those of similar objects reported in the literature, we have found that LSB galaxies display sizes covering a quite extended continous range (reff ~ 0.3 - 4.5 kpc), in contrast to "normal" early-type galaxies, which shows reff ~ 1.0 kpc with a low dispersion. This fact might be pointing to different formation processes for both types of galaxies.

Astrophysical site(s) ofr-process elements in galactic chemodynamical evolution model

Astrophysical site(s) of rapid neutron-capture process ( r -process) is (are) not identified yet. Although core-collapse supernovae have been regarded as one of the possible candidates of the astrophysical site of r -process, nucleosynthesis studies suggest that serious difficulties in core-collapse supernovae to produce heavy elements with mass number of ≳110. Recent studies show that neutron star mergers (NSMs) can synthesize these elements due to their neutron rich environment. Some chemical evolution studies of the Milky Way halo, however, hardly reproduce the observed star-to-star scatters of the abundance ratios of r-process elements (e.g., Eu) in extremely metal-poor stars. This is because of their low rate (∼ 10 −4 yr −1 for a Milky Way size galaxy) and long merger time (≳ 100 Myr). This problem might be solved if the stars in the Galactic halo are consisted of the stars formed in dwarf galaxies where the star formation efficiencies were very low. In this study, we carry out numerical simulations of galactic chemo-dynamical evolution using an N-body/smoothed particle hydrodynamics code. We construct detailed chemo-dynamical evolution model for the Local Group dwarf spheroidal galaxies (dSphs) assuming that the NSMs are the major source of r -process elements. Our models successfully reproduce the observed dispersion in [Eu/Fe] as a function of [Fe/H] if we set merger time of NSMs, ≲ 300 Myr with the Galactic NSM rate of ∼ 10 −4 yr −1 . In addition, our results are consistent with the observed metallicity distribution of dSphs. In the early phase (≲1 Gyr) of galaxy evolution is constant due to low star formation efficiency of dSphs. This study supports the idea that NSMs are the major site of r -process nucleosynthesis.

A MULTIPLE BURST ACCRETION MODEL TO DESCRIBE THE METALLICITY DISTRIBUTIONS AND STELLAR MASS–METALLICITY RELATION FOR LOCAL DWARF GALAXIES

A one parameter model to describe the individual metallicity distributions and stellar mass-metallicity relation for dwarf galaxies is presented. This multiple-burst model is based on an accretion scenario, accomodates the observational constraint between $\overline{z}$ and $\sigma_{z}^{2}$ recently established by Leaman (2012), and predicts a slope consistent with the stellar mass-metallicity relation of Kirby et al (2013) who showed that the local group dwarf spheroidal and dwarf irregular galaxies lie on the same relation. One interpretation of the model is that it describes star formation occuring either in gas rich mergers or at the intersection of colliding gas streams.

Local Group dSph radio survey with ATCA – II. Non-thermal diffuse emission

Our closest neighbours, the Local Group dwarf spheroidal (dSph) galaxies, are extremely quiescent and dim objects, where thermal and non-thermal diffuse emissions lack, so far, of detection. In order to possibly study the dSph interstellar medium, deep observations are required. They could reveal non-thermal emissions associated with the very low level of star formation, or to particle dark matter annihilating or decaying in the dSph halo. In this work, we employ radio observations of six dSphs, conducted with the Australia Telescope Compact Array in the frequency band 1.1–3.1 GHz, to test the presence of a diffuse component over typical scales of few arcmin and at an rms sensitivity below 0.05 mJy beam−1. We observed the dSph fields with both a compact array and long baselines. Short spacings led to a synthesized beam of about 1 arcmin and were used for the extended emission search. The high-resolution data mapped background sources, which in turn were subtracted in the short-baseline maps, to reduce their confusion limit. We found no significant detection of a diffuse radio continuum component. After a detailed discussion on the modelling of the cosmic ray (CR) electron distribution and on the dSph magnetic properties, we present bounds on several physical quantities related to the dSphs, such that the total radio flux, the angular shape of the radio emissivity, the equipartition magnetic field, and the injection and equilibrium distributions of CR electrons. Finally, we discuss the connection to far-infrared and X-ray observations.

THE COMPARATIVE CHEMICAL EVOLUTION OF AN ISOLATED DWARF GALAXY: A VLT AND KECK SPECTROSCOPIC SURVEY OF WLM

Building on our previous spectroscopic and photometric analysis of the isolated Local Group dwarf irregular (dIrr) galaxy WLM, we present a comparison of the metallicities of its RGB stars with respect to the well studied Local Group dwarf spheroidal galaxies (dSphs) and Magellanic Clouds. We calculate a mean metallicity of [Fe/H]$ = -1.28 \pm 0.02$, and intrinsic spread in metallicity of $\sigma = 0.38 \pm 0.04$ dex, similar to the mean and spread observed in the massive dSph Fornax and the Small Magellanic Cloud. Thus, despite its isolated environment the global metallicity still follows expectations for WLM's mass and its global chemical evolution is similar to other nearby luminous dwarf galaxies (gas-rich or gas-poor). The data also show a radial gradient in [Fe/H] of $\rm{d[Fe/H]/dr_{c}} = -0.04 \pm 0.04$ dex $\rm{r_{c}^{-1}}$, which is flatter than that seen in the unbiased and spatially extended surveys of dSphs. Comparison of the spatial distribution of [Fe/H] in WLM, the Magellanic Clouds, and a sample of Local Group dSphs, shows an apparent dichotomy in the sense that the dIrrs have statistically flatter radial [Fe/H] gradients than the low angular momentum dSphs. The correlation between angular momentum and radial metallicity gradient is further supported when considering the Local Group dEs. This chemodynamic relationship offers a new and useful constraint for environment driven dwarf galaxy evolution models in the Local Group.

MEASURING DARK MATTER PROFILES NON-PARAMETRICALLY IN DWARF SPHEROIDALS: AN APPLICATION TO DRACO

We introduce a novel implementation of orbit-based (or Schwarzschild) modeling that allows dark matter density profiles to be calculated non-parametrically in nearby galaxies. Our models require no assumptions to be made about velocity anisotropy or the dark matter profile. The technique can be applied to any dispersion-supported stellar system, and we demonstrate its use by studying the Local Group dwarf spheroidal (dSph) galaxy Draco. We use existing kinematic data at larger radii and also present 12 new radial velocities within the central 13 pc obtained with the VIRUS-W integral field spectrograph on the 2.7m telescope at McDonald Observatory. Our non-parametric Schwarzschild models find strong evidence that the dark matter profile in Draco is cuspy for 20 < r < 700 pc. The profile for r > 20 pc is well-fit by a power law with slope \alpha=-1.0 +/- 0.2, consistent with predictions from Cold Dark Matter (CDM) simulations. Our models confirm that, despite its low baryon content relative to other dSphs, Draco lives in a massive halo.

ON THE CENTRAL HELIUM-BURNING VARIABLE STARS OF THE LeoI DWARF SPHEROIDAL GALAXY

We present a study of short period, central helium-burning variable stars in the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and 51 Cepheids. So far, this is the largest sample of Cepheids and the largest Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with other Local Group dwarf spheroidals, Carina and Fornax, shows that the period distribution of RR Lyrae stars is quite similar, suggesting similar properties of the parent populations, whereas the Cepheid period distribution in LeoI peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a broader range, from 0.5 to 1.78d. Evolutionary and pulsation predictions indicate, assuming a mean metallicity peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids traces a unique mix of Anomalous Cepheids (blue extent of the red--clump, partially electron degenerate central helium-burning stars) and short-period classical Cepheids (blue-loop, quiescent central helium-burning stars). Current evolutionary prescriptions also indicate that the transition mass between the two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different implications of the current findings on the star formation history of LeoI.

FIRST GROUND-BASED CHARGE-COUPLED DEVICE PROPER MOTIONS FOR FORNAX. II. FINAL RESULTS

We present the first entirely ground-based astrometric determination of the proper motion for the Fornax Local Group Dwarf Spheroidal satellite galaxy of the Milky Way, using CCD data acquired with the ESO 3.5 m NTT telescope at La Silla Observatory in Chile. Our unweighted mean from five Quasar fields in the background of Fornax, used as fiducial reference points, leads to $\mu_\alpha \cos \delta=0.62 \pm 0.16$ $\masy$, and $\mu_\delta=-0.53 \pm 0.15$ $\masy$. A detailed comparison with all previous measurements of this quantity seems to imply that there is still no convincing convergence to a single value, perhaps indicating the existence of unnacounted systematic effects in (some of) these measurements. From all available proper motion and radial velocity measurements for Fornax, we compute Fornax's orbital parameters and their uncertainty using a realistic Galactic potential and a Monte Carlo simulation. Properties of the derived orbits are then compared to main star formation episodes in the history of Fornax. All published proper motion values imply that Fornax has recently (200-300 Myr ago) approached perigalacticon at a distance of $\sim$150 kpc. However, the derived period exhibits a large scatter, as does the apogalacticon. Our orbit, being the most energetic, implies a very large apogalactic distance of $\sim 950$ kpc. If this were the case, then Fornax would be a representative of an hypervelocity MW satellite in late infall.

LOCAL GROUP DWARF SPHEROIDALS: CORRELATED DEVIATIONS FROM THE BARYONIC TULLY-FISHER RELATION

Local Group dwarf spheroidal satellite galaxies are the faintest extragalactic stellar systems known. We examine recent data for these objects in the plane of the Baryonic Tully-Fisher Relation (BTFR). While some dwarf spheroidals adhere to the BTFR, others deviate substantially. We examine the residuals from the BTFR and find that they are not random. The residuals correlate with luminosity, size, metallicity, ellipticity, and susceptibility of the dwarfs to tidal disruption in the sense that fainter, more elliptical, and tidally more susceptible dwarfs deviate farther from the BTFR. These correlations disfavor stochastic processes and suggest a role for tidal effects. We identify a test to distinguish between the {\Lambda}CDM and MOND based on the orbits of the dwarf satellites of the Milky Way and how stars are lost from them.