Search

We analyze four epochs of Hubble Space Telescope imaging over 18 yr for the Draco dwarf spheroidal galaxy. We measure precise proper motions for hundreds of stars and combine these with existing line-of-sight (LOS) velocities. This provides the first radially resolved 3D velocity dispersion profiles for any dwarf galaxy. These constrain the intrinsic velocity anisotropy and resolve the mass–anisotropy degeneracy. We solve the Jeans equations in oblate axisymmetric geometry to infer the mass profile. We find the velocity dispersion to be radially anisotropic along the symmetry axis and tangentially anisotropic in the equatorial plane, with a globally averaged value β B ¯ = − 0.20 − 0.53 + 0.28 , (where 1 – β B ≡ 〈 v tan 2 〉 / 〈 v rad 2 〉 in 3D). The logarithmic dark matter (DM) density slope over the observed radial range, Γdark, is − 0.83 − 0.37 + 0.32 , consistent with the inner cusp predicted in ΛCDM cosmology. As expected given Draco's low mass and ancient star formation history, it does not appear to have been dissolved by baryonic processes. We rule out cores larger than 487, 717, and 942 pc at 1 σ, 2 σ, and 3 σ confidence, respectively, thus imposing important constraints on the self-interacting DM cross section. Spherical models yield biased estimates for both the velocity anisotropy and the inferred slope. The circular velocity at our outermost data point (900 pc) is 24.19 − 2.97 + 6.31 km s − 1 . We infer a dynamical distance of 75.37 − 4.00 + 4.73 kpc and show that Draco has a modest LOS rotation, with v / σ = 0.22 ± 0.09 . Our results provide a new stringent test of the so-called "cusp–core" problem that can be readily extended to other dwarfs. [ABSTRACT FROM AUTHOR]

The satellite galaxy Crater II of the Milky Way is extremely cold and exceptionally diffuse. These unusual properties are challenging to understand in the standard model of cold dark matter. We use controlled N -body simulations to investigate the formation of Crater II in self-interacting dark matter (SIDM), where dark matter particles can scatter and thermalize. Taking the orbit motivated by the measurements from Gaia Early Data Release 3, we show a strong self-interacting cross section per particle mass of 60 cm ^2 g ^−1 is favored for Crater II. The simulated SIDM halo, with a 1 kpc core, leads to both a low stellar velocity dispersion and a large half-light radius for Crater II. These characteristics remain robust regardless of the initial stellar distribution.

We analyze four epochs of Hubble Space Telescope imaging over 18 yr for the Draco dwarf spheroidal galaxy. We measure precise proper motions for hundreds of stars and combine these with existing line-of-sight (LOS) velocities. This provides the first radially resolved 3D velocity dispersion profiles for any dwarf galaxy. These constrain the intrinsic velocity anisotropy and resolve the mass–anisotropy degeneracy. We solve the Jeans equations in oblate axisymmetric geometry to infer the mass profile. We find the velocity dispersion to be radially anisotropic along the symmetry axis and tangentially anisotropic in the equatorial plane, with a globally averaged value $\bar{{\beta }_{{\rm{B}}}}=-{0.20}_{-0.53}^{+0.28}$ , (where 1 – ${\beta }_{{\rm{B}}}\equiv \langle {v}_{\tan }^{2}\rangle /\langle {v}_{\mathrm{rad}}^{2}\rangle $ in 3D). The logarithmic dark matter (DM) density slope over the observed radial range, Γ _dark , is $-{0.83}_{-0.37}^{+0.32}$ , consistent with the inner cusp predicted in ΛCDM cosmology. As expected given Draco’s low mass and ancient star formation history, it does not appear to have been dissolved by baryonic processes. We rule out cores larger than 487, 717, and 942 pc at 1 σ , 2 σ , and 3 σ confidence, respectively, thus imposing important constraints on the self-interacting DM cross section. Spherical models yield biased estimates for both the velocity anisotropy and the inferred slope. The circular velocity at our outermost data point (900 pc) is ${24.19}_{-2.97}^{+6.31}\,\mathrm{km}\,{{\rm{s}}}^{-1}$ . We infer a dynamical distance of ${75.37}_{-4.00}^{+4.73}$ kpc and show that Draco has a modest LOS rotation, with $\left\langle v/\sigma \right\rangle =0.22\pm 0.09$ . Our results provide a new stringent test of the so-called “cusp–core” problem that can be readily extended to other dwarfs.

A fundamental question regarding the evolution of dwarf spheroidal galaxies is the identification of the key physical mechanisms responsible for gas depletion. Here, we focus on the study of stellar feedback in isolated dwarf spheroidal galaxies by performing numerical simulations using a modified version of the smoothed-particle hydrodynamics code GADGET-3. The Milky Way satellite Leo II (PGC 34176) in the Local Group was considered as our default model dwarf galaxy. The parameter space for the stellar feedback models was explored to match observational constraints of Leo II, such as residual gas mass, total mass within the tidal radius, star formation history, final stellar mass, stellar ages, and metallicity. Additionally, we examined the impact of the binary fraction of stars, initial mass function, dark matter halo mass, and initial gas reservoir. Many simulations revealed recent star formation quenching due to stellar feedback. In general, the gas depletion, expected star formation history, total mass of stars, and total mass within the tidal radius were adequately reproduced in the simulations when compared to observational estimates. However, there were discrepancies in the distribution of stellar ages and metallicities, which suggested that the cosmic gas infall would play a more complex role in our dwarf spheroidal galaxy than captured by a monolithic infall scenario. Our results suggest that currently quenched dwarf galaxies may not necessarily need to evolve within clusters or groups and that stellar feedback alone could be a sufficient factor in shaping at least some of these galaxies as we observe them today.

A fundamental prediction of the Lambda cold dark matter cosmology is the centrally divergent cuspy density profile of dark matter haloes. Density cusps render cold dark matter haloes resilient to tides, and protect dwarf galaxies embedded in them from full tidal disruption. The hierarchical assembly history of the Milky Way may therefore give rise to a population of “microgalaxies”; i.e., heavily stripped remnants of early accreted satellites, which can reach arbitrarily low luminosity. Assuming that the progenitor systems are dark matter dominated, we use an empirical formalism for tidal stripping to predict the evolution of the luminosity, size, and velocity dispersion of such remnants, tracing their tidal evolution across multiple orders of magnitude in mass and size. The evolutionary tracks depend sensitively on the progenitor distribution of stellar binding energies. We explore three cases that likely bracket most realistic models of dwarf galaxies: one where the energy distribution of the most tightly bound stars follows that of the dark matter, and two where stars are defined by either an exponential density or surface brightness profile. The tidal evolution in the size–velocity dispersion plane is quite similar for these three models, although their remnants may differ widely in luminosity. Microgalaxies are therefore best distinguished from globular clusters by the presence of dark matter; either directly, by measuring their velocity dispersion, or indirectly, by examining their tidal resilience. Our work highlights the need for further theoretical and observational constraints on the stellar energy distribution in dwarf galaxies.

The stellar initial mass function (IMF) describes the distribution of stellar masses that form in a given star formation event. The long main-sequence lifetimes of low-mass stars mean that the IMF in this regime (below ∼ 1 M _⊙ ) can be investigated through star counts. Ultrafaint dwarf galaxies are low-luminosity systems with ancient, metal-poor stellar populations. We investigate the low-mass IMF in four such systems (Reticulum II, Ursa Major II, Triangulum II, and Segue 1), using Hubble Space Telescope imaging data that reaches to ≲ 0.2 M _⊙ in each galaxy. The analysis techniques that we adopt depend on the number of low-mass stars in each sample. We use Kolmogorov–Smirnov tests for all four galaxies to determine whether their observed apparent magnitude distributions can reject a given combination of IMF parameters and binary fraction for the underlying population. We forward model 1000 synthetic populations for each combination of parameters, and reject those parameters only if each of the 1000 realizations reject the null hypothesis. We find that all four galaxies reject a variety of IMFs, and the IMFs that they cannot reject include those that are identical, or similar, to that of the stellar populations of the Milky Way. We determine the best-fit parameter values for the IMF in Reticulum II and Ursa Major II and find that the IMF in Reticulum II is generally consistent with that of the Milky Way, while the IMF in Ursa Major II is more bottom heavy. The interpretation of the results for Ursa Major II is complicated by possible contamination from two known background galaxy clusters.

The Draco Dwarf spheroidal (dSph) galaxy is one of the nearest and the most dark-matter-dominated satellites of the Milky Way. We obtained multiepoch near-infrared (NIR, JHK _s ) observations of the central region of Draco dSph covering a sky area of ∼21′ × 21′ using the WIRCam instrument at the 3.6 m Canada–France–Hawaii Telescope. Homogeneous JHK _s time-series photometry for 212 RR Lyrae (173 fundamental-mode, 24 first-overtone, and 15 mixed-mode variables) and five Anomalous Cepheids in Draco dSph are presented and used to derive their period–luminosity relations at NIR wavelengths for the first-time. The small scatter of ∼0.05 mag in these empirical relations for RR Lyrae stars is consistent with those in globular clusters and suggests a very small metallicity spread, up to ∼0.2 dex, among these centrally located variables. Based on empirically calibrated NIR period–luminosity–metallicity relations for RR Lyrae in globular clusters, we determined a distance modulus to Draco dSph of μ _RRL = 19.557 ± 0.026 mag. The calibrated K _s -band period–luminosity relations for Anomalous Cepheids in the Draco dSph and the Large Magellanic Cloud exhibit statistically consistent slopes but systematically different zero points, hinting at possible metallicity dependence of ∼ − 0.3 mag dex ^−1 . Finally, the apparent magnitudes of the tip of the red-giant branch in I and J bands also agree well with their absolute calibrations with the adopted RR Lyrae distance to Draco. Our recommended ∼1.5% precise RR Lyrae distance, D _Draco = 81.55 ± 0.98(statistical) ± 1.17(systematic) kpc, is the most accurate and precise distance to Draco dSph galaxy.

In this study, we compare the structural parameters of ultradiffuse galaxies (UDGs) to those of other dwarf galaxies and investigate whether UDGs form a distinct population. We observed deep $u^{\prime} $ -, $g^{\prime} $ -, and $r^{\prime} $ -band images (maximum limiting surface brightness [3 σ , 10″ × 10″] $u^{\prime} $ and $g^{\prime} $ : $\approx \ 30\,\mathrm{mag}\,{\mathrm{arcsec}}^{-2}$ ; $r^{\prime} $ : $\approx \,29\,\mathrm{mag}\,{\mathrm{arcsec}}^{-2}$ ) of A1656 (Coma cluster) and A262 with the Wendelstein Wide Field Imager at the 2.1 m Fraunhofer Wendelstein Telescope at the Wendelstein Observatory. We measure $u^{\prime} -g^{\prime} $ and $g^{\prime} -r^{\prime} $ colors and structural parameters using parametric fitting of tens of thousands of potential UDGs and other dwarf galaxies. Cluster members are identified and separated from diffuse background galaxies based on red sequence membership and location in the $u^{\prime} -g^{\prime} $ versus $g^{\prime} -r^{\prime} $ color–color diagram. We find 11 UDGs in A262 and 48 UDGs in A1656. The latter is 6 times more than van Dokkum et al. found in the overlapping region. By comparing the structural parameters of UDGs to non-UDGs in our sample and to spheroidals from the literature, we do not find any separation in all tested parameter spaces. Instead, UDGs form the diffuse end of the already well-known spheroidal population and slightly extend it. Furthermore, we find that the UDG definition used by Koda et al. and Yagi et al. mainly extends the definition by van Dokkum et al. toward ordinary spheroidals.

The recently discovered stellar system Ursa Major III/UNIONS 1 (UMa3/U1) is the faintest known Milky Way satellite to date. With a stellar mass of ${16}_{-5}^{+6}\,{M}_{\odot }$ and a half-light radius of 3 ± 1 pc, it is either the darkest galaxy ever discovered or the faintest self-gravitating star cluster known to orbit the Galaxy. Its line-of-sight velocity dispersion suggests the presence of dark matter, although current measurements are inconclusive because of the unknown contribution to the dispersion of potential binary stars. We use N -body simulations to show that, if self-gravitating, the system could not survive in the Milky Way tidal field for much longer than a single orbit (roughly 0.4 Gyr), which strongly suggests that the system is stabilized by the presence of large amounts of dark matter. If UMa3/U1 formed at the center of a ∼10 ^9 M _⊙ cuspy LCDM halo, its velocity dispersion would be predicted to be of order ∼1 km s ^−1 . This is roughly consistent with the current estimate, which, neglecting binaries, places σ _los in the range 1–4 km s ^−1 . Because of its dense cusp, such a halo should be able to survive the Milky Way tidal field, keeping UMa3/U1 relatively unscathed until the present time. This implies that UMa3/U1 is plausibly the faintest and densest dwarf galaxy satellite of the Milky Way, with important implications for alternative dark matter models and for the minimum halo mass threshold for luminous galaxy formation in the LCDM cosmology. Our results call for multi-epoch high-resolution spectroscopic follow-up to confirm the dark matter content of this extraordinary system.

A recent study found dynamical evidence of a supermassive black hole of ∼3 × 10 ^6 M _⊙ at the center of Leo I, the most distant dwarf spheroidal galaxy of the Milky Way. This black hole, comparable in mass to the Milky Way’s Sgr A*, places the system >2 orders of magnitude above the standard M _• – M _⋆ relation. We investigate the possibility, from a dynamical standpoint, that Leo I’s stellar system was originally much more massive and, thus, closer to the relation. Extreme tidal disruption from one or two close passages within the Milky Way’s virial radius could have removed most of its stellar mass. A simple analytical model suggests that the progenitor of Leo I could have experienced a mass loss in the range 32%–57% from a single pericenter passage, depending on the stellar velocity dispersion estimate. This mass-loss percentage increases to the range 66%–78% if the pericenter occurs at the minimum distance allowed by current orbital reconstructions. Detailed N -body simulations show that the mass loss could reach ∼90% with up to two passages, again with pericenter distances compatible with the minimum value allowed by Gaia data. Despite very significant uncertainties in the properties of Leo I, we reproduce its current position and velocity dispersion, as well as the final stellar mass enclosed in 1 kpc (∼5 × 10 ^6 M _⊙ ) within a factor

We present results from wide-field imaging of the resolved stellar populations of the dwarf spheroidal galaxies Cassiopeia III (And XXXII) and Perseus I (And XXXIII), two satellites in the outer stellar halo of the Andromeda galaxy (M31). Our WIYN pODI photometry traces the red giant star population in each galaxy to ∼2.5−3 half-light radii from the galaxy center. We use the tip of the red giant branch (TRGB) method to derive distances of ( m − M ) _0 = 24.62 ± 0.12 mag (839 ${}_{-45}^{+48}$ kpc, or ${156}_{-13}^{+16}$ kpc from M31) for Cas III and 24.47 ± 0.13 mag (738 ${}_{-45}^{+48}$ kpc, or 351 ${}_{-16}^{+17}$ kpc from M31) for Per I. These values are consistent within the errors with TRGB distances derived from a deeper Hubble Space Telescope study of the galaxies’ inner regions. For each galaxy, we derive structural parameters, total magnitude, and central surface brightness. We also place upper limits on the ratio of neutral hydrogen gas mass to optical luminosity, confirming the gas-poor nature of both galaxies. We combine our data set with corresponding data for the M31 satellite galaxy Lacerta I (And XXXI) from earlier work and search for substructure within the RGB star populations of Cas III, Per I, and Lac I. We find an overdense region on the west side of Lac I at a significance level of 2.5 σ –3 σ and a low-significance filament extending in the direction of M31. In Cas III, we identify two modestly significant overdensities near the center of the galaxy and another at two half-light radii. Per I shows no evidence for substructure in its RGB star population, which may reflect this galaxy’s isolated nature.

We present spectroscopic data for 16,369 stellar targets within and/or toward 38 dwarf spheroidal galaxies and faint star clusters within the Milky Way halo environment. All spectra come from observations with the multiobject, fiber-fed echelle spectrographs M2FS at the Magellan/Clay telescope or Hectochelle at the MMT, reaching a typical limiting magnitude G ≲ 21. Data products include processed spectra from all observations and catalogs listing estimates—derived from template model fitting—of line-of-sight velocity (median uncertainty 1.4 km s ^−1 ) effective temperature (255 K), (base-10 logarithm of) surface gravity (0.59 dex in cgs units), [Fe/H] (0.4 dex) and [Mg/Fe] (0.27 dex) abundance ratios. The sample contains multiepoch measurements for 3720 sources, with up to 15 epochs per source, enabling studies of intrinsic spectroscopic variability. The sample contains 6087 likely red giant stars (based on surface gravity), and 4492 likely members (based on line-of-sight velocity and Gaia-measured proper motion) of the target systems. The number of member stars per individual target system ranges from a few, for the faintest systems, to ∼850 for the most luminous. For most systems, our new samples extend over wider fields than have previously been observed; of the likely members in our samples, 820 lie beyond 2 times the projected half-light radius of their host system, and 42 lie beyond 5 R _half .

It has been suggested that a class of chemically peculiar metal-poor stars called iron-rich metal-poor (IRMP) stars formed from molecular cores with metal contents dominated by thermonuclear supernova nucleosynthesis. If this interpretation is accurate, then IRMP stars should be more common in environments where thermonuclear supernovae were important contributors to chemical evolution. Conversely, IRMP stars should be less common in environments where thermonuclear supernovae were not important contributors to chemical evolution. At constant [Fe/H] ≲ −1, the Milky Way’s satellite classical dwarf spheroidal (dSph) galaxies and the Magellanic Clouds have lower [ α /Fe] than the Milky Way field and globular cluster populations. This difference is thought to demonstrate the importance of thermonuclear supernova nucleosynthesis for the chemical evolution of the Milky Way’s satellite classical dSph galaxies and the Magellanic Clouds. We use data from the Sloan Digital Sky Survey Apache Point Observatory Galactic Evolution Experiment and Gaia to infer the occurrence of IRMP stars in the Milky Way’s satellite classical dSph galaxies η _dSph and the Magellanic Clouds η _Mag , as well as in the Milky Way field η _MWF and globular cluster populations η _MWGC . In order of decreasing occurrence, we find ${\eta }_{\mathrm{dSph}}={0.07}_{-0.02}^{+0.02}$ , ${\eta }_{\mathrm{Mag}}={0.037}_{-0.006}^{+0.007}$ , ${\eta }_{\mathrm{MWF}}={0.0013}_{-0.0005}^{+0.0006}$ , and a 1 σ upper limit η _MWGC < 0.00057. These occurrences support the inference that IRMP stars formed in environments dominated by thermonuclear supernova nucleosynthesis and that the time lag between the formation of the first and second stellar generations in globular clusters was longer than the thermonuclear supernova delay time.

The progenitor systems and explosion mechanisms responsible for the thermonuclear events observationally classified as Type Ia supernovae are uncertain and difficult to uniquely constrain using traditional observations of Type Ia supernova host galaxies, progenitors, light curves, and remnants. For the subset of thermonuclear events that are prolific producers of iron, we use published theoretical nucleosynthetic yields to identify a set of elemental abundance ratios infrequently observed in metal-poor stars but shared across a range of progenitor systems and explosion mechanisms: [Na, Mg, Co/Fe] < 0. We label stars with this abundance signature “iron-rich metal-poor,” or IRMP stars. We suggest that IRMP stars formed in environments dominated by thermonuclear nucleosynthesis and consequently that their elemental abundances can be used to constrain both the progenitor systems and explosion mechanisms responsible for thermonuclear explosions. We identify three IRMP stars in the literature and homogeneously infer their elemental abundances. We find that the elemental abundances of BD +80 245, HE 0533–5340, and SMSS J034249.53–284216.0 are best explained by the (double) detonations of sub-Chandrasekhar-mass CO white dwarfs. If our interpretation of IRMP stars is accurate, then they should be very rare in globular clusters and more common in the Magellanic Clouds and dwarf spheroidal galaxies than in the Milky Way’s halo. We propose that future studies of IRMP stars will quantify the relative occurrences of different thermonuclear event progenitor systems and explosion mechanisms.

We present the stellar parameters and chemical abundances of 30 elements for five stars located at large radii (3.5–10.7 times the half-light radius) in the Sextans dwarf spheroidal galaxy. We selected these stars using proper motions, radial velocities, and metallicities, and we confirm them as metal-poor members of Sextans with −3.34 ≤ [Fe/H] ≤ −2.64 using high-resolution optical spectra collected with the Magellan Inamori Kyocera Echelle spectrograph. Four of the five stars exhibit normal abundances of C (−0.34 ≤ [C/Fe] ≤ + 0.36), mild enhancement of the α elements Mg, Si, Ca, and Ti ([ α /Fe] = +0.12 ± 0.03), and unremarkable abundances of Na, Al, K, Sc, V, Cr, Mn, Co, Ni, and Zn. We identify three chemical signatures previously unknown among stars in Sextans. One star exhibits large overabundances ([X/Fe] > +1.2) of C, N, O, Na, Mg, Si, and K, and large deficiencies of heavy elements ([Sr/Fe] = −2.37 ± 0.25, [Ba/Fe] = −1.45 ± 0.20, [Eu/Fe] < + 0.05), establishing it as a member of the class of carbon-enhanced metal-poor stars with no enhancement of neutron-capture elements. Three stars exhibit moderate enhancements of Eu (+0.17 ≤ [Eu/Fe] ≤ + 0.70), and the abundance ratios among 12 neutron-capture elements are indicative of r -process nucleosynthesis. Another star is highly enhanced in Sr relative to heavier elements ([Sr/Ba] = +1.21 ± 0.25). These chemical signatures can all be attributed to massive, low-metallicity stars or their end states. Our results, the first for stars at large radius in Sextans, demonstrate that these stars were formed in chemically inhomogeneous regions, such as those found in ultra-faint dwarf galaxies.

Elucidating dark matter density profiles in Galactic dwarf satellites is essential to understanding not only the quintessence of dark matter, but also the evolution of the satellites themselves. In this work, we present the current constraints on dark matter densities in Galactic ultrafaint dwarf (UFD) and diffuse galaxies. Applying our constructed nonspherical mass models to the currently available kinematic data of the 25 UFDs and two diffuse satellites, we find that whereas most of the galaxies have huge uncertainties on the inferred dark matter density profiles, Eridanus II, Segue I, and Willman 1 favor cuspy central profiles even when considering effects of a prior bias. We compare our results with the simulated subhalos on the plane between the dark matter density at 150 pc and the pericenter distance. We find that the most observed satellites and the simulated subhalos are similarly distributed on this plane, except for Antlia 2, Crater 2, and Tucana 3, which are less than one-tenth of the density. Despite considerable tidal effects, the subhalos detected by commonly used subhalo finders have difficulty explaining such a huge deviation. We also estimate the dynamical mass-to-light ratios of the satellites and confirm the ratio is linked to stellar mass and metallicity. Tucana 3 deviates largely from these relations, while it follows the mass–metallicity relation. This indicates that Tucana 3 has a cored dark matter halo, despite a significant uncertainty in its ratios.

Dark matter is a key piece of the current cosmological scenario, with weakly interacting massive particles (WIMPs) a leading dark matter candidate. WIMPs have not been detected in their conventional parameter space (100 GeV ≲ M _χ ≲ 100 TeV), a mass range accessible with current Imaging Atmospheric Cherenkov Telescopes. As ultraheavy dark matter (UHDM; M _χ ≳ 100 TeV) has been suggested as an underexplored alternative to the WIMP paradigm, we search for an indirect dark matter annihilation signal in a higher mass range (up to 30 PeV) with the VERITAS γ -ray observatory. With 216 hr of observations of four dwarf spheroidal galaxies, we perform an unbinned likelihood analysis. We find no evidence of a γ -ray signal from UHDM annihilation above the background fluctuation for any individual dwarf galaxy nor for a joint-fit analysis, and consequently constrain the velocity-weighted annihilation cross section of UHDM for dark matter particle masses between 1 TeV and 30 PeV. We additionally set constraints on the allowed radius of a composite UHDM particle.

This paper presents improved constraints on the low-mass stellar initial mass function (IMF) of the Boötes I (Boo I) ultrafaint dwarf galaxy, based on our analysis of recent deep imaging from the Hubble Space Telescope. The identification of candidate stellar members of Boo I in the photometric catalog produced from these data was achieved using a Bayesian approach, informed by complementary archival imaging data for the Hubble Ultra Deep Field. Additionally, the existence of earlier-epoch data for the fields in Boo I allowed us to derive proper motions for a subset of the sources and thus identify and remove likely Milky Way stars. We were also able to determine the absolute proper motion of Boo I, and our result is in agreement with, but completely independent of, the measurement(s) by Gaia. The best-fitting parameter values of three different forms of the low-mass IMF were then obtained through forward modeling of the color–magnitude data for likely Boo I member stars within an approximate Bayesian computation Markov Chain Monte Carlo algorithm. The best-fitting single power-law IMF slope is $\alpha =-{1.95}_{-0.28}^{+0.32}$ , while the best-fitting broken power-law slopes are ${\alpha }_{1}=-{1.67}_{-0.57}^{+0.48}$ and ${\alpha }_{2}=-{2.57}_{-1.04}^{+0.93}$ . The best-fitting lognormal characteristic mass and width parameters are ${M}_{c}={0.17}_{-0.11}^{+0.05}{{ \mathcal M }}_{\odot }$ and $\sigma ={0.49}_{-0.20}^{+0.13}$ . These broken power-law and lognormal IMF parameters for Boo I are consistent with published results for the stars within the Milky Way, and thus it is plausible that Boötes I and the Milky Way are populated by the same stellar IMF.