Ultra-faint Dwarf Research Articles

Dark Matter Halo Properties of the Galactic Dwarf Satellites: Implication for Chemo-dynamical Evolution of the Satellites and a Challenge to Lambda Cold Dark Matter

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.

Discovery of a New Local Group Dwarf Galaxy Candidate in UNIONS: Boötes V

We present the discovery of Boötes V, a new ultra-faint dwarf galaxy (UFD) candidate. This satellite is detected as a resolved overdensity of stars during an ongoing search for new Local Group dwarf galaxy candidates in the UNIONS photometric data set. It has a physical half-light radius of pc, a V-band magnitude of −4.5 ± 0.4 mag, and resides at a heliocentric distance of approximately 100 kpc. We use Gaia DR3 astrometry to identify member stars, characterize the systemic proper motion, and confirm the reality of this faint stellar system. The brightest star in this system was followed up using Gemini GMOS-N long-slit spectroscopy and is measured to have a metallicity of [Fe/H] = −2.85 ± 0.10 dex and a heliocentric radial velocity of v r = 5.1 ± 13.4 km s−1. Boötes V is larger (in terms of scale radius), more distant, and more metal-poor than the vast majority of globular clusters. It is likely that Boötes V is an UFD, though future spectroscopic studies will be necessary to definitively classify this object.

Six More Ultra-faint Milky Way Companions Discovered in the DECam Local Volume Exploration Survey

We report the discovery of six ultra-faint Milky Way satellites identified through matched-filter searches conducted using Dark Energy Camera (DECam) data processed as part of the second data release of the DECam Local Volume Exploration (DELVE) survey. Leveraging deep Gemini/GMOS-N imaging (for four candidates) as well as follow-up DECam imaging (for two candidates), we characterize the morphologies and stellar populations of these systems. We find that these candidates all share faint absolute magnitudes (M V ≥ −3.2 mag) and old, metal-poor stellar populations (τ > 10 Gyr, [Fe/H] < −1.4 dex). Three of these systems are more extended (r 1/2 > 15 pc), while the other three are compact (r 1/2 < 10 pc). From these properties, we infer that the former three systems (Boötes V, Leo Minor I, and Virgo II) are consistent with ultra-faint dwarf galaxy classifications, whereas the latter three (DELVE 3, DELVE 4, and DELVE 5) are likely ultra-faint star clusters. Using data from the Gaia satellite, we confidently measure the proper motion of Boötes V, Leo Minor I, and DELVE 4, and tentatively detect a proper-motion signal from DELVE 3 and DELVE 5; no signal is detected for Virgo II. We use these measurements to explore possible associations between the newly discovered systems and the Sagittarius dwarf spheroidal, the Magellanic Clouds, and the Vast Polar Structure, finding several plausible associations. Our results offer a preview of the numerous ultra-faint stellar systems that will soon be discovered by the Vera C. Rubin Observatory and highlight the challenges of classifying the faintest stellar systems.

Towards ultra metal-poor DLAs: linking the chemistry of the most metal-poor DLA to the first stars

ABSTRACT We present new Keck/HIRES data of the most metal-poor damped Ly α (DLA) system currently known. By targeting the strongest accessible Fe ii features, we have improved the upper limit of the [Fe/H] abundance determination by ∼1 dex, finding [Fe/H] &amp;lt; −3.66 (2σ). We also provide the first upper limit on the relative abundance of an odd-atomic number element for this system [Al/H] &amp;lt; −3.82 (2σ). Our analysis thus confirms that this zabs ≃ 3.08 DLA is not only the most metal-poor DLA but also the most iron-poor DLA currently known. We use the chemistry of this DLA, combined with a stochastic chemical enrichment model, to probe its enrichment history. We find that this DLA is best modelled by the yields of an individual Population III progenitor rather than multiple Population III stars. We then draw comparisons with other relic environments and, particularly, the stars within nearby ultra-faint dwarf galaxies. We identify a star within Boötes I, with a similar chemistry to that of the DLA presented here, suggesting that it may have been born in a gas cloud that had similar properties. The extremely metal-poor DLA at redshift zabs ≃ 3.08 (i.e. ∼2 Gyr after the Big Bang) may reside in one of the least polluted environments in the early Universe.

The Progenitor of the Peculiar Galaxy NGC 3077

We present a study of the structural properties and metallicity distribution of the nearby peculiar galaxy NGC 3077. Using data from our survey of the M81 Group with the Hyper Suprime-Cam on board the Subaru Telescope, we construct deep color–magnitude diagrams that are used to probe the old red giant branch (RGB) population of NGC 3077. We map these stars out to and beyond the nominal tidal radius, which allows us to derive the structural properties and stellar content of the peripheral regions. We show that NGC 3077 has an extended stellar halo and pronounced “S-shaped” tidal tails that diverge from the radial profile of the inner region. The average metallicity of the old population in NGC 3077 is estimated from individual RGBs to be [M/H] = −0.98 ± 0.26, which decreases with the distance from the galaxy center as [M/H] = −0.17 dex . The metallicity of the S-shaped structure is similar to that of the regions lying at r ∼ 4 × R h ( ∼ 30 kpc), indicating that the stellar constituents of the tidal tails have come from the outer envelope of NGC 3077. These results suggest that this peculiar galaxy was probably a rather normal dwarf elliptical galaxy before the tidal interaction with M81 and M82. We also examine the evidence in our data set for the six recently reported ultrafaint dwarf candidates around NGC 3077. We recover a spatial overdensity of sources coinciding with only one of these.

Spectroscopic Analysis of Milky Way Outer Halo Satellites: Aquarius II and Boötes II

In this paper, we present a chemical and kinematic analysis of two ultrafaint dwarf galaxies (UFDs), Aquarius II (Aqu II) and Boötes II (Boo II), using Magellan/IMACS spectroscopy. We present the largest sample of member stars for Boo II (12), and the largest sample of red giant branch members with metallicity measurements for Aqu II (eight). In both UFDs, over 80% of targets selected based on Gaia proper motions turned out to be spectroscopic members. In order to maximize the accuracy of stellar kinematic measurements, we remove the identified binary stars and RR Lyrae variables. For Aqu II, we measure a systemic velocity of −65.3 ± 1.8 km s−1 and a metallicity of [Fe/H] = . When compared with previous measurements, these values display a ∼6 km s−1 difference in radial velocity and a decrease of 0.27 dex in metallicity. Similarly for Boo II, we measure a systemic velocity of km s−1, more than 10 km s−1 different from the literature, a metallicity almost 1 dex smaller at [Fe/H] = , and a velocity dispersion 3 times smaller at km s−1. Additionally, we derive systemic proper-motion parameters and model the orbits of both UFDs. Finally, we highlight the extremely dark-matter-dominated nature of Aqu II and compute the J-factor for both galaxies to aid searches of dark matter annihilation. Despite the small size and close proximity of Boo II, it is an intermediate target for the indirect detection of dark matter annihilation due to its low-velocity dispersion and corresponding low dark matter density.

GaiaData Release 3

Context.RR Lyrae stars are excellent tracers of the oldest stars (ages ≳ 9–10 Gyr) and standard candles for measuring the distance to stellar systems that are mainly composed of an old stellar population. TheGaiaThird Data Release (DR3) publishes a catalogue of full-sky RR Lyrae stars observed during the initial 34 months of science operations. They were processed through the Specific Object Study (SOS) pipeline, which was developed to validate and characterise Cepheids and RR Lyrae stars (SOS Cep&amp;RRL) observed byGaia.Aims.The main steps of the SOS Cep&amp;RRL pipeline are described in the documentation and papers accompanying previousGaiadata releases. For DR3, the pipeline was modified in its process: in addition to theGaiamultiband (G,GBP,GRP) time-series photometry, the epoch radial velocities measured for RR Lyrae and Cepheids with the Radial Velocity Spectrometer (RVS) on boardGaiawere also processed through the pipeline.Methods.The SOS Cep&amp;RRL validation of DR3 candidate RR Lyrae stars relies on diagnostics tools that include the period versusG-amplitude diagram and the period versusϕ21andϕ31parameters of theGlight-curve Fourier decomposition, as defined by a reference sample of bona fide RR Lyrae stars known in the literature (that we named Gold Sample). Great care was devoted to building a large and pure Gold Sample comprising more than 200 000 RR Lyrae stars. The SOS processing led to an initial catalogue of 271779 RR Lyrae stars that are listed in thevari_rrlyraetable of the DR3 archive. A thorough cleaning procedure was then performed to produce a final catalogue of SOS-confirmed DR3 RR Lyrae stars by dropping sources that clearly are contaminants or have an uncertain classification.Results.Multiband time-series photometry and characterisation are published inGaiaDR3 for a clean, validated sample of 270 905 RR Lyrae stars (174 947 fundamental-mode, 93 952 first-overtone, and 2006 double-mode RR Lyrae) that were confirmed and fully characterised by the SOS Cep&amp;RRL pipeline. They are distributed throughout the sky, including variables in 95 globular clusters and 25 Milky Way (MW) companions (the Magellanic Clouds, seven dwarf spheroidal galaxies, and 16 ultra-faint dwarf satellites of the MW). RVS time-series radial velocities are also published for 1096 RR Lyrae and 799 Cepheids of different types (classical, anomalous, and type II Cepheids). Of the 270 905 DR3 RR Lyrae stars, 200 294 are already known in the literature (Gold Sample) and 70 611 are new discoveries byGaia, to the best of our knowledge. An estimate of the interstellar absorption is published for 142 660 fundamental-mode RR Lyrae stars from a relation based on theG-band amplitude, the (G−GRP) colour and the pulsation period. Metallicities derived from the Fourier parameters of the light curves are also released for 133 559 RR Lyrae stars.Conclusions.The finalGaiaDR3 catalogue of confirmed RR Lyrae stars almost doubles the DR2 RR Lyrae catalogue. An increase in statistical significance, a better characterisation of the RR Lyrae pulsational and astrophysical parameters, and the improved astrometry published withGaiaEDR3 make the SOS Cep&amp;RRL DR3 sample the largest, most homogeneous, and parameter-richest catalogue of all-sky RR Lyrae stars published so far in the magnitude range from ⟨G⟩=7.64 mag (the magnitude of RR Lyr itsef, the class prototype) to ⟨G⟩=21.14 mag (the faintest RR Lyrae in the catalogue).

A possible dwarf galaxy satellite-of-satellite problem in ΛCDM

Dark matter clusters on all scales, and it is therefore expected that even substructure should host its own substructure. Using the Extragalactic Distance Database, we searched for dwarf-galaxy satellites of dwarf galaxies, that is, satellite-of-satellite galaxies, corresponding to these substructures of substructure. From investigation of Hubble Space Telescope data for 117 dwarf galaxies, we report the discovery of a previously unknown dwarf galaxy around the ultra-diffuse M96 companion M96-DF6 at 10.2 Mpc in the Leo-I group. We confirm its dwarf-galaxy nature as a stellar overdensity. Modeling its structural parameters with a growth-curve analysis, we find that it is an ultrafaint dwarf galaxy with a luminosity of 1.5 × 105 L⊙, which is 135 times fainter than its host. Based on its close projection to M96-DF6, it is unlikely that their association occurs simply by chance. We compare the luminosity ratio of this and three other known satellite-of-satellite systems with results from two different cosmological sets of ΛCDM simulations. For the observed stellar mass range of the central dwarf galaxies, the simulated dwarfs have a higher luminosity ratio between the central dwarf and its first satellite (≈10 000) than observed (≈100), excluding the Large and Small Magellanic Cloud (LMC/SMC) system. No simulated dwarf analog at these observed stellar masses has the observed luminosity ratio. This cannot be due to missing resolution, because it is the brightest subhalos that are missing. This may indicate that there is a satellite-of-satellite (SoS) problem for ΛCDM in the stellar-mass range between 106 and 108 M⊙, the regime of the classical dwarf galaxies. However, simulated dwarf models at both a lower (&lt; 106 M⊙) and higher (&gt; 108 M⊙) stellar mass have comparable luminosity ratios. For the higher-stellar-mass systems, the LMC/SMC system is reproduced by simulations; for the lower stellar masses, no observed satellite-of-satellite system has been observed to date. More observations and simulations of satellite-of-satellite systems are needed to assess whether the luminosity ratio is at odds with ΛCDM.

Devouring the Milky Way Satellites: Modeling Dwarf Galaxies with Galacticus

Dwarf galaxies are ubiquitous throughout the universe and are extremely sensitive to various forms of internal and external feedback. Over the last two decades, the census of dwarf galaxies in the Local Group and beyond has increased markedly. While hydrodynamic simulations (e.g., FIRE II, Mint Justice League) have reproduced the observed dwarf properties down to the ultrafaints, such simulations require extensive computational resources to run. In this work, we constrain the standard physical implementations in the semianalytic model Galacticus to reproduce the observed properties of the Milky Way satellites down to the ultrafaint dwarfs found in the Sloan Digital Sky Survey. We run Galacticus on merger trees from our high-resolution N–body simulation of a Milky Way analog. We determine the best-fit parameters by matching the cumulative luminosity function and luminosity–metallicity relation from both observations and hydrodynamic simulations. With the correct parameters, the standard physics in Galacticus can reproduce the observed luminosity function and luminosity–metallicity relation of the Milky Way dwarfs. In addition, we find a multidimensional match with half-light radii, velocity dispersions, and mass to light ratios at z = 0 down to M V ≤ −6 (L ≥ 104 L ⊙). In addition to successfully reproducing the properties of the z = 0 Milky Way satellite population, our modeled dwarfs have star formation histories that are consistent with those of the Local Group dwarfs.

On the dearth of C-enhanced metal-poor stars in the galactic bulge

ABSTRACTThe chemical fingerprints of the first stars are retained within the photospheres of ancient unevolved metal-poor stars. A significant fraction of these stellar fossils is represented by stars known as Carbon-Enhanced Metal-Poor (CEMP), $\mathrm{[C/Fe]} \gt +0.7$ and $\mathrm{[Fe/H]} \lt -2$, which are likely imprinted by low-energy primordial supernovae. These CEMP stars are largely observed in the Galactic halo and ultrafaint dwarf galaxies, with values reaching $\rm [C/Fe]=+4.5$. The Galactic bulge is predicted to host the oldest stars, but it shows a striking dearth of CEMP stars with $\rm [C/Fe]\gtrsim +2.0$. Here, we explore the possible reasons for this anomaly by performing a statistical analysis of the observations of metal-poor stars in combination with the predictions of Lambda cold dark matter models. We suggest that the dearth of CEMP stars with high $\mathrm{[C/Fe]}$ is not due to the low statistics of observed metal-poor stars but is the result of the different formation process of the bulge. N-body simulations show that the first star-forming haloes which end up in the bulge are characterized by the highest star formation rates. These rates enable the formation of rare massive first stars exploding as pair-instability supernovae (PISNe), which wash out the signature of primordial faint supernovae. We demonstrate that the mean $\mathrm{[C/Fe]}$ of first stars polluted environments decreases with the increasing contribution of PISNe. We conclude that the dearth of CEMP stars in the Galactic bulge indirectly probes the existence of elusive PISNe, and propose a novel method which exploits this lack to constrain the mass distribution of the first stars.

Evolution of binary stars in the early evolutionary phases of ultra-faint dwarf galaxies

ABSTRACT The dynamics of binary stars provides a unique avenue to gather insight into the study of the structure and dynamics of star clusters and galaxies. In this paper, we present the results of a set of N-body simulations aimed at exploring the evolution of binary stars during the early evolutionary phases of ultra-faint dwarf galaxies (UFD). In our simulations, we assume that the stellar component of the UFD is initially dynamically cold and evolves towards its final equilibrium after undergoing the violent relaxation phase. We show that the early evolutionary phases of the UFD significantly enhance the disruption of wide binaries and leave their dynamical fingerprints on the semimajor axis distribution of the surviving binaries as compared to models initially in equilibrium. An initially thermal eccentricity distribution is preserved except for the widest binaries for which it evolves towards a superthermal distribution; for a binary population with an initially uniform eccentricity distribution, memory of this initial distribution is rapidly lost for most binaries as wider binaries evolve to approach a thermal/superthermal distribution. The evolution of binaries is driven both by tidal effects due to the potential of the UFD dark matter halo and collisional effects associated to binary–binary/single star encounters. Collisional effects are particularly important within the clumpy substructure characterizing the system during its early evolution; in addition to enhancing binary ionization and evolution of the binary orbital parameters, encounters may lead to exchanges of either of the primordial binary components with one of the interacting stars.

The Supersonic Project: The Eccentricity and Rotational Support of SIGOs and DM GHOSts

A supersonic relative velocity between dark matter (DM) and baryons (the stream velocity) at the time of recombination induces the formation of low-mass objects with anomalous properties in the early universe. We widen the scope of the “Supersonic Project” paper series to include objects we term Dark Matter + Gas Halos Offset by Streaming (DM GHOSts)—diffuse, DM-enriched structures formed because of a physical offset between the centers of mass of DM and baryonic overdensities. We present an updated numerical investigation of DM GHOSts and Supersonically Induced Gas Objects (SIGOs), including the effects of molecular cooling, in high-resolution hydrodynamic simulations using the AREPO code. Supplemented by an analytical understanding of their ellipsoidal gravitational potentials, we study the population-level properties of these objects, characterizing their morphology, spin, radial mass, and velocity distributions in comparison to classical structures in non-streaming regions. The stream velocity causes deviations from sphericity in both the gas and DM components and lends greater rotational support to the gas. Low-mass (≲105.5 M ⊙) objects in regions of streaming demonstrate core-like rotation and mass profiles. Anomalies in the rotation and morphology of DM GHOSts could represent an early universe analog to observed ultra-faint dwarf galaxies with variations in DM content and unusual rotation curves.

Finding r-II Sibling Stars in the Milky Way with the Greedy Optimistic Clustering Algorithm

R-process enhanced stars with [Eu/Fe] ≥ +0.7 (so-called r-II stars) are believed to have formed in an extremely neutron-rich environment in which a rare astrophysical event (e.g., a neutron-star merger) occurred. This scenario is supported by the existence of an ultra-faint dwarf galaxy, Reticulum II, where most of the stars are highly enhanced in r-process elements. In this scenario, some small fraction of dwarf galaxies around the Milky Way were r enhanced. When each r-enhanced dwarf galaxy accreted to the Milky Way, it deposited many r-II stars in the Galactic halo with similar orbital actions. To search for the remnants of the r-enhanced systems, we analyzed the distribution of the orbital actions of N = 161 r-II stars in the solar neighborhood by using Gaia EDR3 data. Since the observational uncertainty is not negligible, we applied a newly developed greedy optimistic clustering method to the orbital actions of our sample stars. We found six clusters of r-II stars that have similar orbits and chemistry, one of which is a new discovery. Given the apparent phase-mixed orbits of the member stars, we interpret that these clusters are good candidates for remnants of completely disrupted r-enhanced dwarf galaxies that merged with the ancient Milky Way.

Understanding the origin of CEMP – no stars through ultra-faint dwarfs

ABSTRACT The origin of Carbon Enhanced Metal-Poor (CEMP-no) stars with low abundances of neutron-capture elements is still unclear. These stars are ubiquitous, found primarily in the Milky Way halo and ultra-faint dwarf galaxies (UFDs). To make a major step forward, we developed a data-calibrated model for Böotes I that simultaneously includes all carbon sources: supernovae and asymptotic giant branch (AGB) stars both from first (Pop III) stars, and subsequent normal star formation (Pop II). We demonstrate that each of these sources leaves a specific chemical signature in the gas, allowing us to identify the origin of present day CEMP-no stars through their location in the A(C)–[Fe/H] diagram. The CEMP stars with A(C) &amp;gt; 6 are predominantly enriched by AGB Pop II stars. We identify a new class of moderate CEMP-s stars with A(C) ∼7 and 0 &amp;lt; [Ba/Fe] &amp;lt; + 1, imprinted by winds from AGB stars. True Pop III descendants are predicted to have A(C) &amp;lt; 6 and a constant [C/Mg] with [Fe/H], in perfect agreement with observations in Böotes I and the Milky Way halo. For the first time we now have a complete picture of the origins of CEMP-no stars which can and will be verified with future observations.

Unravelling the mass spectrum of destroyed dwarf galaxies with the metallicity distribution function

ABSTRACTAccreted stellar populations are comprised of the remnants of destroyed galaxies, and often dominate the ‘stellar haloes’ of galaxies such as the Milky Way (MW). This ensemble of external contributors is a key indicator of the past assembly history of a galaxy. We introduce a novel statistical method that uses the unbinned metallicity distribution function (MDF) of a stellar population to estimate the mass spectrum of its progenitors. Our model makes use of the well-known mass–metallicity relation of galaxies and assumes Gaussian MDF distributions for individual progenitors: the overall MDF is thus a mixture of MDFs from smaller galaxies. We apply the method to the stellar halo of the MW, as well as the classical MW satellite galaxies. The stellar components of the satellite galaxies have relatively small sample sizes, but we do not find any evidence for accreted populations with L &amp;gt; Lhost/100. We find that the MW stellar halo has N ∼ 1−3 massive progenitors (L ≳ 108L⊙) within 10 kpc, and likely several hundred progenitors in total. We also test our method on simulations of MW-mass haloes, and find that our method is able to recover the true accreted population within a factor of 2. Future data sets will provide MDFs with orders of magnitude more stars, and this method could be a powerful technique to quantify the accreted populations down to the ultra-faint dwarf mass scale for both the MW and its satellites.

Metal Mixing in the r-process Enhanced Ultrafaint Dwarf Galaxy Reticulum II* *This paper includes data gathered with the 6.5 m Magellan Telescopes, located at Las Campanas Observatory, Chile.

The ultrafaint dwarf galaxy Reticulum II was enriched by a single rare and prolific r-process event. The r-process content of Reticulum II thus provides a unique opportunity to study metal mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find of the stars are r-process-enhanced, with a mean and unresolved intrinsic dispersion σ [Ba/H]<0.20. The homogeneous r-process abundances imply that Ret II’s metals are well mixed by the time the r-enhanced stars form, which simulations have shown requires at least 100 Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultrafaint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield r-process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from [Ba/H] and [Mg/Ca] that the r-enhanced stars in Ret II formed in the absence of substantial pristine gas accretion, perhaps indicating that ≈70% of Ret II stars formed after reionization.

Pegasus W: An Ultrafaint Dwarf Galaxy Outside the Halo of M31 Not Quenched by Reionization

We report the discovery of an ultrafaint dwarf (UFD) galaxy, Pegasus W, located on the far side of the Milky Way–M31 system and outside the virial radius of M31. The distance to the galaxy is kpc, measured using the luminosity of horizontal branch stars identified in Hubble Space Telescope optical imaging. The galaxy has a half-light radius (r h ) pc, mag, and a present-day stellar mass M ⊙. We identify sources in the color–magnitude diagram (CMD) that may be younger than ∼500 Myr, suggesting late-time star formation in the UFD galaxy, although further study is needed to confirm these are bona fide young stars in the galaxy. Based on fitting the CMD with stellar evolution libraries, Pegasus W shows an extended star formation history. Using the τ 90 metric (defined as the timescale by which the galaxy formed 90% of its stellar mass), the galaxy was quenched only Gyr ago, which is similar to the quenching timescale of a number of UFD satellites of M31 but significantly more recent than the UFD satellites of the Milky Way. Such late-time quenching is inconsistent with the more rapid timescale expected by reionization and suggests that, while not currently a satellite of M31, Pegasus W was nonetheless slowly quenched by environmental processes.

Timing the r-process Enrichment of the Ultra-faint Dwarf Galaxy Reticulum II

The ultra-faint dwarf galaxy Reticulum II (Ret II) exhibits a unique chemical evolution history, with % of its stars strongly enhanced in r-process elements. We present deep Hubble Space Telescope photometry of Ret II and analyze its star formation history. As in other ultra-faint dwarfs, the color–magnitude diagram is best fit by a model consisting of two bursts of star formation. If we assume that the bursts were instantaneous, then the older burst occurred around the epoch of reionization, forming ∼80% of the stars in the galaxy, while the remainder of the stars formed ∼3 Gyr later. When the bursts are allowed to have nonzero durations, we obtain slightly better fits. The best-fitting model in this case consists of two bursts beginning before reionization, with approximately half the stars formed in a short (100 Myr) burst and the other half in a more extended period lasting 2.6 Gyr. Considering the full set of viable star formation history models, we find that 28% of the stars formed within 500 ± 200 Myr of the onset of star formation. The combination of the star formation history and the prevalence of r-process-enhanced stars demonstrates that the r-process elements in Ret II must have been synthesized early in its initial star-forming phase. We therefore constrain the delay time between the formation of the first stars in Ret II and the r-process nucleosynthesis to be less than 500 Myr. This measurement rules out an r-process source with a delay time of several Gyr or more, such as GW170817.

Determining satellite infall times using machine learning

ABSTRACT A key unknown of the Milky Way (MW) satellites is their orbital history, and, in particular, the time they were accreted onto the MW system since it marks the point where they experience a multitude of environmental processes. We present a new methodology for determining infall times, namely using a neural network (NN) algorithm. The NN is trained on MW-analogues in the EAGLE hydrodynamical simulation to predict if a dwarf galaxy is at first infall or a backsplash galaxy and to infer its infall time. The resulting NN predicts with 85-per cent accuracy if a galaxy currently outside the virial radius is a backsplash satellite and determines the infall times with a typical 68-per cent confidence interval of 4.4 Gyr. Applying the NN to MW dwarfs with Gaia EDR3 proper motions, we find that all of the dwarfs within 300 kpc had been inside the Galactic halo. The overall MW satellite accretion rate agrees well with the theoretical prediction except for late times when the MW shows a second peak at a lookback time of 1.5 Gyr corresponding to the infall of the LMC and its satellites. We also find that the quenching times for ultrafaint dwarfs show no significant correlation with infall time and thus supporting the hypothesis that they were quenched during reionization. In contrast, dwarfs with stellar masses above 105 M⊙ are found to be consistent with environmental quenching inside the Galactic halo, with star-formation ceasing on average at $0.5^{+0.9}_{-1.2}$ Gyr after infall.

Detailed Chemical Abundances of Stars in the Outskirts of the Tucana II Ultrafaint Dwarf Galaxy* * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

We present chemical abundances and velocities of five stars between 0.3 and 1.1 kpc from the center of the Tucana II ultrafaint dwarf galaxy (UFD) from high-resolution Magellan/MIKE spectroscopy. We find that every star is deficient in metals (−3.6 < [Fe/H] < −1.9) and in neutron-capture elements as is characteristic of UFD stars, unambiguously confirming their association with Tucana II. Other chemical abundances (e.g., C, iron peak) largely follow UFD trends and suggest that faint core-collapse supernovae (SNe) dominated the early evolution of Tucana II. We see a downturn in [α/Fe] at [Fe/H] ≈ −2.8, indicating the onset of Type Ia SN enrichment and somewhat extended chemical evolution. The most metal-rich star has strikingly low [Sc/Fe] = −1.29 ± 0.48 and [Mn/Fe] = −1.33 ± 0.33, implying significant enrichment by a sub-Chandrasekhar mass Type Ia SN. We do not detect a radial velocity gradient in Tucana II ( km s−1 kpc−1), reflecting a lack of evidence for tidal disruption, and derive a dynamical mass of M ⊙. We revisit formation scenarios of the extended component of Tucana II in light of its stellar chemical abundances. We find no evidence that Tucana II had abnormally energetic SNe, suggesting that if SNe drove in situ stellar halo formation, then other UFDs should show similar such features. Although not a unique explanation, the decline in [α/Fe] is consistent with an early galactic merger triggering later star formation. Future observations may disentangle such formation channels of UFD outskirts.