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We present the star formation history (SFH) of the isolated (D ∼ 970 kpc) Local Group dwarf galaxy Wolf-Lundmark-Melotte (WLM) measured from colour-magnitude diagrams (CMDs) constructed from deep Hubble Space Telescope imaging. Our observations include a central (0.5 rh) and outer field (0.7 rh) that reach below the oldest main-sequence turn-off. WLM has no early dominant episode of star formation: 20 per cent of its stellar mass formed by ∼12.5 Gyr ago (z ∼ 5). It also has an SFR that rises to the present with 50 per cent of the stellar mass within the most recent 5 Gyr (z < 0.7). There is evidence of a strong age gradient: the mean age of the outer field is 5 Gyr older than the inner field despite being only 0.4 kpc apart. Some models suggest such steep gradients are associated with strong stellar feedback and dark-matter core creation. The SFHs of real isolated dwarf galaxies and those from the Feedback in Realistic Environment suite are in good agreement for M*(z = 0) ∼ 107-109M☉, but in worse agreement at lower masses (M*(z = 0) ∼ 105-107 M☉). These differences may be explainable by systematics in the models (e.g. reionization model) and/or observations (HST field placement). We suggest that a coordinated effort to get deep CMDs between HST/JWST (crowded central fields) and WFIRST (wide-area halo coverage) is the optimal path for measuring global SFHs of isolated dwarf galaxies.

We explore the radial variation of star formation histories (SFHs) in dwarf galaxies simulated with Feedback In Realistic Environments (FIRE) physics. The sample contains 26 field dwarf galaxies with Mstar = 105–109 M. We find age gradients are common in our dwarfs, with older stars dominant at large radii. The strength of the gradient correlates with overall galaxy age such that earlier star formation produces a more pronounced gradient. The relation between formation time and strength of the gradient is driven by both mergers and star formation feedback. Mergers can both steepen and flatten the age gradient depending on the timing of the merger and SFHs of the merging galaxy. In galaxies without significant mergers, feedback pushes stars to the outskirts. The strength of the age gradient is determined by the subsequent evolution of the galaxy. Galaxies with weak age gradients constantly grow to z = 0, meaning that young star formation occurs at a similar radius to which older stars are heated to. In contrast, galaxies with strong age gradients tend to maintain a constant half-mass radius over time. If real galaxies have age gradients as we predict, stellar population studies that rely on sampling a limited fraction of a galaxy can give a biased view of its global SFH. Central fields can be biased young by Gyrs while outer fields are biased old. Fields positioned near the 2D half-light radius will provide the least biased measure of a dwarf galaxy’s global SFH.

The shape of the low-mass (faint) end of the galaxy stellar mass function (SMF) or ultraviolet luminosity function (UVLF) at |$z \gtrsim 6$| is an open question for understanding which galaxies primarily drove cosmic reionization. Resolved photometry of Local Group low-mass galaxies allows us to reconstruct their star formation histories, stellar masses, and UV luminosities at early times, and this fossil record provides a powerful 'near-far' technique for studying the reionization-era SMF/UVLF, probing orders of magnitude lower in mass than direct HST / JWST observations. Using 882 low-mass (⁠|$M_{\rm star}\lesssim 10^{9}\, \rm {M_\odot }$|⁠) galaxies across 11 Milky Way (MW)- and Local Group-analogue environments from the FIRE-2 cosmological baryonic zoom-in simulations, we characterize their progenitors at |$z=6\!-\!9$|⁠ , the mergers/disruption of those progenitors over time, and how well their present-day fossil record traces the high-redshift SMF. A present-day galaxy with |$M_{\rm star}\sim 10^5\, \rm {M_\odot }$| (⁠|$\sim 10^9\, \rm {M_\odot }$|⁠) had |$\approx 1$| (⁠|$\approx 30$|⁠) progenitors at |$z\approx 7$|⁠ , and its main progenitor comprised |$\approx 100~{{\ \rm per\ cent}}$| (⁠|$\approx 10~{{\ \rm per\ cent}}$|⁠) of the total stellar mass of all its progenitors at |$z\approx 7$|⁠. We show that although only |$\sim 15~{{\ \rm per\ cent}}$| of the early population of low-mass galaxies survives to present day, the fossil record of surviving Local Group galaxies accurately traces the low-mass slope of the SMF at |$z \sim 6 \!-\! 9$|⁠. We find no obvious mass dependence to the mergers and accretion, and show that applying this reconstruction technique to just low-mass galaxies at |$z = 0$| and not the MW/M31 hosts correctly recovers the slope of the SMF down to |$M_{\rm star} \sim 10^{4.5}\, \rm {{\rm M}_{\odot }}$| at |$z \gtrsim 6$|⁠. Thus, we validate the 'near-far' approach as an unbiased tool for probing low-mass reionization-era galaxies. [ABSTRACT FROM AUTHOR]

The oldest stars in the Milky Way (MW) bear imprints of the Galaxy's early assembly history. We use FIRE cosmological zoom-in simulations of three MW-mass disc galaxies to study the spatial distribution, chemistry, and kinematics of the oldest surviving stars (z form ≳ 5) in MW-like galaxies. We predict the oldest stars to be less centrally concentrated at z = 0 than stars formed at later times as a result of two processes. First, the majority of the oldest stars are not formed in situ but are accreted during hierarchical assembly. These ex situ stars are deposited on dispersion-supported, halo-like orbits but dominate over old stars formed in situ in the solar neighbourhood, and in some simulations, even in the galactic centre. Secondly, old stars formed in situ are driven outwards by bursty star formation and energetic feedback processes that create a time-varying gravitational potential at z ≳ 2, similar to the process that creates dark matter cores and expands stellar orbits in bursty dwarf galaxies. The total fraction of stars that are ancient is more than an order of magnitude higher for sight lines away from the bulge and inner halo than for inward-looking sight lines. Although the task of identifying specific stars as ancient remains challenging, we anticipate that million-star spectral surveys and photometric surveys targeting metal-poor stars already include hundreds of stars formed before z = 5. We predict most of these targets to have higher metallicity (-3 < [Fe/H] < -2) than the most extreme metal-poor stars.

We investigate the merger histories of isolated dwarf galaxies based on a suite of 15 highresolution cosmological zoom-in simulations, all with masses of Mhalo ≈ 1010M⊙ (and M* ~ 105 - 107M⊙) at z= 0, from the Feedback in Realistic Environments project. The stellar populations of these dwarf galaxies at z= 0 are formed essentially entirely 'in situ': over 90 per cent of the stellar mass is formed in the main progenitor in all but two cases, and all 15 of the galaxies have > 70 per cent of their stellar mass formed in situ. Virtually all galaxy mergers occur prior to z~ 3, meaning that accreted stellar populations are ancient. On average, our simulated dwarfs undergo five galaxy mergers in their lifetimes, with typical pre-merger galaxy mass ratios that are less than 1:10. This merger frequency is generally comparable to what has been found in dissipationless simulations when coupled with abundance matching. Two of the simulated dwarfs have a luminous satellite companion at z= 0. These ultra-faint dwarfs lie at or below current detectability thresholds but are intriguing targets for nextgeneration facilities. The small contribution of accreted stars makes it extremely difficult to discern the effects of mergers in the vast majority of dwarfs either photometrically or using resolved-star colour-magnitude diagrams (CMDs). The important implication for near-field cosmology is that star formation histories (SFHs) of comparably massive galaxies derived from resolved CMDs should trace the build-up of stellar mass in one main system across cosmic time as opposed to reflecting the contributions of many individual SFHs of merged dwarfs.

The shape of a galaxy's spatially unresolved, globally integrated 21-cm emission line depends on its internal gas kinematics: galaxies with rotationally supported gas discs produce double-horned profiles with steep wings, while galaxies with dispersion-supported gas produce Gaussian-like profiles with sloped wings. Using mock observations of simulated galaxies from the FIRE project, we show that one can therefore constrain a galaxy's gas kinematics from its unresolved 21-cm line profile. In particular, we find that the kurtosis of the 21-cm line increases with decreasing V/σ and that this trend is robust across a wide range of masses, signal-to-noise ratios, and inclinations. We then quantify the shapes of 21-cm line profiles from a morphologically unbiased sample of ~2000 low-redshift, HI-detected galaxies with M star = 107-11 M☉ and compare to the simulated galaxies. At M star ≳ 1010 M☉, both the observed and simulated galaxies produce double-horned profiles with low kurtosis and steep wings, consistent with rotationally supported discs. Both the observed and simulated line profiles become more Gaussian like (higher kurtosis and less-steep wings) at lower masses, indicating increased dispersion support. However, the simulated galaxies transition from rotational to dispersion support more strongly: at M star 108-10 M, most of the simulations produce more Gaussian-like profiles than typical observed galaxies with similar mass, indicating that gas in the low-mass simulated galaxies is, on average, overly dispersion supported. Most of the lower-mass-simulated galaxies also have somewhat lower gas fractions than the median of the observed population. The simulations nevertheless reproduce the observed line-width baryonic Tully-Fisher relation, which is insensitive to rotational versus dispersion support.

We study the z = 0 gas kinematics, morphology and angular momentum content of isolated galaxies in a suite of cosmological zoom-in simulations from the FIRE project spanning Mstar = 106-11M⊙. Gas becomes increasingly rotationally supported with increasing galaxy mass. In the lowest mass galaxies (Mstar < 108M⊙), gas fails to form a morphological disc and is primarily dispersion and pressure supported. At intermediate masses (Mstar = 108-10M⊙), galaxies display a wide range of gas kinematics and morphologies, from thin, rotating discs to irregular spheroids with negligible net rotation. All the high-mass (Mstar = 1010-11M⊙) galaxies form rotationally supported gas discs. Many of the haloes whose galaxies fail to form discs harbour high angular momentum gas in their circumgalactic medium. The ratio of the specific angular momentum of gas in the central galaxy to that of the dark matter halo increases significantly with galaxy mass, from 〈jgas〉/〈jDM〉 ~ 0.1 at Mstar = 106-7M⊙ to 〈jgas〉/〈jDM〉 ~ 2 at Mstar = 1010-11M⊙. The reduced rotational support in the lowest mass galaxies owes to (a) stellar feedback and the UV background suppressing the accretion of high angular momentum gas at late times, and (b) stellar feedback driving large non-circular gas motions. We broadly reproduce the observed scaling relations between galaxy mass, gas rotation velocity, size and angular momentum, but may somewhat underpredict the incidence of disky, high angular momentum galaxies at the lowest observed masses (Mstar = (106- 2 × 107) M⊙). Stars form preferentially from low angular momentum gas near the galactic centre and are less rotationally supported than gas. The common assumption that stars follow the same rotation curve as gas thus substantially overestimates the simulated galaxies' stellar angular momentum, particularly at low masses.

We present a suite of 15 cosmological zoom-in simulations of isolated dark matter haloes, all with masses of Mhalo ≈ 1010M⊙ at z = 0, in order to understand the relationship among halo assembly, galaxy formation and feedback's effects on the central density structure in dwarf galaxies. These simulations are part of the Feedback in Realistic Environments (FIRE) project and are performed at extremely high resolution (mbaryon = 500M⊙, mdm = 2500M⊙). The resultant galaxies have stellar masses that are consistent with rough abundance matching estimates, coinciding with the faintest galaxies that can be seen beyond the virial radius of the Milky Way (M*/M⊙ ≈ 105 - 107). This non-negligible spread in stellar mass at z = 0 in haloes within a narrow range of virial masses is strongly correlated with central halo density or maximum circular velocity Vmax, both of which are tightly linked to halo formation time. Much of this dependence of M* on a second parameter (beyond Mhalo) is a direct consequence of the Mhalo ~ 1010M⊙ mass scale coinciding with the threshold for strong reionization suppression: the densest, earliest-forming haloes remain above the UV-suppression scale throughout their histories while late-forming systems fall below the UV-suppression scale over longer periods and form fewer stars as a result. In fact, the latest-forming, lowest-concentration halo in our suite fails to form any stars. Haloes that form galaxies with M ≳ 2 × 106 M⊙ have reduced central densities relative to dark-matter-only simulations, and the radial extent of the density modifications is well-approximated by the galaxy half-mass radius r1/2. Lower-mass galaxies do not modify their host dark matter haloes at the mass scale studied here. This apparent stellar mass threshold of M ≈ 2 × 106-2 × 10-4 Mhalo is broadly consistent with previous work and provides a testable prediction of FIRE feedback models in Λcold dark matter.

We constrain the evolution of the galaxy stellar mass function from 2 2 × 105M⊙ at z = 0). The most recent studies from ZFOURGE and CANDELS also suggest flatter faint end slopes that are consistent with our results, but with a lower degree of precision. This work illustrates the strong connections between low and high-z observations when viewed through the lens of lambda cold dark matter numerical simulations.

We present novel constraints on the underlying galaxy formation physics (e.g. mass-loading factor, star formation history, and metal retention) at z ≳ 7 for the low-mass (M * ∼ 105 M⊙) Local Group ultrafaint dwarf galaxy (UFD) Eridanus ii (Eri ii). Using a hierarchical Bayesian framework, we apply a one-zone chemical evolution model to Eri ii 's CaHK-based photometric metallicity distribution function (MDF; [Fe/H]) and find that the evolution of Eri ii is well characterized by a short, exponentially declining star formation history (⁠|$\tau _\text{SFH}=0.39\pm _{0.13}^{0.18}$| Gyr), a low star formation efficiency (⁠|$\tau _\text{SFE}=27.56\pm _{12.92}^{25.14}$| Gyr), and a large mass-loading factor (⁠|$\eta =194.53\pm _{42.67}^{33.37}$|⁠). Our results are consistent with Eri ii forming the majority of its stars before the end of reionization. The large mass-loading factor implies strong outflows in the early history of Eri ii and is in good agreement with theoretical predictions for the mass scaling of galactic winds. It also results in the ejection of >90 per cent of the metals produced in Eri ii. We make predictions for the distribution of [Mg/Fe]–[Fe/H] in Eri ii as well as the prevalence of ultra metal-poor stars, both of which can be tested by future chemical abundance measurements. Spectroscopic follow-up of the highest metallicity stars in Eri ii ([Fe/H] > −2) will greatly improve model constraints. Our new framework can readily be applied to all UFDs throughout the Local Group, providing new insights into the underlying physics governing the evolution of the faintest galaxies in the reionization era. [ABSTRACT FROM AUTHOR]

We perform cosmological hydrodynamical simulations to study the formation of proto-globular cluster candidates in progenitors of present-day dwarf galaxies |$(M_{\rm vir} \approx 10^{10}\, {\rm M}_\odot$| at z = 0) as part of the 'Feedback in Realistic Environment' (FIRE) project. Compact (r 1/2 < 30 pc), relatively massive (0.5 × 105 ≲ M ⋆/M⊙ ≲ 5 × 105), self-bound stellar clusters form at 11 ≳ z ≳ 5 in progenitors with |$M_{\rm vir} \approx 10^9\, \mathrm{M}_{\odot }$|⁠. Cluster formation is triggered when at least |$10^7\, \mathrm{M}_{\odot }$| of dense, turbulent gas reaches |$\Sigma _{\rm gas} \approx 10^4\, {\rm M}_\odot \, {\rm pc}^{-2}$| as a result of the compressive effects of supernova feedback or from cloud–cloud collisions. The clusters can survive for |$2-3\, {\rm Gyr}$|⁠ ; absent numerical effects, they could possibly survive substantially longer, perhaps to z = 0. The longest lived clusters are those that form at significant distance – several hundreds of pc – from their host galaxy. We therefore predict that globular clusters forming in progenitors of present-day dwarf galaxies will be offset from any pre-existing stars within their host dark matter haloes as opposed to deeply embedded within a well-defined galaxy. Properties of the nascent clusters are consistent with observations of some of the faintest and most compact high-redshift sources in Hubble Space Telescope lensing fields and are at the edge of what will be detectable as point sources in deep imaging of non-lensed fields with JWST. By contrast, the star clusters' host galaxies will remain undetectable. [ABSTRACT FROM AUTHOR]

We present the method behind HubPUG , a software tool built for recovering systemic proper motions (PMs) of targets (e.g. clusters or resolved galaxies) in Hubble Space Telescope (HST) fields with two epochs of observations by utilizing stars observed by Gaia as a foreground frame of reference. HST PM experiments have typically relied on the use of distant background galaxies or quasi-stellar objects (QSOs) as stationary sources against which to measure PMs. Without consistent profiles, background galaxies are more difficult to centroid, but benefit on-aggregate from their large numbers. QSOs, though they can be fit with stellar point-spread functions, are sparse, with most fields containing none. Historically, the use of stars as references against which to measure PMs would have been difficult because they have individual PMs of their own. However, Gaia has now provided positions and PMs for over 1.4 billion stars, which are much more likely to be well-imaged in the fields around targets versus background sources, have predictable stellar profiles, and require less observing time per-image for good signal-to-noise. This technique allows us to utilize the power of Gaia to measure the PM of targets too faint for Gaia to observe itself. We have recovered PMs for the Milky Way satellites Sculptor and Draco with comparable uncertainties over HST -only and Gaia -only measurements, limited primarily by the current capabilities of the Gaia data. We also show the promise of this method for satellites of M31 with a new PM measurement for Andromeda VII. [ABSTRACT FROM AUTHOR]

We report discovery of a bright, nearby (⁠|$G = 13.8;\, \, d = 480\, \rm pc$|⁠) Sun-like star orbiting a dark object. We identified the system as a black hole candidate via its astrometric orbital solution from the Gaia mission. Radial velocities validated and refined the Gaia solution, and spectroscopy ruled out significant light contributions from another star. Joint modelling of radial velocities and astrometry constrains the companion mass of |$M_2 = 9.62\pm 0.18\, \mathrm{M}_{\odot }$|⁠. The spectroscopic orbit alone sets a minimum companion mass of |$M_2\gt 5\, \mathrm{M}_{\odot }$|⁠ ; if the companion were a |$5\, \mathrm{M}_{\odot }$| star, it would be 500 times more luminous than the entire system. These constraints are insensitive to the mass of the luminous star, which appears as a slowly rotating G dwarf (⁠|$T_{\rm eff}=5850\, \rm K$|⁠ , log  g = 4.5, |$M=0.93\, \mathrm{M}_{\odot }$|⁠), with near-solar metallicity (⁠|$\rm [Fe/H] = -0.2$|⁠) and an unremarkable abundance pattern. We find no plausible astrophysical scenario that can explain the orbit and does not involve a black hole. The orbital period, P orb = 185.6 d, is longer than that of any known stellar-mass black hole binary. The system's modest eccentricity (e = 0.45), high metallicity, and thin-disc Galactic orbit suggest that it was born in the Milky Way disc with at most a weak natal kick. How the system formed is uncertain. Common envelope evolution can only produce the system's wide orbit under extreme and likely unphysical assumptions. Formation models involving triples or dynamical assembly in an open cluster may be more promising. This is the nearest known black hole by a factor of 3, and its discovery suggests the existence of a sizable population of dormant black holes in binaries. Future Gaia releases will likely facilitate the discovery of dozens more. [ABSTRACT FROM AUTHOR]

We use a semi-analytic model for globular cluster (GC) formation built on dark matter merger trees to explore the relative role of formation physics and hierarchical assembly in determining the properties of GC populations. Many previous works have argued that the observed linear relation between total GC mass and halo mass points to a fundamental GC–dark matter connection or indicates that GCs formed at very high redshift before feedback processes introduced non-linearity in the baryon-to-dark matter mass relation. We demonstrate that at |$M_{\rm vir}(z=0) \gtrsim 10^{11.5} \, \mathrm{M}_{\odot }$|⁠, a constant ratio between halo mass and total GC mass is in fact an almost inevitable consequence of hierarchical assembly: by the central limit theorem, it is expected at z  = 0 independent of the GC-to-halo mass relation at the time of GC formation. The GC-to-halo mass relation at |$M_{\rm vir}(z=0) \lt 10^{11.5} \, \mathrm{M}_{\odot }$| is more sensitive to the details of the GC formation process. In our fiducial model, GC formation occurs in galaxies when the gas surface density exceeds a critical value. This model naturally predicts bimodal GC colour distributions similar to those observed in nearby galaxies and reproduces the observed relation between GC system metallicity and halo mass. It predicts that the cosmic GC formation rate peaked at z  ∼ 4, too late for GCs to contribute significantly to the UV luminosity density during reionization. [ABSTRACT FROM AUTHOR]

The Local Group hosts a number of dwarf galaxies that show evidence of periods of little to no star formation. We use a suite of cosmological simulations to study how star formation is reignited in such galaxies. We focus on isolated galaxies at |$z$|  = 0 with halo masses between 9.2 × 108 and 8.4 × 109 M⊙, where star formation is typically shut off by reionization or by supernova feedback. Nearly 20 per cent of these simulated galaxies later restart star formation due to interactions with streams of gas in the intergalactic medium, indicating that this mechanism is relatively common in this mass range and that many isolated dwarfs at |$z$|  = 0 may not have been isolated throughout their histories. While high ram pressure interactions lead to stripping, the encounters that reignite star formation are low density and/or low velocity and thus low ram pressure, resulting in compression of the hot gas in the haloes of our dwarfs. The gas mass bound up in hot haloes can be substantial – at least an order of magnitude greater than the mass contained in H  i. Consequently, we find that dwarfs that have experienced reignition tend to be more H  i -rich and have a higher |$M_{\rm H\, {\small I}}/M_{*}$| ratio at |$z$|  = 0 than galaxies with continuous star formation. Using this fact, we identify galaxies in the Local Volume that might have 'gappy' star formation histories, and can be studied by the Hubble Space Telescope or the James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

We constrain the evolution of the galaxy stellar mass function from 25 M⊙ by combining star formation histories of Milky Way satellite galaxies derived from deep Hubble Space Telescope observations with merger trees from the ELVIS suite of N-body simulations. This approach extends our understanding more than two orders of magnitude lower in stellar mass than is currently possible by direct imaging. We find the faint end slopes of the mass functions to be α = 1.42.0.05+0.07 at z = 2 and α = 1.57-0.06+0.06 at z = 5, and show the slope only weakly evolves from z = 5 to z = 0. Our findings are in stark contrast to a number of direct detection studies that suggest slopes as steep as α = -1.9 at these epochs. Such a steep slope would result in an order of magnitude too many luminous Milky Way satellites in a mass regime that is observationally complete (M* > 2 × 105M⊙ at z = 0). The most recent studies from ZFOURGE and CANDELS also suggest flatter faint end slopes that are consistent with our results, but with a lower degree of precision. This work illustrates the strong connections between low and high-z observations when viewed through the lens of lambda cold dark matter numerical simulations. [ABSTRACT FROM AUTHOR]