Formation History Research Articles

The Outflowing [O ii] Nebulae of Compact Starburst Galaxies at z ∼ 0.5

High-velocity outflows are ubiquitous in compact, massive (M * ∼ 1011 M ⊙), z ∼ 0.5 galaxies with extreme star formation surface densities (ΣSFR ∼ 2000 M ⊙ yr−1 kpc−2). We have previously detected and characterized these outflows using Mg ii absorption lines. To probe their full extent, we present Keck/KCWI integral field spectroscopy of the [O ii] and Mg ii emission nebulae surrounding all of the 12 galaxies in this study. We find that [O ii] is more effective than Mg ii in tracing low surface brightness, extended emission in these galaxies. The [O ii] nebulae are spatially extended beyond the stars, with radial extent R 90 between 10 and 40 kpc. The nebulae exhibit nongravitational motions, indicating galactic outflows with maximum blueshifted velocities ranging from −335 to −1920 km s−1. The outflow kinematics correlate with the bursty star formation histories of these galaxies. Galaxies with the most recent bursts of star formation (within the last <3 Myr) exhibit the highest central velocity dispersions (σ ≳ 400 km s−1), while the oldest bursts have the lowest-velocity outflows. Many galaxies exhibit both high-velocity cores and more extended, slower-moving gas indicative of multiple outflow episodes. The slower, larger outflows occurred earlier and have decelerated as they propagate into the circumgalactic medium and mix on timescales ≳50 Myr.

Spatially Resolved Comparison of SFRs from UV and Hα in GASP Gas-stripped Galaxies

Star formation rates (SFRs) in galaxies offer a view of various physical processes across them, and are measured using various tracers, such as Hα and ultraviolet (UV). Different physical mechanisms can affect Hα and UV emission, resulting in a discrepancy in the corresponding SFR estimates (ΔSFR). We investigate the effects of ram pressure on the SFR measurements and ΔSFR across five galaxies from the GASP survey caught in the late stages of gas stripping due to ram pressure. We probe spatially resolved ΔSFR at pixel scales of 0.5 kpc, and compare disks to tails and regions dominated by the dense gas to diffuse ionized gas (DIG) regions. The regions dominated by dense gas show similar SFR values for UV and Hα tracers, while the regions dominated by the DIG show up to 0.5 dex higher SFR(UV). There is a large galaxy-by-galaxy variation in ΔSFR, with no difference between the disks and the tails. We discuss the potential causes of variations in ΔSFR between the dense gas and DIG areas. We conclude that the dominant cause of discrepancy are recent variations in star formation histories, where star formation recently dropped in the DIG-dominated regions leading to changes in ΔSFR. The areal coverage of the tracers shows areas with Hα and no UV emission; these areas have LINER-like emission (excess in [O i λ6300]/Hα line ratio), indicating that they are ionized by processes other than star formation.

UVCANDELS: Catalogs of Photometric Redshifts and Galaxy Physical Properties

The UltraViolet imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey Fields (UVCANDELS) program provides deep Hubble Space Telescope (HST) F275W and F435W imaging over four CANDELS fields (GOODS-N, GOODS-S, COSMOS, and Extended Groth Strip). We combine this newly acquired UV imaging with existing HST imaging from CANDELS as well as existing ancillary data to obtain robust photometric redshifts and reliable estimates for galaxy physical properties for over 150,000 galaxies in the ∼430 arcmin2 UVCANDELS area. Here, we leverage the power of the new UV photometry to not only improve the photometric redshift measurements in these fields, but also constrain the full redshift probability distribution combining multiple redshift-fitting tools. Furthermore, using the full UV-to-IR photometric data set, we measure the galaxy physical properties by fitting templates from population synthesis models with two different parameterizations (flexible and fixed form) of the star formation histories (SFHs). Compared to the flexible SFH parameterization, we find that the fixed-form SFHs systematically underestimate the galaxy stellar masses, both at the low-mass (≲109 M ⊙) and high-mass (≳1010 M ⊙) end, by as much as ∼0.5 dex. This underestimation is primarily due the limited ability of fixed-form SFH parameterization to simultaneously capture the chaotic nature of star formation in these galaxies.

The HST Large Programme on ω Centauri

We present a study of the white dwarf (WD) cooling sequence (CS) in the globular cluster (GC) Omega Centauri (or NGC 5139; hereafter, ω Cen), the primary goal of a dedicated Hubble Space Telescope (HST) programme. We find that the peak at the termination of the WD CS is located at mF606W = 30.1 ± 0.2 (equivalent to V ∼ 31). The brighter part of ω Cen’s WD CS is consistent with the presence of massive He-core WDs, in agreement with previous HST analyses with ultraviolet and blue filters. Comparative analyses of the WD luminosity function (LF) and theoretical counterparts show that a single-age population for the cluster is compatible with the data. However, an analysis of only the WD LF cannot entirely exclude the possibility of an age range, due to uncertainties in the present-day WD mass function, with a star formation history potentially spanning up to 5 billion years, predominantly comprising stars about 13 Gyr old, with a minority potentially as young as 8 Gyr. This underscores the need for global spectroscopic and photometric investigations that simultaneously include both the WD populations and the previous evolutionary phases, in order to fully understand the cluster’s diverse chemical compositions and ages.

Detecting Detached Black Hole Binaries through Photometric Variability

Understanding the connection between the properties of black holes (BHs) and their progenitors is interesting in many branches of astrophysics. Discovering BHs in detached orbits with luminous companions (LCs) promises to help establish this connection since the LC and BH progenitor are expected to have the same metallicity and formation time. We explore the possibility of detecting BH–LC binaries in detached orbits using photometric variations of the LC flux, induced by tidal ellipsoidal variation, relativistic beaming, and self-lensing. We create realistic present-day populations of detached BH–LC binaries in the Milky Way (MW) using binary population synthesis where we adopt observationally motivated initial stellar and binary properties, star formation history, and the present-day distribution of these sources in the MW based on detailed cosmological simulations. We test detectability of these sources via photometric variability by Gaia and TESS missions by incorporating their respective detailed detection biases as well as interstellar extinction. We find that Gaia is expected to resolve ∼300–1000 (∼700–1500) detached BH–LC binaries with signal-to-noise ratio (SNR) ≥ 10 (1) depending on the photometric precision and details of supernova physics. Similarly, the number of resolved BH–LC binaries with TESS is ∼50–200 (∼140–350). We find that 136−15+15 BH–LC binaries would be common between Gaia and TESS. Moreover, ∼60–70 (∼50–200) BH–LC binaries identifiable using photometry with SNR ≥ 10 may also be resolved using Gaia’s radial velocity (astrometry).

Calibration of the JAGB method for the Magellanic Clouds and Milky Way from Gaia DR3, considering the role of oxygen-rich AGB stars

The JAGB method is a new way of measuring distances in the Universe with the use of asymptotic giant branch (AGB) that are situated in a selected region in a $J$ versus $J- colour--magnitude diagram (CMD), and relying on the fact that the absolute $J$ magnitude is (almost) constant. It is implicitly assumed in the method that the selected stars are carbon-rich AGB stars (carbon stars). However, as the sample selected to determine J $ is purely colour based, there can also be contamination by oxygen-rich AGB stars in principle. As the ratio of carbon-rich to oxygen-rich stars is known to depend on metallicity and initial mass, the star formation history and age--metallicity relation in a galaxy should influence the value of $M_ J $. The aim of this paper is to look at mixed samples of oxygen-rich and carbon-rich stars for the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Milky way (MW) using the Gaia catalogue of long-period variables (LPVs) as a basis. The advantage of this catalogue is that it contains a classification of O- and C-stars based on the analysis of Gaia Rp spectra. The LPV catalogue is correlated with data from the Two Micron All Sky Survey (2MASS) and samples in the LMC, SMC, and the MW are retrieved. Following methods proposed in the literature, we report the mean and median magnitudes of the selected sample using different colour and magnitude cuts and the results of fitting Gaussian and Lorentzian profiles to the luminosity function (LF). For the SMC and LMC, we confirm previous results in the literature. The LFs of the SMC and LMC JAGB stars are clearly different, yet it can be argued that the mean magnitude inside a selection box agrees at the 0.021 mag level. The results of our analysis of the MW sample are less straightforward. The contamination by O-rich stars is substantial for a classical lower limit of $(J- 1.3$, and becomes less than 10&lt;!PCT!&gt; only for $(J- 1.5$. The sample of AGB stars is smaller than for the MCs for two reasons. Nearby AGB stars (with potentially the best determined parallax) tend to be absent as they saturate in the 2MASS catalogue, and the parallax errors of AGB stars tend to be larger compared to non-AGB stars. Several approaches have been taken to improve the situation but finally the JAGB LF for the MW contains about 130 stars and the fit of Gaussian and Lorentzian profiles is essentially meaningless. The mean and median magnitudes are fainter than for the MC samples by about 0.4 mag which is not predicted by theory. We do not confirm the claim in the literature that the absolute calibration of the JAGB method is independent of metallicity up to solar metallicity. A reliable calibration of the JAGB method at (near) solar metallicity should await further data releases, or should be carried out in another environment.

Adrift in the Andaman Sea: &amp;#x2028;Law, Archipelagos and the Making of Maritime Sovereignty

Abstract This essay focuses on the long history of archipelagic formations in the Bay of Bengal as sites of legal experimentation. This history is often narrated beginning with convict transportation and the permanent occupation of the Andaman Islands as a British penal settlement in 1857 and the violent erasure of indigenous cultures that followed it. This essay focuses instead on the hundred years preceding it where the English East India Company experimented with abandoning jurisdiction over the lesser-known islands off the Bengal and Burma coasts and people who lived on them, despite being in the position of a territorial sovereign. These experimentations were recorded most eloquently in legal and administrative records about crime - which included, for example, how assault and “river dacoity” on the deltaic islands of the Sunderbans in lower Bengal were to be dealt with and how men convicted of murder on the islands off the Tenasserim coast in southern Burma were to be prosecuted. In each case, prompted by tensions between the Company and the British Crown, policing and prosecution were abandoned, but this escaped public attention as it took place on the empire’s maritime edges. Although jurisdictional claims were central to the expansionist aims of the British empire in the nineteenth century around the Indian Ocean, these instances offer an alternate account of sovereignty, one where assertions and abandonments were both critical to the making of empires.

Spin–Orbit Alignment of Early-type Astrometric Binaries and the Origin of Slow Rotators

The spin–orbit alignment of binary stars traces their formation and accretion history. Previous studies of spin–orbit alignment have been limited to small samples, slowly rotating solar-type stars, and/or wide visual binaries that not surprisingly manifest random spin–orbit orientations. We analyze 917 Gaia astrometric binaries across periods P = 100–3000 days (a = 0.5–5 au) that have B8-F1 IV/V primaries (M 1 = 1.5–3 M ⊙) and measured projected rotational velocities v sin i. The primary stars in face-on orbits exhibit substantially smaller v sin i compared to those in edge-on orbits at the 6σ level, demonstrating significant spin–orbit alignment. The primaries in our astrometric binaries are rotating more slowly than their single-star or wide-binary counterparts and therefore comprise the slow-rotator population in the observed bimodal rotational velocity distribution of early-type stars. We discuss formation models of close binaries where some of the disk angular momentum is transferred to the orbit and/or secondary spin, quenching angular momentum flow to the primary spin. The primaries in astrometric binaries with small mass ratios q = M 2/M 1 < 0.3 possess even smaller v sin i, consistent with model predictions. Meanwhile, astrometric binaries with large eccentricities e > 0.4 do not display spin–orbit alignment or spin reduction. Using a Monte Carlo technique, we measure a spin–orbit alignment fraction of F align = 75% ± 5% and an average spin reduction factor of 〈S align〉 = 0.43 ± 0.04. We conclude that 75% of close A-type binaries likely experienced circumbinary disk accretion and probably formed via disk fragmentation and inward disk migration. The remaining 25%, mostly those with e > 0.4, likely formed via core fragmentation and orbital decay via dynamical friction.

HISTORY AND CONSTITUTIONALISM OF THE GRAND DUCHY OF LUXEMBOURG: BRIEF NOTES - DOI: 10.12818/P.0304-2340.2024v85p223

The text provides a brief and general approach to the formation history of Luxembourg, emphasizing the constitutional landscape of the country, presented in three sections. The first one highlights the historical and formal events that lead to modern Luxembourg. In the second, the more important constitutional moments in the XIX century are exposed, with commentaries regarding the Constitutions of 1841, 1848, 1856, and 1868. In the third, the political and constitutional reforms of the XX century, considering the I and II World Wars, are commented upon, with the intention to expose some of the main characteristics of the Grand-Duché, such as its openness to the international sphere, constitutional actions for the integration of Europe, and the main normative diploma that shows the compromise with Human and Fundamental rights, considering specific dynamics of the relations of the powers of the state.

Abundant Molecular Gas in the Central Region of Lenticular Galaxy PGC 39535

Lenticular galaxies (S0s) in the local Universe are generally absent of recent star formation and lack molecular gas. In this paper, we investigate one massive (M * ∼ 5 × 1010 M ⊙) star-forming S0, PGC 39535, with the Northern Extended Millimeter Array (NOEMA). Using optical data from the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at the Apache Point Observatory survey, we find star formation mainly concentrates in the central region of PGC 39535. The total star formation rate estimated using extinction-corrected Hα flux is 1.57 M ⊙ yr−1. The results of the NOEMA observation suggest that the molecular gas mainly concentrates in the central regions as a gaseous bar and a ring-like structure, and shows similar kinematics as the stellar and ionized gas components. The total molecular gas mass estimated from CO(1–0) is (5.42 ± 1.52) × 109 M ⊙. We find PGC 39535 lies on the star-forming main sequence but falls below the Kennicutt–Schmidt relation of spiral galaxies, suggesting that the star formation efficiency may be suppressed by the massive bulge. The existence of a second Gaussian component in the CO spectrum of the central region indicates possible gas flows. Furthermore, our analyses suggest that PGC 39535 resides in the center of a massive group and the derived star formation history indicates it may experience a series of gas-rich mergers over the past 2–7 Gyr.

Physical properties of extreme emission-line galaxies at z ∼ 4–9 from the JWST CEERS survey

Context. Extreme emission line galaxies (EELGs) are typically characterized by high equivalent widths (EWs) which are driven by elevated specific star formation rates (sSFRs) in low-mass galaxies with subsolar metallicities and little dust. Such extreme systems are exceedingly rare in the local universe, but the number density of EELGs increases with increasing redshift. Such starburst galaxies are currently strongly presumed to be the main drivers of hydrogen reionization over 5.5 &lt; z &lt; 15, which serves to motivate many of the searches for high-z EELGs. Aims. We aim to characterize the physical properties of a sample of ∼730 EELGs at 4 ≲ z &lt; 9 photometrically selected from the CEERS survey using JWST/NIRCam. We validate our method and demonstrate the main physical properties of a subset of EELGs using NIRSpec spectra. Methods. We create synthetic NIRCam observations of EELGs using empirical templates based on ∼2000 local metal-poor starbursts to select EELGs based on color-color criteria. We study their properties based on SED fitting and flux excess from emission lines in the photometric filters. Results. Our sample of EELGs has a mean stellar mass of 107.84 M⊙ with high sSFRs from SED fitting with a mean value of 10−7.03 yr−1. We consider a delayed-τ model for the star formation history and find our sample of EELGs are young with a mean value of the time after the onset of star formation of 45 Myr. We find that they have similar line ratios to local metal-poor starburst galaxies with high log([OIII]/Hβ) ≳ 0.4−1 which indicates that star formation may be the dominant source of ionization in these galaxies. Based on the photometric fluxes and morphologies, we find an increase of EW([OIII]+Hβ) with sSFR and ΣSFR, and a decrease with age and stellar mass. The sample of EELGs can reach ΣSFR &gt; 10 M⊙ yr−1 kpc−2 which indicate they are strong candidates of LyC leakers. Another indirect indicator is the high values of O32 &gt; 5 that can be reached for some galaxies in the sample. This indicates that they may have the conditions to facilitate the escape of ionizing photons.

Gaia-Sausage-Enceladus star formation history as revealed by detailed elemental abundances. An archival study using SAGA data

The Gaia-Sausage-Enceladus merger was a major event in the history of the Milky Way. Debris from this merger has been extensively studied with full kinematic data from the Gaia mission. Understanding the star formation history of the progenitor galaxy aids in our understanding of the evolution of the Milky Way and galaxy formation in general. We aimed to constrain the star formation history of the Gaia-Sausage-Enceladus progenitor galaxy using elemental abundances of member stars. Previous studies on Milky Way satellite dwarf galaxies show that key elemental abundance patterns, which probe different nucleosynthetic channels, reflect the host galaxy's star formation history. We gathered Mg, Fe, Ba, and Eu abundance measurements for Gaia-Sausage-Enceladus stars from the SAGA database. Gaia-Sausage-Enceladus members were selected kinematically. Inspired by previous studies, we used Fe/Mg Ba/Mg Eu/Mg and Eu/Ba as a function of Fe/H to constrain the star formation history of Gaia-Sausage-Enceladus. We used the known star formation histories and elemental abundance patterns of the Sculptor and Fornax dwarf spheroidal galaxies as a comparison. The elemental abundance ratios of Fe/Mg Ba/Mg Eu/Mg and Eu/Ba all increase with Fe/H in Gaia-Sausage-Enceladus. The Eu/Mg begins to increase at Fe/H -2.0$ and continues steadily, contrasting with the trend observed in the Sculptor dSph galaxy. The Eu/Ba increases and remains high across the Fe/H range, unlike the pattern seen in the Sculptor dSph galaxy, and deviates from the Fornax dSph galaxy at high Fe/H . The Ba/Mg is higher than those of the Sculptor dSph galaxy at the lowest Fe/H and gradually increases, similar to the Fornax dSph galaxy. We constrained three main properties of the Gaia-Sausage-Enceladus star formation history: 1) star formation started gradually, 2) it extended for over 2 Gyr, and 3) it was quenched around Fe/H of $-0.5$, likely when it fell into the Milky Way. We show that the elemental abundance ratios Fe/Mg Ba/Mg Eu/Mg and Eu/Ba can be used to trace the star formation history of a disrupted galaxy when these measurements are available over an Fe/H range that is representative of the progenitor galaxy's stellar population.

3D Morphology and Motions of the Canis Major Region from Gaia DR3

The Canis Major (CMa) region is known for its prominent arc-shaped morphology, visible at multiple wavelengths. This study integrates molecular gas data with high-precision astrometric parameters of young stellar objects (YSOs) from Gaia DR3 to provide the first three-dimensional (3D) insights into the dynamical evolution and star formation history of the CMa region. By utilizing the average distances and proper motions of the YSOs as proxies for those of the molecular clouds (MCs), we confirm the presence of a slowly expanding shell-like morphology in the CMa region, with an estimated radius of 47 ± 11 pc and expansion velocity of 1.6 ± 0.7 km s−1. Further, the dynamical evolution of the shell supports its expansion, with an expansion timescale of ∼4.4 Myr obtained by the traceback analysis assuming constant velocities. Finally, a momentum estimate suggests that at least two supernova explosions are needed to power the observed expanding shell, reinforcing the previous hypothesis of multiple supernova events. This study effectively combines CO data with the astrometric data of YSOs from Gaia, offering significant support for future studies of the 3D morphology and kinematics of MCs.

Scylla. I. A Pure-parallel, Multiwavelength Imaging Survey of the ULLYSES Fields in the LMC and SMC

Scylla is a deep Hubble Space Telescope (HST) survey of the stellar populations, interstellar medium, and star formation in the LMC and SMC. As a pure-parallel complement to the Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) survey, Scylla obtained 342 orbits of ultraviolet (UV) through near-IR imaging of the LMC and SMC with Wide Field Camera 3. In this paper, we describe the science objectives, observing strategy, data reduction procedure, and initial results from our photometric analysis of 96 observed fields. Although our observations were constrained by ULLYSES primary exposures, we imaged all fields in at least two filters (F475W and F814W) and 64% of fields in at least three and as many as seven WFC3 filters spanning the UV to IR. Overall, we reach average 50% completeness of m F225W = 26.0, m F275W = 26.2, m F336W = 26.9, m F475W = 27.8, m F814W = 25.5, m F110W = 24.7, and m F160W = 24.0 Vega mag in our photometric catalogs, which is faintward of the ancient main-sequence turnoff in all filters. The primary science goals of Scylla include characterizing the structure and properties of dust in the MCs, as well as their spatially resolved star formation and chemical enrichment histories. Our images and photometric catalogs, which represent the widest-area coverage of MCs with HST photometry to date, are available as a high-level science product at the Barbara A. Mikulski Archive for Space Telescopes.

Extreme emission line galaxies detected in JADES JWST/NIRSpec – I. Inferred galaxy properties

ABSTRACT Extreme emission line galaxies (EELGs) exhibit large equivalent widths (EW) in their rest-optical emission lines ([O iii]$\lambda 5007$ or H $\alpha$ rest-frame EW$\gt 750$ Å) which can be tied to a recent upturn in star formation rate (SFR), due to the sensitivity of the nebular line emission and the rest-optical continuum to young ($\lt 10$ Myr) and evolved stellar populations, respectively. By studying a sample of 85 star-forming galaxies (SFGs), spanning the redshift and magnitude interval $3 \lt z\lt 9.5$ and $-16\gt $M$\rm _{UV}\gt -21$, in the JWST Advanced Deep Extragalactic Survey (JADES) with NIRSpec/prism spectroscopy, we determine that SFGs initiate an EELG phase when entering a significant burst of star formation, with the highest EWs observed in EELGs with the youngest luminosity-weighted ages ($\lt 5$ Myr) and the highest burst intensity (those with the greatest excess between their current and long-term average SFR). We spectroscopically confirm that a greater proportion of SFGs are in an EELG phase at high redshift in our UV-selected sample ($61\pm 4~{{\ \rm per\ cent}}$ in our $z\gt 5.7$ high-redshift bin, compared to $23^{+4}_{-1}\%$ in our lowest redshift bin $3\lt z\lt 4.1$) due to the combined evolution of metallicity, ionization parameter, and star formation histories with redshift. We report that the EELGs within our sample exhibit a higher average ionization efficiency ($\log _{10}(\xi _\mathrm{ion}^\mathrm{HII}/{\rm erg^{-1}Hz}) =25.5\pm 0.2$) than the non-EELGs. High-redshift EELGs therefore comprise a population of efficient ionizing photon producers. Additionally, we report that 53 per cent (9/17) of EELGs at $z\gt 5.7$ have observed Ly $\alpha$ emission, potentially lying within large ionized regions. The high detection rate of Ly $\alpha$ emitters in our EELG selection suggests that the physical conditions associated with entering an EELG phase also promote the escape of Ly $\alpha$ photons.

Scylla. II. The Spatially Resolved Star Formation History of the Large Magellanic Cloud Reveals an Inverted Radial Age Gradient

The proximity of the Magellanic Clouds provides the opportunity to study interacting dwarf galaxies near a massive host, and spatial trends in their stellar population properties in particular, with a unique level of detail. The Scylla pure parallel program has obtained deep (80% complete to >1 mag below the ancient main-sequence turnoff), homogeneous two-filter Hubble Space Telescope imaging sampling the inner star-forming disk of the Large Magellanic Cloud (LMC), the perfect complement to shallower, contiguous ground-based surveys. We harness this imaging together with extant archival data and fit lifetime star formation histories (SFHs) to resolved color–magnitude diagrams of 111 individual fields, using three different stellar evolutionary libraries. We validate per-field recovered distances and extinctions, as well as the combined global LMC age–metallicity relation and SFH against independent estimates. We find that the present-day radial age gradient reverses from an inside-out gradient in the inner disk to an outside-in gradient beyond ∼2 disk scale lengths, supported by ground-based measurements. The gradients become relatively flatter at earlier look-back times, while the location of the inversion remains constant over an order of magnitude in look-back time, from ∼1 to 10 Gyr. This suggests at least one mechanism that predates the recent intense LMC–Small Magellanic Cloud interaction. We compare observed radial age trends to other late-type galaxies at fixed stellar mass and discuss similarities and differences in the context of potential drivers, implying strong radial migration in the LMC.

Scylla. III. The Outside-in Radial Age Gradient in the Small Magellanic Cloud and the Star Formation Histories of the Main Body, Wing, and Outer Regions

The proximity of the Large and Small Magellanic Clouds (LMC and SMC) provides the opportunity to study the impact of dwarf–dwarf interactions on their mass assembly with a unique level of detail. To this end, we analyze two-filter broadband imaging of 83 Hubble Space Telescope (HST) pointings covering 0.203 deg2 toward the SMC, extending out to ∼3.5 kpc in projection from its optical center. Lifetime star formation histories (SFHs) fit to each pointing independently reveal an outside-in age gradient such that fields in the SMC outskirts are older on average. We measure radial gradients of the look-back time to form 90%, 75%, and 50% of the cumulative stellar mass for the first time, finding δ(τ 90, τ 75, τ 50)/δ R = (0.61 −0.07+0.08 , 0.65−0.08+0.09 , 0.82−0.16+0.12 ) Gyr kpc−1 assuming PARSEC evolutionary models and a commonly used elliptical geometry of the SMC, although our results are robust to these assumptions. The wing of the SMC deviates from this trend, forming 25% of its cumulative mass over the most recent 3 Gyr owing to a best-fit star formation rate that remains approximately constant. Our results are consistent with chemodynamical evidence of a tidally stripped SMC component in the foreground and imply contributions to the observed SFH from multiple previous LMC–SMC interactions. We also compare our SMC SFH with results from a companion study of the LMC, finding that while the two galaxies present different internal, spatially resolved SFH trends, both the LMC and SMC have similar near-constant lifetime SFHs when viewed globally.

The star formation history of the first bulge fossil fragment candidate Terzan 5

Terzan 5 and Liller 1 are the only bulge stellar clusters hosting multi-iron and multi-age stellar populations. They are therefore claimed to constitute a novel class of astrophysical objects: the fossils of massive star-forming clumps that possibly sank to the center of the Milky Way and contributed to the formation of the bulge. This is based on the hypothesis that the ancient clumps were able to retain iron-enriched supernova ejecta, later giving rise to younger and more metal-rich populations. A way to investigate this scenario is reconstructing their star formation histories (SFHs) and proving a prolonged and multiepisode star formation activity. Leveraging ground- and space-based high-resolution images, we derived the SFH of Terzan 5 by employing the color-magnitude diagram fitting routine SFERA. The best-fit solution predicts an old, main peak occurred between 12 and $13\,$Gyr ago that generated $70\,&lt;!PCT!&gt;$ of the current stellar mass, followed by a lower-rate star formation activity with two main additional bursts. These results indicate that Terzan 5, similarly to Liller 1, experienced a prolonged, multiepisode star formation activity, fueled by metal-enriched gas deposited in its central regions, in agreement with the expectations of a self-enrichment scenario in a primordial massive clump.

A Potential Dynamical Origin of the Galactic Disk Warp: The Gaia–Sausage–Enceladus Major Merger

Previous studies have revealed that the Galactic warp is a long-lived, nonsteady, and asymmetric structure. There is a need for a model that accounts for the warp’s long-term evolution. Given that this structure has persisted for over 5 Gyr, its timeline may coincide with the completion of the Gaia–Sausage–Enceladus (GSE) merger. Recent studies indicate that the GSE, the significant merger of our Galaxy, was likely a gas-rich merger and the large amount of gas introduced could have created a profound impact on the Galactic morphology. This study utilizes GIZMO simulation code to construct a gas-rich GSE merger. By reconstructing the observed characteristics of the GSE, we successfully reproduce the disk warp and capture nearly all of its documented features, which align closely with observational data from both stellar and gas disks. This simulation demonstrates the possibility that a single major merger could generate the Galactic warp amplitude and precession. Furthermore, the analysis of the warp’s long-term evolution may offer more clues into the formation history of the Milky Way.

Stellar Models are Reliable at Low Metallicity: An Asteroseismic Age for the Ancient Very Metal-poor Star KIC 8144907

Very-metal-poor stars ([Fe/H] < −2) are important laboratories for testing stellar models and reconstructing the formation history of our galaxy. Asteroseismology is a powerful tool to probe stellar interiors and measure ages, but few asteroseismic detections are known in very-metal-poor stars and none have allowed detailed modeling of oscillation frequencies. We report the discovery of a low-luminosity Kepler red giant (KIC 8144907) with high signal-to-noise ratio oscillations, [Fe/H] = −2.66 ± 0.08 and [α/Fe] = 0.38 ± 0.06, making it by far the most metal-poor star to date for which detailed asteroseismic modeling is possible. By combining the oscillation spectrum from Kepler with high-resolution spectroscopy, we measure an asteroseismic mass and age of 0.79 ± 0.02(ran) ± 0.01(sys) M ⊙ and 12.0 ± 0.6(ran) ± 0.4(sys) Gyr, with remarkable agreement across different codes and input physics, demonstrating that stellar models and asteroseismology are reliable for very-metal-poor stars when individual frequencies are used. The results also provide a direct age anchor for the early formation of the Milky Way, implying that substantial star formation did not commence until redshift z ≈ 3 (if the star formed in situ) or that the Milky Way has undergone merger events for at least ≈12 Gyr (if the star was accreted by a dwarf satellite merger such as Gaia-Enceladus).