Superb arXiv Selection 🐻

We present Very Large Telescope/X-Shooter spectroscopy for the host galaxies of 12 fast radio bursts (FRBs) detected by the Australian SKA Pathfinder (ASKAP) observed through the ESO Large Programme "FURBY", which imposes strict selection criteria on the included FRBs and their host galaxies to produce a homogeneous and well-defined sample. We describe the data reduction and analysis of these spectra and report their redshifts, line-emission fluxes, and derived host properties. From the present sample, this paper focuses on the faint host of FRB ($m_R = 22.53 \pm 0.02$) identified at low redshift ($z=0.1050$). This indicates an intrinsically very low-luminosity galaxy ($L \approx 10^8 L_\odot$), making it the lowest-luminosity non-repeating FRB host to date by a factor of $\sim 3$, and slightly dimmer than the lowest-luminosity host for repeating FRBs. Our SED fitting analysis reveals a low stellar mass ($M_* \approx 10^{8.0} M_\odot$), low star formation rate (${\rm SFR} \approx 0.04 M_\odot \rm yr^{-1}$), and very low metallicity ($12+\log(\text{O}/\text{H})\sim(7.99-8.3)$), distinct from the more massive galaxies ($\log(M/M_\odot) \sim 10$) that are commonly identified for non-repeating FRBs. Its discovery demonstrates that FRBs can arise in among the faintest, metal-poor galaxies of the universe. In turn, this suggests that at least one FRB progenitor channel must include stars (or their remnants) created in very low metallicity environments. This indicates better prospects for detecting FRBs from the high-$z$ universe where young, low-mass galaxies proliferate.

Open clusters have long been used as tracers of Galactic structure. However, without a selection function to describe the completeness of the cluster census, it is difficult to quantitatively interpret their distribution. We create a method to empirically determine the selection function of a Galactic cluster catalogue. We test it by investigating the completeness of the cluster census in the outer Milky Way, where old and young clusters exhibit different spatial distributions. We develop a method to generate realistic mock clusters as a function of their parameters, in addition to accounting for Gaia's selection function and astrometric errors. We then inject mock clusters into Gaia DR3 data, and attempt to recover them in a blind search using HDBSCAN. We find that the main parameters influencing cluster detectability are mass, extinction, and distance. Age also plays an important role, making older clusters harder to detect due to their fainter luminosity function. High proper motions also improve detectability. After correcting for these selection effects, we find that old clusters are $2.97\pm0.11$ times more common at a Galactocentric radius of 13~kpc than in the solar neighbourhood -- despite positive detection biases in their favour, such as hotter orbits or a higher scale height. The larger fraction of older clusters in the outer Galaxy cannot be explained by an observational bias, and must be a physical property of the Milky Way: young outer-disc clusters are not forming in the outer Galaxy, or at least not with sufficient masses to be identified as clusters in Gaia DR3. We predict that in this region, more old clusters than young ones remain to be discovered. The current presence of old, massive outer-disc clusters could be explained by radial heating and migration, or alternatively by a lower cluster destruction rate in the anticentre.

Context. Open clusters are groups of stars formed from the same cloud of gas and cosmic dust. They play an important role in studying stars' formation and evolution and understanding galaxies' structure and dynamics. Aims. The main objective of this work is to identify stars that belong to open clusters using astrometric data from Gaia EDR3 and spectroscopic data from APOGEE DR17. Furthermore, we investigate the metallicity gradients and orbital properties of the open clusters in our sample. Methods. By applying the HDBSCAN clustering algorithm to this data, we identified observed stars in our galaxy with similar dynamics, chemical compositions, and ages. The orbits of the open clusters were also calculated using the GravPot16 code. Results. We found 1987 stars belonging to 49 open clusters, and 941 of these stars have probabilities above 80 % of belonging to open clusters. Our metallicity gradient presents a two-slope shape for two measures of different Galactic center distances, the projected Galactocentric distance, and the guiding center radius to the Galactic Center, as already reported in previous work. However, when we separate the open clusters by age, we observe no significant difference in the metallicity gradient slope beyond a certain distance from the Galactic center. Our results show a shallower gradient for clusters younger than 2 Gyr than those older than 2 Gyr. All our OCs dynamically assemble the Disk-like population very well, and they are in prograde orbits, which is typical for disk-like populations. Some OCs resonate with the Galactic bar at the Lagrange points L4 and L5.

Analyzing filaments and cores in molecular clouds is key to understanding galactic star formation and its environmental dependence. This paper studies the properties and distribution of dense cores and filaments in the Ophiuchus molecular cloud, with a focus on the L1688 hub-filament system (HFS) and its star formation potential. We extracted sources and filaments from Herschel images and a 13.5 arcsec resolution surface density map using the getsf method, identified prestellar cores among the extracted sources, evaluated core mass segregation, and constructed the core mass function (CMF). We derived properties of the filaments from their radial surface density profiles, constructed the filament linear density function (FLDF), and assessed the mass distribution in the L1688 HFS to estimate the core and filament formation efficiencies (CFE, FFE). The results suggest that the filament-dominated core formation is a key mechanism in star formation within the system.

The understanding of mixing processes in stars is crucial for improving our knowledge of the chemical abundances in stellar photospheres and of their variation with evolutionary phase. This is fundamental for many astrophysical issues on all scales, ranging from stellar evolution to the chemical composition, formation and evolution of stellar clusters and galaxies. Among these processes, convective-envelope overshooting is in dire need of a systematic calibration and comparison with predictions from multi-dimensional hydrodynamical simulations. The Red Giant Branch bump (RGBb) is an ideal calibrator of overshooting processes, since its luminosity depends on the maximum depth reached by the convective envelope after the first dredge-up. Indeed, a more efficient overshooting produces a discontinuity in the Hydrogen mass fraction profile deeper in the stellar interior and consequently a less luminous RGBb. In this work, we calibrated the overshooting efficiency by comparing the RGBb location predicted by stellar models with observations of stellar clusters with HST and Gaia photometry, as well as solar-like oscillating giants in the Kepler field. We explored the metallicity range between -2.02 dex and +0.35 dex and found overshooting efficiencies ranging from $0.009^{+0.015}_{-0.016}$ to $0.062^{+0.017}_{-0.015}$. In particular, we found that the overshooting efficiency decreases linearly with [M/H], with a slope of $(-0.010\pm0.006)$ dex$^{-1}$. We suggest a possible explanation for this trend, linking it to the efficiency of turbulent entrainment at different metallicities.

Split main-sequences (MSs) and extended main-sequence turn-offs (eMSTOs) have been observed in nearly all Magellanic Clouds clusters younger than 2 Gyr. More recently, Hubble Space Telescope (HST) ultraviolet photometry uncovered a puzzling new population of UV-absorbed stars, dubbed UVdim, in five Magellanic Clouds clusters aged between 40 and 200 Myr, as well as in one 1.5 Gyr-old cluster. These UVdim stars predominantly lie on the blue MS, which is composed of slow rotators, and their distinct UV properties are believed to stem from dusty circumstellar disks. Although eMSTOs are common in both Magellanic Clouds and Galactic open clusters (OCs) of comparable ages, UVdim stars have not yet been investigated in Galactic OCs. In this work, we fill that gap by combining Swift/UVOT, SkyMapper, and Gaia photometry to extend the search for UVdim stars to 35 Galactic OCs younger than 2 Gyr. By constructing colour-colour diagrams analogous to those employed with HST WFC3/UVIS, we find no evidence of UVdim-like stars in most Galactic open clusters and identify possible UVdim candidates in only five systems. The rarity of UVdim stars in young OCs suggests a potential difference between Magellanic Cloud clusters and their Milky Way counterparts, although the underlying reason remains unclear.

The new generation of optical spectrographs (i.e., WEAVE, 4MOST, DESI, and WST) offer unprecedented opportunities for statistically studying the star formation histories of galaxies. However, these observations are not easily comparable to predictions from cosmological simulations. Our goal is to build a reference framework for comparing spectroscopic observations with simulations and test tools for deriving stellar population properties of galaxies. We focus on the observational strategy of the Stellar Population at Intermediate Redshift Survey (StePS) with the WEAVE instrument. We generate mock datasets of ~750 galaxies at redshifts z = 0.3, 0.5, and 0.7 using the TNG50 simulation, perform radiative transfer with SKIRT, and analyze the spectra with pPXF as if they were real observations. We present the methodology to generate these datasets and provide an initial exploration of stellar population parameters (i.e., mass-weighted ages and metallicities) and star formation histories for three galaxies at z = 0.7 and their descendants at z = 0.5 and 0.3. We find good agreement between the mock spectra and intrinsic ages in TNG50 (average difference $0.2\pm0.3$ Gyr) and successfully recover their star formation histories, especially for galaxies form the bulk of their stars on short timescales and at early epochs. We release these datasets, including multi-wavelength imaging and spectra, to support forthcoming WEAVE observations.

The existence of high galactic latitude molecular clouds has been known for several decades, and studies of their dust and gas distributions reveal complicated morphological structures. Their dynamics involve turbulence even in the absence of internal energy sources such as stars. We study in detail two such clouds, MBM 3 and MBM 16, trying to recover the geometric structure and topology of the gas distribution. In particular, we address the evidence of superthermal asymmetric atomic and molecular line profiles as a result of filament superposition combined with turbulent motions. We use a variety of spectroscopic and imaging archival observations of the gas and dust components. The spectroscopic data set comprises HI 21 cm, 12CO, 13CO, and CH line profiles. We also use archival infrared images to study the dust distribution and temperature. To understand the topology of MBM 3 and MBM 16 we compare molecular and atomic spectra, along with profile decomposition of the HI 21 cm line. Standard tools such as Structure Functions of velocity centroids are used to characterise the turbulence in MBM 3, and channel maps and position-velocity diagrams are employed for elucidating the filament topology of both clouds. The unusually large linewidths previously reported for MBM 3 are due to superposition of individual filaments whose superthermal linewidths are about 1 km/s. In MBM 16, the cloud appears to decompose into two adjacent structures with similar properties. The filaments have a high aspect ratio, with lengths of about 1 pc and widths of about 0.1 pc. In general, the molecular gas is embedded within more extended neutral hydrogen structures. Velocity gradients found within these structures are not necessarily dynamical, convergent flows. Projection effects and topology of the driving flows produce signatures that mimic velocity shears even if they are simply distortions of ordered gas.

Aims. We investigate the gas reservoirs, star formation (SF) properties, and environment of the ultra-diffuse galaxy GAMA526784 to understand its formation history, the efficiency of molecular gas conversion into stars, and the possible role of an interacting companion in shaping its morphology. Methods. We analyse low and high-resolution CO observations to constrain the molecular gas content, compare with HI data, and examine the SF efficiency of GAMA526784. The potential influence of a newly identified nearby dwarf galaxy is assessed using photometric and spatial information. Results. GAMA526784 exhibits a regular HI reservoir (M_HI/M* = 2.88) but only upper limits on its molecular gas mass (M_H2(5sigma)/M* < 0.23). The HI reservoir and CO non-detection can be explained by several mechanisms: (1) predominance of CO-dark H2, invisible to CO observations but contributing to SF; (2) a time delay in HI-to-H2 conversion following a recent interaction; or (3) elevated turbulence inhibiting gas collapse. An identified companion, found at a projected distance of 48 kpc, shows similar colours and lies in the direction of young star clusters in GAMA526784, indicating a possible association. We hypothesise this companion may have triggered the formation of star clusters in GAMA526784 through a high-velocity encounter. Conclusions. Our findings suggest GAMA526784 may have undergone an interaction that influenced its gas reservoirs and SF activity. The presence of a nearby companion agrees with predictions of a high-speed encounter, potentially offering a rare example of such an interaction in progress. We suggest this encounter may have shaped the system's recent evolution. Future observations, particularly targeting molecular gas tracers beyond CO and resolved HI maps, will be crucial in determining the extent of GAMA526784's cold gas reservoir and the nature of its recent star formation.

Hybrid morphology radio sources (HyMoRSs) are a rare subclass of radio galaxies that display a Fanaroff-Riley type I (FR I) morphology on one side of the central supermassive black hole (SMBH) and a type II (FR II) morphology on the other. These unique sources provide valuable insights into the physical mechanisms behind the FR dichotomy. In this study, we report the discovery of thirty-six new HyMoRSs, marking the largest collection of such sources in the southern sky to date, using data obtained from the MeerKAT absorption line survey (MALS). The identified HyMoRSs exhibit moderate radio luminosities, with a median value of 4.4 $\times 10^{24}$ W Hz$^{-1}$ at 1.4 GHz, and are located within the redshift range 0.04 < z < 1.34. To gain further insights into their origin, we also investigate the mid-infrared properties and environments of their host galaxies. Notably, nine out of the thirty-six sources are situated near the centers of galaxy clusters, including one with giant radio jets that extend over 811 kpc. Our analysis reveals that the majority of HyMoRSs are high-excitation radio galaxies (HERGs), hosted by actively star-forming galaxies that exhibit elevated star formation rates. Furthermore, our findings suggest that hybrid morphology may arise from FR II jets being deflected by a dense, cluster-like environment, along with orientation effects that make one jet appear FR I-like. As our candidates are selected through visual inspection of MALS radio maps, higher-resolution follow-up observations are still necessary to confirm the nature of their morphologies.

We use large-scale cosmological simulations to study the prospect of observing Population III (Pop III) bright galaxies with the James Webb Space Telescope (JWST). To quantify the impact of radiative transfer (RT), we compare a simulation that includes moment-based RT with one in which RT is handled approximately. Both simulations include a subgrid model of turbulent mixing, which is essential in tracking the formation of Pop III stars. We find that RT has a minor impact on our results and that the overall star formation rate densities for both simulations are in fair agreement with observations and other simulations. While our overall galaxy luminosity functions are consistent with current high-redshift observations, we predict a drop of a factor of at least 6 in detectable galaxy counts at $z = 14$ as compared to $z = 12$ at $M_{ab}$ $\le$ -16. Modeling Pop III stars according to a lognormal, top-heavy initial mass function (IMF), we find that these stars contribute no more than about 1 percent of the flux of potentially detectable lensed galaxies at $z = 12$-14 with $M_{ab} \le -15$. This is because a top-heavy Pop III IMF results in 99 percent of Pop III stellar mass being recycled within 10 Myr, well before the approx. 30 Myr timescale on which galaxies recover from supernova feedback and heating. These effects conspire to quickly extinguish Pop III star formation, making their detection difficult even for JWST.

We investigate the relationships between infrared excess (IRX=$L_{\rm IR}/L_{\rm UV}$) and Balmer decrement (${\rm H}\alpha/{\rm H}\beta$) as indicators of dust attenuation for 609 ${\rm {H\,{\small II}}}$ regions at scales of $\sim 50-200$ pc in NGC 628, utilizing data from AstroSat, James Webb Space Telescope (JWST) and Multi Unit Spectroscopic Explorer (MUSE). Our findings indicate that about three fifths of the sample ${\rm {H\,{\small II}}}$ regions reside within the regime occupied by local star-forming galaxies (SFGs) along the dust attenuation correlation described by their corresponding color excess parameters $E(B-V)_{\rm IRX} = 0.51\,E(B-V)_{{\rm H}\alpha/{\rm H}\beta}$. Nearly 27$\%$ of the sample exhibits $E(B-V)_{\rm IRX}> E(B-V)_{{\rm H}\alpha/{\rm H}\beta}$, while a small fraction ($\sim 13\%$) displays significantly lower $E(B-V)_{\rm IRX}$ compared to $E(B-V)_{{\rm H}\alpha/{\rm H}\beta}$. These results suggest that the correlation between the two dust attenuation indicators no longer holds for spatially resolved ${\rm {H\,{\small II}}}$ regions. Furthermore, the ratio of $E(B-V)_{\rm IRX}$ to $E(B-V)_{{\rm H}\alpha/{\rm H}\beta}$ remains unaffected by various physical parameters of the ${\rm {H\,{\small II}}}$ regions, including star formation rate (SFR), SFR surface density, infrared luminosity ($L_{\rm IR}$), $L_{\rm IR}$ surface density, stellar mass, gas-phase metallicity, circularized radius, and the distance to galactic center. We argue that the ratio is primarily influenced by the evolution of surrounding interstellar medium (ISM) of the star-forming regions, transitioning from an early dense and thick phase to the late blown-away stage.