Young Stellar Populations Research Articles

A Merger-driven Scenario for Clumpy Galaxy Formation in the Epoch of Reionization: Physical Properties of Clumps in the FirstLight Simulation

Recent JWST observations with superb angular resolution have revealed the existence of clumpy galaxies at high redshift through the detection of rest-frame optical emission lines. We use the FirstLight simulation to study the properties of (sub)galactic clumps that are bright in the [O III] λ5007 line with flux greater than ∼10−18 erg s−1 cm−2, to be detected by JWST. For 62 simulated galaxies that have stellar masses of (0.5–6) × 1010 M ⊙ at z = 5, we find clumps in 1828 snapshots in the redshift range z = 9.5–5.5. The clumps are identified by the surface density of the star formation rate (SFR). About one-tenth of the snapshots show the existence of clumpy systems with two or more components. Most of the clumps are formed by mergers and can be characterized by their ages: central clumps dominated by stellar populations older than 50 Myr, and off-centered clumps dominated by younger stellar populations with specific SFRs of ∼50 Gyr−1. The latter type of young clumps is formed from gas debris in the tidal tails of major mergers with baryonic mass ratios of 1 ≤ q < 4. The merger-induced clumps are short-lived and merge within a dynamical time of several tens of million years. The number density of the clumpy systems is estimated to be ∼10−5 cMpc−3, which is large enough to be detected in recent JWST surveys.

Intense Star Cluster Formation: Stellar Masses, the Mass Function, and the Fundamental Mass Scale

Within the birth environment of a massive globular cluster, the combination of a luminous young stellar population and a high column density induces a state in which the thermal optical depth and radiation pressure are both appreciable. In this state, the sonic mass scale, which influences the peak of the stellar mass function, is tied to a fundamental scale composed of the Planck mass and the mass per particle. Thermal feedback also affects the opacity-limited minimum mass and how protostellar outflows and binary fragmentation modify stellar masses. Considering the regions that collapse to form massive stars, we argue that thermal stabilization is likely to flatten the high-mass slope of the initial mass function. Among regions that are optically thick to thermal radiation, we expect the stellar population to become increasingly top-heavy at higher column densities, although this effect can be offset by lowering the metallicity. A toy model is presented that demonstrates these effects and in which radiation pressure leads to gas dispersal before all of the mass is converted into stars.

Mapping radial abundance gradients with Gaia -ESO open clusters. Evidence of recent gas accretion in the Milky Way disk

Recent evidence from spectroscopic surveys points towards the presence of a metal-poor, young stellar population in the low-alpha , chemically thin disk. In this context, the investigation of the spatial distribution and time evolution of precise, unbiased abundances is fundamental to disentangle the scenarios of formation and evolution of the Galaxy. We study the evolution of abundance gradients in the Milky Way by taking advantage of a large sample of open star clusters, which are among the best tracers for this purpose. In particular, we used data from the last release of the Gaia -ESO survey. We performed a careful selection of open cluster member stars, excluding those members that may be affected by biases in spectral analysis. We compared the cleaned open cluster sample with detailed chemical evolution models for the Milky Way, using well-tested stellar yields and prescription for radial migration. We tested different scenarios of Galaxy evolution to explain the data, namely, the two-infall and the three-infall frameworks, which suggest the chemical thin disk is formed by one or two subsequent gas accretion episodes, respectively. With the performed selection in cluster member stars, we still find a metallicity decrease between intermediate-age ($1&lt;$ Age/Gyr $&lt;3$) and young (Age $&lt;1$ Gyr) open clusters. This decrease cannot be explained in the context of the two-infall scenario, even by accounting for the effect of migration and yield prescriptions. The three-infall framework, with its late gas accretion in the last 3 Gyr, is able to explain the low metallic content in young clusters. However, we have invoked a milder metal dilution for this gas infall episode relative to previous findings. To explain the observed low metallic content in young clusters, we propose that a late gas accretion episode triggering a metal dilution would have taken place, extending the framework of the three-infall model for the first time to the entire Galactic disk.

QSOFEED: Relationship between star formation and active galactic nuclei feedback

Context. Large-scale cosmological simulations suggest that feedback from active galactic nuclei (AGN) plays a crucial role in galaxy evolution. More specifically, outflows are one of the mechanisms by which the accretion energy of the AGN is transferred to the interstellar medium (ISM), heating and driving out gas and impacting star formation (SF). Aims. The purpose of this study is to directly test this hypothesis utilising SDSS spectra of a well-defined sample of 48 low-redshift (z &lt; 0.14) type 2 quasars (QSO2s). Methods. By exploiting these data, we were able to characterise the kinematics of the warm ionised gas by performing a non-parametric analysis of the [OIII]λ5007 emission line. We also constrained the properties of the young stellar populations (YSP; tysp &lt; 100 Myr) of their host galaxies via spectral synthesis modelling. Results. These analyses revealed that 85% of the QSO2s display velocity dispersions in the warm ionised gas phase greater than that of the stellar component of their host galaxies, indicating the presence of AGN-driven outflows. We compared the gas kinematics with the intrinsic properties of the AGN and found that there is a positive correlation between gas velocity dispersion and 1.4 GHz radio luminosity – but not with the AGN bolometric luminosity or Eddington ratio. This either suggests that the radio luminosity is the key factor driving outflows or that the outflows themselves are shocking the ISM and producing synchrotron emission. We found that 98% of the sample host YSPs to varying degrees, with star formation rates (SFRs) of 0 ≤ SFR ≤ 92 M⊙ yr−1, averaged over 100 Myr. We compared the gas kinematics and outflow properties to the SFRs to establish possible correlations that could suggest that the presence of the outflowing gas could be impacting SF, but we found that no such correlation exists. This leads us to the conclusion that on the scales probed by the SDSS fibre (between 2 and 7 kpc diameters), AGN-driven outflows do not impact SF on the timescales probed in this study. However, we find a positive correlation between the light-weighted stellar ages of the QSO2s and the black hole mass, which might indicate that successive AGN episodes lead to the suppression of SF over the course of galaxy evolution.

JWST PRIMER: a new multifield determination of the evolving galaxy UV luminosity function at redshifts z ≃ 9 – 15

ABSTRACT We present a new determination of the evolving galaxy ultraviolet (UV) luminosity function (LF) over the redshift range $8.5&amp;lt; z&amp;lt; 15.5$ using a combination of several major Cycle-1 JWST imaging programmes – Public Release IMaging for Extragalactic Research, JWST Advanced Deep Extragalactic Survey, and Next Generation Deep Extragalactic Exploratory Public Survey. This multifield approach yields a total of $\simeq 370$ arcmin2 of JWST/NIRCam imaging, reaching (5-$\sigma$) depths of $\simeq 30$ AB mag in the deepest regions. We select a sample of 2548 galaxies with a significant probability of lying at high redshift ($p(z&amp;gt; 8.5)&amp;gt; 0.05$) to undertake a statistical calculation of the UV LF. Our new measurements span $\simeq 4$ mag in UV luminosity at $z=9-12.5$, placing new constraints on both the shape and evolution of the LF at early times. Our measurements yield a new estimate of the early evolution of cosmic star-formation rate density ($\rho _{\rm {SFR}}$) confirming the gradual decline deduced from early JWST studies, at least out to $z \simeq 12$. Finally we show that the observed early evolution of the galaxy UV LF (and $\rho _{\rm {SFR}}$) can be reproduced in a ${\rm \Lambda }$cold dark matter Universe, with no change in dust properties or star-formation efficiency required out to $z \simeq 12$. Instead, a progressive trend towards younger stellar population ages can reproduce the observations, and the typical ages required at $z \simeq$ 8, 9, 10, and 11 all converge on $\simeq 380-330$ Myr after the big bang, indicative of a rapid emergence of early galaxies at $z \simeq 12 - 13$. This is consistent with the first indications of a steeper drop-off in $\rho _{\rm {SFR}}$ we find beyond $z \simeq 13$, possibly reflecting the rapid evolution of the halo mass function at earlier times.

Hot Stars, Young Stellar Populations and Dust with Swift/UVOT

Hot Stars, Young Stellar Populations and Dust with Swift/UVOT

Searching for pulsars, magnetars, and fast radio bursts in the sculptor galaxy using MeerKAT

Abstract The Sculptor Galaxy (NGC 253), located in the Southern Hemisphere, far off the Galactic Plane, has a relatively high star-formation rate of about 7 M⊙ yr−1 and hosts a young and bright stellar population, including several super star clusters and supernova remnants. It is also the first galaxy, apart from the Milky Way Galaxy to be associated with two giant magnetar flares. As such, it is a potential host of pulsars and/or fast radio bursts in the nearby Universe. The instantaneous sensitivity and multibeam sky coverage offered by MeerKAT therefore make it a favourable target. We searched for pulsars, radio-emitting magnetars, and fast radio bursts in NGC 253 as part of the TRAPUM large survey project with MeerKAT. We did not find any pulsars during a four-hour observation, and derive a flux density limit of 4.4 μJy at 1400 MHz, limiting the pseudo-luminosity of the brightest putative pulsar in this galaxy to 54 Jy kpc2. Assuming universality of pulsar populations between galaxies, we estimate that detecting a pulsar as bright as this limit requires NGC 253 to contain a pulsar population of ⪞20 000. We also did not detect any single pulses and our single pulse search flux density limit is 62 mJy at 1284 MHz. Our search is sensitive enough to have detected any fast radio bursts and radio emission similar to the brighter pulses seen from the magnetar SGR J1935+2154 if they had occurred during our observation.

Spatially Resolved Analysis of Stellar Populations in NGC 2992: Impact of Active Galactic Nucleus Feedback

In NGC 2992, a galaxy-scale ionized gas outflow driven by active galactic nuclei (AGNs) has long been recognized, yet its impact on the host galaxy has remained elusive. In this paper, we utilize data from the archival Very Large Telescope/MUSE to present a spatially resolved analysis of stellar populations in this galaxy. Two different stellar population templates are employed to fit the stellar continuum, allowing us to determine the light-weighted stellar age, metallicity, the fraction of the young stellar population (age <100 Myr, P Y), and the average age and metallicity of P Y. Our results reveal the presence of a very young stellar population (≤40 Myr) within the dust lane and nearly along the galaxy’s major axis. The light-weighted stellar age and the fraction of P Y show negative trends along the major and minor axes. The average age and metallicity of P Y present positive trends with increasing distance, except along the northern direction of the major axis. Within the circumnuclear region (<1 kpc), the distribution of the young stellar population is spatially anticorrelated with the AGN outflow cone. The highest fraction of P Y is observed at the outskirts of the nuclear radio bubble in the northern region near the nucleus. Considering the coupling efficiency and timescales, we propose that the AGN outflow in this galaxy may exert both negative and positive feedback on its host. Additionally, the star formation and the AGN activities could be attributed to the interaction between NGC 2992 and NGC 2993.

Black Hole Mass and Eddington Ratio Distribution of Hot Dust-obscured Galaxies

Hot dust-obscured galaxies (Hot DOGs) are a rare population of hyperluminous infrared galaxies discovered by the Wide-field Infrared Survey Explorer mission. Despite the significant obscuration of the active galactic nucleus (AGN) by dust in these systems, pronounced broad and blueshifted emission lines are often observed. Previous work has shown that eight Hot DOGs, referred to as blue-excess Hot DOGs (BHDs), present a blue excess consistent with type 1 quasar emission in their UV–optical spectral energy distributions (SEDs), which has been shown to originate from the light of the obscured central engine scattered into the line of sight. We present an analysis of the rest-frame optical emission characteristics for 172 Hot DOGs through UV–mid-IR SED modeling and spectroscopic details, with a particular focus on the identification of BHDs. We find that while the optical emission observed in Hot DOGs is in most cases dominated by a young stellar population, 26% of Hot DOGs show a significant enough blue excess emission to be classified as BHDs. Based on their broad C iv and Mg ii lines, we find that the mass of the black hole M BH in a BHD ranges from 108.7 to 1010 M ⊙. When using the same emission lines in regular Hot DOGs, we find the M BH estimates cover the entire range found for BHDs while also extending to somewhat lower values. This agreement may imply that the broad lines in regular Hot DOGs also originate from scattered light from the central engine, just as in BHDs, although a more detailed study would be needed to rule out an outflow-driven nature. Similar to z ∼ 6 quasars, we find that Hot DOGs sit above the local relation between stellar and black hole masses, suggesting either that AGN feedback has not yet significantly suppressed the stellar mass growth in the host galaxies or that they will be outliers of the relation when reaching z = 0.

AGN populations in the local Universe: Their alignment with the main sequence, stellar population characteristics, accretion efficiency, and the impact of AGN feedback

In this study, we used a sample of 338 galaxies – within the redshift range of 0.02 &lt; z &lt; 0.1 drawn from the Sloan Digital Sky Survey (SDSS) – for which there are available classifications based on their emission line ratios. We identified and selected Compton-thick (CT) AGN through the use of X-ray and infrared luminosities at 12 μm. We constructed the spectral energy distributions (SEDs) for all sources and fit them using the CIGALE code to derive properties related to both the AGN and host galaxies. Employing stringent criteria to ensure the reliability of SED measurements, our final sample comprises 14 CT AGN, 118 Seyfert 2 (Sy2), 82 composite, and 124 low-ionization nuclear emission-line regions (LINER) galaxies. Our analysis reveals that, irrespective of their classification, the majority of the sources lie below the star-forming main sequence (MS). Additionally, a lower level of AGN activity is associated with a closer positioning to the MS. Using the Dn4000 spectral index as a proxy for the age of stellar populations, we observe that, compared to other AGN classes, LINERs exhibit the oldest stellar populations. Conversely, CT sources are situated in galaxies with the youngest stellar populations. Furthermore, LINER and composite galaxies tend to show the lowest accretion efficiency, while CT AGN, on average, display the most efficient accretion among the four AGN populations. Our findings are consistent with a scenario in which the different AGN populations might not originate from the same AGN activity burst. Early triggers in gas-rich environments can create high-accretion-rate supermassive black holes (SMBHs), leading to a progression from CT to Sy2, while later triggers in gas-poor stages result in low-accretion-rate SMBHs like those found in LINERs.

CHAOS. VIII. Far-ultraviolet Spectra of M101 and the Impact of Wolf–Rayet Stars* ∗Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

We investigate the stellar and nebular properties of nine H ii regions in the spiral galaxy M101 with far-ultraviolet (FUV; ∼900–2000 Å) and optical (∼3200–10,000 Å) spectra. We detect significant C iii] λλ1907,1909 nebular emission in seven regions, but O iii] λ1666 only in the lowest-metallicity region. We produce new analytic functions of the carbon ionization correction factors as a function of metallicity in order to perform a preliminary C/O abundance analysis. The FUV spectra also contain numerous stellar emission and P-Cygni features that we fit with luminosity-weighted combinations of single-burst Starburst99 and BPASS models. We find that the best-fit Starburst99 models closely match the observed very-high-ionization P-Cygni features, requiring very hot, young (≲3 Myr), metal-enriched massive stars. The youngest stellar populations are strongly correlated with broad He ii emission, nitrogen Wolf–Rayet (WR) FUV and optical spectral features, and enhanced N/O gas abundances. Thus, the short-lived WR phase may be driving excess emission in several N P-Cygni wind features (λ955, λ991, λ1720) that bias the stellar continuum fits to higher metallicities relative to the gas-phase metallicities. Accurate characterization of these H ii regions requires additional inclusion of WR stars in the stellar population synthesis models. Our FUV spectra demonstrate that the ∼900–1200 Å FUV can provide a strong test bed for future WR atmosphere and evolution models.

Subaru Suprime-Cam Wide-field BVI Stellar Photometry of the M33 Galaxy

We have surveyed the complete extent of the disk of M33—a gas-rich low-mass dwarf spiral galaxy in the Local Group. The B-, V-, and I-passband (the Johnson–Cousins system) CCD images (typical seeing ∼0.″8) were obtained with the Subaru Telescope equipped with the Suprime-Cam mosaic camera. The wide-field (∼1.°0 × 1.°5) catalog of 803,095 (15 ≤ V ≤ 25) starlike objects, measured using the point-spread function and aperture photometry techniques, is presented. We determined the distance modulus of M33 using the tip of the red giant branch (I TRGB = 20.64 ± 0.02) as a reference point of (m − M)0 = 24.63 ± 0.02stat ± 0.06syst (843 kpc). We found young (≲100 Myr) stellar populations residing up to the deprojected radius of ∼10 kpc. The scale length of the young main-sequence (MS) star surface-number density in the range of radial distances from 7 to 9 kpc is 0.53 ± 0.03 kpc. The youngest MS stars (≲15 Myr) reside up to the radius of ∼8 kpc. This distribution of stellar populations may suggest an outside-in scenario of recent star formation in the disk of M33.

A study of interacting galaxies from the Arp-Madore catalog. Triggering of star formation and nuclear activity

We present Gemini Multi-Object Spectrograph (GMOS) spectroscopic observations of 95 galaxies from the Arp Madore (1987) catalog of peculiar galaxies. These galaxies have been selected because they appear to be in pairs and small groups. These observations have enabled us to confirm that 60 galaxies are indeed interacting systems. For the confirmed interacting sample, we have built a matched control sample of isolated galaxies. We present an analysis of the stellar populations and nuclear activity in the interacting galaxies and compare them with the isolated galaxies. We find a median light (mass) fraction of 55&lt;!PCT!&gt; (10&lt;!PCT!&gt;) in the interacting galaxies coming from stellar populations younger than 2 Gyr and 28&lt;!PCT!&gt; (3&lt;!PCT!&gt;) in the case of the isolated galaxies. More than half of the interacting galaxies are dominated by this young stellar population, while in the isolated ones most of the light comes from older stellar populations. We used a combination of diagnostic diagrams (BPTs and WHAN) to classify the main ionization mechanisms of the gas. The interacting galaxies in our sample consistently show a higher fraction of active galactic nuclei (AGN) relative to the control sample, which ranges between 1.6 and 4 depending on the combination of diagnostic diagrams employed to classify the galaxies and the number of galaxies considered. Our study provides further observational evidence that interactions drive star formation and nuclear activity in galaxies and can have a significant impact on galaxy evolution.

Structure and Kinematics of Star-forming Elliptical Galaxies in SDSS-MaNGA

Using “spatially” resolved spectroscopy, we investigated the characteristics and different modes of formation of stars in elliptical galaxies. We identified an unusual population of 59 star-forming elliptical (SF-E) galaxies in SDSS-MaNGA, our primary sample. To identify these rare star-forming ellipticals, we combined GSWLC-A2 containing outputs of stellar population synthesis models with morphological results from the deep-learning catalog and resolved and integrated properties from the MaNGA Pipe3D value-added catalog. We have also constructed two control samples of star-forming spirals (SF-Sps; 2419 galaxies) and quenched ellipticals (Q-Es; 684 galaxies) to compare with our primary sample of SF-Es. Hα emission line flux of SF-Es is similar to spiral galaxies. The D4000 spectral index indicates that SF-Es have a mixture of old and young stellar populations. Mass-weighted stellar age and metallicity for the SF-Es are lower than the Q-Es and 67% of stellar- and gas-velocity maps of the primary sample show signs of kinematic disturbance. All of these indicate that SF-Es have acquired metal-poor gas through recent mergers or interactions with other galaxies and are forming a new generation of stars. Further, we subdivide our primary sample of SF-Es into four classes based on their bulge to total luminosity ratio (B/T) and spin parameter λre . These four classes have their distinct evolutionary history and modes of formation. Based on these results, we suggest that the Hubble diagram does not accurately capture galaxy evolution processes, and we need a revised morphology diagram like the comb morphology diagram to get a complete picture of the galaxy evolution processes.

Gas conditions of a star-formation selected sample in the first billion years

ABSTRACT We present Atacama Large Millimetre/submillimetre Array (ALMA) observations of the [O iii] 88 $\mu {\rm m}$ emission of a sample of thirteen galaxies at $z = 6$ to 7.6 selected as [C ii]-emitting companion sources of quasars. To disentangle the origins of the luminous Oxygen line in the $z\, \gt\, 6$ Universe, we looked at emission-line galaxies that are selected through an excellent star-formation tracer [C ii] with star-formation rates between 9 and 162 $\rm {\rm M}_{\odot }\,yr^{-1}$. Direct observations reveal [O iii] emission in just a single galaxy ($L_\mathrm{[O\, {\small III}]}/L_\mathrm{[C\, {\small II}]}$$\, = 2.3$), and a stacked image shows no [O iii] detection, providing deep upper limits on the $L_\mathrm{[O\, {\small III}]}/L_\mathrm{[C\, {\small II}]}$ ratios in the $z \gt 6$ Universe ($L_\mathrm{[O\, {\small III}]}/L_\mathrm{[C\, {\small II}]}$$\, \lt 1.2$ at $3 \sigma$). While the fidelity of this sample is high, no obvious optical/near-infrared counterpart is seen in the JWST imaging available for four galaxies. Additionally accounting for low-z CO emitters, line stacking shows that our sample-wide result remains robust: The enhanced $L_\mathrm{[O\, {\small III}]}/L_\mathrm{[C\, {\small II}]}$ reported in the first billion years of the Universe is likely due to the selection towards bright, blue Lyman-break galaxies with high surface star-formation rates or young stellar populations. The deep upper limit on the rest-frame 90 μm continuum emission ($\lt 141 \mu$Jy at $3 \sigma$), implies a low average dust temperature ($T_\mathrm{dust} \lesssim 30\,$ K) and high-dust mass ($M_\mathrm{dust} \sim 10^8\, \mathrm{M}_\odot$). As more normal galaxies are explored in the early Universe, synergy between JWST and ALMA is fundamental to further investigate the ISM properties of the a broad range of samples of high-z galaxies.

The Origin of Young Stellar Populations in NGC 1783: Accretion of External Stars

The presence of young stellar populations in the Large Magellanic Cloud cluster NGC 1783 has caught significant attention, with suggestions ranging from it being a genuine secondary stellar generation to a population of blue straggler stars or simply contamination from background stars. Thanks to multiepoch observations with the Hubble Space Telescope, proper motions for stars within the field of NGC 1783 have been derived, thus allowing accurate cluster membership determination. Here, we report that the younger stars within NGC 1783 indeed belong to the cluster and their spatial distribution is more extended compared to the bulk of the older stellar population, consistent with previous studies. Through N-body simulations, we demonstrate that the observed characteristics of the younger stars cannot be explained solely by blue straggler stars in the context of the isolated dynamical evolution of NGC 1783. Instead, accretion of the external, low-mass stellar system can better account for both the inverse spatial concentration and the radial velocity isotropy of the younger stars. We propose that NGC 1783 may have accreted external stars from low-mass stellar systems, resulting in a mixture of external younger stars and blue straggler stars from the older bulk population, thereby accounting for the characteristics of the younger sequence.

UVIT Study of the MAgellanic Clouds (U-SMAC) – I. Recent star formation history and kinematics of the Shell region in the north-eastern Small Magellanic Cloud

ABSTRACT The interactions between the Magellanic Clouds significantly affect the shape and distribution of the young stellar population, particularly in the periphery of the Small Magellanic Cloud (SMC). We present the first far-UV (FUV) map of the north-east SMC-Shell region using the Ultra Violet Imaging Telescope (UVIT) onboard AstroSat. The detected FUV stars are combined with Gaia Early Data Release 3 data to create a FUV–optical catalogue of ∼14 400 stars. FUV–optical colour-magnitude diagrams are used along with isochrones to estimate the stellar ages. The detected stars are formed in multiple episodes. We identified two episodes of star formation (∼60 and ∼260 Myr ago), where the episode at ∼260 Myr is linked to the recent interaction with the Large Magellanic Cloud (LMC) and the episode at ∼60 Myr is linked to the pericentric passage of the SMC around our Galaxy. The median proper motion (PM) and velocity dispersion are found to be similar to the SMC main body, indicating that this region has not experienced significant tidal effects. The FUV stellar surface density and the dispersion in PM suggest that the extent of the inner SMC in the north-east direction to be around 2.2°. We detect arm-like and arc-like structures in the FUV stellar density map, and their kinematics appear to be similar to the SMC main body. These extended outer features are the spatial stellar overdensities formed over multiple episodes of star formation, but without apparent kinematic distinction.

Evidence for very massive stars in extremely UV-bright star-forming galaxies at z ∼ 2.2–3.6

We present a comprehensive analysis of the presence of very massive stars (VMS &gt; 100 M⊙) in the integrated spectra of 13 UV-bright star-forming galaxies at 2.2 ≲ z ≲ 3.6 taken with the Gran Telescopio Canarias (GTC). These galaxies have very high UV absolute magnitudes (MUV ≃ −24), intense star formation (star formation rate ≃100 − 1000 M⊙ yr−1), and metallicities in the range of 12 + log(O/H) ≃ 8.10 − 8.50 inferred from strong rest-optical lines. The GTC rest-UV spectra reveal spectral features indicative of very young stellar populations with VMS, such as strong P-Cygni line profiles in the wind lines N Vλ1240 and C IVλ1550 along with intense and broad He IIλ1640 emission with equivalent width (EW0) ≃ 1.40 − 4.60 Å, and full width half maximum (FWHM) ≃1150 − 3170 km s−1. A Comparison with known VMS-dominated sources and typical galaxies without VMS reveals that some UV-bright galaxies closely resemble VMS-dominated clusters (e.g., R136 cluster). The presence of VMS is further supported by a quantitative comparison of the observed strength of the He II emission with population synthesis models with and without VMS, where models with VMS are clearly preferred. Employing an empirical threshold for EW0 (He II) ≥ 3.0 Å, along with the detection of other VMS-related spectral profiles (N IVλ1486, 1719), we classify nine out of 13 UV-bright galaxies as VMS-dominated sources. This high incidence of VMS-dominated sources in the UV-bright galaxy population (≈70%) contrasts significantly with the negligible presence of VMS in typical LUV∗ LBGs at similar redshifts (&lt; 1%). Our results thus indicate that VMS are common in UV-bright galaxies, suggesting a different initial mass function (IMF) with upper mass limits between 175 M⊙ and 475 M⊙.

On the Roles of Stellar Rotation and Binarity in NGC 2423's Main-sequence Turnoff Region

Research has shown that many young and intermediate-age clusters (younger than ∼2 Gyr) have extended main sequences and extended main-sequence turnoffs (eMSTOs), which cannot be adequately described by a single isochrone. The reason for the extended main sequences is now known, with the most probable cause being the fast rotation of stars. However, a significant fraction of slowly rotating stars form a younger stellar population than their fast-rotating counterparts, leading to speculation that they have undergone rotational mixing processes internally. One speculation is that a considerable number of slowly rotating stars reside in close binary systems, where tidal forces from companion stars are the cause of their rotational deceleration. In this work, we report a relatively old open star cluster in the Milky Way, NGC 2423 (∼1 Gyr old), which exhibits an apparent eMSTO. As anticipated, many characteristics of NGC 2423 indicate that its eMSTO is driven by stellar rotations. Our calculations indicate that if slowly rotating stars commonly have a close companion star, they should exhibit significant differences in radial velocities observationally, and binary systems that can be tidally locked within the age of NGC 2423 should have a mass ratio close to 1. However, none of these predictions align with our observations. Interestingly, among the only two equal-mass binary systems in the observed region for which spectroscopic data could be obtained, we discovered that one of them is a tidally locked binary system. This further suggests the validity of our numerical simulation results.

A JWST/MIRI and NIRCam Analysis of the Young Stellar Object Population in the Spitzer I Region of NGC 6822

We present an imaging survey of the Spitzer I star-forming region in NGC 6822 conducted with the NIRCam and MIRI instruments on board JWST. Located at a distance of 490 kpc, NGC 6822 is the nearest non-interacting low-metallicity (∼0.2 Z ⊙) dwarf galaxy. It hosts some of the brightest known H ii regions in the local universe, including recently discovered sites of highly embedded active star formation. Of these, Spitzer I is the youngest and most active, and houses 90 color-selected candidate young stellar objects (YSOs) identified from Spitzer Space Telescope observations. We revisit the YSO population of Spitzer I with these new JWST observations. By analyzing color–magnitude diagrams constructed with NIRCam and MIRI data, we establish color selection criteria and construct spectral energy distributions to identify candidate YSOs and characterize the full population of young stars, from the most embedded phase to the more evolved stages. In this way, we have identified 140 YSOs in Spitzer I. Comparing to previous Spitzer studies of the NGC 6822 YSO population, we find that the YSOs we identify are fainter and less massive, indicating that the improved resolution of JWST allows us to resolve previously blended sources into multiple objects.