Hubble Space Telescope Research Articles

DAmodel: hierarchical Bayesian modelling of DA white dwarfs for spectrophotometric calibration

ABSTRACT We use hierarchical Bayesian modelling to calibrate a network of 32 all-sky faint DA white dwarf (DA WD) spectrophotometric standards ($16.5 < V < 19.5$) alongside three CALSPEC standards, from 912 Å to 32 $\mu$m. The framework is the first of its kind to jointly infer photometric zero points and WD parameters (surface gravity $\log g$, effective temperature $T_{\text{eff}}$, extinction $A_V$, dust relation parameter $R_V$) by simultaneously modelling both photometric and spectroscopic data. We model panchromatic Hubble Space Telescope Wide Field Camera 3 (HST/WFC3) UVIS and IR photometry, HST/STIS UV spectroscopy, and ground-based optical spectroscopy to sub-per cent precision. Photometric residuals for the sample are the lowest yet yielding $<0.004$ mag RMS on average from the UV to the NIR, achieved by jointly inferring time-dependent changes in system sensitivity and WFC3/IR count-rate nonlinearity. Our GPU-accelerated implementation enables efficient sampling via Hamiltonian Monte Carlo, critical for exploring the high-dimensional posterior space. The hierarchical nature of the model enables population analysis of intrinsic WD and dust parameters. Inferred spectral energy distributions from this model will be essential for calibrating the James Webb Space Telescope as well as next-generation surveys, including Vera Rubin Observatory’s Legacy Survey of Space and Time and the Nancy Grace Roman Space Telescope.

Compact Obscured Nuclei. II. Chaotic Central Dust Structures in NGC 4418

Abstract Compact Obscured Nuclei (CONs) are rare types of galactic nuclei obscured by dense, dusty molecular gas. Dusty structures extending beyond hidden CONs produce observable absorption features revealing connections between CONs and their environments. We analyzed central dust features in the CON galaxy NGC 4418, a Luminous Infrared Galaxy using Hubble Space Telescope images. We mapped dust absorption features, calculated optical depths, and derived limits on associated gas masses from color-ratio images. We find NGC 4418 contains complex, three-dimensional spatially resolved dust structures near its CON having τ V ≈ 2–3. This medium is a candidate for the foreground responsible for the deep mid-infrared silicate absorption toward the CON. Our map of the inner dust structures agrees with molecular studies in suggesting the complex structure of the central molecular gas NGC 4418 reflects the ongoing interaction with its companion and the wind from the CON.

OGLE-2011-BLG-0462: An Isolated Stellar-mass Black Hole Confirmed Using New HST Astrometry and Updated Photometry

Abstract The long-duration Galactic-bulge microlensing event OGLE-2011-BLG-0462 produced relativistic astrometric deflections of the source star, which we measured using Hubble Space Telescope (HST) observations taken at eight epochs over ∼6 yr. Analysis of the microlensing light curve and astrometry led our group (followed by other independent groups) to conclude that the lens is an isolated stellar-mass black hole (BH)—the first and only one unambiguously discovered to date. There have now been three additional epochs of HST observations, increasing the astrometric time baseline to 11 yr. Additionally, the ground-based OGLE data have been updated. We have reanalyzed the data, including the new HST astrometry, and photometry obtained with 16 different telescopes. The source lies only 0 . ″ 4 from a bright neighbor, making it crucial to perform precise subtraction of its point-spread function (PSF) in the astrometric measurements of the source. Moreover, we show that it is essential to perform a separate PSF subtraction for each individual HST frame as part of the reductions. Our final solution yields a lens mass of 7.15 ± 0.83 M ⊙. Combined with the lack of detected light from the lens at late HST epochs, the BH nature of the lens is conclusively verified. The BH lies at a distance of 1.52 ± 0.15 kpc, and it is moving with a space velocity of 51.1 ± 7.5 km s−1 relative to the stars in the neighborhood. We compare our results with those of other studies and discuss reasons for the differences. We also searched for binary companions of the BH at a range of separations, but found no evidence for any.

Hα Variability of AB Aur b with the Hubble Space Telescope: Probing the Nature of a Protoplanet Candidate with Accretion Light Echoes

Abstract Giant planets generate accretion luminosity as they form. Much of this energy is radiated in strong Hα line emission, which has motivated direct imaging surveys at optical wavelengths to search for accreting protoplanets. However, compact disk structures can mimic accreting planets by scattering emission from the host star. This can complicate the interpretation of Hα point sources, especially if the host star itself is accreting. We describe an approach to distinguish accreting protoplanets from scattered-light disk features using “accretion light echoes.” This method relies on variable Hα emission from a stochastically accreting host star to search for a delayed brightness correlation with a candidate protoplanet. We apply this method to the candidate protoplanet AB Aur b with a dedicated Hubble Space Telescope Wide Field Camera 3 program designed to sequentially sample the host star and the candidate planet in Hα while accounting for the light travel time delay and orbital geometry of the source within the protoplanetary disk. Across five epochs spanning 14 months, AB Aur b is over 20 times more variable than its host star; AB Aur’s Hα emission changes by 15% while AB Aur b varies by 330%. These brightness changes are not correlated, which rules out unobstructed scattered starlight from the host star as the only source of AB Aur b’s Hα emission and is consistent with tracing emission from an independently accreting protoplanet, inner disk shadowing effects, or a physically evolving compact disk structure. More broadly, accretion light echoes offer a novel tool to explore the nature of protoplanet candidates with well-timed observations of the host star prior to deep imaging in Hα.

Multiwavelength Study of a Hyperluminous X-Ray Source near NGC 6099: A Strong IMBH Candidate

Abstract We report on the intriguing properties of a variable X-ray source projected at the outskirts of the elliptical galaxy NGC 6099 (d ≈ 139 Mpc). If truly located near NGC 6099, this is a hyperluminous X-ray source that reached an X-ray luminosity L X ≈ a few times 1042 erg s−1 in 2012 February (XMM-Newton data), about 50–100 times brighter than in 2009 May (Chandra) and 2023 August (XMM-Newton). The X-ray spectrum was soft at all three epochs, with a thermal component at kT ≈ 0.2 keV and a power-law photon index >3. Such properties make it a strong candidate for an intermediate-mass black hole (IMBH). We also discovered a point-like, blue optical counterpart (m g,Vega ≈ 24.7 mag, M g,Vega ≈ −11.2 mag), from images taken by the Canada–France–Hawaii Telescope and later confirmed with Hubble Space Telescope observations. The optical continuum can be modeled as stellar emission from a compact star cluster or an X-ray-irradiated accretion disk, consistent with the IMBH scenario. We discuss alternative explanations for the nature of this system. A possible scenario is tidal stripping of an orbiting star, with repeated X-ray outbursts every few years. An alternative possibility is that the thermal X-ray emission seen in 2009 was from shocked gas in the self-intersecting tidal stream during the rising phase of a tidal disruption event, while the 2012 and 2023 emissions were from the fully formed accretion disk.

The Dwarf Irregular Galaxy NGC 6822. II. Young, Intermediate and Old Stellar Populations: Comparison between Theory and Observations

Abstract This paper presents a quantitative analysis of the stellar content in the Local Group dwarf irregular galaxy NGC 6822 by comparing stellar evolution models and observations in color–magnitude diagrams (CMDs) and color–color diagrams (CC-Ds). Our analysis is based on optical ground-based g, r, i photometry, and deep archival Hubble Space Telescope photometry of two fields in the galactic disk. We compared young, intermediate-age, and old stellar populations with isochrones from the BaSTI-IAC library and found that NGC 6822 hosts a quite metal-rich ([Fe/H] = −0.7 to −0.4) young component with an age ranging from 20–100 Myr. The intermediate-age population experienced a modest chemical enrichment between 4 and 8 Gyr ago, while stars older than 11 Gyr have a low metal abundance ([Fe/H] ∼ −1.70). We also identified the asymptotic giant branch (AGB) clump population with a luminosity peak at i ∼ 23.35 mag. Our analysis of both the CMD and the optical–near-IR (NIR)–mid-IR (MIR) CC-Ds of AGB oxygen- and carbon-rich stars, using the PARSEC+COLIBRI isochrones with and without circumstellar dust, reveals that this stellar component exhibits a spread in age from 1–2 Gyr and in metallicity between [Fe/H] = −1.30 and −1.70. The stellar models we used reproduce very well the two distinct color sequences defined by AGB O- and C-rich stars in the various optical–NIR–MIR CC-Ds, suggesting that they are reliable diagnostics to identify and characterize intermediate-age stellar populations. However, we also find that evolutionary prescriptions in the optical i-(r − i) CMDs predict, at fixed color, systematically lower luminosities than observed AGB stars.

Evidence of star cluster migration and merger in dwarf galaxies

Nuclear star clusters (NSCs) are the densest stellar systems in the Universe. These clusters can be found at the centre of all galaxy types but tend to favour galaxies of intermediate stellar mass around 109M⊙ (refs. 1,2). At present, two main processes are under debate to explain their formation: in situ star formation from gas infall3 and migration and merging of globular clusters (GCs) caused by dynamical friction4. Studies5, 6, 7, 8–9 of NSC stellar populations suggest that the former predominates in massive galaxies, whereas the latter prevails in dwarf galaxies, and both contribute equally at intermediate mass. However, until now, no ongoing merger of GCs has been observed to confirm this scenario. Here we report the serendipitous discovery of five dwarf galaxies with complex nuclear regions, characterized by multiple nuclei and tidal tails, using high-resolution images from the Hubble Space Telescope. These structures have been reproduced in complementary N-body simulations, supporting the interpretation that they result from migrating and merging of star clusters. The small detection rate and short simulated timescales (below 100 Myr) of this process may explain why this has not been observed previously. This study highlights the need for large surveys with high resolution to fully map the migration scenario steps.

Lack of Rest-frame Ultraviolet Variability in Little Red Dots Based on HST and JWST Observations

Abstract Variability is a fundamental signature for active galactic nuclei (AGN) activity and serves as an unbiased indicator for rapid instability happening near the center of supermassive black holes (SMBHs). Previous studies showed that AGN variability does not have strong redshift evolution, and scales with their bolometric luminosity and BH mass, making it a powerful probe to identify low-mass, low-luminosity AGNs at high redshift. JWST has discovered a new population of high-redshift galaxies likely hosting moderate accreting BHs (>106 M ⊙)—the little red dots (LRDs; z ∼ 4–10). In this Letter, we study the variability of a sample of 22 LRDs with V-shaped spectral energy distributions in three JWST deep fields that also have reliable Hubble Space Telescope observations in closely paired filters at 1–2 μm (rest-frame UV), with the time difference between 6 and 11 yr. This LRD sample covers a redshift range of 3 < z < 8 with −21.3 < M UV < −18.4. Based on both photometry and imaging difference analyses, we find a mean magnitude difference of ∼0.15 ± 0.26 mag, with none of the LRDs showing photometric variability at 3σ significance. Extrapolation of Sloan Digital Sky Survey quasar variability predicts a magnitude change of order 0.3 mag for our LRD sample. This suggests an upper limit of about ∼30% AGN contribution to the total observed UV light in our sample of LRDs.

Echoes in the Noise: Posterior Samples of Faint Galaxy Surface Brightness Profiles with Score-based Likelihoods and Priors

Abstract Examining the detailed structure of galaxy populations provides valuable insights into their formation and evolution mechanisms. Significant barriers to such analysis are the nontrivial noise properties of real astronomical images and the point-spread function, which blurs structure. Here we present a framework which combines recent advances in score-based likelihood characterization and diffusion model priors to perform a Bayesian analysis of image deconvolution. The method, when applied to minimally processed Hubble Space Telescope data, recovers structures which have otherwise only become visible in next-generation James Webb Space Telescope imaging.

I-band Asymptotic Giant Branch (IAGB) Stars. II. A First Estimate of Their Precision and a Differential Zero Point**Based on observations made with the NASA/ESA Hubble Space Telescope, obtained 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.

Abstract Hubble Space Telescope observations of 92 galaxies that have a strong showing of I-band asymptotic giant branch (IAGB) stars in their color–magnitude diagrams (CMDs) are used to measure the relative offset between the mean apparent I-band magnitudes of the IAGB population and the corresponding apparent I-band magnitudes of the TRGB as measured in the same frames (and CMDs) of those individual galaxies. This first exploratory, large-sample comparison is independent of any extinction (foreground or internal) that may be shared by these two populations. The marginalized luminosity functions used to determine the modal value of the IAGB population are well fit by a single, symmetric Gaussian. The difference in the two apparent magnitudes (in the sense IAGB minus TRGB) is −0.589 mag, with a combined standard deviation of ±0.119 mag. Adopting M I = −4.05 mag for the TRGB stars, the modal absolute magnitude of the IAGB is then calculated to be M I (IAGB) = −4.64 ± 0.12 mag. The ensemble dispersion quoted above gives a standard error on the mean of ±0.012 mag (based on the full sample of 92 galaxies). Independently, the three geometry-based zero-points for I-band AGB stars are found (in Paper I) to be M I = −4.49 ± 0.003 mag in the LMC (4204 stars), M I = −4.67 ± 0.008 mag for the SMC (916 stars), and M I = −4.78 ± 0.030 mag for NGC 4258 (62 stars), leading to a global zero-point (weighted) average of < M I > = −4.64 ± 0.15 mag (stat). The scatter found in the anchors is comparable to the scatter in the field sample discussed here, but the calibration sample is small. The application of this method to galaxies well outside of the Local Group shows that these standard candles can readily be found and measured out to at least 9 Mpc, using already available archival data.

EPOCHS paper – X. Environmental effects on Galaxy formation and protocluster Galaxy candidates at 4.5 < z < 10 from JWST observations

ABSTRACT In this paper, we describe our search for galaxy protocluster candidates at $4.5{<} z {<} 10$ and explore the environmental and physical properties of their member galaxies identified through JWST wide-field surveys within the CEERS (Cosmic Evolution Early Release Science), JADES (JWST Advanced Deep Extragalactic Survey), and PEARLS (Prime Extragalactic Areas for Reionization and Lensing Science) NEP-TDF (North Ecliptic Pole Time-Domain Fields). Combining with Hubble Space Telescope (HST) data, we identify 2948 robust $z>4.5$ galaxy candidates within an area of 185.4 arcmin$^2$. We determine nearest neighbour statistics and galaxy environments. We find that high-z galaxies in overdense environments exhibit higher star formation activity compared to those in underdense regions. Galaxies in dense environments have a slightly increased star formation rate (SFR) at a given mass compared with galaxies in the lower density environments. At the high-mass end, we also find a gradual flattening of the $M_{\star }$-SFR slope. We find that galaxies in high-density regions often have redder ultraviolet slopes than those in low-density regions, suggesting more dust extinction, weaker Ly α emission, and/or a higher damped Ly α absorption. The mass–size relation shows a weak positive correlation between galaxy size and local density. Furthermore, we quantitatively assess the probability of a galaxy belonging to a protocluster candidate. In total, we identified 26 overdensities at $z=5{\!-\!}7$ and estimate their dark matter halo masses. We find that all proto-cluster candidates could evolve into clusters with $M_{\rm halo} > 10^{14}{\rm M}_{\odot }$ at $z = 0$, thereby supporting the theoretical and simulation predictions of cluster formation. Notably, this marks an early search for protocluster candidates in JWST wide field based on photometric data, providing valuable candidates to study cosmic structure formation at the early stages.

The Cosmic Evolution Early Release Science Survey (CEERS)

Abstract We present the Cosmic Evolution Early Release Science (CEERS) Survey, a 77.2 hr Director’s Discretionary Early Release Science Program. CEERS demonstrates, tests, and validates efficient extragalactic surveys using coordinated, overlapping parallel observations with the JWST instrument suite, including NIRCam and MIRI imaging, NIRSpec low- (R ∼ 100) and medium- (R ∼ 1000) resolution spectroscopy, and NIRCam slitless grism (R ∼ 1500) spectroscopy. CEERS targets the Hubble Space Telescope–observed region of the Extended Groth Strip field, supported by a rich set of multiwavelength data. CEERS facilitated immediate community science in both of the extragalactic core JWST science drivers “First Light” and “Galaxy Assembly,” including: (1) the discovery and characterization of large samples of galaxies at z ≳ 10 from ∼90 arcmin2 of NIRCam imaging, constraining their abundance and physical nature; (2) deep spectra of >1000 galaxies, including dozens of galaxies at 6 < z < 10, enabling redshift measurements and constraints on the physical conditions of star formation and black hole growth via line diagnostics; (3) quantifying the first bulge, bar, and disk structures at z > 3; and (4) characterizing galaxy mid-IR emission with MIRI to study dust-obscured star formation and supermassive black hole growth at z ∼ 1–3. As a legacy product for the community, the CEERS team has provided several data releases, accompanied by detailed notes on the data reduction procedures and notebooks to aid in reproducibility. In addition to an overview of the survey and the quality of the data, we provide science highlights from the first two years with CEERS data.

The Transmission Spectrum of the Potentially Rocky Planet L 98-59 c

Abstract We present observations of the 1.35 ± 0.07 Earth radius planet L 98-59 c, collected using Wide Field Camera 3 on the Hubble Space Telescope (HST). L 98-59 is a nearby (10.6 pc), bright (H = 7.4 mag) M3V star that harbors three small, transiting planets. As one of the closest known transiting multi-planet systems, L 98-59 offers one of the best opportunities to probe and compare the atmospheres of rocky planets that formed in the same stellar environment. We measured the transmission spectrum of L 98-59 c, and the extracted spectrum showed marginal evidence (2.1σ) for wavelength-dependent transit depth variations that could indicate the presence of an atmosphere. We forward-modeled possible atmospheric compositions of the planet based on the transmission spectrum. Although L 98-59 was previously thought to be a fairly quiet star, we have seen evidence for stellar activity, and therefore we assessed a scenario where the source of the signal originates with inhomogeneities on the stellar surface. We also see a correlation between transits of L 98-59 c and L 98-59 b collected 12.5 hr apart, which is suggestive (but at <2σ confidence) of a contaminating component from the star impacting the exoplanet spectrum. While intriguing, our results are inconclusive and additional data are needed to verify any atmospheric signal. Fortunately, additional data have been collected from both the HST and James Webb Space Telescope. Should this result be confirmed with additional data, L 98-59 c would be the first planet smaller than 2 Earth radii with a detected atmosphere.

Evidence of a Disk Wind Origin for Fluorescent H2 in Classical T Tauri Stars

Abstract We use far-ultraviolet spectra of 36 T Tauri stars, predominately from the Hubble Space Telescope (HST) ULLYSES program, to examine the kinematic properties of fluorescent H2 emission lines for evidence of disk outflows. Leveraging improvements to the HST Cosmic Origins Spectrograph wavelength solution, we coadd isolated lines within four fluorescent progressions ([v’,J’] = [1,4], [1,7], [0,2], and [3,16]) to improve signal-to-noise ratio (S/N), and we fit each coadded line profile with one or two Gaussian components. Of the high-S/N line profiles (S/N ≥ 12 at the peak of the profile), over half are best fit with a combination of a broad and a narrow Gaussian component. For profiles of the [1,4] and [1,7] progressions, we find a systematic blueshift of a few kilometers per second between the broad and narrow centroid velocities and stellar radial velocities. For the [0,2] progression, we find centroid velocities consistently blueshifted with respect to stellar radial velocities on the order of −5 km s−1 for the single and narrow components, and −10 km s−1 for the broad components. Overall, the blueshifts observed in our sample suggest that the molecular gas traces an outflow from a disk wind in some sources, and not solely disk gas in Keplerian rotation. The low-velocity systematic blueshifts, as well as emitting radii as inferred from line FWHMs, observed in our sample are similar to those observed with optical [O i] surveys of T Tauri stars. We estimate H2 mass-loss rates of 10−9 to 10−11 M ⊙ yr−1, but incomplete knowledge of wind parameters limits comparisons to global models.

The infrared counterpart and proper motion of magnetar SGR 0501+4516

Aims. Soft gamma repeaters (SGRs) are highly magnetised neutron stars (magnetars) notable for their gamma-ray and X-ray outbursts. We used near-infrared (NIR) imaging of SGR 0501+4516 in the days, weeks, and years after its 2008 outburst to characterise the multi-wavelength emission, and to obtain a proper motion from our long temporal baseline observations. Methods. We present short- and long-term monitoring of the IR counterpart of SGR 0501+4516 and a measurement of its proper motion. Unlike most magnetars, the source has only moderate foreground extinction with minimal crowding. Our observations began only ∼2 hours after the first activation of SGR 0501+4516 in August 2008 and continued for ∼4 years, including two epochs of Hubble Space Telescope (HST) imaging. The proper motion constraint was improved using a third HST epoch from 10 years later. Results. The NIR and X-rays faded slowly during the first week, which was followed by a steeper power-law decay. The behaviour is satisfactorily fit by a broken power law. Three epochs of HST imaging with a 10-year baseline allowed us to determine the quiescent level and to measure a proper motion of μ = 5.4 ± 0.6 mas yr−1. This corresponds to a low transverse peculiar velocity of v ≃ 51 ± 14 km s−1 (at 2 kpc). The magnitude and direction of the proper motion rules out supernova remnant HB9 as the birth site. We can find no other supernova remnants or groups of massive stars within the region traversed by SGR 0501+4516 during its characteristic lifetime (∼20 kyr). Conclusions. Our observations of SGR 0501+4516 suggest three possibilities: that some magnetars are significantly older than expected, that their progenitors produce low supernova ejecta masses, or that they can be formed through accretion-induced collapse or low-mass neutron star mergers. Although the progenitor of SGR 0501+4516 remains unclear, we propose that SGR 0501+4516 is the best Galactic candidate for a magnetar formed through a mechanism other than massive star core-collapse.

Lyman Continuum Leakers at z > 3 in the GOODS-S Field: Mergers Dominated

Abstract Understanding the ionizing photon escape from galaxies is essential for studying cosmic reionization. With a sample of 23 Lyman continuum (LyC) leakers (among which eight are high-confidence leakers) at 3 < z < 4.5 in the GOODS-S field, we investigate their morphologies using high-resolution data from the Hubble Space Telescope and the James Webb Space Telescope. We find that 20 of the 23 LyC leakers (seven out of eight high-confidence leakers) show merging signatures via visual inspection, while the remaining three are starbursts. The merger fraction of our sample is significantly higher than that of normal galaxies at similar or higher redshifts. Even when using the nonparametric method, which may miss some mergers in a sample, the merger fraction remains high, especially for the high-confidence leakers. Based on our previous finding that LyC leakers are not necessarily starbursts while some are in the star formation main sequence, we further find that those in the main sequence show merger signatures. Our results suggest that LyC leakers are either starbursts or mergers, both of which can facilitate the LyC photon escape, in addition to generating more LyC photons. Furthermore, we show that high-z LyC leakers are statistically more extended than those selected at low redshift, which exhibit a higher merger fraction as size increases. This is likely due to the observational bias that the spatial resolution limits the detection of high-z compact galaxies, while low redshift LyC leakers are more selected as compact starbursts.

The middle-aged pulsar PSR J1741–2054 and its bow-shock nebula in the far-ultraviolet

Context. Far-ultraviolet (FUV) observations of pulsars allow us to measure surface temperatures of neutron stars and study their thermal evolution. Some pulsars can exhibit FUV bow-shock nebulae (BSNe), providing an additional tool for probing the interstellar medium and studying the pulsar’s properties. The nearby middle-aged gamma-ray pulsar J1741–2054 and its pulsar wind nebula (PWN) have been studied in X-rays, and its BSN has been investigated in the Balmer lines, but they have never been observed in the FUV. Aims. To further study the thermal and magnetospheric emission from PSR J1741–2054 and the BSN properties, we observed them in the FUV range with the Hubble Space Telescope (HST). Methods. We imaged the target in two FUV filters of the HST ACS/SBC detector. We also reanalyzed previous optical observations of the pulsar and its BSN. We fit the pulsar’s FUV-optical spectrum separately and together with its X-ray spectrum. Results. We found that the pulsar’s FUV-optical spectrum consists of a thermal and a nonthermal component. A joint fit of the FUV-optical and X-ray spectra with combinations of the nonthermal and thermal components showed a hard optical nonthermal spectrum with a photon index Γopt ≈ 1.0–1.2 and a softer X-ray component, ΓX ≈ 2.6–2.7. The thermal emission is dominated by the cold component with the temperature kTcold ≈ 40–50 eV and emitting sphere radius Rcold ≈ 8–15 km, at d = 270 pc. An additional hot thermal component, with kThot ∼ 80 eV and Rhot ∼ 1 km, is also possible. Such a spectrum resembles the spectra of other middle-aged pulsars, but it shows a harder (softer) optical (X-ray) nonthermal spectrum. We detected the FUV BSN, the first one associated with a middle-aged pulsar. Its closed-shell morphology is similar to the Hα BSN morphology, while its FUV flux, ∼10−13 erg cm−2 s−1, is a factor of ∼4 higher than the Hα flux. This FUV BSN has a higher surface brightness than the two previously known BSNe.

First detections of CO absorption in the Magellanic Clouds and direct measurement of the CO-to-H2 ratio

Molecular hydrogen (H2) is by far the most abundant molecule in the Universe. However, due to the low emissivity of H2, carbon monoxide (CO) is widely used instead to trace molecular gas in galaxies. The relative abundances of these molecules is expected to depend on both physical (e.g. density) and chemical (e.g. metal enrichment) properties of the gas, making direct measurements in diverse environments crucial. We present a systematic search for CO in absorption towards 34 stars behind H2 gas in the Magellanic Clouds using the Hubble Space Telescope. We report the first two definitive detections of CO absorption in the Large Magellanic Cloud (LMC) and one in the Small Magellanic Cloud (SMC), along with stringent upper limits for the remaining sightlines. Non-detections of CO are consistent with models of low thermal pressures and/or low metallicities while detections at the lower metallicities of the Magellanic Clouds require higher thermal pressures, Pth = 105 − 106 K cm−3 than detections the Milky Way at similar N(H2). Notably, the high density derived from the rotational excitation of CO towards Sk 143 in the SMC suggests full molecularisation of CO in the absorbing cloud, with CO/H2 = 8.3−1.6+2.0 × 10−5 consistent with the standard ratio (3.2 × 10−4) measured in dense molecular gas in the Milky Way, scaled to the SMC’s 0.2 Z⊙ metallicity.

The Structure of the Local Interstellar Medium. VII. Additional New Mg ii, Fe ii, and Mn ii Observations toward Stars within 100 pc

Abstract We present new high-resolution (R ∼ 114,000), near-ultraviolet spectra in the direction of 36 nearby (<100 pc) stars to uncover the structure and composition of the local interstellar medium (LISM). The spectra were obtained using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We model the LISM absorption due to Mg ii, Fe ii, and Mn ii in order to measure new velocities, Doppler broadening parameters, and column densities for interstellar clouds; refine the extent of known clouds; and identify new clouds. Based on our analyses of these 36 new sight lines, we find one to four discrete LISM features for each individual sight line, with the number of components remaining relatively flat until increasing at ≈60 pc. By comparing our final measurements with a dynamical model of the LISM by S. Redfield & J. L. Linsky, we determine that 77% of the predictions made by the kinematic model are confirmed by observed components in the new lines of sight and identify 10 LISM features among eight sight lines that do not match the model. In addition to testing and refining three-dimensional kinematic models of the LISM, the substantially increased number of integrated Mg ii emission fluxes presented here will be valuable for predicting the far-ultraviolet and extreme-ultraviolet emission for these stars. We also find that six stars in our sample show possible circumstellar disk absorption, including 47 UMa, for which a disk has not yet been confirmed.

Kinematics of metallicity populations in Omega Centauri using the Gaia Focused Product Release and Hubble Space Telescope

Context. Omega Cen is the largest known globular cluster in the Milky Way. It is also a quite complex object with a large metallicity spread and multiple stellar populations. Despite a number of studies over the past several decades, the series of events that led to the formation of this cluster is still poorly understood. One of its peculiarities is the presence of a metal-rich population that does not show the phenomenon of light-element anti-correlations (C-N, Na-O, Mg-Al), a trait that is considered characteristic of Galactic globular clusters and present among more metal-poor Omega Cen stars. This leads to speculation that such an anomalous population was accreted by the cluster. Aims. We aim to investigate the kinematics of Omega Cen populations to gain insight into the formation scenario of the cluster. Methods. Using the newly released Gaia FPR and DR3 catalogue, we conducted a detailed kinematical analysis of cluster members within Omega Cen. The cluster members were divided into four metallicity populations, and their mean proper motion in radial and tangential components were compared with each other. We also performed Gaussian-mixture model fitting on the metallicity distribution to estimate the number of populations within our sample and an independent analysis of the Hubble Space Telescope catalogue as confirmation. Results. The mean proper motions (μr and μt) of the metallicity populations do not show any significant differences. It is also not dependent on the approach chosen to determine the number of metallicity populations. We do find a clear signature of rotation in all of the populations (including the metal-rich one) with similar velocities.