Characterizing the mechanisms and galaxy properties conducive to the escape of ionizing (LyC) emission is necessary to accurately model the Epoch of Reionization and identify the sources that powered it. Using Hubble Space Telescope data, the Ly α and Continuum Origins Survey (LaCOS) is the first program to obtain uniform, multiwavelength subkiloparsec imaging for a large sample (42) of galaxies observed in LyC and enable statistically robust studies between LyC and resolved galaxy properties. Here, we characterize the morphology and galaxy merger properties of LaCOS galaxies and investigate their connection with the escape fraction of LyC emission f esc LyC . We find strong anticorrelations between f esc LyC and size ( r 20 , r 50 , and r 80 ) measured in filters containing emission from star-forming regions, and with the asymmetry and clumpiness in F150LP, a filter tracing UV continuum and Ly α . We find that ≥48% of LaCOS galaxies, and ≥41% of LaCOS LyC-emitters are visually classified as galaxy mergers. Galaxies robustly identified as mergers in LaCOS are at advanced stages of interaction, close to coalescence. The f esc LyC properties of robust mergers and low-probability mergers cannot be differentiated statistically, and we only find significant difference between the two populations in terms of their sizes and LyC luminosity: robust mergers having larger values. We conclude that (i) f esc LyC tends to be larger in galaxies with a small number of compact, centrally located, UV-emitting star-forming regions, (ii) mergers at advanced stages of interaction represent a sizable fraction of LyC-emitting samples at z ∼ 0.3, and (iii) mergers can facilitate the escape of LyC photons from galaxies.
Context. Hot subdwarf B (sdB) and O (sdO) type stars are evolved helium-burning objects that lost their hydrogen envelope before the helium flash when their progenitors were close to the tip of the red giant branch (RGB). They populate the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). The mass distribution of canonical hot subdwarfs is expected to peak at the core mass required for helium ignition under degenerate conditions in the 0.45-0.5 M⊙ range. However, non-degenerate helium ignition from intermediate-mass progenitors and non-canonical pathways, such as the merger of helium white dwarfs and delayed helium flashes, are also expected to contribute to the hot subdwarf population. Aims. Using high-quality, homogeneous spectra of 335 hot subluminous star candidates from the Arizona-Montréal Spectroscopic Survey, we aim to improve our understanding of the atmospheric and stellar properties of hot subdwarf stars. Our focus is on the mass distribution of the different types of hot subdwarfs and their connections to the various formation scenarios. Methods. We used large grids of model atmospheres to fit the observed spectra and derived their atmospheric parameters: effective temperature (Teff), surface gravity, and helium abundance. The model grids were further utilized to fit the spectral energy distribution of each star and the Gaia parallax was used to compute the stellar parameters radius, luminosity, and mass. Results. Our spectroscopic sample mostly consists of H-rich sdBs and sdOs, but also contains 41 He-rich sdOs. Additionally, the sample includes 11 intermediate-helium stars and 19 horizontal branch objects with Teff > 14 kK. We detected the presence of helium stratification in six sdB stars with Teff around 30 kK, making them good candidates for also showing 3He enrichment in their atmospheres. Our sdB distribution along the EHB shows a gap near 33 kK, visible in both the Kiel (logg-Teff) diagram and HRD, corroborating previous observations and predictions. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47 M⊙, consistent with the canonical formation scenario of helium ignition under degenerate conditions. Among the H-rich hot subdwarfs, we found no difference between the mass distributions of close binaries and apparently single stars. The He-sdOs have a significantly wider mass distribution than their H-rich counterparts, with an average mass of about 0.78 M⊙. In the HRD, the He-sdOs lie on the theoretical helium main sequence for masses between 0.6 and 1 M⊙. This strongly favors a merger origin for these He-rich objects. We identified a small number of candidate low-mass (<0.45 M⊙) sdBs located below the EHB that might have originated from more massive progenitors. These low-mass sdBs preferentially show low helium abundances. Finally, we identified more than 80 pulsating stars in our sample and found that they fall into well-defined p- and g-mode instability regions.
Studying the remnants of young core-collapse supernovae (SNe) can yield insight into the chemical composition of their progenitors and the geometry of the explosions. The supernova remnant (SNR) G292.0+1.8 is one of only three known oxygen-rich SNRs in the galaxy-remnants of core collapse of which relatively pure fragments of ejecta can be seen. Several dozen ejecta knots from G292.0+1.8 were the subject of a proper motion analysis, based on [O iii] 5007 & Aring; images taken over a 22 yr baseline by P. F. Winkler et al. They determined that the transverse velocities of the filaments are linearly proportional to their distances from a common expansion center; thus, the O-rich filaments have been traveling with little deceleration since the initial SN event, similar to 3000 yr ago. In this paper, we use optical spectra of G292.0+1.8, all taken from the Cerro Tololo Inter-American Observatory, to measure radial velocities for 93 knots. Assuming undecelerated expansion, as indicated by the proper motions, the radial velocity should be proportional to the distance from the center along the line of sight, just as the proper motions are proportional to the transverse distance. Therefore, we can map the three-dimensional structure and kinematics of the SNR. We find that the knots generally follow a broad biconical distribution, suggesting that the SN explosion produced broad jets of ejecta. This structure is similar to that seen in some other young core-collapse SNRs.
New populations of red active galactic nuclei (known as “little red dots”) discovered by JWST exhibit remarkable spectral energy distributions. Leveraging X-ray through far-infrared observations of two of the most luminous known little red dots, we directly measure their bolometric luminosities. We find evidence that more than half of the bolometric luminosity likely emerges in the rest-frame optical, with L _bol / L _5100 = 5, roughly half the value for “standard” active galactic nuclei. Meanwhile, the X-ray emitting corona, UV-emitting blackbody, and reprocessed mid to far-infrared emission are all considerably subdominant, assuming that the far-infrared luminosity is well below current measured limits. We present new bolometric corrections that dramatically lower inferred bolometric luminosities by a factor of 10 compared to published values in the literature. These bolometric corrections are in accord with expectations from models in which gas absorption and reprocessing are responsible for the red rest-frame optical colors of little red dots. We discuss how this lowered luminosity scale suggests a lower mass scale for the population by at least an order of magnitude (e.g., ∼10 ^5 –10 ^7 M _⊙ black holes, and ∼10 ^8 M _⊙ galaxies), alleviating tensions with clustering, overmassive black holes, and the integrated black hole mass density in the Universe.
We present COSMOS2025, the COSMOS-Web catalog of photometry, morphology, photometric redshifts, and physical parameters for more than 700 000 galaxies in the Cosmic Evolution Survey (COSMOS) field. This catalog is based on our James Webb Space Telescope 255 h COSMOS-Web program, which provides deep near-infrared imaging in four NIRCam (F115W, F150W, F277W, F444W) and one MIRI (F770W) filter over the central similar to 0.54 deg(2) (similar to 0.2 deg(2) for MIRI) in COSMOS. These data are combined with ground- and space-based data to derive photometric measurements of NIRCam-detected sources using both fixed-aperture photometry (on the space-based bands) and a profile-fitting technique on all 37 bands spanning 0.3 mu m to 8 mu m. We provide morphology for all sources from complementary techniques including profile fitting and machine-learning classification. We derive photometric redshifts, physical parameters, and non-parametric star formation histories from spectral energy distribution (SED) fitting. The catalog has been extensively validated against previous COSMOS catalogs and other surveys. Photometric redshift accuracy measured using spectroscopically confirmed galaxies out to z similar to 9 reaches sigma(MAD) = 0.012 at m(F444W) < 28 and remains at sigma(MAD) less than or similar to 0.03 as a function of magnitude, color, and galaxy type. This represents a factor of similar to 2 improvement at 26 AB mag compared to COSMOS2020. The catalog is approximately 80% complete at log(M-star/M-circle dot) similar to 9 at z similar to 10 and at log(M-star/M-circle dot) similar to 7 at z similar to 0.2, representing a gain of 1 dex compared to COSMOS2020. COSMOS2025 represents the definitive COSMOS-Web catalog. It is provided with complete documentation, together with redshift probability distributions, and it is ready for scientific exploitation today.
