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Abstract:

JWST observations, when combined with Hubble Space Telescope (HST) data, promise to improve age estimates of star clusters in nearby spiral galaxies. However, feedback from young cluster stars pushes out the natal gas and dust, making cluster formation and evolution a challenge to model. Here, we use JWST+ HST observations of the nearby spiral galaxy NGC 628 to produce spectral energy distribution (SED) templates of compact star clusters spanning 275 nm through 21 μm. These preliminary SEDs capture the cluster stars and associated gas and dust within radii of ≈0 .″ 12–0 .″ 67 (corresponding to ≈6–33 pc at the distance of NGC 628). One important finding is that the SEDs of 1, 2, 3, and 4 Myr clusters can be differentiated in the infrared. Another is that, in 80%–90% of the cases we study, the polycyclic aromatic hydrocarbon (PAH) and Hα emission track one another, with the dust responsible for the 3.3 μm PAH emission largely removed by 4 Myr, consistent with pre-supernova stellar feedback acting quickly on the surrounding gas and dust. Nearly embedded cluster candidates have infrared SEDs that are quite similar to optically visible 1–3 Myr clusters. In nearly all cases, we find there is a young star cluster within a few tenths of an arcsec (10–30 pc) of the nearly embedded cluster, suggesting the formation of the cluster was triggered by its presence. The resulting age estimates from the empirical templates are compatible both with dynamical estimates based on CO superbubble expansion velocities, as well as the TODDLERS models, which track spherical evolution of homogeneous gas clouds around young stellar clusters.

Abstract:

We present measurements of the neutral atomic hydrogen (H i) mass function (HiMF) and cosmic H i density (ΩH I) at 0 ≤ z ≤ 0.088 from the Looking at the Distant Universe with MeerKAT Array (LADUMA) survey. Using LADUMA Data Release 1 (DR1), we analyze the HiMF via a new “recovery matrix” method that we benchmark against a more traditional modified maximum likelihood (MML) method. Our analysis, which implements a forward modeling approach, corrects for survey incompleteness and uses extensive synthetic source injections to ensure robust estimates of the HiMF parameters and their associated uncertainties. This new method tracks the recovery of sources in mass bins different from those in which they were injected and incorporates a Poisson likelihood in the forward modeling process, allowing it to correctly handle uncertainties in bins with few or no detections. The application of our analysis to a high-purity subsample of the LADUMA DR1 spectral line catalog in turn mitigates any possible biases that could result from the inconsistent treatment of synthetic and real sources. For the surveyed redshift range, the recovered Schechter function normalization, low-mass slope, and “knee” mass are ϕ*=3.56−1.92+0.97×10−3 Mpc−3 dex−1, α=−1.18−0.19+0.08 , and log(M*/M⊙)=10.01−0.12+0.31 , respectively, which together imply a comoving cosmic H i density of ΩHI=3.09−0.47+0.65×10−4 . Our results show consistency between recovery matrix and MML methods and with previous low-redshift studies, giving confidence that the cosmic volume probed by LADUMA, even at low redshifts, is not an outlier in terms of its H i content.

Abstract:

The neutron star X-ray binary, EXO 0748−676, was observed regularly by the Rossi X-ray Timing Explorer (RXTE) and XMM–Newton during its first detected outburst (1985–2008). These observations captured hundreds of asymmetric, energy-dependent X-ray eclipses, influenced by the ongoing ablation of the companion star and numerous Type I thermonuclear X-ray bursts. Here, we present the light curves of 22 Type I X-ray bursts observed by RXTE that coincide, fully or partially, with an X-ray eclipse. We identify nine instances where the burst occurs entirely within totality, seven bursts split across an egress, and six cases interrupted by an ingress. All in-eclipse and split bursts occurred while the source was in the hard spectral state. We establish that we are not observing direct burst emission during eclipses since the companion star and the ablated outflow entirely obscure our view of the X-ray emitting region. We determine that the reflected flux from the outer accretion disc, even if maximally flared, is insufficient to explain all observations of in-eclipse X-ray bursts and instead explore scenarios whereby the emission arising from the X-ray bursts is scattered, either by a burst-induced rise in that provides extra material, an accretion disc wind or the ablated outflow, into our line of sight. However, the rarity of a burst and eclipse overlap makes it challenging to determine their origin.