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Abstract:

We present a comparison of observed polycyclic aromatic hydrocarbon (PAH) feature ratios in 19 nearby galaxies with a grid of theoretical expectations for near- and mid-infrared dust emission. The PAH feature ratios are drawn from Cycle 1 JWST observations and are measured for 7224 stellar clusters and 29,176 stellar associations for which we have robust ages and mass estimates from Hubble Space Telescope five-band photometry. Though there are galaxy-to-galaxy variations, the observed PAH feature ratios largely agree with the theoretical models, particularly those that are skewed toward more ionized and larger PAH size distributions. For each galaxy we also extract PAH feature ratios for 200 pc wide circular regions in the diffuse interstellar medium, which serve as a noncluster/association control sample. Compared to what we find for stellar clusters and associations, the 3.3 μm/7.7 μm and 3.3 μm/11.3 μm ratios from the diffuse interstellar medium are ∼0.10–0.15 dex smaller. When the observed PAH feature ratios are compared to the radiation field hardness as probed by the [O iii]/Hβ ratio, we find anticorrelations for nearly all galaxies in the sample. These results together suggest that the PAH feature ratios are driven by the shape and intensity of the radiation field and that the smallest PAHs—observed via JWST F335M imaging—are increasingly “processed” or destroyed in regions with the most intense and hard radiation fields.

Abstract:

Wide, highly eccentric (𝑒 >0.9) compact binaries can naturally arise as progenitors of gravitational wave (GW) mergers. These systems are expected to have a significant population in the mHz band (e.g., ∼3–45 detectable stellar-mass binary black holes with 𝑒 >0.9 in the Milky Way), with their GW signals characterized by “repeated bursts” emitted upon each pericenter passage. In this study, we show that the detection of mHz GW signals from highly eccentric stellar mass binaries in the local universe can strongly constrain their orbital parameters. Specifically, it can achieve a relative measurement error of ∼10−6 for orbital frequency and ∼1% for eccentricity (as 1 −𝑒) in most of the detectable cases. On the other hand, the binary’s mass ratio, distance, and intrinsic orbital orientation may be less precisely determined due to degeneracies in the GW waveform. We also perform mock LISA data analysis to evaluate the realistic detectability of highly eccentric compact binaries. Our results show that highly eccentric systems could be efficiently identified when multiple GW sources and stationary Gaussian instrumental noise are present in the detector output. This work highlights the potential of extracting the signal of “bursting” LISA sources to provide valuable insights into their orbital evolution, surrounding environment, and formation channels.

Abstract:

Active galactic nucleus (AGN) growth in disc-dominated, merger-free galaxies is poorly understood, largely due to the difficulty in disentangling the AGN emission from that of the host galaxy. By carefully separating this emission, we examine the differences between AGNs in galaxies hosting a (possibly) merger-grown, classical bulge, and AGNs in secularly grown, truly bulgeless disc galaxies. We use galfit to obtain robust, accurate morphologies of 100 disc-dominated galaxies imaged with the Hubble Space Telescope. Adopting an inclusive definition of classical bulges, we detect a classical bulge component in per cent of the galaxies. These bulges were not visible in Sloan Digital Sky Survey photometry, however these galaxies are still unambiguously disc-dominated, with an average bulge-to-total luminosity ratio of . We find some correlation between bulge mass and black hole mass for disc-dominated galaxies, though this correlation is significantly weaker in comparison to the relation for bulge-dominated or elliptical galaxies. Furthermore, a significant fraction ( per cent) of our black holes are overly massive when compared to the relationship for elliptical galaxies. We find a weak correlation between total stellar mass and black hole mass for the disc-dominated galaxies, hinting that the stochasticity of black hole–galaxy co-evolution may be higher in disc-dominated than bulge-dominated systems.