Galaxy formation and evolution

Abstract:

Polycyclic aromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM) of near solar metallicity galaxies, where they play a critical role in ISM heating, cooling, and reprocessing stellar radiation. The PAH fraction, the abundance of PAHs relative to total dust mass, is a key parameter in ISM physics. Using JWST and MUSE observations of 42 galaxies from the PHANGS survey, we analyzed the PAH fraction in over 17 000 H  II regions spanning a gas-phase oxygen abundance of 12 + log(O/H) = 8.0–8.8 ( Z ∼ 0.2–1.3 Z ⊙ ), and ∼400 isolated supernova remnants (SNRs). We find a significantly lower PAH fraction toward H  II regions compared to a reference sample of diffuse ISM areas at matched metallicity. At 12 + log(O/H) > 8.2, the PAH fraction toward H  II regions is strongly anti-correlated with the local ionization parameter, suggesting that PAH destruction is correlated with ionized gas and/or hydrogen-ionizing UV radiation. At lower metallicities, the PAH fraction declines steeply in H  II regions and in the diffuse ISM, likely reflecting less efficient PAH formation in metal-poor environments. Carefully isolating dust emission from the vicinity of optically identified supernova remnants, we see evidence of selective PAH destruction from measurements of lower PAH fractions, which is, however, indistinguishable at ∼50 pc scales. Overall, our results point to ionizing radiation as the dominant agent of PAH destruction within H  II regions; metallicity plays a key role in their global abundance in galaxies.

Abstract:

The stellar velocity dispersion ( σ ) of massive elliptical galaxies is a key ingredient in breaking the mass-sheet degeneracy and obtaining precise and accurate cosmography from gravitational time delays. The relative uncertainty on the Hubble constant H 0 is double the relative error on σ . Therefore, time-delay cosmography imposes much more demanding requirements on the precision and accuracy of σ than galaxy studies. While precision can be achieved with an adequate signal-to-noise ratio (S/N), the accuracy critically depends on key factors such as the elemental abundance and temperature of stellar templates, flux calibration, and wavelength ranges. We carried out a detailed study of the problem using multiple sets of galaxy spectra of massive elliptical galaxies with S/N ∼ 30–160 Å −1 , along with state-of-the-art empirical and semi-empirical stellar libraries and stellar population synthesis templates. We show that the choice of stellar library is generally the dominant source of residual systematic errors. We propose a general recipe for mitigating and accounting for residual uncertainties. We show that a sub-percent level of accuracy can be achieved on individual spectra with our data quality, which we subsequently validated with simulated mock datasets. The covariance between velocity dispersions measured for a sample of spectra can also be reduced to sub-percent levels. We recommend this recipe for all applications that require high precision and accurate stellar kinematics. Thus, we have made all the software publicly available to facilitate its implementation. This recipe will also be used in future TDCOSMO collaboration papers.

Abstract:

Context. Gas-phase abundances provide insights into the baryon cycle, with radial gradients and 2D metallicity distributions tracking how metals are built up and redistributed across galaxy disks over cosmic time. Aims. We use a catalog of 22 958 H  II regions across 19 nearby spiral galaxies to examine how precisely the radial abundance gradients can be traced when using only the [N  II ] λ 5755 electron temperature as a proxy for temperature-based, direct method metallicities. Methods. Using 534 direct detections of the temperature sensitive [N  II ] λ 5755 auroral line, we measured gradients in 15 of the galaxies. Leveraging our large catalog of individual H  II regions, we carried out a stacking procedure in bins of the H  II region [N  II ] λ 6583 luminosity and radius to recover stacked radial gradients. Results. We found a good agreement between the metallicity gradients from the stacked spectra and those gradients from individual regions and those from strong-line methods. In addition, particularly in the stacked T e [N  II ] measurements, some galaxies show very low (< 0.05 dex) scatter in metallicities, indicative of a well-mixed ISM. We examined the individual high confidence (S/N > 5) outliers and identified 13 regions across nine galaxies with anomalously low metallicities, although this is not strongly reflected in the strong-line method metallicities. By stacking arm and interarm regions, we found no systematic evidence for offsets in metallicity between these environments, suggesting that enrichment within spiral arms is due to very localized processes. Conclusions. This work demonstrates the potential to systematically exploit the single [N  II ] λ 5755 auroral line for detailed gas-phase abundance studies of galaxies. It provides strong validation of previous results, based on the strong-line calibrations, of a well-mixed ISM across typical star-forming spiral galaxies.

Abstract:

We present combined James Webb Space Telescope (JWST) NIRSpec and MIRI/MRS integral field spectroscopy data of the nuclear and circumnuclear regions of the highly dust obscured Seyfert 2 galaxy NGC 7582, which is part of the sample of active galactic nucleaus (AGN) in the Galaxy Activity, Torus and Outflow Survey (GATOS). Spatially resolved analysis of the pure rotational H lines (S(1)–S(7)) reveals a characteristic power-law temperature distribution in different apertures, with the two prominent southern star-forming regions exhibiting unexpectedly high molecular gas temperatures, comparable to those in the AGN powered nuclear region. We investigate potential heating mechanisms including direct AGN photoionization, UV fluorescent excitation from young star clusters, and shock excitation. We find that shock heating gives the most plausible explanation, consistent with multiple near- and mid-IR tracers and diagnostics. Using photoionization models from the PhotoDissociation Region Toolbox, we quantify the ISM conditions in the different regions, determining that the southern star-forming regions have a high density ( cm) and are irradiated by a moderate UV radiation field ( Habing). Fitting a suite of Paris-Durham shock models to the rotational H lines, as well as rovibrational 1-0 S(1), 1-0 S(2), and 2-1 S(1) H emission lines, we find that a slow ( km s−1) C-type shock is likely responsible for the elevated temperatures. Our analysis loosely favours local starburst activity as the driver of the shocks and circumnuclear gas dynamics in NGC 7582, though the possibility of an AGN jet contribution cannot be excluded.