Galaxy formation and evolution

Abstract:

Context. High-resolution JWST-MIRI images allow us to resolve the multi-scale nature of the emission in nearby star-forming galaxies in great detail, from compact star-forming regions to large-scale diffuse emission, providing new insights into dust emission, its composition, and the surrounding interstellar medium (ISM). Aims. We aim to understand at which scale the different processes driving dust emission in mid-infrared (7.7−21 μm) wavelengths take place and whether we can disentangle emission from dense regions from the one linked to a more diffuse component. Methods. We use and enhance the Constrained Diffusion Decomposition algorithm (CDD), an alternative to wavelet transform decomposition, to disentangle the emission from compact regions from the emission originating from diffuse sources. This allows us to quantify the mid-IR spectral properties of the ISM at intervals within a continuum of physical scales. Results. We find a transition scale for Polycyclic Aromatic Hydrocarbons (PAH) emission around 300 pc, with a weaker PAH fraction at smaller scales, highlighting the destruction of PAHs in H II regions. We also show variations in the PAH fraction in different morphological environments, with a lower fraction in bright and star-forming environments. By studying and comparing the probability distribution functions (PDFs) of H II regions and of diffuse ISM with the PDFs at different scales, we find a similar separation scale around 200 pc at which we observe a transition from a power-law PDF for dense structures to a log-normal distribution for the diffuse ISM. Conclusions. We infer the destruction of PAHs in nebular environments and the existence of a transition scale between stellar feedback in nebular regions and heating in the diffuse ISM. We also show that the CDD enables future applications to study the physical scales of emission.

Abstract:

Little red dots (LRDs) are candidate high-redshift supermassive black holes accreting in dense gas. They remain undetected in X-rays. In previous work, we provided the first quantitative models that reproduce the optical and near-infrared spectra of LRDs with the S IROCCO radiative transfer code, thereby constraining the properties of the surrounding gas. Here, we use these constraints to predict the X-ray attenuation produced by dense gas cocoons, and explore its dependence on Balmer-break strength, metallicity, intrinsic X-ray spectral energy distribution, and observed bandpass as a function of redshift. The X-ray constraints are very tight, requiring extinction by a Compton-thick gas column ( N H ∼ 10 25 cm −2 ) with moderate metallicity (0.05-0.1 Z ⊙ ) and intrinsically weak X-ray emission (the ratio of bolometric to X-ray luminosity is k bol,X ≳ 30), as observed in narrow-line active galactic nuclei with high accretion rates, to make LRDs sufficiently faint to evade detection. Intrinsically bright X-ray emitters as seen in typical broad-line active galactic nuclei would be detected even behind the typical Compton-thick gas columns with modest metallicity that were inferred from the optical spectra. Very low metallicity objects might be detected in X-rays even with low intrinsic X-ray luminosities, suggesting that LRDs are not (currently) chemically pristine.

Abstract:

Context. The spatial distribution of giant molecular clouds (GMCs) at sub-kiloparsec scales encodes information about cloud formation and evolution. However, we still lack a general quantitative characterisation of molecular gas structure at this scale. Aims. We aim to provide a quantitative description of molecular gas structure at 150–1000 pc for a typical star-forming main sequence galaxy. We analyse how GMCs cluster together and how CO emission is spatially correlated with bright GMCs using a sample of 8984 GMCs from 40 galaxies observed by PHANGS-ALMA. Methods. We homogenized our data to a common spatial resolution of 150 pc and a mass sensitivity of 2.5 M ⊙ pc −2 to remove observational bias. We then calculated the nearest neighbour distances, neighbour number density, and two-point correlation functions (2PCFs) for the catalogued GMCs in each galaxy. When analysing the 2PCFs, we generated several control samples that reflect different null hypotheses on large spatial scales. We stacked integrated intensity CO emission profiles around the position of catalogued GMCs to probe the gas distribution on scales between the observational resolution and the typical GMC-GMC spacing. Results. Our measurements of cloud spacing and the number of neighbours show that GMC clustering follows the large-scale gas distribution. Once we accounted for this contribution, the peak excess clustering relative to the null hypothesis in the 2PCF dropped from 1 + ω ∼ 2.3 to 1.3, with the power-law slope flattened from −0.25 to 0. Stacks of CO intensity around local maxima show a strong clustering signal on scales smaller than the typical GMC-GMC separation. We show that this is largely the same signal captured by the ‘GMC size’ measured by CPROPS , with an additional ∼20% of the flux in an extended component beyond 500 pc. We find that our stacked profiles can be fit with a double Gaussian function plus a constant offset. The broad Gaussian component accounts for 70% of the over-density power above the constant background and is stronger around massive and gravitationally bound GMCs. Conclusions. Our measurements yield a general statistical description of the structure of CO emission from ≈150 pc to galactic scales that can serve as a benchmark for simulations of molecular cloud formation and destruction in galaxy disks. Our results indicate that galactic structure exerts a strong influence on the GMC distribution in galaxy disks and that the formation of massive gravitationally bound GMCs is related to strong local gas clustering.

Abstract:

Abstract Blue asymmetries (“Blueshifts”) in the C iv 1550Å emission line are common in luminous quasars. If they are formed in winds, how much energy, momentum and mass do those winds transport? We address this question by considering how much mass must be supplied through the line-forming region to maintain a given density and ionization state. Using a combination of 1D analytic and 2D numerical models, we find that for blueshifted C iv lines to form in a wind, the wind must have mass outflow rates of ~50fV times the accretion rate, where fV ≤ 1 is the volume filling factor accounting for clumping. Our results therefore disfavour line formation in a smooth disc wind and point towards one of two scenarios: either the wind is clumpy, with required clumping factors suggestively close to those in hot star winds; alternatively, if the mass is instead swept up from the ambient medium, the wind need not be clumpy and MHD and radiative winds can provide the original source of momentum and energy. The power of the outflow depends on the square of the terminal velocity of the flow, v∞. If the wind is also the BAL outflow, with v∞ ~ 10, 000 km s−1, the wind power is significant and important for feedback. There are various caveats which moderate our conclusions, motivating i) a better theoretical understanding of wind driving and clump formation physics and ii) improved observational constraints on the physical conditions where the lines are formed.