Galaxy formation and evolution

Abstract:

Cosmic ray (CR) feedback has been proposed as a powerful mechanism for driving warm gas outflows in galaxies. We use cosmological magnetohydrodynamic simulations to investigate the impact of CR feedback on neutral hydrogen (H i) in a 1011 M⊙ dark matter halo at 2 < z < 4. To this end, we postprocess the simulations with ionizing radiative transfer and perform Monte Carlo Lyman-α (Lyα) transfer calculations. CR feedback reduces H i column densities around young stars, thereby allowing more Lyα photons to escape and consequently offering a better match to the Lyα luminosities of observed Lyα emitters. Although galaxies with CR-driven outflows have more extended H i in the circumgalactic medium, two Lyα line properties sensitive to optical depth and gas kinematic—the location of the red peak relative to the Lyα line center in velocity space (vred) and relative strength of the blue-to-red peaks (B/R)—cannot distinguish between the CR-driven and non-CR simulations. This is because Lyα photons propagate preferentially along low H i density channels created by the ionizing radiation, thereby limiting the scattering with volume-filling H i. In contrast, the observed low flux ratios between the valley and peak and the surface brightness profiles are better reproduced in the model with CR-driven outflows because the Lyα photons interact more before escaping, rather than being destroyed by dust as is the case in the non-CR simulation. We discuss the potential cause of the paucity of sightlines in simulations that exhibit prominent red peaks and large vred, which may require the presence of more volume-filling H i.

Abstract:

Atomic hydrogen constitutes the gas reservoir from which molecular gas and star formation in galaxies emerges. However, the weakness of the line means it has been difficult to directly detect in all but the very local Universe. Here, we present results from the first search using the MeerKAT International Tiered Extragalactic Exploration (MIGHTEE) Survey for high-redshift () H i emission from individual galaxies. By searching for 21-cm emission centred on the position and redshift of optically selected emission-line galaxies we overcome difficulties that hinder untargeted searches. We detect 11 galaxies at , forming the first sample of detections with an interferometer, with the highest redshift detection at . We find they have much larger H i masses than their low-redshift H i-selected counterparts for a given stellar mass. This can be explained by the much larger cosmological volume probed at these high redshifts, and does not require any evolution of the H i mass function. We make the first-ever measurement of the baryonic Tully–Fisher relation (bTFr) with H i at and find consistency with the local bTFr, but with tentative evidence of a flattening in the relation at these redshifts for higher-mass objects. This may signify evolution, in line with predictions from hydrodynamic simulations, or that the molecular gas mass in these high-mass galaxies could be significant. This study paves the way for future studies of H i beyond the local Universe, using both searches targeted at known objects and via pure H i selection.

Abstract:

We analyze 99 photometrically selected Little Red Dots (LRDs) at z ≈ 4–8 in the GOODS fields, leveraging ultradeep JADES NIRCam short-wavelength (SW) data. Among the 99 selected LRDs, we examine the morphology of 30. The remaining 69 appear predominantly compact, with sizes ≲400 pc and no extended components even in stacked SW images. However, their unresolved nature may partly reflect current depth limitations, which could prevent the detection of faint diffuse components. Among the 30 morphologically analyzed LRDs, 50% show multiple associated components, while the rest exhibit highly asymmetric structures, despite appearing as single sources. This diversity in rest-frame UV morphologies may point to interactions or strong internal feedback. We find median stellar masses of log10(M⋆/M⊙)=9.07−0.08+0.11 for pure stellar models with AV≈1.16−0.21+0.11 mag, and log10(M⋆/M⊙)=9.67−0.27+0.17 for models including active galactic nuclei (AGNs) with AV≈2.74−0.71+0.55 mag, in line with recent studies suggesting higher masses and dust content for AGN-fitted LRDs. NIRSpec spectra are available for 15 sources, six of which are also in the morphological sample. Broad Hα is detected in 40% (FWHM = 1200–2900 km s−1), and one source shows broad Hβ emission. Emission line ratios indicate a composite nature, consistent with both AGN and stellar processes. Altogether, these results suggest that LRDs are a mixed population, and their rest-frame UV morphology reflects this complexity. Morphological studies of larger samples could provide a new way to understand what drives their properties and evolution.

Abstract:

We present JWST/MIRI spectra from the Medium-Resolution Spectrometer of I Zw 18, a nearby dwarf galaxy with a metallicity of ∼3% solar. Its proximity enables a detailed study of highly ionized gas that can be interpreted in the context of newly discovered high-redshift dwarf galaxies. We derive aperture spectra centered on 11 regions of interest; the spectra show very low extinction, AV ≲ 0.1, consistent with optical determinations. The gas is highly ionized; we have detected 10 fine-structure lines, including [O iv] 25.9 μm with an ionization potential (IP) of ∼55 eV, and [Ne v] 14.3 μm with an IP of ∼97 eV. The ionization state of I Zw 18 falls at the extreme upper end of all of the line ratios we analyzed, but not coincident with galaxies containing an accreting massive black hole (active galactic nucleus). Comparison of the line ratios with state-of-the-art photoionization and shock models suggests that the high-ionization state in I Zw 18 is not due to shocks. Rather, it can be attributed to metal-poor stellar populations with a self-consistent contribution of X-ray binaries or ultra-luminous X-ray sources. It could also be partially due to a small number of hot low-metallicity Wolf−Rayet stars ionizing the gas; a small fraction (a few percent) of the ionization could come from an intermediate-mass black hole. Our spectroscopy also revealed four 14 μm continuum sources, ≳30–100 pc in diameter, three of which were not previously identified. Their properties are consistent with H ii regions ionized by young star clusters.

Abstract:

The high-redshift UV luminosity function provides important insights into the evolution of early galaxies. JWST has revealed an unexpectedly large population of bright (MUV ≲ −20) galaxies at z ≳ 10, implying fundamental changes in the star-forming properties of galaxies at increasingly early times. However, constraining the fainter population (MUV ≳ −18) has been more challenging. In this work, we present the z ≳ 9 UV luminosity function from the JWST Advanced Deep Extragalactic Survey. We calculate the UV luminosity function from several hundred z ≳ 9 galaxy candidates that reach UV luminosities of MUV ∼ −17 in redshift bins of z ∼ 8.5–12 (309 candidates) and z ∼ 12–16 (63 candidates). We search for candidates at z ∼ 16–22.5 and find none. We also estimate the z ∼ 14–16 luminosity function from the z ≥ 14 subset of the z ∼ 12–16 sample. Consistent with other measurements, we find an excess of bright galaxies that is in tension with many theoretical models, especially at z ≳ 12. However, we also find high number densities at −18 ≲ MUV ≲ −17, suggesting that there is a larger population of faint galaxies than expected, as well as bright ones. From our parametric fits for the luminosity function, we find steep faint-end slopes of −2.5 ≲ α ≲ −2.3, suggesting a large population of faint (MUV ≳ −17) galaxies. Combined, the high normalization and steep faint-end slope of the luminosity function could imply that the reionization process is appreciably underway as early as z = 10.