Galaxy formation and evolution

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.

Abstract:

With excellent spectral and angular resolutions and, especially, sensitivity, the JWST allows us to observe infrared emission lines that were previously inaccessible or barely accessible. These emission lines are promising for evaluating the physical conditions in different galaxies. Based on MAPPINGS V photoionization models, we systematically analyze the dependence of over 20 mid-infrared (mid-IR) emission lines covered by MIRI on board JWST on the physical conditions of different galactic environments, in particular narrow-line regions in active galactic nuclei (AGN). We find that mid-IR emission lines of highly ionized argon (i.e., [Ar V] 7.90 and 13.10 μm) and neon (i.e., [Ne V] 14.32 and 24.32 μm, and [Ne VI] 7.65 μm) are effective in diagnosing the physical conditions in AGN. We accordingly propose new prescriptions to constrain the ionization parameter (U), peak energy of the AGN spectrum (Epeak), metallicity ( 12+log(O/H) ), and gas pressure (P/k) in AGN. These new calibrations are applied to the central regions of six Seyfert galaxies included in the Galaxy Activity, Torus, and Outflow Survey as a proof of concept. We also discuss the similarity and difference in the calibrations of these diagnostics in AGN of different luminosities, highlighting the impact of hard X-ray emission and particularly radiative shocks, as well as the different diagnostics in star-forming regions. Finally, we propose diagnostic diagrams involving [Ar V] 7.90 μm and [Ne VI] 7.65 μm to demonstrate the feasibility of using the results of this study to distinguish galactic regions governed by different excitation sources.

Abstract:

Abstract We investigate the clustering of Lyman-break galaxies at redshifts of 3 ≲ z ≲ 5 within the COSMOS field by measuring the angular two-point correlation function. Our robust sample of ~60,000 bright (mUV ≲ 27) Lyman-break galaxies was selected based on spectral energy distribution fitting across 14 photometric bands spanning optical and near-infrared wavelengths. We constrained both the 1- and 2-halo terms at separations up to 300 arcsec, finding an excess in the correlation function at scales corresponding to <20 kpc, consistent with enhancement due to clumps in the same galaxy or interactions on this scale. We then performed Bayesian model fits on the correlation functions to infer the Halo Occupation Distribution parameters, star formation duty cycle, and galaxy bias in three redshift bins. We examined several cases where different combinations of parameters were varied, showing that our data can constrain the slope of the satellite occupation function, which previous studies have fixed. For an MUV-limited sub-sample, we found galaxy bias values of $b_g=3.18^{+0.14}_{-0.14}$ at z ≃ 3, $b_g=3.58^{+0.27}_{-0.29}$ at z ≃ 4, $b_g=4.27^{+0.25}_{-0.26}$ at z ≃ 5. The duty cycle values are $0.62^{+0.25}_{-0.26}$, $0.40^{+0.34}_{-0.22}$, and $0.39^{+0.31}_{-0.20}$,respectively. These results suggest that, as the redshift increases, there is a slight decrease in the host halo masses and a shorter timescale for star formation in bright galaxies, at a fixed rest-frame UV luminosity threshold.