Sai Zhai

Context . Ly α emitters (LAEs) are galaxies with strong Ly α emission, tracing early star formation and ionizing radiation. Their connection to active galactic nuclei (AGNs) is key to understanding the mechanisms behind (extended) Ly α emission. Aims . For this study, we measured the fraction of LAEs identified as radio-emitting AGNs ( f _AGN,radio ) and the fraction of radio sources that exhibit Ly α emission ( f _{Ly α} ) to investigate the connection of radio AGN activity to Ly α emission at 1.88 < z < 3.52. Methods . We identified 928 sources that were detected in both the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) and the LOw Frequency ARray (LOFAR) surveys. These matches were drawn from 55 109 spectroscopically confirmed LAEs and 27 625 radio sources. Results . After applying completeness corrections, we obtain f _AGN,radio = 1.77 ± 0.04% and f _{Ly α} = 18.15 ± 0.14% (for Ly α fluxes above 3×10 ⁻¹⁷ erg s ⁻¹ cm ⁻² and radio flux densities above 0.1 mJy at 144 MHz). The fraction f _AGN,radio increases from 0.4 ± 0.1% at log ₁₀ L _{Lyα,ergs ⁻¹} ≈ 42.6 to 9.7 ± 1.3% at log ₁₀ L _{Lyα,ergs ⁻¹} ≈ 44.5. The fraction f _Lyα rises from 0.7 ± 0.1% at log ₁₀ L _{150,WHz ⁻¹} ≈ 24.6 to 55.8 ± 14.5% for log ₁₀ L _{150,WHz ⁻¹} ≈ 28.3. ‘LAEs with radio AGNs’ and ‘optical AGNs with Ly α emission’ exhibit similar radio luminosity distributions above the AGN threshold ( L _150MHz > 10 ^23.9 W Hz ⁻¹ ), though optical AGNs show systematically higher Ly α luminosities. We find no statistically significant correlations of Ly α luminosity with both radio luminosity and a low-frequency spectral index, suggesting that Ly α emission is regulated by gas properties rather than jet power alone. We also find a positive correlation between the full width at half maximum (FWHM) and luminosity of the Ly α line, suggesting that more luminous sources have broader lines, potentially due to enhanced outflows or velocity dispersions. However, we find no correlation between FWHM and radio size, indicating minimal direct jet-gas interaction in these systems. Conclusions . Our results show that most Ly α emission at 1.88 < z < 3.52 is powered by star formation, with radio AGN activity confined to a small luminous subset (1.77 ± 0.04%). While luminous systems preferentially host radio AGNs, the lack of correlation between Ly α and radio luminosities in individual sources indicates that Ly α emission is governed primarily by host galaxy gas properties rather than direct AGN jet coupling.

The Hobby–Eberly Dark Energy Experiment (HETDEX) is an untargeted ∼540 deg ² spectroscopic survey of Ly α emission in the 1.9 < z < 3.5 Universe. In surface brightness, this survey reaches 1 σ Ly α sensitivities of ∼2–5 × 10 ⁻¹⁸ erg s ⁻¹ cm − 2 arcsec − 2 , allowing large samples of extended Ly α nebulae (LAN) to be studied. We selected a sample of 70,691 Ly α -emitting galaxies (LAEs) with an emission-line signal-to-noise ratio greater than 6 and modeled the Ly α emission as a point-source component with an optional exponential envelope. Half (∼47.5%) of the LAE sample (33,612 objects) exhibits significant extended emission and is best fit by the two-component model. The fraction of resolved sources increases with Ly α flux and luminosity. Their isophotal areas range from 10 to 130 arcsec ² (median 15 arcsec ² ), with integrated Ly α fluxes from 6 to 2000 × 10 ⁻¹⁷ erg s ⁻¹ cm ⁻² (median 20 × 10 ⁻¹⁷ erg s ⁻¹ cm ⁻² ). Comparison between point-spread function-weighted and isophotal flux measurements shows that the HETDEX pipeline underestimates the total Ly α flux by ∼30% on average, reflecting the substantial halo contribution in extended sources. Approximately 420 LANs are found per deg ² over 79.5 deg ² of noncontiguous sky. About 12% of resolved sources show active galactic nuclei signatures and are bright in Ly α and continuum. The remaining 88% span a wide range of morphologies and often lack continuum counterparts. Exponential scale lengths show no strong correlation with Ly α flux or luminosity (median 11.6 ± 1.9 kpc). Only 2.9% of the full S/N > 6 LAE population with ancillary data have radio counterparts, but 64% of those are found to be extended, with the radio fraction increasing with Ly α size. We present a catalog of all modeled sources, with their positions, redshifts, luminosities, and structural parameters for over 70,000 LAEs consisting of 33,000 spatially extended LAN. The catalog can be found at https://hetdex.org/data-results/ and in the online version of this paper.

Peaked-spectrum sources, known for their distinct peaked radio spectra, are a type of radio-loud active galactic nuclei. One subtype, megahertz-peaked-spectrum (MPS) sources, which exhibit a spectral peak at a frequency of a hundred megahertz, have emerged as a potential tool for identifying high-redshift candidates. However, the potential evolutionary link between the fraction of these sources and their redshifts remains unclear and requires further investigation. The recent, high-sensitivity Low Frequency Array (LOFAR) surveys enable statistical studies of these objects down to ultra-low frequencies (< 150 MHz). In this study we first used the multi-radio data to investigate the evolution of spectral index with redshift for 1187 quasars from the 16th SDSS quasar catalog. For each quasar, we analyzed available data from the LOFAR Low Band Antenna at 54 MHz, the High Band Antenna at 144 MHz, and the Very Large Array Faint Images of the Radio Sky at Twenty centimeters at 1.4 GHz. We measured the spectral index (α₅₄¹⁴⁴ and α₁₄₄¹⁴⁰⁰) and find no significant change in their median values with redshift. Extended sources have steeper spectral indices than compact sources, which is consistent with previous findings. Based on the spectral index information, we identified MPS sources using the criteria α₅₄¹⁴⁴ > = 0.1 and α₁₄₄¹⁴⁰⁰ < 0, and analyzed their properties. We find that the fraction of MPS sources is constant with the redshift (0.1 − 4.8), bolometric luminosity (10⁴⁴ − 10⁴⁸ erg/s), and supermassive black hole mass (10⁷ − 10^10.5 M_⊙), which suggests that MPS sources have relatively stable physical conditions or formation mechanisms across various evolutionary stages and environments.

The ${\rm H\, {\small I}}$-rich ultra-diffuse galaxies (HUDGs) offer a unique case for studies of star formation laws as they host low star formation efficiency and low-metallicity environments where gas is predominantly atomic. We collect a sample of six HUDGs in the field and investigate their location in the extended Schmidt law ($\Sigma _{\text{SFR }} \propto \left(\Sigma _{\text{star}}^{0.5} \Sigma _{\text{gas}}\right)^{1.09}$). They are consistent with this relationship well (with deviations of only 1.1σ). Furthermore, we find that HUDGs follow the tight correlation between the hydrostatic pressure in the galaxy mid-plane and the quantity on the x-axis ($\rm log(\Sigma _{star}^{0.5}\Sigma _{gas})$) of the extended Schmidt law. This result indicates that these HUDGs can be self-regulated systems that reach the dynamical and thermal equilibrium. In this framework, the stellar gravity compresses the disc vertically and counteracts the gas pressure in the galaxy mid-plane to regulate the star formation as suggested by some theoretical models.

The circumgalactic medium (CGM) connects the gas between the interstellar medium and the intergalactic medium, which plays an important role in galaxy evolution. We use the stellar mass–metallicity relationship to investigate whether sharing the CGM will affect the distribution of metals in galaxy pairs. The optical emission lines from the Sloan Digital Sky Survey Data Release are used to measure the gas-phase metallicity. We find that there is no significant difference in the distribution of the metallicity difference between two members in star forming–star forming pairs ($\rm \Delta log(O/H)_{diff}$), metallicity offset from the best-fitting stellar mass–metallicity relationship of galaxies in pairs ($\rm \Delta log(O/H)_{MS}$), as compared to ‘fake’ pairs. By looking at and as a function of the star formation rate (SFR), specific SFR, and stellar mass ratio, no difference is seen between galaxies in pairs and control galaxies. From our results, the share of the CGM may not play an important role in shaping the evolution of metal contents of galaxies.