arXiv subjects

Fangxia An

Publications and source records attributed to Fangxia An.

At least 19 recordsLinked to original sources

arXiv2026

Tracing cosmic star formation history through radio continuum spectral energy distribution and non-thermal emission

As a tracer of massive star formation unaffected by dust, the radio continuum emission provides a unique window into the formation of the first stars and galaxies in the Universe. Recent observations show that the integrated rest-frame mid-radio (~1-10 GHz) luminosity of galaxies serves as one of the most robust tracers of the star formation rate (SFR). These studies further demonstrate that the synchrotron spectral index and the shape of the radio spectral energy distribution (SED) evolves with redshift as a consequence of the cosmic evolution of star formation activity. These findings underscore the importance of deep multi-band radio continuum observations in calibrating the SFR of early galaxies and understanding the astrophysical processes governing their assembly and evolution over cosmic time. This chapter presents recent progress in radio SFR calibrations for star-forming galaxies (SFGs) and reviews radio-continuum studies of the cosmic star formation history (SFH). We highlight the transformative potential of SKA AA4, whose broad frequency coverage and high sensitivity will enable well-constrained radio SEDs for SFGs across a wide redshift range.

astro-ph.GA↗

arXiv2026

Tracing the Star Formation History of the Universe through Thermal Free-Free Emission with the SKA

One of the major scientific aims of the SKA is to trace the history of star formation across cosmic time. High-frequency radio surveys are indispensable in this regard, as these are capable of probing thermal free-free emission (FFE) -- the dominant component of the radio continuum of star-forming galaxies above rest-frame frequencies of $\gtrsim25\,$GHz. FFE is a powerful, direct star-formation rate (SFR) indicator, which robustly traces the number of ionizing photons produced by recently formed massive stars in a nearly dust-unbiased manner. In this chapter, we forecast the ability of the SKA to detect FFE in typical star-forming galaxies in the early Universe. Our starting point is the state-of-the-art T-RECS simulation suite of the faint radio sky, to which we apply an ambitious, matched-depth, multi-band AA4 SKA-Mid survey in Bands 1 through 5b, covering an area of $0.25\,\mathrm{deg}^2$ across $0.35 - 15.4\,\mathrm{GHz}$. We predict that such a survey will detect $\sim1.5\times10^4$ star-forming galaxies in all bands out to $z\approx7$, and perform simulations using established fitting techniques to investigate the accuracy with which their thermal FFE can be recovered. We find that thermal fractions ($f_\mathrm{th}$) and synchrotron spectral indices can be constrained in an unbiased manner, and predict uncertainties on the thermal SFRs of $\lesssim 0.1\,\mathrm{dex}$ for galaxies at the knee of the radio luminosity function across redshift. Convolving the distribution of $f_\mathrm{th}$ inferred from the multi-band SKA-Mid survey with wider luminosity function determinations at low radio frequencies will yield robust constraints on the total cosmic star formation rate density out to $z\sim7$.

astro-ph.GA↗

arXiv2026

A Python-Based Peeling Framework for Radio Interferometry: Application to uGMRT 650MHz Imaging

Modern radio interferometric arrays offer high sensitivity, wide fields of view, and broad frequency coverage, but also pose significant data calibration challenges. Standard direction-independent calibration is insufficient to correct direction-dependent effects, such as ionospheric phase distortions and primary beam variations, which produce strong artifacts around bright sources and limit achievable image dynamic range. Built on standard CASA tasks, we present a Python-based direction-dependent calibration and peeling framework, demonstrated using radio continuum imaging data from the upgraded Giant Metrewave Radio Telescope (uGMRT). The framework efficiently subtracts bright-source models and suppresses their associated direction-dependent artifacts, producing significantly flattened backgrounds and improving image fidelity and faint-source detectability. We further introduce an optimized ``model-restoration'' strategy that mitigates direction-dependent artifacts while preserving the flux densities and morphologies of bright sources that are themselves of scientific interest. For fields containing multiple bright sources, sequential application of the framework systematically reduces background noise, thereby increasing sensitivity and faint-source detectability. The framework is Python-based, CASA-compatible, and can be readily applied to other mid- and low-frequency interferometric arrays. The code is publicly released with this paper.

astro-ph.IM↗

arXiv2025

Deblending the MIGHTEE-COSMOS survey with XID+: The resolved radio source counts to $S_{1.4}\approx 5\mu$Jy

Deep radio continuum surveys provide fundamental constraints on galaxy evolution, but source confusion limits sensitivity to the faintest sources. We present a complete framework for producing high-fidelity deblended radio catalogs from the confused MIGHTEE maps using the probabilistic deblending framework XID+ and prior positions from deep multi-wavelength data in the COSMOS field. To assess performance, we construct MIGHTEE-like simulations based on the Tiered Radio Extragalactic Continuum Simulation (T-RECS) radio source population, ensuring a realistic distribution of star-forming galaxies and active galactic nuclei (AGN) for validation. Through these simulations, we show that prior catalog purity is the dominant factor controlling deblending accuracy: a high-purity prior, containing only sources with a high likelihood of radio detection, recovers accurate flux densities and reproduces input source counts down to $\sim 3\sigma$ (where $\sigma = $ thermal noise). On the other hand, a complete prior overestimates the source counts due to spurious detections. Our optimal strategy combines the high-purity prior with a mask that removes sources detected above $50~\mu$Jy. Applied to the $\sim$1.3\,deg$^2$ area of the MIGHTEE-COSMOS field defined by overlapping multi-wavelength data, this procedure yields a deblended catalog of 89,562 sources. The derived 1.4\,GHz source counts agree with independent P(D) analyses and indicate that we resolve the radio background to $\sim 4.8\,\mu$Jy. We also define a recommended high-fidelity sample of 20,757 sources, based on detection significance, flux density, and goodness-of-fit, which provides reliable flux densities for individual sources in the confusion-limited regime.

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arXiv2025

Machine Learning Approaches for Classifying Star-Forming Galaxies and Active Galactic Nuclei from MIGHTEE-Detected Radio Sources in the COSMOS Field

Radio synchrotron emission originates from both massive star formation and black hole accretion, two processes that drive galaxy evolution. Efficient classification of sources dominated by either process is therefore essential for fully exploiting deep, wide-field extragalactic radio continuum surveys. In this study, we implement, optimize, and compare five widely used supervised machine-learning (ML) algorithms to classify radio sources detected in the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE)-COSMOS survey as star-forming galaxies (SFGs) and active galactic nuclei (AGN). Training and test sets are constructed from conventionally classified MIGHTEE-COSMOS sources, and 18 physical parameters of the MIGHTEE-detected sources are evaluated as input features. As anticipated, our feature analyses rank the five parameters used in conventional classification as the most effective: the infrared-radio correlation parameter ($q_\mathrm{IR}$), the optical compactness morphology parameter (class$\_$star), stellar mass, and two combined mid-infrared colors. By optimizing the ML models with these selected features and testing classifiers across various feature combinations, we find that model performance generally improves as additional features are incorporated. Overall, all five algorithms yield an $F1$-score (the harmonic mean of precision and recall) $>90\%$ even when trained on only $20\%$ of the dataset. Among them, the distance-based $k$-nearest neighbors classifier demonstrates the highest accuracy and stability, establishing it as a robust and effective method for classifying SFGs and AGN in upcoming large radio continuum surveys.

astro-ph.GA↗

arXiv2025

Balmer Decrement and IRX Break in Tracing Dust Attenuation at Scales of Individual Star-forming Regions in NGC 628

We investigate the relationships between infrared excess (IRX=$L_{\rm IR}/L_{\rm UV}$) and Balmer decrement (${\rm H}\alpha/{\rm H}\beta$) as indicators of dust attenuation for 609 ${\rm {H\,{\small II}}}$ regions at scales of $\sim 50-200$ pc in NGC 628, utilizing data from AstroSat, James Webb Space Telescope (JWST) and Multi Unit Spectroscopic Explorer (MUSE). Our findings indicate that about three fifths of the sample ${\rm {H\,{\small II}}}$ regions reside within the regime occupied by local star-forming galaxies (SFGs) along the dust attenuation correlation described by their corresponding color excess parameters $E(B-V)_{\rm IRX} = 0.51\,E(B-V)_{{\rm H}\alpha/{\rm H}\beta}$. Nearly 27$\%$ of the sample exhibits $E(B-V)_{\rm IRX}> E(B-V)_{{\rm H}\alpha/{\rm H}\beta}$, while a small fraction ($\sim 13\%$) displays significantly lower $E(B-V)_{\rm IRX}$ compared to $E(B-V)_{{\rm H}\alpha/{\rm H}\beta}$. These results suggest that the correlation between the two dust attenuation indicators no longer holds for spatially resolved ${\rm {H\,{\small II}}}$ regions. Furthermore, the ratio of $E(B-V)_{\rm IRX}$ to $E(B-V)_{{\rm H}\alpha/{\rm H}\beta}$ remains unaffected by various physical parameters of the ${\rm {H\,{\small II}}}$ regions, including star formation rate (SFR), SFR surface density, infrared luminosity ($L_{\rm IR}$), $L_{\rm IR}$ surface density, stellar mass, gas-phase metallicity, circularized radius, and the distance to galactic center. We argue that the ratio is primarily influenced by the evolution of surrounding interstellar medium (ISM) of the star-forming regions, transitioning from an early dense and thick phase to the late blown-away stage.

astro-ph.GA↗

arXiv2025

The Radio Spectral Energy Distribution and Star Formation Calibration in MIGHTEE-COSMOS Highly Star-Forming Galaxies at 1.5 < z < 3.5

Studying the radio spectral energy distribution (SED) of distant galaxies is essential for understanding their assembly and evolution over cosmic time. We present rest-frame radio SEDs of a sample of 160 starburst galaxies at redshifts 1.5 to 3.5 in the COSMOS field, as part of the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) project. MeerKAT observations, combined with archival VLA and GMRT data, allow us to determine the integrated mid-radio (1-10 GHz) continuum (MRC) luminosity and magnetic field strength. A Bayesian method is used to model the SEDs and separate free-free and synchrotron emission. We calibrate the star formation rate (SFR) in radio both directly through SED analysis and indirectly via the infrared-radio correlation (IRRC). With a mean synchrotron spectral index of approximately 0.7, we find that the index flattens with redshift and specific SFR, suggesting that cosmic rays are more energetic in the early universe due to higher star formation activity. The magnetic field strength increases with redshift (B is proportional to (1 + z)^0.7) and with star formation rate (B is proportional to SFR^0.3), indicating a small-scale dynamo as the dominant amplification mechanism. Accounting for SED evolution, the IRRC remains redshift-invariant and does not vary with stellar mass at 1.5 < z < 3.5, though the correlation deviates from linearity. Similarly, we show that SFR estimates based on integrated MRC luminosity are also redshift-invariant.

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arXiv2025

A Multi-Mechanism Hybrid Model of Peaked-Spectrum Radio Sources

The peaked-spectrum (PS) sources exhibit turnover characteristics in their broad radio spectra. However, the mechanism underlying this phenomenon remains elusive. The two most common hypotheses are synchrotron self-absorption (SSA) and free-free absorption (FFA). By incorporating multiple absorption scenarios, we propose a multi-mechanism hybrid (MMH) model, which aligns well with current observational data and provides a good physical explanation. Using the GLEAM survey data, we identified a sample of 4,315 sources with peak frequencies approximately between 72--3000 MHz, most of which are MHz-peaked-spectrum sources (MPS). Our analysis shows that instead of SSA, the FFA is the dominant mechanism in producing the spectral turnover for most of the sources in this sample. The index of the optically thick spectrum alpha_thick has a lower boundary due to the FFA, and the steeper alpha_thick indicates a complex multi-absorption mechanism. In particular, the external FFA produces substantial alpha_thick, which exhibits a weak correlation with the peak frequency. Future ultra-long wavelength observations would also provide data on the spectrum of these sources at even lower frequencies. Determining the absorption mechanism that shaped the spectrum of these sources would be a crucial part of understanding their nature.

astro-ph.GA↗

arXiv2025

The Evolution of Size and Merger Fraction of Submillimeter Galaxies across $1 < z \lesssim 6$ as Observed by JWST

Precise tracking of the growth in galaxy size and the evolution of merger fractions with redshift is vital for understanding the formation history of submillimeter galaxies (SMGs). This study investigates these evolutions over a broad redshift range ($1 < z \lesssim 6$), using a sample of 222 SMGs with a median redshift of $z = 2.61^{+0.89}_{-0.82}$ identified by ALMA and JCMT, enhanced by the advanced imaging capabilities of the JWST/NIRCam and MIRI. We find significant evolution in effective radii ($R_e$) in rest-frame V-band ($R_e \propto (1 + z)^{-0.87 \pm 0.08}$) and near-infrared (NIR) band ($R_e \propto (1 + z)^{-0.88 \pm 0.11}$), with the NIR size evolution resembling that of massive star-forming galaxies at lower redshift. Visual inspections reveal a major merger fraction of $24.3 \pm 3.7\%$ and an interaction fraction of up to $48.4 \pm 11.1\%$. The major merger fraction exhibits an increase from 14.7$\pm9.1$\% at $z = 1$ to 26.6$\pm 8.4$\% at $z = 3$, after which it remains approximately constant across the redshift range $3 < z < 6$. In contrast, the interaction fraction remains relatively stable across the range $2 < z < 5$. Our results indicate that late-stage major mergers are not the primary formation mechanism for SMGs at $z<3$, while interactions appear to play a significant role across the broader redshift range of $1<z<6$. Additionally, HST-based major merger identifications may overestimate the true fraction by a factor of 1.7 at $z \sim 2$. These findings highlight the varying roles of mergers and interactions in driving the formation of massive, dusty star-forming galaxies across different redshifts.

astro-ph.GA↗

arXiv2025

Discovery of a years-delayed radio flare from an unusually slow-evolved tidal disruption event

SDSS J1115+0544 is a unique low-ionization nuclear emission-line region (LINER) galaxy with energetic ultraviolet (UV), optical and mid-infrared outbursts occurring in its nucleus. We present the results from an analysis of multi-wavelength photometric and radio follow-up observations covering a period of ~9 years since its discovery. We find that following a luminosity plateau of ~500 days, the UV/optical emission has decayed back to the pre-outburst level, suggesting that the nuclear outburst might be caused by a stellar tidal disruption event (TDE). In this case, SDSS J1115+0544 could be an unusually slow-evolved optical TDE with longest rise and decline time-scales ever found. Three years later than the optical peak, a delayed radio brightening was found with a 5.5 GHz luminosity as high as ~1.9x10^39 erg/s. Using a standard equipartition analysis, we find the outflow powering the radio emission was launched at t~1260 days with a velocity of beta<~0.1 and kinetic energy of E_K~>10^50 erg. The delayed radio brightening coupled with the disappearing plateau in the UV/optical light curves is consistent with the scenario involving delayed ejection of an outflow from a state transition in the disk. SDSS J1115+0544 is the first TDE displaying both a short-lived UV/optical plateau emission and a late-time radio brightening. Future radio observations of these TDEs in the post-plateau decay phase will help to establish the connection between outflow launching and changes in accretion rate.

astro-ph.HE↗

arXiv2024

A spatially-resolved spectral analysis of giant radio galaxies with MeerKAT

In this study we report spatially resolved, wideband spectral properties of three giant radio galaxies (GRGs) in the COSMOS field: MGTC J095959.63+024608.6 , MGTC J100016.84+015133.0 and MGTC J100022.85+031520.4. One such galaxy MGTC J100022.85+031520.4 is reported here for the first time with a projected linear size of 1.29 Mpc at a redshift of 0.1034. Unlike the other two, it is associated with a brightest cluster galaxy (BCG), making it one of the few GRGs known to inhabit cluster environments. We examine the spectral age distributions of the three GRGs using new MeerKAT UHF-band (544-1088 MHz) observations, and $L$-band (900-1670 MHz) data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey. We test two different models of spectral ageing, the Jaffe-Perola and Tribble models, using the Broadband Radio Astronomy Tools (\textsc{brats}) software which we find agree well with each other. We estimate the Tribble spectral age for MGTC J095959.63+024608.6 as 68 Myr, MGTC J100016.84+015133.0 as 47 Myr and MGTC J100022.85+031520.4 as 67 Myr. We find significant disagreements between these spectral age estimates and the estimates of the dynamical ages of these GRGs, modelled in cluster and group environments. Our results highlight the need for additional processes which are not accounted for in either the dynamic age or spectral age estimations.

astro-ph.GA↗

arXiv2024

MIGHTEE: The Continuum Survey Data Release 1

The MeerKAT International GHz Tiered Extragalactic Exploration Survey (MIGHTEE) is one of the large survey projects using the MeerKAT telescope, covering four fields that have a wealth of ancillary data available. We present Data Release 1 of the MIGHTEE continuum survey, releasing total intensity images and catalogues over $\sim$20 deg$^2$, across three fields at $\sim$1.2-1.3 GHz. This includes 4.2 deg$^2$ over the Cosmic Evolution Survey (COSMOS) field, 14.4 deg$^2$ over the XMM Large-Scale Structure (XMM-LSS) field and deeper imaging over 1.5 deg$^2$ of the Extended Chandra Deep Field South (CDFS). We release images at both a lower resolution (7-9 arcsec) and higher resolution ($\sim 5$ arcsec). These images have central rms sensitivities of $\sim$1.3$-$2.7 $\mu $Jy beam$^{-1}$ ($\sim$1.2$-$3.6 $\mu $Jy beam$^{-1}$) in the lower (higher) resolution images respectively. We also release catalogues comprised of $\sim$144~000 ($\sim$114 000) sources using the lower (higher) resolution images. We compare the astrometry and flux-density calibration with the Early Science data in the COSMOS and XMM-LSS fields and previous radio observations in the CDFS field, finding broad agreement. Furthermore, we extend the source counts at the $\sim$10 $\mu$Jy level to these larger areas ($\sim 20$ deg$^2$) and, using the areal coverage of MIGHTEE we measure the sample variance for differing areas of sky. We find a typical sample variance of 10-20 per cent for 0.3 and 0.5 sq. deg. sub-regions at $S_{1.4} \leq 200$ $\mu $Jy, which increases at brighter flux densities, given the lower source density and expected higher galaxy bias for these sources.

astro-ph.GA↗

arXiv2024

Cosmic Himalayas: The Highest Quasar Density Peak Identified in a 10,000 deg$^2$ Sky with Spatial Discrepancies between Galaxies, Quasars, and IGM HI

We report the identification of a quasar overdensity in the BOSSJ0210 field, dubbed Cosmic Himalayas, consisting of 11 quasars at $z=2.16-2.20$, the densest overdensity of quasars ($17\sigma$) in the $\sim$10,000 deg$^2$ of the Sloan Digital Sky Survey. We present the spatial distributions of galaxies and quasars and an HI absorption map of the intergalactic medium (IGM). On the map of 465 galaxies selected from the MAMMOTH-Subaru survey, we find two galaxy density peaks that do not fall on the quasar overdensity but instead exist at the northwest and southeast sides, approximately 25 $h^{-1}$ comoving-Mpc apart from the quasar overdensity. With a spatial resolution of 15 $h^{-1}$ comoving Mpc in projection, we produce a three-dimensional HI tomography map by the IGM Ly$\alpha$ forest in the spectra of 23 SDSS/eBOSS quasars behind the quasar overdensity. Surprisingly, the quasar overdensity coincides with neither an absorption peak nor a transmission peak of IGM HI but lies near the border separating opaque and transparent volumes, with the more luminous quasars located in an environment with lesser IGM HI. Hence remarkably, the overdensity region traced by the 11 quasars, albeit all in coherently active states, has no clear coincidence with peaks of galaxies or HI absorption densities. Current physical scenarios with mixtures of HI overdensities and quasar photoionization cannot fully interpret the emergence of Cosmic Himalayas, suggesting this peculiar structure is an excellent laboratory to unveil the interplay between galaxies, quasars, and the IGM.

astro-ph.GA↗

arXiv2023

MIGHTEE: multi-wavelength counterparts in the COSMOS field

In this paper we combine the Early Science radio continuum data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey, with optical and near-infrared data and release the cross-matched catalogues. The radio data used in this work covers $0.86$ deg$^2$ of the COSMOS field, reaches a thermal noise of $1.7$ $\mu$Jy/beam and contains $6102$ radio components. We visually inspect and cross-match the radio sample with optical and near-infrared data from the Hyper Suprime-Cam (HSC) and UltraVISTA surveys. This allows the properties of active galactic nuclei and star-forming populations of galaxies to be probed out to $z \approx 5$. Additionally, we use the likelihood ratio method to automatically cross-match the radio and optical catalogues and compare this to the visually cross-matched catalogue. We find that 94 per cent of our radio source catalogue can be matched with this method, with a reliability of $95$ per cent. We proceed to show that visual classification will still remain an essential process for the cross-matching of complex and extended radio sources. In the near future, the MIGHTEE survey will be expanded in area to cover a total of $\sim$20~deg$^2$; thus the combination of automated and visual identification will be critical. We compare redshift distribution of SFG and AGN to the SKADS and T-RECS simulations and find more AGN than predicted at $z \sim 1$.

astro-ph.GA↗

arXiv2023

Radio spectral properties of star-forming galaxies between 150-5000MHz in the ELAIS-N1 field

By combining high-sensitivity LOFAR 150MHz, uGMRT 400MHz and 1,250MHz, GMRT 610MHz, and VLA 5GHz data in the ELAIS-N1 field, we study the radio spectral properties of radio-detected star-forming galaxies (SFGs) at observer-frame frequencies of 150-5,000MHz. We select ~3,500 SFGs that have both LOFAR 150MHz and GMRT 610MHz detections, and obtain a median two-point spectral index of $\alpha_{150}^{610}=-0.51\pm0.01$. The photometric redshift of these SFGs spans $z=0.01-6.21$. We also measure the two-point radio spectral indices at 150-400-610-1,250MHz and 150-610-5,000MHz respectively for the GMRT 610-MHz-detected SFGs, and find that, on average, the radio spectrum of SFGs is flatter at low frequency than at high frequency. At observer-frame 150-5,000MHz, we find that the radio spectrum slightly steepens with increasing stellar mass. However, we only find that the radio spectrum flattens with increasing optical depth at $V$-band at $\nu<1$GHz. We suggest that spectral ageing due to the energy loss of CR electrons and thermal free-free absorption could be among the possible main physical mechanisms that drive the above two correlations respectively. In addition, both of these mechanisms could physically explain why the radio spectrum is flatter at low frequency than at high frequency.

astro-ph.GA↗

arXiv2022

MIGHTEE: Deep 1.4 GHz Source Counts and the Sky Temperature Contribution of Star Forming Galaxies and Active Galactic Nuclei

We present deep 1.4 GHz source counts from $\sim$5 deg$^2$ of the continuum Early Science data release of the MeerKAT International Gigahertz Tiered Extragalactic Exploration (MIGHTEE) survey down to $S_{1.4\textrm{GHz}}\sim$15 $\mu$Jy. Using observations over two extragalactic fields (COSMOS and XMM-LSS), we provide a comprehensive investigation into correcting the incompleteness of the raw source counts within the survey to understand the true underlying source count population. We use a variety of simulations that account for: errors in source detection and characterisation, clustering, and variations in the assumed source model used to simulate sources within the field and characterise source count incompleteness. We present these deep source count distributions and use them to investigate the contribution of extragalactic sources to the sky background temperature at 1.4 GHz using a relatively large sky area. We then use the wealth of ancillary data covering{a subset of the COSMOS field to investigate the specific contributions from both active galactic nuclei (AGN) and star forming galaxies (SFGs) to the source counts and sky background temperature. We find, similar to previous deep studies, that we are unable to reconcile the sky temperature observed by the ARCADE 2 experiment. We show that AGN provide the majority contribution to the sky temperature contribution from radio sources, but the relative contribution of SFGs rises sharply below 1 mJy, reaching an approximate 15-25% contribution to the total sky background temperature ($T_b\sim$100 mK) at $\sim$15 $\mu$Jy.

astro-ph.GA↗

arXiv2022

MIGHTEE: the nature of the radio-loud AGN population

We study the nature of the faint radio source population detected in the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Early Science data in the COSMOS field, focusing on the properties of the radio-loud active galactic nuclei (AGN). Using the extensive multi-wavelength data available in the field, we are able to classify 88 per cent of the 5223 radio sources in the field with host galaxy identifications as AGN (35 per cent) or star-forming galaxies (54 per cent). We select a sample of radio-loud AGN with redshifts out to $z \sim 6$ and radio luminosities $10^{20} < \textrm{L}_{1.4~\textrm{GHz}} / \textrm{W Hz}^{-1} < 10^{27}$ and classify them as high-excitation and low-excitation radio galaxies (HERGs and LERGs). The classification catalogue is released with this work. We find no significant difference in the host galaxy properties of the HERGs and LERGs in our sample. In contrast to previous work, we find that the HERGs and LERGs have very similar Eddington-scaled accretion rates; in particular we identify a population of very slowly accreting AGN that are formally classified as HERGs at these low radio luminosities, where separating into HERGs and LERGs possibly becomes redundant. We investigate how black hole mass affects jet power, and find that a black hole mass $\gtrsim 10^{7.8}~\textrm{M}_\odot$ is required to power a jet with mechanical power greater than the radiative luminosity of the AGN ($L_\textrm{mech}/L_\textrm{bol} > 1$). We discuss that both a high black hole mass and black hole spin may be necessary to launch and sustain a dominant radio jet.

astro-ph.GA↗

arXiv2021

Radio spectral properties of star-forming galaxies in the MIGHTEE-COSMOS field and their impact on the far-infrared-radio correlation

We study the radio spectral properties of 2,094 star-forming galaxies (SFGs) by combining our early science data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey with VLA, GMRT radio data, and rich ancillary data in the COSMOS field. These SFGs are selected at VLA 3GHz, and their flux densities from MeerKAT 1.3GHz and GMRT 325MHz imaging data are extracted using the "super-deblending" technique. The median radio spectral index is $\alpha_{\rm 1.3GHz}^{\rm 3GHz}=-0.80\pm0.01$ without significant variation across the rest-frame frequencies ~1.3-10GHz, indicating radio spectra dominated by synchrotron radiation. On average, the radio spectrum at observer-frame 1.3-3GHz slightly steepens with increasing stellar mass with a linear fitted slope of $\beta=-0.08\pm0.01$, which could be explained by age-related synchrotron losses. Due to the sensitivity of GMRT 325MHz data, we apply a further flux density cut at 3GHz ($S_{\rm 3GHz}\ge50\,\mu$Jy) and obtain a sample of 166 SFGs with measured flux densities at 325MHz, 1.3GHz, and 3GHz. On average, the radio spectrum of SFGs flattens at low frequency with the median spectral indices of $\alpha^{\rm 1.3GHz}_{\rm 325MHz}=-0.59^{+0.02}_{-0.03}$ and $\alpha^{\rm 3.0GHz}_{\rm 1.3GHz}=-0.74^{+0.01}_{-0.02}$. At low frequency, our stacking analyses show that the radio spectrum also slightly steepens with increasing stellar mass. By comparing the far-infrared-radio correlations of SFGs based on different radio spectral indices, we find that adopting $\alpha_{\rm 1.3GHz}^{\rm 3GHz}$ for $k$-corrections will significantly underestimate the infrared-to-radio luminosity ratio ($q_{\rm IR}$) for >17% of the SFGs with measured flux density at the three radio frequencies in our sample, because their radio spectra are significantly flatter at low frequency (0.33-1.3GHz).

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