[Paper Review] The LoTSS view of radio AGN in the local Universe. The most massive galaxies are always switched on

J. Sabater, P. N. Best+19 more|Open Research Online (The Open University)|Nov 13, 2018

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy119 references44 citations

TL;DR

The paper cross-matches LoTSS DR1 with SDSS DR7 to identify local radio AGN, derives their local 150 MHz luminosity function, and shows that the most massive galaxies are always hosting radio-AGN activity, with implications for duty cycle and feedback.

ABSTRACT

This paper presents a study of the local radio source population, by cross-comparing the data from the first data release (DR1) of the LOFAR Two-Metre Sky Survey (LoTSS) with the Sloan Digital Sky Survey (SDSS) DR7 main galaxy spectroscopic sample. The LoTSS DR1 provides deep data (median rms noise of 71 $\mathrmμ$Jy at 150 MHz) over 424 square degrees of sky, which is sufficient to detect 10615 (32 per cent) of the SDSS galaxies over this sky area. An improved method to separate active galactic nuclei (AGN) accurately from sources with radio emission powered by star formation (SF) is developed and applied, leading to a sample of 2121 local ($z < 0.3$) radio AGN. The local 150 MHz luminosity function is derived for radio AGN and SF galaxies separately, and the good agreement with previous studies at 1.4 GHz suggests that the separation method presented is robust. The prevalence of radio AGN activity is confirmed to show a strong dependence on both stellar and black hole masses, remarkably reaching a fraction of 100 per cent of the most massive galaxies ($> 10^{11} \mathrm{M_{\odot}}$) displaying radio-AGN activity with $L_{ m 150 MHz} \geq 10^{21}$W Hz$^{-1}$; thus, the most massive galaxies are always switched on at some level. The results allow the full Eddington-scaled accretion rate distribution (a proxy for the duty cycle) to be probed for massive galaxies. More than 50 per cent of the energy is released during the $\le 2$ per cent of the time spent at the highest accretion rates, $L_{\mathrm{mech}}/L_{\mathrm{Edd}} > 10^{-2.5}$. Stellar mass is shown to be a more important driver of radio-AGN activity than black hole mass, suggesting a possible connection between the fuelling gas and the surrounding halo. This result is in line with models in which these radio AGN are essential for maintaining the quenched state of galaxies at the centres of hot gas haloes.

Motivation & Objective

• Quantify the prevalence of radio AGN in the local universe as a function of stellar and black hole mass.
• Develop and apply a robust, multi-diagnostic method to separate radio AGN from star-formation powered radio emission.
• Derive the local 150 MHz luminosity function for radio AGN and star-forming galaxies.
• Investigate the Eddington-scaled accretion rate distribution (duty cycle) for massive galaxies.
• Assess how stellar mass versus black hole mass drives radio-AGN activity and discuss implications for feedback in hot halos.

Proposed method

• Cross-match LoTSS DR1 150 MHz radio data with SDSS DR7 main galaxy spectroscopic sample over 424 deg^2.
• Use a four-diagnostic scheme (D4000 vs L150/M*, BPT line ratios, L_Halpha vs L150, and WISE colors) to separate radio AGN from star-forming galaxies.
• Calibrate the diagnostic combination by comparing with the H-ATLAS/Gürkan et al. 2018 sample to minimize misclassification.
• Convert to 150 MHz luminosities using a standard spectral index (alpha = 0.7) for cross-diagnostic consistency.
• Aggregate classifications into a final radio-AGN vs SF sample, estimating contamination to be ≤3%.
• Derive the local 150 MHz luminosity function for AGN and SF galaxies and compare with 1.4 GHz results to verify robustness.

Experimental results

Research questions

• RQ1 What is the prevalence of radio AGN among local galaxies as a function of stellar mass and black hole mass?
• RQ2 How does the distribution of Eddington-scaled accretion rates (the duty cycle) look for the most massive galaxies?
• RQ3 Can a robust multi-diagnostic scheme reliably separate radio AGN from star-formation powered emission in deep LOFAR data?
• RQ4 How does the LoTSS-derived 150 MHz luminosity function for AGN compare with previous 1.4 GHz studies?
• RQ5 What are the implications of radio-AGN activity and its duty cycle for feedback in hot gaseous halos of massive galaxies?

Key findings

• 2121 sources are classified as radio AGN and 8494 as SF-dominated radio emitters in the local sample (z < 0.3).
• The prevalence of radio-AGN activity strongly depends on stellar mass and black hole mass, with the most massive galaxies (>10^11 M_sun) showing radio-AGN activity at L150MHz ≥ 10^21 W Hz^-1 in a complete 100% fraction.
• The local 150 MHz luminosity function derived for AGN agrees with previous studies at 1.4 GHz, indicating robustness of the separation method.
• The Eddington-scaled accretion rate distribution for massive galaxies peaks at L_mech/L_Edd ≈ 10^-5, with more than 50% of energy released during the top 2% of time when L_mech/L_Edd > 10^-2.5.
• Stellar mass is found to be a more important driver of radio-AGN activity than black hole mass, suggesting a link between fueling gas and the surrounding halo and supporting halo-quenching models for maintenance of quenched states in hot halos.