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AGN-driven outflows and the AGN feedback efficiency in young radio galaxies

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F.Santoro, C.Tadhunter, D.Baron, R.Morganti, J.Holt

Astrophysics of Galaxies · Submitted 23 Sep 2020 · 115 references

Pith's verdict: Useful, careful outflow energetics for young CSS/GPS radio galaxies; the qualitative low-efficiency result holds, but the 'below 1% in most cases' headline needs tempering because of density upper limits and Lbol systematics.

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arXiv ID2009.11175 (version 1)
Pith IDI67RH5IA, Pith's permanent record of this paper and its review
CitationC.Tadhunter, D.Baron, F.Santoro, J.Holt, R.Morganti. "AGN-driven outflows and the AGN feedback efficiency in young radio galaxies." arXiv:2009.11175 (2020). https://pith.science/paper/I67RH5IA
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The paper

The paper claims

Jets in young radio galaxies drive warm gas outflows with feedback efficiencies below 1 percent.

The reading

The paper sets out to establish that in young, compact radio galaxies the expanding radio jets, not radiation from the accretion disk, are the main agents pushing warm ionised gas out of the host: the outflows have radii ($0.06$–$2$ kpc) similar to the radio sources, and their emission-line structures align with the radio axes. It also claims that when outflow densities are measured with trans-auroral sulphur and oxygen line ratios, which stay sensitive at densities where classical diagnostics saturate, the warm outflows are dense ($\log(n_e/\mathrm{cm}^{-3})\sim3$ to $4.8$), and their kinetic powers are a small fraction of the AGN bolometric luminosity, giving feedback efficiencies below $1\%$ for most of the sample. If true, this calibrates how much of the available accretion energy young jets deposit in the interstellar medium, and it suggests that jet-mode feedback in this early phase can be as energetic per unit luminosity as the radiation-driven winds seen in ultraluminous infrared galaxies.

What carries the argument

The load-bearing machinery is the density diagnostic diagram (DDD), a two-dimensional plot of the trans-auroral line ratios tr[O ii] = [O ii](3726+3729)/(7319+7330) and tr[S ii] = [S ii](4069+4076)/(6717+6731). Because the upper levels of these transitions have high critical densities, the ratios remain sensitive where the classical [O ii] and [S ii] doublets saturate (above roughly $10^{3.5}$ cm$^{-3}$). The observed ratios are compared with grids of photoionisation models computed with Cloudy for different hydrogen densities, reddenings, metallicities, ionisation parameters and AGN spectral shapes; the grid with solar metallicity, a standard AGN SED and the object's empirically estimated ionisation parameter is used to read off $n_e$ and $E(B-V)$. Those densities then feed the standard chain $M_{\rm gas}=L(H\beta)m_p/(n_e\,\alpha_{H\beta}^{\rm eff}\,h\nu_{H\beta})$, $\dot M=M_{\rm gas}v_{\rm out}/r$, $\dot E=\tfrac{1}{2}\dot M(v_{\rm out}^2+3\sigma^2)$, and $F=\dot E/L_{\rm bol}$. The radii entering this chain come from HST [O iii] images, spectro-astrometry of X-shooter data, or, where unresolved, from the radio source extents.

Extended reading notes

Core claim

The central discovery is that the warm ionised gas outflows in nine young radio sources selected from the 2Jy sample are compact ($r\sim0.06$–$2$ kpc), comparable in size to the radio sources and in many cases aligned with the radio axes, which the authors take as evidence that jet-mode AGN feedback is the dominant driving mechanism. A second, quantitative result is that, using the density diagnostic diagram based on trans-auroral [O ii] and [S ii] line ratios compared with new photoionisation model grids, the outflowing gas is dense ($\log(n_e/\mathrm{cm}^{-3})\sim3$ to $4.8$), often denser than the ambient interstellar medium, plausibly compressed by jet-induced shocks. Combining intrinsic H$\beta$ luminosities, these densities, outflow radii and velocities, the mass outflow rates fall in the range $0.4$–$20\,M_\odot$ yr$^{-1}$, and the AGN feedback efficiencies $F=\dot E/L_{\rm bol}$ lie mostly below $1\%$ (from $0.002\%$ to $1\%$), far below the $5$–$10\%$ commonly assumed in feedback models. The warm outflows resemble those of nearby ultraluminous infrared galaxies studied with the same techniques, but are more energetic and have higher efficiencies on average than the general population of local AGN of similar bolometric luminosity, suggesting unusually strong jet-ISM coupling in the early stages of radio source evolution.

Load-bearing premise

The whole calculation hinges on the electron densities read off photoionisation model grids that assume solar metallicity, a standard AGN spectral shape and an ionisation parameter chosen from line ratios; because outflow masses, kinetic powers and feedback efficiencies all scale inversely with density, any systematic error in those model densities shifts the central efficiency result in the opposite direction.

Editorial extensions

If this is right

  • If jet-mode feedback is the dominant driver in these systems, kpc-scale energy injection into the host ISM is already under way while the radio source is younger than about a million years, not only after the jets have inflated large lobes.
  • With warm-outflow feedback efficiencies of 0.002%–1%, the warm ionised phase alone is far too weak to match the 5–10% efficiencies used in classical AGN feedback models, so explaining galaxy-scale effects requires either additional gas phases or a longer duty cycle.
  • Measuring outflow densities with high-density-sensitive trans-auroral ratios rather than assuming $n_e\sim200$ cm$^{-3}$ lowers the derived mass outflow rates and kinetic powers by one to two orders of magnitude, implying that published $\dot M$–$L_{\rm bol}$ relations built on the low-density assumption may overestimate ionised-outflow energetics.
  • The similarity of these CSS/GPS warm outflows to those in ULIRGs, despite different likely drivers, implies that the observable properties of a warm outflow alone cannot uniquely identify its acceleration mechanism.
  • Because the outflows align with the radio axes and have radii matching the radio lobes, high-resolution radio and optical observations of young sources can be used jointly to map where and how jet energy is deposited into the ISM.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Editorial inference: the DDD-inferred densities may apply to the densest line-emitting clumps only; if the bulk of the outflowing mass is at lower density, the true masses, kinetic powers and feedback efficiencies would all be higher than reported, potentially moving some objects above 1%.
  • Editorial inference: the known molecular outflow in PKS 1549–79 carries an order of magnitude more kinetic power than its warm phase, so a testable extension is that when ALMA-quality molecular measurements exist for the other eight sources, the total multi-phase feedback efficiencies will exceed the warm-only values and may reach a few percent.
  • Editorial inference: if jet-ISM coupling is strongest while the source is most compact, feedback efficiency may decline as the radio source expands; this predicts that larger CSS sources should show lower $F$ for fixed $L_{\rm bol}$, a trend this nine-object sample is too small to test but larger samples could check.
  • Editorial inference: because the bolometric luminosities rely on [O iii] calibrations and the authors argue these may overestimate $L_{\rm bol}$ in gas-rich young sources, independent X-ray or mid-infrared luminosity estimates could push the derived efficiencies even lower.
Original abstract

Active galactic nuclei (AGN) feedback operated by the expansion of radio jets can play a crucial role in driving gaseous outflows on galaxy scales. Galaxies hosting young radio AGN, whose jets are in the first phases of expansion through the surrounding interstellar medium (ISM), are the ideal targets to probe the energetic significance of this mechanism. In this paper, we characterise the warm ionised gas outflows in a sample of nine young radio sources from the 2Jy sample, combining X-shooter spectroscopy and Hubble Space Telescope (HST) imaging data. We find that the warm outflows have similar radial extents (~0.06-2 kpc) as radio sources, consistent with the idea that `jet mode' AGN feedback is the dominant driver of the outflows detected in young radio galaxies. Exploiting the broad spectral coverage of the X-shooter data, we have used the ratios of trans-auroral emission lines of [SII] and [OII] to estimate the electron densities, finding that most of the outflows have gas densities ($\log( n_e~cm^{-3})~3-4.8 $), which we speculate could be the result of compression by jet-induced shocks. Combining our estimates of the emission-line luminosities, radii, and densities, we find that the kinetic powers of the warm outflows are a relatively small fraction of the energies available from the accretion of material onto the central supermassive black hole (SMBH), reflecting AGN feedback efficiencies below 1% in most cases. Overall, the warm outflows detected in our sample are strikingly similar to those found in nearby ultraluminous infrared galaxies (ULIRGs), but more energetic and with a high feedback efficiencies on average than the general population of nearby AGN of similar bolometric luminosity; this is likely to reflect a high degree of coupling between the jets and the near-nuclear ISM in the early stages of radio source evolution.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 6,133 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.39 · human participants 0.01 · controlled comparison 0.58 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.94 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Active galactic nuclei (AGN) feedback operated by the expansion of radio jets can play a crucial role in driving gaseous outflows on galaxy scales. Galaxies hosting young radio AGN, whose jets are in the first phases of expansion through the surrounding interstellar medium (ISM), are the ideal targets to probe the energetic significance of this mechanism. In this paper, we characterise the warm ionised gas outflows in a sample of nine young radio sources from the 2Jy sample, combining X-shooter spectroscopy and Hubble Space Telescope (HST) imaging data. We find that the warm outflows have similar radial extents (~0.06-2 kpc) as radio sources, consistent with the idea that `jet mode' AGN feedback is the dominant driver of the outflows detected in young radio galaxies. Exploiting the broad spectral coverage of the X-shooter data, we have used the ratios of trans-auroral emission lines of [SII] and [OII] to estimate the electron densities, finding that most of the outflows have gas densities ($\log( n_e~cm^{-3})~3-4.8 $), which we speculate could be the result of compression by jet-induced shocks. Combining our estimates of the emission-line luminosities, radii, and densities, we find that the kinetic powers of the warm outflows are a relatively small fraction of the energies available from the accretion of material onto the central supermassive black hole (SMBH), reflecting AGN feedback efficiencies below 1% in most cases. Overall, the warm outflows detected in our sample are strikingly similar to those found in nearby ultraluminous infrared galaxies (ULIRGs), but more energetic and with a high feedback efficiencies on average than the general population of nearby AGN of similar bolometric luminosity; this is likely to reflect a high degree of coupling between the jets and the near-nuclear ISM in the early stages of radio source evolution.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Active galactic nuclei (AGN) feedback operated by the expansion of radio jets can play a crucial role in driving gaseous outflows on galaxy scales. background 1.00 0.04 0.06 hedged possibility 0.93
s1 Galaxies hosting young radio AGN, whose jets are in the first phases of expansion through the surrounding interstellar medium (ISM), are the ideal targets to probe the energetic significance of this mechanism. background 0.98 0.07 0.03 asserted 0.99
s2 In this paper, we characterise the warm ionised gas outflows in a sample of nine young radio sources from the 2Jy sample, combining X-shooter spectroscopy and Hubble Space Telescope (HST) imaging data. contribution 0.51 0.97 0.03 asserted 0.97
s3 We find that the warm outflows have similar radial extents (~0.06-2 kpc) as radio sources, consistent with the idea that `jet mode' AGN feedback is the dominant driver of the outflows detected in young radio galaxies. result 0.89 0.95 0.39 hedged possibility 0.56
s4 Exploiting the broad spectral coverage of the X-shooter data, we have used the ratios of trans-auroral emission lines of [SII] and [OII] to estimate the electron densities, finding that most of the outflows have gas densities ($\log( n_e~cm^{-3})~3-4.8 $), which we speculate could be the result of compression by jet-induced shocks. result 0.57 0.97 0.04 speculative 0.62
s5 Combining our estimates of the emission-line luminosities, radii, and densities, we find that the kinetic powers of the warm outflows are a relatively small fraction of the energies available from the accretion of material onto the central supermassive black hole (SMBH), reflecting AGN feedback efficiencies below 1% in most cases. result 0.99 0.98 0.38 asserted 1.00
s6 Overall, the warm outflows detected in our sample are strikingly similar to those found in nearby ultraluminous infrared galaxies (ULIRGs), but more energetic and with a high feedback efficiencies on average than the general population of nearby AGN of similar bolometric luminosity; this is likely to reflect a high degree of coupling between the jets and the near-nuclear ISM in the early stages of radio source evolution. interpretation 0.91 0.68 0.41 hedged possibility 0.76

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.92.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. HST ACS/WFC images of PKS 0023–26 (top) and PKS 1306–09 (bottom). The left-hand panels show the narrow￾band [O iii] images, whereas the right-hand panels show the intermediate-band continuum images. In each case, the dashed line shows the direction of the radio axis. Note that in both cases, the brightest extended [O iii] structures are closely aligned in an￾gle with the radio axes (within ∼10 degrees). Article number, page 4 of 40
Figure 2. Emission-line profiles for PKS 0023–26: First row: [O iii]λλ4958,5007Å (left panel) and Hβ line (right panel) fits. Second row: Hα+[N ii]λλ6548,84Å line fits. Third row: [S ii]λλ6717,31Å (left panel) and [S ii]λλ4069,76Å (right panel) trans-auroral line fits. The latter fit includes also the Hδ line. Fourth row: [O ii]λλ7319,30Å (left panel) and [O ii]λλ3726,29Å (right panel) trans￾auroral line fits. The former fit also includes the [O ii]λ7381Å line which has been modelled with a single Gaussian component. In each of the sub-figures the upper panel shows the best fit (red solid…
Figure 3. Position-velocity diagrams obtained by applying the spectro-astrometry technique to the [O
Figure 4. Locations of our targets in the [O iii]/Hβ vs [N ii]6583/Hα and [O iii]/Hβ vs [S ii]6716,31/Hα BPT diagrams. Filled circles mark the line ratios obtained from total line fluxes while empty circles are related to the line emission of the broad components only. The solid line in both panels is the Kewley et al. (2001) maximum starburst line. The dashed line in the left panel is the semi-empirical Kauffmann et al. (2003) line and has bee used together with the Kewley et al. (2001) line to separate between line ratios due to photoionisation from stars (HII), AGN or a mixture of both…
Figure 5. Figure showing the location of our targets in the density diagnostic diagram (DDD) using the logarithm of the tr[O
Figure 6. AGN bolometric luminosity as a function of the outflow
Figure 7. Mass outflow rate (left panel) and kinetic power of the outflow (right panel) as a function of the AGN bolometric luminosity

Figures reproduced from arXiv:2009.11175 by the authors.

References
How the literature uses it

8 arXiv papers cite this in 11 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 8 sentences without a confident label

Cites it as support 1 paper
Cites it as background 2 papers

What it cites · 6 sentences without a confident label

Compares with 1 paper
Cites as support 1 paper
  • This is based both on statistical studies of large samples of SDSS-selected AGN [ref] and on a growing number of observations of individual objects in which the outflows show detailed morphological associations with the radio lobes on kpc scales (e.g. IC 5063: [ref] ; SDSS J165315.06+232943.0: [ref] ; NGC 613: [ref] ; 3C 273: [ref] ; HE 1353-1917: [ref] ; ESO 428-G14: [ref] ; NGC 5929: [ref] ; NGC 1386: [ref] ; and the targets in the sample of [ref] ).

    A Catalog of 406 AGNs in MaNGA: A Connection between Radio-mode AGN and Star Formation Quenching · Introduction · 0.97

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