Extended ionized-gas structures in Seyfert 2 galaxy Mrk 78

A.A. Smirnova; A.V. Moiseev; D.V. Kozlova

REVIEW 3 major objections 5 minor 18 references

Extended ionized-gas structures in Seyfert 2 galaxy Mrk 78

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read The ionized-gas clouds 12-16 kpc from Mrk 78 are photoionized by its AGN, not by star formation, and the apparent nearby companion is a background galaxy.

desk verdict A modest but useful confirmation that Mrk 78's outer EELR clouds are AGN-photoionized; the evidence is plausible yet lacks error bars and contains a wavelength slip that needs checking. read the letter →

arxiv 1908.01376 v1 pith:IAQ4B5CD submitted 2019-08-04 astro-ph.GA

D.V. Kozlova , A.V. Moiseev , A.A. Smirnova This is my paper · ORCID

classification astro-ph.GA

keywords Seyfert2galaxyMrk78extendedemission-lineregionAGNphotoionizationBPTdiagramionizationconeionizedgascloudsbackground

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports new long-slit and 3D spectroscopy of the Seyfert 2 galaxy Mrk 78 and identifies two ionized-gas clouds, C1 and C2, at projected distances of 12-16 kpc southwest of the nucleus. The integrated line ratios place these clouds on the Seyfert side of standard BPT diagrams, so the paper argues that the dominant ionization source is the AGN rather than star formation. The clouds also fall inside the western ionization cone inferred from inner-region HST data and have low velocity dispersion, suggesting they are cool, photoionized gas accreted from outside the disc rather than AGN outflow. A nearby galaxy candidate is shown to be a distant background object at z≈0.382, leaving the origin of the external gas unresolved.

What carries the argument

The central machinery is the BPT emission-line diagnostic diagram, introduced by Baldwin, Phillips, and Terlevich, using the ratios [N II]/Hα, [O III]/Hβ, and [S II]/Hα, with the AGN/HII boundary branches taken from Kewley et al. (2006). The integrated long-slit spectrum of C1 and C2 supplies the line ratios, while FPI [O III] maps provide the spatial distribution, velocities, and velocity dispersions of the clouds. The Fischer et al. (2011) ionization-cone model, based on inner-region HST kinematics, is overlaid on these maps to test whether the clouds fall within the projected cone.

What would settle it

Re-observe C1 and C2 at higher spectral resolution and signal-to-noise, resolve the [S II] λ6717,6731 doublet, and check for Hβ at 6718 Å from the z=0.382 background galaxy; if the corrected line ratios move off the Seyfert branch onto the HII-region branch, the central claim would be refuted.

Watch

Extended reading notes

Core claim

The paper's central discovery is that the ionized-gas clouds C1 and C2, at 12-16 kpc projected distance southwest of Mrk 78, are photoionized predominantly by Seyfert-type AGN radiation. Using emission-line ratio diagnostics from an integrated long-slit spectrum, the clouds fall on the AGN branch of the BPT diagrams, matching the ionization state of the inner extended emission-line region. Their positions are consistent with the projected borders of the ionization bicone derived from circumnuclear HST data, and their low velocity dispersion shows the gas is dynamically cold rather than part of an AGN-driven outflow. The nearby galaxy SDSS J074240.37+651021.4, once suspected to be a dwarf companion or tidal remnant, is instead a background galaxy at z=0.382, so the origin of the external clouds remains an open question.

Load-bearing premise

The conclusion assumes the faint spectra of the two clouds are measured cleanly enough to trust the line-ratio diagnostics, especially that redshifted Hβ from the distant galaxy behind the clouds does not contaminate the [S II] lines, as the authors themselves note, and that the small signal still gives reliable ratios.

Editorial extensions

If this is right

If the clouds are truly AGN-photoionized, the AGN in Mrk 78 has illuminated gas at projected radii of 12-16 kpc, extending the known EELR to nearly twice the previously confirmed size.
The dynamically cold kinematics imply the external clouds trace inflowing or tidally stripped gas rather than AGN-driven outflow, so such EELRs can serve as tracers of external gas accretion.
The agreement with the ionization cone strengthens the unified-model picture in which the same collimated nuclear radiation shapes both the inner and outer ionized gas.
With the nearby galaxy ruled out as a companion, searches for the origin of the external gas must focus on lower-mass or fully disrupted satellites, or on a previous interaction.
The combined long-slit and 3D observations demonstrate that faint, kiloparsec-scale ionized clouds can be diagnosed even when they are located far outside the host galaxy's stellar disc.

Reading between the lines

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

A decisive test would be to map C1 and C2 with an integral-field spectrograph at higher signal-to-noise: photoionization predicts radial gradients in line ratios and no broad component, while a shock model would produce enhanced [O I]/Hα.
The authors note that Hβ from the z=0.382 background galaxy may blend with the [S II] λ6731 line in the C1 and C2 spectra; modeling or masking this contamination could shift the BPT positions, so a dedicated high-resolution re-observation would test the robustness of the AGN classification.
The retrograde kinematics of the clouds could be compared with minor-merger simulations to see whether such off-plane, AGN-illuminated clouds naturally arise in the observed phase space.
The same observational strategy could be applied to other Seyfert 2 galaxies with known ionization cones to search for similar distant, dynamically cold clouds, potentially revealing a population of accreted gas reservoirs around active galaxies.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Colleague,

This paper reports new long-slit and 3D spectroscopy of the Seyfert 2 Mrk 78, giving the first ionization classification of two outer gas clouds at 12-16 kpc. That's a real increment: previous spectroscopy along PA=90 missed these clouds, so the work fills a gap. The identification of SDSS J074240.37+651021.4 as a background galaxy rather than a companion is also well-supported by the redshift match to SDSS photometry. The BPT classification is standard, and the clouds fall within the ionization cone from Fischer et al. (2011) — a nice consistency check, not a fitted result. I think the central claim is probably right.

That said, the presentation is too thin. No line-flux uncertainties or S/N values are given, so it's impossible to judge how secure the BPT placement is for faint extended clouds. The bigger issue: the paper states that the background galaxy's Hβ at z=0.382 would blend with [S II] λ6731 in the cloud spectrum. That's off by a few Å — Hβ at z=0.382 sits at 6718 Å, much closer to [S II] λ6717. Given 7 Å resolution, contamination of the blue [S II] component is a real possibility, and the integrated cloud spectrum is spatially near the background galaxy. If [S II] is contaminated, the [S II]/Hα ratio is overestimated and the [S II] BPT point shifts toward the Seyfert region. However, the paper also uses the [N II]/Hα-based BPT, which is unaffected by this particular blend, and the diamond appears in the Seyfert zone there too. So the AGN photoionization conclusion probably survives, but the authors need to show that explicitly by providing the [N II] ratios with errors, or deblending the [S II] doublet.

There's also a minor internal slip: they write 6731 when they mean 6717. That's the kind of detail that matters when you're making a claim about spectral overlaps.

Overall, this is a small but legitimate observational step. It doesn't change the big picture, but it does complete the picture for one well-studied galaxy. The paper is for EELR specialists and Mrk 78 aficionados. A serious referee should see it — the missing errors and the contamination question are fixable. I'd recommend peer review with a request for uncertainties and a clear handling of the background galaxy's Hβ.

Best, [Name]

Referee Report

3 major / 5 minor

Summary. This paper presents new 6-m SAO RAS spectroscopic observations of the Seyfert 2 galaxy Mrk 78, combining scanning Fabry-Perot interferometry, MPFS integral-field spectroscopy, and SCORPIO-2 long-slit spectroscopy. The authors report two ionized-gas clouds, C1 and C2, at projected distances of 12-16 kpc southwest of the nucleus, and argue from BPT line-ratio diagrams that these clouds are photoionized by the AGN rather than by star formation. They also show that the clouds fall within the projected ionization-cone borders derived by Fischer et al. (2011) and that the gas is dynamically cold, favoring an external accretion or tidal origin over an AGN outflow. Finally, they identify the nearby galaxy SDSS J074240.37+651021.4 as a distant background object at z=0.382 rather than a dwarf companion.

Significance. If the BPT classification is correct, the paper extends the known EELR of Mrk 78 to 12-16 kpc and provides an interesting consistency check of the biconical AGN illumination model. The use of multiple independent data sets (FPI, MPFS, long-slit) is a strength, and the ionization-cone overlay is properly used as a consistency check rather than a fitted parameter. The central claim, however, rests almost entirely on one integrated long-slit spectrum, so the reliability of the measured line ratios is decisive. The paper would be a useful short contribution if the contamination and uncertainty issues described below are addressed, but as presented the main conclusion is not fully secured.

major comments (3)

[Section 4, Figs. 3 and 4] The paper's statement that the background Hβ 'should be blended with [S II] λ6731 in the C1 and C2 clouds' is not supported by the wavelength scales. At z=0.382, Hβ falls near 6718 Å, whereas Mrk 78's [S II] λ6717 and λ6731 lines appear near 6960-6980 Å in the observed frame given the velocities quoted in Section 3 (10900-11500 km/s). The features that actually threaten the BPT classification are the background [O III] λ5007 line at 7016 Å, within about 40 Å of Mrk 78's [S II] λ6731, and the background [O II] λ3727 line at 5230 Å, within about 40 Å of Mrk 78's [O III] λ5007 at ~5190 Å. Because the Seyfert classification of C1+C2 rests directly on [S II]/Hα and [O III]/Hβ, the authors must quantify or subtract the background galaxy's contribution in the C1/C2 extraction or demonstrate spatially that it is negligible. As written, the paper acknowledges a contamination channel but does not assess its effect on the diagnostic ratios.
[Section 3, Fig. 4] No line-flux uncertainties or signal-to-noise ratios are reported for the integrated C1+C2 spectrum. The diamond marking the C1+C2 ratios in the BPT diagrams has no error bars, and the separation from the Kewley et al. (2006) boundary cannot be judged. Given that the clouds are faint extended structures at 12-16 kpc, the Hβ and [S II] fluxes may be uncertain by several tenths of a dex, which could change the classification. Please provide line fluxes, uncertainties, and detection significances for the relevant emission lines.
[Section 4] The redshift of galaxy G is quoted as z=0.382, but the two identified lines at 5230 Å and 7016 Å imply z≈0.40 if they are [O II] λ3727 and [O III] λ5007, respectively. This discrepancy should be resolved because the predicted observed wavelength of the background Hβ line, and hence the contamination analysis, depends directly on the adopted redshift.

minor comments (5)

[Section 2] The text uses 'Voight' fitting; this should be 'Voigt' fitting.
[Abstract and Introduction] The word 'faction' appears where 'fraction' is intended; please correct this typo.
[Section 3] The phrase 'intergral-field' should be 'integral-field'.
[Section 3, Fig. 3] The extraction apertures used to produce the integrated C1+C2 spectrum and the spectrum of galaxy G are not described; please state the spatial windows along the slit.
[Figure 4] The axis label 'HII Comp AGN' is unclear; please clarify what quantity is plotted along the horizontal axis of the lower panel.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the AGN-ionization claim rests on external BPT classification and prior HST cone geometry, not on fitted inputs or self-citation.

full rationale

The paper's central claim is that the external clouds C1 and C2 are photoionized by the AGN, supported by BPT diagnostic diagrams using emission-line ratios from an integrated long-slit spectrum. BPT classification boundaries (Baldwin et al. 1981; Kewley et al. 2006) are external, literature-based empirical benchmarks, not parameters fitted from the same data that the paper then 'predicts'. The overlay of the Fischer et al. (2011) ionization cone onto the FPI maps is a consistency check derived from independent HST observations of the circumnuclear region, not a fitted input used to define the clouds' ionization state. The authors cite their own previous work (Smirnova et al. 2018; Afanasiev & Moiseev 2005, 2011) only for data-reduction instruments and algorithms, which is not load-bearing for the physics conclusion. The possible blending of background-galaxy H-beta with [S II] lambda6717 is a data-quality and flux-uncertainty concern that could weaken the BPT classification, but it does not make the derivation circular. No self-definitional step, fitted-input-as-prediction, or self-citation chain is present; the analysis is self-contained against external diagnostics. Therefore the circularity score is 0.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

No free parameters or invented entities; the analysis rests on standard empirical diagnostics and previously published geometric models.

assumptions (4)

domain assumption BPT diagnostic diagrams (Baldwin et al. 1981; Kewley et al. 2006) reliably separate HII-region photoionization from AGN photoionization based on [N II]/Hα, [O III]/Hβ, and [S II]/Hα ratios.
Used in Section 3 to classify the ionization state of C1 and C2 as Seyfert-like.
domain assumption Distance to Mrk 78 is 165 Mpc (scale 0.80 kpc/arcsec), taken from NED.
Used in the abstract and Section 1 to convert projected separations to physical units.
domain assumption The bicone opening angle and orientation from Fischer et al. (2011), derived from HST imaging of the inner r<4 arcsec region, can be projected to larger radii.
Used in Section 3 to note that C1 and C2 fall within the projected cone borders.
domain assumption The two weak emission lines in the spectrum of galaxy 'G' are [O II] λ3727 and [O III] λ5007 at z=0.382, rather than other line identifications.
Used in Section 4 to classify SDSS J074240.37+651021.4 as a distant background galaxy.

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Cite this review

Pith. "Pith review of Extended ionized-gas structures in Seyfert 2 galaxy Mrk 78." pith.science (2026). https://pith.science/paper/IAQ4B5CD

@misc{pith2026190801376,
  author       = {Pith},
  title        = {Pith review of: Extended ionized-gas structures in Seyfert 2 galaxy Mrk 78},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/IAQ4B5CD}},
  note         = {Machine review of arXiv:1908.01376}
}

read the original abstract

Search for and study of extended emission-line regions (EELRs) related to AGN in early-type galaxies is interesting to probe the history of nuclear ionization activity and also to understand the process of external gas accretion. In this work, we present observations of the EELR in Mrk 78 obtained at the 6-m Russian telescope using the long-slit and 3D spectroscopy methods. We show that ionized-gas clouds at the 12-16 kpc projected distances from the nucleus are ionized by the AGN radiation. Also we have checked if the galaxy appearing in the optical images in the immediate neighbourhood of Mrk 78 near the external clouds is a dwarf companion or a part of a tidal structure. However, the spectrum of this galaxy, SDSS J074240.37+651021.4, obtained at the 6-m telescope corresponds to the distant background galaxy.

Figures

Figures reproduced from arXiv: 1908.01376 by the authors.

**Figure 1.** Left: the HST/FOC image with the F520M filter, shows the [OIII] emission (see Fischer et al., 2011); right: the MPFS image in the [OIII]λ5007 line with the superimposed continuum isophotes. 1http://ned.ipac.caltech.edu/ [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗

**Figure 2.** Mrk 78 observations. Top left: the SCORPIO R-band image with the long-slit position, a candidate companion galaxy marked as ‘G’; top right: the map of the [OIII]λ4959 emission derived from the FPI data; bottom: the line-of-sight velocity field in the [OIII] emission line (left) and the velocity dispersion map with the marked ionization cone position (right). The colour bars are in the km s−1 , the external gaseous c… view at source ↗

**Figure 3.** Top: identification of the emission lines in the ionized-gas clouds (C1 and C2) and the nearby galaxy (G) long-slit spectra; bottom: the integrated spectrum of both clouds. et al., 2015), the asymmetric distribution of the external [OIII] emission (Unger et al., 1987; Afanasiev & Silchenko, 1991). De Robertis (1987) suggested that the disturbed external isophotes are caused by passing through a companion galaxy and … view at source ↗

Figures from the paper (1 more)

**Figure 4.** Figure 4: BPT diagrams. The separate lines are taken from Kewley et al. (2006). The circles denote the MPFS data for the circumnuclear region, the colour bar shows the projected distance of pixels on the MPFS maps from the nucleus. The diamond shows the line ratio of C1+C2 deriv…

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Works this paper leans on

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, " * write output.state after.block = add.period write newline

ENTRY address archiveprefix author booktitle chapter edition editor howpublished institution eprint journal key month note number organization pages publisher school series title type volume year doi label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts #0 'before.all :=...
[2]

write newline

" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION word.in bbl.in " " * FUNCTION format....
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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

[1] [1]

, " * write output.state after.block = add.period write newline

ENTRY address archiveprefix author booktitle chapter edition editor howpublished institution eprint journal key month note number organization pages publisher school series title type volume year doi label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts #0 'before.all :=...

[2] [2]

write newline

" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION word.in bbl.in " " * FUNCTION format....

[3] [3]

L., Dodonov , S

Afanasiev , V. L., Dodonov , S. N., & Moiseev , A. V., Kinematics of Circumnuclear Regions of Galaxies: 2D Spectroscopy on SAO RAS 6m Telescope . 2001, in Stellar Dynamics: from Classic to Modern , ed. L. P. Ossipkov & I. I. Nikiforov , 103

work page 2001

[4] [4]

Afanasiev, V. L. & Moiseev, A. V., The SCORPIO universal focal reducer of the 6-m telescope. 2005, Astronomy Letters , 31 , 194, DOI: 10.1134/1.1883351

doi:10.1134/1.1883351 2005

[5] [5]

Afanasiev , V. L. & Moiseev , A. V., Scorpio on the 6 m Telescope: Current State and Perspectives for Spectroscopy of Galactic and Extragalactic Objects . 2011, Baltic Astronomy , 20 , 363, DOI: 10.1515/astro-2017-0305

doi:10.1515/astro-2017-0305 2011

[6] [6]

Afanasiev , V. L. & Silchenko , O. K., Gas Kinematics in the Central Regions of Seyfert Galaxies - Part Six - MARKARIAN:34 MARKARIAN:78 and MARKARIAN:270 . 1991, Astrofizicheskie Issledovaniia Izvestiya Spetsial'noj Astrofizicheskoj Observatorii , 33 , 144

work page 1991

[7] [7]

S., Ahumada , R., Almeida , A., et al

Aguado , D. S., Ahumada , R., Almeida , A., et al. , The Fifteenth Data Release of the Sloan Digital Sky Surveys: First Release of MaNGA-derived Quantities, Data Visualization Tools, and Stellar Library . 2019, , 240 , 23, DOI: 10.3847/1538-4365/aaf651

doi:10.3847/1538-4365/aaf651 2019

[8] [8]

A., Phillips , M

Baldwin , J. A., Phillips , M. M., & Terlevich , R., Classification parameters for the emission-line spectra of extragalactic objects . 1981, , 93 , 5, DOI: 10.1086/130766

doi:10.1086/130766 1981