Infrared Spectral Energy Distribution and Variability of Active Galactic Nuclei: Clues to the Structure of Circumnuclear Material
Round 1
Reviewer 1 Report
The manuscript "Infrared Spectral Energy Distribution and Variability of Active Galactic Nuclei: Clues to the Structure of Circumnuclear Material" by Lyu and Rieke is an interesting review about the infrared emission of AGN. I think that it covers fairly well all the most interesting aspects of the topic, and it also provides an comprehensive list of papers in the references to have a complete view on the topic.
The only major thing that is not examined in this review are relativistic jets, and if/how they do contribute to the infrared emission of AGN. In my opinion, this should be dealt with, either by changing the title/introduction of the review to highlight that jetted AGN are not considered, or by adding a specific section. If jets are not considered, though, the term "quasar" should be avoided, as it refers to "quasi-stellar radio source", thus likely jetted. The more appropriate "quasi-stellar object" (QSO) label should be used. If the authors want to keep using "quasar", then they should clarify this point to avoid confusion.
Beside this, I only have very few minor comments that should be addressed.
line 68: is the distinction between quasar/QSO and Seyfert really necessary? I would briefly mention this here already.
line 90 ahead: given that this paragraph goes through the history of observations, I would write the year explicitly, because they are not apparent unless someone checks directly the references.
line 203: SMC acronym not defined
line 342: I would better clarify what the authors mean with "feet" of PAH lines, as it is not obvious for non experts.
Author Response
>>> We thank the referee for the comments. Our replies are given after ">>>". In the associated PDF, we have highlight the changes of the new version.
Referee 1:
The manuscript "Infrared Spectral Energy Distribution and Variability of Active Galactic Nuclei: Clues to the Structure of Circumnuclear Material" by Lyu and Rieke is an interesting review about the infrared emission of AGN. I think that it covers fairly well all the most interesting aspects of the topic, and it also provides an comprehensive list of papers in the references to have a complete view on the topic.
The only major thing that is not examined in this review are relativistic jets, and if/how they do contribute to the infrared emission of AGN. In my opinion, this should be dealt with, either by changing the title/introduction of the review to highlight that jetted AGN are not considered, or by adding a specific section. If jets are not considered, though, the term "quasar" should be avoided, as it refers to "quasi-stellar radio source", thus likely jetted. The more appropriate "quasi-stellar object" (QSO) label should be used. If the authors want to keep using "quasar", then they should clarify this point to avoid confusion.
>> We thank the referee for pointing out this issue. Now we have added a new section 2.3.7 “Contamination from Non-thermal Processes in Radio-loud systems” and discuss this issue in some detail. We keep using the word “quasar” since its association with a radio source has been weaken through the years and many people (like us) think this world is interchangeable with “QSO” (e.g., https://en.wikipedia.org/wiki/Quasar). A footnote 1 is added in the last paragraph of Section 1.
Beside this, I only have very few minor comments that should be addressed.
line 68: is the distinction between quasar/QSO and Seyfert really necessary? I would briefly mention this here already.
>> In the literature, some people treat Seyfert and quasar/QSO differently and most papers just focus on one type (either quasars or just Seyfert-nuclei). Some people use AGN to represent Seyfert nuclei, which is very confusing. So it is critical to define all these terms here.
line 90 ahead: given that this paragraph goes through the history of observations, I would write the year explicitly, because they are not apparent unless someone checks directly the references.
>> We have added the year information for some key references in Section 2.1.
line 203: SMC acronym not defined
>> We have defined this term in Section 2.2.2.
line 342: I would better clarify what the authors mean with "feet" of PAH lines, as it is not obvious for non experts.
>> This has been rephrased in Section 2.2.4.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper "Infrared Spectral Energy Distribution and Variability of Active
Galactic Nuclei: Clues to the Structure of Circumnuclear Material" by Lyu and
Rieke is a very well written and updated review on the material surrounding
the central engine of AGN.
I have mostly minor comments and suggestions (including some additional
references) that the authors may consider in their revised version.
1. Fig. 1: it may be useful to report on the filter of HST images.
2. I would qualify the term 'IR' in Fig. 2 for the polar emission (which is
then described in the main text).
3. Section 2.1: few words here on the radio loud/quiet (or jetted/not-jetted,
as Padovani+) can be useful here.
4. Section 2.2: I would add a sentence on how AGN 1.2-1.9 (i.e., the
intermediate types) are classified. I would also slightly expand the last
sentence about the fact that for very obscured sources the optical spectra
are not available (i.e., too faint? too much dilution of the AGN in the host
galaxy, hence IFU spectroscopy is needed? anything different?)
5. Section 2.2.1: the authors mention the sublimation temperature of graphite
which, however, is properly introduced only at the end of the review. Maybe
some numbers can be anticipated. Furthermore, when the sample of HDD quasars
of Lyu+ work is introduced: I think it can be useful to report on the redshift
and Lbol range.
6. Fig. 5: high and low tau_9.7 micron is not reported (from Sajina+12) in the
figure caption. Besides, it is not totally clear to me what the 'normal' AGN
template is among the few from literature shown in Fig. 3.
7. I would mention - Section 2.2.2? - also the possibility that the host galaxy
has a role in the extinction of the nuclei, as reported and widely discussed in
Goulding+12.
8. In Section 2.2.3, the [OIV] emission is introduced: I would suggest to
slightly expand the part on the implications (AGN radiation field) of the
detection of this line. In Fig. 7 also other lines are reported: it could be
nice to say briefly in the text is any of these can be useful as well ([NeV]
is 'similar' to the [OIV] line, i.e., to infer the presence of an AGN in
most cases; line ratios using other transitions can be indicative of other
kind (star-formation?) of radiation fields).
9. Section 2.2.4: the 11.3 micron PAH emission is described for the link with
the total FIR emission/SFR. A brief mention to the possible use
(or limitations) of the other PAH features can be reported.
10. Sometimes 'warm' color is used (starting from IRAS) but a proper
definition is never reported. It can be useful - as some of the suggestions
reported above/below - for general readers.
11. Section 2.3.1: the importance of Compton-thick AGN can be reported briefly
(i.e., possible links with the obscured growth phase in the AGN-galaxy
co-evolution scenario, mergers, etc.; see also Vito+18 for the higher fraction
of obscured AGN at high redshift which can be relevant in the same context).
Besides, I would move the definition of CT AGN from page 13 to page 6 when
they are introduced [ref. 213] for the first time.
The authors may also decide to mention (not necessarily here) that spotting
these extremely obscured systems can be done also combining the mid-IR vs.
the X-ray information (see Gandhi+09; for an application, see La Caria+19).
12. Section 2.3.2, Hot-Dogs: the authors may also cite Vito+18; Zappacosta+18
(where also deep NuSTAR data were used). I would also expand a bit the
explanation of the properties in terms of possible strong outflows/winds etc.
13. Section 2.4.4: about interacting/merging systems hosting AGN, the authors
may also cite - if they think that it is appropriate - the work by Lanzuisi+19
in the COSMOS field.
14. Section 2.4.5: sSFR, please define.
15. Section 3.4.1: the unification picture of radio-loud AGN can be described
in a few sentences (or it can be described in the broad context of Unification
Schemes for all AGN).
Typo: Section 3.4.2: 'to confirm THE this behaviour'
16. Section 5.2: about clumpy models, the authors may mention the recent
results using NuSTAR data from Marchesi+19. About the torus geometry/structure
using WISE data in relation with other AGN properties, a very recent paper
that the authors may decide to cite is Son, Kim & Ho 2022.
17. Section 5.4: a few words on the MATISSE instrument would help.
Author Response
>>> We thank the referee for the comments. Our replies are given after ">>>". In the associated PDF, we have highlighted the changes of the new version.
Referee 2:
The paper "Infrared Spectral Energy Distribution and Variability of Active
Galactic Nuclei: Clues to the Structure of Circumnuclear Material" by Lyu and
Rieke is a very well written and updated review on the material surrounding
the central engine of AGN.
I have mostly minor comments and suggestions (including some additional
references) that the authors may consider in their revised version.
- Fig. 1: it may be useful to report on the filter of HST images.
>> This is some three-color HST WFC3 images we grabbed from wikipedia. We have given the link of the original source so people can figure out how it was made.
- I would qualify the term 'IR' in Fig. 2 for the polar emission (which is then described in the main text).
>> We have updated the caption of Fig.2
- Section 2.1: few words here on the radio loud/quiet (or jetted/not-jetted, as Padovani+) can be useful here.
>> Now we have a new Section 2.3.7 to discuss the radio loud systems in details.
- Section 2.2: I would add a sentence on how AGN 1.2-1.9 (i.e., the intermediate types) are classified. I would also slightly expand the last sentence about the fact that for very obscured sources the optical spectra are not available (i.e., too faint? too much dilution of the AGN in the host galaxy, hence IFU spectroscopy is needed? anything different?)
>> We have updated the last sentence of the first paragraph of Section 2.2.
- Section 2.2.1: the authors mention the sublimation temperature of graphite which, however, is properly introduced only at the end of the review. Maybe some numbers can be anticipated. Furthermore, when the sample of HDD quasars of Lyu+ work is introduced: I think it can be useful to report on the redshift and Lbol range.
>> The Lyu+ work is a general discussion on the AGN IR SED variations, not limited to a specific sample (the main sample is the PG quasar, but Lyu+ also explored the situations at z=0.5–6.5). In Section 2.3.6, we mentioned “Similar near- to mid-IR SED variations such as WDD and HDD are also seen up to these redshifts \citep{lyu2017b}, further supporting the generally similar behaviors.” Reporting the redshift and Lbol range would not be necessary (as these variations are seen across the parameter space).
- Fig. 5: high and low tau_9.7 micron is not reported (from Sajina+12) in the figure caption. Besides, it is not totally clear to me what the 'normal' AGN template is among the few from literature shown in Fig. 3.
>> We have updated the captions of Fig.4 and Fig. 5.
- I would mention - Section 2.2.2? - also the possibility that the host galaxy has a role in the extinction of the nuclei, as reported and widely discussed in Goulding+12.
>> This has been updated in the 3rd paragraph of Section 2.2.2.
- In Section 2.2.3, the [OIV] emission is introduced: I would suggest to slightly expand the part on the implications (AGN radiation field) of the detection of this line. In Fig. 7 also other lines are reported: it could be nice to say briefly in the text is any of these can be useful as well ([NeV] is 'similar' to the [OIV] line, i.e., to infer the presence of an AGN in most cases; line ratios using other transitions can be indicative of other kind (star-formation?) of radiation fields).
>> We have revised the relevant paragraphs to mention that [OIV] is tracing the NLR. [NeV] is not mentioned in the review, so its description is not relevant.
- Section 2.2.4: the 11.3 micron PAH emission is described for the link with the total FIR emission/SFR. A brief mention to the possible use (or limitations) of the other PAH features can be reported.
>> We have now mentioned in the Section 2.2.4: “The carriers of many aromatic features are destroyed around AGNs [e.g., 289 ]. “
- Sometimes 'warm' color is used (starting from IRAS) but a proper definition is never reported. It can be useful - as some of the suggestions reported above/below - for general readers.
>> We have now added footnotes or words within brackets to make them clear.
- Section 2.3.1: the importance of Compton-thick AGN can be reported briefly (i.e., possible links with the obscured growth phase in the AGN-galaxy co-evolution scenario, mergers, etc.; see also Vito+18 for the higher fraction of obscured AGN at high redshift which can be relevant in the same context). Besides, I would move the definition of CT AGN from page 13 to page 6 when they are introduced [ref. 213] for the first time. The authors may also decide to mention (not necessarily here) that spotting these extremely obscured systems can be done also combining the mid-IR vs.the X-ray information (see Gandhi+09; for an application, see La Caria+19).
>> For almost all different types of AGNs, we typically only mention their names without going into any details of their significance and interpretations. To keep this review focused, we decide not to adopt the suggested revisions.
- Section 2.3.2, Hot-Dogs: the authors may also cite Vito+18; Zappacosta+18 (where also deep NuSTAR data were used). I would also expand a bit the explanation of the properties in terms of possible strong outflows/winds etc.
>> These two references are now added. We do not think the second suggestion is very relevant for the discussed topic.
- Section 2.4.4: about interacting/merging systems hosting AGN, the authors may also cite - if they think that it is appropriate - the work by Lanzuisi+19 in the COSMOS field.
>> We decide not to cite them as they are not critical for the relevant review topic.
- Section 2.4.5: sSFR, please define.
>> Done.
- Section 3.4.1: the unification picture of radio-loud AGN can be described in a few sentences (or it can be described in the broad context of Unification Schemes for all AGN).
>> We have added one sentence to briefly talk about this.
Typo: Section 3.4.2: 'to confirm THE this behaviour'
>> Corrected.
- Section 5.2: about clumpy models, the authors may mention the recent results using NuSTAR data from Marchesi+19. About the torus geometry/structure using WISE data in relation with other AGN properties, a very recent paper that the authors may decide to cite is Son, Kim & Ho 2022.
>> We do not think the first reference (Marchesi+19) is very relevant to the issues we are discussing. The second reference (Son, Kim & Ho 2022) does not contain significant results that would bring much more insights compared with many other papers we decide not to cite due to the space limitations.
- Section 5.4: a few words on the MATISSE instrument would help.
>> we have added a footnote for this instrument.
Author Response File: Author Response.pdf
