Surveying the Onset and Evolution of Supermassive Black Holes at High-z with AXIS

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

the manuscript is very interesting and well written. I have just a few comments and suggestions.

The references should be improved. A high proportion of the cited references belong to the Authors with a self-citation rate of about 40%. While it is obvious in a white paper to cite works in which authors are involved and in this case of the AXIS working group, references from other groups should be added, if possible, in some cases in places of some currently reported. In addition, some discussion reported should be supported with a dedicated citation (see below).

As a general comment, if possible, I suggest to reduce the number of  small paragraphs, not breaking up the text excessively, especially if the topic remains the same (see e.g. L144-156).

The X-ray survey carried out by Chandra is taken as reference for future surveys with AXIS. This is reasonable but a brief paragraph with the comparison between the surveys with Chandra and other current X-ray instruments can be added.

Please double check the acronym, some of them are defined multiple times throughout the paper (e.g. SMBH, PSF, AGN, XLF)

Please use universe or Universe consistently throughout the paper.

I suggest to use $z$ instead of z and to report Chandra, XMM-Newton, NewAthena in italics throughout the paper.

In the Abstract:

- the telescope is the the Vera C. Rubin Observatory, LSST is the survey

- I suggest to specify in the last line "....recently observed by JWST in the early Universe."

In the caption of Figure 1: "...between 10 and 100 solar masses."

L27: I suggest to specify what do you mean here for high and low redshift.

L31: With which facility those SMBH have been detected? It may be useful to specify the information.

L101: 0 < $z$ < 5

L131: it is not completely clear to me because the more luminous sources are more difficult to be detected.

L138: X-rays

L148: A reference could be added here.

L167: this paragraph is linked to previous one, don't go to the end.

L181:  I suggest to remove "Therefore" here.

L205-207: these numbers come from a publication? If yes, please add a reference here.

L226: sources of emission

L230: Galactic

L252-254: please add a reference here

L259: based on [29] simultaneously studied...

L260: please specify what how the 'occupation fractions' are defined, even as a footnote.

In the caption of Figure 3, please add a reference for the statistical accuracy achieved by NuSTAR in studying CT AGN in the local Universe.

L265: z < 8 and [please add a blank space after 8]

L277: "Deep" and "Intermediate" AXIS survey....

L311: what GNz11 means?

L316: please add a reference here.

L355: at high z [z is missing here]

L375: NewAthena

L400: Marchesi et al. [52] should be only [52]. The same applies to the other references in which the format of the journal is not respected.

L432: Guest Observer (GO)

In the caption of Table 1 three reference AXIS surveys are cited, but only 2  are reported in the main Table. Please check.

In the Section 5.2 it is important to add at least a reference for each facility cited.

L457: please define LSST here.

L473: ...to achieve optical identification with follow-up observations

L475-476: some of the source catalogs cited here are already available, please add the references for those catalogs.

L488: here you refer to new or available techniques?

L502: please add a reference for the flux limit of deep JWST surveys.

L505: Please add a reference or a link to the website for Keck MOSFIRE and Suburu PSF

In the caption of Figure 5 0.5-2 keV [there is an extra comma before 2]

Author Response

Dear referee, thanks for your comments. Please find our responses here.

Dear Authors,

the manuscript is very interesting and well written. I have just a few comments and suggestions.

The references should be improved. A high proportion of the cited references belong to the Authors with a self-citation rate of about 40%. While it is obvious in a white paper to cite works in which authors are involved and in this case of the AXIS working group, references from other groups should be added, if possible, in some cases in places of some currently reported. In addition, some discussion reported should be supported with a dedicated citation (see below).

As a general comment, if possible, I suggest to reduce the number of  small paragraphs, not breaking up the text excessively, especially if the topic remains the same (see e.g. L144-156).

The X-ray survey carried out by Chandra is taken as reference for future surveys with AXIS. This is reasonable but a brief paragraph with the comparison between the surveys with Chandra and other current X-ray instruments can be added.

- We added in the introduction:As of the last release of the {\em Chandra Source Catalog}, \cite[CSC,][]{csc21}, over 25 years of operation, {\em Chandra} covered about 700 deg$^2$ of sky and detected about 400,000 sources with only a few hundreds at the faintest fluxes over of a few square arc-minutes of survey area \cite{luo}. A similar number of sources was detected, over 1150 deg$^2$, at around 1 order of magnitude shallower flux limit than {\em Chandra},  by {\em XMM-Newton} as of the latest release of the 4XMM Catalog \cite{4XMM}.

Please double check the acronym, some of them are defined multiple times throughout the paper (e.g. SMBH, PSF, AGN, XLF)

-OK

Please use universe or Universe consistently throughout the paper.
-ok

I suggest to use $z$ instead of z and to report Chandra, XMM-Newton, NewAthena in italics throughout the paper.

- OK

In the Abstract:

- the telescope is the the Vera C. Rubin Observatory, LSST is the survey

R: OK

- I suggest to specify in the last line "....recently observed by JWST in the early Universe."

- OK

In the caption of Figure 1: "...between 10 and 100 solar masses."

- OK

L27: I suggest to specify what do you mean here for high and low redshift.
OK

L31: With which facility those SMBH have been detected? It may be useful to specify the information.

- We added UKIRT, DESI and PanStarrs
L101: 0 < $z$ < 5
-Ok

L131: it is not completely clear to me because the more luminous sources are more difficult to be detected.

- We specified rarer-lower volume density

L138: X-rays

Ok
-
L148: A reference could be added here.

- We rephrased this sentence and addeed refences
 - OK

L167: this paragraph is linked to previous one, don't go to the end.

-OK

L181:  I suggest to remove "Therefore" here.

OK

L205-207: these numbers come from a publication? If yes, please add a reference here.

- Done

L226: sources of emission

- OK
 
L230: Galactic

- Ok 
L252-254: please add a reference here

-added references to Wang et al. (2021) and Bogdan et al. (2024)--basically, papers with  plots with mass as a function of redshift for given Eddington ratios and seed masses.

L259: based on [29] simultaneously studied...
- OK

L260: please specify what how the 'occupation fractions' are defined, even as a footnote.
-An occupation fraction definition was added in parentheses. "The fraction of galaxies that host massive BHs"

In the caption of Figure 3, please add a reference for the statistical accuracy achieved by NuSTAR in studying CT AGN in the local Universe.

- Added .... NuSTAR, that unambiguously provided Compton-thick AGN line-of-sight column density measurements the local Universe [25].

L265: z < 8 and [please add a blank space after 8]
OK

L277: "Deep" and "Intermediate" AXIS survey....

- OK 
L311: what GNz11 means?
- Added Goods North z11

L316: please add a reference here.

- Added

L355: at high z [z is missing here]
-Ok

L375: NewAthena
Ok

L400: Marchesi et al. [52] should be only [52]. The same applies to the other references in which the format of the journal is not respected.

L432: Guest Observer (GO)

OK

In the caption of Table 1 three reference AXIS surveys are cited, but only 2  are reported in the main Table. Please check.

Added the footnote:\footnote{The Wide Serendipitous is not an official AXIS program but it is an estimate of the extragalatic survey sensitivity obtained by grouping all the expected GO observations over the 5 year of mission lifetime.}

In the Section 5.2 it is important to add at least a reference for each facility cited.
- As for many facilities reference publication is not yet availaible of too extensive we added citations and/or URLs as footnotes. 

L457: please define LSST here.
OK

L473: ...to achieve optical identification with follow-up observations

L475-476: some of the source catalogs cited here are already available, please add the references for those catalogs.

There is a collection of more than 500 survey catalogs in Vizie-r that we can use. We added a reference to Vizie-r.

- OK

L488: here you refer to new or available techniques?

- added "mentioned above"

L502: please add a reference for the flux limit of deep JWST surveys.

-OK, added reference to CEERS Survey paper and Maiolino paper on high-z AGN discovery

L505: Please add a reference or a link to the website for Keck MOSFIRE and Suburu PSF
-OK

In the caption of Figure 5 0.5-2 keV [there is an extra comma before 2]

OK

Reviewer 2 Report

Comments and Suggestions for Authors

This is an interesting and well-written paper. However, the entire premise of the paper rests on the assumption that X-rays trace massive black holes reliably and at all redshifts. There is emerging evidence undermining this assumption, e.g., it is being found that many AGNs at high redshift are weak X-ray emitters.

https://ui.adsabs.harvard.edu/abs/2023arXiv231012330L/abstract

https://ui.adsabs.harvard.edu/abs/2024arXiv240500504M/abstract

Some local AGNs are also thought to be intrinsically weak in X-rays, e.g., BAL quasars, so total reliance on X-rays would even undermine the completeness of a local sample, but current work indicates this might become a far greater problem at high redshift.  This issue needs to be discussed in the paper because it is at the heart of the entire treatise.

 

The unqualified statements starting on line 144: “At the same time, distinguishing highly obscured AGNs from normal star-forming galaxies based solely on Near-Mid Infrared observations is indeed challenging, if not virtually impossible. This difficulty arises due to the presence of highly degenerate features shared by both star formation processes (such as dust and narrow spectral lines) and obscured AGNs. This similarity in features makes it extremely challenging to untangle the true nature of a given source using only infrared observations.” Is total nonsense. The authors should read Lyu et al.,

 

https://ui.adsabs.harvard.edu/abs/2023arXiv231012330L/abstract

 where the methods for distinguishing AGNs and star forming galaxies are laid out thoroughly. This is a more thorough treatment than the Yang et al. paper cited here, but that paper also shows how to distinguish the two types, just less thoroughly.

 

 The statements about black holes being overmassive relative to host galaxies (lines 231 – 233) are naïve. This is a complex issue subject to selection biases and a consensus has not been reached. See Li et al.

 https://ui.adsabs.harvard.edu/abs/2024arXiv240300074L/abstract

 This needs to be presented as an unproven hypothesis.

 

 On line 474, the citation of the COSMOS Legacy Survey to predict 95% matching completeness with optical counterparts has a number of problems. First, Lyu et al. (already mentioned) found 34% more AGNs among 111 massive galaxies in the HUDF than had been identified as AGNs previously, even with the 7Ms Chandra data. The cited paper states: “After matching all the X-ray fields to the same astrometric optical frame, 95% of the X-ray sources used for the astrometry correction have a distance to their optical counterpart smaller than 1.4 arcsec.”  This is not the same as saying that 95% of the X-ray sources had an optical counterpart – presumably the sources without optical counterparts were not used for astrometry. The statement in the current text therefore appears to be very misleading.

 

  

Line 576 – do these authors have any idea how faint the very high redshift galaxies being found by JWST really are? Without serious qualification, this statement is very misleading.

 A current problem that the authors might mention is Little Red Dots (LRDs), which are not at such high redshifts and whose nature is a vigorous debate in the literature right now.

Author Response

We thank the referee for the insightful comments. Here we reply to their concerns and highlight the changes made to the white paper. 

This is an interesting and well-written paper. However, the entire premise of the paper rests on the assumption that X-rays trace massive black holes reliably and at all redshifts. There is emerging evidence undermining this assumption, e.g., it is being found that many AGNs at high redshift are weak X-ray emitters.

https://ui.adsabs.harvard.edu/abs/2023arXiv231012330L/abstract

https://ui.adsabs.harvard.edu/abs/2024arXiv240500504M/abstract

Some local AGNs are also thought to be intrinsically weak in X-rays, e.g., BAL quasars, so total reliance on X-rays would even undermine the completeness of a local sample, but current work indicates this might become a far greater problem at high redshift.  This issue needs to be discussed in the paper because it is at the heart of the entire treatise.

 
The unqualified statements starting on line 144: “At the same time, distinguishing highly obscured AGNs from normal star-forming galaxies based solely on Near-Mid Infrared observations is indeed challenging, if not virtually impossible. This difficulty arises due to the presence of highly degenerate features shared by both star formation processes (such as dust and narrow spectral lines) and obscured AGNs. This similarity in features makes it extremely challenging to untangle the true nature of a given source using only infrared observations.” Is total nonsense. The authors should read Lyu et al.,
 
https://ui.adsabs.harvard.edu/abs/2023arXiv231012330L/abstract

 where the methods for distinguishing AGNs and star forming galaxies are laid out thoroughly. This is a more thorough treatment than the Yang et al. paper cited here, but that paper also shows how to distinguish the two types, just less thoroughly.

R: Thanks for pointing this out. Indeed, MIRI is more sensitive than Chandra, and that's why we need AXIS to push surveys 2 dex fainter on a wider FOV. The problem of X-ray weakness in some samples selected by JWST is far from being understood. It is well known that the main biasing factor for AGN selection is intrinsic obscuration at every redshift (see, e.g., Peca et al. 2022). We agree with the referee on the point raised about the findings of Lyu et al., which state: "The intrinsically faint AGNs will not be detected in X-ray even if they are unobscured due to the shallower Chandra detection limits compared to MIRI." These non-detections in the X-ray are likely due to (a) poor performance of Chandra beyond 2" off-axis and (b) sensitivity degradation. AXIS, thanks to its superior average PSF uniformity and effective area, will boost Chandra's sensitivity by two orders of magnitude and place the sources in the detectable range of Fig. 20 of Lyu et al. This is stated at line 134. Moreover, in section 3.3.3, they state: "[Due to obscuration] Therefore, the identification of high-z AGNs is relatively less robust and likely only yields a subset." Without a more powerful X-ray telescope, we cannot complete the selection. This is more important when looking for extremely high-z sources (see, e.g., Natarajan 2017).

The Maiolino paper points out to more sophisticated likely introduced by obscuration. Moreover, according to Vito et al. 2019, in a sample of high-z QSOs detected by Chandra at z > 6, there is no evident evolution of accretion physics in the first Gyr of the Universe while according to Zappacosta 2023 there seems to be. This makes the topic extremwly exciting to exciting to tackle in the X-ray band. 

Furthermore, MIRI at high z becomes a rest-frame optical telescope. Hence, according to the current literature, X-ray selected AGN outnumber optically selected sources in source surface density. As a final point, we also note that MIRI has a limited FOV and its performance is degrading. It is not currently foreseeable to have a survey wide and deep enough to capture the rarer, lower volume density, more luminous sources.

We added the reference to the papers and: expanded the paragraph in this way:
{\bf In particular, for obscured AGN} this similarity in features makes it extremely challenging to untangle the true nature of a given source using only infrared observations.

{\bf In a recent paper, \cite{2024ApJ...966..229L} showed that MIRI can, in principle, disentangle AGN from SFG. However, at z$>$4, its sensitivity to obscuration features significantly decreases as the rest-frame wavelengths sampled are optical-NIR. 

In their sample of MIRI-selected AGN, 80\% are not X-ray detected. This can be due to a large number of Compton-Thick sources or the intrinsic X-ray faintness of high-z AGN at the faint end of the luminosity function. On the other hand, \cite{vito19} found no difference in the spectral shape of high-z QSOs, while \cite{zappa} found steeper than average X-ray spectra. Finally, \cite{2024arXiv240500504M} presented a population of JWST selected. X-ray weak sources where heavy X-ray absorption by clouds with large (Compton-thick) column density and low dust content, such as the Broad Line Region (BLR) clouds, can explain the X-ray weakness.  As for the extreme redshifts that AXIS plans to sample, \cite{PN17} predicts extremely bright X-ray/Optical ratios for the early phase of SMBH growth at $z\sim$9.

This suggests that, in order to understand the nature of accretion at high-z and confirm the expected large fraction of Compton-thick AGN \cite[see e.g.][]{gilli07, treister09, ananna17, peca}, further studies in the X-ray band, pair with other wavelenghts, are necessary.
}

 

- We do not believe that our statement is non sense, using AXIS we will able to achieve depth comparable to JWST (i.e. L_X>42 erg/s at z=6) and peer into obscuration. We removed the sentence 'if not virtually impossible'. Moreover in the following paragraph we state:
" a multi-wavelength approach is crucial. Combining observations from different spectral ranges, such as X-ray, optical, and infrared, allows for a more comprehensive analysis. X-ray surveys, such as those conducted by Chandra or future missions like AXIS, provide valuable information about the presence of AGN activity through the detection of X-ray emissions associated with the accretion onto SMBH

 
 The statements about black holes being overmassive relative to host galaxies (lines 231 – 233) are naïve. This is a complex issue subject to selection biases and a consensus has not been reached. See Li et al.
 https://ui.adsabs.harvard.edu/abs/2024arXiv240300074L/abstract
This needs to be presented as an unproven hypothesis.

R: We agree with the referee. We addded: "However, we caution the reader that this topic is a matter of fierce debate in the literature {\cite[see e.g.][]{li24}}, particularly while methods for measuring SMBH masses at high-z are being refined."
  }

 On line 474, the citation of the COSMOS Legacy Survey to predict 95% matching completeness with optical counterparts has a number of problems. First, Lyu et al. (already mentioned) found 34% more AGNs among 111 massive galaxies in the HUDF than had been identified as AGNs previously, even with the 7Ms Chandra data. The cited paper states: “After matching all the X-ray fields to the same astrometric optical frame, 95% of the X-ray sources used for the astrometry correction have a distance to their optical counterpart smaller than 1.4 arcsec.”  This is not the same as saying that 95% of the X-ray sources had an optical counterpart – presumably the sources without optical counterparts were not used for astrometry. The statement in the current text therefore appears to be very misleading.

- This statement is correct: 95% of Cosmos Legacy X-ray sources have an optical IR counterpart. Civano et al. 2016 and vast literature in the field of X-ray surveys (e.g Brusa et al. 2007 or the recent Merloni et al 2024 eROSITA paper) uses likelihood ratio to find counterparts of X-ray sources.  Also the 7Ms data of Chandra suffer from strong off-axis PSF degeneracy with only the inner arcmin performig at subarcsec resolution. Moreover, Chandra suffers from a 0.5" systematic astrometric uncertainty so the 1.4" arcsec separation found by Lyu et al., while surprising, is not totally unexpected. 
 
  
Line 576 – do these authors have any idea how faint the very high redshift galaxies being found by JWST really are?

- Yes, and AXIS will fill the gap with Chandra.

Without serious qualification, this statement is very misleading. A current problem that the authors might mention is Little Red Dots (LRDs), which are not at such high redshifts and whose nature is a vigorous debate in the literature right now.

- We believe that this was addressed above. LRD are extremely controversial sources and we are still not sure if they really are AGN. We believe that the topic is still too immature for a detailed discussion.

 

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thanks to the authors for addressing my concerns.