The Composite Spectral Energy Distribution of Quasars Is Surprisingly Universal Since Cosmic Noon

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this article, the author discusses the mean and median SED of quasars with redshift <=3 (cosmic noon). The author extends one of their previous analyses that suggested a luminosity-independent SED at z~2 to include a selection of quasars since cosmic noon. The author provides a comprehensive discussion of event selection, followed by corrections due to biases. The author presents the results of this analysis that suggests a universality of the composite SED for the selected sample and ends with discussions of its implications.

I believe this article is well written, with sound arguments and analysis, and a very interesting discussion regarding limitations and an outlook into the future. I recommend publication. There are just some points I would like to address. They are as follows.

Some major comments:

In Section 2, when discussing the data selection, the author includes very detailed information on cuts in various physical observables and refers to "tradeoffs" very often. While these choices all intuitively make sense, as indicated in the article in lines 122-124 for example, it is hard to quantify which choice "the best." It might therefore be helpful to include a comparison between some different choices so as to convince the readers that 1. the arbitrary cutoffs are indeed reasonable and perhaps 2. perturbations in some cutoffs will not lead to significantly different conclusions.

Figure 3: although the ~500A results suffer from low sample size, it would be better to still show them in darker colors. They now easily escape the readers' observation.

Section 2.4: regarding the correction for IGM absorption and correction, is there a way through which the error bands can be shown on the plots? It would be nice to see if the uncertainty varies with redshift or luminosity at all. 

Section 3.3: from the plots I can believe that the current survey prefers a truncated disk model, but the rejection of models that include X-ray emission (Kubota and Done, as mentioned in the text, for example), seems a bit hand wavy. Would it be possible to provide a fit here? Some more discussions will make this argument sounder.

Finally, this might be beyond the scope of this current work, but I suggest a sensitivity analysis based on this work considering future X-ray telescopes regarding effects of more X-ray regime detections' implication on production mechanism. As suggested by this work, the low statistics at the X-ray regime is the limiting factor for the exploration of quasar models. As mentioned in the abstract and in the conclusion (but somehow unfortunately only at these aforementioned places), there exist upcoming telescopes; a natural question therefore arises: what will be the discovery capability and rejection confidence levels for these experiments? If we assume a truncated disk model, for example, what statistics and timeframe may lead us to a discover and/or rejection of other models?

Some minor comments:

Throughout the article, the author prefers passive tense. While the editor and the author should have the last call on this issue, I do believe that using more active tenses will make the article easier to read. For example, lines 1-7 (very beginning of abstract) starts with two long passive sentences. I would make these tenses active instead. 

lines 46-48: comparing [...], we find that [...] (subject of sentence is misassigned). 

line 76: a simple ", and" junction between sections 3 and 4 would be nice

Caption to figures 1 and 2: I think the captions are too long. Take that of figure 1 as an example. I do not see the explanation of the right panel until two thirds way down the caption block. I would consider removing explanatory reasoning such as "Beneath the red dashed stairs [...] the parent quasar sample is likely subject to sample incompleteness" (note additional "the" in the original caption) since the same sentence appears in main text lines 107-108. In general, I would put only necessary description for the plot in the caption and leave discussions and implications to the main text. 

lines 101-102: [...] and the number of quasars [...] as a function of log L_{2200} are shown [as an example]. (the current structure includes "for example" in an awkward location)

lines 175-177: there are still ~25% quasars that [are not] heavily absorbed [...]

lines 239-240: Previously, we compared the model-predicted SED implied the [...]. I didn't understand this sentence. perhaps a missing [by]? i.e. implied by?

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Summary

This paper presents evidence for a universal spectral energy distribution (SED) shape of quasars across redshifts, using data from SDSS and GALEX. The study finds that quasars above a bolometric luminosity of 10^{45.5} ergs^-1 maintain a consistent SED shape from optical to EUV, favoring a truncated disk model over the thin disk model to explain this consistency.

Recommendations for Improvement

Truncated Disk Model and Truncation Radius:

The model employs a variable truncation radius across redshift bins without explaining why this parameter should change over time. To avoid concerns of overfitting, a clear physical rationale for this variability (e.g., linked to redshift-dependent quasar properties like black hole mass or Eddington ratio) is essential.

Clarification of Disk Assumptions:

Expand briefly on how the truncated disk model accounts for the universal SED shape, particularly in the EUV. A concise description of the physical basis for truncation would strengthen the theoretical grounding.

Discussion on Evolutionary Implications:

While the universal SED shape implies stability in emission mechanisms, a brief discussion of how this aligns with known evolution in quasar populations (e.g., accretion rates) would enrich the study’s context.

The study’s findings are valuable, but clarifying the rationale for the variable truncation radius and refining certain explanations would enhance the paper’s robustness and accessibility.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In this paper, the author explores the spectral energy distributions of quasars using photometric data from the Sloan Digital Sky Survey and the Galaxy Evolution Explorer. In particular, mean and median spectral energy distributions for bright quasars in various redshift bins are constructed, correcting for intergalactic medium absorption effects. The key results of the paper are:

 

1) Independence from redshift, bolometric luminosity, black hole mass, and Eddington ratio found in the composite spectral energy distributions, suggesting uniformity in the dust and gas properties of quasar host galaxies since cosmic noon;

 

2) Deviation of the observed spectral energy distributions from predictions of the standard thin disk model;

 

3) Reduction in the expected ionizing radiation for wavelength shorter than 1000 Å.

 

These findings certainly contribute to the understanding of active galactic nuclei physics. The paper is clearly written. I recommend the paper for publication.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf