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Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). X. Discovery of 35 Quasars and Luminous Galaxies at 5.7 $\le$ z $\le$ 7.0

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Pith's one-line read A survey at redshift 6 finds 35 new quasars and galaxies, including a class of narrow-line objects that may be hybrid AGN-starburst sources.

desk verdict SHELLQs X is a solid, incremental survey paper: 35 new high-z objects are secure, and the narrow-line composite is interesting but hinges on Ly-alpha redshifts the authors already warn are approximate. read the letter →

arxiv 1908.07910 v1 pith:A6ABPHEZ submitted 2019-08-19 astro-ph.GA

Yoshiki Matsuoka , Kazushi Iwasawa , Masafusa Onoue , Nobunari Kashikawa , Michael A. Strauss , Chien-Hsiu Lee , Masatoshi Imanishi , Tohru Nagao
show 39 more authors
Masayuki Akiyama Naoko Asami James Bosch Hisanori Furusawa Tomotsugu Goto James E. Gunn Yuichi Harikane Hiroyuki Ikeda Takuma Izumi Toshihiro Kawaguchi Nanako Kato Satoshi Kikuta Kotaro Kohno Yutaka Komiyama Shuhei Koyama Robert H. Lupton Takeo Minezaki Satoshi Miyazaki Hitoshi Murayama Mana Niida Atsushi J. Nishizawa Akatoki Noboriguchi Masamune Oguri Yoshiaki Ono Masami Ouchi Paul A. Price Hiroaki Sameshima Andreas Schulze John D. Silverman Naoshi Sugiyama Philip J. Tait Masahiro Takada Tadafumi Takata Masayuki Tanaka Ji-Jia Tang Yoshiki Toba Yousuke Utsumi Shiang-Yu Wang Takuji Yamashita
This is my paper · ORCID
classification astro-ph.GA
keywords high-redshiftquasarsluminousgalaxiesLy-alphaemissionnarrow-linereionizationHyperSuprime-Camdropoutselectioncompositespectra
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 the discovery of 28 quasars and 7 luminous galaxies at redshifts 5.7 to 7.0 from spectroscopic follow-up of faint, color-selected dropouts in deep multi-band imaging. The new objects bring the survey's spectroscopically confirmed haul to 93 quasars and 31 galaxies over roughly 900 square degrees, extending knowledge of the quasar population to luminosities where quasars and galaxies are comparably common. The paper's central interpretive claim is that 18 quasars with very strong, narrow Ly-alpha emission form a distinct class: their stacked spectrum shows N V 1240 emission like narrow-line quasars, but the Ly-alpha line is often spectrally unresolved, which the authors take as evidence that host-galaxy starlight contributes to the light. These objects may therefore be composites of an active nucleus and a star-forming galaxy. Redshifts remain approximate, with offsets up to about 0.1, because they are measured from Ly-alpha peaks.

What carries the argument

The argument is carried by two linked tools: dropout selection from deep multi-band imaging in the i, z, and y bands, requiring extremely red colors and compact morphology and ranking candidates by a Bayesian probability built from quasar and brown-dwarf spectral energy distribution models; and median-stacked rest-frame composite spectra of the full spectroscopically confirmed sample, normalized to a common absolute magnitude, which expose the narrow-line objects' distinct N V and Ly-alpha pattern. The composites are the mechanism that turns 18 individual low-signal-to-noise spectra into a class-level statement.

What would settle it

Measure near-infrared Mg II 2800 or ALMA [C II] 158 micron emission for a sample of the 18 narrow-line objects and for the new quasars; if the resulting systemic redshifts shift the Ly-alpha peaks by more than about 0.1, or reveal that the N V 1240 feature in the stacked spectrum falls at the wrong rest wavelength, the redshift scale and the composite interpretation would need revision.

Watch

Extended reading notes

Core claim

In its own terms, the paper establishes that the 28 new quasars and 7 new luminous galaxies are genuine high-redshift objects, and it identifies 18 objects in the full sample whose Ly-$\alpha$ emission is luminous (L > $10^{43}$ erg/s), narrow (FWHM < 500 km/s), and usually strong (rest-frame equivalent widths up to about 500 angstroms). The stacked spectrum of these 18 differs from both the broad-line quasar composite and the galaxy composite: it contains a P Cygni-like N V 1240 feature and an asymmetric Ly-$\alpha$ profile, while many individual lines are unresolved at about 250 km/s resolution. The authors interpret this as evidence for a composite nature, an active nucleus plus a significant star-forming host, rather than a pure galaxy or a typical quasar. They also note that the composite spectra are not necessarily representative of the whole high-redshift population, since color selection favors objects with strong Ly-$\alpha$.

Load-bearing premise

The load-bearing premise is that the observed Ly-alpha line peak marks the intrinsic 1216 angstrom wavelength, so each redshift is good to about delta z = 0.1; the paper itself notes that strong neutral-hydrogen absorption by the intergalactic medium can break this assumption.

Editorial extensions

If this is right

  • The survey now contains 93 quasars and 31 galaxies at redshifts 5.7 to 7.0 over about 900 square degrees, expanding the faint-end census used to measure the quasar luminosity function at redshift 6.
  • The 7 newly found galaxies, with absolute magnitudes M1450 around -22 to -25 mag, sit well above the break magnitude of the galaxy luminosity function at z = 6 and can be compared with quasars at matched luminosities.
  • If the 18 narrow-line objects are composites, then a measurable fraction of luminous Ly-alpha sources at these redshifts is powered jointly by an accreting black hole and by star formation in the host galaxy.
  • Accurate systemic redshifts will require follow-up of Mg II 2800 or [C II] 158 micron emission, because the Ly-alpha-based redshifts presented here are only approximate and affect the placement of the rest-frame composite spectra.

Reading between the lines

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

  • If the narrow-line objects really are composite AGN and starburst systems, then the faint-end quasar luminosity function, built from color-selected samples that favor strong Ly-alpha, may systematically miss a population of dusty or weak-line quasars; the paper's observation that weak-line quasars are overrepresented among near-infrared-detected objects already hints in this direction.
  • A direct test would be deep rest-frame optical spectroscopy or imaging of the 18 narrow-line objects with a space telescope: if host stellar light dominates at longer wavelengths, the composite nature is confirmed, whereas pure AGN photoionization models would predict different line ratios.
  • The narrow-line class may represent a short-lived evolutionary phase connecting gas-rich star-forming galaxies and mature quasars; if so, the relative counts of broad-line and narrow-line objects in the same survey could yield an estimate of its duty cycle, though the authors do not make this argument.
  • Because the Bayesian selection algorithm currently omits galaxy spectral energy distribution models, some galaxies may be misclassified as quasars; adding galaxy models and tighter morphological cuts would sharpen the boundary and could change the faint-end luminosity function.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

0 major / 4 minor

Summary. This paper reports the spectroscopic identification of 67 candidate sources from the HSC-SSP survey as part of the SHELLQs program. The authors identify 28 quasars and 7 luminous galaxies at redshifts nominally between 5.7 and 7.0, along with 10 [O III] emitters at z~0.8 and 22 Galactic cool dwarfs. Redshifts are primarily based on Lyα emission peaks or the Gunn-Peterson trough, with uncertainties up to Δz≈0.1. The paper also constructs median rest-frame composite spectra for the full SHELLQs quasar sample, finding that 18 narrow-line Lyα quasars have a stacked spectrum distinct from broad-line quasars and galaxies, showing an N V λ1240 feature and a P Cygni-like profile; the authors propose that these objects may be composites of AGNs and star-forming galaxies.

Significance. If the discoveries hold, this work expands the census of low-luminosity quasars at z>5.7, which is important for measuring the quasar luminosity function and for assessing the quasar contribution to cosmic reionization. The paper is transparent about the approximate nature of Lyα-based redshifts, the possible IGM absorption biases, and the inherent quasar/galaxy classification ambiguity. The direct spectra, the quantified redshift uncertainties, and the acknowledged limitations strengthen confidence in the catalog. The composite spectra of a homogeneous sample are a useful resource, and the narrow-line quasar population is a noteworthy finding that will motivate near-infrared and ALMA follow-up. The central discovery claim is well supported by the presented data; the composite interpretation is appropriately hedged, though a quantitative sensitivity analysis would further improve it.

minor comments (4)
  1. [Abstract and Table 3] The stated redshift range "5.7 ≤ z ≤ 7.0" in the abstract and title is not strictly consistent with Table 3, which lists quasar J113218.15+003800.1 at z=5.66 and quasar J235646.33+001747.3 at z=7.01. Because the redshift uncertainties are Δz≈0.1, please rephrase the range as approximate (for example, 5.7≲z≲7.0) or explicitly note that boundary objects are included.
  2. [Section 4 and Figure 9] The identification of N V λ1240 and the associated mini-BAL profile in the narrow-line composite assumes that the Lyα peaks trace the systemic redshift. The text already notes that this assumption is not always correct, but I recommend adding an explicit statement in the figure caption or discussion that the N V feature is provisional and could be smeared or shifted if the redshift offsets approach Δz≈0.1; a quantitative restacking test with known offsets would be a simple way to assess this.
  3. [Section 4] Many of the narrow-line quasars are spectrally unresolved at the FOCAS resolution (~250 km/s), with FWHM upper limits below 230 km/s, yet the composite shows a narrow but resolved asymmetric Lyα line. Please briefly discuss how the observed upper limits are consistent with the composite width and with the proposed AGN plus star-forming galaxy composite interpretation.
  4. [Figure 8 and text] The phrase "adoptive moments" should be "adaptive moments" in both the text and the figure caption.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the discoveries rest on external follow-up spectra, and the composite-spectrum interpretation, while sensitive to the stated Ly-alpha redshift assumption, does not reduce any claimed result to its input.

full rationale

This is an observational discovery paper, not a derivation or prediction paper. The central claims are the spectroscopic identification of 28 quasars, 7 galaxies, 10 [O III] emitters, and 22 cool dwarfs; these identifications are supported by the presented spectra and external template comparisons (e.g., SpeX prism templates for cool dwarfs; SDSS and Pan-STARRS composites for quasars). The photometric candidate selection uses the authors' own Bayesian probability model from earlier SHELLQs papers, but the discovery claims do not depend on that model once spectroscopy is obtained; the paper explicitly acknowledges imperfect classification and possible galaxy contamination. The redshift determination from Ly-alpha peaks is a measurement assumption with a stated uncertainty of up to Delta z ~ 0.1, and the paper explicitly warns that it is not always correct due to IGM absorption. This is a systematic uncertainty, not a circularity: the composite spectra are built using those redshifts, but the N V 1240 feature and P Cygni-like profile are empirical features that are not fed back into the redshift estimates, and the paper explicitly defers profile analysis until systemic redshifts are available. There is no fitted parameter renamed as a prediction, no uniqueness theorem imported from the authors' prior work, and no equation that reduces to its own input. The self-citations to earlier SHELLQs papers describe the survey's established selection procedure and are not load-bearing for the spectroscopic discoveries reported here. Therefore the circularity score is 0.

Assumptions & free parameters 0 free parameters · 5 assumptions · 0 invented entities

No fitted parameters or invented entities are introduced. The analysis relies on standard cosmology, literature power-law continuum slopes, a pre-existing Bayesian selection model, and the Ly-alpha peak redshift assumption, the last of which is explicitly acknowledged as uncertain.

assumptions (5)
  • domain assumption Adopted cosmology with H0 = 70 km/s/Mpc, Omega_M = 0.3, and Omega_Lambda = 0.7.
    Used to compute luminosities and distances; stated in Section 1.
  • domain assumption Quasar continua follow a power law with slope alpha = -1.5, and galaxy continua follow a power law with beta = -2.0.
    Taken from Vanden Berk et al. 2001 and Stanway et al. 2005; used to estimate M1450 and line properties in Section 4.
  • domain assumption Ly-alpha line peaks trace the systemic redshift of quasars.
    Used throughout Section 4 for redshift determination; the authors explicitly state it is approximate, with possible biases from IGM absorption.
  • domain assumption The Bayesian quasar probability calculation from Matsuoka et al. 2016, based on quasar and brown dwarf SED models and surface densities, is valid for candidate selection.
    Inherited from prior SHELLQs work; used in Section 2 to prioritize candidates, with galaxy models not included.
  • ad hoc to paper Objects with Ly-alpha luminosity greater than 1e43 erg/s and FWHM below 500 km/s are classified as possible narrow-line quasars.
    A working definition taken from previous SHELLQs papers; used in Section 4 and for the narrow-line composite.

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Cite this review

Pith. "Pith review of Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). X. Discovery of 35 Quasars and Luminous Galaxies at 5.7 $\le$ z $\le$ 7.0." pith.science (2026). https://pith.science/paper/A6ABPHEZ

@misc{pith2026190807910,
  author       = {Pith},
  title        = {Pith review of: Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). X. Discovery of 35 Quasars and Luminous Galaxies at 5.7 $\le$ z $\le$ 7.0},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/A6ABPHEZ}},
  note         = {Machine review of arXiv:1908.07910}
}
abstract

We report the discovery of 28 quasars and 7 luminous galaxies at 5.7 $\le$ z $\le$ 7.0. This is the tenth in a series of papers from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The total number of spectroscopically identified objects in SHELLQs has now grown to 93 high-z quasars, 31 high-z luminous galaxies, 16 [O III] emitters at z ~ 0.8, and 65 Galactic cool dwarfs (low-mass stars and brown dwarfs). These objects were found over 900 deg2, surveyed by HSC between 2014 March and 2018 January. The full quasar sample includes 18 objects with very strong and narrow Ly alpha emission, whose stacked spectrum is clearly different from that of other quasars or galaxies. While the stacked spectrum shows N V 1240 emission and resembles that of lower-z narrow-line quasars, the small Ly alpha width may suggest a significant contribution from the host galaxies. Thus these objects may be composites of quasars and star-forming galaxies.

Figures

Figures reproduced from arXiv: 1908.07910 by the authors.

Figure 1
Figure 1. Discovery spectra of the first set of 11 quasars, displayed in decreasing order of redshift. The object name and the estimated redshift (and the designation “NL” for the possible quasars with narrow Lyα emission) are indicated at the top left corner of each panel. The blue dotted lines mark the expected positions of the Lyα and N V λ1240 emission lines, given the redshifts. The spectra were smoothed using inverse-va… view at source ↗
Figure 2
Figure 2. Same as [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. Same as [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗
Figures from the paper (6 more)
Figure 4
Figure 4. Figure 4: Same as [PITH_FULL_IMAGE:figures/full_fig_p011_4.png]
Figure 5
Figure 5. Figure 5: Same as [PITH_FULL_IMAGE:figures/full_fig_p012_5.png]
Figure 6
Figure 6. Figure 6: Same as [PITH_FULL_IMAGE:figures/full_fig_p013_6.png]
Figure 7
Figure 7. Figure 7: Same as [PITH_FULL_IMAGE:figures/full_fig_p014_7.png]
Figure 8
Figure 8. Figure 8: Shape indicators of the quasars (blue dots, includ￾ing previously-known quasars in the HSC survey footprint), narrow-line quasars (blue dots with larger circles), high-z galaxies (green dots), [O III] emitters (green open circles), and Galactic cool dwarfs (red open ci…
Figure 10
Figure 10. Figure 10: Rest-frame composite spectrum of all 16 [O III] emitters at z ∼ 0.8 over the full (top) and lower (bottom) flux scales. The red dotted lines mark the wavelengths of Hγ, [O III] λ4364, He I λ4473, Hβ, [O III] λ4959 and λ5007. number of HSC objects, including quasar can…

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