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The ALMA Spectroscopic Survey in the HUDF: Constraining the Molecular Content at $\log{(M_*/M_\odot)} \sim 9.5$ with CO stacking of MUSE detected $z\sim1.5$ Galaxies

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Hanae Inami, Roberto Decarli, Fabian Walter, Axel Weiss, Chris Carilli, Manuel Aravena, Leindert Boogaard, Jorge González-López
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Gergö Popping, Elisabete da Cunha, Roland Bacon, Franz Bauer, Thierry Contini, Paulo C. Cortes, Pierre Cox, Emanuele Daddi, Tanio Díaz-Santos, Melanie Kaasinen, Dominik A. Riechers, Jeff Wagg, Paul van der Werf, Lutz Wisotzki

Astrophysics of Galaxies · Submitted 22 Sep 2020 · 77 references

Pith's verdict: A careful CO-stacking measurement that extends upper limits to log M* ~ 9.5 at z ~ 1.5, but the claimed flattening of the gas-to-stellar mass ratio is not supported by 3-sigma non-detections.

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arXiv ID2009.10843 (version 1)
Pith IDIVLOZA33, Pith's permanent record of this paper and its review
CitationAxel Weiss, Chris Carilli, Dominik A. Riechers, Elisabete da Cunha, Emanuele Daddi, Fabian Walter, Franz Bauer, Gerg\"o Popping, Hanae Inami, Jeff Wagg, Jorge Gonz\'alez-L\'opez, Leindert Boogaard, Lutz Wisotzki, Manuel Aravena, Melanie Kaasinen, Paulo C. Cortes, Paul van der Werf, Pierre Cox, Roberto Decarli, Roland Bacon, Tanio D\'iaz-Santos, Thierry Contini. "The ALMA Spectroscopic Survey in the HUDF: Constraining the Molecular Content at $\log{(M_*/M_\odot)} \sim 9.5$ with CO stacking of MUSE detected $z\sim1.5$ Galaxies." arXiv:2009.10843 (2020). https://pith.science/paper/IVLOZA33
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The paper

The paper claims

Stacking CO spectra at MUSE redshifts extends molecular gas measurements of z~1.5 galaxies down to log(M*/Msun)~9.5, where the gas-to-stellar mass ratio flattens rather than continuing its steep decline.

The reading

Stacking CO(2-1) spectra at the positions and redshifts supplied by an optical integral-field survey pushes molecular-gas measurements of $z\sim1.5$ galaxies down to stellar masses of $\log(M_*/M_\odot)\sim9.5$, below the reach of individual detections. The paper reports a robust stacked detection at $\log(M_*/M_\odot)=10.0$ and tight $3\sigma$ upper limits at $9.0\lesssim\log(M_*/M_\odot)\lesssim10.0$, and argues that the molecular gas-to-stellar mass ratio declines more gently in this low-mass regime than it does above $\log(M_*/M_\odot)\sim10.5$. Under a metallicity-dependent CO-to-H$_2$ conversion factor, the low-mass constraints are consistent with a plateau in gas fraction, matching simulation predictions and local-galaxy observations. This matters because it extends gas scaling relations into the mass range that dominates the future star formation budget, while demonstrating that stacking two blind volumetric surveys recovers most of the CO emission in the field.

What carries the argument

The load-bearing technique is spectral stacking: sub-cubes are extracted from ALMA Band 3 data at MUSE positions, shifted to the rest frame using the MUSE redshifts, resampled onto a common grid with a Gaussian decimation filter, and averaged without weighting. Line fluxes come from 2D Gaussian fits to coadded moment-0 images, and gas masses follow from the CO(2-1) luminosity via the conversion $M_{\rm gas}=\alpha_{\rm CO}\,r_{21}\,L'_{\rm CO}$, with $r_{21}=0.76$ and a metallicity-dependent $\alpha_{\rm CO}$ that rises from $3.6$ to $7.2$ to $10.8\,M_\odot\,(\mathrm{K\,km\,s^{-1}\,pc^2})^{-1}$ toward lower masses. The mechanism that carries the argument is the redshift overlap between the blind MUSE and ALMA surveys, which turns sub-threshold noise in individual galaxies into a measurable average.

Extended reading notes

Core claim

On the paper's own terms, the central discovery is that the molecular gas content of main-sequence galaxies at $z\sim1.5$ can be measured in aggregate down to $\log(M_*/M_\odot)\sim9.5$ by stacking ALMA spectra at MUSE-selected redshifts. Direct CO(2-1) detections are limited to $\log(M_*/M_\odot)>10.0$, but stacks of 38 and 32 galaxies in the mass bins $8<\log(M_*/M_\odot)\leq9$ and $9<\log(M_*/M_\odot)\leq10$ yield meaningful upper limits, and the gas mass keeps increasing with stellar mass from $\sim9.0$ to $\sim12.0$. The paper finds that, with a metallicity-based conversion factor, the gas-to-stellar mass ratio does not fall as steeply below $\log(M_*/M_\odot)\sim10.0$ as it does above $\sim10.5$, consistent with a plateau; with a constant conversion factor the ratio is consistent with being flat from $\sim9.5$ to $\sim10.5$ before turning down. The paper further shows that stacking recovers about half the cosmic molecular gas density at this epoch, $\rho(\mathrm{H}_2)=(0.49\pm0.09)\times10^8\,M_\odot\,\mathrm{Mpc}^{-3}$, in line with an independent CO-selected estimate.

Load-bearing premise

The entire low-mass interpretation rests on the assumed metallicity-dependent CO-to-H$_2$ conversion factor; if the true factor at $\log(M_*/M_\odot)\sim9.5$ is closer to the high-mass value of 3.6, the apparent plateau disappears and the turnover mass becomes unconstrained.

Editorial extensions

If this is right

  • Gas scaling relations at $z\sim1.5$ now have an anchor at $\log(M_*/M_\odot)\sim9.5$, constraining the low-mass end that individual CO surveys cannot reach.
  • The flat gas-to-stellar mass ratio below $\log(M_*/M_\odot)\sim10.5$ implies that feedback-driven gas depletion is weaker in low-mass galaxies, shifting the balance toward longer depletion times.
  • Recovering most of the CO emission via stacking implies the faint-end slope of the CO luminosity function is at least as flat as the stellar mass function's faint end.
  • The consistency with simulation predictions at the low-mass end sets a target: a tenfold larger sample would confirm or rule out the predicted gas masses.

Reading between the lines

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

  • If the plateau is real, low-mass galaxies at $z\sim1.5$ are not gas-starved; their lower specific star formation must instead arise from longer depletion times, meaning they will keep forming stars over a longer cosmic interval.
  • A sharper test would be to measure CO(1-0) or CO(2-1) for individual galaxies in this mass range and derive $\alpha_{\rm CO}$ directly from oxygen abundances, removing the main freedom in the conversion.
  • The same stacking machinery could be pointed at higher-$J$ CO lines once Ly$\alpha$ systemic offsets are better calibrated, potentially exposing a population of low-excitation, dust-poor gas reservoirs in faint galaxies.
Original abstract

We report molecular gas mass estimates obtained from a stacking analysis of CO line emission in the ALMA Spectroscopic Survey (ASPECS) using the spectroscopic redshifts from the optical integral field spectroscopic survey by the Multi Unit Spectroscopic Explorer (MUSE) of the {\it Hubble} Ultra Deep Field (HUDF). Stacking was performed on subsets of the sample of galaxies classified by their stellar mass and position relative to the main-sequence relation (on, above, below). Among all the CO emission lines, from \cotwoone to CO(6-5), with redshifts accessible via the ASPECS Band~3 and the MUSE data, \cotwoone provides the strongest constraints on the molecular gas content. We detect \cotwoone emission in galaxies down to stellar masses of $\log{(M_*/M_\odot)}=10.0$. Below this stellar mass, we present a new constraint on the molecular gas content of $z\sim1.5$ main-sequence galaxies by stacking based on the MUSE detections. We find that the molecular gas mass of main-sequence galaxies continuously decreases with stellar mass down to $\log{(M_*/M_\odot)}\approx9.0$. Assuming a metallicity-based CO--to--$\rm H_2$ conversion factor, the molecular gas-to-stellar mass ratio from $\log{(M_*/M_\odot)}\sim9.0$ to $\sim10.0$ does not seem to decrease as fast as for $\log{(M_*/M_\odot)}>10.0$, which is in line with simulations and studies at lower redshift. The inferred molecular gas density $\rho{\rm (H_2)}=(0.49\pm0.09)\times10^8\,{\rm M_\odot\,Mpc^{-3}}$ of MUSE-selected galaxies at $z\sim1.5$ is comparable with the one derived in the HUDF with a different CO selection. Using the MUSE data we recover most of the CO emission in our deep ALMA observations through stacking, demonstrating the synergy between volumetric surveys obtained at different wavebands.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 7,541 input tokens.

What the abstract itself states, as probabilities: claims first 0.05 · states limitation 0.46 · human participants 0.01 · controlled comparison 0.20 · randomized assignment 0.02 · states quantitative result 0.99 · compares with prior results 0.96 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

We report molecular gas mass estimates obtained from a stacking analysis of CO line emission in the ALMA Spectroscopic Survey (ASPECS) using the spectroscopic redshifts from the optical integral field spectroscopic survey by the Multi Unit Spectroscopic Explorer (MUSE) of the {\it Hubble} Ultra Deep Field (HUDF). Stacking was performed on subsets of the sample of galaxies classified by their stellar mass and position relative to the main-sequence relation (on, above, below). Among all the CO emission lines, from \cotwoone to CO(6-5), with redshifts accessible via the ASPECS Band~3 and the MUSE data, \cotwoone provides the strongest constraints on the molecular gas content. We detect \cotwoone emission in galaxies down to stellar masses of $\log{(M_*/M_\odot)}=10.0$. Below this stellar mass, we present a new constraint on the molecular gas content of $z\sim1.5$ main-sequence galaxies by stacking based on the MUSE detections. We find that the molecular gas mass of main-sequence galaxies continuously decreases with stellar mass down to $\log{(M_*/M_\odot)}\approx9.0$. Assuming a metallicity-based CO--to--$\rm H_2$ conversion factor, the molecular gas-to-stellar mass ratio from $\log{(M_*/M_\odot)}\sim9.0$ to $\sim10.0$ does not seem to decrease as fast as for $\log{(M_*/M_\odot)}>10.0$, which is in line with simulations and studies at lower redshift. The inferred molecular gas density $\rho{\rm (H_2)}=(0.49\pm0.09)\times10^8\,{\rm M_\odot\,Mpc^{-3}}$ of MUSE-selected galaxies at $z\sim1.5$ is comparable with the one derived in the HUDF with a different CO selection. Using the MUSE data we recover most of the CO emission in our deep ALMA observations through stacking, demonstrating the synergy between volumetric surveys obtained at different wavebands.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We report molecular gas mass estimates obtained from a stacking analysis of CO line emission in the ALMA Spectroscopic Survey (ASPECS) using the spectroscopic redshifts from the optical integral field spectroscopic survey by the Multi Unit Spectroscopic Explorer (MUSE) of the {\it Hubble} Ultra Deep Field (HUDF). contribution 0.84 0.95 0.06 asserted 1.00
s1 Stacking was performed on subsets of the sample of galaxies classified by their stellar mass and position relative to the main-sequence relation (on, above, below). method 0.99 0.94 0.03 asserted 0.98
s2 Among all the CO emission lines, from \cotwoone to CO(6-5), with redshifts accessible via the ASPECS Band~3 and the MUSE data, \cotwoone provides the strongest constraints on the molecular gas content. result 0.86 0.87 0.10 asserted 1.00
s3 We detect \cotwoone emission in galaxies down to stellar masses of $\log{(M_*/M_\odot)}=10.0$. result 0.98 0.96 0.08 asserted 1.00
s4 Below this stellar mass, we present a new constraint on the molecular gas content of $z\sim1.5$ main-sequence galaxies by stacking based on the MUSE detections. contribution 0.90 0.95 0.13 asserted 0.99
s5 We find that the molecular gas mass of main-sequence galaxies continuously decreases with stellar mass down to $\log{(M_*/M_\odot)}\approx9.0$. result 1.00 0.96 0.55 asserted 1.00
s6 Assuming a metallicity-based CO--to--$\rm H_2$ conversion factor, the molecular gas-to-stellar mass ratio from $\log{(M_*/M_\odot)}\sim9.0$ to $\sim10.0$ does not seem to decrease as fast as for $\log{(M_*/M_\odot)}>10.0$, which is in line with simulations and studies at lower redshift. interpretation 0.75 0.79 0.27 hedged possibility 0.91
s7 The inferred molecular gas density $\rho{\rm (H_2)}=(0.49\pm0.09)\times10^8\,{\rm M_\odot\,Mpc^{-3}}$ of MUSE-selected galaxies at $z\sim1.5$ is comparable with the one derived in the HUDF with a different CO selection. result 0.98 0.92 0.10 asserted 1.00
s8 Using the MUSE data we recover most of the CO emission in our deep ALMA observations through stacking, demonstrating the synergy between volumetric surveys obtained at different wavebands. interpretation 0.62 0.95 0.14 asserted 1.00

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from analysis of existing data 0.88.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. The redshift coverages of the MUSE (blue) and ASPECS (green) HUDF surveys. The numbers of ASPECS directly detected sources and MUSE sources with a secure redshift measurement are listed on the right hand side (There are also nine and two sources classified as “stars” and “others”, respectively, Inami et al. 2017). The numbers in parentheses are the MUSE sources whose locations are in the region of LP primary beam response > 50% for each CO emission. The absorption features detected with MUSE include C IV, Fe II, Mg II. The analysis presented in this work focuses on the stacking…
Figure 2. Reduction of standard deviation of stacked spec￾tra (100 realizations) with increasing number of stacked spec￾tra (red region). The stacked spectra used here were ex￾tracted randomly from the ASPECS data cube. The red dotted line represents 1/ √ N, where N is the number of the stacked spectra. lution. The effect of this filter was equivalent to per￾forming a linear convolution of the spectrum with an area-normalized Gaussian of FWHM 63.2 km s−1 . Next, we interpolated the filtered spectrum onto the frequency grid of the stacked spectra. The mean of the resulting spectra was then…
Figure 3. SFR-M∗ relation of our CO(2-1) stacking sam￾ple. The gray filled band shows the main-sequence rela￾tion and its intrinsic scatter for MUSE-detected galaxies (at z = 1.43, Boogaard et al. 2018). The gray solid, dashed and dashed dotted lines are the main-sequence relations from Speagle et al. (2014) (which is used for PHIBSS2 Tacconi et al. 2018, see also Appendix D), Whitaker et al. (2014) and Schreiber et al. (2015), respectively. The blue vertical and diagonal dotted lines show the divisions of the bins that we used for stacking. The numbers at the top left of each grid are the…
Figure 4. Resulting stacked 2D images (left) and 1D spectra (right) in the bins of SSFRs (above, on, and below the galaxy main-sequence, as discussed in §3.2; y-axis) and stellar masses (x-axis). The upper and lower two panels show the results of including and excluding the individually reported direct CO(2-1) detections, respectively (Aravena et al. 2019; Boogaard et al. 2019). The 2D images are coadded between 230.335 ≤ νrest/GHz ≤ 230.741 (∆v = 528 km s−1 ) and the white ellipses indicate the mean of the beam size. The 1D spectrum is extracted from the central spaxel (the red solid line).…
Figure 5. [Left] Molecular gas mass as a function of stellar mass of the main-sequence galaxies in this study. We use αCO = 3.6 M (K km s−1 pc2 ) −1 for galaxies in the log (M∗/M ) > 10 bins and 7.2 and 10.8 for the log (M∗/M ) ∼ 9.5 and 8.5 bins, respectively, to obtain molecular gas mass (see §5.1). We also show the estimates based on a constant αCO = 3.6 M (K km s−1 pc2 ) −1 in the lower mass bins with the dashed symbols for reference. The large green circles are from the stacked CO detections and the downward-pointing arrows are the 3σ upper limits. The squares indicate the ASPECS CO…
Figure 7. Depletion time (tdepl) plotted against SSFR. The symbols are the same as
Figure 8. Individual spectra used for stacking in the bins of 10.0 < log (M ∗ /M ) ≤ 11.0 for the galaxies on the MS galaxies (left) and below the MS (right). The 2D image (moment-0) and 1D spectrum are displayed for each galaxy with the MUSE ID number at the top left in each panel of the 1D spectrum. If the emission has been detected by the blind (Gonz´alez-L´opez et al. 2019) or prior searches (Boogaard et al. 2019), then its ASPECS ID is also shown in parentheses. The lines and symbols are the same as in
Figure 9. The stacked results of CO(3-2), CO(4-3), CO(5-4), and CO(6-5), shown in the same manner as the left panel in
Figure 10. Tentative detections of CO(5-4) of two Lyα emitters. The left panels show the 2D images of CO(5-4) created over the velocity range highlighted in the middle panels. The middle panels depict the ALMA 1D spectra. In the right panels, the MUSE 1D spectra are presented along with the MUSE redshift in the top right corner. The velocity is defined relative to the MUSE redshift with an empirical correction for Lyα systemic redshift (see §4.2; Verhamme et al. 2018)
Figure 11. The same diagrams as
Figure 12. The same diagrams as Figures 6 and 7 in the main text, but lower redshift bins are also shown, following

Showing 11 of the paper's 12 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:2009.10843 by the authors.

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Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

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