REVIEW 2 major objections 4 minor 44 references
Exploring the local black hole mass function below $10^6$ solar masses
T0 review · 2 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read The paper claims that the local number density of black holes between 100,000 and one million solar masses is pinned to within a factor of ten once galaxy counts and X-ray occupation constraints are combined.
desk verdict A transparent, honest first estimate of the low-mass black hole mass function with a real weakness—the occupation fraction is early-type only—but the authors say so plainly and the sub-dex claim should be read as conditional on that morphology assumption. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing element is the convolution in Equation 3, which builds the BHMF as $\Phi(M_{\rm BH}) = \int \Phi(M_\star)\,\lambda_{\rm occ}(M_\star)\,\frac{1}{\sqrt{2\pi\sigma^2}}\exp[-(M_{\rm BH}-(\alpha+\beta M_\star))^2/(2\sigma^2)]\,dM_\star$. Here $\Phi(M_\star)$ is the GAMA galaxy stellar mass function, $\lambda_{\rm occ}$ is the occupation fraction of Equation 1 — a smooth tanh ramp that is nearly zero below $10^7\,M_\odot$ and nearly one above $10^{10}\,M_\odot$, with a transition mass $M_{\star,0}$ constrained by X-ray observations of 326 nearby early-type galaxies — and the Gaussian carries the scatter in the $M_{\rm BH}$–$M_\star$ relation. This object does the work: it turns a galaxy census into a black hole census while suppressing low-mass galaxies that probably lack black holes, which is why the low-mass end of the BHMF is no longer set by the scaling relation alone.
What would settle it
Measure the nuclear black hole occupation fraction in a large, uniformly observed sample of late-type and dwarf galaxies with $M_\star$ between roughly $10^9$ and $10^{10}\,M_\odot$ (for example, a deep X-ray survey of about 5,000 such galaxies with sub-arcsecond resolution). If the occupation fraction at those masses differs substantially from the early-type-derived tanh ramp, the low-mass normalization of the BHMF shifts by the difference and the claimed sub-dex precision in the $10^5$–$10^6$ $M_\odot$ band would be contradicted.
Extended reading notes
Core claim
The paper's central claim is that the low-mass end of the local black hole mass function is empirically accessible despite the lack of a reliable black hole mass proxy. By folding an X-ray-derived occupation fraction into the convolution between galaxy stellar mass and black hole mass, the authors obtain a BHMF whose 68% normalization uncertainty is less than one dex across $10^5$–$10^6$ $M_\odot$, the band where LISA is most sensitive. The median best-guess mass function is approximated by $\log\Phi = -2.13 - 0.098\log(M_{\rm BH}/M_\odot) - 0.00011\,(M_{\rm BH}/M_\odot)$. The paper further reports that different choices of the $M_{\rm BH}$–$M_\star$ scaling relation change the inferred BHMF by only a factor of about three below $10^7$ $M_\odot$, so the occupation fraction, rather than the scaling relation, is the main lever on the low-mass normalization.
Load-bearing premise
The calculation assumes that the occupation fraction measured from 326 early-type galaxies applies to all galaxies at the same stellar mass, even though late-type and dwarf galaxies outnumber early types below $M_\star \sim 10^{10}\,M_\odot$; the authors themselves flag this as a serious drawback in Section 4.
Editorial extensions
If this is right
- The local volume density of black holes near a few times $10^5\,M_\odot$ is bracketed to within about a factor of ten, so rate predictions for low-mass black hole mergers in LISA no longer float freely over this band.
- Within $10^5$–$10^6\,M_\odot$ the 68% normalization uncertainty stays below one dex for each of the scaling relations considered, so the remaining disagreement among black hole mass proxies matters less than feared for the census.
- Below $10^7\,M_\odot$ the occupation-corrected BHMFs built from different scaling relations agree to within a factor of about three, making the low-mass end comparatively stable against the proxy choice.
- The median BHMF is approximated by $\log \Phi = -2.13 - 0.098\log(M_{\rm BH}/M_\odot) - 0.00011\,(M_{\rm BH}/M_\odot)$, a compact form that population-synthesis calculations can adopt directly.
- A future X-ray survey of roughly 5,000 nearby galaxies with uniform, sub-arcsecond coverage could reduce occupation-fraction uncertainties to a few percent and tighten the BHMF further across the LISA band.
Reading between the lines
- If late-type and dwarf galaxies turn out to have a lower nuclear occupation fraction at $M_\star \sim 10^9$–$10^{10}\,M_\odot$ than the early-type-based ramp, the claimed sub-dex normalization in the $10^5$–$10^6\,M_\odot$ band is likely optimistic, since a factor-of-two change in $\lambda_{\rm occ}$ shifts the BHMF normalization by roughly the same factor.
- The same convolution could be run with the redshift-evolving galaxy stellar mass function to produce a $z>0$ BHMF, giving LISA a direct prediction for the redshift distribution of massive black hole mergers; the paper itself only builds the local function.
- The paper's median BHMF, inserted into binary population models, yields concrete predictions for local merger and extreme-mass-ratio inspiral rates that LISA can test once it flies.
- A cleaner test of the low-mass census would come from dynamical black hole mass measurements in dwarf galaxies, which would replace the broad $M_{\rm BH}$–$M_\star$ scatter with actual masses and directly validate the Gaussian convolution.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This Letter estimates the local black hole mass function (BHMF) below 10^6 solar masses by convolving the GAMA galaxy stellar mass function with a black hole occupation fraction derived from Chandra X-ray observations of 326 nearby early-type galaxies and with a log MBH–log Mstar scaling relation fit to the combined AGN plus inactive galaxy sample of Reines & Volonteri (2015). The central quantitative claim is that the 68% normalization uncertainty of the resulting BHMF is less than 1 dex over the range [10^5, 10^6] M_sun, the range most relevant for LISA. The paper also compares with AGN-only and inactive-only scaling relations, provides a Schechter-like fitting formula for the median BHMF, and illustrates how future X-ray surveys with about 5,000 galaxies could shrink the occupation-fraction uncertainty.
Significance. If the result holds, this is one of the first observationally grounded estimates of the BHMF below 10^6 M_sun, with direct implications for LISA event-rate predictions and for seeding models of supermassive black holes. The convolution framework in Equation (3) is transparent, and the use of an empirically constrained occupation fraction rather than an assumed 100% occupation is a genuine step forward. The paper also deserves credit for propagating the posterior uncertainties of the fitted parameters and for providing a simple analytic approximation to the median BHMF. However, the headline uncertainty claim is conditioned on an assumption that the authors themselves describe as a serious drawback, namely that the early-type occupation fraction applies to the full galaxy population used in the convolution; this systematic is not included in the reported uncertainty band.
major comments (2)
- [Section 4, Figure 2] In Section 4 the authors write that using the early-type occupation fraction 'represents a serious drawback for our estimates,' and yet the 68% band shown in Figure 2 and the less-than-1-dex claim in the abstract are computed from Equation (3) using only the lambda_occ posterior derived from 326 early-type galaxies. In the stellar mass range that maps to MBH in [10^5, 10^6] M_sun for the adopted scaling relation (roughly Mstar from 10^9.2 to 10^9.8 M_sun), late-type galaxies dominate the GAMA mass function, so a systematically lower late-type occupation fraction would shift the median Phi by about 0.5 dex for a factor-of-3 difference and by more than the quoted band for a factor-of-10 difference. Because no morphology-dependent systematic is included in the yellow band, the headline normalization uncertainty is conditional on the early-type occupation fraction rather than being an uncertainty on the true local BHMF.
- [Section 3, Equation (3)] Equation (3) adopts a single linear log MBH–log Mstar relation fitted to the Reines & Volonteri sample and then applies it down to the GAMA mass limit, Mstar approximately 10^7.5 M_sun, with no allowance for a change of slope, intercept, or scatter in the dwarf-galaxy regime. The posterior range of alpha, beta, and sigma is propagated, but the unvalidated extrapolation itself is not included in the systematic budget of Figure 2. Since the central result is the normalization in the 10^5–10^6 M_sun range, where this extrapolation is directly relevant, the paper should either justify the extrapolation with low-mass black hole mass estimates and upper limits or add an explicit systematic term.
minor comments (4)
- [Section 3, Equation (3)] Equation (3) writes MBH in the exponent, but the scaling relation is defined for log MBH; please make the logarithm explicit (for example, log MBH - (alpha + beta log Mstar)) to avoid ambiguity.
- [Section 2, Equation (1)] The typeset argument of the tanh in Equation (1) is missing a division: it should be 2.5 |8.9 - log Mstar,0| log(Mstar/Mstar,0), not 2.5 |8.9 - log Mstar,0| log Mstar Mstar,0.
- [Figure 2 caption] The caption of Figure 2 contains a typo ('Gair et at. 2010' should be 'Gair et al. 2010') and also uses 'BHFM' instead of 'BHMF'; both should be corrected.
- [Section 4, right panel of Figure 3] In the discussion of the right panel of Figure 3, the phrase 'the corresponding BHMF uncertainty would shrink to less than about 2' lacks units; please specify whether this is 2 dex or another quantity.
Circularity Check
No circularity: BHMF is an error-propagated convolution of independent inputs; the late-type caveat is a stated systematic limitation, not a circular reduction.
full rationale
The central BHMF, Eq. (3), is an explicit convolution of three independent empirical inputs: the GAMA galaxy stellar mass function (Wright et al. 2017), the occupation fraction posterior from Chandra X-ray data on 326 early-type galaxies (Miller et al. 2015 plus Gallo et al. 2019), and an MBH-Mstar scaling relation refit from the Reines & Volonteri (2015) sample using Kelly (2007). None of these inputs is the target BHMF itself, and the paper does not fit Phi(MBH) to any data. The claimed less-than-1-dex normalization uncertainty is obtained by propagating posterior distributions through Eq. (3), not by inverting or reabsorbing a measurement of the BHMF. The self-citations to Gallo et al. (2019) and Lee et al. (2019) supply separate X-ray observations and contamination assessment routines; they are data and methodology references, not unverified theorems invoked to force the result. The serious drawback that occupation constraints come only from early-type galaxies, while late types dominate the relevant stellar mass range, is explicitly acknowledged in Section 4 and is a legitimate external-validity/systematic concern, but it does not make the derivation circular: the calculation honestly reports the uncertainty conditional on the adopted early-type occupation posterior rather than claiming that posterior was derived from the BHMF. No equation reduces by construction to its own input, and no fitted parameter is renamed as a prediction. The paper is therefore self-contained as an error-propagation exercise, with its main caveat lying outside the scope of circularity.
Assumptions & free parameters
free parameters (5)
- log Mstar,0 (occupation fraction transition mass) =
8.17 (median of posterior)
- alpha (intercept of log MBH versus log Mstar relation) =
8.13 +/- 0.09
- beta (slope of log MBH versus log Mstar relation) =
1.72 +/- 0.14
- sigma (intrinsic scatter in log MBH at fixed Mstar) =
0.61 +/- 0.05
- lambda_occ shape parameters (2.5 and 8.9 in Eq 1) =
chosen by hand
assumptions (6)
- domain assumption The GAMA galaxy stellar mass function of Wright et al. (2017) is accurate and complete down to about 10^7.5 solar masses.
- domain assumption Nuclear X-ray luminosity, after probabilistic X-ray binary subtraction, traces accreting massive black holes through a single LX-Mstar relation with constant intrinsic scatter.
- domain assumption The Bayesian formalism of Miller et al. (2015) correctly maps the observed X-ray detection fraction into the occupation fraction lambda_occ.
- domain assumption The distribution of MBH at fixed Mstar is log-normal with constant scatter sigma in Eq 3.
- ad hoc to paper The combined AGN plus inactive MBH-Mstar relation is linear and can be extrapolated to stellar masses well below the fitted sample.
- ad hoc to paper Early-type galaxy occupation fraction applies to the entire galaxy population used in Eq 3.
Cite this review
Pith. "Pith review of Exploring the local black hole mass function below $10^6$ solar masses." pith.science (2026). https://pith.science/paper/D6QMUALR
@misc{pith2026190902585,
author = {Pith},
title = {Pith review of: Exploring the local black hole mass function below $10^6$ solar masses},
year = {2026},
howpublished = {\url{https://pith.science/paper/D6QMUALR}},
note = {Machine review of arXiv:1909.02585}
}
abstract
The local black hole mass function (BHMF) is of great interest to a variety of astrophysical problems, ranging from black hole binary merger rates to an indirect census of the dominant seeding mechanism of supermassive black holes. In this Letter, we combine the latest galaxy stellar mass function from the Galaxy And Mass Assembly survey with X-ray-based constraints to the local black hole occupation fraction to probe the BHMF below $10^6$ $M_{\odot}$. Notwithstanding the large uncertainties inherent to the choice of a reliable observational proxy for black hole mass, the resulting range of BHMFs yields a combined normalization uncertainty of $\lesssim$1 dex over the $[10^5-10^6]$ $M_{\odot}$ range, where upcoming, space-based gravitational wave detectors are designed to be most sensitive.
Figures
Reference graph
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