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arxiv: 1907.10438 · v2 · pith:CQTH6QLSnew · submitted 2019-07-24 · 🌌 astro-ph.GA

Edge-on HI-bearing ultra diffuse galaxy candidates in the 40% ALFALFA catalog

Min He , Hong Wu , Wei Du , James Wicker , Pin-Song Zhao , Feng-Jie Lei , Ji-Feng Liu This is my paper

Pith reviewed 2026-05-24 16:43 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords ultra-diffuse galaxiesHI-bearing galaxiesedge-on galaxiesALFALFA catalogsurface brightness profilesgalaxy candidateslow-density environments
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The pith

Correcting central surface brightness for face-on view reveals 11 edge-on HUDS candidates in the 40% ALFALFA catalog.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper explores searching for ultra-diffuse galaxies among edge-on HI sources because exponential disks show enhanced central surface brightness when viewed edge-on. From the 40% ALFALFA catalog and SDSS g- and r-band images, the authors select candidates and correct their observed central surface brightness back to face-on values. This process identifies 11 new HUDS candidates. These objects are all blue and HI-bearing, matching other field examples from the 70% ALFALFA catalog while differing from cluster UDGs.

Core claim

After correcting the observed central surface brightness to a face-on perspective, we discover 11 edge-on HUDS candidates. All these newly discovered HUDS candidates are blue and HI-bearing, similar to other HUDS in 70% ALFALFA catalog, and different from UDGs in clusters.

What carries the argument

Inclination correction of observed central surface brightness assuming exponential surface brightness profiles to recover face-on values for edge-on HI sources.

If this is right

  • The edge-on selection plus correction provides an efficient route to find ultra-diffuse galaxies inside HI catalogs.
  • The 11 candidates lie in low-density environments.
  • Their blue colors and HI content match previously reported field HUDS.
  • The candidates differ in color and gas content from UDGs found in clusters.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same selection could be run on the full ALFALFA catalog or other HI surveys to increase the sample size.
  • Targeted imaging or spectroscopy could confirm distances and verify the face-on surface brightness values.
  • The shared traits with other field HUDS point toward formation channels distinct from those in dense environments.

Load-bearing premise

Ultra-diffuse galaxies follow exponential surface brightness profiles so edge-on viewing reliably boosts their apparent central surface brightness enough for selection.

What would settle it

Measuring Sérsic indices for the 11 candidates and finding values far from 1, or obtaining direct face-on photometry showing central surface brightness too bright to qualify as ultra-diffuse.

Figures

Figures reproduced from arXiv: 1907.10438 by Feng-Jie Lei, Hong Wu, James Wicker, Ji-Feng Liu, Min He, Pin-Song Zhao, Wei Du.

Figure 1
Figure 1. Figure 1: Edge-on disk model fitting to the galaxy AGC 202262. These images are the fpC-image after sky subtraction and bright star removal, the model image and the residual image, from left to right respectively. Du et al. (2015) have selected LSBGs with the ratio of b/a > 0.3 from the α.40 catalog in both the g-band and r-band. This set excludes the edge-on galaxies. On the contrary, we use all these remaining cas… view at source ↗
Figure 2
Figure 2. Figure 2: These pictures show the SDSS DR7 images, DECaLS images and Hi-line spectra of the 11 edge-on HUDS candidates in the α.40 catalog. SDSS DR7 images are created by combining the g-/r-/i-band images with blue, green and red colors respectively. The DECaLS images are targets in g-/z-bands and Hi-line spectra are achieved from the NASA Extragalactic Database. These optical images show edge-on disk-like morpholog… view at source ↗
Figure 3
Figure 3. Figure 3: This picture shows the Ag of the edge-on (red dots) and face-on (blue dots) galaxies, which have good qual￾ity Hα and Hβ spectral information and are fainter than −17 absolute mag, with g − r color bluer than 0.4. There are not obvious differences between their distributions. This may in￾dicate that the internal extinction of low luminosity edge-on galaxies does not differ very much from the low luminosity… view at source ↗
Figure 4
Figure 4. Figure 4: Top: (a)-(d): are histograms showing optical properties (Mg, re, µg,0, color g − r) of HUDS candidates compared with samples from the literature. Bottom: (e)-(h): are histogram comparisons of Hi relative properties (ratio of Hi to stellar mass, Hi mass and Hi-line velocity width W50) and the ellipticity. Black represents UDGs of V15a in Coma Cluster. Orange means the total group UDGs found by Rom´an & Truj… view at source ↗
Figure 5
Figure 5. Figure 5: Color-absolute magnitude diagram of UDGs in fields and clusters. Green filled circles are our 11 edge-on HUDS candidates in α.40, blue triangles are HUDS R of L17, orange diamonds are the complete group of UDGs found by Rom´an & Trujillo (2017b); Shi et al. (2017); M¨uller et al. (2018); Merritt et al. (2016) and the big black star is a rough mean value of UDGs in the Coma cluster (V15a). The red solid lin… view at source ↗
read the original abstract

Ultra-diffuse galaxies (UDGs) are objects which have very extended morphology and faint central surface brightness. Most UDGs are discovered in galaxy clusters and groups, but also some are found in low density environments. The diffuse morphology and faint surface brightness make them difficult to distinguish from the sky background. Several previous works have suggested that at least some UDGs are consistent with exponential surface brightness profiles (S\'{e}rsic n ~ 1). The surface brightness of exponential disks is enhanced in edge-on systems, so searching for edge-on systems may be an efficient way to select UDGs. In this paper, we focus on searching for edge-on HI-bearing ultra-diffuse sources (HUDS) from the 40% ALFALFA catalog, based on SDSS g- and r-band images. After correcting the observed central surface brightness to a face-on perspective, we discover 11 edge-on HUDS candidates. All these newly discovered HUDS candidates are blue and HI-bearing, similar to other HUDS in 70% ALFALFA catalog, and different from UDGs in clusters.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The paper searches the 40% ALFALFA catalog for edge-on HI-bearing ultra-diffuse galaxy (HUDS) candidates using SDSS g- and r-band imaging. It applies a face-on correction to the observed central surface brightness under the assumption of exponential (Sérsic n~1) profiles drawn from prior work, identifies 11 new candidates, and reports that they are blue and HI-rich, resembling other field HUDS but unlike cluster UDGs.

Significance. If the deprojection is valid, the result demonstrates an efficient observational route to recover low-surface-brightness field galaxies that are otherwise missed, thereby enlarging the known HUDS population outside clusters and reinforcing environmental differences in UDG properties.

major comments (1)
  1. [Abstract and selection procedure] Abstract and selection procedure: The face-on central surface-brightness correction that defines the 11 HUDS candidates rests on the assumption that these objects follow Sérsic n~1 profiles. The manuscript supplies no Sérsic-index measurement or profile fit for the candidates themselves; if n>1 the inclination boost is weaker, the corrected face-on value is brighter than assumed, and the UDG classification threshold may not be satisfied.
minor comments (1)
  1. [Abstract] The abstract states the count of 11 candidates but does not report the total number of edge-on sources examined or the precise numerical threshold applied after correction; adding these quantities would clarify the selection efficiency.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their thoughtful review and constructive comments on our manuscript. We address the major comment below in a point-by-point manner.

read point-by-point responses

  1. Referee: The face-on central surface-brightness correction that defines the 11 HUDS candidates rests on the assumption that these objects follow Sérsic n~1 profiles. The manuscript supplies no Sérsic-index measurement or profile fit for the candidates themselves; if n>1 the inclination boost is weaker, the corrected face-on value is brighter than assumed, and the UDG classification threshold may not be satisfied.

    Authors: We acknowledge the validity of this point. The assumption of Sérsic n ≈ 1 is explicitly based on prior literature cited in the manuscript, which has found that many UDGs and HUDS are consistent with exponential profiles. No individual Sérsic fits were performed on these 11 candidates, as the selection focused on identifying edge-on systems via the inclination correction under this literature-supported assumption. We agree this constitutes an important caveat for the classification. In revision, we will expand the methods and discussion sections to state the assumption more explicitly, note its implications if n > 1 (i.e., brighter face-on surface brightness and possible non-UDG status), and strengthen the literature references. This will be a textual clarification rather than new measurements. revision: partial

Circularity Check

0 steps flagged

No circularity: pure observational catalog search using external prior assumption

full rationale

The paper conducts an observational search in the ALFALFA catalog, applying a standard face-on correction for edge-on exponential disks drawn from prior literature (not self-citation). No equations derive a result from fitted inputs, no self-definitional loops, no predictions that reduce to the selection criteria by construction, and no load-bearing self-citation chain. The Sérsic n~1 assumption is cited from external previous works as a selection criterion; failure to verify it on the new candidates is a potential correctness issue, not circularity. The derivation chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the domain assumption that UDGs can be treated as exponential disks for selection purposes; no free parameters or invented entities are visible in the abstract.

axioms (1)
  • domain assumption At least some UDGs are consistent with exponential surface brightness profiles (Sérsic n ~ 1)
    Invoked to justify why edge-on systems enhance central surface brightness and enable efficient UDG selection.

pith-pipeline@v0.9.0 · 5747 in / 1213 out tokens · 24500 ms · 2026-05-24T16:43:36.034529+00:00 · methodology

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