AGC 192040: A Giant LSB Galaxy

• AGC 192040 is a giant low surface brightness galaxy characterized by an extended, gas-rich disk and signs of a past merger with a compact elliptical companion.
• Deep MUSE mosaic observations, Legacy Survey imaging, and HI data enabled precise mapping of its regular co-rotating stellar and ionized-gas kinematics and flat gas-phase metallicity gradient.
• The preferred formation scenario involves filamentary gas accretion followed by a prograde merger, distinguishing it from similar systems like UGC 1382.

Searching arXiv for the cited AGC 192040 paper and closely related gLSBG work. AGC 192040 is a giant low surface brightness galaxy (gLSBG) studied in detail through deep MUSE mosaic observations, Legacy Survey imaging, and HI data from the literature. It is characterized in the source study as an Sa galaxy with a prominent disk and a compact elliptical satellite, and is analyzed in conjunction with UGC 1382 to clarify formation channels for very extended, dynamically cold disks (Saburova et al., 29 Sep 2025). The available evidence links AGC 192040 to a massive, gas-rich, chemically weakly evolved outer disk, regular co-rotating stellar and ionized-gas kinematics, a flat gas-phase metallicity gradient, and signs of merger activity several Gyr ago. The preferred formation scenario proposed for this system is gas accretion from a filament followed by an intermediate-mass ratio merger with the companion on a prograde orbit (Saburova et al., 29 Sep 2025).

1. Classification and global properties

AGC 192040 is identified as a giant low surface brightness galaxy with morphological type Sa and a compact elliptical satellite (Saburova et al., 29 Sep 2025). Its distance is given as 192.1 Mpc, and its inclination as 27.7° (Saburova et al., 29 Sep 2025). The galaxy has an HI disk radius of up to 65 kpc, while the optical R-band disk is traced out to 76 kpc (Saburova et al., 29 Sep 2025). In the $r$-band, its exponential disk scale length is $h = 18.5 \pm 1.1$ kpc, and its central surface brightness is $\mu_{0, r} = 24.47 \pm 0.10$ mag arcsec$^{-2}$ (Saburova et al., 29 Sep 2025).

Its baryonic content is strongly gas rich. The stellar mass is reported as $3.46 \times 10^{10}\ M_\odot$, while the HI mass is $5.7 \times 10^{10}\ M_\odot$, nearly twice the stellar mass (Saburova et al., 29 Sep 2025). The rotation velocity listed in the paper summary is $217.4 \pm 13.6$ km/s (Saburova et al., 29 Sep 2025). These measurements place AGC 192040 among the rare, very extended disk systems whose existence is difficult to reconcile with standard merger-driven disk growth scenarios, a point emphasized in the comparative study with UGC 1382 (Saburova et al., 29 Sep 2025).

The disk photometry is represented by the exponential form

$I(r) = I_0 \exp(-r/h),$

with $h = 18.5 \pm 1.1$ kpc in the $r$-band for AGC 192040 (Saburova et al., 29 Sep 2025). This formalization is central to the characterization of the galaxy as a gLSBG rather than a conventional high-surface-brightness spiral.

2. Observational basis and analysis workflow

The principal dataset consists of deep MUSE (Multi-Unit Spectroscopic Explorer) mosaic observations obtained with four MUSE pointings, totaling approximately 12 hours and covering the galaxy in a mosaic configuration (Saburova et al., 29 Sep 2025). These observations were supplemented by Legacy Survey imaging and HI data from the literature (Saburova et al., 29 Sep 2025).

The reduction and analysis procedures reported for AGC 192040 include careful sky subtraction, flux calibration, and precise image alignment using external imaging (Saburova et al., 29 Sep 2025). Spectral binning was performed with Voronoi tessellation, adapted separately for gas and stellar analysis (Saburova et al., 29 Sep 2025). Stellar populations were modeled through full-spectrum fitting with E-MILES SSP models, while emission lines were fit to derive gas kinematics and metallicity diagnostics (Saburova et al., 29 Sep 2025). Photometric decomposition of the Legacy Survey $h = 18.5 \pm 1.1$0-band image was carried out with a Sérsic bulge plus exponential disk, augmented by two Gaussian rings to account for spiral arm structure (Saburova et al., 29 Sep 2025).

For gas-phase abundance analysis, the study used the O3N2 and S-calibration strong-line methods on star-forming regions, and BPT diagrams were applied to exclude non-photoionized or AGN-contaminated regions (Saburova et al., 29 Sep 2025). This methodological combination allowed the authors to construct spatially resolved kinematic and metallicity maps, and to compare the stellar and gaseous components in a uniform framework (Saburova et al., 29 Sep 2025).

A plausible implication is that the study was designed to separate structural regularity from chemical and kinematic signatures of past interaction, rather than to rely on global integrated measurements alone.

3. Stellar and gas kinematics

The stellar velocity field of AGC 192040 is described as showing regular rotation in the main body (Saburova et al., 29 Sep 2025). A distinct “reddish clump,” identified as a northern compact object, rotates more than 100 km/s faster than the disk and exhibits high velocity dispersion together with an SSP-equivalent age of approximately 5 Gyr (Saburova et al., 29 Sep 2025). In the source interpretation, this is suggestive of a disrupted satellite (Saburova et al., 29 Sep 2025). The bulge is characterized as old, metal-rich, and possessing moderate central velocity dispersion (Saburova et al., 29 Sep 2025).

The ionized-gas velocity field is also regular and aligned with the stellar disk in the inner region (Saburova et al., 29 Sep 2025). A notable result is the absence of counter-rotation: unlike UGC 1382, AGC 192040 shows no evidence for counter-rotation between stars and gas, and the two components co-rotate (Saburova et al., 29 Sep 2025). In the outer disk, a warp is observed, consistent with HI maps and interpreted as indicative of tidal interaction or external influences (Saburova et al., 29 Sep 2025).

These measurements are central to the preferred evolutionary scenario. In the comparison presented by the study, UGC 1382 is associated with counter-rotating gas and stars and with multiple retrograde interactions, whereas AGC 192040 is associated with co-rotating gas and stars and a prograde interaction history (Saburova et al., 29 Sep 2025). This suggests that kinematic alignment is treated not merely as a descriptive property, but as a discriminator among formation channels.

4. Chemical properties and radial trends

The mean gas-phase oxygen abundance of AGC 192040 is slightly sub-solar. The reported values are $h = 18.5 \pm 1.1$1 from O3N2 and $h = 18.5 \pm 1.1$2 from S-cal, compared with a solar reference value of 8.69 (Saburova et al., 29 Sep 2025). The radial abundance profile is flat by both calibrations: the O3N2 gradient is $h = 18.5 \pm 1.1$3 dex kpc$h = 18.5 \pm 1.1$4, while the S-cal result is consistent with zero at $h = 18.5 \pm 1.1$5 dex kpc$h = 18.5 \pm 1.1$6 (Saburova et al., 29 Sep 2025).

The study interprets these flat gradients as evidence for efficient mixing, likely associated with past mergers or gas accretion events (Saburova et al., 29 Sep 2025). AGC 192040 is also described as having an integrated metallicity lower than expected for its stellar mass relative to normal disk galaxies, which is presented as being consistent with accretion of metal-poor gas (Saburova et al., 29 Sep 2025).

The metallicity formalism summarized in the source includes the strong-line calibration expression

$h = 18.5 \pm 1.1$7

together with the closed-box chemical-evolution relation

$h = 18.5 \pm 1.1$8

and the effective oxygen yield

$h = 18.5 \pm 1.1$9

These relations are used in the paper to test whether the observed abundance structure is compatible with closed-box evolution or instead implies dilution, accretion, or passive gas reservoirs (Saburova et al., 29 Sep 2025).

5. Stellar populations and the outer-disk gas reservoir

The bulge and inner disk of AGC 192040 are reported to be old, with ages greater than 10 Gyr, and metal-rich, with $\mu_{0, r} = 24.47 \pm 0.10$0 dex (Saburova et al., 29 Sep 2025). By contrast, the outer disk is described as lower metallicity and lower surface density, with evidence for passively evolving, gas-rich outer regions and negligible current star formation (Saburova et al., 29 Sep 2025). Across the disk as a whole, the mean luminosity-weighted age is approximately 4 Gyr, implying that the last major star-formation episode occurred several Gyr ago (Saburova et al., 29 Sep 2025).

The diffuse northern clump is assigned an intermediate age of about 5 Gyr and high velocity dispersion, which the source interprets as a remnant of an accreted or destroyed companion galaxy (Saburova et al., 29 Sep 2025). This kinematically and spectroscopically distinct component is therefore treated as part of the merger evidence rather than as a normal substructure of the present disk.

The effective oxygen-yield analysis leads to a further inference about the outskirts. In the outer disk, $\mu_{0, r} = 24.47 \pm 0.10$1 is reported to be much higher than the canonical yield found for local spirals, and this is interpreted as indicative of a large reservoir of chemically unevolved “passive” gas that is not actively involved in star formation or enriching the ISM (Saburova et al., 29 Sep 2025). The summary states that gas dominates baryonic density at radii $\mu_{0, r} = 24.47 \pm 0.10$2 kpc, but is mostly passive (Saburova et al., 29 Sep 2025). This combination of high HI content, low star-formation efficiency, and weak chemical processing is a defining feature of the system as presented in the source.

6. Formation scenario and comparison with UGC 1382

The paper’s central interpretive claim is that AGC 192040 experienced gas accretion from a filament followed by an intermediate-mass merger with the companion on a prograde orbit (Saburova et al., 29 Sep 2025). Several observational facts are assembled in support of this scenario. First, there are no signatures of recent major mergers such as visible shells or streams, which suggests that any merger occurred several Gyr ago and that the system has since relaxed (Saburova et al., 29 Sep 2025). Second, the outer HI warp is suggestive of tidal interaction or late gas accretion (Saburova et al., 29 Sep 2025). Third, the galaxy is HI-excess, with more gas than stars (Saburova et al., 29 Sep 2025). Fourth, the co-rotation of gas and stars is consistent with a prograde, rather than retrograde, interaction history (Saburova et al., 29 Sep 2025).

The comparison with UGC 1382 serves as a control case within the same study. UGC 1382 hosts a globally counter-rotating gaseous disk, whereas AGC 192040 does not (Saburova et al., 29 Sep 2025). The preferred explanation for UGC 1382 is multiple gas-rich mergers with companions on retrograde orbits; for AGC 192040, the favored explanation is a single gas-accreting merger on a prograde orbit (Saburova et al., 29 Sep 2025). This comparison is important because it frames AGC 192040 not as a generic gLSBG, but as one endpoint in a broader space of giant-disk assembly histories.

Morphological evidence is also incorporated into this interpretation. The study notes long, straight spiral arm segments, especially in the west, and states that such “rows” are usually formed in response to non-stationary, compressive disturbances, likely triggered here by interaction with the compact cE satellite now projected onto the disk (Saburova et al., 29 Sep 2025). This suggests a link between the present morphology and the inferred interaction history, though the wording remains interpretive rather than directly demonstrative.

7. Significance and unresolved issues

AGC 192040 is significant within the gLSBG literature because it combines a giant, diffuse stellar disk, an HI mass exceeding the stellar mass, regular co-rotating gas and stellar kinematics, flat and slightly sub-solar gas-phase metallicity, and evidence for a past merger without obvious recent large-scale disruption (Saburova et al., 29 Sep 2025). In the source synthesis, these properties are most consistent with recent or ongoing gas accretion from the cosmic web rather than purely closed-box chemical evolution (Saburova et al., 29 Sep 2025).

A common simplification would be to treat all giant low surface brightness disks as products of the same merger pathway. The comparison in the study argues against that view by showing that AGC 192040 and UGC 1382, despite both being gLSBGs with compact satellites, are better explained by different evolutionary channels (Saburova et al., 29 Sep 2025). Another potential misconception is that a gas-rich extended disk necessarily implies strong present-day star formation. For AGC 192040, the evidence points instead to low star-formation efficiency and a substantial reservoir of gas in the outskirts that is not actively participating in star formation (Saburova et al., 29 Sep 2025).

The paper does not present AGC 192040 as a fully settled case. The proposed sequence of filamentary gas accretion plus prograde merger is an interpretation grounded in the observed metallicity, kinematic alignment, HI excess, warp, and disturbed substructure, but it remains a scenario rather than a direct reconstruction (Saburova et al., 29 Sep 2025). This suggests that AGC 192040 functions as an empirical benchmark for testing models of giant disk formation in which both smooth gas supply and interaction-driven restructuring play a role.