UGC 1382: Prototypical Giant LSB Galaxy

• The paper identifies UGC 1382 as a prototypical giant low-surface-brightness galaxy with a normal S0 inner region embedded in an enormous, diffuse spiral disk.
• Using deep imaging and IFU spectroscopy (MUSE and MaNGA), the study reveals key kinematic features including a retrograde, globally counter-rotating gaseous disk and a nearly flat metallicity gradient.
• The analysis underscores that multiple gas-rich mergers and retrograde accretion events are central to forming such extreme disk systems, providing benchmarks for galaxy evolution models.

Searching arXiv for recent and foundational papers on UGC 1382 and giant low-surface-brightness galaxies. UGC 1382 is one of the prototypical giant low-surface-brightness galaxies (gLSBGs): an S0 galaxy in its inner regions that is embedded in an enormous, extremely diffuse spiral disk. At a distance of 80 Mpc, with scale 0.39 kpc arcsec$^{-1}$, it combines an apparently normal early-type central body with a giant low-surface-brightness disk of visually estimated radius $R_d \sim 80$ kpc, a configuration that places it among the most extreme known disk galaxies. In the MUSE study of two giant low surface brightness galaxies with compact satellites, UGC 1382 is identified as hosting a globally counter-rotating gaseous disk and as a nearby, well-resolved example in which structure, kinematics, and chemistry jointly encode a complex merger-driven origin (Saburova et al., 29 Sep 2025).

1. Classification, morphology, and global parameters

UGC 1382 is morphologically classified as an S0 (lenticular) galaxy in its inner regions. Deep imaging has revealed that this apparently normal early-type galaxy is embedded in an enormous, extremely diffuse spiral disk. From the decomposition of the $g$-band surface brightness profile, the stellar disk of UGC 1382 consists of an inner high-surface-brightness (HSB) disk with

$$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$

and a giant low-surface-brightness (LSB) disk with

$$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$

The visually estimated disk radius of the LSB component is $R_d \sim 80$ kpc. This is an order of magnitude larger than the typical exponential scale lengths ($\sim 3$–5 kpc) and several times larger than the optical radii of “normal” spiral disks. The central surface brightness of the LSB component is $\sim 4.3$ mag fainter than the canonical Freeman value ($\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$), confirming its status as a low-surface-brightness disk (Saburova et al., 29 Sep 2025).

Property	Value
Distance	80 Mpc
Scale	0.39 kpc arcsec$^{-1}$
Stellar mass	$R_d \sim 80$0
H I mass	$R_d \sim 80$1
Rotation velocity	$R_d \sim 80$2
Inclination	$R_d \sim 80$3
Photometric major-axis PA	$R_d \sim 80$4

A characteristic dynamical mass within radius $R_d \sim 80$5 can be estimated as

$R_d \sim 80$6

and previous work shows that UGC 1382 reaches $R_d \sim 80$7 within the extent of the giant disk—typical for gLSBGs and comparable to massive early-type systems. In this configuration, UGC 1382 encapsulates a central old stellar system embedded in one of the largest known stellar disks, with a very low central surface brightness and a dynamically cold, thin, tightly wound spiral morphology (Saburova et al., 29 Sep 2025).

2. Compact elliptical satellite and system architecture

UGC 1382 hosts a compact elliptical (cE) satellite projected on the outer disk. The satellite was confirmed spectroscopically with SDSS and interpreted as a tidally stripped remnant of a more massive disk galaxy. Using MUSE stellar population fits and Legacy Survey photometry, the paper estimates a satellite absolute $R_d \sim 80$8-band magnitude of $R_d \sim 80$9, a stellar population age $g$0 Gyr, $g$1 dex, and an $g$2-band mass-to-light ratio of $g$3 for a Salpeter IMF and $g$4 for a Kroupa IMF (Saburova et al., 29 Sep 2025).

This implies a current stellar mass of order a few $g$5, and, accounting for stripping with up to $g$6 of the progenitor’s stellar mass lost, the progenitor could have contributed at most $g$7 to the disk of UGC 1382. This is far too small to build the entire giant disk, whose spiral arms alone host $g$8 in stars, implying that multiple satellites and/or additional gas accretion were involved.

The satellite’s line-of-sight velocity shares the sense of rotation of the ionized gas, but is opposite to that of the central stellar body, consistent with a retrograde orbit relative to the inner stellar disk. Within the system-scale architecture, this compact companion therefore functions both as direct evidence of past strong tidal interactions and as a kinematic tracer of the retrograde material associated with the extended gaseous disk. A plausible implication is that the presently observed cE is only one surviving remnant of a broader accretion history rather than the sole source of the giant LSB disk (Saburova et al., 29 Sep 2025).

3. Integral-field spectroscopy and large-scale counter-rotation

UGC 1382 was observed with MUSE at the VLT in wide-field mode with ground-layer AO in the nominal wavelength range. To cover the enormous LSB disk, the authors constructed a mosaic of 11 pointings and 31 observing blocks, with a total integration time of 32.6 h. For 10 fields in the LSB disk, the observing pattern was O–S–O–O–S–O with $g$9 s on-target and $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$0 s sky per OB, rotated by $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$1 between exposures; for the central pointing, the setup was $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$2 s on-target + $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$3 s sky, with no rotation. Seeing varied between $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$4 and $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$5. The final mosaicked data cube has wavelength range 4750–9350 Å, spectral resolution with FWHM of the line spread function $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$6 Å at 7500 Å, pixel size 0.2″, and limiting H$$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$7 surface brightness $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$8 (Saburova et al., 29 Sep 2025).

Data reduction used the pymusepipe wrapper around the standard MUSE DRS, including bias subtraction, flat-fielding, wavelength calibration, geometric corrections, sky subtraction using dedicated blank-sky exposures in each OB, absolute flux calibration, and astrometric alignment via convolution with the $$(\mu_0)_{\rm HSB} = 21.19 \pm 0.10\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm HSB} = 3.73 \pm 0.13\ \mathrm{kpc},$$9-band transmission curve and comparison to Hyper Suprime-Cam $$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$0-band images. The pipeline computes relative offsets and optional normalization factors to correct small transparency variations; these flux renormalizations were not applied here to avoid biasing LSB regions. The authors also re-analysed the MaNGA IFU cube for the central region with the same fitting procedure, both to cross-check kinematics and to extend the stellar population analysis into the very center.

To characterize the ionized gas, the authors built a tailored spatial binning for gas by computing S/N maps in the continuum (5050–5150 Å) and in narrow bands around redshifted H$$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$1 and H$$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$2, masking spaxels with very low S/N, and applying Voronoi binning to reach target S/N $$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$3 for gas kinematics and emission-line fluxes. Full spectrum fitting was done with the NBursts code using E-MILES SSP templates, simultaneously fitting the stellar continuum and emission lines over 4800–6800 Å in the rest frame. Emission lines such as H$$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$4, [O III] $$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$5, [N II] $$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$6, H$$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$7, and [S II] $$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$8 were modeled as Gaussians, with constrained flux ratios within doublets and the Balmer series where appropriate (Saburova et al., 29 Sep 2025).

The H$$(\mu_0)_{\rm LSB} = 25.51 \pm 0.15\ \mathrm{mag\ arcsec^{-2}},\quad h_{\rm LSB} = 26.39 \pm 2.53\ \mathrm{kpc}.$$9 velocity field of UGC 1382, constructed by combining MUSE and MaNGA data, shows a regular rotating disk extending across the entire MUSE mosaic. The key kinematic result is the globally counter-rotating gaseous disk: the kinematic major axes of stellar and gaseous components are offset by $R_d \sim 80$0 in position angle; the stellar major axis is roughly aligned with the photometric major axis at PA $R_d \sim 80$1, while the gas’s kinematic PA is essentially flipped. Thus, with respect to the stellar body, comprising the bulge and inner HSB disk, the extended gaseous disk is globally counter-rotating.

The counter-rotation is not confined to the central few kpc but extends through the entire MUSE-covered region of the giant disk; no radial transition is identified where the gas changes sense of rotation. Within the gas velocity field, the PA changes slightly with radius, indicating mild kinematic twists. The innermost region appears kinematically decoupled from the outer disk; this is plausibly associated with a bar. Beyond this bar-dominated region, the gas velocity field becomes regular, with no strong indication of recent disturbances or discrete kinematic substructures, implying that any merger or accretion events that set up the counter-rotation occurred several Gyr ago and the disk has since dynamically relaxed (Saburova et al., 29 Sep 2025).

4. Stellar kinematics, bar, dust lane, and central stellar populations

Stellar kinematics were extracted using NBursts with E-MILES SSP templates. For the central high-S/N region, the spectrum was fit over 4700–8700 Å in the rest frame, including the Ca II triplet. Stellar radial velocities and dispersions were derived by convolving template SSP spectra with a parametric line-of-sight velocity distribution and minimizing residuals. The stellar velocity field of UGC 1382 in the central few kpc shows regular rotation aligned with the photometric major axis, rotates in the opposite sense to the gaseous disk, and has a central velocity dispersion peak of $R_d \sim 80$2, consistent with a massive bulge or pseudo-bulge. These central stellar kinematics are consistent between MUSE and MaNGA (Saburova et al., 29 Sep 2025).

NBursts also provides SSP-equivalent luminosity-weighted age and metallicity. The bulge stellar population is very old, with age $R_d \sim 80$3 Gyr and metallicity $R_d \sim 80$4 dex. The inner disk, identified with the HSB component, is also old and metal-rich. A few compact regions in the disk show younger ages of $R_d \sim 80$5 Gyr, though these are in low-S/N, low-surface-brightness areas and thus less certain. This central population contrasts with SED-fitting results that suggested a bulge that is younger than the LSB disk; the spectroscopic analysis instead finds a classical old, metal-rich bulge embedded in an extended, more complex disk.

The structural residuals reinforce this view of central complexity. GALFIT modeling of the HSC $R_d \sim 80$6-band image with bulge+disk leaves a prominent bar-like residual. An HST F814W residual image shows a strong dust lane aligned with this bar. The bar length is $R_d \sim 80$7 ($R_d \sim 80$8 kpc). The combination of old central stars, strong dust lane aligned with the bar, and a secondary stellar component oriented perpendicular to the bar hints at a past merger or accretion event that has left a fossil nuclear structure. This suggests that the central S0-like body is not a simple unperturbed early-type system, but the dynamically and chemically old core of a galaxy that subsequently acquired a giant retrograde gaseous disk (Saburova et al., 29 Sep 2025).

5. Chemical structure, metallicity gradients, and effective oxygen yield

To probe chemical structure and search for gas accretion signatures, the authors defined a binning scheme to isolate star-forming clumps in the LSB disk. H$R_d \sim 80$9 and [O III]/[N II]/[S II] maps were used to find clumps, and each clump was checked in the BPT diagram; only regions below the Kewley (2001) “maximum starburst” line, consistent with photoionization by young stars, were retained for metallicity analysis. Gas-phase oxygen abundances were estimated with two strong-line methods: the O3N2 calibration and the S-calibration. O3N2 uses

$\sim 3$0

and the calibrations have a generic form

$\sim 3$1

For UGC 1382, the reported values are

$\sim 3$2

for O3N2, and

$\sim 3$3

for the S-calibration. These mean values are only slightly sub-solar, adopting $\sim 3$4, and the gradients are very shallow. The metallicity profile is essentially flat across tens of kpc in radius (Saburova et al., 29 Sep 2025).

A striking feature in the metallicity map is a localized low-metallicity region with $\sim 3$5 near the top of the disk, coincident with a bright blue star-forming clump visible in optical imaging. This is interpreted as a likely imprint of infall of a metal-poor gas clump triggering local star formation. At the galaxy-wide level, the globally flat metallicity gradient, combined with a slightly sub-solar overall metallicity for a stellar mass $\sim 3$6, places UGC 1382 slightly below the mass–metallicity relation of “normal” disks.

The paper further computes the effective oxygen yield $\sim 3$7 as a function of radius. In the instantaneous-recycling, closed-box model,

$\sim 3$8

and the effective yield is defined as

$\sim 3$9

To compute $\sim 4.3$0 for UGC 1382, gas surface density $\sim 4.3$1 was taken from the H I profile multiplied by 1.3 to account for helium; molecular gas is neglected. Stellar surface density $\sim 4.3$2 for the disk was computed using the two exponential disks and $\sim 4.3$3 colors converted to mass-to-light ratios. From $\sim 4.3$4 and $\sim 4.3$5, the gas mass fraction

$\sim 4.3$6

was obtained, and oxygen abundance $\sim 4.3$7 was derived from $\sim 4.3$8 using $\sim 4.3$9 (Saburova et al., 29 Sep 2025).

The resulting $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$0 profile shows inner regions with $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$1 slightly below or comparable to the canonical value $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$2, while in the outer disk at $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$3 kpc, $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$4 rises significantly above this reference, reaching $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$5, indicating an apparent oxygen overabundance for the measured gas fraction. The authors argue that these high values likely do not imply the accretion of strongly enriched gas. Instead, the underlying assumptions of the closed-box, instantaneous-mixing model fail in the outer disk: at $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$6 kpc, $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$7 is low, $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$8, and, assuming a typical turbulent velocity dispersion $\mu_{0,B}\sim 21.65\ \mathrm{mag\ arcsec^{-2}}$9, the gas scale height in a self-gravitating disk,

$^{-1}$0

exceeds $^{-1}$1 kpc, while the midplane gas volume density is $^{-1}$2. In such thick, diffuse disks, mixing is inefficient and the majority of the gas remains only weakly processed by star formation. Therefore, the high inferred $^{-1}$3 likely reflects that the emission-line-bright fraction of the gas is chemically more evolved than the bulk of the very extended, “passive” metal-poor gas reservoir (Saburova et al., 29 Sep 2025).

6. Star formation regime, merger history, and significance within the gLSBG class

The MUSE data reach extremely low H$^{-1}$4 surface brightness, yet the outer LSB disk of UGC 1382 shows only sparse H II regions embedded in a broadly passive gas disk. In the BPT diagram, the outer points of UGC 1382 lie comfortably within the star-forming locus and below the Kewley (2001) line; there is no evidence for strong shock-excitation or widespread AGN photoionization at large radii. The central few kpc lie above the Kewley line, indicating a composite/AGN-like ionization source. However, unWISE/WISE mid-IR data show a “blue” $^{-1}$5 color, no long-term mid-IR variability and no AGN-typical $^{-1}$6 / $^{-1}$7 colors; any AGN is thus low-luminosity and compact. Despite the large gas reservoir, the star formation is extremely inefficient in the outer disk. Coupled with the large gas fractions and high effective yields at large radii, this supports the idea that the outer gas is “passive”: most of it is not participating in contemporaneous star formation and has evolved very little chemically since accretion (Saburova et al., 29 Sep 2025).

Several independent observational signatures point to a merger-rich history several Gyr in the past. These include the global gaseous counter-rotation, the compact elliptical satellite, central structural complexity with a strong inner bar and aligned dust lane, the flat metallicity gradient with mild dilution, stellar population ages in which the bulge and inner disk are old while the outer disk contains regions with $^{-1}$8–6 Gyr populations, and the lack of bright tidal debris. Deep imaging and the gas metallicity maps do not show obvious shells or tidal tails; numerical work suggests that prominent tidal features in intermediate-mass mergers fade within $^{-1}$9–8 Gyr, hence the absence of such features implies that the last major merger(s) occurred at least several Gyr ago (Saburova et al., 29 Sep 2025).

Based on these constraints, the proposed formation scenario is that UGC 1382 formed its giant, dynamically cold, LSB disk through multiple gas-rich mergers with companions on retrograde orbits. In that scenario, the inner S0 galaxy, comprising the bulge plus HSB disk, formed by more “normal” processes early on. Subsequently, UGC 1382 accreted several gas-rich satellites whose orbital angular momentum was opposite to that of the pre-existing stellar disk; gas stripped from these satellites settled into a coherent retrograde disk; because the satellites were gas-rich and the orbits had high angular momentum, the infalling gas could dissipate, settle into a thin disk, and form the extended, tightly wound spiral structure, rather than violently heat or destroy the existing stellar disk. The cumulative baryonic mass contributed had to be large, because a single cE progenitor cannot supply the $R_d \sim 80$00 stellar plus gas mass of the LSB disk (Saburova et al., 29 Sep 2025).

Within the broader gLSBG context, UGC 1382 demonstrates that a massive halo, a giant low-surface-brightness stellar disk, a globally counter-rotating gaseous component, and nearly flat metallicity gradients can coexist in a system whose present kinematics are regular and dynamically relaxed. The paper identifies it as the outcome of multiple retrograde, gas-rich mergers, in contrast to AGC 192040, which does not host a globally counter-rotating gaseous disk and for which gas accretion from the filament followed by the intermediate-mass ratio merger with the companion on a prograde orbit is proposed. UGC 1382 therefore serves as a benchmark for models of gLSBG formation in which merger geometry, gas richness, and the persistence of chemically under-evolved passive gas all play central roles (Saburova et al., 29 Sep 2025).