---
title: 'NGC 2768: Structure and Kinematics'
url: https://www.emergentmind.com/topics/ngc-2768
type: topic
---

# NGC 2768: Structure and Kinematics

Searching arXiv for recent and relevant papers on NGC 2768 and its structure/kinematics.
NGC 2768 is a nearby early-type galaxy, variously classified as E5, E6, E/S0, and S0 \(1/2\), and commonly treated as a lenticular system with a prominent spheroid and a thin, rapidly rotating disk. At adopted distances of \(D \approx 21.8\)–\(22.15\) Mpc, it has become a benchmark object for linking photometric structure, halo morphology, globular-cluster and planetary-nebula kinematics, and multi-phase gas accretion. Deep optical imaging shows unusually strong outer boxiness and kpc-scale isophote center drifts; kinematic studies resolve a dynamically cold disk embedded in a more pressure-supported bulge; and recent optical and H I work identifies ongoing or recent accretion, including a dwarf-like progenitor candidate and a diffuse, disturbed neutral-hydrogen envelope [2311.09286; 1208.1767; 2507.04703].

## 1. Classification, distance scale, and global parameters

NGC 2768 has been described as lenticular/elliptical, with classifications including E6, E6/S0, E/S0, S0 \(1/2\), and E5 in different catalogs and analyses. It resides in a low-density environment and is cataloged in the Lyon Group of Galaxies as LGG 167; one study further describes it as the brightest member of the loose LGG 167. Adopted distances differ slightly by study: \(D = 21.8\) Mpc in the SLUGGS and extended-kinematics analyses, \(D \approx 21.8\)–22 Mpc in the chromodynamical study, and \(D = 22.15\) Mpc in the HERON analysis. At \(D = 22.15\) Mpc, \(1\) arcsec \(\approx 107\) pc, while the HERON C28 imaging scale is \(0.82\) arcsec pixel\(^{-1}\) and \(88\) pc pixel\(^{-1}\) [1806.11242; 2311.09286; 1310.1979].

Published global parameters also depend on the adopted decomposition and photometric band. Reported effective radii include \(R_e = 6.37\) kpc, \(6.66 \pm 1.3\) kpc, \(6.7\) kpc, and \(7.6\) kpc in different studies. Additional quoted properties include systemic velocity \(v_{\rm sys} = 1353\) km s\(^{-1}\), central stellar velocity dispersion within 1 kpc \(\sigma_{1\rm kpc} = 206\) km s\(^{-1}\), ellipticity \(\epsilon = 0.57\) or \(0.60 \pm 0.03\) depending on dataset, luminosity-weighted stellar age within \(1 R_e\) of \(12.3\) Gyr, stellar mass \(\log(M_\star/M_\odot)=11.21\), and bulge-to-total light ratio \(B/T = 0.71\). The local environment density reported in the wide-field globular-cluster study is \(\rho = 0.31\ {\rm Mpc}^{-3}\) [1806.11242; 1310.1979].

This parameter spread is methodological rather than contradictory. Different works target different tracers, surface-brightness regimes, and structural decompositions, so NGC 2768 is best understood as a flattened S0/early-type system whose measured global properties depend on whether the emphasis is on the stellar light, the bulge–disk decomposition, or the faint outer halo.

## 2. Stellar structure and internal morphology

Near-infrared bulge–disk decomposition shows that NGC 2768 contains a prominent bulge and a thin, highly flattened disk. A 2D decomposition of the 2MASS \(K\)-band image models the disk as exponential and the bulge as Sérsic. The best-fit disk has scalelength \(h = R_d = 0.72\) arcmin, axis ratio \(b/a = 0.29\), and \(K = 8.19\); the bulge has Sérsic index \(n = 4.65\), effective radius \(R_{e,b} = 0.84\) arcmin, axis ratio \(b/a = 0.66\), and \(K = 7.23\). Using \(M/L_K \approx 0.6\), the corresponding stellar masses are \(M_{\rm disk} \approx 3.1 \times 10^{10}\ M_\odot\) and \(M_{\rm bulge} \approx 7.5 \times 10^{10}\ M_\odot\), implying \(B/T_{\rm mass} \approx 0.7\) [1208.1767].

Internal morphological complexity is a recurring theme across datasets. The galaxy shows a dust lane along the minor axis, ionized gas whose inner kinematics differ from the stellar kinematics, rich dust lanes visible in model-subtracted optical images, and a possible vestigial X-shaped bulge feature. The H I study adds that the molecular CO disk is nearly polar, rotating perpendicular to the galaxy’s stellar major axis, and that the HST dust ring is similarly oriented and extends toward the H I overdensity termed Clump A [1806.11242; 2311.09286; 2507.04703].

Taken together, these data define a system with an unambiguously composite structure: a thin stellar disk, a dominant spheroid, and misaligned gas and dust components. This supports the view that the galaxy’s present morphology cannot be reduced to a single-axisymmetric equilibrium component.

## 3. Outer halo, isophotal boxiness, and photometric asymmetry

The HERON survey established that NGC 2768 has one of the more extreme boxy outer morphologies known among nearby early-type galaxies. Deep C28 imaging obtained in October–November 2011 with a broad-band Astrodon Luminance filter \((4000\)–\(7000\ \text{\AA})\), total exposure time \(3 \times 300\) s, \(3.5\) arcsec seeing, and depth \(28.9\) mag arcsec\(^{-2}\) in the luminance/\(r\) band was analyzed with IRAF’s `ellipse`. Deviations from perfect ellipses were parameterized through
\[
r(\theta) = r_0 + \sum_n [A_n \cos(n\theta) + B_n \sin(n\theta)],
\]
with the fourth-order term \(A_4/a\) defining boxy \((A_4/a<0)\) and disky \((A_4/a>0)\) isophotes. In the outer regions of NGC 2768, \(A_4/a \approx -0.04\), a value described as rare in large samples and characteristic of a strongly boxy early-type galaxy [2311.09286].

The same analysis found substantial isophotal asymmetry. Allowing the isophote centers to vary yields offsets up to \(\approx 20\) pixels in both \(x\) and \(y\), corresponding to \(\approx 1.8\) kpc at 88 pc pixel\(^{-1}\), with the strongest shifts beyond \(\approx 200\)–\(300\) arcsec, i.e. \(\approx 21\)–32 kpc at the adopted distance. The HERON work also notes a broader pattern of \(\sim 2\)–4 kpc center shifts in the boxy galaxies NGC 720 and NGC 2768. Its ellipticity profile is consistent with the E5 designation, while the outer envelope extends to a diameter of \(96\) kpc at the \(28\) mag arcsec\(^{-2}\) level, or \(\approx 13 R_e\); the present isophotal analysis reaches roughly six half-light radii above \(3\sigma\) of the sky [2311.09286].

These outer-halo results are notable because large surveys find only \(2\)–\(3\%\) of early-type galaxies to be boxy. In NGC 2768, the combination of \(A_4/a \approx -0.04\), center drifts, and visible substructure places the galaxy within that rare morphological subset while also tying the boxiness to an evidently disturbed halo rather than to a purely static intrinsic shape.

## 4. Globular clusters, planetary nebulae, and stellar kinematics

NGC 2768 has been studied extensively as a tracer-rich S0. Wide-field globular-cluster work based on Subaru/Suprime-Cam \(gri\) imaging and HST/ACS data identified \(978\) photometric globular clusters and \(106\) spectroscopic globular clusters, while the Planetary Nebula Spectrograph provided \(315\) planetary nebulae. The globular-cluster system reaches the background at \(R = 9.9 \pm 0.5\) arcmin, corresponding to \(63 \pm 3\) kpc, or \(\sim 10 R_e\). The total number of globular clusters is \(N_{\rm GC} = 744 \pm 68\), and the specific frequency is \(S_N = 1.3 \pm 0.1\), with
\[
S_N = N_{\rm GC}\,10^{0.4(M_V^T+15)}.
\]
The color distribution is bimodal, with Subaru peaks at \((g-i)=0.819 \pm 0.015\) and \(1.076 \pm 0.017\), separated at \((g-i)=0.96\); blue clusters are more extended than red clusters, with fitted effective radii \(1.83 \pm 0.27\) arcmin and \(1.50 \pm 0.23\) arcmin, respectively [1310.1979].

Spatially, the globular-cluster system is unusually flattened. The total GC distribution has \({\rm PA}=89 \pm 2^\circ\) and \(\epsilon=0.59 \pm 0.03\), closely matching the stellar light \(({\rm PA}=93 \pm 3^\circ,\ \epsilon=0.60 \pm 0.03)\). Blue and red subpopulations are similarly aligned, with \({\rm PA}=90 \pm 3^\circ,\ \epsilon=0.57 \pm 0.04\) for blue clusters and \({\rm PA}=87 \pm 3^\circ,\ \epsilon=0.60 \pm 0.05\) for red clusters. This is consistent with a highly flattened global dynamical structure extending into the halo [1310.1979].

Kinematic decomposition sharpens the contrast between the disk and the spheroid. Using photometric spheroid–disk decomposition and a two-component likelihood model, the chromodynamical analysis found weighted GC counts of \(24.8\) disc-like clusters and \(68.1\) spheroid-like clusters, with \(10\) rejected outliers. No correlation was found between GC color and component membership; both red and blue subpopulations are predominantly spheroid-like. Red GCs display significant inner rotation, with \(V_{\rm rot} \approx 100\) km s\(^{-1}\) at \(R \lesssim R_e\), whereas blue GCs show negligible rotation. For both GC subpopulations, \(V/\sigma \le 1\) at all radii, indicating that random motions dominate over ordered rotation in the GC system as a whole [1806.11242].

The extended-kinematics analysis, which directly compared planetary nebulae, red globular clusters, and starlight, found excellent agreement among the tracers to \(\sim 4 R_e\). In that framework the disk shows a rapidly rising rotation curve reaching \(V_{\rm rot} \approx 270\) km s\(^{-1}\) at a few scalelengths and a strongly declining dispersion profile, while the bulge rotates only mildly, with \(V_{\rm rot} \approx 100\) km s\(^{-1}\) and \(\sigma \approx 120\) km s\(^{-1}\) at large radii. Bulge PNe and bulge starlight follow the same radial density distribution as the red GCs, whereas the disk tracers form a distinct flatter component. The resulting picture is a dynamically cold, rotationally supported disk embedded in a more pressure-supported bulge [1208.1767].

## 5. Accretion signatures in the stellar halo: Pelops and tidal debris

Model-subtracted HERON imaging reveals a prominent plume west of the galaxy center that is interpreted as the progenitor candidate of an ongoing accretion event and is given the name “Pelops.” Fitting the feature with a Sérsic profile using GALFIT yields an absolute magnitude \(M_r \approx -12.2\) mag, effective radius \(R_e = 2.4 \pm 0.3\) kpc, Sérsic index \(n = 0.37 \pm 0.02\), surface brightness at \(R_e\) of \(\mu_e = 22.96 \pm 0.01\) mag arcsec\(^{-2}\), ellipticity \(e = 0.55 \pm 0.01\), and position angle \(88.4^\circ \pm 1.1^\circ\). The adopted Sérsic law is
\[
I(R)=I_e\exp\{-b_n[(R/R_e)^{1/n}-1]\},
\]
with \(b_n \approx 2n-1/3\) [2311.09286].

Pelops is photometrically dwarf-like but structurally unusual. Aperture photometry on SDSS residual images gives dereddened magnitudes \(u_0 = 19.19 \pm 0.26\), \(g_0 = 17.93 \pm 0.14\), \(r_0 = 17.00 \pm 0.09\), \(i_0 = 16.65 \pm 0.08\), and \(z_0 = 16.50 \pm 0.07\) mag, with colors \((g-r)_0 = 0.93 \pm 0.17\) and \((r-i)_0 = 0.35 \pm 0.12\). E-MILES SSP fits at LMC-like metallicity with a Kroupa IMF yield a best-fit age of \(6.5^{+5.5}_{-3.8}\) Gyr, intrinsic reddening \(A_V = 0.15 \pm 0.15\) mag, and \(M/L_r = 1.9^{+1.0}_{-0.9}\), implying a stellar mass of \(\approx 10^7\ M_\odot\). If instead a dwarf-spheroidal-like \(M/L \sim 20\) is assumed, the mass becomes \(\approx 10^8\ M_\odot\). Relative to the host’s disk+bulge mass \(\sim 10^{11}\ M_\odot\), the implied merger is minor, with mass ratios \(\sim 10^{-4}\)–\(10^{-3}\) [2311.09286].

The strongest argument that Pelops is disrupting rather than merely projected arises from its size and associated debris. Its \(R_e\) is larger by a factor of a few than those of typical dwarfs of similar luminosity, placing it near disrupted systems such as NGC 4449B and HCC-087, and near tidally affected Local Group dwarfs such as And XIX and Antlia 2. In GALEX NUV, a vestigial stream or plume appears aligned with Pelops and extends toward the south of NGC 2768; FUV shows no detection. The HERON study argues that this feature is unlikely to be a GALEX ghost because documented ghosts typically sit \(30\)–\(60\) arcsec above or below bright sources along the detector \(y\)-axis, whereas the candidate stream is at \(\sim 140\) arcsec separation, is more extended, and lacks a donut morphology [2311.09286].

Within the limits of the available imaging, these data strongly favor the interpretation that a minor merger is in progress and that the outer optical boxiness is being shaped, at least in part, by ongoing tidal disruption.

## 6. Neutral hydrogen, dark satellite candidates, and group-scale interactions

Deep 21 cm observations with the Five-hundred-meter Aperture Spherical radio Telescope transformed the H I view of NGC 2768. Using the FAST 19-beam receiver in on-the-fly mapping mode, the observations reached an rms of \(0.4\) mJy beam\(^{-1}\) at \(6.7\) km s\(^{-1}\) channels, a \(5\sigma\) column-density threshold of \(6.0 \times 10^{16}\ {\rm cm}^{-2}\) per \(6.7\) km s\(^{-1}\) resolution element, and moment-map contours down to \(1.1 \times 10^{18}\ {\rm cm}^{-2}\). FAST reveals a large, diffuse circumgalactic H I envelope with total mass \(M_{\rm HI,total}=8.1\times10^8\ M_\odot\), more than an order of magnitude above the earlier WSRT estimate because the single-dish data recover very low-surface-brightness large-scale emission missed by the interferometer [2507.04703].

The envelope contains two principal components. One is an H I disk associated with NGC 2768, showing an asymmetric S-shaped position–velocity signature spanning \(\approx 1150\)–\(1520\) km s\(^{-1}\), with centroid \(\approx 1335\) km s\(^{-1}\) around the galaxy’s systemic velocity \(v_{\rm sys} \approx 1353\) km s\(^{-1}\). The other is a high-velocity structure, Clump B, interpreted as a newly discovered satellite galaxy without detected optical counterpart. The peak H I column density of the envelope is offset by about \(5\) arcmin, or \(\approx 32\) kpc at the adopted distance, from the optical center, and the H I disk center is likewise misaligned relative to the optical photometric center by \(\approx 5\) arcmin. The redshifted side of the PV diagram is stronger and more extended, indicating that the gas disk has been disturbed [2507.04703].

Clump B is dynamically significant. FAST gives it \(M_{\rm HI,B}=7.8\times10^7\ M_\odot\), velocity span \(\approx 1570\)–\(1660\) km s\(^{-1}\), and TiRiFiC model parameters \(V_{\rm rot}=58.7\) km s\(^{-1}\), \(\sigma=7.5\) km s\(^{-1}\), and \(R_{\rm HI}\approx9.5\) kpc. The corresponding dynamical mass,
\[
M_{\rm dyn}(R)=\frac{(V_{\rm rot}^2+3\sigma^2)R}{G},
\]
is \(\approx 7.9 \times 10^9\ M_\odot\), exceeding its baryonic content by more than an order of magnitude. No optical or UV counterpart is detected in SDSS, DESI Legacy Surveys, MATLAS, or GALEX; the quoted limits imply \(L_R \lesssim 2.6 \times 10^5\ L_\odot\), \(L_B \lesssim 1.9 \times 10^5\ L_\odot\), and \(M_\star \lesssim 10^6\ M_\odot\). On that basis, the study argues that Clump B is a dark-matter-dominated dwarf satellite rather than a tidal dwarf [2507.04703].

The same H I dataset also links NGC 2768 to its group environment. FAST detects a diffuse H I bridge between NGC 2768 and UGC 4808, a cloud C3 apparently stripped from PGC 2599651 toward Clump B, and a faint gas bridge in PV space between Clump B and the NGC 2768 disk. The collision or close-passage timescale estimated for Clump B relative to NGC 2768 is \(\Delta t \approx 0.38\) Gyr. The paper further states that \(\gtrsim 90\%\) of the gas lies below \(10^{19}\ {\rm cm}^{-2}\), emphasizing that the accretion reservoir is predominantly diffuse [2507.04703].

## 7. Formation scenarios, interpretive synthesis, and unresolved problems

The literature converges on a composite evolutionary picture. Stellar kinematics identify a thin disk with spiral-like \(V/\sigma\) behavior, while the bulge and halo are more pressure-supported; globular clusters and planetary nebulae indicate a dominant spheroidal component and a history in which mergers were important; deep optical imaging shows rare outer boxiness and substantial asymmetry; and both optical and H I observations point to ongoing minor accretion [1208.1767; 1806.11242; 2311.09286; 2507.04703].

Two formation statements recur explicitly. One is that NGC 2768 is a transformed late-type galaxy: the disk reveals a rapidly rising rotation curve, declining velocity dispersion, and a \(V/\sigma\) profile resembling that of a spiral galaxy, while the bulge is more nearly an oblate, pressure-supported spheroid. The second is that mergers and accretion have been very important: the chromodynamical analysis favors an unequal-mass merger origin for the S0 structure; HERON interprets the outer morphology as merger-driven and the Pelops event as an ongoing minor merger; and the FAST study argues that the galaxy is currently undergoing a transition from a spiral galaxy to an S0 through external gas accretion and interactions in a loose group [1208.1767; 1806.11242; 2311.09286; 2507.04703].

These lines of evidence are complementary rather than mutually exclusive. A plausible implication is that NGC 2768 preserves a dynamically cold stellar disk from a late-type progenitor while its halo, gas content, and outer isophotes continue to be reshaped by minor mergers and external accretion. The comparison with NGC 720 in the HERON study underscores that even galaxies with similar \(A_4/a \approx -0.04\) can differ strongly in kinematics and in whether an accreted progenitor is directly identifiable [2311.09286].

Outstanding questions are explicitly identified in the recent work. They include refining the mass and orbit of Pelops; mapping the full extent, geometry, and stellar populations of the tidal debris; obtaining deeper, higher-resolution optical and UV imaging; extending wide-field spectroscopy of planetary nebulae and globular clusters; and recovering higher-resolution H I structure with short-spacing-sensitive interferometric data. Those follow-ups are aimed at constraining the timescale and detailed dynamics of the accretion events and at clarifying how minor mergers imprint boxiness, asymmetry, and gas misalignment in fast-rotating S0/E systems such as NGC 2768 [2311.09286; 2507.04703].

Source: https://www.emergentmind.com/topics/ngc-2768