Nuclear Stellar Disk (NSD) in Galactic Centers

• The Nuclear Stellar Disk (NSD) is a flattened, rotationally supported stellar structure at galaxy centers with well-defined exponential profiles and distinct kinematics.
• It is characterized by bar-driven gas inflow that fuels episodic star formation and exhibits unique chemical abundance patterns, including notable sodium enhancements.
• Observations using photometry, spectroscopy, and X-ray mapping in the Milky Way and other galaxies help reveal its scaling relations, secular evolution, and sensitivity to merger histories.

The Nuclear Stellar Disk (NSD) is a dynamically cold, flattened stellar structure commonly found at the centers of both early- and late-type galaxies, and is especially well-studied in the Milky Way due to its proximity. NSDs reside within the central few hundred parsecs, are embedded in the host’s bulge or bar, and co-exist alongside compact nuclear star clusters (NSCs) and supermassive black holes. Morphologically and kinematically distinct from classical bulges and galactic bars, the NSD in the Milky Way presents a unique laboratory for probing secular galaxy evolution, star formation in extreme environments, and the linkage between nuclear and galactic dynamics.

1. Morphology, Structure, and Scaling Relations

NSDs exhibit highly regular, axisymmetric morphologies with typical exponential scale lengths $R_\mathrm{NSD} \sim 90$–$100$ pc and vertical scale heights $H_\mathrm{NSD} \sim 28$–$50$ pc for the Milky Way, and $R_\mathrm{NSD} \sim 200$–$500$ pc and $M_\mathrm{NSD}\sim10^{8.5}$–$10^{10}~M_\odot$ in external galaxies. The Milky Way NSD’s total mass is $(1.05 \pm 0.10)\times10^9~M_\odot$ (Schultheis et al., 4 Sep 2025), embedded within a larger-scale bulge/bar and enveloping the more compact NSC. The NSD’s surface brightness and mass density follow nearly exponential or broken power-law profiles. Star counts, infrared photometry, and 3D deprojections yield near equality of in-plane and line-of-sight extents (projected radius $\sim150$ pc, LoS depth $\sim330$ pc) and a regular, disk-like shape (Nogueras-Lara, 2022, Sormani et al., 2021, Nezabudkin et al., 6 Jul 2025).

Exponential profile parameters for the Milky Way NSD: | Radial scale-length $R_\mathrm{disc}$ (pc) | Vertical scale-height $H_\mathrm{disc}$ (pc) | Total stellar mass $M_\mathrm{NSD}$ ($M_\odot$) | | ---------------------------------------- | -------------------------------------------- | ---------------------------------------------- | | $88.6^{+9.2}_{-6.9}$ | $28.4 \pm 5.5$ | $(1.05^{+0.11}_{-0.10})\times 10^9$ |

NSDs follow host-mass scaling relations: $$\log M_\mathrm{NSD}=(1.32\pm0.25)\log (M_\mathrm{gal}/10^9 M_\odot)+(7.62\pm0.44)$$ (photometric mass) (Schultheis et al., 4 Sep 2025, Ledo et al., 2010).

2. Kinematics, Orbital Structure, and Dynamical Models

Kinematically, NSDs are dynamically cold and rapidly rotating systems, with $\langle v_\phi \rangle \approx 100$ km s$^{-1}$ and internal velocity dispersions in the range $\sigma_z,\sigma_R,\sigma_\phi \approx 70$–$130$ km s$^{-1}$ (Sormani et al., 2021, Nogueras-Lara et al., 23 Sep 2024, Sormani et al., 2020). The Milky Way NSD exhibits a prominent $\sigma$-drop toward the center and v/σ ratios of $\approx 1.4$, indicating rotation-dominated kinematics consistent with axisymmetry (Schultheis et al., 4 Sep 2025). Self-consistent dynamical models constructed in action space or via axisymmetric Jeans equations robustly recover the NSD potential and distribution function (Sormani et al., 2021, Sormani et al., 2020).

Orbital analyses of the stellar component in the Milky Way using full 6D phase-space and frequency mapping demonstrate that $\sim65$% of stars occupy regular $z$-tube orbits—3D analogs of planar $x_2$ orbits—directly linking NSD orbits with bar-driven gas dynamics (Nieuwmunster et al., 1 Mar 2024). Chaotic and box orbits constitute $\sim20$–$34$% of NSD-like stars but have spatial distributions and frequency correlations consistent with contamination from bar/bulge interlopers rather than intrinsic NSD members (Nieuwmunster et al., 1 Mar 2024, Nogueras-Lara et al., 23 Sep 2024).

3. Chemical Abundances and Stellar Populations

The NSD stellar population is chemically and kinematically distinct from the inner bulge/bar and the NSC. NSD metallicity distributions are bimodal, with a metal-rich peak at $[\mathrm{M/H}]\sim+0.1$ and a metal-poor peak at $[\mathrm{M/H}]\sim-0.2$; this differs from the broader and more metal-poor bulge/bar MDF (Nogueras-Lara et al., 23 Sep 2024, Schultheis et al., 2021). High-resolution spectroscopic studies sampling 18 elements find NSD abundance trends for $\alpha$ (Mg, Si, Ca), iron-peak, and neutron-capture elements that are nearly indistinguishable (within 0.1 dex) from inner-bulge, NSC, and local thick disc stars at similar $[\mathrm{Fe/H}]$ for 17/18 elements. Sodium, however, is significantly enhanced ($[\mathrm{Na/Fe}]\sim+0.5$–$+0.6$ at $[\mathrm{Fe/H}]>0$), greater than thin disc or inner bulge levels—potentially indicating special nuclear starburst conditions, but not globular cluster-like abundance anomalies (Ryde et al., 21 May 2025).

Radial metallicity gradients are mild within the NSD, with $d[\mathrm{M/H}]/dR\sim(3$–$5)\times10^{-4}$ dex pc$^{-1}$, but the NSC–NSD transition over $\sim20$ pc is much steeper, evidencing their distinct star-formation histories (Feldmeier-Krause, 2022, Nogueras-Lara et al., 23 Sep 2024). Metal-rich stars exhibit lower velocity dispersions and rotate faster than metal-poor stars, which more closely resemble bulge interlopers and are dynamically hotter, consistent with an in-situ, inside-out assembly of the NSD (Nogueras-Lara et al., 23 Sep 2024, Schultheis et al., 2021). Population analysis confirms the majority of NSD mass is $\gtrsim8$ Gyr old, with a $\sim$1 Gyr intermediate-age component and minor recent star formation [$\lesssim$5% of mass; e.g., 1 Gyr and $<30$ Myr bursts; (Nogueras-Lara et al., 2023, Nogueras-Lara et al., 2021)]. The stellar age gradient in the NSD—older stars at small radii, more intermediate-age stars at larger radii—is consistent with both chemical and dynamical secular evolution models (Nogueras-Lara et al., 2023).

4. Star Formation, Cluster Dissolution, and the “Missing Cluster Problem”

The NSD has been the most prolific star-forming region of the Milky Way over the past 30 Myr, with a rate of $0.2$–$0.8~M_\odot$ yr$^{-1}$ and an integrated mass of recent stars of order $1$–$3\times10^6~M_\odot$ (Martínez-Arranz et al., 28 Jan 2024, Martínez-Arranz et al., 2023). Yet the combined mass of presently identified young clusters (Arches, Quintuplet, and the Nuclear cluster) is $\lesssim10^5~M_\odot$, significantly below the expected total—a discrepancy termed the “missing cluster problem” (Martínez-Arranz et al., 28 Jan 2024). Tidal field strength near the Galactic Center ($r_t\lesssim1$–$2$ pc for $M_\mathrm{cl}\sim10^4~M_\odot$), frequent molecular-cloud encounters, and high ambient densities deliver disruption timescales $t_\mathrm{dis}\lesssim10$ Myr, efficiently dissolving clusters and dispersing their stars into the field (Martínez-Arranz et al., 28 Jan 2024, Martínez-Arranz et al., 2023).

Recent searches employing spectroscopy, proper motions, and extinction as line-of-sight proxies have identified co-moving groups—lower-mass stellar associations that may represent cluster remnants—bridging the mass gap and partially resolving the cluster formation efficiency. Four such groups (masses $2.7$–$5.9\times10^3~M_\odot$, ages $\lesssim6$ Myr) have been confirmed within the NSD; these contribute to understanding of short cluster lifespans in strong tidal environments (Martínez-Arranz et al., 2023). Only $\sim5$% of the NSD area has been searched at the requisite resolution, so future surveys are expected to further clarify the pathways of cluster dissolution and field-star assembly (Martínez-Arranz et al., 28 Jan 2024).

5. X-ray Emission and Compact Binaries

The NSD exhibits a regular, flattened hard X-ray glow aligned with the Galactic plane, as resolved in the 4–12 keV band by SRG/ART-XC (Nezabudkin et al., 6 Jul 2025). Its spatial profile follows the stellar mass density, with latitudinal and longitudinal scale heights of 20 pc and 100 pc, respectively, matching stellar-density models. The measured X-ray flux is $(6.8^{+0.1}_{-0.3})\times10^{-10}$ erg s$^{-1}$ cm$^{-2}$ ($L_X=(5.9^{+0.1}_{-0.3})\times10^{36}$ erg s$^{-1}$ at 8.178 kpc distance). The mass-normalized X-ray emissivity, $\varepsilon = (5.6^{+0.5}_{-0.7})\times10^{27}$ erg s$^{-1}~M_\odot^{-1}$, is $\sim3.3$ times that of the Galactic ridge, with a centrally concentrated emissivity maximum within a few tens of parsecs. The entire X-ray morphology and luminosity profile are fully consistent with unresolved emission from compact binaries—primarily accreting white dwarfs and coronally active binaries—associated with the old stellar population. This leaves negligible room for a diffuse hot-plasma component (Nezabudkin et al., 6 Jul 2025).

6. Formation Mechanisms, Evolution, and Fragility

Comprehensive dynamical, chemo-kinematic, and star-formation evidence supports the in-situ formation of NSDs via bar-driven gas inflow fueling episodic and continuous star formation in nuclear rings. As the central mass concentration and star formation progress, the NSD expands inside-out, imprinting monotonic gradients in age, metallicity, and $\alpha$-element abundance (Schultheis et al., 4 Sep 2025, Nogueras-Lara et al., 23 Sep 2024, Ryde et al., 21 May 2025). Hydrodynamical simulations and analytical models of barred galaxies reproduce the observed radial growth and dynamical features such as $z$-tube orbit dominance (Nieuwmunster et al., 1 Mar 2024). In unbarred or elliptical galaxies, external gas accretion during minor mergers or star cluster inspiral can—but less commonly does—contribute to NSD mass (Schultheis et al., 4 Sep 2025, Portaluri et al., 2013).

N-body simulations demonstrate that NSDs are fragile with respect to major mergers (mass ratios $>1:3$), which typically destroy photometric and kinematic disk signatures. Conversely, NSDs can survive minor mergers or low-mass accretion events, maintaining kinematic disc-like features (Sarzi et al., 2015). The presence or absence of a thin, rapidly rotating NSD thus provides a constraint on a galaxy’s merger history.

7. Future Prospects and Open Questions

Key outstanding questions include the prevalence of NSDs in unbarred and elliptical galaxies, the merger history required for their survival, and their co-evolution (or lack thereof) with NSCs. Open theoretical problems surround the origin of NSD sodium enhancement, the detailed structure of the inner NSD–NSC transition, and the true metallicity and mass distributions at small scales. Forthcoming facilities—such as MOONS@VLT, JWST NIRCam, ELT HARMONI, and next-generation multiplexed spectroscopic and astrometric surveys—are projected to yield million-star samples with detailed abundances, kinematics, and ages resolving NSD chemo-dynamics throughout the Local Universe (Schultheis et al., 4 Sep 2025). Numerical simulations now reach the requisite resolution to model NSDs’ formation, resilience, and feedback across diverse environments. Progress in these areas is poised to clarify the secular evolution of barred galaxies and the physical links between nuclear disks, bars, and central black holes.