NGC 5746: Diffuse X-ray Halo & Outflows

• Diffuse X-ray emission in NGC 5746 is characterized by a hot halo with gas temperatures around 0.5–0.7 keV, driven by stellar feedback and CGM accretion.
• High-resolution XMM-Newton imaging and spectral dissection reveal biconical, bubble-like structures and anisotropic surface brightness profiles indicative of active galactic outflows.
• Empirical scaling relations link the halo’s X-ray luminosity to star formation rates, highlighting contributions from both wind-driven processes and an extensive circumgalactic medium.

Diffuse X-ray emission in NGC 5746 refers to the spatially extended, predominantly soft X-ray radiation observed in the halo and near-disc region of the massive, edge-on spiral galaxy NGC 5746. This emission provides crucial diagnostics for the hot interstellar and circumgalactic medium (CGM), its origin in stellar feedback-driven outflows, and its connection to star formation activity. Recent deep XMM-Newton observations have established that NGC 5746 hosts a prominent, hot X-ray halo and biconical structures indicative of stellar outflows, challenging previous interpretations that quiescent, non-starburst spirals lack significant hot gas reservoirs (Laktionov et al., 1 Oct 2025).

1. Physical Origin of Diffuse X-ray Emission

Diffuse X-ray emission in NGC 5746 arises from two principal sources: thermalized gas expelled in star-formation-driven outflows and an extended, hot CGM. The thermal outflow is powered by supernovae (SNe) and stellar winds that inject mechanical energy into the ISM, heating it to X-ray emitting temperatures ($kT \sim 0.5-0.7$ keV, corresponding to several million K). The hot CGM, likely a mix of feedback-heated and accreted gas, can emit X-rays via thermal bremsstrahlung and line emission, typically confined to soft X-ray bands.

Hydrodynamical simulations and analytic frameworks indicate that the X-ray luminosity ($L_X$) results from the combination of central star-formation driven winds and the CGM contribution. The wind component's emission is governed by the mass loading factor ($\beta$) and star formation rate (SFR), whereas the CGM component reflects the integrated hot gas reservoir over tens of kiloparsecs (Sarkar et al., 2016).

2. Methods of Spatial and Spectral Dissection

The deep XMM-Newton campaign employed EPIC imaging, which harnesses a broad field of view and superior sensitivity to soft X-rays (<1 keV), surpassing previous observatories such as Chandra. The following methodology was used to isolate and quantify diffuse emission (Laktionov et al., 1 Oct 2025):

• Filtering and CCD anomaly removal using ESAS tasks.
• Point source detection/masking (edetect_chain and “cheese”) to excise compact contributions, yielding data dominated by extended emission.
• Construction of three-color merged images (soft/medium/hard bands) and azimuthal surface brightness profiles to map emission geometry.
• Spectral modeling using a combination of thermal (apec) and powerlaw components, along with nuanced background and instrumental line treatments.

This approach enabled the isolation of faint emission, the identification of biconical/bubble-like structures oriented perpendicular to the disc, and the quantitative measurement of temperatures and luminosities.

3. X-ray Luminosity, Star Formation Rate, and Scaling Relations

Theoretical and empirical investigations have established that $L_{X}$ correlates with SFR, but the relationship displays regime-specific scaling (Sarkar et al., 2016, Laktionov et al., 1 Oct 2025):

• For high SFR ($\gtrsim1$ M$_\odot$ yr$^{-1}$), centrally driven wind models yield $L_X \propto \mathrm{SFR}^2$, supported by the analytic relation:

$$L_{X,\mathrm{C}} \approx 3\times10^{39} \alpha^{-1} \beta^3\, \mathrm{SFR}^2\, R_{100\,\mathrm{pc}}^{-1} \Lambda_{-23}(T,Z)\,\mathrm{erg\,s^{-1}}$$

where $\alpha$ is thermalization efficiency, $\beta$ the mass loading factor, $R_{100\,\mathrm{pc}}$ the central region size, and $\Lambda_{-23}$ the emissivity (Sarkar et al., 2016).

• For low SFR, $\beta$ is constrained by ISM availability and cooling/expansion competition; with $\beta \propto \mathrm{SFR}^{-2/3}$, the $L_X$–SFR dependence flattens and may become nearly SFR-independent.
• The observed empirical relation in NGC 5746’s halo is

$$L_X (\mathrm{erg\,s}^{-1}) \approx 2.61 \times 10^{39} \times \mathrm{SFR} (\mathrm{M_\odot\,yr}^{-1})$$

The measured diffuse X-ray luminosity suggests an SFR in NGC 5746 possibly as high as $2.9\,\mathrm{M_\odot\,yr}^{-1}$, exceeding values inferred from IR diagnostics; this may point to recent or episodic star-forming activity.

4. Plasma Properties and Morphological Structure

Imaging and spectral analysis demonstrate that NGC 5746’s hot gas halo is both extensive and energetic (Laktionov et al., 1 Oct 2025):

• Diffuse soft X-ray emission is detected up to $40$ kpc from the disc, forming a biconical/bubble-like structure perpendicular to the plane. These bubbles show enhanced emission to the east and west.
• The plasma temperature in the halo is measured at $kT \approx 0.56^{+0.08}_{-0.09}$ keV, higher than the typical $\sim 0.2$ keV seen in other spiral galaxies. The disc region is hotter ($kT \approx 0.70^{+0.14}_{-0.18}$ keV), though dominated by non-thermal X-ray binary emission.
• Surface brightness profiles reveal anisotropies, with extended emission preferentially along the bubble axes, supporting the outflow scenario.
• The thermal bubble temperatures ($kT \sim 0.58-0.59$ keV) and evidence for radial temperature gradients are consistent with gas heated by stellar feedback and subsequently transported into the halo.

5. Mass Loading Constraints and Galactic Outflows

The mass loading factor $\beta$, quantifying the ratio of outflowing mass to SFR, critically shapes X-ray properties (Sarkar et al., 2016):

• $\beta$ is bounded by SN/wind injection, ISM mass availability, and the requirement that radiative cooling be subordinate to expansion.
• High $\beta$ values boost central X-ray luminosity ($\propto \beta^3$), but gas supply and cooling times impose strong constraints, particularly in low-SFR contexts.
• In NGC 5746, the limited gas reservoir in the star-forming region may confine $\beta$ to modest values, suggesting that the observed extended, luminous halo is more attributable to accumulated CGM gas than outflows alone.

6. Circumgalactic Medium Contribution and Observational Implications

The CGM’s contribution often dominates the total diffuse X-ray luminosity in massive spirals at low-to-moderate SFR (Sarkar et al., 2016, Laktionov et al., 1 Oct 2025):

• Analytically, the CGM luminosity scales approximately as

$$L_{X,\mathrm{CGM}} \approx 8.6 \times 10^{39} n_{0,-3}^{4/3} r_{c,3} \Lambda_{-23}(T,Z) M_{\mathrm{CGM},10}^{2/3}\,\mathrm{erg\,s^{-1}}$$

where $n_0$ is central CGM density, $r_c$ core radius, and $M_{\mathrm{CGM}}$ mass.

• The extended emission and elevated luminosity observed in NGC 5746 imply that the CGM may account for much of the X-ray budget, especially if central star formation is not uniquely elevated.
• This also explains the large scatter in $L_X$–SFR relations at low SFR and motivates the use of spatially-resolved spectroscopy to disentangle wind and CGM components.

7. Broader Significance and Theoretical Context

The detection of a hot, luminous halo in NGC 5746—a system not classified as a starburst—demonstrates that massive, normal spiral galaxies can host extensive reservoirs of hot gas. Previous claims of a lack of hot halos in quiescent spirals were likely limited by observational sensitivity; deeper exposures reveal features predicted by hydrodynamical simulations and analytic models (Laktionov et al., 1 Oct 2025, Sarkar et al., 2016). This has significant implications for galaxy evolution, metal enrichment, baryon cycling, and the role of the CGM as a repository for galactic feedback.

A plausible implication is that metal abundance measurements (e.g., with forthcoming missions such as NewATHENA) will be essential to distinguish between wind-ejected metal-rich plasma and more pristine accreted gas. Future deep, high-resolution X-ray observations are required to fully map the multi-phase hot gas structure and calibrate its connection to star formation and galaxy halo mass, ideally developing an accurate “fundamental plane” relation among $L_X$, SFR, and stellar mass.

In summation, diffuse X-ray emission in NGC 5746 exemplifies the complex interplay between stellar feedback, ISM and CGM physics, and observational challenges and opportunities in tracing heated baryons in galaxies beyond the starburst regime.