SDSS-V Local Volume Mapper (LVM): Helix Nebula public data, Data Analysis Pipeline data products

TL;DR

This work presents a spatially resolved optical spectroscopic analysis of the Helix Nebula (NGC 7293) using SDSS-V Local Volume Mapper data and the LVM-DAP pipeline. It delivers hexagonally sampled, wide-field emission-line maps across 3600–9800 Å for all major ionic species, enabling direct comparison with classical aperture studies and revealing ionization stratification, faint auroral lines, and a slowly expanding shell kinematic pattern. The two-stage stellar continuum modeling with MaStar-based templates and a dual emission-line analysis (parametric and non-parametric) yields robust, fiber-by-fiber plasma diagnostics with quantified uncertainties, even for very weak lines. The results validate the LVM approach for extended nebulae and demonstrate the pipeline’s ability to recover global and spatially resolved physical conditions, setting the stage for deeper, broader LVM data releases of the Milky Way and Local Volume ionized gas.

Abstract

We present a spatially resolved spectroscopic analysis of the Helix Nebula (NGC 7293) using data from the SDSS-V Local Volume Mapper (LVM), by applying the recently developed LVM Data Analysis Pipeline (LVM-DAP). Covering the full optical range (3600-9800 Å) over a contiguous ~ 0.5 degree field, the LVM data provide the first hexagonally sampled, wide-field emission-line maps of all major ionic species in this archetypal planetary nebula. The resulting flux, kinematic, and line-ratio maps reveal the well-known ionization stratification of the nebula, from the compact He++ core to the bright [O III] ring and the extended low-ionization envelope, enabling a detailed comparison with classical aperture spectroscopy. Owing to the sensitivity and uniform spatial sampling of the LVM, numerous faint auroral and diagnostic lines are detected across the nebula, including [O III] 4363, [N II] 5755, and He I lines, allowing precise measurements of weak-line morphology. The derived radial trends confirm the remarkably low dust content and the overall homogeneity of electron temperature and density across the main ring. Ionized-gas kinematics traced by Hα further support the scenario of a slowly expanding, limb-brightened shell consistent with previous studies. This work demonstrates the diagnostic power of LVM spectroscopy for extended nebulae and highlights its capability to recover both global and spatially resolved physical conditions across complex ionized structures.

Figures (12)

• Figure 1: Color image created combining the WISE W4 (22$\mu$m, red), W3 (12$\mu$, green) and W2 (4.6$\mu$m, blue) band images covering $\sim$0.65° size centered in Helix Nebula odell98. The foot-print of the LVM science IFU fibers included in the exposure delivered in the SDSS DR19 is represented by a set of semi-transparent white circles. The white points indicate the location of previous regions explored in the literature. The regions discussed by odell98 are represented by numbers: (1) 'middle', (2) 'transition', (3) 'ring' and (4) 'arc'. Finally, the regions discussed by henry99 are labeled with the same letters adopted in that article.
• Figure 2: Updated scheme of the LVM-DAP analysis flow for a single fiber spectrum, including the main procedures: (i) derivation of the non-linear parameters of the stellar spectrum (v$_\star$, $\sigma_\star$ and A$_{\rm V,\star}$), (ii) parametric derivation of the properties of the ionized gas emission lines (EL), including the flux intensity (f$_{\rm EL}$), velocity (v$_{\rm EL}$) and velocity dispersion ($\sigma_{\rm EL}$), (iii) stellar component synthesis, i.e., decomposition into a set of RSP templates, and generating a model of the stellar spectrum, (iv) non-parametric derivation of the properties of the emission lines, including the equivalent width for each emission line (EW$_{\rm EL}$), and (v) a re-evaluation of the parametric derivation of the emission lines. The last two analysis were performed over a so-called gas-pure spectrum, i.e., the residual of subtracting the stellar component model from the original spectrum. RND and LM stands for the two methods included in pyFIT3D to fit the parametric models to the EL, as explained in the text. Black boxes indicate in which extension of the DAP data products files the analysis of each module is stored.
• Figure 3: Integrated spectrum of the Helix Nebula across the entire field-of-view of the analyzed LVM pointing (black solid line), together with the results from the LVM-DAP analysis, i.e., the best-fitting stellar model (red solid line), the model of the strongest emission lines fitted with Gaussian functions (blue solid line), the residual after subtraction of the stellar model (cyan solid line), and the combination of both models, including a final correction to the residual shape (yellow solid line). The first three panels, from top to bottom, display the wavelength ranges covered by the blue (1st row), green (2nd row), and infrared (3rd row) arms that compose the LVM spectrograph. Shaded areas indicate the overlapping regions between these arms. The insets in the two bottom rows show zoom-ins on selected wavelength intervals. The panels in the 4th row illustrate the quality of the modeling for the strongest emission lines based on the parametric analysis described in the text. Finally, the bottom row of panels presents three wavelength ranges (3751–3999 Å, 4976–4799 Å, and 8451–8699 Å) that highlight specific spectral features discussed in the text. Three additional insets display close-ups around the auroral lines most commonly used to determine the electron temperature in ionized nebula: [@series O iii O iii O iii] $\lambda4363$, [@series N ii N ii N ii] $\lambda5755$, and [@series S iii S iii S iii] $\lambda6312$. The flux scales in these three insets are identical to facilitate comparison among the lines. In all panels, orange vertical lines indicate the emission lines analyzed using the non-parametric procedure described in the text.
• Figure 4: Example of the analysis performed by LVM-DAP to recover the properties of the ionized gas emission lines. Each panel shows the distribution across the FoV of the LVM IFU of the flux intensity estimated by the weighted-moment procedure for the 20 brightest emission lines shown in the integrated spectrum of the Helix Nebula(Fig. \ref{['fig:spec']}). The emission lines are ordered from the brightest (top-left) to the faintest (bottom-right). The legend in each panel indicates the represented emission line. The remaining detected emission lines are shown in Appendix \ref{['app:fe']}
• Figure 5: Radial profiles of the azimuthally averaged flux intensities in each fiber in absolute values (left-panel), and relative to H$\beta$ (right-panel), for a sub-set of the emission lines analyzed along this study. The filled region in each profile corresponds to the standard deviation across radial bin of 0.5$\arcmin$. Each emission line is represented with a different color and hash-style. The sub-set comprises both prominent hydrogen recombination lines (H$\alpha$, H$\beta$, H$\gamma$), and key collisional excited transitions ($[$@series O ii O ii O ii$]$3737, $[$@series O iii O iii O iii$]$5007, $[$@series N ii N ii N ii$]$6584, $[$@series S II S II S II$]$6731, $[$@series Ar iii Ar iii Ar iii$]$ and $[$@series S III S III S III$]$9069,9531) that trace the ionization structure of the nebula. The profiles reveal the characteristic bright main ring around $\sim$5$\arcmin$–7$\arcmin$ and the decline toward the outer halo, highlighting differences in excitation between high- and low-ionization species. Similar structure is seen in Fig. \ref{['fig:fe']}.