A Galactic Supernova Remnant Candidate at l =25.8, b=+0.2 Revealed by Near-Infrared Imaging and Spectroscopy

TL;DR

The study identifies G25.8+0.2 as a galactic object where near-infrared [Fe II] emission reveals shock-excited gas consistent with a recent supernova remnant, embedded within the larger G26 star-forming complex. Using high-resolution IGRINS spectroscopy in the H- and K-bands, the authors detect two distinct velocity components: a high-velocity, [Fe II]-bright, shock-dominated component and a lower-velocity, Br$\gamma$-bright photoionized component, with extinction toward the source constrained to $A_V \approx 9$–$11$ mag from Brackett-line ratios. The [Fe II] filaments align with portions of the bright radio shell and with adjacent molecular material, and are juxtaposed with centrally filled soft X-ray emission, a morphology characteristic of mixed-morphology SNRs interacting with dense gas. The multiwavelength analysis suggests a young to middle-aged SNR within an active, OB-rich environment, where stellar feedback and SN activity shape the G26 complex. The results emphasize the utility of infrared shock tracers for uncovering obscured SNRs in the inner Galaxy and call for further spectroscopic typing of ionizing stars and comprehensive multiwavelength follow-up to refine the remnant’s age, energetics, and relation to the surrounding massive-star-forming region.

Abstract

We present high-resolution near-infrared spectroscopic observations of the newly identified supernova remnant (SNR) candidate G25.8+0.2 obtained with the Immersion Grating Infrared Spectrograph (IGRINS) on the Harlan J. Smith Telescope. The source was originally discovered in the UKIRT Wide-field Infrared Survey for Fe+ (UWIFE; Lee et al. 2014). Our spectra reveal multiple kinematic components in the [Fe II] 1.644 um emission. The high-velocity components exhibit elevated [Fe II]/Br gamma ratios characteristic of shock excitation, while the low-velocity components are dominated by hydrogen and helium recombination lines and are consistent with photoionized gas, indicating an H II-region origin. G25.8+0.2 lies within the G26 complex, a large (~15'x 30', corresponding to ~28 pc x 57 pc at a distance of 6.5 kpc) star-forming region in the inner Galaxy. The shock-excited [Fe II] filaments closely trace the morphology of the bright radio shell that partially encloses centrally filled soft X-ray emission, strongly suggesting recent supernova activity in this region. We discuss the physical nature of G25.8+0.2 and its relationship to the surrounding G26 star-forming complex. In addition, we derive the extinction toward the source using Brackett-line ratios and constrain the spectral types of the dominant ionizing stars from the He I 2.058 um/Br gamma line ratios.

Figures (5)

• Figure 1: (left) Continuum-subtracted [Fe II] 1.644 ${\mu}$m narrow-band image of G25.8+0.2 region. Red rectangles show slit positions. (right) Panel (a) and (b) are magnified image of the continuum-subtracted [Fe II] 1.644 ${\mu}$m image in left, showing the slit positions of our NIR spectroscopy.
• Figure 2: 1D spectra of identified emission lines. (Upper) slit 1; (lower) slit 2.
• Figure 3: Position-velocity (PV) diagrams of the detected emission lines in slit 1 (left) and slit 2 (right). The top three panels show the [Fe II] 1.644 $\mu$m, He I 2.058 $\mu$m, and Br $\gamma$ 2.166 $\mu$m emission lines. The flux scale of the color bar is in units of $10^{-19}$ W m$^{-2}$. Velocity channels between 90 and 100 km s$^{-1}$ in the He I panels, which are contaminated by OH residuals, are masked. The bottom panel presents contour maps of the [Fe II] (filled blue) and Br $\gamma$ (red) emission over the same position-velocity range.
• Figure 4: 1D spectra of the [Fe II] 1.644 ${\mu}$m, He I 2.058 ${\mu}$m, and Br ${\gamma}$ 2.166 ${\mu}$m lines (from top to bottom) for the two slit positions. For slit 1, two spectra were extracted from the upper and lower sections of the slit, whereas for slit 2, a single spectrum was extracted from the entire slit (see Figure \ref{['fig:2d']}). The black solid lines represent the observed spectra. The blue dotted lines show the individual Gaussian components, and the red solid lines indicate the combined model profiles obtained from the Gaussian fitting. When the spectra are fitted by two Gaussian components (Section 3.1), each component is indicated by a blue dotted line. The artifact at V$_{LSR}$${\sim}$ 90--100 km s$^{-1}$ in the He I spectrum of slit 1, marked with a horizontal bar with vertical end caps, was masked during the Gaussian fitting.
• Figure 5: Observed Brackett-line intensities relative to Br$\gamma$ in the IGRINS spectra of the two slit positions. The dotted lines show the Case B theoretical predictions for $T_{e}=10^{4}$ K and $n_{e}=10^{4}$ cm$^{-3}$Hummer+87, shown for a range of extinctions.