Optical observations and atomic environment of supernova remnant G25.1-2.3

Abstract

The supernova remnant (SNR) G25.1-2.3 was identified in the radio band during the Sino-German $λ$6 cm survey of the Galactic plane. We present a detailed investigation of the optical, HI, and CO emission towards the G25.1-2.3 to better understand its characteristics and environment. In this study, optical spectroscopic data of the remnant and its environment have been analysed for the first time, providing new insights into their emission properties. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and 1.5-m Russian-Turkish Telescope (RTT150) data show variations across the observed regions, with [SII]/H$α$ ranging from 0.16 to 0.83. We identified shock-heated gas in the northern and southern regions and several photoionized regions around the SNR based on their [SII]/H$α$ ratios derived from spectra. The [SII]$λ$6716/$λ$6731 ratio observed in the northern region suggests electron densities ($n_{\rm e}$) ranging from 120 to 1030 cm$^{-3}$, whereas the southern regions show higher values, between 490 and 4500 cm$^{-3}$. The variations in the observed H$α$/H$β$ ratios indicate significant differences in extinction across the regions. H$α$ images obtained using the 1-m Turkish Telescope (T100) reveal optical emission in the northern and southern, characterized by filamentary and diffuse structures. We newly found a hole-like distribution of HI, whose spatial extent is roughly consistent with the diameter of the SNR. Based on radio data, we examine the evolutionary stage of G25.1-2.3 using the surface brightness-diameter ($Σ-D$) relation and the equipartition method.

Figures (13)

• Figure 1: The $\lambda$11 cm radio continuum image Re90 of the area around G25.1$-$2.3. The radio continuum contours scale linearly from 60 to 488 mJy beam$^{-1}$. We show five regions (S1, S2, S3, NW, and N) observed with the 1-m T100 telescope, marked with boxes.
• Figure 2: The continuum-corrected SHASSA Ga01 H$\alpha$ image ($2.6 \times 2.6$ deg$^{2}$) of the SNR's vicinity, highlighting the radio size of the SNR (white box; $80 \times 30$ arcmin$^{2}$) from Ga11, the regions observed by the 1-m T100 telescope (brown boxes; $21 \times 21$ arcmin$^{2}$), and the locations where the LAMOST and RTT150 spectra were extracted, marked with black and red crosses, respectively. The overlaid $\lambda$11 cm radio continuum contours Re90 scale linearly from 60 to 488 mJy beam$^{-1}$.
• Figure 3: The continuum-subtracted H$\alpha$ images for S1, S2, S3, NW, and N regions with the 1-m T100 telescope, starting from the top left, respectively. The bottom-right panel shows a continuum-subtracted H$\alpha$ image focusing on the filament located in the S3 region, obtained with the 1.5-m RTT150 telescope. The slit positions from the RTT150 spectroscopic observations are shown as crosses, and their central coordinates are listed in Table \ref{['tab:Table2']}.
• Figure 4: Top panel: Example spectrum including the H$\alpha$ stellar absorption feature. Bottom panel: Residual spectrum after Gaussian fit. The yellow line shows the Gaussian fit to the stellar absorption. Pink areas refer to fitted sampling regions.
• Figure 5: The LAMOST spectra ($6540-6600$ and $6710-6750$ Å) for the P1, P2, P3, and P4 positions. The residual data in the upper right corner refer to the spectrum after the stellar line is removed (see Fig. \ref{['fig:figure3a']}). The original data refer to the spectrum with detrending only. This is used in the same sense in all figures. All the spectra in these images and the ones that follow are presented in a clockwise order, starting from the top-left.