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Open Access
Issue
A&A
Volume 700, August 2025
Article Number L4
Number of page(s) 4
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202555315
Published online 01 August 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1. Introduction

Narrow emission lines from central narrow emission line regions (NLRs) are fundamental optical spectroscopic characteristics of emission line galaxies, such as active galactic nuclei (AGN) and HII galaxies. Moreover, among emission line galaxies, there is one unique subclass: the emission line galaxies with double-peaked narrow emission lines (DPN galaxies). Since the first DPN galaxy reported in Zhou et al. (2004), more 10000 DPN galaxies have been reported, such as the samples in Comerford et al. (2009), Wang et al. (2009, 2019), Smith et al. (2010), Ge et al. (2012), Maschmann et al. (2020), Zheng et al. (2025).

In order to explain the double-peaked narrow emission lines (DPNELs), two main scenarios have been proposed. First, two independent NLRs related to two galaxies merging as a dual galaxy system (DGS) can normally lead to DPNELs, due to the orbital motions of the two galaxies, as natural products of hierarchical galaxy formation and evolution (Silk & Rees 1998; Cole et al. 2000; Volonteri et al. 2003; Merritt 2006; De Lucia & Blaizot 2007; Satyapal et al. 2014; Zoldan et al. 2019; Yoon et al. 2022; Attard et al. 2024). Second, local dynamic structures (such as radial flows, rotating disks) in NLRs in individual galaxy can also lead to DPNELs (see the discussions in Shen et al. 2011; Fu et al. 2012; Nevin et al. 2016; Comerford et al. 2018; Rubinur et al. 2019).

It is clear that not all the DPN galaxies can be accepted as indicators of DGSs, but it is not clear that the proportion of DPN galaxies are not related to DGSs. Here, an independent method is proposed to determine the proportion of DPN galaxies not related to DGSs. If we assume that DPNELs are related to DGSs, the corresponding spatial separation of the two galaxies in one DGS should be long enough that narrow-line emission properties are self-governed by each individual galaxy in the DGS, otherwise more complicated profiles would be expected rather than apparent double-peaked features. In other words, very weak intrinsic connections could be expected between narrow emission lines from the two galaxies in one DGS leading to DPNELs. Therefore, checking the emission property connections between the redshifted components and the blueshifted components in DPNELs will provide meaningful clues to support or disprove that the DPNELs are related to the proposed DGSs, which is our main objective.

The manuscript is organized as follows. Section 2 presents the main results and necessary discussions on tight flux correlations between the redshifted components and the blueshifted components in DPNELs. The main summary and our conclusions are given in Section 3. In the manuscript, we have adopted the cosmological parameters of H0 = 70 km s−1 Mpc−1, Ωm = 0.3, and ΩΛ = 0.7.

2. Main results and necessary discussions

The DPN galaxies collected from the public sample reported in Ge et al. (2012) were classified into five groups according to the properties of the narrow and broad emission lines. In Ge et al. (2012), through the spectra of the main galaxies in Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) after removing the host galaxy contributions determined by STARLIGHT, the emission lines were measured by multiple Gaussian functions. After checking the best-fitting results to the emission lines by the F-test technique, about 3030 DPN galaxies were collected with apparent DPNELs with significance levels higher than 3σ. Moreover, we did not consider the more than 10000 galaxies collected with asymmetric or top-flat profiles of emission lines in Ge et al. (2012). Meanwhile, in order to check the probable correlation between RB (flux rations of the blueshifted components in DPNELs) and RR (flux rations of the redshifted components in DPNELs), the 1618 of 3030 galaxies were collected from Ge et al. (2012) with the measured line fluxes of the blue- and redshifted emission components in [O III] and in narrow Hα at least five times larger than their corresponding uncertainties. Among the 1618 galaxies, considering the appearance of broad Hα, the first group (G1) includes 56 Type 1 AGN. For the other 1562 galaxies, assuming DGSs leading to the DPNELs, the measured emission fluxes of the blueshifted and the redshifted components in the DPNELs could be applied to determine the classifications of the two individual galaxies in the assumed DGS through the commonly applied BPT diagrams (Baldwin et al. 1981; Kewley et al. 2001, 2006, 2019; Kauffmann et al. 2003; Zhang et al. 2020). Then, the second group (G2) includes 717 galaxies with each individual galaxy classified as a Type 2 AGN. The third group (G3) includes 260 galaxies with one individual galaxy classified as a Type 2 AGN, but the other individual galaxy classified as an HII galaxy. The fourth group (G4) includes 492 galaxies with each individual galaxy classified as an HII galaxy. The fifth group (G5) includes 93 galaxies with one individual galaxy not classified by the BPT diagrams, due to lack of emission components of some narrow emission lines.

Through the reported emission line fluxes of the redshifted and blueshifted components in both narrow [O III] and narrow Hα for the 1618 galaxies included in the five groups, Fig. 1 shows the correlations between RB (flux ratio of the blueshifted component in [O III] to the blueshifted component in narrow Hα) and RR (flux ratio of the redshifted component in [O III] to the redshifted component in narrow Hα). For all 1618 galaxies, the Spearman’s rank correlation coefficient is about 0.65 (Pnull < 10−21). Meanwhile, for the galaxies in G1, G2, G3, G4, and G5, the corresponding Spearman’s rank correlation coefficients are about 0.90 (Pnull ∼ 1.2 × 10−21), 0.88 (Pnull < 10−21), 0.31 (Pnull ∼ 6 × 10−7), 0.63 (Pnull < 10−21), and 0.48 (Pnull ∼ 1.5 × 10−6), respectively.

thumbnail Fig. 1.

Correlations between RR and RB for the SDSS objects with apparent DPNELs in the five groups. In each panel, the solid red line and the dashed red lines show the best-fitting results and the corresponding 3σ confidence bands. In each panel, the open red circles mark the galaxies with larger Rps.

Furthermore, considering the uncertainties in the two coordinates, through the least trimmed squares regression technique (Cappellari et al. 2013; Mahdi & Mohammad 2017), the linear correlations shown in Fig. 1 can be described as

log ( R R ) = 0.10 ± 0.01 + ( 0.82 ± 0.02 ) log ( R B ) ( ALL ) log ( R R ) = 0.12 ± 0.03 + ( 0.86 ± 0.07 ) log ( R B ) ( G 1 ) log ( R R ) = 0.09 ± 0.02 + ( 0.78 ± 0.02 ) log ( R B ) ( G 2 ) log ( R R ) = 0.01 ± 0.09 + ( 0.99 ± 0.16 ) log ( R B ) ( G 3 ) log ( R R ) = 0.06 ± 0.01 + ( 0.86 ± 0.05 ) log ( R B ) ( G 4 ) log ( R R ) = 0.94 ± 1.65 + ( 1.82 ± 1.71 ) log ( R B ) ( G 5 ) $$ \begin{aligned}&\log (R_{R})=0.10\pm 0.01+(0.82\pm 0.02)\log (R_{B}) \ \ \ (\mathrm{ALL})\nonumber \\&\log (R_{R})=0.12\pm 0.03+(0.86\pm 0.07)\log (R_{B}) \ \ \ (\mathrm{G1})\nonumber \\&\log (R_{R})=0.09\pm 0.02+(0.78\pm 0.02)\log (R_{B}) \ \ \ (\mathrm{G2})\nonumber \\&\log (R_{R})=0.01\pm 0.09+(0.99\pm 0.16)\log (R_{B}) \ \ \ (\mathrm{G3})\nonumber \\&\log (R_{R})=0.06\pm 0.01+(0.86\pm 0.05)\log (R_{B}) \ \ \ (\mathrm{G4})\nonumber \\&\log (R_{R})=0.94\pm 1.65+(1.82\pm 1.71)\log (R_{B}) \ \ \ (\mathrm{G5}) \end{aligned} $$(1)

with corresponding 1RMS scatters of about 0.269, 0.276, 0.266, 0.328, 0.201, and 0.436, respectively. It is clear that except for the objects in G5, the determined best-fitting results and the determined Spearman’s rank correlation coefficients can be applied to confirm the robust dependence of RR on RB. For the dependence of RR on RB in the galaxies in G5, although its scatter is about two times larger than those of the dependences of the objects in the other groups and although the determined slope 1.82 is not apparently larger than its corresponding uncertainty 1.71, the Spearman’s rank correlation of about 0.48 can be accepted to support the dependence of RR on RB in the objects in G5. In other words, the dependence of RR on RB is common in the DPN galaxies collected from Ge et al. (2012). However, the linear dependence cannot be expected by the DGSs.

Before proceeding further, it was necessary to check whether there was flux connection between narrow emission lines in two nearby galaxies. In SDSS DR16 (Ahumada et al. 2020), there are 77982 narrow emission line galaxies collected with redshift lower than 0.35, and with the measured emission line fluxes at least five times higher than their uncertainties in the [O III]λ5007 Å and in narrow Hα, and with the median spectral signal-to-noise ratio higher than 10, and with the SDSS pipeline provided subclasses of AGN, STARFORMING, or STARBURST, and with the flux ratio of narrow Hα to narrow Hβ lower than 6. Then, among the collected 77982 narrow emission line galaxies, there are about 13000 Type 2 AGN. Among the 13000 Type 2 AGN, there are 669 couples of AGN (AGN pairs) with redshift differences smaller than 0.001 and with position distances smaller than 30 arcmin. Here, if there is more than one AGN near the target AGN within a position distance smaller than 30 arcmin, the one with the smallest position distance is accepted as the companion AGN to the target AGN. The top panel of Fig. 2 shows the flux correlations in [O III]λ5007 Å and in narrow Hα in the 669 AGN pairs, leading to the corresponding Spearman’s rank correlation coefficients of about −0.01 (Pnull ∼ 78%) and 0.06 (Pnull ∼ 13%) for the correlations on fluxes of [O III]λ5007 Å and on fluxes of narrow Hα, respectively. The bottom panel of Fig. 2 shows the dependence of the flux ratios of [O III]λ5007 Å and of narrow Hα on the position distances in the 669 AGN pairs, leading to the corresponding Spearman’s rank correlation coefficients about 0.03 (Pnull ∼ 40%) and −0.002 (Pnull ∼ 97%) for the correlations on fluxes of [O III]λ5007 Å and on fluxes of narrow Hα, respectively. Therefore, through the collected 669 AGN pairs, there are no narrow emission line flux correlations, nor dependence of narrow emission line flux ratios on position distances.

thumbnail Fig. 2.

Top panel: Flux correlations of narrow emission lines of both [O III]λ5007 Å (in blue) and narrow Hα (in red) in the 669 AGN pairs with a redshift difference smaller than 0.001 and with a position distance smaller than 30 arcmin. Bottom panel: Dependences of narrow emission line flux ratios on position distance. In the bottom panel, the symbols in blue and red show the flux ratios respectively of [O III]λ5007 Å and of narrow Hα in the 669 AGN pairs.

Meanwhile, among the 77982 narrow emission line galaxies in SDSS DR16, based on the same criteria of redshift difference smaller than 0.001 and position distance smaller than 30 arcmin, there are about 3299 pairs of a Type 2 AGN plus a HII galaxy, and 11442 pairs of HII galaxies. The corresponding Spearman’s rank correlation coefficients are not larger than 0.1 for the narrow emission line flux correlations nor for the dependences of narrow emission line flux ratios on position distances. Moreover, a criterion on position distances smaller than 20 arcmin (10 arcmin) have also been applied, leading to smaller number of pairs of narrow emission line galaxies, but leading to the similar conclusions that the corresponding Spearman’s rank correlation coefficients are not larger than 0.1. Here, in addition to the results shown in Fig. 2 for the pairs of Type 2 AGN, there are no plots for correlations and dependences for the other kinds of pairs of narrow emission line galaxies.

Furthermore, as known, in order to detect apparent double-peaked features in narrow emission lines, the peak intensities of each DPNEL should be not very different. Therefore, it is necessary to check the probable effects of the peak intensity ratio on the results shown in Fig. 1. Assuming that the intensity ratio between 0.1 and 10 of two peaks in one DPNEL could lead to detectable double-peaked features, and following the distributions of the flux ratios Rat of [O III] to narrow Hα of the collected narrow emission line galaxies shown in the left panel of Fig. 3 (for AGN, for HII galaxies, and for all the narrow emission line galaxies), the total intensities (flux in arbitrary units) of [O III] and narrow Hα were artificially set to be Rat and 1. Then, random values R3 and Rα were collected from uniform distributions (with 1 as the mean value and [0.1, 10] as the minimum and maximum values) for the intensity ratio of the redshifted component to the blueshifted component in the [O III] and in the narrow Hα, leading the simulated fluxes of the redshifted and blueshifted components in [O III] and narrow Hα to be Rat × R 3 1 + R 3 $ \mathrm{Rat}\times\frac{R_3}{1+R_3} $, Rat × 1 1 + R 3 $ \mathrm{Rat}\times\frac{1}{1+R_3} $, 1 × R α 1 + R α $ 1\times\frac{R_\alpha}{1+R_\alpha} $ and 1 × 1 1 + R α $ 1\times\frac{1}{1+R_\alpha} $. Then, the corresponding Spearman’s rank correlation coefficients could be determined between the simulated RR and RB. We repeated the procedure above 500 times; the distributions of the coefficients are shown in the middle panel of Fig. 3. Meanwhile, if random values of R3 and Rα are not from uniform distributions, but from Gaussian distributions (with 0 as the mean value and 0.5 as the second moment in logarithmic space), the corresponding distributions of correlation coefficients are shown in the right panel of Fig. 3. It is clear that the peak intensity ratios can lead to simulated Spearman’s rank correlation coefficients not larger than 0.3. Therefore, the shown results in Fig. 1 are not due to the narrow range of peak intensity ratios.

thumbnail Fig. 3.

Left panel: Distributions of flux ratio of [O III]λ5007 Å to narrow Hα for the AGN (histogram filled with red lines), the HII galaxies (histogram filled with dark green lines), and all the narrow emission line galaxies (histogram filled with blue lines). Middle panel and right panel: Determined distributions of the Spearman rank correlation coefficient, after accepting the uniform distributions (with 1 as the mean value and [0.1, 10] as the minimum and maximum values) and the Gaussian distributions (with 1 as the mean value and 0.5 as the second moment) for the peak intensity ratios.

Once we accept the robust conclusion that there are no correlations between narrow emission line fluxes nor dependences of narrow emission line flux ratios on position distances in pairs of narrow emission line galaxies, the DGS is not the preferred scenario to explain the results in Fig. 1. However, we expect that the redshifted components and the blueshifted components in DPNELs should have similar physical dynamical environments. Therefore, the results in Fig. 1 can be applied to roughly determine the proportion of DPN galaxies that have their DPNELs not related to DGSs.

Based on all the NG (=669+3299+11442) pairs of narrow-line galaxies in SDSS DR16, NR (≤NG) nonrepeating pairs are randomly collected. For each selected pair, the fluxes in narrow emission lines are re-set to equal values. Then, the corresponding Spearman’s rank correlation coefficient can be estimated through the newly set emission line fluxes in narrow emission lines in the NG pairs. We repeated the procedure above about 2000 times with random values of NR; Fig. 4 shows the determined dependence of Spearman’s rank correlation on the number ratio of NR to NG. In order to find the strong linear dependence with correlation coefficient about 0.65 for all the DPN galaxies, at least 65.5% of the DPN galaxies have their DPNELs not related to the proposed DGSs. Moreover, different NG = 1618 (the number of DPN galaxies in Ge et al. 2012) was also applied, and lead to similar results, which indicates that there are few effects of different NG on the results shown in Fig. 4. Furthermore, when individually considering the DPN galaxies in G1, G2, G4, G3, and G5, the corresponding number ratios are about 91.5%, 88.3%, 62.5%, 47.8% and 30.1%, respectively.

thumbnail Fig. 4.

Dependence of Spearman’s rank correlation coefficient on the number ratio of NR/NG. The symbols in blue and in dark green show the results through all the galaxy pairs and through the randomly collected 1618 galaxy pairs. The horizontal red solid line marks the coefficient to be 0.65 for all the DPN galaxies, and the horizontal red dashed lines from top to bottom show the coefficients to be 0.90, 0.88, 0.63, 0.48, and 0.31 for the galaxies in G1, G2, G4, G3, and G5, respectively.

Before ending the section, we note three additional points. First, there are different Spearman’s rank correlation coefficients for the DPN galaxies in the five groups; however, we have no clear points on the physical origins for the different coefficients and then for the very different expected number ratios of NR to NG. Probably, the physical dynamical environments have more apparent effects on the DPNELs in AGN-related NLRs than on the DPNELs in HII related NLRs. However, the different Spearman’s rank correlation coefficients and the corresponding different number ratios of NR to NG could probably indicate larger effects of intrinsic AGN variability on the detected DPNELs. Second, the results shown in Fig. 2 are only through the pairs of nearby galaxies within 30 (20, and 10) arcmin, probably when the merger process leading to the two NLRs moving close enough, some unknown physical mechanisms could lead to intrinsic connections between the different NLR properties in the two galaxies undergoing the merge process. Third, based on a defined parameter Rps as the ratio of peak separation to the sum of the line width of the two components in each DPNEL, we checked whether more apparent DPNELs with higher Rps can lead to different results. Then, in each group, about half of the galaxies are collected by Rps higher than the mean value of Rps, and shown as open red circles in Fig. 1. The corresponding Spearman’s rank correlation coefficients between RR and RB are about 0.84 (Pnull ∼ 1.5 × 10−6), 0.87 (Pnull < 10−21), 0.40 (Pnull ∼ 2.7 × 10−5), 0.70 (Pnull < 10−21), and 0.44 (Pnull ∼ 8.9 × 10−3) for about half of the objects with higher Rps in G1, G2, G3, G4, and G5, respectively, similar to the results for all the galaxies in each group. Therefore, there are few effects of Rps on our discussed results. Studying an independent large sample of DPN galaxies in the near future could provide further clues to understanding the discussed results.

3. Conclusions

  • Through the large sample of DPNELs in SDSS, strong linear correlations can be found between RR and RB.

  • Through the collected galaxy pairs with redshift differences smaller than 0.01 and position distances smaller than 30arcmin, there are no flux connections between narrow emission lines in the different kinds of galaxy pairs.

  • Considering a narrow range of peak intensity ratios for detecting DPNELs, the estimated correlation coefficients are smaller than the coefficients for the DPNELs in SDSS.

  • The strong linear correlations between RR and RB are robust enough, strongly indicating that the commonly proposed dual galaxy system is not the preferred scenario to explain the reported DPNELs.

  • Through simple simulating results on real flux ratio distributions of [O III] to narrow Hα, more than 65.5% of the DPNELs are not related to DGSs.

  • Considering the DPN galaxies in G1, G2, G4, G3, and G5, more than 91.5%, 88.3%, 62.5%, 47.8%, and 30.1% of the DPNELs are not related to DGSs, respectively.

Acknowledgments

Zhang gratefully acknowledges the anonymous referee for giving us constructive comments and suggestions to greatly improve the paper. Zhang gratefully thanks the kind financial support from GuangXi University and the kind grant support from NSFC-12173020 and NSFC-12373014 and the Guangxi Talent Programme (Highland of Innovation Talents). This manuscript has made use of the data from the SDSS projects. The SDSS-III web site is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration.

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All Figures

thumbnail Fig. 1.

Correlations between RR and RB for the SDSS objects with apparent DPNELs in the five groups. In each panel, the solid red line and the dashed red lines show the best-fitting results and the corresponding 3σ confidence bands. In each panel, the open red circles mark the galaxies with larger Rps.
In the text
thumbnail Fig. 2.

Top panel: Flux correlations of narrow emission lines of both [O III]λ5007 Å (in blue) and narrow Hα (in red) in the 669 AGN pairs with a redshift difference smaller than 0.001 and with a position distance smaller than 30 arcmin. Bottom panel: Dependences of narrow emission line flux ratios on position distance. In the bottom panel, the symbols in blue and red show the flux ratios respectively of [O III]λ5007 Å and of narrow Hα in the 669 AGN pairs.
In the text
thumbnail Fig. 3.

Left panel: Distributions of flux ratio of [O III]λ5007 Å to narrow Hα for the AGN (histogram filled with red lines), the HII galaxies (histogram filled with dark green lines), and all the narrow emission line galaxies (histogram filled with blue lines). Middle panel and right panel: Determined distributions of the Spearman rank correlation coefficient, after accepting the uniform distributions (with 1 as the mean value and [0.1, 10] as the minimum and maximum values) and the Gaussian distributions (with 1 as the mean value and 0.5 as the second moment) for the peak intensity ratios.
In the text
thumbnail Fig. 4.

Dependence of Spearman’s rank correlation coefficient on the number ratio of NR/NG. The symbols in blue and in dark green show the results through all the galaxy pairs and through the randomly collected 1618 galaxy pairs. The horizontal red solid line marks the coefficient to be 0.65 for all the DPN galaxies, and the horizontal red dashed lines from top to bottom show the coefficients to be 0.90, 0.88, 0.63, 0.48, and 0.31 for the galaxies in G1, G2, G4, G3, and G5, respectively.
In the text

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