High energy gamma-ray sources in the VVV survey - III. Spectroscopic confirmation of an AGN at low Galactic latitudes

TL;DR

The paper reports the spectroscopic confirmation of VVV-J181258.71-314346.7 as an AGN behind the Galactic bulge and a counterpart to the unassociated gamma-ray source 4FGLJ1812.8-3144. Using near-infrared spectroscopy with Flamingos-2 on Gemini South, covering 1.1–1.8 μm at $R \sim 1200$, the authors detect Paβ and Fe II emission, yielding a robust redshift of $z = 0.206 \pm 0.001$ and indicating a type-1 AGN with broad-line region gas of roughly $V \sim 1600$–$2400$ km s$^{-1}$. The line diagnostics and broad widths support Seyfert-like activity, affirming the association with the gamma-ray source and the AGN classification. This work demonstrates that near-infrared spectroscopy can penetrate extinction and confusion in the zone of avoidance, enabling a census of AGNs and constraints on unassociated gamma-ray sources in the Galactic plane, and it motivates extending this approach to additional VVV candidates.

Abstract

We aim to spectroscopically confirm the nature of VVV-J181258.71-314346.7, a candidate counterpart to the unassociated gamma-ray source 4FGLJ1812.8-3144. This object was selected based on its near-infrared photometric properties and moderate variability, as part of a broader effort to identify active galactic nuclei (AGN) behind the Galactic bulge and disc. We obtained near-infrared spectra using the Flamingos-2 instrument at Gemini South, covering the $1.1 $--$ 1.8~μ$m range with a spectral resolution of $R \sim 1200$. Standard data reduction procedures were applied, including telluric correction and wavelength calibration. The analysis focused on the identification of emission lines and the estimation of the redshift using cross-correlation techniques and spectral template fitting. Despite a relatively low signal-to-noise ratio, the spectrum reveals the presence of Pa$β$ and Fe\,\textsc{ii} emission lines. The measured redshift is $z = 0.206 \pm 0.001$, which confirms the extragalactic nature of the source. The spectral features such as line ratios and full width at half maximum are consistent with those typically observed in type-1 AGNs, particularly Seyfert 1 galaxies. This study demonstrates the ability of near-infrared spectroscopy to reveal AGNs that are obscured by highly extincted and crowded galactic fields. The confirmation of an AGN at low Galactic latitude ($b\sim -6.5$°) shows that near-IR surveys like VVV can successfully penetrate the zone of avoidance. Extending this approach to additional candidates is crucial for improving the census of AGNs hidden behind the Milky Way, as well as for constraining the population of unassociated gamma-ray sources in these troublesome regions.

Figures (4)

• Figure 1: $K_s$ (red), $H$ (green), and $J$ (blue) colour composite image of the studied object, VVV-J181258.71-314346.7. The field-of-view covers 64 arcsec on a side and the orientation is indicated in the bottom-right corner.
• Figure 2: The top panel displays the spectrum of the studied object VVV-J181258.71-314346.7, while the bottom panel shows the spectrum of the telluric star HD-169044. Flux is presented in arbitrary units, and wavelength is given in Angstroms.
• Figure 3: Telluric-corrected spectrum of the observed object, VVV-J181258.71-314346.7 (upper panel). The shaded gray regions indicate wavelengths affected by telluric absorption features. Flux is presented in arbitrary units, and wavelength is given in Angstroms. The normalized transmission spectrum used for the correction is shown in the lower panel, with a horizontal dashed line at transmission = 1 for reference.
• Figure 4: Zoomed rest-frame spectrum considering the calculated redshift of $z = 0.206$. The black curve shows the observed telluric-corrected spectrum, while the red curve corresponds to the synthetic template used for the cross-correlation. Vertical lines indicate the central wavelengths of the identified emission lines (Fe ii$\lambda12570$, Pa$\beta\lambda12822$, and Fe ii$\lambda13209$) in the rest frame. Horizontal error bars represent the uncertainty in the redshift determination ($\pm 0.001$). The shaded gray regions indicate wavelengths affected by telluric absorption features and are the same as in the previous figure (see Sect. \ref{['sec:observations']}). Flux is in arbitrary units and wavelength is given in Angstroms.