Unveiling a new extragalactic structure hidden by the Milky Way

¹ Departamento de Geofísica y Astronomía, CONICET, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Av. Ignacio de la Roza 590 (O), J5402DCS Rivadavia, San Juan, Argentina
e-mail: dgaldeano@unsj-cuim.edu.ar
² Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
³ Instituto de Astrofísica de La Plata, CONICET–UNLP, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
⁴ Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, 91501-970 Porto Alegre, RS, Brasil
⁵ Instituto de Astrofísica, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Fernandez Concha 700, Las Condes, Santiago, Chile
⁶ Vatican Observatory, V00120 Vatican City State, Italy

Received: 19 September 2022
Accepted: 25 October 2022

Abstract

Context. The zone of avoidance (ZoA) does not allow for clear optical observations of extragalactic sources behind the Milky Way due to the meaningful extinction of the optical emission of these objects. Observations in near-infrared (NIR) wavelengths represent a potential source of astronomical discoveries that support the detection of new galaxies and potentially complete the picture of the large-scale structures in this as-yet poorly explored area of the sky.

Aims. Our aim is to decipher the nature of the overdensity located behind the Milky Way in tile b204 of the VISTA Variables in Vía Láctea (VVV) survey.

Methods. We studied an area of six arcmin around a galaxy concentration located at l = 354.82° and b = −9.81°. We selected five galaxies, taking into account the source distribution on the sky to optimise the requested time for the observations, and we obtained the spectra with Flamingos 2 long-slit spectrograph at Gemini South 8.1-meter telescope. To identify and characterise the absorption features, we fit the galaxies underlying spectrum using the STARLIGHT code together with the IRTF stellar library. In addition, the spectroscopic findings are reinforced using complementary photometric techniques such as red-sequence and photometric redshift estimation.

Results. The mean spectroscopic redshift estimated from the NIR spectra is z = 0.225 ± 0.014. This value presents a good agreement with that obtained from photometric analysis, photoz = 0.21 ± 0.08, and the probability distribution function of the galaxies in the studied region. Also, the red-sequence slope is consistent with the one expected for NIR observations of galaxy clusters.

Conclusions. The redshifts obtained from both, photometric and spectroscopic techniques are in good agreement, allowing for the confirmation of the nature of this structure at z = 0.225 ± 0.014, thereby unveiling a new galaxy cluster, VVVGCl-B J181435-381432, behind the Milky Way bulge.