Unveiling a new structure behind the Milky Way

Abstract: Context. The ZOA does not allow clear optical observations of extragalactic sources behind the Milky Way due to the meaningful extinction of the optical emission of these objects. The observations in NIR wavelengths represent a potential source of astronomical discoveries supporting the detection of new galaxies, completing the picture of the large scale structure in this still little explored area of the sky. Aims. Our aim is to decipher the nature of the overdensity located behind the Milky Way, in the tile b204 of the VVV survey. Methods. We studied an area of six arcmin around a galaxy concentration located at l = 354.82{\deg} and b = -9.81{\deg}. We selected five galaxies taking into account the source distribution on the sky, in order 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 have fitted 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 the confirmation of the nature of this structure at z = 0.225 +- 0.014, unveiling a new galaxy cluster, VVVGCl-B J181435-381432, behind the Milky Way bulge.

• The paper demonstrates the discovery of a hidden galaxy cluster with a mean spectroscopic redshift of 0.225±0.014 using near-infrared observations.
• The study employs Gemini South's Flamingos 2 spectroscopy, VVV survey photometry, red-sequence analysis, and photometric redshift estimation for robust detection.
• The findings underscore the potential of multi-wavelength strategies in mapping obscured cosmic structures, enhancing our understanding of large-scale universe formation.

Overview of "Unveiling a New Extragalactic Structure Hidden by the Milky Way"

The paper "Unveiling a new extragalactic structure hidden by the Milky Way," authored by Galdeano et al., presents a comprehensive study aimed at uncovering a hidden extragalactic structure located behind the Milky Way, specifically in the Zone of Avoidance (ZOA) where optical observations are severely limited due to significant extinction caused by the Galactic dust. This research leverages near-infrared (NIR) observations, which are less affected by dust, enabling the detection of new galaxies and offering a more complete picture of the large-scale structure in these obscured regions of the sky.

Methods and Data

The study focuses on an area with a notable galaxy concentration in the VISTA Variables in the Via Lactea (VVV) survey's tile b204. The researchers utilized the Gemini South 8.1-meter telescope's Flamingos 2 long-slit spectrograph to obtain NIR spectra for a sample of five galaxies chosen based on their spatial distribution and photometric properties, thus optimizing observation time. The spectral data were analyzed with the Starlight code and the IRTF stellar library to identify absorption features indicative of underlying galaxy spectra.

To fortify the spectroscopic findings, the study also employed complementary photometric methods. These included red-sequence analysis, which characterizes the color-magnitude relationship typical of galaxy clusters, and photometric redshift estimation using the EAZY code to assess the redshift distribution of the galaxies within the detected overdensity.

Results

The mean spectroscopic redshift derived from the NIR spectra for the studied galaxies is reported as $z=0.225 \pm 0.014$. This measure aligns well with the photometric analysis, which indicates an average photometric redshift $photoz=0.21 \pm 0.08$. Additionally, the derived red-sequence slope is consistent with expectations for NIR observations of galaxy clusters, supporting the identification of a galaxy cluster, VVVGCl-B J181435-381432, behind the Milky Way bulge.

Discussion and Implications

The discovery and confirmation of an extragalactic structure in the ZOA provide critical insights into the distribution and clustering of galaxies in otherwise unexplored regions, advancing our understanding of the universe's large-scale structures. The ability to detect such structures using NIR observations demonstrates the potential of these methods to explore other obscured and under-studied portions of the sky.

The implications of these findings are both practical and theoretical. Practically, this strengthens the case for employing multi-wavelength surveys to uncover hidden cosmic structures, which could have significant repercussions for our mapping of the universe. Theoretically, it provides empirical data that could refine models of cosmic structure formation and evolution in regions where gravitational interactions with massive hidden structures might influence galaxy dynamics and distribution.

Future Directions

This research exemplifies the necessity for synergistic observational strategies combining different wavelengths and methodologies to probe cosmic structures. Future developments could focus on expanding surveys like VVV using NIR capabilities and combining these with other data, such as X-ray or radio observations, to enhance the detection and characterization of similar hidden structures. Furthermore, increasing spectroscopic samples will improve parameter estimates like velocity dispersion and cluster mass, contributing to a more detailed cosmological understanding.