A Giant Arc on the Sky (2201.06875v2)

Abstract: We present the serendipitous discovery of a Giant Arc on the Sky' at $z \sim 0.8$. The Giant Arc (GA) spans $\sim 1$ Gpc (proper size, present epoch), and appears to be almost symmetrical on the sky. It was discovered via intervening MgII absorbers in the spectra of background quasars, using the catalogues of Zhu \& M\'enard. The use of MgII absorbers represents a new approach to the investigation of large-scale structures (LSSs) at redshifts $0.45 \lesssim z \lesssim 2.25$. We present the observational properties of the GA, and we assess it statistically using methods based on: (i) single-linkage hierarchical clustering ($\sim 4.5\sigma$); (ii) the Cuzick-Edwards test ($\sim 3.0\sigma$); and (iii) power spectrum analysis ($\sim 4.8\sigma$). Each of these methods has distinctive attributes and powers, and we advise considering the evidence from the ensemble. We discuss our approaches to mitigating any {\it post-hoc} aspects of analysing significance after discovery. The overdensity of the GA is $\delta \rho / \rho \sim 1.3 \pm 0.3$. The GA is the newest and one of the largest of a steadily accumulating set of very large LSSs that may (cautiously) challenge the Cosmological Principle, upon which thestandard model' of cosmology is founded. Conceivably, the GA is the precursor of a structure like the Sloan Great Wall (but the GA is about twice the size), seen when the Universe was about half its present age.

• The paper reports the discovery of the Giant Arc, a 1 Gpc large-scale structure at z ~0.8, offering new insights into cosmic inhomogeneities.
• It utilizes Mg II absorption in quasar spectra combined with SLHC, CE test, and PSA methods, achieving statistical significances up to 4.8σ.
• The findings challenge the Cosmological Principle and promote further studies with expanded quasar surveys and improved large-scale structure analyses.

Overview of "A Giant Arc on the Sky"

The paper "A Giant Arc on the Sky," primarily authored by Alexia M. Lopez and colleagues, reports the discovery of an extensive large-scale structure (LSS) named the Giant Arc (GA), located at redshift $z \sim 0.8$. This Giant Arc extends over approximately 1 Gpc in proper size at the present epoch, representing one of the largest cosmic structures identified to date.

Discovery and Methodology

The discovery of the GA occurred serendipitously through the investigation of intervening Mg II absorbers in the spectra of background quasars, utilizing the Mg II doublet line as a tracer for metal-rich gas associated with galaxies. This method offers advantages for studying LSS over substantial redshift ranges ($0.45 \la z \la 2.25$) due to the strength and detectability of the Mg II absorption line, which correlates with properties of galaxy halos.

To assess the significance of the GA in terms of LSS, Lopez et al. employed several statistical methods, including single-linkage hierarchical clustering (SLHC), the Cuzick-Edwards (CE) test, and 2D power spectrum analysis (PSA). The application of these techniques indicated statistical significances of $\sim 4.5\sigma$ for SLHC, $\sim 3.0\sigma$ based on the CE test, and $\sim 4.8\sigma$ for the PSA.

Observational Properties and Implications

The observational analysis of the GA reveals a somewhat symmetrical crescent shape on the sky. However, the paper explores finer observational characteristics, highlighting slight asymmetries in equivalent width and redshift distribution across the arc. Notably, the overdensity of the GA is evaluated as $\delta \rho / \rho \sim 1.3 \pm 0.3$, placing it in line with other prominent cosmos-spanning structures.

The existence of such large structures potentially challenges the Cosmological Principle (CP), which postulates that the universe is homogeneous and isotropic on large scales. The GA, among other LSSs exceeding theoretical homogeneity scales, suggests possible avenues for re-evaluating certain cosmological models or understanding the universe's early structural formation.

Independent Corroboration and Future Directions

Lopez et al. also cross-referenced the GA with independent data, utilizing quasars from the SDSS DR16Q database and DESI galaxy clusters to corroborate its existence through alternative cosmic tracers. The quasi-independent corroboration from quasars bolsters the validity of the structure as a physical entity, independent of bias from Mg II detections alone.

Moving forward, this discovery emphasizes developing more sophisticated analyses to handle the complexities associated with LSS identification. The paper hints at future goals, including expanding the Mg II database and creating new catalogues from subsequent quasar surveys, providing a comprehensive exploration of other possible large LSSs and their implications for cosmic evolution and the CP. The exploration of how similar structures could evolve into prominent features such as the Sloan Great Wall offers intriguing prospects for the universe's dynamic history.

In summary, the identification of the GA and its further analysis points to significant inquiries in cosmology, especially regarding the limits of structural homogeneity and the early universe's formative mechanisms. This research steady advance in cosmological understanding, urging a nuanced view on the universe's large-scale spatial uniformity.