Abell 697 BCG

Abell 697 BCG (short for Abell 697 Brightest Cluster Galaxy), also known as PGC 2079433, is a massive type-cD elliptical galaxy residing as the brightest cluster galaxy in Abell 697 galaxy cluster. It is located in the constellation of Lynx and has a redshift of 0.28, meaning the galaxy is located 3.5 billion light-years away from Earth.

Characteristics
Abell 697 BCG is one of the largest galaxies with a diameter of 885,200 light-years and a central velocity distribution of between the ranges of ә ~ 300–400 km s−1. Such of these velocity distributions are measured as σcl and σ*,BCGs in which the relation found is tight. The σ*,BCG/σcl ratio decreases as a function of σcl indicating its formation is efficient in lower mass halos.

The galaxy is optically luminous and such classfied as a low-excitation radio galaxy with a 1.4 GHz luminosity range that is between 2 × 1023 and 3 × 1025 W Hz−1. Moreover, Abell 697 BCG is a fossil group galaxy that is on average fainter and has a radio-loud (L 1.4 GHz > 1023 W Hz−1) active galactic nucleus. It also contains an extended stellar envelope with an excessive light profile reaching over several hundred kilometers. The light profile is described by a de Vaucouleurs surface brightness law, Ѱ(r) α r1/4, over a large range in its radius and has a massive black hole in its center. Despite offsets from the cluster's X-ray center, it is aligned with its cluster mass distribution.

Researchers theorized Abell 697 BCG was formed through major mergers. As the galaxies of various self-gravitating disks and halo types collide with one after another, dynamical friction along with tidal forces converts orbital kinetic energy into random energy. This allows them merge into an amorphous, triaxial system which resembles an elliptical galaxy like Abell 697 BCG and at the same time, the stellar mass of the central galaxy is built up. According to optical observations presented by researchers from Keck Observatory, Abell 697 BCG has since undergone another recent merger given it shows an asymmetric disturbed structure in cD halo.

The star formation in Abell 697 is estimated to be (SFR). 22+6.2 -5.3% according to researchers who detected it in the far-infrared analysis, with SFR = 1-150 M ⊙ yr−1.

Dark matter halo
The dark matter halo in the galaxy cluster is highly elongated with the mean ellipticity of 0.482 ± 0.028. According to researchers, the position angle of major axis of the dark matter halo and Abell 697 BCG is aligned together with the mean value of alignment angle of 22.2 ± 3.9 deg. However it is more elongated compared to the galaxy as the mean difference of the ellipticity is 0.11 ± 0.03. This research was conducted through sample of 39 massive clusters from Hubble Frontier Field (HFF), Cluster Lensing And Supernova survey with Hubble (CLASH), and Reionization Lensing Cluster Survey (RELICS).

Abell 697
The galaxy cluster where Abell 697 BCG is residing in, is found to be a massive and rich cluster at z=0.282, obtained by M <R_max 0.75 Mpc h−1 = 9.5 x 1014 solar masses of h−1 and M <R_vir = 3.85 Mpc h−1 = 4.5x1015 solar masses h^-1 for the region sampled through spectroscopic data. The cluster is known to have active galactic nucleus activity varying between its cluster center and outskirts. This is caused by ram pressure removing gas from other galaxies infalling into Abell 697 or attributed by increased frequency of galaxy mergers, given several galaxies display disturbed morphologies.

The intracluster medium in Abell 697 is hot, with its X-ray temperature inferring from a broadband (0.7-7.0 keV) spectrum and a hard-band (2.0-7.0 keV). The emission of the intracluster medium is estimated to be measured >=10-30% for its primary hot component while its cool component is TX = 0.5-3.0 keV. Moreover, the cluster contains a magnetic field with strength of 5−10 μG and power-law indices found between n = 1 and n = 4.

Not only to mention, Abell 697 is X-ray luminous, and not fully relaxed as observed by further investigations. This indicates the cluster is undergoing a complex cluster merger. The total radio spectrum of the halo in Abell 697 is found steep with a value of α~325 MHz~1.4 GHz ≈ 1.7-1.8. and further classified as an Ultra Steep Spectrum Radio Halo (USSRH). This indicates steep spectrum halos in merging clusters are caused by emitting electrons accelerated by turbulence. In addition, Abell 697 contains luminous infrared galaxies, showing rapid molecular gas exhaustion, effective in quenching star formations.

The cluster is also known to have a gravitational lens. This provides opportunities for researchers to investigate the mass distribution at the cluster's core and study galaxies located at high redshift. Using a strong lensing model, they found the lensing strength of Abell 697 has an area of ∣μ∣ ≥ 3 for z s = 9, normalized to a lens redshift of z = 0.5. Although there is a possible trend between the lensing strength and large-scale mass of Kendall τ = 0.26 and Spearman r = 0.36, the inner slope measuring 50 kpc ≤ r ≤ 200 kpc of the projected mass-density profile has a higher probability of correlated with its lensing strength. This can set an upper bound on the lensing strength of a cluster. Given the correlation of the effective Einstein area and large ( ≳ 30.″0) radial extent of lensing evidence, the gravitational lens in Abell 697 is powerful since it was able to lens a high-redshift J 125 ≃ 25.7 galaxy candidate, which a single emission-line is detected through Gemini-N/GMOS spectroscopic observations. Such results can help researchers design future observations, more effective, through they can better estimate using clusters as cosmic telescopes.