# Abell 2219 BCG

Abell 2219 BCG (short for Abell 2219 Brightest Cluster Galaxy), also known as LEDA 2285869, is a massive type-cD elliptical galaxy residing as the brightest cluster galaxy (BCG) in the Abell 2219 galaxy cluster located in constellation Hercules. At the redshift of 0.224, the galaxy is around 2.7 billion light-years from Earth.

## Properties

Abell 2219 BCG is one of the largest galaxies. Radio loud and estimated to be 713,400 light-years across in diameter, the galaxy is the most luminous object found. Distinctive from other galaxies located in the same cluster, Abell 2219 BCG is considered to have a narrow distribution of luminosity with a luminosity evolution of estimated dM/dt = 1.09 (1 + z) H0 mag yr -1, based from the Friedmann universe model. This high luminosity indicates, the galaxy was brighter during the past. It also contains an X-ray source at high-galactic latitude.
Moreover, Abell 2219 BCG is aligned together with its parent cluster with a large galactic halo, making it the most massive observed at slightly later cosmic epoch. It contains an extended intracluster light profile, with surface brightness of the ranging between 27.5 mag arcsec-2 at 100 kpc and ~32 mag arcsec-2 at 700 kpc, observed by r-bands. Light profiles tend to reach up to several hundreds of kiloparsecs.
One such possible theory for special properties of BCGs like Abell 2219 BCG, is the galactic cannibalism. As the BCG in Abell 2219, the galaxy tends to merge and engulf its neighboring and satellite galaxies through the process of dynamic friction, becoming more luminous in appearance. As it is a mass-dependent process, these galaxies merge quickly with their central galaxy. From major mergers, comes the building-up of mass in the central galaxy as well as reducing the number of massive satellite galaxies. According to Lauer who looked for evidence of interactions between the BCG and secondary galaxies, the cannibalism rate is estimated to be ~2L$_*$ per 5 × 109 yr.
The recent star formation in Abell 2219 BCG that contains a blue rest-frame ultraviolet color, has an age less than 200 million years old, in which contributes mass fractions of less than one percent. By looking at the young star population, it is said to be fueled by cooling gas flowing out of the intracluster medium.
According to researchers, Abell 2219 BCG has a hot X-ray core with multiple surface brightness edges surrounding it. These galaxy edges are likely shock and cold front origins and corresponding to Mach numbers of ~1.15. Moreover, Abell 2219 BCG is said to be ~180 kpc apart from the second brightest cluster galaxy in Abell 2219, 2MASX J16402369+4642099 or PGC 3870329, classfied a low-power radio galaxy with 1.4 GHz luminosity between 2 × 10$^{23}$ and 3 × 10$^{25}$ W Hz$^{−1}$. These galaxies are yet to cross each other.

## Abell 2219

The galaxy cluster where Abell 2219 BCG is residing, is found to be the hottest (T$_X$ = 9.5 keV) and the most X-ray luminous clusters known with luminosity of L$_X$>= 8 × 10$^{44}$ erg s$^{−1}$, 0.1-2.4 keV at z = 0.2. Twice as bright compared to the Bullet Cluster and Abell 520, it is a massive cluster. It is found to gone through a violent merger-driven shock, due to collisions by other galaxy clusters and by dissipated energy that is going into the intra-cluster medium (ICM) through the injection of turbulence and acceleration of particles.
According to optical and X-ray observations, researchers found clear evidence of a disturbed, elongated morphology for Abell 2219 and with more than one dominant mass clumps. According to the multiwavelength analysis, conducted by Boschin et al. (2004), this suggests a complex merger history in Abell 2219. The system is currently undergoing an in-fall of many clumps aligned together with a galactic filament in the foreground, oriented at an oblique angle with respect to the line of sight.
Abell 2219 is notable to contain a powerful diffuse radio halo measuring (1.99 ± 0.11)M $_{200}$ wide. The radio emission in the halo located inside the central regions of the cluster is dominated by a blend of discrete sources at 1.4 GHz. From Chandra X-ray space telescope observations, researchers detected traces of large-scale and high temperature shock fronts measuring ~ 2 arcmin or ~430 kpc from the cluster core. Looking at observed temperature and density jumps, they found this led to Mach numbers of >1.3 with an estimated shock velocity of ~2500 | $\rm \thinspace km\rm \thinspace s^{-1}\,$ |⁠. Surprisingly, they found that Abell 2219 also has a hot core which is >20 keV. What caused the hot core is unclear.
The cluster appears at a peak in velocity space, whom researchers selected 113 cluster members. They noted it by computing high values for the line-of-sight velocity dispersion, σv= 1438$^{+109}$$_{-86}$ km s$^{−1}$, consistent with the high average X-ray temperature of 10.3 keV. Assuming dynamical equilibrium, the cluster is estimated to be M~2.8× 1015 M⊙ global masses.

## Gravitational lensing

Abell 2219 has two gravitational lensing arcs, as observed by the Infrared Space Observatory at 14.3 μm (hereafter 15 μm). Through these observations, mid-infrared sources are identified in three galaxy cluster members, three foreground galaxies, an Extremely Red Object, a star and two galaxies located at unknown redshift. According to researchers, they obtained a best-fit model and found the value suits within star-forming galaxies.
The infrared luminosities in three cluster members are between ~5.7 x 10$^{10}$ L○ and 1.4 x 10$^{11}$ L○, corresponding to infrared star formation rates between 10 and 24 M○ yr$^{−1}$. For the two cluster galaxies, researchers noted they have with optical classifications are found in the Butcher-Oemler region of the color–magnitude diagrams. The three galaxies in the foreground, whom they found, contain infrared luminosities between the range of 1.5 x 10$^{10}$ L○ and 9.4 x 10$^{10}$ L○. These galaxies have infrared star formation rates of around 3 to 16 M○ yr$^{−1}$. Researchers suggested three clusters contain luminous infrared galaxies (LIRGs). By detecting presence of LIRGs in Abell 2219, this strengthens the bond between them and ongoing cluster merger activity occurring.
A gravitational signal was also detected by researchers who observed at near-infrared wavelengths based on images taken through the 4.2m William Herschel Telescope. This suggests an Einstein radius of θE ~= 13.7$^{+3.9}$$_{-4.2}$ arcsec, 66 percent over the confidence limit. Researchers noted the gravitational signal corresponds to projected velocity dispersion of σv~800 km$^{−1}$, in agreement with constraints from strongly lensed features. They found it has a best-fitting halo scalelength of 125h$^{−1}$ kpc.