Draft:IC 4965

IC 4965
IC 4965 imaged with the Dark Energy Camera on the Víctor M. Blanco Telescope. Diffuse stellar light surrounds the galaxy.
Observation data (J2000 epoch)
Constellation	Pavo
Right ascension	20h 12m 27.3517s
Declination	−56° 49′ 36.177″
Redshift	0.055789±0.000133
Heliocentric radial velocity	16690±38 km/s
Group or cluster	Abell 3667
Apparent magnitude (B)	14.42
Characteristics
Type	S0; D/cD; radio galaxy
Apparent size (V)	1.36′ × 1.09′
Notable features	Brightest cluster galaxy of Abell 3667
Other designations
PGC 64272, LEDA 64272, ESO 186-11, ESO-LV 186-0110, 2MASX J20122726−5649363, WINGS J201227.32−564936.3, 6dFGS gJ201227.3−564936, MRC B2007−569, AM 2008−565

IC 4965, also known as PGC 64272, is a luminous early-type galaxy in the constellation Pavo. It is the brightest cluster galaxy of the massive merging galaxy cluster Abell 3667. Catalogues have classified it as a lenticular galaxy, a D or cD galaxy, and a radio galaxy.

The galaxy has a redshift of approximately 0.05567. Under a flat cosmological model with $H_{0}=70$ km s⁻¹ Mpc⁻¹, $\Omega _{\mathrm {m} }=0.3$, and $\Omega _{\Lambda }=0.7$, this corresponds to a luminosity distance of approximately 249 megaparsecs (810 megalight-years). Its observed dimensions correspond to a projected optical extent of roughly 88 by 71 kiloparsecs (290,000 by 230,000 light-years).

Deep optical observations have revealed an extended envelope of intracluster light around IC 4965 and a low-surface-brightness bridge connecting it with the second principal galaxy of the merging system, LEDA 64210. The bridge extends approximately 400 kiloparsecs (1,300,000 light-years) and is interpreted as stellar material removed during a recent close passage between the two galaxies.

Identification and measurements

IC 4965 is located at the J2000 equatorial coordinates

$\alpha =20^{\mathrm {h} }12^{\mathrm {m} }27.33^{\mathrm {s} },$

$\delta =-56^{\circ }49'36.4''.$

Its Galactic coordinates are approximately

$l=340.868^{\circ },\qquad b=-33.384^{\circ }.$

The galaxy lies near the optical and X-ray center adopted for the principal component of Abell 3667. The precise separation from the cluster center depends on whether the center is defined using the X-ray peak, X-ray centroid, galaxy-density peak, or dominant mass concentration.

Selected observed properties of IC 4965

Property	Value	Source
Right ascension	20h 12m 27.33s
Declination	−56° 49′ 36.4″
Redshift	$0.05567\pm 0.00013$
Recession parameter, $cz$	$16,690\pm 38$ km/s
Apparent B-band magnitude	14.42
Apparent angular dimensions	1.36′ × 1.09′
Position angle	Approximately 134°
Absolute K-band magnitude	Approximately −25.8
Host cluster	Abell 3667

Redshift and distance

Cosmological redshift is defined by

$1+z={\frac {\lambda _{\mathrm {observed} }}{\lambda _{\mathrm {emitted} }}},$

where $\lambda _{\mathrm {observed} }$ is the observed wavelength and $\lambda _{\mathrm {emitted} }$ is the wavelength measured in the emitting galaxy's rest frame.

For a low-redshift object, the approximate recession parameter is

$cz\approx (299\,792.458\ \mathrm {km\,s^{-1}} )(0.05567)\approx 16\,690\ \mathrm {km\,s^{-1}} .$

The relation between angular extent and projected physical size is

$D=\theta D_{\mathrm {A} },$

where $D$ is the transverse physical size, $\theta$ is the angular size in radians, and $D_{\mathrm {A} }$ is the angular-diameter distance. At the redshift of IC 4965, one arcsecond corresponds to approximately 1.08 kpc under the adopted cosmology.

Morphology and luminosity

IC 4965 is an early-type galaxy with a smooth stellar distribution and no prominent spiral arms. The ESO–LV catalogue assigns it a numerical morphological type corresponding to a lenticular or elliptical system, while studies of brightest cluster members describe it as a D/cD galaxy.

A D galaxy is a luminous elliptical-like system surrounded by an extended stellar envelope. The related cD classification is generally applied when the envelope is exceptionally broad and merges gradually with the surrounding intracluster light. In IC 4965, the distinction between the galaxy's outer halo and the diffuse stellar component of Abell 3667 becomes increasingly uncertain at low surface brightness.

Its absolute near-infrared magnitude of approximately

$M_{K}=-25.8$

places it among the most luminous galaxies in the cluster. Near-infrared light is less affected by dust than blue optical light and is commonly used as an approximate tracer of a galaxy's accumulated stellar mass.

The relation between absolute magnitude and luminosity is

${\frac {L_{K}}{L_{K,\odot }}}=10^{-0.4(M_{K}-M_{K,\odot })},$

where $M_{K,\odot }$ is the absolute K-band magnitude of the Sun. The result depends on the adopted photometric system and solar reference value.

Selected regions of Abell 3667 observed with DECam. The panel containing IC 4965 shows the brightest cluster galaxy, its surrounding diffuse light, and nearby members of the infalling group.

The broad stellar envelope surrounding IC 4965 is not completely symmetric. Deep imaging shows an extended plume toward the southeast, enclosing or overlapping the projected position of the nearby galaxy LEDA 64290. Such diffuse structures are associated with tidal stripping and the redistribution of stars during the assembly of brightest cluster galaxies.

Brightest galaxy of Abell 3667

Abell 3667 is a massive, dynamically disturbed cluster at a mean redshift of approximately 0.0553. Spectroscopic observations identified 550 member galaxies and measured a line-of-sight velocity dispersion of

$\sigma _{\mathrm {cluster} }=1056\pm 38\ \mathrm {km\,s^{-1}} .$

The cluster contains two principal galaxy concentrations with mean velocities separated by approximately 500 km/s. Their spatial distribution, together with the disturbed X-ray-emitting intracluster medium, indicates an ongoing merger between two major subclusters.

Labeled deep optical image of Abell 3667. IC 4965 is associated with the principal concentration of galaxies and diffuse intracluster light.

IC 4965 is the dominant galaxy of the primary component. A study of brightest cluster members classified it as a D/cD system and measured a peculiar velocity of approximately 84 km/s relative to the adopted cluster velocity.

The rest-frame peculiar velocity of a cluster galaxy may be written as

$v_{\mathrm {pec} }={\frac {v_{\mathrm {galaxy} }-v_{\mathrm {cluster} }}{1+z_{\mathrm {cluster} }}}.$

For IC 4965, the ratio between peculiar velocity and cluster velocity dispersion is approximately

${\frac {|v_{\mathrm {pec} }|}{\sigma _{\mathrm {cluster} }}}\approx 0.09.$

Its comparatively small line-of-sight velocity offset is consistent with its position as the dominant galaxy of the primary subcluster. It does not imply that Abell 3667 as a whole is dynamically relaxed.

Intracluster light

Intracluster light consists of stars gravitationally bound to the cluster rather than to a single galaxy. It is produced principally through tidal stripping, galaxy interactions, and mergers.

Deep observations of Abell 3667 were obtained with the Dark Energy Camera on the Víctor M. Blanco Telescope. More than 28 hours of observations were combined to reach a depth comparable to several years of imaging planned for the Vera C. Rubin Observatory Legacy Survey of Space and Time.

The extended light surrounding IC 4965 contains a negative color gradient, with the outer regions becoming progressively bluer. This pattern is consistent with the gradual incorporation of stars stripped from less massive satellite galaxies.

Surface brightness in magnitudes per square arcsecond can be written as

$\mu =m+2.5\log _{10}A,$

where:

• $m$ is the integrated apparent magnitude; and
• $A$ is the measured angular area in square arcseconds.

Because the magnitude scale is logarithmic, a greater numerical value of $\mu$ represents fainter emission.

Stellar bridge

A diffuse optical bridge extends approximately 400 kpc between IC 4965 and LEDA 64210, the dominant galaxy of the second merging component. Its surface brightness is generally fainter than

$\mu \gtrsim 26\ \mathrm {mag\,arcsec^{-2}} .$

The bridge has a broad, comparatively uniform color. Its stellar population resembles the disrupted outer regions of LEDA 64210 more closely than the inner regions of either dominant galaxy. It is therefore interpreted as material removed principally from LEDA 64210 during a recent close passage and now being incorporated into the intracluster component around IC 4965.

Deep DECam image of Abell 3667. The faint yellowish light across the center includes the extended envelopes of its dominant galaxies and the stellar bridge connecting the merging components.

The light profiles indicate two concurrent growth mechanisms:

• gradual stripping of stars from lower-mass satellite galaxies, visible in the inner extended envelopes; and
• a more recent major interaction, represented by the disrupted outer profile of LEDA 64210 and the stellar bridge.

The bridge is interpreted as evidence that the two dominant galaxies have completed a recent first passage rather than having fully merged.

Radio source

IC 4965 is associated with the radio source MRC B2007−569. Radio observations classify it as a head–tail radio galaxy, in which synchrotron-emitting plasma from the active nucleus is displaced into a curved or trailing structure by motion relative to the surrounding intracluster medium.

Radio synchrotron emission is commonly described by

$S_{\nu }\propto \nu ^{\alpha },$

where $S_{\nu }$ is the measured flux density at frequency $\nu$ and $\alpha$ is the spectral index.

Observations with the Karoo Array Telescope detected the radio emission from IC 4965 together with the large northwestern and southeastern radio relics of Abell 3667. After subtracting the compact and extended emission attributed to IC 4965, a localized diffuse excess remained around the cluster center.

The excess was identified as a possible radio mini-halo with a flux density of

$S_{1.37\,\mathrm {GHz} }=67.2\pm 4.9\ \mathrm {mJy} ,$

and an estimated 1.4-GHz radio power of

$P_{1.4\,\mathrm {GHz} }=(4.28\pm 0.31)\times 10^{23}\ \mathrm {W\,Hz^{-1}} .$

The diffuse component is distinct from the head-tail radio source associated directly with IC 4965.

Radio and X-ray view of Abell 3667. IC 4965 and its radio source lie in the central cluster region between the two large peripheral radio relics.

The radio morphology of IC 4965 is affected by the dense, magnetized intracluster plasma. The larger radio structures of Abell 3667 trace shock waves, turbulence, and particle acceleration produced by the cluster merger rather than activity within IC 4965 alone.

Discovery and cataloguing

IC 4965 was photographed by American astronomer Royal Harwood Frost at the Harvard College Observatory's Boyden Station in Arequipa, Peru, on 17 May 1904.

Frost used the 24-inch Bruce photographic telescope to survey southern nebulae and star clusters. The object was later included in the second Index Catalogue published by John Louis Emil Dreyer in 1908.

Subsequent surveys assigned numerous catalogue designations:

Catalogue	Designation
Index Catalogue	IC 4965
Principal Galaxies Catalogue	PGC 64272
Lyon–Meudon Extragalactic Database	LEDA 64272
European Southern Observatory catalogue	ESO 186-11
ESO–LV catalogue	ESO-LV 186-0110
Two Micron All-Sky Survey	2MASX J20122726−5649363
WINGS survey	WINGS J201227.32−564936.3
6dF Galaxy Survey	6dFGS gJ201227.3−564936
Molonglo Reference Catalogue	MRC B2007−569

Observation history

Photographic surveys established the galaxy's position, apparent size, and early-type morphology. Optical and near-infrared surveys later measured its luminosity and identified it as the brightest galaxy of Abell 3667.

Spectroscopic surveys measured its redshift and velocity relative to the cluster. These observations placed IC 4965 within the principal galaxy concentration identified in the cluster's two-component velocity structure.

Radio observations identified the active head-tail source MRC B2007−569 and separated its emission from the larger-scale radio halo and relic structures of Abell 3667.

The 2025 DECam observations reached substantially fainter surface-brightness levels than previous wide-field optical imaging. They resolved the extended stellar envelope, southeastern plume, and 400-kpc intracluster bridge associated with the ongoing merger.

Scientific importance

IC 4965 provides an observational example of brightest-cluster-galaxy growth in a dynamically active environment. Its properties connect several components of cluster evolution:

• the assembly of an extended stellar envelope through disrupted satellites;
• the transfer of stars between two dominant galaxies;
• the formation of intracluster light during a major merger;
• the orbital motion of a brightest cluster galaxy relative to its host cluster;
• interaction between an active radio source and the intracluster medium; and
• the eventual combination of the two principal galaxy concentrations in Abell 3667.

The low peculiar velocity of IC 4965 indicates that it is closely associated with the primary subcluster, while the stellar bridge records an interaction with the dominant galaxy of the secondary component. Its head-tail radio morphology traces the response of relativistic plasma to motion and pressure within the merging cluster atmosphere.

The bridge also permits the stellar component of the merger to be compared with independent X-ray and radio observations. The diffuse stars, hot plasma, galaxies, radio relics, and dark-matter concentrations respond differently during a cluster collision and preserve complementary evidence of its geometry and timing.

See also

• Abell 3667
• Brightest cluster galaxy
• cD galaxy
• Galaxy cluster
• Galaxy cluster merger
• Head–tail radio galaxy
• Intracluster light
• Intracluster medium
• Lenticular galaxy
• Radio galaxy

Further reading

• Lin, Yen-Ting; Mohr, Joseph J. (December 2004). "K-band properties of galaxy clusters and groups: Brightest cluster galaxies and intracluster light". The Astrophysical Journal. 617 (2): 879–895. arXiv:astro-ph/0408557. Bibcode:2004ApJ...617..879L. doi:10.1086/425412.

• de Gasperin, F.; Rudnick, L.; Finoguenov, A.; Wittor, D.; et al. (March 2022). "MeerKAT view of the diffuse radio sources in Abell 3667 and their interactions with the thermal plasma". Astronomy & Astrophysics. 659 A146. arXiv:2111.06940. Bibcode:2022A&A...659A.146D. doi:10.1051/0004-6361/202142658.

• Vikhlinin, Alexey; Markevitch, Maxim; Murray, Stephen S. (April 2001). "A moving cold front in the intergalactic medium of A3667". The Astrophysical Journal. 551 (1): 160–171. arXiv:astro-ph/0008496. Bibcode:2001ApJ...551..160V. doi:10.1086/320078.

External links

• IC 4965 at SIMBAD
• Images and media related to IC 4965 at Wikimedia Commons
• Deep DECam images of IC 4965 and Abell 3667 at NSF NOIRLab
• Intracluster-light study of Abell 3667 at the Astrophysics Data System
• Dynamical study of brightest cluster galaxies at the Astrophysics Data System

4965 64272 Category:Lenticular galaxies Category:Radio galaxies Category:Pavo (constellation) Category:Astronomical objects discovered in 1904 Category:Discoveries by Royal Harwood Frost