Arp 220

Arp 220
Name	Arp 220
Type	Merger
Epoch	J2000
Redshift	0.018
Dist ly	250 million
Constellation name	Serpens Caput
Names	UGC 09913, IC 4553

Arp 220 is a nearby ultra-luminous infrared galaxy resulting from a major merger in Serpens Caput. It is one of the nearest and best-studied examples of a merger-driven extreme starburst system and a laboratory for studying galaxy merger evolution, active galactic nucleus activity, and compact star formation. Observations across the electromagnetic spectrum have linked Arp 220 to phenomena studied in Hubble Space Telescope imaging, Atacama Large Millimeter/submillimeter Array mapping, and high-resolution radio interferometry.

Overview

Arp 220 is cataloged in the Atlas of Peculiar Galaxies compiled by Halton Arp and is identified with entries in the Uppsala General Catalogue and the Index Catalogue. The system lies at a distance used in studies comparing local mergers to high-redshift submillimeter galaxy populations and luminous infrared systems such as NGC 6240 and Mrk 231. As a prototypical ultraluminous infrared galaxy, it has been central to work by teams using facilities like Keck Observatory, Very Large Array, Very Long Baseline Array, and the Spitzer Space Telescope.

Structure and Morphology

Arp 220 exhibits a disturbed morphology with dual nuclei embedded in a common envelope, tidal tails, and dust lanes visible in optical imaging by Hubble Space Telescope and ground-based observatories such as Palomar Observatory. Near-infrared adaptive optics observations from Keck Observatory and Gemini Observatory reveal compact stellar concentrations and nuclear disks, while millimeter interferometry from ALMA and the IRAM Plateau de Bure Interferometer resolves dense molecular concentrations. The system is compared morphologically to late-stage mergers like NGC 3256 and Centaurus A in studies of merger remnants.

Starburst Activity and Star Formation

The nuclei host an intense starburst with star-formation rates inferred from far-infrared luminosity comparable to those in starburst galaxy prototypes such as M82 and NGC 253. Infrared photometry from IRAS and spectroscopy from Spitzer Space Telescope and Herschel Space Observatory demonstrate that reprocessed starlight by dust dominates the bolometric output, while near-infrared observations link to young super star clusters observed in systems like Antennae Galaxies. Stellar population analyses reference work on supernova-rich environments exemplified by studies of SN 1993J and radio supernovae in compact starbursts.

Active Galactic Nucleus and Black Holes

Debate over the presence and energetics of one or more obscured active galactic nucleus candidates in Arp 220 involves comparisons with composite systems such as NGC 1068, Circinus Galaxy, and Mrk 231. Hard X-ray constraints from Chandra X-ray Observatory and XMM-Newton and high-resolution infrared spectroscopy from Keck Observatory inform models invoking deeply buried supermassive black hole accretion, sometimes contrasted with pure starburst interpretations used in analyses of Seyfert galaxy populations.

Molecular Gas, Dust, and Infrared Emission

Arp 220 contains extreme concentrations of molecular gas traced by CO and dense tracers (HCN, HCO+) mapped by ALMA, NOEMA, and SMA. Dust continuum observations at submillimeter wavelengths by James Clerk Maxwell Telescope and Herschel Space Observatory reveal high optical depths and compact warm dust components, analogous in some respects to the compact obscured nuclei in NGC 4418. Spectroscopic detections of water masers and molecular absorption lines link to studies of molecular chemistry in environments explored around Orion KL and in the Galactic Center.

Radio Emission and Supernovae

High-resolution radio imaging with the Very Long Baseline Array, European VLBI Network, and Very Large Array has resolved dozens of compact radio sources interpreted as radio supernovae and supernova remnants, drawing parallels to the radio source populations in M82 and radio-bright starbursts. Radio spectral indices, free–free absorption, and synchrotron measurements are compared to models developed for compact starburst nuclei and radio supernova catalogs including objects like SN 1986J.

Kinematics and Dynamics

Kinematic mapping using CO, HI, and ionized gas lines from instruments on ALMA, VLA, and optical integral field units on Keck Observatory and Very Large Telescope have characterized rotating nuclear disks, inflows, and outflows. Dynamical mass estimates reference work on merger-driven central concentration in systems like NGC 7252 and simulations by groups employing codes tested against observations of Antennae Galaxies and numerical frameworks used in studies of Toomre sequence mergers.

Observations and Multiwavelength Studies

Arp 220 has been observed from radio through X-ray energies by facilities including VLBA, VLA, ALMA, IRAM, Spitzer Space Telescope, Herschel Space Observatory, Chandra X-ray Observatory, and XMM-Newton. Multiwavelength campaigns integrate spectroscopy, photometry, and interferometry to disentangle contributions from star formation, dust reprocessing, and any hidden active galactic nucleus, drawing methodological connections to surveys like the Great Observatories All-sky LIRG Survey and comparative analyses with high-redshift submillimeter galaxy samples identified in surveys by SCUBA and Herschel.

Category:Interacting galaxies Category:Ultraluminous infrared galaxies