Arp's Catalog Of Peculiar Galaxies
An Exploration For Observers Compiled By Observers - Anyone Can See The Arps!

Arp 271, NGC 5426 & 5427, 1966 by Halton Arp, made with the finest photographic emulsions available at the time using the 200-inch Hale Telescope.

Arp 271, NGC 5426 &5427, 2003 by Dennis Webb using a homemade Cookbook 245 camera and an Celestron 8-inch Schmidt-Cassegrain telescope.

Halton Arp, photo by Jeff Rowe, copyright 2005

Arp 271 by Halton Arp, 200-inch telescope, 1966

Arp 271 by Dennis Webb,  8-inch telescope,  2003

New on the Arp Page:  Hubble Space Telescope Images of Arps and APOD shows Arps

[Observing the Arps]     [Summary Data] 

This page illuminates the catalog of 338 peculiar galaxy views gathered by Dr. Halton C. Arp in his Atlas of Peculiar Galaxies. The Atlas is a selection of unusual or peculiar galaxies, interacting pairs or larger groups.

As amateur telescopes grow in capability and CCD cameras are becoming more sensitive and affordable, amateur astronomers can grasp the faint peculiar features of these remarkable views. This page provides data to support this amateur pursuit, offering convenient information and contemporary galaxy naming and characteristics.

Dr. Arp compiled the Atlas with photographs he made mainly using the Palomar 200-inch telescope and the 48-inch Schmidt between 1961 and 1966.  The Atlas was published in the Astrophysical Journal Supplement Number 123, Volume 14, November 1966, University of Chicago Press.  A small number of larger scale photographic prints of the Atlas were published in 1966 and 1978.  Caltech presents an online version of the Atlas, including Arp's original tabular data.

Dr. Arp plays a unique role in the debate on the nature of the universe in general and the galaxies in particular, often standing alone in his assessments of observations on the nature of observed redshifts and the cosmological implications.  He continues to research and publish, and is author or co-author of many papers in the journals.   For information about his current work, visit his webpage at 


Observing the Arp Peculiar Galaxies
The Astronomical League encourages observation of the Arps through their Arp Observer's Club, recognizing those who have visually observed or imaged 100 of the peculiar views, 70 individuals as of this writing. 

The Atlas images were acquired with the Hale and Schmidt telescopes of the Carnegie Observatories and best photographic emulsions of the time, presenting very high resolution of faint galactic details.  Using varying enlargements, Dr. Arp was able to uniformly present galaxies independent of their angular size.  He selected the fields to emphasize the features of interest and, in the process, created some remarkable compositions.  These attributes represent three challenges to the amateur observer:

  1. Wide range of field sizes - The Atlas offers 338 identically sized  rectangular images, presenting  field sizes ranging from just over a degree (Arp 318) down to 2.2 arcminutes, with several size groupings in between.  Two thirds of the image fields are 3.6 arcminutes or smaller.  (See figure illustrating most field sizes).  This characteristic is particularly challenging to CCD imagers because of the extreme range of focal lengths required.
  2. Faintness of the features of interest - While the 338 Arps include many bright familiar galaxies, most of the involved galaxies are magnitude 13 or fainter (see figure).  Further, the peculiar features highlighted in the Atlas pictures are often very faint extensions from the brighter usually observed parts of the galaxies.  Most of these features are accessible to CCD imaging, although many will be just above the background noise.  Skilled visual observers with large aperture will find some peculiarity in most.
  3. Framing - While many of the Atlas images are of one disturbed galaxy, most involve multiple galaxies.  It is not obvious from looking at a star chart, what objects and framing Dr. Arp used.  To fully observe what attracted Dr. Arp, one should be familiar with the original Atlas image.

Finding Arps to Observe
The 338 Arp fields range from familiar bright galaxies like M51 down to faint 17th magnitude nearly-anonymous fleaspecks.  Few starcharts or observing guides reference the Arp number, so to find most Arps, the observer must start with a reference that relates more commonly used galaxy names to the Arp number.  Some database programs and online sources answer the question "what is Arp 32?" but observing planning benefits from compact tables listing all of the Arps, their characteristics and locations.  As the galaxies become fainter, different sources will disagree about what galaxy has what name, which causes some confusion. 

Useful Summaries to Help Find Arps:
   Arps in other catalogs: Messier, Herschel 400, and Hickson.
   83 bright Arps (90 galaxies) in the Revised Shapley-Ames.
   All 338 Arp Fields in Arp sequence.
   All Galaxies involved in the 338 Arps by Right Ascension
   Arps Sorted by Constellation.

   Irregularities in identifying the galaxies involved in the Arps.

   Where The Arps Are, a plot of locations in RA and Dec.
   Magnitude Distribution of the brightest galaxies in each Arp.

Note - these data are from the 1999 dataset published on this page.  These data will be revised to be consistent with the Willmann-Bell Arp book, which improves the uniformity of naming and characteristics.

On the subject of older datasets, Nancy Roman recruited the 1996 dataset as file Catalog 7192 in NASA's Astronomical Data Center (ADC).  The Centre de Donn astronomiques de Strasbourg (CDS) now hosts these files as catalog VII/192. The CDS catalogs are mirrored in Japan, Russia, China, India, and the US.

Researchers interested in the Arps
John Hibbard at NRAO studies interacting galaxies:
     Lots of pictures of Arps     Colors of Tidal Tails      HI Rogues
Bill Keel at the University of Alabama studies galaxy interaction.
     Dr. Keel is a contributor to the Hubble Heritage collection.

Arp Dingbats

About this page
This page is the result of a personal quest by Dennis Webb to study and promote Halton Arp's list of peculiar galaxies. Members of the Johnson Space Center Astronomical Society, the Fort Bend Astronomical Club, and the Houston Astronomical Society supported the effort through analysis and data entry. 

This page first developed  in 1995, updated November 12, 2007.
Dennis Webb's Home Page  |  Dennis Webb's Email


 [Amateur Observations]     [References]

Books for Arp enthusiasts:

THe Arp Atlas of Peculiar Galaxies, A Chronicle and Observer's Guide Buy | Samples | Blog

Willmann-Bell's Arp Atlas of Peculiar Galaxies, A Chronicle and Observer's Guide by Dennis Webb and Jeff Kanipe features 400 pages of in-depth analysis of the 338 Arp fields, organized for the observer with a reproduction of the original Atlas, monochrome amateur CCD images of all, finder charts,  interpretive diagrams, visual observations, assessments of galaxy naming difficulties, and a thoroughly researched chronicle of Arp and his Atlas.
Observing the Arp Peculiar Galaxies

Buy | Samples

Alvin Huey's Observing the Arp Peculiar  Galaxies features naked eye finder charts and a detailed page for each of the 338 Arps with local finder chart, observing notes, labeled DSS image and an eyepiece rendition.  456 pages, spiral bound.

















Arp Dingbats

Selected Amateur Arp Observations
Arp observers share their Arp observations on their own webpages:

Visual observers' Arp narratives and sketches:
   Ray Cashs Arp observations
   Bob Hills Visual Arp Observations 
   Ron Muirs Flinthill Deep Sky Guide for the Arps:
   Martin Schoenball 's
   Bruce Scodovas visual Arp observations
   Jim Shields Arp Sampler (also Arp Galaxy Chains) 
CCD Arp images:
   Jim Burnell
   Paul and Liz Downing 
   Ch. Dupriez (en Frans)
   Ivar Hamberg
   Tim Hunter and James McGaha (all 338 Arps)
   Tracy Knauss and Mike Morton
   Richard Miller   (all 338 Arps)
   Maynard Pittendreigh (his separate home page)
   David Ratledge
   Bob Ross
   Dennis Webb
Spectacular amateur color CCD images of Arps
Selected images posted by hard-working astrophotographers:

Arps in your books and software:
Bright Star Atlas locates 20 of the brighter Arps (6%).
Herald-Bobroff ASTROATLAS locates about 210 Arps (63%).
Uranometria 2000.0 (1987) locates 202 Arps (60%).
Uranometria 2000.0 (2001) locates 303 Arps (90%).

Webb Society Deep Sky Observer's Handbook Vol 6, Galaxies
    provides information for visually observing over 70 Arp views.
    Out of print, it is dedicated to Dr. Arp who wrote the preface.
Burnham's Celestial Handbook lists galaxies for 82 Arps (24%).
Night Sky Observer's Guides describe galaxies for 106 Arps (30%).
The Arp Atlas of Peculiar Galaxies, Chronicle and Observing Guide
     offers detailed observational data for all 338 Arps (100%).
Alvin Huey's Observing the Arp Peculiar Galaxies
     offers detailed observational data for all 338 Arps (100%)

Carnegie Atlas of Galaxies (not in print but online) presents 77 Arps.
Vickers' Deep Space CCD Atlas: North (1994) shows 153 Arps.
     Supplementary Arp Index available in HTML or PDF.
     Mr. Vickers offers a later edition but this table may still be usable.
Willmann-Bell's MegaStar and HyperSky include Arp lists (Win).
Starry Night includes the Arp list (Win/Mac).
TheSky Professional edition includes the Arps (Win/Mac).
SkyMap offers an Arp file (Win).
Bartels Scope-Drive offers an Arp data file at their Yahoo Group.
Argo-Navis offers an Arp data file at their Yahoo Group.

Hubble Space Telescope Images of Arps:

       The story of Hubble Heritage's image of Arp 87.