

[Observing the Arps] [Summary Data]

Introduction

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 www.haltonarp.com.

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:

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. 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. 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.

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